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Remote Sensing of the Hydrosphere

Remote Sensing of the Hydrosphere. The Hydrologic Cycle. 70% of Earth is covered by oceans and surface freshwater Residence time varies from seconds to millennia. Ocean Remote Sensing. Sea surface temperature Sea surface height Ocean color Wind and waves.

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Remote Sensing of the Hydrosphere

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  1. Remote Sensing of the Hydrosphere

  2. The Hydrologic Cycle 70% of Earth is covered by oceans and surface freshwater Residence time varies from seconds to millennia

  3. Ocean Remote Sensing • Sea surface temperature • Sea surface height • Ocean color • Wind and waves

  4. Thermal sensors (e.g. AVHRR, MODIS) good spatial resolution and accuracy incomplete spatial coverage long heritage (>20 years) obscured by clouds atmospheric corrections are required measures SST to a water depth of 10 mm Passive microwave sensors (e.g. TRMM, SSM/I) lower spatial resolution and accuracy than w/thermal good spatial coverage long heritage (>20 years) clouds are transparent relatively insensitive to atmospheric effects measures SST to a water depth of 1 mm sensitive to surface roughness (waves) sensitive to heavy precip. Sea Surface Temperature (SST)

  5. Differences in Spatial Coverage AVHRR TRMM Microwave Imager (white pixels are areas of no data)

  6. AVHRR image of Gulf Stream (5 km gridding) MODIS image of Gulf Stream (1 km gridding)

  7. Sea Surface Height • Sea surface height changes with wind patterns, ocean currents, ocean temperature (thermal expansion) • Radar altimeter used to measure changes in sea surface height (timing of radar pulse gives distance between sensor and surface)

  8. Sea Surface Height • TOPEX/POSEIDON • launched in 1992 and still operating • measures SSH between 66o N lat and 66o S lat • C and Ku band altimeters • Sea surface height accurate to 4.2 cm • data available for free from http://podaac.jpl.nasa.gov • Jason-1 (similar to TOPEX/POSEIDON) • launched in December 2001 • some data are now available for free from http://podaac.jpl.nasa.gov

  9. Ocean Color • Ocean color is a measure of biological activity • phytoplankton (produce chlorophyll) • dissolved and particulate matter • Used to measure: • biological productivity • marine optical properties • interaction of winds and currents with ocean biology • human influences on the marine environment • Remote sensing of ocean color uses multispectral data

  10. Ocean Color Sensors • Coastal Zone Color Scanner (CZCS) • October 1978 - June 1986 • 1 km and 4 km gridded data • data products available for free from http://daac.gsfc.nasa.gov/data/dataset/CZCS/index.html • Seaviewing Wide Field-of-view Scanner (SeaWIFS) • since September 1997 • 1 km and 4 km gridded data • data products available for free for scientific use from http://daac.gsfc.nasa.gov/data/dataset/SEAWIFS • Moderate Resolution Imaging Spectroradiometer (MODIS) • since February 2000 • 1 km gridded data • data products available for free from http://daac.gsfc.nasa.gov/data/dataset/MODIS

  11. MODIS chlorophyll data MODIS Quality Assurance data lt. blue=good data dk.blue=questionable purple=cloud red=bad data Revised MODIS chlorophyll data

  12. Ocean Winds • Ocean wind measurements are needed for understanding and predicting severe weather patterns • Winds modulate energy transfer between the atmosphere and ocean • Waves are the expression of momentum transfer from atmosphere to ocean • Surface roughness can be monitored using SAR • “Scatterometry” is a new method of measuring backscattered return from a SAR

  13. Ocean Wind Sensors, I. • NASA Scatterometer (NSCAT) • produced data from Sept 1996 - June 1997 • C-band • 50 km grid resolution • QuikSCAT SeaWinds scatterometer • launched in June 1999 • C-band • 12.5 and 25 km spatial resolution for wind vectors • measures winds of 3- 20 ms-1, with 2 ms-1 accuracy • Data from NSCAT and QuikSCAT are available for free from http://podaac.jpl.nasa.gov

  14. How Scatterometry Works • Microwave radar signal is transmitted to ocean surface • Pulses are scattered by waves • Rougher surface = more scattering • Near-surface wind speed is computed from radar backscatter • Orientation of wind ripples indicate wind direction

  15. Ocean Wind Sensors, II. • Radar Altimeters measure sea surface height • Areas of convergence and divergence are mapped • Examples of radar altimeters: • TOPEX/Poseidon • ERS-1 and ERS-2

  16. TOPEX data show waves developing in Pacific

  17. Two days later, a surfer at Mavericks (Half Moon Bay, California) rides the huge waves

  18. ERS-2 SAR image of waves the next day. SAR measures wave direction and wave height

  19. Lakes • Monitoring changes in surface water extent • Lake Chad

  20. Water level fluctuations in Lake Chad, Nigeria Landsat MSS, 1973 AVHRR, 1997

  21. Rivers • Flooding • extent of surface waters • floodplain mapping

  22. Mekong Delta flooding using multi-temporal MODIS data Mapping the extent of surface waters for large flood events allows improved mapping of floodplains

  23. Southeast Asian Flooding in summer 2000 NASA global flood monitoring system operated through the Dartmouth Flood Observatory Data collected from MODIS 8-day composites Used to locate hardest hit areas for United Nations relief efforts

  24. Tropical Rainfall Monitoring Mission (TRMM) maps areas of heavy precipitation between 50oN and 50oS. Near real time capability (daily maps) Flooding in southern Texas

  25. Snow cover Mapping • Passive microwave • 25 km grid scale • weekly data available since October 1978 • daily data available since January 2000 • AVHRR • 1 km grid scale • weekly data available since October 1966 • MODIS • 500m grid scale • data available since February 2000 • All data available for free from http://nsidc.org/data

  26. Snow covered area from MODIS • daily and 8-day composite global snow cover products • 500 m spatial resolution • available for free from http://nsidc.org/data/mod10_l2.html March 25, 2003 Colorado Blizzard (blue=snow, white=cloud)

  27. MODIS Snow Standard Product, Colorado, 15 Feb 2002 snow cloud no snow 25% of actual snow covered area is incorrectly classified as cloud

  28. MODIS fractional snow cover, Colorado, 15 Feb 2002 90-100% 70-90% 50-70% 30-50% 10-30% 0-10% Vegetation obscures snow

  29. Glaciers • Global Land Ice Monitoring from Space (GLIMS) • Uses ASTER data to map • glacier extent • snow line • glacier topography • Synthetic Aperture Radar • Used to map glacier zones • Interferometry used to map glacier velocity

  30. ASTER image of Patagonian glacier in Chile

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