The Impact of a Superposed Jet's Ageostrophic Circulation on the 1–2 May 2010 Nashville Flooding Event and the Implications for a Warmer Climate

An increase in extreme sensible weather events is a widely anticipated consequence of climate change. Carefully chosen, detailed case study work concerning current-day examples of extreme weather can serve to 1) further elucidate the nature of the synoptic/planetary-scale interactions that conspire to produce extreme weather, and 2) help to develop conceptual subtropical/extratropical models that suggest how such interactions may become more frequent in an altered climate. Interactions between the usually separate subtropical and polar jet streams can result in the production of extreme weather. On occasion, the subtropical and polar jet streams become superposed in the vertical, resulting in a single, intense jet feature with a substantially enhanced secondary circulation that extends through a large depth of the troposphere.

The 1-2 May 2010 Nashville flooding event was an historic event that brought two-day rainfall totals in excess of 33 cm (13 in.) over a large portion of the Tennessee River Valley, with some isolated rural areas tallying upwards of 50 cm (20 in.). The combination of anomalous moisture flux, adequate instability, and a stagnant upper-tropospheric pattern, allowed for several upper-level disturbances and a stationary surface frontal boundary to initiate repeated rounds of thunderstorm activity. While the environment of this event shares characteristics of other high impact flooding events, the presence, in this case, of a superposed jet and its ageostrophic secondary circulation enhanced both the vertical motions and moisture flux into the lower Mississippi River Valley, particularly prior to and concurrent with, the second day of heavy rainfall. The analysis provides insight into the dynamics of superposed jets, important synoptic/planetary-scale features whose frequency may well increase in a warmer climate.