Research On Remote Sensing Methods For The Aerodynamic Roughness Length And Its Application In Evapotranspiration Calculation

The interaction between the land surface and the atmosphere at various temporal and spatial scales is a hot area in the research of meteorology,ecological,hydrology and global change.As a key parameter in land surface models,the aerodynamic roughness length(z0m)plays a crucial role in the process of matter and energy exchange between the land and the atmosphere.The aerodynamic roughness length is defined as the height at which the wind speed becomes zero under neutral conditions.It represents the characteristics of surface and reflects the weakening effect on wind speed resulting from the differences in the underlying surface.However,it has always been the bottleneck to estimate z0_m accurately at regional scale,and there is no unified and widely accepted z0_m estimation model at present.On the basis of the years of accumulated meteorological and flux observations in Hai Basin and Heihe River Basin,remote sensing method is introduced to carry out relevant research on z0_m through the process of analysis,modeling,application and evaluation.The driving factors of z0_m are analyzed and quantified under different underlying surfaces,and then the Hot-darkspot Vegetation Index(HDVI)is developed for z0_m estimation over cropland.The study also finds that the calculation of z0_m directly affects the accuracy of ET,especially in the crop growth peak.This paper includes the following sections:(1)The aerodynamic surface roughness length can be determined iteratively based on wind profile data.Using the Monin-Obukhov similarity,z0_m values can be calculated via the logarithmic wind profile equation.A correlation analysis identifies the dominant factors that influence z0m;then,a factor analysis is performed to identify the different contributions of vegetation indices and meteorological factors to z0_m over different underlying surfaces.Results show that the main driving factors for z0_m include wind speed,wind direction,atmospheric stability,topography and vegetation characteristic,while each factor affects z0_m in different underlying surfaces to different degrees.The z0_m values decrease with increasing wind speed,and to a certain extent,the z0_m values remain relatively low and stable when the wind speeds become high;the z0_m influences from changes in wind direction may depend on the heterogeneity of the topography,wind direction is a main driving factor for z0_m in mountainous areas but can be ignored over flat surfaces underlying homogenous vegetation;z0m presents clear differences under different stability conditions,with higher values under stable conditions and lower values under unstable conditions.Therefore,wind speed,wind direction and atmospheric stability are indispensable for modelling instantaneous z0_m.Factor analysis shows that,for grassland and forest,the first common factors are wind speed and Obukhov length,which are aerodynamic factors(A_factor);their contributions are 44.03% and 42.87%,respectively.The second common factor(the topographic factor(T_factor))is mainly expressed by wind deflection angle,and the contributions are 38.33% and 38.66%,respectively.For cropland,the first common factor is the vegetation factor(V_factor)consisting of NDVI and LAI.The contribution of the vegetation factor is 53.14%.The aerodynamic factor,composed of wind speed and Obukhov length,is a secondary factor that contributes of 30.29%.In conclusion,the vegetation index model used for estimating z0_m is only applicable to the farmland surface.(2)An innovative method for estimation of z0_m over farmland with a new vegetation index,the Hot-darkspot Vegetation Index(HDVI)is developed duing crop growing periods.To obtain this new index,the normalized-difference hot-darkspot index(NDHD)is introduced using a semi-empirical,kernel-driven bidirectional reflectance model with multi-temporal Proba-V 300-m top-of-canopy(TOC)reflectance products.A linear relationship between HDVI and z0_m was found during the crop growth period.Results show that the relationship between HDVI and z0_m is more pronounced than that between NDVI and z0_m for spring maize at Yingke site,with an R2 value that improved from 0.636 to 0.772.At Guantao site,HDVI also exhibits better performance than NDVI,with R2 increasing from 0.630 to 0.793 for summer maize and from 0.764 to 0.790 for winter wheat.HDVI can capture the impacts of crop residue on z0_m,whereas NDVI cannot.(3)The aerodynamic roughness length is applied to the evapotranspiration(ET)simulation by introducing and improving the nonlinear complementary relationship,the z0_m variable is identified though analyzing the sensitivity of the adopted ET calculated model.The global sensitivity analysis reveals that the sensitivity of z0_m is much higher among all input parameters,especially in the crop growth peak,the calculation of z0_m directly affects the accuracy of ET.Therefore,it is significant to develop mature and reliable z0_m calculation model in the research of water and heat flux exchanges between land surface and atmosphere.Although the study of this paper has improved the understanding and estimation accuracy of z0_m,there is still broad space and much potential to be tapped on the research of z0_m.