Clouds Bases

Galo

What are the clouds bases referenced to? AGL or AMSL? Thanks!

Gkikas LGPZ

If you look to a mountainous area (e.g. The Alps), you'll see cloud bases 350ft.
It can't be AMSL, isn't it?
So, cloud bases are AGL.

TZ

It is "above the level of the model orography". So principially it is AGL, but be aware of ECMWF model resolution which is 9km, i.e. terrain is sampled with 9km step. So if you are in mountainous terrain, you can not rely on it, do not use it for planning in such cases.

More about Cloud bases: Can you tell us what do you use it for? Are you a pilot? We are thinking of adding new ECMWF parameter called Ceiling, which is similar to present Cloud base parameter. It may be interesting for you. If you want to know more, you can read this short description: https://software.ecmwf.int/wiki/display/FCST/43r1+new+parameters%3A+Ceiling

What do you think about this parameter? Would it be more useful for you than Cloud base?

stevenmogg

@TZ,

I think ceiling would be useful from pilots perspective. Currently I check cloud base and low cloud to get an idea of the base and amount of cloud. Ceiling seems to combine both these with the >50% filter. I think I would use ceiling first and then maybe base and low cloud if I needed more info.

Gkikas LGPZ

@stevenmogg
I totally agree.
@ivo please have in mind that ECMWF introduced the new parameter "ceiling" that takes into account the base of the lowest cloud layer covering more than 50%.

TZ

@stevenmogg, @meteo-GR

We switched to ECMWF parameter Ceiling, because we found it more usefull.

In Windy it has the same name (cloud base), but now it shows height of clouds where the cloud cover is >50%.

Gkikas LGPZ

@TZ
Thanks a lot! You're awesome!!!

waynemcc

@TZ To be consistent with ATC vocabulary, the current display of Cloud Bases (because they are in AGL) should be renamed Ceilings. I would also suggest adding a separate display called Cloud Bases which would be in MSL.
I have few complaints about the fine Windy product... but the above misuse of AGL for MSL, also occurs with Freezing Level, which currently is AGL and be much more useful if displayed in MSL. When we pick an altitude to fly, we want to know forecasted Freezing Level in MSL.
Thanks for a fine product.
Wayne, pilot

Gabou971

Yep, good point, but I think It would be simplier to use the same feature if you're a pilot or not.

First, it might be a good idea to explain what are height and altitude to other users => height has the ground level for reference (AGL-above ground level). Altitude has the main sea level for reference (MSL)

There are in my opinion two options regarding aeronautical weather data :

-Forecaster point of view: Using charts, he gives the ALTITUDE of clouds (from MSL - main sea level) and doesn't care how you use the info...

-Pilots or controllers point of view: They don't want you to crash and provide the HEIGHT (from AGL - above ground level) of cloud base. So you have an idea of what you're going to see (or not...) while arriving or departing from an airport, whatever the altitude of the airfield.

Considering this, I think that giving the HEIGHT of cloud base is the best option (AGL), which is already the case. It is also good for non pilot users as it provides a useful and easily understandable info.

Regarding Freezing level... It's indeed more useful for pilots when using ALTITUDE (MSL) for other technical reasons. And... It's more logical for pilots but also for other users ... For hikers for exemple. They want to know the freezing ALTITUDE and not height, so they can compare it with the ALTITUDE of a mountain for exemple, because what is depicted on topo charts are ALTITUDEs.

A good solution in my opinion would be to use the words "height" and "altitude"in windy layers names and provide a clear definition of these 2 terms in the help forum section/legend, and obviously use MSL for freezing level and AGL for cloud base.

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?