Originally Posted by
Aurora Australis
The point I think BV is making is that the TAFs do include a forecast of conditions above anemometer height - the part of the TAF that appears to trigger the closure of the airspace is the 56//// turbulence code, typically for turbulence up to 3000'. The rest of the TAF often does not contain anything suggesting conditions that would cause major concern. Until recently, it was just the runway that was closed - aircraft could still transit the zone during the forecast conditions, and regularly did (without falling out of the sky or losing control......).
As BV said, you would expect the METARs to bear some similarity to the conditions forecast in the TAF, yet frequently (e.g. as per the data copied in my post #54) there is very little correlation. The accuracy of the TAFs is presumably assessed for quality control - (I would be interested to see the Met Offices own assessment of how accurate the MPA TAFs are), but how is the accuracy of the turbulence forecast for the layers above the airfield assessed, without allowing aircraft through?
A subjective assessment by the majority of pilots I know who have used this airspace over the past 30 years is that recently, the TAFs often seem extremely pessimistic, and the actuals often bear no resemblance to the forecast.
Once again, at the risk of labouring the point, none of us locally think that inbound aircraft should land regardless of conditions, but just that there should be some flexibility, particularly when the forecast conditions are nowhere near materialising.
Yesterday, the forecast was for severe turbulence, and I have no doubt it did occur, and would not have been a day to mess with - but the actuals were reflecting this, e.g. 340/30g46, and 320/09 tempo 350/20g30. (The southbound airbridge was delayed 24 hours).
Thank you. I must make the point that I retired as a Chief Met Officer in the UK Met Office in 1996 and had never heard of a 56//// group. I would have been startled to be asked to predict quantitatively in a TAF something that could not be measured Everything else in a TAF can be checked against the actual [not necessarily a METAR, but certainly a SYNOP.] The non-critical exception is cloud base above the effective range of the relevant devices. Perhaps memory fails me.
I have searched Google but cannot find chapter and verse for the 56 group coding.
It is well understood that turbulence, other than convective, is a mysterious beast. Our investigations into CAT using aircraft reports and aircraft chasers demonstrated that it is like a shoal of fish in the ocean .......... mobile, transient and fickle. This included CAT nearer the surface than jet streams, but we only came up with generalisations such as coastlines, mountain ridges etc. Rotors and lee waves were acknowledged as very very difficult to predict, especially in dry air with no tell-tale cloud. Our numerical models became so good at jet levels that, as a by-product, CAT at these heights became better understood and better forecast, but far from perfect.
I am left wondering what has changed in 20 years regarding low-level terrain-induced shears and turbulence. Understanding? I doubt it. Measuring? I doubt it. Predicting? I doubt it.