| The variability of East Asian summer monsoon draws meteorologists’ high attention, because the sustainment and development of East Asia and its surrounding area are directly influenced by the temporal and spatial variation of East Asian summer monsoon. Especially, the interannual variation of East Asian summer monsoon will have signification impact on the local temperate and precipitation, which may cause the uncertainty of the occurrence of extreme weather such as heat wave, flood and drought. Therefore, investigation of the natural variability and impact factors (such as the aerosols emitted by human activities) of cloud and precipitation will help to understand the variation characteristics of East Asian summer monsoon, and it also has essential social and economic significance regarding disaster prevention and mitigation.This study investigate the East Asian summer monsoon area (18°-45°N,100°-135°E) by dividing it into7sub-regions:East China (EC), South China (SC), East China Sea(ECS), Sichuan Pen Di (SPD), North China (NC), Diao Yu Dao (DYD), Yellow sea and its surrounding area (YC). By using TRMM satellite data during1998-2013and MOD IS Aerosol Optical Depth (AOD) and cloud data during2000-2013, GPCP precipitation data, NCEP reanalysis data and soundings data, aerosol-cloud-precipitation interaction and the characteristics and cause of the interannual variation of cloud, precipitation and aerosols are diagnosed and studied. The main findings are as follows:(1) The interannual variation and strong-weak monsoon year difference of East Asia summer monsoon circulation and precipitation during1998-2013During recent60and more years, the intensity of East Asian summer monsoon circulation showed an apparent interannual decreasing trend, which was especially clear after1970s, and among all the months June had the most significant interannual decreasing trend. In such a decreasing background, during1998-2013the intensity of East Asian summer monsoon circulation kept decreasing but with large variation. There was a strong-weak year cycle with the same number of strong and weak years. The quasi-biennial and4-7year oscillation were also shown. The difference between the strong-weak monsoon year was revealed in the meridional transport. The East Asian summer monsoon precipitation mainly occurred in850hPa summer monsoon. Seen from500hPa geopotential height, the precipitation over the land and ocean of EC was generally controlled by the ridge of subtropical high in West Pacific. During weak monsoon years, subtropical high reached to the southeast of China, and southwesterlies dominated with weak meridional transport, and the monsoon rain band located at the middle and lower reaches of the Yangtze River with more and stronger precipitation, while less and weaker precipitation occurred in SC due to the control of subtropical high. Generally, the precipitation amount and intensity were in good accordance to the monsoon variation. However, the correlation between precipitation intensity and monsoon index showed regional discrepancy. For the East Asia monsoon area south of40°N in China,"drought in the south and flood in the north" occurred in weak monsoon years, while "flood in the south and drought in the north" occurred in strong monsoon years.(2) The interannual variation of cloud parameters in East Asia summer monsoon area and the characteristics of its strong-weak year differenceFor the cloud macroparameters, from the averaged value over summer season, the cloud macroparameters (cloud top pressure, CTP, and cloud top temperature, CTT) decreased from south to north. These cloud macroparameters were idea indicator of the movement of East Asia summer monsoon and its rain band, and also the shift of subtropical high. During the recent14years (2000-2013), the cloud top height had a decreasing trend, while cloud amount had an increasing trend. There were significant regional difference of the interannual variation of monsoon index of cloud top height and could amount. The cloud top height and cloud amount in East Asia monsoon area were generally controlled by the interannual variation of monsoon, but its correlation with monsoon was not as close as that of precipitation. Besides, its interannual variation also showed significant regional difference.For the cloud microparameters, the cloud effective radius (CER) increased from west to east, the CER over land was apparently smaller than that over ocean. The cloud optical depth (COD) and cloud water path (CWP) decreased from west to east, the values of the two parameters over land were obvious larger than those over ocean. Generally, the movement of monsoon and rain band could be revealed by these cloud microparameters. During the summers of recent14years (2000-2013), the CER, COD and CWP in East Asia monsoon area were all decreasing interannually. There were significant phase and regional difference between the variation of monsoon index and the variation of CER, COD and CWP. Compared to the strong monsoon years, the CER (COD and CWP) was (were) smaller (larger) in weak monsoon years over the area from the SPD to EC region in middle and lower reaches of the Yangtze River, but was (were) larger (smaller) over south and north of the area.For the precipitation cloud parameters, the CER (COD and CWP) of summer precipitation cloud, convective precipitation cloud and stratiform precipitation cloud increased from west to east (from south to north). Their CER over land was significantly smaller than that over ocean. The precipitation cloud CER in the seven sub-regions was well correlated with the CER from MODIS with linear increasing trend. However, there were some regional differences of their magnitudes. They shared similar magnitudes in EC, SPD and NC sub-regions, while for the other sub-regions, the CER from MODIS was larger than the precipitation cloud CER from VISR by different amplitude in different sub-regions. Similarly, the precipitation cloud COD and CWP in the seven sub-regions were well correlated with the COD and CWP from MODIS with linear increasing trend. However, for all the sub-regions, the COD and CWP were apparently larger than those of MODIS, which indicated the thicker cloud and more water content of precipitation cloud.(3) The interannual variation of convective and stratiform precipitation in East Asia summer monsoon area and the characteristics of their strong-weak year differenceFor the precipitation intensity, both convective and stratiform precipitation became stronger along with the progradation of the monsoon and the associated rain band. The precipitation distribution was also intimately related with the progradation of the monsoon. There are good correlations between the intensity of convective/stratiform precipitation and monsoon, but with regional difference. For sub-regions EC, SPD and YS, there were significant negative correlations, while for sub-regions SC, ECS and DYD, there were significant positive correlations.For the area proportion and precipitation contribution, the area proportion and precipitation contribution of summer convective precipitation in East Asia monsoon area decreased from south to north, while the area proportion and precipitation contribution of summer stratiform precipitation in East Asia monsoon area increased from south to north accordingly. Summer stratiform precipitation dominated the Ease Asia monsoon area, but its proportion showed regional difference. Along with the progradation of summer monsoon from south to north, area proportion of convective and stratiform and their contribution to the total precipitation varied accordingly, which mainly indicated by the increase of stratiform precipitation’s area proportion and its contribution to the total precipitation. Therefore, it can be concluded that stratiform precipitation of East Asia summer monsoon was controlled by the large scale summer monsoon circulation. For stratiform (convective) precipitation, the precipitation proportion and contribution were significantly positively (negatively) correlated with monsoon over EC, SC, ECS and DYD, but only had a weak negative (positive) correlation over SPD and YS.For the vertical structure, clear interannual variation can be found in the average precipitation profile and rain top height of both convective and stratiform precipitation. The interannual variation of precipitation profile was mainly presented as the interannual fluctuation of near surface precipitation intensity. The variation also showed regional difference. For both convective and stratiform precipitation, the interannual variability over ocean was larger than that over land. However, no significant interannual variation was found for the vertical stratification structure of both convective and stratiform precipitation. Their corresponding height showed no or little interannual variation. For the convective precipitation, there was significant negative correlation between rain top height and monsoon over sub-regions EC, SCS, SPD and YS, while positive correlation over sub-regions SC and DYD, and almost no correlation over ECS. For the stratiform precipitation, there was negative correlation over EC and SPD, and positive correlation over DYD, and no correlation over other sub-regions.For the difference between strong and weak monsoon year, in the East Asia monsoon area south of40°N in China, there were stronger (weaker) convective and stratiform precipitation, higher (lower) rain top, less (more) convective precipitation proportion and contribution, and more (less) stratiform precipitation proportion and contribution north (south) of the Yangtze River during the weak monsoon years, while the opposite occurred during the strong monsoon years. These results further indicate that stratiform precipitation dominates East Asia monsoon area, and there is a "weak in the south and strong in the north" pattern during weak monsoon years, and a "strong in the south and weak in the north" pattern during strong monsoon years.(4) The statistics of the impact of aerosols on cloud parameters and precipitation intensity over East Asia monsoon area For the relation between aerosols and cloud microparameters, the correlation between them showed large difference among various phases/areas.(1) For the relation between AOD and CER:along with the increase of AOD, CER of water (combined) cloud decreased (increased) and CER of ice cloud showed little variation over EC, CER of water, combined and ice cloud all decreased over SC, CER of water and combined cloud increased and CER of ice cloud first increased then decreased over SPD, CER of water cloud increased and CER of combined and ice cloud showed little variation over NC, CER of water, combined and ice cloud all significantly decreased over ECS, DYD and YS.(2) For the relation between AOD and COD/CWP: along with the increase of AOD, COD/CWP of water, combined and ice cloud increased over EC, COD/CWP of water, combined and ice cloud first increased then decreased over SC, COD/CWP of water (ice) cloud decreased (increased) and COD/CWP of combined cloud showed little variation over SPD, COD/CWP of water, combined and ice cloud all significantly decreased over NC, COD/CWP of water, combined and ice cloud all increased over ECS and DYD, COD/CWP of water (ice and combined) cloud increased (decreased) over YS.(3) Without discriminating the phase condition, the relation between AOD and precipitation cloud parameters is consistent with the relation between AOD and MODIS combined cloud parameters.For the relation between precipitation cloud parameters and surface rain rate (RR), seen from the average of precipitation cloud with various phases, along with the increase of CER, RR increased over land, while decreased over ocean, which is possibly due to the lack of cloud nuclei over ocean so that the rain drop is prevent from growing; along with the intensifying of COD and CWP, RR increased over both land and ocean, and leveled off when it reached a threshold value. The above characteristics also apply to the relation between the CER/COD/CWP of convective/stratiform precipitation cloud and RR.For the relation between aerosol and RR, the correlation of AOD and RR which is concluded from AOD-CER-RR interaction indirectly is consistent with the direct statistics:along with the increase of AOD, RR had an increasing (decreasing) trend over EC, SPD and DYD (SC and YS), while over ECS RR had a relatively smaller increasing trend. Therefore, the impact of aerosols on RR has its local characteristics.(5) The cause of interannual variation trend of precipitationThe interannual variation trend of East Asia summer precipitation and aerosols and East Asia summer monsoon and their relationships are as follows:(1) The atmospheric stratification condition and moisture transportation condition over East Asia monsoon area were controlled by the large scale summer monsoon circulation.(2) During2000-2013and under the background of decreasing intensity of monsoon, there were regional discrepancies for the trend and cause of the variation of summer monsoon, AOD and precipitation. More specifically,①the summer total precipitation amount over the middle and lower reaches of the Yangtze River (which was largely contributed by stratiform precipitation) was mainly controlled by summer monsoon, and RR was modulated by aerosols;②the summer total precipitation amount and RR over SC was mainly controlled by summer monsoon, and the impact of aerosols on RR was not significant;③The total precipitation amount and RR over the ocean area of EC was co-affected by summer monsoon and aerosols;④The total precipitation amount and RR over SPD were mostly regulated by summer monsoon, and the impact of aerosols on RR is trivial;⑤The total precipitation amount and RR over DYD and its souring area were co-affected by summer monsoon and aerosols;⑥The stratiform precipitation over YC was controlled by summer monsoon, while convective precipitation here was modulated by the aerosols. Overall, the atmospheric stratification condition and moisture transportation in East Asia monsoon area were controlled by the summer monsoon (which was an element of natural variability), and then RR, proportion and contribution of stratiform precipitation were changed and total precipitation amount was affected; besides, regional discrepancies existed in the relation between monsoon and AOD and in the relation between AOD and RR, so that the RR was affected by aerosols (which was an element of anthropogenic influence). To sum up, in the background of decreasing trend of East Asia monsoon, the RR and precipitation amount in this area were more affected by monsoon activity, while over some regions aerosols also had their influence on RR. |
PDF Full Text Request