11B.5 An assessment of internal and external forcings in supercell interactions and their impact on storm morphology

Wednesday, 7 November 2012: 2:30 PM
Symphony II (Loews Vanderbilt Hotel)
Ann Syrowski, University of Illinois, Urbana, IL; and B. F. Jewett and R. B. Wilhelmson

This study examined how interactions between supercells can modify the storms' severity, structure, and evolution. A suite of 51 idealized storm interaction simulations containing two initial cells was generated using the Weather Research and Forecasting (WRF) model. The relative positions of the two storm cells were varied for each case, and a single-cell simulation served as the control. Despite each simulated storm forming in identical convective available potential energy (CAPE) and vertical wind shear profiles, a wide array of storm morphologies occurred depending on the initial storm pair orientation. Notably, fifty of the fifty-one two-cell storm simulations produced stronger low-level mesocyclones than the single-cell control case.

Low-level mesocyclone intensification was found to be coupled with unsteady downdraft bursts in the forward flank. Downburst-driven, surface-based rotation centers formed along the forward flank gust front and propagated toward the main updraft. These were stretched immediately prior to merging with the mesocyclone, producing a marked increase in low-level rotation. The discrete rotation centers are approximately one kilometer deep and would often be unobserved by operational radar.

The external forcing associated with the initial cell pair orientation modulated the frequency and location of the internal downbursts that produced the discrete rotation centers along the forward flank gust front. Changing the initial cell pair orientation led to significant variations in storm intensity.

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