1. Probing clouds: why its necessary to use multiple instruments.
Richard Cotton and Steve Abel
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2. Probing clouds: why its necessary to use multiple instruments
Probing clouds: why its necessary to use multiple instruments. Or why we throw the toys out of the pram when one instrument fails.
Some information about cloud physics probes.
Constructing ‘composite’ Particle Size Distributions.
What about bulk water probes?
Examples of recent work that required a range of probes: ice particle density, fallspeed, raindrop size distribution.
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3. A wide range of instruments using different technologies are required to measure cloud particles.
Small ice crystals: forward scattered light (CDP,SID-2h,SID-3).
Habit classification: high resolution video (CPI).
Large snow aggregates: optical ‘shadow’ probes (2D-C,2D-S,CIP-100).
Special tips constructed to deflect fragments of snow that break up on impact of leading edges.
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4. Particle phase using SID-2h
Diameter?
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5. CIP-15 shadow-image probe operated with ‘grey-scale’.
Out of focus image.
Diameter?
Four pixel requirement.
Shattering
Reconstruct full image.
‘Partial’ image
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6. Effective sample area defined by depth of field and photodiode array width. Issue for counting statistics.
CIP-100 (100μm)
2D-S (10μm)
Large error
Low statistics
2D-C (25μm)
SID-2h
CDP
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7. Cloud particle imager (CPI) gives context, but low statistics.
Snow aggregate
Ice crystal
Super-cooled droplet?
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8. Composite particle size spectra using multiple cloud probes (CDP,SID2,2D-S,2D-C,CIP-100).
Small ice crystal mode
Ice to snow transition diameter, D_
Large snow aggregate mode
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9. Ice crystal-aggregate mass fraction.
The current operational version of the MetUM contains only one ice prognostic variable, the ice mass mixing ratio QICE and for each time-step this is split diagnostically between ice crystals and aggregates.
The fraction fAGG that is apportioned to the aggregate part of the PSD is,
fAGG=1-exp(-T0ΔT)QICE/Q0,
Model
Observations
The diagnostic split underestimates the ice aggregate mass observed during these cirrus cases. The ice to snow transition diameter has a mode around 100μm, much less than the often used 300m.
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10. Bulk water probes: Nevzorov.
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11. Total water probe and WVSS-2
Measures the total water (condensed plus vapour) at 64Hz using a Lyman-Alpha absorption hygrometer (0 to 20 g/kg, accuracy +/-0.15g/kg).
Calibrate TWC detector response using WVSS-2 (or GE).
Saturated
In cloud CWC=TWC-QV.
Liquid water
Mixed-phase cumulus cloud.
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12. Effective density of ice varies with particle size.
The ice effective density function, parametrised by the constant density ρ0 for small ice less than Dρ0 diameter and the power b in the mass-dimension relation m(D)=aDb greater than Dρ0.
Prefactor in m=aDb is a=π/6 (Dρ0)3-b ρ0.
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13. Every 10-second particle size spectra, over five cirrus flights.
The ice particle effective density, or mass-diameter relation, is then determined by comparing the directly measured bulk ice water content with that obtained by integration of the composite PSDs.
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14. Is there any bias in the numerical minimisation which used QTWC-QPSD?
Temperature, IWC, Dmm
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15. Rain and cloud droplets
Drizzle in stratocumulus
Drizzle drops
Cloud drops
3B (UK4/UKV)
3C/D (global/climate)
Thompson (WRF)
This study
Heavy frontal rain
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16. Monthly mean surface rain-rate.
Global/NAE parameterization
Observations
New parameterization
UK4/UKV parameterization
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17. Conclusion
Cloud particle ‘diameter’, concentration, shape and phase requires multiple probes to cover size-range with sufficient statistics (using both light scattering and shadowing).
Measurement issues such as definition of size, particle shattering, out of focus images and counting statistics must be considered.
Bulk water content (g /m3) using Nevzorov TWC probe over g /m3 with accuracy ± 10%, using baseline correction.
Bulk water content from Total Water Probe requires hygrogemeter (WVSS-2) for ADC to g /m3, but ± ?%.
The agreement of ice water content from a bulk probe and from integrating the particle size distribution (assuming an ice density function) gives confidence in measurements.
Raindrop PSD parametrisation improves representation of drizzle in MetUm. Soon operational.
Ice particle density being tested in parallel suite.
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