Generation Mechanisms of Mountain-Wave Turbulence in the Upper Troposphere and Lower Stratosphere over Alaska

Generation and evolution mechanisms of mountain-wave turbulence (MWT) encountered by an aircraft in the upper troposphere and lower stratosphere (UTLS) over the mountainous region of Alaska were investigated using high-resolution numerical simulations with four nested domains (Dx 5 9, 3, 1, and 0.2 km). In domain 3, large-amplitude mountain waves were generated by strong cross-barrier flow associated with an intense cyclone. The positive vertical wavenumber squared and the absence of a critical level supported vertical propagation of these waves. However, they broke near the tropopause and within the negative vertical wind shear, with an abrupt change in their vertical wavelength. Shear and static instabilities were induced by mountain-wave breaking, and subsequent turbulence was identified by small Richardson number and large turbulent kinetic energy (TKE). In domain 4, small-scale waves were partially resolved within the turbulent region, resulting in the generation of both subgrid- and resolved-scale TKEs. A TKE budget analysis indicated that the TKE was mainly driven by wind shear, and turbulence was not fully dissipated to smaller scales and advected far downstream by the mean flow. Total TKE and eddy dissipation rates calculated using simulation results and aircraft observation data were comparable in domain 4. Ultimately, the findings of this study will contribute to improving operational MWT forecasts over Alaska by informing the future development and applications of subkilometer-scale numerical weather prediction models.