From: heyms@mmm.mmm.ucar.EDU (Andy Heymsfield)
Date: Sun, 4 Oct 1998 19:58:26 -0600 (MDT)
Subject: ICE Initiative (fwd)

Forwarded message:
From heyms@mmm.mmm.ucar.EDU  Fri Sep 25 03:44:21 1998
From: heyms@mmm.mmm.ucar.EDU (Andy Heymsfield)
Message-Id: <199809250344.VAA25074@bitternut>
Subject: ICE Initiative
To: cooperw@ncar.mmm.ucar.EDU
Date: Thu, 24 Sep 1998 21:44:17 -0600 (MDT)
Cc: heyms1@ncar.mmm.ucar.EDU
X-Mailer: ELM [version 2.4 PL23]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit

Al,

The time is drawing near to the deadline Gabor imposed for our contributions
to the ICE initiative. As you know, Gabor asked us (or we agreed to) comment
on what modes of ice nucleaiton dominate in wave clouds at different
temepratures, and either in mid-continental or west-coast environments.

Data in wave clouds in mid-continental environments support the view 
that activated droplets freeze homogeneously (or ``spontaneously'' 
in the temperature range -35 to -37C. Below these temperatures 
nucleation takes place homogeneously before the
solution droplets can be activated, e.g. solution droplets freeze homogeneously
at sub-water saturations. While there is indications that a heterogeneous ice 
nucleation mechanism may also be active at these temperatures, it pales in
comparison with nucleation via homogeneous freezing.

There have been virtually no measurements in wave clouds below -35$^{\circ}C$ 
in west-coast environements (possiblly with the exception of the New 
Zealanders).  Observed onset relative humidities for ice nucleation 
in cirrus off the west coast during the SUCCESS experiment were 
much higher than comparable cirrus over mid-continental regions, 
suggesting differences in the aerosol abundance and
composition, and emphasizing the need to make measurements in wave cloud 
environments off the west coast. 

Data in wave clouds at temperatures between -20 and -37 C from mid-continental
environments indicate concentrations of active ice nuclei that are lower than
those at warmer temperatures in lower levels. There is no strong indication that
ice nucleus/ice crystal concentrations increase with decreasing temperature. 

Do you want to add more material to this last paragraph. The conclusion
I am left with from the first three paragraphs is that some west-coast
wave cloud sampling at low temperatures might be very useful and
interesting to identify possible differences in chemical composition
and CCN abundance.

Andy