Multi-cell complexes are formed when individual cells join together to form a larger, longer lived, system. The individual cells do not have to be in the same development state, they can all have a different "age" and shape.
In a multi-cell complex, the downdrafts of a thunderstorm create a new thunderstorm. The individual thunderstorms are not longer lived than normal thunderstorms that occur on summer days, however the entire complex is significantly more persistent.
Multi-cell complexes can cause large scale flooding if they move slowly, and can also bring hail (however, the hail doesn’t usually reach the size of that in supercells, only up to 3cm).
Types of Multi-cell complexes
MCS (Mesoscale Convective System)
Mesoscale convective systems are not defined very clearly, and their main criteria is what was described above.
MCC (Mesoscale Convective Complex)
A MCC is usually larger than an MCS and typically has a round or oval shape. MCCs are usually strongest in the night. The following criteria have to be reached for a thunderstorm complex to be considered a MCC:
- The temperature of an area at least 100.000km² large has to be below -32°C or an area of at least 50.000km² has to be below -52°C
- The above has to be fulfilled for at least 6 hours
In multi-cell lines thunderstorms align linearly to one another. The line of thunderstorms can grow to over 100km in length. The intense downdrafts within the line force the warm air located in front of the multi-cell line to rise and therefore form new thunderstorms. Multi-cell lines are clearly visible on the radar. Especially in Germany, there are impressive examples in some summers, with multi-cell lines reaching from one end to the other of the country.
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