Understanding changes in tropical circulations in a future warmer climate using a cloud-resolving model and a conceptual model

The interaction between large-scale tropical circulations and moist convection has been the focus of a number of studies. However, projections of how the large-scale tropical circulation may change under global warming remain uncertain because our understanding of this interaction is still limited.

Here, we use a cloud-resolving model (CRM) coupled with a supra-domain scale (SDS) parameterisation of the large-scale circulation to investigate how tropical circulations driven by sea-surface temperature (SST) gradients change in a future warmer climate. Two popular SDS parameterisation schemes are compared; the weak temperature gradient approximation and the damped-gravity-wave approximation. In both cases, the large-scale vertical velocity is related to the deviation of the simulated density profile from a reference profile taken from the same model run to radiative-convective equilibrium.

We examine how the large-scale vertical velocity profile varies with surface temperature for fixed background profile (relative SST) as well as how it varies with the surface temperature of the reference profile (background SST). The domain mean vertical velocity appears to be very top-heavy with the maximum vertical velocity becoming stronger at warmer surface temperatures. The results are understood using a simple model for the thermodynamic structure of a convecting atmosphere based on an entraining plume. The model uses a fixed entrainment rate and the relative humidity from the cloud-resolving model to predict a temperature profile. The vertical velocities calculated from these predicted temperature profiles is similar to the vertical velocity structures and their behaviour in a warmer climate that we see in the CRM simulations. The results provide insight into large scale vertical velocity structures simulated by SDS parameterisation schemes, providing a stepping stone to understanding the factors driving changes to the large-scale tropical circulation in a future warmer climate.