Large eddy simulations of Arctic stratus clouds

Mixed-phase Arctic stratocumulus clouds are ubiquitous to the region during the summer months. However, despite their prevalence, very little is known about the processes which maintain the cloud. Recent observations have shown that Arctic stratocumulus commonly extend into the temperature inversion which caps the Arctic boundary layer. This is atypical to sub-tropical stratocumulus where the cloud top is found in the vicinity of the inversion base, and unexpected as strong longwave radiative cooling would be
expected to keep the cloud top and inversion base heights in equilibrium. Uniquely to the Arctic, inversions in speci�c humidity are also commonly observed coincident with
temperature inversions, and this is thought to contribute to the clouds' subsistence in the strongly stable inversion layer. In this thesis, observations from the Arctic Summer Cloud Ocean Study (ASCOS) are used to characterize the lower Arctic atmosphere and provide the basis for simulations of stratocumulus cloud encroachment into the Arctic temperature inversion. Observations show that cloud extending into the inversion by more than 100 m was a common
occurrence during ASCOS, which is consistent with measurements made during previous summer field campaigns. Simulations made with the Met Office Large Eddy Model
(LEM) were used to model the encroachment, and results suggest that the depth of encroachment has a high correlation with the humidity inversion strength.
A number of different cloud-inversion regimes were identi�ed from the model simulations. When specific humidity fell of inside the temperature inversion, the high relative humidity of the region just above the inversion base was found to allow encroachment of cloud up to 40 m into the inversion layer. While in the presence of a speci�c humidity inversion the encroachment was larger reaching a maximum of 200 m. The presence of specific humidity inversions and their relationship to the encroaching cloud was determined to be self-sustaining, and the cloud found to remain at a quasi-stable depth for as long as a moisture source is available to replenish the loss of water from ice precipitation. However, encroachment of cloud into the inversion was shown to cause a signi�cant reduction in the buoyant production of TKE at cloud top, which led to turbulence shutting off completely in the clouds with the largest encroachment depth. This caused a thermal adjustment of the inversion layer to the cloud which led a reduction in the encroachment depth. The overall impact of encroachment on boundary layer turbulence was found
to be significant, with TKE reduced by up to 90% in the simulations with the largest encroachment depth.