Numerical Simulation On Weather Effect By Plastic Film Mulching In Arid And Semiarid Area

Arid and semi-arid regions are more easily to be affected by human activities because of the fragile ecosystem and complex underlying surface features there,and the features of underlying surface have important influence on the occurrence and development of weather system[1].At present,laying plastic film mulch on the cropland is a widely used tillage method in arid and semi-arid area of China[2].Such kind of film mulching will influence the local water vapor and energy exchange between soil and the atmosphere,and further affect the regional weather conditions.The changes of regional weather conditions are closely related to the production and life of human beings.No matter flood or drought will threaten the lives and property of people [3].Therefore,it is very important to study the weather effect caused by plastic film mulching in arid and semi-arid areas.In order to study the weather effects of large area plastic film mulching in arid and semi-arid area,we use mesoscale model WRF to simulate the effect of this phenomenon.Firstly,select different micro physics schemes,clouds schemes,land surface process schemes and boundary layer schemes in WRF model to get 14 parameter scheme combinations.Then,carry on the numerical simulation of a heavy rain process at the edge of the Tengger Desert in July 29,2012 using these parameter scheme combinations.Compare the simulation performance of precipitation by different combinations combined with the observation data and regional precipitation,and find the most suitable parameterization schemes combination for arid region.Later,analysis the characteristics of cropland underlying surface mulched by plastic film combining with the observation data of Inner Mongolia plastic film experiment,and build the physical model of mulched cropland underlying surface.Parameterize the character of this physical model,then,couple the mulched cropland underlying surface module to WRF model.Test the simulation performance of the model by comparing the observation and simulation results before and after coupling the plastic film module.Then,choose the precipitation process at the edge of the Tengger Desert in July 29,2012 as an example,compare the temperature,circulation,dynamical condition,water vapor conditions as well as precipitation condition before and after coupling the mulched cropland underlying surface module and investigate weather effects by large area of plastic film mulching.This study draws the following conclusions:(1)By comparing the simulation results by 14 combinations of parameterization schemes,it can be found that there are significant differences between the simulations by these combinations.More than half of the combinations can guarantee the accuracy of 24 h cumulative precipitation,but the locations of heavy rainfall center have different degrees of deviation between different combinations,and the precipitation time lags behind the measured.Consider above,case11,which choose Thompson micro physics scheme,Kain-Fritsch cumulus scheme,MYNN 2.5 level boundary layer parameterization scheme and RUC land surface process,performs well in simulating 24 h accumulated rainfall,single station hourly precipitation,TS score and average error.Case11 can realistically simulate the rainfall process.(2)Comparing the simulating differences of physical quantities between the 14 combinations,we can find that the difference of precipitation is the objective form of physical quantities' difference.The following results can be found through the analysis of weather situation and physical diagnosis.Firstly,this precipitation occurred in the circulation background that short wave trough in 500 hPa westerlies moved eastward and developed,and the intersection of cold and warm current at the shear line on 700 hPa worked as the trigger mechanism of rainstorm.The strongly ascended steam and the structure of low-level convergence and upper-level divergence offered dynamic conditions for this precipitation;and the strong water vapor convergence,thick wet layer and transportation of warm moist current provided abundant water vapor for the precipitation process.Combined with the influence of mesoscale convective system,the rainstorm occurred.(3)Through the analysis of the observation data of Inner Mongolia plastic film experiment,it can be seen that the soil moisture and soil temperature are increased after mulching the plastic film,which proves that the plastic film can preserve heat and water.The soil heat flux and its changing range on plastic film mulching surface are lower than that on the surface without the plastic film.Through the comparison between the simulation and observation,it shows that after laying plastic film,the precipitation is decreased and the occurrence time is delayed,the decline scale of 5cm soil moisture content decreased,soil temperature at 5cm increased as well as sensible heat flux,and latent heat flux decreased.The basic overall variation trend of simulation is consistent with theory.It can be seen that the plastic film mulched underlying surface module can be built reasonably.(4)The influence on the meteorological factors by mulching plastic film in arid and semi-arid area are as follows.It makes the atmospheric stratification above the region near the test site tends to be stable.Before the occurrence of precipitation,the geopotential height at 500 hPa was reduced upon the region near the test site,Heilongjiang area,and the eastern part of Hunan province.And the geopotential height at 500 hPa was increased upon the upstream region of test site,Qinghai-Tibet Plateau region,North China,Southwest China,and south part of Northeastern China.It also inhibits the development of Southwest airflow,which is not conducive to water vapor transport from India Ocean to the region near the test site.The relative vorticity,vertical velocity and surface air relative humidity upon the region near the test site are reduced,while the surface air relative humidity upon Northwest Gansu region is increased.It inhibits the water vapor convergence at lower layers upon test site and weakens the transfer process of water vapor to upper layers.(5)The influence on precipitation by mulching plastic film in arid and semi-arid area are as follows.It decreases the daily precipitation significantly in the region near the test site,Heilongjiang area,Shandong Peninsula,central Gansu,northeast of Qinghai province,and Guangxi region.It increases the daily precipitation in border area between Inner Mongolia,Ningxia and Shaanxi,central Qinghai,central Ningxia,northern Gansu,Western Inner Mongolia,some areas at the junction region between Heilongjiang and Inner Mongolia,Western Fujian and some places in southwestern Yunnan.The effects on temperature,circulation,dynamic conditions and water vapor conditions by plastic film mulched underlying surface are reflected in precipitation differences.