Winds and tides of the Extended Unified Model in the mesosphere and lower thermosphere validated with meteor radar observations
Authors: Griffith Matthew J. , Dempsey Shaun M. , Jackson David R. , Moffat-Griffin Tracy , Mitchell Nicholas J. 
Published: June 2021

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 and middle atmosphere and the thermosphere. An accurate representation of the MLT is thus 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 and 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 h) tide dominates, and the
polar regime, where the semi-diurnal (12 h) 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 and 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 westward rather than eastward, and in austral summer they are larger than observed above 90 km. The ExUM tidal amplitudes here are in
general consistent with observed values, but they 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.