Mesoscale Convective Complex
Encyclopedia
Mesoscale Convective System
A mesoscale convective system 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...
which is defined by characteristics observed in infrared satellite imagery
Satellite imagery
Satellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History :The first images from space were taken on sub-orbital flights. The U.S-launched V-2 flight on October 24, 1946 took one image every 1.5 seconds...
. They are long-lived, nocturnal in formation and commonly contain heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
fall, wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
, hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
, lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
and possibly tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
es.
Size
A mesoscale convective complex has either... 1: an area of cloud top of 100,000 km² or greater with temperature less than or equal to -32 °C, or 2: an area of cloud top of 50,000 km² with temperature less than or equal to -52 °C. Size definitions must be met for 6 hours or greater. Its maximum extent is defined as when cloud shield reaches maximum area. Its eccentricity (minor axis/major axis) is greater than or equal to 0.7 at maximum extent.Development
MCCs commonly develop from the merging of thunderstormThunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically assigned cloud type associated with the...
s into a squall line
Squall line
A squall line is a line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight-line winds, and possibly tornadoes and waterspouts....
which eventually meet the MCC criteria. Furthermore, some MCC formation can be tracked from the plain
Plain
In geography, a plain is land with relatively low relief, that is flat or gently rolling. Prairies and steppes are types of plains, and the archetype for a plain is often thought of as a grassland, but plains in their natural state may also be covered in shrublands, woodland and forest, or...
s in Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
back to the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
. These are called "orogenic" complexes. The characteristics of the meteorological environment that MCCs form in are strong warm air advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
into the formation environment by a southerly low-level jet stream
Jet stream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...
(wind maximum), strong moisture advection
Moisture advection
Moisture advection is the horizontal transport of water vapor by the wind. Measurement and knowledge of atmospheric water vapor, or "moisture", is crucial in the prediction of all weather elements, especially clouds, fog, temperature, humidity thermal comfort indices and...
which increases the relative humidity
Relative humidity
Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the partial pressure of water vapor in the air-water mixture, given as a percentage of the saturated vapor pressure under those conditions...
of the formation environment, convergence of air near the surface, and divergence of air aloft. These conditions are most prominent in the region ahead of an upper level trough. The systems begin in the afternoon as scattered thunderstorms which organize overnight in the presence of wind shear (wind speed and direction changes with height). The probability for severe weather
Severe weather
Severe weather phenomena are weather conditions that are hazardous to human life and property.- Examples Include :Severe weather can occur under a variety of situations, but three characteristics are generally needed: a temperature or moisture boundary, moisture, and , instability in the...
is highest in the early stages of formation, during the afternoon. The MCC persists at its mature and strongest stage overnight and into the early morning in which the rainfall is characterized as stratiform rainfall (rather than convective rainfall which occurs with thunderstorms). Dissipation of the MCC commonly occurs during the morning hours. After dissipation, a remnant mid-level circulation known as a mesoscale convective vortex
Mesoscale convective vortex
A mesoscale convective vortex is a low-pressure center within an mesoscale convective system that pulls winds into a circling pattern, or vortex. With a core only 30 to wide and 1 to deep, an MCV is often overlooked in standard weather analysis. But an MCV can take on a life of its own,...
can initiate another round of thunderstorms later in the day.
Structure
The structure of an MCC can be separated into three layers. The low-levels of the MCC near the surface, the mid-levels in the middle of the troposphereTroposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
, and the upper-levels in the upper-troposphere. Near the surface, the MCC exhibits high pressure, with an outflow boundary
Outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air 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...
, or mesoscale cold front, at its leading edge. This high pressure is caused by the cooling of the air from the evaporation of rainfall (commonly referred to as a cold pool). In the mid-levels (mid-troposphere), the MCC exhibits a cyclonic (counterclockwise) rotating low pressure which is warm compared to the surrounding environment (referred to as a warm core). This mid-level circulation is referred to as a Mesoscale Convective Vortex. The upper-levels contain an anti-cyclonic (clockwise) rotating high pressure which is a sign of divergence of air. This high pressure is colder relative to its surrounding environment. This divergence at upper-levels and convergence of air at the surface along the cool pool's outflow boundary results in rising motion which aids maintenance of the MCC.
Effects and climatology
MCCs produce heavy rainfall which can lead to flooding and other hydrological impacts. MCCs are found in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
during the spring
Spring (season)
Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...
and summer
Summer
Summer is the warmest of the four temperate seasons, between spring and autumn. At the summer solstice, the days are longest and the nights are shortest, with day-length decreasing as the season progresses after the solstice...
months (warm season), the India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
n monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
region, the West Pacific and throughout Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. In particular, the heavy rainfall from MCCs accounts for a significant portion of the precipitation during the warm season in the United States . As the warm season progresses, the favorable regions for MCC formation shift from the southern plains of the United States northward. By July and August, the north-central states become the most favorable. The mid-level low pressure areas of MCCs have also been tracked to the origin of some tropical cyclones, and on rare occasions vice versa.
Notable MCCs
One of the most notable MCCs occurred overnight on 19 July 1977, in western PennsylvaniaPennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
. The MCC resulted in heavy rainfall which led to the disastrous flooding of Johnstown, Pennsylvania
Johnstown, Pennsylvania
Johnstown is a city in Cambria County, Pennsylvania, United States, west-southwest of Altoona, Pennsylvania and east of Pittsburgh. The population was 20,978 at the 2010 census. It is the principal city of the Johnstown, Pennsylvania, Metropolitan Statistical Area, which includes Cambria County...
. The complex was tracked 96 hours back to South Dakota
South Dakota
South Dakota is a state located in the Midwestern region of the United States. It is named after the Lakota and Dakota Sioux American Indian tribes. Once a part of Dakota Territory, South Dakota became a state on November 2, 1889. The state has an area of and an estimated population of just over...
and produced copious amounts of rain throughout the northern United States before producing up to 12 inches (304.8 mm) of rain in Johnstown.
A second notable MCC brought destructive straight-line winds to southern Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, Upstate New York
Upstate New York
Upstate New York is the region of the U.S. state of New York that is located north of the core of the New York metropolitan area.-Definition:There is no clear or official boundary between Upstate New York and Downstate New York...
, Vermont
Vermont
Vermont is a state in the New England region of the northeastern United States of America. The state ranks 43rd in land area, , and 45th in total area. Its population according to the 2010 census, 630,337, is the second smallest in the country, larger only than Wyoming. It is the only New England...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, Connecticut
Connecticut
Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...
, and Rhode Island
Rhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...
on the morning of 15 July 1995. The MCC produced winds in excess of 160 km/h (100 mph) and was responsible for seven deaths, widespread destruction of forests in the Adirondack
Adirondack Mountains
The Adirondack Mountains are a mountain range located in the northeastern part of New York, that runs through Clinton, Essex, Franklin, Fulton, Hamilton, Herkimer, Lewis, Saint Lawrence, Saratoga, Warren, and Washington counties....
and Berkshire Mountains, and over $500 million in property damage.
The formation of large MCCs over the same general area for a large percentage of the nights in April to July 1993 and their tendency to persist well into the next day was a large part of the cause for the flooding in much of the central United States that year.
See also
- Mesoscale convective systemMesoscale Convective SystemA mesoscale convective system 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...
- SupercellSupercellA supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, continuously-rotating updraft. For this reason, these storms are sometimes referred to as rotating thunderstorms...
- Tropical cycloneTropical cycloneA tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
- Convective storm detectionConvective storm detectionConvective storm detection is the meteorological observation of deep, moist convection and consists of detection, monitoring, and short-term prediction. This term includes the minority of storms which do not produce lightning and thunder. Convective storms can produce tornadoes as well as large...
- Mesoscale convective vortexMesoscale convective vortexA mesoscale convective vortex is a low-pressure center within an mesoscale convective system that pulls winds into a circling pattern, or vortex. With a core only 30 to wide and 1 to deep, an MCV is often overlooked in standard weather analysis. But an MCV can take on a life of its own,...
- DerechoDerechoA derecho is a widespread and long-lived, violent convectively induced straight-line windstorm that is associated with a fast-moving band of severe thunderstorms in the form of a squall line usually taking the form of a bow echo...