Climate Quality Observations from Satellite Lidar

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28 April 06

• Climate Quality Observations from Satellite Lidar
• Dave Winker, NASA LaRC, Hampton, VA

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Initial Thoughts

• Propose that lidar can provide climate-quality
  measurements with extremely good stability and
  sufficient accuracy
• Two ways to look at lidar and climate data
  records (CDRs)
• CDRs can be constructed from lidar (cloud,
  aerosol) measurements
• Lidar measurements can be used to intercompare
  with CDRs derived from other instruments/systems
  or assess underlying assumptions of retrievals
• Lidar allows direct measurements of some
  parameters
• Pre-launch characterization, but not calibration
  per se
• self-calibrating in a sense avoids traditional
  calibration concerns

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Primary Space Lidars for Earth, So Far
Focus is on cloud and aerosol measurements Wont
discuss DIAL, Doppler, etc.
LITE (STS-64) 1994 GLAS (ICESat) 2002-? CALIOP (CALIPSO) 2006-9
1064/532/355 nm 1064 nm altimetry 532 nm profiling 1064/532 nm 532 par and perp
57o inclination 94o inclination 180-day precession 98o inclination Sun-synchronous
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Climate Dataset Requirements Clouds

• To directly observe climate change, stable and
  accurate cloud measurements are necessary
• The2002 Satellite Calibration Workshop defined
  cloud measurement requirements for climate
  datasets (NISTIR 7047 Ohring, et al., BAMS,
  2005)

Accuracy Stability
Cloud cover 1 0.3
Cloud height 150 m 30 m
Optical depth 10 2
Ice/water phase N/A N/A
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The response of low-cloud cover to CO2 doubling
For DT 0.2 K per decade, D(cloud cover) 0.2
- 0.4/decade
One IPCC climate model
Change in low cloud amount w/ 2X CO2
Another IPCC climate model
The model-to-model spread is a measure of
uncertainty and it is thought to be largely
governed by uncertain climate feedbacks that
all involve cloud processes.
CMIP year 70 mean of a 1 per year increase in
CO2 minus the control (fixed CO2)
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• 705 km, sun-synchronous orbit
• Three co-aligned instruments
• CALIOP polarization lidar
• 532 nm and , 1064 nm
• 2 x 110 mJ _at_ 20 Hz
• 1-meter telescope
• 0 40 km altitude, 30 - 60 m
• IIR Imaging IR radiometer
• WFC Wide-Field Camera

CALIPSO
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CALIPSO Payload
1-meter receiver telescope
CALIOP
Lidar Transmitter/ boresight system
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Cloud Detection/Height
Cloud and aerosol layers identified by contrast
with molecular background Sea surface establishes
reference for height measurements
Adaptive threshold
btotal (measured)
bair (from model)
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Typical Scene
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CALIOP Layer Detection Sensitivity
Clouds with t 0.01 could be climactically
significant, should be monitored
(assuming 300 m layer and Sc 20)
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Lidar vs. 95 GHz Radar for Cloud Detection

• Lidar is sensitive to the thinnest clouds,
  easily detects water clouds
• Cloud-profiling radar (CPR) is insensitive to
  small droplets and low concentrations of ice
  crystals

Radar-lidar composite from CRYSTAL-FACE (26
July) blue lidar-only, yellow
radar-only, green both
Lidar CPR best approach for cloud base
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2-l Improves Cloud-Aerosol Discrimination
Separation of cloud and aerosol using c
b1064/b532

• To a large degree, cloud and aerosol can be
  separated by scattering strength, but there are
  two problem areas
• There is a region of overlap in scatter
  strength where 2-l measurements are necessary.
• Attenuation by upper layers will cause lower
  cloud layers to be classified as aerosol.

c
Integrated scatter
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Detection of Multiple-layers

• Vertical distribution of highest cloud-top
  vs. all detected cloud tops (Tropics)

LITE 45 of cloudy returns reached surface
75 reach top of boundary layer (2 km) 80
mean cloud cover
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Accuracy is driven by spatial sampling

• Example cloud cover
• Nadir-pointing lidar can achieve required
  accuracies at seasonal-zonal scales
• Cross-track measurements would enable monitoring
  at regional scales

Fowler et al., 2000 4o x 5o, monthly
RMS uncertainty in cloud cover for nadir-only
observations
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Cloud Ice/Water Phase

• The threshold temperature dividing mixed-phase
  and ice clouds is not well known
• Ice/water partitioning is an important modulator
  of the climate sensitivity in climate models
• Direct, vertically resolved observations of
  ice/water phase are needed to address this issue

(Fowler and Randall, 1996 J. Clim. 9, 561)
Back- scatter Depol
Lidar can directly sense particle sphericity -
Backscatter from liquid droplets retains the
incident polarization - lidar depolarization
P /P - Backscatter from ice crystals is
depolarized (typically 20-40)
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Cloud optical depth

• Radiative forcing is most sensitive to changes in
  thin clouds (t lt 1), and clouds with optical
  depths as small as 0.01 need to be monitored
• Lidar provides direct measurement of optical
  depth of thin cirrus
• Optical depth from layer transmittance (for t lt
  3)
• Corrections for multiple scattering may be
  required

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HSRL Direct measurement of AOD
Climate requirement for aerosol optical depth
(AOD) - Accuracy 0.01 ( 7) - Stability
0.005 ( 3) HSRL has molecular and Mie
channels, providing direct measurement of aerosol
attenuation
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Instrument Requirements for Clouds

• The required measurements can be obtained from
  current (CALIPSO), planned (EarthCare) satellite
  lidars and suitably-designed follow-ons
• Vertical resolution 30-150 meters
• Two-wavelengths (for cloud/aerosol
  discrimination)
• Depolarization (for ice/water phase)
• Exact wavelengths and instrument sensitivity need
  not be the same

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Current and Under Development
CALIOP (CALIPSO) ALADIN (ADM/Aeolus) ATLID (EarthCare)
1064 nm 532 nm 532 depolarization 355 nm HSRL X No depolarization X 1064 nm 355 nm HSRL depolarization
Dz 30/60 m Dz 1 km X Dz ?
w/ passive sensors ---- w/ passive sensors
98o inclination, 16-day Sun-synch, 130 PM 97o inclination, 7-day rpt Sun-synchronous Sun Sync polar orbit 1030 AM
2006-2009 2008 - ? 2012 - ?
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Mission Considerations

• Sun-synchronous polar orbit preferred
• Good latitude coverage
• Provides sampling at constant local time of day
• Precessing orbit
• Biases the diurnal cycle into the record
• Rapidly precessing orbits have low inclination
  angles limited global coverage
• Slowly precessing orbits
• Precession through 24 hours in 1 year preserves
  interannual variability but prevents monitoring
  changes on shorter timescales
• Prefer to fly with other instruments
• Provide acquisition of simultaneous, coincident
  CDRs
• Allow intercomparison/cross-calibration
• Due to inherently high stability of lidar
  measurements, overlap between instrument data
  records may not be required
• Needs to be tested

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Issues/Opportunities

• How to relate lidar cloud cover to passive cloud
  cover
• High lidar sensitivity gives higher cloud
  fraction than passive
• How to relate lidar cloud top to passive cloud
  top
• Lidar profiles cloud to optical depth of 3-5 (the
  portion which interacts radiatively in the
  thermal IR)
• Expand definition of cloud height to include
  multilayer cloud height
• Can we stitch together data records collected at
  different equator crossing times?
• Poor spatial sampling of nadir-viewing lidar
• Restricts climate-accuracy monitoring to large
  space-time scales
• Scanning or multi-beam lidar provides improved
  statistics

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Notional Multi-beam Concept

• Simple backscatter lidar in formation with
  NPOESS or other platform
• Accuracy improved by adding multi-beam lidar with
  cross-track coverage
• For cloud monitoring, want independent samples
• Widely spaced measurements are more efficient