Modeled and observed relations between the AVHRR split window temperature difference and atmospheric precipitable water over land surfaces

Atmospheric correction to near-infrared reflectances for land surface studies requires information about precipitable water. A relation between the split window temperature difference (ΔT) and atmosphere precipitable water (W) over land surfaces is developed from the radiative transfer equation considering spectral variation of the emissivity. This relation is found to be essentially linear with slope and intercept being dependent on the emissivity. The observed ΔT (from NOAA-9 AVHRR data for 10.1–11.5 μm and 11.2–12.6 μm channels) and W (from radiosonde data) for 1 year (1988) at 24 globally distributed locations of various surface types (hot and temperate deserts, savannas, temperature deciduous forest, rainforest, boreal forest, etc.) were analyzed to assess the relation between ΔT and W and compared with the theoretical results. These globally distributed data were segregated into specified ranges of visible reflectance (from the AVHRR corresponding to ΔT data) so as to minimize emissivity variations in determining the slope and the intercept of the ΔT-W relationship and in estimating emissivity. Both the theory and the observations demonstrate the feasibility of estimating W from ΔT over vegetated surfaces, although a priori knowledge of emissivity will be needed for a wider application.