Showing papers on "Thunderstorm published in 2020"

The formation, character and changing nature of mesoscale convective systems

Abstract: Mesoscale convective systems (MCSs) describe organized groupings of thunderstorms in the tropics and mid-latitudes that span thousands of square kilometres. While recognized for over a century, the advent of satellite and radar observations, as well as atmospheric-model simulations, has brought about their increased understanding. In this Review, we synthesize current knowledge on MCS formation, climatological characteristics, hazardous weather, predictive capacity and projected changes with anthropogenic warming. Driven by typical deep moist convective processes (moisture, lift and instability) and vertical wind shear, MCS formation occurs preferentially in locations where these ingredients are present and can be maintained by large-scale ascent and the cold pools that they produce. MCSs also generate hazardous weather, including extreme rainfall, flooding, derechos and, sometimes, tornadoes and hail, all of which have substantial economic and societal impacts. Given that MCSs also produce a large fraction of warm-season rainfall, there is critical need for both short-term forecasts and long-term projections, presently challenged by inadequate model resolution. Yet, with continually improving modelling capabilities, as well as greater theoretical basis, it is suggested that MCSs might increase in frequency and intensity under a warming climate. Further modelling progress, in turn, offers improved understanding of MCS characteristics, from their life cycle through to impacts. Mesoscale convective systems are an important source of precipitation in many tropical and mid-latitude regions, but can also produce hazardous weather, such as extreme rain, derechos and tornadoes. This Review discusses the formation of mesoscale convective systems, their hazardous weather, predictive capabilities and projected changes with anthropogenic warming.

Three years of the Lightning Imaging Sensor onboard the International Space Station: Expanded Global Coverage and Enhanced Applications

Abstract: The Lightning Imaging Sensor (LIS) was launched to the International Space Station (ISS) in February 2017, detecting optical signatures of lightning with storm-scale horizontal resolution during bo...

A terrestrial gamma-ray flash and ionospheric ultraviolet emissions powered by lightning

Abstract: Terrestrial gamma-ray flashes (TGFs) are transient gamma-ray emissions from thunderstorms, generated by electrons accelerated to relativistic energies in electric fields. Elves are ultraviolet and optical emissions excited in the lower ionosphere by electromagnetic waves radiated from lightning current pulses. We observed a TGF and an associated elve using the Atmosphere-Space Interactions Monitor on the International Space Station. The TGF occurred at the onset of a lightning current pulse that generated an elve, in the early stage of a lightning flash. Our measurements suggest that the current onset is fast and has a high amplitude—a prerequisite for elves—and that the TGF is generated in the electric fields associated with the lightning leader.

Did ice-charging generate volcanic lightning during the 2016–2017 eruption of Bogoslof volcano, Alaska?

Abstract: The 2016–2017 shallow submarine eruption of Bogoslof volcano in Alaska injected plumes of ash and seawater to maximum heights of ~ 12 km. More than 4550 volcanic lightning strokes were detected by the World Wide Lightning Location Network (WWLLN) and Vaisala’s Global Lightning Dataset (GLD360) over 9 months. Lightning assisted monitoring efforts by confirming ash-producing explosions in near-real time, but only 32 out of the 70 explosive events produced detectable lightning. What led to electrical activity within some of the volcanic plumes, but not others? And why did the lightning intensity wax and wane over the lifetime of individual explosions? We address these questions using multiparametric observations from ground-based lightning sensors, satellite imagery, photographs, acoustic signals, and 1D plume modeling. Detailed time-series of monitoring data show that the plumes did not produce detectable lightning until they rose higher than the atmospheric freezing level (approximated by − 20 °C temperatures). For example, on 28 May 2017 (event 40), the delayed onset of lightning coincides with modeled ice formation in upper levels of the plume. Model results suggest that microphysical conditions inside the plume rivaled those of severe thunderstorms, with liquid water contents > 5 g m−3 and vigorous updrafts > 40 m s−1 in the mixed-phase region where liquid water and ice coexist. Based on these findings, we infer that ‘thunderstorm-style’ collisional ice-charging catalyzed the volcanic lightning. However, charge mechanisms likely operated on a continuum, with silicate collisions dominating electrification in the near-vent region, and ice charging taking over in the upper-level plumes. A key implication of this study is that lightning during the Bogoslof eruption provided a reliable indicator of sustained, ash-rich plumes (and associated hazards) above the atmospheric freezing level.

Anak Krakatau triggers volcanic freezer in the upper troposphere.

Abstract: Volcanic activity occurring in tropical moist atmospheres can promote deep convection and trigger volcanic thunderstorms. These phenomena, however, are rarely observed to last continuously for more than a day and so insights into the dynamics, microphysics and electrification processes are limited. Here we present a multidisciplinary study on an extreme case, where volcanically-triggered deep convection lasted for six days. We show that this unprecedented event was caused and sustained by phreatomagmatic activity at Anak Krakatau volcano, Indonesia during 22–28 December 2018. Our modelling suggests an ice mass flow rate of ~5 × 106 kg/s for the initial explosive eruption associated with a flank collapse. Following the flank collapse, a deep convective cloud column formed over the volcano and acted as a ‘volcanic freezer’ containing ~3 × 109 kg of ice on average with maxima reaching ~1010 kg. Our satellite analyses reveal that the convective anvil cloud, reaching 16–18 km above sea level, was ice-rich and ash-poor. Cloud-top temperatures hovered around −80 °C and ice particles produced in the anvil were notably small (effective radii ~20 µm). Our analyses indicate that vigorous updrafts (>50 m/s) and prodigious ice production explain the impressive number of lightning flashes (~100,000) recorded near the volcano from 22 to 28 December 2018. Our results, together with the unique dataset we have compiled, show that lightning flash rates were strongly correlated (R = 0.77) with satellite-derived plume heights for this event.

Climatology of thunderstorms, convective rainfall and dry lightning environments in Australia

Abstract: The thunderstorm climatology of Australia is examined, including convective rainfall events. Lightning observations are used to train a systematic method for indicating thunderstorm activity, with the method applied to environmental variables obtained from reanalysis data from 1979 to 2016. A range of maps showing seasonal averages in thunderstorm conditions as well as associated rainfall are presented. Long-term climate change trends are also examined, as well as the influence of large-scale drivers such as the El Nino-Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode. Rainfall observations are examined for days on which thunderstorm activity is indicated based on this method, enabling new insight on convection-related rainfall. Low rainfall days are also used to examine the climatology of dry lightning as this is important for understanding the risk of wildfire ignitions. A long-term decrease in thunderstorm activity is indicated for many regions of Australia, as well as some regions of increase. The results also indicate a long-term increase in thunderstorm-related rainfall, noting implications for water availability, design standards and flood risk factors. The findings for northern Australia help provide insight on some aspects of the Australian monsoon, including based on a reduced frequency of days with convective environments as well as indicating an increased intensity of convective rainfall events. An increase in convective rainfall is indicated for both northern and southern Australia, while for non-convective rainfall the results indicate an increase in northern Australia and a decrease in southern Australia. Long-term changes in dry lightning events are also identified, depending on the region and season, noting implications for wildfire management.

The contributions of fronts, lows and thunderstorms to southern Australian rainfall

Abstract: A systematic analysis of the main weather types influencing southern Australian rainfall is presented for the period 1979–2015. This incorporates two multi-method datasets of cold fronts and low pressure systems, which indicate the more robust fronts and lows as distinguished from the weaker and less impactful events that are often indicated only by a single method. The front and low pressure system datasets are then combined with a dataset of environmental conditions associated with thunderstorms, as well as datasets of warm fronts and high pressure systems. The results demonstrate that these weather types collectively account for about 86% of days and more than 98% of rainfall in Australia south of 25° S. We also show how the key rain-bearing weather systems vary throughout the year and for different regions, with the co-occurrence of simultaneous lows, fronts and thunderstorm conditions particularly important during the spring and summer months in southeast Australia.

Hazardous weather affecting European airports: Climatological estimates of situations with limited visibility, thunderstorm, low-level wind shear and snowfall from ERA5

Abstract: The consistently growing demand for airline transportation has resulted in increased air traffic and air operations in airports across the world. According to the International Air Transport Association, forecasts assume that in the year 2036 about 7.8 billion passengers will travel using air transport. In Europe, it is estimated that 16.2 million flights will take place in 2040, 53% more than in 2017. One of the crucial factors that significantly affect air transportation is the weather. In this article, ERA5 reanalysis and a 40-year timeframe (1979–2018) are used to construct the European climatology of environmental proxies associated with hazardous weather conditions causing disruptions in airline traffic. These include situations with limited visibility, thunderstorm, low-level wind shear, and snowfall. Results indicate that northern Europe and mountain zones are the most often affected by unfavorable weather conditions, while southern Europe is the least affected. Winter has the highest estimated number of situations with low-level wind shear and snowfall conditions. Situations with limited visibility are the most frequent in autumn and winter while thunderstorms are the main threat during summer over continental Europe and autumn in the Mediterranean area. Long-term changes of the analyzed proxies indicate that situations with low-level wind shear and snowfall are becoming less frequent while the number of thunderstorms generally increases, particularly over northern, central and south-central Europe. A decrease of situations with limited visibility is observed over majority of Europe with the exception of Scandinavia and parts of central Europe.

Radio Emission Reveals Inner Meter-Scale Structure of Negative Lightning Leader Steps

Abstract: We use the Low Frequency Array (LOFAR) to probe the dynamics of the stepping process of negatively charged plasma channels (negative leaders) in a lightning discharge. We observe that at each step of a leader, multiple pulses of vhf (30-80 MHz) radiation are emitted in short-duration bursts (<10 μs). This is evidence for streamer formation during corona flashes that occur with each leader step, which has not been observed before in natural lightning and it could help explain x-ray emission from lightning leaders, as x rays from laboratory leaders tend to be associated with corona flashes. Surprisingly, we find that the stepping length is very similar to what was observed near the ground, however with a stepping time that is considerably larger, which as yet is not understood. These results will help to improve lightning propagation models, and eventually lightning protection models.

Storms and the Depletion of Ammonia in Jupiter: II. Explaining the Juno Observations

Abstract: Observations of Jupiter's deep atmosphere by the Juno spacecraft have revealed several puzzling facts: The concentration of ammonia is variable down to pressures of tens of bars and is strongly dependent on latitude. While most latitudes exhibit a low abundance, the Equatorial Zone of Jupiter has an abundance of ammonia that is high and nearly uniform with depth. In parallel, the Equatorial Zone is peculiar for its absence of lightning, which is otherwise prevalent most everywhere else on the planet. We show that a model accounting for the presence of small‐scale convection and water storms originating in Jupiter's deep atmosphere accounts for the observations. Where strong thunderstorms are observed on the planet, we estimate that the formation of ammonia‐rich hail (“mushballs”) and subsequent downdrafts can deplete efficiency the upper atmosphere of its ammonia and transport it efficiently to the deeper levels. In the Equatorial Zone, the absence of thunderstorms shows that this process is not occurring, implying that small‐scale convection can maintain a near‐homogeneity of this region. A simple model satisfying mass and energy balance accounts for the main features of Juno's microwave radiometer observations and successfully reproduces the inverse correlation seen between ammonia abundance and the lightning rate as function of latitude. We predict that in regions where ammonia is depleted, water should also be depleted to great depths. The fact that condensates are not well mixed by convection until far deeper than their condensation level has consequences for our understanding of Jupiter's deep interior and of giant‐planet atmospheres in general.

Thunderstorm Downbursts and Wind Loading of Structures: Progress and Prospect

Abstract: In 1961, Davenport published a paper, considered by most a constitutive deed of wind engineering, in which meteorology, micrometeorology, climatology, aerodynamics and structural dynamics were embedded in a homogeneous framework of the wind loading of structures. This framework, known as Davenport chain and based on a wind model coherent with synoptic-scale extra-tropical cyclones, is so limpid and elegant as to become, in the course of the years, a sort of axiom. Between 1976 and 1978 Gomes and Vickery separated thunderstorm from non-thunderstorm winds, determined their extreme wind speed marginal distributions and from them derived a mixed statistical model later extended to other wind phenomena. This viewpoint, dealt with as a milestone in the emerging issue of mixed climatology, proved the impossibility to label a heterogeneous range of phenomena endowed with different velocity fields, frequencies, durations and sizes by the generic term “wind”. Many wind types, in particular tropical cyclones, tornadoes and downslope winds, occur in limited and well-known areas. Extra-tropical cyclones and thunderstorms are natural hazards that affect the whole planet. This paper provides a state-of-the-art about thunderstorm downburst, one of the most spectacular and damaging events produced by nature, and its wind loading of structures. Also in the light of planet's climatology evolution, this topic is a key issue of structural safety and sustainability.

What Does a Convection-Allowing Ensemble of Opportunity Buy Us in Forecasting Thunderstorms?

Abstract: The High Resolution Ensemble Forecast v2.1 (HREFv2.1), an operational convection-allowing model (CAM) ensemble, is an ‘‘ensemble of opportunity’’ wherein forecasts from several independently designed deterministic CAMs are aggregated and postprocessed together. Multiple dimensions of diversity in the HREFv2.1 ensemble membership contribute to ensemble spread, including model core, physics parameterization schemes, initial conditions (ICs), and time lagging. In this study, HREFv2.1 forecasts are compared against the High Resolution Rapid Refresh Ensemble (HRRRE) and the Multiscale data Assimilation and Predictability (MAP) ensemble, two experimental CAM ensembles that ran during the 5-week Spring Forecasting Experiment (SFE) in spring 2018. The HRRRE and MAP are formally designed ensembles with spread achieved primarily through perturbed ICs. Verification in this study focuses on composite radar reflectivity and updraft helicity to assess ensemble performance in forecasting convective storms. The HREFv2.1 shows the highest overall skill for these forecasts,matching subjective real-time impressions fromSFEparticipants. Analysis of the skill and variance of ensemble member forecasts suggests that the HREFv2.1 exhibits greater spread and more effectively samples model uncertainty than the HRRRE or MAP. These results imply that to optimize skill in forecasting convective storms at 1–2-day lead times, future CAM ensembles should employ either diverse membership designs or sophisticated perturbation schemes capable of representing model uncertainty with comparable efficacy.

Aerosol invigoration of atmospheric convection through increases in humidity

Abstract: Cloud-aerosol interactions remain a major obstacle to understanding climate and severe weather. Observations suggest that aerosols enhance tropical thunderstorm activity; past research, motivated by the importance of understanding aerosol impacts on clouds, has proposed several mechanisms that could explain that observed link. Here, we show that high-resolution atmospheric simulations can reproduce the observed link between aerosols and convection. However, we also show that previously proposed mechanisms are unable to explain the invigoration. Examining underlying processes reveals that, in our simulations, high aerosol concentrations increase environmental humidity by producing clouds that mix more condensed water into the surrounding air. In turn, higher humidity favors large-scale ascent and stronger convection. Our results provide a physical reason to expect invigorated thunderstorms in high-aerosol regions of the tropics.

Hailstorm Detection by Satellite Microwave Radiometers

Abstract: Passive microwave measurements from satellites have been used to identify the signature of hail in intense thunderstorms. The scattering signal of hailstones is typically observed as a strong depression of upwelling brightness temperatures from the cloud to the satellite. Although the relation between scattering signal and hail diameter is often assumed linear, in this work a logistic model is used which seems to well approximate the complexity of the radiation extinction process by varying the hail cross-section. A novel probability-based method for hail detection originally conceived for AMSU-B/MHS and now extended to ATMS, GMI, and SSMIS, is presented. The measurements of AMSU-B/MHS were analyzed during selected hailstorms over Europe, South America and the US to quantify the extinction of radiation due to the hailstones and large ice aggregates. To this aim, a probabilistic growth model has been developed. The validation analysis based on 12-year surface hail observations over the US (NOAA official reports) collocated with AMSU-B overpasses have demonstrated the high performance of the hail detection method in distinguishing between moderate and severe hailstorms, fitting the seasonality of hail patterns. The flexibility of the method allowed its experimental application to other microwave radiometers equipped with MHS-like frequency channels revealing a high level of portability.

Climatology of Transient Luminous Events and Lightning Observed Above Europe and the Mediterranean Sea

Abstract: In 1999, the first sprites were observed above European thunderstorms using sensitive cameras. Since then, Eurosprite campaigns have been conducted to observe sprites and other transient luminous events (TLEs), expanding into a network covering large parts of Europe and coastal areas. In 2009 through 2013, the number of optical observations of TLEs reached a peak of 2000 per year. Because of this unprecedented number of European observations, it was possible to construct a climatology of 8394 TLEs observed above 1018 thunderstorm systems and study for the first time their distribution and seasonal cycle above Europe and parts of the Mediterranean Sea. The number of TLEs per thunderstorm was found to follow a power law, with less than 10 TLEs for 801 thunderstorms and up to 195 TLEs above the most prolific one. The majority of TLEs were classified as sprites, 641 elves, 280 halos, 70 upward lightning, 2 blue jets and 1 gigantic jet. The climatology shows intense TLE activity during summer over continental areas and in late autumn over coastal areas and sea. The two seasons peak, respectively, in August and November, separated by March and April with almost no TLEs, and a relative minimum around September. The observed TLE activity, i.e. mostly sprites, is shown to be largely consistent with lightning activity, with a 1/1000 of observed TLE-to-lightning ratio in regions with most observations. The overall behaviour is consistent among individual years, making the observed seasonal cycle a robust general feature of TLE activity above Europe.

Kilometer-scale modeling projects a tripling of Alaskan convective storms in future climate

Abstract: Convective storms produce heavier downpours and become more intense with climate change. Such changes could be even amplified in high-latitudes since the Arctic is warming faster than any other region in the world and subsequently moistening. However, little attention has been paid to the impact of global warming on intense thunderstorms in high latitude continental regions, where they can produce flash flooding or ignite wildfires. We use a model with kilometer-scale grid spacing to simulate Alaska’s climate under present and end of the century high emission scenario conditions. The current climate simulation is able to capture the frequency and intensity of hourly precipitation compared to rain gauge data. We apply a precipitation tracking algorithm to identify intense, organized convective systems, which are projected to triple in frequency and extend to the northernmost regions of Alaska under future climate conditions. Peak rainfall rates in the core of the storms will intensify by 37% in line with atmospheric moisture increases. These results could have severe impacts on Alaska’s economy and ecology since floods are already the costliest natural disaster in central Alaska and an increasing number of thunderstorms could result in more wildfires ignitions.

The role of large-scale dynamics in an exceptional sequence of severe thunderstorms in Europe May–June 2018

Abstract: . Over three weeks in May and June 2018, an exceptionally large number of thunderstorms hit vast parts of western and central Europe, causing precipitation of up to 80 mm and several flash floods. During this time, the large-scale atmospheric circulation, which was characterized by a blocking situation over northern Europe, influenced atmospheric conditions relevant for thunderstorm development. Initially, the southwesterly flow on the western flank of the blocking anticyclone induced the advection of warm, moist, and unstably stratified air masses. Due to a low-pressure gradient associated with the blocking anticyclone, these air masses were trapped in western and central Europe, remained almost stationary and prevented a significant air mass exchange. In addition, the low-pressure gradient led to weak flow conditions in the mid-troposphere and thus to low vertical wind shear that prevented thunderstorms from developing into severe organized systems. Most of the storms formed as local-scale, relatively slow-moving single cells. However, due to the related weak propagation speed, several thunderstorms were able to produce torrential heavy rain that affected local-scale areas and triggered several flash floods. Atmospheric blocking also increased the upper-level cut-off low frequency on its upstream regions, which was up to 10 times higher than the climatological mean. Together with filaments of positive potential vorticity (PV), the cut-offs served as trigger mechanisms for a majority of the thunderstorms. For the 22-day study period, we found that more than 50 % of lightning strikes can be linked to a nearby cut-off low or PV filament. The exceptional persistence of low stability combined with weak wind speed in the mid-troposphere over three weeks has not been observed during the past 30 years.

Thunderstorm Trends over Africa

Abstract: Thunderstorms inflict death and damage worldwide due to lightning, heavy rains, hail, and strong winds. While the effect of global warming on future thunderstorm activity is still debatable...

Classification of VLF/LF Lightning Signals Using Sensors and Deep Learning Methods

Abstract: Lightning waveform plays an important role in lightning observation, location, and lightning disaster investigation. Based on a large amount of lightning waveform data provided by existing real-time very low frequency/low frequency (VLF/LF) lightning waveform acquisition equipment, an automatic and accurate lightning waveform classification method becomes extremely important. With the widespread application of deep learning in image and speech recognition, it becomes possible to use deep learning to classify lightning waveforms. In this study, 50,000 lightning waveform samples were collected. The data was divided into the following categories: positive cloud ground flash, negative cloud ground flash, cloud ground flash with ionosphere reflection signal, positive narrow bipolar event, negative narrow bipolar event, positive pre-breakdown process, negative pre-breakdown process, continuous multi-pulse cloud flash, bipolar pulse, skywave. A multi-layer one-dimensional convolutional neural network (1D-CNN) was designed to automatically extract VLF/LF lightning waveform features and distinguish lightning waveforms. The model achieved an overall accuracy of 99.11% in the lightning dataset and overall accuracy of 97.55% in a thunderstorm process. Considering its excellent performance, this model could be used in lightning sensors to assist in lightning monitoring and positioning.

African Lightning and its Relation to Rainfall and Climate Change in a Convection-Permitting Model

Abstract: Global climate models struggle to simulate both the convection and cloud ice fundamental to lightning formation. We use the first convection‐permitting, future climate simulations for the lightning hot spot of Africa, at the same time utilizing an ice‐based lightning parametrization. Both the model and observations show that lightning over Africa's drier areas, as well as the moist Congo, have more lightning per rainfall than other regions. Contrary to results in the literature, the future projection shows little increase in total lightning (~107 flashes (or 2%) per degree warming). This is a consequence of increased stability reducing the number of lightning days, largely offsetting the increased graupel and updraft velocity driving an increase in lightning per lightning day. The next step is to establish if these results are robust across other models and, if combined with parametrized‐convection models, whether ensemble‐based information on the possible responses of lightning to climate change can be investigated. Plain Language Summary Lightning depends on ascending air in thunderstorms and the collision of cloud ice particles, which charge the thundercloud. Many climate models have too coarse a resolution to reliably capture these processes. We focus on Africa, which has some of the most frequent lightning in the world. We use a model that is much higher resolution than usual, and this allows us to explicitly simulate the deep convection associated with thunderstorms as well as provide more detailed representation of the distribution of cloud ice particles. Our results show that in drier regions, as well as the much wetter Congo, there is relatively more lightning per kilogram of surface rainfall than there is in other parts of the continent. Lightning does increase across the continent under climate change, but by a relatively small amount. This is despite the number of days with lightning decreasing as the lower atmosphere becomes more stable. On days with lightning, there are more lightning flashes because there is an increase in cloud ice and intensity of convection. This study gives much more detailed information about African lightning than previous work. However, it is a single simulation. Future research should look at these results across other climate models.

Modeling the Transmission of Optical Lightning Signals Through Complex 3-D Cloud Scenes

Abstract: Space-based lightning imagers have shown that complex cloud scenes that consist of multiple tall convective features, anvil clouds, and warm boundary cloud layers are illuminated by lightning in ma...

Ambient conditions prevailing during hail events in central Europe

Abstract: . Around 26 000 severe convective storm tracks between 2005 and 2014 have been estimated from 2D radar reflectivity for parts of Europe, including Germany, France, Belgium, and Luxembourg. This event set was further combined with eyewitness reports, environmental conditions, and synoptic-scale fronts based on the ERA-Interim (ECMWF Reanalysis) reanalysis. Our analyses reveal that on average about a quarter of all severe thunderstorms in the investigation area were associated with a front. Over complex terrains, such as in southern Germany, the proportion of frontal convective storms is around 10 %–15 %, while over flat terrain half of the events require a front to trigger convection. Frontal storm tracks associated with hail on average produce larger hailstones and have a longer track. These events usually develop in a high-shear environment. Using composites of environmental conditions centered around the hailstorm tracks, we found that dynamical proxies such as deep-layer shear or storm-relative helicity become important when separating hail diameters and, in particular, their lengths; 0–3 km helicity as a dynamical proxy performs better compared to wind shear for the separation. In contrast, thermodynamical proxies such as the lifted index or lapse rate show only small differences between the different intensity classes.

Changes to the Appearance of Optical Lightning Flashes Observed from Space According to Thunderstorm Organization and Structure

Abstract: Optical lightning observations from space reveal a wide range of flash structure. Lightning imagers such as the Geostationary Lightning Mapper and Lightning Imaging Sensor measure flash appearance by recording transient changes in cloud top illumination. The spatial and temporal optical energy distributions reported by these instruments depend on the physical structure of the flash and the distribution of hydrometeors within the thundercloud that scatter and absorb the optical emissions. This study explores how flash appearance changes according to the scale and organization of the parent thunderstorms with a focus on mesoscale convective systems. Clouds near the storm edge are frequently illuminated by large optical flashes that remain stationary between groups. These flashes appear large because their emissions can reflect off the exposed surfaces of nearby clouds to reach the satellite. Large stationary flashes also occur in small isolated thunderstorms. Optical flashes that propagate horizontally, meanwhile, are most frequently observed in electrified stratiform regions where extensive layered charge structures promote lateral development. Highly radiant "superbolts" occur in two scenarios: embedded within raining stratiform regions or in nonraining boundary/anvil clouds where optical emissions can take a relatively clear path to the satellite.

Backward adaptive brightness temperature threshold technique (BAB3T): A methodology to determine extreme convective initiation regions using satellite infrared imagery

Abstract: Thunderstorms in southeastern South America (SESA) stand out in satellite observations as being among the strongest on Earth in terms of satellite-based convective proxies, such as lightning flash rate per storm, the prevalence for extremely tall, wide convective cores and broad stratiform regions. Accurately quantifying when and where strong convection is initiated presents great interest in operational forecasting and convective system process studies due to the relationship between convective storms and severe weather phenomena. This paper generates a novel methodology to determine convective initiation (CI) signatures associated with extreme convective systems, including extreme events. Based on the well-established area-overlapping technique, an adaptive brightness temperature threshold for identification and backward tracking with infrared data is introduced in order to better identify areas of deep convection associated with and embedded within larger cloud clusters. This is particularly important over SESA because ground-based weather radar observations are currently limited to particular areas. Extreme rain precipitation features (ERPFs) from Tropical Rainfall Measurement Mission are examined to quantify the full satellite-observed life cycle of extreme convective events, although this technique allows examination of other intense convection proxies such as the identification of overshooting tops. CI annual and diurnal cycles are analyzed and distinctive behaviors are observed for different regions over SESA. It is found that near principal mountain barriers, a bimodal diurnal CI distribution is observed denoting the existence of multiple CI triggers, while convective initiation over flat terrain has a maximum frequency in the afternoon.