Sounding forecast shows incorrect data.

tomA27

Whenever the surface pressure is greater than 1000hPa the sounding forecast shows incorrect data. Specifically, it shows the surface temperature and dew point at the 1000hPa level as if this were the surface. In the screenshot below, the model elevation is 112ft whereas the 1000hPa level is 748ft which results in LCL being reported 636ft too high (TCON/CCL are also incorrect). This can be verified quite easily by looking at the data returned by getMeteogramForecastData() which, certainly appears to be exactly the same data as is being displayed by the Sounding forecast.
Please can this be fixed?

nrbray

Tom and I have talked about this for a while. I've been confused in the past by the LCL and CCL numbers not matching what I expect from the chart in the past, and now Tom has pointed out the inconsistency.

May I ask @vicb if the FlyXC sounding could help here? I would dearly like the CCL 'green line' projection to be available in the standard plugin, and larger graphics limited to the layer below ~600hPa.

@tomA27 said in Sounding forecast shows incorrect data.:

Please can this be fixed?

vicb

@nrbray I commented about this here.

TL;DR Plugins have access to data at different level, the lower one being gh-1000. There is a gh-surface but I'm not sure if we should use this for soundings - they are about the air mass not the ground. I'm not exactly sure what sh-surface is. I asked on the thread I linked above but got not reply.

nrbray

Thank you very much for the feedback. Perhaps with @tomA27 we can dig into this. @tomA27 has the calculation in his app so could perhaps show what he used. For me airmass surface (20m AGL) temperature and particularly dewpoint are needed below 1000hPa for any meaningful model so would be disapointed if they were not available.

Without those, I guess surface dewpoint could be extrapolated given geopotential height at 1000hPa, but not surface temperature. Given that CCL would be possible not LCL.

May I also ask if @jancellor has any further insights that could help my understanding?

@vicb said in Sounding forecast shows incorrect data.:

TL;DR Plugins have access to data at different level, the lower one being gh-1000. There is a gh-surface but I'm not sure if we should use this for soundings - they are about the air mass not the ground. I'm not exactly sure what sh-surface is. I asked on the thread I linked above but got not reply.

@jancellor said in Sounding ground elevation never less than 1000hPa:

I hope the temp-surface data can be used. If not, the green thermal line and cloud prediction would have to be removed from the plugin completely, right? Also I think the built-in sounding would have to remove the LCL and CCL.

jancellor

@nrbray sounds like you and Tom know at least as much as me.

As I understand it, both soundings use temperature and dew point from 1000hPa as the basis of LCL, CCL, green thermal line, cloud base, and cloud tops. I believe this is wrong and gives incorrect data. On the other thread I gave an example of 100% humidity at sea in a high pressure location where the LCL should be 0 but it was 204m, and the same error in the plugin (no screenshot). (I don't know what the calculation is when the pressure is less than 1000hPa, though this is uncommon for me personally.)

It seems to me it would be better to switch to some kind of surface data even before it's clear which is strictly correct to use. I know Victor is sceptical about using surface data, but I'm not sure if he agrees that using 1000hPa temp/dp is wrong.

Windy haven't acknowledged much simpler bugs elsewhere for me so I don't wait for any change to the sounding unfortunately.

tomA27

@jancellor @vicb Thank you for your input. I'm sorry that I never discovered your thread before.

The Sounding forecast already uses the surface temperature/dewpoint at the 1000hPa level. (or perhaps it uses the temperature and dewpoint from getPointForecastData which gives the same values). This is plain to see if you display the temperature, dewpoint and LCL separately in another add-in

My understanding is that "surface" means the air in contact with the surface (normally at 2m). Indeed dewpoint-surface, rh-surface, wind-v-surface etc are clearly properties of the air close to the surface rather than the surface itself.

I find it impossible to see any good reason not to use the surface temperature/dewpoint at the surface level (model elevation).

Personally, I see this as quite an important bug in the official sounding forecast.