Shear-driven cloudy boundary layer in the Arctic

We study multiple cloud layers in stably stratified Arctic boundary layers using observation from SHEBA experiment and large-eddy simulations. A stable Arctic cloudy boundary layer events after a passage of a synoptic low was observed on 29 July 1998. The boundary layer gradually developped during six hours and decayed when the upper clouds were advected in. The local dynamic and thermodynamic structure of the boundary layer is determined from aircraft measurements including anlysis of turbulence and cloud microphysics. After the upper cloud layer was advected over the existing cloud layer, the turbulent kinetic energy budget indicates that the cloud layer below 250m is maintained predominantly by large shear production. In this case the cloud top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our large-eddy simulation also demonstrates importance of shear in maintenance of the lower cloudy layer. In addition, we found that once the cloud reaches a certain height depending on the amount of cloud-top cooling, the two sources of production begin to separate in space. Due to the strong shear mixing at the surface, a more strongly stratified layer develops and causes another cloud layer near the surface, which is one of possible mechanisms of formation of multiple cloud layers in the Arctic.