AVHRR SPLIT WINDOW TEMPERATURE DIFFERENCES AND TOTAL PRECIPITABLE WATER OVER LAND SURFACES

Measurements from the thermal infrared split window channels of the AV HRR sensor were investigated for their relationship to the total atmos pheric water vapour amount over land surfaces. The difference in brigh tness temperature between the AVHRR channel 4 and 5 (10-3-11.3 mum and 11.4-12.3 mum respectively) was found to be a linear function of tota l precipitable water, for several stations in differing climatic regim es. For each individual location the total precipitable water was esti mated with a standard error ranging from 0.22 to 0-48 cm for the compl ete range of conditions from wet to dry season or summer to winter. Fo r mid-latitude continental locations there is very little influence of atmospheric aerosols on the relationship while for the African Sahel region the effect of large airborne particulates with a silicate compo nent introduces a significant effect at large values of aerosol optica l depth due to absorption. The influence of spectral emissivity variat ion in the split window region was also observed for arid regions wher e there is a significant quartz component to the soil. It is concluded that for regional retrieval of precipitable water, this technique pro vides sufficient accuracy for application to correction of near-infrar ed satellite data such as AVHRR channel 2 (0.71-0.98 mum), however the site specific relation between T4-T5 and PW needs to be established w ith independent PW measurements.