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

Bärring Lars Lars.Barring at smhi.se
Wed Mar 22 06:47:28 MDT 2017

Dear all,

Some further comments (top posting because my email reader does not easily handle inline comments)

1. I think that it is essential to also capture the temperature threshold used to calculate the GDD. The Wikipedia article suggests that 10 degC is most common (or even standard), but in my experience 5 degC is common, and this is what ETCCDI and ET-SCI are using in their definitions. And it seems that 5 degC is used in the scientific paper cited in Wikipedia article. Hence, a CF standard name should somehow be able to capture which threshold temperature is used.

2.The Wikipedia article mentions that "maximum temperature is usually capped at 30 °C" but this is to my understanding in relation to the simplified calculation using  diurnal midrange, (Tmax+Tmin)/2. as a 'proxy' for daily mean temperature. And it is not clear what is meant by "usually" in this context. For example ETCCDI and ET-SCI  definitions do not impose this upper limit, and their definitions are very well established and have for example been used in several rounds of IPCC assessments. Now, there might be (are) alternative definitions out there, some tweaked for specific purposes that involves all sorts of complexities. But if we are going to work towards defining some standard names I think it would be good to begin with the well-established and widely disseminated definitions, cf. the AMS glossary http://glossary.ametsoc.org/wiki/Growing_degree-day. 

Another aspect to keep in mind is that with modern (since a couple of decades...) measurement equipment it is possible to get higher temporal resolution than daily mean temperature (proper,  or estimated as diurnal mid-range). Hence there is also the very similar concept of growing degree hours, cf. http://glossary.ametsoc.org/wiki/Growing_degree-hour

Preferably, a standard name should be agnostic to the temporal resolution of the input data.

4. I guess that the cell methods, and units, will become clearer once the standard name has been teased out.

Jon, I would be interested to learn what your project colleagues are using.

Finally, to refocus back on the original topic of this thread (that is much broader than growing degree day thresholds dates) : We should not forget that there are many other use cases for a CF mechanism to record the first/last/etc. timing of an event in relation to a reference time. 

Kind regards,

-----Original Message-----
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jon Blower
Sent: den 22 mars 2017 10:34
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Recording "day of year on which something happens"

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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