A numerical simulation of airflow over a mountain was used to characterize the nature of discrete gust patterns for a case of strong stratospheric turbulence documented by the High Altitude Clear-Air Turbulence Project U-2 airplane. The model was used with grid resolution of 1.75 km and 0.25 km, for the horizontal and vertical grid spacing respectively, to simulate the 2-dimensional air flow over the mountain range and the development of wave and turbulent eddy motions in the lower stratosphere. At the onset of turbulence a smaller domain encompassing the turbulence patch encountered by the airplane was selected for more detailed characterization. To focus on the character of gusts produced by wave instability within this subdomain, grid resolutions of 0.30 and 0.25 km, in the horizontal and vertical respectively, were used for further simulation. This resolution describes the scale of transient behavior that would be experienced in a few seconds for subsonic aircraft and in one second, or less, for supersonic cruise aircraft.
Measurements from the airplane taken at an altitude of approximately 17 km show that the mountain wave consisted of a component near 40 km wavelength, some perturbations in the 5 - 8 km wavelength range and predominant gusts at scales near 1 km or less. Closely following the time of turbulence onset in the simulation, both the 1.7 and 0.3 km horizontal grid resolution simulations produced the 5 - 8 km perturbations. In addition the 0.3 km horizontal grid simulation produced transient gusts with scales near 1 km or less. Output of these inviscid simulations were stable long after the onset of turbulence. For comparison, viscous simulations using a module to parameterize sub-grid turbulence reproduced the major 40 km gravity wave component, properly located the turbulence region and statistically quantified the turbulence intensity. However the sub-grid turbulence smoothed the gust perturbations at the small scales of interest to airplane design and clear air turbulence (CAT) forecasting.
The 8th Conference on Aviation, Range, and Aerospace Meteorology