What kind of precip is being forecast?

[scottd]

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[Ted]

I'm going to venture a guess and say SLD, your favorite and mine for turning perfectly good airplanes into lawn darts.

 

[Ted]

Can you be more specific? Details please.

A lawn dart that hits the ground very hard, making a large crater and ending up in many pieces?

The lines aren't perfectly together the whole way down, so I wouldn't necessarily expect it to be very heavy. But they are very much one between about 650 and 700 mbar range, so I would expect precipitation from that. You've told us before that to get SLD/freezing rain you don't necessarily need to have an inversion that goes above freezing. So in that 700 mbar range down to the surface, I'd expect to see freezing rain/SLD.

Above 650 mbar you'd be above the clouds, so that'd be the ideal place to be for those of us with LearBarons. Otherwise, it looks like a pretty not-fun day to fly through that area.

Not sure if that's what you're looking for - I know how to interpret the weather and decide "Hmm. I don't want to go flying." You're the technical guru.

 

[Everskyward]

You've told us before that to get SLD/freezing rain you don't necessarily need to have an inversion that goes above freezing. So in that 700 mbar range down to the surface, I'd expect to see freezing rain/SLD.

I must have missed that discussion. How is it that you can get freezing rain without the temperatures being above freezing at some point?

 

[AdamZ]

Freezing Rain?

 

[Ted]

I must have missed that discussion. How is it that you can get freezing rain without the temperatures being above freezing at some point?

That's a question for Scott, I just remember that had come up.

 

[chucky]

Well, according to this WW2010 page, supercooled rain can form when the clouds where it forms are warmer than -10C, which is the case here. I'm trying to figure out why that is. Is it that the number density of water droplets is too small to actually start the crystallization process?

 

[chucky]

Thanks for the clarification. I'm looking at the two discussions you linked now. I'm still not clear on the physics of the formation - phase changes involving solids are well outside of the realm of physics I work in.

 

[Richard]

As the question pertained to precip at the sfc, I figured drizzle based on divergent lines and wind speed. FZ DZ given the temps.

 

[Everskyward]

The key here is that ice nuclei (similar to condensation nuclei) don't become active until the temperature reaches about -12°C. Once the cloud top temperature becomes colder than this, ice crystals begin to form...which usually happens near the top of the cloud. At that point, the cloud begins to quickly build ice crystals at the expense of supercooled liquid water which generally lessens the structural icing threat. When the cloud top temperature is warmer than -12°C, you can be guaranteed that the cloud is dominated by supercooled liquid water.

Now there are variations to this...sometimes higher (colder) clouds can "seed" these lower clouds with ice crystals starting the crystal process in the lower and warmer cloud deck.

We're going to see this process take place here in Charlotte, NC on Monday and Tuesday. It'll start out as snow and then dryer air will move in aloft (mid-layers of the atmosphere) and effectively cut off the supply of ice crystals changing the precip to freezing drizzle.

The sounding I posted is referred to as non-classical SLD and will likely produce freezing drizzle at the surface. I've also discussed some of this in this FREE e-Tip. One you introduce ice crystals into a supercooled environment, you can change the cloud over to a process that builds snow, not drizzle or rain.

Interesting. I didn't know that.

I'm still not clear on the physics of the formation - phase changes involving solids are well outside of the realm of physics I work in.

I don't understand this part either but I will take for granted that it is true.

 

[Richard]

The key here is that ice nuclei (similar to condensation nuclei) don't become active until the temperature reaches about -12°C.

What do you mean when you say "become active"?

Once the cloud top temperature becomes colder than this, ice crystals begin to form...which usually happens near the top of the cloud. At that point, the cloud begins to quickly build ice crystals at the expense of supercooled liquid water which generally lessens the structural icing threat. When the cloud top temperature is warmer than -12°C, you can be guaranteed that the cloud is dominated by supercooled liquid water.

Now there are variations to this...sometimes higher (colder) clouds can "seed" these lower clouds with ice crystals starting the crystal process in the lower and warmer cloud deck.

This would be an ideal condition when climbing to avoid icing is the proper response, correct?

 

[chucky]

I don't understand this part either but I will take for granted that it is true.

Well, I'm curious now. What I've learned so far is that nucleation is complicated, but that water does not spontaneously nucleate at 0C, but rather at a much colder temperature (wikipedia says -42). Maybe it requires some level of deposition straight from the gas phase.

 

[denverpilot]

This "advice" from magazines, websites, et. al., about climbing in freezing rain is also littered throughout the Instrument Rating written test question pool, having just studied those.

Having taken a couple of Met courses in college, I knew that answer was flawed, but they seem more than happy to pound it into Instrument student's brains, just hard enough to kill them...

 

[Steve]

Without writing a book here, two phase transitions can lead to ice crystal formation: the freezing of a liquid drop or the direct deposition (sublimation) of vapor to the solid phase. Both are nucleation processes, and in principle homogeneous and heterogeneous nucleation are possible. ...

.

Is sublimation the correct term here? If so, its usage in that context is new to me.

btw, we're getting a mixed precip here today sleet/rain/snow

 

[Everskyward]

To sum up in layman's terms. Small droplets of water don't necessarily freeze into ice even if the temperature is below freezing. They need some sort of nucleus to do so. This is especially true when the temperature is greater than -12C. My question is how do you know suitable nuclei are present? I see your discussion about dry air aloft. Are you saying that because there are no ice crystals falling through the warmer (but below freezing) saturated area that it will remain liquid? I didn't figure that ice crystals would fall, so to speak, but were light enough to remain suspended in the air.

 

[Let'sgoflying!]

I would like to put a request out there for all posters of skT diagrams on POA to please put few arrows/circles on them, so those unfamiliar with them could get something out of the posts.
Just a few marks indicating the highlights of each would be helpful.

 

[Everskyward]

You don't, but there are clues that can tell you about this as well.

Here is what I came up with for the local area. It's been snowing all day. I guess it's snow and not freezing drizzle because the temps in the clouds are pretty cold, all the way down to -40C.

No. As the air dries out aloft, it causes the cloud top temperatures to warm...the tops of the clouds become lower. As this happens, the tops go from -25°C (which allows ice crystal production) to -10°C which limits or cuts off the production of ice crystals.

But even if the cloud tops were still cold how would the crystals get to the lower, warmer, part of the cloud in order for those droplets to also freeze?

Edit: Sorry Dave, I don't know how to do that since I linked the picture from another site.

Second edit: OK hopefully fixed. The arrow is supposed to point to the -40C area where the temp and dew point diverge.

 

[Let'sgoflying!]

VERY helpful, thanks Mari.

 

[Everskyward]

Turbulent mixing and precipitation scouring are two examples.

I googled this term but couldn't really come up with anything that looked relevant. Do you mean that the ice crystals at the top of the cloud get heavy enough to fall so that they become nuclei for the droplets in the lower part of the cloud?

But once you introduce ice crystals into a cloud, it tends to build more ice crystals at the expense of supercooled liquid water.

As long as one part of a cloud has ice crystals forming it tends to migrate to the other parts even if the cloud is pretty large both vertically and horizontally? I understand about the mixing. In the Denver example the wind isn't particularly strong although there is some windshear at about 12,000' which would put it just about at the ridge level west of here.

 

[Ted]

Non-classical SLD, that's what I was looking for but didn't know the technical term other than "Turns your plane into a lawn dart."

Thanks for the lesson, Scott.

 

[Everskyward]

Thanks for the lesson, Scott.

Yes, absolutely! I'm still not sure how you determine some of those things from the charts but I'll look at the website.

 

[Ted]

Yes, absolutely! I'm still not sure how you determine some of those things from the charts but I'll look at the website.

I'm working on that, too. Of course, then there's when you look at the charts, formulate a plan, and then the actual weather gives cause to do something different.

For example, on yesterday's flight coming back up north from Houston I had planned to stay pretty low in the above-freezing zone up until my first fuel stop. However, reports of the tops being lower than what I saw on the Skew-Ts caused me to go up higher. Although those reports weren't as accurate as I'd hoped, but I ended up at 15,000 ft. Overall, I think that resulted in a better flight than had I stayed at lower altitudes. Mostly smooth air, a very nice tailwind, and no ice once I got up to altitude.

 

[chucky]

I was just outside, and a little bit of freeze rain or freezing drizzle had started up. Here's HVN's current METAR:
KHVN 270025Z 02015G23KT 3SM -FZRA BR SCT010 OVC016 00/M02 A2962 RMK AO2 UPB2359E00FZRAB00 P0000

Sure enough, here's the nearest forecast sounding:

This thread has prompted me to go out and try to learn more about the physics of clouds and precipitation. Turns out that's a really complicated subject. Who knew?

 

[Richard]

My question about climbing in post #15 had to do with dryer air aloft and colder temps. Therefore, descending to avoid icing was not the appropriate response.