A North American Hourly Assimilation and Model Forecast Cycle: The Rapid Refresh

Abstract

The Rapid Refresh (RAP), an hourly-updated assimilation and model forecast system, replaced the Rapid Update Cycle (RUC) as an operational regional analysis and forecast system among the suite of models at the NOAA National Centers for Environmental Prediction (NCEP) in 2012. The need for an effective hourly-updated assimilation and modeling system for the United States for situational awareness and related decision-making has continued to increase for various applications including aviation (and transportation in general), severe weather, and energy. The RAP is distinct from the previous RUC in three primary aspects: a larger geographical domain (covering North America); use of the community-based WRF-ARW (Weather Research and Forecasting – Advanced Research WRF) model replacing the RUC forecast model; and use of the Gridpoint Statistical Interpolation (GSI) assimilation instead of the RUC three-dimensional variational (3DVar) assimilation. As part of the RAP development, modifications have been made to the community WRF-ARW model (especially in model physics) and GSI assimilation systems, some based on previous model and assimilation design innovations developed initially with the RUC. Upper-air comparison is included for forecast verification against both rawinsondes and aircraft reports, the latter allowing hourly verification. In general, the RAP produces superior forecasts to those from the RUC, and its skill has continued to increase from 2012 up to RAP version 3 as of 2015. In addition, the RAP can improve on persistence forecasts for the 1-3h forecast range for surface, upper-air, and ceiling forecasts.