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

Jim Biard jbiard at cicsnc.org
Wed Mar 22 07:47:13 MDT 2017


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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