Extensive convective outflow in the south of mainland Portugal, June 7th, 2015
2015-11-19 (IPMA)
As previous news published by IPMA on its website, the 7th of June was marked by the occurrence of strong wind gusts in the southern region of mainland Portugal. The first available evidence pointed to occurences with focus on the Lisbon and Setúbal area while further study revealed that it was, in fact, a larger scale phenomena. It affected a huge area comprising the south of Portuguese territory and the Spanish provinces of Extremadura and Andaluzia.
The use of products of numerical weather prediction models as well as remote sensing products (satellite, radar), surface observations and rawinsond, allowed to characterize the atmosphere with a focus in the area located to the southwest of Iberia. Thus, it allowed to diagnose the cause for numerous high wind reports produced in the area of interest in the territory as well as to monitor the propagation of the disturbance over the territory.
Over the Atlantic ocean, particularly from the early hours of that day onwards, the atmosphere supported the development of clouds with vertical extension but with particularly high base; the atmospheric instability was modest but it was available up to high levels. Additionally it was found that the atmosphere had a high content of precipitable water. In this context it would be fair to assume a high probability of development of clouds with great precipitating potential, that is, able to produce large precipitation amounts. However, it was also found the presence of two layers of dry air, the lowest of which coincided with the atmospheric volume located below the cloud base and the highest overlapped the upper third of the mentioned clouds.
All these ingredients were crucial to the events that came to occur. The mixture of dry air pockets with the neighbor cloud mass, at high altitude, generates strong evaporation and sublimation of hydrometeors present at such high levels (water droplets, rain drops and snow and ice crystals). As a consequence an extensive dissipation of the cloud tops can occur, followed by latent heat release from the dry air involved in the process. Given the large vertical extension of the atmosphere in which these phenomena were observed, it was possible to admit the formation of organized and strong descending air currents. These currents would originally be rich in hydrometeors (including rain drops) but, on lower levels, they should have already lost some water content and this would be enhanced by the presence of a dry air layer located between the cloud base and the surface. These descending air currents of cold air (but with no precipitation already) while suffering acceleration towards the surface, became converted into horizontal flows of relatively moist air when hitting the ground. Then, tend to propagate at low levels in turbulent regime. Under favorable conditions, these flows can travel a large distance from the source location.
Under these conditions and during the early afternoon of that day, reflectivity observations performed by the Loulé/Cavalos do Caldeirão radar (L/CC) revealed that in a region located southwest of Cape Sagres, a sudden cloud tops decrease was noticed as illustrated by the animation loop depicted in Figure 1. This phenomenon materialized shortly after by the propagation of what is known in meteorology as a convective outflow – it is an horizontal flow, usually turbulent in character, that was originally produced by an organized downdraft associated with convective activity.
The availability of surface observations in a relatively dense network over land, allowed to identify various instants at which the disturbance affected each location and, thus, the representation in the form of an isochronous map. This map, shown in Figure 2, depicts lines of points that were affected by the phenomenon at approximately the same time. The arrival of the outflow to a certain location is associated to the substitution of the air mass which was previously over it by cooler air (therefore denser, thus heavier) and more humid. This substitution is often accompanied by strong wind gusts. In the current episode, these gusts reached values close to 80 km/h, as Figure 3 illustrates for the case of the Instituto Geofísico do Infante D. Luís (IGIDL, Lisbon). Under favorable conditions, the weather radar systems can identify lines of reflectivity resulting from the convergence processes associated with the propagation of convective outflows. Figure 4 shows the location of the thin reflectivity line observed by the L/CC radar at different times, during the afternoon of June the 7th, jointly with the isochronous map already detailed in Figure 2. It is interesting to confirm, on a comparative analysis, the almost perfect coincidence between a certain isochronous location and the thin reflectivity line location in a nearby instant. Any of these representations depicts the outflow propagation from the southwest to the northeast.
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