Analysis of gravity waves in the tropical middle atmosphere over La Reunion Island (21°S, 55°E) with lidar using wavelet techniques

Abstract

International audienceThe capabilities of the continuous wavelet transform (CWT) and the multiresolution analysis (MRA) are presented in this work to measure vertical gravity wave characteristics. Wave properties are extracted from the first data set of Rayleigh lidar obtained between heights of 30 km and 60 km over La Reunion Island (21°S, 55°E) during the Austral winter in 1994 under subtropical conditions. The altitude-wavelength representations deduced from these methods provide information on the time and spatial evolution of the wave parameters of the observed dominant modes in vertical profiles such as the vertical wavelengths, the vertical phase speeds, the amplitudes of temperature perturbations and the distribution of wave energy. The spectra derived from measurements show the presence of localized quasi-monochromatic structures with vertical wavelengths <10 km. Three methods based on the wavelet techniques show evidence of a downward phase progression. A first climatology of the dominant modes observed during the Austral winter period reveals a dominant night activity of 2 or 3 quasi-monochromatic structures with vertical wavelengths between 1-2 km from the stratopause, 3-4 km and 6-10 km observed between heights of 30 km and 60 km. In addition, it reveals a dominant activity of modes with a vertical phase speed of -0.3 m/s and observed periods peaking at 3-4 h and 9 h. The characteristics of averaged vertical wavelengths appear to be similar to those observed during winter in the southern equatorial region and in the Northern Hemisphere at mid-latitudes

Evaluation of methods for gravity wave extraction from middle-atmospheric lidar temperature measurements

“…Using a typical stratospheric value of N = 0.02 s −1 and a Coriolis parameter for midlatitudes of f = 10 −4 s −1 , the intrinsic periodτ = 2π ω ranges between 5 min and 17 h. It is important to note that the lidar only detects the observed period τ which can be Doppler-shifted to larger or smaller values, depending on local wind conditions. Typical vertical wavelengths of gravity waves measured by ground-based instruments vary between 1 and 17 km (see Chane-Ming et al, 2000, their Table 2). The spatial scales combined with the temporal scales define the spectral requirements on the methods of extracting gravity-wave-induced temperature perturbations.…”

“…Another method which can be applied to vertical profiles is spectral filtering (e.g., Chane-Ming et al, 2000). By applying a high-pass filter to individual temperature profiles, temperature perturbations can be retrieved.…”

“…Perturbation profiles obtained through a fit of polynomial functions to the measured profiles are examined by, e.g., Whiteway and Carswell (1995), Duck et al (2001) and Alexander et al (2011). Mzé et al (2014) apply a variance method in order to determine perturbation profiles, while Chane-Ming et al (2000) use spectral filtering.…”

Gravity-wave effects on tracer gases and stratospheric aerosol concentrations during the 2013 ChArMEx campaign

“…Compact support orthogonal bases with high orders of smoothness, such as the eighth-order Daubechies wavelet, ensures energy conservation and a very good localisation both in space and frequency for the analysis of GWs in the lower and middle atmosphere. Applications of the pyramid algorithm and the eighth-order Daubechies wavelet coefficients are described in Chane Ming et al (1999Ming et al ( , 2000a. In this way, fine structures with λ v < 0.32 km are efficiently removed from resampled profiles.…”

“…Cases, for which aerosol layers are visualised in the UTLS, are analysed by the continuous wavelet transform (CWT) to detect GW signatures (Chane Ming et al, 2000a). The continuous wavelet transform with Morlet complex-valued mother wavelet is applied to perturbation profiles of temperature, horizontal wind speed, ozone and concentration of aerosols to produce altitude-wavelength representations of the modulus of CWT coefficients, also called scalograms.…”