The Rapid Update Cycle (RUC) is an operational mesoscale data assimilation and numerical forecasting system developed by NOAA Research Forecast Systems Laboratory (FSL) in Boulder, CO. The first version of the RUC (RUC1) became operational at the National Centers for Environmental Prediction (NCEP) in 1994. Several major upgrades have been introduced since then, the most recent, RUC13, in June 2005. RUC13 has a 13-km horizontal resolution, 50 vertical levels, and utilizes a 1-h intermittent data assimilation cycle (for general information about RUC see Benjamin et al. 2004a,b). With this latest upgrade the RUC series will be terminated and a new design for rapid updating, called Rapid Refresh (RR), will be introduced. Both the RUC data assimilation system and dynamical model will be replaced by components more suited for the Weather Research and Forecasting model (WRF) framework and/or the Earth System Modeling Framework (ESMF). This paper describes new developments related to the data assimilation aspects of RR. In the present RUC system a three-dimensional variational (3DVAR) scheme (Devenyi and Benjamin 2003, Benjamin et al. 2004a) is used in RUC’s hourly update cycle for analysis of data from diverse sources, including RAOB, METAR, buoy observations, commercial aircraft (ACARS), wind profilers, geostationary (GOES) and polar-orbiting (SSM/I) satellites, ground-based GPS, and radars (VAD winds). In the new RR, following a design decision in June 2005, the Gridpoint Statistical Interpolation (GSI) scheme will be used for data assimilation instead of the current RUC 3DVAR scheme. GSI is under development at NCEP as a new generation of 3DVAR to replace the successful Spectral Statistical Interpolation (SSI) analysis scheme, developed during the early 1990s by Parrish and Derber (1992).
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