Understanding convective organization in a changing climate

Representation of tropical convection in weather and climate prediction models remains an uncertainty. Understanding convective organization---the process in which scattered convection organizes in convective patches surrounded by non-active, dry regions---can lead to better weather and climate predictions. We study the dependence of convective organization on changes in sea surface temperature (SST) in idealized simulations.

We use an idealized cloud resolving model (CRM) utilizing the weak temperature gradient (WTG) approximation, which effectively acts as a parameterization of the effects of the large-scale environment on the local environment. We represent the large-scale environment by imposing steady state radiative convective equilibrium (RCE) temperature and moisture profiles---at respective SSTs---in WTG simulations. WTG simulations exhibit multiple equilibria in precipitation for a range of imposed wind speeds---for the same boundary conditions, moist and dry initialized simulations have either a precipitating or a non-precipitating steady state. We assume a hypothesised analogy between the precipitating and non-precipitating equilibria in WTG simulations to precipitating and non-precipitating regions in convective organisation. Understanding how multiple equilibria in precipitation in WTG simulations depend on SST can lead to better understanding of convective organization in a changing climate.

We find that there are two distinct effects of SST changes on WTG multiple equilibria simulations: effects of changes in the reference environment and effects of surface fluxes as a result of changing wind speeds. First, increasing SSTs produce warmer and moister RCE reference environments. Warmer SSTs exhibit multiple equilibria over a smaller range of wind speeds compared to cooler SSTs. This may suggest that warmer climates favor unorganized convection. Second, the transition time from a dry to a precipitating equilibrium occurs earlier with stronger wind speeds. However, this effect is not SST dependent. Together these results suggest that there is a distinct difference between thermodynamic and dynamics effects of SST changes in WTG simulations, and, by inference, the real atmosphere.