CHARACTERISTICS OF SNOW AND ICE MORPHOLOGICAL FEATURES DERIVED FROM MULTI-POLARIZATION TERRASAR-X DATA

Transcription

1 CHARACTERISTICS OF SNOW AND ICE MORPHOLOGICAL FEATURES DERIVED FROM MULTI-POLARIZATION TERRASAR-X DATA Dana Floricioiu 1, Helmut Rott 2, Thomas Nagler 2, Markus Heidinger 2 and Michael Eineder 1 1 DLR, Remote Sensing Technology Institute (IMF) 2 Enveo IT Innsbruck Austria

2 TerraSAR-X Basic Imaging Modes Stripmap (SM) Θ 1 =20 o High Resolution Spotlight (HS) & Spotlight (SL) ScanSAR (SC) >30 km Θ 2 =45 o 10 km 5-10 km 100 km Stripmap Spotlight (HS & SL) ScanSAR swath width (range) 30 km (single pol.) 15 km (dual pol.) MHz chirp BW azimuth: 5 / 10 km (HS / SL) 100 km (only single pol.) full performance incidence angle range azimuth resolution 3.3 m (single pol.) 6.6 m (dual pol.) 1.1 m / 2.2 m (HS, single / dual pol.) 1.7 m / 3.4 m (SL, single / dual pol.) 17 m (1 look, 4 beams) ground range 150 MHz chirp BW 1.7 m m (@ ) 1.5 m m (@ ) 1.7 m m (@ ) (nom.) product length 50 km 5 km/10 km 150 km polarizations: single HH or VV dual (HH/VV) (HH/HV) or (VV/VH), no cross-pol for SL,HS

3 TerraSAR-X full performance beams and incidence angle ranges* Imaging Mode Polarization Mode Full Performance Beam Configurations Incidence Angle (Look Angle) Range Stripmap single strip_003- strip_ ( ) Stripmap dual stripnear_003 - stripfar_ ( ) Spotlight & High-Resolution Spotlight single & dual spot_010 spot_ ( ) ScanSAR single scan_003 scan_ ( ) *for right looking mode These are the only beams which can be ordered by science users Folie 3

4 Used TerraSAR-X products Radiometric Enhanced (RE) variant Resolution*:7.6 to 10 m (decreased to reduce speckle) 1.5 db radiometric resolution Pixel spacing: 3.5 to 4.5 m No of looks: 5 to 7 ( e.g. 4.4rg x 1.4 az) *depending on the incidence angle Polarization: 1st acquisition HH/HV 2nd acquisition HH/VV one cycle (11 days) later

5 Glaciers zones with characteristic backscatter properties Dry-snow facies Percolation facies Superimposed ice Wet snow facies (in summer) Ice facies. Glacier facies zones during the accumulation period Folie 5

6 Aim of the study Establish relationship between accumulation rate and backscattering on ice sheets and glaciers Snow properties of the dry snow zone Physical properties for medium characterization relevant for radar scattering: grain size and shape sequence, roughness and orientation of internal interfaces External processes determining the snow metamorphism air (and snow) temperature accumulation rate wind intensity and direction Folie 6

7 Dry snow zone Test site in Dronning Maud Land, Antarctica, 4 stations across the Heimefrontfjella Mts. Veststraumen 74.3S 13.7W TerraSAR-X coverage Base Camp 74.7S 12.6W Camp S 11.9W 15 km Amundsen Ice 75.3S 10.3W Folie 7

8 In situ measurements: vertical profiles of snow density Depth hoar layer Multi-year accumulation Veststraumen Base Camp Amundsen Ice Accumulation [kg m -2 a -1 ] Density (0-2 m) [kg m-3] m firn temperature [ C] Elevation a.s.l. [m] Folie 8

9 In situ measurements (1990) : scatterometer at X-band, VV, HH, VH and HV Incidence angle: deg Veststraumen Base Camp Amundsen Ice Folie 9

10 TerraSAR-X Veststraumen γ=σ 0 /cosθ } Scatt. az rg HH,HV,HH-HV

11 TerraSAR-X Base Camp Camp 3 Scatt. az rg HH,HV,HH-HV

12 TerraSAR-X az rg } HH,HV,HH-HV Scatt. Amundsen Ice Details of the bedrock topography?

13 Dry snow zone Site along the traverse between Georg von Neumayer and Kottas camp 74 S 9.5 W TerraSAR-X Kottas camp 74.2S, 9.73W Accumulation [kg m -2 a -1 ] az rg

14 Backscattering profile in azimuth across Heimefrontfjella from North (left) to South (right) TerraSAR-X data , Incid. angle 30 deg. Veststraumen Base camp Camp 3 Amundsen Ice 260 Accumulation [kg m -2 a -1 ] Backscattering profile 40km N of Kottas camp TerraSAR-X , Incid. angle ~40 deg. Accumulation gradient 200 to 300 kg m -2 a -1

15 Percolation zone Test site: Sulztal - glacier in the Austrian Alps TerraSAR-X dual pol VV & VH on 15 January 2008 Winter fine grained snow covers the coarse grained snow (percolation zone) TerraSAR-X σ 0 VV θ i at image center 27 deg σ 0 VV Signature dominated by the scattering in the firn layer Folie 15

16 Modelling of X-Band backscatter from the percolation zone Input values for winter snow layer: Parameters DMRT Model Snow surface RMS height 0.2 cm Snow surface Correlation 8 cm length Grain shape Cylinder Grain equivalent radius Case 1: 0.4 mm Case 2: 0.6 mm Grain axis ratio (b/a) 0.5 Vertical profile of snow density and stratigraphy on Snow depth Snow density SWE Snow temperature 1.67 m 274 kg/m³ mm 7 C Folie 16

17 1st Order RT model calculations compared to TerraSAR-X Case 1: omega 0.4 Case 2: omega 0.5 omega - single scattering albedo TerraSAR-X data Good estimation of both VV and HV backscattering because the RT uses empirical relations for polarization behavior. DMRT underestimates σ 0 HV by several db. Folie 17

18 Model calculation for coarse grained snow (percolation zone) superimposed by 1 m of fine grained winter snow σ 0 of winter snow + coarse layer --- σ 0 of coarse layer At X-band weak sensitivity of the signal to accumulation of winter snow above the older one. Snow parameters DMRT Model Winter snow grain radius 0.2 mm Density 300 kg/m 3 Winter snow depth Grain radius in the percolation layer Grain radius in the winter snow Ellipsoid axis ratio (b/a) 1 m 1.0 mm 0.2 mm 0.5 Folie 18

19 Conclusions (1) For the dry snow facies the X-band backscatter signatures are related to mean accumulation rates of several years. the magnitude of co-and cross-polarized backscatter is inversely related to the accumulation rate. HH and VV polarized σ show very small differences. Co- and cross- polarized backscatter show minor decrease with increasing accumulation rate. The main snow properties of relevance for backscattering are: The sequence of internal layers (in particular the thickness of annual layers and related depth hoar layers). Size and shape of the snow grains in the layers. Azimuth orientation of sastrugi (in katabatic wind zones). Possibilities for combining X-band data with other frequencies for estimation of snow accumulation should be studied. Folie 19

20 Conclusions (2) For the percolation facies the X-band backscatter during the accumulation period (dry snow) is dominated by the signal of the refrozen firn. Accumulation of winter snow results in a small decrease of the total backscatter due to the reduced scattering albedo of the winter snow. At X-band this is a weak signal, increasing towards higher radar frequencies. The main snow properties of relevance for the retrieval of winter snow accumulation are similar as those in the dry snow zone: - Background signature of the frozen firn or ice surface. - Size and shape of the snow grains. - Sequence of internal layers (related to meteorological events). Folie 20