Using TODWL and Optical Particle Counters
to Investigate Aerosol Backscatter Signatures
from Organized Structures in the Marine Boundary Layer
D.A. Bowdle
University of Alabama in Huntsville
G.D. Emmitt and S.A. Wood
Simpson Weather Associates
Working Group on Space-Based Lidar Winds
Frisco, Colorado, June 29 - July 1, 2004
CONTENTS
• EXPERIMENT
• ANALYSIS
• RESULTS
• SUMMARY
MOTIVATION
• Joint research project by ONR and NPOESS IPO
• Investigate data processing issues related to future
space-based wind lidar operations
• Develop calibration/validation procedures for all wind
profiling systems (ground-based, airborne, space-based)
• Conduct basic research on lower tropospheric winds and
aerosols in the marine and continental boundary layers
INSTRUMENTS
Aircraft Platform
Optical Particle Counters
PCASP
0.1-3.0 m
NPS CIRPAS Twin Otter
FSSP
2.5-51 m
CAPS
0.45-118 m
Naval Postgraduate School
Center for Interdisciplinary Remotely-piloted Aircraft Studies
TODWL Transceiver
2.0125 µm, coherent detection
4-6 mJ, 330 nsec (FWHM), 80 Hz
10 cm telescope
two axis scanner, 30 & 120 deg, side door mount
digitization rate 100 MHz
~7-10% total system efficiency
TODWL Scanner
OPERATIONS
Schedule
Location
• Series 1: February 9-15, 2002
Series 1:
• Monterey area & San Joaquin River
• Series 2: March 12-15, 2002
Series 2:
• Monterey area
• Monterey to Boulder via Las Vegas
•Series 3: February 8-21, 2003
Series 3:
• Monterey area, ocean & land
Flight Plans
MBL Database
• Straight and level 50-100 km runs
• 8 flights, approx 30 hours
• Along-wind and cross-wind runs
• multiple scanning patterns
• Multiple altitudes over same ground track
• concentrate on February 20, 2003
• Near surface, near and above inversion
MEASUREMENTS*
scattering
volume
bi
ground track,
heading,
ground velocity
(neglecting
yaw, sideslip)
beam
direction
particle
probes
Vac
Xj = | Vac | D tj
Ri,j = | Vac | ( Dtj + Dti)
Vi
*Backscatter-related scan patterns
• along-track RHI step-and-stare
• forward stare
• nadir stare
beam
direction
(neglecting pitch)
BACKSCATTER EQUATION* - 1
     X ,R,f  K X ,R  
 

2

 1    Da  
 EX

SNR X, R, f      2   
4  R  
 h   B X


2
tar
 tar
 

X, R
For a diffuse atmospheric target, with volume backscatter coefficient b,


c  b X, R
tar  tar X, R 
2
 
 
For calibrations against a hard target, with diffuse reflectance ,

 ht   ht
tar   tar X, R 
 
t ht
Combining the above equations gives a non-dimensionalized formulation:


 
   




R  SNR X, R, f
 EX BX



 
2

Rht  SNR X ht , Rht , fht
 E X ht B X ht
2


 

   
 




2

 X, R, f  K X, R
 
 c  t ht  b X, R 
 



 
2
2






ht
ht

 
 
 X ht , Rht , fht  K X ht , Rht

*generalized from ACLAIM backscatter analysis [Steve Hannon, 1999]
BACKSCATTER EQUATION - 2
When TODWL points straight forward, make the following assumptions:




ˆ
b X, R   b z    X, R ;








ˆ

X,R   exp 2   z R   exp  2  X, r dr 




ˆ
E X  E z ; B X  B z ;  X, R, f   opt z    het z, R, f    het X, R, f
b
0
R
K2
0


0
where ˆhet  1, ˆb  1, ˆ  0
Combine the terms that have no range dependence
Express the backscatter equation using non-dimensional variables

 exp 2   0 z   R  
baseline
ˆ
ˆ
S X, R, f  b 0 z   ˆhet z, R, f   

terms
 exp 2   ht 0  R ht  


 
R






ˆ
ˆ
ˆ

 b X, R   het X, R, f  exp  2     X, r  dr 

0



2

 het z, R, f 
R  SNR X, R, f
ˆhet z, R, f  

SNRˆ X, R, f  2
 het zht , Rht , fht 
Rht  SNR X ht , Rht , fht
perturbation
terms


 






 E z  Bz   
 opt z  
  c  t ht 
ˆ
b 0 z   


  b 0 z 






E
z
B
z

z
2




ht
ht  
ht
ht 

 opt ht 
 

ANALYTICAL APPROACH
For a pulsed coherent 2-m Doppler lidar, analysis of
ABSOLUTE BACKSCATTER VARIABILITY
• requires absolute backscatter calibration at range Rht;
• requires correction for nominal range response function;
• requires correction for atmospheric extinction;
• requires correction for atmospheric refractive turbulence;
• assumes system stability during a given data run;
RELATIVE BACKSCATTER VARIABILITY
• avoids all of the above requirements.
ANALYTICAL METHODS
Dropouts & Anomalies
• exclude wild velocities
(Normalized)
Turbulent Residuals
• for TODWL
• exclude backscatter dropouts
- compute mean V & b at each range
• exclude major pulse tail artifacts
- compute residual V & b at each pixel
• account for aircraft pitch
Filtering
• for OPC, compute mean, residual Nm
Correlation
• 1-s data – good V’ and b’ most ranges
• TODWL time-range plots (Hovmuller)
• 1-s data – poor OPC count statistics
• aerosol time-size plots
• filtered V’ & b’ may not resolve waves
• scale analysis
• filtered OPC improves count statistics
• variance analysis
RESULTS*
SAMPLING CONDITIONS
• sharp inversion ~450 m; winds below inversion NNW ~17 m/s;
RH ~70% at ~30 m, ~90% --> 45% across inversion, ~30% above inversion
• horizontal legs at ~35 m (x1), ~400 m (x1), ~900 m (x3), 1400 m (x1)
HOVMULLER PLOTS IN RADIAL VELOCITY AND SNR
• stratification by aircraft pitch eliminates unphysical “striping”, and markedly
reduces the observed variation along individual coherent features
• stratified plots still exhibit residual non-coherent variation along features
• radial velocity variation across scene up to 8 m/s; along features <1 m/s
• SNR variation across scene (fixed range) up to 6 dB, along features TBD
• promising results from preliminary attempts to correct for atmospheric
attenuation and lidar range response, even before pitch stratification
• velocity-backscatter correlations observed below, at, above inversion
OPTICAL PARTICLE COUNTERS
• large particles, with poor count statistics, often dominate 2-m backscatter
*Planned graphics unavailable due to severe case of Microsoft fever
CONCLUSIONS
ATMOSPHERIC FEATURES
• turbulent waves in aerosol and velocity, multiple scales
• aerosol-velocity correlations will bias DWL LEO winds, even in clear air
• nature & magnitude of bias will depend on shot integration strategy
ANALYTICAL CHALLENGES
• beam elevation offset, pitch fluctuations, altitude fluctuations
• measured vs. modeled absolute backscatter
• OPC operational status
• OPC count statistics
SCIENCE POTENTIAL
• substantial information content remains untapped in TODWL database
RECOMMENDATIONS - 1
INSTRUMENTATION AND OPERATIONS
 TODWL – modify programmed scans to account for pitch offset in mounting
 TODWL – add option for automatic dither in beam elevation
 TODWL – improve frequency, quality of ground-based radiometric calibrations
 OPC – verify PCASP, FSSP, CAPS operational status on every flight
 OPC - add flight-level sensor that has higher volume sampling rate
OPC ANALYSIS METHODS
 replace contiguous-point temporal smoothing by feature-composited averaging
 replace measured size distributions from individual OPC’s by aerosol-modelconstrained composites from FSSP, PCASP, CAPS forward, CAPS backward
 augment composited size distributions using Monte Carlo & Poisson statistics
RECOMMENDATIONS - 2
ANALYSIS POTENTIAL – MEAN CONDITIONS
• Backscatter: model using cabin data (OPC); derive from TODWL
• Attenuation: model using cabin data (OPC, T, RH); derive from TODWL
• Coherence Length: model using cabin data (V, T, RH); derive from TODWL
ANALYSIS POTENTIAL – TURBULENT CONDITIONS
• scale analysis: power spectrum, structure function, autocorrelation
• analysis of variance: composite wave, inter-wave, intra-wave, sensor, sampling
• aerosol microphysics: identify and quantify sources of aerosol variability
ACKNOWLEDGMENTS
• This work was funded by the Office of Naval Research through
the Center for Interdisciplinary Remotely-piloted Aircraft
Studies and by the Integrated Program Office of NPOESS
• SPAWAR and ONR 35/SBIR Program provided the lidar and
supported its integration into the CIRPAS Twin Otter
• IPO co-funded the lidar adaptation to the Twin Otter.
• IPO solely funded the mission planning, flight hours, data
collection, and the post-flight installation of the lidar in a trailer
for inter-flight research.