QUANTIFYING THE EFFECT OF EMISSIVITY ON THE RELATIONS BETWEEN AVHRR SPLIT WINDOW TEMPERATURE DIFFERENCE AND ATMOSPHERIC PRECIPITABLE WATER OVER LAND SURFACES

The effect emissivity on the relations between the NOAA-9 AVHRR infrar ed split window temperature difference (Delta T = difference of bright ness of temperatures for 10.1-11.5 mu m and 11.2-12.6 Delta m bands) a nd atmospheric precipitable water (W), namely, Delta T = alpha + beta W and W = alpha' + beta' Delta T, is quantified in terms of red reflec tance (0.55-0.72 mu m) observed by the AVHRR. Two years (1987 and 1988 ) of AVHRR and radiosonde observations at 44 globally distributed land surface locations have been used in the analysis. Predictions of a ra diative transfer model are tested using these observations, which indi cate possible discrepancies between the predicted and the observed res ults. Estimates of 10.1-11.5 mu m band emissivity have been made by in terpreting the intercept of Delta T-W regression according to the theo retical predictions, and these emissivities were found to be linearly related to the red reflectance. Precipitable water estimated from the observed Delta T and red reflectance (by the AVHRR) for the two years at 11 locations over South America and Europe when compared with the o bserved precipitable water (from radiosonde) gave a mean absolute erro r of 5 mm and a bias of 4 mm, with the explained variance of 79%. Some inherent uncertainties in the data used in the present analysis have been noted.