Radar data assimilation with The Rapid Update Cycle

This presentation will summarize ongoing radar data assimilation efforts with the Rapid Update Cycle (RUC). The RUC utilizes a 1-h update cycle to advance a detailed three-dimensional mesoscale analysis used to initialise a short range forecast. The NCEP operational RUC is slated for a significant upgrade in early Summer 2005, including a horizontal resolution enhancement from 20-km to 13-km and an expansion of the assimilated observation types. Presently, the operational RUC includes assimilation of most conventional observation types, but does not make use of radar data. Experimental versions of the RUC, running at FSL, make use of radar reflectivity mosaic data, but not radial velocity data.

Experiments using a 13-km version of the RUC are being conducted at FSL to examine the impact of WSR-88D radial velocity data on RUC analyses and forecasts. Preliminary work has focused on direct inclusion in the RUC 3DVAR of radial velocity components from low-level scans. Simple strategies have been employed to provide velocity unfolding and other quality control, as well as to provide thinning of the voluminous data set. Two test cases from May 2005 are being used for the experiments, which utilize multiple scans of data from five adjacent radars across the Colorado Front Range and adjacent high plains. In both cases, a cluster of individual storms evolves into a mesoscale convective system. In contrast to storm-resolving model simulations, the interaction of the assimilated radial velocity data with the cumulus parameterization scheme is extremely important for these 13-km experiments. Within the RUC cumulus parameterization, an ensemble of closures and control parameters is used. As part of this research, we will be examining the response variations of the different cumulus parameterization closures to the radial velocity data.

Followup experiments will focus on the use of a separate single-Doppler velocity retrieval to obtain Cartesian wind vectors that can be assimilated within the RUC 3DVAR. Comparisons with the radial velocity assimilation experiments will reveal the degree to which the retrieved cross-beam velocity information (with its associated errors) benefits the forecast. Finally, companion work is planned to assimilate NSSL radar reflectivity data within the RUC cloud analysis. Additional work in this area will examine techniques for using the reflectivity data to modulate the weights of the various cumulus parameterization ensemble closures.