scholarly journals Seasonal Variability in In-situ Supraglacial Streamflow and Drivers in Southwest Greenland in 2016

2020 ◽  
Author(s):  
Rohi Muthyala ◽  
Asa K. Rennermalm ◽  
Sasha Z. Leidman ◽  
Matthew G. Cooper ◽  
Sarah W. Cooley ◽  
...  
Keyword(s):  
Seasonal Variability ◽  
Greenland Ice Sheet ◽  
Shortwave Radiation ◽  
Daily Maximum ◽  
Significant Heterogeneity ◽  
Stream Networks ◽  
Ice Dynamics ◽  
Stream Discharge ◽  
Southwest Greenland

Abstract. Greenland ice sheet surface runoff is evacuated through supraglacial stream networks, which influence surface mass balance as well as ice dynamics. However, in-situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discharge measurements and continuous water level measurements for 62 days spanning 13 June to 13 August 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedented long record of supraglacial discharge capturing both diurnal and seasonal variability. By comparing in situ hydraulic geometry parameters with previous studies, we find that significant heterogeneity exists such that estimating stream discharge using these parameters over ungauged supraglacial catchments could lead to substantial errors. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting (78 % of melt energy). However, during high melt episodes, the shortwave only contributes to 50 % of melt energy. Instead, the relative contribution of longwave radiation, sensible and latent heat fluxes to overall melt increases by 16.5 %, 4 %, and 7 % respectively. Our data also show a seasonal variation in the timing of daily maximum discharge during clear sky days, shifting from 16:00 local time (i.e., 2.75 hours after solar noon) in late June to 14:00 in late July, then rapidly returns to 16:00 in early August coincident with an abrupt drop in air temperature. These changes in peak daily flow timing can be attributed to a changing effective catchment area, resulting in a smaller stream network supplying water to the outlet at the end of the season throughout the melt season. Further work is needed to uncover how widespread rapid shift in the timing of peak discharge is across Greenland supraglacial streams, and thus their potential impact on meltwater delivery to the subglacial system and ice dynamics.

scholarly journals Application of an improved surface energy balance model to two large valley glaciers in the St. Elias Mountains, Yukon

2020 ◽  
pp. 1-16
Author(s):  
Tim Hill ◽  
Christine F. Dow ◽  
Eleanor A. Bash ◽  
Luke Copland
Keyword(s):  
Energy Balance ◽  
Surface Energy ◽  
Surface Energy Balance ◽  
Shortwave Radiation ◽  
Balance Model ◽  
Landsat 8 ◽  
Glacier Surface ◽  
Ice Dynamics ◽  
Surface Melt

Abstract Glacier surficial melt rates are commonly modelled using surface energy balance (SEB) models, with outputs applied to extend point-based mass-balance measurements to regional scales, assess water resource availability, examine supraglacial hydrology and to investigate the relationship between surface melt and ice dynamics. We present an improved SEB model that addresses the primary limitations of existing models by: (1) deriving high-resolution (30 m) surface albedo from Landsat 8 imagery, (2) calculating shadows cast onto the glacier surface by high-relief topography to model incident shortwave radiation, (3) developing an algorithm to map debris sufficiently thick to insulate the glacier surface and (4) presenting a formulation of the SEB model coupled to a subsurface heat conduction model. We drive the model with 6 years of in situ meteorological data from Kaskawulsh Glacier and Nàłùdäy (Lowell) Glacier in the St. Elias Mountains, Yukon, Canada, and validate outputs against in situ measurements. Modelled seasonal melt agrees with observations within 9% across a range of elevations on both glaciers in years with high-quality in situ observations. We recommend applying the model to investigate the impacts of surface melt for individual glaciers when sufficient input data are available.

Supplementary material to "Seasonal Variability in In-situ Supraglacial Streamflow and Drivers in Southwest Greenland in 2016"

2020 ◽  
Author(s):  
Rohi Muthyala ◽  
Asa K. Rennermalm ◽  
Sasha Z. Leidman ◽  
Matthew G. Cooper ◽  
Sarah W. Cooley ◽  
...  
Keyword(s):  
Seasonal Variability ◽  
Supplementary Material ◽  
Southwest Greenland

scholarly journals Shallow firn cores 1989–2019 in southwest Greenland's percolation zone reveal decreasing density and ice layer thickness after 2012

2021 ◽  
pp. 1-12
Author(s):  
Åsa K. Rennermalm ◽  
Regine Hock ◽  
Federico Covi ◽  
Jing Xiao ◽  
Giovanni Corti ◽  
...  
Keyword(s):  
Layer Thickness ◽  
Pore Space ◽  
Greenland Ice Sheet ◽  
Infiltration Capacity ◽  
In Situ Observations ◽  
Percolation Zone ◽  
Over Time ◽  
Southwest Greenland

Abstract Refreezing of meltwater in firn is a major component of Greenland ice-sheet's mass budget, but in situ observations are rare. Here, we compare the firn density and total ice layer thickness in the upper 15 m of 19 new and 27 previously published firn cores drilled at 15 locations in southwest Greenland (1850–2360 m a.s.l.) between 1989 and 2019. At all sites, ice layer thickness covaries with density over time and space. At the two sites with the earliest observations (1989 and 1998), bulk density increased by 15–18%, in the top 15 m over 28 and 21 years, respectively. However, following the extreme melt in 2012, elevation-detrended density using 30 cores from all sites decreased by 15 kg m−3 a−1 in the top 3.75 m between 2013 and 2019. In contrast, the lowest elevation site's density shows no trend. Thus, temporary build-up in firn pore space and meltwater infiltration capacity is possible despite the long-term increase in Greenland ice-sheet melting.

scholarly journals In situ cosmogenic <sup>10</sup>Be–<sup>14</sup>C–<sup>26</sup>Al measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size

2021 ◽  
Vol 17 (1) ◽  
pp. 419-450
Author(s):  
Nicolás E. Young ◽  
Alia J. Lesnek ◽  
Josh K. Cuzzone ◽  
Jason P. Briner ◽  
Jessica A. Badgeley ◽  
...  
Keyword(s):  
Regional Climate ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Model Data ◽  
Climate History ◽  
Surface Mass ◽  
Current Configuration ◽  
Maximum Extent ◽  
Southwest Greenland

Abstract. Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current configuration. Determining the exact dimensions of the Holocene ice-sheet minimum and the duration that the ice margin rested inboard of its current position remains challenging. Contemporary retreat of the GrIS from its historical maximum extent in southwestern Greenland is exposing a landscape that holds clues regarding the configuration and timing of past ice-sheet minima. To quantify the duration of the time the GrIS margin was near its modern extent we develop a new technique for Greenland that utilizes in situ cosmogenic 10Be–14C–26Al in bedrock samples that have become ice-free only in the last few decades due to the retreating ice-sheet margin at Kangiata Nunaata Sermia (n=12 sites, 36 measurements; KNS), southwest Greenland. To maximize the utility of this approach, we refine the deglaciation history of the region with stand-alone 10Be measurements (n=49) and traditional 14C ages from sedimentary deposits contained in proglacial–threshold lakes. We combine our reconstructed ice-margin history in the KNS region with additional geologic records from southwestern Greenland and recent model simulations of GrIS change to constrain the timing of the GrIS minimum in southwest Greenland and the magnitude of Holocene inland GrIS retreat, as well as to explore the regional climate history influencing Holocene ice-sheet behavior. Our 10Be–14C–26Al measurements reveal that (1) KNS retreated behind its modern margin just before 10 ka, but it likely stabilized near the present GrIS margin for several thousand years before retreating farther inland, and (2) pre-Holocene 10Be detected in several of our sample sites is most easily explained by several thousand years of surface exposure during the last interglaciation. Moreover, our new results indicate that the minimum extent of the GrIS likely occurred after ∼5 ka, and the GrIS margin may have approached its eventual historical maximum extent as early as ∼2 ka. Recent simulations of GrIS change are able to match the geologic record of ice-sheet change in regions dominated by surface mass balance, but they produce a poorer model–data fit in areas influenced by oceanic and dynamic processes. Simulations that achieve the best model–data fit suggest that inland retreat of the ice margin driven by early to middle Holocene warmth may have been mitigated by increased precipitation. Triple 10Be–14C–26Al measurements in recently deglaciated bedrock provide a new tool to help decipher the duration of smaller-than-present ice over multiple timescales. Modern retreat of the GrIS margin in southwest Greenland is revealing a bedrock landscape that was also exposed during the migration of the GrIS margin towards its Holocene minimum extent, but it has yet to tap into a landscape that remained ice-covered throughout the entire Holocene.

Greenland land-terminating glaciers velocity trends during the last two decades

2021 ◽  
Author(s):  
Paul Halas ◽  
Jeremie Mouginot ◽  
Basile de Fleurian ◽  
Petra Langebroek
Keyword(s):  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Melting ◽  
Limited Area ◽  
Ice Dynamics ◽  
Basal Sliding ◽  
Surface Melt ◽  
Ice Sheet Dynamics ◽  
The Impact

&lt;div&gt; &lt;p&gt;Ice losses from the Greenland Ice Sheet have been increasing in the last two decades, leading to a larger contribution to the global sea level rise.&amp;#160;Roughly 40% of the contribution comes from ice-sheet dynamics, mainly regulated by basal sliding.&amp;#160;The sliding component&amp;#160;of glaciers has been observed to be strongly related to surface melting, as water&amp;#160;can eventually&amp;#160;reach the bed and impact&amp;#160;the subglacial water pressure, affecting the basal sliding.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;The link between ice velocities and surface melt on multi-annual time scale is still not totally understood&amp;#160;even though it&amp;#160;is of major importance with expected increasing surface melting.&amp;#160;Several studies showed&amp;#160;some&amp;#160;correlation between an increase in surface melt and a slowdown in&amp;#160;velocities, but&amp;#160;there is no&amp;#160;consensus&amp;#160;on those trends.&amp;#160;Moreover&amp;#160;those&amp;#160;investigations&amp;#160;only&amp;#160;presented results&amp;#160;in a limited area over&amp;#160;Southwest&amp;#160;Greenland.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Here we present the ice motion over many land-terminating glaciers on the Greenland Ice Sheet for the period 2000 - 2020. This type of glacier is ideal for studying processes at the interface between the bed and the ice since they are exempted from interactions with the sea while still being relevant for all glaciers since they share the same basal friction laws. The velocity data was obtained using optical Landsat 7 &amp; 8 imagery and feature-tracking algorithm. We attached importance keeping the starting date of our image pairs similar, and avoided stacking pairs starting before and after melt seasons, resulting in multiple velocity products for each year.&amp;#160;&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Our results show similar velocity trends for previously studied areas with a slowdown until 2012 followed by an acceleration.&amp;#160;This trend however does not seem to be observed on the whole ice sheet and is probably&amp;#160;specific&amp;#160;to&amp;#160;this region&amp;#8217;s&amp;#160;climate forcing.&amp;#160;&lt;/p&gt; &lt;/div&gt;&lt;div&gt; &lt;p&gt;Moreover comparison between ice velocities from different parts of Greenland allows us to observe the impact of different climatic trends on ice dynamics.&lt;/p&gt; &lt;/div&gt;

Future changes in heatwaves over Africa at the convection-permitting scale

2021 ◽  
Author(s):  
Cathryn Birch ◽  
Lawrence Jackson ◽  
Declan Finney ◽  
John Marsham ◽  
Rachel Stratton ◽  
...  
Keyword(s):  
Climate Model ◽  
Daily Rainfall ◽  
Shortwave Radiation ◽  
Daily Maximum ◽  
Future Change ◽  
Driving Mechanisms ◽  
The Future ◽  
Convective Scale ◽  
Climate Model Simulations ◽  
The Impact

&lt;p&gt;Mean temperatures and their extremes have increased over Africa since the latter half of the 20th century and this trend is projected to continue, with very frequent, intense and often deadly heatwaves likely to occur very regularly over much of Africa by 2100. It is crucial that we understand the scale of the future increases in extremes and the driving mechanisms. We diagnose daily maximum wet bulb temperature heatwaves, which allows for both the impact of temperature and humidity, both critical for human health and survivability. During wet bulb heatwaves, humidity and cloud cover increase, which limits the surface shortwave radiation flux but increases longwave warming. It is found from observations and ERA5 reanalysis that approximately 30% of wet bulb heatwaves over Africa are associated with daily rainfall accumulations of more than 1 mm/day on the first day of the heatwave. The first ever pan-African convection-permitting climate model simulations of present-day and RCP8.5 future climate are utilised to illustrate the projected future change in heatwaves, their drivers and their sensitivity to the representation of convection. Compared to ERA5, the convection-permitting model better represents the frequency and magnitude of present-day wet bulb heatwaves than a version of the model with more traditional parameterised convection. The future change in heatwave frequency, duration and magnitude is also larger in the convective-scale simulation, suggesting CMIP-style models may underestimate the future change in wet bulb heat extremes over Africa. The main reason for the larger future change appears to be the ability of the model to produce larger anomalies relative to its climatology in precipitation, cloud and the surface energy balance.&lt;/p&gt;

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