Summer Dryness under a Warmer Climate over North America

0450341 EltahirGCMS预测的全球变暖最重要的潜在影响之一是夏季中纬度地区土壤水分干燥(例如Manabe等人，1981；Wetherald和Manabe 1995)。如果这一预测被证明是真的，它将对世界上一些产量最高的地区的水资源和农业产生严重影响。然而，这一预测也是最不确定的预测之一，因为形成中纬度土壤水分气候学的一系列复杂的相互作用和反馈(Seneviratne等人，2002年)。这些相互作用涉及土壤水文学、湿对流和云动力学。这些都是需要在气候模型中更好地描述的过程。土壤水文学在调节土壤水分干燥过程中的作用将通过研究以下假设来研究。如果夏季的蒸发受到辐射能量的限制，以推动地表的液态水转化为水蒸气，那么大气变暖将导致蒸发量的增加。此外，如果蒸发量的增加大于相应的降雨量增加，将导致土壤水分的初始干燥，这可能会被土壤水分-降雨反馈机制放大。然而，如果夏季蒸发量受到土壤水文学的限制，或者如果最初的蒸发量增加小于相应的降雨量增加，任何变暖都将导致很少或根本没有干燥。我们假设，后一种情况在北美更为普遍。在拟议的项目过程中，我们将检验这一假设，以确定土壤水文学是否会在这一全球变化过程中发挥作用，以及在什么条件下。任何关于夏季干旱的研究结果在某种程度上都将取决于陆地表面模型的技术。在拟议的项目中，全球土壤湿度项目第二阶段的模拟将通过与观测结果的比较来评估北美地区。我们的分析将决定陆地模型再现地表水文观测气候的技巧。根据这些结果，更全面的分析将密切关注模型捕获水文异常的机制，以及模型特征是否具有导致高技能的独特性。这一提议有几个明智的优点。我们建议将重点放在水文过程在预测夏季中纬度土壤水分干燥中的作用。我们将在以北美为重点的建模方法中考虑几种潜在的全球变暖情景。我们将测试干燥过程的重要性，并试图确定导致这一过程的物理机制。我们建议使用一个区域气候模型，该模型已经根据遥感数据和其他观测源进行了校准和测试(Pal和Eltahir，2001)。拟议活动预计会产生更广泛的影响，包括更好地理解和减少对一个关键的全球变化进程的不确定性，以及在全球气候变化的一般领域对研究生进行教育。

0450341EltahirOne of the most significant potential impacts of the global warming predictions by GCMs is the dryness of soil moisture in mid-latitudes during summer (e.g. Manabe et al., 1981; Wetherald and Manabe 1995). If this prediction proves to be true, it would have serious implications on water resources and agriculture in some of the most productive regions of the world. However, this prediction is also one of the least certain due to the complex set of interactions and feedbacks that shape the climatology of soil moisture in mid-latitudes (Seneviratne et al., 2002). These interactions involve soil hydrology, moist convection, and cloud dynamics. All are processes that need to be better represented in climate models.The role of soil hydrology in regulating the process of soil moisture drying will be investigated by studying the following hypothesis. If summer evaporation is limited by the availability of radiative energy to fuel the transformation of liquid water at the surface into water vapor, an atmospheric warming would lead to an increase in evaporation. Further, if this increase in evaporation is larger than the corresponding increase in rainfall, it would result in an initial drying of the soil moisture, which could be amplified by the soil moisture-rainfall feedback mechanism. However, if summer evaporation is limited by soil hydrology, or if the initial increase in evaporation is smaller than the corresponding increase in rainfall, any warming would result in little or no drying. We hypothesize that the latter set of conditions are more prevalent over North America. In the course of the proposed project, we will test this hypothesis to determine if, and under what conditions, would soil hydrology play any role in this global change process. Results of any study on summer dryness would depend to some extent on the skill of the land surface model. In the proposed project, the Global Soil Wetness Project Phase 2 simulations will be evaluated over North America by comparing to observations. Our analysis will determine the skill to which the land models reproduce the observed climate of surface hydrology. From these results, a more comprehensive analysis will look closely at the mechanisms by which hydrologic anomalies are captured by the models, and whether there is uniqueness in model features that results in high skill. This proposal has several intellectual merits. We propose to focus on the role of hydrologic processes in the predicted drying of soil moisture in mid-latitudes during summer. We will consider several potential global warming scenarios in a modeling approach that focuses on North America. We will test the significance of the drying process, and we will attempt to identify the physical mechanisms responsible for it. We propose to use a regional climate model that has been calibrated and tested against remote sensing data and other sources of observations (Pal and Eltahir, 2001). The broader impacts anticipated from the proposed activity include a better understanding and reduction in uncertainty about a key global change process, and education of graduate students in the general area of global climate change.