5.1 What is an MCC?

Mesoscale convective complexes (MCCs) are a still larger form of convective organization than we have discussed to this point. It was Maddox in 1980 who called attention to this class of larger convective systems by coining the term MCC. MCCs were the first larger convective system type (other than tropical cyclones) to be identified. A system is classified as an MCC based on its size and duration characteristics as identified in IR satellite imagery. The physical characteristics of an MCC include a general cloud shield with continuously low IR temperatures less than -32°C over an area >= 100,000 km 2, with an interior cold cloud region with temperatures less than -52°C having an area >= 50,000 km². This corresponds to an average diameter of ~ 600 km (or ~320 n mi). These size criteria must be met for at least six hours or longer. Now that meteorological radars are more common, we have learned that many different convective storm structures occur under these enormous anvil shields. Additionally, many MCSs, some of which are quite large and long-lived, never meet the exact size and duration criteria to be labeled an MCC. This is why the term MCS is now more commonly used to describe the entire range of organized convective systems. Nonetheless, because they have been documented and quantified using satellite imagery in otherwise data-sparse regions around the globe, MCCs remain a useful area of study and recognition.

5.2 Favored Synoptic Setting

MCCs are most often observed at night, in areas in which the boundary layer is stable. Observational studies suggest that MCC structure and evolution are more dependent on interactions with large-scale forcing features than the evolution we described for squall line MCSs. Specifically, case studies have shown that MCC initiation is usually associated with a weak large-scale frontal zone and the eastward progression of a weak shortwave trough and associated vorticity maximum in the middle troposphere. MCCs tend to occur on the anticyclonic side of a broad, weak westerly jet stream in a location where a low-level jet is delivering moisture-rich air into the genesis region.

In 2000, authors Laing and Fritsch published a paper examining the environments conducive to MCCs in four regions outside the United States including Africa, Australia, China, and South America. They contrasted the MCC-producing synoptic and mesoscale environments to those in the United States and concluded that the environments are all very similar in terms of both their dynamics and thermodynamics.

5.3 MCC Life Cycle

The life cycle of an MCC is classified as having four stages: 1) Genesis, 2) Development, 3) Mature, and 4) Dissipating. These stages could just as well apply to any MCS with Mature corresponding to the system at its maximum extent. During the genesis portion of an MCC’s life cycle the convective structures that make up the future MCC may include multiple squall lines, bow echoes, or isolated convective cells, each evolving through its own life cycle, with each system contributing to the expanding cold pool and anvil. During the later stages of evolution, however, a large stratiform precipitation region dominates the MCC, as it often does in the later stages of squall line evolution. An observation study of MCCs by McAnelly and Cotton noted that on average MCCs last 10 to 12 hours from storm initiation to the time that they shrink below the minimum size criteria in satellite imagery.

5.4 MCC to MCV

The flow field in the later stages of an MCC is characterized by divergent anticyclonic outflow near the surface and aloft within the anvil, with convergent cyclonic flow at midlevels. Like the northern line-end vortices of squall lines discussed earlier in this module, this mid-level cyclonic flow is often referred to as an MCV (or sometimes MVC as discussed earlier). These can be generated by large MCSs, including MCCs, and are most likely to survive beyond their parent MCS when the environmental vertical wind shear is weak. These conditions are often met in the classic weak synoptic setting for derechoes under a large-scale ridge.

Mid-level cyclonic vortices, as documented by Trier and coauthors in a 2000 paper, are frequently observed to be the focus of subsequent convective outbreaks. In fact, 9 of 16 MCVs they observed in the central U.S. during the convective warm season of 1998 were responsible for initiating subsequent deep convection and heavy precipitation. Thus, MCVs spawned by some MCSs are a significant indicator of the location of possible future convection initiation.

5.5 Elevated Convection

The buoyancy source for any MCS/MCC may come from the boundary layer, as in many warm season, daytime convective scenarios or from an elevated layer of instability above a cooler surface-based inversion. This module has primarily focused on MCSs that feed off boundary-layer-based instability. Elevated convection is an especially common cause of MCCs and nocturnal MCSs that form well north of weak surface fronts. Details of forecasting elevated convection are beyond the scope of this module, but it is important in this context to realize that predicting MCSs that form as a result of elevated layers of instability is an especially difficult forecasting challenge. The cold pool/shear balance concepts discussed for boundary-layer-based convection earlier in this module do NOT apply to systems with elevated buoyancy sources. The curious student is referred to Colman 1990 and Moore and coauthors 2003 to learn more about elevated instability and MCSs.

5.6 Associated Threats

MCCs often last for 6 to 12 hours and are especially known for producing heavy rain and even widespread flooding as shown in this example from July 2003. The quantitative precipitation estimate from this case showed as much as 6 to 8 inches of rain in some places. MCCs are also associated with severe winds, hail, and tornadoes usually during the early phases of their evolution. The July 2003 event also had this tendency, with tornadoes and large hail predominantly occurring during the first half of the system’s evolution, and severe winds characterizing the second half. This makes sense because many nocturnal systems that reach MCC-size and duration criteria evolve from surface-based MCSs that initiated earlier in the day. Because of their size, duration, and extensive lightning MCCs are a significant threat to aviation.

Investigations of MCCs around the globe have concluded that they do indeed occur worldwide over both land and ocean surfaces. Because they are so common and generate so much heat and moisture exchange and are responsible for producing extensive precipitation, MCCs are an important part of the global hydrologic cycle.

5.7 Review Question: Identify Importance of MCCs

Why should we forecasters be concerned about MCCs? Choose all that apply.

a) Occur worldwide over land and ocean
b) Especially known for producing heavy rain and widespread flooding
c) Known for a large area of relatively warm cloud-top temperature
d) Produce tornadoes in their later stages
e) Significant threat to aviation
f) Can produce an MCV that moves downstream and initiates convection at a later time
g) Often caused by the result of elevated convection, which is especially difficult to forecast

Review Question Feedback

All answers are correct except d) “Produce tornadoes in their later stages” and c) “Known for a large area of relatively warm cloud-top temperatures.” MCC systems can produce tornadoes, but usually in the early stages of their life cycle. These deep convective systems are actually characterized by a large area of relatively cold cloud-top temperatures on GOES IR satellite imagery.

5.8 Review Question: Identifying MCCs

Please select the best answer below.
MCCs are uniquely identified by:

a) Very large, bow echo pattern on radar
b) Dense area of strikes in lightning data
c) General cloud shield with continuously low temperatures on IR satellite data

Review Question Feedback

The correct answer is c). MCSs often exhibit a dense area of lightning strikes, while some take on a large bow appearance on radar. However, many do not meet MCC criteria. The physical characteristics of an MCC include a general cloud shield with continuously low IR temperatures less than -32°C over an area >= 100,000 km², with an interior cold cloud region with temperatures less than -52°C having an area >= 50,000 km². This corresponds to an average diameter of ~ 600 km (or ~ 320 n mi).

End of Section 5.0: Mesoscale Convective Complexes