P2D.9 A bottom-up route of tropical cyclone intensity change in vertical wind shear

Thursday, 1 May 2008
Palms ABCD (Wyndham Orlando Resort)
Michael Riemer, NPS, Monterey, CA; and M. T. Montgomery, M. E. Nicholls, K. Emanuel, and B. H. Tang

It is well known that vertical shear of the environmental wind can have a detrimental effect on the intensity of a tropical cyclone. However, the processes that modulate the intensity of a tropical cyclone affected by shear are not well understood. A top-down theory has been proposed in which ventilation of the upper-level warm core results in an increase of surface pressure by hydrostatic arguments. The hypothesized ventilation of the warm core at upper levels, however, is not a consistent feature in the range of experiments performed by different investigators. The development of asymmetries in the storm's convection has been invoked to explain the observed decrease in intensity also. We are not yet satisfied by the proposed casual link of this process. Intense hurricane vortices tend to be resilient and thus the tropical cyclone heat engine may operate at near peak efficiency despite the presence of asymmetries in the vortex core region.

We address here the problem of intensity modulation and demise of tropical cyclones in idealized numerical experiments on an f-plane using a non-hydrostatic mesoscale model. We employ a simple bulk aerodynamic boundary layer scheme in which the surface exchange coefficients of heat and momentum can be prescribed and also 'warm rain' microphysics. After a spin up period in a quiescent environment we impose the vertical shear on the model tropical cyclone. We perform a suite of experiments with shear values ranging from 5 to 15 m/s between 850 and 200 hPa. Upon introduction of the vertical shear, periods of enhanced vortex Rossby wave (VRW) activity are observed whose amplitude appears to scale with the strength of the vertical shear. This is consistent with the theory of vortex resiliency, in which the tilt of the vortex projects onto VRW modes. The periods of enhanced VRW activity are associated with a strong wave number one asymmetry in the boundary layer equivalent potential temperature (theta_e) and a decrease of the near eye wall theta_e in the azimuthal average. Consistent with the heat engine concept the modulation of storm intensity in our experiments is strongly coupled to the evolution of the boundary layer theta_e. As expected, the decrease in intensity is more significant in stronger vertical shear. The evolution of the boundary layer theta_e is moreover found to lead the variation in the strength of the upper-level warm core. For the moderate shear regime we therefore hypothesize that intensity modulation is governed by the evolution of the near core boundary layer theta_e. In contrast with previous work that suggests a direct interaction with the environmental shear flow, the strength of the upper-level warm core is here observed to reflect the storm intensity.

In this presentation we will elaborate on our hypothesis and extend our results into the strong shear regime, in which the tropical cyclone finally looses its vertical coherence and decays.

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