Abstract
Abstract. The Arctic climate is changing; temperature changes in the Arctic are greater than at midlatitudes, and changing atmospheric conditions influence Arctic mixed-phase clouds, which are important for the Arctic surface energy budget. These low-level clouds are frequently observed across the Arctic. They impact the turbulent and radiative heating of the open water, snow, and sea-ice-covered surfaces and influence the boundary layer structure. Therefore the processes that affect mixed-phase cloud life cycles are extremely important, yet relatively poorly understood. In this study, we present sensitivity studies using semi-idealized large eddy simulations (LESs) to identify processes contributing to the dissipation of Arctic mixed-phase clouds. We found that one potential main contributor to the dissipation of an observed Arctic mixed-phase cloud, during the Arctic Summer Cloud Ocean Study (ASCOS) field campaign, was a low cloud droplet number concentration (CDNC) of about 2 cm−3. Introducing a high ice crystal concentration of 10 L−1 also resulted in cloud dissipation, but such high ice crystal concentrations were deemed unlikely for the present case. Sensitivity studies simulating the advection of dry air above the boundary layer inversion, as well as a modest increase in ice crystal concentration of 1 L−1, did not lead to cloud dissipation. As a requirement for small droplet numbers, pristine aerosol conditions in the Arctic environment are therefore considered an important factor determining the lifetime of Arctic mixed-phase clouds.
Highlights
The Arctic is a unique region that is highly sensitive to changes in climate (Curry et al, 1996)
The θ profiles imply that the lower half of the boundary layer transitions towards less stable and the decoupling inversion around 300 m disappears afwww.atmos-chem-phys.net/17/6693/2017/
When cloud droplet number concentration (CDNC) is reduced relative to the reference value in the control simulation, the liquid water path (LWP) time series shows a decrease to around 40 g m−2 with the CDNC 10 cm−3, and to below 10 g m−2 for CDNC set to 2 cm−3 (Fig. 7, black lines)
Summary
The Arctic is a unique region that is highly sensitive to changes in climate (Curry et al, 1996). An unique challenge with modeling Arctic mixed-phase clouds is the observed supercooled liquid layer which acts as a direct link between microphysics and dynamics by cloudtop cooling. This layer forces cloud top cooling which drives the evolution of the cloud by generating a buoyancy-driven vertical overturning (Shupe et al, 2008). Some basic aspects of our simulations follow the model setup of Ovchinnikov et al (2014), such as fixed number concentrations of both CDNC and ice crystal concentration, large-scale subsidence, and a 2 h spin-up period before ice crystal formation The advantage of this simplified approach, having fixed number concentrations of CDNC, is that the microphysical processes are constrained and can be varied in sensitivity experiments. The initial profiles were the same in all simulations except for the moisture profiles of sensitivity experiment SensMoist (see below)
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