NWS Technical Procedures Bulletin No. 490 RUC20 - The 20-km version of the Rapid Update Cycle 11 April 2002 (updated 16 May) Stanley G. Benjamin, John M. Brown, Kevin J. Brundage, Dezső Dévényi, Georg A. Grell, Dongsoo Kim, Barry E. Schwartz, Tatiana G. Smirnova, and Tracy Lorraine Smith, and Stephen S. Weygandt NOAA/OAR Forecast Systems Laboratory, Boulder, CO Geoffrey S. Manikin Environmental Modeling Center, National Centers for Environmental Prediction, Camp Springs, MD Abstract A major revision to the Rapid Update Cycle (RUC) analysis/model system was implemented into operations at the National Centers for Environmental Prediction Center (NCEP) on 17 April 2002. The new RUC version with 20- km horizontal resolution (RUC20) replaces the previous 40-km version of the RUC (RUC40). Summary of RUC20 vs. RUC40 (RUC-2) differences 1. Horizontal resolution The RUC20 has a 20-km horizontal resolution, compared to 40 km for the previous RUC40 (RUC-2). The area covered by the computational grid has not changed. The RUC20 has a 301x225 horizontal grid, compared to 151x113 for the RUC40. 2. Vertical resolution The RUC20 has 50 computational levels, compared to 40 levels for the RUC40, and continues to use the hybrid isentropic-sigma vertical coordinate as in previous versions of the RUC. 3. Improved moist physics Improved quantitative precipitation forecasts have been the primary focus for changes in the RUC20 model, including a major revision in the MM5/RUC mixed-phase microphysics cloud routine, and a new version of the Grell convective parameterization with an ensemble approach to closure and feedback assumptions. The main effect of the microphysics change is to decrease overforecasting of graupel and ice and improve the precipitation type forecast. The new Grell scheme provides in considerable improvement in convective precipitation forecasts from the RUC. 4. Assimilation of GOES cloud-top data The RUC20 includes a cloud analysis that updates the initial 3-d cloud/hydrometeor fields by combining cloud-top pressure data from GOES with the background 1-h RUC hydrometeor field. Cloud clearing and building is done to improve the initial cloud water/ice/rain/snow/graupel fields for the RUC. 5. Better use of observations in analysis The RUC20 assimilates near-surface observations more effectively through improved algorithms for calculating observation-background differences. Assimilation of surface observations is improved by diagnosing background forecasts for surface temperature and dewpoint at 2 m and for winds at 10 m. It is also improved by matching land-