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Abstract(68) HTML(9) PDF (17273KB)(41)
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The analysis of circulation background and trigger mechanism of an extreme precipitation weather process was carried out in the Jilin area on July 13−14, 2017 using the ERA-Interim reanalysis data, conventional meteorological observation data, CMORPH（CPC MORPHing technique）fusion precipitation data, and high-resolution numerical simulation results of WRF（Weather Research and Forecasting）. The observed results are as follows: (1) The large-scale circulation background of precipitation was found in the northward movement of the subtropical high- and cold vortex over Northeast China and mid-latitude front zone. Precipitation occurred between the bottom of the cold vortex and the subtropical high in the straight zonal circulation. Clod trough, low-level shear line, upper-level jet stream, and low-level jet stream are all critical weather systems that influence precipitation. (2) Extreme water vapor transport, blocking, and forced uplift of the low west and east high terrain in the Jilin area are the factors that cause extreme precipitation in the context of high-level divergence and low-level convergence. (3) Dry and cold air intruded into the middle and upper levels, and high-level momentum was transmitted to the low altitude. Thus, the low-level jet was strengthened, which generated an ultra-low-level jet near the surface, enhancing the vertical ascent movement. The intersection of north-south meridional momentum transport strengthened low-level wind convergence shear. The development of convection along the shear line and the uplift of nearby small terrain led to extreme precipitation in Yongji County.
Abstract(96) HTML(21) PDF (20895KB)(48)
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During the Meiyu Period (June–July, JJ) of 2020, the Yangtze–Huaihe River Basin (YHRB) in China experienced record-breaking rainfalls, resulting in severe floods and disasters. These rainfall anomalies were closely related to the extremely strong anomalous anticyclone developed over the western North Pacific (WNPAC), which favored convergence of water vapor over YHRB. The aim of this study is to determine the cause of the record-breaking rainfalls and WNPAC in the Meiyu period of 2020. It was found that a weak Central-Pacific El Niño rapidly decayed in spring and developed to a La Niña in late summer, whereas sea surface temperature (SST) in the tropical Indian Ocean (TIO) and tropical northern Atlantic (TNA) was considerably and persistently high from the previous winter to summer. The results showed that the weak decaying El Niño alone was insufficient to sustain the strong WNPAC in JJ of 2020, whereas the long-lasting warm SST anomalies in the TIO and TNA significantly contributed to the enhancement and westward shift of the WNPAC. The TIO warming intensifies the WNPAC by propagating Kelvin waves eastward and/or modulating the Hadley circulation. The TNA warming can force a westward-extending overturning circulation over the Pacific–Atlantic Oceans, with a sinking branch over the central tropical Pacific, which suppresses the convection activity over there and gives rise to the WNPAC. The TIO and TNA warming significantly contributed to the extremely strong WNPAC in JJ of 2020.
Abstract(59) HTML(11) PDF (4342KB)(22)
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Fog, especially extremely dense fog, has adverse effects on transportation. This paper utilizes the fog droplet spectrum data measured using the FM-100 fog drop spectrometer at the Shouxian National Climate Observatory in January 2019 and the contemporary conventional meteorological observation data to investigate the microphysical characteristics of fog with different intensities. Based on the analysis of the relationships between the visibility (V) and liquid water content (L) and the number concentration (N) of fog droplets and RH (relative humidity), various visibility parameterization schemes were established. The results showed that: (1) With the increase of fog intensity, the water content in fog increased significantly, with average values of 0.003 g m−3, 0.01 g m−3, and 0.09 g m−3 during the periods of fog, dense fog, and extremely dense fog, respectively. When the L was greater than 0.02 g m−3, the proportion of extremely dense fog reached 95%. (2) The N and droplet size increased with the increase in fog intensity. From fog to dense fog, the N increased significantly by 67%, while from dense fog to extremely dense fog, the droplet size increased significantly, and the average diameter (D) and effective radius (Re) increased by 62% and 135%, respectively. When the Re was greater than 4.7 μm, the proportion of extremely dense fog reached 95%. (3) All spectra distributions of the droplet number concentration for fog, dense fog, and extremely dense fog exhibited a bimodal structure with major peaks close to the end of the small particles. The spectrum type of the extremely dense fog was a Deirmendjian distribution, while that of the dense fog and fog was a Junge distribution. The fog water mass concentration spectrum was characterized by multipeaks for extremely dense fog with maximum peak appearing at 21.5 μm, while the dense fog and fog exhibited a bimodal distribution and single peak type, respectively, with maximum peak at 5 μm. (4) Both L and N were inversely correlated with visibility, with L showing the highest correlation coefficient with visibility. Four kinds of visibility parameterization schemes were established using the full sample and segmented method. Test results indicated that the visibility subsection fitting scheme based on L was the best.
Abstract(42) HTML(5) PDF (6576KB)(18)
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Meteorological satellites have provided useful information for improving weather forecasting, environmental monitoring, and short-term climate prediction. In the field of the weather forecast, satellites provide a powerful means for the forecast of typhoons, rainstorms, hail, sandstorms, and other severe weather conditions. In this study, the microstructure of hail clouds was analyzed by satellite observation data based on nearly a decade of hail event records of Shaanxi, Shandong, Guizhou, and Xinjiang. The comparison between the hail cloud and deep convective precipitation cloud characteristics retrieved by polar orbit satellites showed different cloud properties such as cloud top temperature/effective radius and cloud glaciation temperatures. Based on distinct cloud properties between hail clouds and convective clouds, this work summarized the characteristics and further applied them to the FY-4A geostationary satellite, which captures the hail cycle, which occurred on August 16, 2019, in the Shandong area. Results showed that the satellite has the potential to capture a hail cloud during its developing stage and use it as an application of early warning. The hail cloud shows the following characteristics: (1) There are considerable differences in the cloud’s physical characteristics between hail clouds and deep convective precipitation clouds. Microphysical characteristics of hail clouds observed by satellites are shown in three aspects: Glaciation temperature（Tg）is cooler with an average value of −33°C. The hail cloud reaches the glaciation temperature with a smaller effective radius (<40 μm) with an average of 36.9 μm when the clouds are fully glaciated. It also shows that the smaller the reg (effective radius corresponding to glaciation temperature), the stronger the hail cloud. Additionally, hail cloud tops often have a reduction zone of re with increasing height. (2) All the studied areas have consistent cloud properties such as a lower Tg, smaller reg, and a decreased re compared to those of adjacent convective clouds. However, it still showed regional variabilities that indicate the need to establish different indicators for identifying hail clouds for early warning purposes. (3) The case study of the FY-4A geostationary satellite shows that the geostationary satellite can track the evolution of hail clouds. By tracking the hail cloud, the geostationary satellite has a response consistent with that of the polar orbit satellite, providing a method for monitoring and early warning service of hail weather. The geostationary satellite can be used to track the development and evolution of the cloud cluster at any time when the satellite detects a strong hail signal because of the high time resolution. Combining the satellite’s early warning with radar observation, the location of hail occurrence can be determined precisely. (4) Combining the indicators summarized by polar orbit satellites with the FY-4A to track the hail cloud evolution. Four hail storms that occurred in Shaanxi and Shandong were applied for early warnings. Ground observations reported 24 hail events in the two regions, of which the satellite successfully warned 22 times in advance and missed two times. The average early warning time is about two hours before the hail disaster. All of these suggest that the automatic warning of hail by the FY-4A satellite has important practical significance for timely and effective organization and implementation of operational hail mitigation.
Abstract(41) HTML(14) PDF (18255KB)(18)
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Microphysical processes in the Meiyu front rainfall have an important effect on the evolution of precipitation. Based on the WRF (version 3.4.1) model, one Meiyu front heavy rainfall case from 29 to 30 June is analyzed with three different microphysics schemes (Morrison, Thompson, and MY). The main findings are as follows. (1) The general large-scale circulation of the Meiyu rainfall case could be reasonably reproduced by all three experiments with different microphysics schemes, which was consistent with the ERA5 reanalysis data. The local circulation during the Meiyu front heavy rainfall was significantly influenced by microphysical processes and the differences in the local features between different experiments were evident. The local circulation and updraft in the Thompson experiment were stronger than those in the other two schemes. Precipitation in all the model output was overestimated and the hourly rain rate was always greater. The overestimation of the melting of ice phase hydrometeors or the accretion of cloud droplets by raindrops was one of the most important causes of the overestimation of the modelled precipitation. On the whole, the Morrison run performed relatively better. (2) The melting of ice phase hydrometeors and the accretion of cloud droplets by the raindrop were the key source terms to the growth of the raindrop. Moreover, the evaporation process was the most important sink term. On the whole, the raindrop collecting cloud droplet contributed more than the melting of ice phase hydrometeors to the growth of the raindrop. However, for each scheme, the differences of these microphysical process terms led to the difference of the modelled precipitation in distribution. (3) The Thompson run produced the largest amount of melting of ice phase hydrometeors and evaporation (especially in the low level). At the same time, it produced the largest amount of condensation that led to more collection of cloud droplets by raindrops. Therefore, the Thompson run produced the most raindrop and rainfall. The predominant product through the deposition and riming process was snow, and the largest amount of snow was produced. (4) Through the Bergeron process, the Morrison run produced more snow than graupel (ice particles nearly could be neglected), the Thompson run produced predominant snow, and the MY run produced more snow than ice particles (graupel nearly could be neglected). The largest amount of produced ice particles in the MY run through the process led to more ice particles than that in other schemes. (5) The cloud droplet contributed more than the raindrop in the riming process. In the Morrison and Thompson schemes, the amount of graupel collecting cloud droplet was larger than that through other riming processes. Other riming processes contributed to the growth of graupel in different degrees in the Morrison run, while other riming processes could nearly be neglected compared to the graupel collecting cloud droplet. Further, the MY run produced a larger amount of snow growth by deposition. Therefore, the differences of the Bergeron and riming processes in all three schemes led to the differences in the ice phase hydrometeors distribution.
Abstract(154) HTML(23) PDF (26461KB)(69)
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This study investigates the relative contributions of the interdecadal Pacific oscillation (IPO), Atlantic multidecadal oscillation (AMO), and global warming (GW) to the decadal variation of land precipitation in North America from 1934 to 2018. The singular value decomposition (SVD) analysis for the North American land precipitation and sea surface temperature (SST) in the middle and low latitudes reveal that the main SST modes affecting the interdecadal variation of land precipitation are the IPO and AMO, with values of 42.33% and 23.21% in winter and 21.60% and 32.66% in summer, respectively. A linear regression model is then used to analyze the relative contribution of the three signals (IPO, AMO, and GW). For North America, results show that the AMO contributes dominantly, while IPO only plays a secondary role to IPO in summer. However, the opposite behavior is observed in winter. GW is also found to play an important role. In summer, AMO is the primary contributor to the changes in precipitation in Alaska, whereas GW dominates in Canada. Meanwhile, GW, AMO, and IPO are of equal importance in the USA and Mexico. In winter, GW is the primary contributor to the changes in precipitation in Alaska and Canada, whereas IPO dominates in the USA and Mexico. In terms of information flow, this study presents the regions of sensitivity to the three modes. Results are verified using the ECHAM 4.6 model, showing that the Indian Ocean is pivotal in having the AMO and IPO in effect, causing the precipitation variabilities.
Abstract(76) HTML(13) PDF (13044KB)(28)
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From the aspects of unstable energy sources for convective development, uplifting mechanism, and vertical shear of environmental wind, this work studies how the monsoon flow affects the intensity and distribution of typhoon precipitation. For this, CMORPH (CPC morphing technique) satellite data product obtained from the CPC’s (Climate Prediction Center) morphing technique, JRA-55 (55-year-old Japanese reanalysis products), and TBB (blackbody brightness temperature) data from the FY-2E (FengYun-2E) satellite were used, and the strong monsoon-influenced case 0604 “Bilis” and weak monsoon-influenced case 0708 “Sepat,” which have similar paths and cause heavy precipitation in the southern area of the Hunan Province after landfall, were selected. The analysis showed that with a similar underlying surface and topography, the main causes of heavy precipitation of the two cases are different. This is mainly reflected in the factors that induce and maintain convection. The strong convergent updraft and convective instability under the strong monsoon background were the main factors leading to the heavy rain of “Bilis,” while the rainstorm of “Sepat” was mainly due to the local orographic lift and baroclinic instability under the weak monsoon background. Moreover, the precipitation of “Bilis” was obviously asymmetric, which indicated that the monsoon flow could lead to asymmetric precipitation by changing the vertical shear of the environmental wind indirectly.
Abstract(87) HTML(32) PDF (12492KB)(55)
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The WRF (Weather Research and Forecasting) model was used to numerically simulate the heavy snowfall process that occurred on 30 November 2018, in the Ili River valley and the northern slope of the Tianshan mountains and to analyze the vertical velocity and vertical kinetic energy change mechanism of heavy snowfall under complex terrain. Studies have shown that the passage of a cold front increases the surface pressure and the dry air mass in the column, leading to changes in the vertical pressure gradient force and the dry air column buoyancy, which in turn causes the development of vertical motion. The local time variation of vertical velocity mainly depends on the perturbation vertical pressure gradient force, drag force of the water substance, and perturbation dry air buoyancy. When the airflow crosses the north slope of the Tianshan mountains, the perturbation vertical pressure gradient force on the windward slope becomes larger, and the perturbation dry air buoyancy becomes smaller, promoting the upward movement. On the leeward slope, the perturbation vertical pressure gradient force and the perturbation air buoyancy form a downward total force produce sinking acceleration, which results in a strong sinking gale of the leeward slope. The work done by the perturbation vertical pressure gradient force is basically contrary to the work done by the perturbation dry air buoyancy. On the leeward slope, the work items of the perturbation vertical air pressure gradient force and the comprehensive force are to suppress the vertical kinetic energy, while the work item of the perturbation dry air buoyancy and the drag force of the water substance enhances the vertical kinetic energy. In addition, the work items of the perturbation vertical pressure gradient force and the perturbation dry air buoyancy mainly appear in the middle and low layers, the work items of the water material drag force are mainly located in the lower layers, and the comprehensive force work in flat terrain is significantly less than that in complex terrain.
Abstract(125) HTML(11) PDF (10375KB)(53)
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The dominant mode of anomalous precipitation in May over Southwest China and its association with the Arabian Sea monsoon are investigated in this study using the 115 observation stations and Japanese 55-year reanalysis for the period of 1960–2017. The result showed approximately consistent variabilities of the leading mode of May precipitation over Southwest China, exhibiting a close relationship with the Arabian Sea monsoon. However, around the late 1970s, the relationship experienced an interdecadal change. In 1960–1976, the anomalous atmospheric circulations and water vapor transportation associated with the Arabian Sea monsoon were mainly over the Arabian Sea to the Bay of Bengal, showing a weak influence on the May precipitation over Southwest China. However, in 1981–2017, the anomalous Arabian Sea monsoon was related to large-scale atmospheric anomalies from the northern Indian Ocean to the South China Sea, leading to anomalous water vapor and vertical motion over Southwest China. Therefore, the Arabian Sea monsoon can significantly influence May precipitation over Southwest China during 1981–2017. Further analysis indicated that the change in the relationship between the Arabian Sea monsoon and May precipitation over Southwest China could be related to the change in decadal variability of the Arabian Sea monsoon. Over the period before the late 1970s, the Arabian Sea monsoon’s variability was relatively weaker, and its related atmospheric circulation anomalies were also weaker. Whereas, after the late 1970s, the Arabian Sea monsoon’ s variability became stronger, and the related atmospheric circulation anomalies extended more eastward, covering Southwest China. Consequently, the Arabian Sea monsoon can significantly influence the May precipitation over Southwest China after the late 1970s. This result indicated the important role of monsoon variability in precipitation.
Abstract(68) HTML(12) PDF (26881KB)(29)
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The similarities and differences of monsoon circulations between 2016 and 1998 Meiyu periods in the middle and lower reaches of the Yangtze River following the previous strong El Niño years, together with the comparison of their physical mechanisms, are analyzed in this paper. The results show that: (1) The monsoon circulations during the Meiyu concentrated period in 2016 and the two Meiyu periods in 1998 have several similarities: the western Pacific subtropical high (WPSH) is stronger and is positioned westward more than usual; the South Asia high (SAH) is stronger and positioned more eastward; and the southwest monsoon from the Bay of Bengal to the South China Sea is weaker than normal. In addition, a cold trough is maintained from eastern North China to Jianghuai. The WPSH continuously guides the southwest monsoon to the middle and lower reaches of the Yangtze River, forming a strong warm and humid southwest airflow. The airflow converges with the dry and cold air flown from north to south out of the cold trough, causing heavy rainfall under the condition of high-level divergence. (2) For the three periods of Meiyu, they all have a warm ridge over Tibetan Plateau and the nearby area. The similar characteristics of monsoon circulations are due to (a) the positive temperature anomalies at the middle-upper levels over most of the regions from Tibetan Plateau to Jiangnan and South China, which are resulted from the warm advection, the heat sources from the Plateau, and the latent heat of condensation of Meiyu, and (b) the positive sea surface temperature anomaly (SSTA) with strong convection and heat sources over Indonesian archipelago. (3) During the 2016 Meiyu concentrated period, the combined effect of the strongest warm ridge over Tibetan Plateau, the shallowest eastern cold trough, and the greatest and the most northerly positive SSTA leads to the most northerly SAH and WPSH with the most northerly Meiyu rain belt. The end of the Meiyu concentrated period is related to the weakening of cold air and the significant increase in positive SSTA in the seas east of Taiwan and the Taiwan Strait, which caused the super typhoon “Nibert” to land. From the 4th pentad of July, the positive SSTA over the South China Sea, the eastern ocean of the Philippines, and the East China Sea increases rapidly with active convection which lead to a strong and obviously northerly WPSH after July 21, so there is no longer a second Meiyu concentrated period. (4) From the middle July to early August in 1998, the ridge over Tibetan Plateau is weak with negative geopotential height anomalies, which results in the strong cold air over northern Tibetan Plateau. The cold trough in the East is deep. Also, there is no significant changes of positive SSTA in Northwest Pacific. These factors make SAH and WPSH continue to be strong and abnormally southward, which results in the second period of Meiyu.
Abstract(62) HTML(32) PDF (9222KB)(26)
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The microphysical formation mechanism of hailstones is the scientific basis of hail suppression operation. However, the relevant study on this issue in southwestern China is few. A hailstorm case on July 11, 2016, in Yunnan is numerically simulated using the three-dimensional cloud model with hail-bin microphysics developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, and the microphysical formation mechanism of the hailstorm is investigated. The hail cloud developed rapidly with high intensity and is a typical summer hail cloud in the southwest mountainous region. The simulated physical parameters such as rainfall, hailfall, and reflectivity are generally consistent with those observed. The simulated maximum updraft is 28.5 m/s. The microphysical processes of the hailstorm indicate that the main source of hail/graupel embryos is produced by the probability freezing process of supercooled raindrops, which is accounted for 95%, whereas the source via the collision between ice crystals and supercooled raindrops is only accounted for 5%, which are significantly different from other regions in both China and other countries. The diameters of hail/graupel embryos mainly range from 0.3–3.0 mm. The growth of hail/graupel embryos depends on the collision process with the supercooled cloud water, in which the embryos with a diameter less than 0.3 mm are hard to grow. Larger raindrops can directly freeze into larger hail embryos, which can contribute to the formation of hailstones in a short period. A short-lasting accumulation zone of supercooled rainwater occurs in the development process of the hail cloud, and it has little contribution to the growth of hail/graupel embryos.
Abstract(96) HTML(16) PDF (9507KB)(26)
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This paper studies the atmospheric circulation characteristics of the abnormally heavy precipitation events over southern China in the fall of 2016 and the impact of Sea Surface Temperature (SST) based on the precipitation data from the weather stations in China, UK Hadley Center SST, and NCEP/NCAR reanalysis datasets. Results show that the subtropical westerly jet in East Asia was much stronger in the fall of 2016 and southern China was located to the right of the jet stream entrance, which was conducive to an ascending motion. The western Pacific subtropical high was also much stronger than normal, with a larger area and more northwestward location. The anomalous southeasterly winds on the southwest side of the western Pacific subtropical high transported warm and moist air from the tropical Pacific to southern China, leading to heavy precipitation there. Landing typhoons along the coast of southeast China also contributed to heavy precipitation. Further analysis shows that the heavy rainfall event was mainly related to the abnormally higher SST over the equatorial western and southeastern Pacific simultaneously on interannual time scales. These SST anomalies could affect East Asian atmospheric circulation through exciting downstream-propagating teleconnection wave trains or Gill-type atmospheric responses. The above results are further confirmed by a series of numerical model simulations, using Community Atmosphere Model Version 5.3 (CAM 5.3).
Abstract(78) HTML(16) PDF (16395KB)(24)
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In order to improve the performance of high-resolution regional numerical modeling, the impact from spectral nudging (SN) technique driven with the ECMWF data on the fine prediction of super typhoon Lekima (2019) in Zhejiang Province is investigated with the Weather Research and Forecasting (WRF) model, based on both the advantages of high accuracy forecasting data from European Centre for Medium-range Weather Forecasts (ECMWF) and SN technique. The results show that: (1) The improvement by using SN to track Lekima is obvious. For instance, the maximum hourly track error can be reduced by 80 km. Overall the improvement is the most significant for the track and intensity of Lekima (2019) when nudging is adopted above 800 hPa. (2) In Zhejiang Province, the fine predictions of gale and heavy rainfall induced by Lekima (2019) are largely improved through SN technique. Compared to the control experiment, the increased percentage of ETS (Equitable Threat Score) score for the wind stronger than 17.2 m s−1 is about 8% in mean and 20.8% in maximum. At the same time, the increased rates of TS (Threat Score) scores for the rainfall heavier than 50 mm (24 h)−1 and 100 mm (24 h)−1 reach 11.8% and 26.2%, respectively. (3) Further diagnosis illustrates that wind field spectral nudging can amend effectively the west deviation of typhoon track, the over-prediction of strong wind, and the heavy rainfall in southwest of Zhejiang Province, which all are related to the improvements in the atmospheric elements in troposphere, steering flow of Lekima, low-level wind field, and the resulted rainfall enhancement effect by the local topography.
Abstract(114) HTML(16) PDF (18396KB)(72)
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A simultaneous observation method combining a Ka-band polarized radar and an X-band polarized radar, both of which located at the same observation station of the Institute of Atmospheric Physics, Chinese Academy of Sciences, has been designed for the first time and discussed in this paper. The method has been applied to the study of the formation and development of the snowbands during the snowfall process of a frontal cyclone system in Beijing on February 14, 2019, and the lifecycle and vertical structure of the snowbands are analyzed. Results show that the vertical structure of the snowbands has similar layered structures as previously known, however is different from the four-layer structure composed of condensation layer, aggregation layer, riming layer and melting layer of rainbands. The snowbands only contain condensation layer, aggregation layer and riming layer formed from the seeding of the upper layer cloud to the lower layer cloud. Horizontal wind speeds vary from layer to layer, so the three layers of a snowband may not be aligned vertically. Multiple snowbands are continuously generated and developed to maintain the snowfall. Snowflakes are formed continuously in the snowbands until the condensation layer becomes empty, and the cloud is then split from this layer into multi-layers and dissipates from there, respectively. The study has proven that the simultaneous observations by the dual frequency polarized radars is necessary and efficient. It has extended the application capability of the Ka-band and X-band radars for the observation of snowfalls and enriched the understanding of snowbands in frontal cyclone systems.
Abstract(92) HTML(22) PDF (5456KB)(31)
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As one of the main processes affecting cloud and fog, the entrainment-mixing process has an important impact on the cloud/fog life cycle, precipitation formation, radiative transfer, aerosol indirect effect evaluation, and so on. This study discussed the entrainment-mixing mechanisms in a radiation fog from the microphysical and dynamical perspectives, which not only improved the theoretical understanding of entrainment-mixing mechanisms but also revealed the development and dissipation of radiation fog from a new perspective. Using the comprehensive field observational data in Nanjing during the 2006 and 2007 winter, entrainment-mixing mechanisms in nine fog cases were analyzed. First, a radiation fog event during December 10–11, 2007, was studied to understand the microphysical relationships and entrainment-mixing mechanisms during different phases in detail. Results showed that the extreme inhomogeneous entrainment-mixing was found in the mature phase, in which the volume-mean radius slightly changed as the number concentration and liquid water content decreased. The homogeneous entrainment-mixing was found in the rapid dissipation phase, in which all microphysical properties decreased simultaneously with positive correlations. Except for microphysical properties, the scale number was calculated as a dynamical measure for entrainment-mixing mechanisms. In the mature (rapid dissipation) phase, the scale number was small (large), indicating that the extreme inhomogeneous (homogeneous) entrainment-mixing was most likely to occur. The microphysical relationships of the other eight fog events were then examined, which indicated that the volume-mean radius had positive correlations with the liquid water content in general, i.e., homogeneous entrainment-mixing dominated. The research results were helpful to the development of parameterization schemes of entrainment-mixing mechanisms and provided a reference for the simulation and prediction of radiation fog.
Abstract(129) HTML(37) PDF (22397KB)(47)
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A warm-sector rainstorm occurring in Yongji, Jilin Province on July 13, 2017 was simulated utilizing the regional model WRF3.9, and the storm development process was reproduced, which included the stages such as the initiation of mesoscale convective cells and linear convective clusters, organized system development, and the formation of bow echos. Based on the simulation data, the characteristics cloud microphysices for the mesoscale convective systems were analyzed, and their impact on the warm-sector precipitation was discussed. The results show that the precipitation process in the Yongji occurred under favorable conditions in which multi-scale environmental structure was dominated by the northeast cold vortex. The mesoscale systems was mainly a cold cloud systems, in which there was a wide warm zone and a high location of supercooled water, with the coexistence of ice and supercooled water. The "seeding" effect of the coexisting area caused a large amount of graupel. The budget analyses of the mass- and heat-hydrometeors showed that during the triggering and organizing stage of the precipitation system showed that the main source of rainwater was the accretion growth of cloud droplets, and the main sink was the collection of raindrop by ice. During the bow-shaped echo stage, besides the accretion growth of the cloud droplets, the melting of graupel also served as the main source, while the main sinks were the evaporation of rainwater in the lower layer and the collection of rainwater by the graupel in the upper layer. The main heat source of warm-sector precipitation was the latent heat released from condensation of water vapor, and the main heat sink (cooling) was the evaporation of rain and cloud water. Furthermore, during the bow-shaped echo stage, the confluence of the inflow at the frontend with the air inflow at the backend above the cold pad on the ground brought water vapor into the upper layer. The "seeding" effect significantly increased the content of the graupel particles around the height of 8 km above the ground, which coincided with the high temperature area as generated by the releasing of a large amount of latent heat through the condensation of water vapor, causing the melting of a large amount of the graupel and resulting in a strong precipitation system.
Abstract(72) HTML(25) PDF (8356KB)(40)
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Based on observations from an aircraft that directly entered the snowfall cloud and MICPAS (Meteorological Information Comprehensive Analysis and Processing System), radar and satellite data, the macro and micro structure characteristics of a snowfall cloud in Shanxi Province on 29 November 2011, were analyzed. Notably, the radar echo of this snowfall process was mainly between 10 dBZ and 20 dBZ large stratiform cloud echo, inlaid with more than 30 dBZ massive and strong echo. The zero line of the radar radial velocity had a strong “S” curve with a bull’s eye structure. Strong vertical wind shear was observed from the lower level to the upper level. The liquid water content was mainly below 3.2 km, and the maximum value was 0.0697 g m−3. The ice and snow crystal number concentrations N50 (Number concentration of ice and snow crystals with particle diameter greater than 50 μm) and N200 (Number concentration of ice and snow crystals with particle diameter greater than 200 μm) and the ice water content were mainly produced in the upper part of the stratiform mixed-phase snowfall cloud, and the maximum values, which were 188.4 L−1, 33.5 L−1, and 0.121 g m−3, were obtained at approximately −9.3°C. From −14.4°C to −19.7°C, the images of ice crystals were mainly needle, columnar, and irregular, and the ice particles grew mainly by deposition. The images of ice and snow crystals at approximately −9.3°C were mainly dendritic and irregular. The aggregation, collision, and fracture of radial dendritic ice crystals may be the main reason for the high concentration of ice and snow crystals. The distribution of the ice and snow crystal spectra can be well fitted by the exponential form. The spectrum fitting parameters can be expressed by the power function Nos=1.021λ1.684 (Nos and λ represent intercept and slope, respectively. The larger Nos was, the higher the concentration of small particles was. The larger λ was, the higher the proportion of small particle number concentration to total particle number concentration was.), and the correlation coefficient R2 was 0.86. Three inversion times below 3.2 km were detected. The appearance of the inversion layer influenced the cloud microphysical characteristics, decreased the fitting parameter Nos, and inhibited the growth of rimming and deposition. As a result, the variation of the fitting parameters Nos and λ with temperature in this study was inconsistent with that in previous studies. The greater the inversion intensity was, the greater the inhibition effect.
Abstract(44) HTML(14) PDF (11770KB)(18)
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Framework design and observation selection are mainly for meso- and small-scale analyses; hence, kilometer-scale data assimilation (DA) systems often suffer from insufficient large-scale analysis capabilities. This work adds an extra large-scale constraint to the cost-function of the GRAPES (Global/Regional Assimilation and Prediction System) regional 3-km variational DA framework to study the impacts of introducing large-scale information of the global system on the kilometer-scale DA and forecast. Results of numerical experiments in one month show that the introduction of a large-scale constraint can greatly improve the analysis and forecast capabilities of the synoptic situation field, increase the precipitation forecast scores, and reduce the analysis and forecast error of the 2 m temperature and 10 m wind. Furthermore, results of the quantitative precipitation sensitivity tests show that the large-scale constraint of the temperature and humidity field is a crucial factor in improving the precipitation scores. Results also indicate that the humidity field constraint is important for reducing the precipitation false alarm and improving the TS (Threat Score) scores for short-term precipitation forecast, while the temperature field constraint is important for improving the TS scores for longer forecast ranges. In addition, under the condition of introducing the large-scale constraint, the analysis and prediction results of the experiment with the full cycling scheme (no cold start during one month of cycling) are equivalent to that of the experiment with a partial cycle (daily cold start). This laid a good foundation for the GRAPES kilometer-scale system to adopt the full cycling scheme to further simplify the cycle process and reduce the calculation consumption.
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Besides the winter mean temperature, the conspicuous intraseasonal oscillation of East Asia winter air temperature (EAT) also exerts a large influence on human activities and the economy. Based on the station data and the reanalysis data during the period of 1959–2018, the intraseasonal spatiotemporal variability of East Asian winter temperature was analyzed by using the method of season-reliant Empirical Orthogonal Function (S-EOF). The results show that the spatiotemporal variability of EAT is mainly characterized by the intraseasonal phase inversion variation pattern, which means warmer (colder) than normal in early winter and colder (warmer) in late winter. This mode accounts for 31.1% of the temperature variance. It is not a local phenomenon, but instead represents the intraseasonal winter temperature over the entire Asian continent. The circulation evolution of this mode is mainly characterized by the phase inversion of the Eurasian teleconnection pattern (EU) from a positive (negative) phase in early winter (December) to a negative (positive) phase in late winter (February), accompanied by the turning of lower Siberian high and Aleutian low intensity, and the intraseasonal evolution of the subtropical jet. The possible influencing factors which affect the EU phase reversal are as following: First, the circulation over the North Atlantic reverses between early winter and later winter and influences the EU through the North Atlantic heat flux. Second, wide and narrow sea surface temperature (SST) anomalies resulted from ENSO events also contribute to the reversal. In El Niño years, the events of wider (narrower) SST anomalies are more likely to inverse from the phase of warmer (colder) temperature in early winter to colder (warmer) temperature in late winter. When La Niña events occur, the influence is opposite.
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A new Ka/Ku dual-wavelength (Ka- and Ku-band wavelengths are 8.9 mm and 2.2 cm, respectively) cloud radar (DWCR), an upgraded form of the Ka-band CR (cloud radar), was used to observe the vertical structures of cloud and precipitation in South China to improve the observational ability of vertical velocity for air motion (Vair) and microphysical parameters. Based on the study of the dynamic and microphysical parameters of precipitation in South China using the DWCR, this paper presents a retrieval algorithm (DWSZ) for the Vair, raindrop size (DSD), liquid water content (LWC), rain rate (R), and radar wave attenuation correction in rain areas with the reflectivity spectral density (SZ) data of the DWCR. The Vair was studied using the DWSZ and single Ka- and Ku-band CR in a precipitation case in Longmen, Guangdong Province. The Vair and microphysical parameters retrieved by the DWSZ were examined using the DSD data from a disdrometer. The results revealed that the DWSZ provided acceptable Vair, microphysical parameters, and vertical distributions. The attenuation correction for the Ka- and Ku-band reflectivity data minimized the bias between the two reflectivity bands. The DWCR can detect cloud and precipitation with a reflectivity of 0–30 dBZ. In the mixed cloud detected by the DWCR, the updraft and downdraft were found in front of and behind the convective cells. According to the averaged profiles, the Vair and mass-weighted mean diameter (Dm) reached their maximum at 2 km, and the drop number concentration (Nw), LWC, and R increased below this level. Water vapor condensation, raindrop collection, and cloud drop are possibly the main processes. This work revealed the relationships between the microphysical and dynamic processes in this precipitation case, verified the DWCR observational ability, and provided the basis for the development of Ka/Ku-band DWCR, future improvement of single-wavelength CR DWSZ, and observation of finer and more accurate structures of the microphysical and dynamic parameters.
Abstract(240) HTML(7) PDF (8972KB)(44)
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VOCs (volatile organic compounds), key precursors to the ozone pollutant, are indispensable parts of urban air quality modeling. Owing to their complex composition and the lack of monitoring data, understanding of their simulation accuracy is still poor. Here, simulations of VOCs in the Pearl River Delta region were performed using the nested grid air quality model prediction system (NAQPMS) from 21 September to 20 November 2017. First, monitoring of VOC concentrations from eight ground stations of the photochemical assessment monitoring stations was conducted to evaluate the accuracy of key VOC components. The results showed that the model has high simulation accuracy for toluene, ethylene, and xylene with concentration deviation ratios of 0.4%–26.6%, which can well reproduce the trend of daily average concentration and double-peak characteristics of diurnal variation. However, the model has a large simulation deviation for isoprene with strong chemical reaction activity and closed relation to plant emissions. The deviation ratio was nearly 100%, which cannot reproduce the diurnal variation characteristics of high concentration during the day and low concentration at night. The total amount of VOCs emitted by plants in the Pearl River Delta region was relatively high. Hence, ignoring the biological VOC emissions in the current simulation system can be the key reason for this simulation deviation. In addition, the results of simulation evaluation showed that the model still has great uncertainty in the VOC spatial distribution. With these, combining VOC experimental data with simulations to reveal and reduce the uncertainty of VOC simulations is necessary.
Abstract(133) HTML(18) PDF (19302KB)(47)
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This study analyzes the microphysical properties of precipitating and nonprecipitating warm clouds based on seven-flight cloud measurements from 6 November 2014 to 25 December 2014 in Jiangxi Province. The autoconversion threshold function (T) represents the probability of collision–coalescence process occurrence in clouds, which is critical for determining the initial time and intensity of precipitation. The authors found that, in general, T increases with height above the cloud base, with the maximum value occurring in the middle and upper parts of clouds. The occurrence frequency of T>0.6 in the precipitating clouds is larger than that in the nonprecipitating clouds, indicating a stronger collision–coalescence process and a greater probability of raindrops generated by the condensation and collision–coalescence processes in the precipitating clouds. There is a negative relationship between the relative dispersion of cloud droplet size distribution (ε) and the number concentration of cloud droplets (Nc), and this negative relationship becomes more evident with increasing T. Compared with the average radius of cloud droplets (ra), the standard deviation of cloud droplet size distribution (σ) dominates the enhancement of the degree of the negative relationship between ε and Nc.
2021 Issue 4
Display Method:
2021, (4): 1-2.
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2021, 45(4): 697-712.   doi: 10.3878/j.issn.1006-9895.2009.20106
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As a critical system of the East Asian winter monsoon, the Siberian high has an important impact on the winter weather and climate anomalies in Eurasia. Using the National Center for Environment Prediction-Climate Forecast System version 2 (NCEP-CFSv2), this study comprehensively evaluates the seasonal and monthly prediction of the Siberian high intensity during the winter time (November to February). Results show that the NCEP-CFSv2 model can skillfully predict the Siberian high intensity only in November, the reasons for which are that the local thermal process, dynamic process, and Siberian snow cover extent mainly affect the Siberian high intensity in November. In terms of the thermal process, the NCEP-CFSv2 can better reproduce the Siberian high intensity in November and its related surface soil temperature, upward long-wave radiation, and other thermal factors in Siberia. In terms of the dynamic process, the NCEP-CFSv2 can better reproduce the Siberian high strength in November, which is associated with the low-level tropospheric divergent circulation and the sinking movement of the upper and middle layers in the Siberian area. The model also reproduces the relationship between the snow cover extent over Siberia and the Siberian high intensity in November. The thermodynamic process of the Siberian high and snow cover extent in the area are predictability sources of the Siberian High intensity in November, and the NCEP-CFSv2 can reasonably reproduce these predictability sources in November.
2021, 45(4): 713-724.   doi: 10.3878/j.issn.1006-9895.2101.20216
Abstract(682) HTML(117) PDF (7684KB)(309)
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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.
2021, 45(4): 725-745.   doi: 10.3878/j.issn.1006-9895.2010.20171
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In this paper, two earth system models, i.e., CAS-ESM (Earth System Model, Chinese Academy of Sciences) and NCAR CESM (Community Earth System Model, National Center for Atmospheric Research), are used to carry out a series of simulations with different parameterizations and horizontal resolutions, and the daily precipitation characteristics of Eurasia are analyzed. Four 19-year (1998–2016) Atmospheric Model Intercomparison Project simulations have been conducted in this study. They included CESM with the Community Atmosphere Model version 5 (CAM5) physics package at a resolution of 1.9°×2.5°, CAS-ESM with the CAM4 (Lcam4) and CAM5 (Lcam5) packages at a resolution of 1.4°×1.4°, and CAS-ESM with the CAM5 package at a resolution of 0.5°×0.5° (Hcam5). Compared with GPCC (Global Precipitation Climatology Centre) and CMORPH (CPC MORPHing technique), both models competently reproduced the climatological means of precipitation and extreme precipitation. However, the two models overestimated precipitation frequency and underestimated precipitation intensity. CESM produced the smallest biases in the number of heavy rain days, and Hcam5 showed added value for maximum daily precipitation. For CAS-ESM, both the physics package and horizontal resolutions impacted daily precipitation characteristics. Noticeable improvements in precipitation characteristics were demonstrated by Hcam5. The precipitation frequency of Lcam4 was higher than that of Lcam5 in the middle and high latitudes of Eurasia. Over eastern China, the biases of precipitation frequency for Hcam5 were smaller than those of Lcam5, and the high-resolution model distinctly improved the simulation of extreme precipitation. Further, compared with Lcam5, Lcam4 produced more large-scale precipitation and stronger water vapor fluxes over Europe. Over eastern China, improvements in daily precipitation frequency and extreme precipitation related to the reduction in convective precipitation frequency, greater large-scale rainfall, and stronger water vapor fluxes.
2021, 45(4): 746-758.   doi: 10.3878/j.issn.1006-9895.2009.20112
Abstract(200) HTML(38) PDF (31868KB)(96)
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In this paper, the mesoscale weather research and forecasting (WRF) model is used to simulate mountain-valley wind circulation in the region of south Gaoligong Mountains in the dry and wet seasons, in 2016, respectively. The applicability of five layer parameterization schemes (YSU, MYJ, MYNN3, ACM2 and BouLac) under complex underlying surface of the Gaoligong Mountains is compared. The results show that the YSU scheme performs best in temperature simulation. The mean absolute error of wind speed simulated by the ACM2 scheme is the smallest, and the absolute error of wind direction simulated by the MYNN3 scheme is the smallest. The diurnal variation of wind direction simulated by the YSU and MYJ schemes is more consistent with the observation. Valley wind circulation in the southern Gaoligong Mountains appears at 0900 BJT and turns to mountain wind circulation at 1900 BJT. Southerly wind mostly prevails in the daytime and northerly and westerly wind at night. During the day, the airflow converges at the top of the mountain and diverges in the valley. At night, it is opposite. The wind speed in daytime is greater than that at night. In the dry season, the west wind is weak, which is conducive to the development of local circulation in the lower troposphere. In the wet season, the eastward background wind is strong, suppressing the development of local circulation; thus, the height of the boundary layer is shorter than that in the dry season. In the dry season, the westerly wind meets the Gaoligong Mountains, sinks on the west slope, and forms vortices, and then the turbulence on the west side is well mixed, leading to a deep boundary layer. In the wet season, the easterly wind weakens the valley wind on the west side of Gaoligong Mountains, and the height of the boundary layer between Tengchong and Baoshan is similar.
2021, 45(4): 759-776.   doi: 10.3878/j.issn.1006-9895.2010.20121
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Atmospheric ice nucleating particles (INPs) trigger the heterogeneous ice nucleation, which significantly affects the microphysics and radiative properties of ice clouds. For better understanding of the aerosol-cloud interactions and their associated climatic effects, it is crucial to elucidate the abundance, sources, and ice nucleation mechanisms of INPs in the atmosphere. This review compiles the recent progress made in the INP detection techniques, ice nucleation activities and mechanisms of representative aerosol particles, and the developed ice nucleation parameterizations based on the field and lab investigations. The critical challenges and open questions in the atmospheric INP research field are pointed out. Finally, we highlight the urgent needs to promote the ice nucleation studies in China.
2021, 45(4): 777-798.   doi: 10.3878/j.issn.1006-9895.2005.20141
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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.
2021, 45(4): 799-818.   doi: 10.3878/j.issn.1006-9895.2007.20149
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This study first uses the volumetric soil water content data from 732 stations in China, as provided by the National Meteorological Information Center (NMIC) at the China Meteorological Administration (CMA), to evaluate the simulations of the Community Land Model version 4.5 (CLM4.5) driven by Climate Forecast System Reanalysis (CFSR) near surface meteorological data (referred as to CLM4.5-CFSR). Then the CLM4.5-CFSR is used to investigate the spatial-temporal characteristics of soil moisture memory in China region during 1980–2009. The soil moisture memory is calculated by both Pearson correlation method and autocorrelation method. The effects of precipitation frequency, precipitation intensity and near-surface temperature on soil moisture memory are then explored. The results show that CLM4.5-CFSR can reflect the soil moisture changes on the monthly time scale in China. The spatial distributions of soil moisture memory from two methods are similar, but their seasonal characteristics are different. The soil moisture memory duration does not vary very much with soil depth, ranging from 0.85–2.2 months across China with relatively larger magnitude in the northeast of the Inner Mongolia, while smaller in the southwest of Xinjiang Province. In spring, the wetter the soil, the longer the soil moisture memory. When the precipitation frequency is low, it has little effect on soil moisture memory in areas where the evaporation rate is high. When the precipitation intensity is high, soil moisture memory will be shortened by it because it can replenish the soil water rapidly and destroy the initially dry or wet conditions of the soil. Changes in the near-surface temperature will shorten the soil moisture memory through its effect on the soil evaporation. In the future, it is necessary to carry out climate model sensitivity experiments to gain insights on physical mechanisms associated with the conclusions obtained in the current study, which in turn to provide a basis for further improvements of the seasonal and intraseasonal precipitation prediction.
2021, 45(4): 819-832.   doi: 10.3878/j.issn.1006-9895.2104.20159
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Air pollution resulting from regional development is having a significant impact on agroecological regions of North China. To understand the seasonal distribution characteristics of atmospheric PM2.5 (fine particulate matter) pollution in agricultural areas of North China, seasonal PM2.5 analysis was conducted at the Yucheng Comprehensive Experimental Station of the Chinese Academy of Sciences in Yucheng, Shandong Province in July, September, December 2017, and April 2018. Samples were collected and 30 chemical components were determined for each sample. The results showed that the overall concentration of carbonaceous aerosols was 13.11±8.37 μg m−3. The OC (organic carbon) concentration was high in winter and spring. The EC (elemental carbon) concentration was high in autumn and winter. Simultaneously, the OC/EC ratio was significantly low in autumn, indicating that the contribution of secondary carbonaceous aerosols to PM2.5 was small in autumn. The overall concentration of water-soluble ions is highest in winter and lowest in spring. The \begin{document}${\rm{NO}}^-_3$\end{document}/\begin{document}${\rm{NO}}^{2-}_4$\end{document} ratio, representing the contribution of traffic and fixed point sources (such as factories) to atmospheric pollution, is significantly low in summer at 0.69, indicating that the contribution of fixed point sources to air pollution is relatively high in the summer. The trace metals in atmospheric particles are mainly crust elements such as Na, Mg, Al, Ca, K, and Fe. The annual average concentration levels of trace metals such as Co, Cr, Ni, Pb, and As are 0.32±0.24 ng m−3, 5.40±5.42 ng m−3, 10.23±7.46 ng m−3, 42.23±27.75 ng m−3, and 5.66±3.79 ng m−3. The calculated results of the PMF (Positive Matrix Factorization) receptor model show that the main sources of PM2.5 are secondary pollution sources, biomass combustion sources, coal-fired fuel sources, diesel vehicle exhaust, and soil sources with contribution rates of 37.1%, 18.2%, 14.2%, 9.4%, and 7.9%. Pollution from fine particles is affected by the multiple effects of industry and agriculture in the North China agricultural region.
2021, 45(4): 833-850.   doi: 10.3878/j.issn.1006-9895.2009.20160
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Based on the reanalysis data and monthly precipitation data, in this study we investigate the variabilities of the western North Pacific subtropical high indexes (WPSHI) and discuss the relationship between WPSHI and summer precipitation in eastern China, together with the related circulation changes. Results show that WPSHI can be divided into three categories, namely, absolute intensity index (ASI), relative intensity index (RLI), and north-south index (NSI). The upward trend in ASI is associated with the expansion of the area covered by 5870 gpm isolines. The downward trend in RLI accords well with the shrink of the 0 gpm isoline of the eddy geopotential height (EGH). The lack of variations in NSI indicates that the ridge of WPSH oscillates around 25°N. Depicted by circulation anomalies and precipitation patterns, ASI cannot reflect the variation of local relative vorticity of WPSH, but it has close relationship with EAP (East Asia–Pacific) pattern. In this case, ASI has the best correlation with the precipitation in the Yangtze River basin. In the years when the ASI is high, north wind anomalies favor less rainfall in northern China, while the water vapor convergence strengthens the precipitation along the Yangtze River, and vice versa. RLI can better describe the variation of the local relative vorticity of WPSH, but it has a weaker relationship with the EAP pattern and the precipitation in eastern China. In the years when the RLI is high, precipitation center appears in the middle reaches of the Yangtze River. Conversely, drought occurs in the north of Yangtze River with anomalous north winds over China. NSI can describe both the variation of local relative vorticity of WPSH and the EAP pattern and has a high correlation with precipitations in southern and northern China. In the years when the NSI is high, enhanced vapor flux leads to flood in northern China and drought along the Yangtze River. When the NSI is low, the precipitation is trapped in southern China, as resulting from a reversed circulation pattern.
2021, 45(4): 851-862.   doi: 10.3878/j.issn.1006-9895.2008.20161
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Based on regional model simulations of a typical case of an eastward-moving Southwest vortex with the eastern retreat of a subtropical high (case 20150721), this paper discusses the effect of the strength of the subtropical high on the eastward Southwest vortex based on a numerical sensitivity test of a subtropical high enhancement. The main results are as follows: (1) An enhanced subtropical high can be stably maintained for a long time and can have a lasting impact on the development and evolution of the entire Southwest vortex meso-scale system. The area and intensity of precipitation directly depends on the path and intensity of the Southwest vortex. (2) The circulation field first changes when the strength of the subtropical high increases, the invading north wind becomes weaker, and the southwest guided airflow becomes stronger, which eventually results in a faster and weaker Southwest vortex. (3) The change in the circulation field directly affects the transport, convergence, and divergence of the water vapor, and further affects the whole evolutionary process of the Southwest vortex. (4) After the increase in the subtropical high, the faster Southwest vortex results in the deviation between the center of the Southeast vortex and the thermal center in the lower layer, which causes a mismatch between the dynamic and thermal centers and a weakening of the strength of the Southwest vortex.
2021, 45(4): 863-873.   doi: 10.3878/j.issn.1006-9895.2104.20143
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The characteristics of the Mesoscale Convective Complex (MCC) from convective initiation to mature stage over the Sichuan Basin in 2018 were analyzed using high-frequency FY-4A satellite data. The results indicated that the area of MCC convective cloud increased at a rate of 0–50 pixels (15 min)−1 at the convective–initiation stage and at 150–200 pixels (15 min)−1 at the mature stage, finally reaching 7, 000–10, 000 pixels. The maximum temperature gradient is located on the side of the low-level inflow zone at the convective–initiation stage, which is concentrated near the 240 K contour with the most abundant texture in the cloud top, having a maximum value of 30°C–40°C. This feature basically disappears at the mature stage. The variation in the minimum infrared radiation 1 channel (IR1) and water vapor channel (IR3) at the convective–initiation and mature stages are consistent; both rapidly decrease to 190 K at the convective–initiation stage, while the minimum value remains unchanged at the mature stage. The distribution patterns of cloud top cooling rate (R) of IR1 and IR3 are also similar. The significant R, which is close to the 240 K on the side of the low-level inflow zone, reaches −40 K (15 min)−1 at the convective–initiation stage but remains stable from −25 to −10 K (15 min)−1 at the mature stage. The maximum brightness temperature difference (D) is 6–10 K at the convective–initiation stage and 0～6 K at the mature stage. Moreover, R(D) in the low-level inflow zone is the most significant at the convective–initiation stage, remaining steady at the mature stage.
2021, 45(4): 874-888.   doi: 10.3878/j.issn.1006-9895.2011.20164
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The Singular Vectors (SVs) that include the linearized moist physical process in calculations are called Moist SVs (MSVs). The sensitivity study of MSVs to horizontal resolutions and optimization time intervals (OTI) is important for the ensemble forecasting system. Based on the operational version of Global/Regional Assimilation and Prediction System-Global ensemble prediction system (GRAPES-GEPS), which is independently developed by the China meteorological administration’s numerical forecast center, this paper analyzes the characteristics of the subtropical MSVs and their ensemble forecasts under four groups of experiments with different horizontal and temporal resolutions. The characteristics of MSVs in terms of energy norm, energy spectrum and spatial profile are analyzed, and the evaluation of the ensemble forecast with the four groups of experiments is made in terms of isopressure variable scores, precipitation scores, and precipitation probability predictions. An increase in the horizontal resolution of MSVs leads to an increase in the growth rate of their perturbation. The upward propagation of MSV energy is more obvious than the downward propagation with the reduced OTI, which also produces relatively large SV perturbations in the mesoscale ranges. Under different OTIs, the initial MSVs are less similar to each other and their structures are different from each other. From the perspective of ensemble forecasting, the average ensemble perturbed energy with the 24-h OTI increases greatly, and the ensemble spread is improved for the 0- to 96-h prediction, especially for the 2-m temperature and the outlier scores of the near-surface variables. It is further found that increasing the horizontal and temporal resolutions can improve the precipitation probability prediction. The precipitation scores show that at the same spatial resolution, the shorter the OTI, the better the scores, while increasing the horizontal resolution of the MSVs fails to improve the precipitation scores for the light to moderate rains.
2021, 45(4): 889-900.   doi: 10.3878/j.issn.1006-9895.2011.20168
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Based on the NCEP-DOE reanalysis data during 1979–2019, this paper investigated the relationship and correlation mechanisms between the frequency of winter extremely low temperature events in mid-latitude Asia and the abnormal warming over Barents and Kara Seas. The results show that the abnormal warming over Barents and Kara Seas usually corresponds to the enhancement of Siberian high and the frequent occurrence of extreme low temperatures in mid-latitude Asia. It is found that the positive geopotential height anomaly appears over Novaya Zemlya and Urals with the abnormal warming over the Barents and Kara Seas. Significant cold advection and sinking motion anomalies appear on the east and south sides of the positive geopotential height anomaly and the abnormal divergent flow appears near the ground, which makes Siberian high strengthening and extending to Southeast. The diagnosis using thermodynamic equations further indicates that cold advection anomaly caused by the northeast wind on the south side of the Siberian high pressure anomaly, the ascending motion anomaly caused by the easterly wind anomalies passing through the terrain, and the abnormal diabatic cooling caused by radiation, sensible heat, and latent heat result in an increase in the amplitude of the near-surface temperature’s seasonal cycle in mid-latitude Asia, which in turn favors the frequent occurrence of extreme low temperatures.
2021, 45(4): 901-914.   doi: 10.3878/j.issn.1006-9895.2010.20187
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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.
2021, 45(4): 915-930.   doi: 10.3878/j.issn.1006-9895.2103.20205
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Abstract:

Since 1976 Bimonthly

Sponsors by: Institute of Atmospheric Physics, Chinese Academy of Sciences, Chinese Meteorological Society

Editor: Lu Riyu

ISSN 1006-9895

CN 11-1768/O4