Dear Dan,
I agree that setting a threshold depending on how the data were collected does not seem very satisfactory and it wouldn't allow you to combine readings from a variety of instruments into a single data product. The definition of 'greater|less than or equal to' is clearly not the same as 'greater|less than' so I would argue that they are different quantities and should have different standard names. Currently we have only nine 'threshold' names in the table and personally I don't think it's a big problem to add one more of the form
number_of_days_with_lwe_thickness_of_precipitation_amount_at_or_above_threshold. The definition would of course need to make clear the difference from the existing name and in fact the definitions should cross-reference one another to make users aware of both. Do others agree?
Regarding searching the mailing list archives, if I want to find a very specific phrase within the email text I download the plain text file for the appropriate year (available from the main archive page
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/) and use my browser's 'Find' function. It takes a few minutes to download but it can be a useful way of pinpointing the thing you're looking for.
Best wishes,
Alison
------
Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Oxford, Didcot, OX11 0QX, U.K.
> -----Original Message-----
> From: Hollis, Dan [mailto:dan.hollis at metoffice.gov.uk]
> Sent: 03 September 2014 12:55
> To: Gregory, Jonathan; cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Days of rain
>
> Hi Jonathan,
>
> For manually-read rain gauges the advice to the observer is simply to record
> the measurement to one decimal place. For the thresholds of interest this
> seems to me to be equivalent to saying the values have been rounded.
> Therefore 0.2 mm does mean 0.15-0.25 mm, 1.0 mm means 0.95-1.05 mm,
> and 10 mm means 9.95-10.05 mm.
>
> In contrast automated sites use a tipping bucket gauge (in the UK at least)
> which constrains the observations to be multiples of the bucket size. I
> believe that for all the data we use this is a nominal 0.2 mm i.e.
> precipitation totals can be 0.0 mm, 0.2 mm, 0.4 mm etc, and values such as
> 0.1 mm, 0.3 mm, 0.5 mm cannot be reported. Given that all we know is that
> the bucket has tipped (i.e. has become full and caused the mechanism to tip
> and empty the bucket) this implies that an observation of 0.2 mm actually
> means 0.2 <= true value < 0.4 (because the bucket has not yet tipped a
> second time).
>
> For our gridded climate datasets (rainfall total, days of rain etc) we use data
> from both types of gauge without correction or adjustment. I think we can
> be fairly confident that uncertainties in the interpolation process will be
> quite a bit larger than either the observation uncertainty or the differences
> between the two observation types i.e. these types of subtlety are probably
> 'in the noise' and to be honest not something I'd given much thought to.
>
> In conclusion I'm slightly reluctant to specify a threshold that tries to reflect
> how the observations have been gathered, partly because this is not the
> same for all sites and partly because it could change in the future (e.g. if we
> were to adopt a different type of rain gauge). Personally I'd prefer to
> describe what we do to the data once it has been collected, which in the
> case of the 'days of rain' variables is to test if the value is greater than or
> equal to a threshold.
>
> Thoughts?
>
> Regards,
>
> Dan
>
>
>
> -----Original Message-----
> From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf
> Of Jonathan Gregory
> Sent: 01 September 2014 17:42
> To: cf-metadata at cgd.ucar.edu
> Subject: [CF-metadata] Days of rain
>
> Dear Dan
>
> > We have several variables that we describe loosely as 'days of rain'.
> Strictly speaking they are a count (e.g. for a calendar month) of the number
> of days when the 24-hour precipitation total was greater than or equal to a
> threshold. We currently generate grids for three thresholds - 0.2mm, 1.0mm
> and 10.0mm. My intention is to use the following existing standard name:
> >
> >
> number_of_days_with_lwe_thickness_of_precipitation_amount_above_thr
> eshold
> >
> > My only slight problem is that the definition implies 'greater than'
> whereas our variables are 'greater than or equal to' the threshold. Assuming
> the observations have a precision of 0.1 mm ...
>
> I think it depends on how the data have been treated. Are they rounded to
> the
> nearest 0.1 mm? If so, a recorded value of 0.0 mm means an actual value in
> the
> range 0.00-0.05 mm, 0.1 mm means 0.05-0.15 mm, 0.2 mm means 0.15-0.25
> mm, etc.,
> and your threshold of 0.2 mm in recorded precipitation is actually a
> threshold
> of 0.15 mm. That is therefore what I would suggest as the coordinate value
> for
> the threshold.
>
> Best wishes
>
> Jonathan
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