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- Author or Editor: C. P. R. Saunders x
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Abstract
Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer on temperature and cloud liquid water content. Field data upon which the laboratory simulations are based are obtained by increasingly sophisticated airborne and ground-based means. Calculations of electric field growth using experimental charge-transfer data in numerical models of the dynamical and microphysical development of thunderstorms show agreement with observations, although further refinement is required. Some directions for future research are outlined.
Abstract
Recent developments in the area of thunderstorm electrification processes are reviewed. These processes have two main divisions; (a) convective, in which particles charged by ion capture are moved by convection currents to strengthen the electric field in the cloud, and (b) processes involving charge transfer during particle interactions, following which oppositely charged particles move apart in the updraft to form the observed charge centers. Type-b processes are further subdivided into inductive (relying on the preexistence of an electric field) and noninductive charge-transfer mechanisms. Field and laboratory evidence points to the importance of interactions between particles of the ice phase, in the presence of liquid water droplets, in separating electric charge in thunderstorms. Recent experimental studies have investigated the dependence of charge transfer on the size and relative velocity of the interacting particles and have determined the dependence of the sign of the charge transfer on temperature and cloud liquid water content. Field data upon which the laboratory simulations are based are obtained by increasingly sophisticated airborne and ground-based means. Calculations of electric field growth using experimental charge-transfer data in numerical models of the dynamical and microphysical development of thunderstorms show agreement with observations, although further refinement is required. Some directions for future research are outlined.
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Abstract
Laboratory studies of rime growth on a moving rod under conditions of secondary ice crystal production show that the rod acquires a positive charge, equivalent to charge associated with each ejected particle of 5 × 10−4C. Ice crystals produced by seeding also impart a positive charge to the rime, equivalent to a charge per particle of 5 × 10−16C. As the water vapor supply is cut off, the charge sign reverses. The results suggest that the sign of the charge transfer depends on the physical state of the rime surface and its vapor pressure excess or deficit relative to the environment. Charge separation in convective clouds is critically dependent on the changing proportion of graupel and small secondary ice crystals.
Abstract
Laboratory studies of rime growth on a moving rod under conditions of secondary ice crystal production show that the rod acquires a positive charge, equivalent to charge associated with each ejected particle of 5 × 10−4C. Ice crystals produced by seeding also impart a positive charge to the rime, equivalent to a charge per particle of 5 × 10−16C. As the water vapor supply is cut off, the charge sign reverses. The results suggest that the sign of the charge transfer depends on the physical state of the rime surface and its vapor pressure excess or deficit relative to the environment. Charge separation in convective clouds is critically dependent on the changing proportion of graupel and small secondary ice crystals.
Abstract
During the replication of ice crystals by conventional methods the crystals both grow and flocculate; this can lead to erroneous determination of their size distribution and to unreliable analysis of the in-cloud interactions which occurred prior to replication. A study has been made of these difficulties and an improved replication procedure has been used to overcome them.
Abstract
During the replication of ice crystals by conventional methods the crystals both grow and flocculate; this can lead to erroneous determination of their size distribution and to unreliable analysis of the in-cloud interactions which occurred prior to replication. A study has been made of these difficulties and an improved replication procedure has been used to overcome them.
Abstract
Oversampling of optical array probes (OAPs) is described as a novel technique to increase the quality of small-particle data available in cloud microphysics. The slice rate of a 10-μm-resolution grayscale OAP is increased by a factor f with respect to that which would produce images with an aspect ratio of 1:1 for the given air velocity and probe resolution. The Fresnel diffraction pattern is simulated and the probe response calculated for normal (f = 1) and oversampled (f = 10) cases. The increased number of pixels imaged by the OAP permits the fraction of 25%–50% shadowed pixels to be used to obtain a much improved estimate of the actual droplet diameter for droplets between 7 and 50 μm in diameter. The process of oversampling is also found to significantly increase the sample area for the smallest particles. Example laboratory particle size distributions are provided. The oversampling factor for an individual probe is limited by the maximum sampling frequency, the airspeed, and the probe resolution.
Abstract
Oversampling of optical array probes (OAPs) is described as a novel technique to increase the quality of small-particle data available in cloud microphysics. The slice rate of a 10-μm-resolution grayscale OAP is increased by a factor f with respect to that which would produce images with an aspect ratio of 1:1 for the given air velocity and probe resolution. The Fresnel diffraction pattern is simulated and the probe response calculated for normal (f = 1) and oversampled (f = 10) cases. The increased number of pixels imaged by the OAP permits the fraction of 25%–50% shadowed pixels to be used to obtain a much improved estimate of the actual droplet diameter for droplets between 7 and 50 μm in diameter. The process of oversampling is also found to significantly increase the sample area for the smallest particles. Example laboratory particle size distributions are provided. The oversampling factor for an individual probe is limited by the maximum sampling frequency, the airspeed, and the probe resolution.
Abstract
During the 1989 intensive field campaign of the International Cirrus Experiment (ICE) over the North Sea, broadband radiative fluxes were measured in, above, and below cirrus cloud by a number of European meteorological research aircraft. One mission during the campaign was an intercomparison flight in clear air with no cloud above in order to compare, among other things, radiative flux measurements made by the U.K. C-130, the French Merlin, and the German Falcon aircraft. All three aircraft measured shortwave flux (0.3–3 µm) with standard Eppley pyranometers above and below the fuselage. The intercomparison showed agreement between the three aircraft of within 2% for both the upwelling and downwelling shortwave flux components. Using a coincident temperature and humidity radiosonde profile, the downward clear-sky fluxes at the level of the aircraft were also calculated using a variety of different radiation models. Modeled shortwave fluxes were all higher (between 2% and 4%) than the measured values. In addition to shortwave fluxes the C-130 and Merlin also measured near-infrared fluxes (0.7–3 µm) by having additional Eppley pyranometers mounted with red domes over the thermopiles. The near-infrared fluxes measured by the Merlin and C-130 were different because slightly different red-dome filters were used; model calculations show the difference between the measured fluxes was consistent with the different pass band of the filters. Infrared fluxes (4–40 µm) were measured using standard Eppley pyrgeometers on the Falcon and pyrgeometers developed at the Meteorological Research Flight on the C-130; comparisons show no significant differences for the downwelling fluxes but the Falcon upwelling fluxes were 7% higher than the corresponding C-130 values. This latter difference is higher than would be expected for these instruments. The modeled infrared fluxes were up to 9% lower than the C-130 and Falcon measurements.
Abstract
During the 1989 intensive field campaign of the International Cirrus Experiment (ICE) over the North Sea, broadband radiative fluxes were measured in, above, and below cirrus cloud by a number of European meteorological research aircraft. One mission during the campaign was an intercomparison flight in clear air with no cloud above in order to compare, among other things, radiative flux measurements made by the U.K. C-130, the French Merlin, and the German Falcon aircraft. All three aircraft measured shortwave flux (0.3–3 µm) with standard Eppley pyranometers above and below the fuselage. The intercomparison showed agreement between the three aircraft of within 2% for both the upwelling and downwelling shortwave flux components. Using a coincident temperature and humidity radiosonde profile, the downward clear-sky fluxes at the level of the aircraft were also calculated using a variety of different radiation models. Modeled shortwave fluxes were all higher (between 2% and 4%) than the measured values. In addition to shortwave fluxes the C-130 and Merlin also measured near-infrared fluxes (0.7–3 µm) by having additional Eppley pyranometers mounted with red domes over the thermopiles. The near-infrared fluxes measured by the Merlin and C-130 were different because slightly different red-dome filters were used; model calculations show the difference between the measured fluxes was consistent with the different pass band of the filters. Infrared fluxes (4–40 µm) were measured using standard Eppley pyrgeometers on the Falcon and pyrgeometers developed at the Meteorological Research Flight on the C-130; comparisons show no significant differences for the downwelling fluxes but the Falcon upwelling fluxes were 7% higher than the corresponding C-130 values. This latter difference is higher than would be expected for these instruments. The modeled infrared fluxes were up to 9% lower than the C-130 and Falcon measurements.
Abstract
A system is described for measurement and analysis of precipitation particle charge from an aircraft in the highly variable and harsh environment of a convective cloud. A compromise, practical instrument design enables particle charge and sign to be measured with concentrations up to 5 L−1. The system employs two induction rings in series; it is de-iced both on the electrostatic shield and internally. New techniques are described which enable rapid analysis of sequential charge data over a penetration period of 55 s, with rejection of spurious data pulses resulting from particle impaction.
Abstract
A system is described for measurement and analysis of precipitation particle charge from an aircraft in the highly variable and harsh environment of a convective cloud. A compromise, practical instrument design enables particle charge and sign to be measured with concentrations up to 5 L−1. The system employs two induction rings in series; it is de-iced both on the electrostatic shield and internally. New techniques are described which enable rapid analysis of sequential charge data over a penetration period of 55 s, with rejection of spurious data pulses resulting from particle impaction.
