Multicell Storms

Multicell clusters and Lines will often evolve from merging Pulse storms or in situations where an environmental cap is “broken”, multicell clusters develop almost immediately.

  • can evolve into lines or supercell(s)
  • often well organized and long-lived 
Stage 1
Stage 2
Stage 3

Schematic of the evolution of multi-cellular storms

Some large-scale precipitating systems begin as combinations of a number of convective elements. The resulting system, although still feeding on unstable air, takes on a character much different from typical cumulus-scale convection yet it can still be considered a multicell system. Regions of strong convection and heavy showers can become distributed in bands within a larger area of developing stratiform precipitation or the strong convection can be limited to the large system's leading edge with the rest of the system primarily composed of stratiform rain. However, the convective line can, at times, be near the trailing edge as well. When organized in a linear fashion, the multicell convective elements are typically distributed along a band about 20 km (11 nm) wide and hundreds of kilometers long. The bands are related to the low-level “cold pool,” convergence, both low-level and deep layer wind flow and shear, the Earth's topography, and the rotation of elements within the line itself can all affect the structure of the rain areas. The characteristics of precipitation bands have been categorized, but it is not completely understood why precipitation has the strong tendency to become organized into the characteristic scales and patterns that are observed.

Precipitation areas can be grouped into categories of size and lifetime. Observations show that synoptic areas that are larger than 104 km2 (2915 nm2) have lifetimes of one day or longer; large mesoscale areas that range from 103 to 104 km2 (291.5 to 2915 nm2) last several hours; small mesoscale areas that cover 100 to 400 km2 (29 to 116 nm2) last about an hour; and elements that are about 10 km2 (2.9 nm2) in size usually last no longer than half an hour.

Along with the larger mesoscale features, typically the smaller deep moist convective elements have the highest rain rates and supply the major contribution to the total rainfall. Multicell storms are a common occurrence when deep, moist, convection organizes in clusters, lines, or areas. Multicells are defined in this text as a group of cells in close proximity sharing a common cold pool and precipitation area. In Maddox's (1980) classification of the Mesoscale Convective Complex (MCC), multicells could be thought of as belonging to both Linear and Circular types (Figure 7-3). The term Mesoscale Convective System (MCS) (Zipser 1982) includes all precipitation systems 20 to 500 km (11 to 270 nm) wide that contain deep convection. Examples in middle-latitudes are large isolated thunderstorms, squall lines, MCCs, and rain bands. The aerial extent of these systems is often too large to be covered by a single radar. Examination of mosaic maps from a network of radars is required to capture the full extent of many MCSs.

Schematic of Mesoscale Convective Weather Systems Classification
From Maddox (1980).

Source

Federal Meteorological Handbook no. 11: Doppler radar theory and meteorology, chapter 7, December 2005

References

Maddox, R. A., 1980: Mesoscale convective complexes. Bull. Amer. Meteor. Soc., 61, 1374–1387.

Zipser, E. J., 1982: Use of a conceptual model of the life cycle of mesoscale convective systems to improve very-short-range forecasts. Nowcasting, K. Browning, Ed., Academic Press, 191–204.