An Arctic Springtime Mixed-Phase Cloudy Boundary Layer Observed during SHEBA

Abstract

The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixedlayer, mixed-phase cloud with an average temperature of approximately Ϫ20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surfacebased remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 Ϯ 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud's radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m Ϫ2 with a diurnal amplitude of ϳ20 W m Ϫ2 . This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths Ͼ6.

The Influence of Cloud and Surface Properties on the Arctic Ocean Shortwave Radiation Budget in Coupled Models*

“…During the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, a year-long program in the Beaufort Sea, it was found that liquiddominant mixed-phase clouds at SHEBA were very frequent throughout the year and occurred at temperatures as low as Ϫ25° (Intrieri et al 2002;Shupe et al 2006). Cloud scenes containing liquid water strongly dominated the SW cloud effect in all sunlit seasons, while ice-only cloud scenes had very little SW shading effect (Shupe and Intrieri 2004;Zuidema et al 2005). Although the SHEBA conditions cannot be considered as the only "truth" due to high spatial and interannual variability of cloud properties, the averaged mixedphase microphysical properties observed during SHEBA are within a reasonable range of past in situ observations (Shupe et al 2006).…”

New insight into the disappearing Arctic sea ice

“…In addition to warming and increased clouds, more abundant liquid-water-containing clouds [Zuidema et a/., 2005] and increased atmospheric water vapor content [Wang and Key, 2005] also appear to be influential. Trends in satellite-derived observations (Table 1) likely are caused by a combination of increased moisture transport from lower latitudes [Groves and Francis, 2002], as well as by evaporation from additional ice-free areas and from an earlier start to the melt season [Belchansky et al, 2003] .These changes constitute a positive feedback to Arctic warming, augmenting the much antic ipated ice-albedo feedback.…”

Intercomparison of model simulations of mixed‐phase clouds observed during the ARM Mixed‐Phase Arctic Cloud Experiment. I: single‐layer cloud

“…The frequent occurrence of mixed-phase clouds has important implications for the cloud radiative forcing at the surface, since mixed-phase clouds tend to be optically thicker than ice-only clouds (Sun and Shine, 1994;Shupe and Intrieri, 2004;Turner, 2005;Zuidema et al, 2005). The presence of mixed-phase as compared to ice-only clouds may also significantly impact the structure of the boundary layer and large-scale dynamics through the influence of cloud-top radiative cooling (Morrison and Pinto, 2006).…”