Radiative Closure Studies Over the Arctic During Polarstern Expedition PS106

Recent warming in the Arctic has been leading to a dramatical decrease of sea ice/snow in recent years. Several studies suggest that this trend is expected to continue in the upcoming years. The role of clouds in sea ice/atmosphere interactions is still not fully understood, and leads to large uncertainties in predicting the Arctic climate. To improve our understanding of the role of clouds, an unique combination of active and passive remote sensing observations and radiative flux measurements were performed onboard the research vessel POLARSTERN (PS) during the PS106 expedition (25 May - 21 July, 2017). This study builds upon the collected observations and aims to evaluate radiative closure using the single-column Rapid Radiative Transfer Model (RRTMG). Forecast profiles of atmospheric temperature and humidity are combined with vertical profiles of cloud microphysical properties retrieved from Cloudnet-processed ground-based remote sensing measurements to estimate surface radiative fluxes. The simulated radiative fluxes are then compared to in-situ observations to assess our ability to close the radiative budget in an Arctic environment during summer time. This method is applied to specific cases considering single and multiple layers with single and mixed-phase clouds over open water and over sea ice to study the resulting cloud radiative effect. Additionally, the sensitivity of surface radiative fluxes to changes in the cloud properties is investigated and traced back to its origin using error propagation techniques.