Computation of Infrared Transmittances in the Troposphere Using Satellite Values of Precipitable Water: Preliminary Method

Abstract : Transmittance in the thermal infrared may be estimated from various in situ and remote techniques using ground-based or airborne sensors. However, estimating transmittance in the troposphere from data gathered by sensors on space platforms remains a problem, especially over land surfaces. This report presents a preliminary method for computing thermal infrared transmittance over a horizontal, vertical, or slant path. This method uses estimates of precipitable water from satellite data combined with sounding data from satellite or other sources. A brief description of the technique and a brief sensitivity analysis provide a basic understanding of the method. Profiles of specific humidity are computed from actual atmospheric profiles or climatological data and then adjusted according to satellite estimates of total precipitable water. Temperature and humidity data at each pressure level provide input to equations of the types found in LOWTRAN. These equations yield estimates of thermal infrared transmittance for user specified path lengths.