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|Title:||Numerical simulation of a squall line along a cold front|
|Doctoral Committee Chair(s):||Wilhelmson, Robert B.|
|Department / Program:||Atmospheric Science|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Atmospheric Science|
|Abstract:||The 10-11 June 1985 squall line system with trailing stratiform precipitation observed during the Oklahoma PRE-STORM project has been numerically simulated using the three-dimensional cloud resolving nonhydrostatic COMMAS (COllaborative Model for Multiscale Atmospheric Simulation) model.
The mesoscale model simulation successfully predicts the location and timing for the initiation of the cold frontal squall line. With the modified Kuo cumulus parameterization (Geleyn, 1985), the model was also able to simulate the observed rear inflow jet although its intensity was weaker than observed and some of its other general characteristics. However, leading edge convective features and the surface pressure perturbation pattern across the squall line (mesolow ahead, mesohigh just behind, and weak low near the back of the stratiform region) were not simulated. This is related to the limitations in the modified Kuo parameterization and the horizontal resolution.
The COMMAS model, however, with higher resolution and better microphysical parameterizations including the presence of ice, does capture these and other common features of observed squall lines. Other features include a stratiform precipitation region, characteristic relative front-to-rear inflow at low levels over the gust front, both rear and front flow in the upper part of the convection, banded vorticity structure, cyclonic circulation around a midlevel mesoscale convective vortex at the northern end of the line, anticyclonic circulation at the southern end of the line, and supercellular structure at the northern end of the line where a tornado was observed.
Vertical vorticity analysis reveals more detailed structure than in observational studies (e.g., Biggerstaff and Houze, 1991a,b). The vertical vorticity fields are characterized by positive bands in the convective and stratiform regions with a negative band in between up to about 6 km in height. In the upper troposphere, the vertical vorticity field is characterized by a positive vorticity band in the convective region and generally negative vorticity throughout the northern portion of the stratiform region. Convective scale positive-negative vertical volticity bands are embedded within the mesoscale vorticity band structure. In the lower troposphere, the stretching of existing vertical vorticity is the most important factor for generating and maintaining the vertical vorticity over the convective region. The tilting and solenoidal terms are the two main factors responsible for the generation and maintenance of the positive vorticity within the stratiform region and the negative vertical vorticity in between the convective and stratiform region. The baroclinic (solenoidal) term has the same magnitude as those of tilting and stretching terms. This term has not been estimated in previous observational studies.
A strong cyclonic circulation or mesoscale convective vortex (MCV) is located about 120 km behind the north end of the surface gust front. The positive vorticity area is limited approximately to a 50 km by 70 km region and the maximum magnitude of the vertical vorticity ranges between 10f and 20f. Budget analysis shows that the positive vorticity within the MCV region is produced mainly by the tilting and stretching terms. (Abstract shortened by UMI.)
|Rights Information:||Copyright 1996 Zhou, Guangming|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9702726|
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