Titles and authors: I. Contact Nucleation Inside-Out, |
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I. Contact Nucleation Inside-Out
Raymond Shaw
Dept. of Physics
Michigan Tech
Ice forms in atmospheric clouds either homogeneously (pure liquid water
freezing spontaneously) or heterogeneously, meaning the ice forms on some
pre-existing aerosol particle called an ice nucleus. It turn out that
cirrus clouds, and probably even volcanic clouds, are sensitive to the
competing effects of heterogeneous and homogeneous ice nucleation. To
make
matters worse, there are several different types of heterogeneous
nucleation: ice forming on an ice nucleus already immersed in a liquid
droplet, ice forming directly onto the surface of an ice nucleus, or ice
forming when a liquid cloud droplet collides with a dry ice nucleus. The
latter is called contact nucleation and is known to occur at higher
temperatures than the other nucleation modes, given identical ice nucleus
and water droplet. There are at least four hypothesized mechanisms for
this enhanced rate of contact nucleation. We have carried out experiments
in which we observe contact nucleation by an immersed nucleus touching
the
surface of a water droplet, an observation that contradicts all four
hypotheses. We suggest that the experiments instead support the notion
that there are separate volume and surface nucleation rates for water.
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II. Using digital in-line holography to find the three-dimensional distribution
of cloud droplets
by Jacob Fugal, MTU Dept. of Physics
Clustering of cloud droplets influences processes such as radiative transfer
through clouds, precipitation formation, and the remote sensing of clouds.
We have designed and built an instrument we call HOLODEC I (Holographic
Detector for Clouds I) that measures the 3-D position and size of these
cloud droplets using inline (Gabor) holography. HOLODEC I flew nine times
on
the NCAR/NSF C-130 research aircraft (http://raf.atd.ucar.edu
<http://raf.atd.ucar.edu/>
) during August-September 2003. During the
flights we successfully obtained holograms of cloud droplets and as a
surprise to us, ice crystals. I will present a brief instrument overview,
introduction to the holographic method, and a Huygens-Fresnel method of
hologram reconstruction. I will then show holograms and reconstructions
of
calibration drops, and condensation and dust on optical surfaces, ice
crystals, and of course cloud droplets.