Summer Dryness under a Warmer Climate over North America

通过GCM进行的全球变暖预测的最重要的潜在影响之一是夏季中纬度地区土壤水分的干燥（例如Manabe等人，1981; Wetherald and Manabe 1995）。 如果这一预测被证明是正确的，它将对世界上一些生产力最高的地区的水资源和农业产生严重影响。然而，由于形成中纬度土壤水分气候学的一组复杂的相互作用和反馈，这种预测也是最不确定的预测之一（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。所有这些过程都需要在气候模式中得到更好的体现。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。如果夏季蒸发量受到地表液态水转化为水蒸气的辐射能的限制，那么大气变暖将导致蒸发量增加。此外，如果蒸发量的增加大于相应的降雨量的增加，就会导致土壤水分的初始干燥，而土壤水分-降雨反馈机制会放大这种干燥。 然而，如果夏季蒸发受到土壤水文学的限制，或者如果蒸发的初始增加小于相应的降雨量增加，任何变暖都会导致很少或根本没有干燥。我们假设后一组条件在北美更为普遍。在拟议的项目过程中，我们将测试这一假设，以确定是否，以及在什么条件下，将土壤水文在这一全球变化过程中发挥任何作用。 任何关于夏季干燥的研究结果都将在一定程度上取决于陆面模式的技巧。在拟议的项目中，将通过与观测结果进行比较，对全球土壤湿度项目第二阶段的模拟在北美进行评价。我们的分析将确定陆地模式再现观测到的地表水文气候的技巧。 从这些结果，一个更全面的分析将密切关注的机制，水文异常捕获的模型，以及是否有独特的模型功能，导致高技能。 这一建议有几个知识上的优点。我们建议把重点放在水文过程中的作用，预测干燥的土壤水分在中纬度地区在夏季。我们将考虑几个潜在的全球变暖的情况下，以北美为重点的建模方法。我们将测试干燥过程的重要性，并试图找出造成这一过程的物理机制，我们建议使用一个区域气候模型，该模型已根据遥感数据和其他观测来源进行了校准和测试（Einstein和Eltahir，2001年）。预计拟议活动将产生更广泛的影响，包括更好地了解和减少对一个关键的全球变化进程的不确定性，以及对研究生进行全球气候变化一般领域的教育。

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.