A Study On Surface Energy Balance And Effect Of Mesoscale Motions On Turbulence Over A Homogeneous Bare Soil Surface In Arid Area

The surface energy balance closure and the applicability of Monin-Obukhov similarity theory are two key problems that are still unsolved in the development of boundary layer and land surface process.However,the current study of boundary layer and land surface process shows that a variety of complex factors are intertwined together,which complicates the solution of the two problems.Therefore,the energy balance closure problem and the applicability of Monin-Obukhov similarity theory are investigated using the observation data over a homogeneous bare soil surface.Firstly,the energy balance closure problem on homogeneous bare soil surface is discussed.Secondly,the effect of mesoscale motions on the surface energy balance and the Monin-Obukhov similarity is studied.Finally,some common characteristics of the effect of mesoscale motions on turbulence on different surface are analyzed.The main conclusions are summarized as follows:(1)The instrument parallel comparison showed that,in most cases(more than 80%of the samples),the observed biases of wind speed,temperature,relative humidity,radiation fluxes,and turbulent fluxes are small.The observation results of different instruments have good consistency,which can meet the basic requirements of observation research of land surface process.(2)The analysis of surface energy balance on homogeneous bare soil surface shows that the surface energy imbalance is obvious in different seasons.The daily variation peak of energy balance residuals is between 33.6 W/m~2 and 65.8 W/m~2,the OLS closure rate is between 0.75 and 0.79,and the range of EBR closure rate is between 0.83 and 0.95.In the aspect of Monin-Obukhov similarity,the flux-variance relationship for wind speed essentially follows the 1/3 power law,and the flux-variance relationship for temperature basically follows the-1/3 power law when the atmosphere is unstable,but the similarity relationship is scattered when the atmosphere is stable.(3)The analysis of the surface energy balance on the homogeneous bare soil surface shows that the energy balance closure rate depends on the atmospheric state,and the closure rate increases with the intensity of turbulent mixing.The energy balance closure rate is high during the day when the atmosphere is unstable,while the closure rate is low during the night when the atmosphere is stable,which may be due to the effect of mesoscale motions on the turbulent fluxes at night.In addition,the differences in surface soil heat fluxes calculated by different methods also have a significant effect on the energy balance closure,but do not change the dependence of energy balance closure rate and atmospheric state.(4)At night when the atmosphere is stable,the turbulent fluxes may be contaminated by mesoscale motions,resulting in a number of energy balance closure outliers(ie,EBR<0 or EBR>1),and affecting the surface energy balance closure.After mesoscale motions are excluded by MRD method,the number of energy balance closure outliers drops dramatically and the energy balance closure rate increases evidently.However,compared with the case of strong turbulent mixing,the energy balance closure rate is still low when turbulent mixing is weak,even after mesoscale motions are excluded.This is probably due to the fact that the energy transport is not completely undertaken by turbulence at this time,while the mesoscale motions also have an important contribution to the energy transport.Just for this reason,the energy balance closure during the nighttime and weak turbulence periods is often lower than that during the daytime and strong turbulence periods.(5)Mesoscale motions do not follow Monin-Obukhov similarity theory,so the application of similarity theory will be affected once turbulence characteristic values are contaminated by mesoscale motions.This is reflected in the flux-variance relationship and flux-gradient relationship.After mesoscale motions are excluded,the scatter of the flux-variance relationship is reduced.The flux-variance relationship for wind speed still follows the 1/3 power law in stable and unstable conditions,while the flux-variance relationship for temperature is obviously deviated from the-1/3 power law,and the deviation is large in stable conditions.The scatter of the flux-gradient relationship is also reduced,especially in stable conditions.The observed values of the dimensionless velocity gradient and dimensionless temperature gradient are consistent with the estimated values of different universal functions in unstable conditions.But the observed values are smaller than the estimated values of different universal functions in stable condition,and the deviations increase with the atmospheric stability.(6)The effects of mesoscale motions on turbulence on different surfaces have some similar features.Firstly,after mesoscale motions are removed,the scatter of flux-variance relationships on different surfaces is significantly improved.In stable conditions,the dimensionless velocity/temperature standard deviation(?_w/u_*and?_T/T_*)becomes independent of atmospheric stability(?),and the flux-variance relationship for temperature is significantly deviated from-1/3 power law.Secondly,when the contribution of mesoscale motions to flux is large,?_w/u_*,|?_T/T_*|and?will largely deviate from their true values,for most cases,?_w/u_*become smaller,|?_T/T_*|become larger,and?move closer to zero.Thirdly,the effect of mesoscale motions on turbulence is usually larger(small)in stable(unstable)conditions,but the sign of?may be changed by mesoscale motions(from positive to negative),which leads to the flux-variance relationships in unstable conditions become scattered.Fourthly,the removel of mesoscale motions can significantly improve the problem of energy balance closure outliers during the nighttime,and reduce the uncertainty in the analysis of surface energy balance.