Relations And Mechanism Between Winter And Spring Snow Over The Tibetan Plateau And Summer Precipitation In Northwest China

Snow over the Tibetan Plateau(TP)is closely related to weather and climate in China and eastern Asia.Under the climate change,precipitation in Northwest China has changed obviously,what is the relationship between snow over the TP and precipitation in Northwest China,what is the associated mechanism,and whether or not climate models can reproduce such relationship,still need to be investigated further.This thesis analyzed the characteristics of spatial and temporal anomalies of TP snow depth;diabatic heating anomaly caused by the snow anomaly in different regions of TP,and investigated the influence mechanism of TP snow on summer precipitation in Northwest China(NWC)from the perspective of large-scale circulation anomaly,vertical circulation in northern TP and momentum and heat transport.In addition,relation and mechanism of the winter and spring snow anomaly over the TP and summer precipitation in Northwest China are also analyzed.On these bases,the simulation performance of climate models in CMIP6 on the relation between winter and spring snow over the TP and summer precipitation in Norwest China is also examined.The main conclusions of this thesis are as follows:1.From 1979 to 2019,there are differences in the characteristics of changes in snow depth in different regions of TP at different temporal scales.The accumulation and melting rate of snow depth in the eastern TP is faster than that in the western TP,and the accumulation and melting rate of snow depth in the southern TP is faster than that in the central TP.On the seasonal scale,snow depth in winter is largest in eastern TP,while it is smallest in the central TP.In spring,the melting rate of snow depth is largest in eastern TP,while the melting rate of the western snow depth is slower but the snow depth is the largest.On the interannual scale,there are two high value regions for the abnormal changes in winter and spring snow depth,located in the eastern and western TP.The snow depth in the western TP shows a slow detrend,while the decrease in snow depth in eastern TP is not significant.The persistence of snow depth from winter to spring is not consistent in different regions,with longer duration of snow depth anomalies and shorter duration of snow cover anomalies.2.Differences in winter snow depth between the eastern and western TP have discrepancies in their effects on the subsequent summer precipitation in Northwest China,especially in the western part of NWC.When snow depth is more in the eastern TP(SD_E),the summer precipitation is generally more in eastern Xinjiang,southern Shaanxi,and the Hexi Corridor area,and less in northern and western Xinjiang,Inner Mongolia,Gansu,Ningxia and northern Shaanxi.When snow depth is more in the western TP(SD_W),the summer precipitation is more in western Xinjiang and southern Shaanxi,and less in northern and eastern Xinjiang,Gansu,Ningxia and central and western of Inner Mongolia.TP spring snowmelt has a significant impact on summer precipitation in NWC.When spring snowmelt is more abundant in the eastern TP,summer precipitation is more in Gansu and southern Shaanxi,while less in central and western Inner Mongolia,Ningxia,and northern Shaanxi.When there is more spring snowmelt in the western TP,summer precipitation is more in most areas in NWC,and less in the central and southern Xinjiang.3.Winter and spring snow over the TP is significantly correlated to the general circulation associated with summer precipitation in NWC.The statistical diagnosis results indicate that when there is more SD_E,atmospheric temperature of the main body and the surrounding of TP are negative anomalies,and the sinking motion on the northern side of TP is strengthened.The excitation of mid latitude Rossby wave train leads to negative geopotential anomaly on the Mongolian Plateau and its north side.The increase of westerly wind is not conducive to water vapor transport to NWC,resulting in less summer precipitation in most areas of NWC.When there is more SD_W,the main body of TP experiences warm anomalies while the northern side of TP significant cold anomalies.The temperature gradient on the northern side of TP significantly increases,and the subtropical westerly wind strengthens,which is not conducive to the convergence of water vapor in the eastern NWC.At the same time,the anomalous dipole configuration in the Caspian Sea and Baikal Lake regions is conducive to the convergence of water vapor in the western NWC,and there are significant anomalies in the vertical direction of upward movement in the western NWC(80°-90°E)and downward movement in the eastern NWC(90°-100°E),resulting more summer precipitation in the western part of Xinjiang and less in most parts of Gansu,Ningxia,Inner Mongolia,and northern Shaanxi.4.The impacts of diabatic heating over the TP caused by SD_W and SD_E are different.The longer memory of SD_W leads to snow melting becomes slower,leading to an anomalous surface diabatic heating.Based on the CESM model simulation of the impacts of SD_W and SD_E on summer precipitation in NWC,it is found that when SD_W and SD_E is more evident,then results of more precipitation in Shaanxi and less precipitation in Inner Mongolia,northern Xinjiang and Qinghai can be simulated.The anomalous snow cover in the eastern and western parts of TP causes anomalous changes in the diabatic heating of the TP,resulting in anomalous interactions between the TP and atmospheric circulation.From the perspective of energy transfer,the EP flux anomaly reflects the forcing of diabatic heating anomalies caused by snow anomalies in different regions of the TP on the general circulation on both sides of the TP.When SD_W is higher,the divergence of the upper troposphere and 40°N EP flux is stronger,resulting in weaker westerly airflow on the northern TP.In summer,the transfer of zonal and meridional kinetic energy is strong,resulting in water vapor being transported to the southeast of the NWC,but not to the central and northern parts of the NWC.As a result,summer precipitation in the NWC is significantly lower in western Xinjiang,central Gansu,Ningxia,and central Inner Mongolia,while there is an abnormal increase in southern Shaanxi and southern Gansu.5.The coupled CMIP6 models can well simulate the multi-year averaged spatial distribution of winter snow cover and spring snowmelt on the TP.All models can simulate that the alpine region is a snowy area,the hinterland of the plateau and the northern basin are less snowy areas,but all models overestimate the overall level of snow cover in the TP.The CMIP6 model can reproduce the distribution of precipitation in NWC,and can capture the spatial model of the overall plateau anomaly in winter on summer precipitation in most NWC,but it does not well reproduce the relationship between winter-spring snow cover anomalies over eastern and western TP and summer precipitation in NWC.Above results imply that improving the description of TP snow process by CMIP6 model is one of the ways to improve the simulation and prediction of precipitation in NWC.