Understanding the Decrease of Precipitation Extremes at High Temperature and its Implications

近几十年来，包括美国大陆在内的全球大部分地区的极端降水强度（EPI）呈增加趋势。这种趋势通常被认为是温度升高的结果，因为在相同的相对湿度下，较热的空气比较冷的空气含有更多的可降水量。除了趋势之外，EPI在与天气有关的冷暖日之间的温度波动中也显示出类似的变化，因为寒冷日的极端降水事件往往不如温暖日的极端降水事件强烈。但在日常的温度波动中，EPI随温度的增加只维持在一个点上，超过这个点，强度随着温度的进一步升高而下降。因此，了解为何EPI随温度上升到一个阈值而增加，然后在逐日变化中下降，是什么决定了EPI达到峰值的温度，以及EPI-温度关系如何随着温度的长期升高而变化是很有意义的。PI假设在最高温度下EPI的减少是温度对对流云中空气和周围空气之间水分差影响的结果。对流云中上升的空气通常含有饱和的水蒸气，而云周围的环境空气则没有，因此当干燥的环境空气混入云中时，云的活力受到抑制。如果云和周围空气之间的水分差随温度增加而增加，则混合在云中的环境空气的抑制作用会随温度增加而增加，这种增加是在合理的假设下预期的，包括环境相对湿度的最小变化。该奖项下的工作研究了这种水分亏缺机制在抑制由浅云向深对流云发展的作用，从而减少高温下的极端降水。这项工作还考虑了随着气候变暖而发生的epi -温度关系的变化，并假设这种关系在质量上保持不变，但阈值逐渐向更高的温度转移。额外的工作考虑了水分亏缺机制对干旱的可能影响。这项工作是通过观测分析和气候模式模拟相结合的方式进行的。由于极端降水对人类活动的严重影响，以及需要了解极端降水所构成的威胁将如何在不断变化的气候中演变，该项目具有更广泛的影响。PI还在当地学校开展了各种外展活动，包括为初中和高中数学和科学教师举办的为期5天的讲习班。此外，该项目还支持一名研究生和一名博士后研究人员，从而为该研究领域的劳动力发展提供支持。

Recent decades have seen an increasing trend in extreme precipitation intensity (EPI) over much of the globe including the continental US. The trend is generally regarded as a consequence of increasing temperature, as for the same relative humidity warmer air holds more precipitable water than colder air. Apart from the trend, EPI shows similar variation during weather-related temperature fluctuations between warm and cold days, as extreme precipitation events on colder days tend to be less intense than their counterparts on warmer days. But in day-to-day temperature fluctuations the increase of EPI with temperature only holds up to a point, beyond which intensity falls off with further increases in temperature. It is thus of interest to understand why EPI increases with temperature up to a threshold value and then decreases in day-to-day variability, what determines the temperature at which EPI peaks, and how the EPI-temperature relationship changes as a result of long-term increases in temperature.The PI hypothesizes that the reduction in EPI at the highest temperatures is a consequence of the effect of temperature on the moisture difference between air in convective clouds and the ambient air surrounding them. Rising air in convective clouds is generally saturated with water vapor while ambient air surrounding the clouds is not, thus the vigor of clouds is suppressed when drier ambient air is mixed into them. The suppressing effect of ambient air mixed into clouds increases with temperature if the moisture difference between cloud and ambient air increases with temperature, an increase which is expected under reasonable assumptions including a minimal change in ambient relative humidity. Work under this award examines the role of this moisture deficit mechanism in suppressing the development of deep convective clouds from shallow clouds, thereby reducing extreme precipitation at high temperatures. The work also considers the change in the EPI-temperature relationship that occurs as climate warms, with the hypothesis that the relationship remains qualitatively the same but the threshold value shifts to progressively higher temperatures. Additional work considers the possible implications of the moisture deficit mechanism for drought. The work is carried out through a combination of observational analysis and climate model simulations.The project has broader impacts due to the severe consequences of extreme precipitation for human activities and the need to understand how the threats posed by extreme precipitation will evolve in a changing climate. The PI is also engaged in a variety of outreach activities at local schools, including a 5-day workshop for middle- and high-school math and science teachers. In addition, the project supports a graduate student and a postdoctoral researcher, thereby providing for the development of the workforce in this research area.