Sensitivity of Reflectance to Water Vapor and Aerosol Optical Thickness

The atmospheric condition parameters used in the radiative transfer-based atmospheric correction (AC) are often uncertain. This uncertainty propagates to the estimated reflectance. The reflectance, is, however, not equally sensitive to all the parameters. A sensitivity analysis (SA) helps in prioritizing the parameters. The objective of this study was to perform an SA of reflectance to water vapor concentration (wv) and aerosol optical thickness (AOT). SA was performed using the Fourier amplitude sensitivity test (FAST) method, which computes sensitivity indices (SI) of these parameters. Besides variation in the two parameters, we also studied the effect of surface albedo on the SI by quantifying SI for three target surfaces (in the spectral range 0.44-0.96 μm): 1) a dark target (water); 2) a bright target (bare soil); and 3) a target having low albedo in the visible and high albedo in near-infrared range (forest). For AOT, high (≈0.9) SI values were observed at the nonwater absorption wavelengths. For wv, high SI values were observed at wavelengths, where strong absorption features are located and when the surface albedo was high. For the dark target, the effect of AOT was prominent throughout the spectral range. We found that the sensitivity of reflectance to wv and AOT is a function of wavelength, strength of the absorption features, and surface albedo. We conclude that AOT is a more important parameter for dark targets than wv even at the principal absorption feature. For bright targets, the importance of wv and AOT depends on the strength of the absorption feature.