A Comparison of Mountain Waves and Gravity Wave Drag Produced by New Zealand, the Southern Andes, and the Himalayas

Recently, much has been learned about the vertical propagation and attenuation of mountain waves launched by the Southern Alps of New Zealand (NZ) from the Deep Propagating Gravity Wave Experiment (DEEPWAVE) field campaign. Approximately half of NZ mountain wave events propagate beyond 35 km altitude. The other half are nearly completely attenuated in a lower-stratospheric “valve layer,” where frequent wind minima cause mountain waves to steepen and attenuate. Global reanalyses indicate that similar lower-stratospheric wind minima are either frequent or the norm in winter mid-latitudes, suggesting that this valve layer may be common elsewhere. This study seeks to put these results for New Zealand into a more global context.

Continuous, deep (~45 km) 6-km resolution realistic Weather Research and Forecasting (WRF) model simulations are completed for an entire season (nearly three months) encompassing New Zealand, the Southern Andes, and the Himalayas. The Southern Andes may produce the strongest stratospheric momentum fluxes globally according to satellite observations, while some GWD parameterizations suggest the Himalayas may produce the strongest lower-stratospheric GWD globally. Within these realistic WRF simulations, which resolve a significant portion of the gravity wave spectrum, the vertical flux of horizontal momentum and gravity wave drag are quantified. Both quantities are compared between the three major mountain ranges, and GWD is quantitatively compared with the parameterized GWD within the MERRA reanalysis data set.