Rock wall thermal regime characterization in high mountain areas and Rock wall thermal regime characterization in high mountain areas and related permafrost degradation: preliminary data from the Western Alpsrelated permafrost degradation: preliminary data from the Western Alps

IntroductionIntroduction

International Union of Geodesy and Geophysics XXIV Perugia, Italy 2-13 July, 2007

Edoardo Cremonese (1), Umberto Morra di Cella (1), Paolo Pogliotti (2,1), Marco Giardino (2), Stephan Gruber (3).1. ARPA Valle d’Aosta A. O. Cambiamenti Climatici, Italia 2. Dipartimento di Scienze della Terra, GEOSITLAB, Università degli studi di Torino, Italia. 3. Glaciology and Geomorphodynamics Group, Department of Geography, University of Zurich, Switzerland

Contact: paolo.pogliotti@unito.it

Permafrost degradation of ice-filled discontinuities has been hypothesized to be one of the main causes of rockfalls and rock wall instabilities occurred in the last years in high mountain areas, like Mc Ginnis Peak, (Alaska, 2002) and Kolka-Karmadon (Caucasus, 2002) or in the Alps: Brenva Glacier (1997), Matterhorn (2003), Punta Thurwieser (2004), the Drus west face (2005) and Dents du Midi (2006). In the context of global warming, alpine regions are extremely sensitive to increasing temperatures and permafrost has revealed to be a privileged geoindicator of climate change effects. PERMA_TEMP is one of the action of PERMAdataROC project whose main objective is the development of rock wall instability monitoring strategy (more details on poster “The relation of permafrost degradation and slope instabilities in high alpine The relation of permafrost degradation and slope instabilities in high alpine steep rockwalls: the research project PERMAdataROCsteep rockwalls: the research project PERMAdataROC” in this session).

ObjectiveObjectiveThe aim of PERMA_TEMP action is the direct measurement of rock wall thermal regimes, in order to asses the effect of global warming, and consequent permafrost degradation, on rock wall stability. Rock temperature data, in association with meteo-climatic and radiative parameters, will be used to calibrate and validate models of rock wall temperature regimes and their regional distribution in high-mountain areas, aiming to obtain a better understanding of the mechanisms triggering rockfall phenomena.

MethodologyMethodology

Rock walltemperature

Air temperature &relative humidity

Radiometricparameters

Wind speed and direction

Rock surface energy balance

Spatial modellingat regional scale

Signal processing

Transferfunction

Δtime (3-55cm)Δenergy (3-55cm)

TheoreticalThermal conductivity

Topography &Morphology

Lithology

Fracturation density

Geotechnical test

Rock sampling

Structural analysis and statistical elaboration of geologic data

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Compressive strength module

Thermal conductivity laboratory measure

Thermal conductivity variabilityevaluationFunction of...

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InstrumentationInstrumentationMeasured variables at all sites are: rock temperature at different depths (3, 30 and 55 cm), air temperature and relative humidity close to rock-atmosphere interface. Two sites (CC and AdM) have been dedicated to intensive measurements that also include solar radiation (incoming and outgoing short / long wave), wind speed and wind direction. Project’s first year has been mainly dedicated to the development and the improvement of measurement methodology leading to reliable and accurate data: results of this methodological approach are shown in the boxes below.

Solar radiation, wind speed and wind direction

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At Carrel Hut south site (CCS), we installed a combination of a net radiometer (NrLite) and an albedometer (CM7B). Both were installed in vertical position parallel to the rock wall. Measurements at this sites last for more than one year.

At Aiguille du Midì south site (AdMS) we installed an authomatic weather station with a net radiometer (CNR1) which measures all radiometric components (SWin, SWout, LWin, LWout) and a sonic anemometer. All instruments are parallel to rock surface.

In order to asses the uncertainities coming from vertical installation, a one-day intercomparison experiment has been carried out with a CM7B horizontally installed (picture above). The effect of vertical installation

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The anemometer has been oriented so that north corresponds to an ascensional wind.

To install and to maintain this kind of instruments in high mountain environment we have to face big logistical constraints.Measurements can be affected by ice formation and its permanence. During ugly weather days, datalogging has been guaranteed by properly working batteries.

Rock Temperature

CNR1

Anemometer

Air Temperature

CNR1 Anemometer

CM7B+NrLite

CM7B intercomparison

Expected results and developmentsExpected results and developments

The PERMAdataRoc project will officially end on march 2008; data acquisition will continue for the following years aiming to acquire longer and more meaningful temporal series of such parameters.

Acquired data will be used for validation and calibration of mathematical models for distribution, degradation and temperature evolution of rock wall permafrost in global warming context.

Rock temperature data will be treated by spectral analysis methods for appraising rock wall thermal conductivity variability in relation of meteo-climatic parameters and some geological variables like schistosity, lithology and fracturation density.

Future application of thermography techniques will provide fundamental information regarding the spatial distribution of rock surface temperature data.

Thermographic approachThermographic approach

Beside we show some preliminary results obtained from thermographic analysis (FLIR ThermaCAM P640) at Aiguille du Midi site (single day measurements 05/04/07). The main purpose of this approach is obtain surface temperature distribution and to analyze its spatial variability. The picture shows the data collected on northern (upper raw) and southern (lower raw) slopes. Images 1 and 2, displayed with the same temperature range give evidence of the temperature difference between the two expositions: northern slope appears entirely below 0°C, while the southern one side is consistently warmer. The enhanced temperature contrast images (3 and 4) underline the surface temperature spatial variability of both expositions.A first attempt to compare temperature measured by rock temperature sensors and derived by thermographic data is shown in graphic above: preliminary elaborations give encouraging results for future applications of this integrated approach.

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Rock wall temperature

In configuration A sensors cables were exposed to the frost and wind action and many have been damaged. Therefore configuration B has been developed and some comparative tests have been conducted for appraising the differences caused by the new configuration.

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Very weak!! more resistant and reliable!!

Configurations comparison

Aiguille du Midì South Face 3840 m a.s.l

AdMS AdMSsn

Tour Ronde South Face 3510 m a.s.l

CES

Snow / Hoarfrost formation on instruments (see picture beside) can lead to different kind of errors: some instruments show an interruption of data measurements (i.e. sonic anemometer) while others give unreliable data (i.e. radiometer). In order to invalid data from all the sensors during period of hoarfrost formation and permanence, we used anemometer status report.

Rock wall temperature: preliminary data series from Mont Blanc massifRock wall temperature: preliminary data series from Mont Blanc massif

Subsurface isotherms of daily maximum temperatures during winter 2007 at site Aiguille du Midì (beside) and Col d’Entréves (below) in the first 55 cm of depth.

Beside, the three graphs clearly underline the effect of the exposure (1 vs. 2) and that of the snow-cover (2 vs. 3) on the thermal regime of the rock wall. The snow inhibits any oscillation of the isotherms and maintains the temperature few degrees under zero.

Below, eight months of temperature acquisition on CES site shown the big differences between winter and early summer. The minimum temperatures are been reached to the end of January and March as confirmed also by the graph 1 on north face of Aiguille du Midì.

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Radiation measurements on rock wallRadiation measurements on rock wall

Annual course of the Incoming Short Wave and Net Radiation on the southern face of Matterhorn during the year 2006.

The graph show that the intensity of the solar radiation on a vertical wall during the winter it is double in comparison to that during the summer.

Incoming SW and Net Radiation on Matterhorn south face

This is due to the smaller wintry azimuthal angle and on the rock this fact causes a great increase of the daily superficial thermal excursions.

Incoming and outgoing SW and LW and calculated Net radiation at the Aiguille du Midì site.