Upper tropospheric humidity at low latitudes

大气中的水蒸气是最强的“温室气体”(也就是说，它“阻挡”了从地表发出的辐射，导致气温升高)，而且由于二氧化碳浓度的增加，预计的升温幅度大约会增加一倍。这种效应是温度和水蒸气之间正反馈的结果(温度越高，水蒸气浓度越高，温度升高越快)。因此，我们预测未来气候变化的能力在很大程度上也取决于对大气湿度的准确预测。然而，控制大气湿度的过程非常复杂。也许令人惊讶的是，大气湿度似乎在很大程度上受到大尺度大气环流的控制。相反，云过程的细节，例如，由于重力，云中的冰晶和水滴沉积的速度有多快，似乎起到了次要的作用。在这个项目中，我们将把观测到的大气湿度分解为反映大气环流影响的部分和反映“云效应”的部分。我们将尽可能准确地量化这两个领域，这是以前从未做过的。我们利用了欧洲中期天气预报中心提供的所谓的“分析”风场和温度场。“已分析”是指将对温度、风和其他大气变量的大量观测整合到大气的数值模型中，然后该模型的风和温度也是对没有观测数据的地区的最佳可能的大气重建。这些数据使我们能够仅根据大气传输来计算大气湿度。然后，我们将这个湿度场与来自卫星观测的水蒸气观测结果进行比较。我们预计，模式湿度和观测值的计算中的不确定性在大小上类似于云的净效应，因此，简单地计算两个场之间的差异并不能准确地估计“云效应”。因此，我们在研究中补充了对重水的观测。氢化水在冷凝和蒸发过程中的行为与水略有不同，其中一个氢原子被重氢取代。在冷凝时，比例更多的氚化水分子进入凝聚相。其结果是，剩余的水蒸气在氚水中被耗尽，而液滴/冰晶则被浓缩。因此，大气中的氚水蒸气是涉及水的相变的过程的敏感指标。直到最近，人们对重水的观测还非常稀少。然而，对迈克尔逊被动大气探测干涉仪(欧洲ENVISAT卫星上的迈克尔逊干涉仪)的测量应用了新的反演技术，首次实现了覆盖全球的观测和高空间和时间采样。我们将使用这些新的观测结合数值云模型，以比单独测量水蒸气更高的精确度来估计云效应的贡献。我们的分析集中在低纬度地区，因为我们预计那里既有最大的‘云效应’(由于频繁的潮湿对流将大量水向上输送)，也有最强烈的水和氚水蒸气的空间特征(由于明显分隔成频繁对流和下沉区)。在建立了“环流”和“云效应”水汽场的可靠气候学之后，我们将分析场如何随时间演变，以及它们如何共同产生观测到的大气湿度场。了解这两个场在当前气候下的行为，将有助于推断它们可能如何因预期的大气二氧化碳浓度增加而变化。

Water vapour in the atmosphere is the strongest 'greenhouse gas' (i.e. it 'traps' emitted radiation from the surface, leading to an increase in temperature), and roughly doubles expected warming due to increasing CO2 concentrations. This effect is a consequence of a positive feedback between temperature and water vapour (higher temperatures lead to higher water vapour concentrations, which in turn enhance temperature increase). Hence, our ability to predict future climate changes hinges also to a good degree on an accurate prediction of atmospheric humidity. However, the processes that control atmospheric humidity are highly complex. Perhaps surprisingly, atmospheric humidity seems to be largely controlled by the large scale atmospheric circulation. Conversely, details of cloud processes, for example how fast ice crystals and droplets in clouds sediment due to gravity, seem to play a secondary role. In this project we will decompose the observed atmospheric humidity into a part that reflects the effect of the atmospheric circulation, and a part that reflects 'cloud effects'. We will quantify these two fields as accurately as possible, which has not been made before. We make use of so-called 'analysed' wind and temperature fields from the European Centre for Medium-range Weather Forecasts. 'Analysed' means that a large number of observations of temperature, wind and other atmospheric variables are integrated into a numerical model of the atmosphere, and the winds and temperatures of this model then are the best possible reconstruction of the atmosphere also for regions where no observations are available. This data allows us to calculate atmospheric humidity based solely on atmospheric transport. We then compare this humidity field with observations from water vapour from satellite observations. We expect that uncertainties in the calculation of both the model humidity and that from observations are similar in magnitude to the net effect of clouds, such that simply taking the difference between the two fields would not be an accurate estimate of the 'cloud effect'. Hence we complement our study with observations of deuterated water. Deuterated water, where one hydrogen atom is replaced by deuterium, behaves slightly different than water in condensation and evaporation processes. Upon condensation, proportionally more deuterated water molecules move into the condensed phase. The result is that the remaining water vapour is depleted in deuterated water, whereas the droplet/ice crystal is enriched. Hence deuterated water vapour in the atmosphere is a sensitive indicator for processes that involve phase changes of water. Observations of deuterated water were very sparse until recently. However, novel retrieval techniques applied to measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS; on board of the European ENVISAT satellite) allow for the first time observations with global coverage, and high spatial and temporal sampling. We will use these novel observations in combination with numerical cloud models to estimate the contribution of cloud effects with higher accuracy than possible from water vapour measurements alone. Our analysis focusses on low latitudes, because we expect both largest 'cloud effects' there (due to frequent moist convection that transports large amounts of water upwards) and strongest spatial signatures in water and deuterated water vapour (due to the clear separation into regions of frequent convection and subsidence zones). Having established reliable climatologies of both the 'circulation' and 'cloud effect' water vapour field, we will analyse how fields evolve over time, and how they conspire to produce the observed atmospheric humidity field. Understanding the behaviour of these two fields in the current climate will allow deductions how they may change due to expected increases in, e.g., atmospheric CO2 concentrations.