Geographical distributions of mesospheric gravity wave activity before and after major sudden stratospheric warmings observed by Aura/MLS

Abstract

<p><strong>Abstract.</strong> Observations of the global distribution of mesospheric gravity wave activity are rare. To our knowledge there exist only a few articles showing global maps of gravity wave potential energy in the mesosphere derived from observations of the instrument SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) on NASA's satellite TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics). In the present study, we find that the geopotential height (GPH) measurements of the instrument MLS (Microwave Limb Sounder) on NASA's satellite Aura are sensitive to mesospheric gravity waves with horizontal wavelengths between 200 and 1500 km. We apply a data analysis which evaluates the standard deviation of horizontal GPH perturbations at a fixed pressure level and along the orbit of the sounding volume of Aura/MLS. The orographic waves from the Southern Andes in August serve as a test signal for the horizontal resolution and sensitivity of the method. We find enhanced gravity wave activity in the lower, middle, and upper mesosphere in a small region over the Southern Andes. It seems that the horizontal resolution of the mesospheric gravity wave maps provided by Aura/MLS is higher than those of TIMED/SABER. We apply the method to estimate the global distributions of mesospheric gravity wave activity before and after the major sudden stratospheric warmings (SSWs) of January 21, 2006, January 24, 2009, and January 6, 2013 using 30 day intervals of Aura/MLS observations of GPH. It seems that the gravity wave activity in the lower mesosphere over the subtropical convection regions of the summer hemisphere are decreased after the SSW of January 21, 2006. The gravity wave activity in the lower and middle mesosphere over middle and high latitudes (40° N to 70° N) of the winter hemisphere is decreased after the SSW of January 24, 2009. The major SSW of January 6, 2013 is preceded by enhanced mesospheric gravity wave activity over Eurasia at high latitudes (40° N to 60° N). This asymmetric gravity wave activity in the lower mesosphere is coincident with a long-lasting stay of the stratospheric polar vortex mainly in the Eurasian longitude sector before the SSW of January 6, 2013. In case of the SSW 2009 and SSW 2013, the gravity wave activity is enhanced at latitudes poleward of 70° N in the lower and middle mesosphere after the SSWs.</p>

Seasonal evolution of winds, atmospheric tides, and Reynolds stress components in the Southern Hemisphere mesosphere–lower thermosphere in 2019

“…For the case of momentum fluxes, Hocking (2005) proposed a method using typical meteor radars, which was later echoed and reformulated as correlations by Vierinen et al (2019). In this work, we present a brief derivation of the Reynolds stress tensor and show how the different tensor elements are estimated from the meteor radar observations.…”

“…Considering that these radial velocity fluctuations are mostly driven by GW, the Reynolds stresses can be computed by minimizing the following quantity (Hocking, 2005):…”

“…In particular, tides have large amplitudes in the MLT, causing large vertical and temporal shears within a time and altitude bin. Noting that Hocking (2005) and Placke et al (2011b) suggested the use of at least 30 meteors for a successful momentum flux fit, which is often achieved by temporal averaging, the importance of the temporal shear becomes evident.…”

“…Furthermore, at alti-tudes above 94 km, MF radars tend to underestimate the wind speeds, which might lead to some systematic bias in the derived momentum flux (Wilhelm et al, 2017). Hocking (2005) presented a method to obtain the Reynolds stress tensor components from meteor radar observations. Based on this, several studies applied the method to optimize the data analysis as it appeared to be challenging to get the technique implemented (Placke et al, 2011a, b;Andrioli et al, 2013).…”

“…For this purpose, we complement our meteor radar observations with data from the Microwave Limb Sounder (MLS) on board the AURA satellite (Livesey et al, 2006;Schwartz et al, 2008). Furthermore, we utilize a recently developed retrieval algorithm, which builds on the initial momentum flux analysis formulation reported by Hocking (2005). In particular, we introduce a generalized approach to obtain wind variances and momentum fluxes from several meteor radars (many of which are standard low power systems) for the year 2019, which evolved into one of the rare minor stratospheric warming events during September (Yamazaki et al, 2020).…”