Freezing Rain and Ice Pellets

[BeechBoy]

I just landed in Milwaukee and encountered the following weather on the approach. The temperature was above freezing from 8,000 MSL down to about 1,400 MSL (600 AGL) where it dropped to 0C and remained at 0C down to the surface. We started to pick up ice as soon as the temp reached zero but only accumulated a trace.

Here's the temperature profile of the air we flew through on the approach:

6,000 AGL: +2C
3,000 AGL: +4C
2,000 AGL: +2C
600 AGL: 0C
Surface: 0C


After landing I noticed that I was being hit with ice pellets (in addition to the snow) as I was walking in (no jetways for us!). Ice pellets were also mentioned in the past several METARS.

I thought that ice pellets were formed via the following process:

1. Frozen precip falls through a layer of above-freezing air and melts.
2. The liquid precip then falls through a layer of approximately 3,000 feet of below-freezing air where it turns to freezing rain.
3. The freezing rain continues to fall through this layer of below-freezing air and, after another several thousand feet, the freezing rain solidifies into ice pellets.

It was my understanding that at least 6,000 feet of below-freezing air was required to complete the transformation from liquid precip to ice pellets. How is it that ice pellets formed below such a "thick" layer of above-freezing temps in only 600 feet (especially considering that the temps never got below freezing)?

[B767Driver]

Was the temp OAT or TAT?

I looked at the upper air progs for the time of your flight and they all were forecasting a freezing level at the surface with a standard lapse rate up to the 700mb level.

The Ram air rise at each of those altitudes you had recorded is consistent with that of 210kts at 6000', 180kts at 4000' and 140kts at 2000'...and finally a freezing level at the surface.

The strangest icing phenomenon (and weather flying in general) that I've ever encountered has been flying below FL240 around the Great Lakes Region. I think those bodies of water influence upper air temperature, moisture content and icing significantly. It's possible that the upper air was warmer over the lake, influencing the precip type at the shore.

The semi-permanent High that sits over SW MI has a large affect of frontal activity...for years and years I've watched fronts charge over the plains only to fall apart on the east side of L. Michigan.

But I think that same pressure area affects local temperatures between the surface and 10,000 feet as well...possibly pumping some warmer air in the coastal regions of Lake.

I've seen more variations in icing throughout WI, MI and MN than Baskin-Robbins has flavors of ice cream.

[BeechBoy]

The temps listed are SAT. The stuff falling at the surface was a mixture of heavy, wet, snow and ice pellets. I could tell that very little (if any) freezing rain actually fell. The streets were covered in slush but there wan't any "glazing" of trees, poles, cars, streets, etc. that would signify freezing rain.

[B767Driver]

If those were SAT readings, my guess is that some warm air off the lake was affecting local temps below 10,000 in the MKE area. It does seem to occasionally create a local inversion from time to time.

From my inflight experience, it does not take much altitude for precip types to change forms. Mainly just temperature. Right at the freezing level, inflight anyway...it can be a mixture type of precip that gets pretty messy.

[Dugie8]

Keep in mind that water does funky things at "zero degrees". It both melts and freezes, depending on the state it was in.

We did a flight into central OH a couple of nights ago and got the same inversions you had, almost +10 at 12,000 with a steady decrease to +2 at the surface with a nice little flip to -2 or so around 6,000, but we were in the clear there. No icing while in the clouds but the rain was freezing very nicely to the parts of the wing that had fuel in them (on the ground).