Radar observations of winds, waves and tides in the mesosphere and lower thermosphere over South Georgia island (54°S, 36°W) and comparison to WACCM simulations

Abstract

Abstract. The mesosphere and lower thermosphere (MLT) is a dynamic layer of the earth’s atmosphere. This region marks the interface at which neutral atmosphere dynamics begin to influence the ionosphere and space weather. However, our understanding of this region and our ability to accurately simulate it in global circulation models (GCMs) is limited by a lack of observations, especially in remote locations. To this end, a meteor radar was deployed on the remote mountainous island of South Georgia (54° S, 36° W) in the Southern Ocean from 2016 to 2020. The goal of this study is to use these new measurements to characterise the fundamental dynamics of the MLT above South Georgia including large-scale winds, solar tides, planetary waves (PWs) and mesoscale gravity waves (GWs). We first present an improved method for time-height localisation of radar wind measurements and characterise the large-scale MLT winds. We then explore the amplitudes and phases of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) solar tides at this latitude. We also explore PW activity and find very large amplitudes up to 30 ms−1 for the quasi-2 day wave in summer and show that the dominant modes of the quasi-5, 10 and 16 day waves are westward W1 and W2. We investigate wind variance due to GWs in the MLT and use a new method to show an east-west tendency of GW variance of up to 20 % during summer and a weaker north-south tendency of 0–5 % during winter. This is contrary to the expected tendency of GW directions in the winter stratosphere below, which is a strong suggestion of secondary GW (2GW) observations in the MLT. Lastly, comparison of radar winds to a climatological Whole Atmosphere Community Climate Model (WACCM) simulation reveals a simulated summertime mesopause and zonal wind shear that occur at altitudes around 10 km lower than observed, and southward winds during winter above 90 km altitude in the model that are not seen in observations. Further, wintertime zonal winds above 85 km altitude are eastward in radar observations but in WACCM they are found to weaken and reverse to westward. Recent studies have linked this discrepancy to the impact of 2GWs on the residual circulation which are not included in WACCM. These measurements therefore provide vital constraints that can guide the development of GCMs as they extend upwards into this important region of the atmosphere.

Multi‐Step Vertical Coupling During the January 2017 Sudden Stratospheric Warming

“…When these secondary GWs break in the winter mesopause region, they exert an eastward drag. According to Becker and Vadas (2018), this effect can explain the observed predominant eastward wind direction in the wintertime mesopause region at middle to high latitudes during medium-to-strong polar vortex periods, as is found in observations (de Wit et al, 2017;Hindley et al, 2022;Hoffmann et al, 2010;Smith, 2012;Stober, Janches, et al, 2021). In turn, the dissipation of secondary GWs leads to the generation of tertiary GWs that can propagate well into the thermosphere .…”

Improving ionospheric predictability requires accurate simulation of the mesospheric polar vortex

“…It is now known that the polar vortex broadens with increasing altitude into the upper mesosphere. High-top models such as the Whole Atmosphere Community Climate Model (WACCM) properly simulate the mesospheric polar vortex (MPV) up to middle mesospheric altitudes, but fail to reproduce observations above ~80 km (Harvey et al, 2019;Hindley et al, 2022) especially when the vortex is strong (Harvey et al, 2022). Descent in the polar winter mesosphere, depicted by the arrow in Figure 1 marked "circulation", is part of a global wave-driven pole-to-pole circulation characterized by ascent over the summer pole, cross-equatorial flow from the summer to the winter hemisphere, and descent in the winter high latitudes.…”

“…Unfortunately, representation of the polar vortex in the upper mesosphere is generally not accurate in state-of-the-art global models. In fact, in many models the zonal winds blow in the wrong direction in the polar winter upper mesosphere (Eswaraiah et al, 2016;Harvey et al, 2019;Hindley et al, 2022;Lieberman et al, 2015;Liu, 2016;Marsh et al, 2013;Noble et al, 2022;Rüfenacht et al, 2018;Smith, 2012;Yuan et al, 2008;Griffith et al, 2021;McLandress et al, 2006;McCormack et al, 2017;2021;Pedatella et al, 2014;Schmidt et al, 2006;Stober et al, 2021) compared to observations (e.g., Wilhelm et al, 2019) or meteorological analyses that use data assimilation (Eckermann et al, 2018, Stober et al, 2020. Important impacts of this easterly (westward) wind bias are (1) a reduction in the vertical extent of the MPV (Harvey et al, 2019), (2) an increase in the vertical wind shear, which alters the spectrum of GWs and PWs (e.g., Chandran et al, 2013;France et al, 2015), (3) persistent negative meridional potential vorticity gradients at mid-to-high latitudes, which can generate PWs via baroclinic or barotropic instability (e.g., Charney and Stern, 1962), and (4) a reduction in the amplitude of the migrating wavenumber 2 semidiurnal tide (SW2) in Arctic winter (Zhang et al, 2021).…”

Interannual Variability of the 12‐hr Tide in the Mesosphere and Lower Thermosphere in 15 Years of Meteor‐Radar Observations Over Rothera (68°S, 68°W)

“…The full-width-half-maxima for these Gaussian weightings are 2 hr in time and 3 km in height. The center of the Gaussian progresses across the data in 1-hr time and 1-km height steps, giving hourly winds between 75 and 105 km (Hindley et al, 2021).…”