Zbigniew Ulanowski

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...

Self-charging of the Eyjafjallajökull volcanic ash plume

Volcanic plumes generate lightning from the electrification of plume particles. Volcanic plume charging at over 1200 km from its source was observed from in situ balloon sampling of the April 2010 Eyjafjallajokull plume over Scotland. Whilst upper and lower edge charging of a horizontal plume is expected from fair weather atmospheric electricity, the plume over Scotland showed sustained positive charge well beneath the upper plume edge. At these distances from the source, the charging cannot be a remnant of the eruption itself because of charge relaxation in the finite conductivity of atmospheric air.

Calibration of the Cloud Particle Imager Probes Using Calibration Beads and Ice Crystal Analogs: The Depth of Field

Abstract This paper explains and develops a correction algorithm for measurement of cloud particle size distributions with the Stratton Park Engineering Company, Inc., Cloud Particle Imager (CPI). Cloud particle sizes, when inferred from images taken with the CPI, will be oversized relative to their “true” size. Furthermore, particles will cease to be “accepted” in the image frame if they lie a distance greater than the depth of field from the object plane. By considering elements of the scalar theory for diffraction of light by an opaque circular disc, a calibration method is devised to overcome these two problems. The method reduces the error in inferring particle size from the CPI data and also enables the determination of the particles distance from the object plane and hence their depth of field. These two quantities are vital to enable quantitative measurements of cloud particle size distributions (histograms of particle size that are scaled to the total number concentration of particles) in the atm...

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.

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.

Scattering of light from atmospheric ice analogues

New analogues replacing atmospheric ice crystals in light scattering measurements have been developed. These include thin, hexagonal glass fibres and both simple and complex microcrystals resembling cirrus ice, including columns, plates, rosettes and aggregates with a variety of sizes. Results of angle-dependent scattering measurements on the fibres and on levitated crystals are presented, including phase functions, polarization properties and the asymmetry parameter.

Scalar field of nonparaxial Gaussian beams

A family of closed-form expressions for the scalar field of strongly focused Gaussian beams in oblate spheroidal coordinates is given. The solutions satisfy the wave equation and are free from singularities. The lowest-order solution in the far field closely matches the energy density produced by a sine-condition, high-aperture lens illuminated by a paraxial Gaussian beam. At the large waist limit the solution reduces to the paraxial Gaussian beam form. The solution is equivalent to the spherical wave of a combined complex point source and sink but has the advantage of being more directly interpretatable.

Application of neural networks to the inverse light scattering problem for spheres

A new approach suitable for solving inverse problems in multiangle light scattering is presented. The method takes advantage of multidimensional function approximation capability of radial basis function neural networks. An algorithm for training the networks is described in detail. It is shown that the radius and refractive index of homogeneous spheres can be recovered accurately and quickly, with maximum relative errors of the order of 10(-3) and mean errors as low as 10(-5). The influence of the angular range of available scattering data on the loss of information and inversion accuracy is investigated, and it is shown that more than two thirds of input data can be removed before substantial degradation of accuracy occurs.

Observations of Saharan dust layer electrification

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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.