Computation and use of the reflectivity at 3.75 μM from AVHRR thermal channels

Global monitoring of land surface properties uses mainly AVHRR red and near infrared channels through NDVI. The AVHRR channel 3, centered at 3.75μm has been shown to be sensitive to vegetation on a local scale. In order to make use of the 3.75μm signal one has to decouple the thermal component and the reflective component, the former being seen as a correction term the latter as the signal of interest. We present in this paper a formula to compute 3.75μm reflectivity by subtracting the thermal contribution from the total signal and dividing the reflective component from atmospheric transmission and solar irradiance. The thermal contribution is estimated by using infrared channels 4 and 5 as well as NDVI to assess infrared emissivities. The atmospheric transmission is computed with MODTRAN and uses integrated water vapor derived from the Split Window Technique. The first section is devoted to water vapor retrieval which are validated over ocean using coincident SSM/I measurements and over land using sun photometer measurements. The second section present the theoretical background and gives analytical formula as well as an error budget. Section three validates the derived formula over ocean using sun glint observation and over land using FIFE‐87 data set. The last section presents possible application of the formula to aerosol and surface properties retrievals.