Convective storm detection

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Convective storm detection is the observation of deep, moist convection (DMC); this term includes the minority of storms which do not produce lightning and thunder. Convective storms produce tornadoes as well as large hail, strong winds, and flash flooding. The detection of convective storms relies on direct eyewitness observations, for example from storm spotters; and on remote sensing, mostly weather radar. Some in situ measurements are used for direct detection as well, notably, wind speed reports from surface observation stations. It is part of the integrated warning system, consisting of prediction, detection, and dissemination of information on severe weather to the public.[1]

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[edit] History

1960s radar technology detects supercells over the Twin Cities during the 1965 Twin Cities tornado outbreak.
1960s radar technology detects supercells over the Twin Cities during the 1965 Twin Cities tornado outbreak.

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 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 watches and convective outlooks in 1952.[2] In 1953 it was confirmed that hook echoes are associated with tornadoes. By recognizing these radar signatures, meteorologists could detect thunderstorms likely producing tornadoes from dozens of miles away.[3]

[edit] Storm spotting

A Doppler weather radar image indicating the likely presence of a tornado over DeLand, Florida. Green colors indicate areas where the precipitation is moving towards the radar dish, while red areas are moving away. In this case the radar is in the bottom right corner of the image. Strong mesocyclones show up as adjacent areas of bright green and bright red, and usually indicate an imminent or occurring tornado.
A Doppler weather radar image indicating the likely presence of a tornado over DeLand, Florida. Green colors indicate areas where the precipitation is moving towards the radar dish, while red areas are moving away. In this case the radar is in the bottom right corner of the image. Strong mesocyclones show up as adjacent areas of bright green and bright red, and usually indicate an imminent or occurring tornado.

In the mid 1970s, the US National Weather Service (NWS) increased its efforts to train storm spotters to spot key features of storms which indicate severe hail, damaging winds, and tornadoes, as well as damage itself and flash flooding. The program was called Skywarn, and the spotters were local sheriff's deputies, state troopers, firefighters, ambulance drivers, amateur radio operators, civil defense (now emergency management) 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 report to them. The organizations activate public warning systems such as sirens and the Emergency Alert System, and forward the report to the NWS, which does directly disseminate information and warnings through its NOAA Weather Radio All Hazards network.[1] There are more than 230,000 trained Skywarn weather spotters across the United States.[4]

In Canada, a similar network of volunteer weather watchers, called Canwarn, helps spot severe weather, with more than 1,000 volunteers.[5] In Europe, several nations are organizing spotter networks under the auspices of Skywarn Europe[6] and the Tornado and Storm Research Organisation (TORRO) has maintained a network of spotters in the United Kingdom since the 1970s.

Storm spotters are needed because radar systems such as NEXRAD do not detect a tornado, only indications of one. Radar may give a warning before there is any visual evidence of a tornado or imminent tornado, but ground truth from an observer can either verify the threat or determine that a tornado is not imminent. The spotter's ability to see what radar cannot is especially important as distance from the radar site increases, because the radar beam becomes progressively higher in altitude further away from the radar, chiefly due to curvature of Earth, and the beam also spreads out. Therefore, when far from a radar, only high in the storm is observed and the important areas are not sampled, and data resolution also suffers. 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.

[edit] Visual evidence

A rotating wall cloud with rear flank downdraft clear slot evident to its left rear.
A rotating wall cloud with rear flank downdraft clear slot evident to its left rear.

Storm spotters are trained to discern whether a storm seen from a distance is a supercell. They typically look to its rear, the main region of updraft 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.[7]

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, large overshooting top, a hard anvil (especially when backsheared against strong upper level winds), 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.[8]

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. Barring a low-level boundary, tornadogenesis is highly unlikely unless a rear flank downdraft occurs, which is usually visibly evidenced by evaporation of cloud adjacent to a corner of a wall cloud. A tornado often occurs as this happens or shortly after; first, a funnel 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 base and surface.[9]

[edit] Radar

A classic hook echo. The tornado associated with this echo was part of the 1999 1999 Oklahoma tornado outbreak. It reached F5 strength on the Fujita scale.
A classic hook echo. The tornado associated with this echo was part of the 1999 1999 Oklahoma tornado outbreak. It reached F5 strength on the Fujita scale.

Today, most developed countries have a network of weather radars, which remains the main method of detecting signatures likely associated with tornadoes. 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, 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 away. 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.

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.

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 algorithms. Meteorologists first look at the atmospheric environment as well as changes thereof, and once storms develop, storm motion and interaction with the environment.

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, it may trip the mesocyclone detection algorithm (MDA). In reflectivity (precipitation intensity) data, a tight echo gradient (particularly on the inflow area) and a fan shape generally indicate a supercell. 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).

Vertical cross-section through a supercell exhibiting a BWER.
Vertical cross-section through a supercell exhibiting a BWER.

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 precipitation 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.

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). 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 echo 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).

[edit] Satellite and other methods

Additionally, most populated areas of the earth are now visible from the Geostationary Operational Environmental Satellites (GOES), which aid in the nowcasting of severe convective and tornadic storms.[5]

One particular signature associated with very intense convective updrafts and thus severe weather is the Enhanced-V feature on infrared satellite imagery where cold cloud tops form an overshooting top beginning as a point at the updraft and fanning out in a V shape as cloud matter is blown downwind. A similar feature can also be seen on visible satellite imagery.[10]

[edit] See also

[edit] References

  1. ^ a b Doswell, Charles A. III; A.R. Moller and H.E. Brooks (Aug 1999). "Storm Spotting and Public Awareness since the First Tornado Forecasts of 1948". Weather and Forecasting 14 (4): 544–57. doi:10.1175/1520-0434(1999)014<0544:SSAPAS>2.0.CO;2. 
  2. ^ Galway, Joseph G. (Dec 1992). "Early Severe Thunderstorm Forecasting and Research by the United States Weather Bureau". Weather and Forecasting 7 (4): 564–87. American Meteorological Society. doi:10.1175/1520-0434(1992)007<0564:ESTFAR>2.0.CO;2. 
  3. ^ Markowski, Paul M. (Apr 2002). "Hook Echoes and Rear-Flank Downdrafts: A Review". Monthly Weather Review 130 (4): 852–76. doi:10.1175/1520-0493(2002)130<0852:HEARFD>2.0.CO;2. 
  4. ^ What is SKYWARN?. National Weather Service. Retrieved on 2007-02-27.
  5. ^ a b Tornado Detection at Environment Canada. Environment Canada (2004-06-02). Retrieved on 2007-03-16.
  6. ^ Skywarn Europe Retrieved on 2007-05-18
  7. ^ Edwards, Moller, Purpura, et al (2005). Basic Spotters’ Field Guide. National Oceanic and Atmospheric Administration. Retrieved on 2006-11-01.
  8. ^ Doswell, Moller, Anderson, et al (2005). Advanced Spotters' Field Guide. National Oceanic and Atmospheric Administration. Retrieved on 2006-09-20.
  9. ^ Questions and Answers about Tornadoes. A Severe Weather Primer. National Severe Storms Laboratory (2006-11-15). Retrieved on 2007-07-05.
  10. ^ Brunner, Jason C.; S.A. Ackerman, A.S. Bachmeier, and R.M. Rabin (Aug 2007). "A Quantitative Analysis of the Enhanced-V Feature in Relation to Severe Weather". Weather and Forecasting 22 (4): 853–72. doi:10.1175/WAF1022.1. 

[edit] Further reading

  • Church, C.; D. Burgess (Author), C. Doswell (Author), R. Davies-Jones (Editor) (December 1993). The Tornado: Its Structure, Dynamics, Prediction, and Hazards (Geophysical Monograph #79). Washington, DC: American Geophysical Union. ISBN 0875900380. 
  • Doswell, Charles A. III (Editor) (November 2001). Severe Convective Storms (Meteorological Monographs, Vol. 28, No. 50). Boston, MA: American Meteorological Society. ISBN 1878220411. 
  • Grazulis, Thomas P. (July 1993). Significant Tornadoes 1680-1991: A Chronology and Analysis of Events. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 1-879362-03-1. 
  • Kessler, Edwin (September 1988). Instruments and Techniques for Thunderstorm Observation and Analysis (Thunderstorms: a Social, Scientific, and Technological Documentary, Vol 3). Norman, OK: University of Oklahoma Press. ISBN 0806121173. 

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