Testing the Impact of Additional Vertical Levels for the 3km High-Resolution Rapid Refresh

In this talk, we describe recent tests conducted with the 3-km High-Resolution Rapid Refresh (HRRR) in which 14 additional vertical levels are introduced to the sigma level specification within WRF-ARW. The additional levels are largely focused in the low- to mid-levels of the troposphere. These tests were originally motivated by a need for improved resolution of wintertime low-level cold airmasses, but forecast impacts could be wide-ranging.

Here, we present results from case studies in which the HRRRv3 code was otherwise held constant, and GSI and WRF-ARW were run with 14 additional vertical levels. Various degrees of impact are seen for different phenomena, with the largest impact evident in the representation of low-level cold airmasses and the strength of inversions. Forecasts are also evaluated for the speed of shallow cold fronts, the timing of convective initiation, convective evolution, and air quality forecasting in complex terrain.

Stability considerations required several attempts at a sigma level specification in the early stages of this work before a configuration was found which successfully ran without CFL violations. An additional avenue of this work is testing the stability of the level configuration in situations of strong flow over steep terrain, and extreme convective instability. Finally, the runtime cost of such a modification to the HRRR is evaluated.