ABSTRACT

Abstract

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. The RUC20 continues to use the hybrid isentropic-sigma vertical coordinate used 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 to improve the precipitation type forecast. The new Grell scheme results 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 of land-use type between the background and the observation for near-coastal stations. The RUC20 continues to use an optimal interpolation analysis as in the RUC40 implementation of a 3-d variational analysis has been deferred.

6. Improved land-surface physics The RUC20 land-surface model is changed from that of the RUC40. It uses more detailed land-use and soil texture data, in contrast to 1-degree resolution fields used in the RUC40. It includes improved cold-season processes (soil freezing/thawing) and a 2-layer snow model. These changes improve the evolution of surface moisture and temperature and snow cover, which in turn improve forecasts of surface temperature and moisture and precipitation.

7. Lateral boundary conditions The RUC40 used lateral boundary conditions specified from the Eta model initialized every 12 h. The RUC20 adds updates of its lateral boundaries from the 0600 and 1800 UTC Eta runs.

8. Improved post-processing The RUC20 includes improved diagnostic techniques for 2-m temperature and dewpoint, 10-m winds, helicity, visibility, convective available potential energy, and convective inhibition. Most significant improvements in RUC20 fields over those from RUC40 (RUC-2).