Abstract: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.
Idealized high-resolution numerical simulations of tropical cyclogenesis are presented in a model that represents deep convection by a warm rain process only. Starting with an initially weak, cloud-free, axisymmetric warm-cored vortex (maximum wind speed 5 m s −1 at a radius of 100 km), rapid vortex intensification begins after a gestation period on the order of 2 days. From a three-dimensional perspective, the genesis process is similar to that in the rotating convection paradigm for vortex intensification starting with a much stronger initial vortex (V max = 15 m s −1 ). The patterns of deep convection and convectively amplified cyclonic relative vorticity are far from axisymmetric during the genesis period. Moreover, the organization of the cyclonic relative vorticity into a monopole structure occurs at relatively low wind speeds, before the maximum local wind speed has increased appreciably. Barotropic processes are shown to play an important role in helping to consolidate a single-signed vorticity monopole within a few hours near the intensification begin time.The rotating convection paradigm appears adequate to explain the basic genesis process within the weak initial vortex, providing strong support for a hypothesis of Montgomery and Smith that the genesis process is not fundamentally different from that of vortex intensification. In particular, genesis does not require a 'trigger' and does not depend on the prior existence of a mid-level vortex.
We present idealized numerical model experiments to investigate the convective generation of vertical vorticity in a tropical depression. The ambient vertical vorticity is represented by a uniform solid-body rotation. The calculations are motivated by observations made during the Pre-Depression Investigation of Cloud-systems in the Tropics (PREDICT) experiment. A specific aim is to isolate and quantify the effects of low-to mid-level dry air on convective cells that form within a depression and, in particular, on the generation of vertical vorticity in these cells. The results do not support a common perception that dry air aloft produces stronger convective downdraughts and more intense, cold-air outflows therefrom. Indeed, we find that dry air aloft weakens both updraughts and downdraughts, corroborating the recent results of James and Markowski.As in the recent calculations of Wissmeier and Smith, the growing convective cells locally amplify the ambient rotation at low levels by more than an order of magnitude and this vorticity, which is produced by the stretching of existing ambient vorticity, persists long after the initial updraught has decayed. Moreover, significant amplification of vorticity occurs even for clouds of only moderate vertical extent. The maximum amplification of vorticity is relatively insensitive to the maximum updraught strength, or the height at which it occurs, and it is not unduly affected by the presence of dry air aloft. Thus the presence of dry air is not detrimental to the amplification of low-level vorticity, although it reduces the depth through which ambient vorticity is enhanced.Results for a limited number of different environmental soundings indicate that the maximum amplification of vorticity increases monotonically with the strength of the thermal perturbation that initiates the convection, but the amount of increase depends also on the thermodynamic structure of the sounding.
The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation.The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a timedependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 108 and 308N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations.
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