Satellite Microphysical Retrievals for Land-Based Fog with Validation by Balloon Profiling

Abstract

Digital data from the National Oceanic and Atmospheric Administration Advanced very High Resolution Radiometer (AVHRR) satellite instrument provides multispectral images in visible near-infrared and thermal infrared wave bands, which have been utilized to develop retrieval techniques for estimating the droplet effective radius and optical depth of land-based fog. The retrieval methods are based on multiple scattering calculations that simulate the increased near-infrared absorption by fog layers with increasing droplet size and liquid water path. The AVHRR thermal window channels are utilized to remove the effects of thermal emission in the near-infrared band. New instrumentation and field sampling methods have been developed for obtaining detailed vertical profiles of fog droplet size distributions and thermodynamic conditions in fog decks. The in situ measurements derived from the field observations were employed to test the satellite retrieval techniques. Intercomparison shows a close correspondence between field observations and retrieved values of the fog droplet effective radius as well as fog optical depth. Simulated 4-km near-infrared and visible pixel data are also used to test retrievals from GOES-8. The AVHRR and GOES-8 retrievals provide a mapped database of the fog microphysical and depth parameters over the entire region of fog, which may be applied to numerical simulation of fog evolution and pollutant deposition, newcasting of fog visibility hazards, and global monitoring of fog influences on the atmosphere-surface radiation budget.