Nasty non-classical SLD environment in southern Canada tonight

[Scott Dennstaedt, PhD]

Yikes.  Not a good night to be flying near Wiarton Keppel International Airport (CYVV).  Non-classical SLD signature showing up on the forecast sounding.  

[N201MKTurbo]

Because of the warm nose at 850mb, doesn't it look more like classical? Or is it the large area of close temp/dewpoint between 750 and 950mb?

 

I have no idea what I'm talking about...

Edited December 28, 2021 by N201MKTurbo

[HXG]

Thanks for these Scott.

Looks to me like  roughly 4,000 feet thick cloud layer with icing (saturated air with likely liquid water and temperatures just below 0C  that equal dew point). Icing and SLD probability gets worse near the top of the clouds with the unstable lapse rate. Climbing within the clouds will make things worse until you can break out on top, which you better be able to do quickly.
 

A hell no go SkewTLogP for me in my FIKI Mooney Bravo. 

[N201MKTurbo]

Here is some jibber jabber about this stuff...

 

 

 

Two primary mechanisms are responsible for the formation of nonconvective SLD and are diagnosed by CIP: classical and nonclassical. By determining the mechanism and examining the temperature and moisture profiles as well as the observed precipitation type, the sounding CIP was used to assess the SLD potential. Using this approach, only the potential presence of drops with maximum diameters in excess of ~200 microns is assessed. It is not possible to estimate DSD or MVD. Classical SLD occurs when a layer of T >0°C (warm nose) was located between two layers with T <0°C, freezing or liquid precipitation was observed at the surface, and the CTT (cloud top temperature) was less than -12°C (figure 42 (A)). The relatively cold CTT indicates that an ice process was likely to have been active above the warm nose. Snow fell into the warm nose, melted to form liquid precipitation, which then fell into the lower subfreezing layer to form classical SLD, usually in the form of freezing rain (FZRA). The precipitation typically reached the surface in the form of FZRA, ice pellets (PL), or rain (RA), depending on the strength (depth and temperature) of the warm nose and the temperature and relative humidity within and beneath the lower subfreezing layer. 64 Figure 42. Examples of (A) Classical and (B) Nonclassical SLD The warm nose is marked in figure 42(A). Temperature and dewpoint profiles are shown in the left-hand panels. Grey shaded areas indicate cloud vertical extent. SLDPOT profiles (gray lines) are given in the right-hand panels, with the SLD layer tops and bases indicated. Figure 42(B) shows nonclassical (collision-coalescence) icing and SLD are identified for three situations: (1) freezing or liquid precipitation was observed when a classical warm nose was present, but CTT was greater than -12°C, indicating a good chance that an all-liquid process was responsible for the precipitation formation; (2) no warm nose was present, only RA and/or DZ were observed at the surface, and CTT was greater than -12°C; and (3) no warm nose was present and freezing precipitation (FZDZ, FZRA, PL) was observed at the surface. CTT was not a factor in case 3, since the precipitation had to have been formed via collision-coalescence. Each sounding was examined for its SLD potential at every level. Frequencies (percent) were calculated by dividing the number of soundings that had SLDPOT ≥0.4 at any level in the sounding by the total number of soundings examined. Figures 43 and 44 show the full-year geographic distribution of SLD icing frequencies for North America and most of Europe, respectively, using an SLDPOT threshold of 0.4. When considering whether SLDPOT exceeded 0.4 at any level in the column, most of North America has SLD frequencies below 3%, while maximum frequencies are on the order of 5% to 8%. While the frequencies may seem large, recall that they are calculated using a low-to-moderate SLDPOT threshold (0.4) and that they indicate the frequency of occurrence of conditions that are conducive to SLD at any altitude (up to ~10 km [30,000 ft mean sea level (MSL)]), anywhere within 100 km of the sounding site. This represents a 314,000 km3 volume of airspace above a given station. Likewise, values for individual 1-km (~3000 ft) thick altitude ranges represent 31,400 km3 volume of airspace above a given station. Thus, the percentages do not represent point or instantaneous frequencies of SLD (i.e., that SLD would be encountered during 5% to 8% of the time during random flight within the area of peak SLD frequencies). Frequencies of aircraft SLD encounters are expected to be much lower. Note that absolute frequencies depend heavily on threshold choice. The choice of the 0.4 SLDPOT threshold is based upon the experience of the algorithm developers,

 

[0TreeLemur]

For those who don't understand the jargon used in this post (like me):  SLD = Supercooled Large Drops

I work for the National Organization for the Advancement of Acronyms.   I'm fighting a one-person battle to try and change an agency culture that overuses jargon at the expense of understanding by the public that we strive to serve.

[jaylw314]

1 hour ago, 0TreeLemur said:

For those who don't understand the jargon used in this post (like me):  SLD = Supercooled Large Drops

I work for the National Organization for the Advancement of Acronyms.   I'm fighting a one-person battle to try and change an agency culture that overuses jargon at the expense of understanding by the public that we strive to serve.

Wait, there's an organization for the advancement of acronyms???  Why do you work for them?  What kind of concentrated evil is that?? 

[aviatoreb]

1 hour ago, 0TreeLemur said:

For those who don't understand the jargon used in this post (like me):  SLD = Supercooled Large Drops

I work for the National Organization for the Advancement of Acronyms.   I'm fighting a one-person battle to try and change an agency culture that overuses jargon at the expense of understanding by the public that we strive to serve.

I doubt very much sir that you work for the NOAA or you would have understood that the National Advancement of Acronyms has an unfortunate overlap with another government agency.

Edited December 28, 2021 by aviatoreb

[0TreeLemur]

38 minutes ago, jaylw314 said:

Wait, there's an organization for the advancement of acronyms???  Why do you work for them?  What kind of concentrated evil is that?? 

Yes.  It's called NOAA.  Officially, that this stands for another FoLA, the National Oceanic and Atmospheric Administration.

 

[Scott Dennstaedt, PhD]

11 hours ago, N201MKTurbo said:

Because of the warm nose at 850mb, doesn't it look more like classical? Or is it the large area of close temp/dewpoint between 750 and 950mb?

 

I have no idea what I'm talking about...

You can read more about this in my recent blog post.  

[jaylw314]

20 hours ago, 0TreeLemur said:

Yes.  It's called NOAA.  Officially, that this stands for another FoLA, the National Oceanic and Atmospheric Administration.

 

Oooooooh, I get it now! 

[carusoam]

If you can only remember one acronym….  SLD is an important one…

Reminder SLD….  Solid….  What the large drops become after they impact your wings…

Super cooled water, in large drops… is looking for something to initiate freezing into a crystal structure….

Leading edges make perfect crystallization initiators…

Wings pick up ice quickly just by flying into this stuff…

Getting out of this stuff becomes priority one.

PP thoughts only,

-a-

[aviatoreb]

Cool SLD experiments in the comfort of your own home.

we got a good bit of that SLD which made a BUNCH of structure ice - on my car - sitting in the driveway.  Unfortunately my car does not have a tks system.  

[FlyingDude]

 

On 12/28/2021 at 1:10 PM, N201MKTurbo said:

jibber jabber

Hi guys,

In the figures shared by @N201MKTurbo above, the bottom for SLD begin below the cloud base (even at 0.4 micron level). How come?

Thanks.

[N201MKTurbo]

15 minutes ago, FlyingDude said:

 

Hi guys,

In the figures shared by @N201MKTurbo above, the bottom for SLD begin below the cloud base (even at 0.4 micron level). How come?

Thanks.

I believe it is because it is showing rain below the cloud base, indicating freezing rain below the clouds.

 

BTW,

I didn't write that jibber jabber above. I should have given an attribution.

Edited December 29, 2021 by N201MKTurbo

[FlyingDude]

Oh yeah, I missed the RA end DZ on the plots...

[Scott Dennstaedt, PhD]

19 hours ago, N201MKTurbo said:

I didn't write that jibber jabber above. I should have given an attribution.

Yes, here is the reference.  I am very good friends with Ben Bernstein who is one of the authors and he helped co-author my Ice is NOT Nice series.  

Cober, S., Bernstein, B., Jeck, R., Hill, E., Isaac, G., Riley, J., & Shah, A. (2009). Data and analysis for the development of an engineering standard for supercooled large drop conditions. US Department of Transportation.