An Hourly Assimilation–Forecast Cycle: The RUC

TLDR

The Rapid Update Cycle (RUC) is an operational regional analysis–forecast system that uniquely combines an hourly assimilation cycle with a hybrid isentropic–sigma vertical coordinate, enabling observation influence to adapt to potential temperature structure and delivering highly current short‑range forecasts. This study describes the RUC analysis framework in the hybrid coordinate and discusses considerations for high‑frequency cycling. A 20‑km, 50‑level hourly RUC was operationalized in April 2002, and its forecasts were verified against rawinsonde and surface observations. Verification statistics indicate that the hourly assimilation cycle improves short‑range forecasts, even for 1‑hour lead times.

Abstract

The Rapid Update Cycle (RUC), an operational regional analysis–forecast system among the suite of models at the National Centers for Environmental Prediction (NCEP), is distinctive in two primary aspects: its hourly assimilation cycle and its use of a hybrid isentropic–sigma vertical coordinate. The use of a quasi-isentropic coordinate for the analysis increment allows the influence of observations to be adaptively shaped by the potential temperature structure around the observation, while the hourly update cycle allows for a very current analysis and short-range forecast. Herein, the RUC analysis framework in the hybrid coordinate is described, and some considerations for high-frequency cycling are discussed. A 20-km 50-level hourly version of the RUC was implemented into operations at NCEP in April 2002. This followed an initial implementation with 60-km horizontal grid spacing and a 3-h cycle in 1994 and a major upgrade including 40-km horizontal grid spacing in 1998. Verification of forecasts from the latest 20-km version is presented using rawinsonde and surface observations. These verification statistics show that the hourly RUC assimilation cycle improves short-range forecasts (compared to longer-range forecasts valid at the same time) even down to the 1-h projection.