Session 11B.7 Mid-level ventilation's effects on tropical cyclone thermodynamic structure and intensity

Wednesday, 30 April 2008: 2:45 PM
Palms E (Wyndham Orlando Resort)
Brian H. Tang, MIT, Cambridge, MA

Presentation PDF (248.8 kB)

Mid-level ventilation of the inner core of a tropical cyclone with ambient low θe 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 θe 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 θe air overcomes the turbulent flux of θe at the surface, the inner core will become subsaturated, inducing downdrafts that cause the eyewall and eye to adjust to a lower value of θe. 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 θe air in the tropical cyclone boundary layer. Downdraft air is not able to recover to the pre-downdraft values of θe before reaching the inner core, resulting in a flattening of the radial θe gradient and a temporary but significant decrease in maximum wind speed.

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