Preprints
https://doi.org/10.5194/angeo-2021-6
https://doi.org/10.5194/angeo-2021-6

  29 Jan 2021

29 Jan 2021

Review status: a revised version of this preprint was accepted for the journal ANGEO and is expected to appear here in due course.

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

Matthew J. Griffith1, Shaun M. Dempsey2, David R. Jackson3, Tracy Moffat-Griffin4, and Nicholas J. Mitchell2,4 Matthew J. Griffith et al.
  • 1Department of Mathematical Sciences, University of Bath, Claverton Down, Bath, BA2 7AY, UK
  • 2Department of Electronic & Electrical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom
  • 3Met Office, Fitzroy Rd, Exeter, EX1 3PB, United Kingdom
  • 4British Antarctic Survey, High Cross, Madingley Rd, Cambridge, CB3 0ET, United Kingdom

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.

Matthew J. Griffith et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on angeo-2021-6', Anonymous Referee #1, 03 Mar 2021
    • AC1: 'Reply to Reviewers Comments', Matthew Griffith, 23 Mar 2021
  • RC2: 'Comment on angeo-2021-6', Anonymous Referee #2, 13 Mar 2021
    • AC2: 'Reply to Reviewers Comments', Matthew Griffith, 23 Mar 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on angeo-2021-6', Anonymous Referee #1, 03 Mar 2021
    • AC1: 'Reply to Reviewers Comments', Matthew Griffith, 23 Mar 2021
  • RC2: 'Comment on angeo-2021-6', Anonymous Referee #2, 13 Mar 2021
    • AC2: 'Reply to Reviewers Comments', Matthew Griffith, 23 Mar 2021

Matthew J. Griffith et al.

Matthew J. Griffith et al.

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Short summary
There is great scientific interest in extending atmospheric models upwards to include the upper atmosphere. The Met Office’s Unified Model has recently been successfully extended to include this region. Atmospheric tides are an important driver of atmospheric motion at these greater heights. This paper provides a first comparison of winds and tides produced by the new extended model with meteor radar observations; comparing key tidal properties and discussing their similarities and differences.