Showing papers on "Thunderstorm published in 2003"

Global frequency and distribution of lightning as observed from space by the Optical Transient Detector

Abstract: of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of 10:1 The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl km 2 yr 1 in Rwanda, and includes an area of over 3 million km 2 exhibiting flash densities greater than 30 fl km 2 yr 1 (the flash density of central Florida) Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle INDEX TERMS: 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3324 Meteorology and Atmospheric Dynamics: Lightning; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques;

The Advanced Regional Prediction System (ARPS), storm-scale numerical weather prediction and data assimilation

Abstract: In this paper, we first describe the current status of the Advanced Regional Prediction System of the Center for Analysis and Prediction of Storms at the University of Oklahoma. A brief outline of future plans is also given. Two rather successful cases of explicit prediction of tornadic thunderstorms are then presented. In the first case, a series of supercell storms that produced a historical number of tornadoes was successfully predicted more than 8 hours in advance, to within tens of kilometers in space with initiation timing errors of less than 2 hours. The general behavior and evolution of the predicted thunderstorms agree very well with radar observations. In the second case, reflectivity and radial velocity observations from Doppler radars were assimilated into the model at 15-minute intervals. The ensuing forecast, covering a period of several hours, accurately reproduced the intensification and evolution of a tornadic supercell that in reality spawned two tornadoes over a major metropolitan area. These results make us optimistic that a model system such as the ARPS will be able to deterministically predict future severe convective events with significant lead time. The paper also includes a brief description of a new 3DVAR system developed in the ARPS framework. The goal is to combine several steps of Doppler radar retrieval with the analysis of other data types into a single 3-D variational framework and later to incorporate the ARPS adjoint to establish a true 4DVAR data assimilation system that is suitable for directly assimilating a wide variety of observations for flows ranging from synoptic down to the small nonhydrostatic scales.

Ion-aerosol-cloud processes in the lower atmosphere

Abstract: [1] Natural terrestrial radioactivity and cosmic ray ionization lead to the formation of air ions and charged aerosol particles even away from regions of active charge separation, such as in thunderstorms. The natural electrified state of the atmosphere has been studied for over a century; however, the effect of ionization on the physical properties of aerosols and clouds has rarely been studied in its own right except in thunderstorms. Here we review the status of our understanding of atmospheric charged particles and their influence on aerosol and cloud microphysical processes. We also review mechanisms that have been recently proposed to connect variations in the atmospheric ionization rate with variations in global cloudiness and weather systems. We conclude that a mechanism linking cosmic ray ionization and cloud properties cannot be excluded and that there are established electrical effects on aerosol and cloud microphysics. Necessary further work includes measurements of cloud, droplet, and aerosol charging and ion-aerosol conversion, together with modeling of the electrical aspects of nonthunderstorm cloud microphysics.

Gigantic jets between a thundercloud and the ionosphere.

Abstract: Transient luminous events in the atmosphere, such as lighting-induced sprites and upwardly discharging blue jets, were discovered recently in the region between thunderclouds and the ionosphere. In the conventional picture, the main components of Earth's global electric circuit include thunderstorms, the conducting ionosphere, the downward fair-weather currents and the conducting Earth. Thunderstorms serve as one of the generators that drive current upward from cloud tops to the ionosphere, where the electric potential is hundreds of kilovolts higher than Earth's surface. It has not been clear, however, whether all the important components of the global circuit have even been identified. Here we report observations of five gigantic jets that establish a direct link between a thundercloud (altitude approximately 16 km) and the ionosphere at 90 km elevation. Extremely-low-frequency radio waves in four events were detected, while no cloud-to-ground lightning was observed to trigger these events. Our result indicates that the extremely-low-frequency waves were generated by negative cloud-to-ionosphere discharges, which would reduce the electrical potential between ionosphere and ground. Therefore, the conventional picture of the global electric circuit needs to be modified to include the contributions of gigantic jets and possibly sprites.

Effects of charge and electrostatic potential on lightning propagation

Abstract: [1] Three-dimensional lightning mapping observations are compared to cloud charge structures and electric potential profiles inferred from balloon soundings of electric field in New Mexico mountain thunderstorms. For six individual intracloud and cloud-to-ground flashes and for a sequence of 36 flashes in one storm, the comparisons consistently show good agreement between the altitudes of horizontal lightning channels and the altitudes of electric potential extrema or wells. Lightning flashes appear to deposit charge of opposite polarity in relatively localized volumes within the preexisting lower positive, midlevel negative, and upper positive charge regions associated with the potential wells. The net effect of recurring lightning charge deposition at the approximate levels of potential extrema is to increase the complexity in the observed storm charge structure. The midlevel breakdown of both normal intracloud flashes and negative cloud-to-ground flashes is observed to be segregated by flash type into the upper and lower parts of the deep potential well associated with the midlevel negative charge. The segregation is consistent with perturbations observed in the bottom of the negative potential well due to embedded positive charge that was probably deposited by earlier flashes. It is also consistent with an expected tendency for vertical breakdown to begin branching horizontally before reaching the local potential minimum. The joint observations reconcile the apparent dichotomy between the complex charge structures often inferred from balloon soundings through storms and the simpler structures often inferred from lightning measurements.

Moisture plumes above thunderstorm anvils and their contributions to cross-tropopause transport of water vapor in midlatitudes

Abstract: Water vapor in the lower stratosphere may play significant roles in the atmosphericradiative budget and atmospheric chemistry; hence it is important to understand itstransport process. The possibility of water vapor transport from the troposphere to thestratosphere by deep convection is investigated using three-dimensional, nonhydrostatic,quasi-compressible simulations of a Midwest severe thunderstorm. The results show thatthe breaking of gravity waves at the cloud top can cause cloud water vapor to be injectedinto the stratosphere in the form of plumes above a thunderstorm anvil. Meteorologicalsatellites and aircrafts have observed such plumes previously, but the source of watervapor and the injection mechanism were not identified. The present results reveal thatthere are two types of plumes, anvil sheet plumes and overshooting plumes, in thisinjection process and that the process is diabatic. A first-order estimate of this plumetransport of water vapor per day from the upper troposphere to the lower stratosphere wasmade assuming that all thunderstorms behave the same as the one simulated. Other tracechemicals may also be similarly transported by the same mechanism.

Nowcasting Storm Initiation and Growth Using GOES-8 and WSR-88D Data

Abstract: The evolution of cumulus clouds over a variety of radar-detected, boundary layer convergence features in eastern Colorado has been examined using Geostationary Operational Environmental Satellite (GOES) imagery and Weather Surveillance Radar-1988 Doppler (WSR-88D) data. While convective storms formed above horizontal rolls in the absence of any additional surface forcing, the most intense storms initiated in regions above: gust fronts, gust front interaction with horizontal rolls, and terrain-induced stationary convergence zones. The onset of vigorous cloud growth leading to storm development was characterized by cloud tops that reached subfreezing temperatures and exhibited large cooling rates at cloud top 15 min prior to the first detection of 10dBZ radar echoes aloft and 30 min before 35 dBZ. The rate of cloud-top temperature change was found to be important for discriminating between weakly precipitating storms (,35 dBZ) and vigorous convective storms (.35 dBZ). Results from this study have been used to increase the lead time of thunderstorm initiation nowcasts with the NCAR automated, convective storm nowcasting system. This improvement is demonstrated at two operational forecast offices in Virginia and New Mexico.

A review of positive and bipolar lightning discharges

Abstract: Characteristics of lightning discharges that transport either positive charge or both positive and negative charges to the ground are reviewed. These are termed positive and bipolar lightning discharges, respectively. Different types of positive and bipolar lightning are discussed. Although positive lightning discharges account for 10% or less of global cloud-to-ground lightning activity, there are five situations that appear to be conducive to the more frequent occurrence of positive lightning. These situations include 1) the dissipating stage of an individual thunderstorm, 2) winter thunderstorms, 3) trailing stratiform regions of mesoscale convective systems, 4) some severe storms, and 5) thunderclouds formed over forest fires or contaminated by smoke. The highest directly measured lightning currents (near 300 kA) and the largest charge transfers (hundreds of coulombs or more) are thought to be associated with positive lightning. Two types of impulsive positive current waveforms have been obse...

The Environment of Warm-Season Elevated Thunderstorms Associated with Heavy Rainfall over the Central United States

Abstract: Twenty-one warm-season heavy-rainfall events in the central United States produced by mesoscale convective systems (MCSs) that developed above and north of a surface boundary are examined to define the environmental conditions and physical processes associated with these phenomena. Storm-relative composites of numerous kinematic and thermodynamic fields are computed by centering on the heavy-rain-producing region of the parent elevated MCS. Results reveal that the heavy-rain region of elevated MCSs is located on average about 160 km north of a quasi-stationary frontal zone, in a region of low-level moisture convergence that is elongated westward on the cool side of the boundary. The MCS is located within the left-exit region of a south-southwesterly lowlevel jet (LLJ) and the right-entrance region of an upper-level jet positioned well north of the MCS site. The LLJ is directed toward a divergence maximum at 250 hPa that is coincident with the MCS site. Near-surface winds are light and from the southeast within a boundary layer that is statically stable and cool. Winds veer considerably with height (about 1408) from 850 to 250 hPa, a layer associated with warm-air advection. The MCS is located in a maximum of positive equivalent potential temperature ue advection, moisture convergence, and positive thermal advection at 850 hPa. Composite fields at 500 hPa show that the MCS forms in a region of weak anticyclonic curvature in the height field with marginal positive vorticity advection. Even though surfacebased stability fields indicate stable low-level air, there is a layer of convectively unstable air with maximumue CAPE values of more than 1000 J kg21 in the vicinity of the MCS site and higher values upstream. Maximumue convective inhibition (CIN) values over the MCS centroid site are small (less than 40 J kg 21) while to the south convection is limited by large values of CIN (greater than 60 J kg 21). Surface-to-500-hPa composite average relative humidity values are about 70%, and composite precipitable water values average about 3.18 cm (1.25 in.). The representativeness of the composite analysis is also examined. Last, a schematic conceptual model based upon the composite fields is presented that depicts the typical environment favorable for the development of elevated thunderstorms that lead to heavy rainfall.

Effects on the ionosphere due to phenomena occurring below it

Abstract: The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (“meteorological influences”) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. “Ionospheric weather”, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.

Lightning Activity over Land and Sea on the Eastern Coast of the Mediterranean

Abstract: This paper presents a study of the characteristics of lightning activity during the Cyprus low winter storms over the eastern coast of the Mediterranean. The focus is on changes in the nature of thunderstorms crossing the coastline from the sea into the northern and central parts of Israel, as manifested in their electrical activity. It is based on the Lightning Position and Tracking System (LPATS) measurements of lightning ground strikes during four winter seasons between 1995 and 1999. The spatial distribution shows a maximum of lightning ground strikes over Mount Carmel, possibly due to its topographical forcing. The annual variation shows a major maximum in January with two minor peaks, one in November and another in March, which can be explained by changes in the static instability of the atmosphere throughout the rainy period. The average fraction of positive ground flashes was found to be 6% and their average peak current 141 kA. The average peak current of negative ground flashes was 227 kA. Larger frequencies of ground flashes were detected over the sea than over land during the study period. This is probably due to the large heat and humidity fluxes from the sea surface, which destabilize the colder air above and drive cloud convection. The annual distribution shows that during midwinter (December‐January‐ February) there is higher flash density over the sea, while during autumn and spring the flash density is similar above the two regions. The diurnal variation shows that the maximum in maritime lightning activity was at 0500 LST and over land at 1300 LST. The mean peak current of positive ground flashes was higher over land and of negative ground flashes, over the sea.

Observational‐ and modeling‐based budget of lightning‐produced NOx in a continental thunderstorm

Abstract: NO x transport and production by lightning for the 10 July 1996 Stratosphere-Troposphere Experiment-Radiation, Aerosols, and Ozone convective storm is examined using radar, in situ observations and cloud model simulations. Observations and model simulations indicate that most of the NO x produced by the storm was transported out into the anvil. The analyzed NO x flux into the anvil, combined with results from a cloud model simulation, indicate that approximately 60 percent of the NO x transported into the anvil during the observational period is produced by lightning. Lightning flash rate and channel length measurements, obtained using the Office National d'Etudes et de Recherche Arospatiales lightning interferometer, are combined with the NO x budget to give estimates of average lightning NO x production per interferometer flash and per unit flash channel length. The analysis yields production rates of approximately 43.2 moles (2.6 × 10 25 molecules) NO x per interferometer flash and 1.7 x 10 -3 moles (1.0 x 10 21 molecules) NO x per meter of flash channel. These production rates fall within the bounds of rates derived in previous studies using completely different approaches.

Evolution of Cloud-to-Ground Lightning and Storm Structure in the Spencer, South Dakota, Tornadic Supercell of 30 May 1998

Abstract: On 30 May 1998, a tornado devastated the town of Spencer, South Dakota. The Spencer tornado (rated F4 on the Fujita tornado intensity scale) was the third and most intense of five tornadoes produced by a single supercell storm during an approximate 1-h period. The supercell produced over 76% positive cloud-to-ground (CG) lightning and a peak positive CG flash rate of 18 flashes min−1 (5-min average) during a 2-h period surrounding the tornado damage. Earlier studies have reported anomalous positive CG lightning activity in some supercell storms producing violent tornadoes. However, what makes the CG lightning activity in this tornadic storm unique is the magnitude and timing of the positive ground flashes relative to the F4 tornado. In previous studies, supercells dominated by positive CG lightning produced their most violent tornado after they attained their maximum positive ground flash rate, whenever the rate exceeded 1.5 flashes min−1. Further, tornadogenesis often occurred during a lull in C...

Shear Parameter Thresholds for Forecasting Tornadic Thunderstorms in Northern and Central California

Abstract: A study of 39 nontornadic and 30 tornadic thunderstorms (composing 25 tornado “events,” as defined in the text) that occurred in northern and central California during the period 1990–94 shows that stratification of the stronger tornadic events (associated with F1 or greater tornadoes) on the basis of 0–1- and 0–6-km positive and bulk shear magnitudes is justified statistically. Shear values for the weaker F0 events could not be distinguished statistically from the “background” values calculated for the nontornadic (null) thunderstorm events observed during the period. Shear magnitudes calculated for the F1/F2 events suggest that these tornadoes had developed in an environment supportive of supercell convection. Hindcasting the tornado events based upon shear thresholds produced a high probability of detection (POD) and low false alarm ratio (FAR), particularly for the stronger (F1/F2) events. Although the current sample size is limited and the conclusions drawn from it should be considered preli...

Acoustic waves in the upper mesosphere and lower thermosphere generated by deep tropical convection

Abstract: [1] A time-dependent, nonlinear, fully compressible, axisymmetric, f-plane, numerical model is used to simulate the propagation of acoustic waves in the mesosphere and thermosphere by intense deep convection in the troposphere. The simulations show that major convective storms in the tropics launch acoustic waves into the mesosphere-thermosphere directly above the storm centers. The principal feature of the overhead acoustic wave field in the period interval of ∼3 to 5 min is a trapped oscillation below about 80 km altitude with a period of ∼5 min and a nearly vertically propagating wave with about a 3-min period above this height. Acoustic oscillations in the troposphere-mesosphere duct have a standing wave character directly over the storm; the oscillations that propagate off vertical mainly reflect off the top and bottom of the duct resulting in a horizontally propagating acoustic field within the duct. The acoustic oscillations that are mainly confined to the troposphere-mesosphere duct persist for about an hour after the storm has ended. The vertically propagating thermospheric acoustic oscillations are waves propagating upward from the thunderstorm source through the stratosphere-mesosphere. These predominantly 3-min waves are strongly driven for ∼30 min after the storm event and weaken with time thereafter. The vertically propagating thermospheric acoustic waves may be the source of the F-region 3-min oscillations. Intense acoustic disturbances directly above thunderstorms may be responsible for localized heating of the thermosphere.

Narrow Bipolar Events as indicators of thunderstorm convective strength

Abstract: [1] Narrow Bipolar Events (NBEs) are impulsive in-cloud lightning discharges that are commonly detected by both ground-based and satellite-based radio-frequency sensors. In this paper, NBE flash rates are shown to be statistically correlated to cloud-to-ground (CG) lightning flash rates as recorded by an array of electric-field-change sensors located in Florida. In addition, NBE source heights are found to generally increase with increasing NBE flash rates. The inference is that NBE flash rates and source heights are generally driven by the intensity/strength of the convective updraft in thunderstorms. As a consequence, NBEs represent a meteorologically important type of lightning and provide an excellent target of opportunity for future satellite-based very high frequency (VHF) global lightning monitors.

Phase-Correcting Data Assimilation and Application to Storm-Scale Numerical Weather Prediction. Part II: Application to a Severe Storm Outbreak

Abstract: A scheme to correct phase errors in numerical model forecasts using Doppler radar, radiosonde, profiler, and surface data is demonstrated to improve forecasts in a complex severe thunderstorm situation. The technique is designed to directly address forecast phase errors or initial position errors as part of a data assimilation strategy. In the demonstration the phase error correction is applied near the time of initial cell development and the forecast results are compared to the uncorrected forecast and forecasts made using an analysis created at the time of the observations. Forecasts are verified qualitatively for the position of thunderstorm cells and quantitatively for accumulated precipitation. It is shown that the scheme can successfully correct errors in thunderstorm locations and it has a positive influence on the subsequent forecast. The advantage of the phase correction over the control lasts for about 3 h despite storm dissipation and regeneration, and interactions among multiple storms.

Compilation and Discussion of Trends in Severe Storms in the United States: Popular Perception v. Climate Reality

Abstract: The ongoing greenhouse gas buildup and increase in near-surface air temperatures may have an impact on severe weather events in the United States. Output from some numerical modeling simulations suggests that the atmosphere over mid-latitude land areas could become more unstable in the future thereby supporting an increase in convective activity. However, despite the numerical simulation results, empiricists have been unable generally to identify significant increases in overall severe storm activity as measured in the magnitude and/or frequency of thunderstorms, hail events, tornadoes, hurricanes, and winter storm activity across the United States. There is evidence that heavy precipitation events have increased during the period of historical records, but for many other severe weather categories, the trends have been downward over the past half century. Damage from severe weather has increased over this period, but this upward trend disappears when inflation, population growth, population redistribution, and wealth are taken into account.

Numerical modeling of lightning‐produced NOx using an explicit lightning scheme: 1. Two‐dimensional simulation as a “proof of concept”

Abstract: [1] We use the two-dimensional (2D) version of our Storm Electrification Model to test its potential for studying lightning-produced NO x . We assume that NO production is a function of energy dissipation and calculate this value from the electric field before and after each lightning flash. We use a production rate of 9.2 x 10 16 molecules joule -1 to generate the NO. Using a limited set of chemical reactions involving NO, NO 2 , and O 3 , we simulated a small storm with 10 intracloud lightning flashes produced over a 2-min span. Their energy dissipation ranged between 0.024 and 0.28 GJ. The simulation was run an additional 18 min after the cessation of lightning. Our results show that the parameterization produced NO mixing ratios internal to the cloud of the order of 10 ppbv after the most energetic flashes and 1-2 ppbv in the upwind portion of the anvil toward the end of the simulation. These mixing ratios are shown to be comparable to observations in a generic sense. Comparison with the C-shaped profiles developed by Pickering et al. [1998], also using a 2D model, show similarities, but our results are more weighted toward larger values at higher altitudes than those of Pickering et al. This may be due to differences in the length of the simulation, a lack of cloud-to-ground lightning in our work, a lack of reactive chemistry in Pickering et al., or the use by Pickering et al. of the assumption of Price et al. [1997] that intracloud flashes dissipate one tenth the energy of cloud-to-ground flashes. We show, using recent observational data and an analysis of the assumptions of Price et al., that this one tenth energy dissipation assumption is not appropriate. We conclude that our use of an explicit lightning scheme to study NO production at the process level is a viable methodology.

Diurnal variation of surface electric field at a tropical station in different seasons: a study of plausible influences

Abstract: Based on the fair weather days of 1993 surface electric field at Pune (18 32 N, 73 51 E, 550 m ASL) is studied for its diurnal and seasonal variations. The analysis attempts to look into factors like atmospheric pollution and space charge formation in presence of the meteorological factors prevalent in the three seasons that modify the global effects that govern fair weather electric field at the site of measurement. Summer (FMAM), monsoon (JJAS) and winter (ONDJ) are the seasons considered. By and large diurnal variation is with double oscillation. The results show high persistent value of electric field during monsoon. Evening peak is the highest during winter with high atmospheric stability conditions and summer afternoons have the lowest value. Space charge formation depending on the wetness or dryness of the ground is proposed in connection with the high and low values of electric field close to ground. It appears that space charge is equally or more important in contributing to electric field variations as conductivity as for local effects. The generation of positive space charge occurrence in connection with evaporation and negative space charge occurrence with very dry ground bypassing the local component to global one due to large area coverage elsewhere as well, under similar weather conditions are suggested. Possibility of monsoon field showing a glimpse of global activities being a cleaner tropical environment during this period is also discussed. Observational verification of space charge formation and the role it could be playing, on abundantly available, by the then active physical processes plays on thunderstorm electrification based on convective theory is mooted.