A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and generally forms near weather fronts. The type that forms during the warm season over land has been noted across North and South America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.
Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone (ITCZ) or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS which form at high latitudes during the cold season. Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV). Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall. [1]
Mesoscale convective systems are thunderstorm regions which may be round or linear in shape, on the order of 100 kilometres (62 mi) or more across in one direction but smaller than extratropical cyclones, [2] and include systems such as tropical cyclones, squall lines, and mesoscale convective complexes (MCCs), among others. MCS is a more generalized term which includes systems that do not satisfy the stricter size, shape, or duration criteria of an MCC. They tend to form near weather fronts and move into areas of 1000-500 mb thickness diffluence, which are areas where the low to mid level temperature gradient broadens, which generally steers the thunderstorm clusters into the warm sector of extratropical cyclones, or equatorward of warm fronts. They can also form along any convergent zones within the tropics. A recent study found that they tend to form when the surface temperature varies with more than 5 degrees between day and night. [3] Their formation has been noted worldwide, from the Meiyu front in the far East to the deep tropics. [4]
There are four main types of thunderstorms: single-cell, multi-cell, squall line (also called multi-cell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"). Single-cell thunderstorms form in environments of low vertical wind shear and last only 20–30 minutes. Organized thunderstorms and thunderstorm clusters/lines can have longer life cycles as they form in environments of sufficient moisture, significant vertical wind shear (normally greater than 25 knots (13 m/s) in the lowest 6 kilometres (3.7 mi) of the troposphere) [5] ), which aids the development of stronger updrafts as well as various forms of severe weather. The supercell is the strongest of the thunderstorms, most commonly associated with large hail, high winds, and tornado formation.
Precipitable water values of greater than 31.8 millimetres (1.25 in) favor the development of organized thunderstorm complexes. [6] Those with heavy rainfall normally have precipitable water values greater than 36.9 millimetres (1.45 in). [7] Upstream values of CAPE of greater than 800 J/kg are usually required for the development of organized convection. [8]
A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. Their area of cold cloud tops exceeds 100,000 square kilometres (39,000 sq mi) with temperature less than or equal to −32 °C (−26 °F); and an area of cloud top of 50,000 square kilometres (19,000 sq mi) with temperature less than or equal to −52 °C (−62 °F). Size definitions must be met for six hours or greater. Its maximum extent is defined as when the cloud shield, or the overall cloud formation, [9] reaches its maximum area. Its eccentricity (minor axis/major axis) is greater than or equal to 0.7 at maximum extent, so they are fairly round. They are long-lived, nocturnal in formation as they tend to form overnight, and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes. [10]
A squall line is an elongated line of severe thunderstorms that can form along and/or ahead of a cold front. [11] [12] In the early 20th century, the term was used as a synonym for cold front. [13] The squall line contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts. [14] Severe weather, in form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line which bows out the most. [15] Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low-pressure areas are present. [16] Some bow echoes that develop within the summer season are known as derechos, and they move quite fast through large sections of territory. [17] On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low-pressure area that forms behind the mesoscale high-pressure system normally present under the rain canopy, which are sometimes associated with a heat burst. [18] Another term that may be used in association with squall line and bow echoes is quasi-linear convective systems (QLCSs). [19]
A tropical cyclone is a fairly symmetric storm system characterized by a low pressure center and numerous thunderstorms that produce strong winds and flooding rain. A tropical cyclone feeds on the heat released when moist air rises, resulting in condensation of water vapour contained in the moist air. It is fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows, leading to their classification as "warm core" storm systems. [20]
The term "tropical" refers to both the geographic origin of these systems, which form often in tropical regions of the globe, and their formation in Maritime Tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. Depending on their location and strength, tropical cyclones are referred to by other names, such as hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply as a cyclone. Generally speaking, a tropical cyclone is referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan) in the Atlantic and eastern Pacific oceans, a typhoon across the northwest Pacific Ocean, and a cyclone across in the southern hemisphere and Indian Ocean. [21]
Tropical cyclones can produce extremely powerful winds and torrential rain, as well as high waves and damaging storm surge. [22] They develop over large bodies of warm water, [23] and lose their strength if they move over land. [24] This is the reason coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from the strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. [25] They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere.
Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses; [20] From an operational standpoint, a tropical cyclone is usually not considered to become a subtropical cyclone during its extratropical transition. [26]
Lake-effect snow is produced in the winter in the shape of one or more elongated bands when cold winds move across long expanses of warmer lake water, providing energy and picking up water vapor which freezes and is deposited on the lee shores. [27] The same effect over bodies of salt water is called ocean effect snow, [28] sea effect snow, [29] or even bay effect snow. [30] The effect is enhanced when the moving air mass is uplifted by the orographic effect of higher elevations on the downwind shores. This uplifting can produce narrow, but very intense bands of precipitation, which is deposited at a rate of many inches of snow per hour and often brings copious snowfall totals. The areas affected by lake-effect snow are called snowbelts. This effect occurs in many locations throughout the world, but is best known in the populated areas of the Great Lakes of North America. [31]
If the air temperature is not low enough to keep the precipitation frozen, it falls as lake-effect rain. In order for lake-effect rain or snow to form, the air moving across the lake must be significantly cooler than the surface air (which is likely to be near the temperature of the water surface). Specifically, the air temperature at the altitude where the air pressure is 850 millibars (or 1.5 kilometres (0.93 mi) altitude) should be 13 °C (24 °F) lower than the temperature of the air at the surface. [31] Lake-effect occurring when the air at 850 millibars is 25 °C (45 °F) colder than the water temperature can produce thundersnow, snow showers accompanied by lightning and thunder (due to the larger amount of energy available from the increased instability). [32]
A polar low is a small-scale, symmetric, short-lived atmospheric low-pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 metres per second (38 mph). [33]
The time period in the Plains where thunderstorm areas are most prevalent ranges between May and September. Mesoscale convective systems develop over the region during this time frame, with a bulk of the activity occurring between 6 and 9 p.m. local time. Mesoscale convective systems bring 30 to 70 percent of the annual warm season rainfall to the Plains. [34] A subset of these systems known as mesoscale convective complexes lead to up to 10% of the annual rainfall across the Plains and Midwest. [35] Squall lines account for 30% of the large thunderstorm complexes which move through the region. [36]
While most form over the continent, some MCSs form during the second half of August and September over the western Mediterranean. MCS triggering over Europe is strongly tied to mountain ranges. On average, a European MCS moves east-northeast, forming near 3 p.m. local solar time, lasts 5.5 hours, dissipating near 9 p.m. LST. Around 20% of the MCSs over Europe do not form during maximum heating. Their average maximum extent is around 9,000 square kilometres (3,500 sq mi). [37]
Mesoscale convective systems, which can evolve into tropical cyclones, form along areas such as tropical waves or easterly waves which progress westward along monsoon troughs and the Intertropical Convergence Zone in regions of ample low level moisture, convergent surface winds, and divergent winds aloft. This typically occurs north of the equator from Africa across the Atlantic and eastern Pacific oceans, as well as across the northwest and southwest Pacific oceans, from Australia eastward into Oceania, the Indian Ocean, Indonesia, and from southeast Brazil into the southern Atlantic Ocean. It is also noted on occasion in the southeast Pacific Ocean mild to cool ENSO years, outside of El Niño. [38] More intense systems form over land than water. [39]
In the cases of Lake-effect snow and polar lows, the convective systems form over warm water bodies when cold air sweeps over their surface and leads to an increase in moisture and significant vertical motion. This vertical motion leads to the development of showers and thunderstorms in areas of cyclonic flow on the backside of extratropical cyclones. [31] [33]
A mesoscale convective vortex--(MCV)--is a mid-level low-pressure center within an MCS that pulls winds into a circling pattern, or vortex. Once the parent MCS dies, this vortex can persist and lead to future convective development. With a core only 30 miles (48 km) to 60 miles (97 km) and up to 8 kilometres (5.0 mi) deep, [40] an MCV can occasionally spawn a mesoscale surface low-pressure area which appears on mesoscale surface weather analyses. But an MCV can take on a life of its own, persisting for up to several days after its parent MCS has dissipated. [41] The orphaned MCV will sometimes then become the seed of the next thunderstorm outbreak. An MCV that moves into tropical waters, such as the Gulf of Mexico, can serve as the nucleus for a tropical storm or hurricane. [42] A good example of this is Hurricane Barry (2019).
In meteorology, a cyclone is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above. Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale.
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.
An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere as viewed from above. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure.
A mesocyclone is a meso-gamma mesoscale region of rotation (vortex), typically around 2 to 6 mi in diameter, most often noticed on radar within thunderstorms. In the northern hemisphere it is usually located in the right rear flank of a supercell, or often on the eastern, or leading, flank of a high-precipitation variety of supercell. The area overlaid by a mesocyclone’s circulation may be several miles (km) wide, but substantially larger than any tornado that may develop within it, and it is within mesocyclones that intense tornadoes form.
A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase of the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.
A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. Linear thunderstorm structures often contain heavy precipitation, hail, frequent lightning, strong straight-line winds, and occasionally tornadoes or waterspouts. Particularly strong straight-line winds can occur where the linear structure forms into the shape of a bow echo. Tornadoes can occur along waves within a line echo wave pattern (LEWP), where mesoscale low-pressure areas are present. Some bow echoes can grow to become derechos as they move swiftly across a large area. On the back edge of the rainband associated with mature squall lines, a wake low can be present, on very rare occasions associated with a heat burst.
In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather, while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places:
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.
This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting.
A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For instance, cold fronts can bring bands of thunderstorms and cumulonimbus precipitation or be preceded by squall lines, while warm fronts are usually preceded by stratiform precipitation and fog. In summer, subtler humidity gradients known as dry lines can trigger severe weather. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.
A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands in tropical cyclones can be either stratiform or convective and are curved in shape. They consist of showers and thunderstorms, and along with the eyewall and the eye, they make up a tropical cyclone. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.
Mesoscale meteorology is the study of weather systems and processes at scales smaller than synoptic-scale systems but larger than microscale and storm-scale. Horizontal dimensions generally range from around 5 kilometres (3 mi) to several hundred kilometres. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes.
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. These vary depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal phenomena include blizzards (snowstorms), ice storms, and duststorms.
Outflow, in meteorology, is air that flows outwards from a storm system. It is associated with ridging, or anticyclonic flow. In the low levels of the troposphere, outflow radiates from thunderstorms in the form of a wedge of rain-cooled air, which is visible as a thin rope-like cloud on weather satellite imagery or a fine line on weather radar imagery. For observers on the ground, a thunderstorm outflow boundary often approaches in otherwise clear skies as a low, thick cloud that brings with it a gust front.
Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are fed by inflow focused along their cold front and warm fronts. Tropical cyclones require a large inflow of warmth and moisture from warm oceans in order to develop significantly, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the flow of warm and moist air is cut off from thunderstorms and their associated tornadoes, normally by the thunderstorm's own rain-cooled outflow boundary, the storms begin to dissipate. Rear inflow jets behind squall lines act to erode the broad rain shield behind the squall line, and accelerate its forward motion.
A mesovortex is a small-scale rotational feature found in a convective storm, such as a quasi-linear convective system, a supercell, or the eyewall of a tropical cyclone. Mesovortices range in diameter from tens of miles to a mile or less and can be immensely intense.
A severe thunderstorm outbreak, also called a severe weather outbreak or simply a severe outbreak, is an event in which a weather system or combination of weather systems produces a multitude of severe thunderstorms in a region over a continuous span of time. A severe outbreak which is most notable for its tornadoes is called a tornado outbreak. The four kinds of severe weather produced in these outbreaks are tornadoes, severe wind, large hail, and flash flooding.
A line echo wave pattern (LEWP) is a weather radar formation in which a single line of thunderstorms presenting multiple bow echoes forms south of a mesoscale low-pressure area with a rotating "head". LEWP often are associated with a multiple-bow serial derecho and often produce tornadoes, some of which can be strong. The existence of a LEWP on radar means that a serial derecho has developed or is likely to develop soon, much as a hook echo indicates the same for a tornado.
This glossary of meteorology is a list of terms and concepts relevant to meteorology and atmospheric science, their sub-disciplines, and related fields.
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