Microclimate within coarse debris of talus slopes in the alpine periglacial belt and its effect on permafrost

Transcription

1 Permafrost, Phillips, Springman & Arenson (eds) 2003 Swets & Zeitlinger, Lisse, ISBN Microclimate within coarse debris of talus slopes in the alpine periglacial belt and its effect on permafrost T. Herz & L. King Institut für Geographie der JLU, Giessen, Germany H. Gubler ALPUG, Davos, Switzerland ABSTRACT: Investigations of polar permafrost occurrences indicated that different surface cover types have an important influence on the ground thermal regime. In the discontinuous permafrost zone, they are regarded as the decisive factor determining the local permafrost distribution pattern. Similarly, a coarse debris cover typical for alpine periglacial environments can be treated as an independent layer with certain vertical extent and variable amounts of lithospherical (solid material) and atmospherical (air-filled spaces) components. It forms a transition zone between the near ground atmosphere and the lithosphere with microclimatological conditions different from those of well defined surfaces of finer grained substrates. 1 INTRODUCTION Talus slopes covered with coarse debris are wide-spread geomorphological phenomena in the periglacial belt of high mountains. Another phenomenon controlled predominantly by climatic factors is permafrost. It forms and persists in locations where the mean annual surface temperature is below 0 C. Thus, microclimatic energy exchange processes at the earth s surface play a dominant role in permafrost evolution. The distinct topography in high mountains in connection with alternating surface characteristics leads to remarkable differences in the subsurface temperature regime and to correspondingly small-scale permafrost distribution patterns, especially in the zone of discontinuous mountain permafrost. This paper aims to introduce the methodology applied to investigate microclimate in coarse debris and to present first results. 2 RESEARCH REVIEW So far, investigations of microclimate and ground thermal regime in connection with permafrost have been conducted predominantly in polar regions (e.g. Brown 1973, Pavlov 1973, Smith 1975, Romanovsky & Osterkamp 1995). Concerning the influence of various ground cover types on discontinuous permafrost distribution, different vegetation types have been studied besides the dominating role of a snow cover (e.g. Hinkel et al. 1993). In the short history of mountain permafrost research, former studies identified a high probability of permafrost occurrence in non-vegetated debris areas and the favouring role of a coarse debris cover on permafrost existence in general (Haeberli 1975). Following Humlum (1997), a coarse debris cover causes ground cooling in case of a missing or thin snow cover (particularly through the Balch effect) and ground warming, when a thick snow cover is present (because of the release of latent heat during the refreezing of infiltrated meltwater). Furthermore, a cooling effect of coarse debris covers on the thermal regime of the ground was proved in two case studies in the Kunlun Shan, China and in the area of Plateau Mountain, Canada (Harris & Pedersen 1998). Within the coarse blocky active layer of the rock glacier Murtèl-Corvatsch, a coupling of the air chamber system between blocks and the atmosphere through vertical funnels in an early winter snow cover has been described (Keller & Gubler 1993, Bernhard et al. 1998). Also the detailed energy balance studies conducted during the PACE project made advective fluxes in that layer responsible for the measured non-zero energy budget at this site (Mittaz et al. 2000, Hoelzle et al. 2001). 3 BASIC ASSUMPTIONS AND RESEARCH OBJECTIVE The present study is based on the following assumptions: 1 The surface of coarse blocky debris occurrences cannot exactly be indicated but only averaged over a certain area because of the alternation of solid rock material and air filled spaces. Furthermore there is an energy exchange space with a certain vertical extent rather than a definable energy exchange surface. 2 Advective processes play an important role in the energy exchange in areas where coarse debris is 383

2 present, at least during snow free periods and the dominating influence of direct solar radiation is decreased. Considering these thermal properties, the energy exchange within debris occurrences is dominated by atmospheric rather than lithospheric processes. Therefore, they are treated not as a component of the permafrost active layer but as an independent layer similar to a vegetation cover. Accordingly the ground surface is situated at the base of the block layer, where the gaps between blocks are filled with fine material. 3 Advective processes cause a temperature reduction within such block layers. Therefore the mean annual ground surface temperature (MAGST) is remarkably lower compared to the mean annual cover layer surface temperature (MACST) and the heat input into the ground during summer is reduced. Consequently, the focus of the investigations are microclimatic conditions within a coarse blocky debris layer, their dependence of meteorological states and processes in the near ground atmosphere and their effects on the ground thermal regime. The major objective is to quantify the amount of temperature reduction between MACST and MAGST within periglacial block layers both during longer timescales (annual means) and during seasonal and subseasonal periods. 4 RESEARCH AREA AND MEASUREMENT PROGRAMME Research areas are located in the Mattertal, Valais, Swiss Alps. This valley has been researched intensely during the PACE project and the general permafrost distribution is well established (Gruber & Hoelzle 2001). In the study area Grächen-Seetalhorn (46 11 N, 7 51 E), the Ritigraben catchment consists of a block slope, which covers an area of about 1.4 km 2 at an altitude of 2600 up to 2900 m a.s.l. Block sizes at the surface range from 0.5 up to several cubic meters. The block cover shows a micro relief of alternating ridges and little valleys parallel to the slope with an altitudinal difference of up to 4 m. Figure 1 shows the sequence of layers and the corresponding measurement equipment, which was installed in the lower part of the Ritigraben block slope at an altitude of 2615 m a.s.l. Vertical temperature profiles are measured both in the solid component (rock temperatures) and in the atmospheric component (air temperatures in spaces) within the block layer. This instrumentation is expected to give information about advective processes such as infiltration of cold air during clear-sky nights, air circulation between blocks caused by the local wind field or the transport of sensible and latent heat by infiltration of precipitation or meltwater. The measuring equipment was arranged in subsequent horizontal profiles perpendicular to the slope to also register presumed effects of cold air drainage in the deepest parts of the microrelief. The meteorological station is equipped with sensors measuring net radiation, air temperature and relative humidity, surface temperature, snow depth, precipitation and wind speed and direction. The ground thermal regime is measured in a borehole instrumented with a thermistor chain consisting of negative temperature coefficient thermistors (type Yellow Springs Instruments YSI 44006, relative accuracy estimated at 0.02 C according to Isaksen et al. 2001) in depths ranging from 0.1 to 30 m. The described installations measure thermal states and state changes within the block layer depending on those of the near ground atmosphere. Furthermore, the measurement of high resolution temperature profiles through the near ground atmosphere, block layer and near surface ground provides information about the amount of thermal offset caused by the block layer. Similar instrumentations of coarse blocky debris covers are installed in the Gornergrat-Stockhorn area (45 59 N, 7 47 E) above Zermatt, especially in the vicinity of the PACE borehole located on the Stockhorn-Plateau (3410 m a.s.l.). Additionally, comparison measurements of ground temperatures in the transition zone between block covered and non-block covered areas are carried out in both investigation areas. 5 FIRST RESULTS The temperature curves displayed in figures 2 and 3 were recorded by UTL-1-dataloggers (cf. Hoelzle et al. 1999), which were installed in gaps between blocks in different parts of the Ritigraben block slope. None of the loggers was affected by direct solar radiation during the measuring period. Figure 2 shows an example of a vertical air temperature profile through the block layer. According to the nomenclature of figure 1, the surface -logger was installed in the MACST-level, while the 1.8 m - logger represents the MAGST-level, as it was installed in the deepest gap of the air chamber system, which was accessible from the block layer surface. Only the temperature at the surface shows a pronounced daily course, which was caused by the radiation transfer at the block cover surface during both snow free periods (September and October 1999, mid June August 2000). The temperatures constantly decreased from the surface to the bottom of the block layer except for some early morning values. The blocks at the surface were protected against solar radiation by the high albedo of a thin snow cover 384

3 Figure 1. Layer names and schematic illustration of instrumentation. Temperature [ C] Sep Oct Oct Nov Dec Jan Feb 00 Date 29. Mar Apr May 00 surface -1,5 m -1,8 m Figure 2. Air temperatures in the block layer in the deepest part of the Ritigraben block slope (September 1999 August 2000). 27. Jun Jul 00 from the beginning of November until the end of December. During these two months, most of the gaps at the block layer surface remained open and a more or less direct coupling of the block layer atmospherical component with the near ground atmosphere leaded to an effective cooling down of the whole layer. The temperature gradient between surface and bottom was non-uniform but altogether small during this period. The formation of a thicker snow cover during January filled up and closed most of the gaps at the block layer surface. Until March, temperature variations decreased. The temperature gradient through the block layer reversed under the largely closed snow cover and the coldest temperatures were measured near the surface. From mid March on, the surface -logger was affected by processes connected with infiltration and refreezing of meltwater from the snow cover surface, while the two deeper loggers constantly remained at a temperature of nearly 5 C. 385

4 Temperature [ C] Valley (-1,5 m) Ridge (-1,5 m) Temperature [ C] Sep Oct Oct Nov Dec Jan Feb 00 Date Figure 3. Air temperatures in the block layer of the Ritigraben block slope. Comparison of microtopographical positions (September 1999 August 2000). 29. Mar Apr May Jun Jul 00 Depth [m] The snow cover began to thaw from mid April on, leading to a temperature increase in the whole block layer and to subsequent zero curtains from the surface to the bottom. In mid June, the whole block layer was free of snow. The average temperatures during the measurement period displayed in figure 2 were 0.7 C at the surface, 2.2 C in a depth of 1.5 m and 2.3 C in 1.8 m depth. In figure 3, the 1.5 m -curve out of figure 2 is named valley ( 1.5 m) and compared with data recorded by a logger installed at the same depth, but in a ridge position. This ridge is situated at a horizontal distance of about 6 m from the profile displayed in figure 2. It is obvious that the ridge ( 1.5 m) -curve is constantly warmer with maximum temperature deviations of 5 C. Especially the extreme values show these deviations, but also the period under the influence of a thicker snow cover was colder in the valley position. The average temperature over the measurement period recorded in the ridge position was 0.8 C. 6 APPLIED ASPECTS The Ritigraben torrent produced nine debris flows during the 20th century with an increasing frequency since the mid 80s (Rebetez et al. 1997). The starting zone of the last events is situated beneath a slope step, which is only separated from the Ritigraben block slope by a 10 m wide ski run and lies about 50 m away from the instrumentations described in figure 1. The temperature curve displayed in figure 4 indicates that permafrost is present underneath the block slope at present, but is in a relatively warm and therefore possibly unstable state. Consequently, the measurements also contribute to a local hazard assessment and mitigation concept Figure 4. Ground temperatures, Ritigraben block slope, 2615 m a.s.l. ( ). in the context of presumed spermafrost degradation caused by climatic change, as formulated recently in the research strategy of the PACE project (Harris et al. 2001). 7 CONCLUSIONS This study intends to investigate coarse debris occurrences in general and independent from rock glaciers, on which mountain permafrost research in the Alps was mostly focused so far. The first results support the basic assumptions formulated in chapter 3. The presented methodology and the comparison of different types of debris occurrences in two study areas will allow general statements on their influence on the ground thermal regime. ACKNOWLEDGEMENTS UTL-1-dataloggers were funded by the EU project PACE (Contract ENV4-CT ). Borehole drilling and meteorological station at the Ritigraben site were financed by the Canton Valais. The Berg-bahnen

5 Grächen provided valuable logistical support during fieldwork. Last but not least, two anonymous reviewers made valuable suggestions for improvement of the manuscript. REFERENCES Bernhard, L., Sutter, F., Haeberli, W. & Keller, F Processes of snow/permafrost-interactions at a highmountain site, Murtèl/Corvatsch, Eastern Swiss Alps. In 7th International Conference on permafrost, Proceedings, Yellow-knife, June 1998: Brown, R.J.E Influence of climatic and terrain factors on ground temperatures at three locations in the permafrost region of Canada. In 2nd International Conference on permafrost, Proceedings, North American Contribution, July 1973: Gruber, S. & Hoelzle, M Statistical modelling of mountain permafrost distribution: Local calibration and incorporation of remotely sensed data. In Permafrost and periglacial processes 12(1): Haeberli, W Untersuchungen zur Verbreitung von Permafrost zwischen Flüelapass und Piz Grialetsch (Grau-bünden). In Mitteilungen der Versuchsanstalt für Wasser-bau, Hydrologie und Glaziologie, ETH Zürich, no. 17: 221 pp. Harris, C., Haeberli, W., Vonder Mühll, D. & King, L Permafrost monitoring in the high mountains of Europe: The PACE project in its global context. In Permafrost and periglacial processes 12(1): Harris, S.A. & Pedersen, D.E Thermal regimes beneath coarse blocky materials. In Permafrost and periglacial processes 9(2): Hinkel, K.M., Outcalt, S.I. & Nelson, F.E Near-surface summer heat-transfer regimes at adjacent permafrost and non-permafrost sites in central Alaska. In 6th International conference on permafrost, Proceedings, Vol. 1, Beijing, 5 9 July 1993: Hoelzle, M., Wegmann, M. & Krummenacher, B Miniature temperature dataloggers for mapping and monitoring of permafrost in high mountain areas: First experience from the Swiss Alps. In Permafrost and periglacial processes, 10(2): Hoelzle, M., Mittaz, C., Etzelmüller, B. & Haeberli, W Surface energy fluxes and distribution models of permafrost in european mountain areas: An overview of current developments. In Permafrost and periglacial processes 12(1): Humlum, O Active layer thermal regime at three rock glaciers in Greenland. In Permafrost and periglacial processes 8(4): Isaksen, K., Holmlund, P., Sollid, J.L. & Harris, C Three deep alpine-permafrost boreholes in Svalbard and Scandinavia. In Permafrost and periglacial processes 12(1): Keller, F. & Gubler, H Interaction between snow cover and high mountain permafrost, Murtèl/Corvatsch, Swiss Alps. In 6th International conference on permafrost, Proceedings, Vol. 1, Beijing, 5 9 July 1993: Mittaz, C., Hoelzle, M. & Haeberli, W First results and interpretation of energy flux measurements over alpine permafrost. In Annals of glaciology 31: Pavlov, A.V Heat exchange in the active layer. In 2nd International Conference on permafrost, Proceedings, USSR Contribution, Yakutsk, July 1973: Rebetez, M., Lugon, R. & Baeriswyl, P.A Climatic change and debris flows in high mountain regions: The case study of the Ritigraben torrent (Swiss Alps). In Climatic change 36: Romanovsky, V.E. & Osterkamp, T.E Interannual variations of the thermal regime of the active layer and near-surface permafrost in northern Alaska. In Permafrost and periglacial processes 6(4): Smith, M.W Microclimatic influences on ground temperatures and permafrost distribution, Mackenzie Delta. In Canadian journal of earth sciences 12:

6