Electrical Behavior of Downburst-Producing Convective Storms over the High Plains (original) (raw)
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Electrical Behavior of Downburst-Producing Convective Storms over the Western United States
A great body of research literature pertaining to microburst generation in convective storms has focused on thermodynamic factors of the pre-convective environment as well as storm morphology as observed by radar imagery. Derived products based on GOES sounder data have been found to be especially useful in the study of thermodynamic environments. However, addressed much less frequently is the relationship between convective storm electrification, lightning phenomenology and downburst generation. Previous research in lightning production by convective storms has identified that electrification, phenomenology (i.e. flash rate, density), and polarity are dependent upon the thermodynamic structure of the ambient atmosphere, especially vertical moisture stratification. Thus, relevant parameters to describe the thermodynamic setting would include convective available potential energy (CAPE), due to its influence on updraft strength, and cloud liquid water content, due to its relationship...
Microphysical and electrical evolution of a Florida thunderstorm: 1. Observations
Journal of Geophysical Research, 1996
This study deals with the microphysical and electrical evolution of a thunderstorm that occurred on August 9, 1991, during the Convection and Precipitationf Electrification (CAPE) Experiment in eastern Florida. During its approximately 1-hour lifetime, the storm was penetrated several times by the Institute of Atmospheric Sciences' T-28 aircraft at midlevels. It was also penetrated at low and middle-levels by a National Oceanographic and Atmospheric Administration (NOAA) P-3 and scanned by three radars, one of which had multiparameter capabilities, operated by the National Center for Atmospheric Research. Two stages of the storm's evolution are analyzed herein during which the storm grew to produce precipitation and lightning. The first stage, sampled during the first T-28 penetration at 5.25 km (-3øC) and the P-3 at 6.4 km (-10øC), was characterized by a 2-to 3-kin wide updraft (maximum 14 m s '•) with cloud liquid water contents up to 4 g m -3, low concentrations of graupel at -10øC, and small to medium raindrops in concentrations of less than 200 m -3 at-3øC. A downdraft region also existed that was devoid of cloud liquid water, but contained graupel up to 2 mm. Radar data (Zr)•) are consistent with a coalescence-dominated precipitation generation mechanism followed by transport of drops in the updraft to heights with temperatures colder than -7øC, where freezing formed graupel that continued to grow by riming. Electrification during this stage remained weak. The second stage, sampled during the second and third T-28 penetrations and the second P-3 penetration, was characterized at midlevels by a narrower updraft and a more diffuse, broad downdraft separated by a 1-to 2-km wide transition zone. The updraft continued to show significant cloud liquid water (•2 g m -3) with few precipitation particles, while the downdraft had very little cloud liquid with graupel in concentrations >1 e -•. The transition zone shared both updraft and downdraft characteristics. The increase in ice concentration was accompanied by a rapid increase in the electrification of the cloud with peak electric fields reaching-20 kV m -• at T-28 altitude and the detection of lightning by ground-based sensors and pilot report. As time progressed, precipitation particle concentrations reached several per liter at midlevels in both updrafts and downdrafts. The observations are consistent with electrification through a precipitation-based mechanism involving the development of the ice phase. um'esolved. 1978a; Jayaratne et al., 1983' Saunders et al., 1991] that charge
Recent Observations of Thunderstorm Electrification on the High Plains
2001
Coordinated observations from meteorological radars, lightning mapping systems, airborne and balloon-borne instruments, and a mobile meso-network of instrumented automobiles were obtained during the spring-summer of 2000 in the eastern Colorado/western Kansas High Plains during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). Several severe storms, as well as many non-severe storms, were studied during the 8-week project. The focus of the project was to correlate electrical behavior with other storm characteristics, such as storm microphysics, updraft intensity, and size. Most thunderstorms produce predominantly cloud-to-ground lightning that lowers negative charge to ground, with only a small percentage of lightning events lowering positive charge. It has been observed that many severe storms in the study region produce anomalously high percentages of positive lightning. During our field study, we observed both severe and non-severe storms producing predominantly positive cloud-to-ground lightning, so it appears that predominant polarity may depend on other factors in addition to storm severity.
Monthly Weather Review, 1997
The relationships among kinematic, microphysical, and electric field properties within a multicell Florida thunderstorm are investigated using observations from three Doppler radars (one with multiple wavelength and polarization diversity capabilities), four instrumented penetrating aircraft, a surface-based electric field mill network, and other observation facilities. The storm was convectively active for about 1 h and at least five primary cells developed within the storm during this time, one of which went through three consecutive development cycles. The updrafts in this storm were 2-4 km wide, exhibited bubble-like evolution, and had lifetimes of 10-20 min. The maximum updraft determined by the multiple Doppler analysis was about 20 m s Ϫ1 . A differential reflectivity (Z DR ) ''column,'' indicating regions containing millimeter-size raindrops, extending above the freezing level, was associated with each cell during its developing stages. This column reached altitudes exceeding 6 km (Ϫ8ЊC) in the stronger updrafts. As the Z DR columns reached maximum altitude, a ''cap'' of enhanced linear depolarization ratio (LDR) and enhanced 3-cm wavelength attenuation (A 3 ) formed, overlapping the upper regions of the Z DR column. These parameters indicate rapid development of mixed-phase conditions initiated by freezing of supercooled raindrops.
Atmospheric Research, 2005
Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the onedimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms. Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively,
Journal of Geophysical Research, 1996
Precipitation development and electrification in Florida thunderstorms are observed using an instrumented aircraft and a multiparameter radar. A low concentration of raindrops initially develops in the updraft, and these raindrops begin to freeze when they are carried above the 0øC level. High concentrations of ice particles and downdrafts soon appear in the -5 ø to -10øC regions of the cloud, where the aircraft penetrated, as do electric fields in the range of tens of kilovolts per meter. In a cell with relatively weak updrafts, drops start to freeze at temperatures just below 0øC. Although significant electric fields are measured by the aircraft, no lightning is observed in this cell. In more vigorous cells, drops first begin to freeze at temperatures between -5øC and -10øC. The electric fields measured by the aircraft in these cells are similar in magnitude to those in the weaker cell, but lightning is observed in these more vigorous cells. The net charge in convective regions at altitudes just above the aircraft penetration levels, 6-7 km, appears to be negative. relaxation ti•nes, as discussed by Gross [19821. I lowever, Canosa et al. [1993] contend, based on laborator)' work by Canosa Paper number 95JD02931. 0148-0227/96/95 JD-02931 $05.00 and List [1993], that in light rain the inductive process due to drop-drop collisions might result in weak electrification. Airborne particle-charge observations of Gaskell ctal. [19781, Christian et al. [1980], and ^Iarshall and Wren [19821 have detected charges on millimeter graupel particles that were too large to be accounted for by the inductive process. However, laboratory studies such as those of,,tttfitermaur amt .Johnson 11972], Gaskell [1981], and Brooks and Saunders [1994] have shown that inductive charge exchange between conducting spheres representing ice and water drops does occur, in agreement with theory. The charge transfer values are small for moderate electric fields, indicating that this process would only be contributory in the later stages of thunderstorm electrical development. Noninductive charge transfer involves the exchange of charge between interacting ice particles in the presence of cloud liquid water. It has been demonstrated in the laboratory. by Reynolds et al. [1957], Takahashi [ 1978al, Gaskell and Illingwor& [ 1980], Jayaratne et al. [1983], Saunders et al. [1985], Keith and Saunders [1989, 1990], and Saunders et al. [1991 ]. It is generally accepted that this process is operative in clouds. Questions rmnain about the exact physical mechanism responsible tbr charge exchange at the microscopic level, the magnitude of charge exchange, and the sign of charge exchange as a fimction of temperature, cloud water content, and growth state of the riming particle [cf. Saunders and Brooks, 1992; Williams and Zhang, 1993; Saunders, 1994; Williams et al., 1994]. In recent years, several field investigations have involved both remote and in situ measurements geared toward characterizing the microphysical, kinematic, and electrical state of convective clouds in an attempt to shed light on this question of charge separation. The investigators provided observations that can be used to constrain the conditions under which electrification commonly occurs. It is noteworthy that of the several investiga-1599 1600 RAMACHANDRAN ET AL.: PRECIPITATION/F. LECTRIFICATION tions that have been carried out, relatively few have studied sub-a great many of these investigations and conchides that the evitropical storms such as occur along the east coast of Florida. Of dence supports the noninductive riming mechanism as a "fundathose that have, the majority have involved the analysis of radar mental ingredient in the charge separation process." and ground-based electric field data to infer the interior electrical structure of the clouds [e.g., dacobson and Krider, 1976; Krehbiel, 1981; Lhermitte and Williams, 1985; Krehbiel, 1986; Maier and Krider, 1986; Krider, 1989; M.d. glurphy et al., Lightning charge analyses during small CaPE storms, submitted to dournal of Geophysical Research, 1995]. The restilts of these studies have characterized the electrical structure of these storms as basically tripolar in nature (upper positive/main negative/ lower positive) without elucidating the accompanying microphysics to aid in the interpretation of charging mechanisms. A few studies have provided in situ electrical and microphysical characterizations of subtropical and tropical clouds. Takahashi [1978b] reported on in situ, large-hydrometeor charge measurements from two mariti•ne thunderstorms near Ponape, Micronesia, that showed varying degrees of electrification, based on lightning production. In one storm that was smnpled via balloon prior to the initiation of lightning, he found charges of both signs at levels from 0 ø to -60øC with a slight excess of positive charge at the higher temperatures. Negative charges dominated at lower temperatures, reaching a maximum around the -40øC level. Another storm was already producing lighming at the ti•ne of a second balloon ascent. In this storm, Takahashi found a more distinct tripolar structure with net positive space charge below the melting level, net negative charge peaking at around -20øC, and net positive charge at still lower temperatures. He inferred that the upper positive charge was carried by ice crystals and the main negative charge was carried on graupel. He interpreted his findings to be consistent with the noninductive timing mechanism.
A REMOTE MICROPHYSICAL STUDY OF SEVERE WIND-PRODUCING CONVECTIVE STORMS
Convective storms that generate hail, lightning, and damaging winds have been identified as a formidable hazard to life and property. Even more impactful are stronger storms that generate and loft liquid-phase hydrometeors to high altitudes where freezing occurs and collisions between drops, graupel, and ice crystals lead to electrification. Condensate loading, sometimes combined with the lateral entrainment of subsaturated air in the storm middle level, initiates the convective downdraft. The subsequent melting of frozen hydrometeors and subcloud evaporation of liquid precipitation, in conjunction with precipitation loading, result in the cooling and negative buoyancy that accelerate the downdraft in the unsaturated layer. A downburst, in general, is defined as a strong downdraft that induces an outburst of damaging winds at or near the ground, and a microburst as a very small downburst with an outflow diameter of less than 4 km and a lifetime of less than 5 minutes. Previous studies of the microphysical structure of downburst-producing convective storms have entailed analysis of polar and geostationary satellite imagery and derived products, meteorological Doppler radar, and in-situ surface wind observations. The current study expands upon previous analysis by incorporating lower tropospheric vertical wind and temperature profile data generated by the Cooperative Agency Profilers (CAP) system that consists of Boundary Layer Profiler (BLP) instruments, operating at a frequency of 915 MHz, and Radio Acoustic Sounding System (RASS) instruments. In addition, Geostationary Lightning Mapper (GLM) data from Geostationary Operational Environmental Satellite (GOES)-16 will also be displayed and analyzed to better explain the role of lightning in a downburst-producing convective storm. Selected thunderstorm events that demonstrate the physical process of downburst generation as observed simultaneously by the GOES-16 Advanced Baseline Imager (ABI) and GLM, Doppler radar (NEXRAD), and boundary layer profilers will be analyzed in this paper. Vertical sounding profile data from the CAP system has been applied as a supplement to Sounding/Hodograph Analysis and Research Program in Python (SHARPpy)-generated thermodynamic profiles to further study the favorable environment for severe convective storm winds. On the afternoon of 1 August 2017, severe downburst-producing thunderstorms occurred in the United States Mid-Atlantic region and over southern California that resulted in tree damage, downed power lines, and traffic disruptions. As shown in Figure 1, for both of these events, GOES-16 ABI water vapor (WV) – thermal infrared (IR) channel brightness temperature difference (BTD) imagery at 2-km resolution displayed a high level of detail in storm structure and was effective in identifying storm-scale features, including cold cloud tops (red shading) and dry-air intrusions (white arrows). Corresponding GLM imagery displayed lightning events in close proximity to downburst events at the time of downburst occurrence.
Field identification of a unique globally dominant mechanism of thunderstorm electrification
Quarterly Journal of the Royal Meteorological Society, 2007
Two wholly distinct studies involving TRMM-satellite global data were conducted. One involved the relationship between lightning frequency f and brightness temperature, the other between f and ice-water-path. Both studies demonstrate that globally valid relationships exist between f and thundercloud ice-precipitation content, from which it follows that graupel pellets play a crucial role in thundercloud charging. Ground-based field studies provide further support for this conclusion and show that f is also strongly dependent upon the ice crystal content. All these findings are consistent with the non-inductive charging mechanism, but not with any other proposed mechanism of thunderstorm electrification. We conclude that the non-inductive mechanism dominates electric field growth and lightning production in all seasons-for both oceanic and terrestrial thunderstorms-on a global scale.
Satellite-based Very High Frequency (VHF) lightning sensors are limited in sensitivity to a particular (but particularly powerful) in-cloud radio frequency (RF) pulse. Hence, the viability of global storm tracking based on satellite-based VHF sensors largely depends on whether these strong in-cloud pulses can be used as robust and generic indicators of convective strength. The RF pulses observed by satellite are sometimes accompanied by distinctive narrow electric field change pulses (called Narrow Bipolar Events or NBEs) observed by ground-based Very Low Frequency sensors such as the Los Alamos Sferic Array (LASA). A recent preliminary study compared LASA NBE flash rates to cloud-to-ground (CG) flash rates in a small geographical area in central and northern Florida. This study used CG flash rates as a proxy for convective strength, and found that NBE flash rates are proportional to CG flash rates, and, by extension, to convective strength. Here, we extend the earlier studies, usin...
Quarterly Journal of the Royal Meteorological Society, 2003
This paper presents a study about correlation between cloud-to-ground lightning ash (CG) activity, and the dynamics and microphysics of thunderclouds. Data collected during Intense Observation Period (MAP IOP) 2a of the Mesoscale Alpine Programme Special Observing Period (SOP) over the Lago Maggiore Target Area (LMTA) in northern Italy are used. IOP 2a was the most electrically active period during the SOP, representing 75% of the total CG activity. Thunderclouds were strongly vertically developed (the 30 dBZ echo tops were sometimes higher than 12 km) and produced large amounts of rainfall and some hail. Doppler and polarimetric radar data allow us to retrieve the three-dimensional wind and radar-re ectivity elds from two synchronous Doppler radars, and particle-type elds in the thunderclouds from the S-Pol polarimetric radar. Both polarities of CGs are distinguished. Temporal and spatial relationships from the global activity over the LMTA are rst studied. Then, the temporal correlation between CG rates and dynamics or microphysics for ve individual cells is considered. A very strong correlation is observed between CG and the presence of a mixture of graupel and hail, which strongly supports a non-inductive charging mechanism. CG impacts seem to be located underneath large radar-re ectivity values and around the maximum vertical velocities. Two individual cells are studied in more detail, in order to better understand their different electrical behaviours: the rst produced mainly negative CGs, and the second produced 61% positive CGs in the second phase of its lifetime. The association of positive CGs with severe weather and especially with the presence of hail is observed.