12.2
The Local wind of an MCS and a vorticity budget for the mesoscale convective vortex within it
Jason C. Knievel, Colorado State Univ., Fort Collins, CO
Mesoscale convective vortices (MCVs) sometimes act as dynamical links between moist convection on the scales of mesoscale convective systems (MCSs) and of individual convective clouds. A link acts down-scale when a dissipating MCS leaves in its wake an MCV that initiates a fresh outbreak of cumulonimbi. A few times each year, such cumulonimbi lead to a subsequent MCS. Whether or not an MCS produces an MCV, and whether or not an MCV initiates a fresh outbreak of cumulonimbi, depends partially on the local vertical wind shear.
Shear is both conducive and destructive to MCVs and subsequent moist convection. It is conducive to MCVs in that tilting can produce vertical vorticity from vertically sheared winds' horizontal vorticity. Shear is conducive to subsequent moist convection in that it produces differential isentropic flow that may lift air to the level of free convection beneath an MCV. Shear is destructive in that it can disrupt the coherence of the atmosphere's balanced vortical response to an MCS's latent heating, which shortens the life of an existing MCV or prevents one from forming at all. Even if an MCV does form, one that is short-lived is less likely to produce much isentropic ascent. Understanding the role of shear in the narrow range that favors MCVs and subsequent cumulonimbi, and better defining that range, may lead to improved forecasting of some of the MCSs that deliver flooding deluges and damaging winds.
The author will examine the vertical wind shear in and around an MCS and its MCV in the Great Plains on 1-2 August 1996. Data from the NOAA Profiler Network indicate that the shear varied greatly in space and time in the immediate vicinity of the MCS. This suggests that both the production of vertical vorticity through tilting and the local environment's conduciveness to MCVs also varied greatly in space and time. While the MCS dissipated into spiral bands of weakly raining clouds over eastern Oklahoma, new cumulonimbi formed beneath the vortex. One cumulonimbus produced a tornado. The author will examine the role of vertical wind shear in the initiation of those cumulonimbi and compare it with the role played by the surface boundaries and convergence lines detected by the Oklahoma Mesonet.
Session 12, Mesoscale Convective Vortices/Mesoscale Convective Systems
Thursday, 14 September 2000, 3:30 PM-5:00 PM
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