Institute of Atmospheric Physics (IAP) AS CR - Czech Republic
Propagating gust fronts, which occur at the boundary of the cold evaporative outflows of downdraft air from convective storms, can generate new convection and promote long-lasting and organized convective systems. The gust front is an example of the geophysical phenomenon called a density current (DC), which may be defined as a distinctive small-scale frontlike disturbance moving out from a localized source of dense air usually along a horizontal bottom and displacing ambient fluid of lesser density. The morphology and propagation of DCs have been studied extensively through analytical, laboratory and numerical modelling approaches. On the basis of these experiments, several 2-dim. analytical models (sometimes called as propagating regimes) of DCs have been developed. The models describe the behaviour of DCs commensurate with ambient flow and shear. Thus, the models together with the results of analytical studies including the effects of base-state stratification, latent heating and surface drag may provide considerable information also about outflow dynamics.
The presented analyses are an attempt to examine the role of gust fronts in the development and organization of severe convection by means of local NWP model. For occasion, the research version of the non-hydrostatic local NWP model LM DWD is employed. The diagnostic run of the model consists of integration of driving and nested LM model (for details, see the abstract contribution by Řezáčová and Sokol: "Diagnostic studies of severe convective precipitation events by local non-hydrostatic NWP model - a summary of results related to the Czech territory").
Selected convective cases accompanied with well-defined gust fronts and recorded over the Czech Republic (CR) in last few years are numerically studied. For example, the squall line from 1998 associated with torrential rainfalls, which caused flash flood in the north-east of the CR, was characterized by the high degree of the orderliness of convection. At early stages of convective evolution horizontal convergence was dominant at lower and middle levels. Later, probably due to outflows, horizontal divergence prevailed along the ground. However, convection did not weaken and new cells grew. The heavy thunderstorms then lasted 10 to 12 hours. Several situations from 2001 were accompanied with distinct gust fronts, which were well apparent on radar pictures and had no effect in triggering new convection. The integration including these events are in process at present. The prognostic fields of basic thermodynamic quantities as well as the fields of vertical velocities, cloud water content and precipitation are presented at various levels/cross-sections and at different times in order to clarify the effect of outflows in organizing severe convection. The position and movement of gust fronts are determined with the aid of objective techniques based on a simple objective frontal analysis model. If it is possible, the appropriate propagating regimes are subjectively assigned to examined gust fronts according to the properties of ambient flow.
The aim of presented analyses is to assess the capability of the local model to simulate thermodynamic processes associated with outflows from severe or organized convection and gust front development over the Czech territory. The results are obtained in numerical modelling block of national research project "Severe convective events in the region of the CR", which has been running since 2000 under the support of the Czech Grant Agency.