Joel,
There is no current standard for dealing with Local Solar Time (LST). I
assume that the time and timebounds values would be elapsed times
represented as fractions of days in quasi-UTC (the time at longitude 0).
This is valid netCDF, but I'm not at all sure what existing software
packages would do with it. Time is the one dimension that most software
packages assume to be 1-dimensional. Another possible approach would be
to have a notional 1-D time variable that would be set to noon UTC of
each day and rename your time variable in your example to localtime. It
would look something like this:
dimensions:
time = UNLIMITED ; // (1 currently)
latitude = 720 ;
longitude = 1440 ;
bounds = 2 ;
variables:
float time(time) ;
time:units = "days since 1850-1-1 0:0:0" ;
time:long_name = "Time (days)" ;
time:standard_name = "time" ;
time:calendar = "gregorian" ;
float localtime(time, longitude) ;
time:units = "days since 1850-1-1 0:0:0" ;
time:long_name = "Time (days)" ;
time:standard_name = "time" ;
time:bounds = "timebounds" ;
time:calendar = "gregorian" ;
float timebounds(time, longitude, bounds)
float latitude(latitude) ;
latitude:units = "degrees_north" ;
latitude:long_name = "Latitude (deg)" ;
latitude:standard_name = "latitude" ;
float longitude(longitude) ;
longitude:units = "degrees_east" ;
longitude:long_name = "Longitude (deg)" ;
longitude:standard_name = "longitude" ;
short tas(time, latitude, longitude) ;
tas:units = "K" ;
tas:long_name = "Mean daily surface air temperature (K)" ;
tas:standard_name = "air_temperature" ;
tas:coordinates = "localtime" ;
tas:cell_methods = "localtime: mean" ;
tas:_FillValue = -32768s ;
tas:scale_factor = 0.002 ;
tas:add_offset = 273.15 ;
This provides an unambiguous, if notional, time dimension, and still
associates the local time values with the data values.
Grace and peace,
Jim
On 7/28/16 7:58 AM, Mitchelson, Joel wrote:
> Dear cf-metadata community,
>
> We are working on new high resolution daily data sets of climate observations as part of the EUSTACE project ( http://www.eustaceproject.eu/ ), where fields such as surface air temperature are presented on global grids, and the sampling period for each grid box is from midnight to midnight in local solar time. In other words, the UTC time period of each grid box depends on longitude.
>
> It seems that within CF conventions there might be several possible approaches to expressing this, though no specific example appears to be given in the conventions documents (versions 1.6 or 1.7). It would be desirable if our chosen method works nicely with as many existing software tools as possible; and uses established conventions if they exist.
>
> Do conventions for this already exist?
>
> If not, is the example CDL fragment below likely to be easy to use within many existing software tools for working with gridded fields of climate data, or might there be a more widely compatible method?
>
> Many thanks in advance for your kind attention.
>
> Joel.
> --
> Dr. Joel R. Mitchelson Observational Climate Data Software Specialist
> Met Office Hadley Centre FitzRoy Road Exeter Devon EX1 3PB United Kingdom
>
>
> --
> // Example CDL fragment representing a single day of data in one NetCDF file.
> // To represent more than one day multiple files could be used,
> // or else a single file with the localtime dimension set to equal the number of days.
>
> dimensions:
> localtime = UNLIMITED ; // (1 currently)
> latitude = 720 ;
> longitude = 1440 ;
> bounds = 2 ;
>
> variables:
>
> float time(localtime, longitude) ;
> time:units = "days since 1850-1-1 0:0:0" ;
> time:long_name = "Time (days)" ;
> time:standard_name = "time" ;
> time:bounds = "timebounds" ;
> time:calendar = "gregorian" ;
>
> float timebounds(localtime, longitude, bounds)
>
> float latitude(latitude) ;
> latitude:units = "degrees_north" ;
> latitude:long_name = "Latitude (deg)" ;
> latitude:standard_name = "latitude" ;
>
> float longitude(longitude) ;
> longitude:units = "degrees_east" ;
> longitude:long_name = "Longitude (deg)" ;
> longitude:standard_name = "longitude" ;
>
> short tas(localtime, latitude, longitude) ;
> tas:units = "K" ;
> tas:long_name = "Mean daily surface air temperature (K)" ;
> tas:standard_name = "air_temperature" ;
> tas:coordinates = "time" ;
> tas:cell_methods = "time: mean" ;
> tas:_FillValue = -32768s ;
> tas:scale_factor = 0.002 ;
> tas:add_offset = 273.15 ;
>
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Received on Thu Jul 28 2016 - 11:18:14 BST
