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2018 Vol. 23, No. 1

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Capability Assessment of CMIP5 Models in Reproducing Observed Climatology and Decadal Changes in Summer Rainfall with Different Intensities over Eastern China
Zhaohui LIN, Xiaoyu YANG, Chenglai WU, Yuxi WANG, Hong CHEN
2018, 23(1): 1-25. doi: 10.3878/j.issn.1006-9585.2017.16207
The capabilities of 17 CMIP5 models for simulating the intensity distribution of summer rainfall over eastern China are evaluated based on daily observational data. The decadal changes of rainfall with different intensities in the later 1970s and their relative contributions to the decadal change in total rainfall for both observation and model simulations are further analyzed and compared. Observations indicate that the total rainfall is mainly composed of light and medium rainfall over northern and northeastern China, while heavy rainfall account for a large proportion of the total rainfall over southern China and the Yangtze-Huai River basin (YHRB). In general, the CMIP5 models are able to capture the observed spatial distribution of proportion of light and heavy rainfall to total rainfall amount in Eastern China, except for medium rainfall. Most models have a bias toward an overestimation of the light and medium rainfall events, and an underestimation of heavy rainfall events. Therefore, these models overestimate the amount of total rainfall over northern and northeastern China and underestimate the amount of total rainfall over southern China and the YHRB. Our analysis indicates that the observed decadal changes in total rainfall over northern China and the YHRB in the late 1970s are mainly attributed to changes in heavy rainfall events. Over northern China, a few CMIP5 models reproduced the observed decadal decrease in both heavy rainfall and total rainfall. Over the YHRB, several models can reproduce the observed decadal increase in total rainfall. However, these models failed to reproduce the increase in heavy rainfall amount due to the model bias of a severe underestimation of heavy rainfall. The authors also find out that multi-model ensemble technique cannot significantly improve the model performance in simulating the spatial distribution of rainfall intensity, especially its decadal changes.
Interannual and Seasonal Trend Analysis of Vegetation Condition in Xinjiang Based on 1982-2013 NDVI Data
Yonggang MA, Yue HUANG
2018, 23(1): 26-36. doi: 10.3878/j.issn.1006-9585.2017.16116
Based on GIMMS 3g (the third generation Global Inventory Modeling and Mapping Studies Normalized Difference Vegetation Index) data, three preprocessing methods including monthly aggregation, standard anomaly computation, and trend-preserving prewhitening were used to develop six data series. Seasonal trend analysis was applied to extract three seasonal representative factors, i.e. amplitude 0, amplitude 1, and phase 1 to detect the characteristic of seasonal trend. The interannual and seasonal trend analysis was conducted using CMK (Contextual Mann-Kendall) and MK (Mann-Kendall) trend test methods. Land use and cover data was used as an accessory to identify spatial distribution pattern of areas with significant changes. The difference between preprocessing method and trend test method was also discussed. The result shows that:(1) The proportional area of vegetation deterioration is higher than the proportional area of vegetation improvement; the former is mainly located at areas of unused lands and grasslands, and the latter is found over grasslands, unused lands and farmlands. (2) The amplitudes of annual variability show a significant increasing trend mainly over grasslands, unused lands and farmlands in the southern margin of the Tarim basin. (3) Different preprocessing methods have obvious impacts on the result of trend analysis. According to the ability of these methods to extract significant trend information, they are in the sequence of standard anomaly > trend-preserving prewhiting > original data > monthly aggregate. (4) 87.88% of farmlands demonstrates a significant increase trend in annual variation amplitude and 53.31% of farmlands shows a significant trend of delayed onset of growing season.
Estimation and Diurnal Variation Analysis of Clear-Sky Surface Longwave Radiation Fluxes from Himawari08 Satellite
Xiao WU, Min MIN, Lixin DONG
2018, 23(1): 37-46. doi: 10.3878/j.issn.1006-9585.2017.16152
Abstract(2572) PDF (4289KB)(1515)
Based on infrared radiative transfer simulation of 446183 global atmospheric profiles and statistical regression analysis, two retrieval models relating the clear-sky surface downward and upwelling longwave radiation fluxes with the channel observations of Himawari08 satellite are established. By applying the models to the observation data of Himawari08, the two flux products from February 2016 have been processed. Several experiments are conducted to validate the processed products, and results show that the RMSE (Root Mean Squared Error) is 7.9 W/m2 and the correlation coefficient R is 0.9399 when comparing the surface upwelling longwave flux product with that of AQUA/CERES; the RMSE is 14.5 W/m2 and R is 0.9586 when comparing the surface downward longwave flux product with that of AQUA/CERES; the RMSE is 15.34 W/m2 when the satellite estimated downward fluxes are compared with the empirically calculated fluxes by Brunt equation and ground air temperature and humidity observations collected at 2260 Chinese ground meteorological stations; the RMSE is 12.6 W/m2 when comparing the upwelling flux product with the flux derived from land surface temperature product of Himawari08. Two months' products in February 2016 and June 2016 are selected to study the diurnal variation of the two fluxes. The results indicate that the diurnal cycles of the two fluxes are highly associated with the solar heating surface over land with the maxima appearing from 1200 LST to 1400 LST and the minima at around 0400 LST to 0700 LST in winter and summer. The diurnal variation of downward flux is either in the same phase with that of upward flux over most land, or one hour later in some locations. The variation patterns of both flux cycles can be approximated with a half-sine curve and two lines. But over ocean there are no distinct diurnal variation characters.
Characteristics of Extreme Precipitation and Associated Anomalous Circulations over Eastern China during Boreal Summer
Wenbo WU, Qinglong YOU, Dai WANG, Neng RUAN
2018, 23(1): 47-58. doi: 10.3878/j.issn.1006-9585.2017.16204
Abstract(1809) PDF (7213KB)(1262)
Anomalous circulations associated with various extreme precipitation events in summer are investigated from the perspective of atmosphere dynamics using daily precipitation data collected at 438 stations from 1961 to 2014 in eastern China and the NCEP/NCAR reanalysis data. The results demonstrate that in the middle and lower reaches of the Yangtze River, increases in the frequency of extreme precipitation always correspond to weaker than normal East Asian summer monsoon, which is reflected in abnormal 850-hPa wind field and water vapor flux that are favorable for more water vapor transport to the middle and lower reaches of Yangtze River. At 500 hPa, a blocking high persists over the Okhotsk Sea, which is conductive to the cold air moving southward. The East Asian subtropical westerly jet anomalously moves southward at 200 hPa, while the westerly wind anomalies to the south of 30°N promotes divergence development, and the wave activity fluxes are favorable for generating and maintaining wave perturbations in this region. In North China, corresponding to increases in the frequency of extreme precipitation, anomalies of 850-hPa wind field and water vapor flux indicate that the East Asian summer monsoon is stronger than normal, which strengths the water vapor transport to North China. The positive anomaly is located over the Sea of Japan and the negative anomalies are located over Mongolia, Baikal and their vicinity at 500 hPa. East Asian subtropical westerly jet abnormally shifts northward at 200 hPa, while the wave activity fluxes are also favorable for generating and maintaining wave perturbations in this region. All the above results suggest that the occurrence of extreme summer precipitation in East China is closely associated with atmospheric dynamics and energy transmission.
Numerical Analysis of Crucial Conditions for the Formation and Development of Heavy Fogs Based on BJ-RUCv2.0 Forecast System
Qingchun LI, Conglan CHENG, Yizhou ZHANG, Zaiwen WANG
2018, 23(1): 59-71. doi: 10.3878/j.issn.1006-9585.2017.16186
In order to find out the evolution characteristics of successive heavy fog and haze processes and reveal crucial conditions for the formation and development of fogs in Beijing, this paper analyzes the weather evolution characteristics corresponding to the heavy fog and haze case occurred from 26 Jan to 31 Jan 2013. Conventional meteorological data, automatic weather station data, and atmospheric composition observation data are used in this study. The weather conditions that are favorable for the formation and development of fogs are also analyzed. Based on the above analysis, the complicated BJ-RUC v2.0 (Beijing Rapid Updated Cycle forecast system version 2.0) is used to simulate water vapor, momentum, and thermal conditions for fogs. The results indicate that BJ-RUCv2.0 can well simulate the area of advection fog that occurred from the night of 30 Jan to the early night of 31 Jan. However, large errors occurred in the simulation for the fog event accompanied with severe air pollution from the nighttime of 28 Jan to the daytime of 29 Jan. The crucial condition for the formation of the heavy fog is the increase in the atmospheric humidity below 950 hPa, which is attributed to successive southeasterly winds in the near surface layer. The atmospheric stratification stability and the inversion layer are strengthened by the obviously warm advection from 975 hPa to 800 hPa, which is an important factor for the development and maintenance of the heavy fog. In addition, the wind convergence near surface (below 950 hPa) and the wind divergence on the upper layer are favorable for the fog development.
Stronger Warming Amplification over Arid Ecoregions and Its Relationship to Vegetation Cover in China since 1982
Qinghan HUANG, Haishan CHEN, Wenjian HUA
2018, 23(1): 72-82. doi: 10.3878/j.issn.1006-9585.2016.16216
Abstract(1358) PDF (5333KB)(1634)
To analyze vegetation variability and its relationship with summer air temperature variability in China, this study classifies various ecoregions based on the climatological Normalized Difference Vegetation Index (NDVI) values. Results show that there is a decreasing spatial distribution of vegetation from east to west in China. Arid ecoregions are the most obvious areas for summer warming since 1982. The warming rates over arid ecoregions for summer mean air temperature and mean maximum air temperature are 0.6-1.0℃/10 a and that for summer mean minimum air temperature is 0.8-1.4℃/10 a. Furthermore, summer air temperature variations have a significant negative correlation with vegetation conditions since 1982. In other words, the lower the vegetation greenness is, the stronger the warming trend is. It is worth noting that the negative correlation is most significant between mean minimum air temperature and vegetation conditions. In each ecoregion, the long-term trend of temperature is largely affected by changes in albedo and clouds. In the sparse vegetation regions, the increased albedo leads to a decrease in latent heat transport and increases the sensible heat flux, which intensifies the warming trend. In the dense vegetation regions, the decline in albedo leads to increases in evaporative cooling, which slows the rate of increase in the air temperature. Overall, vegetation activities play an important role in regional climate change, especially over the arid regions, where significant changes in surface radiation balance and energy flux amplify the rate of warming.
Abnormal Activities of the Western Pacific Subtropical High without Remarkable SST Anomaly Forcing: A Comparison between 1980 and 1981
Tonghua SU, Feng XUE, Jinxiu LIU, Sisi LÜ, Xiao DONG
2018, 23(1): 83-102. doi: 10.3878/j.issn.1006-9585.2017.16221
Evident sea surface temperature anomalies (SSTAs) were not observed over the Pacific and Indian Oceans in the summer and the preceding winter and spring of 1980 and 1981.Yet intraseasonal variation of the East Asian summer monsoon (EASM) circulation exhibited significant anomalies in both years with large differences. The western Pacific subtropical high (WPSH) experienced a much earlier first jump and an obviously later second jump in 1980, while a near-normal first jump and a notably earlier second jump were found in 1981. It was also noted that the jump processes in both years were influenced by different factors. In 1980, both jumps were induced by the enhancement of tropical western Pacific convection. In 1981, however, both jumps were attributed to the phase-locking of the poleward propagation of Rossby wave trains induced by the intensified convection in the tropical western Pacific and the eastward propagation of Rossby waves in the middle and high latitudes. Compared with the jump processes, the maintenance of stable circulations during pre-and post-jumps was more important. Due to the cooperation of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere at various periods, the extent of the WPSH anomaly in the summer of 1980 was comparable to that in strong El Niño decay years like 1983 and 1998. Specifically, the abnormally strong WPSH in June and August played a much more important role in the remarkable anomaly of summer-mean WPSH. Note that the stronger than normal WPSH in June and August was related to the enhancements of the Mascarene high (MH) and the Australian high (AH), respectively. In addition, the WPSH tended to shift southward in July and August since the blocking high in the Okhotsk Sea persisted for a long time. In contrast, the MH and AH in the summer of 1981 were relatively weak, resulting in a weak influence on the WPSH. Instead, the intraseasonal variation of circulation in the middle and high latitudes played a leading role in the rapid northward advance and subsequent southward retreat of WPSH. In particular, the prevalence of meridional circulation in August led to a weaker WPSH that shifted eastward. As a result, the whole summer-mean WPSH tended to be weaker than normal as well. The case study showed that in the absence of SSTA forcing, special attention should be paid to the influence of the southern hemispheric circulation and circulation in the middle and high latitudes of the Northern Hemisphere on the intraseasonal evolution of the WPSH and associated EASM circulation. On the other hand, both factors are difficult to be used in the seasonal prediction due to their relatively short periods of maintenance.
Possible Linkage between Spring Extreme Precipitation over East China and Surface Sensible Heat Flux over the Eurasian Continent
Disang SHEN, Haishan CHEN
2018, 23(1): 103-112. doi: 10.3878/j.issn.1006-9585.2017.16226
Abstract(1043) PDF (2928KB)(1257)
Based on the ERA-20C reanalysis data and observational precipitation data collected at the 332 stations in eastern China (to the east of 105°E) during 1960-2010, the possible linkage between spring extreme precipitation over eastern China and surface sensible heat flux over the Eurasian Continent is explored. Results show that when weakened (intensified) spring surface sensible heat flux occurs in the middle latitudes of the Eurasian Continent (i.e., regions to the west of Lake Balkhash and to the north of Lake Baikal) while intensified (weakened) sensible heat occurs in eastern China, the spring extreme precipitation is usually more (less) than normal over northern China but less (more) than normal over southern China. Further analysis suggests that weakened sensible heat in the middle latitudes and intensified sensible heat in the lower latitudes tend to weaken spring subtropical westerly and shift it northward, which are favorable for enhanced convective and baoclinic instabilities, leading to more extreme precipitation in North China. However, they tend to decrease baroclinic and convective instabilities and result in less extreme precipitation in South China. On the contrary, intensified sensible heat in the middle latitudes and weakened sensible heat in the lower latitudes tend to have opposite effects.
Analysis on SST Anomaly Structure in the Kuroshio Extension in Winter and Its Relationship to Tropical Pacific SST Variation
Minghao YANG, Liqiong WANG, Ruiting ZUO
2018, 23(1): 113-123. doi: 10.3878/j.issn.1006-9585.2017.17007
Abstract(1313) PDF (2928KB)(1261)
Based on the high-resolution SST dataset from 2003 to 2015 provided by the remote sensing system of NASA, the possible relationship between wintertime SST anomaly (SSTA) characteristics over the Kuroshio Extension in the North Pacific and tropical SST is investigated in this study. The methods used include empirical orthogonal function analysis and time-lagged correlation analysis, etc. The results show that the wintertime SSTA over the Kuroshio Extension presents two types of structural characteristics, i.e., the type of upstream area development and the type of zonal extension. The latter involves two zonal expansion modes with the bifurcation structure and the channeling structure, respectively. The two expansion modes are in opposite phase and in the same phase before and after 2010, respectively. The eastward extension of the warm anomalies was limited to the south of 36°N before 2006, whereas the extension shifted to the northern path during 2006 to 2010 and after 2010. The bifurcation extending mode are closely related to tropical SST variation. Further studies by time-lag correlation analysis and composite analysis indicate diverse impacts on the bifurcation mode of SST over different regions in the tropical Pacific. Niño3 SST index led the bifurcation mode over the Kuroshio Extension SSTA by 1-2 months, while Niño3.4 and Niño4 indexes are correlated with the bifurcation mode at a lead and lag of 3-month respectively, which indicates that the eastern type and central type of El Niño may have the lead and concurrent correlation relationships with the bifurcation mode, respectively, and the latter is more remarkable.
Structures of Multi-layer Clouds in Boreal Summer Based on CPR Radar Measurements
Ruiting TAN, Tao XIAN, Yunfei FU
2018, 23(1): 124-138. doi: 10.3878/j.issn.1006-9585.2017.17033
Characteristics of single-layer and multi-layer clouds including cloud amount, vertical structure of cloud, configuration of cloud type, and cloud water path in the Northern Hemisphere (0°-60°N) in boreal summer (June-July-August) are investigated based on measurements of the Cloud Profile Radar(CPR) during 2007-2010. The results indicate that the cloud amounts for single-, double-and triple-layer are 36.63%, 8.26%, and 1.40%, respectively. The high value areas of cloud amounts are consistent with convective regions with higher cloud top and thicker clouds, while the low value areas are mainly located over the subtropical high zone. The occurrence frequencies of cloud types show that the frequencies of cirrus, medium cloud and low cloud in single-layer cloud are similar, and the upper layers of multi-layer clouds mainly consist of cirrus, while the lower layers mainly consist of stratocumulus. Comparing frequencies of cloud types over ocean and land, the authors found that cirrus and stratocumulus occur more frequently over ocean than over land, while altostratus and altocumulus are emerging more frequently over land. The analyses also show that the values of cloud water path (ice water path and liquid water path) of single-layer clouds are the highest. In the multi-layer cloud, higher layers often have higher values of ice water path and lower values of liquid water path.