Improvements to the CRTM for Cloudy Radiance Calculations

The Community Radiative Transfer Model (CRTM) was developed by the Joint Center for Satellite Data Assimilation (JCSDA) for the primary purpose of supporting satellite radiance assimilation for operational numerical weather prediction. Up to this point, the grand majority of operational data assimilation relies only upon clear sky radiance calculations due to inherent difficulties with assimilating clouds. However, recent work has shown progress in cloudy radiance assimilation and it will very likely become routine in the future.

In performing tests with the CRTM, work at the Cooperative Institute for Research of the Atmosphere (CIRA) found that forward radiance calculations have significant errors for wavelengths that include a solar reflected component, such as 3.9 μm and below. Brightness temperature errors were as large as 50 K for optically thick ice clouds. Satellite-observed radiances in the near infrared (e.g., 3.9 μm) are very sensitive to hydrometeor particle size, so CRTM accuracy for these clouds is critical for diagnosing possible problems with model microphysics schemes. In collaboration with the CRTM team at the JCSDA, a study is currently underway to identify the source of the error in the CRTM and to suggest a fix. This presentation will provide details of the ongoing work and present the most recent findings.