Multiscale large eddy states in weakly stratified planetary boundary layers

Abstract

We discuss observations of two classes of two-dimensional large eddy states in weakly stratified atmospheric boundary layers. One class is characterized by large eddies with a single horizontal scale. The other contains multiscale large eddies, with horizontal wavelengths on the order of the boundary layer depth (as generally supported by spectra), coexisting with scales that are several times this depth (as generally seen in cloud street patterns). We develop nonlinear, coupled evolution equations that govern the temporal evolution of a set of three isolated disturbances of initial eigenmode form in a general unstable flow. These equations model both wave/wave and wave/mean flow interactions. They imply that relatively fast growing instabilities to an unstable mean state are themselves unstable. We suggest that the determining factor in selecting a large eddy state in the boundary layer is the presence or absence of sources of waves to the boundary layer that complement the dynamically or convectively driven boundary layer roll vortices. -from Authors