Gravity waves, land surface and convective initiation

Close analysis of the weather situation among the researchers and forecasters in SWIFT Testbed 3 has been uncovering some of the processes and patterns controlling the development of thunderstorms in West Africa.

By looking at the patterns of clouds and storms as they develop during the day, and relating these to patterns of soil moisture on the underlying land surface, we can see how the atmosphere and surface are interacting.
Big storms disturb the atmosphere to create ‘gravity waves’ moving outwards and causing the air to move up and down as each wave passes. Looking from space, in satellite images, the effects look like disturbances on a water surface – some waves can be seen spreading in circles over hundreds of kilometres away from a storm. When a storm is moving quickly, waves can appear as a ‘ship wake’ behind the storm.

These wave patterns are important because they cause the air to rise and fall, and when the air rises, it can trigger a new thunderstorm system. This is especially likely when the wave passes over a ‘hot spot’ in the surface, such as a mountain or a region of hot, dry soil. On 14th September, we observed many cases where storms generated waves, the waves moved hundreds of kilometres, and triggered new storms over surface hot-spots.

The importance of these results is that they help forecasters to spot patterns indicating the likely location (hot-spots) and timing (gravity waves) controlling the initiation of new storms. In future, we will need to improve out computational forecasting models to incorporate these effects too.

1430 UTC Colour-enhanced IR shows “ship wake” patterns. W1 and W2 are associated
with propagating storms at their “bow”, but W3 (harder to pick out) has no such deep convection at this time.
Storms over the Air mountains in Niger, and on a line to the SW of this (dashed ellipse), were initiated when the wake patter n W 1 met the
Initiations of storms in southern and eastern Ghana (dashed ellipse) are all located on the wave fronts.

Douglas J Parker, University of Leeds, 15.09.2021