The variability of warm cloud droplet radius induced by aerosols and water vapor in Shanghai from MODIS observations

Highlights

• The effect of AOD on CER in Shanghai followed the Twomey effect but also the anti-Twomey effect according to the different LWPs.

• The role of AOD on CER might be overestimated when the LWP was less than 50 g/m².

• The influence of AOD on CER became larger as WV decreased when the LWP was less than 50 g/m².

Abstract

The influence of aerosol optical depth (AOD) on cloud droplet effective radius (CER) is closely related to the liquid water path (LWP) and the total water vapor (WV) in the atmosphere. From a spatial distribution, the interaction between AOD and CER coincided with the anti-Twomey effect in Shanghai. The high-value locations of both the AOD and CER were consistent, both found in central Shanghai. Based on a linear regression analysis of the AOD–CER relationship on a log-log scale, the results showed that the effect of AOD on CER in Shanghai followed not only the Twomey effect but also the anti-Twomey effect. This was mainly due to differing LWPs. When the LWP was less than 50 g/m², CER decreased with increasing AOD at moderate aerosol loading (AOD < 0.6), and then followed by an increase in CER with increasing AOD. Note that, the role of AOD on CER may be overestimated under this situation. Conversely, when LWP ranged from 50 g/m² to 150 g/m², CER decreased with the increase in AOD, agreeing with the Twomey effect. The effect of WV on the interaction between AOD and CER was also investigated and the results indicated that the influence of AOD on CER became larger as WV decreased.

Introduction

Atmospheric aerosols are not only one of the main pollutants affecting air quality, but also an important factor influencing the ecological environment and global climate (Rosenfeld et al., 2008; Wang et al., 2011; Jiang et al., 2013; Lee et al., 2015). Aerosols can act as cloud condensation nuclei or ice nuclei, and have the potential to change the macro- and micro-physical properties of clouds, indirectly impacting the radiation balance and energy budget of the earth-atmosphere system (Leng et al., 2014; Wang et al., 2014; Liu et al., 2018). This is known as the aerosol indirect effect (Twomey, 1977; Albrecht, 1989) and due to the complex interaction between aerosols and clouds, it is still highly uncertain (Koch and Del Genio, 2010; Penner et al., 2011; IPCC, 2014). Global cloud distribution is a major factor in the formation of the earth's climate and clouds cover nearly 50% of the skies (Crane and Barry, 1985); hence, small changes caused by aerosols in clouds can also affect weather and climate. Therefore, studying the interaction between atmospheric aerosols and clouds is one of the most important issues in atmospheric science (Rosenfeld et al., 2014; Koren et al., 2015; Kourtidis et al., 2015; Liu et al., 2019).

Some scholars found, more than 100 years ago, that aerosols could affect the properties of clouds by becoming cloud condensation nuclei (Aitken, 1880). With the rapid development of cities, aerosols produced through anthropogenic activities are increasing. To deepen the understanding of the aerosol indirect effect, several large-scale research projects have been carried out during recent decades. Researchers have studied the mechanism of aerosol-cloud interaction using various advanced observation techniques and methods (Han et al., 1994; Brenguier et al., 2000; Ramanathan et al., 2001; Mao et al., 2002; Trochkine et al., 2003; Zhang, 2007; Ma et al., 2007).

When aerosols enter the clouds and act as cloud condensation nuclei or ice nuclei, the increase in the concentration of the nuclei makes all the hydrometeors' spectra shift to small-scale, resulting in a decrease in the effective radius of cloud particles. This enhances the cloud optical depth and albedo and is known as the first aerosol indirect effect, or the Twomey effect (Twomey, 1977).

To date, there are many studies on the Twomey effect. Huang et al. (2006) analyzed the influence of dust aerosols on ice clouds in East Asia using MODerate resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth's Radiant Energy System (CERES) onboard the Aqua and Terra satellites. The results showed that the diameter of ice cloud particles was inversely proportional to the optical depth of dust aerosols. Su et al. (2008) found that the mean radius of ice cloud particles over the area decreased with the increase of dust aerosol optical depth (AOD). Yuan et al. (2008) considered the effective radius of cloud droplets decreases with the increase of AOD in the Atlantic, Pacific, and Indian Oceans, as well as Amazon basins using the aerosol data from MODIS observations. Based on Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and MODIS data, Costantino and Bréon (2013) analyzed the aerosol indirect effect on a boundary layer of warm clouds in the southeastern Atlantic Ocean. The increase of aerosols led to a decrease in the cloud droplet radius. This increase had little effect on the cloud optical depth, which was mainly due to the decrease of the cloud liquid water content. Conversely, the effective radius of cloud droplets over the East China Sea was inversely proportional to the aerosol optical thickness (Yuan et al., 2008). India and China both have aerosol pollution. In the case of northern India, cloud optical depth and fractional cloud cover are positively correlated with AOD (Kumar, 2013). But there were different conclusions in the studied cities. Five of these cities followed the Twomey effect, while the other three cities had an anti-Twomey effect. Both the Twomey and anti-Twomey effects are also found in eastern China. It has been reported that the effective radius of cloud droplets increased with the increase of AOD in both the southeast (Yuan et al., 2008) and the Yangtze River Delta region (Wang et al., 2014). Using long-term AOD data obtained from MODIS, Wang et al. (2015) found that the influence of AOD on the effective radius of cloud droplets had an inflection point. When AOD was less than 0.4, the effective radius of cloud droplets was negatively correlated with AOD, and when AOD was greater than 0.5, there was a positive correlation between them.

The above researches indicate that aerosols over deserts and oceans mostly follow the Twomey effect, while the Twomey and anti-Twomey effect both exist in densely populated cities. One possible reason is that aerosols and the atmospheric environment over the desert and the ocean are primarily affected by a single source, whereas the atmospheric environment in cities is more complex, being influenced by human activities in addition to natural sources. Another possible reason may be that the impact of aerosols on the micro-physical properties of clouds is over- or under-estimated when the variations in atmospheric water vapor (WV) are not considered (Georgoulias et al., 2015). WV plays an important role in clouds and radiation, precipitation, and land-surface processes (Chahine, 1992; Ramanathan et al., 2001; Gui et al., 2017). Some researchers believe the influence of WV on clouds is much greater than that of aerosols (Kourtidis et al., 2015).

Shanghai, located in the Yangtze River Delta in eastern China, is an international metropolis, with a population of over 24 million and an area of 6430 km². Thus, aerosol environmental and climate effects are important for the sustainable development of the city. Since the implementation of air pollution reduction measures (such as Regulations of Shanghai Municipality on the Prevention and Control of Atmospheric Pollution issued in 2014), the pollution situation is better than several years ago in Shanghai (Shanghai Ecological and Environmental Bulletin, 2018). But there is still a great deal of uncertainty in the study of aerosol indirect effects (Wang et al., 2014; Liu et al., 2017). Whether the first aerosol indirect effect in Shanghai follows the Twomey effect or not is still under review.

In a sense, the Twomey effect is the effect of aerosols on the effective radius of cloud particles. Therefore, our primary goal is to further understand the interaction between aerosol and warm cloud droplet radiuses by considering the liquid water path (LWP) and WV in Shanghai's atmosphere, using MODIS data.