Researchers analyzed the cooling effect of rainfall over the United States and demonstrated its significant impact on extreme urban precipitation in Chicago.

The Science

Rainfall can play a crucial role in cooling surface temperatures. When raindrops hit the ground, they are usually cooler than the surface and lower the ground temperature. The heat transferred from the surface to the rain (Q_P), known as the surface sensible heat flux, occurs due to the temperature difference between the surface and raindrops. This heat transfer is often not included in weather and climate models and calculations of how much heat energy is at the surface. A team of researchers assessed climatological Q_P over the contiguous United States (CONUS) and examined Q_P’s impact on the simulation of an urban extreme precipitation event in Chicago. This heat term can be significant and affect the surface energy budget and extreme precipitation.

The Impact

The study shows Q_P’s impact on climate and extreme precipitation can be significant. Q_P was previously neglected in the heat and energy budget analysis and weather and climate models. Including Q_P in analyses is crucial for accurately measuring how much heat energy is at the surface. Q_P has significantly impacted local weather in extreme precipitation events, making it important to consider in weather and climate models.

Summary

Q_P results from the temperature difference between the surface and the rain droplets. Due to its seemingly negligible nature, Q_P is frequently omitted from weather and climate models. This study undertakes a comprehensive assessment of Q_P across the CONUS utilizing high-resolution reanalysis, observational data, and numerical modeling to examine the influence of Q_P on precipitation and the surface energy budget.

Findings indicate the spatial distribution of Q_P climatology is analogous to that of precipitation, with magnitudes ranging from 2 to 3 Watts per square meter predominantly over the Midwest and Southeast regions. A seasonal analysis of Q_P reveals the highest values occurred during the June to August (JJA) period. Peak Q_P values of approximately 4 W m⁻² were observed during JJA over the Great Plains region.

The team hypothesized Q_P during an extreme precipitation event would be nonnegligible and have a significant impact on the local weather. To test this conjecture, researchers performed high-resolution simulations with and without Q_P during an extreme precipitation event over the Chicago Metropolitan Area. Results show Q_P may be a dominant factor compared to other components of surface heat flux during the zenith of precipitation hours. Also, Q_P has the potential to not only diminish precipitation but also alter and reconfigure the remaining surface energy budget components.

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Funding

This study is supported by the Community Research on Climate and Urban Science project funded by the U.S. Department of Energy’s (DOE) Biological and Environmental Research program under Contract DE-FOA-0002581. Computational resources are provided by the DOE-supported National Energy Research Scientific Computing Center and Argonne Leadership Computing Facility. Support was also received from the Climate Program Office of the National Oceanic and Atmospheric Administration through Grant NA22OAR4310612 and DOE through Grant DE-SC0023059.

References

Tan, H., et al. “Estimation of Surface Sensible Heat Flux Due to Precipitation over CONUS and Its Impact on Urban Extreme Precipitation Modeling.” Journal of Hydrometeorology 25 (3), (2024). https://doi.org/10.1175/JHM-D-23-0068.1.