Study On The Relationships Between The Land Surface Processes Of Qinghai-Xizang Plateau And The Asian Summer Monsoon

As one of the critical members of Asian summer monsoon(ASM) system, the diabatic heating of Qinghai-Xizang plateau(QXP) plays an important role in the Asian summer monsoon. Because of its special geographical location and altitude, the snow cover and frozen ground are the main surface features of the QXP. The changes of land surface processes will inevitably affect on the thermal status property of the QXP, furthermore affect the interaction between the land surface and the air over the QXP. Therefore, studying on the relationships between the land surface processes of the QXP and AMS, not only benefits us to understand the interaction between land and the air over the QXP, but also to expand our comprehension the role of the QXP diabatic heating in the ASM system. Certainly, it is also beneficial to enhance the predicting ability of the summer monsoon rainfall of China.By using sensible and latent heat fluxes, snow cover and frozen ground over the QXP, a lot of researcher have investigated the thermal effects of the QXP on the ASM, and got much meaningful achievements. The relationships between the land surface process of Qinghai-Xizang plateau in spring and the Asian summer monsoon are concerned in this paper. First, by using of observational data of Maqu weather station, we comprehensively analyzed the soil thermal property variation of Qinghai-Xizang plateau, especially in the early stage of Asian summer monsoon onset. Basing on the above results, using the observational data of GAME/Tibet in Shiquan River and Gaize stations, and the reanalysis dataset (NCEP-I, AR-II, ERA40), the effects of the soil moisture and temperature variation, which caused by snow melting and soil thawing processes, on computing the surface diabatic heating of the QXP are further investigated. Then, using the the observational surface and air temperature data, the temporal-spatial changing characteristics of the diabatic heating over the QXP in spring of the early stage of ASM establishment are studyed through the empirical orthogonal function(EOF) and rotated empirical orthogonal function(REOF) methods. The relationships between the diabatic heating of the QXP and the snow cover, soil freezing processes are also discussed. Second, we investigate the relationships between the diabatic heating of the QXP in spring and the intensity of East Asian summer monsoon. Furthermore, the interaction between the diabatic heating and the zonal wind and its propagation characteristics are analyzed. Finally, the influences of the soil moisture variation in spring on the QXP surface thermal regime, atmospheric circulation during the ASM building-up, and eastern China early summer precipitation are also investigated through the RegCM3.0 regional climate model simulation. The major contents and results as follows:(1) The soil hydrothermal features of the QXP in spring are studied by using the station observational data. The results show that the time soil freezing processes occurred is the early stage of ASM onset. In the soil freezing processes, with the soil temperature rises up, the soil moisture sharply increases in a short time. As the soil under the different condition(frozen, frozen-thawing transition, melt), there arethe different relationships between the soil temperature and soil moisture. In spring the soil freezing processes siginificantly influences the soil heat flux gradient distribution in the shallow layer over the QXP.(2) The land surface process impacts on the diabatic heating over the QXP are investigated. In spring, the diurnal freezing-thawing cycle of the soil over the QXP leads to the soil moisture changes, furthermore causes the sensible and latent heat fluxes changes. Therefore, the change of soil moisture, which is caused by the soil diurnal freezing-thawing cycle, has a significant influence on the calculation of surface heating fluxes over the QXP. The soil moisture differences among the observation NCEP-Ⅰ, AR-Ⅱand ERA40 reanalysis data, which are caused by the snow melting and soilthawing processes, are the major reason for the computing error of reanalysis sensible and latent heat fluxes. In spring, due to the impacts of the snow melting and soil thawing processes, there are remarkable differences among the three kinds of reanalysis sensible and latent heat fluxes. It needs to be careful and make correction, while the sensible and latent heat fluxes in reanalysis data are used to diagnosed the diabatic heating anomaly of the QXP in spring.(3) The temporal-spatial characteristics of the diabatic heating of the QXP in spring are analyzed. During the early stage of ASM onset, due to the soil thawing and snow melting process impacts, the interannual anomaly center of diabatic heating of the QXP moves from the valley areas of the central in March to the southeastern the QXP in April. Coming into May, after its range expands, it steadily locates on the southeastern the QXP. Due to the influence of the cooling effect of the snow albedo, the diabatic heating of the QXP in April remarkably decreased from 1960s to 1970s, then increased until to the end of 1990s. The diabatic heating of the QXP in May took on a decrease trend from 20th century 60s to 90s, which closely related to the snow melting and soil thawing processes over the QXP under the global warming background.(4) The relationships between the diabatic heating of the QXP and the intensity of East Asian summer monsoon are studied through EP flux and other methods. In spring of the strong/weak monsoon years, the atmospheric waves that were excitated by the heat forcing of the QXP propagated to the upper layer of troposphere and strengthened. In the spring of strong monsoon years, the atmospheric stationary waves, which located in upper layer of troposphere on the north side region of the QXP at 40-50°N, spread to the high latitude areas and the lower troposphere, and gradually strengthened; but in the weak monsoon years, they only spread to the high latitude areas. The QXP and 40-60°N areas on the north side of the QXP are the remarkable difference regions of EP flux divergence between the strong and weak monsoon years. In the early stage of ASM onset, the diabatic heating anomaly of the QXP in spring has the important influence on the west flow in the middle-high latitude. There is a signicifant negative correlation between the diabatic heating of the QXP and strength of East Asian summer monsoon.(5) The relationships between the diabatic heating of the QXP and the East Asian summer monsoon onset are investigated. The results indicate that, in the spring, the early stage of the East Asian summer monsoon onset, there are 30～60 days, quasi-bi-weekly, and 5-7 days atmospheric oscillation over the QXP. The locations and strength and transmission of the MJO are different in strong and weak monsoon years. The MJO propagates northward in strong monsoon years, but remains in situ in weak monsoon years. In strong monsoon years, the diabatic heating of the QXP prevents low-frequency oscillation and reproduces in the east and west sides of the QXP. But in weak monsoon years, the diabatic heating of the QXP strengthens the MJO over the QXP, and forms the quasi-south-north oscillation centered on thethe QXP. In the strong years, the interactions between the diabatic heating of the QXP and zonal wind MJO are remarkable, and persistent to the South China Sea summer monsoon building up. On the contrary, the interactions between diabatic heating and zonal wind MJO rapidly disappear, while South China Sea summer monsoon breaks out in the weak years.(6) The role of diabatic heating of the QXP in the Asian summer monsoon is investigated through numerical simulation experiments of RegCM3.0, The numerical simulation results show that:the soil moisture increase (decrease) over the QXP in the spring, may lead to weaken(strengthen) the upper tropospheric westerly flow near the East Asian jet region in April, and the south branch airflow of the QXP strengthen (weakened). It also may cause the anomaly of the trough-ridge systems in middle-high latitude areas in the early stage of ASM onset. In addition, the soil moisture increase (decrease) over QXP in the spring may lead to the change of the early summer precipitation over eastern China, such as an increase (decrease) in the amount of the monsoon precipitation over the lower-middle reaches of Yangtze River, Yellow River and the southeast areas of South China, and an decrease (increase) over other areas of South China and Huaihe River areas. All of the above indicating that, through influencing on the adjustment of south branch airflow of the QXP, westerly flow and trough-ridge systems during the early stage of ASM onset, the soil moisture change of the QXP in spring has the significant impact on the early summer precipitation over eastern China.