| Using the GLDAS-CLM soil moisture data from 1979 to 2016 and the trend coefficient method,the annual,seasonal and monthly spatial distribution and temporal evolution of soil moisture in the first layer(0-9.1cm)and the second layer(9.1-49.3cm)of the Tibetan Plateau are analyzed.Using the NCEP-FNL reanalysis data,the temperature of brightness blackbody(TBB)data from Fengyun-2E(FY-2E)meteorological satelite,and the Climate Prediction Center morphing technique(CMORPH)precipitation data,the effect of Tibetan Plateau soil moisture on a plateau vortex occurring from 16 August to 17 August 2014 are analyzed on the numeric a l simulation for a control experiment and four sensitivity experiments by employing the WRF model(version 3.8.1).The results show that:(1)The soil moisture in the second layer of the Tibetan Plateau is significa ntly greater than that in the first layer.The spatial distribution characteristics of soil moisture in the two layers are roughly the same,and both show an increasing distribution feature from northwest to southeast.From 1979 to 2016,the soil moisture at both levels show s a consistent increase trend.In the early 1990 s,the plateau soil moisture had a period of obvious decrease,and then it began to increase rapidly,and it increased steadily and slowly after the 21 st century.The year with the highest soil moisture appeared in 2008 and the minimum year is 1996.(2)The increase and decrease of soil moisture in different regions over time is basically the same in both regions.The soil moisture in the western part and the southern margin of the plateau shows a significant increase with time.The most significant increases are in southern Xinjiang and northern Tibet,and the trend coefficients reach at 0.7 and above,the soil moisture in the eastern,central,and central north parts of the plateau decrease significantly over time,with the most signific a nt reductions occurring in northern Qinghai,Western Sichuan Plateau,and central and eastern Tibet,with a trend coefficient of-0.7 and below.(3)For the first layer,the average soil moisture is the highest in the summer,followed by the fall,and the minimum is in the winter.For the second layer,the average soil moisture is higher in the summer and autumn than that in spring and winter.The average soil moisture in both layers shows a trend of increasing with time,and the average soil moisture in winter and spring shows an increasing tendency compared with summer and autumn.The change amplitude of soil moisture decreases with the increase of soil moisture.(4)The monthly mean soil moisture of two layers is characterized by a sinusoid a l function.The multiple water period of the first layer is from June to October,and the low water period of the first layer is from November to May.The multiple water period of the second layer is from July to November,and the low water period of the second layer is from December to June.The monthly mean soil moisture change curve of the second layer has a time lag compared with the first layer,which has a lag of about one month.(5)The vortex center,the vortex intensity and the vortex precipitation at 500 hPa simulated in the control experiment are successful.(6)The soil moisture plays key roles in the intensity and precipitation of vortex,but play little roles in the vortex nature and the location of the vortex center.When the soil moisture increases(decreases),the plateau vortex increases(decreases),and the precipitation increases(decreases).In a certain range,the more soil moisture increases(decreases),the stronger(weaker)plateau vortex becomes and the more precipitatio n increases(decreases).(7)The soil moisture mainly influences the vortex by changing the surface latent heat flux and the surface sensible heat flux.When the soil moisture increases,the surface latent heat flux increases,and the instability of the air in middle and low atmosphere increases as well,which provides energy for the development of convective systems and increases the convective precipitation,leading to the strengthening of vortex intensity by increasing the release of the condensation heating.On the contrary,the vortex intensity and the precipitation decrease.In the paper,the change of the surface sensible heat flux have little effect on the formation of vortex,hence we just considered its impact on the convective precipitation.When the soil moisture increases,the surface skin temperature gets lower,the surface sensible heat flux decreases,reduce the planetary boundary layer height(PBLH)decreases thus increasing the moist static energy per unit mass of air,which increases the convective precipitation.Otherwise,the convective precipitation decreases. |
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