Convective self-organization is one mechanism that can lead to extreme precipitation events, such as those associated with mesoscale convective systems or even tropical cyclones. It is so far not settled under which exact circumstances convective self-organization occurs and how it might be impacted by ongoing global temperature changes. Yet, flooding resulting from such systems is putting humans at risk and predictions of convective extreme rainfall are still very difficult and imprecise.
Here we discuss conceptual mechanisms that can lead to clustering of
individual convective precipitation cells into larger populations. One key mechanism is that of cold pool interactions by which the density current outflows from existing convective cells can help set off additional convective cells in the surroundings. After presenting some basic interaction mechanisms we examine the role played by the continental diurnal cycle - with temperature ranges exceeding 10K not uncommon in many semi-arid regions. Indeed, our simulations and conceptual modeling show that mesoscale convective systems naturally emerge under the diurnal cycle when cold pool interactions are present. Furthermore, under multiday diurnal forcing, system-scale persistent clustering appears - reminiscent of the celebrated convective self-aggregation. We show that a hysteresis effect emerges, by which "our" diurnal self-aggregation persists when surface boundary conditions are suddenly set constant, i.e. the diurnal cycle remove.
Our results may have implications for the advection of mesoscale convective systems from the continent to the ocean, where they could then further develop into tropical cyclones - such as from the African mainland into the Atlantic Ocean. Time permitting, we give an outlook on ongoing field work in the west African Sahel region within the ERC-project DakE.