Master Thesis Defense by Dana Lüdemann

Title: "Impact of snow albedo parameterization over the Greenland ice sheet on the simulated climate of EC-Earth3"

The albedo of snow and ice has a strong positive feedback loop that controls the amount of solar radiation absorbed by the surface. Snow melt induced by surface warming will lead to a decreased albedo that allows for more solar radiation to reach the surface and will further increase the melting and surface warming. In current climate models, the albedo over perennial snow is kept constant and thus, can not represent this feedback. In the recent past, a more realistic snow albedo parameterization has been incorporated into the global climate model EC-Earth3 over the Greenland ice sheet. To analyze and understand the impact of this new parameterization, three experiments were performed using the atmospheric general circulation model (A-EC-Earth3). Each experiment - (1) a control experiment, (2) an experiment using new albedo scheme and (3) an extreme experiment with a low albedo of 0.6 over perennial snow, contains three members which only differ in their initial state. Locally, the new albedo parameterization introduces a seasonal cycle of albedo over Greenland and Ellesmere Island, with an overall decreased snow albedo compared to the control simulations. The new albedo scheme is responsive to changes in temperature and precipitation, which results in local reductions of the temperature biases of EC-Earth3. However, the albedo scheme shows deficits in the snow albedo estimation over the Greenland ice sheet. The local changes in surface energy fluxes suggest that the lowered albedo leads to a larger uptake of energy by the surface, while highlighting that the applied albedo forcing is small. The remote response in near surface temperature, mean sea level pressure and 500 hPa geopotential height shows no systematic pattern. Moreover, only two out of three ensemble members show a significant response to the albedo forcing. The found anomalies can be explained by the large internal variability of the atmosphere, raising awareness to its role when investigation atmospheric responses to forcings.

Censor: Peter Aakjaer