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Cloud Extinction Probe: calibrations and results of measurement
Alexei Korolev, Alexander Shashkov and Howard BarkerEnvironment Canada
ASR Science Team Meeting, San Antonio, TX, 28-31 March 2011
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Cloud Extinction Probe
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(1) LED λ=0,635μm (2) diffuser (3) condenser (4) pinhole(5) objective(6) cone cube retroreflector
Schematic diagram of the optical unit of the Cloud Extinction Probe
(7) beamsplitter(8) photodetector(9) optical chopper (10) optocouple(11) filter(12) front heated glass
airflow
0
ln21
II
LCEP −=β
Beer’s law
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Specifications of Cloud Extinction Probe
Range: 0.2km-1 <β < 200km-1
Sample area: 60cm2
Rate of sampling: 1.5 m3/sReceiving aperture: 0.6o
Optical base: 2.5 m x 2Data rate: 10HzInsensitive to shatteringNon-coherent illuminationAll-weather operation: -60C<T<+40C, 100mb<P<1000mb
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Calibrations
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The effect of forward scattering on the extinction coefficient measurements
Small particle
Large particle
1
0.5
0ββmeas
D
α
α
Gumprecht and Sliepevich, 1953Deepak and Box, 1978
βmeas≤ β0
The measured extinction coefficient is no higher than the actual extinction coefficient
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• There are no techniques for the calibration of transmissometers and extinctiometers
• Absence of calibrating standards, e.g. monodisperse particle clouds with predetermined concentration
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00
0
SQS
III
=−
I0 I’
I’I
TargetRetro-
reflector
00
00
SQS
IIII
=−
Extinction Probe
single pass double pass
0
0
1)(
SS
II
DQ−
==>
Experimental Schema
Q≈2 extinction efficiencytheoretical value
Instrumental extinction efficiency
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Fixed frequency grid targets
Proposed calibrating technique
DD
2DS0
S
160
π=
SS
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Fixed frequency grid targets (custom made)
2000μm 500μm
62μm125μm
1000μm
250μm
160
π=
SS
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D=30μm
D=250μm
D=125μm
D=1000μm
Fixed frequency grid targets160
π=
SS
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120mm
Glass bead targetsD=1000μm
D=230μm
D=480μm
D=60μm
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microscope
target
retroreflector
illuminator
CEP
high resolutionCCD camera3D-positioningstage
Experimental setup
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Results of calibrations
geometrical optics limit
diffraction limitglass beads
dot grids
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Performance and
results of measurements
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Liquid clouds
∑=
=15
1
2
4 jjjFSSP DnQπβ calculation of the extinction coefficient from FSSP measurements
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Liquid clouds
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= ∑j
jinstr
meas ALAQ
0
β
Qinstr=2 Qcalibr≈ 2
Extinction coefficient measurements in liquid clouds during ISDAC
ISDAC, 26 April 2008
Qinstr=Qcalibr
Qcalibr≈ 2
Extinction coeff. in liquid clouds measured by CEP and particle probes is in agreement with laboratory calibrations.
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Techniques for calculations of the extinction coefficient from 2D imagery
baLA =
∑=j
bjjLnQaβ
∑ ∑=i j
bijijiiLnaQβ
Size-to-area conversion technique (conventional)
L-A parameterization
0.05<a<0.631.4<b<2
multiple particle habits
single habit particles
Range of changes of a and b for different ice particle size ranges
and habits
L particle sizeA particle areaQ≈2 extinction efficiency
Examples of variety of different ice habits
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Techniques for calculations of the extinction coefficient from 2D imagery
Δ
= ∑j
jOAP AtLWU
Qβ
= ∑j
jOAP ALAQ
0
β
L length of the sample areaW width of the sample areaU air speedQ≈2 extinction efficiency
Shadow-Area Technique (Korolev, 2008)
ASSUMPTIONS:1. Ice particles with D<100μm
have low contribution to the extinction coeff.
2. The measured 2D images preserve the aspect ratio of the particle shadowgraphs
ADVANTAGES:1. Free of errors related to partial images2. Does not require multiple 2D probes to
cover entire particle size range
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Ice clouds
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14 April 2008; UTC 20:22 – 22:00
ISDAC, 14 April 2008
Extinction coefficient measurements in ice clouds during ISDAC
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= ∑j
jinstr
D ALAQ
02β
Qinstr=2 Qcalibr=1
Extinction coefficient measurements in ice clouds during ISDAC
ISDAC, 14 April 2008
Qinstr≠ Qcalibr
Qcalibr=1
Extinction coeff. in liquid clouds measured by CEP and particle probes is in agreement with each other, but contradict laboratory calibrations.
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Conclusions1. Extinction coefficient measured by CEP in liquid clouds agrees well with that derived from particle probes in assumption that Q=2. This result is in agreement with the laboratory calibrations.
2. Extinction coefficient measured by CEP in ice clouds agrees well with that derived from 2D probes in assumption that Q=2. This result contradicts the laboratory calibrations.
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Hypothesis #1
Issues with particle size distribution measurements: e.g. shattering, oversizing, etc.
• Antishattering tips were used during ISDAC. Shattering cannot explain factor 2 difference.
• To explain factor 2 error in particle area, the sizing error should be factor 1.4.
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Hypothesis #2
Particle orientation. Extinction coefficient measured in horizontal and vertical directions are different in ice clouds.
CEP measures extinction coeff.in ~horizontal direction
2D probes measure extinction coeff.in vertical direction
Particle projection viewed by CEP Particle projection viewed by 2D probes
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?
Hypothesis #3
Ice particles attenuate light like an ensemble of small particles, rather than one big opaque screen.
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Conclusive remarks :
1.Do we understand ice particle measurements?
2.Do we understand how ice particles scatter light?
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Acknowledgements:ACRF DOE proposal: “Parameterization of Extinction Coefficient in Ice and Mixed-Phase Arctic clouds during the ISDAC Project“proposal # “09-5755“