Observations of Low-Frequency Inertia-Gravity Waves in the Lower Stratosphere over Arecibo

Abstract

Observations of the horizontal wind in the subtropical upper troposphere and lower stratosphere were made with the 430-MHz radar located at Arecibo, Puerto Rico (18.4°N), in May 1982 and April 1983. Both sets of observations displayed a slowly varying, anticyclonically rotating, persistent structure in the wind field just above the tropopause, similar to what would be expected if the oscillations are associated with quasi-inertia period waves. A spectral analysis of the May 1982 data revealed that a wave with a period in the earth-fixed earth-fixed reference frame close to 26 h was present. Our calculations show that the intrinsic wave periods varied from ∼2.5 times the inertial frequency to zero at a critical level near 18-km altitude. The changes in phase between perturbation velocities perpendicular and parallel to the propagation direction were systematic below the height where the intrinsic frequency was equal to the inertial frequency but showed a complicated behavior between that height and the critical level. Hines has recently proposed an explanation of similar observations as being due to orographic waves with intrinsic periods that are short compared to the inertial period. Our analysis indicates discrepancies with his simple explanation, although the data show that the waves may have an orographic source but with the earth-fixed period near 24 h determined by the very regular diurnal fluctuation in the surface winds over Puerto Rico. Thus, the observed waves may be orographically generated but with intrinsic frequencies, determined by the 24-h earth-fixed period, small enough to require the use of the dispersion relations for inertia-gravity waves with Coriolis effect included in order to explain the characteristics of the observations.