[CF-metadata] Storing multiple NWP model runs in a NetCDF - CF file [SEC=UNCLASSIFIED]

From: Timothy Hume <T.Hume>
Date: Thu, 8 Jan 2009 13:03:12 +1100

Hi Karl,

Thanks for the help. I have remedied the situation by simply deleting the optional axis attribute from the scalar coordinate variable called "level". In my files, level isn't really an axis (in the sense it does not have a corresponding dimension); perhaps this is the reason for the prohibition?

I have also made some progress with the IDV viewer being able to display the data (which now pass all the tests in the CF-1.3 compliance checker). The latest version of IDV (24 December) will open the files (an earlier version didn't), but it still doesn't understand the method of identifying times which I use, and only appears capable of displaying a single base time. For the time being I am using ncview, which works perfectly on the files, but using IDV would be nice, because it has some pretty nice extra features.

Thanks,

Tim Hume
Centre for Australian Weather and Climate Research
Australian Bureau of Meteorology
Melbourne
Australia


-----Original Message-----
From: Karl Taylor [mailto:taylor13 at llnl.gov]
Sent: Thursday, 8 January 2009 12:07
To: Timothy Hume
Cc: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Storing multiple NWP model runs in a NetCDF - CF file [SEC=UNCLASSIFIED]

Hi Tim,

Section 4.4 (just before section 4.1) states "The methods of identifying
coordinate types described in this section apply both to coordinate
variables and to auxiliary coordinate variables named by the coordinates
attribute." Since the axis attribute is one of the methods used to
identify a vertical coordinate, it would seem to be allowed for the
level in your file.

On the other hand, in the 4th paragraph of section 5.7, it says that
"The axis attribute is not allowed for auxiliary coordinate variables".
  This seems to contradict the earlier statement.

We need to revise the document to be internally consistent. Does anyone
recall why we would want to prohibit the use of the axis attribute for
auxiliary coordinate variables?

cheers,
Karl

Timothy Hume wrote:
> Hi Karl,
>
> I have just checked one of my files, and get two errors:
>
> The first error is:
>
> ------------------
> Checking variable: forecast
> ------------------
> ERROR (4.4): Invalid units and/or reference time
>
> This is the error I am aware of. My file should become compliant if I switch the "T" axis to basetime.
>
>
> The second error is:
>
> ------------------
> Checking variable: level
> ------------------
> ERROR (4): Axis attribute is not allowed for auxillary coordinate variables.
>
> I was unaware that the axis attribute should not be used for scalar coordinate variables (as describe in Section 5.7). Is this intended? In any case, I don't think this small error should cause the NetCDF viewers I tried (IDV and Joe Sirott's web application) to not work.
>
> Cheers,
>
> Tim.
>
> -----Original Message-----
> From: Karl Taylor [mailto:taylor13 at llnl.gov]
> Sent: Thursday, 8 January 2009 11:03
> To: Timothy Hume
> Cc: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Storing multiple NWP model runs in a NetCDF - CF file [SEC=UNCLASSIFIED]
>
> Hi Tim,
>
> Other than the axis attribute for time, I didn't see any issues. You
> might run the CF compliance checker on the file
> (http:// cf-pcmdi.llnl.gov/conformance), but it might not run if the
> other utilities stumbled. Maybe someone else has some ideas.
>
> cheers,
> Karl
>
>
> Timothy Hume wrote:
>> Hi,
>>
>> I am writing NetCDF files which hold surface fields (2m temperature etc) from multiple NWP model runs (all the runs from the same model for the last month or so). The files are for operational use, so I want them to strictly follow the CF conventions. I am running into a couple of problems, where the conventions don't seem to be ideally suited for storing more than a single model run.
>>
>> Here is what I do:
>>
>> I have four dimensions and associated coordinate variables:
>>
>> basetime: The base time for the model run (units: seconds since 1970-01-01 00:00:00.0 +0:00)
>> forecast: The forecast lead time, relative to the basetime (units: hours)
>> latitude
>> longitude
>>
>> There is no need for a vertical dimension, because I am using surface fields. Never-the-less I make use of a scalar vertical coordinate variable as described in Section 5.7 of the CF-1.3 metadata conventions document.
>>
>> The use of two dimensions to store the time information (basetime and forecast) seems to be a natural way to store multiple NWP model runs, and is the standard way used in the very old NUWG conventions. As far as I can tell, what I am doing is supported by the CF conventions, provided the time axis is taken to be the basetime dimension. This is because Section 4.4 of the conventions specifies the units of the time coordinate must include a reference time. Obviously the units of the forecast coordinate cannot include a reference time, because the reference time varies, and is determined by the value of the basetime coordinate.
>>
>> The difficulty I am encountering is that some applications which read NetCDF/CF files (such as the Unidata IDV and Joe Sirott's very nice web based NetCDF data viewer) seem to choke on my data. I suspect (but am not 100% certain in the case of the IDV) that the reason is because of the way I handle the time information in my files.
>>
>> To illustrate in more detail what my files look like, I am attaching the CDL from an example file. The CDL is non-CF compliant, because I have specified the "T" axis (via the axis variable attribute) to be the forecast coordinate, and the forecast coordinate has invalid units (no reference time). I am planning on switching the "T" axis to the base time coordinate, which as far as I can determine should make the file CF compliant.
>>
>> My question is: is there a better (or more standard) way of storing multiple NWP model runs in a single file than what I am doing?
>>
>> Cheers,
>>
>> Tim Hume
>> Centre for Australian Weather and Climate Research
>> Australian Bureau of Meteorology
>> Melbourne
>> Australia
>>
>> --- Example CDL follows ---
>>
>> netcdf gasp_1p0deg_ocf_t2m_rtdb_opn {
>> dimensions:
>> forecast = 69 ;
>> basetime = UNLIMITED ; // (100 currently)
>> latitude = 96 ;
>> longitude = 121 ;
>> bounds = 2 ;
>> variables:
>> double forecast(forecast) ;
>> forecast:long_name = "Time of model forecast, relative to the basetime" ;
>> forecast:units = "hours" ;
>> forecast:standard_name = "forecast_period" ;
>> forecast:axis = "T" ;
>> forecast:bounds = "forecast_bounds" ;
>> double forecast_bounds(forecast, bounds) ;
>> forecast_bounds:long_name = "forecast interval" ;
>> forecast_bounds:units = "hours" ;
>> int basetime(basetime) ;
>> basetime:long_name = "Model basetime" ;
>> basetime:units = "seconds since 1970-01-01 00:00:00.0 +0:00" ;
>> basetime:calendar = "gregorian" ;
>> basetime:standard_name = "forecast_reference_time" ;
>> double latitude(latitude) ;
>> latitude:long_name = "latitude" ;
>> latitude:units = "degrees_north" ;
>> latitude:bounds = "latitude_bounds" ;
>> latitude:valid_min = -90. ;
>> latitude:valid_max = 90. ;
>> latitude:standard_name = "latitude" ;
>> latitude:axis = "Y" ;
>> double latitude_bounds(latitude, bounds) ;
>> latitude_bounds:long_name = "grid cell latitude boundaries" ;
>> latitude_bounds:units = "degrees_north" ;
>> latitude_bounds:valid_min = -90. ;
>> latitude_bounds:valid_max = 90. ;
>> double longitude(longitude) ;
>> longitude:long_name = "longitude" ;
>> longitude:units = "degrees_east" ;
>> longitude:bounds = "longitude_bounds" ;
>> longitude:valid_min = -360. ;
>> longitude:valid_max = 360. ;
>> longitude:standard_name = "longitude" ;
>> longitude:axis = "X" ;
>> double longitude_bounds(longitude, bounds) ;
>> longitude_bounds:long_name = "grid cell longitude boundaries" ;
>> longitude_bounds:units = "degrees_east" ;
>> longitude_bounds:valid_min = -360. ;
>> longitude_bounds:valid_max = 360. ;
>> float temperature_2m(basetime, forecast, latitude, longitude) ;
>> temperature_2m:long_name = "Air temperature 2m above the surface" ;
>> temperature_2m:units = "K" ;
>> temperature_2m:_FillValue = 9.96921e+36f ;
>> temperature_2m:missing_value = 9.96921e+36f ;
>> temperature_2m:valid_min = 180.f ;
>> temperature_2m:valid_max = 330.f ;
>> temperature_2m:standard_name = "air_temperature" ;
>> temperature_2m:cell_methods = "lat: lon: mean (area weighted)" ;
>> temperature_2m:coordinates = "level" ;
>> double level ;
>> level:long_name = "Height above the surface" ;
>> level:units = "m" ;
>> level:positive = "up" ;
>> level:standard_name = "height" ;
>> level:axis = "Z" ;
>>
>> // global attributes:
>> :Conventions = "CF-1.3" ;
>> :history = "File created by the Gridded OCF data ingest system" ;
>> :institution = "Australian Bureau of Meteorology" ;
>> :source = "model" ;
>> :title = "GASP forecasts of temperature_2m; resolution: 1.0 degree; source: rtdb" ;
>> :topography = "MSAS" ;
>> }
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>>
>
>
>
Received on Wed Jan 07 2009 - 19:03:12 GMT