Extended Abstract (324K)P6.4
Numerical simulation of storm boundary anchoring in a high-CAPE, low-shear environment: Implications for the modulation of convective mode
Adam L. Houston, University of Illinois, Urbana, IL; and R. B. Wilhelmson
Observational analysis of the 27 May 1997 Jarrell, Texas tornadic event has revealed that despite the presence of weak-shear and high-CAPE, attributes traditionally associated with air-mass convection, the storms were long-lived, quasi-unicellular, and in possession of mid-level rotation; characteristics of the supercell. Without a more comprehensive analysis of this event it is presumptive to assume that the storms were indeed supercells (especially in the absence of a generally accepted definition of the supercell). Nevertheless, the importance of this case and the motivation for this research is that the observed convective mode was inconsistent with the behavior of air-mass convection. The Jarrell case was also significant because the storm complex was “anchored” to a preexisting boundary. The purpose of this work is to determine the role of storm-boundary anchoring in the modulation of convective mode in high CAPE, low-shear environments.
Preliminary simulations have shown promise. In the absence of the preexisting boundary, the modeled storm evolved in agreement with the high-CAPE, low-shear archetype; namely, an intense updraft formed in response to the initial thermal perturbation but rapidly decayed as outflow undercut the updraft and failed to regenerate deep convection. On the contrary, in the presence of a preexisting boundary, an anchored storm developed, back-built against the mean-flow, and possessed strong updraft rotation. Consistent with our hypotheses, updrafts appeared to redevelop at the intersection of the storm’s gust front and the preexisting boundary and at the leading edge of an internal bore initiated by the intrusion of the storm’s gust front into the cold air behind the preexisting boundary. The role of the internal bore in these simulations is unique because no formal observations have been made of an internal bore, generated by a storm, forcing updraft redevelopment for the same storm.
Results presented at this conference will come from a new set of simulations utilizing the new ICOMMAS model. ICOMMAS is a child of the COMMAS model and has been designed for the high-resolution simulations necessary for this investigation. These simulations will incorporate a larger domain (to facilitate longer simulation times), simple ice microphysics in place of warm-rain microphysics, and a higher resolution grid (to improve the resolution of convective cells as well as the preexisting boundary structure). Analysis of these simulations will focus on the decomposed pressure fields, derived trajectories, and Eulerian and Lagrangian vorticity tendencies and will be directed at identifying the sources of updraft redevelopment and updraft rotation. The mode of modeled convection will be characterized statistically in terms of updraft strength, updraft steadiness, magnitude of updraft rotation, steadiness of updraft rotation, unicellularity, and the spatial correlation between vertical velocity and vertical vorticity.
Supplementary URL: http://redrock.ncsa.uiuc.edu/~ahous/Supercell-DC/
Poster Session 6, Numerical Modeling of Severe Local Storms
Wednesday, 14 August 2002, 3:00 PM-4:30 PM
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