Response of Convective Thermodynamics to Changing SST in a Radiative Convective Equilibrium Simulation Analyzed using an Isentropic Streamfunction

Climate models predict tropical relative humidity is insensitive to changing sea surface temperatures (SSTs). This result implies that specific humidity increases at a rate in-line with Clausius-Clapeyron scaling of 7% per K. Specific humidity plays an important role in selecting the efficiency with which the atmosphere can extract kinetic energy from heating. Radiative Convective Equilibrium simulations were performed with a cloud resolving model for fixed SSTs ranging from 290 K to 310 K. This provides a simple testbed to analyze convection's response to SST changes.

The technique called MAFALDA is then applied. By averaging and transforming the Eulerian model output into a two-dimensional thermodynamic space -- height and moist static energy (MSE) -- the energetics of convection can be diagnosed from the formation of an isentropic streamfunction. Closed streamlines represent the "thermodynamic cycles" of convection. The work done during a cycle can be identified with kinetic energy generation, water lifting, and a penalty arising from phase changes away from saturation. The relative importance of this penalty is found decrease with deepening convection. Therefore, while the total convective mass flux is found to decrease by ~4% per K, overall kinetic energy generation is found to become more efficient as convection deepens with warmer SSTs. In these thermodynamic coordinates, the shape of the streamlines directly provides useful information. Since MSE is nearly conserved under adiabatic transformations, the diabatic processes acting on a parcel can be inferred from the spacing and slopes of the streamlines. The depth of convection can be inferred from the height of a streamline and the moisture content from its maximum width.