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Wind-Driven Snow Conditions Control the Occurrence Of The Cryosphere, 11, 1351–1370, 2017 https://doi.org/10.5194/tc-11-1351-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License. Wind-driven snow conditions control the occurrence of contemporary marginal mountain permafrost in the Chic-Choc Mountains, south-eastern Canada: a case study from Mont Jacques-Cartier Gautier Davesne1,2, Daniel Fortier1,2, Florent Domine2,3, and James T. Gray1 1Cold Regions Geomorphology and Geotechnical Laboratory, Département de géographie, Université de Montréal, Montréal, Canada 2Centre for Northern Studies, Université Laval, Québec, Canada 3Takuvik Joint International Laboratory, Université Laval and Centre National de Recherche Scientifique, and Département de chimie, Université Laval, Québec, Canada Correspondence to: Gautier Davesne ([email protected]) Received: 13 September 2016 – Discussion started: 18 October 2016 Revised: 9 March 2017 – Accepted: 9 March 2017 – Published: 9 June 2017 Abstract. We present data on the distribution and thermo- the summit characterized by a sporadic herbaceous cover. In physical properties of snow collected sporadically over 4 contrast, for the gentle slopes covered with stunted spruce decades along with recent data of ground surface tempera- (krummholz), and for the steep leeward slope to the south- ture from Mont Jacques-Cartier (1268 m a.s.l.), the highest east of the summit, the MAGST was around 3 ◦C in 2013 summit in the Appalachians of south-eastern Canada. We and 2014. The study concludes that the permafrost on Mont demonstrate that the occurrence of contemporary permafrost Jacques-Cartier, most widely in the Chic-Choc Mountains is necessarily associated with a very thin and wind-packed and by extension in the southern highest summits of the Ap- winter snow cover which brings local azonal topo-climatic palachians, is therefore likely limited to the barren wind- conditions on the dome-shaped summit. The aims of this exposed surface of the summit where the low air tempera- study were (i) to understand the snow distribution pattern ture, the thin snowpack and the wind action bring local cold and snow thermophysical properties on the Mont Jacques- surface conditions favourable to permafrost development. Cartier summit and (ii) to investigate the impact of snow on the spatial distribution of the ground surface temperature (GST) using temperature sensors deployed over the summit. Results showed that above the local treeline, the summit is 1 Introduction characterized by a snow cover typically less than 30 cm thick which is explained by the strong westerly winds interacting The thermal impact of the seasonal snow cover is well-known with the local surface roughness created by the physiogra- as being one of the most critical factors for the spatial distri- phy and surficial geomorphology of the site. The snowpack bution of permafrost, especially in mountainous areas. Sev- structure is fairly similar to that observed on windy Arctic eral studies have been undertaken on this topic in Euro- − tundra with a top dense wind slab (300 to 450 kg m 3/ of pean mountains (Haeberli, 1973; Grüber and Hoelzle, 2001; high thermal conductivity, which facilitates heat transfer be- Luetschg et al., 2008; Farbrot et al., 2011, 2013; Hasler et tween the ground surface and the atmosphere. The mean an- al., 2011; Pogliotti, 2011; Gisnås et al., 2014; Ardelean et nual ground surface temperature (MAGST) below this thin al., 2015; Magnin et al., 2016; Beniston et al., 2017), in Japan ◦ and wind-packed snow cover was about −1 C in 2013 and (Ishikawa, 2003; Ishikawa and Hirakawa, 2000), in the Cana- 2014, for the higher, exposed, blockfield-covered sector of dian Rocky Mountains (Harris, 1981; Lewkowicz and Ednie, Published by Copernicus Publications on behalf of the European Geosciences Union. 1352 G. Davesne et al.: Wind, snow and permafrost in the Chic-Choc Mountains Table 1. Abbreviations used in this paper. (Howe, 1971; Walegur and Nelson, 2003); the second be- neath the 1268 m high summit of Mont Jacques-Cartier, the Abbrev. Definition highest point in south-eastern Canada situated in the Chic- DDFs sum of freezing degree days at the ground surface Choc Mountains (Fig. 1) (Gray and Brown, 1979, 1982). DDFa sum of freezing degree days of the air The latter site, with a well-documented record of geother- DDTs sum of thawing degree days at the ground surface mal data, is the location for the present study on the influ- DDTa sum of freezing degree days of the air ence of the snow regime on marginal permafrost. Given its GST ground surface temperature present ground temperature close to 0 ◦C (Gray et al., 2016), MAAT mean annual air temperature the occurrence and the spatial extent of this mountain per- MAGST mean annual ground surface temperature mafrost body is thought to depend fundamentally on the ex- MGSTw mean winter ground surface temperature MGSTs mean summer ground surface temperature istence of favourable azonal topo-climatic conditions on the nf freezing n factor dome-shaped summit. Similar exposed bedrock or mountain- SD standard deviation top detritus summits where a permafrost body is marginally WeqT winter equilibrium temperature preserved have been reported in Scandinavia (e.g. Farbrot et al., 2011, 2013; Isaksen et al., 2011; Gisnås et al., 2013) and in Japan (Ishikawa and Hirakawa, 2000; Ishikawa and Sawa- gaki, 2001). 2004; Lewkowicz et al., 2012; Bonnaventure et al., 2012; This study deals with the impact of the snowpack on the Hasler et al., 2015), and most recently in the Andes (Apaloo ground surface thermal regime and on the permafrost distri- et al. 2012). The snow cover acts as a buffer layer controlling bution on the Mont Jacques-Cartier summit, with implica- heat loss at the ground interface. It provides either a cooling tions on the potential presence of permafrost for the high- (negative surface thermal offset) or warming (positive sur- est summits of the Appalachians. In order to address this face thermal offset) effect on the ground surface temperature question, it was necessary (i) to develop a qualitative and (GST; see Table 1 for abbreviations used throughout this pa- quantitative characterization of the snow distribution over the per) whose magnitude depends on its depth, duration, tim- summit of Mont Jacques-Cartier and (ii) to quantify the sur- ing, and its thermophysical and optical properties (Brown, face thermal offset induced by the snow on the mean annual 1979; Goodrich, 1982; Zhang, 2005; Ishikawa, 2003; Ling ground surface temperature (MAGST). The experimental de- and Zhang, 2003; Smith and Riseborough, 2002, Hasler et sign included snow thickness sounding, excavation of snow al., 2011; Domine et al., 2015). All these snowpack char- pits for observations of snow stratigraphy and measurement acteristics – and the key parameters that control them, such of density and temperature variations over the vertical pro- as micro-relief, landforms, vegetation, and micro-climate – file, and GST monitoring based on the installation of tem- are strongly variable in space and time (Elder et al., 1991; perature data loggers. The hypothesis tested in this study is Li and Pomeroy, 1997; Mott et al., 2010). The close link that wind-driven, almost-snow-free conditions on the summit between snowpack thickness and ground thermal conditions of Mont Jacques-Cartier explain the occurrence of this con- was used to develop a map to predict the presence or ab- temporary permafrost body due to intense ground heat loss sence of permafrost based on snow–ground interface temper- in winter. Some studies have already mentioned the proba- ature. The measurement of the bottom temperature of snow ble link between the near-snow-free winter conditions of the (BTS) is the most widespread technique used to predict per- rounded summits and the occurrence of permafrost bodies in mafrost in mountain areas. It is well adapted to snowy envi- the Appalachian Mountains (Gray and Brown, 1979, 1982; ronments, such as the Alps where it was developed, because Schmidlin, 1988; Walegur and Nelson, 2003), but the con- a late-winter snowpack more than 80 to 100 cm thick is re- trol induced by the snowpack and its feedback mechanisms quired to consider that the BTS values reflect the ground ther- on the winter GST over the summits of the Appalachians has mal condition and are decoupled from the atmosphere tem- not been quantified so far. The research reported in this pa- perature (Haeberli, 1973; Hoelzle, 1992; Grüber and Hoel- per is the first to have investigated this link in detail for one zle, 2001; Bonnaventure and Lewkowicz, 2008). In environ- such summit dome and to have quantified the multiple influ- ments with thin snowpack, the BTS technique is not applica- ences of the seasonal snowpack on the ground surface ther- ble, which led to the development of new techniques to pre- mal regime. If our hypothesis can be confirmed, it will allow dict permafrost occurrence such as the continuous monitor- the prediction of favourable and unfavourable zones for per- ing of GST using temperature loggers (Hoelzle et al., 1999; mafrost occurrence over the entire dome-shaped summit of Ishikawa and Hirakawa, 2000; Ishikawa, 2003; Gray et al., Mont Jacques-Cartier and on other high Appalachians sum- 2016). mits from fine-scale spatial snow distribution patterns. New In the Appalachian region of eastern North America, the knowledge on the distribution of snow on the highest sum- existence of at least two bodies of contemporary permafrost mits in south-eastern North America is an important prelim- are known with certainty: one of these occurs beneath the inary step in modelling the regional spatial distribution of 1606 m high summit of Mount Washington in New England permafrost, the evolution of the ground thermal regime, and The Cryosphere, 11, 1351–1370, 2017 www.the-cryosphere.net/11/1351/2017/ G.
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