Why does a tropical cyclone need an “eye” and what happens there? Cyclone. Part-2
The “eye” of a tropical cyclone is visible and what is happening there? Cyclone. Part-2.
What is the wind direction in the cyclone itself?
The previous article examined the issue of those natural forces that generate powerful and destructive tropical cyclones (hurricanes) and slightly smaller tornadoes in the atmosphere.
https://habr.com/ru/articles/832582/
It turned out that the main force is the thermal convective movement of air from the warm surface of the water upward.
When ascending convective currents arise over large areas in thin layers of the atmosphere, separate toroidal closed air circulation flows arise – “Benard cells”. (See Fig. 1.)
Fig. 1. Contact of the “Benard cells” by the edges with the same directed upward movement of the convective flow. It is interesting that the flow diagram (on the right) incorrectly shows the direction of circulation, which contradicts the text on the picture. The correct direction would be the fall of the flows at the edges of the toroidal vortex and the rise of the hot flow in the center of the toroidal cell. The indicated direction of the toroidal flow swirl is characteristic of an anticyclone with a cold flow in the center.
When such a separate convective “Benard cell” intensifies in the atmosphere, a tornado (at sea) or a tornado (on land) occurs.
But if there are many convective “Benard cells” in the atmospheric layer and somewhere in the general uniform layer one particularly large “Benard cell” appears, then the entire convective field can unite into another, more powerful structure – a “tropical cyclone” (hurricane).
A cyclone is the result of the coalescence of many convective “Benard cells” around the hottest spot of water in the ocean.
The Earth is a sphere, and therefore it is heated by the Sun unevenly.
The intensity of heating by the sun is maximum at midday in the region of the equator (autumn and spring) or at the latitude of the tropics (in winter – the southern tropic, in summer – the northern tropic).
The power of solar heat per unit surface Ez is determined by the constant power of the flow from the sun Ec and the angle A of the surface inclination to this flow:
Ez = Ec*sinA
Where Ec=1 kW/m2 is the power of sunlight at the earth's surface level after passing through the atmospheric layer, sinA is the sine of the angle of incidence of the sun's rays on the earth, angle A=90 degrees at noon at the equator at the equinox (September 21 and March 21) or on the day of the solstice on June 22 at the Northern Tropic (see Fig. 2.)
Fig. 2. Distribution of heat on the Earth's surface on the day of the summer solstice, June 22. The maximum heat output from the sun is 1 kW/m2 in the area of the Northern Tropic, at the North Pole – 0.4 kW/m2, beyond the Antarctic Circle, the complete darkness of the “polar night” – 0 kW/m2.
The maximum heating of water in the ocean occurs in the equatorial zone between the Northern Tropic and the Southern Tropic. At the same time, the flow of very warm water forms ocean currents from the equator to higher latitudes, skirting the continents (see Fig. 4.)
Fig. 4. Approximate diagram of ocean currents on Earth.
Warm equatorial currents have an inconstant position of maximum temperature, since the zone of maximum water heating shifts away from the equator when the seasons change from winter to summer.
In summer, the warmest part of the trade wind current shifts towards the Northern Tropic, and in winter, towards the Southern Tropic.
Thus, in summer, the most powerful ascending convective currents in the “Benard cells” from warm ocean water arise north of the equator.
At some point, one giant “Benard cell” grows excessively and begins to suck in the surrounding smaller convective cells.
This is how a “tropical cyclone” is born between the equator and the tropics in the strip of ocean water most heated by the sun. (See Fig. 5-7.)
Fig. 5. The life cycle of the cyclone coincides with the route of the South Trade Wind Current from Africa to the Gulf of Mexico. The cyclone lived from July 18 to 27, which is the warmest summer time in the tropics of the Northern Hemisphere. The cyclone weakens over Cuba, and then gains strength again in the hot waters of the Gulf of Mexico.
Fig. 6. The life cycle of a cyclone coincides with the route of the equatorial countercurrent in the Pacific Oceanwhere is it bends northeast over Mexico. The cyclone lived from June 30 to July 7, which is the warmest summer time in the tropics of the Northern Hemisphere.
Fig. 7. The life cycle of the cyclone coincides with the route of the East Australian warm current from the equator to New Zealand (the northern edge of the island at the bottom of the picture). The cyclone lived from February 1 to 19, which is the warmest “summer” season in the Southern Hemisphere.
Organizing air flows in a combination of several “Benard cells”
When one convective “Benard cell” swells to such an extent that it draws into its circulation the smaller convective cells surrounding it, a new, more complex circulation system of air currents is created.
Thus, small “Benard cells” receive an additional general direction of movement towards the center of the main cell.
At the same time, the circulation in the small cell not only does not stop, but even intensifies as it moves to warmer waters closer to the center of the “main cell”.
This is how a complex spiral movement of air towards the center of the future cyclone arises, with the axis of the spiral located horizontally in the direction of the center of the cyclone, and the circular circulation is carried out due to the convective flow of the peripheral “Benard cell”.
The “Benard cells” arranged in a row towards the center of the cyclone are combined into “Benard cords” (see Fig. 8-9).
Fig. 8. Formation of “Benard cells” in a thin layer of heated medium (left), cylindrical convective flows “Benard cords” (right).
Fig.9. Cylindrical convective flows “Benard cords” (top) and toroidal flows in “Benard cells” (bottom) in a thin layer of heated medium (l- cyclone, g- anticyclone).
The result is a single Cyclone, as a set of spiral-convective air flows of the “Benard cord” type converging towards a common center.
As the spiral flow moves toward the center of the cyclone, the wind speed increases (see Fig. 10-11.)
Fig. 10. Distribution of ascending and descending flows inside the cyclone as a whole and in individual spiral arms. The graph also shows an increase in wind strength as one approaches the center of the cyclone.
Fig. 11. A section of a cyclone showing the change in the nature of clouds in the zone of the low wind toward the center of the cyclone (cumulus-nimbus clouds) and in the zone of the reverse stratospheric wind from the center of the cyclone (cirrus clouds).
The two pictures of cyclone sections have a common problem, namely that the section is made along the radius.
This cross-section is misleading and does not allow one to correctly construct the true shape of the air flows in the individual horizontal spiral jets of the “Benard cords”.
To correctly model air flows in a cyclone, it is necessary to add a series of cuts along concentric circles to the radial section.
Thus, a section along the circumference will give a completely different picture than a section along the radius of the cyclone.
Wind squalls at sea
A squall is a sudden increase in wind, lasting from several seconds to several hours.
Squalls are possible even in clear weather from a single cloud or without any obvious visible source (blue thermal).
A squall occurs when a stable convective vortex of the Benard cell or Benard cord type moves over the water surface. (See Fig. 12.)
13Fig. 12. Scheme of the emergence of a local wind “squall”.
Squall structure of a cyclone
If we consider the speed and direction of the wind inside the Cyclone only at the sea surface, we will get a system of multidirectional squalls (see Fig. 13.)
Fig. 13. Direction of wind in a cyclone near the surface. The wind near the surface is clearly not directed towards the center of the cyclone, but somewhere far past the center. Nevertheless, the entire mass of air above the sea on average moves precisely towards the center of the cyclone.
If you look at the wind direction in the higher layers under low rain clouds, then the wind direction there will be different than at the sea surface, namely: past the center of the cyclone on the other side.
In total, the wind flows over the sea and the flows in the clouds in one “Benard cord” are on average directed toward the “eye of the cyclone” with a slight shift to the side due to the Coriolis acceleration, directed to the right in the Northern Hemisphere. (See Fig. 14.)
Fig. 14. Wind direction inside individual pairs of “Benard cords”: red arrows are lower hot flows in a separate “cord”, upper ones are cooled flows in the same “cord”. A – a pair of counter-rotating “cords” with a sharp bend about the low-pressure zone in the “eye of the cyclone”, which, when turning about the “eye of the cyclone”, change their position relative to each other (the left one becomes the right one). In the eye zone, the “Benard cord” bends both sideways and rearranges itself along the echelon with an upward rise. B – lower layer of flows entering the cyclone; B – upper layer of the cyclone: reverse flow of air from the cyclone in the stratosphere.
The bending of the “cord” about the “eye of the cyclone” and its release to the upper level for drainage back to the periphery of the cyclone is necessary to maintain the continuity of the medium. At the same time, the contact of the “cord” with the low-pressure zone in the “eye of the cyclone” creates a centripetal acceleration in the cord for its bending in space.
Below is a section along the “Benard cord” as a component of the cyclone. The air moves in a spiral (on the section, the spiral looks like a wavy line) from the periphery to the “cyclone eye”, gradually accelerating at each cycle of “heating + humidification/cooling + drying”. (See Fig. 15.)
Fig. 15. Development of small clouds from separate “Benard cells” into a continuous “Benard cord” inside the cyclone. There is a gradual increase in the amplitude and speed of the ascending-descending spiral flows in the “cord” as they move toward the “eye of the cyclone” as the water temperature under the cyclone increases. A-Upper diagram – the “cord” from the “eye” goes further with a layer of cirrus clouds to the opposite edge of the cyclone (without a sharp turn back). B-Lower diagram – this is the usual version with a sharp turn of the air flow from the “eye” in the opposite direction.
The height of the cloud cover in each section of the “cord” determines the rarefaction near the water and the acceleration of the horizontal wind near the water under this cloud: the higher the cloud, the higher the wind speed near the water.
If you move along the “cord” from the outer edge to the “eye of the cyclone”, you will constantly feel a strong wind on one side and from behind, with a constant increase in its speed, but without changing direction.
To feel the “squall” and the change in wind direction inside the cyclone, you need to cross the boundaries of the “cords”. (See Fig. 16.)
Fig. 16. Cylindrical section of a cyclone, which cuts individual “Benard cords” across. When viewed from above, the cloud row of one pair of “cords” looks like a separate ridge of clouds, limited on the sides by a patch of clear sky (on the periphery of the cyclone) or a depression in the cloudiness (closer to the “eye of the cyclone”).
Movement of ships and aircraft through a cyclone
When a ship moves through a cyclone across the “Benard cords”, the boundaries of the “cords” are crossed by the ship’s course with a sharp change in wind direction at the boundaries of individual “Benard cords”.
In cyclone zones where updrafts dominate, the horizontal wind speed decreases sharply.
This cyclic change in wind strength from hurricane to moderate wind at the boundary of the “cords” is perceived as a succession of “squalls” from different sides, and when the wind speed in the ascending currents at the boundary of the cords decreases, the rain increases sharply. (See Fig. 17.)
Fig. 17. Cyclone section with intersection of individual turns of “Benard cords” near the “cyclone eye”. Also visible is the presence of counter-currents of air in different parts of the vortex “Benard cords”. In the center of the cyclone eye, a low-pressure zone appears, where descending dry and cold air from the stratosphere is sucked in.
It is interesting that the cloud height in the area around the “eye of the cyclone” is indicated as being up to 16 km.
The cumulonimbus clouds reaching altitudes of 16 km at the center of the cyclone pose a major hazard to commercial turbojet aircraft, which typically cannot fly above 12 km.
Thus, a civil aircraft will be at risk of disaster if it encounters powerful vortices in the upper part of the cyclone at the aircraft's maximum flight altitude.
Normal cumulonimbus clouds are located no higher than 9 km, and airplanes fly calmly in a clear blue sky at flight levels of 10-12 km, well above the clouds, where there is no shaking from “turbulence” from meeting emissions of ascending “thermals” in the clouds. (see Fig. 18.).
Fig. 18. Location of clouds by altitude in the Earth’s atmosphere.
The appearance of “Benard cords” in real cyclones
The previously described single “Benard cords” can be clearly distinguished as furrows in the fog in photographs of cyclones from space (see Fig. 19-22).
Fig. 19. View of the “eye of the cyclone” from space. The mounds of cumulonimbus clouds arranged in spiral rows around the “eye of the cyclone” are visible. It is evident that the shaft of cumulonimbus dense clouds around the “eye of the cyclone” rises to the level of cirrus clouds in the stratosphere.
Fig. 20. View of the wall of the “cyclone eye” from the inside of an airplane. You can see mounds of cumulonimbus clouds arranged in spiral rows around the “cyclone eye”. The flow of cold dry air from the stratosphere in the center of the “cyclone eye” causes intense formation of foggy haze over the sea in almost complete calm.
Fig. 21. View of the “eye of the cyclone” from space. The mounds of cumulonimbus clouds arranged in spiral rows around the “eye of the cyclone” are visible. Beyond the “eye of the cyclone” the upper layers of the reverse current clouds transform from dense cumulonimbus clouds into translucent cirrus clouds.
Fig. 22. View of the “eye of the cyclone” from space. The mounds of cumulonimbus clouds arranged in spiral rows around the “eye of the cyclone” are visible.
Finally, an interesting short video about the anomalous tropical cyclone of this year 2024.
It traces its traditional route along the South Atlantic Trade Wind Current with a sharp attenuation toward the Yucatan Peninsula, and a sudden dash north to Texas with a gain in strength in the warm pool of the Gulf of Mexico.
https://dzen.ru/video/watch/668e9713768fc17b65c58805?clid=1400&rid=3383007232.1172.1722523378134.30563
Differences between the “eye of the cyclone” and the point where the tornado touches the ground.
A cyclone is a more complex vortex structure than a tornado.
That is why they have different ways of separating flows from the ground and transitioning to the upper return flow.
If there is ultra-low pressure at the water surface in the “eye of the cyclone”, then the bending of the ground air flows in the tornado causes the appearance of a shock zone of air braking near the ground with increased pressure in it.
This is clearly visible in the video, when a small tornado passes over a car and a trailer: it lifts them off the ground with a very slight rotation and does not carry them away (see video. 1:50-2:09 and 04:55-05:07)
That is, excess pressure formed under the bottom of the car from air flows converging from all sides along the ground towards the “heel of the tornado”.
This high-pressure shock braking zone was at some point forced underneath the car, lifting it off the ground for a few seconds.
At the same time, in the “cloud cone” of the vortex, a strong vacuum is observed with the precipitation of foggy condensation, and in the ground “patch” a strong excess pressure is created (capable of lifting a car off the ground).
The lower and upper cones of the Tornado (whirlwind) have a strong difference in pressure, and therefore are separated from each other by a bridge of all-round compression (wind knot-noose). This local compression between two zones with different pressure is created by sharply bending flows colliding with each other on the axis of the vortex near the ground. (See Fig. 23.)
Fig. 23. Tornado diagram at the point of impact of air flows converging at one point along the ground. In this case, to turn the air flows from the horizontal direction to the vertical direction upwards, they need a zone of excess pressure near the ground. It is this excess pressure in the “tornado heel” that creates centripetal acceleration, bending the air flows.
When carefully examining photographs of tornadoes and whirlwinds, these features of the structure of vortices can be traced by indirect signs (see Fig. 24-26).
Fig.24. A good shot of a tornado held up to the sun, so that the internal structure of the “foggy cone” is visible. The cone is pinched by a “knot” before reaching the surface of the sea.
Fig. 25. The best frame of the tornado, where the color of the dust and the size of the pollutant fractions highlight individual structures of the vortex.
Fig.26. Stages of development of one tornado. The “heel knot” on the sea surface is present at all stages of tornado development.
On the Internet, there is a wide variety of versions of the structure of a tornado vortex in the form of graphic “explanatory diagrams”.
Moreover, in all the families there are strong differences in some key points.
That is, there is still no single, generally accepted explanation for the mechanism of a tornado. (See Fig. 27-32)
Fig. 27. A picture from the Internet on the same topic. The converging flows to the lower patch are shown, but the node with the pinched end of the “foggy cone” from the cloud is hidden in a cloud of dust near the ground. There is another question here: Where and why does the air descend along the “foggy cone” from the cloud?
Fig.28. The converging flows to the lower patch are clearly shown, as if with pincers pulling the lower end of the foggy cone from the cloud. Well, here is the question: Where and why does the air descend along the “foggy cone” from the cloud?
Fig.29. I am showing a picture of air flows drawn by someone else over a real photo of a tornado. This is an example of the fact that the model of flows in a tornado that I am proposing has occurred to other smart people. The sharp curls near the ground are the dust being knocked out by high pressure from under the “heel of the vortex”.
Fig.30. Traditional tornado design, where the details of the surface contact unit are usually hushed up as “an insignificant detail”. The specified speed of up to 200 m/s is more likely to be vertical speed than circular speed. The circular component of the speed in the tornado cord is several times smaller than the vertical component.
Fig.31. A popular but WRONG diagram of a tornado, where a suction funnel with a strong vacuum on the ground surface itself is shown in the contact zone of the vortex with the ground surface. The pressure of 0.4 atm in the cord and cone of the tornado is also questionable: How is this strong pressure drop in the cone separated from the low-hanging cloud above the tornado? Moreover, it must be taken into account that the pressure in the cloud itself is much higher than 0.4 atm.
Fig.32. Another interesting version of the tornado vortex devices. The tornado heel is highlighted with a separate drawing on a large scale (on the right). However, it is not very clear here by what forces the air flow from the cloud first falls down to the ground in the center of the vortex, and then sharply turns around and rushes along the “foggy cone” back up?
