Lidar studies of the nighttime sodium layer over Urbana, Illinois: 2. Gravity waves

Abstract

On 34 nights between December 1980 and May 1986, 171 monochromatic gravity waves were observed in the mesospheric Na layer at Urbana, Illinois, using a lidar system. The characteristics of the waves are compared with radar measurements of gravity wave activity and theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (λz) range from 2 km to 17 km, the observed periods (Tob) range from 25 min to 800 min and the horizontal wavelengths (λx) range from 20 km to almost 3000 km. The most apparent seasonal characteristic of the waves is the absence in summer of periods greater than 200 min and horizontal wavelengths greater than 400 km. The wave kinetic energy distributions are approximately proportional to kz−3, kx−15/14 and ƒ−5/3. Because the average amplitude growth length for the data is only 19 km and linear saturation theory predicts a kz−2 dependence for the kinetic energy distribution of saturated waves, the observed waves are not strongly damped and appear to be influenced more by dissipation rather than by saturation effects. Both λz and λx show a strong tendency to increase with Tob. The measured power law relationships are approximately λz ∝ Tob5/9 and λx ∝ Tob14/9. The wave induced mean flow acceleration ranges from 0 to −200 m s−1 day−1 and the average value is −27.2 m s−1 day−1. The effective viscosity limiting wave amplitudes ranges from 0 to 140 m² s−1 and the average value is 37 m² s−1.