UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2005 Meeting


Section: Atmosphere and Earth Science; Environment Protection


Title:
On the sensitivity of cloud albedo to aerosol concentration and spectral dispersion


Authors:
Gabriela Iorga*, Sabina Stefan**


Affiliation:
* Department of Physics, Faculty of Chemistry, University of Bucharest,

Regina Elisabeta Avenue, No. 4-12, 70346 - Bucharest, Romania

giorga@gw-chimie.math.unbuc.ro

** Department of Atmospheric Physics and Earth Sciences, Faculty of Physics, University of Bucharest,

P.O. Box MG-11, 76900, Bucharest-Magurele, Romania

sabina_stefan@yahoo.com


E-mail
giorga@gw-chimie.math.unbuc.ro


Keywords:
aerosol size distribution, spectral dispersion, effective radius, liquid water content, cloud albedo


Abstract:
Both the enhancement of aerosol number concentration and the change of shape of the cloud droplet size distribution on a regional scale can modify the cloud reflectivity. This work is focussed on the role that pre-cloud aerosol (number concentration and spectral dispersion) plays in cloud reflectivity. Rural and marine aerosols have been described by log-normal aerosol size distributions, and the cloud condensation nuclei were obtained based on Abdul-Razzak`s activation parameterisation. The spectral dispersion effect in the parameterisation of cloud effective radius had been added. Cloud albedo including dispersion and specified aerosol properties has been estimated. The sensitivity of cloud albedo to changes in cloud droplet number concentration in connection to the changes in liquid water content and the cloud droplet effective radius has been investigated. We obtained higher values of effective radius and cloud albedo when dispersion is taken into account. For the same liquid water path, clouds over rural areas have more and smaller cloud droplets and thus a higher cloud albedo than marine clouds. The calculated cloud albedo scaled by a parameter, which is a function of the relative dispersion of cloud droplet spectra, shows better agreement with the estimated cloud albedo from measured effective radius in SCAR-B project than the cloud albedo calculated without scaling, for clouds over rural areas. In cleaner conditions of marine clouds, albedo is much higher than that inferred using an effective radius-liquid water content relationship proposed by Reid et al (1999) from ASTEX project. The low correlation coefficient of the parameterization may also play a role within. Selective Reference: Reid, J.S., Hobbs, P.V., Rangno, A.L., Hegg, D.A., 1999: Relationships between cloud droplet effective radius, liquid water content, and droplet concentration for warm clouds in Brazil embedded in biomass smoke, J. Geophys. Res., 104 (D6), 6145-6153.