1 / 44

READING AND MODELLING

READING AND MODELLING. Review about mass balance modelling:

kalare
Download Presentation

READING AND MODELLING

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. READING AND MODELLING Review about mass balance modelling: Greuell, W., and C. Genthon, 2004: Modelling land-ice surface mass balance. In Bamber, J.L. and A.J. Payne, eds. Mass balance of the cryosphere: observations and modelling of contemporary and future changes. Cambridge University Press. Mass balance model that includes sub-surface module: http://www.phys.uu.nl/%7Egreuell/massbalmodel.html

  2. REMOTE SENSING OF GLACIERS Karthaus, September 2005 Wouter Greuell Institute for Marine and Atmospheric Research Utrecht (IMAU) Utrecht University, the Netherlands Retrieval of: - surface velocity - surface topography - glacier facies - surface albedo

  3. WAVE LENGTHS AND RETRIEVABLE INFORMATIONGlaciology

  4. ORBIT (LANDSAT 7) altitude = 705 km inclination = 98˚ No data beyond 82 ˚N and S orbit period = 99 min ground-track speed ~ 6.7 km/s crosses equator at 9:45 AM local time (for optical sensors)

  5. SCANS LANDSAT TM (1, 2 and 3) total field of view: 11.6˚ scan mirror oscillates once per 33 msec 6 detectors per band i.e. six contiguous lines for each mirror semi-oscillation

  6. COVERAGE LANDSAT 7 orbit repeated after 16 days

  7. SARINTERFERO-METRY InSAR velocity field topography

  8. SIDE-LOOKING RADAR(REAL-APERTURE RADAR) Active sensor ! Emission of a short pulse: tp = 30 ns Across-track resolution obtained by time-resolving the signal if tp = 30 ns, = 35˚ then Rr = 8 m Along-track resolution is poor if H = 800 km,  = 24 cm, L = 10 m = 35˚ then Ra = 24 km

  9. SYNTHETIC APERTURE RADAR (SAR) Increase along track resolution pulse repetition frequency: 1000 Hz satellite speed: 6 km s-1 Every 6 m a sample is taken Therefore, 4000 measurements are taken within 24 km (the original along-track resolution) Every measurement contains the information from 4000 ground elements of 6 m, but each ground element is sampled 4000 times ...... computer time-demanding procedure (called focusing) with complex numbers

  10. PRINCIPLE OF SAR INTERFEROMETRY Orbit A Image A Orbit B Image B • Use 2 images (A and B) from repeat orbits (typically no more than a few 100 m apart = d) • Use phase () • Range (R; distance satellite to pixel) = n (integer number) *  (6 cm) + /2 *  • So phase gives some info about range (but n is unknown!) • Take difference of phases from two images for each pixel = difference in range from two orbits ( but (nA-nB)  is unknown!) • Make image of phase difference (= interferogram) d pixel i pixel i+1 • Contributions to interferometric signal: • Differences in positions orbits • Surface topography • Surface displacements

  11. RAW INTERFEROGRAM Contours (colours) connecting points of equal phase difference are called fringes

  12. THE INTERFEROMETRIC LIMIT signal from a pixel is the sum of hundreds of elementary targets dR = difference in path length for extreme ends of one pixel phase from pixel is random is removed by differencing two signals from the same pixel Targets must remain stable between image acquisitions (e.g. 3 days) Pixel must not stretch or shrink by more than a fraction of  from one image to the other 2 L ( sin 1 - sin 2 ) <  orbital separation (d) should be < 1 km if d too large: incoherence

  13. ALTITUDE OF AMBIGUITY = shift in altitude of the surface corresponding to a phase shift of 2π in the interferogram ha: altitude of ambiguity R: range from satellite to target l: wavelength q: angle of incidence d: horizontal separation of trajectories d q R ha If interested in topography: large d If interested in displacements: small d

  14. DIGITAL ELEVATION MODEL glacier free terrain in Alaska a b prior (a) and after (b) removal of orbital effect Note: phase needs to be unwrapped. Tie points needed!

  15. SEPARATE VELOCITY FROM TOPOGRAPHIC FIELD Option 1: create synthetic interferogram from known topography and und subtract this from measured interferogram Option 2: differential interferometry: use two interferograms and assume constant velocity

  16. EXAMPLE OF VELOCITY FIELD Bagley Ice Field Alaska Date 1: topography (h) and velocity (v) Date 2: h + v h only Date 2: v only

  17. ESTIMATE SURFACE VELOCITY Limitation! Calculated velocity = velocity along line connecting the target with the satellite • Extra info: • another interferogram • assume surface parallel flow • assume flow along the surface gradient • assume flow along valley walls

  18. DETECTING GLACIER FACIES

  19. GLACIER FACIES

  20. SNOW LINE FROM NEAR-INFRARED IMAGERY Morteratschgletscher TM band 4 (800 - 900 nm) 24 June 1999

  21. EFFECTS ON THE RADAR SIGNAL amplitude!

  22. THE RESULTING RADAR SIGNAL Atmosphere some absorption by clouds with water droplets, but no scattering All images useful Water in snow or on ice strong absorption use winter images Most glacier surfaces are rough backscattered signal depends on shape of roughness elements Volume scattering increases with concentration of large ( > 1 cm ) inhomogeneities

  23. AN EXAMPLE: FACIES ON KONGSVEGEN (SVALBARD) FROM SAR 1-4 ice 5-6 supimp. ice 8-9 snow

  24. RADAR ALTIMETRY (PRINCIPLE) Principle: - emittance of a short (tp = 3 ns) pulse - detection of the return - determination of the travel time (Tt) - calculation of the distance to the surface (H)

  25. RADAR ALTIMETRY: RANGE RESOLUTION H = 0.5 c tp H ≈ 0.5 m

  26. RADAR ALTIMETRY: FOOTPRINT The footprint is “pulse-limited” (and not “beam limited”) Footprint (x) = diameter of circle when rear front hits surface From H >> ctp, it follows: x ≈ 2400 m

  27. SLOPE-INDUCED ERROR Technique works only when slope < 1 degree

  28. CHANGE IN ELEVATIONAT CROSSING POINTS Only at crossing point of ascending and descending tracks, because repeat tracks are too far apart (a few km)

  29. ELEVATION CHANGE ANTARCTICA Period: 1992 - 1996 No orbits beyond about 81 ˚S Only measurements when slope < 1˚

  30. ELEVATION CHANGE WITH AIRBORNE LASER • Same principle as radar altimeter, but: • flight lines are repeated exactly, leading to info along entire flight lines • Footprint ≈ 1 m • Direction of reflection known with large accuracy (no problem over steep terrain) • but • Total length of flight lines limited Elevation changes Greenland 1997-2003

  31. TOPOGRAPHY FROM RADAR, LASER AND SAR

  32. SUM UP Orbits, swath, resolution SAR interferometry for surface velocity and topography Glacier facies with optical sensors and SAR Altimetry with radar (laser and SAR)

  33. SOME READING Introduction to remote sensing: Rees, W.G., 2001: Physical principles of remote sensing. Cambridge University Press, Cambridge (U.K.), 343 pp. Review about remote sensing of snow and ice: König, M., J.-G. Winther and E. Isaksson, 2001: Measuring snow and glacier properties from satellite. Rev. Geophys., 39 (1), 1-27. Review about SAR interferometry: D. Massonnet and K. L. Feigl, 1998: Radar interferometry and its application to changes in the Earth’s surface. Rev. Geophys., 36 (4), 441-500. Paper about using SAR interferometry to derive glacier velocity field: Fatland, D.R. and C.S. Lingle, 1998: Analysis of the 1993-95 Bering Glacier (Alaska) surge using differential SAR interferometry. J. Glaciol., 44 (148),532-546.

  34. SIGNALS ARE AVERAGED for ERS-1 over 50 returns Real frequency: 20 Hz Distance of info along track = 330 m

  35. RANGE WINDOW Signal is sampled within short time interval (relative to pulse repetition time) in order to reduce data volume = range window Half-power point = retrack point = mean surface elevation within footprint for Gaussian distribution of slopes Onboard tracker tries to predict travel time of next return in order to place range window correctly When signal is missed altogether: loss of lock Sensor goes into “acquisition mode”: no data for a few seconds

  36. MEASURED RETURN SIGNALS • every signal is mean of 50 returns • every sixth signal is shown • flight over margin Greenland ice sheet • 40-48: coast • 106-232: loss of lock in rugged terrain • 238-274: ice sheet

  37. PULSE REPETITION FREQUENCY For ERS-1: 1 kHz Pulses are 10-3 s apart, compare to pulse duration of 3 ns

  38. ERROR SOURCES • Relevant for changes in ice-elevation measurements: • Atmospheric • a) dry atmosphere • b) wet atmosphere • c) ionosphere • Orbit • Variation in sub-surface properties (from which depth is the signal reflected?) • Slope (see next slide) • Note also that changes in snow (ice) density without changes in elevation do not affect volume, but they do affect mass and sea level

  39. DETERMINATION OF THE SURFACE TEMPERATUREBLACK-BODY RADIATION

  40. BRIGHTNESS TEMPERATURE This temperature is called the brightness temperature Satellite sensors are calibrated on-board with blackbodies of known temperatures The real surface temperature is several degrees centigrade higher than the brightness temperature due to absorption in the atmosphere

  41. ATMOSPHERIC WINDOWS AVHRR bands 4 and 5 are situated in the atmospheric window between 10.3 and 12.5 µm

  42. SPLIT WINDOW AND DUAL VIEW • Difference real surface - brightness temperature varies with: • amount of greenhouse gases along atmospheric path • concentration of gases (e.g. water vapour) • surface elevation If this is unknown: Ts = a0 + a1 TB1 + a2 TB2 where Ts: surface temperature ai: constants TBi: brightness temperatures obtained from different sensors Split window: brightness temperatures from two different spectral bands Dual view: brightness temperatures from two different angles Equation optimized by means of measurements or calculations

  43. SAR INTERFEROMETRY= differencing the phases of two SAR images phase = range = distance between satellite and ground target difference in phase = difference in range (between 2 images) is not absolute, but relative ! • Contributions to interferometric signal: • differences in orbital trajectories • surface topography • surface displacements

More Related