Testing of The New WRF-ARW Hybrid Vertical Coordinate In RAP and HRRR

Abstract

A new hybrid vertical coordinate (Klemp 2011, Mon. Wea. Rev.), shown to be superior to the current sigma coordinate used in the Advanced Research version of the Weather Research and Forecast (WRF-ARW) model in flow over idealized terrain, has very recently been implemented into WRF-ARW by developers at NCAR. Preliminary real-data tests at NCAR have shown encouraging results in the sense that solutions show less evidence of small-scale features over terrain than with the current sigma coordinate. The WRF-ARW is the dynamical core used by the Rapid Refresh (RAP) and the High Resolution Rapid Refresh (HRRR) running operationally at the National Weather Service’s National Centers for Environmental Prediction. Recognizing that use of this new hybrid coordinate has the potential to improve the accuracy of operational NWP forecasts by the RAP and HRRR over areas of complex terrain such as the western US and Alaska, it is being evaluated, both separately and in conjunction with other physics upgrades, in advanced experimental versions of these models being run by NOAA’s Earth System Research Laboratory, Global Systems Division. We will show forecast examples of the impact of use of this coordinate in RAP, HRRR, an experimental version of the HRRR over Alaska, and a 750m grid-spacing nest inside the HRRR over the Pacific Northwest being run in conjunction with the Department of Energy Wind Forecast Improvement Project Version 2. Objective verification using controlled experiments on retrospective periods will also be discussed.