Observations of horizontal and vertical wave number spectra of gravity wave motions in the stratosphere and mesosphere over the mid‐Pacific

Abstract

We present vertical and horizontal wave number spectra of density perturbations in the upper stratosphere (25–40 km) and the upper mesosphere (∼80–105 km) inferred from ∼45 hours of airborne Na/Rayleigh lidar observations in the vicinity of Hawaii. The mean density variance is (0.92%)2 for the upper stratosphere and (6.1%)2 and for the upper mesosphere. The mean vertical shear variance and Richardson's number for the upper mesosphere are (6.1%/km)2 and 0.69, respectively. The observed vertical wave number m and horizontal wave number k spectra exhibit power law dependencies. The mean slopes of the vertical wave number spectra are −2.5 for the upper stratosphere and −3.1 for the upper mesosphere. The mean slope of the horizontal wave number spectra is −1.9 for the upper mesosphere. The mean characteristic vertical and horizontal wavelengths inferred from the spectra are 13.4 and 5810 km, respectively. In all cases the magnitudes of the upper stratosphere m spectra are more than an order of magnitude smaller than those of the upper mesosphere at all observable scales. In the stratosphere the m spectra exhibit significant energy at low wave numbers that are less than the values expected for the characteristic wave numbers. This suggests the presence of gravity wave sources capable of generating significant energy at long vertical scales. The source is believed to be geostrophic adjustment of the jet stream. A large‐amplitude, highly coherent, quasi‐monochromatic, short horizontal scale (λh ∼ 16 km) oscillation in the Na layer was observed on the March 22 flight. The oscillation resembled short horizontal scale structures often observed in OH airglow images and was found to be consistent with a gravity wave interpretation.