Abstract
Most land surface models require information on aerodynamic roughness length and its temporal and spatial variability. This research presents a practical approach for determining the aerodynamic roughness length at fine temporal and spatial resolution over the landscape by combining remote sensing and ground measurements. The basic framework of Raupach, with the bulk surface parameters redefined by Jasinski et al., has been applied to optical remote sensing data collected by the HJ-1A/1B satellites. In addition, a method for estimating vegetation height was introduced to derive the aerodynamic roughness length, which is preferred by users over the height-normalized form. Finally, mapping different vegetation classes was validated taking advantage of the data-dense field experiments conducted in the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) project. Overall, the roughness model performed well against the measurements collected at most HiWATER flux tower sites. However, deviations still occurred at some sites, which have been further analyzed.
Highlights
Aerodynamic roughness length z0m is a surface parameter that scales the vertical profile of the horizontal component of wind speed and characterizes the ability of the surface to absorb momentum from airflow
The coefficients in Equation (7) for different vegetation types were obtained by least squares regression analysis using filed leaf area index (LAI) measurements collected at the 12 eddy covariance (EC) sites and one vegetable site, and HJ-1A/1B Normalized Difference Vegetation Index (NDVI) data for the pixel of the corresponding site
The current research presents a practical approach for generating regional vegetation aerodynamic roughness lengths with fine temporal and spatial resolutions by combining remote sensing and ground measurements
Summary
Aerodynamic roughness length z0m is a surface parameter that scales the vertical profile of the horizontal component of wind speed and characterizes the ability of the surface to absorb momentum from airflow. The roughness length is the vertical length scale of the logarithmic wind profile, which is established when the air flow attains equilibrium (i.e., constant vertical momentum flux) with the underlying homogeneous surface under neutral conditions [2]. Z0m is estimated from turbulence measurements by using the eddy covariance (EC) technique [1,3,4] These methods can produce credible results when the underlying surface is relatively homogeneous. The horizontal scale of variability of the land surface may lead to conditions where no constant (momentum) flux layer, i.e., no logarithmic wind profile, is established. Under such conditions the aerodynamic roughness length becomes an effective land surface property which requires complex measurements over large areas
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