An analysis of dropsonde-observed relative humidity in the Tropical Storm Gabrielle (2013) pregenesis disturbance suggests the presence of a layer of dry air that is being advected into the disturbance from the north. The focus of this study is on this and similar dry air layers, termed dry air inflow pathways (DAIPs), that are characterized by unidirectional disturbance-relative flow drawing dry air into a tropical disturbance. It is hypothesized that DAIPs act to prevent the establishment of persistent deep convection by importing midlevel dry air into the core convective regions of a tropical disturbance and that, furthermore, only once the deep convection is isolated from the DAIP can the deep convection become persistent. This hypothesis is subdivided into a five-step narrative for testing. The five-step narrative is as follows: first, the DAIP imports dry air from the environment into the tropical disturbance. Second, the imported dry air reaches the core deep convection of the tropical disturbance. Third, this dry air is then entrained into the core deep convection. Fourth, the dilution of the core deep convection updrafts by entrained dry air reduces the updraft buoyancy of the core deep convection. Finally, the reduced buoyancy inhibits the persistence of the core deep convection.

Both observations and numerical models are used to test the proposed narrative. The observational analysis includes a detailed examination of DAIPs in three Atlantic tropical disturbances using data collected by dropsondes and satellites. The second hurricane nature run (HNR2; Nolan et al. 2013; Nolan and Mattocks 2014), a simulation of the full life cycle of a tropical cyclone, is used to demonstrate that the DAIP dry air within the region of core deep convection is actually reaching the cloud-environment interface of that deep convection. The final three steps of the narrative are tested using several idealized CM1 (cloud model 1; Bryan and Fritsch 2002) simulations in which deep convection is exposed to DAIPs with varying altitudes and wind speeds.

The analyses presented here show that DAIPs can have a considerable impact on the evolution of deep convection. The findings suggest that the presence of a DAIP will inhibit the persistence of deep convection even in environments typically characterized by strong low-level forcing (e.g., the core of a tropical storm). DAIPs are found to weaken the convective updrafts by encouraging dry air entrainment, which reduces updraft buoyancy. Additionally, the results suggest that the midlevel vortex may be important in protecting the core deep convection from hostile environmental influence (e.g., DAIPs). The manuscript concludes with an evaluation of the present study in the context of past studies and provides suggestions for further work on the topic of DAIPs.