Are you familiar with the definition of a supercell? It seemed to me that this is something from the field of mathematics or nuclear physics. Maybe there is such a thing, but we will now talk about natural phenomena.
The cause of such phenomena as thunderstorms, heavy rain, and squalling wind intensification are monocellular and multicellular cumulonimbus clouds, which quite often pile up in the sky in the summer season. A monocell is one single cumulonimbus cloud that exists independently of the others. A multi-cell is already a cluster (accumulation) of mono-cells, which are united by one anvil. That is, when one cell decays, then another nucleus near it, or nucleation occurs simultaneously. These complexes can occupy an area from several tens to several hundred thousand km2.
The latter are called Mesoscale Convective Clusters (MCC). They are capable of causing powerful squalls, heavy hail and heavy rainfall. However, they are nothing special - just an accumulation of powerful cumulonimbus clouds. But there is an atmospheric formation that produces even more severe weather conditions, including a tornado and it is called a supercell. Their formation conditions and structure are fundamentally different from ordinary cumulonimbus clouds. And this article is just about these amazing, rare and exciting objects of the atmosphere.
Monocells and multicells
To begin with, consider the processes of formation of conventional monocells. On a clear summer day, the sun heats up the underlying surface. As a result, thermal convection occurs, which leads to the emergence of "embryos" of a future thunderstorm - flat cumulus clouds (Cu hum.), The height of which does not exceed 1 km. They are usually generated by chaotically rising volumes of heated air - thermals in the form of bubbles. In this case, the resulting cloud will last for some time (tens of minutes) and eventually dissolve without passing to another stage of development. It is a different matter when the emerging thermal takes the form of not a bubble, but a continuous stream of air. At the same time, in places from which the air has risen, a rarefaction is formed. It is filled with air from the sides. Above, on the contrary, excess air tends to spread to the sides. At some distance, the air traffic closes. As a result, a convective cell is formed.
Moreover, Cu hum. passes into cumulus medium or cumulus powerful clouds (Cu med., Cu cong.), the height of which is already up to 4 km. A cumulus flat cloud will pass into a medium cloud, and then into a powerful one, or it will end its evolution, remaining at the first stage, depends only on the state of the atmosphere in a given place and at a given time. The main factors contributing to the growth of convective clouds are a sharp drop in temperature with height in the background atmosphere, as well as the release of heat during phase transitions of moisture (condensation, freezing, sublimation), which requires a sufficiently high content of water vapor in the air. A limiting factor is the presence of layers in the atmosphere in which the temperature drops slightly with height, up to isotherm (temperature does not change with height) or inversion (warming with height). Under favorable conditions, Cu cong.turns into a cumulonimbus Cb cloud, which causes showers, thunderstorms and hail. But in any case, a cumulonimbus cloud appears initially as Cu hum, and not spontaneously.
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A distinctive feature of this cloud is the icy summit, which has reached the inversion layer (the height Cb is determined by the level of condensation and the level of convection - respectively, the lower and upper boundaries of the cloud. In tropical latitudes, the height of these clouds can reach 20 km and break through the tropopause). It is called an anvil and is a layer of dense cirrus clouds developed in the horizontal plane. At this time, the cloud reached its maximum development. At the same time, along with ascending streams in the cloud, descending streams are formed as a result of precipitation. Falling precipitation cools the surrounding air, it becomes denser and begins to descend to the surface (we observe this process on the ground like a squall) more and more blocking the updrafts, which are very necessary for the existence of the cloud. And any downdraft has a detrimental effect on cloud genesis.
Thus, a cloud that has grown to stage Cb immediately signs its own death warrant. Studies show that downdrafts in its lower part and in the sub-cloud layer have a particularly strong effect - from under the cloud, figuratively speaking, the foundation is knocked out. As a result, the final stage of the existence of Cb begins - its dissipation. At this stage, only downdrafts are observed under the cloud, completely replacing the ascending ones; precipitation gradually weakens and stops, the cloud becomes less dense, gradually passing into a layer of dense cirrus clouds. This is where his existence ends. Thus, the cloud goes through all stages of evolution in about an hour: the cloud grows in 10 minutes, the stage of maturity lasts about 20 - 25 minutes, and dissipation occurs in about 30 minutes.
A monocell is a cloud that consists of one convective cell, but most often (in about 80% of cases) multi-cells are observed - a group of convective cells at different stages of development, united by one anvil. During multi-cell thunderstorm activity, the descending streams of cold air of the "parent" cloud create ascending streams that form the "daughter" thunderclouds. However, it must be remembered that all cells can never be simultaneously at the same stage of development! The lifetime of multi-cells is much longer - on the order of several hours.
Supercell. Basic concepts
A supercell is a very powerful convective monocell. The process of its formation and structure is very different from ordinary cumulonimbus clouds. Therefore, this phenomenon is of great interest to scientists. The interest lies in the fact that an ordinary monocell under certain conditions turns into a kind of "monster" that can exist for about 4 - 5 hours practically unchanged, being quasi-stationary and generate all dangerous weather phenomena. The diameter of a supercell can reach 50 km or more, and its height often exceeds 10 km. The ascending velocity inside the supercell reaches 50 m / s and even more. As a result, hail often forms with a diameter of 10 cm or more. Below we will consider the formation conditions, dynamics, and structure of the supercell.
The main factors necessary for the formation of a supercell are wind shear (change in wind speed and direction with height in the layer 0 - 6 km), the presence of a jet stream at low levels, and strong instability in the atmosphere when "explosive convection" is observed. Initially, the cloud has the characteristics of a monocell with direct ascending streams of warm and humid air, but then at a certain height wind shear and / or a jet stream is observed, which begins to spiral the ascending stream and tilts it slightly from the vertical axis. In the first figure, a red thin arrow shows a wind shear (jet stream), a wide arrow - an updraft.
As a result of its contact with the jet stream, it begins to spiral in a horizontal plane. Then the ascending stream, rotating in a spiral, gradually transforms from horizontal to more vertical. This can be seen in the second figure. Ultimately, the updraft takes on an almost vertical axis. At the same time, the rotation continues, and it is so powerful that it eventually breaks through the anvil, forming a dome above it - a towering crown. The appearance of this dome indicates powerful updrafts that are capable of breaking through the inversion layer. This rotating column is the "heart" of the supercell and is called a mesocyclone. Its diameter can range from 2 to 10 km. The towering crown just indicates the presence of a mesocyclone.
The long lifespan and stability of the supercell is associated with the following. Due to the mesocyclone, precipitation occurs slightly away from the updraft, and therefore the downdrafts are also observed to the side (mainly on both sides of the mesocyclone). In this case, both streams (descending and ascending) coexist with each other - they are friends: going down, the former displaces warm air upward, and does not block its access to the cell, thereby further enhancing the ascending flow. And the more powerful the updraft, the stronger the precipitation, which causes even larger downdrafts, which more and more force the surface air upward. And if the cell is likened to a wheel, it turns out that precipitation in such a situation, as it were, spins this wheel. It is as a result of this that the supercell is able to exist for many hours,expanding during this time by tens of kilometers in width and length, generating large hail, heavy rainfall and often tornadoes. At this time, 3 mini-fronts appear at the surface of the earth: 2 cold ones in the area of downdrafts, and a warm one in the area of ascending ones (see Fig. 1). That is, a miniature cyclone appears, the "embryo" of which is precisely the same mesocyclone.
As mentioned above, tornadoes arise not only in supercells, but also in ordinary mono- and multi-cells. However, there is a major difference: in a supercell, precipitation and tornadoes are observed simultaneously, and in mono- and multi-cells - first a tornado, and then precipitation, and in the area where the tornado was observed. This is due to the absence of an obvious shift in the space of the upper "crystallogenic" part of the cloud, and the lower part into which warm air flows. In addition, in supercells there is usually a jet stream above the apex, which carries the displaced air away from the cloud, as a result of which a very elongated anvil is observed (see Fig. 1), while in a normal cell, the cold air displaced by warm, descends along the edges and thereby additionally blocks "power". Therefore, tornadoes in such cells are short-lived, weak,and are rarely at a stage greater than a funnel cloud.
It should be noted that supercells are both large and small, with a low or high towering crown, and can form anywhere, but mainly in the central states of the United States - on the Great Plains. In Europe and Russia, they are extremely rare, and there is only one type - HP supercells. The classification will be discussed below. Supercells are always associated with significant wind shear and high CAPE values - an indicator of instability. For supercells, the vertical shear limit starts at 20 m / s in the 0-6 km layer.
All supercells produce harsh weather conditions (hail, squalls, rainstorms), but only 30% or fewer of them generate tornadoes, so one must try to distinguish tornado-generating supercells from more "calm" ones.
A powerful shift in the 0-6 km layer (long hodograph) and sufficient buoyancy are necessary for the formation of a powerful mesocyclone. The formation of a supercell under the condition of a significant curvature of the hodograph in the 0-2 km layer promotes the development of a tornado. However, the development of a tornado depends on the dynamic structure of the storm. There must be strong updraft and vertical rotation for a strong mesocyclone and tornado development. The horizontal eddy caused by vertical shear is decisive in the formation of the mesocyclone.
Supercells are generally classified into 3 types. But not all supercells clearly correspond to a specific species and often pass from one species to another in the course of their evolution. All types of cells generate harsh weather conditions.
Classic supercell - That is, it is the ideal supercell, which contains almost all of the above elements, both on the radar and visual. Instability indices for this type are: CAPE: 1500 - 3500 J / kg, Li from -4 to -10. But in nature, such cells are quite rare; the other two types are more often observed.
LP (Low Precipitation) type supercell. This class of supercells has a small area with low precipitation (rain, hail), separated from the updraft. This type can be easily recognized by the sculpted cloud grooves at the base of the updraft, and sometimes has the appearance of being "hungry" in comparison to the classic supercell. This is because they form along the so-called. dry lines (when warm and humid air is observed near the surface, which wedges, like a cold front, under hotter and drier air, since the latter is less dense), having little available moisture for its development, despite a strong wind shear … Such cells usually quickly collapse without changing into other types. They typically generate weak tornadoes and hail less than 1 inch in size. Due to the lack of heavy rainfall,this type of cell has weak radar reflections without a clear hook echo, even though a tornado is actually being observed at the time. The thunderstorm activity of such a cell is significantly lower compared to other types, and lightning is predominantly intra-cloud (IC), and not between cloud and ground (CG). These supercells are formed at CAPE equal to 500 - 3500 J / kg and Li: -2 - (-8). Such cells are found mainly in the central states of the United States during the spring and summer months. They have also been observed in Australia. Such cells are found mainly in the central states of the United States during the spring and summer months. They have also been observed in Australia. Such cells are found mainly in the central states of the United States during the spring and summer months. They have also been observed in Australia.
Supercell type HP (High Precipitation). This type of supercell has much higher precipitation than other types, which can completely surround the mesocyclone. Such a cell is especially dangerous, since it can contain a powerful tornado, which is visually hidden behind a wall of precipitation. HP supercells often cause flooding and severe downbarsts, but are less likely to form large hail than other types. It was noted that these supercells generate more IC and CG discharges than other types. The CAPE index for these supercells is 2000 - 7000 J / kg or more, and Li should be below -6. Such cells move relatively slowly.
After 4 years of unsuccessful searches, photographer Mike Olbinski found what he was looking for. On June 3, near Booker, Texas, he saw that very rare rotating supercell.
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