Ten Years of Aerosol Effects on Single-Layer Overcast Clouds over the US Southern Great Plains and the China Loess Plateau

Hongru Yan,Tianhe Wang

doi:10.1155/2020/6719160

Abstract

Using almost 10 years of observations of clouds and aerosols from the US Southern Great Plains (SGP) atmospheric observatory and the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) in China, the impact of aerosols on single-layer overcast clouds over continental land for different regimes were investigated. Atmospheric conditions at the two sites were first compared in an attempt to isolate the influence of aerosols on cloud properties from dynamic and thermodynamic influences. Cloud types and amounts are similar at the two sites. The dominant aerosol types at the SGP and SACOL sites are sulphate and dust, respectively, with greater aerosol optical depths (AODs) and absorption at the SACOL site. Aerosol first indirect effect (FIE) ranges from 0.021 to 0.152 and from −0.078 to 0.047 at the SGP and SACOL sites, respectively, when using the AOD below cloud base as CCN proxy. Although differences exist, the influence of meteorological conditions on the FIE at the two sites is consistent. FIEs are easily detected under descending motion and dry condition. The FIE at the SGP site is larger than that at the SACOL site, which suggests that the cloud albedo effect is more sensitive under relatively cleaner atmospheric conditions and the dominating aerosol at the SACOL site has less hygroscopicity. The radiative forcing of the FIE over the SGP site is −3.2 W m−2 for each 0.05 increment in FIE. Cloud durations generally prolong as aerosol loading increases, which is consistent with the hypothesis of the aerosol second indirect effect. The negative relationship between cloud duration time and aerosol loading when aerosol loading reaches a large value further might suggest a semidirect effect.

Highlights

Since the late nineteenth century, evidences that aerosol particles are essential for forming cloud droplets have been reported by serving as cloud condensation nuclei (CCN) [1,2,3,4]
Twomey [6] hypothesized that, under fixed cloud liquid water content conditions, cloud particle sizes decrease with an increasing number of CCN. e reflection of solar radiation off the cloud is enhanced due to more but smaller cloud droplets in the cloud. is has been referred to as the aerosol first indirect effect (FIE, [7, 8])
A better understanding requires techniques that can combine accurate aerosol and cloud property parameters over an extended period of time to distinguish the impact of aerosols from the natural cloud variability. Given these uncertainties and difficulties, the goal of this paper is to evaluate and compare aerosol effects on the microphysical, macrophysical, and radiative properties of clouds and to assess their dependence on meteorological conditions by analyzing long-term surface observations made at the US Southern Great Plains (SGP) atmospheric observatory and the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL)

Summary

Introduction

Since the late nineteenth century, evidences that aerosol particles are essential for forming cloud droplets have been reported by serving as cloud condensation nuclei (CCN) [1,2,3,4]. Twomey [6] hypothesized that, under fixed cloud liquid water content conditions, cloud particle sizes decrease with an increasing number of CCN. E reflection of solar radiation off the cloud is enhanced due to more but smaller cloud droplets in the cloud. Is has been referred to as the aerosol first indirect effect (FIE, [7, 8]). A greater number of smaller cloud droplets reduces the precipitation efficiency and increases the cloud liquid water path (LWP), enhancing the cloud lifetime [9]. Is phenomenon is referred to as the cloud lifetime effect [7, 9] or the second indirect effect [8]. Aerosols that strongly absorb solar radiation can generate local heating that, in turn, changes the relative humidity (RH) and the stability of the troposphere and thereby influences cloud formation and lifetime. is is referred to as the semidirect effect [15]

Objectives

Methods

Results