We will define a mesoscale convective system as a cloud and precipitation system with a spacial scale from 20 to 500 km and a temporal scale from 3 to 12 hours that includes deep convection during some part of its lifetime. This "multicellar storm or group of interacting storms ... suggests some organization in its forcing" (Chappell 1986). The MCS typically lasts longer than any individual thunderstorm cell that is part of it.
Zipser (1982) states that a MCS is characterized by an upper tropospheric cloud shield of outflow air. This cloud shield is the basis for satellite identification of these systems. An example is shown at the right. The merged outflow from the convective downdrafts that make up a MCS dominate the surface conditions beneath the MCS cloud shield. Mesoanalysis of surface data usually indicates the presence of a rain-cooled meso-high and a convectively-induced outflow boundary. MCSs are frequently referred to as "organized convection" due to their persistent nature and some evidence of mesoscale forcing (Maddox 1983).
A MCS is classified as either circular or linear based on the eccentricity seen on satellite imagery. Large and persistent circular MCSs which satisfy the criteria in the table below are called Mesoscale Convective Complexes (MCC) (Maddox 1980). Smaller or less persistent circular systems are referred to as "convective clusters" or by the generic term, MCS.
Linear MCSs typically have a large eccentricity with a cloud shield that is longer than it is wide. The best example of a linear MCS is a squall line. Another example includes the derecho (Johns and Hirt 1987).
Depending upon the environment in which they form, MCSs can produce severe weather or flash flooding or both. The convection, in any case, is a hazard to aviation. Zipser (1982) states that "it is important to distinguish between organized convection and severe convection, as there are many examples of one without the other."
|Criteria for Classification as a MCC|
|A Mesoscale Convective Complex (MCC) is a MCS that meets the following|
satellite-based criteria established by Maddox (1980)
A. cloud shief with continously low IR temperature < -32oC must have an area > 100,000 km2 (half the size of the State of Kansas)|
B. interior cold cloud region with temperature < -52oC must have an area > 50,000 km2
|Duration:||size definitions A and B must be met for a period of 6 hours or more|
|Maximum Extent:||contiguous cold-cloud shiefd (IR temperature < -32oC) reaches maximum size|
|Shape:||eccentricity (minor axis/major axis) > 0.7 at time of maximum extent|
Be sure that a convective system meets both the spacial and duration criteria before calling that system a MCC. Most MCSs do not meet MCC criteria and should not be called a MCC.
|Examples of Mesoscale Convective Systems|
1245 UTC 11 May 2002
2115 UTC 11 May 2002
Individual thunderstorm cells tend to travel with the mean wind in the cloud bearing layer. Operationally, this motion can be approximated by the mean 700-500 mb wind. Severe thunderstorms frequently move to the right of the mean wind direction and slower than the mean wind speed. The movement of convective clusters as seen on satellite imagery have been observed to be both faster and slower than the mean wind speed. They typically travel to the right of the upper tropospheric wind direction.
We will define propagation as the apparent movement of convective clusters as the result of preferred new cell development on one flank of the storm. Individual cells within the cluster may still move with the mean wind but the cluster as an identifiable system moves with its own motion. Specifically, "satellite-observed MCS propagation refers to the translation of the most active convective portion of the MCS (the so-called meso-beta elements, MBEs) due to the formation, development, or merger of newly formed convective clusters" (Juying and Scofield 1989). This basis for determining MCS movement is preferred to movement of the satellite-based cloud-top centroid due to the tendency of the cloud-top to expand downwind. This downwind expansion is usually not in the same direction as the active cell formation.
|Example of Mesoscale Beta Elements|
10 May 2002
10 May 2002
MBEs are best observed using a satellite loop where the active elements can be easily seen. Looking at a corresponding radar image can also help identify these elements. Compare the satellite and radar images in the above example and pick out the MBEs.
Propagation can be classified into four types (Juying and Scofield 1989):
Anticipation of backward, quasi-stationary, or regenerative propagation is important in flash flood situations.
Shown below are two sets of satellite and radar images that allow you to compare the meso-beta elements from the satellite image with the corresponding radar image. In both cases note the good spacial correlation between the stronger thunderstorms on the radar images (in red) and the meso-beta elements in the satellite imagery (blues, browns and golds).