The forecast of the storms violent one is the part of the operational Météorologie which tries to envisage the development, the intensity, the type of danger and the zones affected by storms being able to give gross Grêle, Vent S destructors, Tornade S and torrential Pluie S.

Definition of a violent storm

The definition of the criteria of the various phenomena associated with a violent storm varies from one country to another and sometimes even from one area to another inside certain countries. This is due to the morphology of the ground, the type of occupation of the grounds, the concentration of the population and all other factors being able to influence the human life, animal and vegetable.

In general, it is considered that a storm is violent if it gives one or more following items:

• Grêle of 2 Cm of diameter or more what is the cause of serious damage to the dwellings, cultures, people and animals.
• Winds of 90 km/h or more in sudden gusts which can damage the structures.
• Tornades
• sudden Pouring rain which would cause floods.

Exceptions:

• Certains country regards the rate of flash S as being a criterion of warning but like any produced storm of the Foudre, this criterion is not really indicating of the violence of the storm.
• Certaines areas considers that hail of less than 2 cm is also a criterion of warning because of the possible hazards to the cultures: fruit trees, vine, etc
• the criterion of quantity of rain is variable according to the geography and the type of vegetation since the water run-off varies largely according to the places. Certain countries coordinate the forecast of the storms with their hydrological system of measurement S. They consider to send a weather Alerte only when the rain made reach critical levels with the rivers of an area and not with the passage of a violent storm.

History

The storms violent one are as old as the world and the men always tried to guard themselves against their effects. If one passes over the incantations and other readings of oracle S of the Antiquité, research more organized to include/understand the storms occurs starting from the Renaissance by observations. For example, a faithful follower of the weather data acquisition, the British governor John Winthrop, written in his notes of July 1643, that a sudden strong gale in the North-East of the Massachusetts and on the coast of the New Hampshire uprooted trees, fills the air with dust, raised a public edifice of Newbury and killed an Amerindian . Even if this description could be connected to a downward Rafale or a line of grain, it could be well the first description in the history of a tornado.

The population is lost in conjecture in connection with these “terrible swirls”. In July 1759, following a terrible tornado passing to Leicester, Massachusetts, a descendant of written the Winthrop governor:

It seems difficult to me to find a cause adequate for this phenomenon, to show how a small volume of air can be put in so fast rotation. I would not dare to venture me to put forth an assumption .

The August 14th 1773 , professor Samuel Williams is the first in America to give not only one description but objective data of winds. He writes that a marine waterspout was formed on the coastal river Merrimack, in the south of Salisbury (Massachusetts), and transformed himself into tornado by touching ground. Right before its appearance, of violent gusts of winds coming from south-west blew on the area lasting 4 minutes before a fast change with the west-north-west. Two minutes later, the wind became calm and the sky became very dark.

Research in Météorologie became more systematic starting from the XIXe century as well as work on the explanation of the storms. In the Years 1880, the Corps of the engineers of the American army, which was in load of the weather service incipient from this country, organized a team of 2000 volunteers to document all the cases of tornadoes on the center and is of the United States. One drew from them the weather owners from surface favorable to the generation of the storms tornadic and the Corps tried to make the first predictions. It was not very conclusive and the National Weather Service, which succeeded the Corps , decided not to mention until in 1938 the possibility of this phenomenon in its alarms weather of storms violent one.

With the birth of the Aviation, the research of the requirements to the formation of tornadoes and storms violent one was given to the day order in the Années 1920 and 1930. The development of the Radiosondage started to give more information on the vertical structure of the atmosphere what allowed to recognize the factors Thermodynamique and the synoptic releases of altitude necessary to the release of the clouds convectifs.

All information thus joined together was colligées and interpreted by researchers like A.K. Showalter and J.R. Fulks in the United States. Using this work and their own observations, the officers weather E.J. Fawbush and R.C. Miller, Tinker air base ( Tinker Air Force Bases ) US Air Force with Oklahoma City, could successfully predict for the first time the occurrence of a tornado on the basis the March 25th 1948 in evening.

Since this time, world research in meteorology made it possible to better include/understand the storms. The arrival of the models of numerical Prévision of time made it possible to simulate the behavior of the atmosphere on an increasingly fine scale and we now have models of which the resolution approaches that the storms (less than 10 km in diameter). The models also make it possible to produce algorithms which give an idea of the violent potential of the storms. The forecast remains however still an interaction between the computer data and the experiment of the Meteorologist.

Anatomy of a storm

Thermodynamics

See also: Storm, the Lightning

The clouds convectif S are formed in a mass of unstable air where there is availability of heat and moisture on low level and the drier air and cold in altitude. A piece of air which one raises decreases by temperature (T) and pressure (P) with altitude according to the law of perfect gases ($PV\; =\; nRT$). In an unstable atmosphere, it reaches a level where it becomes hotter than the surrounding air: the “free Level of convection” (NCL). It undergone then the Pushed of Archimedes and rises freely until its temperature is again in balance with the surrounding temperature.

When the piece rises, it cools up to its point of dew, on a level called “Niveau of condensation per rise” (NCA) and the Steam which it contains starts to condense. This level can be reached before or after the NCL. Condensation releases a certain quantity of heat, the Latent heat, provided to water at the time of sound evaporation. It results a notable reduction from it from the rate of cooling of the ascending mass of air what increases the push of Archimedes by increasing the difference in temperature between the piece and the environment. The base of the convectif cloud will be at the NCA whereas its top is on the level of balance or slightly higher because of the inertia of the percelle one.

This upswing, that one calls the free convection, is a process liberator of energy, and the potential energy (Potential Énergie of Convection Available) stored in the unstable atmosphere transforms into kinetic energy of displacement. Storms are obtained when the released kinetic energy makes it possible to reach at least an altitude where the temperature is under -20 °C whereas it is above zero close to the ground. Indeed, the movement of the droplets of Nuage S and Précipitation allows D, to tear off electrons by collision and creates a potential difference electric in the bottom and the top of the cloud what will give possibly the lightning.

Stopper

An unstable atmosphere often comprises a zone of inversion of temperature, i.e. a thin layer of air where the temperature increases with the altitude which inhibits the convection temporarily. A piece of air rising through this layer will be colder than the air which surrounds it and will tend to be pushed back downwards. The Inversion is thus very stable, it prevents any upswing and restores balance. Energy necessary to overcome this inversion is called Énergie of inhibition of the convection.

During the day, when the ground is heated by the Sun, the air imprisoned under this inversion is heated even more and can also become wetter because of evaporation. If the zone of inversion is locally eroded by mixtures with the sub-base or so of the phenomena with large scales raise it in block, the surfacing become very unstable spouts out violently at certain places. The air on the surface of the ground runs out then horizontally towards these points of eruption and forms high clouds of storm.

Dynamic releases

Even in the presence of favorable factors Thermodynamic S, an ascending current appears only if the unstable air in the vicinity of the ground is thorough until the free convection. In a uniform mass of air and without movement, the warming alone can be enough but in general there exist releases which will make it possible to concentrate the activity stormy:

• a local inversion can attenuate or to even disappear completely if a power Courant-jet of altitude is on in the sector because inside the jet-stream, of the particularly intense winds, blowing to several hundred kilometers per hour, move in the direction of the current while driving back to the bottom the air in front of them and while aspiring to the top the air behind them. This phenomenon of ascending Aspiration, if it is sufficiently strong, can dissipate an inversion and support the formation of storms or the intensification of the storms in progress.

• the same thing can occur with a jet-stream of low level but in this case, it is about convergence of mass on the left of the jet which forces the air piled up to go up as a pot that one presses at his base.

• Of the local effects like the rise forced air along a slope by weather phenomena with large scales or breezes of sea which brings humid air towards an unstable zone.

• the passage of a Face cold, where cold air and dense advances in a hotter area, cutting through a path under the hot air by raising it.

One locates the zones of potential of storms by initially analyzing the potential Thermodynamique S of the mass of air using diagrams like the Téphigramme or of cuts through the charts of analysis produced by the models of numerical Prévision of time.

Then, the forecaster locates the position where one obtains the maximum of dynamic releases in the mass of air. The chart of right-hand side is the analysis of the dynamic components with 00 hour YOU, on March 26th, 1948 (historical chart), which shows that a very great number of them are found above the Oklahoma at this time (grayed contour).

Analyzes violent potential

Once located the development area of storms, the forecaster must evaluate the potential of these storms. This last depends on three things:

• moisture available.

• potential energy of convection available (EPCD).
• the shearing of the winds in and under the cloud.

Indeed, it is the combination from these three items which will determine the type of storm like its potential to produce violent time. One sees in the table of right-hand side how the various types of storms are in connection with the supplied energy and linear shearing. It however misses in this diagram the effect of the change of management of the wind with altitude and the moisture of which it is necessary to hold account for some of the phenomena.

Torrential rain

The wetter the mass of air is, the more the quantity of steam to be condensed will be large. If the EPCD is weak, the generated cloud will be of weak vertical extension and little of this moisture will change into rain. So on the other hand, the supplied energy is large but the change of the winds with altitude is strong, condensed moisture will be found far from its point of formation.

Thus, the storms which give torrential rains will thus tend to be found in a mass of unstable and wet air. However, in all the cases, there will be little shearing of the winds. The whole gives a very intense storm which moves slowly. One can calculate water available for condensation thanks to the equations of thermodynamics in this case and evaluate the potential of accumulation of rain under the storm.

A particular case of storms to very strong rainfall is that of the complexes convectifs of méso-scale. One CCM is a stormy unit being generally formed in end-of-day starting from dispersed storms and which reaches its apogee lasting during the night whereas it is organized in a broad circular zone. After its formation, it derives in flow from altitude and gives mainly intense precipitations causing of the floods on broad areas. The CCM develop under weak a atmospheric Circulation anticyclonic, in front of a barometric Creux of altitude, in a Masse of very unstable air and with a weak shearing of the winds with altitude. The recognition of the owner of atmospheric circulation is thus important in this case, in addition to the thermodynamic potential.

See also: Complex convectif of méso-scale

Hail

See also: Hail

In the case of the storms of hail, the EPCD must be more important than in the case of the torrential rains so that the formed drops can reach a level where they will freeze. The shearing of the winds must be larger also of such kinds that hail it master key the maximum of time and zones in the cloud before falling down. Finally, the level of congelation must be with a height where the grêlon will not melt completely before reaching the ground. Various algorithms make it possible to evaluate the size of the grêlon.

Tornado

See also: Tornado

When the winds undergo a strong change or Cisaillement in the vertical, en direction and in intensité, that induces a rotation movement around a horizontal axis. When this tube of winds in rotation enter in interaction with fort running the ascending of a violent storm, this rotation around the horizontal axis will rock and become a rotation around a vertical axis and will create a Mésocyclone.

According to a fundamental law of physics, the kinetic moment of a mass of air compared to its vertical axis of rotation is preserved. This kinetic moment is equal to the product of the momentum (mass multiplied by speed) by the distance to the axis. The ascending current by vertically stretching the tube of air in rotation thus increases rotation by decreasing the diameter of the mésocyclone with approximately two to six kilometers.

This Mésocyclone, of which the foot is at an altitude of one kilometer and the top almost at the top of the storm, will be concentrated even more by local reasons of winds in the cloud with a diameter not exceeding a kilometer. If the shearing of the winds under the storm is favorable, one will witness a last concentration which can give a tornado of only a few hundreds of meters but with winds exceeding 100 km/h.

To envisage such a phenomenon, it is thus necessary to know shearing in the low levels and the possibility of its concentration. One uses for that the calculation of the Hélicité of the mass of air under the 3 Kilomètre S of altitude and his relation with the EPCD.

Downward gusts

See also: Rafale downward

A last violent phenomenon is that of the downward gusts. When a storm is gorged with rain and in a relatively dry environment in altitude, the heart of precipitation can attract the dry air in the cloud while going down. This last being colder than the cloud, it undergoes the push of Archimedes downwards. This movement of cold air and dryness as well as the mass of rain which goes down give gusts which can reach 200 km/h under certain conditions.

The analysis of the absolute Humidity, EPCD and the Téphigramme shows the potential for this kind of violent time. So moreover, one notices a Courant-jet of low level in the sector of the storm, one can think of his folding back by the downward gust, which increases it by as much.

Lines of grain, grain in arc and Derecho

See also: Line of grains, Grain in arc, Derecho

If the shearing of the winds is important but linear, i.e. the winds increase with altitude but more or less in the same direction, the storms which will be formed will tend to be linked by forming a line. So moreover, there is a jet-stream of low level with angle of this line, it will be folded back towards the ground by the downward current of the storms. The Front of gusts thus creates will be propagated in front of the line of storms. It is the vertical structure which one sees in the image of right-hand side, with the part the top.

In the part of bottom, one sees two possibilities of form of the line of grain. If the direction of the wind of surface (with before) and that of the jet-stream (with the back) symmetrical but are opposed, a right line of storms is obtained. When the EPCD exceeds 1000 J/kg, the associated faces of gusts can give strong winds. On the other hand, if flows are assymetric, a line in arc is obtained. This type of line can comprise points of rotation, as shown in the head of the diagram, where tornadoes can be formed in addition to the violent gusts along the line.

The forecaster must thus evaluate the potential energy and the structure of the winds to recognize this type of storms violent one.

Forecasts

Once the analysis of the violent potential made, the forecaster must envisage the displacement of the masses of air and the releases of storms. Until the advent of the computers and numerical forecasting models of time, it could only extrapolate the displacement of these characteristics that with the former history. I.e. it followed the displacement of the systems, the jet-streams, etc starting from the data taken at every 6 hours in altitude and of the data of surface at every hour.

Since the Years 1970, the weather models appeared and gradually improved. Until very recently, these models had a resolution of more than 10 km, which did not make it possible to solve the scale of the storms. These operational models however make it possible to envisage the displacement of the releases of longer-term storms that only extrapolation. For a few years, semi-operational models with less than 10 km of resolution have allowed paramétriser the convection, i.e. to use the equations of fine scales which directly simulate the behavior of the unstable masses of air and the storms. The forecaster can thus see the storms which the model develops as if it looked at an image in three dimensions with the radar. However, these models are very expensive in data-processing time and can be rolled only for limited short periods and fields.

The meteorologist thus makes his analysis, looks at where the models move its lucky finds and looks at the models on fine scale to refine his forecast. However, it must always be wary of the results of the models which are prone to errors of forecast. Finally, it obtains a zone thus where the storms are probable and of the subfields where they can be violent one. It then tries to delimit smaller zones using its knowledge of the local effects which can concentrate the convection: break lake, warming of the slopes of mountains, convergent valleys, etc

After all this work, the forecaster will send charts such this one to warn the population of the possible hazards. He will send bulletins of the weather type day before thereafter if the convection starts to materialize.

Monitoring

Thereafter, it follows the formation of storms thanks to the weather Radars, with the satellite and other data of observations. It sends alert when the potential gives signs to be carried out.

Here a list of the indices that the forecaster seeks on the images satellaires:

• It observes the top of a storm of this type by Meteorological satellite. If one notices a continuation characteristic of ascending “bubbles”, made up of clouds which rise between two and four kilometers above the higher level of the principal cloud before falling down in the cloudy mass, that indicates that the ascending current in the cloud is particularly intense. All the elements will reach an exceptional level then.
• the temperature of the top of the cloud also indicates its vertical extension. The meteorologist can deduce the level from it from development of the storm by using his thermodynamic analysis.
• In the case of lines of storms, the shape of the back edge with notches indicates to him that the jet-stream of the mean levels is going down towards the ground what will increase the downward gusts.
• One can also note the position of the face of gusts around the storm thanks to the formation of cumuli which will be able to become new cumulonimbus.
• One also notes the dome of cold air which goes down from the storm by the total release behind the cumuli. It is a stable zone which is unfavourable with the convection.

Here a list of the indices that the forecaster seeks on the images of the radars:

• It looks at the data of the radars and compares the vertical and horizontal owner echoes to see whether it corresponds to the structure of a storm supercellulaire, multicellular with overhangs, line of grain, etc (see Orage).
• It also has the assistance of the algorithms associated with its program with radar data processing to draw its attention to certain points more difficult to follow like the presence of Mésocyclone S.

Moreover:

• It supervises the rate of the lightning. If it witnesses a strong variation of the rate of the lightning with a group of storms which has violent potential or a change of negative with positive, it can conclude their fast development from it (gone up rate) or the moment from their greater potential (descent of the rate).
• It looks at the stations of surface for an index on the concentration of moisture, the changes of winds, etc in order to refine its zone or the storms should move.
• It notes all information which it receives from voluntary observers or Chasseurs of storms to supplement its mental image of the situation.

Alarm

If the potential is realized and that the storms start to be organized according to the known owners that the forecaster will have located thanks to his monitoring, it will send weather alarms for areas in valley of the storms. This one according to diffused by the media. The people and the authorities, as the police services, will take certain measurements which can go until the evacuation.

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