Meteosat-11 monitors squall lines over the Gulf of Guinea on 18 June 2018.
By Gore Bi Tra Olivier and Edoh Kodjo Gboneh Gratien (Aerial Navigation Safety in Africa and Madagascar)
The powerful convective systems that often cause most damage to the Gulf of Guinea region are squall lines (SQL). Well known as mesoscale convective system, they consist of a number of small storms at different phases of development joined together by curved line north-south oriented. They sometimes grow rapidly. These SQL mostly, and particularly those which concern gulf of Guinea region, initiate over RCA/Cameroun/south Nigeria during the rainy season (from April to July) and propagate all along the whole Gulf of Guinea, mainly in its southern parts.
It’s clear that due to the rotation aspect they are less strong that those which develop and propagate over the Sahel region; nevertheless, they produce more rain, mainly when they move very slowly or when they are quite stationary.
Their evolution can be well traced using satellite products and applications, as is illustrated in this case of a convective system from 18–20 June, which swept along the coast of the Gulf of Guinea from Port Harcourt through Lagos to Freetown.
This storm cluster evolved, strengthened and brought torrential rains to cities, causing severe flooding in places (see table). In Abidjan, for example, torrential rains were reported to have fallen continuously overnight for seven hours. As a result the water level rose up to 2.5 m in some places; several casualties, and at least 20 dead bodies were reported.
|Monrovia||20/06/2018||---||3–6 km||50 km/h|
|San Pédro||19/06/2018||54 mm||4–6 km||22 km/h|
|Abidjan||19/06/2018||40 mm||5–8 km||30 km/h|
|20/06/2018||15 mm||6–10 km||26 km/h|
|Accra||19/06/2018||5 mm||8 km||46 km/h|
|Lomé||18/06/2018||11 mm||3–8 km||33 km/h|
|19/06/2016||32mm||6–8 km||30 km/h|
The squall line presented on Figure 2 showed intense convective activity and virga (an observable streak or shaft of precipitation falling from a cloud that evaporates before reaching the ground), as can be seen in the animation.
Figure 2: Meteosat-11 infrared animation, 17 June 17:42 UTC–19 June 23:42 UTC
The environment was very humid (see Figure 1, top right, click to expand). Multi-cellular thunderstorms (progressing in the westward direction) evolved into a monsoon-flow of south west origin, reaching the 800 hPa level, which fed the system.
The low-level was favourable to the wind convergence and, obviously, to the generation of deep convection downstream (Figure 3).
According to the vertical wind profile, the wind speed decreases with altitude and produced a shear which helps to reinforce convection (Figure 4).
It is probably the case, the presence of dry air at middle altitude aided the strengthening of the downdraft, and, obviously, the interaction between this one and the wind shear was favourable to maintaining the life of the convective cluster, and, also, its displacement westward. There was also a relatively high value of CAPE all along its trajectory.
The combination of conditions allowed the system to spread westward to the reaches of Sierra Leone, while maintaining its structure. This convection is typical in western Africa during the rainy season from April to September.