Radar Observations of Gravity Waves over Jicamarca, Peru during the CADRE Campaign

Dennis M. Riggin and David C. Fritts

Colorado Research Associates/NWRA, Boulder

Clinton D. Fawcett and Erhan Kudeki

Department of Electrical and Computer Engineering, University of Illinois, Urbana

Matthew H. Hitchman

Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison

We present results obtained with the mesosphere-stratosphere-troposphere (MST) radar at Jicamarca, Peru, from three 12-day experiments in January 1993, March 1994, and August 1994. Horizontal and vertical velocities were measured over ranges spanning the lower part of the stratosphere and most of the mesosphere. In the stratosphere, the fluctuating part of the wind field was found to be dominated by inertia-gravity waves. Sinusoids of different period were fit to the velocity time series using a least squares procedure. The dominant periods of the inertia-gravity wave motions were found to be 1.5 days for the January 1993 experiment and 2.1 days for the August 1994 experiment. For the January 1993 experiment, the amplitudes and phases of the inertia-gravity wave oscillations were extracted for the vertical as well as the horizontal components. The vertical amplitude of the 1.5-day period wave was small (<0.1 meters per second), but measureable with the Jicamarca radar. The intrinsic periods of the inertia-gravity waves were inferred from the fits using two different methods. The first method predicted intrinsic period using the orbital ellipses traced out in time by fits to the horizontal winds. The second method used information taken from the vertical as well as the horizontal fits. The values of intrinsic period made using the second method were found to have much less scatter than the values inferred solely fromthe orbital ellipses. The momentum flux estimates in both the stratosphere and mesosphere were found to depend significantly on the exact methodology used. A technique was adopted wereby estimates were formed only when radial velocities were measured simultaneously on both beams of a coplanar beam pair. Most of the total wave stress was usually contributed by waves at periods greater than or equal to four hours in the stratosphere and greater than or equal to one hour in the mesosphere. In the stratosphere, localized layers of enhanced momentum flux were sometimes observed. The obvious anticorrelation between the shear of the mean wind and the momentum flux in these layers suggests taht they were caused by in situ generation waves by the Kelvin-Helmholtz instability, rather than gravity waves propagating from lower levels. At short periods, momentum flux spectra in the stratosphere and mesosphere showed numerous positive and negative peaks. A correlation analysis revealed that the high-frequency peaks were associated with discrete wave packets. The short-scale waves associated with these packets were fairly isotropic in their direction of propagation, and due to cancellation they contributed little net momentum.

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