Mid-level ventilation's effects on tropical cyclone thermodynamic structure and intensity

Mid-level ventilation of the inner core of a tropical cyclone with ambient low θ_e air has a profound effect on the steady state structure and intensity of a tropical cyclone. The Carnot heat engine idealization of a tropical cyclone is modified to account for the effects of mid-level ventilation, namely by adding a sink of θ_e in the boundary layer and reducing the thermodynamic efficiency of the system. Both act to reduce the steady state intensity below the potential intensity.

The effects of ventilation on the time evolving intensity and structure of a tropical cyclone are further investigated using an axisymmetric hurricane model with parameterized eddy mixing at mid-levels. If the eddy mixing is sufficiently vigorous such that the import of low θ_e air overcomes the turbulent flux of θ_e at the surface, the inner core will become subsaturated, inducing downdrafts that cause the eyewall and eye to adjust to a lower value of θ_e. The net consequence is a shallower and weaker tropical cyclone. Moreover, marked weakening on smaller timescales also occurs due to convective downdrafts originating outside the radius of maximum winds acting as a source of low θ_e air in the tropical cyclone boundary layer. Downdraft air is not able to recover to the pre-downdraft values of θ_e before reaching the inner core, resulting in a flattening of the radial θ_e gradient and a temporary but significant decrease in maximum wind speed.