Content area
Abstract
The Arctic is of vital importance to the Earth’s climate. Clouds in the Arctic play an important role for Earth’s albedo, and thus also the energy budget of the region. In particular, Arctic low-level mixed-phase stratus clouds can persist for long periods of time before dissipating quickly. This research is conducted with the aim to investigate how Arctic clouds’ properties and existence react to changes in aerosol concentration both above and below clouds. Observational data from recent field campaigns are used to run semi-idealized large eddy simulations of these clouds. Experiments are conducted by slowly decreasing or keeping constant aerosol concentrations above and below clouds separately to understand their respective roles in controlling cloud properties. Results show that with a decrease of aerosol concentration below the cloud layer, the cloud dissipates faster with more liquid and ice precipitation. Solely decreasing aerosol concentration above the cloud layer also helps dissipate the cloud but is not as influential as that below cloud. The influence of holding aerosol concentration constant below cloud is substantial. Constantly high aerosol concentration below cloud suppresses the formation of precipitation as well as lowers in-cloud relative humidity to enhance the Wegener–Bergeron–Findeisen process, which helps the cloud dissipate by glaciation. Nonetheless, for these simulations specifically, decreasing aerosol concentration below cloud makes the cloud dissipate faster than keeping aerosol concentration constant. Expectedly, decreasing aerosol concentration both above and below cloud leads to the fastest dissipation. It seems that the cloud does not maintain itself when aerosol concentration drops below around 7 cm-3 and cloud droplet concentration below 30 cm-3. Further than that, increasing ice nuclei concentration in the domain only speeds up the dissipation process but does not alter the underlying mechanism that leads to it. Lastly, these simulated low-level clouds that are originally capped by inversion layer can occasionally overshoot the boundary layer top to inject cloud into the inversion layer. If conditions are favorable, the main cloud body and the overshooting cloud layer can subsequently merge into one, whose top extends into the inversion layer. Such structures are supported by observations, but their formation has not been previously explained.
