Upper tropospheric humidity algorithm assessment

This study assesses a common analytic expression used to compute upper tropospheric humidity (UTH) from satellite brightness temperatures at 6.7 μm. Vertical averaging methods for UTH were found to have significant impact on the algorithm. The temperature lapse parameter β had more latitudinal variance than previously found; however, large vertical gradients in midlatitude profiles degraded UTH retrievals. The scaled reference pressure parameter P0 did sufficiently improve the radiance to humidity relationship with exception to midlatitude profiles where P0 exceeds 1.7. A new method for computing the reference pressure parameter is presented using HIRS channel 6 observations. Error analysis of radiance to humidity relationship indicates that absolute UTH errors increase with increasing humidity with largest errors of 15% per 1 K bias between simulated and observed brightness temperatures.