Land-atmosphere Interaction And Its Impacts On The Regional Climate In Arid And Semiarid Regions In China

Energy and water exchanges between the land surface and atmosphere have an important impact on climate change.Strong interactions exist between human activities,underlying surfaces,and climate change in the arid and semiarid regions in China,which are suffering the shortage of water resources,ecological fragility and frequent dusty weather.Researches on the landatmosphere interaction and its impacts on the regional climate in this region have a great significance on improving the simulations of the regional climate,predicting the regional climate change in the future,and selecting regional sustainable development strategies.The observational stations are less distributed in arid and semiarid regions,limiting the research on the land atmosphere interactions over these regions.Moreover,the impact of land surface on temperature and precipitation remains controversial and uncertain.To acquire high quality surface turbulent fluxes over complicated terrain,turbulent data from the Semi-Arid Climate and Environment Observatory of Lanzhou University?SACOL?are used.The applicabilities of different coordinate rotations are analyzed and a parameterization scheme is provided in integral turbulent characteristics test?ITC?.The acquiring of high quality surface turbulent fluxes provide basis for subsequent researches on the land atmosphere interactions.At the same time,the effects of soil moisture,net radiation,and vapor pressure deficit on available energy partitioning are compared using data from SACOL and cropland and degraded-grassland in Tongyu.To assess the impacts of initial soil moisture and the vegetation fraction on the diurnal temperature range?DTR?in arid and semiarid regions in China,three simulations using the WRF model are conducted by modifying the soil moisture,surface emissivity and vegetation fraction.Moreover,MODIS data and observational data from SACOL and the Shapotou Desert Research and Experimental Station?SPD?are also used in this research.The impact strength and way of atmospheric moisture fluxes and soil moisture on local precipitation are compared in the arid and semiarid regions and humid and semihumid regions in China.?1?The overall quality after data processing and quality control including sonic temperature correction,coordinate rotations,WPL correction?correction for density fluctuations?,stationarity test and ITC test show that about 45%-62%of the total data are of high quality for friction velocity?_*u?,66-68%for sensible heat flux,62-65%for latent heat flux and 52-54%for CO₂ flux.The proportion of the high quality of_*u obtained by DR is 17%higher than PF,while the proportion of high quality data of the latter three kinds of fluxes obtained by PF is 2%-3%higher than DR.The differences between PF and FPF are mainly in_*u.Comparing the three coordinate rotations in the dominant wind direction,DR still obtain the best quality of_*u.The use of DR is recommended in the complicated terrain for reducing calculation and improving the data quality.?2?The Bowen ratio decrease rapidly with increasing soil moisture when soil is dry but is insensitive to the changes in soil moisture whensoil is wet.Net radiation and vapor pressure deficit affect the sensitivity of the Bowen ratio to soil moisture under dry conditions and the soil moisture threshold above which the Bowen ratio become insensitive to soil moisture.The Bowen ratio increase with net radiation,while the Bowen ratio will decrease with net radiation at a high level of net radiation,especially underdrought conditions.Reduced soil moisture enhanced the effects of the net radiation and vapor pressure deficit on available energy partitioning.The effects of the vapor pressure deficit on Bowen ratio are nonlinear,which depend on the relative strength of the positive and negative impacts of vapor pressure deficit on the latent heat flux under different soil and net radiation conditions.?3?The soil moisture affects the daily maximum temperature?Tmax?and daily minimum temperature?Tmin?by altering the distribution of available energy between sensible and latent heat fluxes during the day and by altering the surface emissivity at night.Reduced soil wetness can increase both the Tmax and Tmin,but the effect on the DTR is determined by the relative strength of the effects on Tmax and Tmin.Observational data from the SACOL and SPDsuggest that the magnitude of the soil moisture effect on the distribution of available energy during the day is larger than that on surface emissivity at night.When the soil moisture increase0.1m³/m³,Tmax,Tmin and DTR decrease 3.77?,2.10?and 1.67?in SACOL,while in SPD Tmax,Tmin and DTR decrease 6.50?,3.52?and 2.98?.In other words,soil moisture has a negative effect on the DTR.Changes in the vegetation fraction modify the surface radiation and the energy budget.Due to the depth of the daytime convective boundary layer,the temperature in daytime is affected less than in nighttime by the radiation and energy budget.Increases in surface emissivity and decreases in soil heating resulting from increased vegetation fraction mainly decrease Tmin,thereby increasing the DTR.The effects of soil moisture and vegetation fraction on both Tmax and Tmin are the same,but the effects on DTR are the opposite.?4?The variation of moisture flux convergence is more important than that of soil moisture for precipitationvariation in arid and semiarid and semihumid and humid regions.The moisture flux convergence mainly influences the precipitation efficiency and the soil moisture mainly influences the recycling ratio.The precipitation efficiency and the recycling ratio are both greater in wet regions than drought regions.The precipitation changes are decomposed into direct and indirect effects.Soil moisture affects precipitation mainly by indirectly changing the precipitation efficiency in both regions.The effects of soil moisture on precipitation are strongest under the condition of high moisture flux convergence in humid and semihumid regions.On the one hand,the soil moisture influences the precipitation through direct and indirect effects,and through changing the moisture flux convergence on the other hand.The moisture flux convergence affects precipitation both directly through changing moisture inflow and indirectly through changing the precipitation efficiency in the arid and humid regions.However,the moisture flux convergence affects precipitation through indirectly changing the precipitation efficiency in the semiarid regions and through directly changing the moisture inflow in the semihumid regions.