Abstract
Abstract. Observations suggest that processes maintaining subtropical and Arctic stratocumulus differ, due to the different environments in which they occur. For example, specific humidity inversions (specific humidity increasing with height) are frequently observed to occur near cloud top coincident with temperature inversions in the Arctic, while they do not occur in the subtropics. In this study we use nested LES simulations of decoupled Arctic Mixed-Phase Stratocumulus (AMPS) clouds observed during the DOE Atmospheric Radiation Measurement Program's Indirect and SemiDirect Aerosol Campaign (ISDAC) to analyze budgets of water components, potential temperature, and turbulent kinetic energy. These analyses quantify the processes that maintain decoupled AMPS, including the role of humidity inversions. Key structural features include a shallow upper entrainment zone at cloud top that is located within the temperature and humidity inversions, a mixed layer driven by cloud-top cooling that extends from the base of the upper entrainment zone to below cloud base, and a lower entrainment zone at the base of the mixed layer. The surface layer below the lower entrainment zone is decoupled from the cloud mixed-layer system. Budget results show that cloud liquid water is maintained in the upper entrainment zone near cloud top (within a temperature and humidity inversion) due to a down gradient transport of water vapor by turbulent fluxes into the cloud layer from above and direct condensation forced by radiative cooling. Liquid water is generated in the updraft portions of the mixed-layer eddies below cloud top by buoyant destabilization. These processes cause at least 20% of the cloud liquid water to extend into the inversion. The redistribution of water vapor from the top of the humidity inversion to its base maintains the cloud layer, while the mixed layer-entrainment zone system is continually losing total water. In this decoupled system, the humidity inversion is the only source of water vapor for the cloud system, since water vapor from the surface layer is not efficiently transported into the mixed layer. Sedimentation of ice is the dominant sink of moisture from the mixed layer.
Highlights
Arctic mixed-phase stratocumulus (AMPS) are observed to occur approximately 45 % of the time on the North Slope of Alaska, with a significant increase in occurrence during spring and fall transition seasons (Shupe, 2011)
In this paper we use high-resolution nested large eddy simulation (LES) simulations to quantify the processes involved in the maintenance and persistence of a single-layer, decoupled AMPS that was observed near Barrow, Alaska, during Indirect and SemiDirect Aerosol Campaign (ISDAC) on 8 April 2008
A mean temperature inversion of 5 K and humidity inversion of 0.4 g kg−1 with a base at 1.2 km was simulated in the LES at 20 Z, similar to the radiosounding taken at 17:34 Z that showed a mean temperature inversion of 4 K and humidity inversion of 0.5 g kg−1 with a base at 1.1 km
Summary
Arctic mixed-phase stratocumulus (AMPS) are observed to occur approximately 45 % of the time on the North Slope of Alaska, with a significant increase in occurrence during spring and fall transition seasons (Shupe, 2011). Solomon et al.: Moisture and dynamical interactions maintaining decoupled Arctic mixed-phase stratocumulus maintenance of these clouds, and that the relative contributions by these mechanisms may differ from spring to fall. This idea is supported by mesoscale model simulations of AMPS observed during the fall Mixed-Phase Arctic Cloud Experiment (MPACE, Morrison et al, 2008), where liquid water paths (LWPs) in AMPS during periods of open water were found to be less sensitive to changes in cloud condensation nuclei (CCN) than for clouds in modeling studies over sea ice (e.g., Pinto, 1998; Harrington et al, 1999; Jiang et al, 2001).
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