Clouds Bases

Gkikas LGPZ

@Gabou971
Freezing level, (named Zero Degree Level (DEG0L) by ECMWF with ID=228024),
is the height of level (counted from 'surface')
where the temperature passes from positive to negative values.
https://www.ecmwf.int/en/forecasts/datasets/set-i#I-i-a_fc

Model's "surface" (named Orography(Z) by ECMWF with ID=129)
over rough terrain may vary a lot compared with actual altitude from MSL.
https://www.ecmwf.int/en/forecasts/datasets/set-i#I-i-a

It might be a good idea to have a Freezing Level chart using altitude from MSL,
where FRZ LVL = DEG0L+Z

idefix37

@Gabou971
Excellent presentation and analysis of these issues of height and altitude. It would be a good idea to clarify them.
I am not a pilot, but nevertheless interested to have the Freezing altitude expressed not by the height AGL, but by the altitude above MSL. This would be a useful mean to get a better idea of the rain-snow limit in mountain, which global models are unable to provide given their horizontal and vertical low resolution.

Gkikas LGPZ

... also, I want to point out that,
acording to "Database Ref. Manual for ICON"
( https://www.dwd.de/SharedDocs/downloads/DE/modelldokumentationen/nwv/icon/icon_dbbeschr_aktuell.html ), page 33

for ICON model, the Height of 0 degree Celsius isotherm (short name HZEROLOC)
has MSL as a reference.

Thus, the frz. alt. from ECMWF and frz. alt. from ICON, are different!

61335858-e106-430f-9697-500c18cc8b96-εικόνα.png
Nuremberg's altitude is 314m

Gabou971

ahh nice about ICON I didn't check it because my area is only covered by GFS and ECMWF.

Yeah I think those changes wouldn't be hard to make... But that's a non-developer point of view :)

ivo

@Gkikas-LGPZ this is important info thank you

idefix37

@Gkikas-LGPZ
OK your example in Nuremberg seems to demonstrate that ICON 0°C isotherm is MSL, which is confirmed by the Ref. Manual.

But how do you explain that this morning in Greece ICON shows on Windy a Freezing « altitude » at 0m in mountains of 1800m alt. approx. and, at close distance, in low land of 100m alt. approx., a Freezing « altitude » at 1300m.

Switching to ECMWF we see the same figures at same places, but just with a lower resolution map.
For me both ICON and ECMWF on Windy seem to display a Freezing HEIGHT AGL.

Gkikas LGPZ

@idefix37
It seems that you are right!
So, I can give you those two explanations:

DWD ICON ref. manual is wrong
Windy uses a different parameter!

I hope Windy team will investigate that.

Gkikas LGPZ

@ivo
Please check if Windy uses HZEROLOC parameter for frz. alt.(ICON).
Read the comments above.

Gkikas LGPZ

@Tomber42
You speak German fluently. Isn't it?
I'd be grateful if you ask DWD if there is a mistake in their reference manual
https://www.dwd.de/SharedDocs/downloads/DE/modelldokumentationen/nwv/icon/icon_dbbeschr_aktuell.html
(description of HZEROCL, Table 7.5, page 46 on Ref. Manual version 1.2.6).
Above msl or above surface?

Tomber42

I'm at vacation tomorrow, so I write a text for you here and you can send it to dwd

Sehr geehrte Damen und Herren,
Ich hätte eine Frage zur Dokumentation des Icon Modell. In der Doku "insert link to dokument " sprechen Sie bei der Beschreibung von HZEROCL, Tabelle 7.5, Seite 46,Version 1.2.6 von MSL oder über Grund Höhe?

Mit freundlichen Grüßen
"your name"

Gkikas LGPZ

@Tomber42
@idefix37
@ivo

DWD answered:
" the freezing level (HZEROCL) is provided in m ASL,
but it is set to zero (rather than the height of the model topography)
if the freezing level is at the surface.

However, I see that this should be changed in our diagnostic
calculation, as well as the scanning direction (which should be top-down
rather than bottom-up) in order to detect the uppermost freezing level."

So, dear idefix37, I think it answers your/our question!

btw how does AROME handle the subject?

idefix37

@Gkikas-LGPZ said in Clouds Bases:

@Tomber42
@idefix37
@ivo

I think it answers your/our question!

btw how does AROME handle the subject?

Yes, I think... but to clarify the "Freezing altitude" layer on Windy, should we understand that this level is counted as Altitude above sea level, but when this level intersects terrain located at a higher altitude , then this level is measured in relation to the ground itself, as Height above the ground.
So this layer should be called FREEZING LEVEL, as sometime it is measured as ALTITUDE above sea level (asl), sometime as HEIGHT above ground.

In Windy, if you check the Freezing level with AROME it switches to ECMWF. So this layer is not available with this model

In the example below (left map) the Freezing level is at 2400m in West of the Alps and at 0m just above the high summits, as described here above for ICON.
On same locations, the Freezing altitude from AROME data on Meteociel (Right map) is shown at 2700-2600m even when this altitude intersects mountains at 3000 or 4000m.

So there are different ways to display the Freezing level as we could try to summarise with this sketch:

I am more familiar with Freezing altitude asl, which is much more precise for outdoors activity in mountains. The Height above ground is too coarse due to the low altitude resolution of the orographic model integrated in the model itself.

Anyway, at least, it clarifies the way how this layer must be interpreted...

Thanks for your investigation

Alain

dirk001

@TZ Hi there, I just checked this old post and it seems like Windy cloud base can still be < 50% cloud cover so you never changed over to ECMWF Ceiling? or am I missing something here in New Zealand? Lots of days when cloud base green around 2000m but cloud cover only say 20%.

jpsurin

@TZ yes ceiling is really useful for pilots . it would be great to have this parameter available.

TZ

@jpsurin It is already available as "Cloud base" layer.