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The Multi-Scale Dynamics of the 29-30 June 2012 "Super Derecho"
AuthorShourd, Kacie Nicole
AdvisorKaplan, Michael L
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The 29-30 June 2012 “super” derecho was and is perhaps the most prolific derecho to occur in contemporary times, and yet was almost entirely unforecasted. While many of the synoptic-scale precursors to derecho events are understood, the multi-scale nature of the dynamical processes involved, which can help to distinguish derecho producing events versus non-derecho events, remain much more elusive. Using both observed datasets and high resolution (meso-beta/gamma scale) WRF-ARW simulations, the sequence of adjustments and accelerations which ultimately set up the pre-derecho environment are examined up to two weeks ahead of the 29 June event. Planetary scale Rossby wave breaking occurred on 15-16 June, almost two weeks prior to the super derecho, resulting in the development and intensification of a strong high pressure system and deep mixed layer over the complex terrain of the western U.S. Seven days after the initial Rossby wave break (~23 June), daily record- breaking temperatures began to dominate much of the central U.S. as the mixed layer/high pressure continued to strengthen via the development of mountain-plains solenoids to enhance mixing and persistent, hot, dry flow off of the Pacific around the ridge. Rossby wave train amplification continued to occur from 16 June through 26 June ahead of another Rossby wave break on 26 June. This wave break was positively tilted, reinforcing the anticyclonic nature of the mixed layer environment. The 26 June wave break was crucial for detaching the mixed layer from the western U.S. elevated plateau, creating an elevated mixed layer which was rapidly deformed, and propagated downstream to set up the derecho environment between 27-29 June. On 28 June, a mass imbalance at the elevated mixed layer front resulted in highly ageostrophic accelerations in the mid-levels of the atmosphere, generating an along-stream midlevel mesoscale jetlet which ultimately assisted in the movement of the elevated mixed layer and associated mesoscale front downstream across the Midwest and Mid-Atlantic. These two features also worked to mutually strengthen one another as they moved downstream. On the morning of 29 June, a well-defined corridor of both potential static instability and lowered inertial stability (or negative isentropic potential vorticity) was set up across the Midwest and Mid-Atlantic states. This, plus strong capping (to suppress convection within this environment ahead of the super derecho), a divergent polar jet entrance region to the north, and the highly unbalanced midlevel jetlet set the stage for the triggering of this prolific severe convective event driven by a strong low-level mesoscale anticyclone. The WRF-ARW simulations replicated the subsequent, narrow corridor of imbalance/instability and eventual derecho remarkably well, providing an exceptional opportunity to examine the cascade of dynamics which occurred and ultimately led to this $2.9+ billion disaster.