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 the addition of measurements at 85 GHz is quantified. Atmospheric emission is simulated from radiosonde data at frequencies commonly used by ground-based water-vapor radiometers (22.235 and 31.65 GHz) and also at 85.5 GHz. Retrieval algorithms are developed from opacities based on full Mie extinction by cloud droplets and under an assumption that ice effects are not significant for downwelling emission. The algorithms use either three frequencies or only the lower two. The inclusion of 85-GHz information significantly improves 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 accuracy of a retrieval algorithm trained by Rayleigh approximation opacities and including the additional uncertainty is shown to provide estimates with error levels similar to those from the algorithm trained with full Mie opacities