Abstract
Abstract. Large wind shears around the mesopause region play an important role in atmospheric neutral dynamics and ionospheric electrodynamics. Based on previous observations using sounding rockets, lidars, radars, and model simulations, large shears are mainly attributed to gravity waves (GWs) and modulated by tides (Liu, 2017). Based on the dispersion and polarization relations of linear GWs and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature data from 2002 to 2019, a method of deriving GW-perturbed wind shears is proposed. The zonal-mean GW-perturbed shears have peaks (13–17 ms−1 km−1) at around the mesopause region, i.e., at z = 90–100 km at most latitudes and at z = 80–90 km around the cold summer mesopause. This latitude–height pattern is robust over the 18 years and agrees with model simulations. The magnitudes of the GW-perturbed shears exhibit year-to-year variations and agree with the lidar and sounding rocket observations in a climatological sense but are 60 %–70 % of the model results in the zonal-mean sense. The GW-perturbed shears are hemispherically asymmetric and have strong annual oscillation (AO) at around 80 km (above 92 km) at the northern (southern) middle and high latitudes. At middle to high latitudes, the peaks of AO shift from winter to summer and then to winter again with increasing height. However, these GW-perturbed shears may be overestimated because the GW propagation direction cannot be resolved by the method and may be underestimated due to the observational filter, sampling distance, and cutoff criterion of the vertical wavelength of GWs.
Highlights
In the mesosphere and lower thermosphere (MLT), large horizontal winds and their vertical shears have been revealed from more than 500 wind profiles observed by sounding rockets (Larsen, 2002; Larsen and Fesen, 2009) and from ground-based lidar and radar observations (Larsen and Fesen, 2009; Oppenheim et al, 2009, 2014; Yue el al., 2010)
Comparing with the annual oscillation (AO) and semi-annual oscillations (SAO) in the gravity waves (GWs) square temperature amplitude (GWSTA) and absolute momentum flux (GWMF) presented by Chen et al (2019), we find that the AO and SAO of GW-perturbed shears agree with GWSTA and GWMF on the aspects of phase shifts and hemispheric asymmetry (Figs. 2 and 3 of Chen et al, 2019)
To fully explore the differences of magnitudes of S derived under the assumptions of f/ωj = 0 and f/ωj ≈ 0, we show the GW-perturbed shears for f/ωj ≈ 0 in the same manner as those in Figs. 4–9 such that we can judge whether the assumptions need to be made
Summary
In the mesosphere and lower thermosphere (MLT), large horizontal winds and their vertical shears (or more precisely vertical wind shears, shears for short) have been revealed from more than 500 wind profiles observed by sounding rockets (Larsen, 2002; Larsen and Fesen, 2009) and from ground-based lidar and radar observations (Larsen and Fesen, 2009; Oppenheim et al, 2009, 2014; Yue el al., 2010). The large winds and shears are associated with tides and GWs and play an important role in forming the middle-latitude sporadic E layers and driving the equatorial electrojet (Mathews, 1998; Hysell et al, 2002; Arras et al, 2009; Haldoupis, 2012; Shinagawa et al, 2017; Arras and Wickert, 2018; Jacobi et al, 2019; Yu et al, 2019), in controlling atmospheric stabilities and the propagation of GWs, and in transporting and mixing tracers locally and/or globally (Fritts et al, 2004; Liu 2007, 2017; Yue et al, 2013; Stevens et al, 2014).
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