Exploring the hysteresis of tropical diurnal self-aggregation under realistic wind shear conditions

Convective self-aggregation (CSA) is when tropical deep convection self-organizes in radiative-convective equilibrium (RCE) simulations whereby the moisture field spontaneously separates into strongly convecting and dry, subsiding, subregions.   Departing from the classical, constant sea surface temperature, RCE setup, a recent study found that including a diurnal cycle to mimic surface temperature variations between night and day over tropical land, e.g. the Sahel region, enables the onset of CSA. In contrast, corresponding simulations with constant surface temperature, which might emulate the atmosphere over the ocean, showed no strong aggregation (Kruse et al., 2025, Jensen et al., 2022, Haerter et al., 2020). Furthermore, once formed, the “diurnal self-aggregation” remained in place, when surface temperature was then set constant, suggesting a hysteresis effect. 

We here explore how this diurnal effect is modified by wind shear - a crucial ingredient for realistic Sahelian conditions. We conduct idealized cloud resolving simulations of the tropical atmosphere using the System for Atmospheric Modeling (SAM), version 6.11 (Khairoutdinov and Randall, 2003). The simplified boundary conditions include an irrotational RCE atmosphere with doubly periodic lateral boundaries. We mimic conditions over (i) land by prescribing diurnal sinusoidal surface temperature oscillations and (ii) over the ocean by prescribing a constant sea surface temperature. To mimic wind shear we nudge towards  a  prescribed wind profile which is based on ERA5 data from tropical northern Africa. 

In simulation runs (Kruse, 2024), we observe that the level of aggregation and other variables oscillate over time. With our simulations we investigate how these frequencies relate to the chosen domain. Additionally, we explore whether not only the simulations with a diurnal cycle but also simulations with realistic wind shear and a constant sea surface temperature show CSA after multiple weeks. Our analysis has relevance for the understanding of convective clustering over tropical land and the persistence of such clusters when advected over the tropical ocean - thus harboring conclusions for tropical cyclogenesis. 

Haerter, Jan O., Bettina Meyer, and Silas Boye Nissen. "Diurnal self-aggregation." NPJ Climate and Atmospheric Science 3.1 (2020): 30.

Jensen, Gorm G., Romain Fiévet, and Jan O. Haerter. "The diurnal path to persistent convective self‐aggregation." Journal of Advances in Modeling Earth Systems 14.5 (2022): e2021MS002923.

Khairoutdinov, Marat F., and David A. Randall. “Cloud Resolving Modeling of the ARM Summer 1997 IOP: Model Formulation, Results, Uncertainties, and Sensitivities.” Journal of the Atmospheric Sciences 60, no. 4 (2003): 607–25. 

Kruse, Irene L. “Chasing the Storms. A Simulation and Observation-Based Exploration of  Mesoscale Convective Systems and Cold Pools, from the Midlatitudes to the Tropics.” PhD Thesis, University of Copenhagen, 2024.

Kruse, Irene L., Romain Fiévet, and Jan O. Haerter. "Tipping to an aggregated state by mesoscale convective systems." Journal of Advances in Modeling Earth Systems 17.3 (2025): e2024MS004369.