HR: 0800h
AN: A31A-0013    [Abstracts]
TI: Hurricane Formation in Diabatic Ekman Turbulence
AU: * Schecter, D A
EM: schecter@nwra.com
AF: NorthWest Research Associates, PO Box 3027, Bellevue, WA 98009-3027, United States
AU: Dunkerton, T J
EM: tim@nwra.com
AF: NorthWest Research Associates, PO Box 3027, Bellevue, WA 98009-3027, United States
AB: This paper examines a simple representation of turbulent flow in the tropical troposphere, which occasionally produces a hurricane. In this paradigm, the flow is essentially two-dimensional (2D) turbulence under the cooperative influence of Ekman pumping and deep cumulus convection. After an incubation period, diabatic Ekman pumping can supercede ideal 2D mechanisms of self-organization, such as vortex merger. A strong cyclone-anticyclone asymmetry can develop, with very intense convective cyclones dominating the system. Diabatic Ekman Turbulence (DET), as described above, is readily studied with a 3-layer model of the troposphere. The model used here includes a frictional boundary layer (BL), a lower troposphere (LT), and an upper troposphere (UT). The parameterizations of surface fluxes, deep convection and radiative cooling are similar to those used by Ooyama in his seminal study of axisymmetric tropical cyclone intensification [Ooyama, K., J. Atmos. Sci., 26, 3 (1969)]. The results given below are from numerical simulations in a 2000km-by-2000km periodic box. In all simulations, the initial turbulence is concentrated in the BL and LT, whereas the UT starts at rest. In general, we find that DET freely evolves into a Frictional Radiative Convective Equilibrium (FRCE), in which there is an approximate balance between convective energy input and energy output by surface drag and radiation. The predictability of the FRCE that emerges from random noise varies with sea-surface temperature (SST), the Coriolis parameter f, and the ratio C_E/C_D, in which C_E and C_D are surface-exchange coefficients for moist entropy and momentum, respectively. At low values of these control parameters, DET tends to dissipate. As the control parameters increase to typical tropical values, the FRCE bifurcates into a metastable synoptic-scale gyre or a hurricane. At higher values of the control parameters, the FRCE is always a hurricane. The hurricane that emerges from DET is realistic in several ways. During rapid intensification, the hurricane typically develops polygonal eyewalls and mesovortices. In the FRCE, the hurricane exhibits moderate intensity oscillations that resemble eyewall breakdown and regeneration cycles. The time-averaged intensity of the hurricane increases with the SST and the ratio of surface-exchange coefficients C_E/C_D [ibid; Emanuel, K.A., J. Atmos. Sci., 43, 585 (1986)].
DE: 3265 Stochastic processes (3235, 4468, 4475, 7857)
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3367 Theoretical modeling
DE: 3374 Tropical meteorology
DE: 3379 Turbulence (4490)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting