RESURGENCE IN ICE NUCLEI MEASUREMENT RESEARCH Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ice nucleation instruments. Through comparison to laboratory cloud simulations, these ice nuclei Measurements have provided increased confidence in our ability to quantify primary ice formation by atmospheric aerosols. (Page 1623)

Mechanisms of thunderstorm electrification are presented and discussed.

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.

Collisions between vapour-grown ice crystals and a riming target, representing a graupel pellet failing in a thunderstorm, were shown by Reynolds, Brook and Gourley to transfer substantial charge, which they showed to be adequate to account for the development of charge centres leading to lightning in thunderstorms. Related experiments by Takahashi and Jayaratne et al. determined that the sign of charge transferred is dependent on the cloud liquid water content and on cloud temperature. There are marked differences between the results of Takahashi and Jayaratne in the details of the dependence they noted of the sign of graupel charging on cloud water and temperature. More recently, Pereyra et al. have shown that results somewhat similar in form to those of Takahashi are obtained by modifying the experimental technique used to prepare the clouds of ice crystals and supercooled water droplets used in the experiments.In order to help resolve the reason for the differences in charge transfer results in various studies, work has continued in the Manchester laboratory with a modified cloud chamber in which the cloud conditions of the crystals and droplets may be controlled independently. Results indicate a profound effect on the charge sign of the particle growth conditions in the two clouds involved. For example, by suitable adjustments to the water contents of the two clouds, graupel is charged negatively by rebounding ice crystal collisions at higher cloud water contents than have been noted previously. It is suggested that the most important influence on charge sign is the relative diffusional growth rate of the two ice surfaces at the moment of impact and that this is affected by an increase in cloud supersaturation experienced by the ice crystals during the cloud mixing process just prior to collision. A range of cloud conditions is used in the present work in order to help determine the reasons for the various results reported previously.Examination of some thunderstorm observations in the context of the present results points to the importance of mixing on the sign of the charge transferred during particle collisions when two cloud regions of different histories mix together.

WeatherVolume 60, Issue 6 p. 173-174 Letter Unusual lightning Clive Saunders, Corresponding Author Clive Saunders School of Earth, Atmospheric and Environmental Sciences, University of ManchesterSchool of Earth, Atmospheric and Environmental Sciences, University of ManchesterSearch for more papers by this author Clive Saunders, Corresponding Author Clive Saunders School of Earth, Atmospheric and Environmental Sciences, University of ManchesterSchool of Earth, Atmospheric and Environmental Sciences, University of ManchesterSearch for more papers by this author First published: 29 December 2006 https://doi.org/10.1256/wea.245.04AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume60, Issue6June 2005Pages 173-174 RelatedInformation

Numerical studies have been made of the importance of cloud saturation to the sign of charge transfer during graupel/ice crystal interactions in thunderstorms. Previous laboratory studies led to the idea that the diffusional growth rates of the interacting ice surfaces may influence the sign of the charge transferred during brief collisional contact. The ice crystals grow by vapour diffusion in a supersaturated environment while the graupel surface grows by diffusion under low accretion rate conditions, but will sublimate when heated sufficiently by riming. The graupel surface is also influenced, even under net sublimation conditions, by the vapour released to it from droplets freezing on its surface. In a cloud, the diffusional growth rates are also affected by ventilation when the supercooled droplets and their local environment flow past the riming surface.

A one-dimensional numerical model is used to study the charge density developed by the non-inductive crystal/graupel collision process along the path of rising thermals in convective clouds. The model employs the concept of successive thermals rising through the same volume with two regions identified: (i) an updraught simulated by successive ascending thermals and (ii) a non-active cloud mass formed from the debris of previously risen and stopped thermals. The microphysical processes utilise bulk parameterisation, while the charge transfer sign and magnitudes are based on laboratory studies.The model results show multilayered charge distributions, the exact distribution being sensitive to updraught velocity and details of the cloud microphysics. The study demonstrates that the cloud particle collisional charge transfer mechanism is able to account for the detection of more than three vertically distributed cloud layers with alternating polarity, as noted by Stolzenburg et al. [J. Geophys. Res. 103 (1998) 14059; J. Geophys. Res. 103 (1998) 14079; J. Geophys. Res. 103 (1998) 14097] and others during balloon ascents through thunderstorms. The analysis also reveals that the data recorded during balloon ascents could be due in some clouds to the relative motion of the balloon with respect to the cloud updraught. (C) 2003 Published by Elsevier B.V.

A two-dimensional lightning frequency model was applied to two case studies of thunderclouds: 9th July 1981, the CCOPE case, and 19th July 1991, the CaPE case. Factors influencing lightning activity were discussed. Although there was a strong link between updraft and lightning frequency, the relationship was not distinct; the initial environmental conditions, the updraft speed and more significantly the graupel number concentration were shown to have a large effect. The model computations suggested the importance of the 2-mm diameter graupel particles. No definite relationship between lightning frequency and cloud ice content could be established in this study. The results demonstrated the limitations of the current charge transfer parameterisation scheme used, with a more detailed structure being required. However, the model was capable of reproducing a realistic cloud structure and lightning activity using a relatively simple dynamical framework.

Laboratory experiments, in which vapour grown ice crystals interact with riming graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively.

Ice crystal clouds have an influence on the radiative budget of the earth; however, the exact size and nature of this influence has yet to be determined. A laboratory cloud chamber experiment has been set up to provide data on the optical scattering behaviour of ice crystals at a visible wavelength in order to gain information which can be used in climate models concerning the radiative characteristics of cirrus clouds. A PMS grey-scale probe is used to monitor simultaneously the cloud microphysical properties in order to correlate these closely with the observed radiative properties. Preliminary results show that ice crystals scatter considerably more at 90° than do water droplets, and that the halo effects are visible in a laboratorygenerated cloud when the ice crystal concentration is sufficiently small to prevent masking from multiple scattering.

Avila and Caranti [1994] measured electric charge transferred when 100 /zm frozen ice spheres collide with a timing target. This work is relevant to thunderstorm electrification caused by the interactions of ice crystals and small graupel with falling graupel pellets. Avila and Caranti note that charges are transferred in two modes, one in which ice fragments from the timing target carry charge away and one in which no fragments are observed. We argue here that there is no requirement for fracturing of a rimer surface or fragment production in thunderstorm electrification processes involving the collisions of ice crystals with a timing target as studied by Jayaratne et al., [1983] and Saunders et al., [1991]. Furthermore, the results to be discussed here lead to the conclusion that ice crystals impacting with frost growths on a timer are unable to remove these surface features. A consequence of this analysis is that temperature gradients along features on the rime surface, as invoked by Avila and Caranti, are not necessarily the cause of crystal/graupel charging in thunderstorms. The fragmentation of rime was studied by Griggs and Choularton [1986] who used glass beads of diameter 485 #m to simulate ice particles impacting a rime target. They used rime densities appropriate to small graupel pellets in thunderstorms, from 0.15 to 0.5 gcm -3, corresponding to temperatures of-15 ø to -3 øC. At -3 øC the velocity required to break the rime was around 70 m s -, while at -15øC the velocity needed was around 30 m s -. It was concluded that rime breakup during collisions with ice particles in thunderstorms is unlikely. The authors also studied the conditions required to break ice crystals. Dendritic crystals were the most fragile; they noted that a 3-mm-long dendrite growing on a substrate could be broken off by an airstream at only 2 m s -. Any vapor deposits on the riming target studied by Avila and Caranti would only have been tens of microns in length, but there is the possibility that the charged fragments they observed originated as vapor-grown deposits (frost) on the rime surface and were removed by the impact of 100-/tm ice spheres at 5 m s -. Also of relevance to this debate is the work of Jayaratne and Griggs [1991] who blew fragments of ice off a timed target with a high-speed airstream. They formed a rime deposit on the front o f a rod target then directed an air jet at the rime with the rime deposit upwind, downwind, or at 90 ø to the air jet, while noting any charge transfer to the target. When the air jet impinged on the front or the side of the

Journal of Geophysical Research: AtmospheresVolume 101, Issue D5 p. 9533-9535 CommentariesFree Access Comment on “A laboratory study of static charging by fracture in ice growing by riming” by Eldo E. Avila and Giorgio M. Garanti Rohan Jayaratne, Rohan JayaratneSearch for more papers by this authorSu Ling Peck, Su Ling PeckSearch for more papers by this authorClive Saunders, Clive SaundersSearch for more papers by this author Rohan Jayaratne, Rohan JayaratneSearch for more papers by this authorSu Ling Peck, Su Ling PeckSearch for more papers by this authorClive Saunders, Clive SaundersSearch for more papers by this author First published: 01 April 1996 https://doi.org/10.1029/95JD03066Citations: 6AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Avila, E. E., G. M. Caranti, A laboratory study of static charging by fracture in ice growing by riming, J. Geophys. Res., 99, 10611– 10620, 1994. Baker, B., M. B. Baker, E. R. Jayaratne, J. Latham, C. P. R. Saunders, The influence of diffusional growth rates on the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones, Q. J. R. Meteorol. Soc., 113, 1193– 1215, 1987. Baker, M. B., J. G. Dash, Mechanism of charge transfer between colliding ice particles in thunderstorms, J. Geophys. Res., 99, 10621– 10626, 1994. Gaskell, W., A. J. Illingworth, Charge transfer accompanying individual collisions between ice particles and its role in thunderstorm electrification, Q. J. R. Meteorol. Soc., 106, 841– 854, 1980. Griggs, D. J., T. W. Choularton, A laboratory study of secondary ice particle production by the fragmentation of rime and vapour-grown ice crystals, Q. J. Meteorol. Soc., 112, 149– 163, 1986. Jayaratne, E. R., Temperature gradients in ice as a charge generation process in thunderstorms, Atmos. Res., 29, 247– 260, 1993. Jayaratne, E. R., D. J. Griggs, Electric charge separation during the fragmentation of rime in an airflow, J. Atmos. Sci., 48, 2492– 2495, 1991. Jayaratne, E. R., C. P. R. Saunders, J. Hallett, Laboratory studies of the charging of soft-hail during ice crystal interactions, Q. J. R. Meteorol. Soc., 109, 609– 630, 1983. Keith, W. D., C. P. R. Saunders, Light emission from colliding ice particles, Nature, 336, 362– 364, 1988. Keith, W. D., C. P. R. Saunders, Further laboratory studies of the charging of graupel during ice crystal interactions, J. Atmos. Sci., 25, 445– 464, 1990. Saunders, C. P. R., W. D. Keith, R. P. Mitzeva, The effect of liquid water on thunderstorm charging, J. Geophys. Res., 96, 11007– 11017, 1991. Citing Literature Volume101, IssueD527 April 1996Pages 9533-9535 ReferencesRelatedInformation

A laboratory investigation of the collection efficiency of a target riming by the collection of supercooled water droplets has shown that the target, which represents a soft-hailstone inside a thundercloud, collects more droplets than theory indicates. The additional growth is due to enhanced collection on the “feathers of rime” that grow on the target surface. The effect is greatest when the rime density is low, which occurs with low droplet impact speeds and at cold temperatures.

Abstract The density of rime ice accreted on a target moving through a cloud of supercooled water droplets has been measured in order to determine whether the rime density, or other characteristics, are modified by radial forces. Two similar targets in the same cloud were rotated on frames that provided the same speed but different radial accelerations to each target. The mass accreted and the rime density were found to be significantly reduced under high radial forces. A relationship was determined between rime density, the mean volume droplet size, the speed, the temperature and the acceleration. These results are important to the design of liquid-water content measurement equipment, particularly for laboratory use, where the mass of ice accreted by a target on a rotating frame is determined. The results also have relevance to laboratory measurements of electric charge transfer between riming targets and re-bounding ice crystals.