Temporal And Spatial Variation Characteristics Of Dry, Wet, Hot And High Temperature In Summer In The Middle And Lower Reaches Of The Yellow River And Their Relationship With Soil Moisture Anomalies

Based on daily CN05.1 meteorological data,NCEP/NCAR reanalysis data,ERA5 reanalysis data of soil moisture,the variations of the summer dry and wet extreme hightemperature(EHT)in the middle and lower reaches of the Yellow River(MLRYR)from 1961 to 2016 were investigated in this thesis.Atmospheric circulation characteristics and anomalies associated with the two types of EHTs were first discussed.In addition,the influence of soil moisture anomalies on the two types of EHTs was analyzed.The main conclusions are as follows.(1)The MLRYR experiences more frequent wet and dry EHT.From 1961 to 1980 s,the frequency of dry EHT indicated a declining trend,while an increase is observed afterwards.On the other hand,the frequency of wet EHT is the opposite with increasing trends in the early 1990 s and then decreased afterwards.The duration of dry EHT is slightly longer than that of wet EHT.In terms of the intensity of high temperature,the intensity of wet EHT is greater than that of dry EHT,but the intensity of dry EHT shows an obvious upward trend.The main spatial distribution of the two types of high temperatures is both located in the central and southern parts of MLRYR.(2)The continental high(CH)and the Northwest Pacific subtropical high(NWPSH)were found to be the most important circulation systems that affect the dry and wet EHTs in MLRYR.When the CH is to the west(east)and the NWPSH is to the south(north),it is more conducive to the occurrence of dry(wet)EHT.When dry EHT occurs,it is mainly due to the increase of temperature caused by downdraft which is caused by the CH,while wet EHT is more due to the north of the NWPSH,which increases the water vapor and relative humidity,resulting in the increase of the thermal index.In the case of dry EHT,the zonal "+-+" geopotential height anomaly appears over central-eastern Asia,while in the case of wet EHT,it is the opposite.Moreover,a teleconnection wave train like the Silk Road pattern from the midlatitudes could affect the dry EHT days,while wet EHT days are more likely to be affected by a teleconnection wave train from high latitudes.(3)The thermal difference between the two types of EHTs in MLRYR mainly comes from the vertical temperature variation.Although the diabatic heating term is positive for both types of EHT,but the increase of diabatic heating in dry EHT is due to the increase of sensible heat flux and long wave radiation flux,while in wet EHT is due to the increase of latent heat flux.When dry EHT occurs,the sky is cloudless,and more solar radiation reaches the ground.This is the opposite when it is wet EHT,the increase of cloud cover strengthens the absorption and reflection of solar radiation,and the temperature does not rise significantly.(4)The frequency changes of two types of EHTs in summer in MLRYR have a significant correlation with the soil moisture anomalies in the early spring and summer of the Northeast Plain.When the soil moisture in spring in the Northeast Plain is dry(wet),the area is also dry(wet)in summer due to its persistence,the sensible heat(latent heat)flux anomalies increase correspondingly,the positive anomaly of geopotential height(water vapor)in MLRYR,which leads to more frequency of dry(wet)EHT in this area.