Mountain wave energy and momentum fluxes from mesoscale model output

Gravity wave energy and momentum flux, two key parameters commonly measured or estimated in previous mountain wave studies, are simple functions of perturbation velocity components and perturbation pressure. In idealized studies, these perturbations are defined to be relative to some mean state. In the actual atmosphere, however, simply demeaning variables over a domain with length scales into the synoptic scale (i.e., ~1000+ km) to find perturbations retains large scale perturbations unrelated to gravity waves. This study focuses on calculating perturbation quantities from mesoscale model output by removing large scale gradients via Gaussian high-pass filtering in order to calculate flux quantities. The dependence of areally integrated energy flux on the filter length scale was investigated. These flux diagnostics were applied to deep (up to 50 km altitude), high resolution Weather Research and Forecasting model simulations of multiple wind events impacting New Zealand throughout the 09-12 July 2011 period in addition to a case observed during the DEEPWAVE-NZ field project. Areas of enhanced vertical energy and zonal momentum fluxes collocated with distinct topographic features were often noted, which have been visualized as vertical flux “towers.” These towers illustrate the 3-D extent and vertical propagation of mountain waves, in some cases terminating near the tropopause and in others extending upwards of 45 km.