Characterization of errors in cirrus simulations from a cloud resolving model for application in ice water content retrievals

Data available from the Atmospheric Radiation Measurement-Unmanned Aerospace Vehicle (ARM-UAV) Spring 1999 experiment are used in this study to estimate errors in cirrus simulations from a 3D Cloud Resolving Model (CRM). The performance of the model, heritage of the CSU Regional Atmospheric Modeling System (RAMS) is assessed by direct comparison of modeled and observed fields. Results show that the CRM succeeds in placing the cloud at approximately the correct altitude, but consistently overestimates the Ice Water Content (IWC). A statistical approach is introduced and applied to quantify average model bias under the assumption of bias-free observations. An error covariance matrix associated with simulated fields is also computed, and used to identify model strengths and deficiencies. Model fields are then used in the context of an optimum estimation retrieval of IWC from a combination of radar and radiometric observations. The retrieval is based on the knowledge of an a priori profile and relative error covariance to ensure algorithm convergence and stability. RAMS average Ice Water Content, corrected for the bias, and the related error covariance matrix derived in this Study are used to provide this a priori information to the retrieval. (C) 2001 Elsevier Science B.V. All rights reserved.