Analysis And Numerical Simulation Of Summer Micrometeorological Characteristics Of Maqu Wetland In The Source Area Of ??the Yellow River

The Maqu wetland is one of the areas most affected by climate change,and its ecological environment is facing a serious threat.In this paper,the alpine meadow underlying surface in the Maqu wetland was chosen,and the observed data in summer2015 were analyzed.Then,the Yellow River source region was chosen as the simulated area,and 4 experiments driven by 4 different land surface process schemes in WRF mesoscale numerical model were designed to evaluate the effect of the surface and near-surface meteorological elements in the Yellow River source region.Besides,the errors of each simulation were calculated.The distributions of meteorological elements in the simulation area were analyzed by utlizing the data from the automatic meteorological observation station and GLDAS dataset.The CO₂flux and density data observed in Maqu alpine meadow underlying surface was collected to analyze the daily and monthly variation,as well as the carbon emission and absorption rate in the fine and cloudy days.And the CO₂ flux simulated by WRF-Noah-MP was evaluated according to the observed ones.The main conclusions are as following:?1?Comparison of the observed and simulated surface and near-surface elements:the air temperature and relative humidity of Maqu alpine meadow underlying surface in summer shows obvious diurnal variational characteristics which shows widely difference between day and night.And the average values were 9.3?and 75.4%,respectively.During the day,the wind speed was significantly greater than that in the night,and the average wind speed was 2.8 m·s^-1,and the wind direction was dominated by western.There were many rainy days in summer,which were mainly in September.The cumulative precipitation was 241.9 mm.The soil temperature showed quasi sine variations.Comparing with the shallow soil layer,the temperature of deeper soil rose later,the average temperature of the 5-cm and 10-cm soil were 14.4and 13.9?,respectively.The moisture of shallow soil layer increases rapidly after precipitations,while the deeper soil layer increases later,the soil moisture of the 5-cm and 10-cm layers were 0.21 and 0.19 m³·m^-3.The WRF model can describe the variations of the air temperature,relative humidity,soil temperature and moisture,while the simulated air temperature and soil temperature were low.However the simulated relative humidity and soil moisture were large.The correlation coefficients between simulated and observed temperature were greater than 0.94,WRF-RUC showed the minimum root mean square error and bias.Comparing with the other three experiments,the relative humidity simulated by WRF-Noah showed the the minimum root mean square error and bias,which were 5.0%and 3.4%,respectively.The simulated dominant wind direction was contrary to the observed.?2?Surface radiation balance and albedo observation results:in summertime the downward shortwave radiation reached about 1200 W·m^-2 on the fine days in Maqu,which was close to those in Lake Ngoring lakeside region,and larger than those in Nagchu region.The downward longwave radiation changed between 217.0³83.7W·m^-2.The maximum upward longwave and shortwave radiation were 520.7 W·m^-2and 210.2 W·m^-2 respectively.In summertime the average albedo of Maqu alpine meadow were 0.20,which was close to that of Lake Ngoring lakeside,Nagchu,Haibei alpine meadow,and larger than that of Litang alpine meadow.?3?Comparison of observed and simulated surface energy balance components:the peak value of net radiation in Maqu alpine meadow reached about 845 W·m^-2.The ground surface heat fluxes changed between-57.6¹14.6 W·m^-2,and average value was 1.8 W·m^-2.Both net radiation and ground surface heat flux showed negative values during the night,at this time,the land surface is transmitting energy to the atmosphere.The average latent and sensible heat flux were 55.7 W·m^-2 and 25.5W·m^-2,the energy transmission was dominated by the latent heat flux.Both latent and sensible heat flux showed negative values during the night,which indicated the land surface is transmitting energy to the atmosphere.During the summertime the average energy closure ratio was only 0.67,which was mainly caused by the observation deviation,thermal storage of vegetation canopy,advection and mesoscale convection.The WRF model can describe the daily variation of energy fluxes,while the simulated negative sensible and latent heat fluxes were not enough.The net radiation and ground surface heat fluxes simulated by WRF-Nosh-MP showed good conformity with the observed data.?4?The distributions of the land surface and near-surface elements:all the land surface schemes can well present the distributions of air temperature,relative humidity,soil temperature and moisture except WRF-RUC.Compared with the GLDAS,four simulated land surface temperature,soil temperature and sensible heat flux were relatively larger.Due to the high altitude,the surface temperature of the plateau is low.The latent heat fluxes simulated by WRF-Noah and WRF-Noah-MP showed good conformity with those of GLDAS.?5?Observed and simulated CO2 fluxes of ecosystem:the observed average CO2flux of Maqu alpine meadow was-0.10 mg·m^-2·s^-1 in summer,performed as a slight“carbon sink”,and the carbon absorption was about 13 hours.There was little difference between fine and cloudy days,while the rate of carbon emission on cloudy days was significantly greater than that of fine days.The variation tendency of CO₂density corresponds to CO₂ flux,and the average CO₂ density was 517.20 mg·m^-3.The WRF-Noah-MP can describe the variation of CO2 flux in Maqu alpine meadow ecosystem.