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doi: 10.3878/j.issn.1006-9895.2012.20186
Abstract:
A return radiosonde trajectory forecast method based on high resolution numerical weather prediction modeling is proposed in this paper. The trajectory forecast method is intended to be used for simulating the return radiosonde network observations and supporting evaluations of the observation impacts on numerical weather predictions. Meanwhile, the trajectory forecast system will also in plan to be used for supporting the return radiosonde application in target observation experiments in the future. A return radiosonde trajectory forecast system is established by embedding return radiosonde’s trajectory equations of ascent stage, drift stage and descend stage into numerical weather prediction model GRAPES (Global/Regional Analysis and PrEdiction System), a regional model with a 3 km horizontal resolution. A dynamic equation of parachutes in descend stage is also considered in this system. By using the trajectory prediction system, the preliminary test of the return sounding trajectory prediction is carried out for 63 successful real observations of return radiosondes. The results show that the forecast trajectories using this trajectory forecast system are reasonable and reliable, with the average error of track prediction less than 40 km in 6 hours.
A return radiosonde trajectory forecast method based on high resolution numerical weather prediction modeling is proposed in this paper. The trajectory forecast method is intended to be used for simulating the return radiosonde network observations and supporting evaluations of the observation impacts on numerical weather predictions. Meanwhile, the trajectory forecast system will also in plan to be used for supporting the return radiosonde application in target observation experiments in the future. A return radiosonde trajectory forecast system is established by embedding return radiosonde’s trajectory equations of ascent stage, drift stage and descend stage into numerical weather prediction model GRAPES (Global/Regional Analysis and PrEdiction System), a regional model with a 3 km horizontal resolution. A dynamic equation of parachutes in descend stage is also considered in this system. By using the trajectory prediction system, the preliminary test of the return sounding trajectory prediction is carried out for 63 successful real observations of return radiosondes. The results show that the forecast trajectories using this trajectory forecast system are reasonable and reliable, with the average error of track prediction less than 40 km in 6 hours.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2010.20187
Abstract:
Based on the T799L91 global numerical weather forecast model and its four-dimensional variational assimilation system, the assimilation process of Yunhai-2 occultation data was constructed, and a one-month assimilation-forecast experiment for Yunhai-2 occultation data was carried out in July 2019. The evaluation of the assimilation forecast results for East Asia, Northern Hemisphere and Southern Hemisphere shows that: In the global numerical weather forecast model, assimilating Yunhai-2 occultation data can effectively improve the accuracy of the forecast field. With the extension of the forecast time and the increase of the forecast days, the improvement range gradually increases. With the longer time efforts of the 72-hour, 120-hour and 168-hour forecasting, the assimilation effects of Yunhai-2 occultation data and GPS (Global Positioning System) occultation data are similar in the middle and early days, while the assimilation of Yunhai-2 occultation data is gradually better than GPS occultation data in the middle and later days. With the increase of the forecasting days, the advantage of the joint assimilation of Yunhai-2 occultation data with GPS occultation data gradually appears.
Based on the T799L91 global numerical weather forecast model and its four-dimensional variational assimilation system, the assimilation process of Yunhai-2 occultation data was constructed, and a one-month assimilation-forecast experiment for Yunhai-2 occultation data was carried out in July 2019. The evaluation of the assimilation forecast results for East Asia, Northern Hemisphere and Southern Hemisphere shows that: In the global numerical weather forecast model, assimilating Yunhai-2 occultation data can effectively improve the accuracy of the forecast field. With the extension of the forecast time and the increase of the forecast days, the improvement range gradually increases. With the longer time efforts of the 72-hour, 120-hour and 168-hour forecasting, the assimilation effects of Yunhai-2 occultation data and GPS (Global Positioning System) occultation data are similar in the middle and early days, while the assimilation of Yunhai-2 occultation data is gradually better than GPS occultation data in the middle and later days. With the increase of the forecasting days, the advantage of the joint assimilation of Yunhai-2 occultation data with GPS occultation data gradually appears.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2005.20141
Abstract:
In this paper, we extensively examine the impacts of El Niño events on boreal summer rainfall over the East Asian Monsoon and South Asian Monsoon (SAM) regions and their associated mechanisms. To date, the various impacts of an El Niño event on the Tibetan Plateau (TP) regional seasonal and monthly rainfall and circulation have not been systematically examined. Based on the timing of the El Niño decay with respect to the boreal summer season and 1950–2018 sea surface temperature (SST) data, the El Niño decay phases are classified into two types: (1) early decay and (2) late decay. If the El Niño decays to below the threshold before spring, a La Niña sea surface temperature anomaly (SSTA) pattern usually develops during summer with increasing anomaly amplitudes from June to September. This causes an enhanced westward shift in the Walker circulation with a strong ascending branch over the tropical Indian Ocean (TIO) and the SAM, and induces concurrent heavy rainfall over the SAM and southwestern TP areas from July to September. Meanwhile, the developing La Niña SSTA forces a response by the anomalous North Western Pacific anticyclone (NWPAC), an anomalous cyclonic circulation over the Arabian Sea, and anticyclonic circulation over the Western Asian region. These induce a strengthening southerly wind anomaly, enhance water vapor transport to the Indian and TP regions from the TIO, and thus increase summer precipitation over northern India and the southwestern TP. In contrast, if El Niño decays below the threshold after September, the eastern Pacific El Niño SSTA pattern and the strong SST warming over the TIO persists into June to July, then gradually weakens from August to September. This causes an anomalous ascending branch of the Walker circulation over the eastern TIO with a weak ascending branch over the western TIO and SAM, an anomalous eastwardly extended NWPAC, an anomalous anticyclonic circulation over the Arabian Sea, and cyclonic circulation over the Western and Central Asian region, which induce a strengthening westerly wind anomaly and reduces water vapor transport over the TP. The above responses result in deficient rainfall and warm surface temperatures in the central and northern SAM regions in June, but relatively increased rainfall and cool surface air temperature over most of the SAM region during August and September. This coincides with dryness over northeastern India and the southwestern TP in June, and then increasing precipitation over northwestern India and the western TP in September. Our results confirm that a decaying El Niño has a significant impact on summer seasonal and monthly precipitation and temperature over the TP, which may explain the positive correlation between Indian and southwestern TP precipitation recently discussed in some studies. Our results also suggest that differences in the El Niño decay phase have strong impacts on the seasonal and intraseasonal rainfall over the SAM region and the southwestern TP.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2005.20120
Abstract:
In this paper, the existing criteria of EOF (Empirical Orthogonal Function) modes temporal stability are improved, and a scheme for electing stable high correlation prediction signals is proposed. Moreover, the temporal stability of EOF modes, the temporal and spatial characteristics, and the key signals to predictable modes of summer precipitation anomaly percentage from 1981 to 2016 in Southwest China are analyzed. Hence, a multi-factor descending dimension prediction model is established. The results show that the first nine modes are stable within 3 years lead and in nearly 10 years for climate prediction. They account for nearly 70% of the variance contribution rate and primary to summer precipitation anomaly percentage in Southwest China. The optimal prediction signals and the corresponding equations for primary PCs (Principal Component) are selected and built with the stable high correlation concept and the optimal subset regression method. These equations are recognized for having good capabilities for PC fitting. Their complex correlation coefficients range from 0.62 to 0.84, all passing the significance test of 99.99%, and their sign coincidence rates are greater than 69%. The prediction model built on that basis has a good hindcast skill for the spatial distribution, variation trend, and abnormal level of summer precipitation in Southwest China. Its mean ACC (Anomaly Correlation Coefficient) score is 0.58. The TCCs (Temporal Correlation Coefficient) pass the 90% significance test in the whole region except for sporadic stations and pass the 99.9% significance test in most areas. While the mean PS (Prediction Score) score is 84, the mean PS score is 87.1 for precipitation in the most abnormal 10 years. According to forecast test in 13 years (1971–1980 and 2017–2019), this model’s mean PS score is 72, and the mean PS score is 77, which is higher than that of published forecasts in 2017–2019.
In this paper, the existing criteria of EOF (Empirical Orthogonal Function) modes temporal stability are improved, and a scheme for electing stable high correlation prediction signals is proposed. Moreover, the temporal stability of EOF modes, the temporal and spatial characteristics, and the key signals to predictable modes of summer precipitation anomaly percentage from 1981 to 2016 in Southwest China are analyzed. Hence, a multi-factor descending dimension prediction model is established. The results show that the first nine modes are stable within 3 years lead and in nearly 10 years for climate prediction. They account for nearly 70% of the variance contribution rate and primary to summer precipitation anomaly percentage in Southwest China. The optimal prediction signals and the corresponding equations for primary PCs (Principal Component) are selected and built with the stable high correlation concept and the optimal subset regression method. These equations are recognized for having good capabilities for PC fitting. Their complex correlation coefficients range from 0.62 to 0.84, all passing the significance test of 99.99%, and their sign coincidence rates are greater than 69%. The prediction model built on that basis has a good hindcast skill for the spatial distribution, variation trend, and abnormal level of summer precipitation in Southwest China. Its mean ACC (Anomaly Correlation Coefficient) score is 0.58. The TCCs (Temporal Correlation Coefficient) pass the 90% significance test in the whole region except for sporadic stations and pass the 99.9% significance test in most areas. While the mean PS (Prediction Score) score is 84, the mean PS score is 87.1 for precipitation in the most abnormal 10 years. According to forecast test in 13 years (1971–1980 and 2017–2019), this model’s mean PS score is 72, and the mean PS score is 77, which is higher than that of published forecasts in 2017–2019.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2008.20134
Abstract:
Ground-based microwave radiometers (MWRs) have been widely used in recent years due to the fact that they provide atmospheric temperature and humidity profiles with high temporal resolutions. The quality of the multi-channel brightness temperature (TB) is the measure used to determine the quality of retrieved atmospheric profile products. In general, periodic absolute calibration using liquid nitrogen best maintains the quality of TB observations, but this operation is complex and difficult. As an auxiliary tool, radiative transfer model can be used to determine the TB quality of MWRs. Using observations from the Beijing radiosonde and two RPG MWRs located at the Beijing Observatory (GXT) in Beijing and the Xianghe site (XH) in Hebei, we evaluated three radiative transfer models—MonoRTM, ARTS, and MWRT—by comparing their simulations with those of the corresponding TBs observed at both sites. The results show that for most of the 14 MWR channels the simulations of the three models are highly consistent with the observed TBs (i.e., correlation coefficients up to 0.99), but for the temperature channels ch8 (51.26 GHz) and ch9 (52.28 GHz), there was a significant absolute deviation (approximately 4–5 K) between the simulated and observed TBs, and the correlation coefficient decreased significantly (<0.80), which indicates that the model simulation at these two channels must be improved. Of the three models, MonoRTM showed obvious systematic deviation at temperature channels ch8, ch9, and ch10 (53.86 GHz), particularly at ch8, which had a bias of up to 5K. The ARTS model generated the worst simulation at the water vapor channel ch1 (22.24 GHz). The MWRT simulations were relatively more stable and closer to the corresponding TB observations at 14 channels, with the least systematic deviation. In addition, the locations of the radiosonde measurements differed from that of the MWR site, which had a significant impact on the simulations for the MWR’s water vapor channels. Comparisons of the observed and simulation TBs at both sites indicates that the observation quality of the water vapor channels for the MWR at GXT must be improved.
Ground-based microwave radiometers (MWRs) have been widely used in recent years due to the fact that they provide atmospheric temperature and humidity profiles with high temporal resolutions. The quality of the multi-channel brightness temperature (TB) is the measure used to determine the quality of retrieved atmospheric profile products. In general, periodic absolute calibration using liquid nitrogen best maintains the quality of TB observations, but this operation is complex and difficult. As an auxiliary tool, radiative transfer model can be used to determine the TB quality of MWRs. Using observations from the Beijing radiosonde and two RPG MWRs located at the Beijing Observatory (GXT) in Beijing and the Xianghe site (XH) in Hebei, we evaluated three radiative transfer models—MonoRTM, ARTS, and MWRT—by comparing their simulations with those of the corresponding TBs observed at both sites. The results show that for most of the 14 MWR channels the simulations of the three models are highly consistent with the observed TBs (i.e., correlation coefficients up to 0.99), but for the temperature channels ch8 (51.26 GHz) and ch9 (52.28 GHz), there was a significant absolute deviation (approximately 4–5 K) between the simulated and observed TBs, and the correlation coefficient decreased significantly (<0.80), which indicates that the model simulation at these two channels must be improved. Of the three models, MonoRTM showed obvious systematic deviation at temperature channels ch8, ch9, and ch10 (53.86 GHz), particularly at ch8, which had a bias of up to 5K. The ARTS model generated the worst simulation at the water vapor channel ch1 (22.24 GHz). The MWRT simulations were relatively more stable and closer to the corresponding TB observations at 14 channels, with the least systematic deviation. In addition, the locations of the radiosonde measurements differed from that of the MWR site, which had a significant impact on the simulations for the MWR’s water vapor channels. Comparisons of the observed and simulation TBs at both sites indicates that the observation quality of the water vapor channels for the MWR at GXT must be improved.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2008.20117
Abstract:
The Changbai Mountains are located in the southeastern part of Jilin Province. Rainfall has a crucial effect on the forest ecosystem and water resource in the Changbai Mountains, which is a national key ecological function area. Based on the monthly precipitation data from 47 stations in Jilin Province and the ERA-Interim monthly reanalysis data from 1979 to 2016, we investigated the climate characteristics of summer precipitation at the Tianchi station in the Changbai Mountains and circulation anomalies associated with the interannual variation of rainfall at this station and compared the Tianchi station with the other stations in Jilin Province. Results show that summer precipitation at the Tianchi station and its interannual variation are approximately two times larger than that at other stations in Jilin Province. On one hand, the circulation anomalies associated with the interannual variation of precipitation at the Tianchi station are consistent with those for the entire Jilin Province. The circulation anomalies that favor more rainfall at the Tianchi station and the entire Jilin Province are characterized by a cyclonic anomaly over Northeast Asia and an enhanced East Asian jet stream in June and an enhanced western Pacific subtropical high and a northward-displaced East Asian jet stream in July and August. On the other hand, precipitation at the Tianchi station is associated with unique circulation anomalies, which are quite different or nearly opposite to precipitation in the entire Jilin Province, and can be attributed to the terrain of the Changbai Mountains. The results indicate that climate variability in the mountains can be related to the complicated and varying patterns of large-scale atmospheric circulation anomalies.
The Changbai Mountains are located in the southeastern part of Jilin Province. Rainfall has a crucial effect on the forest ecosystem and water resource in the Changbai Mountains, which is a national key ecological function area. Based on the monthly precipitation data from 47 stations in Jilin Province and the ERA-Interim monthly reanalysis data from 1979 to 2016, we investigated the climate characteristics of summer precipitation at the Tianchi station in the Changbai Mountains and circulation anomalies associated with the interannual variation of rainfall at this station and compared the Tianchi station with the other stations in Jilin Province. Results show that summer precipitation at the Tianchi station and its interannual variation are approximately two times larger than that at other stations in Jilin Province. On one hand, the circulation anomalies associated with the interannual variation of precipitation at the Tianchi station are consistent with those for the entire Jilin Province. The circulation anomalies that favor more rainfall at the Tianchi station and the entire Jilin Province are characterized by a cyclonic anomaly over Northeast Asia and an enhanced East Asian jet stream in June and an enhanced western Pacific subtropical high and a northward-displaced East Asian jet stream in July and August. On the other hand, precipitation at the Tianchi station is associated with unique circulation anomalies, which are quite different or nearly opposite to precipitation in the entire Jilin Province, and can be attributed to the terrain of the Changbai Mountains. The results indicate that climate variability in the mountains can be related to the complicated and varying patterns of large-scale atmospheric circulation anomalies.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2008.20146
Abstract:
In this study, we developed a regional ensemble prediction system [Global and Regional Assimilation and Prediction Enhanced System-Regional Ensemble Prediction System (GRAPES-REPS V3.0)] with a horizontal resolution of 10 km, which was put into operation by the Numerical Weather Prediction Center of the China Meteorological Administration in 2019. The background field of the GRAPES-REPS V3.0 is the GRAPES global model. The initial perturbation and model perturbation methods used are the ensemble transform Kalman filter and the stochastic perturbation parameterization tendencies, respectively. The system was run in real time for the 2019 summer season (July through September) and its results were compared with those obtained by the GRAPES-REPS V2.0 and ECMWF global ensemble prediction systems by statistical verification and case analysis to objectively and comprehensively evaluate its precipitation forecast skill and forecast uncertainties. We also analyzed the physical mechanism responsible for the forecast uncertainties regarding meso-scale intense precipitation. The study results, which provide a basis for diagnosing regional ensemble prediction systems and developing ensemble forecast methods, can be summarized as follows: (1) GRAPES-REPS V3.0 model obtained better precipitation Equitable Threat Scores (ETS) than the GRAPES-REPS V2.0 model in terms of the forecast lead times and rainfall classes with more equal rainfall members. The probability forecast fractions skill scores were also better. As such, the precipitation forecast skills of the GRAPES-REPS V3.0 model are better overall than those of the GRAPES-REPS V2.0 model. (2) The ensemble mean precipitation bias and ETS scores of the GRAPES-REPS V3.0 for light rain and rainstorm are better than those obtained by the ECMWF global ensemble forecast system, and the probability forecast skill of the two models is comparable. (3) The case studies show that different ensemble prediction systems capture precipitation forecast uncertainties by describing the uncertainties of meso-scale physical quantities. At an initial lead time, the circulation patterns of the GRAPES-REPS V3.0 regional ensemble prediction system and the ECMWF global ensemble prediction system are similar. However, with the evolution of the forecast lead time, the GRAPES-REPS V3.0 ensemble prediction model better describes the meso-scale dynamic and thermal fields, with more accurate rainfall locations and magnitudes and a better probabilistic forecast result. (4) Compared with the ECMWF model, the ensemble members of the GRAPES-REPS V3.0 model can effectively forecast the occurrence, development, and extinction of rainfall processes. As such, the GRAPES-REPS V3.0 model shows higher skill in forecasting precipitation during the Chinese flood season.
In this study, we developed a regional ensemble prediction system [Global and Regional Assimilation and Prediction Enhanced System-Regional Ensemble Prediction System (GRAPES-REPS V3.0)] with a horizontal resolution of 10 km, which was put into operation by the Numerical Weather Prediction Center of the China Meteorological Administration in 2019. The background field of the GRAPES-REPS V3.0 is the GRAPES global model. The initial perturbation and model perturbation methods used are the ensemble transform Kalman filter and the stochastic perturbation parameterization tendencies, respectively. The system was run in real time for the 2019 summer season (July through September) and its results were compared with those obtained by the GRAPES-REPS V2.0 and ECMWF global ensemble prediction systems by statistical verification and case analysis to objectively and comprehensively evaluate its precipitation forecast skill and forecast uncertainties. We also analyzed the physical mechanism responsible for the forecast uncertainties regarding meso-scale intense precipitation. The study results, which provide a basis for diagnosing regional ensemble prediction systems and developing ensemble forecast methods, can be summarized as follows: (1) GRAPES-REPS V3.0 model obtained better precipitation Equitable Threat Scores (ETS) than the GRAPES-REPS V2.0 model in terms of the forecast lead times and rainfall classes with more equal rainfall members. The probability forecast fractions skill scores were also better. As such, the precipitation forecast skills of the GRAPES-REPS V3.0 model are better overall than those of the GRAPES-REPS V2.0 model. (2) The ensemble mean precipitation bias and ETS scores of the GRAPES-REPS V3.0 for light rain and rainstorm are better than those obtained by the ECMWF global ensemble forecast system, and the probability forecast skill of the two models is comparable. (3) The case studies show that different ensemble prediction systems capture precipitation forecast uncertainties by describing the uncertainties of meso-scale physical quantities. At an initial lead time, the circulation patterns of the GRAPES-REPS V3.0 regional ensemble prediction system and the ECMWF global ensemble prediction system are similar. However, with the evolution of the forecast lead time, the GRAPES-REPS V3.0 ensemble prediction model better describes the meso-scale dynamic and thermal fields, with more accurate rainfall locations and magnitudes and a better probabilistic forecast result. (4) Compared with the ECMWF model, the ensemble members of the GRAPES-REPS V3.0 model can effectively forecast the occurrence, development, and extinction of rainfall processes. As such, the GRAPES-REPS V3.0 model shows higher skill in forecasting precipitation during the Chinese flood season.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2007.19236
Abstract:
Although the nonstaggered grid has better physical consistency than the staggered grid, it is still not widely used, mainly because of its poor accuracy for simulating the geostrophic adjustment process under the second-order accuracy difference scheme. However, for higher-order finite-difference schemes, the validity of the above conclusion is still unknown. In this paper, we present a theoretical analysis and a numerical test for shallow water equation under high-order difference schemes. The analysis and test results are as follows. (1) For a low wavenumber, the dispersion of a staggered grid does not change with the accuracy of the difference scheme, while the dispersion of nonstaggered grid is significantly improved, and the dispersion of both grids become very close under the fourth-order scheme. (2) The maximum frequency of nonstaggered grid still exists under high-order difference schemes, and it moves toward a higher wavenumber as the accuracy of the difference scheme increases. The frequency of the staggered grid monotonically increases with the wavenumber and gets closer to the real solution under high-order difference schemes. (3) When the high-frequency noise is removed by explicitly adding a diffusion term, the pros and cops to grid staggering choices diminish with high-order schemes. In general, the nonstaggered grid is an attractive choice for the discretization of the dynamical frame in numerical models under high-order difference scheme.
Although the nonstaggered grid has better physical consistency than the staggered grid, it is still not widely used, mainly because of its poor accuracy for simulating the geostrophic adjustment process under the second-order accuracy difference scheme. However, for higher-order finite-difference schemes, the validity of the above conclusion is still unknown. In this paper, we present a theoretical analysis and a numerical test for shallow water equation under high-order difference schemes. The analysis and test results are as follows. (1) For a low wavenumber, the dispersion of a staggered grid does not change with the accuracy of the difference scheme, while the dispersion of nonstaggered grid is significantly improved, and the dispersion of both grids become very close under the fourth-order scheme. (2) The maximum frequency of nonstaggered grid still exists under high-order difference schemes, and it moves toward a higher wavenumber as the accuracy of the difference scheme increases. The frequency of the staggered grid monotonically increases with the wavenumber and gets closer to the real solution under high-order difference schemes. (3) When the high-frequency noise is removed by explicitly adding a diffusion term, the pros and cops to grid staggering choices diminish with high-order schemes. In general, the nonstaggered grid is an attractive choice for the discretization of the dynamical frame in numerical models under high-order difference scheme.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2101.20136
Abstract:
In order to make better medium-term and extended-range forecast of continuous backflow rainstorms, wavelet analysis and Lanczos time filter were used to analyze the relationship between daily precipitations and atmospheric intraseasonal oscillations during the first rainy season in Guangdong in 2019. The mean circulations of two backflow continuous rainstorms over southwest of Guangdong during April and May and their low-frequency propagation characteristics were analyzed and compared with the frontal-type continuous rainstorms over north of Guangdong during June. The results show that the two continuous backflow rainstorms near Yangjiang from April to May and the precipitation during the first rainy season exhibited a quasi-23-day periodic oscillation. They were continuous backflow rainstorms with or without the influence of cold air, and their corresponding atmospheric circulation fields and low-frequency propagation were significantly different. During the continuous backflow rainstorms on 12–14 April, there were stable “west blocking” and “east blocking” at the middle and high latitudes at 500 hPa, which made cold air continuously moving southeast from the south of the high pressure that was transformed over the East China Sea. The convergence asymptote of relatively dry and cold strong southeast wind and the warm humid southerly wind from the mid-southern South China Sea by Indochina Peninsula was formed near Yangjiang at 925 hPa. During the continuous rainstorms on 23–26 May, there was a stable high-pressure ridge over the regions of eastern China, East China Sea, and Yellow Sea at 500 hPa. Because of such, Guangdong for a long time was under the southwestern airflow coming from the plateau trough, and on the ground was in the southwestern position of the degenerative high-pressure ridge from the East China Sea, in front of the trough from Beibu Bay to the southwest of China. At 925 hPa, the convergence asymptote was formed from the strong southerly winds coming from the Bay of Bengal centered in Yangjiang and the southeast winds coming from the east of the Pearl River Estuary. The low-frequency northeast wind (Southeast wind) from the south (southwest) of low frequency anticyclone in the central China (east of the East China Sea) was gradually strengthened southward, merging at the southwest of Guangdong with the enhanced low-frequency northerly (southerly) winds coming westward from the western Pacific near 140°E (eastward from the Bay of Bengal), which in turn encountered with (without) low-frequency eastern wind from the north of low frequency cyclone propagating northward from the mid-northern South China Sea. This finally resulted in the continuous backflow rainstorms with (without) obvious influence of cold air. But the frontal-type continuous rainstorm during June was affected by convergence between cold air induced by deep East Asian trough and southwesterly from Bengal Bay. The enhanced low-frequency southwesterly propagated southwards and eastwards to Guangdong from mid China and Bengal Bay. It converged with the low-frequency cold dry northeasterly winds from the periphery of the mid-latitude anticyclone at the south of Yangtze River or the north of South China Sea, which led to the happening of the continuous rainstorms during 9–13 June.
In order to make better medium-term and extended-range forecast of continuous backflow rainstorms, wavelet analysis and Lanczos time filter were used to analyze the relationship between daily precipitations and atmospheric intraseasonal oscillations during the first rainy season in Guangdong in 2019. The mean circulations of two backflow continuous rainstorms over southwest of Guangdong during April and May and their low-frequency propagation characteristics were analyzed and compared with the frontal-type continuous rainstorms over north of Guangdong during June. The results show that the two continuous backflow rainstorms near Yangjiang from April to May and the precipitation during the first rainy season exhibited a quasi-23-day periodic oscillation. They were continuous backflow rainstorms with or without the influence of cold air, and their corresponding atmospheric circulation fields and low-frequency propagation were significantly different. During the continuous backflow rainstorms on 12–14 April, there were stable “west blocking” and “east blocking” at the middle and high latitudes at 500 hPa, which made cold air continuously moving southeast from the south of the high pressure that was transformed over the East China Sea. The convergence asymptote of relatively dry and cold strong southeast wind and the warm humid southerly wind from the mid-southern South China Sea by Indochina Peninsula was formed near Yangjiang at 925 hPa. During the continuous rainstorms on 23–26 May, there was a stable high-pressure ridge over the regions of eastern China, East China Sea, and Yellow Sea at 500 hPa. Because of such, Guangdong for a long time was under the southwestern airflow coming from the plateau trough, and on the ground was in the southwestern position of the degenerative high-pressure ridge from the East China Sea, in front of the trough from Beibu Bay to the southwest of China. At 925 hPa, the convergence asymptote was formed from the strong southerly winds coming from the Bay of Bengal centered in Yangjiang and the southeast winds coming from the east of the Pearl River Estuary. The low-frequency northeast wind (Southeast wind) from the south (southwest) of low frequency anticyclone in the central China (east of the East China Sea) was gradually strengthened southward, merging at the southwest of Guangdong with the enhanced low-frequency northerly (southerly) winds coming westward from the western Pacific near 140°E (eastward from the Bay of Bengal), which in turn encountered with (without) low-frequency eastern wind from the north of low frequency cyclone propagating northward from the mid-northern South China Sea. This finally resulted in the continuous backflow rainstorms with (without) obvious influence of cold air. But the frontal-type continuous rainstorm during June was affected by convergence between cold air induced by deep East Asian trough and southwesterly from Bengal Bay. The enhanced low-frequency southwesterly propagated southwards and eastwards to Guangdong from mid China and Bengal Bay. It converged with the low-frequency cold dry northeasterly winds from the periphery of the mid-latitude anticyclone at the south of Yangtze River or the north of South China Sea, which led to the happening of the continuous rainstorms during 9–13 June.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2101.20216
Abstract:
The Northwest China, which is adjacent to the Qinghai-Tibet Plateau, is the largest Eurasian arid region. Its precipitation responses to the global climate changes, to the arid environment, and to the climate change in the Qinghai-Tibet Plateau region are special significance. Based on the observational data of the daily precipitations and monthly temperatures at 144 stations in Northwest China from 1961 to 2018, the characteristics and trends of the precipitation change in Northwest China are analyzed. The results showed that: (1) During nearly the last 60 years, 92% of the stations in Northwest China observed an increasing trend of annual precipitations, while less than 10% of the stations in southeastern Gansu observed a decreasing trend. (2) On the seasonal scale, precipitations at more than 75% stations showed an increasing trend in spring, summer, and autumn, however the most significant is that almost all the stations had an increasing trend of precipitations in winter. The increase of precipitations in autumn and winter was relatively small, which reflected the influence of winter wind on the precipitation in Northwest China. (3) Another feature is that since the beginning of the 21st century, the summer and annual precipitations in Northwest China have maintained a quasi-3 a cycle. The spring and autumn cycles have several stages, while the winter precipitation has a relatively stable cycle of nearly 3 a. Therefore, natural cycle changes do not contribute much to the increase of the precipitation. The precipitation in Northwest China has indeed increased over the past 60 years, especially since the beginning of 21st century when the precipitation has been continuously increasing. However, the increased precipitation amount is limited, which is not enough to change the arid and semi-arid climate characteristics in the region.
The Northwest China, which is adjacent to the Qinghai-Tibet Plateau, is the largest Eurasian arid region. Its precipitation responses to the global climate changes, to the arid environment, and to the climate change in the Qinghai-Tibet Plateau region are special significance. Based on the observational data of the daily precipitations and monthly temperatures at 144 stations in Northwest China from 1961 to 2018, the characteristics and trends of the precipitation change in Northwest China are analyzed. The results showed that: (1) During nearly the last 60 years, 92% of the stations in Northwest China observed an increasing trend of annual precipitations, while less than 10% of the stations in southeastern Gansu observed a decreasing trend. (2) On the seasonal scale, precipitations at more than 75% stations showed an increasing trend in spring, summer, and autumn, however the most significant is that almost all the stations had an increasing trend of precipitations in winter. The increase of precipitations in autumn and winter was relatively small, which reflected the influence of winter wind on the precipitation in Northwest China. (3) Another feature is that since the beginning of the 21st century, the summer and annual precipitations in Northwest China have maintained a quasi-3 a cycle. The spring and autumn cycles have several stages, while the winter precipitation has a relatively stable cycle of nearly 3 a. Therefore, natural cycle changes do not contribute much to the increase of the precipitation. The precipitation in Northwest China has indeed increased over the past 60 years, especially since the beginning of 21st century when the precipitation has been continuously increasing. However, the increased precipitation amount is limited, which is not enough to change the arid and semi-arid climate characteristics in the region.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2009.20126
Abstract:
TRP (Tropical cyclone Remote Precipitation), usually a high-impact weather phenomenon, is difficult to forecast accurately. In this paper, ground and sounding observation data, radar remote sensing data, and NCEP re-analysis data at 0.5°×0.5° resolution and 6-h intervals were used to analyze the physical process of the long-distance rainstorm in the Yangtze River Delta region associated with typhoon Mangosteen during its landfall in Guangdong Province in 2018. The results show that: (1) The TRP event occurred under the control of the western North Pacific subtropical high, and was affected by typhoon’s inverted trough in the lower layer. (2) The heavy rain in the first stage was mainly caused by strong convective instability, which was triggered by a convergence line in the typhoon Mangosteen inverted trough in the lower troposphere. Mangosteen poloidal outflow at upper levels merged into the mid-latitude westerly wind over the Yangtze River Delta region, thus strengthening upper divergence and upward movement in the rainstorm area. In the second stage, convective instability was weakened, but a low pressure vortex had been formed at the north end of the typhoon’s inverted trough at 850 hPa and a short-wave trough developed subsequently at the edge of the subtropical high at 500 hPa, so that dynamic conditions for a rainstorm were more favorable. (3) The three heavy rainfall centers in the Yangtze River Delta region were located at the mouth of the Yangtze River, along the coast of Jiaxing on the north coast of Hangzhou Bay and along the coast of Ningbo, all of which were near the land-boundary. (4) The diagnosis of vorticity budgets on the remote rainstorm area suggest that the initial disturbance of the rainstorm was mainly forced by the horizontal convergence and divergence terms near the surface, and then a low pressure vortex at 850 hPa developed due to the horizontal convergence and divergences terms combined with the twisting term. These were related to the lower-level southerly jet between the inverted trough and the subtropical high. Finally, vorticity was increased upward to the mid-layers (700–500 hPa), attributed to vertical ascending motion, resulting in the development of a short-wave trough at 500 hPa at the edge of the subtropical high. (5) In the formation of the long-distance rainstorm, a lower layer southerly jet formed between typhoon Mangosteen and the western North Pacific subtropical high to transport water vapor to the Yangtze River Delta area, which was similar to a typical TRP event. The difference is that the initial disturbance in a typical TRP is generally provided by the westerly trough in mid-latitudes, while in this rainstorm, it was mainly provided by Mangosteen’s inverted trough and the low-level southerly jet. Vorticity was then transported upward, leading to the development of a short-wave trough.
TRP (Tropical cyclone Remote Precipitation), usually a high-impact weather phenomenon, is difficult to forecast accurately. In this paper, ground and sounding observation data, radar remote sensing data, and NCEP re-analysis data at 0.5°×0.5° resolution and 6-h intervals were used to analyze the physical process of the long-distance rainstorm in the Yangtze River Delta region associated with typhoon Mangosteen during its landfall in Guangdong Province in 2018. The results show that: (1) The TRP event occurred under the control of the western North Pacific subtropical high, and was affected by typhoon’s inverted trough in the lower layer. (2) The heavy rain in the first stage was mainly caused by strong convective instability, which was triggered by a convergence line in the typhoon Mangosteen inverted trough in the lower troposphere. Mangosteen poloidal outflow at upper levels merged into the mid-latitude westerly wind over the Yangtze River Delta region, thus strengthening upper divergence and upward movement in the rainstorm area. In the second stage, convective instability was weakened, but a low pressure vortex had been formed at the north end of the typhoon’s inverted trough at 850 hPa and a short-wave trough developed subsequently at the edge of the subtropical high at 500 hPa, so that dynamic conditions for a rainstorm were more favorable. (3) The three heavy rainfall centers in the Yangtze River Delta region were located at the mouth of the Yangtze River, along the coast of Jiaxing on the north coast of Hangzhou Bay and along the coast of Ningbo, all of which were near the land-boundary. (4) The diagnosis of vorticity budgets on the remote rainstorm area suggest that the initial disturbance of the rainstorm was mainly forced by the horizontal convergence and divergence terms near the surface, and then a low pressure vortex at 850 hPa developed due to the horizontal convergence and divergences terms combined with the twisting term. These were related to the lower-level southerly jet between the inverted trough and the subtropical high. Finally, vorticity was increased upward to the mid-layers (700–500 hPa), attributed to vertical ascending motion, resulting in the development of a short-wave trough at 500 hPa at the edge of the subtropical high. (5) In the formation of the long-distance rainstorm, a lower layer southerly jet formed between typhoon Mangosteen and the western North Pacific subtropical high to transport water vapor to the Yangtze River Delta area, which was similar to a typical TRP event. The difference is that the initial disturbance in a typical TRP is generally provided by the westerly trough in mid-latitudes, while in this rainstorm, it was mainly provided by Mangosteen’s inverted trough and the low-level southerly jet. Vorticity was then transported upward, leading to the development of a short-wave trough.
, Available online ,
doi: 10.3878/j.issn.1006-9895.2007.19247
Abstract:
Using daily precipitation data from eastern China, in this study, we investigated the interdecadal shift in the summer extreme precipitation (SEP) and analyzed the characteristics before and after the climate shift. We also analyzed the cause from the perspective of different responses to global warming between land and ocean, which leads to circulation adjustments. The results show that the SEP in eastern China exhibited an obvious interdecadal shift around 1990, after which it became a positive anomaly. Compared with the SEP before the shift, the distributions of major modes shifted south, the intensity of SEP was enhanced, and the contribution rate to summer precipitation increased in both South and East China, whereas in North China all of these factors exhibited the opposite change. The change in the temperature difference between land and ocean caused by the strong positive anomaly of the sea surface temperature in the western Pacific warm pool is one of the important factors driving this shift, leading to the interdecadal adjustment of the East Asia summer monsoon system. In the low–middle latitudes, the intensity of the summer monsoon weakened, the West Pacific subtropical high strengthened and moved south, and the South China Sea high was also enhanced. In the middle–high latitudes, this cyclonic anomaly was broken and the East Asia Trough strengthened. Under the influence of this circulation adjustment, the water vapor decreased (increases) and vertical motion weakened (is enhanced) in North China (South and East China). Thus, the SEP decreased in North China and increased in South and East China.
Using daily precipitation data from eastern China, in this study, we investigated the interdecadal shift in the summer extreme precipitation (SEP) and analyzed the characteristics before and after the climate shift. We also analyzed the cause from the perspective of different responses to global warming between land and ocean, which leads to circulation adjustments. The results show that the SEP in eastern China exhibited an obvious interdecadal shift around 1990, after which it became a positive anomaly. Compared with the SEP before the shift, the distributions of major modes shifted south, the intensity of SEP was enhanced, and the contribution rate to summer precipitation increased in both South and East China, whereas in North China all of these factors exhibited the opposite change. The change in the temperature difference between land and ocean caused by the strong positive anomaly of the sea surface temperature in the western Pacific warm pool is one of the important factors driving this shift, leading to the interdecadal adjustment of the East Asia summer monsoon system. In the low–middle latitudes, the intensity of the summer monsoon weakened, the West Pacific subtropical high strengthened and moved south, and the South China Sea high was also enhanced. In the middle–high latitudes, this cyclonic anomaly was broken and the East Asia Trough strengthened. Under the influence of this circulation adjustment, the water vapor decreased (increases) and vertical motion weakened (is enhanced) in North China (South and East China). Thus, the SEP decreased in North China and increased in South and East China.
2021 Issue 2
Display Method:
2021, 45(2): 245-256.
doi: 10.3878/j.issn.1006-9895.2006.19167
Abstract:
There are significant precipitation stages that bring changes during the rainy period known as Meiyu. It is important to study the key stable circulation components that lead to the continuous heavy rainfall of this period to enable the analysis and prediction of Meiyu precipitation. Using NCEP/DOE Reanalysis II data from 1979 to 2016, the key stable components of the circulation system during the 2016 Meiyu period were extracted from the geopotential height field, wind field, and relative humidity field. The spatial structure, evolutionary characteristics, and longer-term background factors were analyzed to provide a basis for extended period forecasts of the periodic heavy rainfall in the Meiyu area. The results suggest that (1) the “tri-pole” pattern is the key system that maintained the continuous heavy rainfall in the Meiyu area in 2016. This “tri-pole” pattern in the stable components of the geopotential height field corresponds to the Ural Mountains blocking high, the Okhotsk Sea blocking high, the Southeast Asia high, and the West Pacific subtropical high. The structure of this configuration is conducive to the convergence of warm and cold air in the Meiyu area to generate persistent heavy rainfall. (2) In the middle and high latitudes, there is a relatively deep negative anomaly between two deep positive-anomaly areas. On one hand, this is conducive to the southward transport of cold air. On the other hand, the stability of the relative humidity field indicates that the water vapor transport and convergence of the northern branch of the air flow play an important role in the continuous precipitation of the Meiyu area. (3) The positive-anomaly area in the middle and low latitudes mainly occurs in the middle and high troposphere with zonal distribution features. However, there are two positive areas on both the eastern and western sides of the Meiyu area, which continue down to the lower level and may strengthen the convergence and transport of warm and humid air in the south of the Meiyu area. (4) The analysis of the evolution of the stable component revealed that the establishment and evolution of the “tri-pole” pattern led to different precipitation-stage characteristics in the Meiyu area. (5) The stable circulation components on a longer time scale (60 d) provide important background circulation for the key stable components of the “tri-pole” pattern during the period of continuous heavy rainfall.
There are significant precipitation stages that bring changes during the rainy period known as Meiyu. It is important to study the key stable circulation components that lead to the continuous heavy rainfall of this period to enable the analysis and prediction of Meiyu precipitation. Using NCEP/DOE Reanalysis II data from 1979 to 2016, the key stable components of the circulation system during the 2016 Meiyu period were extracted from the geopotential height field, wind field, and relative humidity field. The spatial structure, evolutionary characteristics, and longer-term background factors were analyzed to provide a basis for extended period forecasts of the periodic heavy rainfall in the Meiyu area. The results suggest that (1) the “tri-pole” pattern is the key system that maintained the continuous heavy rainfall in the Meiyu area in 2016. This “tri-pole” pattern in the stable components of the geopotential height field corresponds to the Ural Mountains blocking high, the Okhotsk Sea blocking high, the Southeast Asia high, and the West Pacific subtropical high. The structure of this configuration is conducive to the convergence of warm and cold air in the Meiyu area to generate persistent heavy rainfall. (2) In the middle and high latitudes, there is a relatively deep negative anomaly between two deep positive-anomaly areas. On one hand, this is conducive to the southward transport of cold air. On the other hand, the stability of the relative humidity field indicates that the water vapor transport and convergence of the northern branch of the air flow play an important role in the continuous precipitation of the Meiyu area. (3) The positive-anomaly area in the middle and low latitudes mainly occurs in the middle and high troposphere with zonal distribution features. However, there are two positive areas on both the eastern and western sides of the Meiyu area, which continue down to the lower level and may strengthen the convergence and transport of warm and humid air in the south of the Meiyu area. (4) The analysis of the evolution of the stable component revealed that the establishment and evolution of the “tri-pole” pattern led to different precipitation-stage characteristics in the Meiyu area. (5) The stable circulation components on a longer time scale (60 d) provide important background circulation for the key stable components of the “tri-pole” pattern during the period of continuous heavy rainfall.
2021, 45(2): 257-272.
doi: 10.3878/j.issn.1006-9895.2005.19225
Abstract:
The Liupan Mountain area is a water-conservation forest base in northwestern China; however, drought and lack of rain restrict the agricultural and economic development of this region. Through the weather research and forecasting model, a precipitation process that occurred in the Liupan Mountain area in southern Ningxia on August 21, 2018, was simulated as the basis of further research on enhancing the technology of artificial precipitation in this area. Based on observational data, a favorable circulation situation was analyzed, and the microphysical structure and precipitation formation mechanism in the precipitation cloud system were investigated. Results show that the weather system of this precipitation process developed in the dynamic field of high-altitude trough associated with low vortex, and the low vortex featured a slower movement compared with the high-altitude trough because of the blocking effect of the Liupan Mountain terrain. The vertical structure of cloud showed a remarkable “seeding-feeding” stratified structure; however, the vertical microstructure differed in different parts of the cloud system. This resulted in differences in the contributions of cold and warm-cloud processes to precipitation. In addition, the precipitation on the east windward side of the Liupan Mountain area was stronger than that on the west. Rain water was mainly produced by the melting of graupel and collection of cloud water. The accretion of supercooled rain was the main process of graupel growth. The layer of cloud water on the windward slope was deep, having high water content. This promoted the process of rain accretion by graupel in the supercooled layer and provided abundant cloud water for the coalescence growth process of raindrops, which enhanced both cold cloud and warm-cloud precipitation processes. The terrain had an impact on clouds development and precipitation formation; when the terrain height was reduced, the cloud water decreased, and consequently, the warm-cloud processes weakened, which also affected the graupel growth process.
The Liupan Mountain area is a water-conservation forest base in northwestern China; however, drought and lack of rain restrict the agricultural and economic development of this region. Through the weather research and forecasting model, a precipitation process that occurred in the Liupan Mountain area in southern Ningxia on August 21, 2018, was simulated as the basis of further research on enhancing the technology of artificial precipitation in this area. Based on observational data, a favorable circulation situation was analyzed, and the microphysical structure and precipitation formation mechanism in the precipitation cloud system were investigated. Results show that the weather system of this precipitation process developed in the dynamic field of high-altitude trough associated with low vortex, and the low vortex featured a slower movement compared with the high-altitude trough because of the blocking effect of the Liupan Mountain terrain. The vertical structure of cloud showed a remarkable “seeding-feeding” stratified structure; however, the vertical microstructure differed in different parts of the cloud system. This resulted in differences in the contributions of cold and warm-cloud processes to precipitation. In addition, the precipitation on the east windward side of the Liupan Mountain area was stronger than that on the west. Rain water was mainly produced by the melting of graupel and collection of cloud water. The accretion of supercooled rain was the main process of graupel growth. The layer of cloud water on the windward slope was deep, having high water content. This promoted the process of rain accretion by graupel in the supercooled layer and provided abundant cloud water for the coalescence growth process of raindrops, which enhanced both cold cloud and warm-cloud precipitation processes. The terrain had an impact on clouds development and precipitation formation; when the terrain height was reduced, the cloud water decreased, and consequently, the warm-cloud processes weakened, which also affected the graupel growth process.
2021, 45(2): 273-286.
doi: 10.3878/j.issn.1006-9895.2005.19231
Abstract:
Based on the reanalysis data provided by the National Centers for Environmental Prediction/National Center for Atmospheric Research and the daily rainfall data obtained at 2374 stations by the China Meteorological Administration, the precipitation moisture transport was found to have increased during the 1993–2002 period and decreased during the 2003–2012 period over South China. In this study, the characteristics of this precipitation transport were compared using a moisture transport budget equation, and the reason for the two decadal changes in the summer precipitation in South China was determined. Our results show that the increase (decrease) in summer precipitation during the 1993–2002 (2003–2013) period in South China was mainly influenced by the local circulation of the vertical air stream, eastward (westward) movement of the South Asia high, and the westward (eastward) extension of the West Pacific subtropical high (WPSH), which resulted in a strengthening (weakening) of the moisture transport from the Philippines and the southwest side of the WPSH and a strong convergence (divergence) of moisture in the lower layer over South China. The interdecadal variation of the summer precipitation of South China is mainly related to anomalies in the dynamic divergence of the moisture transport caused by changes in wind speed. It is also affected by an anomaly of the thermodynamic divergence of the moisture transport caused by a change in the specific humidity and an anomaly in the subseasonal-scale eddies generated by synoptic-scale eddies. In addition, we found the moisture transport anomalies to be closely related to the circulation and sea-surface-temperature anomalies.
Based on the reanalysis data provided by the National Centers for Environmental Prediction/National Center for Atmospheric Research and the daily rainfall data obtained at 2374 stations by the China Meteorological Administration, the precipitation moisture transport was found to have increased during the 1993–2002 period and decreased during the 2003–2012 period over South China. In this study, the characteristics of this precipitation transport were compared using a moisture transport budget equation, and the reason for the two decadal changes in the summer precipitation in South China was determined. Our results show that the increase (decrease) in summer precipitation during the 1993–2002 (2003–2013) period in South China was mainly influenced by the local circulation of the vertical air stream, eastward (westward) movement of the South Asia high, and the westward (eastward) extension of the West Pacific subtropical high (WPSH), which resulted in a strengthening (weakening) of the moisture transport from the Philippines and the southwest side of the WPSH and a strong convergence (divergence) of moisture in the lower layer over South China. The interdecadal variation of the summer precipitation of South China is mainly related to anomalies in the dynamic divergence of the moisture transport caused by changes in wind speed. It is also affected by an anomaly of the thermodynamic divergence of the moisture transport caused by a change in the specific humidity and an anomaly in the subseasonal-scale eddies generated by synoptic-scale eddies. In addition, we found the moisture transport anomalies to be closely related to the circulation and sea-surface-temperature anomalies.
2021, 45(2): 287-299.
doi: 10.3878/j.issn.1006-9895.2005.19198
Abstract:
On Aug. 7, 2015, a broken-line convective system appeared in the northwest of North China, then moved to the southeast and collided with a multi-cell system over the plains of Beijing, which eventually organized to form a strong squall line that caused local flash flooding, wind gusts, and large hailstones to fall over the Beijing area. Based on multiple data sources, our analyses indicate that the squall line formation process had three stages: First was the development and movement of a broken-line convective system in the upstream, followed by the regeneration and consolidation of multiple cells over the plains, and then the organization of a squall line once the upstream broken-line convective system had crossed over the mountains and merged to form multi-cell storms over the plains. During the second stage, local convection was triggered just north of the city by the inhomogeneous temperature distribution combined with local convergence. Along with a cold pool and expansion of the inhomogeneous temperature area, the regenerated convection propagated south ward due to the southward intensified temperature gradient. At the squall-line development stage, the dynamic structure was characterized by two strong inflows—a mid-tropospheric rear inflow at a height of 4500–5000 m and another strong inflow at the squall line moving in a low-level direction perpendicular to the orientation of the squall line. These two inflows induced separate vertical clockwise circles in front of and behind the squall line. The vertical circulation in front of the squall line was continuously intensified as the rear and front inflows were enhanced, which corresponded to a strengthening of the vertical wind shear. This dynamic process was advantageous to an ambient vertical shear in the squall-line organization, which was also a significant factor in the rapid movement and development of the squall line. When the rear inflow disappeared, the frontal vertical circulation weakened and the squall line gradually dispersed. Third, regarding the thermodynamic structure, a stronger cold pool appeared with a temperature disturbance of −8°C at a depth of 1.5 km when the upstream convective system merged with the multi-cell system over the plain. As a result, the upward motion was strengthened at the leading edge of the meso-β temperature gradient. This favored the intensification and development of the squall line. Lastly, a long gust front was induced with the strong squall line.
On Aug. 7, 2015, a broken-line convective system appeared in the northwest of North China, then moved to the southeast and collided with a multi-cell system over the plains of Beijing, which eventually organized to form a strong squall line that caused local flash flooding, wind gusts, and large hailstones to fall over the Beijing area. Based on multiple data sources, our analyses indicate that the squall line formation process had three stages: First was the development and movement of a broken-line convective system in the upstream, followed by the regeneration and consolidation of multiple cells over the plains, and then the organization of a squall line once the upstream broken-line convective system had crossed over the mountains and merged to form multi-cell storms over the plains. During the second stage, local convection was triggered just north of the city by the inhomogeneous temperature distribution combined with local convergence. Along with a cold pool and expansion of the inhomogeneous temperature area, the regenerated convection propagated south ward due to the southward intensified temperature gradient. At the squall-line development stage, the dynamic structure was characterized by two strong inflows—a mid-tropospheric rear inflow at a height of 4500–5000 m and another strong inflow at the squall line moving in a low-level direction perpendicular to the orientation of the squall line. These two inflows induced separate vertical clockwise circles in front of and behind the squall line. The vertical circulation in front of the squall line was continuously intensified as the rear and front inflows were enhanced, which corresponded to a strengthening of the vertical wind shear. This dynamic process was advantageous to an ambient vertical shear in the squall-line organization, which was also a significant factor in the rapid movement and development of the squall line. When the rear inflow disappeared, the frontal vertical circulation weakened and the squall line gradually dispersed. Third, regarding the thermodynamic structure, a stronger cold pool appeared with a temperature disturbance of −8°C at a depth of 1.5 km when the upstream convective system merged with the multi-cell system over the plain. As a result, the upward motion was strengthened at the leading edge of the meso-β temperature gradient. This favored the intensification and development of the squall line. Lastly, a long gust front was induced with the strong squall line.
2021, 45(2): 300-314.
doi: 10.3878/j.issn.1006-9895.2005.19235
Abstract:
The area south of the Yangtze River is highly impacted by heatwave disasters. Previous studies have shown the effects of sea surface temperature anomalies (SSTAs) in several crucial sea regions (e.g., the tropical Atlantic, the North Indian Ocean, and the tropical central eastern Pacific) on summer mean air temperature anomalies over southern China on interannual or interdecadal time scales. However, little research exists focusing on the impacts of SSTAs in these key sea regions on the occurrences and maintenances of heatwave events over the area south of the Yangtze River on the intraseasonal time scale. For this reason, using observational data in gauge stations in China, the National Centers for Environmental Prediction–National Center for Atmosphere Research (NCEP/NCAR) reanalysis, and the National Oceanic and Atmospheric Administration (NOAA) SSTs, the present study focuses on the potential contribution of SSTAs in the tropical Atlantic to heatwave events over the area south of the Yangtze River by analyzing two summer heatwave events (occurred during July 21 to 31 and August 15 to 25) in 2016. In addition, based on the composites of multiple heatwave events during 1981–2016, the present study further explores a possible physical link explaining the effect of the intraseasonal evolution of SSTAs in the tropical Atlantic on heatwave events over the area south of the Yangtze River. The results indicate that the development and maintenance of warmer SSTAs in the tropical western Atlantic are conducive in stimulating a relatively stable Rossby wave train over Eurasia on the intraseasonal time scale. As a result, a deep high-pressure anomaly governs East Asia and its coastal areas, resulting in long-term persistent (≥8 days) heatwave events over the area south of the Yangtze River. This phased increase and maintenance of SSTAs in the tropical Atlantic and associated steady Rossby wave train are prior to the long-term persistent heatwave events, and the latter might occur in a month since the SSTs in the tropical Atlantic are significantly enhanced. On the intraseasonal time scale, there is a clearly phased increase in the SSTs in the North Indian Ocean, around 10 days after the significant increase in the SSTs in the tropical Atlantic Ocean. This implies that in addition to directly exciting the Rossby wave train across Eurasia, it is probable that SSTAs in the tropical Atlantic may affect the occurrence and maintenance of the heatwave events through modulating the phased variation of SSTAs in the North Indian Ocean on the intraseasonal time scale. Furthermore, during the period of El Niño decay and transition to La Niña, the synergistic intraseasonal variations accompanied with cooler SSTAs in the tropical central eastern Pacific and warmer SSTAs in the North Indian Ocean may also contribute to the long-term persistent heatwave events. The intraseasonal variations in SSTAs in the three key sea regions seem to be able to be used as precursory signals for the heatwave events. Nevertheless, the specific process explaining the intraseasonal effects of SSTAs in the three key regions, especially the joint effect of SSTAs in these regions and related physical process, requires further investigation in the future.
The area south of the Yangtze River is highly impacted by heatwave disasters. Previous studies have shown the effects of sea surface temperature anomalies (SSTAs) in several crucial sea regions (e.g., the tropical Atlantic, the North Indian Ocean, and the tropical central eastern Pacific) on summer mean air temperature anomalies over southern China on interannual or interdecadal time scales. However, little research exists focusing on the impacts of SSTAs in these key sea regions on the occurrences and maintenances of heatwave events over the area south of the Yangtze River on the intraseasonal time scale. For this reason, using observational data in gauge stations in China, the National Centers for Environmental Prediction–National Center for Atmosphere Research (NCEP/NCAR) reanalysis, and the National Oceanic and Atmospheric Administration (NOAA) SSTs, the present study focuses on the potential contribution of SSTAs in the tropical Atlantic to heatwave events over the area south of the Yangtze River by analyzing two summer heatwave events (occurred during July 21 to 31 and August 15 to 25) in 2016. In addition, based on the composites of multiple heatwave events during 1981–2016, the present study further explores a possible physical link explaining the effect of the intraseasonal evolution of SSTAs in the tropical Atlantic on heatwave events over the area south of the Yangtze River. The results indicate that the development and maintenance of warmer SSTAs in the tropical western Atlantic are conducive in stimulating a relatively stable Rossby wave train over Eurasia on the intraseasonal time scale. As a result, a deep high-pressure anomaly governs East Asia and its coastal areas, resulting in long-term persistent (≥8 days) heatwave events over the area south of the Yangtze River. This phased increase and maintenance of SSTAs in the tropical Atlantic and associated steady Rossby wave train are prior to the long-term persistent heatwave events, and the latter might occur in a month since the SSTs in the tropical Atlantic are significantly enhanced. On the intraseasonal time scale, there is a clearly phased increase in the SSTs in the North Indian Ocean, around 10 days after the significant increase in the SSTs in the tropical Atlantic Ocean. This implies that in addition to directly exciting the Rossby wave train across Eurasia, it is probable that SSTAs in the tropical Atlantic may affect the occurrence and maintenance of the heatwave events through modulating the phased variation of SSTAs in the North Indian Ocean on the intraseasonal time scale. Furthermore, during the period of El Niño decay and transition to La Niña, the synergistic intraseasonal variations accompanied with cooler SSTAs in the tropical central eastern Pacific and warmer SSTAs in the North Indian Ocean may also contribute to the long-term persistent heatwave events. The intraseasonal variations in SSTAs in the three key sea regions seem to be able to be used as precursory signals for the heatwave events. Nevertheless, the specific process explaining the intraseasonal effects of SSTAs in the three key regions, especially the joint effect of SSTAs in these regions and related physical process, requires further investigation in the future.
2021, 45(2): 315-332.
doi: 10.3878/j.issn.1006-9895.2009.19240
Abstract:
A new convective–stratiform separation technique based on the hourly precipitation obtained from CMPAS (China Meteorological Administration multisource precipitation analysis system) and radar reflectivity mosaics data obtained from CMARMOS (China Meteorological Administration radar mosaic operation system) is presented in this paper. The technique, which is based on fuzzy logic, is developed to improve the cloud analysis scheme in the Gridpoint Statistical Interpolation (GSI) assimilation system (referred to as the CUST scheme). The improved scheme was tested in a severe Mei-yu rain that occurred on 19 June 2019, in Zhejiang Province. Several hourly-cycle assimilation experiments were performed using the WRF (weather research and forecasting) model and the GSI assimilation system to analyze the impact of the new scheme on the precipitation simulation, and the scheme was compared with other schemes. The results showed the following: (1) The new convective–stratiform separation technique accurately separated the convective–stratiform cloud, and it can be used as a discriminating factor to improve the cloud analysis scheme in the GSI assimilation system. (2) The CUST scheme adopted the convective cloud analysis scheme in the convective region and the stratiform cloud analysis scheme in the non-convective region, which reduced the false alarm rate in the simple convective cloud scheme and the underestimation in the simple stratiform cloud scheme; this effectively improved the simulation of short-term precipitation. (3) The CUST scheme showed significant improvement in the initial stage of the model (within 6 hours or even 3 hours), and the improvement of small-level rain was greater than that of heavy rain. (4) Compared with the hybrid cloud analysis scheme (referred to as the CSW scheme) based on the convective scale velocity determined by the surface sensible heat and latent heat fluxes, the CUST scheme showed a more reasonable result for the convective–stratiform cloud partition and precipitation simulation, which indicates a good application prospect.
A new convective–stratiform separation technique based on the hourly precipitation obtained from CMPAS (China Meteorological Administration multisource precipitation analysis system) and radar reflectivity mosaics data obtained from CMARMOS (China Meteorological Administration radar mosaic operation system) is presented in this paper. The technique, which is based on fuzzy logic, is developed to improve the cloud analysis scheme in the Gridpoint Statistical Interpolation (GSI) assimilation system (referred to as the CUST scheme). The improved scheme was tested in a severe Mei-yu rain that occurred on 19 June 2019, in Zhejiang Province. Several hourly-cycle assimilation experiments were performed using the WRF (weather research and forecasting) model and the GSI assimilation system to analyze the impact of the new scheme on the precipitation simulation, and the scheme was compared with other schemes. The results showed the following: (1) The new convective–stratiform separation technique accurately separated the convective–stratiform cloud, and it can be used as a discriminating factor to improve the cloud analysis scheme in the GSI assimilation system. (2) The CUST scheme adopted the convective cloud analysis scheme in the convective region and the stratiform cloud analysis scheme in the non-convective region, which reduced the false alarm rate in the simple convective cloud scheme and the underestimation in the simple stratiform cloud scheme; this effectively improved the simulation of short-term precipitation. (3) The CUST scheme showed significant improvement in the initial stage of the model (within 6 hours or even 3 hours), and the improvement of small-level rain was greater than that of heavy rain. (4) Compared with the hybrid cloud analysis scheme (referred to as the CSW scheme) based on the convective scale velocity determined by the surface sensible heat and latent heat fluxes, the CUST scheme showed a more reasonable result for the convective–stratiform cloud partition and precipitation simulation, which indicates a good application prospect.
2021, 45(2): 333-354.
doi: 10.3878/j.issn.1006-9895.2011.19248
Abstract:
As affected by stable atmosphere, there had been ten fog events in Jinan from 19 December 2016 to 9 January 2017, during which the minimum value of visibility was below 50 m. The low visibility caused by the continuous fog events brought serious negative impacts to industrial and agricultural activities as well as to people’s daily live. In this paper, based on the measurements from fog drop spectrometers, automatic weather stations, and conventional meteorological instruments during the 10 fog events, the fogs’ microphysical characteristics were analyzed, microphysical processes and intensity were deduced, and their effects on visibility were discussed. The results were as follows: (1) The droplet spectrum distribution was different with different thicknesses of the winter fogs, changing from mono-modal to multi-modal as fog became thicker. (2) The droplet number concentration (Nc) had a close inverse correlation with visibility (V), while the liquid water content (LWC) and relative dispersion of the droplet size distribution (S) did not have a definite inverse relationship with V. (3) The temperature of air had an impact on the microphysical processes. The activation and condensational growth (or droplet evaporation) processes played leading roles in the whole winter fog lifecycle. (4) The collection processes arose in the development and maturation stages of the fogs, but not or very infrequently in the formation and weakening stages of the fogs. (5) The results from the autoconversion rate show that the collection rarely occurred in the fogs with V>200 m, while it was very weak or intermittent in the heavy fogs with 100 m≤V<200 m. The collection mainly occurred in the extremely dense fogs with V<50 m and heavy fogs with 50 m≤V<100 m, while it was more frequent and stronger in the heavy fogs with 50 m≤V<100 m than in the extremely dense fog with V<50 m. A larger number of small fog droplets led to poorer visibility in the extremely dense fog with V<50 m, however the maximum values of the droplet properties such as Nc, LWC appeared in the heavy fogs with 50 m≤V<100 m, in which these quantities showed the biggest variations as well. (6) The variations may be related to the more frequent and stronger collection in the heavy fogs with 50 m≤V<100 m, in which collision–coalescence and collision-fragmentation played a role in the biggest variations of microphysical quantities. (7) The trend values of calculated visibility based on the observational data from fog drop spectrometers agreed well with the actual trend values, however the calculated visibility values themselves were much greater than the actual ones, mainly caused by the large amount of aerosol particles in the fogs. In polluted fogs, observational data from fog drop spectrometers are not enough to estimate the visibility of fogs. The influence of aerosol particles on visibility must be considered at the same time.
As affected by stable atmosphere, there had been ten fog events in Jinan from 19 December 2016 to 9 January 2017, during which the minimum value of visibility was below 50 m. The low visibility caused by the continuous fog events brought serious negative impacts to industrial and agricultural activities as well as to people’s daily live. In this paper, based on the measurements from fog drop spectrometers, automatic weather stations, and conventional meteorological instruments during the 10 fog events, the fogs’ microphysical characteristics were analyzed, microphysical processes and intensity were deduced, and their effects on visibility were discussed. The results were as follows: (1) The droplet spectrum distribution was different with different thicknesses of the winter fogs, changing from mono-modal to multi-modal as fog became thicker. (2) The droplet number concentration (Nc) had a close inverse correlation with visibility (V), while the liquid water content (LWC) and relative dispersion of the droplet size distribution (S) did not have a definite inverse relationship with V. (3) The temperature of air had an impact on the microphysical processes. The activation and condensational growth (or droplet evaporation) processes played leading roles in the whole winter fog lifecycle. (4) The collection processes arose in the development and maturation stages of the fogs, but not or very infrequently in the formation and weakening stages of the fogs. (5) The results from the autoconversion rate show that the collection rarely occurred in the fogs with V>200 m, while it was very weak or intermittent in the heavy fogs with 100 m≤V<200 m. The collection mainly occurred in the extremely dense fogs with V<50 m and heavy fogs with 50 m≤V<100 m, while it was more frequent and stronger in the heavy fogs with 50 m≤V<100 m than in the extremely dense fog with V<50 m. A larger number of small fog droplets led to poorer visibility in the extremely dense fog with V<50 m, however the maximum values of the droplet properties such as Nc, LWC appeared in the heavy fogs with 50 m≤V<100 m, in which these quantities showed the biggest variations as well. (6) The variations may be related to the more frequent and stronger collection in the heavy fogs with 50 m≤V<100 m, in which collision–coalescence and collision-fragmentation played a role in the biggest variations of microphysical quantities. (7) The trend values of calculated visibility based on the observational data from fog drop spectrometers agreed well with the actual trend values, however the calculated visibility values themselves were much greater than the actual ones, mainly caused by the large amount of aerosol particles in the fogs. In polluted fogs, observational data from fog drop spectrometers are not enough to estimate the visibility of fogs. The influence of aerosol particles on visibility must be considered at the same time.
2021, 45(2): 355-368.
doi: 10.3878/j.issn.1006-9895.2007.19254
Abstract:
Based on the reanalysis data from NCEP/NCAR, rainfall data from TRMM (Tropical Rainfall Measuring Mission), and database from RSMC (Regional Specialized Meteorological Centre) best-track, this paper presents an evaluation of the precipitation distribution and large-scale circulation of tropical cyclone (TC) Usagi during its extratropical transition (ET). Based on this, a numerical experiment was conducted, in which the upper-level trough and ridge were modified using piecewise potential vorticity inversion to consider the influence of the trough and ridge on the asymmetric precipitation of the TC during ET process. The results are as follows: (1) Precipitation from Usagi during ET is concentrated on the left side of the path, accompanied by an adjustment in the meridional-to-zonal circulation, with the subtropical high retreating southward, while weakening and being propelled westward. (2) The asymmetric distribution of TC precipitation is directly related to the relative strength of the cold and warm fronts, the water vapor transport, and the area of falling upper-level cold air. (3) When the circulation adjustment is strengthened, the baroclinity of the system is enhanced. This means that the increased westerly, as indicated by the increased circulation index, promotes an increase in the LOT (left of track) rainfall.
Based on the reanalysis data from NCEP/NCAR, rainfall data from TRMM (Tropical Rainfall Measuring Mission), and database from RSMC (Regional Specialized Meteorological Centre) best-track, this paper presents an evaluation of the precipitation distribution and large-scale circulation of tropical cyclone (TC) Usagi during its extratropical transition (ET). Based on this, a numerical experiment was conducted, in which the upper-level trough and ridge were modified using piecewise potential vorticity inversion to consider the influence of the trough and ridge on the asymmetric precipitation of the TC during ET process. The results are as follows: (1) Precipitation from Usagi during ET is concentrated on the left side of the path, accompanied by an adjustment in the meridional-to-zonal circulation, with the subtropical high retreating southward, while weakening and being propelled westward. (2) The asymmetric distribution of TC precipitation is directly related to the relative strength of the cold and warm fronts, the water vapor transport, and the area of falling upper-level cold air. (3) When the circulation adjustment is strengthened, the baroclinity of the system is enhanced. This means that the increased westerly, as indicated by the increased circulation index, promotes an increase in the LOT (left of track) rainfall.
2021, 45(2): 369-378.
doi: 10.3878/j.issn.1006-9895.2006.19255
Abstract:
The stratiform clouds with embedded convective cells is an important precipitation system. Precise knowledge on the microphysical structure of the clouds can be very useful for the development of the weather numerical prediction model and the cloud water resource. In our study, a time-variant threshold method to identify the shattered fragments is proposed to study the different influence of the shattered particles on the microphysical measurement in the stratiform cloud region and the convection cloud region. Then the shattered impact on the particle spectrum distribution (PSD), particle number concentration, and ice water content measurement is analyzed. It is found that the influence of the shattered artifacts on the PSD can be in the two ends (<500 μm and >1000 μm). In the stratiform area the influence on the small end is less than 300 μm, while in the convective area it is less than 500 μm. On average, the shattered particles have 20% higher impact on the PSD in the convective region than that in the stratiform region. The shattered fragments can increase the particle number concentration by 4.56 times in the stratiform area while in the convective area the value is 8.47 times, which is nearly twice that of the stratiform area. The shattered artifacts can also overestimate the ice water content in the convective region more than 30% than that in the stratus.
The stratiform clouds with embedded convective cells is an important precipitation system. Precise knowledge on the microphysical structure of the clouds can be very useful for the development of the weather numerical prediction model and the cloud water resource. In our study, a time-variant threshold method to identify the shattered fragments is proposed to study the different influence of the shattered particles on the microphysical measurement in the stratiform cloud region and the convection cloud region. Then the shattered impact on the particle spectrum distribution (PSD), particle number concentration, and ice water content measurement is analyzed. It is found that the influence of the shattered artifacts on the PSD can be in the two ends (<500 μm and >1000 μm). In the stratiform area the influence on the small end is less than 300 μm, while in the convective area it is less than 500 μm. On average, the shattered particles have 20% higher impact on the PSD in the convective region than that in the stratiform region. The shattered fragments can increase the particle number concentration by 4.56 times in the stratiform area while in the convective area the value is 8.47 times, which is nearly twice that of the stratiform area. The shattered artifacts can also overestimate the ice water content in the convective region more than 30% than that in the stratus.
2021, 45(2): 379-392.
doi: 10.3878/j.issn.1006-9895.2011.20102
Abstract:
The characteristics and formation of “train effect” in the spiral cloud belt of typhoon “Fitow” (No. 1323) are analyzed in this paper. The results show a “train effect” occurred during persistent rainfall over the south bank of the Qiantangjiang Bay in Zhejiang. According to the stable position of the rain belt, This rain process can be divided into two “train effect”. Each “train effect" last about 3–4 h, and the space span is 1–2 longitude distance. The rainstorm area shows the characteristics of band, with high precipitation efficiency. The precipitation exceeds 25 mm h−1 and propagates forward linearly. The average reflectivity of radar echo in the spiral cloud belt of typhoon with over 35 dBZ also shows a linear band structure. The trend of precipitation belt and radar echo deviated from the typhoon center by more than 25° to the right. From the formation of the “train effect”, the direction of the high-altitude guided airflow and the direction of the surface meso-scale disturbance convergence zone are consistent with the movement direction of the convective cell in the “train effect” providing the basis for its nearly linear arrangement. The meso-scale disturbance convergence or vortex formed along the coast provides important conditions for the occurrence or development of convection in the spiral cloud belt in coastal areas. This convergence also forms the beginning and maintenance mechanism for the rapid growth and subsequent development of convective clouds, playing an important role in the formation, development, and maintenance of the “train effect”.
The characteristics and formation of “train effect” in the spiral cloud belt of typhoon “Fitow” (No. 1323) are analyzed in this paper. The results show a “train effect” occurred during persistent rainfall over the south bank of the Qiantangjiang Bay in Zhejiang. According to the stable position of the rain belt, This rain process can be divided into two “train effect”. Each “train effect" last about 3–4 h, and the space span is 1–2 longitude distance. The rainstorm area shows the characteristics of band, with high precipitation efficiency. The precipitation exceeds 25 mm h−1 and propagates forward linearly. The average reflectivity of radar echo in the spiral cloud belt of typhoon with over 35 dBZ also shows a linear band structure. The trend of precipitation belt and radar echo deviated from the typhoon center by more than 25° to the right. From the formation of the “train effect”, the direction of the high-altitude guided airflow and the direction of the surface meso-scale disturbance convergence zone are consistent with the movement direction of the convective cell in the “train effect” providing the basis for its nearly linear arrangement. The meso-scale disturbance convergence or vortex formed along the coast provides important conditions for the occurrence or development of convection in the spiral cloud belt in coastal areas. This convergence also forms the beginning and maintenance mechanism for the rapid growth and subsequent development of convective clouds, playing an important role in the formation, development, and maintenance of the “train effect”.
2021, 45(2): 393-406.
doi: 10.3878/j.issn.1006-9895.2012.20162
Abstract:
An aircraft observation on aerosol and shallow cumulus clouds in Xinzhou was carried out by the Weather Modification Office of Shanxi Province on 15 August 2014. In this paper, the microphysical properties of aerosols, CCN (cloud concentration nuclei), shallow cumulus clouds, and their interactions in North China are analyzed in detail, based on the airborne cloud physics data. Main results are as following: (1) In this case, the height of the boundary layer is about 3600 m, and the aerosol particle concentration (Na) near surface can reach 2500 cm−3. The vertical profiles of Na, aerosol effective diameter (Da) and CCN number concentration are obviously different under different stratification conditions. (2) The main source of CCN is aerosol particles in accumulation mode, Aegean mode or nuclear mode. The aerosol AR (activation rate) through the vertical layer does not change much under the 0.2% supersaturation condition, while decreases with height under the 0.4% supersaturation condition. (3) HYSPLIT4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) model analysis shows that aerosols below 2 km mainly come from local urban emissions, which are composed of fine particulate pollutants. Above 2 km, aerosols mainly come from deserts in northwestern China and Mongolia, which are composed of submicron sand and dust. They can be potential IN (ice nuclei) due to their low solubility. (4) The physical characteristics of two adjacent shallow cumulus clouds (Cu-1 and Cu-2) are also analyzed. Cu-1 is loose with a lot of entrainment. The cloud base height and cloud thickness are about 4500 m and 600 m, respectively. The LWC (liquid water content) in Cu-1 is basically maintained at 0.5 g m−3, while the average Nc (cloud particle concentration) is 278.3 cm−3 and Dc (cloud effective diameter) is overall within 15 μm. The maximum Nd (drizzle droplet number concentration) is 0.002 cm−3, with almost no precipitation particles in Cu-1. The particle spectrum width increases with height, and ED (effective diameter) is mainly concentrated within 30 μm. Cu-2 is much denser than Cu-1, with cloud base height at 3900 m and cloud thickness of 1200 m. There is plenty of supercooled water in Cu-2, and LWC is over 1 g m−3 at multiple areas. Ice crystals appear near the cloud top, and particle growth states change from condensation to mixed phase directly. The horizontal distribution of particles in Cu-2 is uneven, and Nc at the same height differs greatly, with the maximum value up to 1240 cm−3. Dc increases with height. The particle spectral width expands with height, up to 1100 μm, and the spectral pattern changes from a single peak to multi-peaks. The images of precipitation particles and ice crystals are mostly graupel, needle-shaped, and plate-shaped.
An aircraft observation on aerosol and shallow cumulus clouds in Xinzhou was carried out by the Weather Modification Office of Shanxi Province on 15 August 2014. In this paper, the microphysical properties of aerosols, CCN (cloud concentration nuclei), shallow cumulus clouds, and their interactions in North China are analyzed in detail, based on the airborne cloud physics data. Main results are as following: (1) In this case, the height of the boundary layer is about 3600 m, and the aerosol particle concentration (Na) near surface can reach 2500 cm−3. The vertical profiles of Na, aerosol effective diameter (Da) and CCN number concentration are obviously different under different stratification conditions. (2) The main source of CCN is aerosol particles in accumulation mode, Aegean mode or nuclear mode. The aerosol AR (activation rate) through the vertical layer does not change much under the 0.2% supersaturation condition, while decreases with height under the 0.4% supersaturation condition. (3) HYSPLIT4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) model analysis shows that aerosols below 2 km mainly come from local urban emissions, which are composed of fine particulate pollutants. Above 2 km, aerosols mainly come from deserts in northwestern China and Mongolia, which are composed of submicron sand and dust. They can be potential IN (ice nuclei) due to their low solubility. (4) The physical characteristics of two adjacent shallow cumulus clouds (Cu-1 and Cu-2) are also analyzed. Cu-1 is loose with a lot of entrainment. The cloud base height and cloud thickness are about 4500 m and 600 m, respectively. The LWC (liquid water content) in Cu-1 is basically maintained at 0.5 g m−3, while the average Nc (cloud particle concentration) is 278.3 cm−3 and Dc (cloud effective diameter) is overall within 15 μm. The maximum Nd (drizzle droplet number concentration) is 0.002 cm−3, with almost no precipitation particles in Cu-1. The particle spectrum width increases with height, and ED (effective diameter) is mainly concentrated within 30 μm. Cu-2 is much denser than Cu-1, with cloud base height at 3900 m and cloud thickness of 1200 m. There is plenty of supercooled water in Cu-2, and LWC is over 1 g m−3 at multiple areas. Ice crystals appear near the cloud top, and particle growth states change from condensation to mixed phase directly. The horizontal distribution of particles in Cu-2 is uneven, and Nc at the same height differs greatly, with the maximum value up to 1240 cm−3. Dc increases with height. The particle spectral width expands with height, up to 1100 μm, and the spectral pattern changes from a single peak to multi-peaks. The images of precipitation particles and ice crystals are mostly graupel, needle-shaped, and plate-shaped.
2021, 45(2): 407-423.
doi: 10.3878/j.issn.1006-9895.2006.20108
Abstract:
Based on a set of twenty-first-century climate change projections by a regional climate model (RegCM4) at a 25-km grid spacing driven by five global models, future climate change over Xinjiang in Northwest China under the middle and high representative concentration pathways of 4.5 and 8.5 (RCP4.5 and RCP8.5) is investigated. Results show the multi-RegCM4 ensemble (ensR) clearly determines both the spatial distributions and the amounts of mean temperature and precipitation, along with extreme temperature and precipitation. In the future, the temperature and precipitation over Xinjiang are projected to rise or increase continuously, especially under RCP8.5 compared with RCP4.5. Moreover, by the end of the twenty-first century under RCP8.5, the regional mean increases in annual temperature and precipitation will be as 4.9°C and 28% (102 mm), respectively. Increases of temperature and precipitation extremes are also reported as measured by different indices, indicating more heat waves, less cold spells, and more extreme precipitation in the future. Specifically, for temperature, greater increase of annual minimum of daily minimum temperature (TNn) is found compared to annual maximum of daily maximum temperature (TXx). By the end of the twenty-first century under RCP8.5, the increase of regional mean TXx and TNn over Xinjiang will be 4.9°C and 5.8°C, respectively. As for extreme precipitation indices, an increase of RX1day by 29% (5 mm) and a decrease of CDD by 10 days are found. The change of snow cover shows spatial differences, with a general decrease except a large percentage increase in the western Tarim Basin. A 13% reduction is reported for the regional mean snow coverage at the end of the twenty-first century under RCP8.5. The total runoff and soil moisture are projected to increase; however, more hydrological droughts in northern Xinjiang are projected to occur. The ensR show consistencies among the ensemble members, but there exist differences concerning the change of amount and the sign in some cases. Overall, considering the analyzed variables, a “warmer and more humid” tendency of climate as observed in the late decades will be expected in Xinjiang in the future. However, this may not change the fact of the dominance of arid and semiarid climate over the region. In addition, hydrological droughts are also projected to increase in the future. Thus, high attention still needs to be paid for the availability and risks on water resources over the region.
Based on a set of twenty-first-century climate change projections by a regional climate model (RegCM4) at a 25-km grid spacing driven by five global models, future climate change over Xinjiang in Northwest China under the middle and high representative concentration pathways of 4.5 and 8.5 (RCP4.5 and RCP8.5) is investigated. Results show the multi-RegCM4 ensemble (ensR) clearly determines both the spatial distributions and the amounts of mean temperature and precipitation, along with extreme temperature and precipitation. In the future, the temperature and precipitation over Xinjiang are projected to rise or increase continuously, especially under RCP8.5 compared with RCP4.5. Moreover, by the end of the twenty-first century under RCP8.5, the regional mean increases in annual temperature and precipitation will be as 4.9°C and 28% (102 mm), respectively. Increases of temperature and precipitation extremes are also reported as measured by different indices, indicating more heat waves, less cold spells, and more extreme precipitation in the future. Specifically, for temperature, greater increase of annual minimum of daily minimum temperature (TNn) is found compared to annual maximum of daily maximum temperature (TXx). By the end of the twenty-first century under RCP8.5, the increase of regional mean TXx and TNn over Xinjiang will be 4.9°C and 5.8°C, respectively. As for extreme precipitation indices, an increase of RX1day by 29% (5 mm) and a decrease of CDD by 10 days are found. The change of snow cover shows spatial differences, with a general decrease except a large percentage increase in the western Tarim Basin. A 13% reduction is reported for the regional mean snow coverage at the end of the twenty-first century under RCP8.5. The total runoff and soil moisture are projected to increase; however, more hydrological droughts in northern Xinjiang are projected to occur. The ensR show consistencies among the ensemble members, but there exist differences concerning the change of amount and the sign in some cases. Overall, considering the analyzed variables, a “warmer and more humid” tendency of climate as observed in the late decades will be expected in Xinjiang in the future. However, this may not change the fact of the dominance of arid and semiarid climate over the region. In addition, hydrological droughts are also projected to increase in the future. Thus, high attention still needs to be paid for the availability and risks on water resources over the region.
2021, 45(2): 424-434.
doi: 10.3878/j.issn.1006-9895.2010.20110
Abstract:
As global warming intensifies, the growing season changes correspondingly across China. Earlier, pertinent studies of the growing season have only covered short-term periods, used low-resolution horizontal data, and focused on limited regions. Based on these premises, the authors investigated the climatological trends of the start of the growing season (GSS), end of the growing season (GSE), and growing season length (GSL) across China during 1961–2018. The authors used daily mean temperature data from a high-resolution horizontal gridded dataset CN05.1 (0.25°×0.25°). The relationship between the changes in the growing season and seasonal average temperature was also analyzed. The results indicated that the national average GSS and GSE were 31 March and 29 October, respectively, and GSL was 212 d across China during 1961–2018. Spatially, annual average GSS was delayed from southeast to northwest, whereas, GSE showed the opposite trend; GSL decreased from southeast to northwest. In generally, the trends in the national average growing season displayed an early start [−1.3 d (10 a)−1], a late end [0.9 d (10 a)−1 ], and a long length [2.2 d (10 a)−1] across China from 1961 to 2018. Finally, both the advance of the national average GSS and extension of the GSL were mainly related to increased spring temperatures, whereas, the delay in the GSE originated from increased autumn temperatures.
As global warming intensifies, the growing season changes correspondingly across China. Earlier, pertinent studies of the growing season have only covered short-term periods, used low-resolution horizontal data, and focused on limited regions. Based on these premises, the authors investigated the climatological trends of the start of the growing season (GSS), end of the growing season (GSE), and growing season length (GSL) across China during 1961–2018. The authors used daily mean temperature data from a high-resolution horizontal gridded dataset CN05.1 (0.25°×0.25°). The relationship between the changes in the growing season and seasonal average temperature was also analyzed. The results indicated that the national average GSS and GSE were 31 March and 29 October, respectively, and GSL was 212 d across China during 1961–2018. Spatially, annual average GSS was delayed from southeast to northwest, whereas, GSE showed the opposite trend; GSL decreased from southeast to northwest. In generally, the trends in the national average growing season displayed an early start [−1.3 d (10 a)−1], a late end [0.9 d (10 a)−1 ], and a long length [2.2 d (10 a)−1] across China from 1961 to 2018. Finally, both the advance of the national average GSS and extension of the GSL were mainly related to increased spring temperatures, whereas, the delay in the GSE originated from increased autumn temperatures.
2021, 45(2): 435-455.
doi: 10.3878/j.issn.1006-9895.2009.20111
Abstract:
In order to study the turbulent characteristics of the boundary layer and its effects on the transport of momentum, heat, and water vapor in the Source Region of the Yellow River (SRYR), northeast of the Tibetan Plateau, large eddy simulations are performed for the first time to investigate the fine structure of turbulence in the convective boundary layer (CBL) over two different underlying surfaces (grassland and lake) in the SRYR. GPS soundings and eddy covariance data observed during a field experiment in the Ngoring Lake Basin in summer 2012 are used. It shows that the averaged structures of CBLs over the grassland and lake are in good agreement with the observations, but the characteristics of the turbulent structures over both surfaces show large differences. The budget of turbulent energy, spatial-temporal distribution of turbulent properties, and the structural features of turbulent eddies above the grassland are consistent with the thermal-driven CBL over the land. Organized convective rolls presented at the top of the CBL over the lake. There is larger turbulent intensity at the top of CBL over the lake due to strong entrainment, while the same occurs in the surface layer over the grassland. It is found that the simulated results are sensitive to the horizontal resolution over both surfaces. The denser horizontal resolution applied over the lake helps to improve accuracy in simulation of turbulent kinetic energy and turbulent fluxes of the surface layer and the entrainment layer, and the contributions of various scale’s waves to turbulent fluxes are fully simulated as well. The grid spacings of 100–200 m are recommended to simulate the fine turbulent structure over the grassland if the simulated time is considered.
In order to study the turbulent characteristics of the boundary layer and its effects on the transport of momentum, heat, and water vapor in the Source Region of the Yellow River (SRYR), northeast of the Tibetan Plateau, large eddy simulations are performed for the first time to investigate the fine structure of turbulence in the convective boundary layer (CBL) over two different underlying surfaces (grassland and lake) in the SRYR. GPS soundings and eddy covariance data observed during a field experiment in the Ngoring Lake Basin in summer 2012 are used. It shows that the averaged structures of CBLs over the grassland and lake are in good agreement with the observations, but the characteristics of the turbulent structures over both surfaces show large differences. The budget of turbulent energy, spatial-temporal distribution of turbulent properties, and the structural features of turbulent eddies above the grassland are consistent with the thermal-driven CBL over the land. Organized convective rolls presented at the top of the CBL over the lake. There is larger turbulent intensity at the top of CBL over the lake due to strong entrainment, while the same occurs in the surface layer over the grassland. It is found that the simulated results are sensitive to the horizontal resolution over both surfaces. The denser horizontal resolution applied over the lake helps to improve accuracy in simulation of turbulent kinetic energy and turbulent fluxes of the surface layer and the entrainment layer, and the contributions of various scale’s waves to turbulent fluxes are fully simulated as well. The grid spacings of 100–200 m are recommended to simulate the fine turbulent structure over the grassland if the simulated time is considered.
2021, 45(2): 456-470.
doi: 10.3878/j.issn.1006-9895.2011.20169
Abstract:
Based on the monthly mean NOAA outgoing longwave radiation (OLR) data and surface heat fluxes derived from ERA-5 reanalysis data from 1980 to 2019, the spatial and temporal distributions of OLR and convection over the Tibetan Plateau (TP) and the relationship between surface heat fluxes and convection were investigated. Results showed that the minimum value of OLR was located in the middle of the TP and increased outward. The OLR (convection) was lower (stronger) in the eastern plateau than in the western region. In the past 40 years, the OLR (convection) increased (decreased) gradually with a period of 6 a and 2–3 a over the TP. However, the tendency for convective activity had significant differences among different regions and seasons. A growing trend of convective activity occurred in boreal summer, while it weakened in the other seasons. The convection had a weakening trend in the vicinity of the Three Rivers’ Headstream region but increased in the north of the Himalayas on the TP. In the boreal summer, the surface latent heat flux was generally stronger than the surface sensible heat flux. The evolution of convection was significantly related to both surface sensible and latent heat fluxes. The surface sensible heat flux was highly negatively correlated to the evolution of convective activity, while there was an east-west dipole pattern of correlation between the surface latent heat flux and convection. A positive correlation between the surface latent heat flux and the evolution of the convection in the western plateau was observed, while a negative correlation was evident with that in the eastern region.
Based on the monthly mean NOAA outgoing longwave radiation (OLR) data and surface heat fluxes derived from ERA-5 reanalysis data from 1980 to 2019, the spatial and temporal distributions of OLR and convection over the Tibetan Plateau (TP) and the relationship between surface heat fluxes and convection were investigated. Results showed that the minimum value of OLR was located in the middle of the TP and increased outward. The OLR (convection) was lower (stronger) in the eastern plateau than in the western region. In the past 40 years, the OLR (convection) increased (decreased) gradually with a period of 6 a and 2–3 a over the TP. However, the tendency for convective activity had significant differences among different regions and seasons. A growing trend of convective activity occurred in boreal summer, while it weakened in the other seasons. The convection had a weakening trend in the vicinity of the Three Rivers’ Headstream region but increased in the north of the Himalayas on the TP. In the boreal summer, the surface latent heat flux was generally stronger than the surface sensible heat flux. The evolution of convection was significantly related to both surface sensible and latent heat fluxes. The surface sensible heat flux was highly negatively correlated to the evolution of convective activity, while there was an east-west dipole pattern of correlation between the surface latent heat flux and convection. A positive correlation between the surface latent heat flux and the evolution of the convection in the western plateau was observed, while a negative correlation was evident with that in the eastern region.
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doi: 10.3878/j.issn.1006-9895.2101.20228
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doi: 10.3878/j.issn.1006-9895.2104.21011
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doi: 10.3878/j.issn.1006-9895.2103.20205
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doi: 10.3878/j.issn.1006-9895.2103.20254
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doi: 10.3878/j.issn.1006-9895.2101.20201
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doi: 10.3878/j.issn.1006-9895.2101.20240
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doi: 10.3878/j.issn.1006-9895.2008.19249
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doi: 10.3878/j.issn.1006-9895.2008.20157
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doi: 10.3878/j.issn.1006-9895.2011.20164
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doi: 10.3878/j.issn.1006-9895.2102.20173
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doi: 10.3878/j.issn.1006-9895.2012.20195
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doi: 10.3878/j.issn.1006-9895.2101.20239
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doi: 10.3878/j.issn.1006-9895.2011.20181
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doi: 10.3878/j.issn.1006-9895.2009.20101
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doi: 10.3878/j.issn.1006-9895.2010.20116
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doi: 10.3878/j.issn.1006-9895.2007.20128
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doi: 10.3878/j.issn.1006-9895.2008.20137
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doi: 10.3878/j.issn.1006-9895.2007.20144
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Since 1976 Bimonthly
Supervisor: Chinese Academy of Sciences
Sponsors by: Institute of Atmospheric Physics, Chinese Academy of Sciences, Chinese Meteorological Society
Editor: Lu Riyu
Email: dqkx@mail.iap.ac.cn
dqkx@post.iap.ac.cn
ISSN 1006-9895
CN 11-1768/O4
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