Saharan rainfall climatology and its relationship with surface cyclones

The Sahara is the largest and driest of the hot deserts on Earth, with regions where rainfall reaches the surface on average less than once a year. Water resources are scarce, and rainfall tends to occur sporadically in space and time. While rain is a precious resource in the Sahara, heavy precipitation events (HPEs) in the desert have the potential to trigger flash floods on the barren soil. Because of the sparse rainfall monitoring network and the relatively poor performance of global models in representing rainfall over the Sahara, the analysis of Saharan HPEs has primarily relied on case studies. Therefore, general rainfall characteristics of Saharan HPEs are unexplored, and the prevailing weather conditions enabling such rainfall are unknown. To tackle this problem, we utilised satellite-derived precipitation estimations (IMERG) spanning 21 years (2000–2021) to identify ∼42⋅103 small (>103km2) to large (<106km2) HPEs in the Sahara and to extract their rainfall properties, and atmospheric reanalyses (ERA5) to examine the corresponding meteorological conditions in which they develop. Three case studies illustrate the relevance of cyclones for exceptionally large HPEs, including one in the driest region of the Sahara. Saharan HPEs occur, on average, every second day. They are more common in summer than in the other seasons, occur most frequently in the southern Sahara, and exhibit a clear convectively-driven diurnal cycle. Winter events have, on average, larger spatial extent, longer duration, and are characterised by larger areas exhibiting more extreme rainfall in terms of return periods. Autumn HPEs are concentrated in the western Sahara, while events in the north of the desert and in its driest core in the northeast occur mainly in winter and spring. In these regions, north of the Tropic of Cancer, events are highly associated with surface cyclones. HPEs that were associated with cyclones are characterised by larger spatial extent and rainfall volume. Considering that weather and climate models often depict synoptic-scale weather systems more accurately than rainfall patterns, the association of Saharan HPEs with surface cyclones and other synoptic-scale systems can aid in comprehending the effects of climate change in the desert. Furthermore, it underscores the potential for higher predictability of these events.