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

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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.