Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Page 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 3 Table of Contents –Introduction –Research Questions –Data –Preliminary Results –Preliminary Conclusions

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 ASAR Data & Products © European Space Agency (2009)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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-2 Data & Products © MacDonald, Dettwiler and Associates Ltd. (2010)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 selected for the study Source:

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 9

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 10

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 12 Measured/Modeled Parameters Used AbbreviationDescription UnitsSource LWCLayered volumetric liquid water content [%]SNOWPACK TSLayered snow temperature [°C]SNOWPACK HSSnow depth [cm]IMIS TAAir temperature [°C]IMIS

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 13 Early Spring Time Melting (ASAR RTC) Station Färmel (1970 m.a.s.l.) Apr :08:32 MESZ Apr :14:24 MESZ May :25:55 MESZ Envisat ASAR Data & Products © European Space Agency (2009)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 14 Early Spring Time Melting (Station Färmel, Apr 2009)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 15 Early Spring Time Melting (ASAR RTC) Station Färmel (1970 m.a.s.l.)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 16 Disappearance of the Snow Cover (ASAR RTC) Station First (2110 m.a.s.l.) Apr :20:23 MESZ May :22:57 MESZ Jun :20:25 MESZ Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 17 Disappearance of the Snow Cover (Station First, Apr 2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 18 Disappearance of the Snow Cover (ASAR RTC) Station First (2110 m.a.s.l.)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 20 Preliminary Results for Station Anniviers (2850 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 21 Preliminary Results for Station Nara (2070 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 22 Preliminary Results for Station Jaun (1716 m.a.s.l)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 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

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 25 Radiometric Terrain Correction (Small, 2011)  Estimation of the local illuminated area for each point in image using known acquisition geometry of SAR image in combination with a digital height model (DHM)  Improved interpretability of RTC imagery in comparison to conventionally normalized scenes based on ellipsoid or local incident, even if imagery originates from different sensors or acquired in different modes  Generally applicable to all SAR imagery, given highly accurate orbit state vectors 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 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 26 ASAR WS VV Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 27 ASAR WS VV Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 28 ASAR WS VV Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 29 ASAR WS VV Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 30 RADARSAT-2 SCNB VV RADARSAT-2 Data & Products © MacDonald, Dettwiler and Associates Ltd. (2012)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 31 RADARSAT-2 SCNB VV RADARSAT-2 Data & Products © MacDonald, Dettwiler and Associates Ltd. (2012)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 32 RADARSAT-2 SCNB VV RADARSAT-2 Data & Products © MacDonald, Dettwiler and Associates Ltd. (2010)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 33 RADARSAT-2 SCNB VV RADARSAT-2 Data & Products © MacDonald, Dettwiler and Associates Ltd. (2010)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 34 Measured parameters at IMIS locations AbbreviationDescription Units HSSnow depth [cm] TAAir temperature [°C] TSSTemperature of snow surface [°C] TS1/TS2/TS3Temperature of snow at 25/50/100 cm [°C] TS0Ground temperature [°C] RSWRReflected short-wave radiation [W/m 2 ] RHRelative humidity [%] Wind speed (average/peak) [m/s] Wind direction [°C]

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 35 Disappearance of the snow cover (ASAR RTC) Station Nara (2070 m.a.s.l.) Apr :20:23 MESZ May :17:12 MESZ Jun :20:25 MESZ Envisat ASAR Data & Products © European Space Agency (2008)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 36 Disappearance of the snow cover (ASAR RTC)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 37 Disappearance of the snow cover (ASAR RTC)

Dept. of Geography / Remote Sensing Laboratories 5 Feb 2014Relating SAR backscatter to in-situ measurements and models, Christoph RohnerPage 38 Preliminary results for selected stations