Maintenance of springtime Arctic mixed-phase stratocumulus in nested LES simulations

Arctic mixed-phase stratocumulus (AMPS) are observed to occur approximately 45% of the time per year on the North Slope of Alaska, with a significant increase in occurrence during the spring and fall transition seasons. Due to the presence of liquid water in these clouds, they play an important role in the structure of the Arctic atmospheric boundary layer and surface energy budget. AMPS are typically observed to persist for days in both the spring, when the Arctic Ocean is essentially ice covered, and fall, when the open ocean produces large fluxes of heat and moisture into the atmospheric boundary layer. The persistence of AMPS under both strong and weak surface conditions may be an indication that the mechanisms that maintain these clouds differ during spring and fall. However, there are also studies that indicate that a similar mechanism may be operating in spring and fall to maintain AMPS, since in cases with and without open water, for example, Pinto (1998) observed turbulent mixing and updrafts within the boundary layer that were forced by cloud top radiative cooling.

AMPS have not been studied as extensively as stratocumulus that occur in regions of the descending branch of the Hadley circulation over relatively cool subtropical oceans. Observations indicate that the processes that maintain subtropical and Arctic stratocumulus differ, due to the different environments in which they occur. For example, in the Arctic, humidity inversions are frequently observed to occur at cloud top, causing turbulence to entrain moist air into the cloud layer, while in the subtropics subsidence at cloud top mixes dry air into the cloud layer, capping the cloud layer and limiting entrainment.

In this presentation we present results from nested LES simulations of AMPS during the ARM DOE Indirect and Semidirect Effect of Aerosols Campaign (ISDAC). Budgets of cloud water, cloud ice, vapor, and equivalent potential temperature are used to quantify the processes that maintain the AMPS. A conceptual mixed-layer model of AMPS is proposed and contrasted with mixed-layer model studies of subtropical stratocumulus.