Dry Air, Midlevel Flow, and the Persistence of Deep Convection in Atlantic Tropical Disturbances

This study examines the combined effects of wind shear and dry air on the pregenesis convective evolution and the subsequent changes in the kinematic and thermodynamic structure of tropical disturbances. Specifically of interest are features, known here as 'dry air intrusion pathways (DAIPs),' that are characterized by collocated layers of midlevel dry air and vertically-unidirectional horizontal winds. We hypothesize that DAIPs can act to prevent the establishment of persistent deep convection by importing midlevel dry air into the convective regions of a tropical disturbance. Since DAIPs occur at midlevels and deep convection in a tropical disturbance tends to cluster around the low-level center, it is expected that a deep, vertically aligned vortex will protect the core deep convection of a tropical disturbance from the negative effects of a DAIP. Finally, we also hypothesize that DAIPs located at a lower altitude and/or associated with a stronger horizontal wind will have a greater negative impact on the deep convection than will DAIPs located at a higher altitude and/or associated with a weaker wind.

To test our hypotheses, we employ a combination of observations and numerical simulations. Dropsonde and satellite observations are used to document the DAIPs and identify patterns in the convective evolution, especially regarding the persistence of deep convection in the presence of a DAIP. Model fields from the Second Hurricane Nature Run (HNR2), provided by Dave Nolan's group, are used to understand how a DAIP evolves in time as well as the origin of the dry air within a DAIP. Finally, a series of idealized simulations examine the impact DAIPs have on deep convective towers.

Our findings suggest that, in the absence of a vertically-aligned vortex, deep convection in the presence of a DAIP will typically collapse during the diurnal convective minimum. Conversely, when the deep convection is protected from DAIPs by a vertically-aligned vortex, deep convection is more often observed to persist through the diurnal convective minimum. Trajectory calculations using the HNR2 data suggest that the dry air within the DAIP primarily originates from the environment, although there is some local contribution from subsidence. Finally, analysis of the idealized simulations suggest that deep convection exposed to a DAIP will have reduced upward vertical mass flux and frozen precipitation mass. Since the upward vertical mass flux and frozen precipitation mass play important roles in the spin up of the low- and midlevel vortices, respectively, reductions in these values can have important upscale implications. This presentation will focus on the observational portion of the study and will conclude by considering the implications of our findings with regard to the future evolution of the parent tropical disturbance.