Feasibility of snow water equivalent retrieval by means of groundbased and spaceborne SAR interferometry

Transcription

1 Feasibility of snow water equivalent retrieval by means of groundbased and spaceborne SAR interferometry, Helmut Rott, Markus Heidinger ENVEO, Innsbruck, Austria Guido Luzi, Giovanni Macaluso, Daniele Mecatti, Linhsia Noferini University of Florence, Florence, Italy Rudolf Sailer, Philipp Jörg, Andreas Schaffhauser, Reinhard Fromm BFW, Innsbruck, Austria

2 Contents SWE Retrieval by means of INSAR Variable Density SWE Retrieval Algorithm GBSAR Test site and data sets Examples for SWE Retrieval Coherence First results of SWE retrieval by means of ALOS PALSAR Conclusions

3 INSAR SWE Retrieval - Backgrounds Total interferometric phase difference:! =! + flat +! topo +! dis +! atm +! snow! noise Phase shift due to dry snow accumulation: ( 2 ) $! = % 2k $ d cos" % # % sin" snow s i i e = r s r 3 s (Mätzler,1996) SWE = d s <r s > For low incidence angles :!# 1.6k cos" snow =! i SWE

4 Sensitivity of Df on SWE L S C Df =2p, q=40 : SWE SD (0.1 g/cm3) SD (0.3 g/cm3) L-Band: 120 mm 1.20 m 0.40 m C-Band: 28 mm 0.28 m 0.09 m

5 SWE Df relationship SWE = d s <r s > = const. C-Band with SWE = 20 mm L-Band with SWE = 20 mm At high incidence angles: with increasing r, expected Df reduces

6 SWE Df relationship Constant snow height d s = 10 cm, variable r d s = SWE / <r s > = const.

7 Variable Density (VD) SWE Retrieval Algorithm

8 GB-SAR Measurements Wattener Lizum Observation geometry Tarntaler Scharte / Wattener Lizum Met Station GB SAR Met Station

9 GB-SAR System GBSAR System - CWSF (continuous-wave step-frequency) radar - VNA as transceiver unit - two fixed antennas (Rx, Tx) - length of linear horizontal rail is 1.3 m - discrete position increments for scanning the synthetic aperture. GBSAR Setup Polarization Central frequency (GHz) Band (MHz) Wavelength (cm) Time sampling Range resolution Azimuth resolution C band VV cm 30 min 7.5 m rad S-band VV min 7.5m rad Winter 2005/06 and 06/07: several time series of several days duration are acquired

10 2757 m Meteo Station Plateau (2350m) slope area 2050 m

11 Time series of Interferometric Phase Feb 2006 Meteorological and Snow Conditions Snow depth Time series of Df C-Band P3 Df P7 Temperature

12 SWE Retrieval at selected Points Feb 2006 Time series of SWE for point 3 and point 7 relative to the initial state, using densities of 0.1, 0.2 g/cm! and the VD-SWE algorithm. For VD-SWE an initial snow pack density of 0.3 g/cm! is assumed.

13 Time series of Interferometric Phase March 2006 Meteorological and Snow Conditions Time series of Df C-Band Snow depth P7 Temperature P3

14 SWE Retrieval at selected Points March 2006 Time series of SWE for point 3 and point 7 relative to the initial state, using densities of 0.1, 0.2 g/cm! and the VD-SWE algorithm. For VD-SWE an initial snow pack density of 0.3 g/cm! is assumed.

15 Temporal decorrelation due to snow accumulation snow density e 0.1 g/cm g/cm g/cm3 1.53! z represents standard deviation of phase delay within pixel due to snow fall, for distributed scatterer [mm] Model calculation of temporal decorrelation due to snow in dependence on the surface roughness

16 Temporal decorrelation slope area Coherence map after 6 hours Coherence map after 18 hours Feb 2007 Range (max 2km) Cross-Range (400m) C band S band

17 ALOS PALSAR Scenes of Winter LOS Telfs Telfs Leutasch Telfs Innsbruck Kühtai Kühtai Umhausen Pitztaler Gletscher Obergurgl

18 PALSAR Data Set PALSAR data Interferometric Pairs 30 Sep Nov 2006 Bper 74 m 31 Dec Feb 2007 Bper 594 m Full Pol Mode (LookA 21 deg)

19 ALOS Palsar Coherence - 46 Day Period L-Band HH 30 Sep Nov 2006 Bper 74 m 31 Dec Feb 2007 Bper 594 m Innsbruck 560m Umhausen 1030 m Kühtai 2040m Pitztaler Gletscher 3340 m Obergurgl 1913 m LOS

20 ALOS Palsar Coherence - 46 Day Period L-Band HH 30 Sep Nov 2006 Bper 74 m 31 Dec Feb 2007 Bper 594 m Innsbruck 560m Umhausen 1030 m Kühtai 2040m Pitztaler Gletscher 3340 m Obergurgl 1913 m LOS

21 Topo-flattened Interferograms L-Band HH 30 Sep Nov 2006 Bper 74 m 31 Dec Feb 2007 Bper 594 m 0/0 cm 0rad 0/0 cm 0rad Innsbruck 560m 0/2 cm 0.2 rad 34/79 cm 4.4 rad Kühtai 2040m Snow Height Date1/2 0/0 cm 0 rad Umhausen 1030 m 5/8 cm 0.3 rad Derived Df 0 20 cm 2 rad 0/5 cm 0.5 rad 2p Pitztaler Gletscher 3340 m 70/120 cm 4.6 rad LOS Obergurgl 1913 m 25/45 cm 2 rad

22 Conclusions - The dry snow SWE retrieval from the interferometric phase requires an estimate of the mean density of the snow pack. The developed interferometric snow water equivalent retrieval algorithm with variable snow density (VD-SWE algorithm) provides a robust and reliable basis for SWE retrieval from interferometric phase time series. The algorithm does not require information from other sources. - The error estimates point out that possible errors in SWE due to errors in estimating the snow density, snow temperature and the state of the initial snow pack are of the order of a few per cent maximum. - First studies with limited data set of ALOS PALSAR data show that coherence in snow covered areas is at least partly preserved over a 46 days period. The observed phase shifts are in the expected range, but the available data set limits the study to small accumulation rates because of the dry winter 2006 / Further analysis of the data set is ongoing.