HORIZONTAL WIND FLUCTUATIONS IN THE STRATOSPHERE DURING LARGE-SCALE CYCLOGENESIS

The meteorological measurement system (MMS) on the U-2 aircraft measured pressure, temperature, and the horizontal wind during a cyclogenesis event over western United States on April 20, 1984. The aircraft flew above the jet core and traversed in a vertical plane normal to the jet stream axis at four altitudes, making measurements on both the cyclonic and anticyclonic sides of the jet. Horizontal wind data were processed by digital filters to permit analyses of large-scale gradients and of small-scale fluctuations. The mean horizontal wind in the stratosphere decreases monotonically with altitude. Superimposed on the mean stratospheric wind is a perturbation wind vector, which is an elliptically polarized wave with an amplitude of 4 to 10 m s-1 and a vertical wavelength of 2 to 3 km. The perturbation wind vector rotates anticyclonically (clockwise) with altitude and produces alternating advection in the plane of the aircraft flight path. This differential advection folds surfaces of constant tracer mixing ratio and contributes to the observed tracer laminar structures and inferred cross-jet transport.