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  1. Abstract

    This study investigates cloud formation and transitions in cloud types at Summit, Greenland, during 16–22 September 2010, when a warm, moist air mass was advected to Greenland from lower latitudes. During this period there was a sharp transition between high ice clouds and the formation of a lower stratocumulus deck at Summit. A regional mesoscale model is used to investigate the air masses that form these cloud systems. It is found that the high ice clouds form in originally warm, moist air masses that radiatively cool while being transported to Summit. A sensitivity study removing high ice clouds demonstrates that the primary impact of these clouds at Summit is to reduce cloud liquid water embedded within the ice cloud and water vapor in the boundary layer due to vapor deposition on snow. The mixed-phase stratocumulus clouds form at the base of cold, dry air masses advected from the northwest above 4 km. The net surface radiative fluxes during the stratocumulus period are at least 20 W m−2 larger than during the ice cloud period, indicating that, in seasons other than summer, cold, dry air masses advected to Summit above the boundary layer may radiatively warm the top of themore »Greenland Ice Sheet more effectively than warm, moist air masses advected from lower latitudes.

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  2. Abstract. Snowfall is the major source of mass for the Greenland ice sheet (GrIS) but the spatial and temporalvariability of snowfall and the connections between snowfall and mass balance have so far been inadequatelyquantified. By characterizing local atmospheric circulation and utilizing CloudSat spaceborne radarobservations of snowfall, we provide a detailed spatial analysis of snowfall variability and its relationshipto Greenland mass balance, presenting first-of-their-kind maps of daily spatial variability in snowfallfrom observations across Greenland. For identified regional atmospheric circulation patterns, we show that thespatial distribution and net mass input of snowfall vary significantly with the position and strength ofsurface cyclones. Cyclones west of Greenland driving southerly flow contribute significantly more snowfall thanany other circulation regime, with each daily occurrence of the most extreme southerly circulation patterncontributing an average of 1.66 Gt of snow to the Greenland ice sheet. While cyclones east of Greenland,patterns with the least snowfall, contribute as little as 0.58 Gt each day. Above 2 km on the ice sheet wheresnowfall is inconsistent, extreme southerly patterns are the most significant mass contributors, with up to1.20 Gt of snowfall above this elevation. This analysis demonstrates that snowfall over the interior ofGreenland varies by up to a factor of 5 depending on regional circulation conditions. Usingmore »independentobservations of mass changes made by the Gravity Recovery and Climate Experiment (GRACE), we verify that thelargest mass increases are tied to the southerly regime with cyclones west of Greenland. For occurrences of thestrongest southerly pattern, GRACE indicates a net mass increase of 1.29 Gt in the ice sheet accumulation zone(above 2 km elevation) compared to the 1.20 Gt of snowfall observed by CloudSat. This overall agreementsuggests that the analytical approach presented here can be used to directly quantify snowfall masscontributions and their most significant drivers spatially across the GrIS. While previous research hasimplicated this same southerly regime in ablation processes during summer, this paper shows that ablation massloss in this circulation regime is nearly an order of magnitude larger than the mass gain from associatedsnowfall. For daily occurrences of the southerly circulation regime, a mass loss of approximately 11 Gt isobserved across the ice sheet despite snowfall mass input exceeding 1 Gt. By analyzing the spatialvariability of snowfall and mass changes, this research provides new insight into connections between regionalatmospheric circulation and GrIS mass balance.« less
  3. Abstract. Clouds warm the surface in the longwave (LW), and this warming effect can be quantified through the surface LW cloud radiativeeffect (CRE). The global surface LW CRE has been estimated over more than2 decades using space-based radiometers (2000–2021) and over the 5-year period ending in 2011 using the combination of radar, lidar and space-basedradiometers. Previous work comparing these two types of retrievals has shown that the radiometer-based cloud amount has some bias over icy surfaces. Here we propose new estimates of the global surface LW CRE from space-based lidarobservations over the 2008–2020 time period. We show from 1D atmosphericcolumn radiative transfer calculations that surface LW CRE linearly decreases with increasing cloud altitude. These computations allow us toestablish simple parameterizations between surface LW CRE and five cloud properties that are well observed by the Cloud-Aerosol Lidar and InfraredPathfinder Satellite Observations (CALIPSO) space-based lidar: opaque cloud cover and altitude and thin cloud cover, altitude, and emissivity. We evaluate this new surface LWCRE–LIDAR product by comparing it to existingsatellite-derived products globally on instantaneous collocated data atfootprint scale and on global averages as well as to ground-based observations at specific locations. This evaluation shows good correlationsbetween this new product and other datasets. Ourmore »estimate appears to be animprovement over others as it appropriately captures the annual variabilityof the surface LW CRE over bright polar surfaces and it provides a datasetmore than 13 years long.« less
  4. Abstract. This study presents the first full annual cycle (2019–2020) of ambient surface aerosol particle number concentration measurements (condensationnuclei > 20 nm, N20) collected at Summit Station (Summit), in the centre of the Greenland Ice Sheet (72.58∘ N, −38.45∘ E; 3250 ma.s.l.). The mean surface concentration in 2019 was 129 cm−3, with the 6 h mean ranging between 1 and 1441 cm−3. The highest monthly mean concentrations occurred during the late spring and summer, with the minimum concentrations occurring in February (mean: 18 cm−3). High-N20 events are linked to anomalous anticyclonic circulation over Greenland and the descent of free-tropospheric aerosol down to the surface, whereas low-N20 events are linked to anomalous cyclonic circulation over south-east Greenland that drives upslope flow and enhances precipitation en route to Summit. Fog strongly affects particle number concentrations, on average reducing N20 by 20 % during the first 3 h of fog formation. Extremely-low-N20 events (< 10 cm−3) occur in all seasons, and we suggest that fog, and potentially cloud formation, can be limited by low aerosol particle concentrations over central Greenland.
  5. Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over theGreenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievalswith respect to remaining ground-clutter issues. Comparing CloudSatobservations to the GrIS topography (obtained from airborne altimetrymeasurements during IceBridge) we find that at the edges of the GrISspurious high-snowfall retrievals caused by ground clutter occasionallyaffect the operational snowfall product. After correcting for this effect,the height of the lowest valid CloudSat observation is about 1200&thinsp;mabove the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple,empirical correction to account for precipitation processes occurringbetween the height of the observed CloudSat reflectivities and the snowfallnear the surface. Using the height-corrected, clutter-cleared CloudSatreflectivities we next evaluate various ZS relationships in terms ofsnowfall accumulation at Summit through comparison with weekly stake fieldobservations of snow accumulation available since 2007. Using a set of threeZS relationships that best agree with the observed accumulation at Summit,we then calculate the annual cycle snowfall over the entire GrIS as well asover different drainage areas and compare the derived meanmore »values and annualcycles of snowfall to ERA-Interim reanalysis. We find the annual meansnowfall over the GrIS inferred from CloudSat to be 34±7.5&thinsp;cm&thinsp;yr−1liquid equivalent (where the uncertainty is determined by the range invalues between the three different ZS relationships used). In comparison,the ERA-Interim reanalysis product only yields 30&thinsp;cm&thinsp;yr−1 liquid equivalentsnowfall, where the majority of the underestimation in the reanalysisappears to occur in the summer months over the higher GrIS and appears to berelated to shallow precipitation events. Comparing all available estimatesof snowfall accumulation at Summit Station, we find the annually averagedliquid equivalent snowfall from the stake field to be between 20 and 24&thinsp;cm&thinsp;yr−1, depending on the assumed snowpack density and from CloudSat 23±4.5&thinsp;cm&thinsp;yr−1. The annual cycle at Summit is generally similar betweenall data sources, with the exception of ERA-Interim reanalysis, which showsthe aforementioned underestimation during summer months.

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  6. Abstract. Radiation fogs at Summit Station, Greenland (72.58&thinsp;N,38.48&thinsp;W; 3210&thinsp;m&thinsp;a.s.l.), are frequently reported by observers. Thefogs are often accompanied by fogbows, indicating the particles are composedof liquid; and because of the low temperatures at Summit, this liquid issupercooled. Here we analyze the formation of these fogs as well as theirphysical and radiative properties. In situ observations of particle size anddroplet number concentration were made using scattering spectrometers near 2 and 10&thinsp;m height from 2012 to 2014. These data are complemented bycolocated observations of meteorology, turbulent and radiative fluxes, andremote sensing. We find that liquid fogs occur in all seasons with thehighest frequency in September and a minimum in April. Due to thecharacteristics of the boundary-layer meteorology, the fogs are elevated,forming between 2 and 10&thinsp;m, and the particles then fall toward the surface.The diameter of mature particles is typically 20–25&thinsp;µm in summer.Number concentrations are higher at warmer temperatures and, thus, higher insummer compared to winter. The fogs form at temperatures as warm as −5&thinsp;C, while the coldest form at temperatures approaching −40&thinsp;C. Facilitated by the elevated condensation, in winter two-thirds offogs occurred within a relatively warm layer above the surface when thenear-surface air was belowmore »−40&thinsp;C, as cold as −57&thinsp;C,which is too cold to support liquid water. This implies that fog particlessettling through this layer of cold air freeze in the air column beforecontacting the surface, thereby accumulating at the surface as ice withoutriming. Liquid fogs observed under otherwise clear skies annually imparted1.5&thinsp;W&thinsp;m−2 of cloud radiative forcing (CRF). While this is a smallcontribution to the surface radiation climatology, individual events areinfluential. The mean CRF during liquid fog events was 26&thinsp;W&thinsp;m−2, andwas sometimes much higher. An extreme case study was observed toradiatively force 5&thinsp;C of surface warming during the coldest partof the day, effectively damping the diurnal cycle. At lower elevations ofthe ice sheet where melting is more common, such damping could signal a rolefor fogs in preconditioning the surface for melting later in the day.

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