Statistical characterization of the sea-breeze physical mechanisms through in-situ and satellite observations

The physical mechanisms that take place under sea-breeze (SB) conditions in the Palma basin (Mallorca island, western Mediterranean sea) are analyzed through the inspection of data from automatic weather stations (AWS) during the period 2009-2017. Satellite-derived land-surface and sea-surface temperatures (LST and SST, respectively) are used to compute the surface thermal gradient (LST-SST) in the Palma basin. It presents a clear daily and seasonal cycles, and the daytime hourly averages for each month are always positive and maximal during summer time.

 

A method similar to the one proposed by Borne et al. (1998) is used to select the SB events from AWS data in Palma basin. Although the thresholds are valid for the SB features in the Palma basin, the method can be applied to other regions. SB conditions are mainly reported during the warm months of the year (from April to September) and specially during summer time. Results from the statistical analysis show that the SB follows diurnal cycle already described in previous works. Among the characteristics founded, the morning veering strongly depends on the speed and direction of the large-scale winds. Besides, weak wind conditions are reported during the morning veering and once the SB starts the cold advection from the sea counteracts the radiative heating and the 2 m-temperature levels off. The maximum surface thermal gradient under SB conditions is larger than 5ºC and it is often reported several hours after the morning veering. About two hours later the maximum wind speed is formed, strongly influenced by the amount of soil moisture. The thermal winds formed inside the basin also condition the propagation of the SB inland. The statistical analysis shows that the SB features in Mallorca depart from the idealized conditions due to the complex terrain of the island.