Extreme humid heat waves pose significant risks to human health and the ecosystem. Against the backdrops of global warming, the intensity and frequency of future humid heat waves will be further increased. When humid heat waves occur, the soil is wetter than normal. Accordingly, the planetary boundary layer is shallower, and the accumulation of water vapor near the surface increases humidity. How does the land surface atmospheric process and radiation change during light rain days affect extreme humid waves? Current GCMs commonly suffer from excessive occurrence of light rain. However, the impact of this model bias on the simulated humid heat waves remains uncertain.

To address the above problem, Associate Professor Wang Yong’s Research Group of the Department of Earth System Science (DESS), Tsinghua University, following their finding that light rain determines long-term aerosol wet deposition (Wang et al., 2016; Wang et al., 2021b), has further discovered that light rain exacerbates extreme humid heat by the enhancement of the total enthalpy flux and the shallowing of the planetary boundary layer. Based on their previous work of developing a stochastic convection scheme to reduce excessive light rain in GCMs (Wang et al., 2016; Wang et al., 2021b), it is shown that with reduced light rain, the underestimations of humid heat waves in energy-limited regions and overestimations in water-limited regions are largely alleviated. The related research results have recently been published in Nature Communications as a paper titled “Light Rain Exacerbates Extreme Humid Heat”.

The research has found that the intensity of humid heat waves is significantly correlated with light rain frequency during the warm season, rather than with moderate-to-heavy rainfall or total precipitation. The probability of light rain occurring on any given day during the extremely humid heat events and the day before such events shows that more than 50% of humid heat waves are accompanied by light rain, except over dry lands such as the north side of the Sahara Desert. The large-scale atmospheric states during humid heat waves with light rain and without rain are almost identical, but the former is more intense than the latter (Figure 1). This is because, compared with the case of no rain, although the surface downward shortwave radiation is reduced in the case of light rain, the increase of the total surface enthalpy flux and the decrease of planetary boundary layer height significantly enhance the extreme humid heat.

Figure 1. (a-c) Temporal correlation coefficients between wet bulb globe temperature (WBGT, heat stress index) and the frequency of light rain (a), moderate-to-heavy rainfall (b), and total precipitation (c). (d) Probability of extreme humid heat days and the day before with light rain. (e) Differences in the intensity of extreme humid heat between light rain and non-rainy cases.

It is found in the research that extreme humid heat with light rain lasts longer than rain-free extremes. However, intermittent light rain is more effective than consecutive light rain in prolonging the duration of humid heat waves (Figure 2). This is because light rain replenishes soil moisture, and to a certain extent, strong radiation maintains evaporation on subsequent rain-free days, providing water vapor stably for a longer period.

Figure 2. (a) Difference in the duration of consecutive events between light-rain and non-rainy cases. (b) Correlation coefficients between consecutive humid heat waves and light rain Gini index of light rain (indicating the distribution of light rain, the larger the index, the more uneven the distribution of light rain). (c-d) Days of consecutive events and rate of latent heat flux change as functions of Gini indices in the tropics (c) and mid-latitudes (d).

By reducing excessive light rain in the GCM, CAM5, increased evapotranspiration in energy-limited, low latitudes reduces the negative bias in the intensity of extreme humid heat, while decreased evapotranspiration in water-limited, mid-latitudes alleviates the positive bias in the intensity of extreme humid heat (Figure 3). These findings advance the understanding of the impact of reduced light rain on humid heat waves in a warming world.

Figure 3. After suppressing excessive light rain, the changes in (a) the Gini index of light rain and (b) WBGT during the warm season, and (c) their changes over corresponding evapotranspiration regimes (the closer the horizontal coordinate is to 1, the more evapotranspiration is constrained by water, and the closer the vertical coordinate is to 1, the more evapotranspiration is constrained by energy).

Zhang Zhanjie, a doctoral student from the DESS, Tsinghua University, is the first author. The co-corresponding authors are Wang Yong, an associate professor from the DESS, Tsinghua University, and Guang J. Zhang, a professor from the Scripps Institution of Oceanography, University of California, San Diego. Co-authors include Xing Cheng, a special research assistant from the Aerospace Information Research Institute, Chinese Academy of Sciences, Xia Wenwen, a postdoctoral fellow from the Institute of Atmospheric Physics, Chinese Academy of Sciences, and Yang Mengmiao, an associate professor from the School of Geographical Sciences, Fujian Normal University. The research was supported by the Key Research and Development Projects of the Ministry of Science and Technology and the National Natural Science Foundation of China.

Full-text link:

Zhang, Z., Wang, Y.^*, Zhang, G.J.^*, Xing, C., Xia, W. and Yang, M., (2024). Light rain exacerbates extreme humid heat. Nature Communications, 15(1), 7326. https://www.nature.com/articles/s41467-024-51778-9

Related research articles:

Wang, Y., Xia, W., Liu, X., Xie, S., Lin, W., Tang, Q., ... & Zhang, G. J.^* (2021a). Disproportionate control on aerosol burden by light rain. Nature Geoscience, 14(2), 72-76.

Wang, Y., Zhang, G. J.^*, Xie, S., Lin, W., Craig, G. C., Tang, Q., & Ma, H. Y. (2021b). Effects of coupling a stochastic convective parameterization with the Zhang–McFarlane scheme on precipitation simulation in the DOE E3SMv1. 0 atmosphere model. Geoscientific Model Development, 14(3), 1575-1593.

Wang, Y.^*, Zhang, G. J., & Craig, G. C. (2016). Stochastic convective parameterization improving the simulation of tropical precipitation variability in the NCAR CAM5. Geophysical Research Letters, 43(12), 6612-6619.

Written by Zhang Zhanjie and Wang Yong

  Edited by Wang Jiayin

Reviewed by Zhang Qiang