Removing spurious inertial instability signals from gravity wave temperature perturbations using spectral filtering methods

Abstract

Abstract. Gravity waves are important drivers of dynamic processes in particular in the middle atmosphere. To analyse atmospheric data for gravity wave signals, it is essential to separate gravity wave perturbations from atmospheric variability due to other dynamic processes. Common methods to separate small-scale gravity wave signals from a large-scale background are separation methods depending on filters in either the horizontal or vertical wavelength domain. However, gravity waves are not the only process that could lead to small-scale perturbations in the atmosphere. Recently, concerns have been raised that vertical wavelength filtering can lead to misinterpretation of other wave-like perturbations, such as inertial instability effects, as gravity wave perturbations. In this paper we assess the ability of different spectral background removal approaches to separate gravity waves and inertial instabilities using artificial inertial instability perturbations, global model data and satellite observations. We investigate a horizontal background removal (which applies a zonal wavenumber filter with additional smoothing of the spectral components in meridional and vertical direction), a sophisticated filter based on 2D time–longitude spectral analysis (see Ern et al., 2011) and a vertical wavelength Butterworth filter. Critical thresholds for the vertical wavelength and zonal wavenumber are analysed. Vertical filtering has to cut deep into the gravity wave spectrum in order to remove inertial instability remnants from the perturbations (down to 6 km cutoff wavelength). Horizontal filtering, however, removes inertial instability remnants in global model data at wavenumbers far lower than the typical gravity wave scales for the case we investigated. Specifically, a cutoff zonal wavenumber of 6 in the stratosphere is sufficient to eliminate inertial instability structures. Furthermore, we show that for infrared limb-sounding satellite profiles it is possible as well to effectively separate perturbations of inertial instabilities from those of gravity waves using a cutoff zonal wavenumber of 6. We generalize the findings of our case study by examining a 1-year time series of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) data.

Propagation Paths and Source Distributions of Resolved Gravity Waves in ECMWF-IFS analysis fields around the Southern Polar Night Jet

“…The small- circulation model data (Fetzer and Gille, 1994;Preusse et al, 2002;Baumgarten et al, 2017;Ehard et al, 2015;Ern et al, 2018). In particular, a horizontal scale separation is an effective mean to isolate gravity wave fluctuations, which has been shown recently by Strube et al (2020).…”

“…A cut-off wavenumber of 18 was applied in the low-pass filter to characterise the background. A zonal wavenumber 6 to 8 should in principle have sufficed to describe the Rossby waves for the targeted analysis altitude of 25 km and hence to isolate the gravity wave perturbations (Strube et al, 2020). However, we use the same scale separation to define the background atmosphere of temperature, winds and pressure from 0 to 45 km altitude for the ray tracing described below.…”

Atmospheric Gravity Waves in Aeolus Wind Lidar Observations

“…(2008), Wright, Hindley, Moss, and Mitchell (2016), Strube et al. (2020), and Krisch et al. (2020), among others.…”

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

“…For our study, it is important that this 2D spectral approach is capable of effictively removing all global-scale waves that are important in the tropics, such as inertial instabilities in the tropical stratosphere and stratopause region (e.g., Rapp et al, 2018;Strube et al, 2020), and different equatorial wave modes in the stratosphere (e.g., Ern et al, 2008) and in the mesosphere and mesopause region (e.g., Garcia et al, 2005;Ern et al, 2009). In particular, Kelvin waves contribute significantly to the temperature variances in the tropics and are difficult to remove by other techniques because they can have very short wave periods, and their vertical wavelengths are in the same range as that of small scale gravity waves.…”