Matthew Bailey

A comprehensive habit diagram for atmospheric ice crystals : Confirmation from the laboratory, AIRS II, and other field studies

Abstract Recent laboratory experiments and in situ observations have produced results in broad agreement with respect to ice crystal habits in the atmosphere. These studies reveal that the ice crystal habit at −20°C is platelike, extending to −40°C, and not columnar as indicated in many habit diagrams found in atmospheric science journals and texts. These diagrams were typically derived decades ago from laboratory studies, some with inherent habit bias, or from combinations of laboratory and in situ observations at the ground, observations that often did not account for habit modification by precipitation from overlying clouds of varying temperatures. Habit predictions from these diagrams often disagreed with in situ observations at temperatures below −20°C. More recent laboratory and in situ studies have achieved a consensus on atmospheric ice crystal habits that differs from the traditional habit diagrams. These newer results can now be combined to give a comprehensive description of ice crystal habits ...

Growth Rates and Habits of Ice Crystals between −20° and −70°C

Abstract A laboratory study of ice crystal growth characteristics at temperatures between −20° and −70°C has been performed at ice supersaturations and pressures comparable with those in the atmosphere using a horizontal static diffusion chamber. Maximum dimension, projected area, and volume growth rates, in addition to habit frequency, have been measured for individual habit types as functions of temperature, ice supersaturation, and air pressure. It was found that from −20° to −40°C and at ice supersaturations in excess of 2%, the most frequent habits observed were platelike polycrystals and plates, the complexity of forms increasing with increasing supersaturation. Columns appear with low frequency in this temperature range for all supersaturations. At low ice supersaturation (1%–2%), the habit consists of thick plates, compact polycrystals, and occasional short columns and is the region with the highest frequency of pristine crystals capable of producing halos. Just colder than −40°C, there is a marke...

A brief review of the problem of lightning initiation and a hypothesis of initial lightning leader formation

[1]xa0A brief review of hypothesized mechanisms of lightning initiation is presented, with the suggestion that these mechanisms provide an incomplete picture of lightning initiation. This is followed by two ideas: (1) a combination of previously hypothesized lightning initiation mechanisms as a means for local intensification of the thundercloud electric field, and (2) a process for the formation of a hot lightning leader channel that is analogous to the space leader phase of the laboratory negative stepped leader. Thundercloud electric field observations have consistently yielded peak values that are an order of magnitude weaker than the dielectric strength of air. Various mechanisms have been proposed to explain how lightning can initiate in such weak electric fields, including hydrometeor-initiated positive streamers and cosmic ray-initiated runaway breakdown. The historically favored positive streamer mechanisms are problematic due to requiring electric fields two to three times larger than peak observed fields. The recently favored runaway breakdown mechanisms appear capable of developing in conditions comparable to peak observed fields although it is not clear how these diffuse discharges can lead to creation of a lightning leader. This paper proposes a solution whereby runaway breakdown and hydrometeor-initiated positive streamer systems serve to locally intensify the electric field. Following this local field intensification, it is hypothesized that formation of the initial lightning leader channel is analogous to the formation of a space leader in a laboratory negative stepped leader.

NOx production in laboratory discharges simulating blue jets and red sprites

[1]xa0Sources of middle atmosphere nitrogen oxides, including transport from the troposphere and production in situ by energetic electrons, are currently not well characterized. Production of nitrogen oxides (NOx) in the middle atmosphere by transient luminous events (TLEs), such as red sprites and blue jets has previously been estimated from satellite observations and modeling studies. This is the first laboratory attempt to estimate NOx production by TLEs, following studies that have confirmed electrical similarities between laboratory discharges and TLEs. A pressure-controlled chamber and high-voltage power supplies simulated middle atmosphere discharges. Chemiluminescence NOx analyzers sampled NOx following the completion of the chamber discharges, which was used to calculate total NOx production for each discharge as well as NOx per ampere of current and NOx per Joule of discharge energy. Three different production efficiencies in NOx/J as a function of pressure pointed to three different production regimes: one for tropospheric pressures (100–500 mb), one for stratospheric pressures (1–100 mb), and one for upper stratospheric to mesospheric pressures (no greater than 1 mb). Discharges at jet-like pressures are measured to produce 1.7 × 1016 to 6.40 × 1017 molecules of NOx per discharge, while discharges at sprite-like pressure produce 6.97 × 1013 to 8.57 × 1013 molecules of NOx per discharge. Blue jets were calculated to produce 1.7 × 1022 to 7.4 × 1026 molecules of NOx, while red sprites were calculated to produce 6.8 × 1023 to 6.3 × 1027 molecules of NOx. On the basis of global sprite frequency estimates global annual NOx production by sprites is estimated to be between 7 × 1023 and 2 × 1028 molecules per second.

Laboratory and In Situ Observation of Deposition Growth of Frozen Drops

The water vapor deposition growth of frozen drops with diameter greater than 100 mm has been studied in a thermal diffusion chamber. For varying periods of time, it was found that frozen drops experience spherical growth. The characteristic time of spherical growth depends on supersaturation, temperature, and drop size, and it varies from minutes to tens of minutes. The average rate of frozen drop growth agrees well with that predicted by the Maxwellian growth equation for ice spheres. Observations in natural clouds conducted with a cloud particle imager probe has yielded evidence that frozen drops may retain spheroidal shapes for at least 15‐20 min under conditions close to saturation over water. These observations are in agreement with the laboratory experiments. The observation of frozen drops in natural clouds may be correlated to freezing drizzle generated by overlying cloud layers that may lead to hazardous in-flight icing.

Light-scattering properties of plate and column ice crystals generated in a laboratory cold chamber

Angular scattering properties of ice crystal particles generated in a laboratory cloud chamber are measured with a lightweight polar nephelometer with a diode laser beam. This cloud chamber produces distinct plate and hollow column ice crystal types for light-scattering experiments and provides a controlled test bed for comparison with results computed from theory. Ice clouds composed predominantly of plates and hollow columns generated noticeable 22 degrees and 46 degrees halo patterns, which are predicted from geometric ray-tracing calculations. With the measured ice crystal shape and size distribution, the angular scattering patterns computed from geometrical optics with a significant contribution by rough surfaces closely match those observed from the nephelometer.

First Results from the Alliance Icing Research Study II

The field phase of the Second Alliance Icing Research Study (AIRS II) was conducted from November 2003 to February 2004, with the main center of interest being near Mirabel, Quebec. The AIRS II project operational objectives are to: a) develop techniques/systems to remotely detect, diagnose and forecast hazardous winter conditions at airports, b) improve weather forecasts of aircraft icing conditions, c) better characterize the aircraft-icing environment and d) improve our understanding of the icing process and its effect on aircraft. In order to support the operational objectives, the following science objectives are being addressed to: a) investigate the conditions associated with supercooled large drop formation, b) determine conditions governing cloud glaciation, c) document the spatial distribution of ice crystals and supercooled water and the conditions under which they co-exist, and d) verify the response of remote sensors to various cloud particles, and determine how this can be exploited to remotely determine cloud composition. Five research aircraft were involved in the field project. These aircraft flew special flight operations over a network of ground in-situ and remote-sensing meteorological measurement systems, located at Mirabel, Quebec. Data were collected to evaluate some prototype airport weather forecasting systems, which use satellite and surface-based remote sensors, PIREPS, and numerical forecast models. The project will also be used in North America and Europe to further develop numerical forecast models, and forecast systems, which predict aircraft icing over large areas. AIRS II is an exciting collaborative effort involving approximately 26 government and university groups from Canada, the United States and Europe. It will assist in providing the aviation community better tools to avoid aircraft icing, and to improve the efficiency of airport operations.

Ice Crystal Linear Growth Rates from −20° to −70°C: Confirmation from Wave Cloud Studies

AbstractAs a result of recent comprehensive laboratory and field studies, many details have been clarified concerning atmospheric ice crystal habits below −20°C as a function of temperature, ice supersaturation, air pressure, and growth history. A predominance of complex shapes has been revealed that is not reflected in most models, with symmetric shapes often incorrectly emphasized. From the laboratory study, linear (maximum dimension), projected area, and volume growth rates of complex and simple habits have been measured under simulated atmospheric conditions for temperatures from −20° to −70°C. Presently, only a few in situ cases of measured ice crystal growth rates are available for comparison with laboratory results. Observations from the Interaction of Aerosol and Cold Clouds (INTACC) field study of a well-characterized wave cloud at −24°C are compared with the laboratory results using a simple method of habit averaging to derive a range of expected growth rates. Laboratory results are also compare...

Development of x‐ray and extreme ultraviolet (EUV) optical devices for diagnostics and instrumentation for various surface applications

New methods of surface-sensitive instrumentation for the diagnostics of extreme ultraviolet (EUV), soft x-ray (SXR) and x-ray radiation have been developed. In particular, these methods of instrumentation are of great importance for studies of the interaction of electron, ion and photon beams with matter (atoms, molecules, ions, clusters, surfaces, micro and submicron structures). Such collision studies are based on both glass capillary converters (GCCs) and multilayer mirrors (MLMs), gratings and crystals. The optical GCC device provides guiding, focusing and polarization analysis of short-wavelength radiation with a large bandwidth, and the MLM, grating and crystal optical elements are used for dispersing, focusing and polarization-sensitive studies of radiation within a more narrow bandwidth. In particular, we report here on the development of optical diagnostic devices, such as very compact EUV and SXR spectrometers, for measurements of polarization and spectral characteristics of short-wavelength radiation. Our high-throughput and high resolution compact spectrometers for the SXR range are based on grazing incidence spectrometers, MLMs and a sliced multi-layer grating (SMG) novel type of dispersion element. Furthermore, we will utilize a Schwarzschild objective with a spatial resolution of ∼0.1-0.3 pm for imaging surfaces following ion-surface interaction.

Investigation of excitation and ionization excitation of He following electron and proton impact using EUV polarimetry

A detailed investigation of excitation of He (1s2) 1S to He (1snp) 1P degree(s) (n equals 2 - 5) states and ionization-excitation of He (1s2) 1S to He (2p) 2P degree(s) and He (3p) 2P degree(s) states in e- + He and H+ + He collision systems is presented for a wide range of projectile velocities (2.2 a.u. < v < 6.9 a.u.). Specifically new experimental data are presented on measurements of the degree of linear polarization for excitation and excitation-ionization of He following proton impact in the extreme ultraviolet (EUV) wavelengths. These measurements have been performed using a characterized molybdenum/silicon multilayer mirror polarimeter (MLM) whose polarization characteristics have been optimized for EUV emission of He and He+. Furthermore, the proton experimental results are compared with theoretical polarization data using the first Born approximation and recent atomic orbital close coupling (AOCC) calculations for the excitation process. A comprehensive comparison of experimental data for negatively and positively charged projectiles at equal impact velocities is also given in order to elucidate differences in the collision mechanisms of two electron targets. It is important to note that these results are relevant for astrophysical diagnostics such as solar flares.