The Impact of Environmental Midlevel Dry Air Inflow on Deep Convection Prior to Tropical Cyclogenesis

Environmental midtropospheric dry air and large values of environmental vertical wind shear (typically measured as a bulk quantity) are often seen as being unfavorable for tropical cyclogenesis.  The present study aims to examine the combined effects of wind shear (in terms of variations in the vertical profile of the horizontal wind) and dry air on the pregenesis convective evolution and the subsequent changes in the kinematic and thermodynamic structure of the tropical disturbance using both observations and idealized and nonidealized simulations.  Initial observational analyses of dropsondes have revealed the presence of midlevel dry air flow layers, hereafter referred to as dry air inflow pathways (DAIPs), that act to draw environmental midlevel dry air into the disturbance and, in cases with a sufficient horizontal distance between the low- and midlevel vortices, into the immediate environment of the core deep convection.  We hypothesize that the dry air imported by these DAIPs can act to offset the midtropospheric convective moistening that previous studies have suggested is important for maintaining the persistent deep convection necessary for tropical cyclogenesis.

In addition to the observational analyses, we will also be making use of both idealized and nonidealized model simulations to test our hypothesis.  The simultaneous evolution of the deep convection, DAIP, and parent tropical disturbance will be examined using a hurricane nature run (David Nolan, personal communication), while the impacts of the DAIP on the deep convection will be investigated using the CM1 cloud model.  Early results of model work confirm that the dry air originates from the environment and indicate that both the dry air and the storm-relative flow associated with the DAIP play an important role in modulating the convective evolution.  This presentation will cover the results of a small number of demonstrative cases from the observational analyses as well as preliminary results of both the idealized and nonidealized model simulations.