Hi Jonathan, et al.,
Legal air-quality standards often apply to multiple time-averaging lengths
simultaneously, eg in California the law imposes both a 1-hour limit of
0.09ppm, and an 8-hour limit of 0.07ppm (see
http://www.arb.ca.gov/research/aaqs/aaqs2.pdf). The UK also uses multiple
averaging times, 1-hour, 8-hour, and annual (see
http://uk-air.defra.gov.uk/documents/National_air_quality_objectives.pdf).
Hence, it is likely that there will be different averaging times in a file for
the same chemical.
Hence, there is a need in principle to be able to specify the three relevant
time quantities:
1) the length of the running-mean kernel (1-hour, 8-hour, annual),
2) the time interval within which the maximum of the running mean is searched
for (daily, monthly, yearly),
3) the point/time in the running-mean kernel that is used to decide whether an
8-hour mean falls within the time interval for #2, since an 8-hour mean will
often include values from outside the time interval.
In practice #3 is often not mentioned because there is usually a diurnal cycle
to air-quality that helps to avoid confusion in given instances (ozone peaks in
the afternoon, and smoke particles peak at night).
I assume that similar issues apply to daily temperature maximums and peak
precipitation rates, which could be important when comparing data and models
with different time-steps. Although in these cases I expect people implicitly
rely on information about the model timestep to implicitly describe the
averaging kernel.
Best wishes,
Philip
-----------------------------------------------------------------------
Dr Philip Cameron-Smith, [email protected], Lawrence Livermore National Lab.
-----------------------------------------------------------------------
> -----Original Message-----
> From: [email protected] [mailto:cf-metadata-
> [email protected]] On Behalf Of Jonathan Gregory
> Sent: Thursday, October 20, 2011 10:40 AM
> To: Schultz, Martin
> Cc: [email protected]
> Subject: Re: [CF-metadata] daily maximum of running 8-hour means
>
> Dear Martin
>
> Yes, I agree, "interval" is an ambiguous word, unfortunately.
>
> > YES - there will be a need for this. Current air quality data bases -
> the largest are probably the American AQS and the European AirBase
> systems - do store hourly data and "daily max 8-hour mean" values, and
> you want to be able to differentiate between those automatically
>
> OK. The distinction between those two can already be made like this:
>
> - hourly data have coordinates spaced hourly, and cell_methods with
> "mean"
> or "point" (depending on wheher they were hourly time-means or
> instantaneous
> values at hourly intervals).
>
> - daily max of running means have coordinates spaced daily, and
> cell_methods
> with "maximum".
>
> What we *can't* record in a standardised way is the extra information
> that the
> maximum was calculated from running means. We need to do this if there
> are also
> daily maxima calculated from other kinds of subdaily data.
>
> I apologise if this is frustrating. I am trying only to clarify what is
> needed
> in practice!
>
> Best wishes
>
> Jonathan
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