The Temporal Behavior of Numerically Simulated Multicell-Type Storms. Part I. Modes of Behavior

Abstract

The temporal behavior of mature multicellular model storms, created in an experiment that varied the vertical wind shear layer depth, is examined 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 evaporationally chilled subcloud cold-air pool. Each moves upward and rearward within the storm as it intensifies, matures, and decays and becomes replaced by a new cell development. As a result, the storms oscillate in time with respect to updraft intensity and the generation of condensation and surface rainfall. A few model storms oscillate in a simply periodic fashion during maturity, generating a series of nearly identical cells separated by a nearly constant period. Other storms are still periodic but in a more complex fashion, manifesting repeat cycles consisting of two or more cells. Several simulations appear quite aperiodic. Spectral analyses of temporal statistics reveal the existence of a fundamental period of oscillation in every (simple or complex) periodic case. Further, this period varies little among the simulations in the present experiment and averages about 15 minutes, a realistic cell production period according to observations. In this paper, the authors examine the various modes of mature storm behavior, laying the foundation for a discussion of the forcings and factors responsible for determining the period and temporal behavior of multicell-type storms to come in future work.