Numerial Simulations of Convective Events – The Effect of Propagating Gust Fronts Kaspar and Müller Institute of Atmospheric Physics ASCR, Prague, CR

- collision with other gust fronts and convergence lines (Wilson and Schreiber, 1986) Interaction with environmental air – convection initiation - interaction with ambient vertical shear (Thorpe et al., 1980; Rotunno, 1988; Moncrieff and Liu, 1999) - interaction with mesoscale oscillations (Crook et al., 1990) Gust front = an advancing surface boundary of the outflow of thunderstorm downdrafts cooled by evaporation. Nowcasting, numerical and analytical studies. (Droegemeier and Wilhelmson, 1987; Liu and Moncrieff, 1996b) H H – head height Simpson (1972) gust front Motivation

Methodology (Kaspar, 2003) LM COSMO thermodynamic data, 2.8km (Doms and Schättler, 1999) Model for the Objective Analysis of Gust Fronts (OAGF) LOCATION 3-dim. position (thermal def.) head height MORPHOLOGY MOVEMENT speed vector VERTICAL SHEAR regime of propagation (Liu and Moncrieff, 1996a) STABILITY potential instability, LCL

Propagating regime Upshear movement Downshear movement Steering-level (h s ) regime Downshear movement with an overturning updraft Vertical shear conditions Relative flow Convection initiation (a)PI and a low LCL all regimes (b)PS and / or a high LCL the steering-level and downshear prop. regime

OA gust fronts + surface precipitation rates [mm/h]OA gust fronts + vertical velocities [m/s]OA gust fronts + potential temperature [K] Case study validation tests Radar Skalky Zmax 13UTC14UTC15UTC

Case study Height of OA gust heads + RSM [dBZ] (Haase and Crewell, 2000) Downshear propagating regime c 0 =7.1m/s H =1180m PI + decreasing LCL

Case study 22/ squall line (Salek, 2000) 01UTC02UTC03UTC Skalky Zmax OA gust fronts + surface precipitation rates [mm/h]

Case study 22/ OA gust heads + RSM [dBZ] Steering-level regime c 0 =5.1m/s H=1343m Potential stability + increasing LCL

Conclusions The validation tests confirmed the applicability of the LM COSMO-OAGF chain in the case of both isolated and multicellular convection. The propagating gust fronts had the potential for convection initiation in both presented case studies. -2 July 2000: favourable vertical shear, humidity and stability conditions -22 / 23 July 1998: favourable vertical shear conditions -the both case studies are included in a paper accepted for Atmospheric Research (2006)

The tuning and verification of the OAGF will continue. -radar data assimilation -locating procedures based on the analysis of wind field … The products of the OAGF are assumed to be used in formulating decision criteria. -total index quantifying the potential of a gust front to trigger new convection Outlooks Acknowledgement: GA ASCR B , GACR 205/04/0114 DWD (LM and RSM codes), CHMI (radar pictures) Acknowledgement: GA ASCR B , GACR 205/04/0114 DWD (LM and RSM codes), CHMI (radar pictures)

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