Geoscience Reference
In-Depth Information
distribution data sets arising from different locations, times, and sampling meth-
ods, and the broad range of data provided strong evidence that bimodal distribu-
tion occurs as a ubiquitous feature of atmospheric aerosols in general, though the
causal processes and mechanisms were unclear. Semiquantitative explanation of
the observed fine particle dynamics provided the scientific support for the bimodal
concept and became the basis of regional dynamically coupled gas-aerosol mod-
els. As pointed out by Whitby (
1978
) and Junge (
1963
)an actual size distribution
comes from the sum of single modes. There is an equivalency between the optical
properties of a combination of several modes and a representative single mode.
From previous work, it can reasonably be assumed that aerosol size distributions
follow a lognormal distribution (Tanre et al.
1996
). Physical size distributions can
be characterized well by a trimodal model consisting of three additive lognormal
distributions (Whitey
2007
). Typically, the planetary boundary layer (PBL) aerosol
is combination of three modes corresponding to Aitken nuclei, accumulation mode
aerosols, and coarse aerosols, the shape of which is often modeled as the sum of
lognormal modes (Whitey
2007
; Chen et al.
2009
). In a nutshell, the bimodal distri-
bution concept states that the atmospheric aerosol mass is distributed in two distinct
size ranges, fine and coarse and that each aerosol mode has a characteristic size
distribution, chemical composition, and optical properties (Husar
2005
).
3.3
Cloud Drop Size Distribution
3.3.1
Cloud Microphysics and Associated Cloud
Dynamical Processes
Prupaccher and Klett (
1997
) have summarized the current state of knowledge of
cloud microphysical processes as follows. One principal continuing difficulty is
that of incorporating, in a physically realistic manner, the microphysical phenom-
ena in the broader context of the highly complex macrophysical environment of
natural clouds. Mason (Mason
1957
) also refers to the problem of scale in cloud
microphysics. Cloud microphysics deals with the growth of particles ranging from
the characteristic sizes of condensation nuclei (≤ 10
−2
μm) to precipitation particles
(≤ 10
4
μm for raindrops, ≤ 10
5
μm for hailstones). This means we must follow the
evolution of the particle size spectrum, and the attendant microphysical processes
of mass transfer, over about seven orders of magnitude in particle size. Similarly,
the range of relevant cloud-air motions varies from the characteristic size of tur-
bulent eddies which are small enough to decay directly through viscous dissipation
(≤ 10
−2
cm), since it is these eddies which turn out to define the characteristic shear-
ing rates for turbulent aerosol coagulation processes, to motion on scales at least as
large as the cloud itself (> 10
5
cm). Thus, relevant interactions may occur over at
least seven orders of magnitude of eddy sizes. A complete in-context understanding
of cloud microphysics including dynamic, electrical, and chemical effects is not yet
Search WWH ::
Custom Search