Tropical High Altitude Clouds and their Impact on Stratospheric Humidity

云在地球气候系统中起着关键作用，通过调节入射和输出的辐射、化学和动力过程。热带高海拔地区的冰云，即所谓的热带对流层顶层，尤为重要，因为这是空气从对流层缓慢上升到干燥的平流层的主要区域。因此，这些冰云影响平流层的水蒸气含量，而水蒸气含量本身就是平流层的辐射和化学过程的主要驱动因素，例如臭氧消耗。这些云要么具有对流性质，出现在对流过冲的云塔中，要么通过大规模上升流和冷却在原位形成不可见的卷云。尽管后者经常发生，但人们对确切的微物理形成机制以及如何维持它们知之甚少。然而，先前使用各种不同机制的建模工作未能与观测到的特性一致。该项目旨在通过研究热带对流层顶云的形成、维持和出现频率，提高我们对其对平流层湿度影响的认识。一套最先进的数值模型将用于模拟热带对流层顶层的云层，利用它们的特殊优势。这些模式是天气研究与预报（WRF）模式、全球气溶胶过程模式（GLOMAP）和澳大利亚社区气候与地球系统模拟器（ACCESS）。首先，将讨论与亚可见卷云的形成和维持有关的问题。第二步将评估不可见卷云和过冲对流对平流层湿度的影响。将研究直接影响（如向平流层注入冰粒）和间接影响（如动力过程的变化）。为了估计净效应，这两种云类型的出现频率将从达尔文站点的一套互补的地面遥感观测资料和国际卫星云气候学项目的卫星观测资料中得出。我在博士期间分析的机载原位测量数据将有助于约束和测试模型模拟。更好地了解与热带对流层顶云层有关的复杂过程将改善它们在数值模式中的表现，从而提高模式预测的质量。由于高度不确定的冰云过程，这将提高我们限制气候预测的能力。此外，了解这些云对平流层湿度的影响将有助于更好地量化它们对气候的影响。

Clouds play a key role in the Earth's climate system by regulation of the incoming and outgoing radiation, chemical and dynamical processes. Ice clouds at high altitudes in the tropics, the so called tropical tropopause layer, are particularly important since this is the main region where air ascends slowly from the troposphere into the dry stratosphere. Thus, these ice clouds affect the stratospheric water vapour content which in itself is a main driver of radiative and chemical processes, e.g. ozone depletion, there. These clouds can either be of convective nature, and occur in convective overshooting cloud turrets, or they form in situ by large scale upwelling and cooling as subvisible cirrus. Although the latter occur frequently, little is known about the exact microphysical formation mechanisms and how they can be maintained. Previous modelling efforts using various different mechanisms, however, have failed to agree with the observed properties. This project aims to improve our knowledge of the impact clouds in the tropical tropopause layer have on stratospheric humidity, by studying their formation, maintenance, and occurrence frequencies.A set of state-of-the-art numerical models will be used to simulate the clouds in the tropical tropopause layer, taking advantage of their particular strengths. These models are the Weather Research and Forecasting (WRF) Model, the GLObal Model of Aerosol Processes (GLOMAP), and the Australian Community Climate and Earth-System Simulator (ACCESS). First, the questions related to the formation and maintenance of subvisible cirrus will be addressed. In a second step the impact of subvisible cirrus and overshooting convection on the stratospheric humidity will be assessed. Both the direct effects (e.g. injection of ice particles into the stratosphere) and indirect effects (e.g. change in dynamical processes) will be studied. In order to estimate the net effect, occurrence frequencies of both cloud types will be derived from a complementary set of ground based remote sensing observations from the Darwin site and satellite observation from the International Satellite Cloud Climatology Project. The data of airborne in situ measurements which I analysed during my PhD will help to constrain and test the model simulations.A better understanding of the complex processes related to the clouds in the tropical tropopause layer will improve their representation in numerical models and thus, enhance the quality of model predictions. This will improve our ability to constrain climate predictions due to highly uncertain ice cloud processes. Additionally, knowing the impact of these clouds on stratospheric humidity will enable an improved quantification of their climate impact.