Recent Rapid Refresh Enhancements to Improve Forecast Guidance for Aviation Weather Hazards and Improve Initial Fields for High Resolution Rapid Refresh Forecasts

The initial version of the Rapid Refresh (RAP) mesoscale analysis and prediction system was implemented at the National Centers for Environmental Prediction in May of 2012. Numerous enhancements to the Rapid Refresh analysis and model system have been made since the RAP code was frozen for the initial NCEP implementation. Analysis changes have included 1) adaptive application of surface moisture increments throughout the depth of the model boundary layer, 2) updating of soil temperature and moisture fields based upon the lowest model level analysis increments, 3) limits on the innovation magnitudes and more accurate accounting for model vs. observed terrain elevation in precipitable water assimilation, and 4) conservation of virtual potential temperature when moistening/saturating regions of cloud building/removal. WRF-ARW model enhancements have included an upgrade from version 3.2 to 3.3.1 changes to some of the options for the numerics. These changes comprise RAP version 2 slated for operational implementation at NCEP in 2013. The combined effect of these changes has been a substantial improvement in the RAP analyses and predictions, especially for the convective storm environment. Associated with this has been a sharp improvement in the skill of HRRR model forecasts. The HRRR runs as a nest within the RAP and provides detailed guidance on thunderstorms and other small-scale weather phenomena.

At the conference, I will first describe these RAP improvements, then show quantitative assessments of retrospective GSD experimental RAP forecasts compared with the operational RAP run at NCEP, followed by qualitative case study comparisons. These improvements in RAP analyses and short-range forecasts include much better depiction of the convective storm environment, including improvement in the sometimes too dry RAP boundary layer structure and associated CAPE analysis. These changes are especially important for the Storm Prediction Center, where forecasters closely scrutinize convective storm environment fields (T, Td, CAPE, etc.) on an hourly basis. Finally, I will show retrospective results and case study examples of how RAP enhancements have led to improvements in the nested HRRR runs. These RAP-related HRRR improvements have included in significant reduction in the over prediction of storms and improved storm structure, especially the ability to predict bow echoes.