Ghislain Picard1, Laurent Arnaud1, Carlo Carmagnola2, Nicolas Champollion1, Eric Lefebvre1, Q. Libois1, Florent Domine3, Marie Dumont2, Samuel Morin2

Transcription

1 "Alpine Snowpack Specific Surface Area Profiler" (ASSSAP): a new instrument to retrieve snow specific surface area with a 1 cm resolution using infrared reflectance Ghislain Picard1, Laurent Arnaud1, Carlo Carmagnola2, Nicolas Champollion1, Eric Lefebvre1, Q. Libois1, Florent Domine3, Marie Dumont2, Samuel Morin2 1 : Laboratoire de Glaciologie et Géophysique de l'environnement (LGGE, UJF/CNRS) 2 : CNRM-GAME, CEN (Météo-France - CNRS) 3 : Takuvik Joint International Laboratory, CNRS and Université Laval, Québec (QC), Canada Snow grain size meeting Grenoble April 2013

2 Context Passive and active microwave satellites are sensitive to grain size in dry snow. many talks today z Scattering = f(scatterers size) Thermal emission

3 Context & chronology In 2006 : Measurements by NIR photography at Dome C Martin Schneebeli Laurent Arnaud DMRT-ML model Results were promising, but: Need to fit a unknown grain size for depths >3m Brucker, L. et al. Modeling time series of microwave brightness temperature at Dome C, Antarctica, using vertically resolved snow temperature and microstructure measurements, Journal of Glaciology, 57(201), , 2011

4 Context & chronology Towards a profiler for the Antarctic : POSSSUM Key concepts : - work in a drilled borehole up to 20 m depth - in-situ measurements : no sampling, minimal perturbation - active/controlled illumination at an optimal wavelength - rapid, high-resolution,... At the same time: Gallet and Domine traditional method to measure accurately reflectance on sample DUFISSS 130mm diameter

5 Context & chronology Towards a profiler for the Antarctic : POSSSUM Key concepts : - work in a drilled borehole up to 20 m depth - in-situ measurements : no sampling, minimal perturbation - active/controlled illumination at an optimal wavelength - rapid, high-resolution Hole Snow in the sphere Too large Extreme sensitivity to the distance Need a specific development: POSSSUM 130mm diameter

6 Context & chronology Towards a profiler for shallow snowpits (Alps, Arctic,...) : ASSSAP Key concepts : - work for 1-2 m depth - compatible with POSSSUM - much lighter than POSSSUM POSSSUM ASSSAP DUFISSS

7 Outline - Measurement principle - POSSSUM and applications - ASSSAP and applications

8 Measurement principle Relationship between albedo and SSA : Albedo = f (wavelength) SSA = f (Albedo) 100m2kg-1 20m2kg-1 5m2kg-1 The choice of the wavelength is important

9 Measurement principle - Penetration depth versus wavelength Balance : resolution / averaging 5 m2kg-1 20 m2kg m2kg nm E-folding depth (cm) POSSSUM / ASSSAP / DUFISSS uses 1310 nm laser diode Sensitivity (relative unit) Relationship between albedo and SSA : - Sensitivity versus wavelength 0.7cm

10 Measurement principle How to measure reflectance in an irregular borehole with a probe hung by a cable? Two lasers: 1310nm and 805nm Laser 805 nm Laser 1310 nm SWIR photodiodes NIR photodiode SWIR photodiodes Snow face

11 Measurement principle How to measure reflectance in an irregular borehole with a probe hung by a cable? 2 problems : 1) Irregular snow face and probe tilt 2) distance Ex: +20 and -20 Laser 805 nm Laser 1310 nm Laser 805 nm Laser 1310 nm Solution: Couples of photodiodes with same zenith angle but opposite azimuth Solution: Two wavelengths: - 1 highly sensitive to SSA + perturbing factors - 1 less sensitive to SSA + perturbing factors Geometrical factors are wavelength independant

12 Measurement principle Algorithm : 6 SWIR photodiode currents SWIR NIR or Visible 1-2 NIR photodiode currents Calibration: pseudo-reflectance Calibration: pseudo-reflectance Average opposite angles + anisotropy Average opposite angles + anisotropy Distance correction (geometrical model) SWIR Albedo First guess: d=0 (no correction) d Distance correction (geometrical model) NIR Albedo SSA using Kokhanovsky et Zege 2004 NIR Albedo Kokhanovsky et Zege 2004 <?> 2 3 iterations are sufficient Outputs: SSA (z), d(z),...

13 Measurement principle Distance correction : Diode signal as a function of the distance Distance snow - cylinder line=theory symbols=measurements Optimum distance=13mm (by construction) Correction possible in a very large range, up to 8 cm from the optimum position!

14 Measurement principle Distance correction, a real-life example : Without correction With correction

15 Measurement principle POSSSUM and ASSSAP software : Processing, quality control, data exploration Different steps of the algorithm can be visualized

16 POSSSUM and applications Protocole in short: 1. Drill a hole: e.g. 2 hours for 8 meters. 2. descent+ascent Posssum: 25 minutes for 8 meters Distance coder (1mm accuracy) 25m long cable Tripod Anti-torque POSSSUM Measurement window Boring tool Snow face in the hole is refreshed just before measurements Centering elements POSSSUM is currently on a boat from Antarctica no show.

17 POSSSUM and applications First comparisons in 2009 : between POSSSUM and DUFISSS Very encouraging! between POSSSUM and CH4 adsorption

18 POSSSUM and applications Inter-comparaison in the Alps in 2009 : POSSSUM, CH4, Tomo (F. Flin), DUFISSS,... Contrasting results: - sampling exactly at the same position is important - weather conditions matter e.g. high humidity or near freezing point snow are unfavorable

19 POSSSUM and applications Inter-comparaison at Dome C : Different years, different sites

20 POSSSUM and applications Deep profiles at Dome C in Antarctica: Measured on samples Profile measured in the hole The driller/boring tools tear out the grains clean face The driller/boring tools cut the grains fine powder on the face In practice, POSSSUM in a drilled hole is limited to ~ 8-10m (for Dome C typical cohesion)

21 ASSSAP and applications - Cable replace by guides (better controlled distance) - Driller is a polycarbonate cylinder - Max 2 m depth - no boring tool 2 flavors: - ASSSAP 1 (25 kg) - ASSSAP 2 (10 kg, plug and play)

22 ASSSAP and applications Convenient and fast operations: 10 minutes (hole+profile) ~120 profiles taken at Dome C during a summer season (Q. Libois and L. Arnaud) Carlo Carmagnola talks

23 ASSSAP and applications ASSSAP horizontal protocole: temporal evolution of surface SSA exactly at the same position The guides are in place for the season 3 horizontal transects (20cm long) obtained at Dome C in 2010

24 Concluding remarks Many measurements have been with POSSSUM : ~ 50 profiles at Dome C (two of them are 80 m deep). ~ 10 profiles in Antarctica on scientific traverses. ~ 10 profiles in Baffin island Many measurements have been done with ASSSAP 1 and 2: ~ hundreds of profiles and horizontal transects (Alps, Antarctica, Greenland) - Objective quality control. How to acertain a measurement is accurate within the % as found during the first comparison? Many comparisons POSSSUM IRIS or ASSSAP DUFISSS have been done and sometimes unexplained discrepencies up to 30 % are obtained (not a single cause). - Long term stability and absolute reference. Calibration. International standart? Depth (in cm) 2 major issues :

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26 "Alpine Snowpack Specific Surface Area Profiler" (ASSSAP): a new instrument to retrieve snow specific surface area with a 1 cm resolution using infrared reflectance Ghislain Picard1, Laurent Arnaud1, Carlo Carmagnola2, Nicolas Champollion1, Eric Lefebvre1, Q. Libois1, Florent Domine3, Marie Dumont2, Samuel Morin2 1 : Laboratoire de Glaciologie et Géophysique de l'environnement (LGGE, UJF/CNRS) 2 : CNRM-GAME, CEN (Météo-France - CNRS) 3 : Takuvik Joint International Laboratory, CNRS and Université Laval, Québec (QC), Canada Snow grain size meeting Grenoble April 2013

27 Context Passive and active microwave satellites are sensitive to grain size in dry snow. many talks today z Scattering = f(scatterers size) Thermal emission

28 Context & chronology In 2006 : Measurements by NIR photography at Dome C Martin Schneebeli Laurent Arnaud DMRT-ML model Results were promising, but: Need to fit a unknown grain size for depths >3m Brucker, L. et al. Modeling time series of microwave brightness temperature at Dome C, Antarctica, using vertically resolved snow temperature and microstructure measurements, Journal of Glaciology, 57(201), , 2011

29 Context & chronology Towards a profiler for the Antarctic : POSSSUM Key concepts : - work in a drilled borehole up to 20 m depth - in-situ measurements : no sampling, minimal perturbation - active/controlled illumination at an optimal wavelength - rapid, high-resolution,... At the same time: Gallet and Domine traditional method to measure accurately reflectance on sample DUFISSS 130mm diameter

30 Context & chronology Towards a profiler for the Antarctic : POSSSUM Key concepts : - work in a drilled borehole up to 20 m depth - in-situ measurements : no sampling, minimal perturbation - active/controlled illumination at an optimal wavelength - rapid, high-resolution Hole Snow in the sphere Too large Extreme sensitivity to the distance Need a specific development: POSSSUM 130mm diameter

31 Context & chronology Towards a profiler for shallow snowpits (Alps, Arctic,...) : ASSSAP Key concepts : - work for 1-2 m depth - compatible with POSSSUM - much lighter than POSSSUM POSSSUM ASSSAP DUFISSS

32 Outline - Measurement principle - POSSSUM and applications - ASSSAP and applications

33 Measurement principle Relationship between albedo and SSA : Albedo = f (wavelength) SSA = f (Albedo) 100m2kg-1 20m2kg-1 5m2kg-1 The choice of the wavelength is important

34 Measurement principle - Penetration depth versus wavelength Balance : resolution / averaging 5 m2kg-1 20 m2kg m2kg nm E-folding depth (cm) POSSSUM / ASSSAP / DUFISSS uses 1310 nm laser diode Sensitivity (relative unit) Relationship between albedo and SSA : - Sensitivity versus wavelength 0.7cm

35 Measurement principle How to measure reflectance in an irregular borehole with a probe hung by a cable? Two lasers: 1310nm and 805nm Laser 805 nm Laser 1310 nm SWIR photodiodes NIR photodiode SWIR photodiodes Snow face

36 Measurement principle How to measure reflectance in an irregular borehole with a probe hung by a cable? 2 problems : 1) Irregular snow face and probe tilt 2) distance Ex: +20 and -20 Laser 805 nm Laser 1310 nm Laser 805 nm Laser 1310 nm Solution: Couples of photodiodes with same zenith angle but opposite azimuth Solution: Two wavelengths: - 1 highly sensitive to SSA + perturbing factors - 1 less sensitive to SSA + perturbing factors Geometrical factors are wavelength independant

37 Measurement principle Algorithm : 6 SWIR photodiode currents SWIR NIR or Visible 1-2 NIR photodiode currents Calibration: pseudo-reflectance Calibration: pseudo-reflectance Average opposite angles + anisotropy Average opposite angles + anisotropy Distance correction (geometrical model) SWIR Albedo First guess: d=0 (no correction) d Distance correction (geometrical model) NIR Albedo SSA using Kokhanovsky et Zege 2004 NIR Albedo Kokhanovsky et Zege 2004 <?> 2 3 iterations are sufficient Outputs: SSA (z), d(z),...

38 Measurement principle Distance correction : Diode signal as a function of the distance Distance snow - cylinder line=theory symbols=measurements Optimum distance=13mm (by construction) Correction possible in a very large range, up to 8 cm from the optimum position!

39 Measurement principle Distance correction, a real-life example : Without correction With correction

40 Measurement principle POSSSUM and ASSSAP software : Processing, quality control, data exploration Different steps of the algorithm can be visualized

41 POSSSUM and applications Protocole in short: 1. Drill a hole: e.g. 2 hours for 8 meters. 2. descent+ascent Posssum: 25 minutes for 8 meters Distance coder (1mm accuracy) 25m long cable Tripod Anti-torque POSSSUM Measurement window Boring tool Snow face in the hole is refreshed just before measurements Centering elements POSSSUM is currently on a boat from Antarctica no show.

42 POSSSUM and applications First comparisons in 2009 : between POSSSUM and DUFISSS Very encouraging! between POSSSUM and CH4 adsorption

43 POSSSUM and applications Inter-comparaison in the Alps in 2009 : POSSSUM, CH4, Tomo (F. Flin), DUFISSS,... Contrasting results: - sampling exactly at the same position is important - weather conditions matter e.g. high humidity or near freezing point snow are unfavorable

44 POSSSUM and applications Inter-comparaison at Dome C : Different years, different sites

45 POSSSUM and applications Deep profiles at Dome C in Antarctica: Measured on samples Profile measured in the hole The driller/boring tools tear out the grains clean face The driller/boring tools cut the grains fine powder on the face In practice, POSSSUM in a drilled hole is limited to ~ 8-10m (for Dome C typical cohesion)

46 ASSSAP and applications - Cable replace by guides (better controlled distance) - Driller is a polycarbonate cylinder - Max 2 m depth - no boring tool 2 flavors: - ASSSAP 1 (25 kg) - ASSSAP 2 (10 kg, plug and play)

47 ASSSAP and applications Convenient and fast operations: 10 minutes (hole+profile) ~120 profiles taken at Dome C during a summer season (Q. Libois and L. Arnaud) Carlo Carmagnola talks

48 ASSSAP and applications ASSSAP horizontal protocole: temporal evolution of surface SSA exactly at the same position The guides are in place for the season 3 horizontal transects (20cm long) obtained at Dome C in 2010

49 Concluding remarks Many measurements have been with POSSSUM : ~ 50 profiles at Dome C (two of them are 80 m deep). ~ 10 profiles in Antarctica on scientific traverses. ~ 10 profiles in Baffin island Many measurements have been done with ASSSAP 1 and 2: ~ hundreds of profiles and horizontal transects (Alps, Antarctica, Greenland) - Objective quality control. How to acertain a measurement is accurate within the % as found during the first comparison? Many comparisons POSSSUM IRIS or ASSSAP DUFISSS have been done and sometimes unexplained discrepencies up to 30 % are obtained (not a single cause). - Long term stability and absolute reference. Calibration. International standart? Depth (in cm) 2 major issues :

50