Evaluating Direct and Semi Direct Effect of Aerosol on Subseasonal Prediction Using a Coupled UFS Model with and without Prognostic Aerosols

The interactions between aerosols, radiation and clouds are one of the key climate  uncertainties despite recent improvements in observational systems and model complexity. Here we investigate the impacts of aerosol direct and semi-direct effects on subseasonal predictions using NOAA’s coupled Unified Forecast System (UFS) – specifically, the coupled atmosphere (FV3), ocean (MOM6), and sea ice (CICE6) model combined with the GOCART aerosol component based on the GEFS-Aerosols model. We perform experiments with 32-day long integrations initialized in May and September from 2003 to 2019. Two more parallel sets of experiments are carried out as well, using either modeled climatological aerosol concentrations or zero aerosol concentration in lieu of GOCART. We found in the multi-year simulations, the estimated aerosol optical depth from the UFS-GOCART model is in good agreement with the satellite observations. The radiative forcing of the total aerosol direct effect shows negative, while the impact on precipitation is not obvious. In addition, the UFS using the modeled climatological aerosol concentrations is able to capture most of the radiative forcing seen in the UFS-GOCART experiments. This suggests a possible alternative of replacing the costly chemistry module with the modeled aerosol concentration climatology in the subseasonal applications.