Improvements of the Driver of ORCHIDEE

These improvements will concern first the specificities of the driver (its capacity to read forcing files), not its performances that will be improved hopefully by the use of the IOServer. The focus is mainly on the use of forcing files at a 6 or 3-hourly time resolution (such as re-analysis), for which the driver interpolates the meteorological fields to calculate half-hourly fields.

Reminder

The driver of ORCHIDEE is expected to read forcing files with a very specific temporal structure. All time information are defined on UTC time. The fields read at a certain time step t in the NetCDF file correspond to:

• the instantaneous value at t+dt of scalar variables : Temperature, Wind Speed, Air Pressure and Specific Humidity.
• the mean value between t and t+dt for fluxes : Long and Short Wave incoming Radiation and the Precipitation.

Where dt is the time step of the forcing file.

AD (26/5/16): Note that, in the above case, the time at which we read the forcing is t+dt. Let's introduce an index itau of the forcing records: itau=1 is the first record in the nc file for all variables, etc. Imagine we are currently reading the record itau_n, coming juste after the record itau_nm1. These are the notation of dim2driver/read2dim, and we have itau_n=itau_nm1+1. The "time_stamps" corresponding to these two records are separated by dt (the dt between two records, and not dt_sechiba: dt=split*dt_sechiba). To make the link with the above notations of time, the time stamp of the record itau_n is t+dt, and the one of of itau_nm1 is t.

Now we need to remember where we are in the code, and what we need the forcing for. When we read itau_n, with values corresponding to t+dt, sechiba hasn't yet run between t and t+dt. It has run (or been initialized) up to t, with forcing controlled by the forcing records itau_nm1 and before. So when we read the record itau_n corresponding to the time stamp t+dt, we know the state of sechiba at t, the forcing values at t (from itau_nm1), and we want to correctly define the forcing variables over each dt_sechiba between t and t+dt (dt=split*dt_sechiba).

For me, these are general principles which need to be true whichever the forcing if we decide to read together all the variables that have the same time stamp. The fact that fluxes are averages over a period of length dt after or before the time stamp can be dealt with in the above general framework, at the initialisation stage (but the latter is an opinion).

Interpolation

• Instantaneous fields and the Long-Wave Radiation are then linearly interpolated for each model time-step t' (typically 1/2 hour) based on the values available at t and t+dt with t < t' < t+dt. Should we not change that for long-wave radiation so that we have one consistent procedure for scalars and another for fluxes ?
• The Precipitation field is downscaled to model's temporal resolution using a step distribution function with one parameter (nb_spread, that is the number of half-hourly time steps over which we spread the precipitation). nb_spread can vary from 1 (all precipitations over a forcing time period (3 ou 6 hours) are put on the first model time step) to SPLIT_DT (number of integration time steps with a forcing time period, in that case, the precipitation are distributed uniformly). By default, nb_spread is set to 1. This default value is not appropriate for all climates and it would make sense to have typical values for various regions of the world (see thesis of T. d'Orgeval.).
• The Short-Wave radiation is interpolated using a function distribution that corresponds to the solar angle distribution over a forcing time period and conserving the average flux for the [t,t+dt] interval. Under low incidence angles this interpolation gives at times strange values, this needs to be checked and improved.

Proposed correction (Jan 26/05/2016)

The graphic below explains probably better the above description.

The following changes in nomenclature are used in the figure : t has become Tf and thus the forcing considered is between Tf and Tf+1 for the interpolation toward t'.

The figure is especially meant to illustrate that the interpolation for fluxes assumes that the interval of validity of the fluxes is assumed to be between Tf-1/2 and Tf+1/2 or, t-dt/2 and t+dt/2.

Add an explicit time information within the forcing files

As described above, the driver of ORCHIDEE is relatively rigid because it makes strict assumptions one temporal specification for the forcing files. At first, we do not expect to develop a more flexible driver but we would like to add explicit time information in the driver in order to avoid misuse. We suggest to add a time_bounds attribute to the time variables stored in the Netcdf forcing file according to the CM convention : ​http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/cf-conventions.html

Each file will contain at least 2 time axes, one for the scalar variables and one for the flux variables. Each of the time axes will have its "time_bounds" variable in order to describe the interval each value describes. A time_op attribute (average, point) for each variable could be also added. The time specification of a forcing file (based on time_bounds and time_op attributes) should be read by the driver, in order to check for consistency.

Questions that remain:

• How to implement in detail such a solution without perturbing too much the day-to-day usage of ORCHIDEE.
• It would be most easily managed if we build a library of forcing files available to all at IPSL and which contains only forcing data sets with all the information.
• Should we try to support old and incomplete forcing files and for how long ?
• The improved driver should use all the extra time information in the forcing files but this might require some changes to IOIPSL.

Restart the last forcing time step read

As the driver is built now, the first time step read (for the Temperature for instance) is valid for t+dt. This means that for a yearly file at a 6-hourly time resolution, the first temperature read is for "1-JAN 06:00".
For a simulation starting from scratch (no restart), the driver uses the first value for the second day ("2-JAN 00:00") in order to interpolate with the first one readed (and to calculate half-hourly values between "1-JAN 00:00" and "1-JAN 06:00". At the end of a simulation, the last forcing file read is not written in the restart file of the driver.
When we perform monthly runs using a yearly forcing file, it is managed within the driver to use the appropriate time step in the forcing file but when we perform yearly runs using a yearly file, we process as for a run starting from scratch (while we could use the last values read during the last run).
We suggest to restart the last time step read of the meteorological variables in order to be make possible this.

Once the improved driver knows about the time interval over which the forcing is valid, then we can test if the restart date is correct and we can dump the next restart at the end of the time interval.

Remove the reading of the watchout files

In ORCHIDEE, there is the possibility to write in an output file, the meteorological fields used during the simulation. It is a feature useful when doing a coupled simulation (ORCHIDEE coupled to LMDz) in order to be capable to re-use this 'forcing' in off-line mode. These output files are named WATCHOUT files. These WATCHOUT files are not based on the same template than the standard forcing files read by ORCHIDEE.
Consequently, there are specific reading for these WATCHOUT files.
We suggest to :

• either generate the WATCHOUT files in ORCHIDEE under the format than a standard forcing file
• or, develop a specific tool that convert the WATCHOUT file into the same format than a standard forcing file. This tool will be launch as a pre-treatment.