Showing papers on "Thunderstorm published in 2009"

Convective-scale warn-on-forecast system: A vision for 2020

Abstract: The National Oceanic and Atmospheric Administration's (NOAA's) National Weather Service (NWS) issues warnings for severe thunderstorms, tornadoes, and flash floods because these phenomena are a threat to life and property. These warnings are presently based upon either visual confirmation of the phenomena or the observational detection of proxy signatures that are largely based upon radar observations. Convective-scale weather warnings are unique in the NWS, having little reliance on direct numerical forecast guidance. Because increasing severe thunderstorm, tornado, and flash-flood warning lead times are a key NOAA strategic mission goal designed to reduce the loss of life, injury, and economic costs of these high-impact weather phenomena, a new warning paradigm is needed in which numerical model forecasts play a larger role in convective-scale warnings. This new paradigm shifts the warning process from warn on detection to warn on forecast, and it has the potential to dramatically increase warning lead ...

Preliminary Development and Evaluation of Lightning Jump Algorithms for the Real-Time Detection of Severe Weather

Abstract: Previous studies have demonstrated that rapid increases in total lightning activity (intracloud + cloud-to-ground) are often observed tens of minutes in advance of the occurrence of severe weather at the ground. These rapid increases in lightning activity have been termed "lightning jumps." Herein, we document a positive correlation between lightning jumps and the manifestation of severe weather in thunderstorms occurring across the Tennessee Valley and Washington D.C. A total of 107 thunderstorms were examined in this study, with 69 of the 107 thunderstorms falling into the category of non-severe, and 38 into the category of severe. From the dataset of 69 isolated non-severe thunderstorms, an average peak 1 minute flash rate of 10 flashes/min was determined. A variety of severe thunderstorm types were examined for this study including an MCS, MCV, tornadic outer rainbands of tropical remnants, supercells, and pulse severe thunderstorms. Of the 107 thunderstorms, 85 thunderstorms (47 non-severe, 38 severe) from the Tennessee Valley and Washington D.C tested 6 lightning jump algorithm configurations (Gatlin, Gatlin 45, 2(sigma), 3(sigma), Threshold 10, and Threshold 8). Performance metrics for each algorithm were then calculated, yielding encouraging results from the limited sample of 85 thunderstorms. The 2(sigma) lightning jump algorithm had a high probability of detection (POD; 87%), a modest false alarm rate (FAR; 33%), and a solid Heidke Skill Score (HSS; 0.75). A second and more simplistic lightning jump algorithm named the Threshold 8 lightning jump algorithm also shows promise, with a POD of 81% and a FAR of 41%. Average lead times to severe weather occurrence for these two algorithms were 23 minutes and 20 minutes, respectively. The overall goal of this study is to advance the development of an operationally-applicable jump algorithm that can be used with either total lightning observations made from the ground, or in the near future from space using the GOES-R Geostationary Lightning Mapper.

Forecasting Lightning Threat Using Cloud-Resolving Model Simulations

Abstract: Two new approaches are proposed and developed for making time and space dependent, quantitative short-term forecasts of lightning threat, and a blend of these approaches is devised that capitalizes on the strengths of each. The new methods are distinctive in that they are based entirely on the ice-phase hydrometeor fields generated by regional cloud-resolving numerical simulations, such as those produced by the WRF model. These methods are justified by established observational evidence linking aspects of the precipitating ice hydrometeor fields to total flash rates. The methods are straightforward and easy to implement, and offer an effective near-term alternative to the incorporation of complex and costly cloud electrification schemes into numerical models. One method is based on upward fluxes of precipitating ice hydrometeors in the mixed phase region at the-15 C level, while the second method is based on the vertically integrated amounts of ice hydrometeors in each model grid column. Each method can be calibrated by comparing domain-wide statistics of the peak values of simulated flash rate proxy fields against domain-wide peak total lightning flash rate density data from observations. Tests show that the first method is able to capture much of the temporal variability of the lightning threat, while the second method does a better job of depicting the areal coverage of the threat. Our blended solution is designed to retain most of the temporal sensitivity of the first method, while adding the improved spatial coverage of the second. Exploratory tests for selected North Alabama cases show that, because WRF can distinguish the general character of most convective events, our methods show promise as a means of generating quantitatively realistic fields of lightning threat. However, because the models tend to have more difficulty in predicting the instantaneous placement of storms, forecasts of the detailed location of the lightning threat based on single simulations can be in error. Although these model shortcomings presently limit the precision of lightning threat forecasts from individual runs of current generation models,the techniques proposed herein should continue to be applicable as newer and more accurate physically-based model versions, physical parameterizations, initialization techniques and ensembles of forecasts become available.

Shifting the diurnal cycle of parameterized deep convection over land

Abstract: [1] In most atmospheric circulation models used for climate projections, cloud and convective processes are not explicitly resolved but parameterized. Such models are known to produce a diurnal cycle of continental thunderstorms in phase with insolation, while observed precipitation peaks in late afternoon. We propose a new approach which corrects this long standing bias of parameterized convection. In this approach, deep convection triggering and intensity are controlled by sub-cloud processes: here boundary layer thermals and gust fronts, and potentially orography or surface heterogeneities. The representation of the diurnal cycle of deep convection is greatly improved in 1D mode, with rainfall maximum delayed from midday to late afternoon, provided parameterizations account for the key role played by shallow cumulus in preconditioning deep convection and by gust fronts in the self-sustaining of thunderstorms in the afternoon.

3D Convective Storm Identification, Tracking, and Forecasting—An Enhanced TITAN Algorithm

Abstract: Storm identification, tracking, and forecasting make up an essential part of weather radar and severe weather surveillance operations. Existing nowcasting algorithms using radar data can be generally classified into two categories: centroid and cross-correlation tracking. Thunderstorm Identification, Tracking, and Nowcasting (TITAN) is a widely used centroid-type nowcasting algorithm based on this paradigm. The TITAN algorithm can effectively identify, track, and forecast individual convective storm cells, but TITAN tends to provide incorrect identification, tracking, and forecasting in cases where there are dense cells whose shape changes rapidly or where clusters of storm cells occur frequently. Aiming to improve the performance of TITAN in such scenarios, an enhanced TITAN (ETITAN) algorithm is presented. The ETITAN algorithm provides enhancements to the original TITAN algorithm in three aspects. First, in order to handle the false merger problem when two storm cells are adjacent, and to isola...

Recent trends of thunderstorm and hailstorm frequency and their relation to atmospheric characteristics in southwest Germany

Abstract: In the context of climate change, it is of particular interest whether extreme events connected to severe thunderstorms have been increasing in number or intensity over the past few decades. Due to their small horizontal extent, such events are not entirely and uniquely captured by current observation systems. To obtain comprehensive information about the convective activity in the state of Baden-Wurttemberg (southwest Germany), we analysed different datasets for the period 1974–2003. They comprise thunderstorm days detected at synoptic stations, hail damage data from a building insurance company, large-scale circulation and weather patterns, and convective indices derived from radiosonde observations at 12:00 UTC. While the annual number of thunderstorm days remained almost unchanged in the mean, hail damage and hail days significantly increased in the last three decades. Both damage and additional radar data indicate that the majority of hail days can be attributed to three specific circulation patterns. Two of the three patterns associated with the preferable occurrence of hail show a significant increase. Most of the commonly used convective indices that depend upon surface temperature and moisture reveal a positive trend regarding both the annual extreme values and the number of days above/below specific thresholds. A relationship was established between the indices and the annual number of hail damage days, yielding correlation coefficients between 0.65 and 0.80. In contrast to this, indices derived from temperature and moisture at higher levels exhibit either a negative or no significant trend. It is shown that the trend directions of the indices may be attributed to differential temperature and moisture stratification in the various atmospheric layers. The significant positive trends of both surface temperature and water vapour can be concisely expressed by an increase in wet-bulb potential temperature. This indicates the presence of warmer parcels throughout the whole troposphere during convection. Copyright © 2009 Royal Meteorological Society

Mesoscale Convective Systems over Western Equatorial Africa and Their Relationship to Large-Scale Circulation

Abstract: This study examines mesoscale convective systems (MCSs) over western equatorial Africa using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. This region experiences some of the world’s most intense thunderstorms and highest lightning frequency, but has low rainfall relative to other equatorial regions. The analyses of MCS activity include the frequency of occurrence, diurnal and annual cycles, and associated volumetric and convective rainfall. Also evaluated is the lightning activity associated with the MCSs. Emphasis is placed on the diurnal cycle and on the continental-scale motion fields in this region. The diurnal cycle shows a maximum in MCS count around 1500–1800 LT, a morning minimum, and substantial activity during the night; there is little seasonal variation in the diurnal cycle, suggesting stationary influences such as orography. Our analysis shows four maxima in MCS activity, three of which are related to local geography (two orographic and one over Lake Victoria)....

The severe hazards analysis and verification experiment

Abstract: During the springs and summers of 2006 through 2008, scientists from the National Severe Storms Laboratory and students from the University of Oklahoma have conducted an enhanced severe-storm verification effort. The primary goal for the Severe Hazards Analysis and Verification Experiment (SHAVE) was the remote collection of high spatial and temporal resolution hail, wind (or wind damage), and flash-flooding reports from severe thunderstorms. This dataset has a much higher temporal and spatial resolution than the traditional storm reports collected by the National Weather Service and published in Storm Data (tens of square kilometers and 1–5 min versus thousands of square kilometers and 30–60 min) and also includes reports of nonsevere storms that are not included in Storm Data. The high resolution of the dataset makes it useful for validating high-resolution, gridded warning guidance applications. SHAVE is unique not only for the type of data collected and the resolution of that data but also for how the...

Dust mobilization and transport in the northern Sahara during SAMUM 2006 - a meteorological overview

Abstract: The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5-km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper-level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct cut-off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.

Relationships between Lightning Location and Polarimetric Radar Signatures in a Small Mesoscale Convective System

Abstract: On 19 June 2004, the Thunderstorm Electrification and Lightning Experiment observed electrical, microphysical, and kinematic properties of a small mesoscale convective system (MCS). The primary observing systems were the Oklahoma Lightning Mapping Array, the KOUN S-band polarimetric radar, two mobile C-band Doppler radars, and balloonborne electric field meters. During its mature phase, this MCS had a normal tripolar charge structure (lightning involved a midlevel negative charge between an upper and a lower positive charge), and flash rates fluctuated between 80 and 100 flashes per minute. Most lightning was initiated within one of two altitude ranges (3–6 or 7–10 km MSL) and within the 35-dBZ contours of convective cells embedded within the convective line. The properties of two such cells were investigated in detail, with the first lasting approximately 40 min and producing only 12 flashes and the second lasting over an hour and producing 105 flashes. In both, lightning was initiated in or nea...

Relationships among Lightning, Precipitation, and Hydrometeor Characteristics over the North Pacific Ocean*

Abstract: Lightning data from the Pacific Lightning Detection Network (PacNet) and Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite were compared with TRMM precipitation radar products and latent heating and hydrometeor data. Three years of data over the central North Pacific Ocean were analyzed. The data were divided into winter (October–April) and summer (June–September) seasons. During the winter, the thunderstorms were typically embedded in cold fronts associated with eastward-propagating extratropical cyclones. Summer thunderstorms were triggered by cold upper-level lows associated with the tropical upper-tropospheric trough (TUTT). Concurrent lightning and satellite data associated with the storms were averaged over 0.5° × 0.5° grid cells and a detection efficiency correction model was applied to quantify the lightning rates. The results of the data analysis show a consistent logarithmic increase in convective rainfall rate with increasing lightning rates. Mo...

Human fatalities from wind-related tree failures in the United States, 1995–2007

Abstract: There were 407 deaths from wind-related tree failures in the United States, 1995–2007. The most common cause of the deadly fallen tree was a thunderstorm (41%), followed by nonconvective high winds (35%), tropical cyclones (14%), tornadoes (7%), and snow and ice (3%). Most (62%) of the deaths were males while the median age was 44 years. The most common location of the fatality was in a vehicle struck by the tree or a vehicle that crashed into a downed tree on the road (44%), followed by persons outdoors (38%), in mobile homes (9%), and in frame houses (9%). Persons killed by wind-related tree failures during tropical cyclones and tornadoes were more commonly at home (40%) when struck than those killed at home by thunderstorm and nonconvective high winds (13%). Seasonality of the deaths varied by weather type with deaths in thunderstorms clustered during May–August, nonconvective high winds October–April, tropical cyclones August–October, tornadoes in April and November, and snow and ice December–April. Regional patterns result from frequency of the wind events, population density, and tree cover. Suggestions are made for hazard reductions.

Passive Microwave Brightness Temperatures as Proxies for Hailstorms

Abstract: The Tropical Rainfall Measuring Mission (TRMM) satellite has been used to infer distributions of intense thunderstorms. Besides the lightning measurements from TRMM, the radar reflectivities and passive microwave brightness temperatures have been used as proxies for convective vigor. This is based on large graupel or hail lofted by strong updrafts being the cause of high–radar reflectivity values aloft and extremely low brightness temperatures. This paper seeks to empirically confirm that extremely low brightness temperatures are often accompanied by large hail at the surface. The three frequencies examined (85, 37, and 19 GHz) all show an increasing likelihood of hail reports with decreasing brightness temperature. Quantification is limited by the sparsity of hail reports. Hail reports are common when brightness temperatures are below 70 K at 85 GHz, 180 K at 37 GHz, or 230 K at 19 GHz.

Properties of the X-ray emission from rocket-triggered lightning as measured by the Thunderstorm Energetic Radiation Array (TERA)

Abstract: [1] The Thunderstorm Energetic Radiation Array (TERA) is located at the University of Florida, Florida Tech International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. The array includes forty-five 7.6-cm-diameter NaI/photomultiplier tube detectors enclosed in 24 separate aluminum boxes that shield the detectors from light, moisture, and RF noise. The array covers the ∼1 km2 ICLRT facility, centered on the rocket launch tower, used to trigger lightning. From 2005 to 2007, TERA recorded seven rocket-triggered lightning flashes. In this paper we present an analysis of the X-ray emission of three of these flashes. The X-ray emission is observed to occur during the dart leader phase of each stroke, just prior to the time of the return stroke. Significant X-rays are observed on all the detectors to a distance of 500 m from the lightning channel for times up to 200 μs prior to the start of the return stroke. Using Monte Carlo simulations to model the X-ray propagation, we find that the energetic electrons that emit the X-rays have a characteristic energy of about 1 MeV for one particular dart-stepped leader event. The X-ray emission for all three events has a radial fall off proportional to [exp (−r/120)]/r and is most consistent with the energetic source electrons being emitted isotropically from the leader. It is also found that the X-ray and energetic electron luminosities of the leader channel decreases with increasing height above the ground. These results help shed light onto the mechanism for producing energetic radiation from lightning. For instance, a characteristic energy of 1 MeV is not consistent with the relativistic runaway electron avalanche mechanism, suggesting that so-called cold runaway electrons, produced by very strong electric fields, dominate the production of the X-rays.

Lightning initiation in the anvils of two supercell storms

Abstract: [1] Previous studies of lightning in anvil clouds have reported that flashes began in or near the storm core and propagated downwind into the anvil. It had been thought that flashes could not be initiated far downwind in the anvil, because anvil charge was thought to be produced mainly in the storm's deep updraft and to decrease with distance into the anvil. Here we report observations of the in-cloud development of lightning flashes in the anvils of two supercell storms, including the first observations of flashes that began in the anvil 30–100 km from the cores of the storms and propagated upwind back toward the cores. Interaction between charge regions in the two converging anvils of adjoining storms appeared to cause some of the distant flash initiations, but a local charging mechanism in the anvil likely also contributed to the flash initiations. All flashes that struck ground beneath the distant anvil transferred negative charge to ground instead of the positive charge usually transferred to ground there, an apparent consequence of the parent storm having an inverted-polarity electrical structure.

Quantification of the troposphere-to-ionosphere charge transfer in a gigantic jet

Abstract: Gigantic jets emerge from the top of thunderstorms and extend all the way to the ionosphere at altitudes of 90 km. Simultaneous video images and magnetic field measurements of a gigantic jet demonstrate an electric charge transfer between the thunderstorm and the ionosphere that is comparable to that observed in cloud-to-ground lightning.

Lightning characteristics observed by a VLF/LF lightning detection network (LINET) in Brazil, Australia, Africa and Germany

Abstract: . This paper describes lightning characteristics as obtained in four sets of lightning measurements during recent field campaigns in different parts of the world from mid-latitudes to the tropics by the novel VLF/LF (very low frequency/low frequency) lightning detection network (LINET). The paper gives a general overview on the approach, and a synopsis of the statistical results for the observation periods as a whole and for one special day in each region. The focus is on the characteristics of lightning which can specifically be observed by this system like intra-cloud and cloud-to-ground stroke statistics, vertical distributions of intra-cloud strokes or peak current distributions. Some conclusions regarding lightning produced NOx are also presented as this was one of the aims of the tropical field campaigns TROCCINOX (Tropical Convection, Cirrus and Nitrogen Oxides Experiment) and TroCCiBras (Tropical Convection and Cirrus Experiment Brazil) in Brazil during January/February 2005, SCOUT-O3 (Stratospheric-Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere) and TWP-ICE (Tropical Warm Pool-International Cloud Experiment) during November/December 2005 and January/February 2006, respectively, in the Darwin area in N-Australia, and of AMMA (African Monsoon Multidisciplinary Analyses) in W-Africa during June–November 2006. Regional and temporal characteristics of lightning are found to be dependent on orographic effects (e.g. S-Germany, Brazil, Benin), land-sea breeze circulations (N-Australia) and especially the evolution of the monsoons (Benin, N-Australia). Large intra-seasonal variability in lightning occurrence was found for the Australian monsoon between the strong convection during build-up and break phases and the weak active monsoon phase with only minor lightning activity. Total daily lightning stroke rates can be of comparable intensity in all regions with the heaviest events found in Germany and N-Australia. The frequency of occurrence of such days was by far the largest in N-Australia. In accordance with radar observed storm structures, the intra-cloud stroke mean emission heights were found distinctly different in Germany (8 km) as compared to the tropics (up to 12 km in N-Australia). The fraction of intra-cloud strokes (compared to all strokes) was found to be relatively high in Brazil and Australia (0.83 and 0.82, respectively) as compared to Benin and Germany (0.64 and 0.69, respectively). Using stroke peak currents and vertical location information, lightning NOx (LNOx) production under defined standard conditions can be compared for the different areas of observation. LNOx production per standard stroke was found to be most efficient for the N-Australian and S-German thunderstorms whereas the yield from Brazilian and W-African strokes was nearly 40% less. On the other hand, the main NO contribution in Brazil was from intra-cloud (IC) strokes whereas in Benin it was due to cloud-to-ground (CG) components. For the German and Australian strokes both stroke types contributed similar amounts to the total NO outcome.

Development of a Long-Range Lightning Detection Network for the Pacific: Construction, Calibration, and Performance*

Abstract: The waveguide between the earth’s surface and the ionosphere allows very low-frequency (VLF) emissions generated by lightning, called sferics, to propagate over long distances. The new Pacific Lightning Detection Network (PacNet), as a part of a larger long-range lightning detection network (LLDN), utilizes this attribute to monitor lightning activity over the central North Pacific Ocean with a network of ground-based lightning detectors that have been installed on four widely spaced Pacific islands (400–3800 km). PacNet and LLDN sensors combine both magnetic direction finding (MDF) and time-of-arrival (TOA)-based technology to locate a strike with as few as two sensors. As a result, PacNet/LLDN is one of the few observing systems, outside of geostationary satellites, that provides continuous real-time data concerning convective storms throughout a synoptic-scale area over the open ocean. The performance of the PacNet/LLDN was carefully assessed. Long-range lightning flash detection efficiency (D...