How to model enhanced firn densification due to strain softening

2021 ◽  
Author(s):  
Falk Oraschewski ◽  
Aslak Grinsted
Keyword(s):  
Strain Softening ◽  
Accumulation Rate ◽  
Strain Rates ◽  
Ice Streams ◽  
North East ◽  
The North ◽  
General Importance ◽  
Model Extension ◽  
Horizontal Strain ◽  
Power Law Creep

<p>Most classical firn densification models merely consider temperature and accumulation rate as variable input parameters. However, in locations with high horizontal strain rates, such as the shear margins of ice streams, a reduced firn thickness can be observed. This is explained by an enhancement of power-law creep due to the effect of strain softening, which is not yet captured by existing firn models. We present a model extension that corrects the densification rate, predicted by any classical, climate-forced firn model, for the effect of strain softening caused by horizontal strain rates. With the presented model firn densities measured along a cross-section of the North-East Greenland ice stream (NEGIS) are reproduced with good agreement, validating the accuracy of the developed model. The results further indicate the general importance of considering strain rates in firn densification modeling and pave the way for the development of a firn model that inherently uses temperature, accumulation rate and horizontal strain rates as forcing parameters.</p>

scholarly journals Modelling of firn densification in the presence of horizontal strain rates

2021 ◽  
Author(s):  
Falk M. Oraschewski
Keyword(s):  
Strain Softening ◽  
Accumulation Rate ◽  
Pure Shear ◽  
Greenland Ice Sheet ◽  
Strain Rates ◽  
Ice Streams ◽  
North East ◽  
The North ◽  
Horizontal Strain ◽  
Power Law Creep

The densification of polar firn that is subjected to horizontal strain rates is studied. A model for the enhanced densification of the firn by strain softening is developed. Strain softening describes an acceleration of power-law creep in the presence of high horizontal strain rates, which was suggested to explain the occurrence of exceptionally thin firn in the shear margins of ice streams. With the model the effect of strain softening is compared to other strain-driven densification mechanisms, like pure shear and strain heating, and to potential variations of temperature and accumulation rate. Thereby, strain softening is identified to dominate firn densification at high strain rates. A recorded density profile along a cross-section of the North-East Greenland ice stream (NEGIS) is reproduced with the presented model with good agreement in the shear margins. There, the thinning of the firn correlates with the location and magnitude of the shear margin troughs, which indicates that their formation is caused by strain softening. In regions with low strain rates the model overestimates the densification rate. Because of a particularly strong sensitivity of the model to low strain rates and the presence of non-zero strain rates on large parts of the Greenland Ice Sheet (GrIS), it is suggested that empirically tuned densification models already implicitly consider moderate horizontal strain rates. Besides the temperature and the accumulation rate, the effective horizontal strain rate is therefore proposed as a third forcing parameter, that needs to be considered in the development of a physics-based firn densification model.

scholarly journals Modeling enhanced firn densification due to strain softening

2021 ◽  
Author(s):  
Falk M. Oraschewski ◽  
Aslak Grinsted
Keyword(s):  
Strain Softening ◽  
High Strain ◽  
Accumulation Rate ◽  
Ice Core ◽  
Strain Rates ◽  
Overburden Pressure ◽  
North East ◽  
The North ◽  
Horizontal Strain ◽  
Power Law Creep

Abstract. In the accumulation zone of glaciers and ice sheets snow is transformed into glacial ice by firn densification. Classically, this processes is assumed to solely depend on temperature and overburden pressure which is controlled by the accumulation rate. However, exceptionally thin firn layers have been observed in the high-strain shear margins of ice streams. Previously, it has been proposed that this firn thinning can be explained by an enhancement of firn densification due to the effect of strain softening inherent to power-law creep. This hypothesis has not been validated, and the greater firn densities in the presence of horizontal strain rates have not yet been reproduced by models. Here, we develop a model that corrects the firn densification rate predicted by classical, climate-forced models for the effect of strain softening. With the model it is confirmed that strain softening dominates the firn densification process when high strain rates are present. Firn densities along a cross section of the North-East Greenland ice stream (NEGIS) are reproduced with good agreement, validating the accuracy of the developed model. Finally, it is shown that strain softening has significant implications for ice core dating and that it considerably affects the firn properties over wide areas of the polar ice sheet, even at low strain rates. Therefore, we suggest that, besides temperature and accumulation rate, horizontal strain rates should generally be considered as a forcing parameter in firn densification modelling.

scholarly journals Baeothryon alpinum (L.) T.V. Egorova (Cypercaeae) in the Polish Lowlands: distribution, population decrease and implications for conservation

2011 ◽  
Vol 79 (3) ◽  
pp. 215-223
Author(s):  
Paweł Pawlikowski
Keyword(s):  
Lake District ◽  
Southern Poland ◽  
North East ◽  
The North ◽  
Population Decrease ◽  
General Importance ◽  
Adjacent Part ◽  
Minor Losses ◽  
First Time ◽  
North Western

A total of 47 localities of <em>Baeothryon alpinum</em>, hosting population of more than 100 000 shoots, were recorded in the lowland part of Poland during field surveys in the years 2003-2009. Among them were 25 populations discovered for the first time. Out of 57 sites of the species known from literature and unpublished (including herbarium) sources, 35 were not confirmed during the survey, 27 of them being definitely extinct. <em>B. alpinum</em> shows a clear pattern of distribution in Poland, with three main areas of occurrence: 1) the north-easternmost Poland (Lithuanian Lake District with the adjacent parts of the Masurian Lake District and the upper Biebrza river valley in North Podlasie Lowlands), which is part of the species boreal main range; 2) scattered localities in north-western Poland; 3) mountain mires at higher altitudes in the Sudetes and Tatra mountains and the adjacent part of southern Poland. The main aggregation of localities is found in Augustów Forest (including the Sejny Lakeland and Wigry National Park), and in the Góry Sudawskie region with adjacent areas. The biggest Polish population in the "Kobyla Biel" fen near Augustów consisted of several dozens of thousands of shoots. The Lithuanian Lake District is an area of general importance for the conservation of <em>B. alpinum</em> in Poland. The species is threatened, first of all, due to secondary succession (mires overgrowing with shrubs, trees and reed) and requires conservation measures as well as establishing nature reserves in places where it occurs. The degree that <em>B. alpinum</em> decreases in number is strikingly different in particular regions of Poland - it has lost most of its localities in north-western Poland and in Masurian Lake District, while in the Lithuanian Lake District and the upper Biebrza valley there are minor losses only. Depending on the region (from the west to the east and from the south-west to the north-east), the species should be given extinct or critically endangered (regions of north-western and southern Poland), endangered (Masurian Lake District), vulnerable (North Podlasie Lowlands) and near threatened (Lithuanian lake District) status. Although the disappearance of the populations beyond the species main range is a common phenomenon, the presented pattern is man-related and connected with differences in land management.

scholarly journals The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers

2000 ◽  
Vol 46 (152) ◽  
pp. 75-87 ◽  
Author(s):  
N. A. Nereson ◽  
C. F. Raymond ◽  
R.W. Jacobel ◽  
E. D. Waddington
Keyword(s):  
Accumulation Rate ◽  
Small Time ◽  
Internal Layer ◽  
Internal Layers ◽  
West Antarctica ◽  
Ice Streams ◽  
Accumulation Pattern ◽  
The North ◽  
Siple Dome ◽  
Radio Echo Sounding

AbstractThe spatial distribution of accumulation across Siple Dome, West Antarctica, is determined from analysis of the shapes of internal layers detected by radio-echo sounding (RES) measurements. A range of assumed accumulation patterns is used in an ice-flow model to calculate a set of internal layer patterns. Inverse techniques are used to determine which assumed accumulation pattern produces a calculated internal layer pattern that best matches the shape of internal layers from RES measurements. All of the observed internal layer shapes at Siple Dome can be matched using a spatially asymmetric accumulation pattern which has been steady over time. Relative to the divide, the best-fitting accumulation pattern predicts 40% less accumulation 30 km from the divide on the south flank of Siple Dome and 15–40% more accumulation 30 km from the divide on the north flank. The data also allow the possibility for a small time variation of the pattern north of the divide. The mismatch between the calculated and the observed layer shapes is slightly reduced when the accumulation rate north of the divide is higher in the past (> 5kyr BP) than at present. Sensitivity tests show that the predicted change in the spatial accumulation pattern required to cause the slight Siple Dome divide migration (inferred from previous studies) would be detectable in the internal layer pattern if it persisted for > 2 kyr. Our analysis reveals no evidence that such a change has occurred, and the possible change in accumulation distribution allowed by the data is in the opposite sense. Therefore, it is unlikely that the Siple Dome divide migration has been caused by a temporal change in the spatial pattern of accumulation. This conclusion suggests the migration may be caused by elevation changes in Ice Streams C and D at the boundaries of Siple Dome.

The upper 2121 m at EastGRIP - Results from physical properties of NEGIS

2020 ◽  
Author(s):  
Nicolas Stoll ◽  
Ilka Weikusat ◽  
Johanna Kerch ◽  
Jan Eichler ◽  
Wataru Shigeyama ◽  
...  
Keyword(s):  
Grain Size ◽  
Ice Core ◽  
Rapid Evolution ◽  
Ice Cores ◽  
Flow Behaviour ◽  
Ice Streams ◽  
High Resolution Data ◽  
North East ◽  
The North ◽  
Ice Stream

&lt;div&gt; &lt;div&gt; &lt;div&gt; &lt;p&gt;Here we present the ice microstructure and CPO (c-axes fabric) data from the upper 2121 m of the EastGRIP ice core, an on-going deep drilling project on the North East Greenland Ice Stream. Understanding ice flow behaviour of fast flowing ice streams is crucial for accurate projections of future global sea level rise, but is still poorly understood due to e.g. missing observational fabric data from ice streams.&lt;/p&gt; &lt;p&gt;The presented CPO patterns found at EastGRIP show (1) a rapid evolution of c-axes anisotropy compared to deep ice cores from less dynamic sites, (2) a CPO evolution towards a strong vertical girdle and (3) CPO patterns that have not previously been directly observed in ice. Furthermore, data regarding grain properties (e.g. grain size) and indications of dynamic recrystallization, already at shallow depths, are presented.&lt;/p&gt; &lt;p&gt;The ice CPO shows a clear evolution with depth. In the first measurements at 111 m depth a broad single maximum distribution is observed, which transforms into a crossed girdle CPO (196-294 m). With increasing depth, an evolution towards a vertical girdle c-axes distribution occurs. Below 1150 m the CPO evolves into a vertical girdle with a higher density of c-axes oriented horizontally, a novel CPO in ice. These CPO patterns indicate a depth-related change in deformation modes, from vertical compression to extensional deformation along flow.&lt;/p&gt; &lt;p&gt;Grain size values are similar to results from other Greenlandic deep ice cores. Grain size evolution is characterized by an increase until 500 m depth, a decrease until 1360 m depth and mainly constant values in the Glacial. These findings are accompanied by indications of an early onset of dynamic recrystallisation e.g. irregular grain shapes, protruding grains and island grains.&lt;/p&gt; &lt;p&gt;The presented high-resolution data enable, for the very first time, a detailed and data- based look into a fast-flowing ice stream and are an important step towards a better understanding of the rheology of ice and its flow behaviour.&lt;/p&gt; &lt;/div&gt; &lt;/div&gt; &lt;/div&gt;

Ras al Khaimah: further archaeological discoveries

Antiquity ◽  
1976 ◽  
Vol 50 (200) ◽  
pp. 216-222
Author(s):  
Beatrice De Cardi
Keyword(s):  
United Arab Emirates ◽  
The State ◽  
Focused Attention ◽  
North East ◽  
The North ◽  
History Of ◽  
The City

Ras a1 Khaimah is the most northerly of the seven states comprising the United Arab Emirates and its Ruler, H. H. Sheikh Saqr bin Mohammad al-Qasimi, is keenly interested in the history of the state and its people. Survey carried out there jointly with Dr D. B. Doe in 1968 had focused attention on the site of JuIfar which lies just north of the present town of Ras a1 Khaimah (de Cardi, 1971, 230-2). Julfar was in existence in Abbasid times and its importance as an entrep6t during the sixteenth and seventeenth centuries-the Portuguese Period-is reflected by the quantity and variety of imported wares to be found among the ruins of the city. Most of the sites discovered during the survey dated from that period but a group of cairns near Ghalilah and some long gabled graves in the Shimal area to the north-east of the date-groves behind Ras a1 Khaimah (map, FIG. I) clearly represented a more distant past.

Lichens in the North-East regions of Cyprus

1999 ◽  
Vol 110 (5-6) ◽  
pp. 455-463 ◽  
Author(s):  
S. Güvenç ◽  
Ş Öztürk
Keyword(s):  
North East ◽  
The North

TBE in Italy

2019 ◽  
Author(s):  
Valentina Tagliapietra ◽  
Flavia Riccardo ◽  
Giovanni Rezza
Keyword(s):  
Case Series ◽  
East Part ◽  
Mountainous Regions ◽  
North East ◽  
The North ◽  
Tick Borne Encephalitis ◽  
Low Incidence

Italy is considered a low incidence country for tick-borne encephalitis (TBE) in Europe. Areas at higher risk for TBE in Italy are geographically clustered in the forested and mountainous regions and provinces in the north east part of the country, as suggested by TBE case series published over the last decade.

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