Squall line experiments: low-shear results

60 minutes

120 minutes

225 minutes

Simulation n325 - frontal forcing - vertical magnification=3; movie (mpeg, 1.5 MB) | movie (QuickTime, 6.8 MB)

A series of simulations were carried out in which a two-dimensional front (modeled previously) was used to initiate convection in a 3D cloud model. Given the linear form of the forcing, a squall line develops. Simulations were performed with:

• Frontal forcing, random perturbations (only) - most realistic
• Frontal forcing, random + 5 thermals - emphasize cell-cell interaction
• No frontal forcing, random + 5 thermals - isolate role of the front

The simulations shown here represent work in progress. These results were presented at the Mesoscale Processes conference (Reading, England, Sept. 9-13).

Our initial work concerned low-shear, greater instability environments. As such, the strongest convection occurred early in the simulation, and the squall lines began to decay late in the simulation since the wind shear was insufficient for the amount of instability (CAPE) in the sounding.

It was found that without the front present the initial splitting cells later interfered with one another in a destructive sense. The right- and left-moving split cells moved apart and towards their neighbors, and the resulting interaction left short-lived cells at the leading edge of the front. With the front in place, this interaction did not occur, and new cell growth continued along the leading edge of the squall line. The presence of the front led to a longer-lived squall line.

The development of the front is done with a 2D version of the cloud model; some of earlier idealized 2D simulations are discussed here. The evolution of the vertical velocity field (domain: 4000 km wide by 18 km high) is depicted here (mpeg, 225k) | (MooV, 1.9 MB).

All of the images below are 90 minutes into each simulation. The vertical scale is exaggerated by a factor of 8 to enhance the view of individual cells in the line. Click on any image to see an animation (mpeg).

Movies - 3D Squall Line Experiments

Forcing	Frontal		Isolated
Perturbation	Random	Random+Bubble
Simulation	n325	n324	n322
Cloud+Rain water	mpeg (2.0 MB)	mpeg (3.0 MB)	mpeg (2.8 MB)
Vertical Velocity contours + Cloud+Rain (transparent)		mpeg (1.6 MB)	mpeg (1.5 MB)
Meridional Vorticity surface + Cloud+Rain (transparent)		mpeg (1.3 MB)	mpeg (1.8 MB)
Meridional Vorticity (below 2.5 km)+ Cloud+rain (transparent)		mpeg (1.5 MB)	mpeg (1.2 MB)
Vertical Vorticity (below 2.5 km)		mpeg (1.7 MB)	mpeg (1.3 MB)

Images from these fields every 30 minutes may be found in the following pages:
Cloud | Theta | Cell | Meridional vorticity | Vertical vorticity

Further information regarding this work may be found (for now) in the conference preprint for the 1996 Severe Local Storms conference (San Francisco) and the corresponding preprint for the 1996 Mesoscale Processes conference (Reading).

Thanks go out to Crystal Shaw for her help visualizing these simulations, including use of the SGI Explorer package.