Sensitivity of developing tropical cyclones to initial vortex depth and the height of environmental dry air

Understanding the ways in which a developing tropical cyclone (TC) interacts with its environment is fundamental to diagnosing predictability. A useful approach to assessing how these interactions affect predictability is to examine the sensitivity to perturbing the TC environment. However, aspects of the TC vortex itself also influence the TC-environment interactions affecting predictability. As such, this study uses a series of idealized, convection-resolving simulations to investigate how both the depth of an initial TC-like vortex and the vertical level of a dry air perturbation in the nearby environment alter the structure and intensity evolution of a developing TC. Balanced initial vortices with three different depths are subject to environmental dry air perturbations that completely encircle the vortices. The minimum mixing ratio associated with dry air perturbations occurs at either 700-hPa (“high”) or within the boundary layer (“low”). Each simulation is integrated for 192 hours.

In almost all simulations, the shallow vortex exhibits a marked delay in intensification relative to the mid and deep vortices, which follow strikingly similar intensity trajectories. This result suggests that if the initial TC depth is greater than a certain threshold, it has significantly less influence on the intensification of the TC. During the rapid intensification stage, dry air perturbations cause a contraction of the primary circulation through the depth of the eye wall, and a migration of the peak boundary layer moist flux convergence toward smaller radii. TC intensity appears to be more sensitive to low-level dry air, as evident in interruptions of rapid intensification for the mid and deep vortices. This is further investigated by incrementally shifting the dry air perturbation to intermediate vertical levels between the boundary layer and 700-hPa. Finally, the results suggest initial vortex depth and the vertical level of dry air no longer play a role in the predictability of TC structure and intensity beyond 120 hours.