First Science Team Meeting
Notes from Discussion Group D: Indirect Aerosol Effect
Moderator: Peter Hobbs; Rapporteur: William Rossow
Discussion outline:
What can we say about this topic now?
What are the key parameters?
How do we get the key parameters?
Discussion was stimulated by comments from the modelers, the
satellite observers, and the in situ observers; there was not time
for discussion about needed laboratory measurements. A summary of
the "key" parameters essentially led to a list of nearly all
parameters, viz., aerosol properties (number and mass density, size
distribution, vertical and geographic distribution, composition as
a function of size, physical constants such as activation
efficiency and hygroscopy, and optical constants such as real and
imaginary index of refraction and shape), cloud properties (layer
thickness, droplet size distribution, number density and effective
radius, cloud water content and path, cloud cover fraction and top
and base locations, cloud optical properties such as albedo and
emissivity, cloud particle phase, and presence of precipitation,
particularly drizzle), and atmospheric properties (humidity
profile, vertical velocity, horizontal winds, especially in the
boundary layer and at the surface). A particular modeling concern
was to obtain information on aerosol sources. Particular satellite
remote sensing concerns were how to identify and treat so-called
"mixed pixels" (i.e., inhomogeneous fields-of-view), how to deal
with lack of information about vertical distributions, and what
limitations on high-resolution retrievals are caused by use of
plane-parallel radiation models instead of 3-D models. Particular
concerns for in situ measurements were to add wind (especially
vertical velocity) measurements, to add "broken cloud" cases, and
to conduct more Lagrangian measurement campaigns.
The discussion illustrated some of the difficulties in defining the
actual "indirect effect" problem and limiting the scope of initial
investigations. The figure below shows one representation of the
complexity of the problem and suggests two first research steps.
Recommendation 1: Since models approaching the complexity of the
system in the figure are a new capability that has not be very
thoroughly explored, initial model studies should focus on
identifying the most critical parameters within our current
understanding. Emphasis should also be placed on determining
relationships among these critical parameters and the modeled
properties of aerosols and clouds.
Recommendation 2: Available ground-based, aircraft and satellite
datasets should be examined to develop a large number of
"in-and-out-of-plume" case studies by combining observations of
aerosol and cloud properties with wind datasets to identify
instances where these properties can be compared with and without
a significant aerosol source for the area.
