Dry Air, Midlevel Flow, and the Establishment of Persistent 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 structure of the tropical disturbance. Previous studies suggest that convection acts to gradually moisten the middle troposphere until sufficient moisture is present to support persistent deep convection; we hypothesize that variations in the vertical structure of the wave-relative wind profile can import midtropospheric dry air that acts to offset this midtropospheric convective moistening in the pregenesis convective environment thereby delaying or preventing the establishment of persistent deep convection.

A variety of in situ and remote sensing observations will be used to test our hypothesis and examine the convective and kinematic evolution of several tropical disturbances. Three-dimensional fields of moisture (obtained from dropsonde profiles and the satellite-based Atmospheric Infrared Sounder) and horizontal wind (obtained from dropsonde profiles and satellite-based atmospheric motion vector winds) will be analyzed in a comoving framework in order to identify pathways by which dry aim may be gaining access to core convective features. This presentation will focus on the simultaneous evolution of moisture, wind, and convection in a small number of demonstrative cases observed during the PREDICT, GRIP, and HS3 field campaigns.