Effects of scale coupling frequency on convective organization in the superparameterized Community Atmospehre Model

Abstract:

The vertical structure of convective mass flux is revealed to be sensitive to global model time step (scale coupling frequency) in the uncoupled Superparameterized Community Atmosphere Model 3.0 (SPCAM3.0). As time step decreases, deep tropical convection becomes more bottom-heavy, with an associated decrease in gross moist stability. These changes in convective organization are associated with intensified tropical rainfall extremes and weakened shortwave and longwave cloud forcing in the model, showing that global model time step provides a useful tuning knob in superparameterized GCMs with effects that are distinct from normal GCMs. Interestingly, the weak temperature gradient approximation is more rigorously satisfied with a smaller time step. Although it is unknown what underlying physical processes ultimately orchestrate this shift in the nature of convective organization, circumstantial evidence is presented that may implicate a planetary scale wave – convection interaction mechanism.