Hi Jennifer,
I'm afraid there are a few things I don't understand from your message.
Here are my questions/answers:
> 1. The absolute time axis ("time") has to span all the ensemble members
> -- thus it should have 18 time steps, beginning 1feb2000, incremented by
> 1 month. I know this leads to a lot of missing data, but in the
> GrADS/GDS environment, time is incompressible and the 18 members must
> all fit into the 5D grid. I noted that you didn't actually provide any
> axis values for your "time" dimension, but if that is the dimension for
> your data variable, it ought to be explicitly defined.
I used the way recommended in CF to encode forecasts. It uses a single
dimension for time ("time" in the file) with the variable with
standard_name "forecast_reference_time" indicating the forecast start
date and the variable with standard_name "forecast_period" indicating
the time when the forecast verifies. Both variables are referenced to an
exact date.
What do you mean by "you didn't actually provide any axis values for
your "time" dimension"? Does the time dimension need to be a variable
too? I thought dimension names can be independent from the names of the
variables that use them.
When you propose that the ensemble members should have 18 time steps, I
don't really understand what you mean.
> 2. The variable you call "reftime" is what we think of as the initial
> time of each ensemble -- exact naming of this to be determined by CF
> consensus. This variable should have dimension "ensemble" not "time"
> with the first 9 values referring to 1feb2000 and the 2nd 9 values
> referring to 1feb2001.
Please, could you confirm that the way a variable is named in the NetCDF
file is not relevant and what is actually meaningful is the standard name?
Why does reftime need to have dimension "ensemble" when it refers to the
start of the forecasts? In an ensemble forecast, all members are
expected to have the same start date and span the same lead time.
> 3. A new variable, called something like "ensemble_length" (once again,
> I defer to CF lexicon) has dimension "ensemble" and gives the number of
> time steps in each ensemble, in this case all elements will have a value
> of 6.
As I understand it, this concept of "ensemble_length" is the same as
"forecast_period" in the file.
> 4. I'm not sure what the variable time_bnd is used for.
This is based on the CF cells concept and it's used to indicate the
operation performed in the leadtime variable (which is the one that uses
the cells "time_bnd") as referred to in the physical variable attribute
"cell_methods". In the example, they give the limits of the month.
If you want to discuss this more in detail over the phone, let me know
where and when I can give you a call.
Best regards,
Paco
--
________________________________________
Francisco J. Doblas-Reyes
European Centre for Medium-Range
Weather Forecasting (ECMWF)
Shinfield Park, RG2 9AX
Reading, UK
Tel: +44 (0)118 9499 655
Fax: +44 (0)118 9869 450
f.doblas-reyes at ecmwf.int
_______________________________________
Received on Thu Nov 02 2006 - 10:05:26 GMT
