The Diurnal Path to Persistent Convective Self‐Aggregation.

Clustering of tropical thunderstorms constitutes an important climate feedback because it influences the radiative balance. Convective self‐aggregation (CSA) is a profound modeling paradigm for explaining the clustering of tropical oceanic thunderstorms. However, CSA is hampered in the realistic limit of fine model resolution when cold pools—dense air masses beneath thunderstorm clouds—are well‐resolved. Studies on CSA usually assume the surface temperature to be constant, despite realistic surface temperatures varying significantly between night and day. Here we mimic the diurnal cycle in cloud‐resolving numerical experiments by prescribing a surface temperature oscillation. Our simulations show that the diurnal cycle enables CSA at fine resolutions, and that the process is even accelerated by finer resolutions. We attribute these findings to vigorous combined cold pools emerging in symbiosis with mesoscale convective systems. Such cold pools suppress buoyancy in extended regions (∼100 km) and enable the formation of persistent dry patches. Our findings help clarify how the tropical cloud field forms sustained clusters under the diurnal forcing and may have implications for the origin of extreme thunderstorm rainfall and tropical cyclones. Plain Language Summary: Computer simulations of tropical clouds suggest that thunderstorms can aggregate into large‐scale clusters even under homogeneous boundary conditions. This process, known as convective self‐aggregation (CSA), is observed in idealized simulations. Yet, the underlying mechanisms are considered relevant for real‐world atmospheric phenomena, such as tropical cyclones, and for climate feedbacks. Realism has been questioned as studies find that increasing model resolution hampers or prevents CSA. However, here we demonstrate that increasing resolution accelerates CSA if models account for surface temperature oscillations related to the natural diurnal cycle. We explain this rapid CSA with rain‐induced density currents in the atmospheric boundary layer, known as cold pools. While our current setup is still strongly idealized, these results may encourage a shift in the CSA paradigm in favor of more realistic simulations in the future. Key Points: Diurnal surface temperature oscillations can enable convective self‐aggregation in idealized simulations―especially at fine grid resolutionMechanistically, strong mesoscale cold pools dry the boundary layer, which later induces irreversible dry patches in the free troposphereOnce formed, such dry patches can persist and intensify even if the diurnal cycle is later removed [ABSTRACT FROM AUTHOR]