Thermodynamic Characteristics of Downdrafts in Tropical Cyclones as Seen in Idealized Simulations of Different Intensities

The thermodynamic effect of downdrafts on the boundary layer and nearby updrafts are explored in idealized simulations of category-3 and category-5 tropical cyclones (TCs) (Ideal3 and Ideal5). In Ideal5, downdrafts underneath the eyewall pose no negative thermodynamic influence because of eye-eyewall mixing below 2- km altitude. Additionally, a layer of higher theta(e) between 1- and 2-km altitude associated with low- level outflow that extends 40 km outward from the eyewall region creates a thermodynamic shield'' that prevents negative effects from downdrafts. In Ideal3, parcel trajectories from downdrafts directly underneath the eyewall reveal that low-theta(e) air initially moves radially inward allowing for some recovery in the eye, but still enters eyewall updrafts with a mean theta(e) deficit of 5.2 K. Parcels originating in low-level downdrafts often stay below 400 m for over an hour and increase their ue by 10-14 K, showing that air-sea enthalpy fluxes cause sufficient energetic recovery. The most thermodynamically unfavorable downdrafts occur similar to 5 km radially outward from an updraft and transport low-theta(e) midtropospheric air toward the inflow layer. Here, the low-theta(e) air entrains into the updraft in less than 5 min with a mean theta(e) deficit of 8.2 K. In general, ue recovery is a function of minimum parcel altitude such that downdrafts with the most negative influence are those entrained into the top of the inflow layer. With both simulated TCs exposed to environmental vertical wind shear, this study underscores that storm structure and individual downdraft characteristics must be considered when discussing paradigms for TC intensity evolution.

Automated summary

In idealized simulations of category-3 and category-5 tropical cyclones, the thermodynamic effects of downdrafts on the boundary layer and nearby updrafts were explored. It was found that in category-5 cyclones, downdrafts had no negative thermodynamic influence due to a thermodynamic shield created by eye-eyewall mixing, while in category-3 cyclones, downdrafts had a significant negative impact on nearby updrafts. The study highlights the importance of considering storm structure and individual downdraft characteristics when discussing paradigms for tropical cyclone intensity evolution.