Determination of regional surface heat fluxes over heterogeneous landscapes by integrating satellite remote sensing with boundary layer observations

Keywords : satellite remote sensing, surface layer observations, atmospheric boundary layer observations, land surface variables, vegetation variables, land surface heat fluxes, validation, heterogeneous landscape, GAME/Tibet, CAMP/Tibet, HEIFE, AECMP'95, DHEX, NOAA/AVHRR, Landsat-5(7) TM(ETM) Arid areas such as the Heihe River Basin and high elevation areas as the Tibetan Plateau with a heterogeneous landscape are characterized by extreme gradients in land surface properties such as wetness and roughness which have a significant but local impact on the Atmospheric Boundary Layer (ABL). Observation of the actual extent of these areas and their properties is essential to understand the mechanisms through which heterogeneous land surfaces may have a significant impact on the structure and dynamics of the overlying ABL. The latter applies specifically to energy and water fluxes. Progress in this research area requires spatial measurements of variables such as surface hemispherical reflectance, radiometric surface temperature, vegetation fractional cover, Leaf Area Index ( LAI ) and local aerodynamic and thermodynamic roughness lengths. It also requires the measurements of Normalized Difference Vegetation Index ( NDVI ), Modified Soil Adjusted Vegetation Index ( MSAVI ). Imaging radiometers on-board satellites can provide useful estimates of most of these variables. Using these variables in combination with the Surface Layer (SL) and ABL observations we are able to derive the distribution of land surface heat fluxes over heterogeneous landscapes. Based on the analysis of the land surface heterogeneity and its effects on the overlying air flow, SL observations, ABL observations and satellite Remote Sensing (RS) measurements [Landsat-5(7) TM (ETM) and NOAA/AVHRR measurements] , three parameterization methodologies, which are called RS approach (RS + SL-assumptions), Tile approach (RS + SL-observations) and Blending height approach (RS + SL-observations + ABL-observations) are developed and demonstrated to estimate the surface heat flux densities over heterogeneous landscapes . The RS approach (see Figures 3.4 and 5.1) uses satellite measurements in combination with assumptions on the SL at and below a reference height of about 2m . The Tile approach (see Figures 3.4, 3.5 and 7.1) uses satellite measurements in combination with SL observations at and below a reference height of about 20m . The Blending height approach (see Figures 3.4, 3.6 and 7.2) uses satellite measurements in combination with SL and ABL observations at and below the blending height of about 200m . The approaches were applied to heterogeneous areas: the HEIhe basin Field Experiment (HEIFE) , the Arid Environment Comprehensive Monitoring Plan, 95 (AECMP'95), the Global Energy and Water cycle EXperiment Asian Monsoon Experiment on the Tibetan Plateau (GAME/Tibet), the Coordinated Enhanced Observing Period Asia-Australia Monsoon Project on the Tibetan Plateau (CAMP/Tibet) and the DunHuang EXperiment (DHEX) . The distributions of NDVI , MSAVI , vegetation fractional cover, LAI , surface reflectance, surface temperature, net radiation flux, soil heat flux, sensible heat flux and latent heat flux have been determined over five different heterogeneous areas. These estimates have been compared with independent ground measurements of flux densities. At the validation sites relative deviations ( ) were less than 10 % . The results derived from the Blending height approach were also compared over the HEIFE area with that derived from the RS approach. The results clearly show that the Tile approach and the Blending height approach using satellite measurements in combination with SL observations and ABL observations provided much better estimates of heat flux densities than the RS approach .