A new version of the Rapid Update Cycle (RUC) with 20-km resolution and significant changes in model and assimilation techniques is being implemented at NOAA’s National Centers for Environmental Prediction (NCEP) in summer 2001. The RUC, a high-frequency mesoscale analysis and forecast model system, has become a widely used source for short-range weather forecasting guidance in the United States, especially for aviation, severe- storm, and public-forecasting applications. This paper describes the primary components of the 20-km RUC and how it differs from the 40-km RUC (Benjamin et al. 1999) that has run at NCEP since April 1998. The overall goals for the 20-km RUC implementation have been the following: • Take advantage of increased computer power at NCEP by using increased spatial resolution • Improve RUC performance for quantative precipitation forecasting, especially in the warm season • Improve RUC initial conditions, which are especially important given its niche for short-range forecasts. These goals are realized in the 20-km RUC by incorporating improved modeling and data assimilation techniques, assimilating new observation data, and eliminating bugs. The primary model changes in the 20-km RUC model concern its treatment of convection, explicit clouds using mixed-phase microphysics, and land-surface processes. The key assimilation changes are the introduction of a three-dimensional variational (3dVAR) analysis in the RUC native hybrid isentropic-sigma vertical coordinate and assimilation of GOES cloud-top data to modify RUC hydrometeor fields. In the following sections, we give more detail on resolution, assimilation, and model changes, and then present some recent results.
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