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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. H – head height. Simpson (1972). H. gust front. Motivation. - PowerPoint PPT Presentation
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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
- 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 heightSimpson (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 heightMORPHOLOGY
MOVEMENT speed vector
VERTICAL SHEAR regime of propagation (Liu and Moncrieff, 1996a)
STABILITY potential instability, LCL
Propagating regimeUpshear movementDownshear movement
Steering-level (hs) regimeDownshear movement with an overturning updraft
Vertical shear conditions
Relative flow1
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
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 SkalkyZmax
13UTC 14UTC 15UTC
Case study 2.7.2000Height of OA gust heads + RSM [dBZ] (Haase and Crewell, 2000)
Downshear propagating regime
c0=7.1m/sH =1180m
PI + decreasing LCL
Case study 22/23.7.1998- squall line (Salek, 2000)
01UTC 02UTC 03UTC
SkalkyZmax
OA gust fronts + surface precipitation rates [mm/h]
Case study 22/23.7.1998OA gust heads + RSM [dBZ]
Steering-level regime
c0=5.1m/sH=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 B3042404, GACR 205/04/0114DWD (LM and RSM codes), CHMI (radar pictures)
thank you for your attention
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.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.
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