Numerical simulations of a THORPEX clear-air-turbulence event

As a part of the 2003 Pacific THORPEX Operational Science Test (TOST), several research flights were conducted by the NASA ER-2 and the NOAA Gulfstream IV (G-4). During February 22 and 23, Clear Air Turbulence (CAT) was detected by the G-4 at an altitude of about 12.5 km. The flight-track time series indicated a range of vertical velocities between - 5 and + 5 m/sec for a duration of about 20 minutes. The flight path was in the region of the northerly jet, to the west of an upper-troposphere trough. The atmospheric dynamics associated with this CAT encounter are investigated using a coupled multi-nested coarse resolution (81,27,9 km) COMAPS model to drive the higher resolution Clark-Hall model down to 250 m resolution. This allows investigation of the dynamical linkages between the larger resolvable scale motions and the small scale turbulence motions that effect aircraft. In this case the large scale showed the presence of a very large amplitude waves with very large potential vorticity gradients. In this environment gravity waves or inertial-gravity waves in the vicinity of the jet were also present and contributed to the observed turbulence.