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.

中文翻译：

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经流星雷达观测验证的中层和低热层扩展统一模型的风和潮汐

中层和低层热层 (MLT) 是一个关键区域，必须在旨在包括低层和中层大气与热层之间耦合的一般环流模型 (GCM) 中准确再现。因此，MLT 的准确表示对于改进气候建模和开发整个大气模型非常重要。这是因为这些高度的大气波特别大，因此它们携带的能量和动量是整个大气层气候现象的重要驱动力，影响陆地和空间天气。扩展统一模型 (ExUM) 是最近开发的气象局统一模型的版本，该模型已扩展为对 MLT 建模。ExUM 在 MLT 中模拟大气风和潮汐的能力目前未知。在这里，我们展示了 ExUM 对风和潮汐的首次研究。我们与 2006 年 80 至 100 年之间的风潮和潮汐流星雷达观测结果进行了比较 公里超过两个雷达站 – Rothera (68 ^∘  S, 68 ^∘  W) 和 Ascension Island (8 ^∘  S, 14 ^∘ W) W)。选择这些位置来研究两种非常不同的潮汐状况 - 赤道状况，其中昼夜 (24 小时) 潮汐占主导地位，以及极地情况，其中半日 (12 小时) 潮汐占主导地位。这项研究的结果表明，ExUM 能够再现大气风和潮汐，这些风和潮汐捕获了流星雷达观测中的许多关键特征，例如纬向和经向风的最大值和最小值、潮汐幅度随着高度的增加而增加，以及潮位随着高度的增加而减少。特别是在赤道地区，背景风、潮汐振幅和潮汐相位的一些基本特征被很好地捕捉到，但在细节上存在显着差异。在极地地区，差异更为明显。 公里。这里的ExUM潮幅大体上与观测值一致，但也大于90公里以上的观测值 在南方的夏天。在这种情况下，潮汐阶段通常可以很好地复制。我们提出，极地区域背景风的偏差是由于在 MLT 中缺乏产生向东通量的原位重力波产生的结果。这项研究的结果表明，ExUM 具有很好的模拟 MLT 大气风和潮汐的自然能力，但该地区的模型物理还有改进的空间。这突出表明需要修改该地区模型中使用的物理参数化方案——例如非地形光谱重力波方案——以改进极地环流等方面。为此，我们对可以实施的更改提出具体建议，以提高 MLT 中 ExUM 的准确性。