Assimilation of NEXRAD radial velocity data to enhance mesoscale modeling of dispersion near New Orleans

Trajectory and dispersion calculations using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) have been computed from high resolution meteorological model output with WSR-88D radar data assimilation. Four experiments were conducted for two different periods: January 2009 and June 2009. The Rapid Update Cycle (RUC-20) is used as a control experiment. Three Weather Research Forecast (WRF) experiments employ different model domain sizes and assimilate NEXRAD radial velocity data. The hourly analyses of the RUC were used for WRF initialization. Two domains were set in the WRF simulations: a regional domain of 4 km resolution and 381X301 grid points that covers the northern Gulf of Mexico and the south central U.S, and a nested domain with 1 km resolution and349X349 grid points centered over the location of the National Weather Service NEXRAD station in Slidell, LA (KLIX). The three-dimensional variational data assimilation system of WRF is implemented after the radial velocity data are preprocessed by an automatic technique. The preprocessing algorithm first de-aliases the radial velocities, then applies some statistical information to remove ground clutter and other noises. Differences of the trajectories and dispersions among the different experiments are examined. A total of 960 12-h simulations starting at 00, 06, 12, and 18 UTC were performed to analyze statistical characteristics. Significant differences in trajectory end-points using different model and model grid resolutions were noted, especially from within the atmospheric boundary layer (ABL). The largest discrepancies among the Lagrangian trajectories occurred during the afternoon hours when ABL turbulence was most active. The assimilation of NEXRAD radial velocity alters the low level wind field resulting speed and direction wise trajectory difference.