Multiple strong, rapid convection developed on the collision points of two convective outflow boundaries, on 19 January 2015.
28 February 2022
19 January 2015
A very intense and broad convection episode occurred over wide regions of Botswana in mid-January.
Initial convective cells formed in the early morning over central Botswana, but the outflow boundary acted as a trigger for convection initiation all over the country later in the afternoon.
Most important is to notice the convective outflow boundaries that were forming around the convective cells. They were connected to the horizontal outflow of the cold air that originated from the downdraft area of convective cells/clusters.
In the Meteosat-10 high resolution visible (HRV) animation, 19 January, 04:00–16:00 UTC, they are visible as curved bands of clouds spreading out from the centre of the convective area.
It is interesting to look for the areas where the two outflow boundaries meet, because these are often the areas of further intense and rapid convective development.
One of such can be seen in the central-west part of Botswana where two big outflow boundaries, from the main convective system, meet. This rather ´explosive´ convection can be easily observed in the zoomed-in HRV animation, 19 January, 04:00–16:00 UTC.
On the HRV image (Figure 1) further intersections of convective outflow boundaries can be seen. The orange circled areas are strong convection, which corresponds well to the meeting point/line of two outflow boundaries. These boundaries are approximated with the blue lines.
Since the area behind the outflow boundary (also known as gust front) acts like a cold pool of air this can be seen using the infrared 10.8µm channel which measures the surface temperatures, provided no clouds are obscuring the view to the ground.
The colour-coded IR 10.8 image (Figure 2) clearly shows that the surface temperature dropped down by approximately 10K behind the convective boundary layer.
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