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Connecting Sensors: SSM/I and QuikSCAT -- the Polar A Train
NSIDC’s 30th Anniversary, 25 October 2006
Introduction• Remote sensing in the microwave portion of the electromagnetic
spectrum have found many applications in cryospheric research.
• Nearly 30 years of continuous and consistent PM observations allow for studies on changes in several cryospheric variables. 1978 – 1987: SMMR 1987 – present: successive SSM/I 2002 – present: AMSR-E
• Active observations (e.g. scatterometers) also started in the late 1970s, but the record is not continuous or consistent. 6/78 - 9/78: SASS – Ku Band (Seasat) 6/96 – 5/97: NSCAT – Ku Band (ADEOS I) 2/92 – 1/01: EScat – C Band (ERS-1/2) 7/99 – present: SeaWinds on QuikSCAT (Ku Band) 12/02 – present: SeaWinds on ADEOS II (Ku Band)
NSIDC’s 30th Anniversary, 25 October 2006
Science with Passive Microwave
Sea ice mapping
NSIDC’s 30th Anniversary, 25 October 2006
Science with Passive Microwave
Greenland Melt
Steffen and Huff
Since 1979, the area that experiences melt has increased at a rate of ~18%/decade.
NSIDC’s 30th Anniversary, 25 October 2006
Science with Passive Microwave
-0.8
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0
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1 2 3 4 5 6 7 8 9 10 11 12
Month
Tre
nd
(m
illio
n s
q-k
m/d
ec
ad
e) Eurasia
North America
Significant trends in late spring and early summer indicating decreasing snow cover
Trend (million sq-km/decade)
Changes in northern hemisphere snow cover
NSIDC’s 30th Anniversary, 25 October 2006
Science with Active MicrowaveAntarctic Iceberg Tracking
• QuikSCAT tracks Antarctic icebergs (used operationally at NIC) Icebergs have high backscatter compared to sea ice and open
ocean (nearly 55% of all iceberg locations reported by NIC are based on QuikSCAT).
Iceberg Tracks from 1978 & 1992-2002
1999
NSIDC’s 30th Anniversary, 25 October 2006
Science with Active Microwave
• Time series analysis of the radar backscatter can be used to determine the onset of melt and re-freezing.
• Note: melt days over open ocean are exaggerated since as the ice edge retreats each pixel previously representing sea-ice now resides over open ocean and accumulates melt days as the summer season progresses.
NSIDC’s 30th Anniversary, 25 October 2006
Science with Active Microwave
• Ice extent is easily defined using active microwave data (and doesn’t require a tuning)
Ice Extent
NSIDC’s 30th Anniversary, 25 October 2006
Passive vs. Active• Both passive and active remote sensing offer:
Large spatial coverage; High temporal resolution; Ability to penetrate clouds; Derive similar geophysical variables (e.g. snow and ice extent,
sea surface wind vectors, melt, etc).• Passive microwave pros:
Long consistent time-series of observations.• Passive microwave cons:
Variation of surface emissivity and atmospheric water vapor create problems for algorithms.
• Active microwave pros (Scatterometers): Less sensitivity to weather effects.
• Active microwave cons (Scatterometers): Short time-scale.
NSIDC’s 30th Anniversary, 25 October 2006
Scatterometer Climate Record Pathfinder (SCP)
• SCP has been generating high resolution scatterometer imagery to support cryospheric studies from QuikSCAT, NSCAT, ERS and SASS). http://www.scp.byu.edu/
• Current products include: Global high resolution data (2.225 and 4.5 km/pixel); Antarctic ice berg tracking (daily at 2.225 km/pixel); Ice motion (daily at 25-km); Ice extent (daily at 2.225 km/pixel).
• NSIDC recently acquired sea ice extents from QuikSCAT spanning 1999-2004. Daily at 2.225 and 4.5 km/pixel for Arctic and Antarctic
NSIDC’s 30th Anniversary, 25 October 2006
REaSON CAN Project
• New research being done: Incorporation of ADEOS-II Seawinds to ensure continuous
Ku-band time-series; Addition of sea ice motion products and sea ice extent maps
derived from combined active/passive microwave data sets; Addition of seasonal melt-freeze products from combined
active/passive microwave data sets; Seasonal change maps of Greenland and Antarctica from
combined active/passive data; Integrated seasonal change maps of land ice/sea ice/cold
regions land cover; Antarctic iceberg drift database continuation.
NSIDC’s 30th Anniversary, 25 October 2006
Blending Active and Passive Microwave
• Generation of enhanced resolution SSM/I and AMSR brightness temperatures to match enhanced resolution scatterometer datasets. Enhanced resolution images are produced by
combining all passes in a single day to improve the spatial resolution of the data at the expense of temporal resolution.
NSIDC currently has enhanced resolution SSM/I data from 1995-2005 at pixel size of 8.9 km for 19-37 GHz and 4.45 km for 85 GHz.
NSIDC’s 30th Anniversary, 25 October 2006
Examples of SSM/I HIRZ
HIRZ
85V
January 1 2004
NSIDC’s 30th Anniversary, 25 October 2006
Image Artifacts
Artifacts arise from Tb changes from pass to pass during the day
7 September 2002
NSIDC’s 30th Anniversary, 25 October 2006
AMSR Local Time of Day
NSIDC’s 30th Anniversary, 25 October 2006
Animation of AMSR-E High Resolution Tbs
36.5 V
AMSR-E 36.5V from 19 June to 6 October 2002
NSIDC’s 30th Anniversary, 25 October 2006
Melt Onset/Freeze-up• Passive and active microwave
radiometers respond to liquid water content of snow Increase in passive microwave Tbs
because of decrease in volume scattering (thus increased emissivity)
Decrease in backscatter because of decrease in volume scattering
Before melt
After melt
13.9 GHz normalized radar cross section at 40o incidence angle before (top) and after (bottom) a major melt event. Note the dramatic change in backscatter over the sea ice and along the periphery of the Greenland ice sheet (http://www.scp.byu.edu).
Enhanced resolution NSCAT
May 22-27
June 18-23June 18-23
NSIDC’s 30th Anniversary, 25 October 2006
Arctic Wide Melt Onset/Freeze• B. Holt and K. McDonald are
developing melt onset/freeze-up across the polar region (includes sea ice and land) using a synthesis of Ku-band and passive microwave data.
Transect that extends from interior Alaska across the Chukchi and Beaufort Seas (inclusive of boreal forest, tundra, the Beaufort coastline, seasonal and multiyear sea ice.
2001
Melt onset
Freeze-up
Broken/thin ice floes
Brooks Range
NSIDC’s 30th Anniversary, 25 October 2006
Case Study, Antarctic Peninsula
• Study by Kunz and Long (2006) developed melt detection from QuikSCAT and compared to SSM/I
• QuickSCAT method: PR ratio (v
o – ho) together with h
o
Melt classification is determined using ML estimate of ice state
• Passive microwave method: HR = Tb(19H) – Tb(37H) (Anderson, 1987)
Tb-Tb(19V) > Tbdry + (1-)Tb
wet (Ashcroft and Long, 2006)
XPGR = Tb(19H) – Tb(37V) (Abdalati and Steffen, 2001) Tb(19H) + Tb(37V)
NSIDC’s 30th Anniversary, 25 October 2006
Melt Detection: Antarctica
When QuikSCAT backscatter decreases, there is usually a rise in SSM/I brightness temperatures
Kunz and Long (2006)
NSIDC’s 30th Anniversary, 25 October 2006
Melt Detection: Antarctica• Melt onset dates from
QuikSCAT are usually a few days earlier than those detected by XPGR and Tb-.
• Better consistency is found between QuikSCAT and Tb-than with XPGR.
• While radiometer data tend to be more sensitive to melt onset, it easily saturates, and is sensitive to weather.
Kunz and Long (2006)
NSIDC’s 30th Anniversary, 25 October 2006
Ice Extent
• Both active and passive are useful for detecting the ice edge.
• However, passive microwave is affected by weather effects. Thus the reason for selecting thresholds for the ice edge
• Scatterometer ice edge does not require tuning but it doesn’t infer ice concentration. V/H of SeaWinds does show sensitivity to ice
concentration and ice type (at medium-to-high concentrations)
NSIDC’s 30th Anniversary, 25 October 2006
Ice Extent Differences
QuikSCAT Ice Extent SSM/I Ice Concentration
1 January 2001
NSIDC’s 30th Anniversary, 25 October 2006
Ice Extent Differences
Seasonal difference between QuikSCAT and SSM/I ice extents – match better during austral winter
Antarctic Ice Extent (2001)
0.00E+00
2.00E+06
4.00E+06
6.00E+06
8.00E+06
1.00E+07
1.20E+07
1.40E+07
1.60E+07
1.80E+07
2.00E+07
0 50 100 150 200 250 300 350 400
Day of Year
Ext
ent
(sq
-km
)
QuikSCAT
SSM/I
In general, QuikSCAT shows more Antarctic sea ice (~600,000 sq-km) during austral summer and 3,000 sq-km more during austral winter.
NSIDC’s 30th Anniversary, 25 October 2006
Ice Extent Blending Ideas• Fusion of scatterometer and radiometer should
enable improved characterization of the ice edge, ice concentration and type.
• Preliminary results by D. Long suggest scatterometry has improved accuracy during ice-edge advance from thermodynamic growth.
• Scatterometer data can help define the ice edge when weather effects the passive microwave signal (make the ice edge less patchy).
• Scatterometer can detect melting well and therefore flag the use of different concentration tables for melting ice.
NSIDC’s 30th Anniversary, 25 October 2006
Ice Motion
• Work by Zhao et al. (2002) and Liu et al (1999) found that regions for which sea ice motion derived from passive microwave was erratic or impossible, ice motion could be determined with scatterometer data (and vice versa).
• Scatterometry doesn’t experience the problems caused by water vapor in SSM/I 85 GHz channel. Thus, scatterometry data can be used to fill in poorly
tracked regions.
NSIDC’s 30th Anniversary, 25 October 2006
Blended SSM/I and QuikSCAT Ice Motion
• Merged Arctic sea ice drift map Provides more
complete coverage than from a single data source
• RMS difference of satellite results from buoy speed is less than 3 cm/s
http://www.scp.byu.edu/data/Quikscat/IceMo/Quikscat_icemotion.html
NSIDC’s 30th Anniversary, 25 October 2006
Summary
• Fusion of active and passive microwave observations shows great promise in polar climate and change studies.
• This is a relatively new research area, with much work still to be done.
• The availability of enhanced resolution passive and active microwave observations will greatly facilitate research.
• Future blended products will include sea ice extent, ice type, melt onset/freeze-up and ice motion. Data will be made available at NSIDC