Estudio de 7 situaciones sinópticas durante el año 2002 unidas a galernas o probabilidad de ocurrencia de las mismas

Abstract

The analysis of the generation and development of a series of galernas has shown the presence of intense winds from the SW at relatively low altitudes, some hours before the triggering of a typical summer galerna. It is now also known that the mechanism of its generation and development includes other factors, such as the advection of a cold air mass in the marine boundary layer, which runs parallel to the coast from W to E, and a significant foehn on the coast of the Basque Country before noon. These results are contained in the report "Analysis and diagnosis of severe weather episodes in the Basque Country" (http://hdl.handle.net/10810/55032). Here, it is intended to use the meteorological data from the coastal meteorological surface stations of the region and the profiler radar of Punta Galea, located on the Basque shoreline, to establish a scheme for evaluating the capacity of a micro-front to deepen and generate a violent galerna in the Basque coast. At a synoptic level, all the galerna events in this report present a cold front over the Galician coast at noon, in addition to a characteristic configuration of the pressure centers: a depression over the west coast of Ireland and high pressures over Western Europe and Western Mediterranean, that block westerly flow over the continent (becomes more S by the western end of the anticyclone) causing larger E-W temperature differences. However, it has been proven that this configuration of the synoptic pressure centers and the position of the fronts is not sufficient for a correct assessment of galerna risk. It is also necessary to know the near frontal activity (either from a synoptic or from any other sub-synoptic front). This activity could be assessed at the local level, using the Punta Galea radar wind profiler output during a time window of several hours before the solar noon: intense SW winds (greater than 10-12 ms-1) at all the levels in the surveillance time interval are indicators of the approach of a cold front with sufficient activity to cause a galerna event. This characteristic, together with an appreciable solar activity, capable of making the boundary layer over the Basque coastal land mass sufficiently unstable (more likely between April and September) and the presence of a mass of cold air trapped to the north of the Cantabrian Mountain Range, over the sea, running parallel to the coast from W to E, are sufficient ingredients for the generation of a galerna. In this regard, it is important to highlight the fact that for the three frontal galernas, out of the seven possible analyzed events, the galerna fronts that penetrates into the Basque Country are always located ahead of the synoptic front, and the latter is not always detected in the surface stations (May 20). The temperature differences between the south-westerlies blowing ahead of the galerna front and the westerlies at the rear easily exceeds 10°C in the warm season, but the W-E thermal contrasts cannot be used only by themselves to make an adequate assessment of galerna risk: the May 24, June 16 and August 17 events are clear examples. Only the combination of the factors described and their proper monitoring could lead to a valid operational prediction.