Statistics and Physics of Stratospheric Gravity Wave Attenuation over New Zealand

Abstract:

The DEEPWAVE field project took place over the New Zealand region during June and July of 2014 and was focused on observing orographic and non-orographic gravity waves from their source regions in the troposphere to attenuation regions in the stratosphere, mesosphere, and thermosphere. An important preliminary finding of this project is that many mountain wave events are attenuated in a 15-20km “valve layer” in the lower stratosphere, characterized by weak winds and non-linearity. This valve layer strongly attenuates about half of New Zealand mountain wave events, reducing wave momentum fluxes by as much as 90% and producing a maximum in momentum flux divergence. This work further characterizes this lower-stratospheric mountain wave attenuation and seeks to understand the physics of actual wave attenuation events “reproduced” within 6- and 2-km resolution realistic WRF simulations. Local attenuation diagnostics, such as Richardson Number, stratification, and the non-linearity ratio, are used to characterize the size and 3-D distribution of attenuation regions and to diagnose dissipation mechanisms. Potential vorticity (PV) is also used as a diagnostic to identify attenuation regions and also to trace the influences of these regions downstream. Preliminary work has revealed that mountain wave attenuation over New Zealand is spatially inhomogeneous, generates PV in dipoles, and that lateral shear instabilities cause lateral mixing 1000s of kilometers downstream of the attenuation regions.