Arctic clouds - interactions with radiation and thermodynamic structure

Abstract: Clouds play in important role in the climate system through their interaction with radiation. Globally, clouds tend to cool the Earth by reflecting solar radiation and shading the surface. Over the Arctic, clouds tend to have the opposite impact, where they instead warm the surface through the cloud greenhouse effect because the surface is generally quite reflective. The magnitude and overall effect of clouds on the surface varies significantly with the surface, cloud and thermodynamic characteristics and can have large impacts on the energy budget at the surface.Low-level central-Arctic stratus clouds interact with the thermodynamics in a manner differently than sub-tropical stratus. Observations from several Arctic observatories indicate that these clouds penetrate and persist within stable temperature inversion structures, rather than being limited to the base of the stable layer as observed in the subtropics. It is hypothesized that such interactions with the thermodynamics can impact for example the cloud phase, lifetime, and their relationship with the sub-cloud layer and surface. Analysis indicates both the thermodynamic setting and the cloud properties affect the vertical location of the cloud top relative to inversion base. Hypothetical longwave radiative impacts resulting from liquid water redistributions are identified and discussed.Clouds primarily influence the energy at the surface via interactions with radiation. Measurements from the central Arctic suggest that the transition of season from melting to freezing was largely determined by the presence, or absence, of liquid-containing clouds and the incumbent cloud longwave warming effect. The components affecting the cloud-radiative forcing are described with relation to the energy budget and the change of season. Additionally, the influence of altering cloud condensation nuclei as a mechanism for limiting cloud liquid water is shown to have strong influences on surface temperature and lower atmospheric stability.Finally, regional climate models, RCMs, are evaluated against an annual dataset to assess the ability of RCMs to represent cloud and radiation processes in the Arctic. It is shown that both inter-model and model-observation spread are rather significant. Biases in the cloud representations yield distinct biases in the radiative fluxes, and can result in significant local climate variations solely through these parameters.