The Production of the Vertical Superposition of the Polar and Subtropical Jets during the May 2010 Nashville Flood

The May 2010 Nashville Flood was an historic, two-day event that brought record rains to many locations in Tennessee, southern Kentucky, and northern Mississippi. While this event shares many characteristics with other high-impact flooding events, the presence, in this case, of a relatively rare vertical superposition of the polar and subtropical jets is notable. Observational work by Defant and Taba (1957) demonstrates that, in such a superposition, the upper tropospheric and lower stratospheric baroclinicity in the vicinity of the jet is intensified. Consequently, a superposed jet possesses an anomalously deep layer of vertical shear and is often attended by a strengthening of its transverse, ageostrophic secondary circulation. Recent work by Winters and Martin (2014) demonstrates that this intensified circulation was responsible for a majority of the increase in poleward moisture flux into the southern Mississippi River Valley observed prior to the second day of heavy rainfall characterizing the Nashville event.

The present study adopts a piecewise potential vorticity (PV) perspective to investigate the dynamical mechanisms that conspired to produce the vertical superposition of the polar and subtropical jets during the May 2010 Nashville Flood. From this perspective, the analysis focuses on the life cycles of the originally separate jet structures. It is apparent that both internal jet dynamics and latent heat release from lower tropospheric convection over the southern Mississippi River Valley were essential in the formation of the superposed jet structure via diabatic as well as dynamic restructuring of the tropopause. In order to pinpoint the dominant mechanism fostering jet superposition, flow patterns associated with the polar and subtropical jets as well as latent heat release are isolated via the piecewise PV inversion.