Large‐scale organization of tropical convection in two‐dimensional explicit numerical simulations: Effects of interactive radiation

We examine interactions among radiative processes, water vapour, clouds, and the multi‐scale organization of tropical convection in two‐dimensional idealized cloud‐resolving simulations using planetary‐scale horizontal domains. We extend our earlier study where radiative cooling was prescribed. Therein, deep convection spontaneously organized into two primary scales: westward travelling mesoscale convective systems on a scale of a few hundred kilometres and the eastward‐propagating envelopes of convection spanning thousands of kilometres. These envelopes represented large‐scale convectively coupled gravity waves, two‐dimensional non‐rotating analogues of equatorially trapped Kelvin waves. Interactive radiation introduces a new mechanism of large‐scale convective organization. Weak overturning circulations, steered by the mean wind and of a few thousand kilometres in scale, gradually develop. This overturning is a manifestation of the baroclinic response to horizontal gradients of radiative heating established between the moist and dry regions, which causes a spontaneous positive feedback among large‐scale dynamics, water vapour, clouds, and radiation. The mechanism is as follows: the convective systems maintain the humidity (vapour and cloud) in the ascending branch which, in turn, maintains the differential radiative heating. Conversely, the large‐scale overturning provides the large‐scale dynamical forcing that maintains the deep convection. This quantifies recent idealized studies that applied convective parametrizations. The radiatively driven overturning and the deep convection are modulated by large‐scale convectively coupled gravity waves responsible for the large‐scale envelopes reported earlier. A series of sensitivity tests, including the effects of environmental shear, demonstrate the robustness of these results.