Radar mapping of snow melt over mountain glaciers in High Mountain Asia Mentor: Tarendra Lakhankar Collaborators: Nir Krakauer, Kyle MacDonald and

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1 Radar mapping of snow melt over mountain glaciers in High Mountain Asia Mentor: Tarendra Lakhankar Collaborators: Nir Krakauer, Kyle MacDonald and Nick Steiner

2 How are Glaciers Formed? Glaciers are formed on the coldest parts of the earth, either at ground level or on the mountains. They contain snow that has turned into ice after more snow has fallen over it and compressed it into hard ice. Glaciers are important indicators of global warming and climate change in several ways. Melting ice sheets contribute to rising sea levels. As ice sheets in Antarctica and Greenland melt, they raise the level of the ocean.... Large additions of fresh water also change the ocean ecosystem.

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4 Assessing Climate-Induced Change in River Flow in High Mountain Asia Improve hydrologic forecasting of rivers using remote sensing information, in-situ data and modeling. Assess links between climate change, river flow, and hydroelectric power generation and downstream biodiversity (fish) Assess climate change impacts on the economic value of hydropower and fisheries. Link with economic estimation to support water resource planning. Trishuli Sub-basin Hydropower: * 5 operational hydropower plants, 70 Megawatts capacity * 17 generation licenses, 422 Megawatts capacity * Survey licenses: additional 16 plants, 620 Megawatts total capacity

5 Observations [monthly] Remote Sensing Instruments and Data Products Advanced Scatterometer (ASCAT) Sentinel-1 SAR The Advanced Scatterometer (ASCAT) is a C-Band scatterometer Backscatter gridded at 5.25 km, repeat coverage every 2 days Spatially enhanced product (BYU-SCP) Using VV, ascending product Sentinel-1 SAR is a C-band synthetic aperture radar Backscatter gridded at 30 m, repeat coverage 6 or 12 day Inferometric wide beam (IW) product, radiometrically terrain corrected, Using VV, ascending product

6 normalized backscatter [db] C-Band Backscatter Signature Over Land (ASCAT) Thaw Bulk landscape thaw results in liquid water at the surface, increased reflectivity, and surface scatter at C-Band Seasonal snow influence at C-Band at VV polarization is minimum Land surfaces will brighten with thaw

7 normalized backscatter [db] C-Band Backscatter Signature Over Snow (ASCAT) Tha w Snowmelt Bulk landscape thaw of snow increases its liquid water content, greatly reducing the penetration depth, leading to decreased volume scatter from snow at C-Band Deep, dry, snowpacks are bright and will darken with snowmelt

8 Backscatter (db) Landsat 8 (Nov. 2017) S1-SAR Dec-Jan Backscatter Hillshade (ASTER) a a a b b b a. Glacier Ice High frequency repeat SAR observations allow for time-series analysis to determine snowmelt and discriminate frozen/thawed state Snowmelt Thaw b. Snow-Field c. Non- Glaciated Area

9 normalized backscatter [db] normalized backscatter [db] Scaling of the C-Band Radar Snowmelt Response Sentinel-1 SAR Sentinel-1 SAR Sentinel-1 SAR Average Sentinel-1 SAR Average ASCAT ASCAT Sentinel-1 (S-1) backscatter averaged (N=3x10 4 ) to ASCAT scale (gray +- 1 standard deviation) Aggregated S-1 show a similar seasonal response compared to ASCAT, differences in magnitude Clear radar snowmelt signatures at high resolution are dampened or obscured in subpixel mixtures

10 Freeze/Thaw and Snowmelt Algorithms Backscatter Time-Series Data ASCAT S1 Wavelet Transform And Multiscale Analysis Seasonal Reference States *Winter, JF *Summer, JA Dry Snow / Snowmelt Frozen / Thawed S1 ASCAT S1 SMAP Algorithm A Linear Discriminant Analyses where Wavelet Fails Fixed Threshold B Melt Duration Thawed Duration Classifier Training Data Thresholding FT t = A - SMAP F/T Algorithm (σ 0 t σ 0 ) winter σ 0 0 summer σ winter >.5 B - Fixed Threshold Algorithm SM t = σ 0 0 t < σ winter 3 [db]

11 Quality Control Areas without a clear seasonal contrast in backscatter are masked in Sentinel-1 and ASCAT products Z-Score (Z) is used to quantify how well classes are resolved, seasonally 0 Z = abs(σ nonfrozen σ frozen 0 0 )/s(σ frozen QC Criteria: Z > 2 and elevation > 700 [m. asl.] )

12 Freeze/Thaw and Snowmelt Classes and Masking Sentinel-1 SAR ASCAT Scatterometer Freeze/Thaw (FT) or Snowmelt (SM) is determined by the sign of the seasonal backscatter response Areas are masked where there is no clear seasonal response (Zscore<2) Sentinel-1 SAR enables distinction of FT/SM in complex terrain Freeze/Thaw Dominant (Surface Scatter) Snow Dominant (Volume Scatter)

13 ASCAT Freeze/Thaw and Snowmelt Product Average melting and thawed days during Gandaki Basin, Trishuli Sub-Basin Seasonal Melting Area

14 Seasonal Freeze/Thaw and Snowmelt Transitions Monthly cumulative thaw and melt duration from ASCAT, 2009

15 Cumulative melt [days] Cumulative thaw [days] Freeze/Thaw and Snowmelt, Trishuli Basin, 2017 Small regions of melting dominate seasonal backscatter response in ASCAT data Change in seasonal surface reflectivity from liquid water lesser magnitude than volume scatter change

16 ASCAT Count (n) Product Comparison Correlation with Aggregation Distribution of Melt Durations S1 SAR ASCAT Sentinel SAR Count (n) Melt Duration (days) Products show a moderate to high degree of correlation at comparable scales. At 6 km Spearman s R:.7 High degree of error in comparison between products (RMSE: ~60 days) **Sensitivity of ASCAT backscatter change to snowmelt in any part of the coarse resolution product triggers melt signature (3dB)

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