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: [email protected] STFC Rutherford Appleton Laboratory R25, 2.22 Harwell Oxford, Didcot, OX11 0QX, U.K. > -----Original Message----- > From: Hollis, Dan [mailto:[email protected]] > Sent: 03 September 2014 12:55 > To: Gregory, Jonathan; [email protected] > 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:[email protected]] On Behalf > Of Jonathan Gregory > Sent: 01 September 2014 17:42 > To: [email protected] > 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 > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata > _______________________________________________ > CF-metadata mailing list > [email protected] > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
