High and Dry: New Observations of Tropospheric and Cloud Properties above the Greenland Ice Sheet

Cloud and atmospheric properties strongly influence the mass and energy budgets of the Greenland Ice Sheet (GIS). To address critical gaps in the understanding of these systems, a new suite of cloud- and atmosphere-observing instruments has been installed on the central GIS as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project. During the first 20 months in operation, this complementary suite of active and passive ground-based sensors and radiosondes has provided new and unique perspectives on important cloud–atmosphere properties. High atop the GIS, the atmosphere is extremely dry and cold with strong near-surface static stability predominating throughout the year, particularly in winter. This low-level thermodynamic structure, coupled with frequent moisture inversions, conveys the importance of advection for local cloud and precipitation formation. Cloud liquid water is observed in all months of the year, even the particularly cold and dry winter, while annual cycle observations indicate that the largest atmospheric moisture amounts, cloud water contents, and snowfall occur in summer and under southwesterly flow. Many of the basic structural properties of clouds observed at Summit, Greenland, particularly for low-level stratiform clouds, are similar to their counterparts in other Arctic regions. The ICECAPS observations and accompanying analyses will be used to improve the understanding of key cloud–atmosphere processes and the manner in which they interact with the GIS. Furthermore, they will facilitate model evaluation and development in this data-sparse but environmentally unique region.

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Nocturnal Continental Low-Level Stratus over Tropical West Africa: Observations and Possible Mechanisms Controlling Its Onset

Monthly Weather Review ◽

10.1175/mwr-d-11-00172.1 ◽

2012 ◽

Vol 140 (6) ◽

pp. 1794-1809 ◽

Cited By ~ 37

Author(s):

Jon M. Schrage ◽

Andreas H. Fink

Keyword(s):

West Africa ◽

Ground Level ◽

Static Stability ◽

Cloud Formation ◽

Cloud Base ◽

Near Surface ◽

Low Level ◽

Stratiform Clouds ◽

Low Level Jet ◽

Potential Factors

Abstract Some spatiotemporal characteristics and possible mechanisms controlling the onset of the widespread, low-level nocturnal stratiform clouds that formed during May–October 2006 over southern tropical West Africa are investigated using cloudiness observations from surface weather stations, data from various satellite platforms, and surface-based remote sensing profiles at Nangatchori in central Benin. It is found that the continental stratus is lower than the maritime stratus over the Gulf of Guinea and persists well into the noon hours. For the study period, a clear seasonal cycle was documented, as well as a dependence on latitude with the cloudiest zone north of the coastal zone and south of approximately 9°N. It is also shown that nonprecipitating clear and cloudy nights observed at Nangatchori in central Benin often reflect clearer and cloudier than normal conditions over a wide region of southern West Africa. At Nangatchori, on average the stratus developed at 0236 UTC (about local time) with an extremely low cloud base at 172 m (above ground level) when averaged over all cloudy nights. About 2–3 h before cloudiness onset, a distinct nighttime low-level jet formed that promoted static destabilization and a low Richardson number flow underneath it. The ensuing vertical upward mixing of moisture that accumulated under the near-surface inversion after sunset caused the cloud formation. It is argued that a strong shear underneath the nighttime low-level jet is the major process for cloud formation, but the low-level static stability and the time scale of the shear-driven mixing are other potential factors.

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Estimating near-surface climatology of multi-reanalyses over the Greenland Ice Sheet

Atmospheric Research ◽

10.1016/j.atmosres.2021.105676 ◽

2021 ◽

pp. 105676

Author(s):

Wuying Zhang ◽

Yetang Wang ◽

Paul C.J.P. Smeets ◽

Carleen H. Reijmer ◽

Baojuan Huai ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Near Surface

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Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland

Journal of Glaciology ◽

10.3189/002214310793146232 ◽

2010 ◽

Vol 56 (198) ◽

pp. 601-613 ◽

Cited By ~ 148

Author(s):

Ian M. Howat ◽

Jason E. Box ◽

Yushin Ahn ◽

Adam Herrington ◽

Ellyn M. McFadden

Keyword(s):

Ice Sheet ◽

Drainage System ◽

Surface Air Temperature ◽

Greenland Ice Sheet ◽

Flow Speed ◽

High Temporal Resolution ◽

Sea Ice Concentration ◽

Seasonal Behavior ◽

Near Surface ◽

Front Position

AbstractRecent studies indicate that the dynamics of fast-flowing, marine-terminating outlet glaciers of the Greenland ice sheet may be sensitive to climate and ocean forcing on sub-annual timescales. Observations of seasonal behavior of these glaciers at such high temporal resolution, however, are currently few. Here we present observations of front position, flow speed, near-surface air temperature and ocean conditions for six large marine-terminating glaciers in the Uummannaq region of West Greenland, to investigate controls on short-term glacier dynamics. As proposed by other studies, we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange. Our data suggest that high sea-surface temperature, anomalously low sea-ice concentration and reduced melange formation in early 2003 have triggered multi-year retreat of several glaciers in the study area, which is consistent with other regions in Greenland. Of the stable glaciers, only Rink Isbræ exhibits a seasonal speed variation that correlates with variations in front position, with the others undergoing mid-summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution. Drainage of supraglacial lakes and water-filled crevasses results in substantial decreases in speed (40–60%) on fast-flowing glaciers. Our results demonstrate that attempts to model ice-sheet evolution must take into account short-timescale flow dynamics resulting from drainage events and oceanographic conditions.

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PROMICE automatic weather station data

10.5194/essd-2021-80 ◽

2021 ◽

Author(s):

Robert S. Fausto ◽

Dirk van As ◽

Kenneth D. Mankoff ◽

Baptiste Vandecrux ◽

Michele Citterio ◽

...

Keyword(s):

Surface Energy Balance ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Weather Station ◽

Automatic Weather Station ◽

Atmospheric Conditions ◽

Production Chain ◽

Near Surface ◽

Surface Mass ◽

Future Assessment

Abstract. The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) has been measuring climate and ice sheetproperties since 2007. Currently the PROMICE automatic weather station network includes 25 instrumented sites in Greenland.Accurate measurements of the surface and near-surface atmospheric conditions in a changing climate is important for reliablepresent and future assessment of changes to the Greenland ice sheet. Here we present the PROMICE vision, methodology,and each link in the production chain for obtaining and sharing quality-checked data. In this paper we mainly focus on thecritical components for calculating the surface energy balance and surface mass balance. A user-contributable dynamic webbaseddatabase of known data quality issues is associated with the data products at (https://github.com/GEUS-PROMICE/PROMICE-AWS-data-issues/). As part of the living data option, the datasets presented and described here are available atDOI: 10.22008/promice/data/aws, https://doi.org/10.22008/promice/data/aws (Fausto and van As, 2019).

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The darkening of the Greenland ice sheet: trends, drivers, and projections (1981–2100)

The Cryosphere ◽

10.5194/tc-10-477-2016 ◽

2016 ◽

Vol 10 (2) ◽

pp. 477-496 ◽

Cited By ~ 81

Author(s):

Marco Tedesco ◽

Sarah Doherty ◽

Xavier Fettweis ◽

Patrick Alexander ◽

Jeyavinoth Jeyaratnam ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Aerosol Deposition ◽

Near Surface ◽

Multispectral Data ◽

Warming Climate ◽

Air Temperatures ◽

Increasing Trends ◽

Snow And Ice

Abstract. The surface energy balance and meltwater production of the Greenland ice sheet (GrIS) are modulated by snow and ice albedo through the amount of absorbed solar radiation. Here we show, using space-borne multispectral data collected during the 3 decades from 1981 to 2012, that summertime surface albedo over the GrIS decreased at a statistically significant (99 %) rate of 0.02 decade−1 between 1996 and 2012. Over the same period, albedo modelled by the Modèle Atmosphérique Régionale (MAR) also shows a decrease, though at a lower rate ( ∼ −0.01 decade−1) than that obtained from space-borne data. We suggest that the discrepancy between modelled and measured albedo trends can be explained by the absence in the model of processes associated with the presence of light-absorbing impurities. The negative trend in observed albedo is confined to the regions of the GrIS that undergo melting in summer, with the dry-snow zone showing no trend. The period 1981–1996 also showed no statistically significant trend over the whole GrIS. Analysis of MAR outputs indicates that the observed albedo decrease is attributable to the combined effects of increased near-surface air temperatures, which enhanced melt and promoted growth in snow grain size and the expansion of bare ice areas, and to trends in light-absorbing impurities (LAI) on the snow and ice surfaces. Neither aerosol models nor in situ and remote sensing observations indicate increasing trends in LAI in the atmosphere over Greenland. Similarly, an analysis of the number of fires and BC emissions from fires points to the absence of trends for such quantities. This suggests that the apparent increase of LAI in snow and ice might be related to the exposure of a "dark band" of dirty ice and to increased consolidation of LAI at the surface with melt, not to increased aerosol deposition. Albedo projections through to the end of the century under different warming scenarios consistently point to continued darkening, with albedo anomalies averaged over the whole ice sheet lower by 0.08 in 2100 than in 2000, driven solely by a warming climate. Future darkening is likely underestimated because of known underestimates in modelled melting (as seen in hindcasts) and because the model albedo scheme does not currently include the effects of LAI, which have a positive feedback on albedo decline through increased melting, grain growth, and darkening.

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Supercooled liquid fogs over the central Greenland Ice Sheet

Atmospheric Chemistry and Physics ◽

10.5194/acp-19-7467-2019 ◽

2019 ◽

Vol 19 (11) ◽

pp. 7467-7485

Author(s):

Christopher J. Cox ◽

David C. Noone ◽

Max Berkelhammer ◽

Matthew D. Shupe ◽

William D. Neff ◽

...

Keyword(s):

Radiative Forcing ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Extreme Case ◽

Supercooled Liquid ◽

Surface Radiation ◽

Radiative Properties ◽

Small Contribution ◽

Near Surface ◽

Radiation Fogs

Abstract. 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 µm 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.

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Depth and properties of freshwater export from the Greenland Ice Sheet modulated by ice-ocean processes

10.5194/egusphere-egu2020-12976 ◽

2020 ◽

Author(s):

Donald Slater ◽

Fiamma Straneo

Keyword(s):

Large Scale ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Regional Variability ◽

Near Surface ◽

Fjord Circulation ◽

Ocean Reanalyses ◽

Freshwater Export ◽

Ocean Processes

<p>Freshwater export from the Greenland Ice Sheet to the surrounding ocean has increased by 50% since the early 1990s, and may triple over the coming century under high greenhouse gas emissions. This increasing freshwater has the potential to influence both the regional and large-scale ocean, including marine ecosystems. Yet quantification of these impacts remains uncertain in part due to poor characterization of freshwater export, and in particular the transformation of freshwater around the ice sheet margin by ice-ocean processes, such as submarine melting, plumes and fjord circulation. Here, we combine in-situ observations, ocean reanalyses and simple models for ice-ocean processes to estimate the depth and properties of freshwater export around the full Greenland ice sheet from 1991 to present. The results show significant regional variability driven primarily by the depth at which freshwater runoff leaves the ice sheet. Areas with deeply-grounded marine-terminating glaciers are likely to export freshwater to the ocean as a dilute mixture of freshwater and externally-sourced deep water masses, while freshwater from areas with many land-terminating glaciers is exported as a more concentrated mixture of freshwater and near-surface waters. A handful of large glacier-fjord systems dominate ice sheet freshwater export, and the vast majority of freshwater export occurs subsurface. Our results provide an ice sheet-wide first-order characterization of how ice-ocean processes modulate Greenland freshwater export, and are an important step towards accurate representation of Greenland freshwater in large-scale ocean models.</p>

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Partitioning of melt energy and meltwater fluxes in the ablation zone of the west Greenland ice sheet

The Cryosphere ◽

10.5194/tc-2-179-2008 ◽

2008 ◽

Vol 2 (2) ◽

pp. 179-189 ◽

Cited By ~ 106

Author(s):

M. van den Broeke ◽

P. Smeets ◽

J. Ettema ◽

C. van der Veen ◽

R. van de Wal ◽

...

Keyword(s):

Ice Sheet ◽

Greenland Ice Sheet ◽

Snow Accumulation ◽

Ablation Zone ◽

The West ◽

Near Surface ◽

Surface Mass ◽

West Greenland ◽

Ice Melt ◽

June July

Abstract. We present four years (August 2003–August 2007) of surface mass balance data from the ablation zone of the west Greenland ice sheet along the 67° N latitude circle. Sonic height rangers and automatic weather stations continuously measured accumulation/ablation and near-surface climate at distances of 6, 38 and 88 km from the ice sheet margin at elevations of 490, 1020 and 1520 m a.s.l. Using a melt model and reasonable assumptions about snow density and percolation characteristics, these data are used to quantify the partitioning of energy and mass fluxes during melt episodes. The lowest site receives very little winter accumulation, and ice melting is nearly continuous in June, July and August. Due to the lack of snow accumulation, little refreezing occurs and virtually all melt energy is invested in runoff. Higher up the ice sheet, the ice sheet surface freezes up during the night, making summer melting intermittent. At the intermediate site, refreezing in snow consumes about 10% of the melt energy, increasing to 40% at the highest site. The sum of these effects is that total melt and runoff increase exponentially towards the ice sheet margin, each time doubling between the stations. At the two lower sites, we estimate that radiation penetration causes 20–30% of the ice melt to occur below the surface.

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Drifting snow measurements on the Greenland Ice Sheet and their application for model evaluation

The Cryosphere ◽

10.5194/tc-8-801-2014 ◽

2014 ◽

Vol 8 (2) ◽

pp. 801-814 ◽

Cited By ~ 10

Author(s):

J. T. M. Lenaerts ◽

C. J. P. P. Smeets ◽

K. Nishimura ◽

M. Eijkelboom ◽

W. Boot ◽

...

Keyword(s):

High Frequency ◽

Climate Model ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Size Distributions ◽

Near Surface ◽

Drifting Snow ◽

Surface Climate ◽

Meteorological Observations ◽

Regional Atmospheric Climate Model

Abstract. This paper presents autonomous drifting snow observations performed on the Greenland Ice Sheet in the fall of 2012. High-frequency snow particle counter (SPC) observations at ~ 1 m above the surface provided drifting snow number fluxes and size distributions; these were combined with meteorological observations at six levels. We identify two types of drifting snow events: katabatic events are relatively cold and dry, with prevalent winds from the southeast, whereas synoptic events are short lived, warm and wet. Precipitating snow during synoptic events disturbs the drifting snow measurements. Output of the regional atmospheric climate model RACMO2, which includes the drifting snow routine PIEKTUK-B, agrees well with the observed near-surface climate at the site, as well as with the frequency and timing of drifting snow events. Direct comparisons with the SPC observations at 1 m reveal that the model overestimates the horizontal snow transport at this level, which can be related to an overestimation of saltation and the typical size of drifting snow particles.

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Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling

10.5194/tc-2017-277 ◽

2018 ◽

Author(s):

Achim Heilig ◽

Olaf Eisen ◽

Michael MacFerrin ◽

Marco Tedesco ◽

Xavier Fettweis

Keyword(s):

Water Content ◽

Liquid Water ◽

Pore Space ◽

Regional Climate ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Liquid Water Content ◽

Water Infiltration ◽

Near Surface ◽

Percolation Zone

Abstract. Increasing melt over the Greenland ice sheet (GrIS) recorded over the past years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness are observable in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779° N/46.2856° W) at 2120 m a.s.l. The radar is capable to monitor quasi-continuously changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well, for both, timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg/m2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.

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