Turbulence Generation Mechanisms of a Mountain Wave Turbulence Event in the Upper Troposphere and Lower Stratosphere over Alaska, USA

Given the situation that air traffic between East Asia and North America has been increasing, Mountain Wave Turbulence (MWT) in the Upper Troposphere and Lower Stratosphere (UTLS) becomes more important especially over Alaska. On 30 Dec 2012, a commercial aircraft encountered moderate-or-greater (MOG)-level turbulence with observed vertical acceleration of 1.87 g (g = 9.81 m s^-2) at about z = 10 km over the southern mountainous region of Alaska. To understand the turbulence generation mechanisms of this event, we conducted high-resolution numerical simulations using the WRF model with four nested domains with the finest horizontal grid spacing of 200 m. In synoptic-scale flow, a strong southeasterly surface flow, induced by a strong surface cyclone moving from the Bering Sea, generated large-amplitude mountain waves across the southwest-northeast oriented mountains in southern Alaska. Also, a southerly jet stream embedded in the eastern part of a well-developed upper-level trough provided a background condition without a mean critical level. However, near the tropopause, the background wind speed (U) decreased and the Brunt-Vӓisӓlӓ frequency (N²) increased rapidly along the height, causing incident waves to break down and generate local turbulence in the UTLS.

In domain 3 (Δx = 1 km), large-amplitude mountain waves with λ_x = 20 km were generated and propagated vertically. Large perturbations of the projected U and potential temperature induced shear and static instabilities, and then produced a wave-induced critical level (U < 0) in the UTLS. Finally, the amplitudes decreased above the tropopause, implying the wave breaking near the tropopause. Therefore, nonzero subgrid-scale turbulent kinetic energy (TKE) was generated at the altitude of the MWT encounter. In domain 4 (Δx = 200 m), turbulence with large perturbations of wind velocities was resolved partially and resolved-scale TKE was produced. Turbulence then propagated downstream due to the nonlinear effect of mountain waves and TKE advection. Also, it was found that the shear production played the dominant role in generating subgrid- and resolved-scale TKE.

We conducted sensitivity tests of the planetary boundary layer parameterization schemes in the WRF model that take into account subgrid-scale vertical mixing in free atmosphere, which plays an important role in the intensity and distribution of UTLS turbulence in this case. Subgrid-scale TKE was much higher than resolved-scale TKE in the MYNN than in the MYJ scheme in all domains due to the dissipative nature of the MYNN scheme. Finally, we compared three-dimensional wind velocities, TKE, and eddy dissipation rate (EDR) calculated using the aircraft observation data and the simulation results. From the comparison, it was found that observed TKE and EDR could not be explained fully by either subgrid- or resolved-scale TKE and EDR, so both should be considered to analyze the intensity of turbulence.

Key Words: Mountain wave turbulence, WRF model, Alaska

Acknowledgment: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2019R1I1A2A01060035). Also, this work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-00310.