Posted by: @cathoderay

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I have at least answered my question about the ERA5 data gridding: it is 'regridded', at a horizontal resolution of 0.25° x 0.25° lat/long or 17 x 11 miles but it is based on NWP (numerical weather prediction) methodology which as the weather Johnnies are forever telling us is based on the laws of physics but still manages to get the forecasts wrong, often very wrong, and I have no doubt the same uncertainties apply to the so called ERA5 reanalysis. What is not clear, at least on first glance, is how many observations are used, and where they are taken.

There are other problems. The first is temporal resolution. I think the ERA5 data is probably run every 12 hours - so how do they get the hourly data? The other is the nature of the modelling and gridding means that the effective resolution is more like 4-5 grid lengths, say 75 x 50 miles (or maybe a bit less - but it is still hardly a fine resolution). 

What all this means is that the ERA5 data is not observed data, it is in effect a forecast of what the weather was like in the past. The bizzareness of this notion should be more than enough to make one's sceptical muscle twitch.      

degreedays.net has figures from actual weather stations.  Personally I would pick a close one unless in a frost hollow.

Posted by: @jamespa

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degreedays.net has figures from actual weather stations

Yes, but @jon used ERA5 data, which is based on NWP modelling. I'm not sure we know how well the modelled outside air temperature (OAT)  corresponded to the actual OAT. One way of checking it would be to compare the ERA5 data with data from a known observation station, but that would only be for that location. Weather is a most slippery thing.

Posted by: @cathoderay

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Posted by: @jamespa

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degreedays.net has figures from actual weather stations

Yes, but @jon used ERA5 data, which is based on NWP modelling. I'm not sure we know how well the modelled outside air temperature (OAT)  corresponded to the actual OAT. One way of checking it would be to compare the ERA5 data with data from a known observation station, but that would only be for that location. Weather is a most slippery thing.

All agreed.  I was suggesting degreedays.net as an (alternative) source of data that appears to come from actual, identifiable, weather stations

Posted by: @cathoderay

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Yes, but @jon used ERA5 data, which is based on NWP modelling.

Did Jon opt to use ERA5 data...?

... or was it Claude's choice?

@jon - please join in here.

This is a discussion topic rather than a thread where you can drop data and leave James and Cathode Ray to pore over it.

You are just as likely as anyone else to spot where the analysis is going wrong!  🙂 

@transparent Sorry for the silence, I'm working today but will reply properly this evening (yes, ERA5 was Claude's choice, but I can go back to degreedays.net and I also have my own temperature sensor figures).

Posted by: @jamespa

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I was suggesting degreedays.net as an (alternative) source of data that appears to come from actual, identifiable, weather stations

Yes, I got that, but it also has dispersed data. Suppose you live in Guildford, which weather station do you use, and how much credibility do you give it for your local weather?

Guildford is a University town. If Jon were there, there a reasonably high probability that the local data he'd want is already made available for use by University of Surrey students.

Apart from their undergraduate degrees, they do considerable research work on Climate Change. Each Research Team has a web-page with contact details at the bottom.

Tell them what you're attempting, and give them the URL of this Forum Topic. You might find there's an overlap with something already being looked into.