Aerosol-radiation interactions on global to local scales

Black carbon (BC) aerosols absorb solar radiation and thus heat the atmosphere. This process occurs on a short time scale and can influence clouds, precipitation, and boundary layer meteorology. Global climate models (GCMs) strongly disagree in their representation of such rapid adjustments, adding to the uncertainty in the effective radiative forcing (ERF) due to BC. Disagreements are at least partly caused by differences in parametrization of the highly regional and potentially non-linear rapid adjustments, which are poorly resolved in coarse resolution GCMs. Knowledge of how the rapid adjustments depend on model resolution is therefore important.

Here we explore the dependence of rapid adjustments on model resolution by performing a set of idealized experiments using a GCM, the Community Earth System Model version 2 (CESM2) with fixed sea-surface temperatures, downscaled by a regional climate model, the Weather Research and Forecasting (WRF), for five years at 45 km horizontal resolution over East and South Asia and at 15 km resolution covering East China. To ensure a sufficient climate response in the models, we perturb BC emissions by a factor of ten, and compare the results to separate simulations with a fivefold increase in (scattering) sulfate emissions and a doubling of CO₂ concentrations.

Preliminary results indicate similar BC-induced responses between CESM2 and 15 km WRF simulations in terms of tropospheric temperature and humidity, and mean precipitation, but strong dependence on model and/or resolution in the cloud response to BC. Potential differences in seasonal and extreme precipitation will be examined, and we plan to explore finer scales using the WRF model in Large Eddy Simulation (LES) mode down to 100 m horizontal resolution.