Showing papers on "Thunderstorm published in 1996"

Physical Processes Involved in the 1988 Drought and 1993 Floods in North America

Abstract: An analysis of the spring–summer 1988 drought and 1993 floods over North America reveals a reversal in the sign of anomalies in several fields. Large sea surface temperature anomalies of opposite signs existed in the tropical Pacific with strong La Nina conditions in 1988 and a mature El Nino in 1993. The distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered, implying a large-scale switch in the anomalous tropical heating and forcing of extratropical quasi-stationary waves in the atmosphere, influencing the subtropical jet stream over the North Pacific and across North America. In 1988 the jet stream and the closely related storm track of high-frequency disturbances in the upper troposphere were displaced into Canada well north of the normal location—the farthest north of any year from 1979 to 1993. In 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location a...

Elves : Lightning-induced transient luminous events in the lower ionosphere

Abstract: Observations of optical phenomena at. high alti- tude a, bove thunderstorms using a multichannel high-speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Field Station (40040 ' N, 104o.56 ' W), Colorado as part of the SPRITES'95 campaign from 15 June to August 6, 1995. These newneasurements indicate that diffuse optical flashes with a duration of < I ms and a hori- zontal scale of-.- 100-300 km occur at 75-105 km altitude in the lower ionosphere just after the onset of cloud-to-ground lightning discharges, but preceding the onset of sprites. Here we designate these events as 'alves" to distinguish them from 'i'ed sprites" . This finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges as suggested by several authors.

A multiparameter radar case study of the microphysical and kinematic evolution of a lightning producing storm

Abstract: We examine the co-evolving microphysical, kinematic, and electrical characteristics of a multi-cell thunderstorm observed on 21 May 1993 along the Front Range of Colorado using data collected with the 11 cm, multiparameter, CSU-CHILL Doppler radar. The measured polarimetric variables provide information on the size, shape, orientation, and thermodynamic phase of hydrometeors. Recent modeling and observational advances in weather radar polarimetry now permit the inference of bulk-hydrometeor types and mixing ratios, and the measurement of precipitation rate in mixed-phase (i.e., hail and rain) environments. We have employed these and other radar techniques, such as dual-Doppler analyses, to investigate the correlation between the convective life cycle of a multi-cell storm and the evolution of lightning type and flash rate.

Stratosphere‐troposphere exchange in a midlatitude mesoscale convective complex: 1. Observations

Abstract: On June 28, 1989, a severe thunderstorm over North Dakota developed into a squall line and then into a mesoscale convective complex (MCC) with overshooting tops as high as ∼14 km and a cirrus anvil that covered more than 300,000 km2. In this paper we describe the trace gas concentrations prior to, in, and around the storm; paper 2 presents numerical simulations. Observations of O3 and θeq unaffected by upstream convection for at least 3 days prior to the flights placed the undisturbed tropopause between 10.7 and 11 km. The anvil outflow, sampled at altitudes of 10.8 to 12.2 km, extended well into what used to be the stratosphere. Air inside the anvil was characterized by notably low concentrations of O3 and high CO relative to the out-of-cloud environment. Elevated concentrations of NO and NOy, due to lightning and upward transport, were observed in the anvil. A tongue of air with tropospheric characteristics lay above stratospheric air, showing that extensive stratosphere-troposphere exchange had occurred. The effects of this mechanism on atmospheric budgets of trace species depend on the fate of the air that enters the anvil and on the frequency of MCCs. Assuming that the symmetry was cylindrical and that the material transported during the observations at the east edge of the anvil was representative of the entire cirrus anvil cloud, we estimate a minimum flux of 2 × 1010 g of O3 into the troposphere and a maximum flux of 3–7 × 1013 g of H2O into the stratosphere. This is a greater flux of water than the stratospheric water budget can support, and thus most of this water must return to the troposphere; the ice crystals were of sufficient size to have substantial settling velocity. If, however, even a small fraction of the mass of such anvils remains in the stratosphere, then convective transport of reactive tropospheric trace species such as NOy, CO, and NMHC may dominate the chemistry of the lower stratosphere in this midlatitude region. More detailed estimates of the fluxes, taking into account the rear anvil as well, are presented in the companion paper.

Initial results from simultaneous observation of X‐rays and electric fields in a thunderstorm

Abstract: With an X ray detector designed for flight on a free balloon, we obtained a sounding of X ray intensity and electric-field strength in a mesoscale convective system (MCS) near Norman, Oklahoma, in the spring of 1995. The balloon passed through a region of high electric field strength, at which time an increase in X ray intensity of about 2 orders of magnitude occurred, lasting for approximately 1 min. The X ray intensity returned to background levels at the time of a lightning flash that reduced the electric field strength measured at the balloon. This observation suggests that the production mechanism for the X rays we observed is related to the storm electric field and not necessarily to lightning discharge processes.

Structure and Evolution of the 22 February 1993 TOGA COARE Squall Line: Numerical Simulations

Abstract: In this study a numerical cloud model is used to simulate the three-dimensional evolution of an oceanic tropical squall line observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment and investigate the impact of small-scale physical processes including surface fluxes and ice microphysics on its structure and evolution. The observed squall line was oriented perpendicular to a moderately strong low-level jet. Salient features that are replicated by the model include an upshear-tilted leading convective region with multiple updraft maxima during its linear stage and the development of a 30-km scale midlevel vortex and associated transition of the line to a pronounced bow-shaped structure. In this modeling approach, only surface flukes and stresses that differ from those of the undisturbed environment are included. This precludes an unrealistically large modification to the idealized quasi-steady base state and thus allows us to more easily isolate effects of ...

On the production of active nitrogen by thunderstorms over New Mexico

Abstract: In July and August of 1989 the National Center for Atmospheric Research (NCAR) Sabreliner jet aircraft was used to probe electrically active and inactive convective storms over west central New Mexico to examine the production of odd nitrogen in the middle and upper troposphere by thunderstorms. In the anvil outflow or cloud top region of active and nonactive storms, the majority of flights showed that O 3 was reduced relative to the extracloud air owing to transport of ozone-poor air from lower altitudes. A similar result was found for active nitrogen (NO x ) and total odd nitrogen (NO y ) in nonelectrically active storms, but the reduction in NO y was also enhanced by removal of soluble constituents during convective transport. Examples of efficient removal from the gas phase are described. There was no evidence of O 3 production by lightning discharges. Indeed, O 3 was a good tracer over the lifetime (∼1 hour) of the storms. During the active-to-mature stage of air mass thunderstorms, large enhancements in active nitrogen were observed in the anvil altitude region (9-11.8 km) and, in one case, in the midlevel outflow (near 7 km) of a dissipating thunderstorm. Two thunderstorms allow good estimates of the NO x production by lightning within or transport to the upper altitude region (8-11.8 km). Thunderstorms of August 12 and August 19 yield amounts in the range of 253-296 kg(N) and 263-305 kg(N), respectively. If, as an exercise, these amounts are extrapolated to the global scale on the basis of the number of cloud-to-ground and intracloud lightning flashes counted or estimated for each storm and a global flash frequency of 100 s -1 the result is 2.4-2.7 and 2.0-2.2 Tg(N)/yr. Alternatively, an estimate for the two storms made on the basis of the average number of thunderstorms that occur per day globally (44,000) yields amounts in the range of 4.1-4.7 and 4.2-4.9 Tg(N)/yr, respectively. These estimates only apply to the production or transport of lightning generated NO x in or to the altitude region between 8 km and the top of the thunderstorm anvil (∼11.8 km in these studies). Since in some large-scale models, lightning-generated NO x is equally distributed by mass into each tropospheric layer, our estimates are roughly equivalent to those model runs that use a global source strength of about twice our estimate for the upper altitude region. In several flights where the region below the base of thunderstorms was examined, no large enhancements in odd nitrogen which could be clearly attributed to lightning were observed. Apparently, the aircraft was not in the right place at the right time. Thus no estimate of the NO x production by lightning that remains below ∼8 km could be made.

Stratosphere‐troposphere exchange in a midlatitude mesoscale convective complex: 2. Numerical simulations

Abstract: Mixing across the tropopause due to intense convective events may significantly influence the atmospheric chemical balance. Stratosphere-troposphere exchange acts as an important natural source of O3 in the troposphere, and a source of H2O, HCs, CFCs, HCFCs, and reactive nitrogen in the stratosphere. The redistribution of atmospheric trace gases produces secondary radiative, dynamical and climate effects, influencing lower stratospheric temperatures and the tropopause height. During the 1989 North Dakota Thunderstorm Project, a severe storm which evolved into a mesoscale convective complex (MCC) on June 28–29 showed the unusual feature of an anvil formed well within the stratosphere and produced strong vertical mixing of atmospheric trace gases including H2O, CO, O3 and NOy as discussed by Poulida et al. [this issue] in Part 1 of this paper. In this paper the two-dimensional NASA Goddard Cumulus Ensemble (GCE) model was employed to simulate this convective storm using observed initial and boundary conditions. The sensitivity to the domain size, initial and boundary conditions, stability, and time resolution are evaluated. Synoptic-scale moisture convergence, simulated by moist boundary inflow, influences significantly the storm intensity, spatial structure, and trace gas transport, and produces a storm that reintensifies after the initial decay, mimicking the observed behavior of the MCC. The deformation of the tropopause documented with aircraft observations was qualitatively reproduced along with transport of stratospheric ozone downward into the troposphere, and the transport of trace species from the boundary layer upward into the stratosphere. If the chemistry and dynamics of this storm are typical of the roughly 100 MCCs occurring annually over midlatitudes, then this mechanism plays an important role in CO, NOy, and O3 budgets and could be the dominant source of H2O in the lower stratosphere and upper troposphere over midlatitudes.

The atmosphere is

Abstract: The study of atmospheric electricity, as a science, has been involved with the electrical behaviour of the lower atmosphere for over two centuries. In particular, phenomena like the influence of fair weather on the sign and strength of the earth’s electric field and the behaviour of thunderstorms were studied in depth. However, the electrical behaviour of the atmosphere when confronted with different pollutants, like pollen, spores, fog, combustion products, etc., was often left aside as being too complex.

Storm-Relative Winds and Helicity in the Tornadic Thunderstorm Environment

Abstract: Environmental flow relative to tornado-producing thunderstorms is examined through the use of the large tornado proximity sounding dataset compiled at the University of Missouri. It is believed that the 184 soundings gleaned from this collection represent the largest, most restrictive database of its kind with observed storm velocities, as determined from microfilm of conventional National Weather Service radar. Using these storm velocities, mean storm-relative wind profiles were derived for the entire data sample and sample subsets based on tornadic intensity, strength of the mean environmental flow, magnitude of CAPE, and direction of storm motion with respect to the mean environmental wind vector. Although it is apparent that a number of tornadoes occur independent of the larger-scale flow, the mean storm-relative wind profiles suggest that there is a preferred storm-relative flow structure for tornadic thunderstorms. Tornadic intensity in association with this structure appears to strengthen as 1 ) the magnitude of storm-relative helicity grows through an increasingly deep layer of the lower through midtroposphere and 2 ) mid- and upper-level storm-relative winds strengthen while possessing decreasing directional variability at their respective heights above ground level (4‐1 2 km AGL ).

The Vaison-La-Romaine Flash Flood: Mesoscale Analysis and Predictability Issues

Abstract: On the morning and early afternoon of 22 September 1992, a flash flood (220 mm of rain in 3 h) occurred in the city of Vaison-La-Romaine, located in southeastern France, causing numerous casualties and considerable property damage. It was generated by a combination of several mesoscale convective systems ahead of a slow-moving cold front associated with a cutoff low. The large-scale setting and a mesoscale analysis of the case, together with estimates of radar-derived rain accumulations, are presented. The mesoscale analysis demonstrates the complexity of the case, which involved five precipitation systems. Orographic influences generated a cold pool and focused convective cell development in a confined area, enabling two precipitation systems to be quasi-stationary. Precipitation forecasts by different versions of two models (a fine mesh version of an operational limited area model, and an operational stretched global model) are summarized. They demonstrate the extent to which realistic rainfall...

Evidence for continuing current in sprite‐producing cloud‐to‐ground lightning

Abstract: Radio atmospherics launched by sprite-producing positive cloud-to-ground lightning flashes and observed at Palmer Station, Antarctica, exhibit large ELF slow tails following the initial VLF portion, indicating the presence of continuing currents in the source lightning flashes One-to-one correlation of sferics with NLDN lightning data in both time and arrival azimuth, measured with an accuracy of ±1° at ∼12,000 km range, allows unambiguous identification of lightning flashes originating in the storm of interest Slow-tail measurements at Palmer can potentially be used to measure continuing currents in lightning flashes over nearly half of the Earth's surface

Multiple-Parameter Radar Observations of Isolated Florida Thunderstorms during the Onset of Electrification

Abstract: A prime objective of the Convection and Precipitation/Electrification experiment was to Study the electrification of Florida thunderstorms in greater depth using the National Aeronautics and Space Administration/United States Air Force electric field mill network; a small fleet of aircraft; the dual-frequency, dual-polarization CP-2 radar of the National Center for Atmospheric Research (NCAR), and other radars including the NCAR Doppler CP-3 and CP-4. Analyses of three small, isolated storms on 19 July 1991 suggest that the onset of electrification coincides with the appearance of significant volumes of differential reflectivity, indicative of liquid raindrops larger than 2-mm diameter, above the −7°C level accompanied by the nearly simultaneous appearance of significant depolarization, initially associated with the freezing of these drops. Although the relationship of the onset of electrification to various aspects of the radar reflectivity factor Z are more ambiguous, a rapid increase in the al...

Multispectral, High-Resolution Satellite Observations of Plumes on Top of Convective Storms

Abstract: Multispectral, high-resolution imagery from the Advanced Very High Resolution Radiometer of NOAA polar orbiting satellites is used to analyze the cloud-top structure of convective storms that develop a cirrus feature above the anvil, referred to as a plume, whose origin remains unclear. Images from the radiometer's channels 2, 3, and 4 and a combination of any two of these suggest a relationship between the emergence of such plumes and a source of small ice particles (diameter around 3.7 µm, channel 3 wavelength) at the cloud top. Unique observations of deep convective storms over Europe are presented and discussed. The paper does not provide an exhaustive explanation of the phenomenon but contributes original material to the study of convective storm cloud-top structure, which is far from being completely described.

Time-varying ice crystal orientation in thunderstorms observed with multiparameter radar

Abstract: Repeated changes associated with lightning have been observed with multiparameter radar in the echoes from the tops of Florida thunderstorms. These lightning-related radar signatures are interpreted as changes in the orientation of ice crystals being preferentially aligned parallel to the in-cloud electric field. The changes occur at intervals on the order of 10 s and are easily observed in the signatures of the differential propagation phase shift and the linear depolarization ratio which are sensitive to propagation effects caused by the oriented ice crystals. The orientation of ice crystals aloft has been previously observed using circularly polarized radar while the simultaneous differential phase shift and linear depolarization measurements reported were obtained with a dual-linear polarized radar. The observations indicate crystal orientation angles greater than 450 and occasionally near vertical. In one case, the crystals were found to be oriented in a layer near radar cloud top spanning a 20-km range and 3 km in depth.

The ERICA IOP 5 Storm. Part III: Mesoscale Cyclogenesis and Precipitation Parameterization

Abstract: Modeling studies have consistently shown the importance of latent heat release in explosive marine cyclogenesis. However, a systematic evaluation of precipitation parameterization in the simulation of marine cyclones has been rare in the literature. This paper is the third in a series of modeling studies on the ERICA IOP 5 storm. The objective is to assess the performance of various subgrid-scale cumulus parameterization and resolvable-scale microphysics schemes in the simulation of the storm using the Penn State–NCAR mesoscale model MM5 at grid resolutions of 20 and 60 km. Emphasis is placed on the intensity, distribution, and character of precipitation and on the mesoscale low pressure centers embedded within the synoptic-scale cyclone. Principal findings are as follows. The distribution and intensity of precipitation, its partitioning into grid-resolvable and subgrid-scale portions, the atmospheric thermodynamic structure in the precipitation region, and the evolution of mesoscale low pressure...

Ground‐based search for X rays generated by thunderstorms and lightning

Abstract: A series of ground-based experiments have been performed to investigate gamma ray/X ray count rate increases that are associated with thunderstorm and lightning activity. NaI scintillation detectors were configured in a long-timescale channel to monitor long-term count rate variations (on timescales of 1–104 s) as a function of time, pressure, temperature, humidity, electric field, rainfall, and general thunderstorm activity and in a short-timescale channel that was triggered by an electric field change meter to monitor correlations of individual counts (on timescales of 10−7–10−2 s) with particular phases of a lightning flash. Long-timescale results typically show count rate increases of a few percent to as much as 100% above background levels during thunderstorm activity and are likely due primarily to gamma ray emissions from radon daughter-ion decay as the daughter ions are precipitated to the ground by rainfall. The production of bremsstrahlung X rays by a thunderstorm runaway electron mechanism cannot be ruled out by this data, but the data indicate that if the mechanism is in operation, the bremsstrahlung flux at the ground is at best intermittent and/or barely discernible above background levels. Short-timescale results do not show any evidence for the production of X rays by individual lightning flashes. However, these results are inconclusive since only 10 cases of lightning flashes within a 1 km distance were recorded. The results of the overall study are compared to previous studies which claim positive correlations of count rate increases with thunderstorm and lightning activity. They are also discussed in terms of runaway electron acceleration in thunderstorm electric fields and in terms of the runaway air breakdown mechanism for lightning initiation. A review of the physics of and previous studies of X ray emissions from thunderstorms and lightning is presented in the introduction.

Enhancement of Tropical Ocean Evaporation and Sensible Heat Flux by Atmospheric Mesoscale Systems

Abstract: The enhancement of monthly averaged evaporation by atmospheric mesoscale systems is estimated from long-term hourly observations of surface meteorological data from the Tropical Ocean Global Atmosphere (TOGA) Tropical Atmosphere Ocean (TAO) buoy moorings over the equatorial Pacific Ocean and a bulk aerodynamic flux algorithm developed as a result of the TOGA Coupled Ocean–Atmosphere Response Experiment (COARE). It is shown that mesoscale enhancement is due primarily to the lack of wind steadiness on subsynoptic timescales and is associated with periods of significant precipitation. The magnitude of the mesoscale enhancement of monthly averaged sea surface evaporation is found to be ∼10% or less of the total. During occasional periods with weak and variable winds over the western Pacific warm pool and the other major precipitation zones in the equatorial Pacific, the mesoscale enhancement of monthly averaged evaporation can reach 30% of the total evaporation. A similar result is obtained for mesos...

The Characteristics of Lightning Occurrence in Southern Germany

Abstract: For the first time, the temporal and spatial distributions of lightning in southern Germany are studied. Data is taken from a newly installed lightning location system (LPATS). The data base covers the years 1992-1994. Inside the observational are of 500 km × 430 km a mean value of 634,000 cloud-to-ground lightning strokes per year is detected. This corresponds to a lightning frequency of 1.2 per minute and a spatial density of 2.8 yr -1 km -2 . Most of the lightning events are confined to the summer months May-August with a mean number of 29 thunderstorm days per year. The diurnal cycle of lightning activity peaks at 1600-1700 local time with a subsequent slow decrease towards the minimum in the morning hours. A secondary maximum is found between 2100 and 2200 which is shown to be a propagation effect from distant thunderstorm source areas. The spatial lightning distribution confirms the well-known preference of certain areas for the development of thunderstorms. Such preferred areas are the leeward side of the Black Forest, the Allgau and other parts of the Alpine foreland. Here lightning densities of more than 10 yr -1 km -2 may occur. A comparison with routinely gathered thunderstorm data yields a good agreement.

The use of dual channel circular-polarization radar observations for remotely sensing storm electrification

Abstract: Observations of thunderstorms with a dual channel circularpolarization radar have provided dramatic indications of the buildup of the electric field inside the storms and of the sudden collapse of the field at the time of lightning. The indications are obtained by coherently correlating the simultaneous returns in the right- and left-hand circular polarization channels of the radar, and follow up on the pioneering observations of this type by Hendry and McCormick (1976). The correlation is estimated and displayed in real time and the results enable one to predict when a storm has the potential for producing a lightning discharge, and often to anticipate the occurrence of individual discharges. The observations detect the presence of electrically aligned particles, believed to be small ice crystals, which are aligned by the electrostatic field of the storm. The aligned particles cause the radar signal to become progressively depolarized as it propagates through an alignment region, giving rise to correlated right- and left-circular polarization echoes. The alignment direction can be determined from the phase of the correlation and is found to be predominantly vertical, indicating a similar electric field orientation. Weaker horizontal alignment is often observed immediately following lightning discharges, consistent with the idea that the aligned particles are ice platelets which fall with horizontal orientation due to aerodynamic forces. The observations have been found to reveal the onset of strong electrification in developing storms and to indicate when decaying storms no longer have the potential to produce lightning. By compensating for signal-to-noise effects, the variation of the depolarization with range can be determined. This provides detailed pictures of the alignment regions which could be used as tracers of ice crystal populations in storms. The pictures also show the spatial variation of the alignment directions, raising the possibility of remotely mapping the storm electric field structure. Finally, the depolarization rate results readily enable one to distinguish between liquid and solid precipitation in the storms.

Models of lightning-produced sprites and elves

Abstract: Three different types of optical phenomena have been observed at high altitude above thunderstorms: an enhanced airglow (“elves”) at roughly ∼90 km; a reddish glow (“sprites”) from 50 to 90 km; and an upward moving, bluish emission (“jets”) below 40 km. A likely explanation for some or all of these phenomena is gas breakdown caused by the electromagnetic fields of lightning discharges. This paper examines the connection between these fields and breakdown at high altitude, using both analytic models and numerical simulations. Included in the calculations are the radiation fields from the lightning return stroke and the quasi-static fields from the continuing current. The different nature of the two fields is shown to produce two distinct types of breakdown, with characteristics similar to those of elves and sprites. Also mentioned is a third breakdown mechanism which may account for blue jets.

Dual multiparameter radar observations of intense convective storms: The 24 June 1992 case study

Abstract: Data from two multiparameter radars are used to diagnose some microphysical characteristics of intense convective storms, in particular, the 24 June 1992 case near Fort Collins and Greeley, Colorado. Dual-polarization and dual-frequency radar measurements from the CSU-CHILL and NCAR/CP-2 radars provided the basis for microphysical interpretations. Supporting in-situ measurements were provided by several T-28 aircraft penetrations of updraft regions.

On abandoning the thunderstorm tripole-charge paradigm

Abstract: We reexamine the original, half-century-old, in situ measurements of Simpson and colleagues used to justify the tripole model of charge structure of a thunderstorm that became the paradigm for the charge structure of mature thunderstorms. We find that the original measurements violate the criteria of the tripole model in 49% of the cases. We also reexamine our soundings of electric field in thunderstorms and mesoscale convective systems in the same way as the original soundings were. Our soundings do not fit the tripole model in at least 86% of the storms. Even when we consider only the major features in the electric field profiles, our data show noncompliance with the tripole model in 45% of the cases. We conclude that we must abandon the paradigm of the tripole-charge structure of thunderstorms.

High-resolution passive microwave observations of convective systems over the tropical Pacific ocean

Abstract: This paper presents high-resolution passive microwave measurements obtained in the western Pacific warm pool region. These measurements represent the most comprehensive such observations of convection over the tropical oceans to date, and were obtained from the Advanced Microwave Precipitation Radiometer (AMPR) aboard the NASA ER-2 during the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. The AMPR measures linearly polarized radiation at 10.7, 19.35, 37.1, and 85.5 GHZ. Nadir brightness temperature scatterplots suggest that the three lower frequencies respond primarily to emission/absorption processes. Strong ice scattering is relatively rare, as absolute magnitudes of the ice-scattering signature do not approach those measured in strong convection over land. This is apparently related to the reported weaker updraft velocities over tropical oceans, which would create and suspend relatively smaller graupel or hail particles in the upper cloud. Observations withi...

Lightning charge analyses in small Convection and Precipitation Electrification (CaPE) experiment storms

Abstract: A least squares method for analyzing multiple, ground-based measurements of electric field changes produced by lightning has been applied to seven small thunderstorms that occurred on July 19 and August 9, 1991, during the Convection and Precipitation Electrification (CaPE) experiment. Two of the storms produced little or no cloud-to-ground (CG) lightning, and a third produced CG lightning only during its early stages. A total of 79 flashes were analyzed on July 19 and 315 on August 9. About 58% of these discharges could be fitted to either a point charge (Q) or a point dipole (P) model, and in this sample, the spatial pattern of the Q and P solutions was consistent with a tripole or double dipole charge pattern in the cloud. In cases where there was little or no CG lightning, the inferred region of upper positive charge was quite close to the inferred negative region. Comparisons of the locations of upper P solutions with measurements of radar reflectivity at S band show that the P solutions tended to cluster at altitudes where the reflectivity was between 25 and 35 dBZ. Comparisons of Q (and P) model solutions with the locations of CG flash strike points, determined using a network of wideband direction finders, showed an average horizontal displacement of 3.9 km with a standard deviation of 3.3 km. This displacement is consistent with the expected random development of a stepped-leader channel from the altitude of the negative Q region to the ground.

A Comparison Of WSR-88D Reflectivities, SSM/I Brightness Temperatures, and Lightning for Mesoscale Convective Systems in Texas. Part I: Radar Reflectivity and Lightning

Abstract: This is the first part of a two part study. Part I compares radar data from the League City, Texas, WSR-88D and cloud-to-ground (CG) lightning data for a set of eight mesoscale convective systems (MCSs), which occur at various stages of development along the upper Texas gulf coast. Vertical profiles of radar reflectivity (VPRR) as well as plan views and vertical cross sections are constructed to characterize the structure and relative strength of each MCS. The VPRR are also compared with similar profiles from tropical oceanic MCSs. The data show that in all the majority of negative CG lightning flashes are located near high-reflectivity convective cores (>35 dBZ) in the mixed-phase region (0°C ≤T≥ −20°C). Growing or mature MCSs typically had larger negative flash counts and higher percentages of negative lightning (≥80%) associated with convective core than MCSs at later stages of their life cycle. Comparison of the median VPRR for the various MCSs showed that although each case had high-reflecti...

Initiation of an Elevated Mesoscale Convective System Associated with Heavy Rainfall

Abstract: A mesoscale convective system (MCS) developed during the morning hours of 6 June 1993 and moved across northern and central Missouri, resulting in a narrow swath of excessive rainfall (>150 mm). The MCS developed well north of a surface warm front above a cool, stable boundary layer and moved east–southeast across the state. Although some features of the synoptic environment agree with the frontal flash flood composite model, predicting the elevated thunderstorms that composed the MCS posed a unique forecasting challenge. This paper first describes the diagnostic parameters of the prestorm environment that would have been helpful to predict the initiation of the MCS and the resultant locally excessive precipitation. Attention is then drawn to the MCS itself via IR satellite and WSR-88D imagery. Finally, the similarities and differences of this episode to previous studies of flash flooding and elevated thunderstorms are noted, and a summary of key parameters useful in the anticipation of this type...

Relationships between Tornado Intensity and Various Wind and Thermodynamic Variables

Abstract: Statistical relationships between tornado intensity and various stability and wind-related parameters are determined from a dataset of more than 200 tornado proximity soundings. Soundings were obtained from two sources: the University of Missouri and from analyses. The surface to 600 hPa wind shear, streamwise vorticity, storm velocity, and storm-relative environmental helicity are shown to be best correlated with tornado intensity. A multiple regression analysis produced an equation relating tornado intensity to the lifted index and the surface to 600 hPa wind shear. This relationship could be used operationally to predict tornado intensity. Mean temperature, dewpoint temperature, and wind profiles are presented for each tornado intensity category and these may be useful as input to numerical thunderstorm simulations. Progressively cooler temperatures and increasing wind speed and vertical wind shear are evident with increasing tornado intensity. A modified form of the bulk Richardson number is ...