A study of the impact of synoptic weather conditions and water vapor on aerosol–cloud relationships over major urban clusters of China

K. Kourtidis,G. Alexandri,S. Rapsomanikis,A. K. Georgoulias,S. Stathopoulos

doi:10.5194/acp-15-10955-2015

K. Kourtidis, G. Alexandri + Show 3 more

Open Access

https://doi.org/10.5194/acp-15-10955-2015

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Abstract

Abstract. The relationships between aerosol optical depth (AOD), cloud cover (CC), and cloud top pressure (CTP) over three major urban clusters in China are studied under different sea level pressure (SLP) and water vapor (WV) regimes using a decade (2003–2013) of MODIS satellite-retrieved data. Over all urban clusters, for all SLP regimes, CC is found to increase with AOD, thus pointing out that the CC dependence on AOD cannot be explained by synoptic covariation, as approximated by SLP, alone. WV is found to have a stronger impact on CC than AOD. This impact is more pronounced at high aerosol load than at low aerosol load. Hence, studies of AOD–CC relationships, based on satellite data, will greatly overestimate the AOD impact on CC in regions where AOD and WV have similar seasonal variations, while they will probably underestimate the AOD impact in regions where AOD and WV have opposite seasonal variations. Further, this impact shows that the hydrological cycle interferes with the aerosol climatic impact and we need to improve our understanding of this interference. Our results also suggest that studies attributing CTP long-term changes to changes in aerosol load might have a WV bias.

Highlights

Aerosols are known to impact the formation, optical properties, and life cycle of clouds (e.g., Ramanathan et al, 2001; Lohmann and Feichter, 2005; Tao et al, 2012; Boucher et al, 2013), either by increasing the cloud droplet number concentration and simultaneously decreasing the droplet size with a fixed water content, known as the first indirect effect (Twomey, 1974), or by suppressing precipitation formation, enhancing at the same time the cloud cover and cloud lifetime, known as the second indirect effect (Albrecht, 1989)
The results from both satellites are similar, with the highest values of aerosol optical depth (AOD), cloud cover (CC), and water vapor (WV) occurring during the summer months, while cloud top pressure (CTP) is higher during winter and lower during summer over all five cities
We investigated the AOD–CC relationship under different synoptic conditions using sea level pressure (SLP) data, and under different clear-sky WV contents

Summary

Introduction

Aerosols are known to impact the formation, optical properties, and life cycle of clouds (e.g., Ramanathan et al, 2001; Lohmann and Feichter, 2005; Tao et al, 2012; Boucher et al, 2013), either by increasing the cloud droplet number concentration and simultaneously decreasing the droplet size with a fixed water content, known as the first indirect effect (Twomey, 1974), or by suppressing precipitation formation, enhancing at the same time the cloud cover and cloud lifetime, known as the second indirect effect (Albrecht, 1989). By scattering or absorbing solar and terrestrial short-wave radiation (direct effect), aerosols affect temperature on the Earth’s surface perturbing the vertical temperature structure (Haywood and Boucher, 2000; Menon et al, 2002). Absorption in the atmosphere may impact clouds by perturbing the vertical temperature structure (semi-direct effect, Ackerman et al, 2000). Increased numbers of cities of different sizes and intensive urbanization are prominent features in these regions, which extend over hundreds of kilometers.

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