Instrument Limitations and Uncertainties

Paul Lawson, Dave Rogers, Andy Heymsfield, Darrel Baumgardner,Olaf Stetzer, Greg McFarquhar, Linnea Avallone, Markus Petters

Workshop on In Situ, Airborne Instrumentation:Addressing and Solving Measurement Problems in ICE Clouds 

Seaside, Oregon, USA, 25‐27 June 2007

Out of focus

Depth of Field

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In focus

Center of Focus

Single Particle Scattering Probes(FSSP, CAS, MASP, CDP, PDPA, SID)

Considerations1. Forward Scattering Probes are Theoretically Designed and Calibrated for

Homogeneous Spherical Particles

2. Measurements Fall into Regime with (Oscillating) Mie Scattering Signal

3. Small DOF and Sample Volume Limits Measurements (due to samplingstatistics) to ~ 100 m in Natural Clouds.

4. Forward Scattering Probes Cannot Discriminate Between Non-Spherical (ice) and Spherical (water) Particles in Mixed-Phase Clouds.

5. PDPA Provides Excellent Discrimination Between Spherical and Non-Spherical Particles, but does not (currently) Measure Non-spherical Particles.

6. CAS and MASP have Demonstrated some Ability to Discriminate Between Non-Spherical and Spherical Particles in Mixed-Phase Clouds.

7. SID Discriminates Between Spherical and Non-Spherical Particles and Provides Some Particle Shape Information.

8. Shattering of Ice Crystals on Probe Inlets Affects all Probes

9. Airflow and Icing can Potentially Affect Measurements

Theoretical Response to Mie Scattering

Mie Scattering Theory Assumes Spherical Particles with a Known Homogeneous Refractive Index.

Ice Particles are (typically) not Spherical and are Inhomogeneous (i.e., a mixture of ice and air), so their Bulk Refractive Index is not Constant.

Highly Accurate Ice Particle Sizing is Impossible

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How Accurately Can Scattering Probes Size Ice Particles?

Comparison with a 10-m Imaging Probe (2D-S) Suggest that “Ballpark” Agreement May be Possible if “Shattering” is not an Issue

Answer: Unknown

Recommendations

Laboratory Calibrations of the Sizing Performance of Forward Scattering Probes using Analogs (e.g., pollen, calcite, etc.), and Icing Tunnel/Cloud Chamber Experiments.

Analysis and Evaluation of the Ability of the SID (and 3V-CPI – Described Later) to Accurately Discriminate Spherical from Non-Spherical Particles and Measure Size Distributions of Partricles < 100 m.

How Accurately can Scattering Probes Count Ice Particles?Considerations

1. Small sample volume of FSSP, CDP, MASP, CAS and SID compromises sampling statistics in some ice clouds.

How Accurately can Scattering Probes Count Ice Particles?Considerations (Continued)

2. Small ice (< 50 m) is generally solid and not too branched, so one would expect one count per particle.

3. Relatively low (< 10 cc) particle concentrations in ice cloud makes coincidence errors negligible.

4. Historically, FSSP measurements have been contaminated with shattering, so assessment of accuracy is not well quantified.

5. Laboratory tests (Nagel and Maixner) with a large (hundreds of microns diameter) spinning wire produced counts in bins 9 – 12, so spurious counts from large (> 50 m) ice could also be possible.

6. PMS FSSP instruments had inherent 6 s deadtime, which reduced the effects of shattering.

7. DMT electronics upgrade (SPP-100) removed deadtime so that shattered particles were more likely to be counted.

8. Fast FSSP, Fast CDP and SID measure particle arrival times and can reduce effects of shattering.

How Accurately can Scattering Probes Count Ice Particles?Considerations (Continued)

9. FSSP with sample tube removed and CDP reduce, but do not eliminate effects of shattering.

How Accurately can Scattering Probes Count Ice Particles?Considerations (Continued)

10. SID (and 3V-CPI – described later) provides most comprehensive picture of scattered light, and may be capable of both discrimating and counting spherical and non-spherical particles.

Summary Limitations and Uncertainties of Single Particle Scattering Probes

1. No Quantitative Assessment of Ice Particle Sizing. Qualitative Assessment Suggests “Ballpark” Agreement with 2D-S.

2. Quantitative Assessment of Ice Particle Counting Awaits Proper Laboratory, Cloud Chamber, Icing Tunnel Evaluations, and Possible Comparisons with SID and 3V-CPI.

3. Small DOF and Sample Volume Limits Measurements to ~ 50 m in Natural Clouds

4. Small Effect from Coincidence Errors in Clouds with < 10 cm-3.5. Pure Forward Scattering Probes (FSSP, CDP) Cannot Discriminate

Between Spherical and Non-Spherical Particles. Forward and Backscatter Probe (CAS) may have some Capability. PDPA and SID (and perhaps MASP) can Discriminate, but Particle Counting Performance still not well Quantified.

6. Shattering of Ice Crystals on Probe Inlets and Tips Needs to be Handled.

7. Airflow and Icing can Affect Instrument Response. CFD and IcingModels can help, but cannot Predict Particle Flow in Complex Geometries.

Optical Imaging Probes(2D-C, 2D-P, CIP, PIP, CPI, 2D-S, HVPS, 3V-CPI, VIPS, Cloud Scope, HOLODEC)

Considerations1. Time Response of Linear Optical Array Probes Limits Performance of

some Instruments to Particles < ~ 100 m. 2. Out-of-Focus (Diffracted) Images are Improperly Sized by Imaging Probes

and must be Corrected using Software. 3. Discrete Pixel Size Must be Considered in Counting and Sizing Particles. 4. Particle Shape and Habit Identificaton Requires High-Quality Images (e.g.

CPI)5. Particle Concentration Measurements are Problematical for Triggered

Instruments (e.g., CPI)6. Collection Efficiency and Shattering using Impactor Instruments (e.g.,

VIPS).7. Sampling Statistics may be poor for Large Particles.8. Shattering of Ice Crystals on Probe Tips and Inlets9. Airflow and Icing can Potentially Affect Measurements

Time Response of Linear OAP‘s

2D-C and 2D-S on NCAR C-130 @103 m s-1 During RICO (Lawson et al. 2006)Drizzle with Max D=120 m

Drizzle with Max D=150 m

Errors Due to Counting and Re-sizing Out-of-Focus Images: DOF = ± c r2 / (Korolev 2007)

Depth of Field (DOF), and therefore sample volume, are a function of r2. Corrections for sample volume of small (< ~100 m) particles is not precise due to variability in instruments, instrument instability, laser inhomogeneity, particle geometry, time response, pixel discretation, etc.

Uncertainties in Identifying Particle Phase and Habit Phase and habit identification of particles < ~ 100 m requires high-resolution

images with multiple gray levels. CPI, VIPS and HOLODEC are possible candidates.

The 3V-CPI Combines the 2D-S and CPI, Providing 3 Views of the Same Particle. The 2D-S, which also Operates Independently, Doubles as the Particle Detection System for the 450 frame s-1 CPI Camera

3V-CPI

Uncertainties in Identifying Particle Phase and Habit (Continued)Discussion

1. The CPI provides excellent shape recognition and ice particle habit recognition. It is generally possible to distinguish ice from water drops that are larger than about 30 m. However, the CPI does not provide good quantitative size distributions, due to variance in triggering senstivity (not uncertainties in DOF and sample volume, as has been reported in the literature).

2. The 3V-CPI is a new instrument that combines the CPI and 2D-S. It uses the 2D-S electro-optics to trigger the CPI and provides good quantitative size distributions. The performance of the 3V-CPI is just now starting to be evaluated.

3. The HOLODEC reconstructs out-of-focus (diffracted) images using digital inline holography. Reconstructed images are not nearly as well defined as in-focus images. Basic ice shapes are identifiable for larger crystals, but discrimination cloud drops from small ice is unlikely.

4. The VIPS and Cloud Scope are impactor devices. The VIPS provides some shape information. Larger (> 300 m) ice crystals shatter upon impact. There are also problems with artifact pieces from shattering and in the oil used on the VIPS tape. Collection efficiencies are still somewhat of a controversial issue. Processing of Cloud Scope data is extremely time consuming.

Uncertainties in Sampling Precipitation Particles2D-P: 200 m pixels 32 photodiodes. Max viewing area: 6.4 mmPIP: 100 m pixels 64 photodiodes. Max viewing area: 6.4 mmHVPS: 150 m pixels 128 photodiodes. Max viewing area: 19.2 mm

Sample volumes of precipiation imaging probes (2D-P, PIP, HVPS) are rarely large enough to provide adequate sampling statistics for the largest particles.

Images of particles < ~ 300 m that are outside the DOF are over sized by the 2D-P and PIP

Uncertainties Resulting from Ice Particle Shattering

History

Cooper (1978): Developed particle inter-arrival time algorithm to remove shatterers. Code has since been used in several 2D-C applications. However, inability of 2D-C to image particles < ~ 100 m casts doubts on usefullness of the 2D-C application.Field et al. (2003): Revived interest in inter-arrival time algorithm as applied to fast FSSP data. Showed that previous FSSP measurements of small ice may have been misleading.Korolev high-speed video: Showed visual evidence that shattering produces hundreds to thousands of small ice fragments, some percentage of which will reach the sample volumes of scattering and imaging probes.Korolev new probe tip design based on icing tunnel and AIIE campaign: New probe tip design and elimination of FSSP inlet tube will reduce, but not eliminate effects of shattering.Jensen et al (2009), Baker et al. (2009), Lawson et al. (2010a,b,c), : Data from RICO, TC4, ISDAC and SPartICus field projects show that new (Korolev) 2D-S probe tips reduce the amount of shattered particles, but not nearly as effectively as the inter-arrival time algorithm.

Shattering Removal Using Particle Arrival TimesShattering Removal Using Particle Arrival Times

2D-S Images of Dendrites Below Cloud Base (ISDAC)

2D-S Images of Dendrites in Mixed-Phase Just Above Cloud Base (ISDAC)

Shattering on the CAS during TC4

Lack of Shattering on the Fast FSSP and 2D-S during SPartICus

Shattering Analyzed Using Standard and Modified 2D-S Probe Tips in AIIEModified Tips Prevent Some Shattering, but Arrival Time Algorithm

Removes Far More (Shattered) ParticlesShattering on 2D-S Appears to Behave Differently than 2D-C and CIP (as Reported by Korolev in AIIE)

No Inter-arrival Time Algorithm

No Inter-arrival Time Algorithm With Inter-arrival Time Algorithm

With Inter-arrival Time Algorithm

100 L-1

20 L-1

2D-S and Fast FSSP Measurements of Average Particle Concentration (with Shattered Particles Removed) in

SPartICus Cirrus are less than some Previous Measurements(From Jensen 2010)

Combined FSSP, CPI and 2DCombined FSSP, CPI and 2D--C Size C Size Distributions (without Shattered Distributions (without Shattered Particles Removed) in MidParticles Removed) in Mid--Latitude Latitude Cirrus (Lawson et al. 2006)Cirrus (Lawson et al. 2006)

2D2D--S Size Distributions (with S Size Distributions (with Shattered Particles Removed) in Shattered Particles Removed) in MidMid--Latitude Cirrus (Latitude Cirrus (SPartIcusSPartIcus))

846 L-1

968 L-1

2170 L-1

2671 L-1

2576 L-1

366 L-1

44 L-1

Summary and Recommendations Limitations and Uncertainties of Optical Imaging Probes

1. Time response of (linear array) optical imaging probes has not been adequately documented (except for the 2D-S). A comprehensive laboratory evaluation of time response using pulsed LED’s and high-speed (200 m s-1) spinning disks is needed. Laboratory evaluation of DOF of optical imaging probes using drop and bead generators.

2. Comprehensive airborne statistical evaluations of the shatteringproperties of all cloud particle probes are needed. This should include probes with and without modified tips, application of arrival time algorithms, high-speed (jet) aircraft, investigations of regions with onlycloud drops, all small ice, only large ice and different different ice particle habits. Simultaneous comparison with bulk IWC devices,including heated vessels and CVI instruments, is recommended. This would be and expanded version of the AIIE field program conducted in 2009 with the NRC Convair.

Bulk IWC Probes(Nevzorov, DRI, SEA, CVI, CSI, CLH, HT, EC)

Basically there are two principles of operation:

• Collection of water drops and ice particles in a small heated vessel (e.g., cone, open cylinder) and measurement of power needed to compensate for cooling due to convective dry air flow and melting/evaporation of water drops and ice particles. Total water content is directly related to the power required to maintain the heated vessel at a constant temperature (i..e, electrical resistance) minus the power required to compensate for the dry convective air flow (Nevzorov, SEA Multicylinder, DRI Cylinder).

• Confinement of ice and water particles through an inlet (usually a counter-flow virtual impactor), followed by heating of the sample air producing melting and evaporation of water drops/ice particles. Total condensed ice and water content is computed by measuring the resulting water vapor concentration and subracting the water vapor concentration of the ambient air (Twohy CVI, DMT Cloud Spectrometer and Impactor (CSI), University of Colorado closed-path laser hygrometer (CLH), Harvard University Lyman-a total water photofragment-fluorescence hygrometer (HT), Environment Canada Iso-kinetic TWC (EC)).

Bulk IWC MeasurementsLimitations and Uncertainties

Heated Vessel1. Heat transfer can be accurately calulated theoretically for flow around an

infinite cylinder, but not for finite “hollow” cylinders and cones, which leads to errors in the dry air term that are mainly a function of airspeed and position.

2. Small drops and ice crystals can bounce out of the vessels. Large drops and ice crystals splash or shatter and are not completely vaporized.

3. Calibration and/or removal of the baseline clear air term, either using autozeroing in electronics, or manual zeroing, produces uncertainties that increase with increasing airspeed, and are on the order of 5 to 50 mg m-3.

4. The lantent heat of fusion, which cannot be accounted for in measurements of mixed-phase clouds, can produce up to about a 15% error.

5. There is no rigorous way to calibrate heated vessel technology for measurement of TWC or IWC.

Bulk IWC MeasurementsLimitations and Uncertainties (Continued)

CVI Technique1. An inherent limitation of the CVI evaporation/water vapor measurement

technique is a saturation level that is a function of flow rate, which alsoimpacts cut size (EC is developing a device with a 10 g m-3 capability. Incomplete evaporation of large ice particles and hysteresis upon cloud exit due to water vapor residue may result in errors in IWC measurements.

2. Heymsfield et al. (2006)is estimate CSI uncertainty (1) at about 11% at water contents of 50 to 1000 mg m-3, increasing to 15% at 5 mg m-3, and to 23% at 2.5 mg m-3 and smaller The components contributing to the CSI uncertainty are the water vapor measurement (5%), enhancement factor uncertainty (10%), and a clear-air baseline offset.

3. Weinstock et al. (2006) estimate HT IWC uncertainty (1) with contributions from particle sampling (15%), total water and water vapor measurements (5% each), hysteresis (5%), and ice crystal evaporation (5%), which yields a 1uncertainty of 17% in TWC measurement.

4. Davis et al., (2007) estimate CLH IWC 1 uncertainty at approximately 11% at values above 5 mg m-3 and increases to about 50% at values at and below 1 mg m-3. Components are 5% water vapor measurement, 5% ambient water vapor, 10% enhancement factor.

5. Scatter plot comparisons of CSI, CLH and HT during MidCiX showed very good agreement for 50 < IWC < 200 mg m-3 (Davis et al. 2007)

Bulk IWC MeasurementsLimitations and Uncertainties (Continued)

CVI Technique5. Scatter plot comparisons of CSI, CLH and HT during MidCiX

showed very good agreement for about 50 < IWC < 200 mg m-3

(Davis et al. 2007)

Overall SummarySize Distributions of ice particles smaller than 10 m can currently only be measured with scattering probes, which suffer errors (mainly) from unknown scattering properties and shattering. SID is the best technological solution, but needs jto be quantified.Size Distributions of ice particles from 10 m to 50 m can be measured with the 2D-S and possibly the VIPS. 2D-S DOF and sizing corrections for particles from 10 to 30 m need more lab calibration. Shattering on impact and collection efficiency is a concern with the VIPS.Size Distributions of ice particles > 50 m to a few mm can be measured with the 2D-S, and possibly the CIP and fast 2D-C, but lab evaluations of the CIP and fast 2D-C have not been done. Habit Information is best provided by CPI. VIPS and HOLODEC also provide some information.Bulk IWC measurements appear to provide good results for 50 < IWC < 200 mg m-3, which covers cirrus, but not SVC, anvils or frontal clouds. New instruments are being developed for higher IWC. 2D-S and Twohy CVI currently provide the best agreement for IWC > 0.2 g m-3.Laboratory Calibrations of 2D-S and other optical imaging probes will take place this summer using GKSS equipment at SPEC.Shattering needs to be investigated systematically with carefully planned airborne programs, building on the Canadian AIIE (Korolev) field program.

(Paul‘s) Summary Statement

We are still a long way from being able to confidently measure particle size distributions and IWC in all types of clouds. New in situ cloud particle instrumentation

can improve these measurements, but only if the instruments themselves are thoroughly evaluated, first in the laboratory, and then carefully evaluated in each

field application.

Unfortunately, the lab studies have not been done and there is not a great deal of incentive to support careful

analysis of results from field campaigns.