  Clustering of tropical thunderstorms constitutes an important climate
feedback because it influences the heat radiated to space. Convective
self-aggregation (CSA) is a profound modelling 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, even over the sea. Here we mimic
oscillating surface temperatures in cloud resolving numerical experiments and
show that, in the presence of a diurnal cycle, CSA is enabled by high
resolutions. We attribute this finding to vigorous combined cold pools emerging
in symbiosis with mesoscale convective systems. Such cold pools suppress
buoyancy in extended regions ($\mathbf{\sim 100}$ km) and enable the formation
of persistent dry patches. Our findings help clarify how the tropical cloud
field forms sustained clusters under realistic conditions and may have
implications for the origin of extreme thunderstorm rainfall and tropical
cyclones.
