Forecast improvement from radar data assimilation within the Rapid Update Cycle, Rapid Refresh, and High Resolution Rapid Refresh forecasts initialized with RUC/RR grids

In November 2008, a diabatic digital filter initialization (DFI)-based procedure for assimilating radar reflectivity data was implemented in the NCEP operational hourly updating Rapid Update Cycle (RUC). This marked the first operational use of WSR-88D mosaic reflectivity data in an NCEP model. This radar reflectivity assimilation procedure has also been ported to the experimental real-time Rapid Refresh (RR) system, which is in final testing for a planned implementation at NCEP in 2010. Fields from the RUC (with the radar assimilation procedure) are also being used to initialize the real-time High Resolution Rapid Refresh (HRRR, Weygandt et al, this conference) resulting in significantly improved forecasts for convective systems. More recent work with Rapid Refresh system has focused on evaluating the impact of hourly assimilation of radial velocity data and consideration of adjustments to the assimilation procedure to maximize the forecast improvement from the radial velocity data.

In this presentation we will describe the radar reflectivity assimilation algorithm as it is implemented for both the RUC and the Rapid Refresh, including minor modifications to improve the retention of cloud and hydrometeor data and enhancements to make use of lightning data as proxy reflectivity data. We will then illustrate the forecast improvement from the radar assimilation for both the RUC and Rapid Refresh with composite skill score measures and case study examples. Next we will document the related improvement in HRRR convective forecasts from use of the RUC / Rapid Refresh fields with the radar assimilation. We will provide details on a minor, but important, modification to the HRRR initialization procedure that provides the most direct transfer of radar information from the RUC / RR grid to the HRRR. We will also illustrate in detail how this radar information evolves from the RUC / RR scale (13-km) to the HRRR scale (3-km) during the first few hours of the HRRR forecast. The final portion of the talk will provide an update on ongoing work to further improve the RR and HRRR forecasts by augmenting the reflectivity assimilation with radial velocity assimilation.