scholarly journals Strong Summer Atmospheric Rivers Trigger Greenland Ice Sheet Melt through Spatially Varying Surface Energy Balance and Cloud Regimes

2020 ◽  
Vol 33 (16) ◽  
pp. 6809-6832 ◽  
Author(s):  
Kyle S. Mattingly ◽  
Thomas L. Mote ◽  
Xavier Fettweis ◽  
Dirk van As ◽  
Kristof Van Tricht ◽  
...  
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Heat Fluxes ◽  
Water Vapor Transport ◽  
Turbulent Heat ◽  
Atmospheric Rivers ◽  
Turbulent Heat Fluxes

ABSTRACTMass loss from the Greenland Ice Sheet (GrIS) has accelerated over the past two decades, coincident with rapid Arctic warming and increasing moisture transport over Greenland by atmospheric rivers (ARs). Summer ARs affecting western Greenland trigger GrIS melt events, but the physical mechanisms through which ARs induce melt are not well understood. This study elucidates the coupled surface–atmosphere processes by which ARs force GrIS melt through analysis of the surface energy balance (SEB), cloud properties, and local- to synoptic-scale atmospheric conditions during strong summer AR events affecting western Greenland. ARs are identified in MERRA-2 reanalysis (1980–2017) and classified by integrated water vapor transport (IVT) intensity. SEB, cloud, and atmospheric data from regional climate model, observational, reanalysis, and satellite-based datasets are used to analyze melt-inducing physical processes during strong, >90th percentile “AR90+” events. Near AR “landfall,” AR90+ days feature increased cloud cover that reduces net shortwave radiation and increases net longwave radiation. As these oppositely signed radiative anomalies partly cancel during AR90+ events, increased melt energy in the ablation zone is primarily provided by turbulent heat fluxes, particularly sensible heat flux. These turbulent heat fluxes are driven by enhanced barrier winds generated by a stronger synoptic pressure gradient combined with an enhanced local temperature contrast between cool over-ice air and the anomalously warm surrounding atmosphere. During AR90+ events in northwest Greenland, anomalous melt is forced remotely through a clear-sky foehn regime produced by downslope flow in eastern Greenland.

scholarly journals The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes

2002 ◽  
Vol 6 (1) ◽  
pp. 85-100 ◽  
Author(s):  
Z. Su
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Surface Energy Balance ◽  
Heat Fluxes ◽  
Physical Parameters ◽  
Turbulent Heat ◽  
Evaporative Fraction ◽  
Balance System ◽  
Turbulent Heat Fluxes

Abstract. A Surface Energy Balance System (SEBS) is proposed for the estimation of atmospheric turbulent fluxes and evaporative fraction using satellite earth observation data, in combination with meteorological information at proper scales. SEBS consists of: a set of tools for the determination of the land surface physical parameters, such as albedo, emissivity, temperature, vegetation coverage etc., from spectral reflectance and radiance measurements; a model for the determination of the roughness length for heat transfer; and a new formulation for the determination of the evaporative fraction on the basis of energy balance at limiting cases. Four experimental data sets are used to assess the reliabilities of SEBS. Based on these case studies, SEBS has proven to be capable to estimate turbulent heat fluxes and evaporative fraction at various scales with acceptable accuracy. The uncertainties in the estimated heat fluxes are comparable to in-situ measurement uncertainties. Keywords: Surface energy balance, turbulent heat flux, evaporation, remote sensing

scholarly journals Dependence of Eemian Greenland temperature reconstructions on the ice sheet topography

2013 ◽  
Vol 9 (6) ◽  
pp. 6683-6732
Author(s):  
N. Merz ◽  
A. Born ◽  
C. C. Raible ◽  
H. Fischer ◽  
T. F. Stocker
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Large Scale ◽  
Surface Energy Balance ◽  
Climate Model ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Lapse Rate

Abstract. The influence of a reduced Greenland ice sheet (GrIS) on Greenland's surface climate during the Eemian interglacial is studied using a comprehensive climate model. We find a distinct impact of changes in the GrIS topography on Greenland's surface air temperatures (SAT) even when correcting for changes in surface elevation which influences SAT through the lapse rate effect. The resulting lapse rate corrected SAT anomalies are thermodynamically driven by changes in the local surface energy balance rather than dynamically caused through anomalous advection of warm/cold air masses. The large-scale circulation is indeed very stable among all sensitivity experiments and the NH flow pattern does not depend on Greenland's topography in the Eemian. In contrast, Greenland's surface energy balance is clearly influenced by changes in the GrIS topography and this impact is seasonally diverse. In winter, the variable reacting strongest to changes in the topography is the sensible heat flux (SHFLX). The reason is its dependence on surface winds, which themselves are controlled to a large extent by the shape of the GrIS. Hence, regions where a receding GrIS causes higher surface wind velocities also experience anomalous warming through SHFLX. Vice-versa, regions that become flat and ice-free are characterized by low wind speeds, low SHFLX and anomalous cold winter temperatures. In summer, we find surface warming induced by a decrease in surface albedo in deglaciated areas and regions which experience surface melting. The Eemian temperature records derived from Greenland proxies, thus, likely include a temperature signal arising from changes in the GrIS topography. For the NEEM ice core site, our model suggests that up to 3.2 °C of the annual mean Eemian warming can be attributed to these topography-related processes and hence is not necessarily linked to large-scale climate variations.

Time-Domain Reflectometry Observations of Meltwater Percolation and Retention in the Firn Layer of the Greenland Ice Sheet

2020 ◽  
Author(s):  
Samira Samimi ◽  
Shawn Marshall ◽  
Michael McFerrin
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Time Domain ◽  
Liquid Water ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Time Domain Reflectometry ◽  
Latent Heat Release ◽  
Near Surface

<p>Mass loss from the Greenland Ice Sheet has increased in recent decades due to significant increases in surface melt and runoff. The fraction of summer melt retains as a liquid water or refreezes as it percolates into the underlying cold firn, acting as a buffer to the summer runoff. There are challenges to quantifying both infiltration and refreezing of meltwater in this complex heterogeneous cold firn and to understand the spatial variability of these processes. In this study we present continuous in situ measurements of near-surface temperature and dielectric permittivity, a proxy for volumetric water content, using TDR (Time Domain Reflectometry) methods in the percolation zone of the southern Greenland Ice Sheet. We established two observation sites near Dye 2 in April, 2016, excavating firn pits to depths of 2.2 and 5.3 m. The two sites are 650 m apart to quantify the percolation and refreezing of meltwater and to observe the spatial variability of these processes through summer 2016. Thermistor arrays were used to track the thermal signature of meltwater penetration in firn, through the effects of latent heat release when meltwater refreezes. Through the addition of TDR probes, we attempt to directly quantify meltwater volume as well as hydraulic conductivity of the near-surface snow and firn. An automatic weather station (AWS) configured for surface energy balance monitoring was also installed. AWS data were used to calculate the surface energy balance and model meltwater production. The melting front, characterized by 0°C conditions and direct evidence of liquid water, penetrated to a depth of between 1.8 and 2.1 m in summer 2016; at depths of 2.1 m and greater, temperatures remained below 0°C, there was no evidence of abrupt warming (i.e. latent heat release), and dielectric permittivities remained at their background levels. Meltwater penetrated several thick ice layers, but not until temperatures reached the melting point at these depths, implying that ice layers may transition to a permeable ‘slush’ layer, given enough conductive and latent heating, permitting progressive penetration of meltwater to depth. Firn temperatures (sub-zero conditions below ~2 m) appear to have been the main barrier to deep penetration of meltwater during summer 2016.</p>

Contribution of turbulent heat fluxes to surface ablation on the Greenland ice sheet.

2020 ◽  
Author(s):  
Maurice Van Tiggelen ◽  
Paul Smeets ◽  
Carleen Reijmer ◽  
Brice Noël ◽  
Jakob Steiner ◽  
...  
Keyword(s):  
Surface Roughness ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Turbulent Fluxes ◽  
Heat Fluxes ◽  
Model Parameters ◽  
Turbulent Heat ◽  
Surface Ablation ◽  
Turbulent Heat Fluxes

<p>Over ice sheets and glaciers, the turbulent heat fluxes are, next to the radiative fluxes, the second largest source of energy driving the ablation. In general, most (climate) models use a bulk turbulence parametrization for the estimation of these energy fluxes. Recent work suggest that the turbulent heat fluxes might be greatly underestimated by such models. Unfortunately, only a few direct and long-term observations of turbulent fluxes are available over ice sheets to evaluate their inclusion in models. </p><p>In this study, we developed a vertical propeller eddy-covariance method to continuously monitor the sensible heat fluxes over the Greenland ice sheet (GrIS). We quantify its contribution to surface ablation using three years of data from the K-transect, located in the western ablation area of the GrIS. The direct flux measurements are also compared to those from several bulk turbulence models, and to a high-resolution regional climate model (RACMO2), in order to quantify modelling uncertainty.</p><p>The differences between observations and models highlight the need for upgrading the bulk turbulence parameterizations and especially the model parameters, such as the surface roughness lengths. We also find that during short but extreme warm events, the turbulent heat fluxes become the largest source for surface ablation. Typical for such intense events on the K-transect are fast changes in wind direction, which cause changes in the surface roughness parameters due to the anisotropic feature of the ice hummocks. These parameters are critical for modelling the turbulent fluxes in bulk parameterizations, but are often variable and unknown. We conclude with drone topography measurements to better constrain the surface roughness locally, and discuss methods to improve the modelling of turbulent surface fluxes on the whole GrIS.</p>

scholarly journals Diurnal land surface energy balance partitioning estimated from the thermodynamic limit of a cold heat engine

2018 ◽  
Author(s):  
Axel Kleidon ◽  
Maik Renner
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Thermodynamic Limit ◽  
Land Surface ◽  
Radiative Forcing ◽  
Surface Energy Balance ◽  
Heat Engine ◽  
Turbulent Fluxes ◽  
Heat Fluxes ◽  
Turbulent Heat

Abstract. Turbulent fluxes strongly shape the conditions at the land surface, yet they are typically formulated in terms of semi-empirical parameterisations that make it difficult to derive theoretical estimates of how global change impacts land surface functioning. Here, we describe these turbulent fluxes as the result of a thermodynamic process that generates work to sustain convective motion and thus maintains the turbulent exchange between the land surface and the atmosphere. We first derive a limit from the second law of thermodynamics that is equivalent to the Carnot limit, but which explicitly accounts for diurnal heat storage changes in the lower atmosphere. We then use this limit of a cold heat engine together with the surface energy balance to infer the maximum power that can be derived from the turbulent fluxes for a given solar radiative forcing. The surface energy balance partitioning estimated from this thermodynamic limit requires no empirical parameters and compares very well with the observed partitioning of absorbed solar radiation into radiative and turbulent heat fluxes across a range of climates, with correlation coefficients r2 ≥ 95 % and slopes near one. These results suggest that turbulent heat fluxes on land operate near their thermodynamic limit on how much convection can be generated from the local radiative forcing. It implies that this type of approach can be used to derive first-order estimates of global change that are solely based on physical principles.

scholarly journals Long-term surface energy balance of the western Greenland ice sheet and the role of large-scale circulation variability

2020 ◽  
Author(s):  
Baojuan Huai ◽  
Michiel R. van den Broeke ◽  
Carleen H. Reijmer
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Large Scale ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Climate Variables ◽  
Large Scale Circulation ◽  
Near Surface ◽  
Surface Climate

Abstract. We present the surface energy balance (SEB) of the west Greenland ice sheet (GrIS), using an energy balance model forced with hourly observations from nine automatic weather stations (AWS) along two transects: the K-transect with seven AWS in the southwest and the T-transect with two AWS in the northwest. Modeled and observed surface temperatures for non-melting conditions agree well, with RMSEs of 1.1–1.6 K, while reasonable agreement is found between modeled and observed 10-day cumulative ice melt. Absorbed shortwave radiation (Snet) is the main energy source for melting (M), followed by the sensible heat flux (Qh). The multi-year average seasonal cycle of SEB components show that Snet and M peak in July at all AWS. The turbulent fluxes of sensible (Qh) and latent heat (Ql) decrease significantly with elevation, and the latter becomes negative at higher elevations, partly offsetting Qh. Average June, July, August (JJA) albedo values are  0.7 for the higher stations. The near-surface climate variables and surface energy fluxes from reanalysis products ERA-interim, ERA5 and the regional climate model RACMO2.3 were compared to the AWS values. The newer ERA5 product only significantly improves on ERA-interim for albedo. The regional model RACMO2.3, which has higher resolution (5.5 km) and a dedicated snow/ice module, unsurprisingly outperforms the re-analyses for (near-) surface climate variables, but the reanalyses are indispensable to detect dependencies of west Greenland climate and melt on large-scale circulation variability. We correlate ERA5 with the AWS data to show a significant positive correlation of western GrIS summer surface temperature and melt with the Greenland Blocking Index (GBI), and weaker and opposite correlations with the North Atlantic Oscillation (NAO). This analysis may further help to explain melting patterns in the western GrIS from the perspective of circulation anomalies.

scholarly journals Surface energy balance observed at the SIGMA-A site on the northwest Greenland ice sheet

2021 ◽  
Vol 83 (2) ◽  
pp. 143-154
Author(s):  
Satoshi HIROSE ◽  
Teruo AOKI ◽  
Masashi NIWANO ◽  
Sumito MATOBA ◽  
Tomonori TANIKAWA ◽  
...  
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
A Site

scholarly journals Dependence of Eemian Greenland temperature reconstructions on the ice sheet topography

2014 ◽  
Vol 10 (3) ◽  
pp. 1221-1238 ◽  
Author(s):  
N. Merz ◽  
A. Born ◽  
C. C. Raible ◽  
H. Fischer ◽  
T. F. Stocker
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Large Scale ◽  
Surface Energy Balance ◽  
Climate Model ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Lapse Rate

Abstract. The influence of a reduced Greenland Ice Sheet (GrIS) on Greenland's surface climate during the Eemian interglacial is studied using a set of simulations with different GrIS realizations performed with a comprehensive climate model. We find a distinct impact of changes in the GrIS topography on Greenland's surface air temperatures (SAT) even when correcting for changes in surface elevation, which influences SAT through the lapse rate effect. The resulting lapse-rate-corrected SAT anomalies are thermodynamically driven by changes in the local surface energy balance rather than dynamically caused through anomalous advection of warm/cold air masses. The large-scale circulation is indeed very stable among all sensitivity experiments and the Northern Hemisphere (NH) flow pattern does not depend on Greenland's topography in the Eemian. In contrast, Greenland's surface energy balance is clearly influenced by changes in the GrIS topography and this impact is seasonally diverse. In winter, the variable reacting strongest to changes in the topography is the sensible heat flux (SHF). The reason is its dependence on surface winds, which themselves are controlled to a large extent by the shape of the GrIS. Hence, regions where a receding GrIS causes higher surface wind velocities also experience anomalous warming through SHF. Vice-versa, regions that become flat and ice-free are characterized by low wind speeds, low SHF, and anomalous low winter temperatures. In summer, we find surface warming induced by a decrease in surface albedo in deglaciated areas and regions which experience surface melting. The Eemian temperature records derived from Greenland proxies, thus, likely include a temperature signal arising from changes in the GrIS topography. For the Eemian ice found in the NEEM core, our model suggests that up to 3.1 °C of the annual mean Eemian warming can be attributed to these topography-related processes and hence is not necessarily linked to large-scale climate variations.

scholarly journals The seasonal cycle and interannual variability of surface energy balance and melt in the ablation zone of the west Greenland ice sheet

2011 ◽  
Vol 5 (2) ◽  
pp. 377-390 ◽  
Author(s):  
M. R. van den Broeke ◽  
C. J. P. P. Smeets ◽  
R. S. W. van de Wal
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Interannual Variability ◽  
Seasonal Cycle ◽  
Surface Energy Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Shortwave Radiation ◽  
Ablation Zone ◽  
The West

Abstract. We present the seasonal cycle and interannual variability of the surface energy balance (SEB) in the ablation zone of the west Greenland ice sheet, using seven years (September 2003–August 2010) of hourly observations from three automatic weather stations (AWS). The AWS are situated along the 67° N latitude circle at elevations of 490 m a.s.l. (S5), 1020 m a.s.l. (S6) and 1520 m a.s.l. (S9) at distances of 6, 38 and 88 km from the ice sheet margin. The hourly AWS data are fed into a model that calculates all SEB components and melt rate; the model allows for shortwave radiation penetration in ice and time-varying surface momentum roughness. Snow depth is prescribed from albedo and sonic height ranger observations. Modelled and observed surface temperatures for non-melting conditions agree very well, with RMSE's of 0.97–1.26 K. Modelled and observed ice melt rates at the two lowest sites also show very good agreement, both for total cumulative and 10-day cumulated amounts. Melt frequencies and melt rates at the AWS sites are discussed. Although absorbed shortwave radiation is the most important energy source for melt at all three sites, interannual melt variability at the lowest site is driven mainly by variability in the turbulent flux of sensible heat. This is explained by the quasi-constant summer albedo in the lower ablation zone, limiting the influence of the melt-albedo feedback, and the proximity of the snow free tundra, which heats up considerably in summer.

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