Convective storm detection is the
meteorological
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
observation, and short-term prediction, of deep moist
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
(DMC). DMC describes atmospheric conditions producing single or clusters of large
vertical extension clouds ranging from
cumulus congestus to
cumulonimbus, the latter producing
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...
s associated with
lightning
Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...
and
thunder
Thunder is the sound caused by lightning. Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning pr ...
. Those two types of clouds can produce severe weather at the surface and aloft.
The ability to discern the presence of deep moist convection in a storm significantly improves meteorologists' capacity to predict and monitor associated phenomena such as
tornado
A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, alt ...
es, large
hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
, strong
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
s, and heavy
rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
leading to
flash flood
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing ...
ing. It relies on direct eyewitness observations, for example from
storm spotter
Storm spotting is a form of weather spotting in which observers watch for the approach of severe weather, monitor its development and progression, and actively relay their findings to local authorities.
History
Storm spotting developed in the ...
s; and on
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
, especially
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...
. Some
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
measurements are used for direct detection as well, notably,
wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
reports from
surface observation
Surface 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 automate ...
stations. It is part of the ''integrated warning system'', consisting of prediction, detection, and dissemination of information on
severe weather
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmos ...
to users such as emergency management, storm spotters and chasers, the media, and the general public.
History
![1965May06 1919](https://upload.wikimedia.org/wikipedia/commons/d/df/1965May06_1919.jpg)
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.
However, with the advent of weather radar, areas near a local office could get advance warning of severe weather. The first public
tornado warnings were issued in 1950 and the first
tornado watch
A tornado watch ( SAME code: TOA) is a severe weather watch product issued by national weather forecasting agencies when meteorological conditions are favorable for the development of severe thunderstorms capable of producing tornadoes. In addit ...
es and
convective outlooks in 1952.
In 1953 it was confirmed that
hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivit ...
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 Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
(NWS) increased its efforts to train
storm spotter
Storm spotting is a form of weather spotting in which observers watch for the approach of severe weather, monitor its development and progression, and actively relay their findings to local authorities.
History
Storm spotting developed in the ...
s to identify and report key features of storms which indicate severe hail, damaging winds, and tornadoes, as well as damage itself and
flash flood
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing ...
ing. The program was called
Skywarn, and the spotters were local
sheriff's deputies
In the United States, a sheriff is an official in a county or independent city responsible for keeping the peace and enforcing the law. Unlike most officials in law enforcement in the United States, sheriffs are usually elected, although so ...
,
state troopers,
firefighter
A firefighter is a first responder and rescuer extensively trained in firefighting, primarily to extinguish hazardous fires that threaten life, property, and the environment as well as to rescue people and in some cases or jurisdictions als ...
s,
ambulance drivers,
amateur radio operator
An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way personal communications with other amateur operators on radio frequencies assigned to the amateur radio service. Amateur radio operators ...
s,
civil defense
Civil defense ( en, region=gb, civil defence) or civil protection is an effort to protect the citizens of a state (generally non-combatants) from man-made and natural disasters. It uses the principles of emergency operations: prevention, mit ...
(now
emergency management
Emergency management or disaster management is the managerial function charged with creating the framework within which communities reduce vulnerability to hazards and cope with disasters. Emergency management, despite its name, does not actual ...
) spotters,
storm chasers, 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 they report to them. The organizations activate public warning systems such as
sirens and the
Emergency Alert System
The Emergency Alert System (EAS) is a national warning system in the United States designed to allow authorized officials to broadcast emergency alerts and warning messages to the public via cable, satellite, or broadcast television, and bot ...
, and forward the reports to the NWS, which does directly disseminate
information and warnings through its
NOAA Weather Radio All Hazards network.
There are more than 230,000 trained Skywarn weather spotters across the United States.
In
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
, a similar network of volunteer weather watchers, called
Canwarn
CANWARN, acronym for CANadian Weather Amateur Radio Network, is an organized severe weather spotting and reporting program organized and run by the Meteorological Services Division of Environment Canada. What CANWARN members do is called ground tr ...
, helps spot severe weather, with more than 1,000 volunteers.
In Europe, several nations are organizing spotter networks under the auspices of
Skywarn Europe Skywarn Europe is a network of volunteer storm spotters throughout Europe based on the U.S. Skywarn program (a similar program, Canwarn operates in Canada).
Its members attend severe weather training courses and deliver real-time observations of ...
and the
Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...
since the 1970s.
Storm spotters are needed because radar systems such as
NEXRAD, and satellite images, do not detect tornadoes or hail, only indications that the storm has the potential.
[Chance Hayes, National Weather Service Wichita, Kansas. "Storm Fury on the Plains." Storm Spotter Training. 4H Building, Salina, Kansas. 22 Feb. 2010. Lecture.] Radar and satellite data interpretation will usually give a warning before there is any visual evidence of such events, but
ground truth 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
![Wall cloud12 - NOAA](https://upload.wikimedia.org/wikipedia/commons/1/18/Wall_cloud12_-_NOAA.jpg)
Storm spotters are trained to discern whether a storm seen from a distance is a
supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms ( ...
.
They typically look to its rear, the main region of
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
and
inflow.
Under the updraft is a rain-free base, and the next step of
tornadogenesis is the formation of a rotating
wall cloud. 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 tower features such as a hard and vigorous updraft tower, a persistent and/or large
overshooting top, a hard anvil (especially when
backsheared against strong upper level
wind
Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ...
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 forward flank precipitation core from the wall cloud. Tornadogenesis is most likely at the interface of the updraft and
forward 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
mesocyclone
A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation (vortex), typically around in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank (back ...
. Barring a low-level boundary, tornadogenesis is highly unlikely unless a
rear flank downdraft
The rear flank downdraft (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. Large hail ...
occurs, which is usually visibly evidenced by evaporation of
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly after; first, a
funnel cloud
A 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 (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water su ...
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 monitor all areas of a storm and their surroundings.
Radar
Today, most developed countries have a network of
weather radar
Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...
s, which remains the main method of detecting signatures likely associated with tornadoes and other severe phenomenons as
hail
Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
and
downburst
In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. Capable of pro ...
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
![Radar-algorithme eng](https://upload.wikimedia.org/wikipedia/commons/3/33/Radar-algorithme_eng.gif)
In short-term prediction and detection of tornadoes,
meteorologist
A meteorologist is a scientist who studies and works in the field of meteorology aiming to understand or predict Earth's atmospheric phenomena including the weather. Those who study meteorological phenomena are meteorologists in research, while t ...
s integrate radar data with reports from the field and knowledge of the meteorological environment. Radar analysis is augmented by automated detection systems called
algorithm
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...
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) with a
tilted updraft
A tilted updraft (also known as a tilted storm) is a thunderstorm which is not vertically erect. This happens as a result of unidirectional wind shear, or a change in wind speed with height. In such an environment, the top of the updraft is push ...
. 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
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
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
reflectivity
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
(precipitation intensity) data, a tight echo gradient (particularly on the inflow area) and a fan shape generally indicate a
supercell
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms ( ...
. 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
velocity
Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...
, including radial
direction (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
vorticity
In continuum mechanics, vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along wi ...
, it may trip the
mesocyclone
A mesocyclone is a meso-gamma mesoscale (or storm scale) region of rotation (vortex), typically around in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank (back ...
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 (TVS) or the tornado detection algorithm (TDA). TVS is then an extremely strong mesocyclone found at very low level and extending over a deep layer of the thunderstorm, 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. Polarimetric radar can discern meteorological and nonmeteorological and other characteristics of hydrometeors that are helpful to tornado detection and nowcasting. Nonmeteorological reflectors co-located with a couplet, can confirm that a tornado has likely occurred and lofted debris. An area of high reflectivity, or debris ball, may also be visible on the end of the hook. Either the polarimetric data or debris ball are formally known as the
tornado debris signature
A tornadic debris signature (TDS), often colloquially referred to as a debris ball, is an area of high reflectivity on weather radar caused by debris lofting into the air, usually associated with a tornado. A TDS may also be indicated by dual-polar ...
(TDS). The
hook echo
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone, resulting in a curved feature of reflectivit ...
feature is formed as the
RFD 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-88D 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. Dual-
polarization radar, being implemented to the US
NEXRAD network, may provide enhanced warning of tornadoes and severe winds and hail associated with the hook echo due to distinct precipitation drop characteristics. Polarimetric radar boosts precipitation observation and prediction, especially rainfall rates, hail detection, and distinguishing precipitation types. Proposed radar technologies, such as
phased array and CASA, would further improve observations and forecasts by increasing the temporal and spatial resolution of scans in the former as well as providing low-level radar data over a wide area in the latter.
In certain atmospheric environments,
wind profiler
A wind profiler is a type of weather observing equipment that uses radar or sound waves ( SODAR) to detect the wind speed and direction at various elevations above the ground. Readings are made at each kilometer above sea level, up to the extent o ...
s may also provide detection capabilities for tornadic activity.
Hail, downburst and downpour
![Coupe verticale VIL](https://upload.wikimedia.org/wikipedia/commons/7/72/Coupe_verticale_VIL.png)
Hail forms in a very intense
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
in a supercell or a multicellular thunderstorm. As for tornadoes, BWER detection and a tilted updraft are indicative of that updraft but does not lead to predict hail. The presence of a
hail spike in the reflectivity pattern is an important clue. It is an area of weak reflectivity extending away from the radar immediately behind a thunderstorm with hail. It is caused by radiation from the radar bouncing from hailstone to hailstone or the ground before being reflected back to the radar. The time delay between the backscattered radiation from the storm and the one with multiple paths causes the reflectivity from the hail to appear to come from a farther range than the actual storm.
However, this artefact is visible mostly for extremely large 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. One 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 liquid'', 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 flood
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing ...
.
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 Service
The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...
meteorologists have found that the VIL density, that is to say VIL divided by the maximum height of the 18
dBZ 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
downburst
In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. Capable of pro ...
, too. A convective
downdraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
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 energy
In physics, the kinetic energy of an object is the energy that 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 stated velocity. Having gained this energy during its acc ...
of the parcel on descending to the surface and is found to be the negative
CAPE
A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck.
History
Capes were common in medieval Europe, especially when combined with a hood in the chaperon. Th ...
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 front
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...
s 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
![40723Z IRsat](https://upload.wikimedia.org/wikipedia/commons/0/0c/40723Z_IRsat.jpg)
Most populated areas of the earth are now well covered by
weather satellite
A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or ...
s, which aid in the
nowcasting of severe convective and tornadic storms.
These images are available in the
visible and
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
domains. The infrared (IR: 10-13
µm) images permit estimation of the top height of the clouds, according to the
air mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to l ...
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 and subsequent traits of 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
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
, so thunderstorms are only seen after being well developed. It is, however, useful in
convective storm prediction
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
, as it illustrates the placement and movement of air masses and of moisture, as well as
shortwaves and areas of vorticity and lifts.
Severe storms have a very strong
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
. The rising air parcels in that column accelerate and will overshoot the
equilibrium level (EL) before being pulled back by negative buoyancy. This means the cloud tops will reach higher levels than the surrounding cloud in the updraft region. This
overshooting top 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 top
Cloud Top () is a 30-story, tall residential twin skyscraper complex located in Xinyi Special District, Taipei, Taiwan. The complex used to be the tallest residential skyscrapers in Taipei when completed in 1997. Built under strict requirement ...
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
multicellular
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially un ...
storms and
squall line
A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accom ...
s, the mid-level
jet stream is often intersecting the line and its dry air introduced into the cloud is negatively unstable. This results in 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
downdraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
s at the surface. These kinds of lines often 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 thunderstorm, the surface cold pool of air associated with the downdraft will stabilize the air and form a cloud-free area that will end along the
gust front
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...
. This mesoscale front, when moving into a warm and unstable air mass, will lift it and
cumulus clouds appear on satellite pictures. This line is likely the point of further convection and storms, especially if it coincides with fronts from other thunderstorms in the vicinity. One can notice it at the leading edge of a squall line, in the southeastern quadrant of a typical supercell (in the northern hemisphere), or different regions around other thunderstorms. They may also be visible as an
outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...
hours or days after convection and can pinpoint areas of favored thunderstorm development, the possible direction of movement, and even likelihood for tornadoes. The speed of forward movement of the outflow boundary or gust front to some degree modulates the likelihood of tornadoes and helps determine whether a storm will be enhanced by its presence or the inflow be choked off thus weakening and possibly killing the storm. Thunderstorms may move along slow-moving or stationary outflow boundaries and tornadoes are more likely; whereas fast-moving gust fronts in many cases weaken thunderstorms after impact and are less likely to produce tornadoes—although brief tornadoes may occur at the time of impact. Fast-moving gust fronts may eventually decelerate and become slow-moving or stationary outflow boundaries with the characteristic "agitated area" of cumulus fields previously mentioned.
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
lightning
Lightning is a naturally occurring electrostatic discharge during which two electrically charged regions, both in the atmosphere or with one on the ground, temporarily neutralize themselves, causing the instantaneous release of an average ...
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 frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...
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. Coming advances in research and observations should improve forecasts of severe weather and increase warning time.
Personal lightning detection systems are also available, which may provide strike time, azimuth, and distance. In addition,
lightning prediction system
A lightning prediction system is a type of Lightning detection, lightning detection equipment that determines when atmospheric conditions likely to produce lightning strikes and sounds an alarm, warning those nearby that lightning is imminent and ...
s are available and used mostly by parks and other outdoor recreational facilities, or meteorologists contracted to provide weather information for them.
See also
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National Severe Storms Laboratory
The National Severe Storms Laboratory (NSSL) is a National Oceanic and Atmospheric Administration (NOAA) weather research laboratory under the Office of Oceanic and Atmospheric Research. It is one of seven NOAA Research Laboratories (RLs).
NSSL ...
(NSSL)
*
Storm Prediction Center
The Storm Prediction Center (SPC) is a US government agency that is part of the National Centers for Environmental Prediction (NCEP), operating under the control of the National Weather Service (NWS), which in turn is part of the National Oceani ...
(SPC)
*
Warning Decision Training Branch
The Warning Decision Training Division (WDTD), known as the Warning Decision Training Branch until April 1, 2015, is one of three training organizations in the NWS Training Division which also includes the Forecast Decision Training Branch and the ...
(WDTB)
*
Robert C. Miller, a pioneer in the field of forecasting tornadoes
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Tropical cyclone observation
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Weather forecasting
Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th cent ...
References
Further reading
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* {{cite book, last=Kessler , first=Edwin , author-link=Edwin Kessler , title=Instruments and Techniques for Thunderstorm Observation and Analysis (Thunderstorms: a Social, Scientific, and Technological Documentary, Vol 3) , publisher=University of Oklahoma Press , date=Sep 1988 , location=Norman, OK , isbn=0-8061-2117-3
External links
Tornado Detection(
NSSL)
NOAA Hazardous Weather TestbedTornado Warning Guidance(NW
Warning Decision Training Branch
Supercell Structure and Dynamics(
NWS)
Tornadogenesis cascade paradigman
(
San Francisco State University
San Francisco State University (commonly referred to as San Francisco State, SF State and SFSU) is a public research university in San Francisco. As part of the 23-campus California State University system, the university offers 118 different ...
)
An Introduction to Storm Observation and Reporting(NWS)
An Online Meteorology Guide: Severe Storms(
University of Illinois
The University of Illinois Urbana-Champaign (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Univer ...
)
Weather Glossary for Storm Spotters(NOAA Technical Memorandum NWS SR-145)
Thunderstorms(NWS JetStream Online Weather School)
Severe weather and convection
Storm
Tornado
Weather prediction
Radar meteorology
Weather warnings and advisories
Storm chasing
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