P4.4 The presence of thermal wind balance in MCS environments

Tuesday, 28 October 2008
Madison Ballroom (Hilton DeSoto)
Benjamin C. Baranowski, North Carolina State University, Raleigh, NC; and M. D. Parker

Mid-latitude mesoscale convective systems (MCS) are commonly found in the vicinity of strong surface temperature gradients. Owing to the linear lifting associated with frontal boundaries, one would expect MCSs to be oriented nearly parallel to such gradients. Also, wind shear, oriented parallel to the convective system, would result from thermal wind balance. Only recently have such line parallel shear or such environmental heterogeneties been incorporated into idealized studies of MCSs. This study classifies the orientation of linear convective

systems relative to surface fronts and determines the degree to which environmental wind profiles ahead of the convection are in thermal wind balance.

Composite radar images are used to identify MCSs in the central United States from 1 April to 15 May in 2006 and 2007. 92 systems meet our MCS criteria. Each system had their general characteristics, such as duration and orientation recorded. Pre-line wind profiles for each

case are retrieved every hour from operational mesoscale model output. Also, the characteristics of the initiation mechanism as well as the shallow and deep layer shear directions are recorded for each case.

Preliminary results show that MCSs generally acquire the orientation of the boundary that initiates them, and usually maintain this orientation as they move away from the boundary. Mean wind profiles show that ageostrophic shear is dominant in the boundary layer, owing to frictional effects. Ageostrophic shear can also be important in the upper troposphere in association with jet streaks. The middle troposphere is mainly dominated by geostrophic shear. Composite diagrams depict environments conducive for the development of convection in the vicinity of the initiation mechanism, with geostrophic shear generally decreasing downstream of such features. As the MCS moves away from the

boundary, the reduced lifting from the boundary as well as the decrease in geostrophic shear will have implications for storm evolution and maintenance.

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