LIMITATIONS IN ESTIMATING SURFACE SENSIBLE HEAT FLUXES FROM SURFACE AND SATELLITE RADIOMETRIC SKIN TEMPERATURES

The objective of this study is to demonstrate that by use of simple ph ysical techniques it is possible to obtain useful skill in retrieving sensible heat fluxes by the radiometric method but only on a site-spec ific basis. Sensible heat fluxes measured by Bowen ratio systems at tw o sites during the First International Satellite Land Surface Climatol ogy Project (ISLSCP) Field Experiment (FIFE) 1987 and FIFE 1989 are co mpared to sensible heat fluxes estimated from radiometric measurements of skin temperature taken by pyrgeometers mounted 1.75 m above the su rface. An experimental version of the biosphere-atmosphere transfer sc heme (Ex-BATS), capable of reproducing statistically reliable estimate s of the measured heat fluxes, is used to show that good agreement can be obtained between the model-diagnosed surface temperatures and the skin temperatures obtained from the pyrgeometers. However, because of biases in the radiometer values which are small in an absolute sense b ut large in terms of the differential temperatures between surface and atmosphere, use of such temperatures in modified bulk formula express ions gives rise to significant biases in the sensible heat fluxes. By reconciling radiometric estimates of the sensible heat fluxes and the fluxes measured by the Bowen ratio systems, a mean effective emissivit y considered in the form of a site-specific calibration constant can b e calculated for each of the two sites. Using the new emissivities, co rrected radiometric surface temperatures are obtained that remove the biases at each site, demonstrating that the radiometric method is feas ible but only on a site-specific basis. The corrected skin temperature s are then compared to the advanced very high resolution radiometer (A VHRR)-derived split window estimates of skin temperature obtained on 3 0 clear-sky days during the 1987 experimental period. The two data set s are found to be well-correlated but also with an underlying bias. Di fferences between the satellite estimated temperatures and the correct ed skin temperatures are attributed to imperfections in the coefficien ts used in the split window equation and residual cloud contamination, as well as considerable differences in the scenes viewed by the pyrge ometers and satellite. These differences lead to significant errors in retrieving heat fluxes from the raw satellite skin temperature estima tes. The estimates can be improved after applying a regression between the corrected radiometric skin temperatures obtained for the radiomet ers and the satellite-derived temperatures, although because of the re sidual cloud effects and scene mismatch, the final RMS error is of the order of 65 W m(-2). An independent application of the regression fit between the radiometer temperatures and the AVHRR-derived temperature s derived from the 1987 data to sensible heat flux calculations for fo ur clear-sky days in 1989 indicates poor skill, reinforcing the notion that for satellite applications the radiometric approach is also only feasible on a site-specific basis.