The High-Resolution Rapid Refresh: Recent Model and Data Assimilation Development Towards an Operational Implementation in 2014

The 13-km Rapid Refresh (RAP) and 3-km convective-allowing High-Resolution Rapid Refresh (HRRR) are hourly updating weather forecast models that use a specially configured version of the Advanced Research WRF (ARW) model and assimilate many novel and most conventional observation types on an hourly basis using Gridpoint Statistical Interpolation (GSI). Included in this assimilation is a procedure for initializing ongoing precipitation systems from observed radar reflectivity data, a cloud analysis to initialize stable layer clouds from METAR and satellite observations, and special techniques to enhance retention of surface observation information.

The HRRR is run hourly out to fifteen forecast hours over a domain covering the entire conterminous United States using initial and boundary conditions from the hourly-cycled RAP and is available in real-time to operational forecasters in both the private and public sectors. The HRRR is currently (August 2013) scheduled for operational implementation at NCEP in 2014.

Recent development of the 2013 HRRR has focused on (1) introduction of 3-km HRRR data assimilation for analysis of storm-scale information and (2) enhancement of HRRR model physics for improved land surface and boundary layer prediction using the updated Mellor-Yamada-Nakanishi-Niino (MYNN) parameterization scheme and higher resolution Rapid Update Cycle (RUC) land-surface model.

In this presentation, we will focus on changes to the real-time HRRR configuration including the establishment of 3-km data assimilation to incorporate storm-scale information using Gridpoint Statistical Interpolation (GSI) that includes sub-hourly 3-km radar data assimilation during a pre-forecast hour, 3-D variational assimilation of conventional observations and a 3-km non-variational cloud and precipitating hydrometeor analysis using radar reflectivity observations to retrieve rain and snow mixing ratios. Observed radar reflectivities are used as a proxy for HRRR model latent heating specification that replaces the model microphysics latent heating during four 15-min periods of a cycled pre-forecast hour with an emphasis on forcing observed convective structures from higher reflectivity regions. Special attention is given to continuity of convective-scale structures, originating from an accurate storm-scale analysis (initial condition), during much of the free forecast period (several hours).

We will also highlight improvements in the HRRR model physics including the MYNN PBL scheme, RUC LSM and Thompson microphysics along with more accurate numerical options in the dynamic core that translate into improved prediction at the surface, boundary layer and free atmosphere. HRRR analysis and forecast improvements, particularly in the first few forecast hours, will be documented with retrospective and real-time verification statistics and case studies from 2012 and 2013.