A Prognostic-Stochastic and Scale-Adaptive Cumulus Convection Closure for Improved Tropical Variability and Convective Gray-Zone Representation in NOAA’s Unified Forecast System (UFS)

A new prognostic closure is incorporated and evaluated within NOAA's Unified Forecast System (pre-operational prototypes of 13 km GFSv17 and 25 km GEFSv13). Departing from previous assumptions of negligible subgrid area fraction and statistical quasi-equilibrium at steady state, the new parameterization targets aspects that are typically not addressed in traditional cumulus convection parameterizations. The new closure introduces a prognostic evolution of the convective updraft area fraction based on a moisture budget. Together with the buoyancy-driven updraft vertical velocity, it effectively completes the cloud-base mass flux. The updraft area fraction allows for a scale adaptive strength of the cloud-base mass-flux. Furthermore, the closure (and convective initiation) accounts for stochasticity and incorporates a representation of subgrid convective organization using self-organizing properties of cellular automata, features that have been shown to be important for improving tropical variability in NOAA’s Unified Forecast System.