Convectively Coupled Equatorial Wave Simulations Using the ECMWF IFS and the NOAA GFS Cumulus Convection Schemes in the NOAA GFS Model

There is a longstanding challenge in numerical weather and climate prediction to accurately model tropical wave variability, including convectively coupled equatorial waves (CCEWs) and the Madden Julian Oscillation. The difficulty stems from the complex nature of the interaction between cumulus convection and the large-scale tropical circulation. For sub-seasonal prediction, the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) has been shown to be superior to the NOAA Global Forecast System (GFS) in simulating tropical variability, suggesting that the ECMWF model is better at simulating the interaction between cumulus convection and the large-scale tropical circulation. In this study, we experiment with the cumulus convection scheme of the ECMWF IFS in a research version of the GFS for the purpose of understanding which aspects of the IFS cumulus convection scheme outperform those of the GFS convection scheme in the tropics. It is found that the IFS cumulus convection scheme produces significantly different tropical moisture and temperature tendency profiles from those simulated by the GFS convection scheme when it is coupled with other physics schemes in the GFS physics package. We show that a consistent treatment of the interaction between parameterized convective plumes in the GFS Planetary Boundary Layer (PBL) and the IFS convection scheme in the GFS physics is required in order for the GFS to replicate the tropical temperature and moisture profiles simulated by the IFS model. We further show that the GFS model with the IFS convection scheme, and the consistent treatment between convective plumes in PBL and convection schemes, produces much more organized convection in the tropics, and tends to generate tropical waves that propagate more coherently than the original GFS due to better simulated interaction between low level convergence and precipitation.