A Study On Data Assimilation Of A Heavy Precipitation In Xi'an Area And Effects Of The Underlying Surface

Urbanization may has a certain impact on regional climate and local weather.In order to explore the influence of urbanization on Xi'an climate and the effects of the underlying surface,this paper analyses the temperature and precipitation in Xi'an area firstly,and then radar data assimilation is carried out for a heavy rainfall case.The role of urban underlying surface in the precipitation process is discussed through numerical experiments with or without urban underlying surface.The main results are as follows:First,in order to quantify the impact of urbanization on the climate of Xi'an,this paper selects the observation data of the city station(Xi'an),the suburban station(WuGong)and the statistics of the grid points in Xi'an from July to August of 1961-2008.The Mann-Kendall mutation tests and sliding t-tests show that average precipitation in Xi'an area increased from 1980 to 2008 but not significantly.The frequency of precipitation in Xi'an station is less than that of WuGong station,but the probability of heavy precipitation is larger,especially after 1980s.There have two temperature mutations in the summer of Xi'an in the past 50 years,which showing a trend of‘negative-positive-negative'.At any stage,the temperature in Xi'an station is higher than regional average temperature or suburban temperature,and daily range of urban temperature decreases the most.Secondly,using the mesoscale forecasting model WRF and its 3DVar assimilation system,cyclical assimilation experiments of doppler radar data are conducted for a storm occurred in Xi'an area on August 3,2015.The results of cycle assimilation schemes show that circulation assimilation radar data improves wind field and water vapor field,also adjusts the temperature field indirectly.After assimilation,the rainstorm area and intensity have been improved,and the TS(True Shooting)and ETS(Effective True Shooting)scores of 10-25 mm predicted by circulation assimilation schemes have increased by 0.188 and 0.187compared with the unassimilated experiments respectively.Finally,two experiments with or without urban surface are carried out to explore the impact of urban surface on forecast of heavy rainfall.And the differences between experiments are compared through wind fields,thermal conditions and water vapor fields during the heavy rainfall.Before the precipitation occurs,2-m temperature above ground in urban area increases by 1-3 ~oC compared with NoCITY experiment.The average convective available potential energy(CAPE)is larger and the convective inhibition energy(CIN)is smaller.The vertical gradient of the pseudo-specific temperature is also more intensive than that in NoCITY test area.Urban surface reduces low-level wind speed and helps forming a low-pressure center,which may promote the convergence and increase ascending motion in the lower layers.For the water vapor conditions,the water vapor mixing ratio and cloud water mixing ratio of700-300 hPa in CITY experiments are stronger,and the water vapor flux also has stronger convergence over the urban area.In a word,urban surface enhances precipitation over Xi'an area.If the urban surface is substituted by cultivated land,the precipitation area would drift towards downwind(south of Xi'an).