There are at least three reasons for which case studies of the most severe thunderstorms generating the phenomena listed above are of priority interest: the typical organization of these cases may help discover new physical processes and interactions as well as better understand the whole convective system; the data will be available for numerical studies; and finally all the other less severe cases are more or less derived from these extreme ones.
The presentation in a same session of case studies relevant to one or the other of these phenomena (hail, flood, lightning, wind) may highlight the physical differences in the storm environment and organization leading to one type of damage or another.
The session is devoted to Frank Ludlam, who has remarkably well described and explained the main processes of cumulonimbus convection in Chapter 8 of his pioneering book, entitled ?Clouds and Storms? (The Pennsylvania State University Press) published three years after his death in 1977.
Papers are invited in all areas listed in the session title.
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Tornadoes are an enigmatic creation that hides, shifts, retreats, reforms, changes shape, multiplies, changes in size and intensity, often by the minute. Identifying and counting of tornadoes is further complicated since some occur in pairs, others produce multiple vortices, and others form and dissipate several times in a few miles. Most tornadoes are small, short-lived phenomenon but five percent become large, long lasting, and very dangerous. All these factors make defining the climatology of tornadoes difficult.
A downburst is a strong downdraft from a thunderstorm that induces an outburst of damaging winds at the surface. The winds travel outward from where they reach the ground, being between 100 yards and 10 miles wide. These high winds are often quite damaging and are often confused as being the result of a tornado. Squall lines and mesoscale groups of storms occasionally initiate a large-scale wave of high winds at the surface and these sweep over hundreds of square miles. Bow echoes are bow-shaped areas of convection in a squall line that produce long swaths of high winds labeled as derechoes. Their climatology is not well defined.
Squall line tornadoes often occur in families and move at speeds of 35 mph or higher. Tornadoes from such storms are typically larger and longer lasting than tornadoes created by other synoptic weather conditions. Tornadoes are also created by supercell thunderstorms which are the most intense, well organized, and longest-lasting thunderstorm. Supercells often create families of tornadoes along their track. A tornado outbreak is when a family of 6 or more tornadoes are spawned by the same weather system and are in close time sequence of an hour or less.
Some thunderstorms that form in unstable warm air masses also cause tornadoes, but these are usually small, short-lived events. Another special form of tornadoes comes from storms in derechoes, which is a large area of severe thunderstorms that create clusters of downbursts and generate extensive damage from straight-line winds but also can spawn tornadoes.
Tornadoes in the Northern Hemisphere occur most frequently in the spring season. However, tornadoes occur in all months of the year. Their average duration is 20 minutes, although a few long-track storms have existed for 2 to 3 hours. Tornadoes are most frequent during afternoon and early evening hours, but can occur at anytime during the day. The average size of a tornado track 12 is miles long and 200 yards wide. A few record setting tornadoes have had tracks over 200 miles and widths of one mile. Forward speeds average 35 to 40 mph. The preferred direction of tornado movement is from southwest to the northeast.
The greatest tornado outbreak in the U.S. and Midwest occurred on April 3-4, 1974, and produced 63 tornadoes. Most tornado damages and loss of life have occurred from a few large, long-lasting storm systems. Annual average property losses resulting from tornadoes in the U.S. is $458 million. Flying debris is a primary cause of deaths and injuries to humans. The average annual number of deaths due to tornadoes in the U.S. is 65.
Works on tornado cases are welcome in this session
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Hailstorms remain a serious problem, mainly for agriculture but also for property and aviation interests, in many areas of the world. The main factors leading to hail formation have been known for many years, but that has not yielded high skill in forecasting the time and location of its occurrence, nor an accepted method for its prevention or mitigation.
The forecasting problem is a difficult one. Currently, the forecast is made following a forecast of expected thunderstorm characteristics, based on dynamic and thermodynamic considerations, but without consideration of such microphysical factors as ice-forming nuclei or condensation nuclei. Such information is not generally available to the forecaster. Potential effects of storm electrification have received scant attention.
Perhaps the most important application for high hail forecasting skill would be the development of predictor variables for dealing with the enormous variability exhibited by hailstorms in randomized hail prevention experiments. There are several believable (by at least some scientists) hypotheses for preventing or mitigating hailfalls, but it has not yet been possible to demonstrate the effectiveness of any of them, due to the high cost and difficulty of performing well-designed statistical experiments in the field.
The topics of major interest for the session on hail are:
1. physics of hail, including growth conditions and mechanisms and the role of ice and condensation nucleation;
2. climatology of hailfall;
3. description of recent hailstorm disasters (crop damage and property damage);
4. surface hailfall measurements, with emphasis on new results and improvements in measurement methods;
5. studies of the relationship between hailfall characteristics and crop and property damage;
6. radar detection of hail;
7. forecasting of hailfall, at various time scales including seasonal;
8. results, techniques and concepts for hail mitigation.
In this session papers concerning the above topics are invited.