Gradient wind is the actual wind as the wind changes with altitude. Here it is probably necessary in my own words, or I did not find the Appearance of the centers of snow charges
1. Basic concepts and definitions
SNOW CHARGES (SNOW CHARGES), according to the well-known classic Meteorological Dictionary 1974. editions [ 1 ] - it is: "... the name of short-term, intense rainfall in the form of snow (or snow pellets) from cumulonimbus clouds, often with snow flurries."
And in the Meteoslovar - POGODA.BY glossaries [ 2 ]: “ Snow "charges"- very intense snowfalls, accompanied by a sharp increase in wind during their passage. Snow "charges" sometimes follow each other at short intervals. They are commonly seen behind cyclone lines and on secondary cold fronts. The danger of snow “charges” is that visibility drops sharply to almost zero when they pass”
In addition, this intense and dangerous weather phenomenon for aviation is also described in the modern Electronic Training Manual "Aviation and Weather" [ 3 ] as: shower sleet and snow with rain), which look like "snow shots" - fast-moving zones of very intense snowfall, literally "collapse" of snow with a sharp decrease in visibility, often accompanied by snow squalls (snow storms) near the surface of the Earth.
A snow charge is a powerful, bright and short-term (usually lasting only a few minutes) weather phenomenon, which, according to emerging weather conditions, is very dangerous not only for flights of light aircraft and helicopters at low altitudes, but also for all types of aircraft (aircraft) in the lower layer atmosphere during takeoff and initial climb, as well as during landing approach. This phenomenon, as we will see below, sometimes even causes an accident (accident). It is important that while maintaining the conditions for the formation of snow charges in the region, their passage can be repeated in the same place!
To improve aircraft flight safety, it is necessary to analyze the causes of the occurrence of snow piles and the meteorological conditions in them, show examples of relevant accidents, and also develop recommendations for the flight control staff and the meteorological service of flights in order to avoid accidents, if possible, in the conditions of the passage of snow charges.
2. Appearance of the centers of snow charges
Since the most dangerous snowballs in question are not so common, in order to understand the problem it is important that all aviators have the correct (including visual) ideas about this powerful natural phenomenon. Therefore, at the beginning of the article, a video example of a typical passage of such a snow charge near the Earth's surface is offered for viewing.
Rice. 1 Approaching the snow charge zone. The first frames from the video, see: http://rutube.ru/video/728d027f45b8ae5356c962f70f40d6dd/
For interested readers, some video episodes of the passage of snow charges near the Earth are also offered for viewing:
and others (see Internet search engines).
3. The process of formation of centers of snow charges
From the point of view of the meteorological situation, the typical conditions for the occurrence of winter storm centers are similar to those that occur during the formation of powerful centers of showers and thunderstorms in summer - after a cold invasion has occurred and, accordingly, the conditions for dynamic convection have arisen. At the same time, cumulonimbus clouds are quickly formed, which give pockets of heavy rainfall in summer in the form of intense rain (often with thunderstorms), and in the cold season - in the form of pockets of heavy snow. Typically, such conditions during cold advection are observed in the rear of cyclones - both behind the cold front and in the zones of secondary cold fronts (including and near them).
Let us consider a diagram of a typical vertical structure of the center of a snow charge at the stage of maximum development, which is formed under a cumulonimbus cloud under conditions of cold advection in winter.
Rice. 2 General scheme of the vertical section of the center of the snow charge at the stage of maximum development (A, B, C - AP points, see paragraph 4 of the article)
The diagram shows that intense heavy rainfall falling from a cumulonimbus cloud "entrains" air, resulting in a powerful downward air flow, which, approaching the Earth's surface, "spreads" away from the source, creating a squally increase in wind near the Earth (in mainly - in the direction of movement of the focus, as in the diagram). A similar phenomenon of “entrainment” of the air flow down by falling liquid precipitation is also observed in the warm season, creating a “gust front” (squall zone) that occurs as a pulsating process ahead of a moving thunderstorm cell—see the literature on wind shear [4].
Thus, in the zone of passage of an intense source of snow charge, the following weather phenomena dangerous for aviation, fraught with accidents, can be expected in the lower layers of the atmosphere: powerful descending air currents, squally wind increases near the Earth, and areas of sharp deterioration in visibility in snow precipitation. Let us consider separately these weather phenomena with snow loads (see paragraphs 3.1, 3.2, 3.3).
3.1 Powerful descending air currents in the center of the snow charge
As already mentioned, in the boundary layer of the atmosphere, the process of formation of areas of strong descending air flows caused by intense precipitation can be observed [4]. This process is caused by the entrainment of air by falling precipitation, if these precipitations have a large size of elements that have an increased fall rate, and a high intensity of these precipitations is also observed (“density” of flying precipitation elements). In addition, it is important in this situation that the effect of "exchange" of air masses along the vertical is observed - i.e. occurrence of sections of compensatory air flows directed from top to bottom, due to the presence of sections of ascending currents during convection (Fig. 3), in which areas of precipitation play the role of a "trigger" of this powerful vertical exchange.
Rice. 3 (this is a copy of Fig. 3-8 from [ 4 ]). Formation of downdraft during maturation stage b), entrained by heavy rainfall (red box).
The power of the emerging downward air flow due to the involvement of falling intense rainfall directly depends on the size of the falling particles (elements) of precipitation. Large particles of precipitation (Ø ≥5 mm) usually fall at velocities of ≥10 m/s and, therefore, large wet snow flakes develop the highest falling speed, since they can also have dimensions > 5 mm, and, unlike dry snow, they have a much lower "sail". A similar effect occurs in summer in the center of intense hail fall, which also causes a powerful downward air flow.
Therefore, in the center of the “wet” snow charge (flakes), the “capture” of air by precipitation increases sharply, leading to an increase in the velocity of the downward air flow in precipitation, which in these cases can not only reach, but even exceed their “summer” values at strong showers. At the same time, as is known, vertical flow velocities from 4 to 6 m/s are considered “strong”, and more than 6 ms are considered “very strong” [4].
Large wet snow flakes usually occur at slightly positive air temperatures, and therefore it is obvious that it is precisely such a temperature background that will contribute to the emergence of strong and even very strong descending air flows in the snow charge.
Based on the foregoing, it is quite obvious that in the zone of a snow charge at the stage of its maximum development (especially with wet snow and positive air temperature), both strong and very strong vertical air flows can occur, which pose an extreme danger to flights of any type of aircraft.
3.2 Wind squalls near the Earthnear the center of the snow charge.
The descending flows of air masses, which were mentioned in paragraph 3.1 of the article, approaching the Earth's surface, according to the laws of gas dynamics, begin to sharply “flow” horizontally away from the source in the boundary layer of the atmosphere (up to heights of hundreds of meters), creating a squally wind increase ( Fig.2).
Therefore, near the storm centers near the Earth, “gust fronts” (or “gusts”) arise - squall zones that propagate from the source, but are “asymmetric” horizontally relative to the location of the source, since they usually move in the same direction as the focus horizontally (Fig. 4).
Fig.4 The structure of the gust front (gusts) propagating from the storm source in the boundary layer of the atmosphere in the direction of the source movement
Such a “windy” squally gust front usually appears suddenly, moves at a fairly high speed, passes through a specific area in just a few seconds and is characterized by sharp squally wind intensifications (15 m/s, sometimes more) and a significant increase in turbulence. The gust front “rolls back” from the source boundary as a process pulsating in time (either appearing or disappearing), and at the same time, the squall near the Earth caused by this front can reach a distance of up to several kilometers from the source (in summer with severe thunderstorms - more than 10 km).
Obviously, such a squall near the Earth, caused by the passage of the gust front near the source, poses a great danger to all types of aircraft in flight in the boundary layer of the atmosphere, which can cause an accident. An example of the passage of such a gust front under conditions of a polar mesocyclone and in the presence of snow cover is given in the analysis of the helicopter accident on Svalbard [5].
At the same time, in the conditions of the cold season, there is an intense "filling" of the airspace with flying snowflakes in a snow flurry, which leads to a sharp decrease in visibility in these conditions (see below - paragraph 3.3 of the article).
3.3 A sharp decrease in visibility in a snow loadand with a snow squall near the Earth
The danger of snow charges also lies in the fact that visibility in the snow in them usually decreases sharply, sometimes to the point of almost complete loss of visual orientation during their passage. The sizes of snow charges vary from hundreds of meters to a kilometer or more.
When the wind intensifies near the Earth at the boundaries of the snow charge, especially near the source - in the zone of the gust front near the Earth, a rapidly moving "snow flurry" arises, when in the air near the Earth there can be, in addition to intense snow falling from above, also snow raised wind from the surface (Fig. 5).
Rice. 5 Snow flurry near the Earth in the vicinity of the snow charge
Therefore, the conditions of a snow squall near the Earth are often a situation of complete loss of spatial orientation and visibility only up to a few meters, which is extremely dangerous for all modes of transport (both ground and air), and in these conditions the probability of accidents is high. Ground vehicles in a snow squall can stop and “wait out” such emergency conditions (which often happens), but the aircraft is forced to continue moving, and in situations of complete loss of visual orientation, this becomes extremely dangerous!
It is important to know that during a snow squall near the source of a snow charge, the moving zone of loss of visual orientation during the passage of a snow squall near the Earth is quite limited in space and usually only 100–200 m (rarely more), and outside the snow squall zone, visibility usually improves.
Visibility becomes better between snow layers, and therefore, away from the snow layer - often even at a distance of hundreds of meters from it and further, if there is no approaching snow squall nearby, the snow zone can even be seen in the form of some moving "snow column". This is very important for prompt visual detection of these zones and their successful "bypass" - to ensure flight safety and alert aircraft crews! In addition, snow charge zones are well detected and tracked by modern meteorological radars, which should be used for meteorological support of flights around the airfield area in these conditions.
4. Types of accidents with snow charges
It is obvious that aircraft that fall into the conditions of a snow charge in flight experience significant difficulties in maintaining flight safety, which sometimes leads to the corresponding accidents. Let us further consider three such typical APs selected for the article - these are cases in t.t. A, B, C ( they are marked in Fig. 2) on a typical diagram of the center of a snow charge at the stage of maximum development.
BUT) On February 19, 1977, near the village of Tapa, the Estonian SSR, the AN-24T aircraft, when landing at a military airfield, being on the glide slope, after passing the DPRM (long-range reference radio marker), already at an altitude of about 100 m above the runway (runway), fell into a powerful snow charge in conditions of complete loss of visibility. At the same time, the aircraft suddenly and sharply lost altitude, as a result of which it touched a high chimney and fell, all 21 people. on board the aircraft were killed.
This accident obviously happened when the aircraft hit the downstream in the snow at some height above the surface of the earth.
AT) January 20, 2011 helicopter AS - 335 NRA-04109 near Lake Sukhodolskoye, Priozersky District, Leningrad Region. flew at low altitude and in the visibility of the Earth (according to the case file). The general meteorological situation in this case, according to the meteorological service, was as follows: the flight of this helicopter was carried out in cyclonic conditions of overcast weather with heavy rainfall and deterioration in visibility in the rear of the secondary cold front ... precipitation was observed in the form of snow with rain, with the presence of individual rainfall zones . Under these conditions, during the flight, the helicopter “bypassed” the centers of heavy precipitation (they were visible), but when trying to descend, it suddenly hit the “edge” of the snow charge, abruptly lost altitude and fell to the ground when the wind increased near the Earth in a snow squall. Fortunately, no one was killed, but the helicopter was seriously damaged.
Conditions of the actual weather at the accident site (according to the protocols of interrogation of witnesses and victims): “... this happened in the presence of pockets of precipitation in the form of snow with rain ... in mixed precipitation ... which worsened horizontal visibility in the area of heavy snowfall ….” This accident obviously happened in t. In accordance with Fig. 2, i.e. in the place where near the vertical boundary of the snow charge zone has already formed snow flurry.
WITH) April 6, 2012 helicopter "Agusta" at the lake. Yanisyarvi, Sortavalsky district of Karelia, when flying at an altitude of up to 50 m in calm conditions and with the visibility of the Earth, at a distance of about 1 km from the center of snowfall (the center was visible to the crew) experienced turbulence in a snow squall that had flown near the Earth and, the helicopter, sharply losing altitude, hit the ground. Fortunately, no one died, the helicopter was damaged.
An analysis of the conditions of this accident showed that the flight took place in a cyclone trough near a rapidly approaching and intense cold front, and the accident occurred almost in the most frontal zone near the Earth. The weather diary data during the passage of this front through the airfield zone show that during its passage near the Earth, powerful pockets of cumulonimbus clouds and heavy rainfall (charges of wet snow) were noted, and also wind intensification near the Earth was observed up to 16 m/s.
Thus, it is obvious that this accident occurred, although outside the snowfall itself, which the helicopter did not hit, but it ended up in the area into which a snow flurry suddenly and at high speed “burst” caused by a distant snowstorm. charge. Therefore, there was a throw of the helicopter in the turbulent zone of the gust front, when a snow flurry came up. In Fig. 2, this is point C - the outer zone of the snow squall boundary, “rolling back” as a gust front near the Earth from the source of the snow charge. Hence, and it's very important that the snow charge zone is dangerous for flights not only within this zone itself, but also at a distance of kilometers from it - beyond the limits of the fall of the snow charge itself near the Earth, where the gust front formed by the nearest center of the snow charge and causing a snow flurry can "rush"!
5. General conclusions
In winter, in the zones of passage of cold atmospheric fronts of various types near the Earth's surface and immediately after their passage, cumulonimbus clouds usually appear and centers of solid rainfall fall in the form of heavy snow (including snow "flakes"), snow grains, showers of wet snow or snow with rain. When heavy snow falls, sharp deterioration in visibility can occur, up to a complete loss of visual orientation, especially in a snow squall (with wind intensification) near the Earth's surface.
With a significant intensity of the processes of formation of heavy rainfall, i.e. with a high "density" of the fallout of elements in the focus, and with an increased size of the falling out solid elements (especially "wet"), the rate of their fall increases sharply. For this reason, there is a powerful effect of "entrainment" of air by falling precipitation, as a result of which a strong downward air flow can occur in the center of such precipitation.
The air masses in the downward flow that has arisen in the source of solid rainfall, approaching the Earth's surface, begin to "spread" away from the source, mainly in the direction of the movement of the source, creating a zone of snow squall, rapidly spreading for several kilometers from the boundary of the source - similar to the summer the gust front that occurs near powerful summer thunderstorm centers. In the zone of such a short-term snow squall, in addition to high wind speeds, strong turbulence can be observed.
Thus, snow plumes are dangerous for aircraft flights as a sharp loss of visibility in precipitation, as well as strong downdrafts in the snow plume itself, as well as a snow squall near the source near the Earth’s surface, which is fraught with corresponding accidents in the snow layer zone.
In connection with the extreme danger of snow charges for the operation of aviation, in order to avoid the accidents caused by them, it is necessary to strictly follow a number of recommendations for both the flight control staff and the operational workers of the Hydrometeorological Support for Aviation. These recommendations were obtained on the basis of an analysis of accidents and materials associated with snow charges in the lower atmosphere in the airfield area, and their implementation reduces the likelihood of an accident in the area of snow charges.
For employees of the Hydrometeorological Service that ensures the operation of the aerodrome, in weather conditions conducive to the occurrence of snow charges in the area of the aerodrome, it is necessary to include in the formulation of the forecast for the aerodrome information about the possibility of the appearance of snow charges in the area of the aerodrome and the probable timing of this phenomenon. In addition, it is necessary to include this information in consultations with aircraft crews at the appropriate time periods for which snow is forecast to occur.
For the period of the predicted occurrence of snow charges in the area of the aerodrome, the forecaster on duty to identify the actual appearance of snow charges, it is necessary to monitor the information he has from meteorological radars, and also regularly request the dispatch service (according to visual data from the control tower - control tower, airfield services and information from the sides VS) about the actual appearance of pockets of snow charges in the airfield area.
Upon receipt of information about the actual occurrence of snow charges in the aerodrome area, immediately prepare an appropriate storm warning and submit it to the aerodrome control service and enter this information into broadcast weather alerts for aircraft crews located in the aerodrome area.
Air traffic control service for the period predicted by weather forecasters for the appearance of snow charges in the airfield area, the appearance of snow charges should be monitored according to radar data, visual observations of the control tower, information from airfield services and aircraft crews.
In the event of the actual appearance of snow piles in the area of the aerodrome, the weather forecaster should be informed about this and, if the relevant data are available, prompt provision of aircraft crews with information on the location of snow piles on the glideslope and on the climb trajectory after liftoff during takeoff should be started. It is necessary to recommend to the aircraft crews, if possible, to avoid the aircraft falling into the snow charge zone, as well as the snow squall near the Earth in the vicinity of the snow charge.
Aircraft crew when flying at low altitude and receiving an alert from the controller about the likelihood or presence of snowballs, you should carefully monitor for their visual detection in flight.
When pockets of snow charges are detected in flight in the lower layers of the atmosphere, it is necessary, if possible, to “go around” them and avoid getting into them, adhering to the rule: DO NOT ENTER, DO NOT APPROACH, LEAVING.
The dispatcher should be immediately informed about the detection of pockets of snow charges. At the same time, if possible, an assessment should be made of the location of the centers of snow charges and snow squalls, their intensity, size and direction of displacement.
In this situation, it is quite acceptable to refuse to take off and/or land due to the detection of a source of intense snow charge or a snow squall, detected on the course ahead of the aircraft.
Literature
- Khromov S.P., Mamontova L.I. Meteorological dictionary. Gidrometeotzdat, 1974.
- Meteorological dictionary - a glossary of meteorological terms POGODA.BY http://www.pogoda.by/glossary/?nd=16
- Glazunov V.G. Aviation and Weather. Electronic textbook. 2012.
- A guide to low-level wind shear. Doc.9817 AN/449 ICAO International Civil Aviation Organization, 2005. http://aviadocs.net/icaodocs/Docs/9817_cons_ru.pdf
- Glazunov V.G. Meteorological examination of the Mi-8MT crash at the Barentsburg heliport (Svalbard) on March 30, 2008
- Automated meteorological radar complex METEOR-METEO-CELL. ZAO Institute of Radar Meteorology (IRAM).
Many newcomers to yachting have heard of the "baseball cap law" which is used in some way by experienced sailors in maritime navigation. It should be said in advance that this law has nothing to do with either headgear or marine equipment in general. The “law of the baseball cap” in marine slang is the baric law of the wind, discovered at one time by a member of the Imperial St. Petersburg Academy of Sciences Christopher Buys-Ballot, often referred to in the English manner as Bais-Ballot. This law explains an interesting phenomenon - why the wind in the northern hemisphere in cyclones turns clockwise - that is, to the right. Not to be confused with the rotation of the cyclone itself, where the air masses rotate counterclockwise!
Academician H. H. Buys-Ballot
Buys-Ballot and the baric wind law
Buys-Ballot was an outstanding Dutch scientist of the mid-19th century, who studied mathematics, physics, chemistry, mineralogy and meteorology. Despite such a wide range of hobbies, he became famous precisely as the discoverer of the law, later named after him. Buys-Ballot was one of the first to actively implement active cooperation between scientists from different countries, nurturing the ideas of the World Academy of Sciences. In Holland, he created the Institute of Meteorology and a warning system for impending storms. In recognition of his services to world science, along with Ampère, Darwin, Goethe and other representatives of science and art, Buys-Ballot was elected a foreign member of the St. Petersburg Academy of Sciences.
As for the actual law (or “rule”) of Bays-Ballot, then, strictly speaking, the first mention of the barric wind law dates back to the end of the 18th century. It was then that the German scientist Brandis first made theoretical assumptions about the deviation of the wind relative to the vector connecting areas with high and low pressure. But he could not prove his theory in practice. It was only in the middle of the 19th century that Academician Buys-Ballot was able to establish the correctness of Brandis's assumptions. Moreover, he did it purely empirically, that is, through scientific observations and measurements.
The essence of the Bays-Ballo law
Literally, the “Bays-Ballo law”, formulated by the scientist in 1857, is as follows: “The wind near the surface, except for subequatorial and equatorial latitudes, deviates from the baric gradient by a certain angle to the right, and in a southerly direction - to the left.” The baric gradient is a vector showing the change in atmospheric pressure in the horizontal direction over the surface of the sea or flat land.
barric gradient
If you translate the Bays-Ballo law from scientific language, then it will look like this. In the earth's atmosphere there are always areas of high and low pressure (we will not analyze the reasons for this phenomenon in this article, so as not to get lost in the wilds). As a result, air flows from an area of higher pressure to an area of lower pressure. It is logical to assume that such a movement should go in a straight line: this is the direction and shows the vector called "baric gradient".
But here the force of the Earth's motion around its axis comes into play. More precisely, the force of inertia of those objects that are on the surface of the Earth, but are not connected by a rigid connection with the earth's firmament - the "Coriolis force" (emphasis on the last "and"!). Such objects include water and air of the atmosphere. As for water, it has long been noticed that in the northern hemisphere, rivers flowing in a meridional direction (from north to south) wash away the right bank more, while the left remains low and relatively even. In the southern hemisphere, the opposite is true. Another academician of the St. Petersburg Academy of Sciences, Karl Maksimovich Baer, was able to explain this phenomenon. He derived the law according to which flowing water is influenced by the Coriolis force. Not having time to rotate along with the solid surface of the Earth, the flowing water, by inertia, “presses” against the right bank (in the southern hemisphere, respectively, against the left), as a result, washing it away. Ironically, Baer's law was formulated in the same 1857 as the Bays-Ballo law.
In the same way, under the action of the Coriolis force, moving atmospheric air is deflected. As a result, the wind begins to deviate to the right. In this case, as a result of the action of the friction force, the deflection angle is close to a straight line in the free atmosphere and less than a straight line near the Earth's surface. When viewed in the direction of the surface wind, the lowest pressure in the northern hemisphere will be on the left and slightly ahead.
Deviations in the movement of air masses in the northern hemisphere under the influence of the force of the Earth's rotation. The baric gradient vector is shown in red, pointing straight from the high pressure region to the low pressure region. The blue arrow is the direction of the Coriolis force. Green - the direction of wind movement, deviating under the influence of the Coriolis force from the baric gradient
Use of the Bays-Ballo law in maritime navigation
The need to be able to apply this rule in practice is indicated by many textbooks on navigation and maritime affairs. In particular, Samoilov's "Marine Dictionary" published by the People's Commissariat of the Navy in 1941. Samoilov gives an exhaustive description of the baric law of the wind in relation to seafaring practice. His instructions may well be adopted by modern yachtsmen:
“... If the ship is located in close proximity to areas of the world ocean, where hurricanes often occur, it is necessary to monitor the barometer readings. If the barometer needle starts to drop and the wind gets stronger, then the possibility of a hurricane is high. In this case, it is necessary to immediately determine in which direction the center of the cyclone is located. To do this, sailors use the Base Ballo rule - if you stand with your back to the wind, then the center of the hurricane will be located about 10 points to the left of the gybe in the northern hemisphere, and the same amount to the right - in the southern hemisphere.
Then you need to determine in which part of the hurricane the ship is located. To determine the location as soon as possible, a sailing ship must immediately drift, and a steam ship must stop the car. After that, it is necessary to make observations of the change in the wind. If the wind direction gradually changes from left to right (clockwise), then the vessel is on the right side of the path of the cyclone. If the direction of the wind changes in the opposite direction, then to the left. In the case when the direction of the wind does not change at all, the ship is directly in the path of the hurricane. To move away from the center of a hurricane in the northern hemisphere, you need to do the following:
* transfer the ship to starboard tack;
* at the same time, if you are to the right of the center of the cyclone, then you should lie in close-hauled;
* if on the left or in the center of the movement - to the backstay.
In the southern hemisphere, the opposite is true, except when the ship is in the center of an advancing cyclone. It is necessary to follow these courses until the ship leaves the path of the center of the cyclone, which can be determined by the barometer that has begun to rise.
And our website wrote about the rules for avoiding tropical cyclones in the article "".
24. Baric wind law
Experience confirms that the actual wind near the earth's surface always (with the exception of latitudes close to the equator) deviates from the baric gradient by some sharp angle to the right in the Northern Hemisphere and to the left in the Southern. From here follows the so-called baric law of the wind: if in the Northern Hemisphere you stand with your back to the wind, and face where the wind blows, then the lowest pressure will be on the left and somewhat in front, and the highest pressure will be on the right and somewhat behind.
This law was found empirically in the first half of the 19th century. Base Ballo and bears his name. In the same way, the actual wind in the free atmosphere always blows almost along isobars, leaving (in the Northern Hemisphere) low pressure on the left, i.e. deviating from the baric gradient to the right by an angle close to a right one. This provision can be considered as an extension of the baric wind law to the free atmosphere.
The baric wind law describes the properties of the actual wind. Thus, the patterns of geostrophic and gradient air movement, i.e. under simplified theoretical conditions, they are mostly justified under more complex actual conditions of the real atmosphere. In the free atmosphere, despite the irregular shape of the isobars, the wind direction is close to the isobars (it deviates from them, as a rule, by 15-20°), and its speed is close to the speed of the geostrophic wind.
The same is true for streamlines in the surface layer of a cyclone or anticyclone. Although these streamlines are not geometrically regular spirals, they are nevertheless helical in nature and in cyclones they converge towards the center, and in anticyclones they diverge from the center.
Fronts in the atmosphere are constantly created such conditions when two air masses with different properties are located one next to the other. In this case, these two air masses are separated by a narrow transition zone called a front. The length of such zones is thousands of kilometers, the width is only tens of kilometers. These zones are inclined relative to the earth's surface with height and can be traced upwards for at least several kilometers, and often to the very stratosphere. In the front zone, when moving from one air mass to another, the temperature, wind and air humidity change dramatically.
Fronts that separate the main geographic types of air masses are called main fronts. The main fronts between arctic and temperate air are called arctic, between temperate and tropical air - polar. The division between tropical and equatorial air does not have the character of a front; this division is called the intertropical convergence zone.
The width of the front in the horizontal direction and its thickness in the vertical direction are small in comparison with the dimensions of the air masses separated by it. Therefore, idealizing the actual conditions, it is possible to represent the front as an interface between air masses.
At the intersection with the earth's surface, the frontal surface forms the front line, which is also briefly called the front. If we idealize the frontal zone as an interface, then for meteorological quantities it is a discontinuity surface, because a sharp change in the frontal zone of temperature and some other meteorological quantities acquires the character of a jump on the interface.
The frontal surfaces pass obliquely in the atmosphere (Fig. 5). If both air masses were stationary, then the warm air would be located above the cold one, and the surface of the front between them would be horizontal, parallel to the horizontal isobaric surfaces. Since the air masses move, the surface of the front can exist and be preserved, provided that it is inclined to the level surface and, therefore, to the sea level.
Rice. 5. Front surface in vertical section
The theory of frontal surfaces shows that the angle of inclination depends on the velocities, accelerations and temperatures of the air masses, as well as on the geographic latitude and on the acceleration of free fall. Theory and experience show that the angles of inclination of the frontal surfaces to the earth's surface are very small, on the order of minutes of arc.
Each individual front in the atmosphere does not exist indefinitely. Fronts are constantly emerging, sharpening, blurring and disappearing. The conditions for the formation of fronts always exist in certain parts of the atmosphere, so fronts are not a rare accident, but a constant, everyday feature of the atmosphere.
The usual mechanism for the formation of fronts in the atmosphere is kinematic: fronts arise in such fields of air movement that bring together air particles with different temperatures (and other properties),
In such a field of motion, horizontal temperature gradients increase, and this leads to the formation of a sharp front instead of a gradual transition between air masses. The process of front formation is called frontogenesis. Similarly, in motion fields that move air particles away from each other, already existing fronts can be blurred, i.e. turn into wide transition zones, and the large gradients of meteorological values that existed in them, in particular temperature, will be smoothed out.
In a real atmosphere, the fronts, as a rule, are not parallel to the air currents. The wind on both sides of the front has components normal to the front. Therefore, the fronts themselves do not remain in the same position, but move.
The front can move either towards colder air or towards warmer air. If the front line moves close to the ground towards colder air, this means that the wedge of cold air is receding and the space vacated by it is taken by warm air. Such a front is called a warm front. Its passage through the place of observation leads to a change in the cold air mass to a warm one, and, consequently, to an increase in temperature and to certain changes in other meteorological quantities.
If the front line moves towards warm air, this means that the cold air wedge is moving forward, the warm air in front of it is receding, and is also being forced upward by the advancing cold wedge. Such a front is called a cold front. During its passage, the warm air mass is replaced by a cold one, the temperature drops, and other meteorological quantities also change dramatically.
In the region of fronts (or, as they usually say, on frontal surfaces), vertical components of the air velocity arise. The most important is the particularly frequent case when warm air is in a state of ordered upward movement, i.e. when, simultaneously with the horizontal movement, it also moves upwards above the wedge of cold air. It is with this that the development of a cloud system above the frontal surface, from which precipitation falls, is connected.
On the warm front, the upward movement covers powerful layers of warm air over the entire frontal surface, the vertical velocities here are of the order of 1 ... 2 cm / s with horizontal velocities of several tens of meters per second. Therefore, the movement of warm air has the character of an upward sliding along the frontal surface.
The upward sliding involves not only the layer of air immediately adjacent to the frontal surface, but also all the overlying layers, often up to the tropopause. As a result, an extensive system of cirrostratus, altostratus - nimbostratus clouds arises, from which extensive precipitation falls. In the case of a cold front, the upward movement of warm air is limited to a narrower zone, but the vertical velocities are much greater than on a warm front, and they are especially strong in front of a cold wedge, where warm air is displaced by cold air. It is dominated by cumulonimbus clouds with showers and thunderstorms.
It is very important that all fronts are connected with troughs in the baric field. In the case of a stationary (slowly moving) front, the isobars in the hollow are parallel to the front itself. In the cases of warm and cold fronts, the isobars take the form of the Latin letter V, intersecting with the front lying on the axis of the trough.
When the front passes, the wind in a given place changes its direction clockwise. For example, if the wind is southeast ahead of the front, then behind the front it will change to south, southwest or west.
Ideally, the front can be represented as a geometric discontinuity surface.
In a real atmosphere, such an idealization is admissible in the planetary boundary layer. In reality, the front is a transitional zone between warm and cold air masses; in the troposphere, it represents a certain area called the frontal zone. The temperature at the front does not experience a discontinuity, but changes sharply inside the front zone, i.e. The front is characterized by large horizontal temperature gradients, an order of magnitude greater than in air masses on both sides of the front.
We already know that if there is a horizontal temperature gradient that closely coincides in direction with the horizontal baric gradient, the latter increases with height, and with it the wind speed increases. In the frontal zone, where the horizontal temperature gradient between warm and cold air is especially large, the baric gradient strongly increases with height. This means that the thermal wind makes a large contribution and the wind speed at heights reaches high values.
With a sharply pronounced front above it in the upper troposphere and lower stratosphere, a strong air current, parallel to the front, is generally observed, several hundred kilometers wide, with speeds from 150 to 300 km/h. It's called a jet stream. Its length is comparable to the length of the front and can reach several thousand kilometers. The maximum wind speed is observed on the axis of the jet stream near the tropopause, where it can exceed 100 m/s.
Higher in the stratosphere, where the horizontal temperature gradient reverses, the baric gradient decreases with altitude, the thermal wind is opposite to the wind speed, and it decreases with altitude.
Near Arctic fronts, jet streams are found at lower levels. Under certain conditions, jet streams are observed in the stratosphere.
Usually, the main fronts of the troposphere - polar, arctic - run mainly in the latitudinal direction, with cold air located at higher latitudes. Therefore, the jet streams associated with them are most often directed from west to east.
With a sharp deviation of the main front from the latitudinal direction, the jet stream also deviates.
In the subtropics, where the temperate troposphere is in contact with the tropical troposphere, a subtropical scab current arises, the axis of which is usually located between the tropical and polar tropopauses.
The subtropical jet stream is not rigidly associated with any front and is mainly a consequence of the existence of an equator-pole temperature gradient.
The jet stream opposite to the flying aircraft reduces the speed of its flight; the associated jet stream increases it. In addition, strong turbulence can develop in the jet zone, so taking into account jet flows is important for aviation.
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2. Coriolis force
3. Friction force: 4. Centrifugal force:
16. Baric wind law in the surface layer (friction layer) and its meteorological consequences in a cyclone and anticyclone.
Baric wind law in the friction layer : under the influence of friction, the wind deviates from the isobar towards low pressure (in the northern hemisphere - to the left) and decreases in magnitude.
So, according to the baric law of the wind:
In a cyclone, circulation is carried out counterclockwise, near the ground (in the friction layer) there is a convergence of air masses, upward vertical movements and the formation of atmospheric fronts. Cloudy weather prevails.
In the anticyclone, there is counterclockwise circulation, air mass divergence, downward vertical movements, and the formation of large-scale (~1000 km) uplifted inversions. Cloudless weather prevails. Stratified clouds in the sub-inversion layer.
17. Surface atmospheric fronts (AF). Their formation. Cloudiness, special phenomena in the X and T AF zone, occlusion front. AF movement speed. Flight conditions in the AF area in winter and summer. What is the average width of the precipitation zone on T and X AF? Name the seasonal differences in the NR for HF and TF. (see Bogatkin p.159 - 164).
Surface atmospheric fronts AF – a narrow sloping transition zone between two air masses with different properties;
Cold air (more dense) lies under warm
The length of the AF zones is thousands of km, the width is tens of km, the height is several km (sometimes up to the tropopause), the angle of inclination to the earth's surface is several arc minutes;
The line of intersection of the frontal surface with the earth's surface is called the front line
In the frontal zone, temperature, humidity, wind speed, and other parameters change abruptly;
The process of front formation is frontogenesis, destruction is frontolysis
Travel speed 30-40 km/h or more
The approach cannot (most often) be noticed in advance - all the clouds are behind the front line
Heavy rainfall with thunderstorms and squally winds, tornadoes are typical;
Clouds replace each other in the sequence Ns, Cb, As, Cs (to increase the tier);
The zone of clouds and precipitation is 2-3 times less than that of the TF - up to 300 and 200 km, respectively;
The width of the precipitation zone is 150-200 km;
The height of the NGO is 100-200 m;
At a height behind the front, the wind picks up and turns to the left - wind shear!
For aviation: poor visibility, icing, turbulence (especially in HF!), wind shear;
Flights are prohibited until the passage of the HF.
HF of the 1st kind - a slowly moving front (30-40 km/h), a relatively wide (200-300 km) zone of cloudiness and precipitation; the height of the upper boundary of the clouds in winter is small - 4-6 km
Type 2 HF - fast moving front (50-60 km/h), narrow cloud width - several tens of km, but dangerous with developed Cb (especially in summer - with thunderstorms and squalls), in winter - heavy snowfalls with a sharp short-term deterioration in visibility
Warm AF
The movement speed is less than that of the HF-< 40 км/ч.
Approach can be seen in advance by the appearance in the sky of cirrus, and then cirrostratus clouds, and then As, St, Sc with NGO 100 m or less;
Dense advective fogs (winter and transitional seasons);
Cloud basis - layered forms clouds formed as a result of the rise of warm air at a speed of 1-2 cm / s;
vast area about cages - 300-450 km with a cloud zone width of about 700 km (maximum in the central part of the cyclone);
At heights in the troposphere, the wind increases with height and turns to the right - wind shear!
Particularly difficult conditions for flights are created in the zone of 300-400 km from the front line, where cloudiness is low, visibility is worse, the possibility of icing in winter, and thunderstorms in summer (not always).
Front of occlusion –
combination of warm and cold frontal surfaces
(in winter it is especially dangerous with icing, ice, freezing rain)
For an addition, read the textbook Bogatkin pp. 159 - 164.
GRADIENT WIND In the case of curvilinear isobars, centrifugal force occurs. It is always directed towards the convexity (from the center of the cyclone or anticyclone towards the periphery). When there is a uniform horizontal movement of air without friction with curvilinear isobars, then 3 forces are balanced in the horizontal plane: the force of the baric gradient G, the force of the Earth's rotation K and the centrifugal force C. Such a uniform steady horizontal movement of air in the absence of friction along curvilinear trajectories is called a gradient wind. The gradient wind vector is directed tangentially to the isobar at a right angle to the right in the northern hemisphere (to the left in the southern hemisphere) relative to the baric gradient force vector. Therefore, in a cyclone - a counterclockwise vortex, and in an anticyclone - clockwise in the northern hemisphere.
Mutual arrangement of acting forces in case of gradient wind: a) cyclone, b) anticyclone. A is the Coriolis force (in the formulas it is denoted by K)
Let us consider the influence of the curvature radius r on the gradient wind velocity. For a large radius of curvature (r > 500 km), the curvature of the isobars (1/ r) is very small, close to zero. The radius of curvature of a straight rectilinear isobar is r → ∞ and the wind will be geostrophic. Geostrophic wind is a special case of gradient wind (at С = 0). With a small radius of curvature (r< 500 км) в циклоне и антициклоне при круговых изобарах скорость градиентного ветра определяется следующими уравнениями: В циклоне уравновешиваются силы G = K + C: или В антициклоне К = G + С: Поэтому в циклоне: или
In the anticyclone: or That is, In the center of the cyclone and anticyclone, the horizontal baric gradient is equal to zero, i.e. Hence, G = 0 as a source of motion. Therefore, = 0. The gradient wind is an approximation to the real wind in the free atmosphere of a cyclone and anticyclone.
Gradient wind speed can be obtained by solving a quadratic equation - in a cyclone: - in an anticyclone: curvature r ≤ 500 km) on the isobaric surface, the following relationships between gradient and geostrophic winds are used: For cyclonic curvature ≈ 0.7 For anticyclonic curvature ≈ 1,
With a large curvature of isobars near the Earth’s surface (1/ r) → ∞ (radius of curvature r ≤ 500 km): with cyclonic curvature ≈ 0.7 with anticyclonic curvature ≈ 0.3 mean radius of curvature 500 km< r < 1000 км, — а также при большой кривизне изобар (r < 500 км) в быстро перемещающихся барических образованиях.
LAW OF THE WIND The relationship between the direction of the surface wind and the direction of the horizontal baric gradient was formulated in the 19th century by the Dutch scientist Bais-Ballo in the form of a rule (law). LAW OF THE WIND: Looking downwind, low pressure will be to the left and somewhat ahead, and high pressure will be to the right and somewhat behind (in the northern hemisphere). When drawing isobars on synoptic maps, the direction of the wind is taken into account: the direction of the isobar is obtained by turning the wind arrow to the right (clockwise) by about 30 -45 °.
REAL WIND Real air movements are not stationary. Therefore, the characteristics of the actual wind near the earth's surface differ from the characteristics of the geostrophic wind. Consider the real wind in the form of two terms: V = + V ′ – ageostrophic deviation u = + u ′ or u ′ = u — v = + v ′ or v ′ = v – We write the equations of motion without taking into account the friction force:
INFLUENCE OF THE FRICTION FORCE ON THE WIND Under the influence of friction, the speed of the surface wind is, on average, two times less than the speed of the geostrophic wind, and its direction deviates from the geostrophic towards the baric gradient. Thus, the actual wind deviates near the earth's surface from the geostrophic to the left in the northern hemisphere and to the right in the southern hemisphere. Mutual disposition of forces. Rectilinear isobars
In a cyclone, under the influence of friction, the wind direction deviates towards the center of the cyclone; in an anticyclone, from the center of the anticyclone to the periphery. Due to the influence of friction, the wind direction in the surface layer is deviated from the tangent to the isobar towards low pressure by an average angle of approximately 30° (over the sea by about 15°, over land by about 40 -45°).
WIND CHANGE WITH ALTITUDE The force of friction decreases with height. In the boundary layer of the atmosphere (friction layer), the wind approaches the geostrophic wind with height, which is directed along the isobar. Thus, with height, the wind will increase and turn to the right (in the northern hemisphere) until it is directed along the isobar. The change in wind speed and direction with height in the boundary layer of the atmosphere (1-1.5 km) can be represented by a hodograph. A hodograph is a curve connecting the ends of vectors depicting the wind at different heights and drawn from the same point. This curve is a logarithmic spiral called the Ekman spiral.
CHARACTERISTICS OF THE WIND FIELD OF THE CURRENT LINE A streamline is a line, at each point of which the wind velocity vector is directed tangentially at a given moment of time. Thus, they give an idea of the structure of the wind field at a given time (instantaneous velocity field). Under gradient or geostrophic wind conditions, streamlines will coincide with isobars (isohypses). The actual wind velocity vector in the boundary layer is not parallel to isobars (isohypses). Therefore, the streamlines of the real wind cross the isobars (isohypses). When drawing streamlines, not only the direction, but also the speed of the wind is taken into account: the greater the speed, the denser the streamlines are.
Examples of streamlines near the Earth's surface in a surface cyclone in a surface anticyclone in a trough in a ridge
AIR PARTICLES TRAJECTORIES Particle trajectories are the paths of individual air particles. That is, the trajectory characterizes the movement of the same air particle at successive points in time. Particle trajectories can be approximated from successive synoptic maps. The trajectory method in synoptic meteorology makes it possible to solve two problems: 1) to determine from where an air particle will move to a given point in a certain period of time; 2) determine where the air particle will move from a given point in a certain period of time. Trajectories can be built on AT maps (more often on AT-700) and on surface maps. A graphical method for calculating the trajectory using a gradient ruler is used.
An example of constructing the trajectory of an air particle (where the particle will move from) on one map: A - forecast point; B is the middle of the particle path; C - the starting point of the trajectory Using the lower part of the gradient ruler, the distance between the isohypses determines the speed of the geostrophic wind (V, km/h). The ruler is applied with the lower scale (V, km / h) along the normal to the isohypses approximately in the middle of the path. On a scale (V , km / h) between two isohypses (at the point of intersection with the second isohypse) determine the average speed V cp.
Gradient ruler for latitude 60˚ Next, determine the path of the particle for 12 h (S 12) at a given transfer rate. It is numerically equal to the particle transfer velocity V h. The path of the particle in 24 h is S 24 = 2· S 12; the path of the particle in 36 hours is equal to S 36 = 3 · S 12 . On the upper scale of the ruler, the path of the particle from the forecast point in the direction opposite to the direction of the isohypse is plotted, taking into account their bending.