GREAT - Greenhouse gases, aerosols and lower atmospheric turbulence

It is a well-established fact that an increase in atmospheric concentrations of CO₂ and other greenhouse gases leads to higher surface temperatures on Earth. However, increasing concentrations of greenhouse gases, and particles (e.g., soot), has many other impacts on the Earth's climate. There is a particularly large knowledge gap in the literature regarding how greenhouse gases and particles affect atmospheric turbulence near the Earth's surface. GREAT will substantially increase our scientific understanding on this issue.

Turbulence is a topic of high societal relevance, entangled as it is with human health, physical damage to infrastructure, and the carbon cycle.

Soot particles are hazardous air pollutants and key contributors to climate change. Observations have now shown that soot particles can reduce turbulence in the lower atmosphere. These new findings could have implications for cloud formation and human health through air pollution.

A driving question for the GREAT project is whether global climate models, our best tools for understanding and projecting air quality and climate change, are able to represent these processes adequately? Do elevated concentrations of sulphate particles and CO₂ concentrations show similar connections with turbulence? GREAT seeks to answer these questions by using high-quality observations from several sites and sources in combination with atmospheric modelling tools ranging from global to microscale.

Results from a range of global climate models show that soot particles strongly influence the stability of the lower atmosphere (Myhre et al., 2018, Nat. Comm.), and this could be an important indicator of changes in turbulence. In contrast to other climate drivers (e.g., CO₂), soot particles strongly increase the stability of the lower atmosphere and this leads to reduction of the sensible heat flux. Sensible heat is the transfer of heat from the surface to the atmosphere (without phase changes) and is one of the most uncertain factors in the global energy budget. Model results further show that soot particles lead to a general reduction in surface winds, a finding that is in line with the observed reduction in lower atmospheric turbulence due to soot.

The effect of increased CO₂ concentrations in the atmosphere has been studied using the regional model Weather Research and Forecasting (WRF) with different horizontal resolutions (50 km and 10 km). The results show that for important indicators of turbulence (e.g., vertical velocity and boundary layer height), changes due to CO₂ increase could be highly dependent on model resolution (Hodnebrog et al., 2021, Clim. Dyn.). In regions dominated by convection, which largely governs the boundary layer height and influences turbulence, results even show different sign of change between 50 and 10 km resolution. This is due to the improved representation of convection in the model simulation with high resolution. These findings indicate that global climate models, and even regional models with coarse resolution (~50 km), do not represent the effect of climate change on turbulence adequately.

Numerical model experiments with the global Community Earth System Model (CESM) show clear differences in how individual climate drivers influence stability and turbulence in the lower part of the atmosphere (Stjern et al., in prep.). Regions with high level of air pollution are being investigated in more detail using the models WRF and WRF-Chem.