The Multiscale Dynamics of the 29 June 2012 Super Derecho

Abstract

The 29–30 June 2012 “super” derecho was, up until the 10 August 2020 “Iowa Derecho”, the most prolific derecho of modern times. While many of the synoptic-scale precursors to derecho events are understood, the multi-scale dynamics which likely distinguish derecho-producing events versus non-derecho events remain much more elusive. Using both observations and high-resolution WRF-ARW simulations, the sequence of adjustments that ultimately set up the pre-29 June derecho environment are examined. Planetary scale Rossby wave breaking occurred almost exactly two weeks before the super derecho on 15–16 June 2012 resulting in the development and intensification of a strong high-pressure system and mixed layer over the complex terrain of the western United States. A week 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. A second Rossby wave break on 26 June was crucial for detaching the mixed layer from the western U.S. elevated plateau, creating an elevated mixed layer that was rapidly deformed and propagated downstream to set up the derecho environment between 27–29 June. On 28 June, flow imbalance at the elevated mixed layer front resulted in highly ageostrophic circulations in the mid-levels, generating an along-stream mid-level jetlet which ultimately moved the elevated mixed layer and associated mesoscale front downstream across the Midwest and Mid-Atlantic. On the morning of 29 June, a well-defined corridor of both potential static instability and lowered inertial stability was set up across the Midwest and Mid-Atlantic states. This along with strong capping, a divergent polar jet entrance region to the north, and the highly imbalanced mid-level jetlet set the stage for this prolific severe convective event.