Sea-breeze Characteristics Over Complex Terrain: an Evaluation From Observational Data and WRF Simulations

The sea breeze has been widely studied due to its influence, amongst others, on air quality and pollution, the formation of convective systems and the forecasting of maximum temperature in locations close to the shore during hot spells. The sea breeze and its characteristics are well studied in the Spanish Mediterranean coast, but not so in the Cantabrian coast. This study is centered in the Basque coast (Northern Spain), a region characterized by its complex topography, which has been proved to impact significantly the evolution of the sea-breeze circulation. On the one hand, observational data from two topographically contrasting sites are employed: one is located in a valley opened to the sea, and the other one, in a closed and irregular valley close to the shore. Additionally, numerical simulations are performed using the Weather Research and Forecast (WRF) model.

An objective and systematic selection method is used to detect sea-breeze days from a database of two summer months. From the obtained sea-breeze days it is inferred that the wind direction after the onset of the onshore flow is highly influenced by the shape of the valley. On the other hand, the sea-breeze intensity is well correlated with the sea-land temperature gradient in the station located in the open valley, whereas the closed valley does not show such a correlation, apparently due to the influence of upslope flows.

Numerical simulations are performed for two cases: a day selected as a sea breeze in both locations and a late-sea-breeze case, i.e., a day in which the shift to an onshore wind direction occurs after the weakening of the convective mixing. For the latter, four sensitivity experiments are performed: varying the Planetary Boundary Layer (PBL) scheme and the grid analysis nudging option. This case results of great interest: on the one hand offshore winds blow during daytime making temperatures reach extremely high values on the coast (up to 40 ºC); and on the other hand, simulations considering the grid analysis nudging reproduce an unreal (not observed) onset of the sea breeze and the subsequent drop of the temperature. Thereby, by comparison with the rest of the simulations, they allow to evaluate the influence of the passage of the sea-breeze front in other variables mainly related to turbulence. Furthermore, the choice of the PBL scheme also alters the onset time of the onshore flow as well as the vertical structure of the density current. 6.22.237 on 2-10-2016-->