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The impact of dry midlevel air on the outer circulation of tropical cyclones is investigated in idealized simulations with and without a moist envelope protecting the inner core. It is found that a dry midlevel layer away from the cyclone center can broaden the outer primary circulation and thus the overall destructive potential at both developing and mature stages. The midlevel outer drying enhances the horizontal gradient of latent heating in the rainbands and drives the expansion of the outer circulation. The moist convection at large radii is suppressed rapidly after the midlevel air is dried in the outer rainbands. An enhanced horizontal gradient of latent heating initiates a radial–vertical overturning circulation anomaly in the rainbands. This anomalous overturning circulation accelerates the radial inflow of the main secondary circulation, increases the angular momentum import, and thus increases the cyclone size. The dry air, mixed into the boundary layer from the midtroposphere, is “recharged” by high enthalpy fluxes because of the increased thermodynamical disequilibrium above the sea surface. This recharge process protects the eyewall convection from the environmental dry-air ventilation. The proposed mechanism may explain the continuous expansion in the tropical cyclone outer circulation after maturity, as found in observations.

Impact of Dry Midlevel Air on the Tropical Cyclone Outer Circulation