draft text - Oct 8, 1998

ICE - Nucleation Working Group
  ****Discussion Topics****
WAVE CLOUD EXPERIMENT.

W6. What laboratory experiments make sense as complements - in
preparation and in evaluation - to the field experiment?



Wave clouds offer one of the best opportunities for making direct
comparisons between atmospheric observations of cloud microphysics and
laboratory experiments. The wave cloud forming process is forced ascent
of stably stratified air. Dissipation occurs as the air descends.
Vertical mixing is strongly suppressed, and the airflow is smooth,
steady, primarily horizontal and isentropic. Gradients of scalar
atmospheric properties along the isentropes are generally small
(temperature, humidity, aerosols, etc.). The laboratory analog is cloud
formation in a steady expansion (pressure reduction) and cloud
dissipation in smooth compression. The conceptual picture of air
continually flowing into the upstream edge of the cloud and out the
downstream edge, with air parcels spending a few hundred seconds in
cloud is supported by many field studies. This picture suggests that
parcel theory should provide an accurate representation of the
microphysics. To the extent that the important variables can be
identified and accurately simulated, laboratory experiments will
reproduce the fundamental processes of droplet and ice formation and
growth.

A variety of laboratory experiments should be performed. Some
experiments address the question that lab and aircraft measurements are
equivalent. For example, airborne instruments and sampling methods for
measuring droplets can be inherently different from those used in the
lab. The differences might impose systematic biases that confound lab -
aircraft comparisons. Any such differences should be characterized.

The ultimate goal of the lab experiments is testing the basic
scientific understanding of the microphysics in cloud forming and
dissipating processes. In the following table, the experiments are
listed according to increasing complexity, where "complexity" refers to
the amount of information needed to describe the variables. It does not
necessarily refer to the difficulty of performing the experiment. The
first experiments are cloud formation on monodisperse CCN of simple
chemistry. The last experiments involve mixed phase clouds with natural
aerosol.  The primary response variables are identified and must be
measurable in both the field and lab studies.

As part of this research, detailed microphysical modeling should
be performed for the field observations and the laboratory experiments.
These modeling studies are important for planning the lab studies, for
identifying the important measurements, linking the airborne and
laboratory observations, and testing fundamental understanding and
descriptions.

QUESTIONS-
What about other possible factors, such as electrical charge, time
scale (residence time), trace organic components, gases, etc. ?  
What are the biggest obstacles, such as detecting ice at small sizes ?  
What laboratory facilities and equipment are currently available to perform
such experiments ? Should these be identified ?  
And what capabilities must be developed ?