17 Ascent of sea spray in the hurricane boundary layer in the presence of strong rain

Monday, 7 July 2014
Jacob Shpund, Hebrew University, Jerusalem, Israel; and A. Khain and J. Zhang

Sea -spray droplets with radii up to several hundred microns may dramatically affect microphysics and dynamics of deep convective clouds in the eye wall of tropical cyclones (TC). However, the strong rain-shaft takes place in the inner TC core. The question arises whether strong rain can prevent the penetration of sea spray into clouds or not. To answer this question, generation and transport of sea spray in the hurricane boundary layer was simulated using a hybrid Lagrangian-Eulerian model of the hurricane boundary layer with a detailed description of microphysical processes. In the model about 2000 adjacent parcels (volumes) move within a turbulent -like velocity field. The turbulence intensity, as well as the intensity of sea -spray production near the ocean surface are determined by background wind speed. To investigate the role of rain in scavenging of sea spray, rain fluxes of different intensities were assumed at the upper boundary of the computational area (z=400m). The simulations show that rain, even a very intense one, is unable to prevent the penetration of large sea-spray to the cloud base of hypothesized deep convective cloud. The paradoxical effect is related to existence of large eddies (rolls) in the hurricane BL. Raindrops falling from above prefer to fall within the zones of downdrafts formed by the large eddies, while spray droplets ascent in the updrafts related to the large eddies. As a result, raindrops and sea -spray drops turned out to be spatially separated. The scavenging efficiency decreases with the increase in the intensity of the large eddies (rolls). Since the intensity of large eddies is linearly related to the wind speed, efficiency of scavenging decreases with the wind speed.
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