1. Active Microwave Physics and Basics 1 Simon Yueh JPL, Pasadena, CA August 14, 2014

2. How Deep Can the Radio Waves Penetrate 10 to17 GHz microwave can penetrate dry snowpack with a broad range of depth (1 to 5 m) 2 Experiment, Radio Laboratory, Helsinki University of Technology in 1987 Theoretical simulations from bicontinuous medium/NMM3D, Xu et al, 2012 FrequencyPenetratio n Depth 10 GHz (X)~5 m 14 GHz (Ku) ~1 m 18 GHz (K)~0.5 m 37 GHz (Ka) ~0.1 m 0.01m 0.1m 1m 10m

3. Radar Sounding of Snow Surface Scattering Surface scattering dominates at near nadir looking Early demonstration by late Prof. Hal Boyne (CSU) Current Status – A well-developed tool for probing the snow stratigraphy – Marsahll et al., ground-based FMCW Radar – Gogineni et al., aircraft-based Snow Radar Courtesy of Boyne What is the resolution? – ΔR=Range resolution=C/2B – ΔH=H(1/cosθ-1) for rough interface Beamwidth (2θ) and height (H) – Horizontal resolution=2Hθ – limited by beamwidth ΔR ΔH BΔR 1 GHz15 cm 5 GHz3 cm HΔH 1000 m, 10deg3.8m 10 m, 5 deg1 cm

4. Off-nadir Looking Radar Volume Scattering SAR processing can achieve horizontal resolution of a few meters from space Backscatter contributions: Volume, surface, and interaction terms. Observed backscatter coefficient σ° : At off-nadir angles (30-50 degrees incidence angles) Volume scattering starts to dominate Surface scattering diminishes Main parameters for snow backscatter: Dry snow Snow water equivalent Grain size (d) Density (ρ) Soil background signal Wet snow Liquid water content (radar signal does not penetrate)

5. One example of data and theory More data acquired through CLPX2, SnowScat and SnowSAR campaigns Snow SnowSCAT backscatter time series σvv with 40∘ incidence angle against SWE. Data taken from at Sodankylä between 12/28 /2010 and 03/01/2011. Simulated radar backscatter using the DMRT/QCA for snow volume scattering at three frequencies. All three frequencies show response to snow water equivalent for moderate and large grain size.

6. SAR Snow Tomography Side-looking radar with multiple baselines Snow stratigraphy - Metamorphism and environmental factors create complex layering structures in the snow pack SAR Tomography will provide insight into snow and ice – Lack of comprehensive theoretical development and experimental testing for snow SAR Tomography – Tested for 3-D forest canopy mapping – Coherence and multiple baselines – Demosntrated by GB-SAR, K Morrison of Cranfield U. Measurements at Reynolds Creek study site, 200 meters from tower - 116 manual probe depth measurements. (Marshall et al. of BSU) Lel n r dr Height (m) Slant Range (m) Polarimetric tomographic profile over a forested area using DLR’s E-SAR system at L-band [Moreira et al., IEEE GRS magazine, 2013].

7. Recent campaigns covering main snow regimes Churchill, Canada, Tundra (Near-)Coincident Ku-band and X-band scatterometers and SAR used Sodankylä, Finland, Taiga Innsbruck, Austria, Alpine Colorado, USA Alpine/Tundra/ Taiga/Prairie Inuvik, Canada, Tundra Kuparuk, Alaska, Tundra

8. Radar backscatter versus SWE – from Sodankylä, Finland, Taiga Backscatter versus observed SWE, Sodankylä, Finland, SnowScat measurements for winter I, for winter II  radiative transfer model calculation for 3 different values of grain size SnowScat measurements at 40° for two winters

9. Radar backscatter versus SWE – from Rocky Mountain, Colorado Backscatter for VV, HH, and VH polarizations shows sensitivity to SWE for three sampling sites Yueh et al., Airborne Ku-band Polarimetric Radar Remote Sensing of Terrestrial Snow Cover, IEEE TGRS, Vol. 47, No. 10, 3347-3364, 2009. NASA/JPL POLSCAT measurements