Shift of daily rainfall peaks over the Beijing–Tianjin–Hebei region: An indication of pollutant effects?

Aerosol impacts on diurnal variations of rainfall over the Beijing–Tianjin–Hebei region are investigated by comparing the observations between polluted and clean days in early summer (June 1 to July 20) before the rainy season begins. The 2002–2012 hourly rainfall measurements in conjunction with environmental data reveal that during high‐pollution days the peak of heavy rainfall shifted ~4 hr earlier while the peak of light rainfall delayed by 2–3 hr. Detailed analyses of meteorological conditions associated with the shifts suggest that the former is mainly caused by enhanced lower‐level aerosol heating during the daytime which increases the lower‐troposphere instability and facilitates the convective rainfall occurrence, and the latter is related to decreases of conversion of cloud droplets into raindrops associated with aerosol increases. Nevertheless, conducting model sensitivity simulations to better understand aerosol–cloud microphysics–precipitation interactions is warranted. A schematic diagram for aerosol impacts on the diurnal variation of rainfall over Beijing–Tianjin–Hebei region. Solid arrows are verified results; blank arrows denote the results need to be verified further.