Long-term heat exchanges over a Mediterranean lagoon

Heat exchanges between a medium-sized Mediterranean lagoon and the atmosphere are investigated at various time scales using long-term observations including 9 months of eddy covariance measurements. Turbulent heat fluxes are assessed using both eddy covariance and aerodynamic methods with a relative accuracy estimated at 15%. At time scales ranging from the diurnal to the seasonal cycle, heat exchanges appear to be dominated by evaporation, and most of the heat release of the lagoon occurs on time scales between 2 and 20 days. Seasonal variations are mainly due to the effects of net radiation and air temperature, while daily and short-term fluctuations of the sensible heat flux are influenced by air temperature and wind. In the short term, low relative humidity is more efficient than high wind speeds to produce strong peaks of evaporation (latent heat flux more than 350 W m(-2)). Episodes of Tramontane, a regional orography-induced wind, are shown to be responsible for 84% of the cooling of the lagoon surface (for cooling of more than 0.5 degrees C) and for 70% of the heat loss due to evaporation. Short-term heat transfers between the lagoon and the atmosphere are largely punctuated by the two regional winds. The Tramontane generates pulses of evaporation lasting 1 to 4 days, while Marin episodes result in very low or negative latent heat fluxes and negative sensible heat fluxes corresponding to heat transfer from the atmosphere to the lagoon.