Downstream development during the extratropical transition of tropical cyclones: observational evidence and Influence on storm evolution

Observational evidence is presented that during Extratropical Transition (ET) of Tropical Cyclones (TCs), Downstream Development (DD) frequently commences to the west of the storms at upper tropospheric levels prior to ET. It is seen as a wave train of amplifying troughs and ridges developing eastward in time. Case studies obtained from previously-documented ET events, Hurricanes Michael, Maria and Wilma, are used to illustrate the process. A non-ET event, Hurricane Rita, is also discussed.

The authors illustrate that the DD results in very rapid changes to the environment of the storm. A critical flow change appears to be the development of a large-scale, low- to mid- level trough, sandwiched between two developing anticyclones, which captures the storm. The trough and associated wrap-around flow seemingly hold the storm upright, allowing it to withstand the increasing wind shear associated with the approaching upper trough, until the storm can reach the favourable equatorward entrance region of the upper jet.

To evaluate the mechanism, a series of simulations have been run ranging in complexity from high-resolution, full physics integrations (to confirm that the simulations reproduce the ET) through to coarse-resolution, no-latent-heating runs with the TC vortex removed from the initial condition. The latter is used as a first approximation to examine the environment of the storm. The authors show that the dry dynamics of the evolving environment can establish the wrap-around flow and the development of the low- to mid- level trough. The simulations suggest that the development is associated with the maturing DD event, with a short period of environmental descent and anticyclonic tendency, followed by a period of environmental ascent and cyclonic tendency. We hypothesise that the net result is to (a) produce a modulation in the vertical motion field (and hence to embedded convection), allowing the boundary layer to moisten via (i) sustained surface fluxes, and (ii) horizontal moisture advection via the developing anticyclone to the east, (b) produce the low-level, large-scale trough that captures the storm, and (c) increase potential for more intense convective activity once the inhibition passes. These characteristics are consistent with proposed conceptual models of ET (e.g., Klein et al. 2000) and with the enhanced vertical mass flux diagnosed by Davis et al. (2007).