Showing papers on "Thunderstorm published in 2005"

The 29 June 2000 Supercell Observed during STEPS. Part II: Lightning and Charge Structure

Abstract: This second part of a two-part study examines the lightning and charge structure evolution of the 29 June 2000 tornadic supercell observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). Data from the National Lightning Detection Network and the New Mexico Tech Lightning Mapping Array (LMA) are used to quantify the total and cloud-to-ground (CG) flash rates. Additionally, the LMA data are used to infer gross charge structure and to determine the origin locations and charge regions involved in the CG flashes. The total flash rate reached nearly 300 min−1 and was well correlated with radar-inferred updraft and graupel echo volumes. Intracloud flashes accounted for 95%–100% of the total lightning activity during any given minute. Nearly 90% of the CG flashes delivered a positive charge to ground (+CGs). The charge structure during the first 20 min of this storm consisted of a midlevel negative charge overlying lower positive charge with no evidence of an upper positiv...

Three Years of TRMM Precipitation Features. Part I: Radar, Radiometric, and Lightning Characteristics

Abstract: During its first three years, the Tropical Rainfall Measuring Mission (TRMM) satellite observed nearly six million precipitation features. The population of precipitation features is sorted by lightning flash rate, minimum brightness temperature, maximum radar reflectivity, areal extent, and volumetric rainfall. For each of these characteristics, essentially describing the convective intensity or the size of the features, the population is broken into categories consisting of the top 0.001%, top 0.01%, top 0.1%, top 1%, top 2.4%, and remaining 97.6%. The set of “weakest/smallest” features composes 97.6% of the population because that fraction does not have detected lightning, with a minimum detectable flash rate of 0.7 flashes (fl) min−1. The greatest observed flash rate is 1351 fl min−1; the lowest brightness temperatures are 42 K (85 GHz) and 69 K (37 GHz). The largest precipitation feature covers 335 000 km2, and the greatest rainfall from an individual precipitation feature exceeds 2 × 1012 k...

Climatological Estimates of Daily Local Nontornadic Severe Thunderstorm Probability for the United States

Abstract: The probability of nontornadic severe weather event reports near any location in the United States for any day of the year has been estimated. Gaussian smoothers in space and time have been applied to the observed record of severe thunderstorm occurrence from 1980 to 1994 to produce daily maps and annual cycles at any point. Many aspects of this climatology have been identified in previous work, but the method allows for the consideration of the record in several new ways. A review of the raw data, broken down in various ways, reveals that numerous nonmeteorological artifacts are present in the raw data. These are predominantly associated with the marginal nontornadic severe thunderstorm events, including an enormous growth in the number of severe weather reports since the mid-1950s. Much of this growth may be associated with a drive to improve warning verification scores. The smoothed spatial and temporal distributions of the probability of nontornadic severe thunderstorm events are presented in...

Lightning-generated NOX and its impact on tropospheric ozone production: A three-dimensional modeling study of a Stratosphere- Troposphere Experiment: Radiation, Aerosols and Ozone (STERAO-A) thunderstorm

Abstract: [1] A three-dimensional cloud-scale chemical transport model has been used to simulate trace gas transport, lightning NO production, and photochemical ozone production in the 12 July 1996 storm observed during the Stratosphere-Troposphere Experiment: Radiation, Aerosols and Ozone (STERAO-A) field experiment. The model is driven by meteorological fields from a nonhydrostatic cloud-resolving model (see Stenchikov et al., 2005). An assumption that both cloud-to-ground and intracloud flashes produce 460 moles NO/flash on average yielded the best comparison with the profile of NO observed in the storm anvil. Scenarios in which the NO production of an intracloud flash was 75 to 100% of the production of a cloud-to-ground flash best matched the column NOx mass computed from observations. Additional ozone production attributable to lightning NO within the storm cloud during the lifetime of the storm was very small (∼2 ppbv). However, simulations of the photochemistry over the 24 hours following the storm show that an additional 10 ppbv of ozone production can be attributed to lightning NO production in the upper troposphere. Convective transport of HOx precursors led to the generation of a HOx plume, which substantially aided the downstream ozone production. Soluble species mixing ratios in the simulated cloud were all within a factor of two of observations.

Cassini Imaging Science: initial results on Saturn's atmosphere.

Abstract: The Cassini Imaging Science Subsystem (ISS) began observing Saturn in early February 2004. From analysis of cloud motions through early October 2004, we report vertical wind shear in Saturn's equatorial jet and a maximum wind speed of ∼375 meters per second, a value that differs from both Hubble Space Telescope and Voyager values. We also report a particularly active narrow southern mid-latitude region in which dark ovals are observed both to merge with each other and to arise from the eruptions of large, bright storms. Bright storm eruptions are correlated with Saturn's electrostatic discharges, which are thought to originate from lightning.

Location accuracy of VLF World-Wide Lightning Location (WWLL) network: Post-algorithm upgrade

Abstract: An experimental VLF World-Wide Lightning Location (WWLL) network has been developed through collaborations with research institutions across the globe The aim of the WWLL is to provide global real-time locations of lightning discharges, with >50% CG flash detection efficiency and mean location accuracy of The detection efficiency of the WWLL is also considered In the selected region the WWLL detected ~13% of the total lightning, suggesting a ~26% CG detection efficiency and a ~10% IC detection efficiency Based on a comparison between all WWLL good lightning locations in February-April 2004, and the activity levels expected from satellite observations we estimate that the WWLL is currently detecting ~2% of the global total lightning, providing good locations for ~5% of global CG activity The existing WWLL network is capable of providing real-time positions of global thunderstorm locations in its current form

The Electrical Structure of Two Supercell Storms during STEPS

Abstract: Balloon soundings were made through two supercell storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) in summer 2000. Instruments measured the vector electric field, temperature, pressure, relative humidity, and balloon location. For the first time, soundings penetrated both the strong updraft and the rainy downdraft region of the same supercell storm. In both storms, the strong updraft had fewer vertically separated charge regions than found near the rainy downdraft, and the updraft’s lowest charge was elevated higher, its bottom being near the 40-dBZ boundary of the weak-echo vault. The simpler, elevated charge structure is consistent with the noninductive graupel–ice mechanism dominating charge generation in updrafts. In the weak-echo vault, the amount of frozen precipitation and the time for particle interactions are too small for significant charging. Inductive charging mechanisms and lightning may contribute to the additional charge regions found at lower a...

The 29 June 2000 Supercell Observed during STEPS. Part I: Kinematics and Microphysics

Abstract: This is a two-part study that addresses the kinematic, microphysical, and electrical aspects of a severe storm that occurred in western Kansas on 29 June 2000 observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field campaign. In this first part, polarimetric and Doppler radar data are used along with a simple particle growth model to examine the evolution of the kinematic and microphysical properties of the storm from its earliest developing phase through its mature and dissipating phases. During its severe stage, the storm exhibited frequent positive cloud-to-ground lightning strikes, very large (5 cm) hail, and a tornado. Doppler-derived winds, radar reflectivity, and hydrometeor classifications from the polarimetric data over a nearly 4-h period are presented. It is shown that updraft velocity and vertical vorticity had to reach magnitudes of at least 10 m s 1 and 10 2 s 1 and occupy major portions of the storm before it could produce most of the observed severe storm characteristics. Furthermore, the establishment of cyclonic horizontal flow around the right flank of the updraft core was essential for hail production. Most of the largest hail grew from near millimeter-sized particles that originated in the mid- to upper-level stagnation region that resulted from obstacle-like flow of environmental air around the divergent outflow from the upper part of the updraft. These recycling embryonic particles descended around the right flank of the updraft core and reentered the updraft, intermingling with other smaller particles that had grown from cloud base along the main low-level updraft stream.

Cluster Analysis of Surface Winds in Houston, Texas, and the Impact of Wind Patterns on Ozone

Abstract: The city of Houston, Texas, is near a complex coastline and numerous petrochemical plants, the combination of which plays a large role in Houston’s air pollution events. It has long been known that the thermally driven afternoon onshore flow (sea breeze or gulf breeze) transports ozone-rich air inland. As a way of quantifying the role of the gulf breeze in Houston’s high-ozone events, cluster analysis of hourly averaged surface winds from a regional network of meteorological sensors was performed for 27 summer days of 2000, with the dates coinciding with the Texas Air Quality Study 2000 (TexAQS 2000). Hourly averaged winds were partitioned into 16 independent clusters, or wind patterns, while simultaneously keeping track of the maximum ozone in the network for each hour. Clusters emerged that represented various wind patterns, including thermally driven flows, stagnant winds, and a thunderstorm outflow. All clusters were used to assess which wind patterns were most likely to be coincident with th...

The lower positive charge center and its effect on lightning discharges on the Tibetan Plateau

Abstract: [1] Electrical characteristics of thunderstorms on the central Tibetan Plateau at an altitude of 4508 m have been studied. The evolution of surface electric (E) field and the E field changes produced by lightning flashes under a representative thunderstorm revealed a tripole charge structure with a larger-than-usual lower positive charge center (LPCC). The storms appear to begin with the lower dipole of a normal tripole structure, rather than with the upper dipole followed by the development of a weaker lower positive charge. The flash rate is quite low and the average value is usually 1 fl/min. The IC flashes were usually polarity-inverted and occurred in the lower dipole. The large LPCC did not cause positive CG flashes to occur during the whole storm lifetime, and only negative CG flashes were observed in the late stage of the storm.

Initialization and Numerical Forecasting of a Supercell Storm Observed during STEPS

Abstract: The feasibility of initializing a numerical cloud model with single-Doppler observations and predicting the evolution of thunderstorms has been tested using an observed case of a supercell storm during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). Single-Doppler observations from the Weather Surveillance Radar-1988 Doppler (WSR-88D) at Goodland, Kansas, are assimilated into a cloud-scale numerical model using a four-dimensional variational data assimilation (4DVAR) scheme. A number of assimilation and short-range numerical prediction experiments are conducted. Both the assimilation and prediction results are compared with those of a dual-Doppler synthesis. The prediction results are also verified with reflectivity observations. It is shown that the analysis of the wind field captures the major structure of the storm as revealed by the dual-Doppler synthesis. Thermodynamical and microphysical features retrieved through the dynamical model show consistency with expectatio...

Distribution system reliability assessment due to lightning storms

Abstract: Lightning is a significant cause of faults and outages in many electric power systems and is one of the major causes of poor system reliability. Predictive assessment of distribution reliability indices can be used to identify areas that have poor reliability so that appropriate changes in system design can be implemented. The assessment of distribution system performance under lightning conditions requires modeling of storm characteristics and system response. In this paper, a Monte Carlo simulation for evaluating distribution system reliability under lightning storm conditions is presented. The results from a practical distribution system show the importance of detailed modeling of storm characteristics and simulation of the system response in assessing distribution system reliability during lightning storms.

Raman Lidar Measurements During the International H2O Project. 2; Instrument Comparisons and Case Studies

Abstract: The amount of water vapor in the atmosphere helps to determine the likelihood that severe storms may develop. The concentration of water vapor, though, is highly variable in space and time. And yet small changes in water vapor concentration over a short period of time or over a short spatial distance can determine whether a storm may or may not develop. Therefore, in order to improve the ability to forecast severe weather such as thunderstorms it is important to measure water vapor in the atmosphere with high spatial and temporal resolution. One of the most attractive research tools for measuring water vapor in the atmosphere with high spatial and temporal resolution is a Raman lidar. A Raman lidar consists of a laser transmitter, a telescope receiver and optics and electronics for processing opticand electronic signals. A laser pulse is emitted into the atmosphere and it interacts with molecules in the atmosphere causing them to become excited and to emit, through the Raman process, photons of different wavelength than emitted by the laser. The molecule that emitted these emitted. This is the way that a Raman lidar identifies water vapor molecules in the atmosphere. can be identified based on the wavelength of the photons One of the great challenges in Raman lidar measurements has been to make useful daytime measurements of the water vapor profile under bright daytime conditions. In this first of two papers, we describe the instrumentation and analysis of the first documented Raman lidar that is able to measure water vapor in the daytime with sufficient quality to permit the study of developing storm systems.

Low-Frequency Atmospheric Acoustic Energy Associated with Vortices Produced by Thunderstorms

Abstract: An infrasonic observatory collocated with the Colorado State University CHILL radar during the summer of 1995 permitted unique comparisons between severe storm kinematics and detected acoustic energy at subaudible frequencies near 1 Hz. Radar observations of a velocity couplet aloft (evolving into a tornado) showed a circulation maximum descending for about 30 min while moving to the east. The detected infrasound followed the trend of these observations. A model of sound radiated from vortex systems predicts frequencies in the range observed. These data are interpreted in the context of past infrasonic observations. An ongoing study comparing regional tornado and funnel sightings with archived infrasonic data has identified over 100 cases to date where the infrasonic signals occurred at the time of, and from the direction of, the vortices. For some of these cases, the distances were greater than 100 km. The author and his associates continue to collect datasets to permit further evaluation of infrasonic detection methods.

Improving Convective Precipitation Forecasting through Assimilation of Regional Lightning Measurements in a Mesoscale Model

Abstract: A technique developed for assimilating regional lightning measurements into a meteorological model is presented in this paper. The goal is to assess the effectiveness of cloud-to-ground (CG) lightning information for improving the convective precipitation forecasting. The main concept of the technique is that utilizing real-time location, timing, and flash-rate data retrieved from a long-range lightning detection network, a regional/mesoscale meteorological model is informed about the deep moist convection spatiotemporal development and intensity. This information is then used to nudge the model-generated humidity profiles to empirical profiles as a function of the observed lightning intensity. The empirical humidity profiles are assumed to be representative of convective regimes since they have been produced on the basis of atmospheric soundings obtained during thunderstorm days. Case studies from three thunderstorm developments in a warm-season environment over the Mediterranean are used to investigate the relationship between lightning density and different empirical humidity profiles, and consequently demonstrate the impact of the technique on model precipitation forecasts. Results show that assimilation of lightning data can significantly improve the model’s prediction accuracy of convective precipitation in the assimilation period, while maintaining the improvement in short-range (up to 12 h) forecasts compared to the control case. The approach is general enough to be applied to any mesoscale model, but with an expectable varying degree of success. Its advantage when applied in an operational setting is that real-time lightning data responding promptly to the occurrence of convection would continuously get assimilated to update the moist state of the atmosphere in the model.

Pulsed wall jet simulation of a stationary thunderstorm downburst, Part A: Physical structure and flow field characterization

Abstract: A pulsed wall jet has been used to simulate the gust front of a thunderstorm downburst. Flow visualization, wind speed and surface pressure measurements were obtained. The characteristics of the hypothesized ring vortex of a full-scale downburst were reproduced at a scale estimated to be 1:3000.

Aircraft observations of the influence of electric fields on the aggregation of ice crystals

Abstract: Aircraft observations of ice-crystal size and habit distributions in the cirrus outflow from deep convection at several geographic locations are reported. In situ measurements were made in the outflow from maritime thunderstorms near Kwajalein, part of the Marshall Islands and of thunderstorms with more continental aerosol concentrations both in the United States and near Darwin, Australia over the Tiwi Islands. Images of chain-like aggregates of small ice crystals, some with plate-like shapes were observed with a state-of-the-art microphysics probe in the outflow regions of continental storms that were typically highly electrified, displaying lightning. The ‘chains’ were not found in the outflow regions of maritime storms that are typically less electrically active. The striking similarity between these images and previous laboratory measurements of ice aggregation in electric fields are remarked upon. This evidence is used to support the theory that chain aggregates of ice crystals may be common in fully glaciated regions of continental thunderstorms, where ice-particle number densities are high, and their presence is due to the electric field alignment of ice crystals with subsequent enhancement of the aggregation process by dipole induction resulting in short-range attractive inter-particle forces. It is not confirmed where in the storm the aggregates were typically formed; however, in the Darwin thunderstorms they were noted to occur with the highest frequency towards the cirrus outflow base when the cirrus base altitude was high, and generally decreased in frequency with increasing distance from the storm. The potential consequences of electrically enhanced aggregation in continental storms and related electric field mechanisms along with the role of homogeneous freezing in intense thunderclouds are discussed. Copyright © 2005 Royal Meteorological Society

Lightning-induced intensification of the ionospheric sporadic E layer

Abstract: A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s. Several mechanisms have been proposed to explain this connection, and evidence from modelling as well as various types of measurements demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events, that the ionospheric 'sporadic E' layer--transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation--can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.

Lightning-induced intensification of the ionospheric sporadic E layer

Abstract: A connection between thunderstorms and the ionosphere has been hypothesized since the mid-1920s. Several mechanisms have been proposed to explain this connection, and evidence from modelling as well as various types of measurements demonstrate that lightning can interact with the lower ionosphere. It has been proposed, on the basis of a few observed events, that the ionospheric 'sporadic E' layer--transient, localized patches of relatively high electron density in the mid-ionosphere E layer, which significantly affect radio-wave propagation--can be modulated by thunderstorms, but a more formal statistical analysis is still needed. Here we identify a statistically significant intensification and descent in altitude of the mid-latitude sporadic E layer directly above thunderstorms. Because no ionospheric response to low-pressure systems without lightning is detected, we conclude that this localized intensification of the sporadic E layer can be attributed to lightning. We suggest that the co-location of lightning and ionospheric enhancement can be explained by either vertically propagating gravity waves that transfer energy from the site of lightning into the ionosphere, or vertical electrical discharge, or by a combination of these two mechanisms.

Previsão de tempestades severas utilizando-se parâmetros convectivos e modelos de mesoescala: uma estratégia operacional adotável no brasil?

Abstract: SEVERE STORMS FORECASTING UTILIZING CONVECTIVE PARAMETERS AND MESOSCALE MODELS: AN OPERATIONAL STRATEGY ADOPTABLE IN BRAZIL? With the goal of inducing further discussion on the issue of severe weather forecasting in Brazil, this review article conducts an updated description of atmospheric parameters conceived to identify atmospheric environments favorable for the development of severe convective storms (thunderstorms capable of producing large hail, strong surface winds and tornadoes). The computation of such atmospheric indices — that highlight conditions of strong convective instability and vertical wind shear — making use, among other sources of information, of the output of mesoscale models represents an operational strategy of possible adoption in Brazil. In this context, this article discusses indications that the occurrence of severe thunderstorms in Brazil is not as rare as typically assumed, and explores the fact that some severe weather parameters originally developed for the mid-latitudes of the Northern Hemisphere (i) may be useful for the prediction of convective systems in Brazil, particularly for the southern part of the country, and (ii) can serve as a conceptual basis for the development of severe weather indices more suitable for the tropical regions of our country. These points should justify a greater attention to the topic of severe convection in Brazil, on both research and operational environments.

Thermal circulation in South Bavaria climatology and synoptic aspects

Abstract: The stronger diurnal temperature variation of air within Alpine valleys compared to the adjacent plain causes a thermal wind system, which in summer often propagates far north into the foreland of South Bavaria. Observations at 39 surface stations in South Bavaria between 1996-2000 are used to derive climatological surface wind fields for weather conditions with strong solar irradiance at ground level. It is shown that the thermal wind system propagates about 100 km into the Alpine foreland in 42 % of all days from April through September during which daily total global radiation exceeds 20 MJ m -2 . The thermal wind system also develops into the vertical direction starting at ground level. Between 1500-1600 CET the inflow layer increases up to 1 km above ground near the Alpine margin and up to 2 km within valleys of the Northern Alps causing a pronounced development of convective clouds. Here we propose the term Alpine Pumping for this well-defined dynamical process. During afternoon subsiding air motion suppresses convective cloud formation north of the Alpine margin. A classification by the 500 hPa geostrophic wind shows that the thermal wind system in the Alpine foreland is stronger influenced by the large-scale synoptic flow than valley wind systems within the Alps. A regional peculiarity can be observed in the Allgau under conditions with weak or southerly synoptic-scale flows. Under these conditions north-westerly slope-winds converge in the Allgau with north-easterly winds prevailing in the eastern part of the Alpine foreland. The well-known higher thunderstorm frequency of this region could be a consequence of the convergence of both flow patterns.