The Dependence of Cloud Particle Size on Non-Aerosol-Loading Related Variables Page: 1 of 6
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Fifteenth ARM Science Team Meeting Proceedings, Daytona Beach, Florida, March 14-18, 2005
The Dependence of Cloud Particle Size on
Non-Aerosol-Loading Related Variables
H. Shao and G. Liu
Florida State University
An enhanced concentration of aerosol may increase the number of cloud drops by providing more cloud
condensation nuclei (CCN), which in turn results in a higher cloud albedo at a constant cloud liquid
water path. This process is often referred to as the aerosol indirect effect (AIE). Many in situ and
remote sensing observations support this hypothesis (Ramanathan et al. 2001). However, satellite
observed relations between aerosol concentration and cloud drop size are not always in agreement with
the AIE. Based on global analysis of cloud effective radius (re) and aerosol number concentration (Na)
derived from satellite data, Sekiguchi et al. (2003) found that the correlations between the two variables
can be either negative, or positive, or none, depending on the location of the clouds. They discovered
that significantly negative re - Na correlation can only be identified along coastal regions of the
continents where abundant continental aerosols inflow from land, whereas Feingold et al. (2001) found
that the response of re to aerosol loading is the greatest in the region where aerosol optical depth (-a) is
the smallest. The reason for the discrepancy is likely due to the variations in cloud macroscopic
properties such as geometrical thickness (Brenguier et al. 2003). Since re is modified not only by
aerosol but also by cloud geometrical thickness (H), the correlation between re and Ta actually reflects
both the aerosol indirect effect and dependence of H. Therefore, discussing AIE based on the re - Ta
correlation without taking into account variations in cloud geometrical thickness may be misleading.
This paper is motivated to extract aerosols' effect from overall effects using the independent
measurements of cloud geometrical thickness, Ta and re.
To detect the true signal from the aerosol indirect effect, one must obtain the correlation between re and
Ta at the same H. In order to fix H, we turn to the following analytical model. For an adiabatic cloud, re
may be related to cloud drop number concentration (N,) and H by (Brenguier et al. 2003)
re ac N,-'13H 13(1)
Since entrainment may dilute the cloud water and cause the cloud departing from an adiabatic cloud, we
write this equation in a more general form for marine stratus:
r = AON O"H,
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Shao, H. & Liu, G. The Dependence of Cloud Particle Size on Non-Aerosol-Loading Related Variables, article, March 18, 2005; United States. (digital.library.unt.edu/ark:/67531/metadc781196/m1/1/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.