Modelling micro- and macrophysical contributors to the dissipation of an Arctic mixed-phase cloud during the Arctic Summer Cloud Ocean Study (ASCOS)

摘要

The Arctic climate is changing; temperature changes in the Arctic are greaterthan at midlatitudes, and changing atmospheric conditions influence Arcticmixed-phase clouds, which are important for the Arctic surface energy budget.These low-level clouds are frequently observed across the Arctic. They impactthe turbulent and radiative heating of the open water, snow, andsea-ice-covered surfaces and influence the boundary layer structure.Therefore the processes that affect mixed-phase cloud life cycles areextremely important, yet relatively poorly understood. In this study, wepresent sensitivity studies using semi-idealized large eddy simulations(LESs) to identify processes contributing to the dissipation of Arcticmixed-phase clouds. We found that one potential main contributor to thedissipation of an observed Arctic mixed-phase cloud, during the Arctic SummerCloud Ocean Study (ASCOS) field campaign, was a low cloud droplet numberconcentration (CDNC) of about 2 cm. Introducing a high icecrystal concentration of 10 L also resulted in clouddissipation, but such high ice crystal concentrations were deemed unlikelyfor the present case. Sensitivity studies simulating the advection of dry airabove the boundary layer inversion, as well as a modest increase in icecrystal concentration of 1 L, did not lead to cloud dissipation.As a requirement for small droplet numbers, pristine aerosol conditions inthe Arctic environment are therefore considered an important factordetermining the lifetime of Arctic mixed-phase clouds.