The perfect storm

Yet, predicting how soil moisture, vegetation, eva poration and rainfall interact with each other is fraught with uncertainty. Thanks to measurements over the past decades, we now understand how climate affects soil moisture and evaporation from the land. Less clear has been the extent to which the suppressed evaporation driven by the dry soils in turn influences rainfall. Recent measurements by the African Monsoon Multidisciplinary Analysis (AMMA) indicate that suppressed evaporation provides a potentially powerful positive feedback: the land can amplify and prolong dry periods, leading to more intense droughts.

Weather prediction and climate models have traditionally provided much of the insight into storm generation. However, we still missed some of the pieces of the jigsaw: detail of the processes going on within storms and detail about the huge variations in land conditions across a region. Within AMMA, we addressed this lack of basic knowledge with an observational campaign designed to measure the atmospheric response to different land conditions. We exploited a characteristic of the Sahelian wet season, the tendency for storms to occur typically every three or four days. These storms travel west across the region, depositing typically tens of millimetres of rain on the land in a swath that is often hundreds of kilometres across (Figure 1).

As anticipated, our aircraft data showed increased atmospheric temperature and reduced humidity in the areas where it hadn’t rained. This was evident even when the dry areas were only five kilometres across, a surprising result given that turbulent eddies tend to mix the lower atmosphere rather effectively during the daytime. We also found that where dry soils met wet soils, the differences in temperature were large enough to affect the winds, in a manner analogous to sea breezes. Anyone who has ever sat on a beach watching the clouds will recognise the importance of such breezes for cloud development, and sometimes, sharp showers. Although generally unwelcome on the beach, the triggering of afternoon showers in the Sahel plays a critical role for the hydrology of the region. Some of these storms grow very rapidly, moving from a single cloud to a major storm covering an area the size of Wales in a matter of two or three hours. These systems produce the vast majority of Sahelian rainfall.

We had the good fortune to witness one such transformation during a research flight over northern Mali. We had set out that afternoon to take measurements above an interesting soil moisture pattern, anticipating calm conditions as had been forecast. But when we approached our target area, the radar indicated clouds directly ahead of us, forcing us to radically change our flight plan. Two hours later I was looking out of the airplane window and marvelling at the towering collection of cumulo-nimbus clouds that reached over 15 km above the ground. I was left wondering how a centimetre or two of water in the soil could possibly influence such a powerful phenomenon. Subsequent analysis of satellite data showed that soil water did indeed have a crucial role in the triggering of this storm. The storm subsequently travelled over 1500 km to reach the Atlantic Ocean. It had developed precisely where the breeze theory predicted, right along the boundary between previously wet and dry soils (Figure 2). It provided a powerful illustration of how rain on one day could trigger new storms in the region.