The effect of 3D radiative transfer on climate

日常天气的共同经验表明，云的存在或不存在对地面温度有深远的影响，因为云在白天阻挡来自太阳的入射辐射，并在夜间捕获地表发出的热红外辐射。更长时间尺度上同样如此,因此它是至关重要的如果我们预测平均表面温度的变化在未来世纪,气候系统的计算机模型能够准确反映云与辐射交互的方式。此外，云可以随着全球变暖而变化，这反过来又影响它们与辐射的相互作用，而这种“反馈”是气候预测不确定性的最大原因之一。看一眼明亮的白色积云，你就会知道太阳辐射可以从云的侧面反射出去，所以当你得知目前所有的气候模型只允许辐射通过云的顶部和底部进入或离开时，你可能会感到惊讶。这种简化可能导致模型中的云层只拦截了现实中入射太阳辐射的一半，这可能导致地表温度的巨大误差，从而反馈给天气和气候。热红外辐射也存在大量的偏差。要精确计算辐射如何与复杂的云场相互作用，通常需要昂贵的“蒙特卡罗”计算，其中模拟了数百万个光子的路径。然而，PI最近设计了一种新的方法来计算通过大气的辐射传输，其中包括通过云层的辐射通量，但比蒙特卡罗快许多个数量级。因此，它适合在气候模式中实施。在这个项目中，新方法将被充分开发并在英国广泛使用的气象局气候模型中实施，同时也是政府间气候变化专门委员会（IPCC）使用的模型之一。高分辨率的卫星图像将用于描述云的结构，为我们的方法提供必要的信息。我们将对新方法进行严格的蒙特卡罗计算测试，然后进行全局计算，以确定目前估计云与辐射相互作用程度的误差大小。然后，我们将进行气候模拟，以确定这对全球变暖的影响程度。我们将探索新方法的其他应用。例如，人们担心飞机尾迹与太阳和红外辐射的相互作用会对气候产生影响，特别是考虑到预计未来十年航空旅行将迅速增加。在最近的一篇论文中，PI已经表明，由于忽略了进入和离开轨迹一侧的辐射，目前对辐射与轨迹相互作用方式的计算存在很大的误差。在这个项目中，我们将使用我们的新代码更准确地计算航迹对辐射的全球影响，这将引起航空业和政策制定者的兴趣。我们对英国气象局气候模型的修改将在未来用于气候和天气预报，以及NERC社区的气候研究。

Common experience with the day-to-day weather reveals that the presence or absence of clouds has a profound impact on surface temperature, by the way clouds block the incoming radiation from the sun during the day and trap thermal infrared radiation emitted by the surface at night. This is no less true on much longer timescales, and so it is crucial if we are to predict changes to average surface temperatures over the next century that computer models of the climate system are able to accurately represent the way clouds interact with radiation. Moreover, clouds can change in response to global warming, which in turn affects their interaction with radiation, and this 'feedback' is one of the largest causes of uncertainty in climate predictions. A glance at a brilliant white cumulus cloud will tell you that solar radiation can be reflected off the side of the cloud, so it may be surprising to learn that all current climate models only allow radiation to enter or leave through the cloud top and base. This simplification can lead to a field of clouds in the model intercepting only half the incoming solar radiation as in reality, potentially resulting in large errors in surface temperature that could feed back on weather and climate. Substantial biases are also present for thermal infrared radiation. To calculate accurately how radiation interacts with a complex cloud field normally requires expensive 'Monte Carlo' calculations, where the path of millions of individual photons are simulated. However, the PI has recently devised a new method to calculate the transfer of radiation through the atmosphere that includes the flux of radiation through cloud sides, but is many orders of magnitude faster than Monte Carlo. Hence it is suitable for implementing within a climate model. In this project the new method will be developed fully and implemented in the Met Office climate model, which is widely used within the UK, as well as being one of the models used by the Intergovernmental Panel on Climate Change (IPCC). High resolution satellite images will be used to characterise the structure of clouds to provide the necessary information for our method. We will test the new method rigorously against full Monte Carlo calculations and then perform global calculations to determine the size of the error in current estimates of how much clouds interact with radiation. Then we will perform climate simulations to determine how much this affects global warming. We will explore other applications of our new method. For example, there is concern over the climate effect of aircraft contrails via their interaction with solar and infrared radiation, particularly given the rapid increase in air travel that is projected over the next decade. In a recent paper, the PI has shown that there are large errors in current calculations of the way radiation interacts with contrails because the radiation entering and leaving the side of the contrail is neglected. In this project we will use our new code to make much more accurate calculations of the global effect of contrails on radiation, which will be of interest to the airline industry and policy makers. Our changes to the Met Office climate model will be available for both climate and weather forecasting in the future, as well as being available for climate research within the NERC community.