Self-Aggregation of Convection in Long Channel Geometry

Cloud cover and relative humidity in the Tropics are strongly influenced by organized atmospheric convection, which occurs across a range of spatial and temporal scales. One mode of organization that is found in idealized numerical modelling simulations is self-aggregation, a spontaneous transition from randomly distributed convection to organized convection despite homogeneous boundary conditions. We explore the influence of domain geometry on the mechanisms, growth rates and length-scales of self-aggregation of tropical convection. We simulate radiative–convective equilibrium with the System for Atmospheric Modeling (SAM), in a non-rotating, highly elongated three-dimensional (3D) channel domain of length >104 km, with interactive radiation and surface fluxes and fixed sea-surface temperature varying from 280–310 K. Convection self-aggregates into multiple moist and dry bands across this full range of temperatures. As convection aggregates, we find a decrease in upper tropospheric cloud fraction but an increase in lower tropospheric cloud fraction; this sensitivity of clouds to aggregation agrees with observations in the upper troposphere but not in the lower troposphere. An advantage of the channel geometry is that a separation distance between convectively active regions can be defined; we present a theory for this distance based on boundary layer. We find that surface fluxes and radiative heating act as positive feedback mechanisms, favouring self-aggregation, but advection of moist static energy acts as a negative feedback, opposing self-aggregation, for nearly all temperatures and times. Early in the process of self-aggregation, surface fluxes are a positive feedback at all temperatures, shortwave radiation is a strong positive feedback at low surface temperatures but weakens at higher temperatures and longwave radiation is a negative feedback at low temperatures but becomes a positive feedback for temperatures greater than 295–300 K. Clouds contribute strongly to the radiative feedback, especially at low temperatures.