Hi all,
Here is the third in a series of questions relating to our work on converting gridded UK observations data to NetCDF...
As many of you will know, climatological observations made in the UK have traditionally been gathered at 0900 GMT. These include maximum temperature, minimum temperature and precipitation amount.
We follow the guidance given in the WMO Guide to Climatological Practices which states:
"Precipitation amounts and maximum temperatures noted at an early morning observation should be credited to the previous calendar day" (p2-13)
The implication is that minimum temperatures should be credited to the calendar day of the observation. This all makes sense as the max will typically occur mid-afternoon whereas the min will generally occur around dawn.
So, for an arbitrary calendar day (e.g. 13 August 2013) the cell bounds would be:
Minimum temperature: 2013-08-12 09:00, 2013-08-13 09:00 (cell_method: minimum)
Max temp and precip: 2013-08-13 09:00, 2013-08-14 09:00 (cell_method: maximum and sum respectively)
We also follow the WMO guidance regarding the calculation of daily mean temperatures:
"the recommended methodology for calculating average daily temperature is to take the mean of the daily maximum and minimum temperatures" (p4-18)
The problem we have is how to describe the daily mean temperature, given that it is the mean of values from two different 24-hour periods.
One possibility is to simply interpret it as an estimate of the true mean temperature for the calendar day in question, and thus specify the bounds as midnight-midnight i.e.
2013-08-13 00:00, 2013-08-14 00:00 (cell_method: mean)
For consistency we propose specifying the value of the time coordinate to be the same for all variables i.e. 2013-08-13 09:00 (which is 09:00 on the calendar day to which the observations are credited - it is the end point of the minimum temperature bounds, the start point of the maximum temperature bounds, and part way through the proposed mean temperature bounds).
One colleague wondered whether it would be better (less confusing to the user) to consider all of the values to be estimates for the calendar day and therefore give all of the variables (max, min, mean and precip) the same midnight-to-midnight bounds (even though the actual observation period is different to this).
If anyone else has had to tackle this type of issue I would be very interested to know what approach you followed. Alternatively if anyone can give advice on how to correctly describe a mean value calculated in the way I have described that would be much appreciated.
Thanks,
Dan
PS We will also need to store monthly means and climatological averages of all these variables. I thought I'd start by asking about the daily quantities and see where the discussion led...
Dan Hollis Climatologist
Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United Kingdom
Tel: +44 (0)1392 886780 Fax: +44 (0)1392 885681
E-mail: dan.hollis at metoffice.gov.uk Website:
http://www.metoffice.gov.uk
For UK climate and past weather information, visit
http://www.metoffice.gov.uk/climate
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