Winds and Tides of the Extended Unified Model in the Mesosphere and Lower Thermosphere Validated with Meteor Radar Observations

Abstract

Abstract. The Mesosphere and Lower Thermosphere (MLT) is a critical region that must be accurately reproduced in General Circulation Models (GCMs) that aim to include the coupling between the lower & middle atmosphere and the thermosphere. An accurate representation of the MLT is important for improved climate modelling and the development of a whole atmosphere model. This is because the atmospheric waves at these heights are particularly large, and so the energy and momentum they carry is an important driver of climatological phenomena through the whole atmosphere, affecting terrestrial and space weather. The Extended Unified Model (ExUM) is the recently developed version of the Met Office's Unified Model which has been extended to model the MLT. The capability of the ExUM to model atmospheric winds and tides in the MLT is currently unknown. Here, we present the first study of winds & tides from the ExUM. We make a comparison against meteor radar observations of winds and tides from 2006 between 80 and 100 km over two radar stations – Rothera (68° S, 68° W) and Ascension Island (8° S, 14° W). These locations are chosen to study tides in two very different tidal regimes – the equatorial regime, where the diurnal (24 hour) tide dominates, and the polar regime, where the semi-diurnal (12 hour) tide dominates. The results of this study illustrate that the ExUM is capable of reproducing atmospheric winds and tides that capture many of the key characteristics seen in meteor radar observations, such as zonal & meridional wind maxima and minima, the increase in tidal amplitude with increasing height, and the decrease in tidal phase with increasing height. In particular, in the equatorial regime some essential characteristics of the background winds, tidal amplitudes and tidal phases are well captured, but with significant differences in detail. In the polar regime, the difference is more pronounced. The ExUM zonal background winds in austral winter are primarily eastward rather than westward, and in austral summer are larger than observed above 90 km. The ExUM tidal amplitudes here are in general consistent with observed values, but are also larger than observed values above 90 km in austral summer. The tidal phases are generally well replicated in this regime. We propose that the bias in background winds in the polar regime is a consequence of the lack of in-situ gravity wave generation to generate eastward fluxes in the MLT. The results of this study indicate that the ExUM has a good natural capability for modelling atmospheric winds and tides in the MLT, but that there is room for improvement in the model physics in this region. This highlights the need for modifications to the physical parameterization schemes used in the model in this region – such as the non-orographic spectral gravity wave scheme – to improve aspects such as polar circulation. To this end, we make specific recommendations of changes that can be implemented to improve the accuracy of the ExUM in the MLT.

Evaluation of Polar Winter Mesopause Wind in WACCMX+DART

“…Unfortunately, there is a limitation to using WACCM as a tool to study these processes in the polar winter mesopause region because it has a well‐known easterly (westward) wind bias in the polar winter upper mesosphere (e.g., 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; Zhang et al., 2021). This bias is not unique to WACCM, and other comprehensive high‐top general circulation models with parameterized GWs show the same deficiencies in simulating the observed zonal wind structure (e.g., Wilhelm et al., 2019) in the winter upper mesosphere (e.g., Griffith et al., 2021; McCormack et al., 2017, 2021; McLandress et al., 2006; Pedatella, Fuller‐Rowell, et al., 2014; Schmidt et al., 2006). However, not all high‐top models exhibit the easterly wind bias to the same degree.…”

Analysis of migrating and non-migrating tides of the Extended Unified Model in the mesosphere and lower thermosphere

“…For instance, there have been extensive groundbased observations made of tides in the MLT, in many cases made by meteor or MF (medium-frequency) radars (e.g. Murphy et al, 2007;Davis et al, 2013;Hibbins et al, 2019;Liu et al, 2020;Pancheva et al, 2021;Dempsey et al, 2021;Griffith et al, 2021). These radar observations usually offer excellent height and time resolution and are well suited to studies of tidal variability on timescales ranging from dayto-day to decadal -but observations made from a single site yield only the amplitudes, phases, and vertical wavelengths of the superposition of migrating and non-migrating tides and cannot resolve the observed tidal oscillations into individual components.…”

“…A summary of several of the recent key non-mechanistic high-top GCMs is given in Griffith et al (2021). Here, we simply note that a number of such models exist including the following: (i) the Whole Atmosphere Model (WAM; Akmaev et al, 2008;Fuller-Rowell et al, 2008), (ii) the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X; Liu et al, 2010Liu et al, , 2018, (iii) the extended Canadian Middle Atmosphere Model (eCMAM; Beagley et al, 2000), (iv) the Groundto-topside model of the Atmosphere and Ionosphere for Aeronomy (GAIA; Fujiwara and Miyoshi, 2010;Jin et al, 2012, and references therein), (v) the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA; Schmidt et al, 2006;Meraner and Schmidt, 2016), (vi) the upperatmosphere extension of ICON (Borchert et al, 2019), (vii) the Entire Atmosphere GLobal model (EAGLE; Klimenko et al, 2019), (viii) the HIgh Altitude Mechanistic general Circulation Model (HIAMCM; Becker and Vadas, 2020), (ix) the Coupled Middle Atmosphere-Thermosphere-2 (CMAT-2; , (x) the University of Leipzig Middle and Upper Atmosphere Model (MUAM; Pogoreltsev, 2007;Pogoreltsev et al, 2007;Suvorova and Pogoreltsev, 2011), and (xi) the whole-atmosphere Kyushu GCM (Miyoshi and Fujiwara, 2008;Miyoshi and Yigit, 2019).…”

“…In the context of these various high-top models, the new Extended Unified Model (ExUM; Griffith et al, 2020Griffith et al, , 2021 extends the standard UM (Unified Model; Walters et al, 2019) to the lower thermosphere. The model itself and its development for the lower thermosphere is described further in Sect.…”

“…2.1, but we highlight here that the ExUM does not make the hydrostatic assumption and uses the deep-atmosphere equations of motion, making it a good candidate for modelling atmospheric tides. Griffith et al (2021) investigated the ability of the ExUM to reproduce the observed winds and diurnal and semidiurnal tides of the MLT and compared them with meteor-radar observations at characteristic equatorial and polar locations (Ascension Island (8 • S, 14 • W) and Rothera (68 • S, 68 • W), respectively). The study demonstrated that, although there are biases in the model tidal fields, they nevertheless capture many essential features of the observed tides.…”