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.