Multi-scale Modelling of Mesospheric Metals (4M)

每天大约有50吨星际尘埃进入地球大气层。尘埃颗粒以11至72公里/S的速度与空气分子相撞，导致大部分颗粒闪热、融化和蒸发。这会产生金属原子，然后出现在大约75到110公里的层中。自20世纪70年代以来，利用地基激光雷达技术已经观测到了Na、Fe、Ca和Ca+层。最近，已经可以使用卫星上的光学仪器在全球范围内观测金属层，包括镁和镁+。解释这些大气观测的需要促进了对这些金属及其离子在大气中发生反应的实验室研究，以及随之而来的局部尺度大气模型的发展。中间层金属层是研究高层大气化学和动力学的非常有用的探测器。这是因为这些层的宽度只有几公里，由快速的光化学过程控制，对重力波和潮汐等动力过程以及O、H和O等大气成分的变化非常敏感。当夏季高纬度地区的温度降至150K以下时，夜光云形成约83公里，由于快速吸收冰面上的金属，导致金属层大量枯竭。在高纬度冬季，极涡上空经向环流的汇聚似乎导致金属浓度的显著增加。太阳质子事件也会对金属层造成显著的扰动。最后，控制大气层高度的化学物质在很大程度上是由大气压力驱动的，因此大气层中二氧化碳和甲烷等温室气体增加导致的全球变冷对大气层高度非常敏感。这项提议的目标是生产首个全球四种金属模型--钠、铁、钙和镁。这些金属在中间层中的行为都截然不同。我们将把这些金属的化学成分插入到最先进的大气环流模型--全大气化学气候模型(WACCM)中，该模型是美国国家大气研究中心在过去十年里开发的。这个大气环流模型从地球表面延伸到140公里，包括所有与金属相互作用的中性和电离成分。为了准备这个项目，我们最近在英国一线的国家超级计算服务Hector(高端计算Terascale资源)上安装并运行了WACCM。对金属层进行建模还需要输入每种金属因消融星际尘埃而被注入大气层的速率，作为高度、季节、纬度和一天中时间的函数。这些注入速率将使用我们新的化学消融模型，结合流星输入函数的天文模型来计算。一名项目学生将首次使用ENVISAT上的SCIAMACHY仪器检索铁和铁+观测的全球数据集。这将补充地面激光雷达对铁的测量。WACCM对金属层密度、峰值高度、日变化和季节变化的预测将与观测数据库进行比较。WACCM的中层风速和温度也将与测量结果进行比较。这些比较将使WACCM中间层得到优化，例如通过更高的垂直分辨率和更好的重力波处理。然后，我们将调查太阳周期和气候变化对所有四种金属分布的可能影响，例如，作为未来观测的指南。最后，改进后的模式将用于研究中间层对平流层臭氧和气候的影响。

Roughly 50 tonnes of interplanetary dust enters the earth's atmosphere every day. The dust particles collide with air molecules at speeds between 11 and 72 km/s, causing most of the particles to flash heat, melt and evaporate. This produces metal atoms which then appear in layers between about 75 and 110 km. The Na, Fe, Ca and Ca+ layers have been observed since the 1970s using the ground-based lidar technique. Recently it has become possible to observe the metal layers, including Mg and Mg+, on a global basis using optical instruments on satellites. The need to explain these atmospheric observations has stimulated laboratory studies of the reactions which these metals and their ions undergo in the atmosphere, and the consequent development of local scale atmospheric models. The mesospheric metal layers are extremely useful probes of the chemistry and dynamics of the upper atmosphere. This is because the layers, whose widths of just a few km are controlled by fast photochemical processes, are very responsive to dynamical processes such as gravity waves and tides, and changes in atmospheric constituents such as O, H and O3. Noctilucent clouds, which form around 83 km when the temperature falls below 150 K during summer at high latitudes, cause substantial depletion of the metal layers because of rapid uptake of the metals on the ice surfaces. During winter at high latitudes, convergence of the meridional circulation over the polar vortex appears to cause a substantial increase in the metal concentrations. Solar proton events also cause significant perturbations to the metal layers. Lastly, the chemistry which controls the heights of the layers is largely driven by atmospheric pressure, and so the layer heights are sensitive to global cooling caused by increasing greenhouse gases such as CO2 and CH4 in the middle atmosphere. The objective of this proposal is to produce the first global model of four metals - sodium, iron, calcium and magnesium. These metals all behave quite differently in the mesosphere. We will insert the chemistry of these metals into a state-of-the-art general circulation model, the Whole Atmosphere Chemistry Climate Model (WACCM), which has been developed at the US National Center for Atmospheric Research over the past decade. This general circulation model extends from the earth's surface to 140 km, and includes all the neutral and ionized constituents with which the metals interact. In preparation for this project, we have recently installed and run WACCM on the UK's front-line national supercomputing service HECToR (High-End Computing Terascale Resource). Modelling the metal layers also requires as input the rates at which each metal is injected into the atmosphere from ablating interplanetary dust, as a function of height, season, latitude and time-of-day. These injection rates will be calculated using our new Chemical Ablation Model, combined with an astronomical model of the meteor input function. A project student will retrieve, for the first time, a global data set of Fe and Fe+ observations, using the SCIAMACHY instrument on ENVISAT. This will supplement the ground-based lidar measurements of Fe. The WACCM predictions of the metal layer densities, peak heights, and diurnal and seasonal variability will be compared with the observational data base. The WACCM mesospheric winds and temperatures will also be compared with measurements. These comparisons will enable the WACCM mesosphere to be optimised e.g. through higher vertical resolution and better treatment of gravity waves. We will then investigate the likely impact of the solar cycle and climate change on the distributions of all four metals, for instance as a guide to future observations. Finally, the improved model will be used to study the impact of the mesosphere on stratospheric ozone and climate.