MLS Ice Water Content and Horizontally Finite Clouds

An analysis of uncertainties in retrievals of ice water content (IWC) from the Earth Observing System (EOS) Microwave Limb Sounder (MLS) on the NASA Aura satellite is described. Errors in IWC retrievals due to variability in cloud shape and size, and in the relative position of cloud and sensor are investigated through analysis of data from simulations of MLS observations of high-altitude clouds. These errors are found to be of the same magnitude as estimated errors based on comparison of EOS MLS IWC with CloudSat IWC retrievals for observations at 147 hPa, while at 100 and 215 hPa the estimated errors are respectively smaller and larger than the CloudSat estimates. The errors due to assumptions regarding the particle size distribution are also considered and are found to be potentially much larger than those due to geometrical factors, especially for observations at higher tangent heights. Retrieval of the total ice mass within the instrument field-of-view is also considered and uncertainties in this quantity are found to be broadly comparable to uncertainties in IWC. Ice mass is an additional cloud parameter which may be retrieved from MLS measurements.