Evelyn Hesse

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.

Scattering from long prisms computed using ray tracing combined with diffraction on facets

A new model suitable for rapid computation of scattering on faceted dielectric objects such as ice crystals is presented. It combines ray tracing with diffraction on flat facets. The model allows retaining the ray nature of the internal field by calculating the diffraction component using an approximation for the far field direction of the Poynting vector. While this approach is similar to methods using the uncertainty principle, it does not require the use of angular distributions of diffracted rays, which leads to negligible computational overheads with respect to pure ray tracing. Results showing angle-dependent scattering computed for long hexagonal prisms, including phase functions and degree of linear polarization, are presented and compared with other models and with measurements on hexagonal fibers.

Ice crystal habits from cloud chamber studies obtained by in-line holographic microscopy related to depolarization measurements

We investigate hydrometeor habits at the AIDA chamber with a newly developed in-line holographic microscope HOLographic Imager for Microscopic Objects (HOLIMO). Sizes and habits of ice crystals and droplets in a mixed-phase cloud experiment are related to relative humidity with respect to ice (RH(ice)), temperature (T), and experiment time. This experiment is initiated with supercooled water drops. As a result, ice crystals within a maximum particle diameter size range of 2 to 118 microm (average size of 19 microm) are detected and 63% of them reveal regular habits. The observed particle habits match those predicted for a given RH(ice) and T. Two different growth modes emerge from this cloud. The first one appears during water injection and reveals mainly optical particle sizes in the range of 5 to 250 microm. The second mode grows to sizes of 5 to 63 microm, just after the particles of the first one fall out. It is found that an increasing aspect ratio chi of maximum length over thickness from 2 to 20 as obtained by HOLIMO corresponds to a decreasing linear depolarization ratio from 0.1 to 0.04, as independently obtained by depolarization measurements.

Stability characteristics of cylindrical fibres in an electrodynamic balance designed for single particle investigation

An electrodynamic balance (EDB) of the double ring double disc type has been designed for studying single microparticles. It allows electrostatic particle injection and recovery as well as alignment of elongated particles. The stability characteristics of cylindrical fibers and their alignment in this EDB are investigated. Angular as well as translational oscillations are observed, each associated with separate but analogous stability criteria.

Characterizing ice particles using two-dimensional reflections of a lidar beam

We report a phenomenon manifesting itself as brief flashes of light on the snows surface near a lidar beam. The flashes are imaged and interpreted as specular reflection patterns from individual ice particles. Such patterns have a two-dimensional structure and are similar to those previously observed in forward scattering. Patterns are easiest to capture from particles with well-defined horizontal facets, such as near-horizontally aligned plates. The patterns and their position can be used to determine properties such as ice particle shape, size, roughness, alignment, and altitude. Data obtained at Summit in Greenland show the presence of regular hexagonal and scalene plates, columns, and rounded plates of various sizes, among others.

Using Machine Learning Techniques to Recover Prismatic Cirrus Ice Crystal Size from 2-Dimensional Light Scattering Patterns

In this paper, we present a prediction model developed to identify particles size of ice crystals in clouds. The proposed model combines a Feed Forward Multi-Layer Perceptron neural network with Bayesian regularization backpropagation and other machine learning techniques for feature reduction with Principal Component Analysis and rotation invariance with Fast Fourier Transform. The proposed solution is capable of predicting the particle sizes with normalized mean squared error around 0.007. However, the proposed network model is not able to predict the size of very small particles (between 3 and 10 \({\upmu }\)m size) with the same precision as for the larger particles. Therefore, in this work we also discuss some possible reasons for this problem and suggest future points that need to be analysed.