Transcript Part1: Land Cover Change and Part 2: 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
Sea Surface Temperature (SST)
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.
Differences in Spatial Coverage
AVHRR
TRMM
Microwave
Imager
(white pixels are areas of no data)
AVHRR image of Gulf Stream
(5 km gridding)
MODIS image of Gulf Stream
(1 km gridding)
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)
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
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
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
MODIS
chlorophyll
data
lt. blue=good data
dk.blue=questionable
purple=cloud
red=bad data
Revised MODIS
chlorophyll data
MODIS
Quality
Assurance
data
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
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
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
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
TOPEX data show waves developing in Pacific
Two days later, a surfer at Mavericks (Half Moon Bay,
California) rides the huge waves
ERS-2 SAR image of waves the next day.
SAR measures wave direction and wave height
Lakes
• Monitoring changes in surface water extent
– Lake Chad
Water level
fluctuations in Lake
Chad, Nigeria
Landsat MSS, 1973
AVHRR, 1997
Rivers
• Flooding
– extent of surface waters
– floodplain mapping
Mekong Delta
flooding using
multi-temporal
MODIS data
Mapping the
extent of surface
waters for large
flood events
allows improved
mapping of
floodplains
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
Tropical Rainfall Monitoring Mission (TRMM) maps
areas of heavy precipitation between 50oN and 50oS.
Near real time capability (daily maps)
Flooding in southern Texas
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
• 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/m
od10_l2.html
March 25, 2003
Colorado Blizzard
(blue=snow, white=cloud)
MODIS Snow Standard Product, Colorado, 15 Feb 2002
snow
cloud
no snow
25% of
actual
snow
covered
area is
incorrectly
classified
as cloud
MODIS fractional snow cover, Colorado, 15 Feb
2002
90-100%
70-90%
50-70%
30-50%
10-30%
0-10%
Vegetation
obscures snow
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
ASTER
image of
Patagonian
glacier in
Chile