Heavy precipitation events where there’s no rain: Saharan rainfall climatology and its relationship with cyclones

The Sahara is the largest and perhaps the driest desert in the world. This desert however, has not always been this dry. In fact, it is presumed that a few thousand years ago it was much wetter. Moreover, it is projected that, by the end of the 21^st century, the Sahara will exhibit the strongest relative increase in precipitation outside the polar regions. To better grasp this information, however, we need to answer some questions: wetter than what? What is the present-day rainfall climatology of the Sahara and what are the synoptic conditions during rainstorms in the desert?

Currently, rainstorms in the Sahara are considered a rare phenomenon. However, rain-bearing cyclones intruding from wetter neighboring regions are possible, and can lead to heavy precipitation events (HPEs) which cause hazardous desert floods. When rainfall occurs, the chances for it to be observed and measured at the ground are close to zero due to the scarcity of rain gauges and the small scale of the precipitation systems. Consequently, the characteristics of rainfall during Saharan rainstorms were seldom analyzed, especially at the scale of the whole desert. In this study, we use high-resolution satellite precipitation estimates (IMERG) and meteorological reanalysis (ERA5) to (a) identify thousands of HPEs that occurred over the Sahara in the past 21 years, (b) characterize rainfall properties during these events, and (c) identify the governing atmospheric conditions on HPE-days, with a focus on surface cyclones.

Our results show that HPEs may occur throughout the entire Sahara. Summer events happen mainly in the southern Sahara. They tend to be short-lived (on average ~12 h) and small in size (~8000 km²), with high-intensity convective rainfall. Conversely, winter HPEs occur primarily in the northern and western parts of the desert, they are longer (~16 h) and larger (15,000 km²) and produce higher rainfall volumes with lower rainfall intensities. When associated with cyclones (29% of events), HPEs exhibit 15% lower rainfall intensities, and 46% higher volumes. This is likely due to a much greater (+64%) areal extent. Our analysis compensates the small number of events at each location with the huge area of the desert, so that a HPE is observed on average every second day. The high-resolution datasets we use enable us to characterize small-size events, with substantial implications for the local scales. Hopefully, such an analysis can serve as a starting point to cope with natural hazards and better understand the future of HPEs in the Sahara.