Richard Greenaway

Ice Particle Interarrival Times Measured with a Fast FSSP

Abstract Ice particle interarrival times have been measured with a fast forward scattering spectrometer probe (FSSP). The distribution of interarrival times is bimodal instead of the exponential distribution expected for a Poisson process. The interarrival time modes are located at ∼10−2 and ∼10−4 s. This equates to horizontal spacings on both the centimeter and meter scales. The characteristics of the interarrival times are well modeled by a Markov chain process that couples together two independent Poisson processes operating at different scales. The possibility that ice crystals shattering on the probe tip causes the bimodal interarrival times is explored and cannot be ruled out. If the observations are indicating real spacings of particles in clouds, then the observations show very localized (centimeter scale) concentrations of ∼100 s cm−3 embedded within an average concentration of typically ∼1 cm−3. If the localized high concentrations are produced by the ice crystals shattering, then the concentrat...

The Ability of the Small Ice Detector (SID-2) to Characterize Cloud Particle and Aerosol Morphologies Obtained during Flights of the FAAM BAe-146 Research Aircraft

Abstract The Small Ice Detector mark 2 (SID-2), which was built by the University of Hertfordshire, has been operated by the Met Office on the Facility for Atmospheric Airborne Research (FAAM) BAe-146 aircraft during a large number of flights. The flights covered a wide range of atmospheric conditions, including stratocumulus, altocumulus lenticularis, cirrus, and mixed-phase cumulus clouds, as well as clear-sky flights over the sea and over desert surfaces. SID-2 is a laser scattering device that provides in situ data on cloud particle concentration and size. SID-2 also provides the spatial light scattering data from individual particles to give some information on the particle shape. The advantage of SID-2 is that it can characterize the cloud particle shape for particle sizes less than the resolutions of the more usual commercially available ice crystal imaging probes. The particle shape characteristics enable, for example, small just-nucleated ice particles to be discriminated from supercooled water d...

Discrimination of micrometre-sized ice and super-cooled droplets in mixed-phase cloud

Original article can be found at: http://www.sciencedirect.com/science/journal/13522310 Copyright Elsevier Ltd. DOI: 10.1016/S1352-2310(00)00377-0 [Full text of this article is not available in the UHRA]

Classifying atmospheric ice crystals by spatial light scattering

We describe preliminary results from an optical scattering instrument designed to assess the shapes and sizes of microscopic atmospheric cloud particles, especially the smallest ice crystals, that can profoundly affect cloud processes and radiative properties. The new instrument captures high-resolution spatial light scattering patterns from individual particles down to approximately 1 microm in size passing through a focused laser beam. Its significance lies in the ability of these patterns to provide morphological data for particle sizes well below the optical resolution limits of current cloud particle probes.

Light scattering from deformed droplets and droplets with inclusions. I. Experimental results

We provide experimental results from the scattering of light by deformed liquid droplets and droplets with inclusions. The characterization of droplet deformation could lead to improved measurement of droplet size as measured by commercial aerodynamic particle-sizing instruments. The characterization of droplets with inclusions can be of importance in some industrial, occupational, and military aerosol monitoring situations. The nozzle assembly from a TSI Aerodynamic Particle Sizer was used to provide the accelerating flow conditions in which experimental data were recorded. A helium-neon laser was employed to generate the light-scattering data, and an externally triggered, pulsed copper vapor laser provided illumination for a droplet imaging system arranged orthogonal to the He-Ne scattering axis. The observed droplet deformation correlates well over a limited acceleration range with theoretical predictions derived from an analytical solution of the Navier-Stokes equation.

Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements

The knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed-phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional lightscattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed-phase and cirrus clouds. The patterns are analysed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light-scattering properties and also similar causes. Overall, the in situ data are consistent, with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic air masses than in a continental, polluted one. Overall, the roughness and complexity are expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivity.

Light scattering from deformed droplets and droplets with inclusions. II. Theoretical treatment

We provide theoretical results from the scattering of light by deformed liquid droplets and droplets with inclusions. With improved instrumentation and computer technologies available, researchers are able to employ two-dimensional angular optical scattering as a tool for analyzing such particle systems and which then could be applied in industrial, occupational, and military aerosol measurement. We present numerically calculated spatial light-scattering data from various droplet morphologies. We describe characteristic features of the theoretical data and compare these with the experimental results.

Laser diffractometer for single-particle scattering measurements

A laser diffractometer (polar nephelometer) constructed for single-particle studies is described. It takes advantage of an array of optical fibres and a scanning disc to allow the use of a single photomultiplier tube for the detection of scattered light. A background subtraction scheme makes the instrument especially valuable in cases where background scattering exceeds the scattering from the observed particle, for example for living cells in aqueous media. The system can be easily integrated with particle trapping equipment such as laser tweezers or an electrodynamic balance.

A test of cirrus ice crystal scattering phase functions

Original article can be found at: http://www.agu.org/journals/gl/ Copyright American Geophysical Union. DOI: 10.1029/2003GL017482 [Full text of this article is not available in the UHRA]

Retrieving the size of particles with rough surfaces from 2D scattering patterns

Normal 0 Scattered intensity measurement is a commonly used method for determining the size of small particles. However, it requires calibration and is subject to errors due to changes in incident irradiance or detector sensitivity. Analysis of two-dimensional scattering patterns offers an alternative approach. We test possible techniques, including morphological image processing operations, on a diverse range of particles. It is found that the average surface area of intensity peaks is inversely proportional to particle size.