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Evolution of potential vorticity associated with mesoscale convective systems




Hertenstein, Rolf F. A., author

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In recent years, there has been much interest in cyclonic and anticyclonic circulations which form in association with Mesoscale Convective Systems (MCSs). Using data from the simulations of two MCSs, we investigate the evolution of potential vorticity (PV) associated with these systems. It is found that a positive and negative PV couplet forms at mid levels early during the MCS lifecycle. As the system matures, the couplet is replaced by a dominant positive PV anomaly. At upper levels, a negative PV anomaly dominates and its size depends on the mode of convection. It is found that the MCS with tilted and organized internal flow branches has less of an effect at upper levels than does the MCS with more randomly-organized upright convection. The analysis performed considers Ertel's PV equation including diabatic and frictional effects. The flux form of this equation is also considered. A budget study of the various terms in the PV equation has been completed over the lifetime of the simulated MCSs. We find that caution must be exercised when applying traditional large-scale approximations of PV to mesoscale systems. For instance, horizontal vorticity contributions to both PV and to the diabatic heating may not be negligible and may even locally dominate. A main result is that the vertical part of the diabatic heating term is a major contribution to the mid-level positive anomaly as anticipated. The negative anomaly forms due to both horizontal and vertical parts of the diabatic heating term. We also investigate the possible role of turbulent eddies in the dissipation of the negative mid-level PV anomaly. We compare our results to those obtained by nonlinear balance simulations as well as primitive equation simulations of idealized MCSs. In our analysis and comparisons with previous studies, we focus on blending PV and vorticity concepts where possible. Based on the results, a new conceptual model of PV evolution in MCSs is presented.


March 21, 1996.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1996.

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Convection (Meteorology)


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