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Mesoscale Convective System (MCS)
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Mesoscale Convective System (MCS) - A region where several thunderstorms have merged to create a continuous area of convective instability where cloud with a horizontal extent greater than 100 km (54 nm) exists. Such systems normally continue to exist for several hours. They can occur in both tropical and temperate latitudes.
Within a MCS is a cluster of multi-celled thunderstorms called Mesoscale Convective Complex (MCC). Such a complex would normally be expected to reach its peak intensity at night having formed during the afternoon and evening. They can reform the next night as well, by way of the same processes of the initial formation. As a guideline, the United States National Weather Service defines a MCC as having an area of cloud top of at least 100,000km² at a temperature of -32° C or less or an area of cloud top of at least 50,000km² at a temperature of -52° C or less. In both cases, they generally have a ratio of minor to major axes of at least 0.7.
MCC observations increased with satellite imagery, in particular thermal infrared IR imagery, due to the size of the storm being too small for synoptic weather charts but too large for individual weather stations. The description of squall lines is based on the radar echo, whereas the description of the MCC is based on satellite imagery. MCCs are responsible for much of the precipitation in midwestern US.
During the night, cooling aloft through long wave radiation loss from the thunderstorm tops help intensify the MCC development. Added to that is the condition of a low level jet that advects moist air poleward in the planetary boundary layer, resulting in a sub-synoptic-scale Earth-atmosphere energy balance.
MCCs can last for time scales of 12 hours, and in the extreme case can last into the realm of 3 days. The individual thunderstorms in the MCC last longer than typical single cell thunderstorms: they can last for about one or two hours.
MCCs can contain squall lines and/or multicell and/or supercell thunderstorms, They generally form in areas of warm humid air, but do not need the dynamics of frontal systems to develop. However, a thermal low condition tends to be an important ingredient for MCCs to form (convergence below and divergence aloft). The active regions (areas of instability) of polar jet streams invites these conditions, as does warm air advection in the lower troposphere. An upper short-wave trough located to the westward of development tends to be associated with MCC development.
Weather conditions associated with MCC, correspondingly, are hail, damaging winds, tornadoes, intense rainfall.
MCC development is generally similar in both the Northern and Southern Hemisphere. The cloud shield was found to be roughly 50% to 60% larger in South American mid-latitudes (ie. Argentina) than over North American mid-latitudes. During periods of El Nino, MCCs tend to be greater in number than during non-El-Nino (La Nina) years.
In the Subtropical regions, subtropical jetstreams in the warm regions which curve anticyclonically seem to aid warm moist air advection into the MCC region. There are about 400 MCC occurrences around the world annually, and are concentrated over land masses, and mainly to the lee of north-south aligned mountain ranges in areas of westerly winds. As a result, more MCCs form over the northern hemisphere than over the southern hemisphere, and are more prevalent in the United States of Amercia's Midwest and Argentina's Patagonia regions.
MCCs over north-central-eastern Atlantic regions during the Northern Hemisphere Autumn are linked to initial hurricane formation in these areas.
Additionally to the severe weather associated with MCCs, the MCC also creates “Mesohighs”, regions of cold air pools resulting from the downdrafts of multiple thunderstorms, creating a thermal high pressure area, but is not to be considered an anticyclone due to its lack of anticyclonic circulation. The winds in the Mesohigh flow away from the centre of the high pressure area. Weather not associated with MCCs (a “break in the storm and rain”) may occur with a Mesohigh. Squall lines in the MCC may also create Mesohighs.
- Danielson, E.W., Levin, J., Abrams, E., 2003: Meteorology, 2nd Edition, McGraw Hill.
- Djuric, D., 1994: Weather Analysis, Prentice Hall, Inc.
- Barry, R.G., Carleton, A.M., 2001: Synoptic and Dynamic Climatology, Routledge.