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We provide an updated analysis of instrumental Greenland monthly temperature data to 2019, focusing mainly on coastal stations but also analysing ice‐sheet records from Swiss Camp and Summit. Significant summer (winter) coastal warming of ~1.7 (4.4)°C occurred from 1991–2019, but since 2001 overall temperature trends are generally flat and insignificant due to a cooling pattern over the last 6–7 years. Inland and coastal stations show broadly similar temperature trends for summer. Greenland temperature changes are more strongly correlated with Greenland Blocking than with North Atlantic Oscillation changes. In quantifying the association between Greenland coastal temperatures and Greenland Ice Sheet (GrIS) mass‐balance changes, we show a stronger link of temperatures with total mass balance rather than surface mass balance. Based on Greenland coastal temperatures and modelled mass balance for the 1972–2018 period, each 1°C of summer warming corresponds to ~(91) 116 Gt·yr−1 of GrIS (surface) mass loss and a 26 Gt·yr−1 increase in solid ice discharge. Given an estimated 4.0–6.6°C of further Greenland summer warming according to the regional model MAR projections run under CMIP6 future climate projections (SSP5‐8.5 scenario), and assuming that ice‐dynamical losses and ice sheet topography stay similar to the recent past, linear extrapolation gives a corresponding GrIS global sea‐level rise (SLR) contribution of ~10.0–12.6 cm by 2100, compared with the 8–27 cm (mean 15 cm) "likely" model projection range reported by IPCC in 2019 (SPM.B1.2). However, our estimate represents a lower limit for future GrIS change since fixed dynamical mass losses and amplified melt arising from both melt‐albedo and melt‐elevation positive feedbacks are not taken into account here. [ABSTRACT FROM AUTHOR]

In recent decades, the Greenland ice sheet (GrIS) has frequently experienced record melt events, which have significantly affected surface mass balance (SMB) and estimates thereof. SMB data are derived from remote sensing, regional climate models (RCMs), firn cores and automatic weather stations (AWSs). While remote sensing and RCMs cover regional scales with extents ranging from 1 to 10 km, AWS data and firn cores are point observations. To link regional scales with point measurements, we investigate the spatial variability of snow accumulation (bs) within areas of approximately 1–4 km 2 and its temporal changes within 2 years of measurements. At three different sites on the southwestern GrIS (Swiss Camp, KAN-U, DYE-2), we performed extensive ground-penetrating radar (GPR) transects and recorded multiple snow pits. If the density is known and the snowpack dry, radar-measured two-way travel time can be converted to snow depth and bs. We spatially filtered GPR transect data to remove small-scale noise related to surface characteristics. The combined uncertainty of bs from density variations and spatial filtering of radar transects is at 7 %–8 % per regional scale of 1–4 km 2. Snow accumulation from a randomly selected snow pit is very likely representative of the regional scale of 1–4 km 2 (with probability p=0.8 for a value within 10 % of the regional mean for KAN-U, and p>0.95 for Swiss Camp and DYE-2). However, to achieve such high representativeness of snow pits, it is required to determine the average snow depth within the vicinity of the pits. At DYE-2, the spatial pattern of snow accumulation was very similar for 2 consecutive years. Using target reflectors placed at respective end-of-summer-melt horizons, we additionally investigated the occurrences of lateral redistribution within one melt season. We found no evidence of lateral flow of meltwater in the current climate at DYE-2. Such studies of spatial representativeness and temporal changes in accumulation are necessary to assess uncertainties of the linkages of point measurements and regional-scale data, which are used for validation and calibration of remote-sensing data and RCM outputs. [ABSTRACT FROM AUTHOR]

Radiation fogs at Summit Station, Greenland (72.58 ∘ N, 38.48 ∘ W; 3210 m a.s.l.), are frequently reported by observers. The fogs are often accompanied by fogbows, indicating the particles are composed of liquid; and because of the low temperatures at Summit, this liquid is supercooled. Here we analyze the formation of these fogs as well as their physical and radiative properties. In situ observations of particle size and droplet number concentration were made using scattering spectrometers near 2 and 10 m height from 2012 to 2014. These data are complemented by colocated observations of meteorology, turbulent and radiative fluxes, and remote sensing. We find that liquid fogs occur in all seasons with the highest frequency in September and a minimum in April. Due to the characteristics of the boundary-layer meteorology, the fogs are elevated, forming between 2 and 10 m, and the particles then fall toward the surface. The diameter of mature particles is typically 20–25 µ m in summer. Number concentrations are higher at warmer temperatures and, thus, higher in summer compared to winter. The fogs form at temperatures as warm as -5 ∘ C, while the coldest form at temperatures approaching -40 ∘ C. Facilitated by the elevated condensation, in winter two-thirds of fogs occurred within a relatively warm layer above the surface when the near-surface air was below -40 ∘ C, as cold as -57 ∘ C, which is too cold to support liquid water. This implies that fog particles settling through this layer of cold air freeze in the air column before contacting the surface, thereby accumulating at the surface as ice without riming. Liquid fogs observed under otherwise clear skies annually imparted 1.5 W m -2 of cloud radiative forcing (CRF). While this is a small contribution to the surface radiation climatology, individual events are influential. The mean CRF during liquid fog events was 26 W m -2 , and was sometimes much higher. An extreme case study was observed to radiatively force 5 ∘ C of surface warming during the coldest part of the day, effectively damping the diurnal cycle. At lower elevations of the ice sheet where melting is more common, such damping could signal a role for fogs in preconditioning the surface for melting later in the day. [ABSTRACT FROM AUTHOR]

ABSTRACT Here, we present an analysis of monthly, seasonal, and annual long-term precipitation time-series compiled from coastal meteorological stations in Greenland and Greenland Ice Sheet ( GrIS) ice cores (including three new ice core records from ACT11D, Tunu2013, and Summit2010). The dataset covers the period from 1890 to 2012, a period of climate warming. For approximately the first decade of the new millennium (2001-2012) minimum and maximum mean annual precipitation conditions are found in Northeast Greenland (Tunu2013 c. 120 mm water equivalent (w.e.) year−1) and South Greenland (Ikerasassuaq: c. 2300 mm w.e. year−1), respectively. The coastal meteorological stations showed on average increasing trends for 1890-2012 (3.5 mm w.e. year−2) and 1961-2012 (1.3 mm w.e. year−2). Years with high coastal annual precipitation also had a: (1) significant high number of precipitation days ( r2 = 0.59); and (2) high precipitation intensity measured as 24-h precipitation ( r2 = 0.54). For the GrIS the precipitation estimated from ice cores increased on average by 0.1 mm w.e. year−2 (1890-2000), showing an antiphase variability in precipitation trends between the GrIS and the coastal regions. Around 1960 a major shift occurred in the precipitation pattern towards wetter precipitation conditions for coastal Greenland, while drier conditions became more prevalent on the GrIS. Differences in precipitation trends indicate a heterogeneous spatial distribution of precipitation in Greenland. An Empirical Orthogonal Function analysis reveals a spatiotemporal cycle of precipitation that is linked instantaneously to the North Atlantic Oscillation and the Atlantic Multidecadal Oscillation and with an ∼6 years lag time response to the Greenland Blocking Index. [ABSTRACT FROM AUTHOR]

ABSTRACT The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a 'heat dome' over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994-2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future 'average' summers projected this century. [ABSTRACT FROM AUTHOR]

The article discusses a study which examines the generation of near-surface wind field of Greenland ice sheet. Topics discussed the use of PROMICE data to obtain insight on the interaction between gravity-driven katabatic winds, radiation and air temperature, Greenland ice sheet's average near-surface air temperature, and the World Meteorological Organization (WMO) Global Cryosphere Watch.

Correlation analysis of Greenland coastal weather station temperatures against the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) indices for the summer season (when Ice Sheet melt and runoff occur) reveals significant temporal variations over the last 100 years, with periods of strongest correlations in the early twentieth century and during recent decades. During the mid-twentieth century, temperature changes at the stations are not significantly correlated with these circulation indices. Greenland coastal summer temperatures and Greenland Ice Sheet (GrIS) runoff since the 1970s are more strongly correlated with the Greenland Blocking Index (GBI) than with the NAO Index (NAOI), making the GBI a potentially useful predictor of ice-sheet mass balance changes. Our results show that the changing strength of NAOI-temperature relationships found in boreal winter also extends to summer over Greenland. Greenland temperatures and GrIS runoff over the last 30-40 years are significantly correlated with AMO variations, although they are more strongly correlated with GBI changes. GrIS melt extent is less significantly correlated with atmospheric and oceanic index changes than runoff, which we attribute to the latter being a more quantitative index of Ice Sheet response to climate change. Moreover, the four recent warm summers of 2007-2010 are characterised by unprecedented high pressure (since at least 1948-the start of the NCEP/NCAR reanalysis record) in the tropospheric column. Our results suggest complex and changing atmospheric forcing conditions that are not well captured using the NAO alone, and support theories of an oceanic influence on the recent increases in Greenland temperatures and GrIS runoff. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article discusses a study of the supraglacial water budget of a moulin basin in the Sermeq Avannarleq ablation zone on the western margin of the Greenland ice sheet in August 2009. A Campbell Scientific SR-50 sonic sensor and a Geopacks MFP51 Flowmeter were utilized to record water surface height and stream velocity, respectively. The mass balance of water and ice within an idealized crevasse over multiple melt seasons was described using a simple one-dimensional model. The dominant input term to the basin was meltwater production. Results also show negligible rainfall, evaporation and crevasse refreezing/storage terms in the August water budget.

The article focuses on a study on the annual glaciohydrology cycle on the ablation zone of the ice sheet in Greenland using a hydrology model. Under the study, a one-dimensional (1-D) hydrology model has been developed to assess the hydrologic cycle of the tidewater glacier Sermeq Avannarleq. It notes that the modeled hydraulic head tend to oscillate near flotation which is said to be caused by the dependence of conduit activity and discharge on the relative difference presented by water and ice pressures. Findings suggest that the behavior of the terminal 50 kilometers (km) of the Sermeq Avannarleq flowline is similar to that of an alpine glacier.

The article discusses a study on the methodology for estimating the onset, duration and volume of meltwater runoff from a localized basin of the Greenland ice sheet (GrlS) using satellite observations of sediment plume length in the Russell Glacier-Watson River-Kangerlussuaq Fjord system. Moderate Resolution Imaging Spectroradiometer (MODIS) was used to generate satellite imagery. It mentions the influence of wind stress on the structure and distribution of buoyant coastal plumes over daily and seasonal timescales. The Watson River and the Ørkendalen river are cited as the major river systems that transport meltwater from the GrlS into the fjord. It concludes that plume formation is an alternative metric to the melt season to estimate runoff onset and duration.

The article discusses the study which examines the temporal variability of surface snow and glacier melt flux and runoff at the ablation area of Jakobshavn Isbræ, West Greenland. It states the modeled interannual runoff variability has resulted to a value that yields a cumulative runoff at the Jakobshavn Glacier terminus towards the freshwater flux. It notes that the surface runoff forms a minor part of the overall Jakobshavn freshwater flux that originates from the surface runoff which has a moisture that changes phases into solid, liquid, and vapor throughout the year as part of the various physical processes.

The article presents a study which examines the impact of snow densification and meltwater retention in the surface mass balance (SMB) module of the large-scale ice-sheet models of Greenland. It says that simple parameters were used to calculate surface melt, snow densification, and meltwater retention. It adds that it aims to improve the SMB estimates for past, present, and future scenarios of the Greenland ice sheet (GrIS). It shows that coupling the meltwater-retention and snow densification processes achieves a separation of volume and mass changes of the surface layer. Moreover, key factors for controlling the quantity and spatial distribution of meltwater runoff include in depth onset of melt, mean annual near-surface air temperature, and the mean density of the annual snow layer.

The article presents a study which measures the surface mass balance (SMB) of the 1995-2007 Greenland Ice Sheet (GrIs), including freshwater flux with the use of the state-of-the-art snow evolution modeling system called SnowModel. It cites that model inputs from meteorological observations from 26 meteorological stations located on the GrIS and in costal Greenland were used. Further, the study aims to improve the quantitative understanding of the GrIS surface melt extent in 2007.

SnowModel, a physically based snow-evolution modeling system that includes four submodels—MicroMet, EnBal, SnowPack, and SnowTran-3D—was used to simulate variations in Greenland [including the Greenland Ice Sheet (GrIS)] surface snow and ice melt, as well as water balance components, for 1995–2005. Meteorological observations from 25 stations inside and outside the GrIS were used as model input. Winter and summer mass balance observations, spatial snow depth observations, and snowmelt depletion curves derived from time-lapse photography from the Mittivakkat and Zackenberg glacierized catchments in East Greenland were used to validate the performance of SnowModel. Model results compared well with observed values, confirming the robustness of the model. The yearly modeled GrIS interior nonmelt area differs from satellite observations by a maximum of ∼68 000 km2 (or ∼6%) in 2004, and the lowest uncertainties (<8000 km2, or <1%) occur for the years with the smallest (2005) and most extensive (1996) nonmelt areas. Modeled surface melt occurred at elevations reaching 2950 m MSL for 2005, while the equilibrium line altitude (ELA) fluctuates from 1640 to 600 m MSL. The modeled interannual variability in the nonmelt area also agrees with observation records (R2 = 0.96), yielding simulated GrIS nonmelt covers of 71% for 1996 and 50% for 2005. On average, the simulated nonmelt area decreased ∼6% from 1995 to 2005; this trend is similar to observed values. An average surface mass balance (SMB) storage of 138(±81) km3 yr-1, a GrIS loss of 257(±81) km3 yr-1, and a runoff contribution to the ocean of 392(±58) km3 yr-1 occurred for the period 1995–2005. Approximately 58% and 42% of the runoff came from the GrIS western and eastern drainage areas, respectively. The modeled average specific runoff from the GrIS was 6.71 s-1 km-2 yr-1, which, over the simulation period, represents a contribution of ∼1.1 mm yr-1 to global sea level rise. [ABSTRACT FROM AUTHOR]

The authors attribute significantly increased Greenland summer warmth and Greenland Ice Sheet melt and runoff since 1990 to global warming. Southern Greenland coastal and Northern Hemisphere summer temperatures were uncorrelated between the 1960s and early 1990s but were significantly positively correlated thereafter. This relationship appears to have been modulated by the North Atlantic Oscillation, whose summer index was significantly (negatively) correlated with southern Greenland summer temperatures until the early 1990s but not thereafter. Significant warming in southern Greenland since ∼1990, as also evidenced from Swiss Camp on the west flank of the ice sheet, therefore reflects general Northern Hemisphere and global warming. Summer 2003 was the warmest since at least 1958 in coastal southern Greenland. The second warmest coastal summer 2005 had the most extensive anomalously warm conditions over the ablation zone of the ice sheet, which caused a record melt extent. The year 2006 was the third warmest in coastal southern Greenland and had the third-highest modeled runoff in the last 49 yr from the ice sheet; five of the nine highest runoff years occurred since 2001 inclusive. Significantly rising runoff since 1958 was largely compensated by increased precipitation and snow accumulation. Also, as observed since 1987 in a single composite record at Summit, summer temperatures near the top of the ice sheet have declined slightly but not significantly, suggesting the overall ice sheet is experiencing a dichotomous response to the recent general warming: possible reasons include the ice sheet’s high thermal inertia, higher atmospheric cooling, or changes in regional wind, cloud, and/or radiation patterns. [ABSTRACT FROM AUTHOR]

Turbulence data collected over a total of 25 days during two summers are used to describe processes responsible for the nonstationarity of turbulent sensible heat fluxes at Summit, Greenland. A stationarity test shows that about 40% of the data are classified as nonstationary. Three main factors are explored to account for the large fraction of nonstationary runs: (1) intermittency of turbulence in stable conditions, (2) changes in net all-wave radiation in response to cloud forcing, and (3) diurnal trends in stability. A classification procedure that accounts for the intermittent nature of turbulence shows that during stable, nonstationary conditions 50% of the total sensible heat flux is realized in 22% of the sampling time. Intermittency often occurs at Summit during periods characterized by weak and irregular horizontal winds in combination with strong stability. Rapid changes in net all-wave radiation in response to cloud forcing results in nonstationarity during unstable conditions. Between 0930–1130 and 1900–1930 UTC turbulent heat fluxes are not only small in magnitude but also typically change sign, with nonstationarity during these periods often as high as 65%. These results should help resolve some of the present uncertainties in obtaining reliable fluxes at this site, in particular under stable atmospheric conditions. [ABSTRACT FROM AUTHOR]

Regional climate model runs using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesocale Model modified for use in polar regions (Polar MM5), calibrated by independent in situ observations, demonstrate coherent regional patterns of Greenland ice sheet surface mass balance (SMB) change over a 17-yr period characterized by warming (1988–2004). Both accumulation and melt rates increased, partly counteracting each other for an overall negligible SMB trend. However, a 30% increase in meltwater runoff over this period suggests that the overall ice sheet mass balance has been increasingly negative, given observed meltwater-induced flow acceleration. SMB temporal variability of the whole ice sheet is best represented by ablation zone variability, suggesting that increased melting dominates over increased accumulation in a warming scenario. The melt season grew in duration over nearly the entire ablation zone by up to 40 days, 10 days on average. Accumulation area ratio decreased by 3%. Albedo reductions are apparent in five years of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived data (2000–04). The Advanced Very High Resolution Radiometer (AVHRR)-derived albedo changes (1988–99) were less consistent spatially. A conservative assumption as to glacier discharge and basal melting suggests an ice sheet mass loss over this period greater than 100 km3 yr-1, framing the Greenland ice sheet as the largest single glacial contributor to recent global sea level rise. Surface mass balance uncertainty, quantified from residual random error between model and independent observations, suggests two things: 1) changes smaller than approximately 200 km3 yr-1 would not satisfy conservative statistical significance thresholds (i.e., two standard deviations) and 2) although natural variability and model uncertainty were separated in this analysis, the magnitude of each were roughly equivalent. Therefore, improvements in model accuracy and analysis of longer periods (assuming larger changes) are both needed for definitive mass balance change assessments. [ABSTRACT FROM AUTHOR]

The main characteristic features of stable atmospheric flows over a large mountain plateau are summarised and then compared with mesoscale and synoptic scale numerical simulation, meteorological analysis, satellite imagery, and surface observations for the cases of flows over Southern Greenland for four wind directions. The detailed features are identified using the concepts and scaling of stably stratified flow over large mountains with variations in surface roughness, elevation, and heating. For westerly and easterly winds detached jets form at the southern tip, where coastal jets converge, which propagate large distances across the ocean. Near coasts katabatic winds can combine with barrier jets and wake flows generated by synoptic winds. Note how the approach flow rises/falls over southern Greenland for easterly/westerly winds, leading in both cases to more cloud on the western side. Some conclusions are drawn about the large-scale influences of these flows; detached jets in the atmosphere; air-sea interaction; formation of low pressure systems. For accurate simulations of these flows, mesoscale models are necessary with resolutions of order of 20 km or less. [ABSTRACT FROM AUTHOR]

Presents a mesoscale simulation model of the atmospheric state and katabatic winds around Greenland. Verification of the simulation model; Description of the model; Comparison of the verification results with observations from automatic weather stations and instrumented aircrafts.

Examines the accuracy of Advanced Very High Resolution Radiometer (AVHRR)-derived surface temperatures over the Greenland ice sheet. Information on the method used in this study; Discussion on the spatial and temporal variability of these surface temperatures; What the results of the study indicate.

The article models the surface mass balance of the Greenland ice sheet (GrlS) in large-scale ice-sheet models using temperature standard deviation parameterizations in relation with positive degree-day (PDD) approach. values of the coefficients. Data shows the annual cycle of the temperature standard deviation from three automatic weather stations (AWS) on the GrlS located at different elevations, in the dry snow zone, near the equilibrium line and in the ablation zone. The limiting influence of the surface temperature over a melting snow and ice surface explains the smaller values of temperature during summer. Results show the altitudinal dependence of standard deviation on minor influences by the latitude and longitude as indicated by their coefficients and root-mean-square difference

Observes temperature in the summit region by the Central Greenland Automatic Weather Station. Combination of near-surface and satellite temperature information; Development of multilayer temperature record for the Greenland Ice Sheet summit region.

Abstract: The sudden drainage of supraglacial lakes has been previously observed to initiate surface-to-bed hydrologic connections, which are capable of enhancing basal sliding, in regions of the Greenland Ice Sheet where ice thickness approaches 1km. In this study, we develop a robust algorithm, which automatically detects and tracks individual supraglacial lakes using visible satellite imagery, to document the evolution of a population of West Greenland supraglacial lakes over ten consecutive melt seasons. Validation tests indicate that the algorithm is highly accurate: 99.0% of supraglacial lakes can be detected and tracked and 96.3% of reported lakes are true supraglacial lakes with accurate lake properties, such as lake area, and timing of formation and drainage. Investigation of the interannual evolution of supraglacial lakes in the context of annual melt intensity reveals that during more intense melt years, supraglacial lakes drain more frequently and earlier in the melt season. Additionally, the lake population extends to higher elevations during more intense melt years, exposing an increased inland area of the ice sheet to sudden lake drainage events. These observations suggest that increased surface meltwater production due to climate change will enhance the spatial extent and temporal frequency of lake drainage events. It is unclear whether this will ultimately increase or decrease the basal sliding sensitivity of interior regions of the Greenland Ice Sheet. [Copyright &y& Elsevier]

Abstract: The most practical way to get spatially broad and continuous measurements of the surface temperature in the data-sparse cryosphere is by satellite remote sensing. The uncertainties in satellite-derived LSTs must be understood to develop internally-consistent decade-scale land surface temperature (LST) records needed for climate studies. In this work we assess satellite-derived “clear-sky” LST products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and LSTs derived from the Enhanced Thematic Mapper Plus (ETM+) over snow and ice on Greenland. When possible, we compare satellite-derived LSTs with in-situ air temperature observations from Greenland Climate Network (GC-Net) automatic weather stations (AWS). We find that MODIS, ASTER and ETM+ provide reliable and consistent LSTs under clear-sky conditions and relatively-flat terrain over snow and ice targets over a range of temperatures from −40 to 0 °C. The satellite-derived LSTs agree within a relative RMS uncertainty of ~0.5 °C. The good agreement among the LSTs derived from the various satellite instruments is especially notable since different spectral channels and different retrieval algorithms are used to calculate LST from the raw satellite data. The AWS record in-situ data at a “point” while the satellite instruments record data over an area varying in size from: 57×57 m (ETM+), 90×90 m (ASTER), or to 1×1 km (MODIS). Surface topography and other factors contribute to variability of LST within a pixel, thus the AWS measurements may not be representative of the LST of the pixel. Without more information on the local spatial patterns of LST, the AWS LST cannot be considered valid ground truth for the satellite measurements, with RMS uncertainty ~2 °C. Despite the relatively large AWS-derived uncertainty, we find LST data are characterized by high accuracy but have uncertain absolute precision. [Copyright &y& Elsevier]

Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming. [ABSTRACT FROM AUTHOR]

Tower-based measurements of hydrogen peroxide (H[sub 2]O[sub 2]) and formaldehyde (HCHO) exchange were performed above the snowpack of the Greenland ice sheet. H[sub 2]O[sub 2] and HCHO fluxes were measured continuously between 16 June and 7 July 2000, at the Summit Environmental Observatory. The fluxes were determined using coil scrubber-aqueous phase fluorometry systems together with micrometeorological techniques. Both compounds exhibit strong diel cycles in the observed concentrations as well as in the fluxes with emission from the snow during the day and the evening and deposition during the night. The averaged diel variations of the observed fluxes were in the range of +1.3 × 10[sup 13] molecules m[sup -2]s[sup -1] (deposition) and -1.6 × 10[sup 13] molecules m[sup -2]s[sup -1] (emission) for H[sub 2]O[sub 2] and +1.1 × 10[sup 12] and -4.2 × 10[sup 12] molecules m[sup -2]s[sup -1] for HCHO, while the net exchange per day for both compounds were much smaller. During the study period of 22 days on average (0.8[sup +46, sub -4.3]) × 10[sup 17] molecules m[sup -2] of H[sub 2]O[sub 2], were deposited and (7.0[sup +12.6, sub -12.2] 10[sup 16] molecules m[sup -2] of HCHO were emitted from the snow per day. A comparison with the inventory in the gas phase demonstrates that the exchange influences the diel variations in the boundary layer above snow covered areas. Flux measurements during and after the precipitation of new snow shows that <16% of the H[sub 2]O[sub 2] and more than 25% of the HCHO originally present in the new snow were available for fast release to the atmospheric boundary layer within hours after precipitation. This release can effectively disturb the normally observed diel variations of the exchange between the surface snow and the atmosphere, thus perturbing also the diel variations of corresponding gas-phase concentrations. [ABSTRACT FROM AUTHOR]

The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400m above ground. The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between ∼9.00 and 18.00 h local time with the formation of shallow mixing heights of ∼70 250 m above the surface. The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37-76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A ∼0.1-3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime. Science Ltd. All rights reserved. [ABSTRACT FROM AUTHOR]

Understanding surface melt over the Greenland Ice Sheet (GrIS) is important for evaluating the effect of the changing global climate on humanity, and for forecasting sea level rise. We demonstrate a recently developed L-band passive microwave algorithm for the retrieval of snow liquid water and snow and firn density over the GrIS. The retrievals are performed using brightness temperatures from the ESA SMOS satellite. The density retrievals show potential for mapping the equilibrium line altitude of the ice sheet by determining where retrieved densities exceed typical seasonal snowpack density, thus indicating the presence of firn. We integrate snow liquid water retrievals over area and time to obtain water column maps and daily estimates of the surface liquid water present over Greenland. The integrated annual surface liquid water retrieved over the entire ice sheet compares on the same order as modeled estimates of total meltwater available in current literature. • L-band brightness temperatures from SMOS can quantify melt over Greenland • Retrieved firn/snow density can be used to map equilibrium line altitude (ELA) • Retrieved liquid water content can be used to calculate total liquid water • First known direct measurement of a surface mass balance component [ABSTRACT FROM AUTHOR]

We demonstrate a novel method to retrieve snow liquid water content and density over a site in the ablation zone of the Western Greenland Ice Sheet from L-band radiometer data measured by the Soil Moisture and Ocean Salinity (SMOS) satellite. Previous demonstrations using ground-based close-range radiometry separately retrieved snow density and snow wetness over frozen and thawed ground. We apply similar techniques over the ice sheet to simultaneously retrieve snow density and wetness at the location of "Swiss Camp" from June 2010 through August 2018 at nearly daily temporal resolution. Achieved results are compared to in-situ air temperature data and to a well-known 19 GHz and 37 GHz passive-microwave melt characterization technique known as the cross-polarized gradient ratio (XPGR). The L-band based snow wetness retrievals often detect the onset of seasonal melt earlier than the XPGR algorithm without the need for empirically tuned thresholds. We also demonstrate the performance of the SMOS based snow wetness retrievals based on error statistics compared with an air temperature melt proxy. By applying temporal averaging to the SMOS based snow density retrievals, we achieve reasonable agreement with in-situ observations from May 2014 and May 2018. The demonstrated retrieval algorithm shows potential as a future SMOS data product for ice-covered regions of the cryosphere. • Snow water content and density can be retrieved simultaneously from passive L-band. • Emission model based retrievals over Greenland ice sheet ablation zone. • Uses soil moisture and ocean salinity satellite multi-angle data. • Agreement with air temperature proxy, manual density, and 19/37 GHz method. [ABSTRACT FROM AUTHOR]

The Greenland land ice's accelerating mass loss contributes to 15-30% of the current sea level rise and mostly originates from increased surface melting and runoff. About 80% of the ice sheet is covered by perennial snow, a.k.a. firn, that retains part of the seasonal surface melt. Firn meltwater retention depends on its physical and thermal state which can be estimated from a firn model driven by weather station observations. We find that increasing air temperatures have driven increasing melt and firn heating at nine GC-Net stations between 1998 and 2015. Accounting for deep preferential meltwater percolation is found to not only to the comparison of simulated firn temperatures to observations modulate the calculated melt and hinder the transmission of heat from the firn to the atmosphere by routing the meltwater at depth where the released latent heat need more time to be conducted back to the surface. We find that, in spite of increasing firn heat influx, the firn refreezing capacity is stable at all sites. At the two warmest sites, firn densification led to the loss of 13-15% of the firn retention capacity. The denser near-surface firn however requires more energy per unit volume to be brought to melting resulting in a stable refreezing capacity at these sites. We find either constant or slightly increasing (≤ 5 %) refreezing capacity and retention capacity at colder sites. [ABSTRACT FROM AUTHOR]