Benno Blumenthal wrote:
> This is not just a model problem -- current meters once upon a time
> calculated average speed and instantaneous direction, which messed up
> calculations of component velocities. Also, thermisters have thermal
> mass, also introducing a lag with respect to other "simultaneously"
> measured quantities.
>
> We are just beginning to describe data better -- this is no time to
> perpetuate mistakes.
Jonathan Gregory wrote:
> The CF convention also provides cell_methods, which is an attribute of the
> data variable, to indicate how the data values are descriptive of the variation
> within cells (instantaneous, mean, integral, etc.). This information is an
> important complement to the coordinate information.
>
Cell methods applied to data variables are - or could be - the most
clear and efficient way to describe sampling schemes of observational
data - they just don't (yet) go far enough.
There's generally one clock in an instrument, and the single value it
records at each time step may need to be interpreted differently with
regard
to each measured variable - but is that a reason to generate separate time
variables for each one? I don't think that can be justified in most cases,
certainly not for long time series data sets; problems arise when there's
clock drift and times have to be adjusted - it seems much safer to keep a
single time variable, and extrapolate times for specific measured
variables using some kind of standard attributes.
As Benno mentioned, some instruments - not just old current meters, but
new ones, and most anemometers - still record a single direction and an
averaged speed. New instruments also record component velocities,
calculated with more frequently sampled, unrecorded direction, but
we carry along the recorded, sub-sampled directions as a check on compass
health.
Also, in real time data streams, the sample scheme preferred by most or
all
real time data aggregators has winds accumulated only over the last 10
minutes of an hour, while the other parameters are measured continuously
and averaged.
It would be useful to us to expand the cell methods, or to define
another attribute,
with standard terms to describe the position of a data variable measurement
relative to the time of the record. This seems to me to be preferable
to suggesting
that everyone use center time, and much more efficient than generating
multiple
time variables.
I don't see why this would in any way compromise the description of
observational
data, as long as there is a standard agreed upon for it.
Cheers - Nan
--
*******************************************************
* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 *
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Received on Wed Nov 18 2009 - 07:28:50 GMT
