Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
A study of range resolution effects on accuracy and
precision of velocity estimates Applications of ship-based 2µm Doppler lidar
data
to space-based lidar performance Sara C. Tucker*, Alan Brewer, Mike
Hardesty, Scott Sandberg, Ann Weickmann*, Dan
LawOptical Remote Sensing Group, Chemical Sciences Division
(CSD)Earth System Research Laboratory, NOAA
*Also with: Cooperative Institute for Research in Environmental Science
University of Colorado, Boulder, CO, NOAA/ESRL/CSDWorking Group on Space-Based Lidar Winds, Snowmass, Colorado, July 17-21, 2007
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
HRDL Winds Characterizations of the Gulf of Mexico and Galveston Bay
• Aerosol measurements: used to determine the expected levels of return signal available in this region
– Closure in aerosol studies at 355 nm using ozone profiling lidar (OPAL), cavity ring-down, and in-situ instruments. Will attempt to scale the backscatter and extinction numbers to HRDL wavelength for comparison studies.
– Comparisons with CALIPSO and HSRL
• Winds and turbulence information: used to determine the potential performance, including errors, based on sample rate/volumes, etc, in space-based Doppler lidar measurements.
• Cloud coverage: used to determine the percentage of time a satellite can make measurements at each altitude in this area.
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
HRDL wind and aerosol products for understanding Marine
Boundary LayersComposite products
• Horizontal mean wind profiles
• Profiles of relative aerosol strength and aerosol layering
• Vertical winds and vertical mixing/turbulence statistics
• Horizontal (near surface) mixing/turbulence statistics
• Aerosol and mixed layer (i.e. Boundary layer) heights
• Wind speed and directional shear profiles
Individual Scan Products
• Boundary layer dynamic features: rolls, surface streaks, thunderstorm outflows, etc
• Ship/oil-platform plume detection
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Objective: To study the effect of variability in the small-scale wind fields, and mean wind shear, on expected performance.
• Reprocess NOAA’s High Resolution Doppler Lidar (HRDL) TexAQS 2006 measurements with 500m range gates and then,
• look at accuracy and precision of velocity estimates as compared to 30 m products.
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Accuracy and Precision
• Accuracy – how far off is the mean? Bias.• Precision – what is the standard deviation of the
measurements?• Averaging more ACFs or Spectra typically means better
precision – but may not mean the results are accurate.
High accuracy, low precision
Low accuracy, high precision
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
System and processing parameters
Typical processing• 10 lag ACF• 30 m range gate • =10 averaged
ACF/gate 1000 ACF/estimate
Reprocessing• 10 lag ACF• 501 m range gate • =167 averaged
ACF/gate 16,667 ACF/estimate
• 3 m sampling (10 ns)• 200 ns PW: 30 m• 100 pulse averaging• Scanning: 5 deg/sec
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
PPI Scan at 45° Elevation: With Shear
Mean of 30m data – 500m RG data
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
PPI Scan at 45° Elevation: With Shear
Closest 30m data – 500m RG data
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Resulting Wind Profiles
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
PPI Scan at 45° Elevation: With Shear
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Instrument and atmospheric variance profiles
Atmospheric vertical variance
Wideband SNR
Instrument precision
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
500 m Range Gate Study: Preliminary Results
• Average SNR usually about the same as 500 m range gates except in cases of strong turbulence and/or approaching “saturation.”
• Precision Improvements:
– 16.67 x more points (for a total of 16,667) should yield ~4X improvement in precision – for same SNR.
– We see ~2X improvement in precision. In other words, instrument “variance” drops by an average factor of ~4 instead of 16.67.
• Profile Wind speed “error”, Mean: -0.03 m/s, Std. Dev: 0.50 m/s.• Profile Wind direction “error”, Mean: 0.34°, Std. Dev: 4.24°.
• Next for the 500 m. range gate study
– Closer look at differences in atmospheric variance estimates – are we underestimating instrument variance?
– This study assumed full azimuth scanning at 45° - what happens if we only have 2 stare angles?
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
HRDL-TexAQS 2006: Relative 2µm Aerosol Backscatter
Major Saharan dust events
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
2µm backscatter
• True, HRDL was not calibrated for aerosol for TexAQS.
• HRDL avg. power constant throughout experiment (within 5% error on power-meter measurement).
• HRDL provided “relative” aerosol layer info during the experiment.
• In-situ measurements of particle size distribution, composition, absorption, extinction, etc. available
• Aerosol backscatter is affected by:– Humidity
– Composition
– Distributions/ Concentration
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Surface area size distributions and HRDL SNR
A B C D E
Image credit: D. Coffman, PMEL, NOAA
Integrated 2-10µm surface areaHRDL SNR at 215 m altitude
Correlation ~0.9
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Particle size distributions: concentrations and backscatter
Likely hard
target returns
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Backscatter dependence on RH and Particle Solubility
Strong dependence on RH
Some dependence on Solubility
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
2µm Backscatter: Caveats
• CNR fit depends on:– Refractive turbulence– Transmission/extinction (estimated in Mie models)
• Ship plume – strong refractive turbulence• Possible long-term system changes due to high vibration• Mie scattering models still “young”
– Particle refractive index is highly composition dependent): Incorporate variable mass fractions of Ammonium Sulfate, Sea Salt, and Dust
Lidar Working Group on Space-Based Winds, Snowmass, Colorado, July 17-21, 2007
Continuing work• Boundary Layer Heights – 600 m over Gulf
• Streak Analysis and integration of
HRDL data with models
• Comparisons of HRDL with in-situ-based calculations of Backscatter, then…
• Compare HSRL and CALIPSO and characterize the relationship between 1 and 2 micron backscatter in this area.
• Extend the process to other areas.