1.8 The 20-km Rapid Update Cycle – Overview and Implications for Aviation Applications Stanley G. Benjamin,John M. Brown, Kevin J. Brundage 1 , Dezso Devenyi 2 , Georg A. Grell 2 , Dongsoo Kim 2 , Tatiana G. Smirnova 2 , Tracy Lorraine Smith 1 , Barry E. Schwartz, Stephen S. Weygandt NOAA Research – Forecast Systems Laboratory Boulder, Colorado 80305 USA Geoffrey S. Manikin SAIC at NOAA/NWS/NCEP/EMC, Camp Springs, Maryland Figure 1. Terrain elevation for 40-km RUC-2 (RUC40), above, and the RUC20 (20-km), below. ------------------------------------------------------------------------------- Corresponding author address: Stan Benjamin, NOAA/FSL, R/E/FS1, 325 Broadway, Boulder, CO 80305 Benjamin@fsl.noaa.gov , http://ruc.fsl.noaa.gov 1 – In collaboration with the Cooperative Institute for Research in the Atmosphere (CIRA), Colorado State Univ., Ft. Collins, Colorado 2 - Cooperative Institute for Research in Environmental Sciences (CIRES), Univ. of Colorado/NOAA Research - Forecast Systems Laboratory, Boulder, Colorado 1. INTRODUCTION A new version of the Rapid Update Cycle (RUC) is being implemented into operations at the National Centers for Environmental Prediction (NCEP) in April 2002. This new version includes a doubling of horizontal resolution (40-km to 20-km), an increased number of computational levels (40 to 50), and improvements in the analysis and model physical parameterizations. A primary goal of the 20-km RUC (or RUC20) is improvement in warm-season and cold- season quantitative precipitation forecasts. Improvements in near-surface forecasts and cloud forecasts have also been targeted. The RUC20 provides improved forecasts for these variables, as well as for wind, temperature, and moisture above the surface. The RUC20 will benefit general public forecasting as well as aviation and severe-weather forecasting. The RUC20 continues to produce new analyses and short-range forecasts on an hourly basis, with forecasts out to 12 h run every 3 h. The implementation of the RUC20 in 2002 follows previous major implementations of a 60-km 3-h cycle version in 1994 and a 40-km 1-h cycle version (known as RUC-2 or RUC40) in 1998. In this paper, we present an overview of these changes in the RUC20 and an assessment of their likely impact on aviation applications. 2. HORIZONTAL AND VERTICAL RESOLUTION CHANGES The RUC20 occupies the same spatial domain as the previous 40-km RUC-2, as shown in Fig. 1. The RUC20 grid points are still a subset of the AWIPS Lambert conformal grid (AWIPS/GRIB grid 215 for 20- km) used as a distribution grid by the National Weather Service. Direct use of the AWIPS grid reduces the number of distribution grids for the RUC. The AWIPS grid ID for the RUC20 is 252, compared to 236 for the RUC40. The RUC20 domain size is 301 x 225 grid points (compared with 151 x 113 for RUC40). The 20-km grid spacing used by the RUC20 provides better resolution of variations of terrain elevation, leading to improved forecasts of topographically induced low-level circulations, mountain waves, and orographic precipitation. It also allows better resolution of land-water boundaries and 1