Using the Weak Temperature Gradient Approximation to Diagnose Multiple Equilibria across a range of Sea Surface Temperatures

The weak temperature gradient (WTG) approximation is a useful tool for parameterizing the influence of the large scale environment on convection in limited domain simulations. In nature, gravity waves redistribute heating anomalies to maintain weak horizontal temperature gradients in the tropics. In models, this effect is achieved by creating a WTG vertical velocity that acts to counteract heating anomalies.

The WTG approximation has been used to investigate convective organization by utilizing the analogy between multiple equilibria in precipitation and moist and dry regions of organized convection. Multiple equilibria refer to parameters which permit either a dry or precipitating steady state, depending on intial moisture conditions. Properties of the dry and precipitating states are consistent with those of dry and precipitating regions in larger domain simulations of convective self-aggregation, including circulations which result in the upgradient transport of moisture (with negative values of gross moist stability in the dry regions).

We investigated the properties of dry and precipitating steady states over sea surface temperatures (SSTs) ranging from 290 to 310 K. Our results suggest stronger convection as well as an increased likelyhood for convective organization at higher SSTs.