6 1/2" dia.Hail stones?

dynam @vianet.on.ca (Ken S. Tucker) wrote in message < >...

Yes.  

It's a supercell.  Without any real proof, my guess would be that almost
all hail of 5 cm diameter and larger is associated with rotating storms.



I'm not an expert on hail growth.  There have been numerical
studies that have suggested that, in good conditions (probably
not the best possible), you can go from a few mm diameter to
4.5 cm in less than 15 minutes.  You want a lot of liquid water
content and a broad updraft.  The peak updraft strength may not
be all that critical, but you want a large area of strong
updraft.  As the stone moves across the updraft, it can pick
up a lot of water quickly.  The rotation likely serves to
increase the residence time.  

Moves across more than hovers.

The problem is the mixed phase in any particular radar scan.
The research polarmetric radars do a pretty good job, it seems,
of differentiating between hail and rain and mixed phase stuff,
but there are all kinds of issues in getting good velocity
estimates out.  You get the best velocity estimates if you've
got homogeneous scatterers.  Put in a few softball-sized hail
stones in the range bin and you get a lot of signal from those,
mixed in with signal from the raindrops, and the errors associated
with trying to figure out what's horizontal vs. vertical motion
and a unambiguous velocity range of ~20-25 m/s, and it's a hard
interpretation problem.  Not insoluble, perhaps, especially with
some new radars in development, but it's a hard problem.

Implicit for the meterorological community.  Adiabatic expansion
and compression is something we take for granted.

From what some people who know more about it than I do have
told me, that looks like what happens.  We don't have a whole
lot of cases, and there's probably a great deal more research
into smaller size ranges that, in toto, cause more damage, so
I'm not sure how well developed the knowledge is.  The Nebraska
hail is likely to spur some work.

Hurricanes are out of my line, but the current understanding is
that the energy source for hurricanes is the difference in
sea surface temperature and the near surface air temperature.
Effectively, you've got a potential energy gradient and, if
you put the right conditions, you can tap that difference.  On
land, the best analogy is probably the dust devil, which gets its
energy from the difference in temperature between a hot surface
and a not-so-hot layer of air above it.  The vertical temperature
gradient in the atmosphere before things become mechanically
unstanble is g/Cp~10 K/km.  On a clear, dry day in summer over a
dark field, the ground can heat up enough, so that the gradient
in the lowest few meters greatly exceeds this.

Harold
--
Harold Brooks
Head, Mesoscale Applications Group
NOAA/National Severe Storms Laboratory, Norman, OK

[snip]

Fascinating Harold, comments inserted...



Paraphasing ...((correct me if I'm wrong)),
(water vapor) + (air) = (liquid water) + (warmer air),
*note at bottom.

Is this a description of a "super-cell", if not, say so and I'll learn it
from the net.

Right, thanks for the correction!

The "recycling hypothesis" may be applicable to monster hail,
Would the "one pass up and down" hypothesis create monster
hail, and how long do you (and other experts) generally estimate
it take to form monster hail?

So instead of "recycling" it hovers but begins to drop as ice
accumulates, that's also interesting.



It would be really nice if some radar frequency reflected off ice,
but was comparatively transparent to water, then doppler effects
(speeds) could be isolated.

Thanks, there seems to be little empathsis on the decompression
of rising air as it reduces in pressure. I mention because the
principle of refrigration is based on decompressing compressed
gas. Is that a significant factor, and so simple it is implicit?

Not fun, but the aerodynamics "all on one side" is quite
interesting, it almost sounds like falling monster hail evolves
a shape that stabilizes it's orientation.

Thanks for the corrections, comments and ancedotes.

*This is a bit off topic, I was reading about James Watt's
first steam engines awhile back. They worked by condensation,
not by steam pressure. Probably because engineering for
a vacuum was easier than for pressure given the state of
engineering then. Anyway, a cylinder would be filled with
steam, then a bucket of cold water would be thrown on
to the cylinder forcing condensation, vacuum and sucking
force. I'm not qualified to ascertain the truth of this account,
but it made me think about weather, especially hurricanes.
   As warm moist air is drawn under a cloud (toward the
eye), a significant component of the atmosphere liquifies,
by shady cooling, (naturally I refer to the water vapor
component), but a large quantity of the atmospheric volume
literally disappears - forming rain, and maintains a lowered
pressure in that region, like James Watts vacuum steam engine.
   So all a hurricane needs is to have a source of highly humid
air, (as found on tropic oceans) to fuel itself. Usually when
a hurricane like phenomena hits land they lose this fuel.

Regards
Ken S. Tucker

dynam @vianet.on.ca (Ken S. Tucker) wrote in message < >...



The initial conversion from water vapor to liquid produces cloud,
which consists of very small droplets that have a very small
terminal velocity, so that they are easily suspended.  
A critical component is that the environment of the storm tends
to cool off rapidly with height-typically in the Plains of the
US at about 8 K/km, so that the the air inside the storm
cools off more slowly since energy is involved in converting
the gas to liquid (~6.5 K/km).  The buoyancy accelerates the air
upward.  Also, for anything producing really large hail (~5 cm
diameter), the storms almost certainly form in an environment
in which the horizontal winds in the atmosphere change speed
and direction with height in the lowest few km (vertical wind
shear).  The produces a rotating updraft and the interaction
between the shear and the rotating updraft produces a positive
upward-directed perturbation pressure force that accelerates
the updraft even more.  It also means that relatively light
rain drops get blown away from the updraft, so that precipitation
loading is less of a problem than if the updraft wasn't rotating.

Freezing, just like condensation, warms the air, relative to
the environment.  (Think of the total "dry air + water" system-
cool the water part, the air part warms.)  The water stays
liquid well below 0 C, unless it hits some kind of condensation
nuclei-dust or a frozen raindrop or something.

The recycling hypothesis fell out of favor with hail researchers
in the late 1970s, as a result of measurements and numerical
modelling as part of the National Hail Reduction Experiment.
One pass up and down is all that's necessary, along with
horizontal motion.  As the stone moves, relative to the air,
through regions of low liquid water content, the water freezes
almost immediately on contact, leading to opaque layers.  As
the stone goes through higher liquid water content regions,
it gets a layer of liquid water that can't freeze as fast as
the water accumulates, producing a clear layer.  When the stone's
terminal velocity becomes greater than the updraft speed, it
falls.  It may well pick up more water as it falls, continuing
to grow until it reaches warmer temperatures, where melting
begins.  Melting can produce a liquid layer on the outside,
which makes the relative humidity in the lower layers important
in determining how much melting and reduction in size takes
place.  

Significant hail falls produce quite a bit of cooling at the
ground.  There was an event in August of '94 in northern
Oklahoma where people were treated for hypothermia after a
big hailstorm.  Air temperatures at 2 meters above the ground
dropped from ~40 C to 7 C before recovering a couple of hours
later to ~25-30 C.  Large hail also melts slowly.  I recall
driving through a t