Intermediate and High Resolution Numerical Simulations of the Transition of a Tropical Wave Critical Layer to a Tropical Depression

Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from the cyclonic Kelvin cat¹s eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis that typifies the trade wind belt. The cyclonic critical layer is thought to be important to tropical cyclogenesis because its cat¹s eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing gyre and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. The entire sequence is likened to the development of a marsupial infant in its mother¹s pouch, and for this reason has been dubbed the ³marsupial paradigm.²

Here we conduct the first multi-scale test of the marsupial paradigm by revisiting the problem of the transformation of an idealized African easterly wave-like disturbance into a tropical storm vortex. An idealized multiply-nested configuration of the Weather Research and Forecasting (WRF) model is employed using an initial zonal flow consistent with the observed zonal flow during Phase III of GATE and with NCEP analyses over the western Atlantic. An analysis of the evolving winds, equivalent potential temperature, and relative vertical vorticity is presented from coarse (28 km), intermediate (9 km) and high resolution (3.3 km) simulations. The results are found to support key elements of the marsupial paradigm by demonstrating the existence of a vorticity dominant region with minimal strain within the critical layer pouch that contains strong cyclonic vorticity and high saturation fraction. This localized region within the pouch serves as the ³attractor² for an upscale ³bottom up² development process while the wave and pouch move together.

Implications of these findings are discussed in relation to a newly proposed field experiment for the most active period of the Atlantic hurricane season in 2010/2011 that is to be conducted collaboratively between the NOAA and the NSF.