A Method to Retrieve the Reflectivity Signature at 3.75 μm from AVHRR Data

Global monitoring of land surface properties has primarily relied on the AVHRR red and near-infrared channels through use of the NDVI. The AVHRR Channel 3, centered at 3.75 μm, has been shown to be sensitive to vegetation on a local scale. A method to separate the reflected and emitted components in this channel has been developed. The 3.75-μm reflectivity is computed by subtracting the thermal contribution from the total signal and dividing the remaining signal component by atmospheric transmission and solar irradiance. The thermal contribution is estimated by using thermal infrared Channels 4 and 5 as well as NDVI to estimate infrared surface emissivities. The atmospheric transmission is computed with MODTRAN2 and uses integrated water vapor derived from the Split Window Technique. The formula derived are validated over ocean using sun glint observations and land using the FIFE-87 data set. Despite the uncertainties inherent to the procedure we adopted, quantitative use of the derived reflectance at 3.75 μm appears possible.