THE TEMPORAL BEHAVIOR OF NUMERICALLY SIMULATED MULTICELL-TYPE STORMS .1. MODES OF BEHAVIOR

The temporal behavior of mature multicellular model storms, created in an experiment that varied the vertical wind shear layer depth, is exa mined herein. These storms form new cells at low levels on the storm's forward side, in or near the forced lifting zone at the edge of the e vaporationally chilled subcloud cold-air pool. Each moves upward and r earward within the storm as it intensifies, matures, and decays and be comes replaced by a new cell development. As a result, the storms osci llate in time with respect to updraft intensity and the generation of condensation and surface rainfall. A few model storms oscillate in a s imply periodic fashion during maturity, generating a series of nearly identical cells separated by a nearly constant period. Other storms ar e still periodic but in a more complex fashion, manifesting repeat cyc les consisting of two or more cells. Several simulations appear quite aperiodic. Spectral analyses of temporal statistics reveal the existen ce of a fundamental period of oscillation in every (simple or complex) periodic case. Further, this period varies little among the simulatio ns in the present experiment and averages about 15 minutes, a realisti c cell production period according to observations. In this paper, the authors examine the various modes of mature storm behavior, laying th e foundation for a discussion of the forcings and factors responsible for determining the period and temporal behavior of multicell-type sto rms to come in future work.