Showing papers on "Thunderstorm published in 2006"

Where are the most intense thunderstorms on earth

Abstract: The instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite have been observing storms as well as rainfall since December 1997. This paper shows the results of a systematic search through seven full years of the TRMM database to find indicators of uncommonly intense storms. These include strong (> 40 dBZ) radar echoes extending to great heights, high lightning flash rates, and very low brightness temperatures at 37 and 85 GHz. These are used as proxy variables, indicating powerful convective updrafts. The main physical principles supporting this assertion involve the effects of such updrafts in producing and lofting large ice particles high into the storm, where TRMM's radar easily detects them near storm top. TRMM's passive microwave radiometer detects the large integrated ice water path as very low brightness temperatures, while high lightning flash rates are a physically related but instrumentally independent indicator. The geographical locations of these very intense convective storms ...

3DVAR and Cloud Analysis with WSR-88D Level-II Data for the Prediction of the Fort Worth, Texas, Tornadic Thunderstorms. Part I: Cloud Analysis and Its Impact

Abstract: In this two-part paper, the impact of level-II Weather Surveillance Radar-1988 Doppler (WSR-88D) reflectivity and radial velocity data on the prediction of a cluster of tornadic thunderstorms in the Advanced Regional Prediction System (ARPS) model are studied. Radar reflectivity data are used primarily in a cloud analysis procedure that retrieves the amount of hydrometeors and adjusts in-cloud temperature, moisture, and cloud fields, while radial velocity data are analyzed through a three-dimensional variational (3DVAR) scheme that contains a mass divergence constraint in the cost function. In Part I, the impact of the cloud analysis and modifications to the scheme are examined while Part II focuses on the impact of radial velocity and the mass divergence constraint. The case studied is that of the 28 March 2000 Fort Worth, Texas, tornado outbreaks. The same case was studied by Xue et al. using the ARPS Data Analysis System (ADAS) and an earlier version of the cloud analysis procedure with WSR-88...

Performance Assessment of the World Wide Lightning Location Network (WWLLN), Using the Los Alamos Sferic Array (LASA) as Ground Truth

Abstract: The World Wide Lighting Location Network (WWLLN) locates lightning globally, using sparsely distributed very low frequency (VLF) detection stations. Due to WWLLN’s detection at VLF (in this case ∼10 kHz), the lightning signals from strong strokes can propagate up to ∼104 km to WWLLN sensors and still be suitable for triggering a station. A systematic evaluation of the performance of WWLLN is undertaken, using a higher-frequency (0–500 kHz) detection array [the Los Alamos Sferic Array (LASA)] as a ground truth during an entire thunderstorm season in a geographically confined case study in Florida. It is found that (a) WWLLN stroke-detection efficiency rises sharply to several percent as the estimated lightning current amplitude surpasses ∼30 kA; (b) WWLLN spatial accuracy is around 15 km, good enough to resolve convective-storm cells within a larger storm complex; (c) WWLLN is able to detect intracloud and cloud-to-ground discharges with comparable efficiency, as long as the current is comparable;...

3DVAR and Cloud Analysis with WSR-88D Level-II Data for the Prediction of the Fort Worth, Texas, Tornadic Thunderstorms. Part II: Impact of Radial Velocity Analysis via 3DVAR

Abstract: In this two-part paper, the impact of level-II Weather Surveillance Radar-1988 Doppler (WSR-88D) radar reflectivity and radial velocity data on the prediction of a cluster of tornadic thunderstorms in the Advanced Regional Prediction System (ARPS) model is studied. Radar reflectivity data are used primarily in a cloud analysis procedure that retrieves the amount of hydrometeors and adjusts in-cloud temperature, moisture, and cloud fields, while radial velocity data are analyzed through a three-dimensional variational (3DVAR) data assimilation scheme that contains a 3D mass divergence constraint in the cost function. In Part I, the impact of the cloud analysis and modifications to the scheme are discussed. In this part, the impact of radial velocity data and the mass divergence constraint in the 3DVAR cost function are studied. The case studied is that of the 28 March 2000 Fort Worth tornadoes. The addition of the radial velocity improves the forecasts beyond that experienced with the cloud analys...

"Sprites, Elves and Intense Lightning Discharges"

Abstract: Contributing Authors. Preface.- Introduction to the Physics of Sprites, Elves and Intense Lightning Discharges M.J. Rycroft. 1.1 Basic Properties of the Atmosphere. 1.2 Risk Theory of Electrical Phenomena Occurring in the Atmosphere. 1.3 The Properties of Sprites, Elves and Intense Lightning Discharges. 1.4 Introduction to Theories and Numerical Modelling of Sprites. 1.5 Conclusions. Acknowledgements.- The Meteorology of Transient Luminous an Introduction and Overview W.A. Lyons. 2.1 Introduction. 2.2 Observations of Convective Phenomena. 2.3 A Brief History of TEE Observations. 2.4 Characteristics of TLE-Parent Lightning and Storms. 2.5 Research Frontiers Acknowledgments.- The Microphysical and Electrical Properties of Sprite-Producing Thunderstorms E.Williams and Y. Yair. 3.1 Introduction. 3.2 The Non-Inductive Charging Process in Thunderclouds. 3.3 Cloud Scale Charge Structure Possible with the Non-Inductive Mechanism. 3.4 The Electrical Structure Inside Sprite-Producing Storms in Summer-time. 3.5 The Electrical Structure inside Sprite-Producing Storms in Wintertime. 3.6 Gaps in Knowledge and Future Needs. Acknowledgments.- Global Thunderstorm Activity C. Price. 4.1 The Earth's Energy Balance. 4.2 The General Circulation of the Atmosphere. 4.3 Frontal Thunderstorms in Mid-Latitude Regions. 4.4 Global Observations of Lightning. 4.5 The Global Atmospheric Electric Circuit. 4.6 Future Directions.- Imaging Systems in the Research Thomas H. Allin et al. 5.1 Introduction to Low Light Imaging. 5.2 The Spritewatch Systems. 5.3 Conclusions.- Spacecraft Based Studies of Transient Luminous Events S.B. Mende et al. 6.1 Introduction. 6.2 FORMOSAT-2 Satellite and the ISUAL Instrument. 6.3 Initial Observations with ISUAL. 6.4 Summary. Acknowledgments.- Observations of Sprites From Space at the Nadir: The LSO (Lightning and Sprite Observations) Experiment on Board of the International Space Station E. Blanc et al. 7.1 Introduction. 7.2 Spectral Differentiation of Sprite and Lightning Emissions. 7.3 Experiment. 7.4 Observations. 7.5 Perspectives.- Remote Sensing of the Upper Atmosphere by VLF C.J. Rodger and R.J. McCormick. 8.1 Ionospheric Conductivity. 8.2 Sources of VLF Electromagnetic (EM) Waves. 8.3 VLF Propagation in the Earth-Ionosphere Waveguide. 8.4 Relaxation of High-Altitude Ionospheric Modifications. 8.5 Summary.- Measurements of Lightning Parameters from Remote Electromagnetic Fields S.A. Cummer. 9.1 Background and Motivation. 9.2 Remote Lightning Parameter Measurements. 9.3 Data Analysis Techniques. 9.4 Summary.- Location and Electrical Properties of Sprite-Producing Lightning from a Single Elf Site Y. Hobara et al. 10.1 Introduction. 10.2 Locating Distant ELF Sources and Quantifying their Electrical Properties. 10.3 Winter TLEs and Associated Electromagnetic Phenomena in Japan. 10.4 Conclusion. Acknowledgments.- Calibrated Radiance Measurements with an Air-Filled Glow Discharge Tube: Applicat

Numerically Simulated Electrification and Lightning of the 29 June 2000 STEPS Supercell Storm

Abstract: A three-dimensional dynamic cloud model incorporating airflow dynamics, microphysics, and thunderstorm electrification mechanisms is used to simulate the first 3 h of the 29 June 2000 supercell from the Severe Thunderstorm Electrification and Precipitation Study (STEPS). The 29 June storm produced large flash rates, predominately positive cloud-to-ground lightning, large hail, and an F1 tornado. Four different simulations of the storm are made, each one using a different noninductive (NI) charging parameterization. The charge structure, and thus lightning polarity, of the simulated storm is sensitive to the treatment of cloud water dependence in the different NI charging schemes. The results from the simulations are compared with observations from STEPS, including balloon-borne electric field meter soundings and flash locations from the Lightning Mapping Array. For two of the parameterizations, the observed “inverted” tripolar charge structure is well approximated by the model. The polarity of th...

Raindrop Size Distribution Measurements in Tropical Cyclones

Abstract: Characteristics of the raindrop size distribution in seven tropical cyclones have been studied through impact-type disdrometer measurements at three different sites during the 2004–06 Atlantic hurricane seasons. One of the cyclones has been observed at two different sites. High concentrations of small and/or midsize drops were observed in the presence or absence of large drops. Even in the presence of large drops, the maximum drop diameter rarely exceeded 4 mm. These characteristics of raindrop size distribution were observed in all stages of tropical cyclones, unless the storm was in the extratropical stage where the tropical cyclone and a midlatitude frontal system had merged. The presence of relatively high concentrations of large drops in extratropical cyclones resembled the size distribution in continental thunderstorms. The integral rain parameters of drop concentration, liquid water content, and rain rate at fixed reflectivity were therefore lower in extratropical cyclones than in tropical cyclones. In tropical cyclones, at a disdrometercalculated reflectivity of 40 dBZ, the number concentration was 700 100 drops m 3 , while the liquid water content and rain rate were 0.90 0.05 g m 3 and 18.5 0.5 mm h 1 , respectively. The mean mass diameter, on the other hand, was 1.67 0.3 mm. The comparison of raindrop size distributions between Atlantic tropical cyclones and storms that occurred in the central tropical Pacific island of Roi-Namur revealed that the number density is slightly shifted toward smaller drops, resulting in higher-integral rain parameters and lower mean mass and maximum drop diameters at the latter site. Considering parameterization of the raindrop size distribution in tropical cyclones, characteristics of the normalized gamma distribution parameters were examined with respect to reflectivity. The mean mass diameter increased rapidly with reflectivity, while the normalized intercept parameter had an increasing trend with reflectivity. The shape parameter, on the other hand, decreased in a reflectivity range from 10 to 20 dBZ and remained steady at higher reflectivities. Considering the repeatability of the characteristics of the raindrop size distribution, a second impact disdrometer that was located 5.3 km away from the primary site in Wallops Island, Virginia, had similar size spectra in selected tropical cyclones.

Methane storms on Saturn's moon Titan

Abstract: Saturn's moon Titan and the Earth are the only Solar System worlds where rain reaches the surface. Titan's rain may not be much like ours, as it's methane rain, and the atmospheric cycles of water and methane are very different. Two papers this week offer clues as to what a rainy day on Titan might be like. Hueso and Sanchez-Lavega use a numerical model to show that severe methane convective storms accompanied by intense precipitation may occur in certain conditions. These storms would be comparable to flash flood events on Earth. Tokano et al. present methane distribution and temperature data from instruments on board the Huygens probe. Huygens recently took images of landscapes suggestive of rivers or lake-beds, but the camera did not show any liquid. The new data point to the presence of weak drizzle-like rain. In contrast to clouds observed by telescopes or the Cassini spacecraft, the barely visible clouds encountered by Huygens are widespread, suggesting that rainfall occurs globally and may affect Titan's surface structures. Calculations show that severe methane convective storms accompanied by intense precipitation may occur on Titan under the right environmental conditions. Such rainfalls on Titan's surface are comparable to flash flood events on Earth. The presence of dry fluvial river channels and the intense cloud activity in the south pole of Titan over the past few years1,2,3 suggest the presence of methane rain. The nitrogen atmosphere of Titan therefore appears to support a methane meteorological cycle that sculptures the surface and controls its properties1,4. Titan and Earth are the only worlds in the Solar System where rain reaches the surface, although the atmospheric cycles of water and methane are expected to be very different5. Here we report three-dimensional dynamical calculations showing that severe methane convective storms accompanied by intense precipitation may occur in Titan under the right environmental conditions. The strongest storms grow when the methane relative humidity in the middle troposphere is above 80 per cent, producing updrafts with maximum velocities of 20 m s-1, able to reach altitudes of 30 km before dissipating in 5–8 h. Raindrops of 1–5 mm in radius produce precipitation rainfalls on the surface as high as 110 kg m-2 and are comparable to flash flood events on Earth6.

Lightning flashes conducive to the production and escape of gamma radiation to space

Abstract: [1] Gamma radiation observed in space has been associated with lightning flashes in thunderstorms. These special flashes do not appear to be the large and energetic positive ground flashes that also produce sprites. Considerations of gamma ray attenuation in air indicate that such flashes may not produce gamma radiation at sufficient altitude to enable their escape to space. High-altitude intracloud lightning, most prevalent in the tropics where the tropopause is also high, may be a necessary source.

Surface In Situ Observations within the Outflow of Forward-Flank Downdrafts of Supercell Thunderstorms

Abstract: In the long-standing conceptual model of a supercell thunderstorm, the forward-flank downdraft (FFD) and its associated negative buoyancy originate from precipitation loading and the latent chilling of air due to the melting and evaporation of precipitation. The horizontal buoyancy gradient within the outflow of the FFD has been identified as an important source of low-level, streamwise vorticity in three-dimensional numerical simulations of supercells. These simulations have demonstrated that the formation of low-level mesocyclones is critically dependent on the baroclinic generation of horizontal vorticity within the FFD outflow. Despite the implied dynamical importance of the FFD outflow in the evolution of supercell thunderstorms, only a very limited number of thermodynamic observations have been obtained within FFD outflow. The range of thermodynamic conditions within FFD outflow is not well known, nor is it known whether any systematic relationship exists between the thermodynamic character...

Lightning-rainfall relationships in Mediterranean winter thunderstorms

Abstract: [1] Space-based instruments on the Tropical Rain Measuring Mission (TRMM) have been used to study rainfall and lightning over the central and eastern Mediterranean Sea. Data from six winters (1998 until 2003) were analyzed. Rainfall amounts increase during the winter months, with the maximum precipitation occurring during December, while lightning activity has a maximum during November. Analysis of seasonal rainfall and lightning activity showed a strong correlation with ENSO events. Instantaneous (90 second) analysis of the rain and lightning in individual storms reveals a strong correlation between rain rate and total flash rate. Monthly and seasonal correlation coefficients between rainfall and lightning were found to vary between 0.81 and 0.98, with the rainfall yield (kg/flash) found to vary between 2.5 × 108 and 9.7 × 108 kg/flash. Due to these high correlations we suggest the possibility to use lightning data over the Mediterranean Sea as a proxy for instantaneous rain rate in thunderstorms.

A review of the initiation of precipitating convection in the United Kingdom

Abstract: Recent severe weather events have prompted the European scientific community to assess the current understanding of convective processes with a view to more detailed and accurate forecasting. The initial development of convective cells remains one of the least understood aspects and one in which limited research has taken place. The important processes can be split into three main areas: boundary-layer forcing, upper-level forcing and secondary generation. This paper is a review of the mechanisms responsible for the initiation of precipitating convection in the United Kingdom; i.e. why convective clouds form and develop into precipitating clouds in a particular location. The topography of the United Kingdom has a large influence on the initiation of convection. Boundary-layer forcings determine the specific location where convection is triggered within larger regions of potential instability. These latter regions are created by mesoscale or synoptic-scale features at a higher level such as dry intrusions and mesoscale vortices. Second-generation cells are those formed by the interaction of outflow from convective clouds with the surrounding environmental air. Large, long-lived thunderstorm complexes can develop when new cells are repeatedly triggered on one side of the system. Current and future field campaigns along with the development of high-resolution modelling will enable these processes to be investigated in more detail than has previously been achieved. © Royal Meteorological Society, 2006. Contributions by P. A. Clark and M. E. B. Gray are Crown Copyright.

Radar and Damage Analysis of Severe Bow Echoes Observed during BAMEX

Abstract: This study examines damaging-wind production by bow-shaped convective systems, commonly referred to as bow echoes. Recent idealized numerical simulations suggest that, in addition to descending rear inflow at the bow echo apex, low-level mesovortices within bow echoes can induce damaging straight-line surface winds. In light of these findings, detailed aerial and ground surveys of wind damage were conducted immediately following five bow echo events observed during the Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) field phase. These damage locations were overlaid directly onto Weather Surveillance Radar-1988 Doppler (WSR-88D) images to (i) elucidate where damaging surface winds occurred within the bow-shaped convective system (in proximity to the apex, north of the apex, etc.), and then (ii) explain the existence of these winds in the context of the possible damaging-wind mechanisms. The results of this study provide clear observational evidence that low-level mesovortices wit...

Electrification and Lightning in an Idealized Boundary-Crossing Supercell Simulation of 2 June 1995*

Abstract: A nonhydrostatic cloud model with electrification and lightning processes was utilized to investigate how simulated supercell thunderstorms respond when they move into environments favorable for storm intensification. One model simulation was initialized with an idealized horizontally varying environment, characteristic of that observed across an outflow boundary in the west Texas Panhandle on 2 June 1995 with larger convective available potential energy (CAPE) and wind shear on the boundary’s cool side. That simulation was compared with a control simulation initialized without the boundary. The simulated rightmoving supercell rapidly increased in updraft strength and volume, low-level rotation, radar reflectivity, and 40-dBZ echo-top height as it crossed the boundary, whereas the supercell that did not cross the boundary failed to intensify. For the same kinematic and microphysical evolution and the same inductive charging parameterization, four noninductive (NI) charging parameterizations were tested. In all four cases, there was a general tendency for the charge regions to be lofted higher within the updraft after crossing the boundary. Once the precipitation regions between the main storm and a secondary storm started merging farther on the cool side of the boundary, a gradual deepening and strengthening of the lowest charge regions occurred with relatively large increases in hail and graupel volume, charging rates, charge volume, charge density, and intracloud and cloud-to-ground (CG) flash rates. The negative charge present on graupel within the downdraft appeared to have a common origin via strong NI charging within the midlevel updraft in all four NI cases. Positive channels were more consistent in coming closer to the ground with time compared to negative channels within this graupel and hail-filled downdraft (four of four cases). Those NI schemes that also set up a positive dipole (three of four cases) or inverted tripole (two of four cases) above the downdraft had downward-propagating positive channels that reached ground as positive CG (CG) flashes. The best overall performance relative to the 2 June 1995 CG lightning observations occurred within one of the rime-accretion-rate-based schemes and the Gardiner scheme as parameterized by Ziegler.

Can Lightning Observations be Used as an Indicator of Upper-Tropospheric Water Vapor Variability?

Abstract: Lightning activity in thunderstorms is closely related to the intensity of vertical updrafts indeep convective clouds that also transport large amounts of moisture into the upper troposphere. Small changes in the amount of upper-tropospheric water vapor (UTWV) can have major implications for the Earth's climate. New evidence is presented showing a strong connection between the daily variability of tropical lightning activity and daily uppertropospheric water vapor concentrations from the NCEP-NCAR reanalysis. Our results over the African continent show that the NCEP upper-tropospheric water vapor peaks one day after intense lightning activity in the Tropics. Given the many caveats related to the NCEP UTWV product over Africa, these results need to be interpreted with caution. However, since global lightning activity can be monitored from a few ground stations around the world via the Schumann resonances, we suggest the possible use of continuous lightning observations for studying the daily varia...

Spatial distribution and frequency of lightning activity and lightning flash density maps for Australia

Abstract: [1] The spatial distribution and frequency of lightning activity in Australia have been analyzed using lightning data obtained by ground-based lightning detection instruments denoted CIGRE-500 and CGR3 and by NASA satellite-based instruments denoted OTD and LIS. The geographical distribution of lightning incidence is described by a map of total lightning flash density, Nt (i.e., cloud-to-ground and intracloud flashes). A high level of lightning activity, Nt > 10 km−2yr−1, is observed in the northern parts of Australia, and a decrease in total flash density occurs southward to Nt < 5 km−2yr−1 in the central and southern parts of Australia. The peak lightning occurrence is in the northwestern part of the Australian continent with Nt values up to about 35 km−2yr−1 centered around 16°S 126°E. A reduction in Nt by a factor of about 10 for a change in latitude from 10°S to 40°S was found, which is in agreement with the earlier studies. The data from all the sources were used to estimate the cloud flash-to-ground flash ratio, Z, which at the studied localities was found to be in a range of values from 0.75 to 7.7. We concluded that for the range of latitude over Australia the most representative value of Z is about 2 ± 30%, and it is relatively independent of latitude. We used this to develop a map of average annual lightning ground flash density, Ng, the first for Australia. Ng varies from over 6 km−2yr−1 in the northern parts of Australia to about 1 km−2yr−1 and below in the southern parts.

Cloud‐to‐ground lightning and surface rainfall in warm‐season Florida thunderstorms

Abstract: the lagged rain volume concurrent with CG flashes ranged from 0.43 � 10 4 to 4.9 � 10 4 m 3 /CG flash, and the mean was 1.9 � 10 4 ± 1.7 � 10 4 m 3 /CG flash. For the five storms that produced 12 or more flashes and had significant lags, a plot of the optimum lag time versus the total number of CG flashes shows a linear trend (R 2 = 0.56). The number of storms is limited, but the lag results do indicate that large storms tend to have longer lags. A linear fit to the lagged rain volume vs. the number of concurrent CG flashes has a slope of 1.9 � 10 4 m 3 /CG flash (R 2 = 0.83). We conclude that warm-season Florida thunderstorms produce a roughly constant rain volume per CG flash and that CG lightning can be used to estimate the location and intensity of convective rainfall in that weather regime.

Cloud-resolving simulations of intense tropical Hector thunderstorms: Implications for aerosol–cloud interactions

Abstract: SUMMARY The Hector thunderstorm is studied with a numerical cloud-resolving model. Special attention is given to modelling the mixed-phase and glaciated cloud microphysical processes (along with the implications of aerosols) and their influence on the resulting microphysical and dynamical storm structure. Radiative impacts are also calculated. Simulations are performed for a typical storm case from the EMERALD-II convective cloud experiment in November and December 2002. It is found that, for intense thunderstorms, aerosol indirect effects are generally modified from recently proposed theoretical considerations. Specifically, the proposed ‘glaciation’ indirect effect, resulting from increasing ice nuclei concentrations, is small for intense convection. More importantly, increasing ice number concentrations results in a ‘collection’ indirect effect (where aggregation and accretion processes lead to precipitation) rather than the ‘glaciation’, Bergeron–Findeisen process. There is a ‘thermodynamic’ indirect effect for Hector, as increasing the cloud droplet number concentration from maritime to continental values resulted in a suppression of the heterogeneous freezing process. However, for extreme continental cases, liquidand raindrop freezing by collection processes acquires higher importance; hence there is an optimal value for strong cumulonimbus development. The ‘glaciation’ indirect effect is found to be similar to increasing the rate of ice production by the Hallett–Mossop process. Another aspect of this study shows that there is a significant impact of microphysics on cloud dynamics, and so studying aerosol–cloud effects must also consider dynamical feedback, a strong component of which arises from the latent heat released during homogeneous freezing. The important indirect effects may be well described by recent theory for smaller, more common stratiform and cumulus clouds; however, in the tropics, the importance of Hector-type storms cannot be ignored as they, and other similar storms, provide a mechanism for the production of widespread cirrus and the release of a large amount of precipitation.

Severe Convective Wind Environments

Abstract: Nontornadic thunderstorm winds from long-lived, widespread convective windstorms can have a tremendous impact on human lives and property. To examine environments that support damaging wind producing convection, sounding parameters from Rapid Update Cycle model analyses (at 3-hourly intervals) from 2003 were compared with 7055 reports of damaging winds and 377 081 occurrences of lightning. Groundrelative wind velocity was the most effective at discriminating between damaging and nondamaging wind convective environments. Steep surface-based lapse rates (a traditional damaging wind parameter) generally did not discriminate between damaging and nondamaging wind convective environments. Other parameters, such as convective available potential energy, humidity aloft, and lapse rates aloft were moderately discriminating. This paper presents a composite damaging wind algorithm in which the two most discriminatory parameters were combined, yielding more skill than any individual parameter. Damaging wind environments are then examined further through a selection of cases that highlight common severe wind ingredients and failure modes. A primary result is that, even in seemingly favorable environments, when the winds at the top of the inflow layer were either parallel to the convective line or blowing from warm to cold over a front, damaging winds were less likely. In the former case, it appears that the downdraft winds and the cold pool’s gust-front-normal flow are not additive. In the latter case, it appears that convection becomes elevated and does not produce downdrafts that reach the surface. Combining the most discriminatory severe wind parameters with knowledge of these severe wind failure modes may help to improve the situational awareness of forecasters.

Observations of Convection Initiation “Failure” from the 12 June 2002 IHOP Deployment

Abstract: Observations of the development of cumulus convection, which reached depths of several kilometers but failed to develop into sustained, precipitating, cumulonimbus clouds—an event the authors term “convection initiation failure”—are presented from the 12 June 2002 International H2O Project (IHOP) case. The investigation relies heavily on remote and in situ data obtained by mobile, truck-borne Doppler radars, mobile mesonets, mobile soundings, and stereo cloud photogrammetry. Data collection was focused in northwestern Oklahoma near the intersection of an outflow boundary and dryline. Thunderstorms developed along the dryline during the late afternoon approximately 40 km east of the domain intensively observed by the ground-based observing systems. Farther west, within the region of dense observations analyzed herein, cumulus congestus clouds formed along an outflow boundary. Multiple-Doppler wind syntheses revealed that the boundary layer vertical velocity field was dominated by thermals rather t...

Generation of neutrons in giant upward atmospheric discharges

Abstract: It has been shown that the efficiency of the reactions of the fusion of deuterons in the atmosphere is very low; therefore, nuclear fusion cannot be responsible for the generation of neutrons by lightning. The generation of neutrons in thunderstorm fields is attributed to photonuclear reactions in giant upward atmospheric discharges over thunderclouds.

The microphysical and electrical properties of sprite-producing thunderstorms

Abstract: Sprites and elves are caused by exceptional lightning flashes in storms distinguishable in both size and structure from ordinary thunderstorms. An ice-based mechanism for charge separation known as the non-inductive process is capable of explaining macroscopic features of both ordinary and sprite-producing thunderstorms. Accordingly, the microphysical basis for this mechanism is reviewed. A key distinction for the sprite-producing storm in summer is the development of a large and laterally-extended reservoir of positive charge in the lower portion of the cloud, favorable to the occurrence of positive ground flashes with exceptional charge moments (>500 C·km). The electrical structure of spriteproducing storms in summertime is presently better resolved than wintertime

Synoptic and Mesoscale Analysis of a High-Latitude Derecho Severe Thunderstorm Outbreak in Finland on 5 July 2002

Abstract: On 5 July 2002, a rapidly propagating bow echo formed over eastern Finland causing severe wind damage in an exceptionally large area. The Ministry of the Interior’s Emergency Response Centers received nearly 400 thunderstorm-related wind damage reports. The 5 July 2002 case is the highest-latitude derecho that has ever been documented. The bow echo developed ahead of a northeastward-moving 500-hPa trough inside of the warm sector of a secondary low and moved north-northwestward on the eastern (warm) side of the quasi-stationary front. The leading edge of the bow echo was oriented perpendicular to the low-level southerly wind shear and the convective system propagated along the 850-hPa equivalent potential temperature ridge with a speed that was close to the maximum wind throughout the troposphere. It is particularly noteworthy that the synoptic pattern was oriented about 90° counterclockwise when compared with the typical synoptic pattern associated with warm season derechos in the United States....

Structure of Highly Sheared Tropical Storm Chantal during CAMEX-4

Abstract: NASA's 4th Convection and Moisture Experiment (CAMEX-4) focused on Atlantic hurricanes during the 2001 hurricane season and it involved both NASA and NOAA participation. The NASA ER-2 and DC-8 aircraft were instrumented with unique remote sensing instruments to help increase the overall understanding of hurricanes. This paper is concerned about one of the storms studied, Tropical Storm Chantal, that was a weak storm which failed to intense into a hurricane. One of the practical questions of high importance is why some tropical sto~ins intensify into hurricanes, and others remain weak or die altogether. The magnitude of the difference between the horizontal winds at lower levels and upper altitudes in a tropical storm, i.e., the wind shear, is one important quantity that can affect the intensification of a tropical storm. Strong shear as was present during Tropical Storm Chantal s lifetime and it was detrimental to its intensification. The paper presents an analysis of unique aircraft observations collected from Chantal including an on-board radar, radiometers, dropsondes, and flight level measurements. These measurements have enabled us to examine the internal structure of the winds and thermal structure of Chantal. Most of the previous studies have involved intense hurricanes that overcame the effects of shear and this work has provided new insights into what prevents a weaker storm from intensifying. The storm had extremely intense thunderstorms and rainfall, yet its main circulation was confined to low levels of the atmosphere. Chantal's thermal structure was not configured properly for the storm to intensify. It is most typical that huricanes have a warm core structure where warm temperatures in upper levels of a storm s circulation help intensify surface winds and lower its central pressure. Chantal had two weaker warm layers instead of a well-defined warm core. These layers have been related to the horizontal and vertical winds and precipitation structure and have helped us learn more about why this storm didn't develop.

Ionospheric Alfvén resonator excitation due to nearby thunderstorms

Abstract: [1] A theory of midlatitude Ionospheric Alfven Resonator (IAR) excitation due to random cloud-to-ground lightning discharges is developed. Electromagnetic wave radiated from the lightning discharges penetrates into the ionosphere, thereby exciting the shear Alfven and magnetosonic waves in the F region of ionosphere. The IAR arises due to wave reflection from the Alfven velocity gradients in the topside ionosphere. Typically, the ionospheric resonance cavity accumulates the shear Alfven wave energy with periods from 1 s to a few tenths of seconds. To proceed analytically, a suitably idealized plane-stratified model of the medium was used that ignores the magnetic field line curvature and dip angle but includes plasma conductivity variations with altitude. The thunderstorm centers distributed around a ground-recording station is assumed to be statistically independent sources of the lightning activity, which is a stochastic Poisson process. The lightning onset time and the current moment is supposed to be a random value, while the shape and duration of return strokes are deterministic. Model calculations of the IAR spectrum due to nearby thunderstorm activity were applied to interpret ULF observation made at Karimshino station (52.94°N, 158.25°E) in Kamchatka peninsula. It is shown that the sharp impulses which are in one-to-one correspondence with the appearance of the spectral resonance structure (SRS) in dynamic spectrograms can be the result from nearby lightning discharges followed by impulse IAR excitation. The correlation functions and power spectra of the IAR due to random lightning discharge process is studied both analytically and numerically. We found that the nearby thunderstorms in the range of 1000–2000 km make a main contribution to the SRS signature of the midlatitude IAR, whereas the remote/tropic thunderstorm activity is of minor importance. It is not inconceivable that there may exist other permanent mechanisms of the midlatitude IAR excitation, for example, the high-altitude neutral wind in the E region might also be operative.

New findings about the influence of smoke from fires on the cloud-to-ground lightning characteristics in the Amazon region

Abstract: [1] In the last decade many authors have found evidence on the influence of smoke from fires on the cloud-to-ground (CG) lightning characteristics, both in the lifetime scale of individual storms and on a climatological scale. In general, the observations indicate that the peak current and percentage of positive flashes increase in thunderstorms injected with smoke, while the negative peak current decreases. This article investigates the validity of these findings in thunderstorms injected with large concentrations of smoke from fires in the Amazon region, comparing CG lightning characteristics in the dry (polluted) and wet (clean) seasons, as well as using a new methodology, based on results of a numerical simulation model of atmospheric transport of tracers coupled to a biomass burning emission model. The results confirm the previous findings and, in addition, give a first estimation of the sensitivity of these parameters to changes in the smoke concentration.

GOES Climatology and Analysis of Thunderstorms with Enhanced 3.9-μm Reflectivity

Abstract: By combining observations from the Geostationary Operational Environmental Satellite (GOES) 3.9- and 10.7-μm channels, the reflected component of the 3.9-μm radiance can be isolated. In this paper, these 3.9-μm reflectivity measurements of thunderstorm tops are studied in terms of their climatological values and their utility in diagnosing cloud-top microphysical structure. These measurements provide information about internal thunderstorm processes, including updraft strength, and may be useful for severe weather nowcasting. Three years of summertime thunderstorm-top 3.9-μm reflectivity values are analyzed to produce maps of climatological means across the United States. Maxima occur in the high plains and Rocky Mountain regions, while lower values are observed over much of the eastern United States. A simple model is used to establish a relationship between 3.9-μm reflectivity and ice crystal size at cloud top. As the mean diameter of a cloud-top ice crystal distribution decreases, more solar r...