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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.

Despite the widespread use of intraarterial thrombolytic therapy for peripheral arterial occlusive disease, a randomized study comparing its efficacy with that of operative intervention has never• been performed. This study evaluates the potential of intraarterial urokinase infusion to provide clinical benefits in patients with acute peripheral arterial occlusion. Methods: Patients with limb-threatening ischemia of less than 7 days' duration were randomly assigned to intraarterial catheter-directed urokinase therapy or operative intervention. Anatomic lesions unmasked by thrombolysis were treated with balloon dilation or operation. The primary end points of the study were limb salvage and survival.Results: A total of 57 patients were randomized to the thrombolytic therapy group, and 57 patients were randomized to the operative therapy group. Thrombolytic therapy resulted in dissolution of the occluding thrombus in 40 (70%) patients. Although the cumulative limb salvage rate was similar in the two treatment groups (82% at 12 months), the cumulative survival rate was significandy improved in patients randomized to the thrombolysis group (84% vs 58% at 12 months, p = 0.01). The mortality differences seemed to be primarily attributable to an increased frequency of in-hospital cardiopulmonary complications in the operative treatment group (49% vs 16%, P = 0.001). The benefits of thrombolysis were achieved without significant differences in the duration of hospitalization (median 11 days) and with only modest increases in hospital cost in the thrombolytic treatment arm (median $15,672 vs $12,253, P = 0.02).Conclusions: Intraarterial thrombolytic therapy was associated with a reduction in the incidence of in-hospital cardiopulmonary complications and a corresponding increase in patient survival rates. These benefits were achieved without an appreciable increase in the duration of hospitalization and with only modest increases in hospital cost, suggesting that thrombolytic therapy may offer a safe and effective alternative to operation in the initial treatment of patients diagnosed with acute limb-threatening peripheral arterial occlusion.

The Greenland Ice Sheet (GIS) has experienced increased surface melt extent and duration over 41 recent decades (Mernild et al. 2011). Increases of GIS surface melt and subsequent enhanced 42 glacial flow (Zwally et al. 2002) contribute to rising sea levels and increased fresh water flux into 43 regional oceans. The surface energy budget modulates surface melting, and thus also the mass 44 balance of the GIS, by controlling the surface temperature. Clouds exert a significant influence on 45 the net radiative flux at the surface (Walsh and Chapman 1998), therefore impacting the surface 46 energy budget. 47The shortwave and longwave radiative effect of clouds, or cloud radiative forcing (CRF), can be 48 quantified by comparing the actual surface radiative flux to the flux during an equivalent clear-sky 49 scene. In general, Arctic clouds have a warming effect on the surface, except for a period in the 50 summer when the sun is highest and surface albedo is lowest (Curry and Ebert 1992; Intrieri et al. 51 2002; Kay and L'Ecuyer 2013). However, unlike many other Arctic locations, the central Green-52 land surface is covered by snow throughout the year, maintaining a high surface albedo in all sea-53 sons. Snow surfaces limit the ability of clouds to reduce absorbed solar radiation because clouds 54 cannot substantially raise the planetary albedo. Moreover, surface-based inversions are a com-55 mon feature above ice-covered surfaces, thus low-level clouds often emit at temperatures much 56 warmer than the surface. Under these conditions, longwave radiative forcing typically dominates 57 and clouds increase the downwelling flux compared to clear skies by 45-95 W m −2 (Shupe and 58 Intrieri 2004; Sedlar et al. 2011; de Boer et al. 2011). Additionally, optically thin clouds can act to 59 maximize the combined shortwave and longwave radiative effects to produce maximum warming 60 at the surface. A notable case in central Greenland occurred when low-level liquid-bearing clouds 61 forced the surface temperature above the melting point of snow in July 2012 (Bennartz et al. 2013). 62 87 and integrated ice-cloud thickness. 88 2. Instrumentation 89 A comprehensive and integrated suite of instruments, measuring the atmospheric state above 90 Summit Station in conjunction with radiative fluxes at the surface, provides linkages between 91 cloud properties and the surface radiation budget. For all data sets a time-averaging window of 92 three hours is used to remove the effects of inhomogeneities in the atmospheric scene while still 93 resolving the diurnal cycle. 94 a. ICECAPS measurements 95 The Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Sum-96 mit (ICECAPS) project is a field campaign, in operation since May 2010, designed to characterize 97 atmospheric properties at Summit similar to observational activities at other Arctic sites such as 98 Barrow, Alaska (Stamnes et al. 1999) and Eureka, Canada. The instrument suite is designed to 99 provide complementary information on atmospheric, ...