Overview
A mesocyclone is a powerful vortex column (usually appearing as a funnel cloud) that arises from the surface of the water. Some occur with cumulus congestus clouds, some with cumulonimbus clouds, and some with cumulonimbus clouds. They are generally tornadoes, not supercell thunderstorms, which occur at the surface.
They are usually weaker than tornadoes that develop over land, but they can produce mesocyclone-induced tornadoes. Most mesocyclones do not pick up water and appear as small, relatively weak tornadic winds that develop on the surface of the water. They are mostly produced in tropical and subtropical regions, but they also occur in Europe, Australia, New Zealand, the Great Lakes, Antarctica, and even Asia, including South Korea. Although uncommon, they have also been known to occur in Salt Lake, Utah. Lava flows associated with lake-effect snow precipitation bands have been observed, but are not common.
Generation
Thunderstorms exist on a microscopic scale, with the parent clouds that generate them being less than 2 kilometers across. The clouds from which they develop can be anything from cumulus clouds that don’t seem to pose any threat to very powerful super-cells.
Some mesocyclones are tornadic with powerful vortices, but most are much weaker and are caused by a variety of atmospheric dynamics. They are usually generated by the development of upper-level mesocyclones in atmospheric conditions with high levels of water vapor. The theory is that it is lifted and rotated by horizontal wind shear near the surface, and when the lower-level shear coincides with a developing cumulus or cumulonimbus cloud, the vortex extends vertically and meets the parent cloud.
Weak tornadoes known as land sprouts have been known to develop in a similar way. More than one mesocyclone may occur simultaneously nearby. In Lake Michigan, up to nine land sprouts have been reported simultaneously.
Forms
Twisters can be categorized into two types: non-tornadic twisters, which are the majority of twisters, and tornadic twisters, which are similar to tornadoes. Non-tornadic mesocyclones are either tornadoes on the surface of the water or tornadoes on land that have moved to the surface of the water, so they have the same characteristics as tornadoes on land.
They are often accompanied by strong storms and include high winds, storm surge, hail, and lightning. On the other hand, non-tornadic thunderstorms often occur along the flat cloud surface of developing cumulus clouds. These are usually not accompanied by thunderstorms. While tornadoes start in cumulonimbus clouds, non-tornadoes start at the surface and gradually rise to connect with the cloud base.
Non-tornadic mesocyclones are formed in low winds, so their movement is small. They can cause damage if they make landfall, but unlike tornadoes, they don’t last long and rarely penetrate far inland.
Non-tornadic mesocyclones
A tornado that occurs unrelated to the strong swirling updrafts (tornadoes) that accompany a severe thunderstorm is known as a “non-tornadic” or “fair-weather tornado,” and these are the most common types of tornadoes.
Non-tornadic thunderstorms occur over water near land and are found with thick, strongly developing convective cumulus clouds with a flat cloud base. They develop and dissipate rapidly, with a life cycle of 20 minutes or less. The intensity is EF0 or less on the Fujita scale, and the wind speed is 30 m/s or less. Typical travel speeds are small because the accompanying clouds remain horizontally static or are generated by soft convective activity instead of settling/contracting between colliding air masses.
Non-tornadic mesocyclones are very similar in shape and structure to terrestrial mesocyclones and exhibit the same characteristics as terrestrial mesocyclones when over land. They are most commonly seen in tropical and subtropical climates, and can occur up to 400 times per year in the Florida Keys.
Tornadoes
A “tornadic melt,” or more accurately, a “tornado on the water,” is a phenomenon that occurs over the ocean using the same mechanisms that produce tornadoes on land: mesocyclonic activity.
Tornadoes that move from land to water are also classified as tornadic mesocyclones. Since the majority of mesocyclone thunderstorms occur inland in the United States, actual tornadic outbreaks are much less common than non-tornadic outbreaks. However, in the Mediterranean region, such as the Adriatic and Aegean seas, tornadoes can account for as much as half of all thunderstorms.
Snow avalanches
A wintertime snowmelt (also known as a snow devil, icespout, ice devil, etc.) is a very rare phenomenon that occurs in a snow squall (a short, small-scale snowstorm).
The term “winter waterspout” is used to distinguish this rare phenomenon, which occurs during the winter months, from more common waterspouts. Little is known about this phenomenon, and there are only six photographs of it, but there are several criteria that must be met for a winter waterspout to form.
The water surface must be warm enough to create a steam fog and the air above it must be very cold. Similar to the conditions for more efficient lake-effect snowfall, winds focusing down the axis of long lakes are likely to enhance wind convergence and facilitate the development of winter meltwater.
Distribution of occurrence
The majority of meltwaters occur in the equatorial regions, but they also occur in temperate climatic zones around the world. Frequent areas include the west coast of Europe, the British Isles, and parts of the Mediterranean and Baltic Seas. They occur on all water surfaces.
A number of meltwaters have also been reported to occur in freshwater bodies such as the Great Lakes and the St. Lawrence River. In the Great Lakes, they are common from late summer to early fall, with more than 66 in a seven-day period in 2003. Lava flows occurred more frequently within about 100 kilometers of the coast than further inland. They occur frequently along the southeastern coast of the United States, especially in southern Florida and the Florida Keys.
In Europe, about 160 are reported to occur annually, with the highest number of 60 in the Netherlands, followed by 25 in Spain and 15 in the United Kingdom. European meltwater events are most common in the summer. In the Northern Hemisphere, they peak in September. The east coast of Australia has also been known to experience frequent auroral events.
The life cycle of a thunderstorm
Stage 1 appears as a circular bright disk on the surface of the water, surrounded by a large dark area of indistinct shape. The second stage is more clearly developed, with light and dark clouds originating from dark spots on the surface of the water that begin to intersect and form a spiral shape. In the third stage, circular dark bubbles called cascades appear around the dark area (with a center that looks like an eye). In stage 4, a funnel cloud is visible, connecting the surface of the water to the cloud base.
Funnel clouds can be hundreds of meters high, sometimes creating wave matrices with visible trajectories and movements. In the fifth and final stage, the funnel and bubble rotation weakens as the supply of warm, moist air to the plume diminishes, and the plume ends its life.
Maritime safety
Plumes have long been recognized as a serious maritime safety threat. Powerful plumes pose a serious threat to ships, aircraft, and people. People are advised to keep a safe distance when they see them and to listen to weather reports. The National Weather Service issues special marine warnings when meltwater is likely to occur or is found offshore, and tornado warnings when landfall is expected.
However, except in a few unusual cases, mesocyclones do not cause significant property damage or loss of life. The 1555 Malta tornado is the earliest record of a deadly mesocyclone. It struck the Grand Harbor of Valletta, the capital of Malta, reportedly sinking four galleys and many smaller vessels and causing hundreds of casualties. The Sicilian Tornado of 1851 was a twin tornado that made landfall in western Sicily, devastating coastal areas before returning to the sea and disappearing. There have also been suggestions that the Mary Celeste incident of the unexplained disappearance of sailors in the Atlantic Ocean on December 5, 1872, was caused by a maelstrom.
Threat to marine life
Depending on how fast the tornado’s whirlwind is spinning, the objects in the water that can be sucked up vary, but fish, frogs, and even turtles as deep as 30 centimeters out of the water have been reported to be sucked up.
These objects can even ascend to the clouds they are attached to. Even if the vortex stops, the fish in the clouds will follow the updrafts and downdrafts within the clouds and remain in the clouds until the updrafts can no longer lift the fish. If the same situation were to occur when the clouds moved over land, we would see the curious phenomenon of fish falling from the sky.
The fate of the fish depends on how long they are trapped in the clouds, but in most cases they fall dead. Even areas as far away as 160 kilometers from the coast can experience fish rain. Even freshwater fish can get sucked up. However, fish rains are not a frequent weather phenomenon.