The Possible Role of Symmetric Instability in the Formation of Precipitation Bands

Abstract

Fifteen cases of banded and nonbanded precipitation not associated with surface frontal regions are presented. Results from the linear perturbation and parcel theories of symmetric instability are compared to the observed properties of these bands. Symmetric instability can explain many of the features of the bands considered in this study: all of the bands are aligned parallel to the thermal wind; strong shear and near-neutral static stabilities are observed when bands occur; multiple bands have a wavelength that is related to the depth of the unstable region and the slope of moist isentropic surfaces. However, the linear theory of symmetric instability assumes a basic state of unidirectional flow and thermal wind balance, while the observations indicate that in some cases these conditions are not met. This study supports the hypothesis that symmetric instability may be responsible for precipitation bands, but the comparison between theory and observations is hampered by the inability of the present theory to account for ageostrophic shear and curved flow, and our inability to access geostrophy via sparse soundings.