Instabilities and oscillations accompanying boundary layer flow over a stratified cold pool: Numerical simulations of dynamics in the Arizona Meteor Crater

We have employed a very high resolution spectral element (SE) code for numerical studies of boundary layer flow within and above an azimuthally symmetric representation of the Arizona Meteor Crater in support of the Meteor Crater Experiment (METCRAX) performed in October 2006. We assessed the responses of a uniformly stratified boundary layer and crater cold pool to boundary layer winds that were started impulsively, ramped up to a steady profile, and oscillatory in time. Our simulations employed a turbulent boundary layer model in order to allow higher near-surface winds, given METCRAX observations. Model resolution was sufficiently high to allow highly unstable shear flows and realistic shear instability structures, though at Reynolds numbers far below real values.

Results are highly dependent on the forcing conditions. Impulsive forcings at lower and higher boundary layer wind speeds lead to shear layers at the top of the cold pool that are unstable to Kelvin-Helmholtz (KH) instability, and to KH vortex pairing and wavelength doubling for stronger boundary layer winds and lower initial Richardson numbers. These instabilities drive mixing and expansion of the shear layer until the downstream mean Richardson number approaches Ri ~0.25. For stronger forcing, this shear penetrates well into the crater and leads to substantial scouring of cold pool air. Ramped forcing leads to similar responses at the top of the cold pool, but also to significant excitation of seiches within the cold pool. Oscillatory forcings yield quite different results, notably the lack of significant shear instability at the cold pool top and very strong seiche forcing within the cold pool. The structures of the seiche responses depend on forcing frequency and lead, in most cases, to significant ejection of air from deep in the cold pool into the external boundary layer for relatively weak forcing. Upon cessation of forcing, seiche motions exhibit a spectrum of discrete responses, with the relative distributions of seiche energies reflecting initial forcing frequencies to a significant degree.