Understanding and Testing the Aerosol Radiative Forcing Responses of a Global Model for the North Atlantic Region as Part of ACSIS.

Climate variability in the North Atlantic region has been shown to potentially influence important processes such as hurricane activity and droughts. Global model simulations have identified aerosol-cloud interactions as a main driver of this variability. However, aerosol-cloud interactions represent one of the main uncertainties in global climate forcing suggesting that caution is needed in interpreting the results from coarse resolution, highly parameterized global models. We aim to test the robustness of the response using observations and high-resolution modelling.

We investigate the types of meteorological situations in which the surface indirect forcing in the North Atlantic region is the strongest, and its seasonality, using the UK Met Office UKCA global model in nudged mode (i.e., using realistic meteorology) in order to prioritize conditions that should be targeted for detailed model evaluation and improvement. The results show that the indirect forcing is strongest in the month of August for the year examined; reasons for this are explored. The highest mean forcing was found to occur when the low-altitude cloud coverage was 30%, but such cloud coverage occurs quite rarely in the model. Overall, situations with 100% low-altitude cloud coverage were found to contribute the most to the indirect forcing. Mid and high-altitude clouds were found to have limited impact. The realism of the model cloud coverage (along with other cloud properties such as liquid water path, cloud droplet concentrations and cloud heights) vs observations is also explored using the COSP satellite simulator since the correct simulation of cloud properties, location and timing is important for determining the indirect response.

Ongoing work is exploring the use of more sophisticated meteorological classification using clustering algorithms and the quantification of indirect effects relative to cyclone locations using cyclone centre compositing. Sets of case studies will be simulated using a nested high resolution version of the global model (based on the same physical framework) in order to further test and improve the global model indirect aerosol response.