Abstract

The mesoscale convective complex (MCC) is a common and particularly well-organized class of meso-&α scale storm systems over the central United States. As observed by infrared (IR) satellite, the typical MCC's 10–12 h evolution displays a fairly consistent sequence of events, including the monotonic areal expansion of its anvil from its formation to its maximum size, followed by the monotonic shrinkage of the colder cloud top areas as the system weakens and dissipates. Primarily within the growth phase of this cycle, a characteristic IR signature reflects the MCC in its most intense, mesoconvective stage, which lasts ∼4 h and during which the coldest cloud top area reaches its largest extent.

Hourly precipitation data have been analyzed for 122 MCC cases that were selected from June–August 1977–83 and screened to insure a reasonable conformity with the typical IR life cycle. On average. these systems produced a rainfall volume of 3.46 km 3 during their life cycle, over an area of 3.20×105km2and at an average depth of 10.8 mm. Relative to a normalized, IR-defined life cycle, the averaged trends of hourly rainfall area, intensity, and volume all have well-defined growth/ decay cycles, but with significantly staggered maxima. Average rainfall intensity (R), and the proportion of measurable reports due to convective intensifies, attain maxima early in the life cycle. Hourly rainfall volumes (ν) are more symmetrically distributed in time, with the maximum occurring near the largest anvil size (based on −54°C IR threshold). Active rainfall area (A) continues to increase until ∼1 h after maximum anvil size. The IR-defined, intense mesoconvective stage corresponds to that portion of the life cycle from maximum R to maximum A, and is so termed because of the large areal extent and volumetric rate of convective precipitation intensities. A large area of stratiform precipitation is generated during this stage; it persists and becomes increasingly dominant as convective activity subsides during the latter stages of the life cycle. Averaged mappings of the precipitation data show that throughout the MCC life cycle, the heaviest rainfall tends to be displaced 50–100 km south of the cloud-shield centroid, while the stratiform pattern tends to be more MCC-centered.

A statistical analysis of these precipitation characteristics, derived individually for each case, provides an estimate of the natural interstorm variability for typical summertime MCCS. A comparison of various composite subsets of the sample reveals several interesting tendencies: 1) smaller, less-organized systems tended to be “drier” than similar-sized but better-organized MCCS; 2) large systems were "rainier” than smaller ones through much of the life cycle, not only in terms of A and V, as expected, but also in terms of R; 3) large systems tended to be “rdnice” in the eastern part of the sample domain than in the western part, but this was not so for small systems; and 4) the eastern systems: both large and small, had a more coherent and intense core of heavy precipitation through their life cycle than the western systems.

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