1. Numerial Simulations of Convective Events – The Effect of Propagating Gust Fronts Kaspar and Müller (kaspar@ufa.cas.cz) Institute of Atmospheric Physics ASCR, Prague, CR

2. - 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

3. 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

4. Propagating regime Upshear movement Downshear movement Steering-level (h s ) regime Downshear movement with an overturning updraft Vertical shear conditions Relative flow 1 2 3 1 2 3 Convection initiation (a)PI and a low LCL all regimes (b)PS and / or a high LCL the steering-level and downshear prop. regime

5. OA gust fronts + surface precipitation rates [mm/h]OA gust fronts + vertical velocities [m/s]OA gust fronts + potential temperature [K] Case study 2.7.2000 - validation tests Radar Skalky Zmax 13UTC14UTC15UTC

6. Case study 2.7.2000 Height of OA gust heads + RSM [dBZ] (Haase and Crewell, 2000) Downshear propagating regime c 0 =7.1m/s H =1180m PI + decreasing LCL

7. Case study 22/23.7.1998 - squall line (Salek, 2000) 01UTC02UTC03UTC Skalky Zmax OA gust fronts + surface precipitation rates [mm/h]

8. Case study 22/23.7.1998 OA gust heads + RSM [dBZ] Steering-level regime c 0 =5.1m/s H=1343m Potential stability + increasing LCL

9. 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)

10. 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 B3042404, GACR 205/04/0114 DWD (LM and RSM codes), CHMI (radar pictures) Acknowledgement: GA ASCR B3042404, GACR 205/04/0114 DWD (LM and RSM codes), CHMI (radar pictures)

11. thank you for your attention

12. References: Crook, N.A., Carbone, R.E., Moncrieff, M.W., Conway, J.W., 1990. The generation and propagation of a nocturnal squall line. Part II: Numerical simulations. Mon. Weather Rev. 118, 50-65. Doms, G., Schättler, U., 1999. The Nonhydrostatic Limited-Area Model LM of DWD. Part I: Scientific Documentation, DWD, Offenbach, Germany, 172 pp., available at http://www.cosmo-model.org.http://www.cosmo-model.org Droegemeier, K.K., Wilhelmson, R.B., 1987. Numerical simulation of thunderstorm outflow dynamics. Part I: Outflow sensitivity experiments and turbulence dynamics. J. Atmos. Sci. 44, 1180-1210. Haase, G., Crewell, S., 2000. Simulation of radar reflectivities using a mesoscale weather forecast model. Water Resources Research 36, 2221-2231. Hewson T.D., 1998. Objective fronts. Meteorol. Appl. 5, 37-65. Kaspar, M., 2003. Analyses of gust fronts by means of limited area NWP model outputs. Atmos. Res. 67-68, 333-351. Liu, C., Moncrieff, M.W., 1996a. A numerical study of effects of ambient flow and shear on density currents. Mon. Weather Rev. 124, 2282-2303. Liu C., Moncrieff, M.W., 1996b. An analytical study of density currents in sheared, stratified fluids including the effects of latent heating. J. Atmos. Sci. 53, 3303-3312. Moncrieff, M.W., Liu, C., 1999. Convection initiation by density currents: role of convergence, shear, and dynamical organization. Mon. Weather Rev. 127, 2455-2464. Rotunno, R., Klemp, J.B., Weisman, M.L., 1988. A theory for strong, long-lived squall lines. J. Atmos. Sci. 45, 463-485. Salek, M., 2000. Torrential rainfalls in the foothills of the Orlicke hory Mts. on the 22 and 23 July 1998 from the viewpoint of remote sensing and numerical model results (in Czech with English summary). Meteor. Bul. 53, 4-15. Simpson, J.E., 1972. Effects of the lower boundary on the head of a gravity current. J. Fluid Mechs. 53, 759-768. Thorpe, A. J., Miller, M.J., Moncrieff, M.W., 1980. Dynamical models of two-dimensional downdraughts. Q. J. R. Meteorol. Soc. 106, 463-484. Wilson, J. W., Schreiber, W.E., 1986. Initiation of convective storms at radar-observed boundary-layer convergence lines. Mon. Wea. Rev. 114, 2516-2536.