Combined Use of Vegetation Density, Friction Velocity, and Solar Elevation to Parameterize the Scalar Roughness for Sensible Heat

Abstract

Monin-Obukhov similarity was used to calculate sensible heat fluxes (Hc) at an array of up to 20 surface flux measurement sites on five days in 1987 and 1989 during the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment by means of spatially distributed radiometric surface temperatures from an airborne platform and ground-based data. To use Monin-Obukhov similarity, a parameterization for the scalar roughness, as a function of spatially varying leaf area index (LAI) and friction velocity (u?), was developed from a previous, simpler parameterization. LAI was found to be significant, but the range of u? was too small to ascertain its significance. The parameterization was found to produce sensible heat flux values that had correlations around 0.8 with the spatially distributed sensible heat flux measurements on four of the days, but on a day with high, uniform soil moisture content, the correlation was only 0.226. It is argued that the high soil moisture values indirectly resulted in relatively larger significance of noise in the surface?air temperature difference, which reduced the reliability of the calculated sensible heat fluxes. In addition, constants in the parameterization from one day may not necessarily be applicable to other days. This may be due to factors such as solar elevation and instrument view angle. It is proposed and verified that the differences between dates can be resolved in a spatially averaged sense by accounting for the effects of seasonal variation in solar elevation on the vertical distribution of canopy temperatures. This produced a correlation of 0.973 between measured and calculated sensible heat fluxes when all dates were considered simultaneously.