Comparison of WRF 4D-Var with 3D-Var for radar data assimilation

A 4D-Var (4-dimensional variational) radar data assimilation (DA) system was recently developed for the Advanced Research WRF model. The 4D-Var system is compared with its corresponding 3D-Var (3-dimensional variational) system using a Great Plains squall line observed during the International H2O Project. Two 3D-Var schemes are used in the comparison: a standard 3D-Var radar DA that is the same as the 4D-Var except for the exclusion of the constraining dynamical model and an enhanced 3D-Var that includes a scheme to assimilate an estimated in-cloud humidity field. The comparison is made by verifying their skills in 0-6 hour quantitative precipitation forecast (QPF) against Stage IV analysis, as well as in wind forecasts against radial velocity observations. The results indicate that 4D-Var substantially improves the QPF skill over the standard 3D-Var for the entire 6-hour forecast range and over the enhanced 3D-Var for most forecast hours. The analyses and forecasts from the 4D-Var and 3D-Var schemes are further compared by examining the meridional wind, horizontal convergence, low-level cold pool, and mid-level temperature perturbation, using analyses from VDRAS (Variational Doppler Radar Analysis System) as references. The diagnoses of these fields suggest that the 4D-Var analyzes the low-level cold pool, its leading edge convergence, and mid-level latent heating in closer resemblance to the VDRAS analyses than the 3D-Var schemes.