Improvements and complications involved with adding an 85-GHz channel to cloud liquid water radiometers

The improvement in cloud-liquid estimates by a microwave radiometer with th e addition of measurements at 85 GHz is quantified. Atmospheric emission is simulated from radiosonde data at frequencies commonly used by ground-base d water-vapor radiometers (22.235 and 31.65 GHz) and also at 85.5 GHz, Retr ieval algorithms are developed from opacities based on full Mie extinction by cloud droplets and under an assumption that ice effects are not signific ant for downwelling emission. The algorithms use either three frequencies o r only the lower two. The inclusion of 85-GHz information significantly imp roves liquid-water path estimates at all levels of integrated liquid water. The Rayleigh approximation is shown to be valid for most cloudy conditions . Uncertainty in the calculated opacities due to varying cloud droplet-size distributions and liquid-water content profiles is quantified. The accurac y of a retrieval algorithm trained by Rayleigh approximation opacities and including the additional uncertainty is shown to provide estimates with err or levels similar to those from the algorithm trained with full Mie opaciti es.