Marek Kašpar

Analyses of gust fronts by means of limited area NWP model outputs

Two convective cases accompanied with propagating gust fronts and recorded over the Czech Republic (CR) in the year 2000 are numerically studied. The experimental integration of the Lokal Modell of the German Weather Service (LM DWD) is used in computations with the horizontal resolution 2.8 km. The prognostic fields of basic thermodynamic quantities as well as the fields of vertical velocities and precipitation rates are presented at various levels/cross-sections and at different times in order to clarify the effect of the cold evaporative outflow on the organization of severe convection. The position and movement of an outflow leading edge (a gust front) are determined with the aid of objective techniques based on a simple model of objective frontal analysis. The techniques include both mathematical and graphical tools. They allow for the automatic identification of gust fronts in atmospheric cross-sections by means of one locating equation and two masking criteria. The equation and criteria employ the moist potential temperature as an input thermal parameter. The appropriate regimes of outflow propagation, which may provide information about outflow dynamics, are assigned subjectively to examined gust fronts according to the properties of ambient flow. As compared with radar, the LM DWD runs prove to forecast precipitation in nearly the right subregions. The results of the numerical study confirm that the LM DWD, together with the objective techniques for gust front analyses, are capable of simulating thermodynamic processes associated with convective outflows. Thereby, they can represent the potential for improving the nowcasting and short-range forecasting of hazardous weather phenomena.