Why do model tropical cyclones grow progressively in size and decay in intensity after reaching maturity?
Smith, Roger K.
Montgomery, Michael T.
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The long term behaviour of tropical cyclones in the prototype problem for cyclone intensification on an f -plane is examined using a nonhydrostatic, three-dimensional, numerical model. After reaching a mature intensity, the model storms progressively decay while both the inner core size, characterized by the radius of the eyewall, and size of the outer circulation, measured for example by the radius of gale force winds, progressively increase. This behaviour is explained in terms of a boundary-layer control mechanism in which the expansion of the swirling wind in the lower troposphere leads through boundary-layer dynamics to an increase in the radii of forced eyewall ascent as well as to a reduction in the maximum tangential wind speed in the layer. These changes are accompanied by changes in the radial and vertical distribution of diabatic heating. As long as the aggregate effects of inner-core convection, characterized by the distribution of diabatic heating are able to draw absolute angular momentum surfaces inwards, the outer circulation will continue to expand. The quantitative effects of latitude on the foregoing processes are investigated also. The study provides new insight on the factors controlling the evolution of size and intensity of a tropical cyclone. It provides also a plausible, and arguably simpler, explanation for the expansion of the inner-core of Hurricane Isabel (2003) than that given previously.