Statistical characterization of high-to-medium frequency mesoscale gravity waves by lidar-measured vertical winds and temperatures in the MLT

Abstract

We present the first statistical study of gravity waves with periods of 0.3–2.5h that are persistent and dominant in the vertical winds measured with the University of Colorado STAR Na Doppler lidar in Boulder, CO (40.1°N, 105.2°W). The probability density functions of the wave amplitudes in temperature and vertical wind, ratios of these two amplitudes, phase differences between them, and vertical wavelengths are derived directly from the observations. The intrinsic period and horizontal wavelength of each wave are inferred from its vertical wavelength, amplitude ratio, and a designated eddy viscosity by applying the gravity wave polarization and dispersion relations. The amplitude ratios are positively correlated with the ground-based periods with a coefficient of ~0.76. The phase differences between the vertical winds and temperatures (φW−φT) follow a Gaussian distribution with 84.2±26.7°, which has a much larger standard deviation than that predicted for non-dissipative waves (~3.3°). The deviations of the observed phase differences from their predicted values for non-dissipative waves may indicate wave dissipation. The shorter-vertical-wavelength waves tend to have larger phase difference deviations, implying that the dissipative effects are more significant for shorter waves. The majority of these waves have the vertical wavelengths ranging from 5 to 40km with a mean and standard deviation of ~18.6 and 7.2km, respectively. For waves with similar periods, multiple peaks in the vertical wavelengths are identified frequently and the ones peaking in the vertical wind are statistically longer than those peaking in the temperature. The horizontal wavelengths range mostly from 50 to 500km with a mean and median of ~180 and 125km, respectively. Therefore, these waves are mesoscale waves with high-to-medium frequencies. Since they have recently become resolvable in high-resolution general circulation models (GCMs), this statistical study provides an important and timely reference for them.