MesoZ

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      http://www.met.utah.edu - Department of Meteorology, The University of Utah
      http://www.met.psu.edu  - Department of Meteorology, The Pennsylvania State University

METEO 6140 WIKIPEDIA PROJECT DRAFT

MESOSCALE CONVECTIVE COMPLEX (MCC)

A mesoscale convective complex (MCC) is a unique kind of Mesoscale Convective System which is defined by characteristics observed in infrared satellite imagery. They are long-lived, nocturnal in formation and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes. The definition of an MCC as observed in infrared satellite imagery is:

• Size: A) Area of cloud top with temperature less than or equal to -32°C is 100,000km² or greater and B) area of cloud top with temperature less than or equal to -52°C is 50,000km² or greater
• Duration: Size definitions met for 6 hours or greater
• Maximum Extent: When cloud shield reaches maximum area
• Shape: Eccentricity (minor axis/major axis) is greater than or equal to 0.7 at maximum extent

MCCs commonly develop from the merging of thunderstorms into a Squall line which eventually meet the MCC criteria. Furthermore, MCC formation can be tracked from the plains in Colorado back to the Rocky Mountains. These are called "orogenic" complexes. The characteristics of the meteorological environment that MCCs form in are: strong warm air advection into the formation environment by a low-level jetstream (maximum in wind speed in the low-levels of the atmosphere) coming from warm air to the south, strong moisture advection which increases the relative humidity 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 a 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 is highest in the early stages of formation, during the afternoon. The MCC then 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). Disspitation of the MCC commonly occurs around sunrise.

The structure of an MCC can be separated into three layers. The low-levels of the MCC near the surface, the mid-levels of the MCC in the middle of the troposphere and third, the upper-levels of the MCC in the upper-troposphere. Near the surface, the MCC exhibits high pressure. This high pressure is caused by the cooling of the air due to the evaporation of rainfall (commonly referred to as a cold pool). In the mid-levels (mid-troposphere), the MCC exhibits a cyclonic (anti-clockwise) rotating low pressure which is warm compared to the surrounding environment (referred to as a warm core). The upper-levels contain an anti-cyclonic (clockwise) rotating high pressure which is a sign of divergence of air. This high pressure is also colder relative to its surrounding environment (referred to as a cold core). This divergence at upper-levels, and convergence of air at the surface, results in rising motion which thus aids maintenance of the MCC.

MCCs produce heavy rainfall which can lead to flooding and other hydrological impacts. Where in the world are MCCs commonly observed? MCCs are found in the United States during the spring and summer months (warm season), the Indian monsoon region, the West Pacific and throughout Africa and South America. In particular, the heavy rainfall from MCCs account 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.

One of the most recent notable MCCs occurred overnight on 19-20 July 1977 in western Pennsylvania. The MCC resulted in heavy rainfall which led to the disastrous flooding of Johnstown, Pennsylvania. The complex was tracked 96 hours back to South Dakota and produced copious amounts of rain throughout the northern United States before producing up to 12 inches of rain in Johnstown.

See also

• Supercell
• Mesoscale Convective System

External links and sources

• Forecasting MCC's - Hydrometeorological Prediction Center
• MCC description - Pennsylvania State University
• MCC description - University of Illinois
• MCC page - UCAR
• NOAA Glossary

Category:Mesoscale meteorology Category:Midlatitude weather

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A PROPOSAL FOR THE METEO 6140 WIKIPEDIA PROJECT

Mesoscale Convective Complexes

One of the more frequent events of organized convection (mesoscale convective systems, MCSs) are mesoscale convective complexes (MCCs). Not only do they account for a significant portion of seasonal rainfall for regions across the United States, MCCs are also the cause of high wind events, flood events and at times, severe weather events. Currently a stub in Wikipedia, additional information can be provided to expand on the lack of information already present in Wikipedia on this important meteorological event. Also included can be the addition of historical MCC events, such as the Johnstown Flood of 1977. This subject is particularly interesting because MCCs are dynamically (i.e. the vertical distribution of vorticity) one of the most interesting mesoscale phenomena, are long-lasting and nocturnal in occurrence. The characteristic long timescale of the mesoscale convective vortex (MCV) within a mature and decayed MCC is particularly interesting as while it is not always convectively active during the day, the presence of the MCV in the following days (under the right conditions) can lead to the formation of another MCC. An MCV from an MCC has even been associated with the initiation of tropical storm development. I am particularly interested as the northeastern United States (particularly Pennsylvania, where I grew up) commonly sees MCC events in the spring and can produce a large amount of the rainfall we see during that season. In addition, my undergraduate university, Penn State, is very near the location of one of the most well-known MCC events, the '77 Johnstown flood, as mentioned above. A number of peer-reviewed journal articles are available on the subject in the atmospheric sciences and will provide the best information on this mesoscale phenomena. Many of them present a climatology of MCC events in the United States, as well as climatology of MCCs for other countries around the world including those in central and southern Africa, India, and eastern South America. Here are a few links for other MCC-related material. A part of the COMET module on Mesoscale Convective Systems: http://meted.ucar.edu/mesoprim/severe2/print_version/_p_5.0MCCs.htm. Forecasting tips from the Hydrometeorological Prediction Center: http://www.hpc.ncep.noaa.gov/research/amsQPF_mcs1_and_mcs2/sld001.htm