Mid-level vorticity in mesoscale convective systems
Date
1996
Authors
King, Ronnie G., author
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Abstract
The objective of this study has been to examine and document the development of mid-level Mesoscale Convective Vortices (MCVs) within Mesoscale Convective Systems (MCSs) and Mesoscale Convective Complexes (MCCs) using the Central Plains Wind Profiler Demonstration Network (WPDN). Nine MCSs from the summer of 1993 were picked for this study based on their formation and lifetime spent over the WPDN. Bartels and Maddox's (1991) climatological study of MCVs for 1981 - 1988 estimated that less than 5% of MCSs exhibit a vortex whose clouds persist long enough after the dissipation of the MCSs' high-level obscuring cirrus cloud to become apparent in visible satellite imagery. This low estimate of MCVs in MCSs leads to the question of how many MCSs produce MCVs. Some researchers state that the MCV is an inherent part of the MCC circulation (Velasco and Fritsch, 1987; Menard and Fritsch, 1989). The nine MCSs studied here do support the theory that the MCV forms in most, if not all, large MCSs. Each of the nine cases developed to varying degrees a maximum of relative vorticity of 10-4s-1 or greater in the mid-tropospheric levels near the freezing level. This suggests the importance of latent heat processes enhancing the inflow and horizontal convergence which in tum produces vorticity (Johnson et al., 1995) suggesting that the latent heat release, melting and evaporative cooling in the stratiform region are the primary contributors to the circulation spinup. This finding is consistent with modeling results (Zhang and Fritsch, 1988) as well as theoretical studies of Hertenstein and Schubert (1991). This finding is also somewhat supported by the results of comparing the average time rate of change of relative vorticity during the period of maximum increase in the mid-levels to the average stretching ( convergence production) term that gives a ratio of 1.16 or 116%. This suggests the stretching term is playing a significant role in the production of vorticity but since the ratio is greater than one the change in vorticity is greater than the stretching term; therefore the tilting term must be contributing also. This result is consistent with other observational studies (Bartels and Maddox, 1991; Johnson and Bartels, 1992) that have found the mid-level convergence production (stretching term) to be the primary producer of mid-level vorticity.
Description
Summer 1996.
Also issued as author's thesis (M.S.) -- Colorado State University, 1996.
Also issued as author's thesis (M.S.) -- Colorado State University, 1996.
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Subject
Convection (Meteorology)
Atmospheric circulation