February 29, 2024

Significant Impact of Rainfall’s Cooling Effects on the Surface on Local Weather Simulation

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

Rain on city street.

Rainfall causes heat transfer from the surface to the rain due to temperature differences. Rainfall’s effect on weather and climate simulations was neglected previously.

[Courtesy Adobe Stock.]

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 (QP), 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 QP over the contiguous United States (CONUS) and examined QP’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 QP’s impact on climate and extreme precipitation can be significant. QP was previously neglected in the heat and energy budget analysis and weather and climate models. Including QP in analyses is crucial for accurately measuring how much heat energy is at the surface. QP has significantly impacted local weather in extreme precipitation events, making it important to consider in weather and climate models.

Summary

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

Findings indicate the spatial distribution of QP 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 QP reveals the highest values occurred during the June to August (JJA) period. Peak QP values of approximately 4 W m−2 were observed during JJA over the Great Plains region.

The team hypothesized QP 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 QP during an extreme precipitation event over the Chicago Metropolitan Area. Results show QP may be a dominant factor compared to other components of surface heat flux during the zenith of precipitation hours. Also, QP has the potential to not only diminish precipitation but also alter and reconfigure the remaining surface energy budget components.

Principal Investigator

Cristina Negri
Argonne National Laboratory
[email protected]

Program Manager

Sally McFarlane
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Urban Integrated Field Laboratories
[email protected]

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.