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

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