8.1 An evaluation of the new hybrid vertical coordinate in the RAP and HRRR

Beck, Jeff, Cooperative Institute for Research in the Atmosphere (CIRA), National Oceanic and Atmospheric Administration/Earth Systems Research Laboratory/Global Systems Division (NOAA/ESRL/GSD) and Developmental Testbed Center (DTC), John M. Brown, NOAA/ESRL/GSD, Joseph Olson and Tatiana G. Smirnova, Cooperative Institute for Research in the Environmental Sciences and NOAA/ESRL/GSD, Trevor Alcott, Stan Benjamin and Curtis Alexander, NOAA/ESRL/GSD, Jimy Dudhia, Dave Gill, Wei Wang and Joe Klemp, National Center for Atmospheric Research

While the terrain-following sigma coordinate has been successfully implemented in modeling frameworks for years, small-scale horizontal and vertical accelerations are known to occur over areas of steep terrain, introducing error into the model equations. In an effort to remedy the introduction of these model-coordinate artifacts within the WRF-ARW model, a collaborative effort between NCAR/MMM, NOAA/ESRL/GSD, and the DTC was undertaken to introduce and evaluate a new hybrid vertical coordinate based on Klemp (2011, MWR). Results from a number of RAP and HRRR retrospective simulations will be presented indicating that the new hybrid vertical coordinate is sensitive to resolution as well as jet-level wind speeds. Verification of RAP wind speed, temperature and relative humidity RMSE/bias show a neutral to slightly negative impact during synoptically active events. However, a reduction in RMSE/bias was found for cases where upper-level winds were weaker. For the HRRR, a September retrospective simulation illustrated no discernable difference in these metrics between the hybrid and original vertical coordinate. Future work will also be discussed, including gravity wave drag methods to mitigate excessive wind speeds.