Coastal-breeze simulation with the WRF model: analysing the sensitivity to land/sea surface temperature changes

Coastal breezes are mesoscale winds formed in mid-latitude regions under fair-weather synoptic conditions, i.e., weak surface-pressure gradient and low winds. They are characterized by a typical wind-direction reversal twice per day, with onshore winds during daytime and offshore during the night. The impacts of these winds are broad and varied: they transport humidity and pollutants (among other physical properties) in the coastal region; they can initiate convection (and even trigger the formation of storms), and they also drive some of the surface oceanic currents formed close to the coast, among others. In this sense, recent surface current observations collected by the Gulf of Cádiz radar system point at the coastal breezes as important contributors to explain the behaviour of the surface currents variability in this region. From a societal point of view, the coastal breezes are crucial for wind power industry, air-quality forecasts, maritime sports, and simply for the refreshing impact that they cause in some warm areas in summer. Hence, a correct understanding of their physical characteristics is needed to correctly forecast them and to be able to investigate their future trends.

These phenomena are formed due to the pressure gradient associated with the temperature difference between the sea and the land surfaces. Therefore, both systems (the ocean and the atmosphere) are involved on the final characteristics that specific breeze events will have, impacting on the variability observed in the strength, duration, and vertical/horizontal extension of the breezes. In this work, we use the Weather Research and Forecasting (WRF) mesoscale model to investigate how the changes in the surface affect the features of this phenomenon through the design of different sensitivity experiments. These include artificial changes in the land and sea surface temperature, but also the use of more realistic (and higher resolution) data to initialise the model. Besides, other experiments are designed to investigate the impact of different technical aspects of the model on the correct simulation of the physical processes, such as the use of different vertical and horizontal resolution, the choice of the planetary-boundary-layer scheme, or the activation or not of typical filters (smoothing) within the model.