Ž .
Atmospheric Research 53 2000 15–27 www.elsevier.comrlocateratmos
A numerical model of the cloud-topped planetary boundary-layer: influence of the physico-chemical
properties of aerosol particles on the effective radius of stratiform clouds
Andreas Bott
Institut fur Physik der Atmosphare, Johannes Gutenberg-UniÕersitat Mainz D-55099 Mainz, Germany
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Abstract
The present investigation deals with the impact of different aeorosol types on the value of the effective radius of stratiform clouds. In several numerical sensitivity studies with the microphysi-
cal stratus model MISTRA, the aerosol composition was linearly changed from pure maritime to pure rural aerosol particles. Numerical results show that in environments with pure maritime
aerosols, the effective radius is usually much larger than in continental regions. Values of the effective radius are also time-dependent with minima and maxima during day and night,
respectively. In precipitating clouds, the effective radius might achieve values larger than 30 mm. In continental regions, the effective radius is relatively small with values below 10 mm. The
numerical results also show that a small contribution of the rural aerosol to the total aerosol number concentration already has a strong impact on the value of the effective radius. According
to these findings, it is recommended to use in large-scale models the lower continental values of the effective radius in all regions over the ocean which are affected by continental air masses.
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2000 Elsevier Science B.V. All rights reserved.
Keywords: Cloud microphysics; Radiation; Boundary-layer clouds; Effective radius of clouds
1. Introduction
Ž . Clouds play an important role for the earth’s radiation budget because 1 they reflect
Ž . efficiently the incoming solar radiation and 2 they emit thermal radiation back to space
Tel.: q49-6131-392862; fax: q49-6131-393532. Ž
. E-mail address: bottmail.uni-mainz.de A. Bott .
0169-8095r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 1 6 9 - 8 0 9 5 9 9 0 0 0 4 9 - 6
at a temperature different from the earth’s surface temperature. The difference between the radiation fields of a clear sky and a cloudy atmosphere is commonly referred to as
cloud radiative forcing. This radiative forcing is of particular interest for studies of the Ž
. global climate. By comparing 19 different general circulation models, Cess et al. 1990
found that a proper treatment of clouds is essential for producing reliable climate predictions.
The investigation of the impact of clouds on the global climate was the objective of Ž
. Ž .
the Earth Radiation Budget Experiment ERBE Barkstrom, 1984 and of the Interna-
Ž . Ž
. tional Satellite Cloud Climatology Project
ISCCP Rossow and Schiffer, 1991 .
Ž .
Ramanathan et al. 1989 found from the ERBE data a negative radiative forcing of Ž
. clouds, i.e., a net cooling of the atmosphere. Harrison et al. 1990 , also utilizing the
ERBE data set, pointed out that among different cloud types, cirrus and marine stratiform clouds have the strongest impact on the radiation balance. For the interpreta-
tion of satellite measurements, it is essential to know several cloud microphysical parameters, such as the cloud water content and the effective radius r of cloud droplets.
e
Ž .
According to Hansen and Travis 1974 , the latter quantity is defined by:
`
3
r n r d r
Ž .
H
w
r s 1
Ž .
e
`
2
r n r d r
Ž .
H
w
Ž . whereby n
r is the spectral number distribution of cloud droplets with radius r. Due
w
to the lack of better information r is often assigned a constant value independent of the
e
Ž .
microphysical structure of the clouds. Rossow et al. 1989 used r s 10 mm to infer
e
cloud optical thickness from the ISCCP data. However, they mentioned that this Ž
. assumption yields an error in the retrieved data of about 20. Han et al. 1994
investigated the ISCCP data in order to obtain a more detailed information on cloud microphysical parameters. They found that the effective cloud droplet radius is not
constant but it depends on the type of clouds, on the geographical region as well as on the season of the year. For a particular cloud type, r is also varying during the day.
e
They also showed that the background aerosol number concentration plays an important role for the microphysical structure of the clouds. With increasing number concentra-
tions of the aerosol particles, the reflectivity of the cloud is also increasing whereby at the same time, the effective radius is decreasing. These findings are in accordance with
Ž .
observations of Albrecht 1989 . Ž
. Iwasaki and Kitagawa 1998, personal communication found clear evidence for a
link between the aerosol type and the radiative forcing of clouds. By including in the global numerical weather prediction model of the Japan Meteorological Agency the
different aerosol types over the ocean and over the land, they obtained a distinct improvement of the model results regarding the prediction of the Asian summer
monsoon.
The goal of the present paper is to improve our knowledge of the impact of the physico-chemical properties of aerosol particles on the effective radius of stratiform
clouds. For this, some numerical sensitivity studies with the microphysical stratus
model MISTRA have been performed. The sensitivity studies consist of five different model runs. In each of the model runs, all initial data are the same with the only
exception that the number concentration and the chemical composition of the aerosol particles differ in each model run. In Section 2, a short summary of the governing model
equations of MISTRA will be given. In Section 3, the numerical results of the five model studies are presented while Section 4 summarizes the findings of the sensitivity
studies.
2. Description of MISTRA
