Mesoscale Weather Prediction with the RUC Hybrid Isentropic-Sigma Coordinate Model and Data Assimilation System

The NOAA Rapid Update Cycle (RUC) is a mesoscale atmospheric data analysis and prediction system configured with a hybrid isentropic-sigma vertical coordinate and run operationally at the National Centers for Environmental Prediction (NCEP). The RUC model is the only quasi-isentropic forecast model running operationally in the world. Primary users include the aviation, severe weather, and general forecast communities, including National Weather Service Forecast Offices. The RUC is distinguished from other hybrid isentropic forecast systems by its application at a fairly high horizontal resolution (10-20 km), utilization of a high-frequency (hourly) analysis update cycle, and use of a continuous vertical coordinate formulation that allows purely isentropic model levels to extend into the lower troposphere. The 2003 operational version of the RUC model and data assimilation system is described herein, complete with a discussion of the analysis formulation and model numerics and physical parameterizations. Within the 3DVAR analysis, use of the quasi-isentropic coordinate system for the analysis increments allows the influence of observations to be adaptively shaped by the potential temperature structure around the observation. Within the model, use of the hybrid θ−σ coordinate reduces the cross-coordinate vertical transport compared to a pure σ coordinate system. Examination of a 36-h forecast East Coast cyclogenesis case illustrates the detailed yet coherent nature of the potential vorticity, moisture, and vertical velocity structures produced by the quasi-isentropic RUC model.

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