On 10 July a low cloud vortex was spotted off the northwest coast of Morocco.
By Stephanie Ball (MeteoGib) and Jochen Kerkmann (EUMETSAT)
In my work as a meteorologist, based in Gibraltar, my attention has been drawn by the power of satellite imagery and what it reveals of the more intricate cloud features and processes taking place within the boundary layer. Luckily, I happen to find myself in a perfect location where the flow of wind around the Rock of Gibraltar, through the Strait and surrounding mountains gives rise to some very interesting clouds and features.
True Color RGB
True Color RGB (zoomed in)
On the 10 July one such feature had my colleagues on Twitter deep in discussion — a low cloud vortex spotted off the northwest coast of Morocco (Figure 1 and zoomed in on Figure 2).
While low cloud vortices are not a rare occurrence in this area (in fact they are quite frequent in summer) this one was quite striking because it was an almost perfect circular shape.
The presence of such cloud betrays an underlying feature which is the presence of turbulent wind eddies and, eventually in some cases, the development of a mesoscale low. While in most cases it is the wind which generates these features, the interaction of cold currents can also come into play, which must certainly be the case with those found off the northwest of Morocco.
In most cases they will originate as a layer of low cloud or fog running along the west coast of Morocco, developing over the cold upwelling area to the northwest of Morocco, and north of the Canaries current, where the Cape Ghir filament sometimes extends further north along the coast.
They tend to develop when stable easterly or levanter conditions are affecting the Strait of Gibraltar, and are more visible during the summer months when high ThetaW (wet-bulb potential temperature) air overlies the area and cold upwellings become more pronounced due to Ekman transfer of water off the coast of Morocco.
As the turbulent eddies begin to develop off the Moroccan coast and to the lee of the Rif Mountains, they may also have some interaction with the accelerated flow of the levanter passing through the narrows of the Strait and emerging as a divergent flow from the Western Strait. Before long a cloud swirl becomes evident as a mesoscale low-pressure centre begins to form, as seen in the MODIS image from 14 July (Figure 3).
On 10 July, conditions were ripe, with a strong levanter running through the Strait of Gibraltar and with a high ThetaW of 22–24 °C (Figure 4) overlying Sea Surface Temperatures (SST) of around 22 °C (Figure 5), which helps to explain the longevity of this vortex, which continued well into the day.
These wind eddies are normally quasi-stationary and most vortices will remain close to the coast, but at times they may be pushed slowly some way out to sea, whether due to the interaction with the movement of cold currents or by wind flow, eventually dissipating as they move away from the more ideal conditions or environment in which they originally formed.
With the case of 10 July, gradual entrainment of drier, warmer air from Morocco may have been behind the eventual development of the closed vortex observed, and made such a fascinating image.
On the Meteosat-10 Natutral Colour RGB animation, 10 July 05:00–18:00 UTC, the progression of the cloud patch (swirl) close to Morocco can be clearly seen.
On the Meteosat-10 High Resolution Visible (HRV) image (Figure 11) from the same day dust can be clearly seen among the cloud. The dust was triggered over Algeria by strong convective systems (thunderstorms) and then blown westward by the strong Levante winds.
Examples of some other interesting imagery observed in the Mediterranean region via MODIS imagery (source: EOSDIS Worldview), including Gibraltar levanter with rotor clouds developing downwind across the Bay of Gibraltar; lenticular cloud over the Sierra de Tejeda, Southern Spain; a ship's trail off the west coast of Morocco, and an amazing close up of the intricate Kármán Vortices which frequently develop off the Canary Islands.
Previous case studies
Forced orographic convection - Strait of Gibraltar (8 March 2014)
Von Kármán vortex streets to the lee of the Canary Islands (8 June 2005)