Showing papers on "Thunderstorm published in 2002"

Simulated three‐dimensional branched lightning in a numerical thunderstorm model

Abstract: Received 8 December 2000; revised 27 August 2001; accepted 7 September 2001; published 2 May 2002 [1] Lightning discharges are simulated by using a stochastic dielectric breakdown model within a numerical thunderstorm model with extensive parameterizations of electrification mechanisms. The lightning model simulates the macroscopic bidirectional extension of discharges as a step-by-step stochastic process. Discharge channels are propagated on a uniform grid, and the direction of propagation (including diagonals) for a particular step is chosen randomly, with the probability for choosing a particular direction depending on the net electric field. After each propagation step the electric fields are recomputed via Poisson’s equation to account for the effect of the conducting channel. The lightning parameterization produces realistic looking, three-dimensional, branched lightning discharges. A variety of lightning types have been produced, including intracloud discharges, negative cloud-to-ground (CG) lightning, and positive CG lightning. The model simulations support the hypothesis that negative CG flashes occur only when a region of positive charge exists below the main negative charge region. Similarly, simulated positive CG flashes were found to occur only in regions of storms where the two significant charge layers closest to ground had roughly a ‘‘normal dipole’’ structure (i.e., positive charge above negative). INDEX TERMS: 3300 Meteorology and Atmospheric Dynamics; 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3324 Meteorology and Atmospheric Dynamics: Lightning; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; KEYWORDS: lightning, thunderstorm electrification, numerical thunderstorm model

Relationships between Convective Storm Kinematics, Precipitation, and Lightning

Abstract: Combined multiparameter radar, dual-Doppler, thermodynamic sounding, and lightning observations of 11 thunderstorms (6 from the midlatitudes, 5 from the Tropics) are examined. The thunderstorms span a wide spectrum of intensities, from weak monsoontype to severe tornadic, and include both unicellular and multicellular convection. In general, the kinematically strongest storms featured lower production of negative cloud-to-ground lightning (typically ,1 min21 flash rates for large portions of the storms’ lifetimes) when compared with more moderate convection, in accord with an elevated charge mechanism. The only significant differences between intense storms that produced predominately positive cloud-to-ground (CG) lightning for a significant portion of their lifetimes (PPCG storms) and intense storms that produced little CG lightning of any polarity (low-CG storms) was that PPCG storms featured much larger volumes of significant updrafts (both.10 and .20 m s21) and produced greater amounts of precipitation (both rain and hail). Otherwise, peak updrafts and vertical airmass fluxes were very similar between the two types of storms, and both types were linked by anomalously low production of negative CG lightning. PPCG effects in storms may result from an elevated region of negative charge (reducing negative CG flash rates) combined with enhanced net positive charge regions created by the larger volume of significant updrafts.

Cloud‐to‐ground lightning characteristics over Houston, Texas: 1989–2000

Abstract: [1] Cloud-to-ground (CG) lightning detected by the National Lightning Detection Network (NLDN) indicates a relatively high flash density over Houston, Texas, for the 12-year period 1989–2000. A significant enhancement of 45% in the flash density is observed compared to the nearby surrounding areas. The strength of the enhancement varies on the basis of season and time of day, with the greatest increases occurring during the summer (58%) and during the 0900–1800 LT time periods in each season. Observations indicate that large lightning events (defined as days with >100 flashes in a geographic region that includes Houston and nearby rural areas) were responsible for the climatological lightning anomaly and that increased thunderstorm initiation was not the most significant cause of the enhancement. A decrease (−12%) in the percentage of positive flashes is observed over the city. Higher negative median peak currents along the coast and well into the Gulf of Mexico were also discovered. Several explanations for our observations are suggested. The urban heat island and increased cloud condensation nuclei concentrations, especially from industrial pollution, are speculated to be significant factors in creating lightning enhancement. Pollution effects are speculated to cause a change in a thunderstorm's charge distribution, which can affect the polarity of CG flashes. The potential effect of the nearby coastal Gulf salt water on the calculated peak current is examined. Variations in multiplicity values across the region are observed but not explained.

Climatic Properties of Tropical Precipitating Convection under Varying Environmental Conditions

Abstract: A clustering algorithm is used to define the radiative, hydrological, and microphysical properties of precipitating convective events in the equatorial region observed by the Tropical Rainfall Measuring Mission (TRMM) satellite. Storms are separated by surface type, size, and updraft strength, the latter defined by the presence or absence of lightning. SST data and global reanalysis products are used to explore sensitivity to changes in environmental conditions. Small storms are much more numerous than mesoscale convective systems, and account for fairly little of the total rainfall but contribute significantly to reflection of sunlight. Lightning storms rain more heavily, have greater cloud area, extend to higher altitude, and have higher albedos than storms without lightning. Lightning is favored by a steep lower-troposphere lapse rate and moist midlevel humidity. Storms occur more often at SST ≥ 28°C and with strong upward 500-mb mean vertical velocity. In general, storms over warmer ocean wat...

Airborne Measurements of NOx, Tracer Species, and Small Particles during the European Lightning Nitrogen Oxides Experiment

Abstract: [1] Airborne in situ measurements of NO, NO2, NOy, CO, CO2, O3, J(NO2), and CN were performed in European thunderstorms during the field experiment EULINOX in July 1998. The measurements in the upper troposphere show enhanced NOx (= NO + NO2) concentrations within thunderstorms and their outflow at horizontal scales from 300 m to several 100 km. The maximum NO mixing ratio measured inside a thundercloud close to lightning (the aircraft was also hit by a small lightning strike) was 25 ppbv. A regional NOx enhancement of 0.5 ppbv over central Europe could be traced back to a thunderstorm event starting ∼24 hours earlier over Spain. The fractions of NOx in thunderclouds which are produced by lightning and convectively transported from the polluted boundary layer are determined by using CO2 and CO as tracers for boundary layer air. The analyses show that on average about 70% of the NOx increase measured in the anvil region was found to result from production by lightning and about 30% from NOx in the boundary layer. Thunderstorms are also strong sources of small particles. The peak CN concentrations measured within thunderstorm outflows (>30,000 particles STP cm−3) were distinctly higher than in the polluted boundary layer. The amount of NOx produced per thunderstorm and NO produced per lightning flash was estimated. The results imply that the annual mean NOx budget in the upper troposphere over Europe is dominated by aircraft emissions (0.1 TgN yr−1) in comparison to lightning production (∼0.03 TgN yr−1). On the global scale, NOx produced by lightning (mean 3 TgN yr−1) prevails over aircraft-produced NOx (0.6 TgN yr−1).

Possibly inverted-polarity electrical structures in thunderstorms during STEPS

Abstract: [1] In full and partial vertical profiles of the electric field in thunderstorms during the Severe Thunderstorm Electrification and Precipitation Study in 2000, we find evidence for inverted-polarity electrical structures in the convective region of thunderstorms. The evidence consists of polarities opposite from normal (1) in the peaks in the profile of the vertical component of the electric field, Ez, and (2) in the vertical sequence of inferred charges. In storms that possibly are inverted in polarity, the lowest peak in Ez inside the cloud is negative, followed by a positive peak farther up. Near storm top, the uppermost peak is negative, and the uppermost inferred charge is positive. The sounding data do not conclusively prove that inverted-polarity thunderstorms exist, but they support it. If additional data corroborate that such storms do exist, it will require modification of at least some aspects of our conceptual models of storm electrification.

The 8 June 1995 McLean, Texas, Storm. Part II: Cyclic Tornado Formation, Maintenance, and Dissipation

Abstract: On 8 June 1995, scientists participating in the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) collected a unique dataset with the Electra Doppler Radar (ELDORA). The ELDORA observations document the sequential life cycles of storm-scale circulations associated with three large tornadoes in a supercell thunderstorm near McLean, Texas. A qualitative description of the evolution of the storm was provided in Part I of this paper. During the first stage of development of each storm-scale circulation, interaction of the updraft with the environmental low-level horizontal vorticity produced a vorticity column that increased in intensity with height. As the vortex matured, vorticity increased greatly at low levels (i.e., below 2 km AGL) and exceeded that aloft. Each tornadic vortex was located near the rear side of the updraft, where the surrounding low-level horizontal vorticity was modified locally, most likely by weak baroclinity within the storm. Tilting of low-level horizo...

Environmental Characteristics of Convective Systems during TRMM-LBA

Abstract: In this paper, data collected from 51 days of continual upper atmospheric soundings and TOGA radar at ABRACOS Hill during the TRMM-LBA experiment are used to describe the mean thermodynamic and kinematic airmass properties of wet season convection over Rondonia, Brazil. Distinct multi-day easterly and westerly lower tropospheric wind regimes occurred during the campaign with contrasting airmass characteristics. Westerly wind periods featured modest CAPE (1000 J/kg), moist conditions (>90% RH) extending through 700 mb and shallow (900 mb) speed shear on the order of 10(exp -4)/s. This combination of characteristics promoted convective systems that featured a relatively large fraction of stratiform rainfall and weak convection nearly devoid of lightning. The environment is very similar to the general airmass conditions experienced during the Darwin, Australia monsoon convective regime. In contrast, easterly regime convective systems were more strongly electrified and featured larger convective rain rates and reduced stratiform rainfall fraction. These systems formed in an environment with significantly larger CAPE (1500 J/kg), drier lower and middle level humidities (< 80% RH) and a wind shear layer that was both stronger (10(exp -3)/s) and deeper (700 mb). The larger CAPE resulted from strong insolation under relatively cloud-free skies (owing to reduced column humidity) and was also weakly capped in the lowest 1-2 km, thus contributing to a more explosive growth of convection. The time series of low- and mid-level averaged humidity exhibited marked variability between westerly and easterly regimes and was characterized by low frequency (i.e., multi-day to weekly) oscillations. The synoptic scale origins of these moisture fluctuations are examined, which include the effects of variable low-level airmass trajectories and upper-level, westward migrating cyclonic vortices. The results reported herein provide an environmental context for ongoing dual Doppler analyses and numerical modeling case studies of individual TRMM-LBA convective systems.

A Lightning Prediction Index that Utilizes GPS Integrated Precipitable Water Vapor

Abstract: The primary weather forecast challenge at the Cape Canaveral Air Station and Kennedy Space Center is lightning. This paper describes a statistical approach that combines integrated precipitable water vapor (IPWV) data from a global positioning system (GPS) receiver site located at the Kennedy Space Center (KSC) with other meteorological data to develop a new GPS lightning index. The goal of this effort is to increase the forecasting skill and lead time for prediction of a first strike at the KSC. Statistical regression methods are used to identify predictors that contribute skill in forecasting a lightning event. Four predictors were identified out of a field of 23 predictors that were tested, determined using data from the 1999 summer thunderstorm season. They are maximum electric field mill values, GPS IPWV, 9-h change in IPWV, andK index. The GPS lightning index is a binary logistic regression model made up of coefficients multiplying the four predictors. When time series of the GPS lightning index are plotted, a common pattern emerges several hours prior to a lightning event. Whenever the GPS lightning index falls to 0.7 or below, lightning occurs within the next 12.5 h. An index threshold value of 0.7 was determined from the data for lightning prediction. Forecasting time constraints based on KSC weather notification requirements were incorporated into the verification. Forecast verification results obtained by using a contingency table revealed a 26.2% decrease from the KSC’s previous-season false alarm rates for a nonindependent period and a 13.2% decrease in false alarm rates for an independent test season using the GPS lightning index. In addition, the index improved the KSC desired lead time by nearly 10%. Although the lightning strike window of 12 h is long, the GPS lightning index provides useful guidance to the forecaster in preparing lighting forecasts, when combined with other resources such as radar and satellite data. Future testing of the GPS lightning index and the prospect of using the logistic regression approach in forecasting related weather hazards are discussed.

Modeling Maximum Hail Size in Alberta Thunderstorms

Abstract: A one-dimensional steady-state cloud model was combined with a time-dependent hail growth model to predict the maximum hailstone size on the ground. Model runs were based on 160 proximity soundings recorded within the Alberta Hail Project area for three summers between 1983 and 1985. The forecast hail sizes were verified against reports of maximum hail size gathered from a high-density observation network within the project area. The probability of detection (POD), the false-alarm ratio (FAR), and the Heidke skill score (HSS) were computed for the hail model forecasts and were compared with the skill scores for a nomogram method developed to forecast hail size in Alberta. The hail model was skillful in forecasting hail (POD = 0.85, FAR = 0.26, HSS = 0.64). On days with hail larger than 2 cm in diameter, the hail model performed slightly better (POD = 0.90, FAR = 0.40, HSS = 0.67). Analysis of the skill scores and hail-size forecasts suggests that employing a coupled cloud and hail model noticeabl...

Numerical simulations of radar rainfall error propagation

Abstract: [1] The primary advantage of radar observations of precipitation compared with traditional rain gauge measurements is their high spatial and temporal resolution and large areal coverage. Unfortunately, radar data require vigorous quality control before being converted into precipitation products that can be used as input to hydrologic models. In this study we coupled a physically based atmospheric model of convective rainfall with an active microwave radiative transfer model to simulate radar observation of thunderstorms. We used the atmospheric model to simulate a well-documented tornadic supercell storm that occurred near Del City, Oklahoma, on 20 May 1977. We then generated radar observations of that storm and used them to evaluate the propagation of radar rainfall errors through distributed hydrologic simulations. This physically based methodology allows us to directly examine the impact of radar rainfall estimation errors on land-surface hydrologic predictions and to avoid the limitations imposed by the use of rain gauge data. Results indicate that the geometry of the radar beam and coordinate transformations, due to radar-watershed-storm orientation, have an effect on radar rainfall estimation and runoff prediction errors. In addition to uncertainty in the radar reflectivity versus rainfall intensity relationship, there are significant range-dependent and orientation-related radar rainfall estimation errors that should be quantified in terms of their impact on runoff predictions. Our methodology provides a tool for performing experiments that address some operational issues related to the process of radar rainfall estimation and its uses in hydrologic prediction.

Upstream influence of numerically simulated squall‐line storms

Abstract: SUMMARY The squall line’s impact on its upstream environment is examined using traditional cloud and simplieed (parametrized moisture) models. The study was motivated by the need to explain signie cant differences between the two dynamical frameworks. In both, the erst environmental response to the convection consists of a rapidly propagating gravity wave characterized by deep tropospheric subsidence. This gravity wave accelerates the lowlevel storm ine ow in its wake, with the largest effect seen near the surface. In the more sophisticated model, however, this wave is eventually followed by a shorter vertical wavelength feature, one possessing lower-tropospheric ascent. This second gravity wave, absent from the simplieed model runs, substantially alters the upstream environment yet again. The gentle low-level uplift establishes the ‘cool/moist tongue’ ofairthat has been found to stretch well ahead of the storm insimulations.Between thesecond wave and the main storm updraught, the accelerated ineow is shifted from the ground to the middle troposphere where it helps to push dry air into the convecting region. This subsequent environmental adjustment responds to the establishment of a small yet persistent area of weak cooling during the early mature phase. Located in the middle troposphere at the upstream edge of the main cloud mass, this cooling is the combined effect of cloud-water evaporation and the ascent of subsaturated air. Though the cooling that excites it is of relatively small magnitude (<2 K), this wave’s effect is dramatic and signie cant. A crude ex to the moisture parametrization served to establish a qualitatively similar tongue-like feature in the simplie ed model simulations, bringing the results from the two dynamical frameworks more into line.

Dynamic response of structures to thunderstorm winds

Abstract: A study of the dynamic response of structure to different types of wind is reported in this paper. Response to large-scale (monsoon wind) and small-scale winds (thunderstorm) of a SDOF structure is investigated. The monsoon wind is observed to be relatively stationary, and structure response predicted by the gust response factor (GRF) method is reasonably accurate. However, for thunderstorm events, the wind processes are observed to be non-stationary, and predictions using GRF method very much under-estimate the actual response. A modification to the GRF method is proposed for thunderstorm winds whereby a running-mean procedure is introduced. The fluctuation about the running-mean is observed to be more stationary. The result obtained from the running-mean GRF method is more accurate. For the thunderstorm events studied, the best predictions are obtained for a running-mean period of around 60 s.

Influences of the Local Environment on Supercell Cloud-to-Ground Lightning, Radar Characteristics, and Severe Weather on 2 June 1995*

Abstract: Radar, cloud-to-ground (CG) lightning characteristics, and storm reports were documented for 20 long-lived supercell thunderstorms that occurred during a 6-h period in the west Texas Panhandle on 2–3 June 1995. These thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary. Storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low-level rotation, lower maximum heights for the 40-dBZ echo top, and had the largest negative CG flash rates. The largest negative flash rate was produced as each storm was gradually weakening. In contrast, out of 11 boundary-crossing storms, several important radar-based measurands increased unambiguously after storms crossed the boundary: 40-dBZ echo-top height in 5 cases, radar reflectivity above the environmental freezing level in 6 cases, and low-level mesocyclone strength in 9 cases. Trends of the first two measurands were ambiguous for 4 of 11 cases aff...

A model for large‐scale convective storms in Jupiter

Abstract: [1] Large-scale convective storms are a common phenomenon in Jupiter's atmosphere. They are apparent in ground-based and spacecraft images and may strongly affect the dynamics of the global atmosphere as well as the energy transport in the meteorological layer. In this paper we analyze the outburst of a large convective storm system (core ∼5000 km) in the South Equatorial Belt (SEB) that was observed at high spatial and temporal resolution by the Voyager 1 spacecraft in 1979. We use a two-dimensional (2-D) model to study the interaction between cloud material brought up by moist convection and the environmental wind. Aided by previous 3-D models of Jovian storms, we can draw several qualitative and quantitative conclusions. The evolution of this storm can be characterized by three phases: (1) onset of the perturbation, well reproduced by the growth of a single-cell storm; (2) an expanding phase in which the number of convective cells increases to ∼200, with updraft velocities limited to 50 m/s; and (3) a relatively sudden suppression of the convective activity leading to the disruption of large structures by the environmental wind. Furthermore, we interpret the observations of the inner bright core as a well-defined anticyclonic vortex, with darker cloud material preferentially left southwest of the system. Finally, we show that ∼1016 W are released by such a storm over its life cycle of 12 days and that the direct formation of a large-scale anticyclonic vortex after the moist convective source has been removed is prevented by the environmental wind shear.

A Re-analysis of Recorded Extreme Wind Speeds in Region A

Abstract: In this paper, recent analyses of extreme wind speeds for structural design, for locations in Region A, covering most of the populated area of Australia, are described. The method of ‘peaks over th...

Method and apparatus for short-term prediction of convective weather

Abstract: A method and apparatus for forecasting the likely occurrence of convective weather events, such as thunderstorms. An image filter is used to identify areas of interest within a meteorological image that are likely to contain convective weather. The image filter and an image difference processor identify sub-image regions within the meteorological image that are likely to experience a growth and/or decay of weather events. The classification filter classifies sub-image regions within the meteorological image into a number of predetermined storm categories. The meteorological images are filtered using matched filters, features within the filtered images are tracked, and the resulting track vectors are combined according to the storm classification. The meteorological image, interest image, growth/decay image, classification image, and combined vectors are processed to produce the short-term forecast.

Observable signatures of a convectively generated wave field over the tropics using Indian MST radar at Gadanki (13.5°N, 79.2°E)

Abstract: [1] An experiment was conducted in the month of June 2000 to study the atmospheric dynamical behavior in the troposphere and lower stratosphere in the presence of a cloudy and moist background environment due to the onset of the summer monsoon over Indian peninsula. The cardinal objective was to observe the gravity wave signatures in the convective environment. We successfully monitored the development, maximizing and weakening of convective phenomena within a time span of 3–4 hours on 21–22 June 2000. The vertical wind in the troposphere during this period was found about 6–8 m/s. Variations of the radar returned signal power for the vertical beam with height and time show the continuous growth of high turbulence in the ambient air, ascending as high as the tropopause. The satellite cloud images exhibited the temporal growth and movement of cloud clusters over the region of the radar site during this period.

Gravity wave characteristics over Tromelin Island during the passage of cyclone Hudah

Abstract: [1] The time and spatial evolution of gravity-wave characteristics are analysed using wavelets in vertical profiles of temperature and winds at Tromelin Island (15.53°S, 54.31°E) during the passage of the intense tropical cyclone Hudah in the Southern Ocean Indian Basin in 2000. Inertia-gravity waves were observed in the upper troposphere and the lower stratosphere with dominant vertical wavelengths of 1.5–3 km, horizontal wavelengths <2000 km and periods of 0.6–1.6 days. Large amounts of gravity-wave energy were detected during landfalls of the tropical cyclone. The distribution of total energy indicates that mesoscale convective structures such as tropical cyclones are important gravity-wave sources in the upper troposphere.

Using Landsat to Identify Thunderstorm Damage in Agricultural Regions

Abstract: During 12 and 18 August 1999, severe thunderstorms produced damaging winds and hail that caused an estimated $50 million in damage to agriculture in west-central Illinois. Landsat-7 imagery was obtained to determine the arealextent of damage and produce a crop damage dollar estimate. The normalized difference vegetation index (NDVI) was calculated for images taken “before” and “after” the severe thunderstorm events in order to examine the changes in NDVI, or vegetation vigor. A differenced image was also produced by subtracting the NDVI of the two images. Landsat-7 data was found to be useful for identifying the areal extent of severe thunderstorm damage in west-central Illinois. In comparing the detection of damage produced by high winds and hail, it was found that hail damage was considerably easier to identify. This is due to the fact that large hail typically destroys the crops while high winds blow over corn plants that can remain rooted and survive. Additionally, county estimates of dollar ...

Combined Wind Profiler/Polarimetric Radar Studies of the Vertical Motion and Microphysical Characteristics of Tropical Sea-Breeze Thunderstorms

Abstract: An experiment combining wind profiler and polarimetric radar analyses of intense, but shallow, tropical thunderstorms has been performed. These storms are important as they are very common along many tropical coasts and islands and are sometimes the precursors to large intense multicellular storms such as occur over the Tiwi Islands north of Darwin, Australia. All the storms sampled had a similar structure, with intense updrafts on the periphery of the cells producing significant-sized hail and downdrafts in the storm center. The hail concentrations are relatively small, but have a large effect on the radar reflectivity and polarimetric measurands because of the size (10–20 mm). It is this hail melting that produces characteristic ZDR columns in the polarimetric radar data.

Cloud-Resolving Simulations of Mesoscale Vortex Intensification and Its Effect on a Serial Mesoscale Convective System

Abstract: The authors performed numerical simulations of the 27–28 May 1998 mesoscale convective vortex (MCV) and mesoscale convective system (MCS) to understand how the MCV modulated the convection, and how the convection intensified the vortex. The fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model (MM5) was used, initialized with the analysis from the Rapid Update Cycle version 2 (RUC-2) and nested to achieve 1.5-km grid spacing covering the MCS. The model successfully simulated a north–south convective band that formed near the center of the MCV in the early evening. The simulation then correctly reoriented the convection along a roughly east–west line in response to northward transport of warm, conditionally unstable air within the nocturnal low-level jet. Balanced vertical motion was found to contribute substantially to mesoscale lifting and thermodynamic destabilization, which localized the convection. Horizontal transport of moist, uns...

The Cool-Season Tornadoes of California and Southern Australia

Abstract: Examples of cool-season tornadic thunderstorms in California and southern Australia are examined. Almost one-half of the reported Australian tornadoes and the majority of those in California occur in the cool season. It is shown that in both areas the typical synoptic pattern shows an active midlatitude trough just upstream, with a strong jet streak aloft. In both areas the tornadic thunderstorms occur with weak to moderate levels of thermodynamic instability in the lower troposphere but with extremely high values of low-level positive and bulk shear. Statistical tests on null cases (nontornadic thunderstorms) in the Central Valley of California indicate that large values of 0–1-km shear provide a discriminator for more damaging (F1–F3) tornadoes, whereas bulk measures of buoyancy, such as CAPE, do not. Australian case studies and tornado proximity soundings show similar characteristics. A “cool-season tornadic thunderstorm potential” diagnostic for Australian conditions, based on regional NWP an...