Soil Water - Climate Feedbacks in Europe in the 21st Century (SWELTER-21)

虽然计算机对未来气候的预测一致认为，温室气体浓度的上升将在21世纪使地球表面变暖，但对于这将如何影响欧洲等地区的气候，尤其是考虑到降雨量的变化时，人们的共识较少。总的来说，北欧可能会变得更潮湿，尤其是在冬季，而中欧和南欧预计将经历更干燥、更炎热的夏天。尽管很难有把握地预测区域降雨量的变化，但这些都是气候变化的关键特征，决策者目前需要对此进行规划。例如，英国未来可能经历更频繁的夏季干旱，这对新水库的规划和建设、农业和关键栖息地的维护都有影响。我们对水循环的预测存在不确定性，原因是气候模型中对关键过程的描述不充分。在考虑未来发生干旱的可能性时，一个关键领域是大气和土地之间的关系。在夏季，土壤干燥，这在欧洲的许多地区限制了土壤水分的蒸发进入大气。当这种情况发生时，陆地表面吸收的太阳能的分配就会发生变化；用于蒸发的能量更少，用于直接加热大气的能量更多。这种“反馈”可能会影响云和降雨的发展，特别是当地夏季风暴的发生，这些风暴在下午发展。此外，当土壤在大范围内干燥时，例如2003年西欧和中欧大部分地区发生的情况，土地蒸发的缺乏可能会影响到更大规模的天气系统。由于春季蒸发量的增加，预计气温升高将在21世纪末的夏季产生明显干燥的土壤条件。因此，我们预计更干燥的土壤将在夏季更早的时候开始反馈到大气中，并在目前潮湿的更北方地区。我们对土壤湿度和降雨量之间的反馈如何运作缺乏详细的知识，这是预测未来几十年干旱可能性的主要不确定性之一。研究表明，2003年的干旱可能预示了未来的形势，干旱的春季土壤与随后的干旱和相关热浪有关。这意味着，未来的欧洲夏季可能会变得更加多变，因为干旱会被有利的土壤条件所“锁定”。这个项目将使用最先进的陆地表面和大气的计算机模型，并结合从卫星获得的最新数据汇编，以提高我们对土壤湿度如何影响降雨的理解。我们将利用干旱时期的观测，直接了解土壤水分下降时气温如何上升。我们将使用这些知识来更好地表示英国气象局气候模型中陆地上的蒸发量。我们还将研究在受干旱影响的地区，相对于相对潮湿或干燥的地形，云和风暴优先发展的地方。这将使我们能够预测干旱土壤抑制降雨从而延长干旱的情况。从我们的详细观察中，我们将评估和改进气候模型及其对土壤湿度反馈的表示。这些改进将为英国气象局对欧洲气候变化的最新预测提供依据。我们还希望提高气象局预测即将到来的夏季是否炎热干燥的能力，因为这些季节性预测使用的是相同的计算机模型。无论在哪种时间尺度上，预测的任何改进都将对英国经济产生直接好处。

Whilst computer predictions of future climate agree that rising concentrations of greenhouse gases will warm the earth's surface over the 21st century, there is less concensus concerning how this will affect the climate in a region such as Europe, in particular when considering changes in rainfall. Broadly speaking, northern Europe is likely to get wetter, particularly during the winter, and central and southern Europe are expected to experience drier and hotter summers. Although regional changes in rainfall are difficult to predict with confidence, these are critical features of a changed climate which decision makers need to plan for now. For example, the likelihood of the UK experiencing more frequent summer droughts in the future has implications for the planning and building of new reservoirs, for agriculture and for the maintenance of key habitats. Uncertainty in our predictions of the water cycle arises from inadequate representation of key processes in climate models. When considering the likelihood of future droughts, one key area is the relationship between the atmosphere and the land. During summer, soils dry, which in many parts of Europe limits evaporation of soil moisture into the atmosphere. When this happens, there is a change in the partition of solar energy absorbed by the land surface; less energy is used for evaporation and more is used to warm the atmosphere directly. This 'feedback' can affect the development of clouds and rainfall, especially the occurrence of local summertime storms which develop during the afternoon. Furthermore, when soil dries out over a large region, as happened for example across much of Western and Central Europe during 2003, the lack of land evaporation can affect much larger-scale weather systems. Warmer air temperatures are expected to produce notably drier soil conditions in the summers of the late 21st century through increased spring-time evaporation. As a result, we would expect the drier soils to start to feed back on the atmosphere earlier in the summer, and in more northerly regions which are currently wet. Our lack of detailed knowledge about how this feedback between soil wetness and precipitation operates provides one of the major uncertainties in predicting the likelihood of droughts in the coming decades. Studies have suggested that the drought of 2003 may have illustrated the shape of things to come, with dry spring soils implicated in the drought and associated heatwave which followed. This implies that future European summers could become more variable from year to year, as droughts become 'locked in' by favourable soil conditions. This project will use a state-of-the-art computer model of the land surface and atmosphere combined with new compilations of data obtained from satellites to improve our understanding of how soil wetness influences rainfall. We will use observations from periods of drought to see directly how temperatures rise as soil water declines. We will use this knowledge to better represent evaporation over land in the UK Met Office climate model. We will also examine where, within drought-affected regions, clouds and storms preferentially develop, over relatively wetter or drier landscapes. This will allow us to predict the conditions where dry soils suppress rainfall, thus prolonging drought. From our detailed observations we will evaluate and improve climate models and their representation of soil wetness feedbacks. These improvements will feed into new Met Office predictions of climate change for Europe. We would also expect to improve the Met Office capability to predict whether the forthcoming summer will be hot and dry, as these seasonal predictions use the same computer model. Any improvements in prediction on either time scale would have direct benefits for the UK economy.