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[1] We present an analysis of surface fluxes and cloud forcing from data obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, conducted in the Beaufort and Chuchki Seas and the Arctic Ocean from November 1997 to October 1998. The measurements used as part of this study include fluxes from optical radiometer sets, turbulent fluxes from an instrumented tower, cloud fraction from a depolarization lidar and ceilometer, and atmospheric temperature and humidity profiles from radiosondes. Clear-sky radiative fluxes were modeled in order to estimate the cloud radiative forcing since direct observation of fluxes in cloud-free conditions created large statistical sampling errors. This was particularly true during summer when cloud fractions were typically very high. A yearlong data set of measurements, obtained on a multiyear ice floe at the SHEBA camp, was processed in 20-day blocks to produce the annual evolution of the surface cloud forcing components: upward, downward, and net longwave and shortwave radiative fluxes and turbulent (sensible and latent heat) fluxes. We found that clouds act to warm the Arctic surface for most of the annual cycle with a brief period of cooling in the middle of summer. Our best estimates for the annual average surface cloud forcings are À10 W m À2 for shortwave, 38 W m À2 for longwave, and À6 W m À2 for turbulent fluxes. Total cloud forcing (the sum of all components) is about 30 W m À2 for the fall, winter, and spring, dipping to a minimum of À4 W m À2 in early July. We compare the results of this study with satellite, model, and drifting station data.

[1] The temporal distributions of cloudiness, vertical distribution of cloud boundary heights, and occurrence of liquid phase in clouds are determined from radar and lidar data sets collected from October 1997 to October 1998 during the Surface Heat Budget of the Arctic Ocean (SHEBA) project. The radar/lidar combination was necessary for comprehensive cloud detection over a variety of physical conditions and is significantly more detailed (5-9 s temporal resolution, 30-40 m vertical resolution) than measurements made by surface observers or satellites. The combined measurements revealed that clouds were almost continuously present, with an annual average occurrence of 85%, and displayed an overall annual trend of a cloudier summer and clearer winter. A monthly averaged cloud occurrence maximum of 97% was observed in September and a minimum of 63% was observed in February. Monthly averaged lowest cloud base heights were between 0.25 and 1.0 km above ground level (agl) and monthly averaged highest cloud top heights were between 2.5 and 5.5 km agl, and displayed no significant seasonal variation. The number of cloud layers was typically 1 or 2, with the summer months tending to be multilayered. The lidar utilized depolarization ratios to detect liquid water; the percentage of lidar-observed clouds containing liquid was 73% for the year. The least amount of liquid water phase was observed during December in 25% of the lidardetected clouds and the maximum was observed during July in 95% of the lidar-detected clouds. Liquid was distributed in a combination of all-liquid and mixed phase clouds, and was detected at altitudes as high as 6.5 km agl and at temperatures as low as À34°C.

A year/long ice camp centered around a Canadian icebreaker frozen in the arctic ice pack successfully collected a wealth of atmospheric, oceanographic, and cryospheric data.

Research involving a yearlong drift with the ice pack in the Arctic Ocean witnessed surprisingly thin ice at the start and even thinner ice at the end. Also, the extent of open water during the summer of 1998 in the Beaufort and Chukchi Seas was the greatest of the past 2 decades. As the ice is melting from under your feet there is an understandable tendency to blame global warming. But the project, known as the Surface Heat Budget of the Arctic Ocean (SHEBA), though motivated by climate change, was not designed to detect global warming. Definitive climate change pronouncements can not be made based on a single experiment.

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.

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