A METHOD TO RETRIEVE THE REFLECTIVITY SIGNATURE AT 3.75 MU-M FROM AVHRR DATA

Global monitoring of land surface properties has primarily relied on t he AVHRR red and near-infrared channels through use of the NDVI. The A VHRR Channel 3, centered at 3.75 mu m, has been shown to be sensitive to vegetation on a local scale. A method so separate the reflected and emitted components in this channel has been developed. The 3.75-mu m reflectivity is computed by subtracting the thermal contribution from the total signal and dividing the remaining signal component by atmosp heric transmission and solar irradiance. The thermal contribution is e stimated 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 oc ean using sun glint observations and land using the FIFE-87 data set. Despite the uncertainties inherent to the procedure we adopted quantit ative use of the derived reflectance at 3.75 mu m appears possible. Pu blished by Elsevier Science Inc., 1998.