DISCUSSION 
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ical properties of Cumulus clouds, J. Met., 10, 
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Discussion 
(Relating to the two immediately 
preceding papers.) 
Dr. Helmut Weickmann—Did you use in your 
computations nuclei distributions that had ac- 
tually been measured? 
Dr. M. Neiburger—This certainly would be 
desirable. I should have mentioned that when we 
started we were not very particular about the 
conditions we chose, because we did not realize 
what a big job it was for the electronic digital 
computer. We decided we would start with a 
composite using Junge’s size distributions for the 
small ones and then some different values of 
Lodge or Woodcock for the big ones for a rough 
mean of a continental aerosol. But since there is 
never any individual aerosol which is actually 
like any of these models we used, it would make 
more sense to use some real ones. The trouble is 
that nobody has taken simultaneous measure- 
ments over the whole size range from the very 
small nuclei to the large ones. It is simple to in- 
corporate measured size distributions into the pro- 
gram, if they are available. 
Dr. Weickmann—Is it important to know the 
size distribution down to the Aitken range of 
about 10~-® microns? 
Dr. Neiburger—Yes, it affects the size in which 
the cutoff occurs, because there are so many of 
those. They do pick up water until the critical 
supersaturation is reached which is then deter- 
mined by the larger sizes. 
Dr. A. Goetz—What determines the size of the 
second maximum? You had it between ten and 
twelve microns. What parameter would have to 
be changed in order to shift its size, decrease it, 
or increase it? 
Dr. Neiburger—The rate of cooling is one fac- 
tor which is very important which would mean 
the vertical velocity in this case; others are rela- 
tive numbers of large and small nuclei. 
Dr. W. E. Howell—I realize that it is extremely 
hard to predict from the initial nucleus distri- 
bution what kind of cloud one is going to end 
up with. I remember I failed completely on 
that point. But I would like to raise the question 
whether the initial conditions that you have chosen 
do actually conform with the trade Cumulus and 
the Cumulonimbus. I believe you started with 
updraft velocity of 130 cm/sec in the condensa- 
tion layer of the trade Cumulus; but going back 
to Dr. Malkus’ paper, I find an estimate of 40 
cm/sec from the flight that went exactly through 
the base of a trade Cumulus, and 108 cm/sec on 
the flight that went 130 m above the cloud base; 
and these were the velocities in the center of the 
updraft, not average across the whole base of the 
cloud. I think a reduction in the velocity would 
make a considerable change in the computation, 
tending toward a much smaller drop number and 
larger drop size for the trade Cumulus. Alterna- 
tively with the Cumulonimbus, where you have 
used an updraft velocity of 30 em/sec, it seems 
from our experience in flying light aircraft for 
water seeding of clouds that 200 or 300 cm/sec 
in the updraft at the base of the cloud would be 
more realistic. Again the change would be in the 
direction of reversing the difference that you com- 
puted between the trade Cumulus and Cumu- 
lonimbus, giving the Cumulonimbus a much 
higher droplet number at least in the initial con- 
densation stage. What happens later on, when 
coalescence takes over, may be different, of 
course. 
Some of us are more interested in the liquid 
water content as a function of the drop size than 
we are in the drop number as such, for studying 
icing, for example. I think you can transform 
your graphs to show liquid water content by 
drawing the base line as a line of slope minus 6 
on those graphs, which immediately transforms 
your double mode to a single mode which corre- 
sponds to the volume median drop size. 
Dr. Netburger—Taking the last point first, I do 
not know whether I got them into the version 
of the figures projected here, but we have some 
diagrams in which there are curves showing the 
