Dept. of Geography / Remote Sensing Laboratories

5 Feb 2014 Page 1

Relating SAR backscatter to in-situ measurements and models of snow liquid water content

Christoph Rohner (UZH), Tobias Jonas (WSL-SLF), David Small (UZH)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 2

Introduction

- Only few studies investigate the relation SAR modeled/measured snow parameters on time series (e.g. Longépé et al., 2009)1

- Knowledge of agreement between radar imagery and modeled snow water equivalent important for integration of SAR products in snow-hydrological models

- We have 10 years of ASAR C-Band backscatter (time-series) of Switzerland and wish to test connections to in-situ snow station measurements

1 Longépé, N., Allain, S., Ferro-Famil, L., Pottier, E., & Durand, Y. (2009). Snowpack Characterization in Mountainous Regions Using C-Band SAR Data and a Meteorological Model. IEEE Transactions on Geoscience and Remote Sensing, 47, 406–418.

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 4

Research Questions

- Which parameters influence the SAR based wet snow retrieval and what is the relative sensitivity to each?

- What are the relations between the parameters measured and modeled at IMIS/MeteoSwiss and wet snow signals retrieved from Envisat ASAR/RADARSAT-2?

- Is it possible to retrieve information about the sensitivity of the SAR backscatter to in-situ/modeled information on the locally known depth of the melting process?

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 5

Data

Remote Sensing data

- Envisat ASAR (Radiometrically Terrain Corrected)1

- RADARSAT-2 (Radiometrically Terrain Corrected)1

Meteorological/snow-hydrological parameters

- IMIS measurements (SLF)

- Physical SNOWPACK model (SLF)

- Gridded data sets (MeteoSwiss)

1 Small, D. (2011). Flattening Gamma: Radiometric Terrain Correction for SAR Imagery. IEEE Transactions on Geoscience and Remote Sensing, 49, 3081–3093.

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 6

Envisat ASAR (C-Band, WS, VV pol)

- ASAR WSM product has nominal 150 m resolution, swath width 400 km

- > 300 RTC scenes of Switzerland between 2007 – 2012 used

Envisat ASARData & Products

© European SpaceAgency (2009)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 7

RADARSAT-2 (C-Band, SCNB, VV/VH)

- SCNB product has nominal 50 m resolution, swath width 300 km

- 4 dual-pol (VV/VH) RTC scenes of Switzerland between 2010 – 2012 used

RADARSAT-2Data & Products

© MacDonald, Dettwilerand Associates Ltd. (2010)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 8

Intercantonal Measurement and Information System (IMIS)- Operated by the Swiss Institute for Snow and Avalanche Research (SLF)

- Meteorological/snow parameters measured at 30 minute intervals

- > 100 automated measurement stations throughout the Swiss Alps

- Stations operate autonomously using battery/solar panel

- 38 representative stations selectedfor the study

Source: www.wsl.ch

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 9

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 10

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 11

Physical SNOWPACK Model (Lehning et al., 2002)

- Finite-element based physical snowpack model

- Snow is modeled as a three-phase (ice/water/air) porous medium

- Data input based on IMIS measurements of meteorological and snow parameters

Lehning, M., Bartelt, P., Brown, B., Fierz, C., & Satyawali, P. (2002). A physical SNOWPACK model for the Swiss avalanche warning. Cold Regions Science and Technology, 35, 123–184.

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 12

Measured/Modeled Parameters Used

Abbreviation Description Units

Source

LWC Layered volumetric liquidwater content

[%] SNOWPACK

TS Layered snow temperature [°C] SNOWPACK

HS Snow depth [cm] IMIS

TA Air temperature [°C] IMIS

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 13

Early Spring Time Melting (ASAR RTC)Station Färmel (1970 m.a.s.l.)

Apr 23 200823:08:32 MESZ

Apr 26 200823:14:24 MESZ

May 2 200823:25:55 MESZ

Envisat ASAR Data & Products © European Space Agency (2009)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 14

Early Spring Time Melting (Station Färmel, Apr 2009)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 15

Early Spring Time Melting (ASAR RTC)Station Färmel (1970 m.a.s.l.)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 16

Disappearance of the Snow Cover (ASAR RTC)Station First (2110 m.a.s.l.)

Apr 29 200823:20:23 MESZ

May 18 2008 23:22:57 MESZ

Jun 03 200823:20:25 MESZ

Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 17

Disappearance of the Snow Cover (Station First, Apr 2008)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 18

Disappearance of the Snow Cover (ASAR RTC)Station First (2110 m.a.s.l.)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 19

Preliminary Results for Selected Stations

Non-trivial to establish connection between areal radar backscatter and in situ station LWC (point observation combined with model)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 20

Preliminary Results for Station Anniviers (2850 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 21

Preliminary Results for Station Nara (2070 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 22

Preliminary Results for Station Jaun (1716 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 23

Preliminary Conclusions

- Early stage of the study

- Promising preliminary results found for certain stations, but no simple connection visible in first juxtaposition of datasets

- Deeper understanding of multivariate combined data set will hopefully reveal further connections in the coming months

- Connections between LWC, SWE, date, temperature, dry reference backscatter, residual topography

- Issues concerning the properties of the data sets (point vs. areal) need to be addressed

- Results based on a wider field of stations await further processing (in progress)

Dept. of Geography / Remote Sensing Laboratories

Relating SAR backscatter to in-situ measurements and models, Christoph Rohner5 Feb 2014 Page 24

Acknowledgments

Thanks to- ESA for the provision of the Envisat ASAR data- CSA/MDA for the provision of the RADARSAT-2 data- SLF for the provision of the IMIS/SNOWPACK data