Sensitivity of BRDF, NDVI and Wind Speed to the Aerodynamic Roughness Length over Sparse Tamarix in the Downstream Heihe River Basin

Abstract

Abstract:The aerodynamic roughness length (z0m) is an important parameter that affects the momentum and energy exchange between the earth surface's and the atmosphere. In this paper, the gradient wind speed data that were observed from May to October, 2014, at the Si Daoqiao station, which is located in an area of sparse Tamarix in the downstream region of the Heihe River Basin (HRB), are used to evaluate the sensitivity of the moderate-resolution imaging spectroradiometer (MODIS) near-infrared (NIR) bi-directional reflectance distribution function (BRDF) R, the MODIS/Landsat 8 normalized difference vegetation index (NDVI), and the wind speed at 5 m in comparison with the field-measured z0m. The results indicate that the NIR BRDF_R_MODIS and the NDVI MODIS /NDVI Landsat8 are less sensitive indicators of the z0m over sparse Tamarix areas (R 2 : 0.0045 for NIR BRDF_R MODIS ; R 2 : 0.0342 for NDVI MODIS ; and, R 2 : 0.1646 for NDVI Landsat8 ), which differs significantly from the results obtained by previous studies for farmlands and grasslands. However, there is a nearly linear correlation between the wind speed at 5 m and the z0m at the time scale of the NDVI Landsat8 acquisitions (R 2 : 0.3696). Furthermore, the combination of the NDVI and wind speed at 5 m can significantly improve this correlation (R 2 : 0.7682 for NDVI MODIS ; R 2 : 0.6304 for NDVI Landsat8 ), whereas the combination of the NIR BRDF_R MODIS and wind speed at 5 m still has a low correlation (R 2 : 0.0886). Finally, the regional z0m of the oasis in the downstream region of the HRB was determined using Landsat 8 surface reflectance data and the wind speed data at the Si Daoqiao station, which properly reflect the temporal evolution of the z0m in that region. The parameterization scheme proposed in this paper has great potential to be applied to evapotranspiration, land surface, and hydrologic model simulations of sparse Tamarix at Si Daoqiao site in the future.

Scale Invariance of Albedo‐Based Wind Friction Velocity

“…Satellite remote sensing provides the ability to produce area-weighted or integrated estimates of land surface characteristics across pixels (e.g., 500 × 500 m). Approaches tested to resolve areal z 0 and u s* include radar backscatter coefficients (Greeley et al, 1991(Greeley et al, , 1997Marticorena et al, 2004Marticorena et al, , 2006, surface shadowing (Chappell et al, 2010;Chappell & Heritage, 2007;Cho et al, 2012), and sensor bidirectional reflectance distribution function (BRDF) parameters (Xing et al, 2018). Using data from wind tunnel experiments, Chappell and Webb (2016) showed that aerodynamic sheltering can be approximated from land surface albedo normalized to remove surface spectral effects.…”