Sensitivity of Mountain Waves and Lee-Side Winds to Upstream Conditions

It is well known that the prediction of topographically-forced phenomena such as mountain waves is very sensitive to the properties of the upstream flow, namely the cross-barrier wind speed and static stability. Mountain waves are an example of a class of threshold phenomena that may develop or change regimes (e.g., breaking, nonlinear, etc.) when the basic properties of the upstream flow change through relatively small changes induced by the synoptic-scale or mesoscale flow. In this study, we focus on mountain waves generated by the Southern Alps in New Zealand, in part motivated by satellite observations that suggest this region in winter contains some of the largest stratospheric gravity wave (GW) amplitudes on the planet. Additionally, this gives us an opportunity to utilize ground-based and research aircraft observations from the DEEP propagating gravity WAVE program (DEEPWAVE) that took place in New Zealand in 2014. This region is unique in that strong surface and upper-level winds in winter permit GWs to propagate to very high altitudes. Given the large-amplitude GWs that propagate routinely into the middle atmosphere, the region offers an ideal natural laboratory for studying the multi-scale sensitivity of GWs.

In this study, the nonlinear, adjoint and tangent linear models for the atmospheric portion of the nonhydrostatic Coupled Atmosphere/Ocean Mesoscale Prediction System (COAMPS) are used to explore the mesoscale sensitivity and predictability characteristics associated with airflow impinging on the Southern Alps. We analyze results from idealized and real world simulations during the DEEPWAVE period in 2014. Results indicate that the short term forecast (0-48h) cross-mountain winds and mountain wave response are very sensitive to the upstream initial state. On the synoptic-scale, rapid growth associated with baroclinic waves impact the stability and cross barrier wind speed upstream of the New Zealand Alps. On the smaller scales, the mountain waves are predominantly influenced by the wave launching conditions (upstream stratification, crest-level wind shear) and modulated by the boundary layer properties.