[CF-metadata] Recording "day of year on which something happens"

Jon Blower j.d.blower at reading.ac.uk
Wed Mar 22 03:33:35 MDT 2017

Dear all,

Thanks again for the helpful replies to my last summary email. I’ll pick up on the points here:

 1. I realise that my use of the word “threshold” may have been confusing in this context. I was following the precedent set by previous standard names. The variable would record the day of the year (or growing season) on which the “threshold” number of degree days is attained. The possible values of this threshold are stored in a coordinate variable. This is very different from the “threshold” temperature that is used in the calculation of the “growing degree day” parameter itself.

 2. I’m not at all an expert here, but my understanding is that there are various possible ways to calculate GDDs. Lars has helpfully pointed out that ET-SCI and ETCCDI definitions exist, and I’ll pass these on to the project team – maybe that’s what the team are using. But anyway, I’m not totally sure that “integral_of_air_temperature_excess_wrt_time” is strictly accurate in all cases, since it’s not always simply a question of integrating some “delta-T” over time. The Wikipedia article points out some ways in which GDD is not a strict integral (e.g. in some cases it is considered that there is a maximum number of GDDs that can be meaningfully attained in a day).

 3. David correctly pointed out the “Northern Hemisphere chauvinism” in my proposal. Our project is focused on Europe, but it is quite correct to consider how the same approach might apply to the southern hemisphere growing season. 

 4. I’m still not convinced about using “sum” in cell_methods. This might be appropriate if the variable in question were GDD, but the variable is actually a _time_ at which we reach a certain number of GDD. We are not summing time, so I’m not sure that using “sum” is right. Happy to be corrected on this though, maybe I’ve misunderstood the intention.

I think I need to discuss these issues within the project to work out exactly what we’re actually going to be recording – I’ll do this and report back our thoughts. 

Many thanks again,

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