Heavy pollution suppresses light rain in China: Observations and modeling

Abstract

[1] Long-term observational data reveal that both the frequency and amount of light rain have decreased in eastern China (EC) for 1956-2005 with high spatial coherency. This is different from the trend of total rainfall observed in EC, which decreases in northern EC and increases in southern EC. To examine the cause of the light rain trends, we analyzed the long-term variability of atmospheric water vapor and its correlation with light rain events. Results show very weak relationships between large-scale moisture transport and light rain in EC. Because of human activities, pollutant emission has increased dramatically in China for the last few decades, leading to a significant reduction in visibility between 1960 and 2000. Cloud-resolving model simulations over EC show that aerosols corresponding to polluted conditions can significantly increase the cloud droplet number concentration (CDNC) and reduce droplet sizes compared to pristine conditions. This can lead to a significant decline in raindrop concentration and delay raindrop formation because smaller cloud droplets are less efficient in the collision and coalescence processes. Together with weaker convection, the precipitation frequency and amount are significantly reduced in the polluted case in EC. Satellite data also reveal higher CDNC and smaller droplet size over polluted land in EC relative to pristine regions, which is consistent with the model results. Observational evidences and simulations results suggest that the significantly increased aerosol concentrations produced by air pollution are at least partly responsible for the decreased light rain events observed in China over the past 50 years.

Change of precipitation intensity spectra at different spatial scales under warming conditions

“…The estimated quasi-constant changing rate for the bottom 10% bin over the ocean is a 5.8%/K increase, while that over land has an opposite trend, a decrease by 7.9%/K. This difference may be partly due to more aerosol particles over land that tend to suppress light precipitation [8]. With respect to the smaller magnitudes of other categories over land than those over the ocean, we hypothesize that this is associated with limited available water vapor over land, which is affected by local evapotranspiration, especially the transportation from the ocean by atmospheric circulation.…”

“…Over land, the decrease of light precipitation may be connected with human activities, ranging from increased anthropogenic aerosols from air pollution to changed land-surface coverage. In the past, many studies attributed the decreased light precipitation to increased aerosols [6,8]. These studies argued that increased aerosols can significantly increase the cloud droplet number concentration and reduce droplet sizes, leading to a significant decline in raindrop concentration.…”

“…Meanwhile, studies on light precipitation over China also have made substantial achievements and have found a significant decrease of light precipitation in the last several decades [17,18]. Some studies have attributed the decrease of light precipitation to increased temperature [17,19], while others have argued that the increased aerosol pollution has led to the reduction of light precipitation through cloud microphysical processes [8]. However, more detailed analyses need to be made regarding changes and their possible explanations in the whole precipitation spectra, ranging from light to extremely heavy, on different spatial scales.…”

“…Heavy precipitation usually forms in fastgrowing convective clouds, which are intimately connected with robust moisture rising caused by dynamic convergence, orographic uplifting or surface heating [2][3][4]. In contrast, low-intensity precipitation is more associated with cloud microphysical processes, which are affected by aerosols, moisture content and so on [5][6][7][8].…”

Multiplatform analysis of upper air haze visibility in downtown Beijing

“…The dense haze layer can evidently alter regional radiation and the hydrological cycle. Then the near-surface visibility will be further impaired due to radiative feedback (Gao et al, 2015;Qian et al, 2009). Tao et al (2014) presented the formation and variation of thick haze layers are mostly associated with regional transport and moist airflows.…”