Convective storm detection is the observation of deep, moist convection (DMC); this term includes the minority of
stormsA thunderstorm, also known as an electrical storm, a lightning storm, a hailstorm, or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically-assigned cloud type associated with the...
which do not produce
lightningLightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
and
thunderThunder is the sound made by lightning. Depending on the nature of the lightning and distance of the listener, it can range from a sharp, loud crack to a long, low rumble . The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a...
. Convective storms produce
tornadoA tornado is a violent, dangerous, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud...
es as well as large
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
, strong
windWind is the flow of air or other gases that compose an atmosphere . On Earth, wind consists of the bulk movement of air...
s, and flash flooding. The detection of convective storms relies on direct eyewitness observations, for example from
storm spotterA storm spotter is a specific type of weather spotter who actively maintains a visual watch of the development and progression of specific weather events while actively relaying important information to the local weather agency in a timely manner.- History :...
s; and on remote sensing, mostly
weather radarA weather radar, or weather surveillance radar , is a type of radar used to locate precipitation, calculate its motion, estimate its type , and forecast its future position and intensity....
. Some
in situIn situ is a Latin phrase meaning in the place. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may work but interference from...
measurements are used for direct detection as well, notably,
wind speedWind speed is the speed of wind, the movement of air or other gases in an atmosphere. It is a scalar quantity, the magnitude of the vector of motion....
reports from
surface observationSurface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using...
stations. It is part of the
integrated warning system, consisting of prediction, detection, and dissemination of information on
severe weatherSevere weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
to the public.
History
Rigorous attempts to warn of tornadoes began in the United States in the mid-20th century. Before the 1950s, the only method of detecting a tornado was by someone seeing it on the ground. Often, news of a tornado would reach a local weather office after the storm.
But, with the advent of
weather radarA weather radar, or weather surveillance radar , is a type of radar used to locate precipitation, calculate its motion, estimate its type , and forecast its future position and intensity....
, areas near a local office could get advance warning of severe weather. The first public
tornado warningA tornado warning is an alert issued by government weather services to warn an area that a tornado may be imminent. It can be issued after either a tornado or funnel cloud has already been spotted, or if there are radar indications that a tornado may be possible...
s were issued in 1950 and the first
tornado watchA tornado watch is issued when weather conditions are favorable for the development of severe thunderstorms that are capable of producing tornadoes. A tornado watch therefore implies that it is also a severe thunderstorm watch...
es and convective outlooks in 1952. In 1953 it was confirmed that
hook echoThe hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on a weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell...
es are associated with tornadoes. By recognizing these radar signatures, meteorologists could detect thunderstorms likely producing tornadoes from dozens of miles away.
Storm spotting
In the mid 1970s, the US
National Weather ServiceThe National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
(NWS) increased its efforts to train
storm spotterA storm spotter is a specific type of weather spotter who actively maintains a visual watch of the development and progression of specific weather events while actively relaying important information to the local weather agency in a timely manner.- History :...
s to identify and report key features of storms which indicate severe hail, damaging winds, and tornadoes, as well as damage itself and
flash floodA flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers and streams. It is caused by heavy rain associated with a thunderstorm, hurricane, or tropical storm. Flash floods can also occur after the collapse of an ice dam, or a human structure, such as a dam, for example, the...
ing. The program was called
SkywarnSKYWARN is a program of the United States' National Weather Service . Its mission is to collect reports of localized severe weather. These reports are used to aid forecasters in issuing and verifying severe weather watches and warnings and to improve the forecasting and warning processes and the...
, and the spotters were local
sheriff's deputiesA sheriff is in principle a legal official with responsibility for a county. In practice, the specific combination of legal, political, and ceremonial duties of a sheriff varies greatly from country to country....
,
state troopersState police are a type of sub-national territorial police force, particularly in Australia and the United States. Some other countries have analogous police forces, such as the provincial police in some Canadian provinces, while in other places, the same responsibilities are held by national...
,
firefighterFirefighters, or firemen, are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car accidents, collapsed and burning buildings and other such situations...
s, ambulance drivers,
amateur radio operatorAn amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...
s,
civil defenseCivil defense, civil defence or civil protection is an effort to prepare non combatant's for military attack. It uses the principles of emergency operations: prevention, mitigation, preparation, response, or emergency evacuation, and recovery...
(now
emergency managementEmergency management is the discipline of dealing with and avoiding risks. It is a discipline that involves preparing for disaster before it occurs, disaster response , as well as supporting, and rebuilding society after natural or human-made disasters have occurred...
) spotters,
storm chaserStorm chaser can refer to:*Storm chasing, the pursuit of any severe weather condition.*Storm chaser , a term used by insurance companies, municipal building departments, local contractors, and the general public to describe roofing and siding contractors from outside a local area that come to help...
s, and ordinary citizens. When severe weather is anticipated, local weather service offices request that these spotters look out for severe weather, and report any tornadoes immediately, so that the office can issue a timely warning.
Usually spotters are trained by the NWS on behalf of their respective organizations, and report to them. The organizations activate public warning systems such as
sirensA civil defense siren is a mechanical or electronic device for generating sound to provide warning of approaching danger and sometimes to indicate when the danger...
and the
Emergency Alert SystemThe Emergency Alert System is a national warning system in the U.S. put into place in 1997, superseding the Emergency Broadcast System and the CONELRAD System and is jointly coordinated by the Federal Communications Commission , Federal Emergency Management Agency , and the National Weather Service...
, and forward the reports to the NWS, which does directly disseminate
information and warningsSevere weather terminology is different around the world, varying between regions and countries. These are articles which explain terminology in various parts of the world.*Severe weather terminology *Severe weather terminology...
through its
NOAA Weather Radio All HazardsNOAA Weather Radio All Hazards is a network of radio stations broadcasting continuous weather information directly from a nearby National Weather Service office. It is operated by the NWS, an agency of the National Oceanic and Atmospheric Administration within the United States Department of...
network.
There are more than 230,000 trained Skywarn weather spotters across the United States.
In
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
, a similar network of volunteer weather watchers, called
CanwarnThe Canwarn program is an organized severe weather spotting and reporting program organized and run by the Meteorological Services Division of Environment Canada....
, helps spot severe weather, with more than 1,000 volunteers.
In Europe, several nations are organizing spotter networks under the auspices of Skywarn Europe
and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the
United KingdomThe United Kingdom of Great Britain and Northern Ireland is a sovereign state located off the northwestern coast of continental Europe. It is an island country, spanning an archipelago including Great Britain, the northeastern part of Ireland, and many small islands...
since the 1970s.
Storm spotters are needed because radar systems such as
NEXRADNEXRAD or Nexrad is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce...
, and satellite images, do not detect tornadoes or hail, only indications that the storm has the potential. Radar and satellite data interpretation will usually give a warning before there is any visual evidence of such events, but
ground truthGround truth is a term used in cartography, meteorology, analysis of aerial photographs, satellite imagery and a range of other remote sensing techniques in which data are gathered at a distance. Ground truth refers to information that is collected "on location." In remote sensing, this is...
from an observer can either verify the threat or determine it is not imminent. The spotter's ability to see what these remote sensing devices cannot is especially important as distance from a radar site increases, because the radar beam becomes progressively higher in altitude further away from the radar, due to curvature of Earth and the spread of the beam with distance. Therefore, when far from a radar, only precipitations and velocities high in the storm are observed. The important areas might not then be sampled or the resolution of the data might be poor. Also, some meteorological situations leading to tornadogenesis are not readily detectable by radar and on occasion tornado development may occur more quickly than radar can complete a scan and send the batch of data.
Visual evidence
Storm spotters are trained to discern whether a storm seen from a distance is a
supercellA supercell is a thunderstorm that is characterized by the presence of a mesocyclone; a deep, continuously-rotating updraft. Of the four classifications of thunderstorms , supercells are the overall least common and have the potential to be the most severe...
. They typically look to its rear, the main region of updraft and
inflowInflow may refer to:*Capital inflow - an economic term describing capital flowing into a particular economy.*Inflow - In hydrology, the source of the water in a body of water...
. Under the updraft is a rain-free base, and the next step of
tornadogenesisTornadogenesis is the process by which a tornado forms. There are many types of tornadoes, and each type of tornado can have several different methods of formation. Scientific study is ongoing, as some aspects of tornado formation remain a mystery....
is the formation of a rotating
wall cloudA wall cloud, or pedestal cloud, is a cloud formation associated with thunderstorms. It is a marked lowering typically beneath the rain-free base portion of a deep cumulus cloud , and indicates the area of primary and strongest updraft which condenses into cloud at altitudes lower than that of...
. The vast majority of intense tornadoes occur with a wall cloud on the backside of a supercell.
Evidence of a supercell comes from the storm's shape and structure, and
cloud towerCumulonimbus is a type of cloud that is tall, dense, and involved in thunderstorms and other intense weather. It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold front in a squall line. They create lightning through the heart of the cloud...
features such as a hard and vigorous updraft tower, a persistent, large
overshooting topAn overshooting top is a dome-like protrusion that shoots out of the top of the anvil of a thunderstorm. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the thunderstorm may be severe....
, a hard anvil (especially when backsheared against strong upper level
windWind is the flow of air or other gases that compose an atmosphere . On Earth, wind consists of the bulk movement of air...
s), and a corkscrew look or striations. Under the storm and closer to where most tornadoes are found, evidence of a supercell and likelihood of a tornado includes inflow bands (particularly when curved) such as a "beaver tail", and other clues such as strength of inflow, warmth and moistness of inflow air, how outflow- or inflow-dominant a storm appears, and how far is the front flank precipitation core from the wall cloud. Tornadogenesis is most likely at the interface of the updraft and front flank downdraft, and requires a balance between the outflow and inflow.
Only wall clouds that rotate spawn tornadoes, and usually precede the tornado by five to thirty minutes. Rotating wall clouds are the visual manifestation of a
mesocycloneA mesocyclone is a vortex of air, approximately 2 to 10 km in diameter , within a convective storm....
. Barring a low-level boundary, tornadogenesis is highly unlikely unless a
rear flank downdraftThe rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes...
occurs, which is usually visibly evidenced by evaporation of
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly after; first, a
funnel cloudA funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud...
dips and in nearly all cases by the time it reaches halfway down, a surface swirl has already developed, signifying a tornado is on the ground before condensation connects the surface circulation to the storm. Tornadoes may also occur without wall clouds, under flanking lines, and on the leading edge. Spotters watch all areas of a storm, and the
cloud baseThis article refers to meteorology, for the airborne base of Captain Scarlet see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...
and surface.
Radar
Today, most developed countries have a network of
weather radarA weather radar, or weather surveillance radar , is a type of radar used to locate precipitation, calculate its motion, estimate its type , and forecast its future position and intensity....
s, which remains the main method of detecting signatures likely associated with tornadoes and other severe phenomenons as
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
and
downburstA downburst is created by an area of significantly rain-cooled air that, after hitting ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water...
s. Radar is always available, in places and times where spotters are not, and can also see features that spotters cannot, in the darkness of night and processes hidden within the cloud as well as invisible processes outside the cloud.
Tornadoes
In short-term prediction and detection of tornadoes, meteorologists integrate radar data with reports from the field and knowledge of the meteorological environment. Radar analysis is augmented by automated detection systems called
algorithmIn mathematics, computing, linguistics, and related subjects, an algorithm is an effective method for solving a problem using a finite sequence of instructions. Algorithms are used for calculation, data processing, and many other fields....
s. Meteorologists first look at the atmospheric environment as well as changes thereof, and once storms develop, storm motion and interaction with the environment.
An early step in a storm organizing into a tornado producer is the formation of a weak echo region (WER). This is an area within the thunderstorm where precipitation should be occurring but is "pulled" aloft by a very strong updraft. The weak echo region is characterized by weak reflectivity with a sharp gradient to strong reflectivity above it and partially surrounding the sides. The region of the
precipitationIn meteorology, precipitation is any product of the condensation of atmospheric water vapor that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel...
lofted above the WER is the echo overhang consisting of precipitation particles diverging from the storm's summit that descend as they are carried downwind. Within this area, a bounded weak echo region (BWER) may then form above and enclosing the WER. A BWER is found near the top of the updraft and nearly or completely surrounded by strong reflectivity, and is indicative of a supercell capable of cyclic tornadogenesis. A mesocyclone may descend or a tornado may form in the lower level of the storm simultaneously as the mesocyclone forms.
In
reflectivityIn photometry and heat transfer, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
(precipitation intensity) data, a tight echo gradient (particularly on the inflow area) and a fan shape generally indicate a
supercellA supercell is a thunderstorm that is characterized by the presence of a mesocyclone; a deep, continuously-rotating updraft. Of the four classifications of thunderstorms , supercells are the overall least common and have the potential to be the most severe...
. A V-notch or "flying eagle echo" tend to be most pronounced with intense classic supercells, the type of supercell that produces most of the strongest, largest, and longest lived tornadoes. This is not to be confused with an inflow notch; which is a lower level indentation in the precipitation where there is little to no reflectivity, indicative of strong, organized inflow and a severe storm that is most likely a supercell. The rear inflow notch (or weak echo channel) occurs to the east or north of a mesocyclone and hook echo. Forward inflow notches also occur, particularly on high-precipitation supercells (HP) and quasi-linear convective systems (QLCS).
In the United States and a few other countries, Doppler capable weather radar stations are used. These devices are capable of measuring the radial
velocityIn physics, velocity is the rate of change of position. It is a vector physical quantity; both speed and direction are required to define it. In the SI system, it is measured in meters per second: or ms-1. The scalar absolute value of velocity is speed...
, including radial
directionDirection is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...
(towards or away from the radar) of the winds in a storm, and so can spot evidence of rotation in storms from more than a hundred miles (160 km) away. A supercell is characterized by a mesocyclone, which is usually first observed in velocity data as a tight, cyclonic structure in the middle levels of the thunderstorm. If it meets certain requirements of strength, duration, and
vorticityVorticity is a concept used in fluid dynamics. In the simplest sense, vorticity is the tendency for elements of the fluid to "spin."More formally, vorticity can be related to the amount of "circulation" or "rotation" in a fluid.The average vorticity in a small region of fluid flow is equal to the...
, it may trip the mesocyclone detection algorithm (MDA). Tornadic signatures are indicated by a cyclonic inbound-outbound velocity couplet, where strong winds flowing in one direction and strong winds flowing in the opposite direction are occurring in very close proximity. The algorithm for this is the tornadic vortex signature (
TVSA tornado vortex signature, or tornadic vortex signature, , is a Doppler weather radar detected rotation algorithm that indicates the likely presence of a strong mesocyclone that is in some stage of the process of tornadogenesis...
). TVS often also forms first in the middle levels of the thunderstorm and may descend and tighten into a tornado. The TVS is smaller and found at lower level than the MDA, and usually is the tornado cyclone not the actual tornadic circulation. The TVS is, however, indicative of a likely tornado or an incipient tornado. The couplet and TVS typically precede tornado formation by 10–30 minutes but may occur at nearly the same time or precede the tornado by 45 minutes or more. The
hook echoThe hook echo is one of the classical hallmarks of tornado-producing supercell thunderstorms as seen on a weather radar. The echo is produced by rain, hail, or even debris being wrapped around the supercell...
feature is formed as the
RFDThe rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes...
occludes precipitation around the mesocyclone and is also indicative of a probable tornado (tornadogenesis usually ensues shortly after the RFD reaches the surface).
After the implementation of the
WSR-88DNEXRAD or Nexrad is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration within the United States Department of Commerce...
network in the U.S., the probability of detection of tornadoes increased substantially, the average lead time rose from four minutes to thirteen minutes, and a 2005 NOAA report estimates that as a result of improved warnings that there are 45 percent fewer fatalities and 40 percent fewer injuries annually.
Hail, downburst and downpour
Hail forms in a very intense updraft in a supercell or a multicellular thunderstorm. As for tornadoes, BWER detection is indicative of that updraft but does not lead to predict hail. What is needed is a knowledge of the water content in the thunderstorm, the freezing level and the height of the summit of the precipitation. On way of calculating the water content is to transform the reflectivities in rain rate at all levels in the clouds and to sum it up. This is done by an algorithm called
Vertically integrated liquidVertically integrated liquid is an estimate of the total mass of precipitation in the clouds. The measurement is obtained by observing the reflectivity of the air which is obtained with weather radar...
, or VIL. This value represent the total amount of liquid water in the cloud that is available. If the cloud would rain out completely, it would be the amount of rain falling on the ground and one can estimate with VIL the potential for
flash floodA flash flood is a rapid flooding of geomorphic low-lying areas - washes, rivers and streams. It is caused by heavy rain associated with a thunderstorm, hurricane, or tropical storm. Flash floods can also occur after the collapse of an ice dam, or a human structure, such as a dam, for example, the...
.
However, the reflectivities are greatly enhanced by hail and VIL is greatly overestimating the rain potential in presence of hail. On the other hand,
National Weather ServiceThe National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...
meteorologists have found that the VIL density, that is to say VIL divided by the maximum height of the 18
dBZdBZ stands for decibels of Z. It is a meteorological measure of equivalent reflectivity of a radar signal reflected off a remote object. The reference level for Z is 1 mm
6 m
-3, which is equal to 1 μm
3...
in the cloud, is a good indicator of the presence of hail when it reach 3.5. This is a crude yes/no index and other algorithms have been developed involving VIL and the freezing level height. More recently, dual polarization of weather radar have shown promising direct detection of hail.
VIL can be used to estimate the potential for
downburstA downburst is created by an area of significantly rain-cooled air that, after hitting ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water...
, too. A convective downdraft is linked to three forces in the vertical, namely perturbation pressure gradient force, buoyancy force and precipitation loading. The pressure gradient force was neglected as it has significant effect only on the updraft in supercells. With this assumption and other simplifications (e.g. requiring the environment of the air parcel to be static on the time scale of the downdraft). The resulting momentum equation is integrated over height to yield the
kinetic energyThe kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its...
of the parcel on descending to the surface and is found to be the negative
CAPEIn meteorology, convective available potential energy , sometimes, simply, available potential energy , is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere...
of a dry air parcel injected into the storm, plus de motion of the convective cell. S. R. Stewart, from NWS, has published in 1991 an equation relating VIL and the echo tops that give the potential for surface gust using this concept. This is a predictive result that gives a certain lead time. With the Doppler velocity data, the meteorologist can see the downdraft and gust fronts happening, but since this a small scale feature, detection algorithms have been developed to point convergence and divergence areas under a thunderstorm on the radar display.
Satellite imagery
Most populated areas of the earth are now well covered by
weather satelliteA weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while...
s, which aid in the nowcasting of severe convective and tornadic storms. These images are available in the
visibleThe visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 750 nm...
and
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
domains. The infrared (IR: 10-13
µmA micrometre or micron is one millionth of a metre,or equivalently one thousandth of a millimetre....
) images permit estimation of the top height of the clouds, according to the
air massIn meteorology, an air mass is a large volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or...
soundings of the day, and the visible (VIS: 0.5-1.1 µm) ones will show the shape of the storms by its brightness and shadow produced. Meteorologists can extract information about the development stage of a thunderstorms by recognizing specific signatures in both domains. Visible imagery permits the most detailed imagery whereas infrared imagery has the advantage of availability at night. Sensors on satellites can also detect emissions from water vapor (WV: 6-7 µm), but mostly in the middle to upper levels of the
troposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75 percent of the atmosphere's mass and 99 percent of its water vapor and aerosols....
, so thunderstorms are only seen after being well developed. It is, however, useful in convective storm prediction, as it illustrates the placement and movement of air masses and of moisture, as well as
shortwavesA shortwave or shortwave trough is an embedded kink in the trough / ridge pattern. This is the opposite of longwaves, which are responsible for synoptic scale systems, although shortwaves may be contained within or found ahead of longwaves and range from the mesocale to the the synoptic scale...
and areas of vorticity and lift.
Severe storms have a very strong updraft. The rising air parcels in that column accelerate and will overshoot the
equilibrium levelIn meteorology, the equilibrium level , or level of neutral buoyancy , is the height at which a rising parcel of air is at a temperature of equal warmth to it. This means that unstable air is now stable when it reaches the equilibrium level and convection stops...
before being pulled back by negative buoyancy. This mean the cloud tops will reach high levels than the surrounding cloud in the updraft region. This
overshooting topAn overshooting top is a dome-like protrusion that shoots out of the top of the anvil of a thunderstorm. When an overshooting top is present for 10 minutes or longer, it is a strong indication that the thunderstorm may be severe....
will be noticeable by a colder temperature region in the thunderstorm on infrared images. Another signature associated with this situation is the Enhanced-V feature where the cold
cloud topThe cloud top is the highest altitude of the visible portion of the cloud. It is traditionally expressed either in metres above the Earth surface, or as the corresponding pressure level in hectopascal .-Measurement:The cloud top is where the snow, rain and sleet come from.Cloud top...
s forming at the overshooting top fan out in a V shape as cloud matter is blown downwind at that level. Both features can be seen on visible satellite imagery, during daytime, by the shadows they cast on surrounding clouds.
In
multi-cellularA multicellular thunderstorm cluster is a thunderstorm that is composed of multiple cells, each being at a different stage in the life cycle of a thunderstorm. It looks like several anvils clustered together. A cell is an updraft/downdraft couplet...
storms and
squall lineA squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts...
s, the mid-level
jet streamJet streams are fast flowing, narrow air currents found in the atmosphere of planets at the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on earth are westerly winds...
is often intersecting the line and its dry air introduced into the cloud is negatively unstable. This results in a drying of the cloudy air in the region where the jet plunge groundward. On the back edge of the line, this shows as clear notches where one can find stronger downdrafts at the surface. These kind of lines will have a very characteristic undulating pattern caused by the interference of the gusts fronts coming from different parts of the line.
Finally, in any type of thunderstorms, the cold pool of air associated the downdraft will stabilize the air and form a cloud free area which will end along the
gust frontAn outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump...
. This front, moving into a warm and unstable air mass, will lift it and
cumulus cloudCumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...
s appear on satellite pictures. This line is likely the point of further convection and storms. One can notice it at the leading edge of a squall line, in the southeastern quadrant of a supercell (northern hemisphere), or different regions around other thunderstorms. They may also be visible as an
outflow boundaryAn outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump...
hours or days after convection and can pinpoint areas of favored thunderstorm development, possible direction of movement, and even likelihood for tornadoes.
Lightning detection
Usually in conjunction with data sources such as weather radar and satellites, lightning detection systems are sometimes utilized to pinpoint where thunderstorms are occurring (and to identify
lightningLightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
hazard). Currently, most lightning data provided in real-time is from terrestrial sources, specifically, networks of ground-based sensors, although airborne sensors are also in operation. Most of these only provide latitude & longitude, time, and polarity of cloud-to-ground strikes within a limited range. Increasing in sophistication and availability, and affording data for a very wide area, are satellite-based lightning detectors which initially included optical sensors indicating flash rates and horizontal location but now
radio frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
receivers that can identify intra-cloud flashes with the addition of altitude, as well.
Lightning data is useful in suggesting intensity and organization of convective cells as well trends in thunderstorm activity (particularly growth, and to a lesser degree, decay). It is also useful in the early stages of thunderstorm development. This was especially true when visible and infrared satellite data was delayed, but continues to be useful in detecting thunderstorms in stages of development before there is a substantial radar signature or for areas where radar data is lacking.
Personal lightning detection systems are also available, which may provide strike time, azimuth, and distance. In addition,
lightning prediction systemA lightning prediction system detects atmospheric conditions likely to produce lightning strikes in a certain area and sounds an auditory alert, warning those nearby that lightning is imminent and giving them the chance to find safety before the storm actually impacts the area...
s are available and used mostly by parks and other outdoor recreational facilities, or meteorologists contracted to provide weather information for them.
See also
- National Severe Storms Laboratory
The National Severe Storms Laboratory is a National Oceanic and Atmospheric Administration weather research laboratory located at the National Weather Center in Norman, Oklahoma....
- Storm Prediction Center
The Storm Prediction Center , located in Norman, Oklahoma, is part of the National Centers for Environmental Prediction , operating under the control of the National Weather Service , which in turn is part of the National Oceanic and Atmospheric Administration of the United States Department of...
- Robert C. Miller
Col. Robert C. Miller, USAF , was an American meteorologist, whom pioneered severe convective storms forecasting and applied research, developing an empirical forecasting method, identifying many features associated with severe thunderstorms, a forecast checklist and manuals, and is known for the...
, a pioneer in the field of forecasting tornadoes
External links