Category: Hallett and Mossop

Don’t really need me anymore.  Everyone’s on top of this ” incoming” now, set to begin in the area overnight on Thursday, the one you’ve  been reading about here since maybe last October I think.  So, feeling sad today, also because it looks like its going to be a bit too warm for snow, which I think I mentioned about a dozen times. Maybe I will take it out on you by boring you with a science story, one about ice in clouds…but one featuring such stalwarts as Sir Basil Mason, Stan Mossop, John Hallett, Pete Hobbs, Alexei Korolev, and others.  Interested now?

But first, a few nice cloud shots from yesterday so you don’t get too mad at me for boring you first:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cloud ice science story

(drink some coffee, maybe take an extra swig of an “energy drink” if venturing forward)

Kind of takes the fun out of it when other people are saying what you want to say by yourself, lilke today’s forecast for Friday’s storm.  Kind of like being second when you publish “new” results behind other researchers who “got in” a little a head of you (like Korolev et al.-with Hallett!) did in 2004 reporting the FIRST image of a shattered frozen drop they said.

Drop shattering during freezing; what about it?

It was thought not to happen in natural clouds after that embarrassing episode back in the 1960s when the Great Knighted, Sir B. J. Mason¹ and his student, Swinbank (1960), reported drops exploded with they froze.  Liquid centers tried to get out of the ice shell as the drop froze from outside in, as you would expect, but then blew up when the freezing water expanded inside the shell.  Looked pretty good.

There was only one thing wrong, their findings weren’t valid for real clouds.

They put too much CO2 in their cloud chamber (that’s right, the very SAME stuff that’s supposed to make the earth warmer and warmer year after year but has been sitting around lately, about 15 years actually, not doing anything) and that CO2 in the experiments turned out to make the outer ice shell real weak, and also the CO2 came out of solution in the water in the liquid center to make matters worse by expressing gas through the shell.  I wonder how many people have done that?

This was found out by researchers in my very own group before I got there, Jim Dye and Peter Hobbs, a few years later.  When real air was used, the drops didn’t explode.  So, down that hypothesis went that exploding drops caused a lot of ice to form in natural clouds.

End of story?  Nope.

Later, Hobbs with grad student, Abdul Alkezweeny, repeated the experiments with freezing drops, but this time instead them just sitting there, had them rotate as they froze and they DID shatter some, but not a lot!  This was back in 1968.

But no one was reporting images of shattered drops.

In those days,  there was a HUGE amount of unexplained ice in clouds.  Cloud chambers on the ground and in aircraft, found that little ice formed until the air IN THE CHAMBER was at least as cold as -20 C (-4 F), but instrumented aircraft repeatedly found tremendous amounts of ice in clouds that had never been colder than -10 C (14 F).  Hence, an enigma.

But the explanation that a few drops exploded, sending out thousands of ice shards never gained any ground because there was never any observational evidence that it happened.  Instead, an Australian researcher, originally from South Africa, Stanly C. Mossop, with John Hallett, discovered in 1974 that a bar moving through a cloud chamber between -2.5 C and -8 C, caused ice splinters to eject from SOME of the little drops hitting the bar and freezing on it.  But the drops had to be at least 24 microns in diameter, fairly large for cloud droplets, or nothing happened.  Also, if they moved the bar too fast or too slow, nothing happened.  So, there were a lot of criteria involved in this process, temperature range, drop sizes, speed.

So, the Hallett-Mossop riming-splintering hypothesis was born.  They assumed the bar, moving at the fall speeds of soft hail, showed what soft hail did inside clouds:  multiply ice content!

It was an exciting time to see that the mystery of all that ice in clouds at higher temperatures was finally explained, not needing, shattered drops or anything else.

But there were some problems.  In the early days, it was thought that this process, to raise the ice concentrations in clouds much, would take as long as 1-2 hours because it was a “cascade” process.  The few first splinters had to grow to sizes there they fell fast enough to bump into drops and cause ice splinters to eject.  Well, that wasn’t right.  Natural clouds formed ice MUCH faster than that, as you here in Arizona know so well.

The experiments continued and it was found that shattering helped this process (assuming it occured, but even more important was the freezing of drizzle and raindrops.  When those froze, they became instant rimers, splintering objects, and so the time for a cloud, but one having drizzle and raindrops in it, and in the right temperature zone, just between -2.5 and -8 C, was cut down to minutes, something like 10-20, to get ice concentrations from about 1 per cubic meter, to tens of thousands per cubic meter, a real rain cloud.

Except for a single image of a drop half by a researcher using a cloud camera with a glider in the 1970s, no one had reported a shattered drop.  Then along come Korolev et al. (with the great Hallett!) in 2004 reporting shattered drop images in a Canadian frontal band using an advanced cloud camera.  They wrote that it was the FIRST images ever reported of shattered drops.  Rangno and Hobbs (2005) also reported images of shattered drops in clouds around the Marshall Islands, thinking at the time that they were going to be first in line, and then discovered the Korolev et al. report.  It was a sad day to find that reference, as a researcher that was thinking about the glorious days ahead, the keynote addresses to important conferences, that would result from being first in line with something and then other people would always have to reference you.

As Ecclesiastes wrote, their is hardly anything new under the sun if you’re slow going about it.

Published another paper on shattered drops back in ’08.  But, found they didn’t SEEM to be making a big contribution to the ice content in clouds, less than 10%.  You can go here to see that I didn’t make that part up. That was kind of sad finding, too.  You want what you find to be HUGE, and it wasn’t so huge as I hoped.

So, riming and splintering remains our best, most accepted explanation for the great amounts of ice in clouds that aren’t so cold, though the author and Hobbs, have mostly found it wasn’t powerful enough to account for the speed of ice development.  Only the author’s friends, Stith et al (2004),  have reported a lot of ice that couldn’t be explained by the riming-splintering mechanism as have  R&H over the years.

But it would be so great if others confirmed the Stith et al findings.

The End for now.

 

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1Wiki doesn’t do a very good job, and doesn’t even list his outstanding updated, Physics of Clouds text published in 1972, the “bible” of cloud physics in those days!  Unbelievable.

²Riming: Think of what happens to an airframe in a liquid drop cloud at below freezing temperatures.  HELL, here’s a photo by the author from the author-occupied Lear Jet 35 flying in supercooled clouds over Saudi Arabia, 2006,  The weapon-looking things under the wings image precipitation particles like raindrops and snowflakes using laser beams with light sensitive diodes at the other end, one that when shadowed, give you a two dimensional image of what went through the laser beam.