Extended Abstract (92K)2.6
The degree of balance in a midlatitude, continental mesoscale convective vortex
Jason C. Knievel, NCAR, Boulder, CO; and D. S. Nolan, J. P. Kossin, and R. H. Johnson
Mesoscale convective vortices (MCVs) are generally considered to be balanced, or nearly balanced, circulations. However, few published empirical studies address the question of balance, and none of these includes an explicit evaluation of the simplest forms of balance that can widely apply to MCVs: hydrostatic and gradient balance. Whether all MCVs are balanced or whether the degree of balance within them varies considerably from one vortex to the next has implications for the simulation and forecasting of severe weather, especially excessive rainfall and violent wind.
In this paper the authors present an evaluation of the hydrostatic and gradient balance in an MCV generated by a mesoscale convective system (MCS) in the central Plains of the United States on 1 August 1996. For the evaluation, the authors first azimuthally averaged the observed tangential wind around the vortex, then diagnosed from this average wind a core temperature profile in balance with it. Finally, the diagnosed profile was compared with the observed profile in the core of the MCV.
The comparison reveals that the MCV of 1 August 1996 was not balanced, even late in its lifecycle. However, the extent of the imbalance was not extreme. The diagnosed, balanced temperature profile and the observed profile display the same gross structure: a cool core surmounted by a warm core. The differences between the two profiles are in the depths and magnitudes of the temperature perturbations. Specifically, the observed cool layer in the MCV was too strong and too shallow to balance the vortex's tangential wind. Stated another way, the observed tangential wind was too weak and its highest wind speeds were at too great an altitude to balance the distribution of mass in the MCV's core.
The authors go on to speculate about some of the possible reasons for the apparent imbalance. Among these are imperfections in the methods of analysis; insufficiencies in the relatively scant thermodynamical data available for the vortex; and the fact that heating and cooling from cumulonimbi were continuing to force the vortex during the period of analysis, perhaps not allowing the MCV to become balanced, even if that was its tendency.
Session 2, Mesoscale Convective Systems II
Monday, 12 August 2002, 1:30 PM-3:00 PM
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