Observational Evidence Reveals Growing Spatial Scales of Compound Occurrence of Humid Heat Stress-Extreme Rainfall in India

Extreme heat under the warming environment has direct societal implications for developing and highly urbanized populations. Previous studies have shown that extreme wet-bulb temperature, a multivariate measure of temperature-humidity, has led to anomalously high convective inhibition and increased precipitation intensity over the tropics. However, little is known about the linkage of humid heat stress characteristics, such as duration and peak heat stress, versus the sub-daily precipitation extremes over urban and periurban locations in the tropics. Leveraging ground-based meteorological records of around five decades from the 27 hydrometric observatories of the India Meteorological Department, I investigate the compound occurrence of humid heat stress versus sub-daily precipitation extremes across the Indian subcontinent (4 - 40° latitude and 65 - 100° longitude). Here heatwaves are identified when three or more consecutive days of extreme wet-bulb temperature, T_w, is above the 90^th percentile daily variable threshold for each day of the year.  I show the impact of heat stress and its duration on sub-daily precipitation extremes using a novel conditional probabilistic approach. The risk of sub-daily precipitation extremes at each urbanized location is modelled considering the nonstationarity of underlying drivers. The relative timings between each driver and duration overlap between heatwaves and above-average precipitation extremes (wet spells) are also shown. The results show that the extremal upper dependence between peak T_w and sub-daily precipitation extremes are significantly positive and lies in the range of 0.12 to ≥0.20 across the central northeast region that housed part of the Indo-Gangetic Plains. More than 40% of sites report the most coinciding occurrence of humid-heat stress versus sub-daily precipitation extreme, where each of these drivers readily overlapped each other with a lag time of fewer than two days. Further, I show that considering the magnitude of heat stress as a 10-year return period, even a moderate increase in duration will increase the probability of sub-daily precipitation extremes by a range of 1.7 to 20%, with a notable increase across coastal cities. These results are supported by the physically consistent theory suggesting an increase in sub-daily rainfall extremes in response to climate warming over lands of the tropics because of the combination of “positive thermodynamic” and “dynamic contributions.” The observational evidence of increased sub-daily precipitation extremes in response to humid heat stress would help stakeholders and international organizations build resilient strategies to mitigate the impacts of such consecutive hazards.