OBJECTIVES

The objective of this page is to present and discuss the existing coupling scheme between ORC and LMDZ and to propose new solution to improve physical/numerical consistency in the coupling.

The main points concern i) the treatment of the Evaporation and how the "beta" factor (soil water stress) is treated as well as how the potential evaporation is calculated and ii) the treatment of the radiation in ORC (longwave and shortwave) to ensure energy conservation when coupled with LMDz.

Several documents have been produced and are usefull to understand such coupling and the major points:

• General Coupling with GCM: ​Article on Coupling
• Updated-note on the coupling by Polcher: ​Updated note (Polcher)
• Note by Dufresne and Ghattas on coupling with LMDZ: ​Dufresne & Ghattas
• New note by Fuxing on recent implementations: ​Fuxing note

EVAPORATION

Evaporation during coupling (beta problem)

1. Problem:

The derivation of the surface temperature as it is done in ORCHIDEE-TRUNK is not consistent with the surface energy budget equation proposed by Polcher (Polcher et al. 1998).

In the trunk of ORCHIDEE, the stress factor beta is missing in the denominator in the sensitivity of the latent heat flux to the surface temperature at the old time step. This is coded in enerbil as:

larsub_old = chalsu0 * vbeta1 * (un - vbeta5) * (peqBcoef -  qsol_sat) / (zikq - peqAcoef)

This is not consistent with the surface energy budget equation and can lead to energy conservation problem.

2. Action decided

It has been decided to re-introduce the beta and take the following formulation for the ORCHIDEE-TRUNK:

larsub_old = chalsu0 * vbeta1 * (un - vbeta5) * (peqBcoef -  qsol_sat) / (zikq - vbeta1 * (un - vbeta5)* peqAcoef)

Note that the correction does not change significantly the results of ORC-LMDZ simulations.

Overestimation of Potential Evaporation

However, the implementation of the potential evaporation formula based on a bulk formulation that consider a moist surface {ETP=rau/ra*[qsat(T*)-qa]} is not satisfactory;

Currently the code considers qstat(Ts) and not qsat(T*), the temperature of a moist surface and thus over-estimates the evaporation: The temperature Ts is too warm on average than T*.

In the present version, this is tackled by correcting the bare soil evaporation afterwards with the Milly approach.

Jan proposed improved approaches based on the Penman-Monteith method to calculate the ETP (formulation that provide smaller ETP than the bulk formulation that is used). They have been developed by Anais Barella during her PhD thesis and tested in forced mode. The correct surface energy budget equation for the implicit coupling is not yet available and work has to be done before implementing this approach in Orchidee.trunk. ​http://www.lmd.polytechnique.fr/~intro/Files/2014_These_Barella.pdf).

It is stressed by all the participants that improvements on the stress factor are important and that Anais work is a first step in this direction. However, more work is needed and a clear formulation for the new approach must be well documented.

Note that other GCM groups also face the same issues and have used various approaches not to overestimate ETP.

RADIATION

In order to save computing time, the time step of the radiation code is longer than other components of the model (rest of the atmospheric physics and LSM). If the radiation budget of each component is updated at a shorter time step, the energy is not conserved. This is the case in the ORCHIDEE.trunk version where the LWnet (lwup) is calculated separately in LMDZ and ORCHIDEE. An option which favors the energy conservation (practically the LWup is updated only when radiative code is activated) has been introduced by Fuxing and Jean-Louis.

It is recognized that none of the option is satisfactory and that improvement is needed. However, in the mean time it is required that both option are available in Orchidee.trunk to be able to check for energy conservation problems (the option favoring energy conservation has to be committed in the trunk).

To go further some work is necessary on the LMDZ and ORCHIDEE sides as well as on the interface.

• LMDZ is currently implementing a new radiation code (RRTM) for the SW radiation, which will allow to increase the number of spectral bands for the albedo (expecting that the spectral bands are consistent with the one adopted for the albedo calculation in Orchidee). A post-doc student is working at LMD on the implementation of the new radiative code.
• ORCHIDEE will give to LMDZ an averaged (over the time-steps at which the radiation is not called) radiative temperature, emissivity and albedo
• The interface variables have to be redefined (net LW and SW radiation, in several bands ?, emissivity, albedo, radiative temperature) but this has to be documented very clearly. It is stressed that these developments can suppress the backward compatibility.