A BALLOON-BORNE PARTICLE SIZE, IMAGING, AND VELOCITY PROBE FOR IN SITU MICROPHYSICAL MEASUREMENTS

A balloon-borne instrument known as the PArticle Size, Image, and Velocity (PASIV) probe has been developed at the National Severe Storms Laboratory to provide in situ microphysical measurements in storms. These observations represent a critical need of microphysical observations for use in lightning studies, cloud microphysics simulations, and dual-polarization radar validation. The instrument weighs approximately 2.72 kg and consists of an HD video camera, a camera viewing chamber, and a modified Parsivel laser disdrometer mounted above the camera viewing chamber. Precipitation particles fall through the Parsivel sampling area and then into the camera viewing chamber, effectively allowing both devices to sample the same particle stream. The data are collected onboard for analysis after retrieval. Taken together, these two instruments are capable of providing a vertical profile of the size, shape, velocity, orientation, and composition of particles along the balloon path within severe weather. The PASIV probe has been deployed across several types of weather environments including thunderstorms, supercells, and winter storms. Initial results from two cases in the Deep Convective Clouds and Chemistry Experiment are shown that demonstrate the ability of the instrument to obtain high temporal and spatial resolution observations of the particle size distributions (PSD) within convection. The ability to resolve the PSD into different particle habits and compare to observed radar and model analysis values is also demonstrated

Measurements of Differential Reflectivity in Snowstorms and Warm Season Stratiform Systems

The organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.United States. Federal Aviation Administration (Air Force Contract FA8721-05-C-0002

Multi-scale process studies in the tropics: results from lightning observations

March 1997.Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1997.Includes bibliographical references.Cloud-to-ground (CG) lightning and meteorological observations collected in the tropics were analyzed to address the following question: What do observations of lightning tell us about processes occurring over multiple scales in the tropical atmosphere? An emphasis was placed on the analysis of observations collected over the western Pacific warm-pool during TOGA COARE. Large-scale observations from COARE suggest that the occurrence of lightning over the western Pacific Ocean is a sensitive function of both the magnitude and vertical distribution of convective available potential energy (CAPE). Small variations in the marine boundary layer humidity were highly correlated to variations in the CAPE and/or boundary-layer 8w. In tum, small increases (O[0.5° C]) in the boundary-layer 8w, were associated with disproportionate increases in lightning activity. The diurnal cycle of CG lightning exhibited a pronounced maximum (minimum) around 2 a.m. (12 p.m.) local-time. Diurnal cycles of CAPE, convective and total precipitation exhibited similar diurnal cycles, but were weaker in amplitude. Over cloud-scales, upward-building 30 dBZ reflectivity cores extended to elevations colder than -10°C in lightning-producing tropical oceanic convection. Additionally, mean updraft strengths (when observed) in several Lightning-producing cases exceeded 6 m s-1 near the -10°C level. These observations support the hypothesis that updraft magnitudes between the 0°C and -10°C levels in tropical convection must exceed the terminal fall-speed of millimeter sized liquid and frozen drops in order to provide the requisite hydrometeor mass to electrification processes in the cold regions of the cloud. To investigate the coupling between cloud-scale electrification, kinematics, microphysics, and the large-scale thermodynamic environment, a one-dimensional cloud-model with a four­class bulk-microphysical ice scheme and a parameterization for non-inductive charging processes, was used to simulate tropical convection. In the cloud-simulations, convective heating profiles associated with lightning (non-lightning) producing convection were associated with a more pronounced upper-level (low-level) heating peak and an increased (decreased) contribution by ice-processes to the total surface rainfall. Since the rainfall process and lightning production become increasingly more correlated as contributions from the ice-phase to the total rainfall increase, we investigated the correlation between rainfall and lightning over large spatial and temporal scales for many different rainfall regimes. The results indicate that CG lightning flash density and rainfall are well correlated in warm-season rainfall regimes where highly electrified convection is prolific. In certain situations, it may be possible to use CG-lightning flash density to diagnose warm-season monthly rainfall totals, or differentiate between rainfall regimes.Sponsored by the National Aeronautics and Space Administration under grant NGT-30268

Ice Particle Size and Roughness from Novel Techniques - In Situ Measurements and Validation

The roughness of ice crystals, defined by small-scale surface roughness and large scale complexity, in high-altitude cloud, has been studied due to its important influence on the radiative properties of ice cloud. The Small Ice Detector 3 (SID-3) created at the University of Hertfordshire was used to measure the characteristics of individual ice crystals in situ. These are supplemented by a range of meteorological in situ measurements, including temperature, relative humidity, and wind velocity to investigate the influence of atmospheric conditions on ice crystal roughness/complexity. Since the method of roughness retrieval was novel, for atmospheric ice particles, laboratory experiments were setup to test and improve the characterization techniques. Criteria were set as a result of the laboratory experiments which data was expected to meet for it to be deemed reliable. These criteria and techniques were applied to data collected in situ on research aircraft. A range of degrees of ice crystal roughness were observed over five flights from two campaigns based out of Scotland in 2012 and 2015 (PIKNMIX and CIRCCREX). When all the flights were combined the majority of particles (51%) were categorised as lightly rough; the second most common roughness type was moderately rough (39%). Smooth particles made up 10% of the total particles, and <0.02% were classed as severely rough. When considering a wave-cloud case separately, a similar range of roughness values were seen, however, smooth particles were only observed at the cloud leading-edge where nucleation was expected to occur during the only straight level run of the aircraft to probe this region. During the same wave-cloud flight smooth particles were more common in supersaturated regions and moderately rough crystals were more common in subsaturated regions, suggesting that crystals are more likely to tend towards rougher values when observed in subsaturated environments (a statistical T-test showed this hypothesis to be statistically significant). It was found that due to limitations associated with instantaneous measurements, it was challenging to observe how ice particle roughness evolved in situ, since the history of the individual crystals was unknown in most cases. Orographic cloud, however, was found to provide a more robust estimation of crystal evolution as a consequence of having sharp-leading edges where nucleation events were expected to occur, and since crystals then follow streamlines, the distance from the sharp-leading edge can act as a proxy for time since nucleation