Observations and numerical simulations of inertia-gravity waves and shearing instabilities in the vicinity of a jet stream

We present a case study of inertia-gravity wave generation by an intense jet stream / upper-level frontal system that was observed on 18 February 2001, north of Hawaii. Using the observations from the NOAA Gulfstream-IV research aircraft, and high resolution numerical simulations, we examine the characteristics and dynamics of inertia-gravity waves generated by the jet / front system. Aircraft in-situ observations and 17 closely spaced dropsondes elucidate the detailed mesoscale structure of this system, including its associated inertia-gravity waves and clear-air turbulence. Results from a multiply nested numerical model show inertia-gravity wave development above the developing jet coinciding with upper-level frontogenesis. The modeled inertia-gravity waves have horizontal wavelengths of about 150 km and vertical wavelengths of about 2.5 km, and are consistent with estimates of gravity wave characteristics inferred from a hodograph analysis of the dropsonde data. As these inertia-gravity waves propagate through the highly sheared flow above the jet stream, they perturb the background wind shear and stability, and create bands of reduced and increased Richardson numbers. These bands of reduced Richardson numbers are regions of likely Kelvin-Helmholtz instability and a possible source of the clear-air turbulence that was observed.