No abstract

No abstract

[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.

Melting of the world's major ice sheets can affect human and environmental conditions by contributing to sea-level rise. In July 2012, an historically rare period of extended surface melting was observed across almost the entire Greenland ice sheet, raising questions about the frequency and spatial extent of such events. Here we show that low-level clouds consisting of liquid water droplets ('liquid clouds'), via their radiative effects, played a key part in this melt event by increasing near-surface temperatures. We used a suite of surface-based observations, remote sensing data, and a surface energy-balance model. At the critical surface melt time, the clouds were optically thick enough and low enough to enhance the downwelling infrared flux at the surface. At the same time they were optically thin enough to allow sufficient solar radiation to penetrate through them and raise surface temperatures above the melting point. Outside this narrow range in cloud optical thickness, the radiative contribution to the surface energy budget would have been diminished, and the spatial extent of this melting event would have been smaller. We further show that these thin, low-level liquid clouds occur frequently, both over Greenland and across the Arctic, being present around 30-50 per cent of the time. Our results may help to explain the difficulties that global climate models have in simulating the Arctic surface energy budget, particularly as models tend to under-predict the formation of optically thin liquid clouds at supercooled temperatures--a process potentially necessary to account fully for temperature feedbacks in a warming Arctic climate.

Arctic mixed-phase cloud macro- and microphysical properties are derived from a year of radar, lidar, microwave radiometer, and radiosonde observations made as part of the Surface Heat Budget of the Arctic Ocean (SHEBA) Program in the Beaufort Sea in 1997–98. Mixed-phase clouds occurred 41% of the time and were most frequent in the spring and fall transition seasons. These clouds often consisted of a shallow, cloud-top liquid layer from which ice particles formed and fell, although deep, multilayered mixed-phase cloud scenes were also observed. On average, individual cloud layers persisted for 12 h, while some mixed-phase cloud systems lasted for many days. Ninety percent of the observed mixed-phase clouds were 0.5–3 km thick, had a cloud base of 0–2 km, and resided at a temperature of −25° to −5°C. Under the assumption that the relatively large ice crystals dominate the radar signal, ice properties were retrieved from these clouds using radar reflectivity measurements. The annual average ice particle mean diameter, ice water content, and ice water path were 93 μm, 0.027 g m−3, and 42 g m−2, respectively. These values are all larger than those found in single-phase ice clouds at SHEBA. Vertically resolved cloud liquid properties were not retrieved; however, the annual average, microwave radiometer–derived liquid water path (LWP) in mixed-phase clouds was 61 g m−2. This value is larger than the average LWP observed in single-phase liquid clouds because the liquid water layers in the mixed-phase clouds tended to be thicker than those in all-liquid clouds. Although mixed-phase clouds were observed down to temperatures of about −40°C, the liquid fraction (ratio of LWP to total condensed water path) increased on average from zero at −24°C to one at −14°C. The observations show a range of ∼25°C at any given liquid fraction and a phase transition relationship that may change moderately with season.

ABSTRACT:Results are presented from an intercomparison of single-column and cloud-resolving model simulations of a cold-air outbreak mixed-phase stratocumulus cloud observed during the Atmospheric Radiation Measurement (ARM) programme's Mixed-Phase Arctic Cloud Experiment. The observed cloud occurred in a well-mixed boundary layer with a cloud-top temperature of −15 • C. The average liquid water path of around 160 g m −2 was about two-thirds of the adiabatic value and far greater than the average mass of ice which when integrated from the surface to cloud top was around 15 g m −2 .Simulations of 17 single-column models (SCMs) and 9 cloud-resolving models (CRMs) are compared. While the simulated ice water path is generally consistent with observed values, the median SCM and CRM liquid water path is a factor-of-three smaller than observed. Results from a sensitivity study in which models removed ice microphysics suggest that in many models the interaction between liquid and ice-phase microphysics is responsible for the large model underestimate of liquid water path.Despite this underestimate, the simulated liquid and ice water paths of several models are consistent with observed values. Furthermore, models with more sophisticated microphysics simulate liquid and ice water paths that are in better agreement with the observed values, although considerable scatter exists. Although no single factor guarantees a good simulation, these results emphasize the need for improvement in the model representation of mixed-phase microphysics.