Strong gusty winds in the areas of Lisbon and Setúbal: June 7, 2015
2015-06-09 (IPMA)
Last June 7, 2015, mainland Portugal was under the influence of a depression centered in Morocco and an upper level trough, which promoted atmospheric instability conditions in the region. In the afternoon, a cloud band was formed as a result of high base convection over the districts of Lisbon, Setúbal, Évora and Beja. This convective activity resulted from instability available only above 3500 m of altitude having been developed over an extremely dry air layer. In the cloud band, the content of precipitable water was though quite high, ensuring precipitating potential.
This context is favorable to the occurrence of dry downburst phenomena. In localized areas of clouds, as usual, currents of precipitation are produced. If these, along its downward path, find extremely dry air in the vicinity, they undergo evaporation and, therefore, cooling. If, as was the case, there is a fairly extensive dry air layer, the cooling mechanism can be very pronounced. Accordingly, the downdraft becomes denser (and therefore heavier) and gradually accelerates towards the surface (Fig. 1). As it touches the ground, the downward flow acquires a typical divergent motion pattern, capable of producing secondary circulations and, hence, strong winds in its path (Fig. 2). These circulations can raise dust and debris, making them visible in their path, sometimes being mistaken for other type of phenomena, such as tornados. The downburst, although it is a sudden phenomenon, can affect the same place for a reasonably long period, even tens of minutes, and propagates some distance away from the starting point.
The evaluation of the observations provided by IPMA surface network suggests that several downbursts have developed in association with the passage of the mentioned cloud band. Consequently, several different locations of the Lisbon and Setúbal areas were affected during the afternoon of June 7. The values of maximum wind gust stated in Table 1 are representative only of the locations where they were measured, being possible that higher values may have affected other locations.
We draw the attention to the fact that this type of phenomenon is not that uncommon in Portugal. Naturally, if it occurs in densely populated areas, as it was the case here, it allows it to be witnessed by a large number of people.
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