Evaluation of Two Momentum Control Variable Schemes and Their Impact on the Variational Assimilation of Radar Wind Data:Case Study of a Squall Line

Different choices of control variables in variational assimilation can bring about different influences on the analyzed atmospheric state. Based on the WRF model＇s three-dimensional variational assimilation system, this study compares the be- havior of two momentum control variable options-streamfunction velocity potential （ψ-χ） and horizontal wind components （U-V）-in radar wind data assimilation for a squall line case that occurred in Jiangsu Province on 24 August 2014. The wind increment from the single observation test shows that the ψ-χ control variable scheme produces negative increments in the neighborhood around the observation point because streamfunction and velocity potential preserve integrals of velocity. On the contrary, the U-V control variable scheme objectively reflects the information of the observation itself. Furthermore, radial velocity data from 17 Doppler radars in eastern China are assimilated. As compared to the impact of conventional observation, the assimilation of radar radial velocity based on the U-V control variable scheme significantly improves the mesoscale dynamic field in the initial condition. The enhanced low-level jet stream, water vapor convergence and low-level wind shear result in better squall line forecasting. However, the ψ-χ control variable scheme generates a discontinuous wind field and unrealistic convergence/divergence in the analyzed field, which lead to a degraded precipitation forecast.

THE EFFECT OF ISOLATED TOPOGRAPHY ON WEST FLOW

With the Fourier-expanding expression of isolated topography,the forcing of isolated topography is studied in this paper.In the initial condition of steady uniform west flow in β-plan channel model,the approximate solution of quasi-geostrophic potential vorticity equation is obtained.The results indicate that a series of steady topographic standing and transient waves can be produced by the forcing of isolated topography.The wave can move eastward or westward according to the wave scales.The most slowly Rossby wave is the strongest wave.So is the standing wave corresponding to the wave number.The main action of topography is to produce these strong waves.It is suggested that action of topography is one of the main causes to pro- duce low frequency oscillation and long-range steady weather.