Momentum Transport in Mesoscale Convective Systems

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Title: Momentum Transport in Mesoscale Convective Systems
Author: Mahoney, Kelly Marie
Advisors: Anantha Aiyyer, Committee Member
Sandra Yuter, Committee Member
Gary Lackmann, Committee Chair
Matthew D. Parker, Committee Member
Abstract: The transport of horizontal momentum by vertical motions within a mesoscale convective system (MCS) affects storm dynamics, sensible weather, and the connection between the system and its surrounding environment. Earlier works have examined this process for a number of purposes, but understanding of its significance to both MCS motion and the generation of convectively-driven surface winds remains incomplete. This study describes the convective momentum transport (CMT) process both qualitatively and quantitatively; this is pursued through the analysis of quasi-idealized numerical simulations. Momentum budgets illustrate that the motion of a numerically-simulated MCS is significantly impacted by CMT within the MCS. Vertical advection of the perturbation wind is found to contribute largely to the momentum field at the leading edge of the cold pool, which is the region in which the resulting accelerated winds drive system motion. Results also show that the pressure gradient acceleration and, to a lesser degree the vertical advection of the background environmental wind, contribute to the acceleration of rear-to-front-directed momentum in the middle- to rearward portions of the storm, thereby generating and reinforcing transport of the perturbation flow into the cold pool and accelerating the MCS. The second part of this dissertation uses a series of experimental simulations to examine the sensitivity of CMT, MCS motion, and surface wind speed generation to environmental humidity and microphysical processes. Results reveal modest changes in MCS motion, but marked differences in the generation of convectively-driven surface winds. Drier air at mid-levels increases descent within the trailing stratiform region and enhances CMT; this slightly increases average MCS speed by ~1 ms-1, but produces a much larger number of severe surface winds. CMT is also shown to be a contributing factor to the occurrence of severe surface winds produced via the favorable superpositioning of a descending rear inflow jet and the low-level circulation associated with gust front mesovortices. The potential for a descending rear inflow jet to cause strong surface winds at locations away from the leading edge of the gust front is discussed as well. While such surface wind patterns may occur in a variety of storm environments, it is shown that the additional downward motion imparted by decreasing the relative humidity of the mid-levels leads to additional acceleration through CMT and contributes to an increase in occurrence of strong to severe surface winds. Reducing evaporation yields the most marked decrease in both MCS motion and strength of surface wind speeds, followed by the removal of melting and sublimation, respectively. The challenge of completely isolating the contribution of “cold pool dynamics†(i.e., density current propagation) from “CMT-forced†MCS motion is also discussed. Avenues for future work are outlined, with a focus on adding or improving the representation of CMT in existing cumulus parameterization schemes, incorporating CMT into conceptual models of both MCS motion and severe surface wind generation, and further exploring of the sensitivity of CMT to a greater variety of storm environments and kinematic profiles.
Date: 2009-08-27
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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