Resumen de Nocturnal coastal fronts in the Mediterranean basin
The main goal of this thesis is to investigate the formation of nocturnal coastal fronts (hereafter CF) in the Mediterranean basin producing nocturnal offshore precipitation near the coastline. After sunset the cooled inland air moves to the coastline driven by drainage winds as a density current. When this air encounters the warm and wet sea-air a CF is formed. Therefore the warm air mass is lifted and when ascending clouds may form producing in some cases precipitation. The influence of the land-sea thermal difference, the shape of the coastline, the sea surface temperature (SST) and the depth of the cold air mass associated to the coastal front (H) on the coastal front formation is investigated. This has been done following methodology consisting in three steps. In the first step events of nocturnal precipitation associated to CFs are detected by using the Tropical Rain Measurement Mission (TRMM) Satellite, reflectivity radar network images from ground radar, and clouds recorded by satellite. The second step consists in analyzing the synoptic situation associated to these possible CF, in order to discard those in which the precipitation is related to synoptic features. Finally, the WRF-ARW model is used for the simulation of these CF. Chapter 2 is devoted to study the role of the land-sea temperature difference. If the thermal gradient is large enough, drainage wind speed can be of the order of the prevailing wind speed. In this case, a line of convergence is simulated offshore and above it several precipitation cells occur forming a quasi-stationary line of precipitation. Relatively strong vertical motions are simulated in these coastal fronts, providing upward moisture and heat that produce convective clouds and precipitation. Chapter 3 focuses on role of the shape of the coastline. Offshore rainbands during nighttime have been simulated and analyzed in concave coastal areas in the Mediterranean basin. Drainage winds drive cold inland air offshore from many areas along the concave coastline. The cold inland air collides offshore with the opposite inland cold drained air from the opposite coastline areas, converging offshore. Clouds and precipitation are simulated following the concave coastline. In Chapter 4, the influence of the SST on the CFs is analyzed. A couple of CFs occurred at the end of the summer and early autumn, when the SST presents its larger values, are analysed. These CFs show the heaviest and widest rainband within the observed and simulated CFs. The role of H is analyzed in Chapter 5 for the all the studied CFs. The hourly and averaged values of H during the nighttime for each CF have been estimated. In the CFs, the cold air mass acts as a moving orographic barrier to the warmer prevailing flow. For this reason some theoretically parameters that accounts the blockage, the triggering convection, and the deceleration that the cold air mass offers to the prevailing maritime air mass are estimated. Finally, from the averaged values of sea-air and land-air temperatures, a cloud bands forecasting index is proposed.
