Lars,
In truth, we haven't even attempted to find or request standard names
for these. We used units of "days" for the counts of days. The freeze
date variables used units of "days since 1900-01-01 00:00:00". Things
are stable enough now that we need to get back to addressing the
standard name questions.
Jim
On 3/22/17 11:24 AM, Bärring Lars wrote:
Dear Jim, all,
This list is very interesting and helpful. I think that most of them
if not all are similar, in structure at least, to the ones we are
working on. To what extent you already managed to find a CF
description to describe the them?
I am asking because the ones that we are working on right now are
similar to your [Modified] Growing Degree Days and First/Last Freeze
Date, as well as the Pth Percentile X. The other ones I like to think
that we have covered, even though the actual proof of having test
files that pass the checker remains.
Thanks & Kind regards,
Lars
*From:*CF-metadata [mailto:[email protected]] *On
Behalf Of *Jim Biard
*Sent:* den 22 mars 2017 14:47
*To:* [email protected]
*Subject:* Re: [CF-metadata] Recording "day of year on which something
happens"
Hi.
Here are descriptions of a few different products I am working on
right now. This may help broaden the discussion a bit.
* Consecutive Dry Days - the maximum span of days over a given
interval (a year in my case) where less than 0.01 inches of
precipitation fell.
* Consecutive Wet Days - the inverse of consecutive dry days. The
maximum span of days where at least 0.01 inches of precipitation fell.
* Cooling Degree Days - the sum over an interval (a year in my case)
of the positive differences between daily average temperature in
deg F and 65 deg F.
* Heating Degree Days - the inverse of cooling degree days.
* Growing Degree Days - the sum over an interval (a year in my case)
of the positive differences between daily average temperature in
deg F and 50 deg F.
* Modified Growing Degree Days - the sum over an interval (a year in
my case) of the positive differences between modified daily
average temperature (average of Tmin and Tmax where Tmin values
below 50 deg F are set to 50 deg F and Tmax values above 86 deg F
are set to 86 deg F) in deg F and 50 deg F.
* First Freeze Date - The date of the first day in the interval from
Aug 1 of the year to April 30 of the next year when the daily
minimum temperature was 0 deg C or less.
* Last Freeze Date - The date of the last day in the interval from
Nov 1 of the previous year to July 31 of the year when the daily
minimum temperature was 0 deg C or less.
* Growing Season Length - The number of days between the first and
last freeze dates for the year.
* 1st Percentile X - The first percentile value of X (done for Tmax,
Tmin, and precip) over a 30-year interval.
* 99th Percentile X - The 99th percentile value of X (done for Tmax,
Tmin, and precip) over a 30-year interval.
* Days of X Above Threshold Y - The number of days in an interval (a
year in my case) where the value X (Tmax, Tmin, precip) was
greater than a threshold value Y.
* Days of X Below Threshold Y - The number of days in an interval (a
year in my case) where the value X (Tmax, Tmin, precip) was less
than a threshold value Y.
* Precip Sum Above Threshold Y - The sum over an interval (a year in
my case) of the quantity of daily precip above threshold Y.
The growing degree days and days of X above/below threshold are all
cases of one of
* Days of X Above Threshold Y
* Days of X Below Threshold Y
The cooling degree and heating degree days, and precip sum above
threshold Y are all cases of one of
* X Sum Above Threshold Y
* X Sum Below Threshold Y (The negative differences are all inverted
to become positive differences.)
Grace and peace,
Jim
On 3/22/17 5:33 AM, Jon Blower wrote:
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,
Jon
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