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Marine aerosols, heavy precipitation and flash-flooding


Despiau Serge

The marine aerosols generation at the sea surface results from the wind drag effects. Film and jet droplets derive from air bubbles entrained below the surface by the breaking waves that then rise to the surface and burst. This process is active from wind speeds about 4 m/s and produce droplets between 0.5 to 50 µm. For wind speeds about 10 m/s, spume droplets are tore from the wave crest, producing larger droplets from 20 to 500 µm. All these droplets compose the function source dF/dr, number produced per square meter of surface, second and micrometer increment in droplet radius. Various relations have been proposed for this function but there are still strong uncertainties about it .
After their introduction in the air layer above the sea the droplets exchange heat and moisture depending on their initial size, temperature of water and air, thin air layer humidity and their "life time" above the sea. The behavior of the spume droplets differs from that of film an jet. All decrease in size, the smallest evaporate quickly and participate few to heat and moisture exchange but most of them are transported into the boundary layer and may act as CCN. Some spume droplets may fall down to the sea by sedimentation but, due to their initial size, they are more efficient in terms of heat and moisture exchange. The spume latent heat flux may represents 10% of the total turbulent flux for a 10 m/s wind speed and 10 to 40% for 15 to 18 m/s wind speeds; the sensible heat flux is estimated to 10% of the total flux at 15 m/s. Nevertheless, large uncertainties remain to quantify more precisely their effects.
HYMEX, in a first time, should be an opportunity to try to answer to three main questions.
Are the marine aerosol heat and moisture exchange linked to the Mediterranean meteorological conditions that prevail to the development of heavy precipitation capable to modify significantly this development ?
Are the marine aerosols injected in the boundary layer able to modify, through their possible role of CCN, significantly the cloud development and water content ?
Are these information useful as such for modeling purposes ?
In a second time, HYMEX should be used to try to asses the knowledge of the function source, using alternative methods, and that of the transport of these marine aerosols in the specific context of the development of heavy precipitation events in the Mediterranean area.