Uncertainty and Impact of Aerosol-Cloud Interaction in the Unified Forecast System

NOAA is collaborating with the US weather and climate science community to develop the fully coupled Unified Forecast System (UFS) for both research and operations across different temporal and spatial scales. Active development and evaluation are underway to improve the representation of aerosols and its interaction with radiation and clouds in UFS applications. Aerosols, acting as cloud condensation nuclei and ice nuclei, affect cloud droplet number concentration and size, cloud lifetime and consequently cloud radiative properties. Up to now, the impact of aerosols on clouds has not been included in UFS global applications for operation. In this study we activate the interactions of aerosols with clouds in the Thompson double moment microphysics scheme used by the UFS-based Global Forecast System (GFS) application. MERRA2 aerosol climatologies instead of prognostic aerosols are employed for this study to reduce the complexity and uncertainty in aerosol prediction itself. GFS free forecasts at the 13-km horizontal resolution for the summer of 2020 were conducted to investigate the uncertainty of the representation of aerosol-cloud interaction and the associated impact on GFS medium-range weather forecasts. The experiment with aerosol-cloud interaction produced an overall larger number concentration of cloud droplets and cloud liquid mixing ratio, and larger number concentration of cloud ice and ice mixing ratio in the low-middle troposphere, but less above the upper troposphere. The relationships between aerosol optical depth and cloud droplet number concentration were analyzed and compared with MODIS retrievals. In addition, the relationships between aerosol loading and optical properties with liquid water path, shortwave and long wave radiation were examined. CCPP single column model was also used to help understand the uncertainty of aerosol-cloud interaction algorithms employed by the UFS.