Microphysics effects of anthropogenic aerosols on urban heavy precipitation over the Pearl River Delta, China

Highlights

• Centralized distribution of rainfall and higher rain rate found in polluted condition.

• Larger area of convective-type precipitation occurred in polluted condition.

• Larger radii raindrops with smaller number concentration found in polluted condition.

• Centralized distribution and greater mass of melting hail led to centralized rainfall.

Abstract

In this study, the Weather Research and Forecasting model coupled with chemistry was used to simulate cloud microphysics processes of a mesoscale urban heavy precipitation event over the Pearl River Delta, China. Two intensities of anthropogenic emissions were considered: the E2010 simulation adopted the level and distribution of present-day pollution, while the E1960 simulation adopted the aerosol condition before urbanization. The modeling reproduced the precipitation process and the results showed a more centralized distribution of precipitation and a rain rate that was 20% higher at the mature stage in E2010. A larger area of convective-type precipitation occurred in E2010 (51.2%) than in E1960 (42.3%), but less stratiform- and mixed-type precipitation occurred in E2010 (7.6% and 7.3%, respectively) than in E1960 (12.1% and 9.1%, respectively). In comparison with E1960, E2010 produced larger quantities of cloud droplets and ice-phase particles. Additional release of condensation/deposition latent heating promoted vertical upward motion and convection in clouds, which further promoted the riming process between hail and raindrops/cloud droplets, as well as the colliding, merging, and collecting processes between raindrops and droplets that ultimately generated raindrops with larger radii (600–700 μm) but smaller number concentrations. Furthermore, hydrometeors convert to precipitation more effectively in polluted conditions. In E2010, the centralized distribution and greater mass magnitude of hail and melting hail resulted in a focused distribution of rainfall. In E1960, the decentralized distributions and smaller mass magnitude of hail and melting hail led to a dispersed distribution of rainfall.

Introduction

There are now likely more land regions experiencing increasingly heavy precipitation events (IPCC, 2013) related to the increasing quantities of anthropogenic aerosols (Zhao and Wu, 2018) produced by human activities such as industry, vehicles, and biomass burning (Albriet et al., 2010; Penner et al., 1992; Wu et al., 2005). Anthropogenic aerosols can serve as cloud condensation nuclei (CCN) and/or ice nuclei that directly and indirectly affect the dynamics, thermodynamics, and microphysics of clouds and the precipitation process (Khain and Pokrovsky, 2004; Tao et al., 2012; Wan et al., 2013), which is termed “aerosol–cloud interaction.”

A consensus is lacking regarding the quantitative evaluation of the effects of aerosol–cloud interaction on precipitation (Khain et al., 2008). Khain et al. (2010) showed that clouds developing in conditions with continental-type aerosols tend to have stronger vertical velocity and attain higher levels. Consequently, the process of smaller particles falling over longer distances through dry air leads to enhanced evaporation and sublimation and thus to suppression of rainfall. Furthermore, suppressed autoconversion of rain and reduced warm rain were also reported to cause the suppression of rainfall in polluted conditions, but the ice-phase process was found insensitive to the CCN number concentration (Heikenfeld et al., 2019; Lim and Hong, 2012). Studies have reported that high aerosol loading tends to suppress precipitation in stratocumulus and small cumulus clouds, but that it is also responsible for the significant trend of increase in heavy precipitation events and rainstorms induced by deep convection clouds and convective systems owing to aerosol-induced invigoration (Jirak and Cotton, 2006; Yuan, 2011). Aerosol-induced cloud invigoration is the process whereby an increase of aerosols deepens convective clouds owing to coupling between the microphysics and the dynamics of cloud systems (Koren et al., 2010). It is widely accepted that invigoration of deep convective clouds is caused primarily by the thermodynamic effects of aerosols. Lee et al. (2014) reported that deposition latent heating contributed the most to the invigoration of clouds, while Miltenberger et al. (2018) showed that condensation latent heating increased with aerosol concentrations, which resulted in the increase of vertical velocities and enhancement of convective precipitation. Importantly, the latent heating rates might perform differently with increased CCN concentration in different microphysics schemes. For example, Lebo and Seinfeld (2011) showed that the latent heating rates in the bin microphysics scheme of the Weather Research and Forecasting (WRF) model increased significantly with CCN number concentration, while the bulk microphysics scheme showed little change. However, Fan et al. (2013) reported that the microphysical effects of aerosols cause dramatic increase in the cloud cover, cloud top height, and cloud thickness at the mature and dissipation stages, even without the thermodynamic invigoration of convection. Moreover, aerosols can also change the spatial distribution of precipitation through microphysics–dynamics feedback (Liu et al., 2018).

The Pearl River Delta (PRD), located in Guangdong Province in South China, has become the greatest urban agglomeration in the world. The generation of large quantities of anthropogenic aerosols in this region is associated with the rapid increase of urbanization and widespread infrastructure construction (Liu et al., 2012). Furthermore, this area experiences heavy rain events annually that are attributable to typhoons, frontal passages, and warm sector heavy rain (Wu et al., 2019). Induced by increasing concentrations of aerosols, enhanced westerly winds contribute to greater westerly moisture fluxes that can change the precipitation generated in the PRD region (Zhao and Wu, 2018). Observations and simulations suggest that elevated aerosol loading can suppress light and moderate precipitation but enhance heavy precipitation and lightning in the PRD region (Fu and Li, 2014; Wang et al., 2011). Statistically, precipitation enhancement occurs in 70% of the PRD region in polluted conditions (Guo et al., 2016). However, owing to the difficulty in distinguishing aerosol–cloud interaction from meteorological covariations and uncertainties in satellite retrievals (Nishant et al., 2019; Zhang et al., 2005), previous studies have not been able to explore fully the mechanism of aerosol microphysical effects on clouds and precipitation using conventional observational data. Numerical model simulation is a useful tool that can help us understand the specific mechanisms of aerosol microphysical effects. Nevertheless, the effects of anthropogenic aerosols on the clouds and local mesoscale heavy rain processes in the PRD region are complex and remain uncertain in terms of the microphysics mechanisms. Thus, this paper discusses the microphysics mechanisms of aerosol effects on the clouds in a heavy rain process based on model simulation.

Aerosol–cloud interaction has been studied extensively using global climate and regional meteorological models, in which the CCN number concentrations and distribution patterns, as a proxy for aerosols, are modified to explore the dynamics, thermodynamics, and microphysics responses of clouds (Cheng et al., 2010; Xiao et al., 2016). However, accurate simulation of the complex feedback mechanisms between aerosols and the atmosphere requires a fully coupled model such as the WRF model coupled with chemistry (WRF-Chem), which is considered a representative online model (Grell et al., 2005) that has been used in some numerical studies (Yang et al., 2016). As chemistry and meteorology are fully coupled in WRF-Chem, the primary pollutant aerosols can produce secondary aerosols through photolysis, chemical reaction, advection and convection, and wet and dry deposition (Chapman et al., 2009; Fast et al., 2006). The CCN number concentration does not increase linearly with the input aerosol concentrations in WRF-Chem (Jiang et al., 2016). Therefore, WRF-Chem provides a more authentic representation with which to explore the impacts of pollutant aerosols on cloud microphysics and precipitation.

The objective of this study was to simulate precisely the mesoscale convective precipitation process of an event that occurred in the PRD region on 21 February 2019, and to explore the effects of anthropogenic aerosols on the cloud microphysics during the precipitation event using anthropogenic emission inventories from before urbanization and for the present day.