Representing cloud inhomogeneity and overlap in a General Circulation Model

日常天气的共同经验表明，云的存在或不存在对地表温度有着深远的影响，云在白天阻挡来自太阳的辐射，并在夜间捕获地表发出的热红外辐射。在更长的时间尺度上，这一点同样正确，因此，如果我们要预测下一个世纪平均地表温度的变化，气候系统的计算机模型能够准确地表示云与辐射相互作用的方式，这一点至关重要。看一眼天空就会发现，云的结构从很小的尺度到很大的尺度都有，但由于计算机能力有限，气候模型中的云通常需要在几百公里的距离上被视为水平均匀的。已知这会导致入射太阳辐射穿透到表面，反射回太空，并在云中吸收的部分中产生重大误差。因此，这不仅会导致地表温度的误差，而且会导致天气系统的性质和地球仪周围降水的分布的误差。挑战在于以某种方式表示小尺度的云结构，而不会使模型变慢，以至于它不能在几十年到几百年的气候时间尺度上运行。一种创新的新方法已经开发出的建议者以前计算云与辐射相互作用的方式。它能够包括云结构的影响，但在计算机运行时间只有适度的增加。在该项目中，新方法将在英国气象局气候模型中实施，该模型在英国广泛使用，也是政府间气候变化专门委员会（IPCC）使用的模型之一。通常情况下，这种模型的变化将是一项重大的任务，但因为我们已经开发了我们的方法在“爱德华-斯林戈”辐射代码，这实际上是相同的代码处理辐射在气象局模型，这将是简单的转移我们的方法到模型中，可以很容易地在这个项目的短时间内实现。然后，我们将使用不同的云结构表示（包括标准的“统一”表示）对模型进行大约10年的运行，以研究对地表温度，降水和特定类型天气系统表示的影响。在项目结束时，博士后将访问气象局，在最新的业务模型版本上实施新方法，从而确保它可用于未来的天气和气候预报。

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. A glance into the sky will tell you that clouds have structure from very small to very large scales, but due to limited computer power it is usually necessary to treat clouds in climate models as horizontally uniform over distances of as much as several hundred kilometres. This is known to cause substantial errors in the fraction of incoming solar radiation that penetrates down to the surface, that is reflected back to space, and that is absorbed within the cloud. This therefore leads to errors not only surface temperature but also the nature of weather systems and the distribution of precipitation around the globe. The challenge is to somehow represent the small-scale cloud structure without slowing down the model so much that it cannot be run over climate timescales of decades to centuries. An innovative new method has been developed previously by the proposers to compute the way clouds interact with radiation. It is able to include the effects of cloud structure but with only a modest increase in computer run-time. In this project the new method will be 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). Normally such a model change would be a major undertaking, but because we have developed our method on the 'Edwards-Slingo' radiation code, which is actually the same code as deals with radiation within the Met Office model, it will be straightforward to transfer our method into the model, and can easily be achieved within the short timescale of this project. We will then proceed to perform runs of the model for periods of around 10 years with different representations of cloud structure (including the standard 'uniform' representation), to investigate the effect on surface temperature, precipitation and the representation of specific types of weather system. Towards the end of the project, the postdoc will visit the Met Office to implement the new method on the latest operational model version, thereby ensuring that it is available for both weather and climate forecasting in the future.