Probing clouds: why its necessary to use multiple instruments. Richard Cotton and Steve Abel © Crown copyright Met Office

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. © Crown copyright Met Office

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. © Crown copyright Met Office

Particle phase using SID-2h Diameter? © Crown copyright Met Office

CIP-15 shadow-image probe operated with ‘grey-scale’. Out of focus image. Diameter? Four pixel requirement. Shattering Reconstruct full image. ‘Partial’ image © Crown copyright Met Office

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 © Crown copyright Met Office

Cloud particle imager (CPI) gives context, but low statistics. Snow aggregate Ice crystal Super-cooled droplet? © Crown copyright Met Office

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 © Crown copyright Met Office

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. © Crown copyright Met Office

Bulk water probes: Nevzorov. © Crown copyright Met Office

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. © Crown copyright Met Office

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. © Crown copyright Met Office

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. © Crown copyright Met Office

Is there any bias in the numerical minimisation which used QTWC-QPSD? Temperature, IWC, Dmm © Crown copyright Met Office

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 © Crown copyright Met Office

Monthly mean surface rain-rate. Global/NAE parameterization Observations New parameterization UK4/UKV parameterization © Crown copyright Met Office

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 0.001-1.0 g /m3 with accuracy ± 10%, using baseline correction. Bulk water content from Total Water Probe requires hygrogemeter (WVSS-2) for ADC to 0.1-10 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. © Crown copyright Met Office