HR: 0800h
AN: A21C-0644    [Abstracts]
TI: A Numerical Investigation of Feedback Processes in the Droplet Evaporation Layer for Tropical Cyclones
AU: * Bianco, L
EM: laura.bianco@noaa.gov
AF: NOAA/ESRL/PSD, 325 Broadway, Mail Stop: PSD3, Boulder, CO 80305-3328,
AU: Bao, J
EM: Jian-Wen.Bao@noaa.gov
AF: NOAA/ESRL/PSD, 325 Broadway, Mail Stop: PSD3, Boulder, CO 80305-3328,
AU: Fairall, C W
EM: Chris.Fairall@noaa.gov
AF: NOAA/ESRL/PSD, 325 Broadway, Mail Stop: PSD3, Boulder, CO 80305-3328,
AU: Michelson, S A
EM: Sara.A.Michelson@noaa.gov
AF: NOAA/ESRL/PSD, 325 Broadway, Mail Stop: PSD3, Boulder, CO 80305-3328,
AB: Numerical experiments using simple, idealized models indicate that the ratio of enthalpy bulk transfer coefficient to surface drag coefficient must be in the range of 0.75 to 1.25 in order for simulated hurricanes to be of realistic intensity. However, within the range in which reliable direct flux measurements exist, the observed value of this ratio over the sea is 0.70 at low winds speeds, but decreases significantly below 0.70 with increasing wind speed. One possibility for this discrepancy is the failure of the traditional bulk transfer coefficients to take into account the effect of sea-spray. High winds generate large re-entrant sea-spray droplets which tend to increase the sea-air enthalpy transfer and, thus, have positive feedback to the intensification of hurricanes. On the other hand, sea spray is generated at the expense of the momentum, which in effect causes the turbulent momentum flux to decrease. The parameterization of the feedback effect of sea spray to the overall air-sea momentum and enthalpy fluxes is still a subject of research. In this study we use the explicit sea-spray model of Kepert et al. (1999) coupled with the 1-D Mellor-Yamada turbulence mixing model to investigate the feedback effect. The spray model is capable of simulating the evaporation and dispersion of saline water droplets of various sizes. There is full coupling among the spray- droplet microphysics, turbulence mixing, and droplet transport. The Fairall-Banner parameterization is used in the sea spray droplet source term (which predicts the size spectrum of sea spray produced by the ocean in terms of wind speed, surface stress, and wave properties) along with direct dynamic and thermal feedback. The characteristics of the way in which evaporating droplets of various sizes modifies the turbulence mixing near the surface, which in turn affects further droplet evaporation, are summarized and discussed.
DE: 3307 Boundary layer processes
DE: 3329 Mesoscale meteorology
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3355 Regional modeling
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting