The electrification of water drops on freezing or f4elting at terminal velocity in air

Measurements on the electrification on freezing of individual water drops of diameter between 3 and 5 mm supported by an air stream have shown that the freezing behaviour of the drops is temperature-dependent. Above -10 C the freezing progressed uniformly throughout the drop from a single point. Below -10 C the outer surface of the drop froze rapidly with the exception of a small area at the top of the drop. Freezing then progressed uniformly from the base of the drop upwards. Mo shattering or electrification of the drops was observed during their freezing. These results are contrasted with those of previous workers who observed the freezing of drops suspended on fibres. Measurements of the electrification of ice spheres supported by an air stream have indicated that the sign of the charge acquired by the spheres due to riming is temperature-dependent. When the sphere is rimed by droplets at temperatures above -10 C it acquires a negative charge, and v/hen the riming droplets are at temperatures below -10 C the ice sphere acquires a positive charge. An explanation is proposed for this effect in terms of the observed freezing behaviour of individual water drops. It I suggested that this effect could lead to thunderstorm electrification of the observed polarity. Measurements on the electrification of melting ice spheres supported by an air stream indicated that the sign of the charge acquired by the spheres is dependent on whether or not water is flung off the charge was negative. This may explain the discrepancies between the laboratory and field measurements of MacCready and Proudfit

Non-inductive charging of tropical convection in high and low CAPE environments

May 29, 1992.Includes bibliographical references.Sponsored by National Science Foundation ATM-9015485

High resolution simulations of the microphysics and electrification in hurricane-like vortices over warm ocean and at landfall.

Cloud-to-ground (CG) lightning bursts in the eyewall of mature tropical cyclones (TCs) are believed to be good indicators of imminent intensification of these systems. While numerous well-documented observational cases exist in the literature, no modeling studies of the electrification processes within TCs have been made so far. At present, little is known about the evolution of charges and subsequent electrification in mature TCs. Towards this goal, a numerical cloud model featuring a 12-class bulk microphysics scheme and a three-dimensional branched lightning module is utilized to simulate the evolution of the microphysics fields and subsequent electrical activity in an idealized hurricane like vortex over ocean (OCEAN case). In a separate experiment (LAND case), two simulations were carried out a slightly coarser resolution. The first simulation was similar than the OCEAN case, while in the second simulation, a simplistic landmass was introduced in the domain in order to investigate the effect of reduced sensible and moisture flux and enhanced drag on the TC's dynamics, microphysics and electrification.Preliminary results of the OCEAN TC case showed that the highest total lightning flash rate were primarily found within the eyewall but seldom within the stronger cells forming the outer rainbands where updraft speeds rarely exceeded 10 m s-1 and 15 m s-1, respectively, consistent with observations. As expected, these regions of the storm were generally characterized by moderate total graupel mixing ratio (> 0.5 g kg -1) and moderate cloud water content (> 0.2 g kg-1). Using the Saunders and Peck non-inductive (NI) charging scheme and moderate inductive charging settings, the inner eyewall region exhibited a normal tripole charge structure (a mid-level negative charge layer amidst two positive charges regions) while a normal dipole (a positive charge region atop a negative charge region at mid-levels) was observed in the outer eyewall stratiform region and in the strongest cells forming the outer rainbands. The charges forming the normal dipole in the outer eyewall were generated within the eyewall via NI charging in the mixed-phase region at mid-levels (near the -15° C isotherm).In summary, despite producing quantitatively different results, the qualitative aspects of the simulated squall line dynamics, microphysics and lightning were overall similar. This suggested that the hurricane simulations presented in this study could still provide a good and useful qualitative insight of the storm's dynamical, microphysical and electrical properties.The simulated tropical squall line exhibited many features consistent with observations. In particular, the updraft speeds were generally much weaker than their continental counterparts, which was in turn consistent with relatively shallow 30 dBZ echo tops and lower content of graupel and supercooled water droplets within the mixed phase layer. This general reduction of graupel and supercooled water was partly caused by a rapid depletion of liquid water by enhanced warm rain processes ahead of the line, in agreement with previous studies. The stratiform region was almost exclusively composed of light ice crystals and snow aggregates, with discrete regions, however, containing small amounts of graupel (∼ 0.1-0.3 g kg-1) All of these factors combined resulted in a system producing overall little lightning.In the LAND TC experiment the landfalling storm was, as expected, much weaker (higher surface pressure, weaker winds) and less organized than the storm evolving over ocean. The weaker landfalling storm was associated with smaller eyewall total updraft mass flux and shallower echo tops (particularly 30 dBZ and greater) in turn consistent with smaller total graupel volume aloft and an overall smaller total lightning activity. Perhaps the most interesting finding of the LAND experiment was that several +CG flashes were produced after landfall, which was not observed in the control simulation over ocean. This indicated that, as suggested by observations, there exists a qualitative difference in the storm electrical behavior after landfall, which as we showed, was directly linked to its change in kinematical and microphysical fields. Observational studies, however, showed that this difference in lightning behavior over land versus over ocean varied from case to case, and therefore could not be generalized. (Abstract shortened by UMI.)Before carrying out these experiments, however, it was necessary to test the reliability of the model in maritime tropical environment. For this purpose, an additional idealized high-resolution simulation of a well-documented TOGA COARE squall line case was carried out. Moreover, for a single microphysical and electrical evolution, the latter experiment was carried out at three additional horizontal grid spacings to determine how the storm's dynamical, microphysical and electrical properties responded to these changes

Relationships between kinematics, microphysics, and lightning in high plains storms observed during the severe thunderstorm electrification and precipitation study

Summer 2006.Includes bibliographical references (pages 168-169).The Severe Thunderstorm Electrification and Precipitation Study (STEPS) was established to improve our understanding of electrification mechanisms and lightning in High Plains storms. In particular, STEPS focused on investigating anomalous positive cloud-to-ground (CG) lightning, which had been documented to occur more often in this region than in the rest of the U.S. Radar and lightning observations of four storms observed during the STEPS field campaign are analyzed and discussed. The four cases include a predominantly positive CG-producing (PPCG) supercell on 29 June, a supercell on 3 June that produced no CG lightning of either polarity, a negative CG-producing multicellular storm on 19 June, and a PPCG multicellular storm on 22 June. Data from multiple Doppler radars have been synthesized to calculate the three-dimensional wind field, polarimetric radar variables have been combined with thermodynamic soundings to estimate hydrometeor types throughout the echo volumes, and Lightning Mapping Array (LMA) data have been sorted into flashes and studied to determine the flash rates and charge structure for several hours of each storm's lifetime. The purpose of this study is to determine what features are unique for storms that produce predominantly positive CG lightning, and attempt to reveal the processes that lead to this behavior. The 29 June supercell produced large amounts of hail and frequent positive CG lightning, as well as exhibited a large volume of strong (> 10 m S-1) updraft and a deep region of cyclonic vertical vorticity. The charge structure of the 29 June supercell was inverted, with a main region of positive charge centered near 8 km MSL with a negative charge layer above and below. The inferred charge structure in the 3 June case consisted of an inverted dipole, with positive charge beneath upper-level negative charge. A lower negative charge layer was not detected in 3 June and may have been the reason for the lack of CG lightning. This case produced some hail, but not as much hail volume as 29 June, and the updraft volume and cyclonic vertical vorticity were dramatically lower as well. The 19 June multicellular storm exhibited a normal charge structure, with main negative charge centered at 7 km MSL, and positive charge layers above and below, and therefore produced mostly negative CG lightning. The storm produced negligible hail, and had very weak and shallow updrafts, yielding near zero values of strong updraft volume. The 22 June multicellular storm exhibited both inverted and normal charge structures in different regions of the storm complex. The volume of strong updraft was very high, similar to that of 29 June, and the storm produced ample amounts of hail. Both positive and negative CG lightning was observed in this storm complex, but the majority of the CG lightning was of positive polarity. The results indicate that PPCG storms tend to have larger updrafts (both wider and larger in volume), which is consistent with previous studies. Large updrafts and enhanced vertical vorticity also play an important role in the production of large hail. Furthermore, low-level negative charge (below a larger region of positive charge) was observed in the cases that produced positive CG lightning, which may be the impetus needed for the flash to come to ground. This lower negative charge, in essence, represents the lowest charge layer of an inverted tripolar charge structure. The charge structures observed during the production of negative CG lightning were a normal tripole (with negative charge situated between upper and lower positive charge layers) on 19June and an inverted dipole (with negative charge above positive) in the anvil on 22 June. Cloud-to-ground flash rates (of either polarity) decreased when either the lower charge layer of the corresponding tripolar structure was absent, or when the low-level charge layer exhibited an enhanced number of LMA sources, in which case intra-cloud (IC) discharges seemed to be preferred between the two lowest charge layers of the tripole

Polarimetric Radar Convective Cell Tracking Reveals Large Sensitivity of Cloud Precipitation and Electrification Properties to CCN

Hypotheses have been proposed for decades about cloud condensation nuclei (CCN) aerosol effect on delaying the warm rain process, invigorating deep convective cloud vertical development, and enhancing mixed-phase process. Observational support has been only qualitative with mixed results due to lack of regional measurements of CCN, while simulations have not produced a robust consensus. Quantitative assessment of these relationships became possible with the advent of CCN retrievals from satellites; when combined with measurements by polarimetric radar and Lightning Mapping Array (LMA), tracking convective cells observed on radar and examining precipitation properties throughout the cells’ life cycle has permitted the study of the impact of CCNs on cloud and precipitation characteristics. We composited more than 2000 well-tracked cells in the Houston region and stratified them by CCN, convective available potential energy (CAPE) and urban/rural locations. The analyzed cell properties include reflectivity (Z), differential reflectivity (ZDR) and LMA data. The results show that added CCN to deep convective clouds delays the initiation of precipitation by up to 20 minutes. Added CCN invigorate the convection until saturation near CCN = 1000 cm^-3; increasing CCN from ~400 to an optimum of ~1000 cm^-3 increases lightning activity by an order of magnitude. A further increase of CCN decreases lightning rates. Adding CAPE enhances lightning only under low CCN <500 cm^-3. Urban area enhances lightning for the same CCN only under low CCN conditions. Urban heat island cannot explain this observation. In summary, CAPE is essential for the initiation of deep convection. It has been believed that CAPE and lightning are positively related. This is indeed the case when CAPE is low. But when CAPE is high, which means that deep convection is already in progress, aerosols dominate the lightning activity. These insights lead to refinement of the physical hypotheses which provide impetus for a field campaign in the Houston area

A Modeling Study on the Sensitivities of Atmospheric Charge Separation According to the Relative Diffusional Growth Rate Theory to Nonspherical Hydrometeors and Cloud Microphysics

Collisional charge transfer between graupel and ice crystals in the presence of cloud droplets is considered the dominant mechanism for charge separation in thunderclouds. According to the relative diffusional growth rate (RDGR) theory, the hydrometeor with the faster diffusional radius growth is charged positively in such collisions. We explore sensitivities of the RDGR theory to nonspherical hydrometeors and six parameters (pressure, temperature, liquid water content, sizes of ice crystals, graupel, and cloud droplets). Idealized simulations of a thundercloud with two‐moment cloud microphysics provide a realistic sampling of the parameter space. Nonsphericity and anisotropic diffusional growth strongly control the extent of positive graupel charging. We suggest a tuning parameter to account for anisotropic effects not represented in bulk microphysics schemes. In a susceptibility analysis that uses automated differentiation, we identify ice crystal size as most important RDGR parameter, followed by graupel size. Simulated average ice crystal size varies with temperature due to ice multiplication and heterogeneous freezing of droplets. Cloud microphysics and ice crystal size thus indirectly determine the structure of charge reversal lines in the traditional temperature‐water‐content representation. Accounting for the variability of ice crystal size and potentially habit with temperature may help to explain laboratory results and seems crucial for RDGR parameterizations in numerical models. We find that the contribution of local water vapor from evaporating rime droplets to diffusional graupel growth is only important for high effective water content. In this regime, droplet size and pressure are the dominant RDGR parameters. Otherwise, the effect of local graupel growth is masked by small ice crystal sizes that result from ice multiplication

Atmospheric Conditions Associated with Lightning during Snow and Ice Events

The purpose of this research was to find the atmospheric mechanisms associated with lightning in snow and ice events. The specific mechanisms that were examined were low-level wind shear, upper level divergence, surface temperature, low-level temperature, the -10 ° C level, and precipitable water. A chi-squared dependency test showed the strong association of low-level wind shear to each precipitation type (snow, sleet/freezing rain, rain) in two separate studies. Surface temperature appeared to have a relationship to lightning in all precipitation categories, while no significant relationship to lightning in all precipitation categories, while no significant relationship was found with upper level divergence, the -10 ° C level, or the precipitable water. From examination of the vertical soundings, temperatures above freezing are found in the low levels for all precipitation types meaning that different types of hydrometeors are present in the clouds. The mixing of these due to the turbulent effects of low-level shear may explain how the thunderclouds (mostly stratiform) are charged. Graupel and snow pellet interaction are also believed to be mechanisms for cloud charging

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

The electrification of stratiform anvils

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997.Includes bibliographical references (p. 225-234).by Dennis J. Boccippio.Ph.D