GROUP and Objectives

• Persons: Tao, Jan, Frederique, Catherine, Fuxing, Philippe P., Jean Louis, Josefine
• Objectives / roadmap : Possible roadmap (to be refined and detailed)
  • Modify the coupling of the new multi-layer snow with soil to be fully implicit. => date ? The current version in MICT works (couple and force mode) but is not fully implicit and the energy conservation at each time step should be checked! The work to make it fully implicit needs to be quantified with Tao (Anticipated to be relatively straightforward)

Meeting on discretization and snow coupling issues (26 Fev 2015)

Persons: Catherine, Agnes, Frederique, Joséfine, Jean Louis, Gerhard, Tao, Fuxing, Jan, Philippe

Location: Jussieu

Snow coupling

Presentation by Catherine and Tao (MICT version)

We had time only for a short presentation of the new snow module implementation: ​https://forge.ipsl.jussieu.fr/orchidee/attachment/wiki/Meetings/CMIP6/Physic/Orchidee_snow_CMIP6.pdf

Some features :

• the number of layers of snow module (ES) is always 3
• when there is a snow fall the whole grid is covered by snow ; only the albedo is calculated with a fractional snow cover (standard linear fractional cover)
• After "Enerbil" the "ES" module update the snow temp profile and accounts for melt, freezing, sublimation,... It then computes the coeficient for the next time step that is given to "Enerbil": the equivalent capacity "C" term that account in an implicit way the ground heat flux (here the flux through the snow pack); Then he provides "Thermosoil" with the lowest snow layer temperature. Thermosoil use that temperature to update its temperature profile and provide back a surface flux for "ES" for the next time step.
• It seems thus that the scheme is not fully implicit with respect to the "flux from ground to the snow" that is provided in an explicit way from the previous time step value.
• With the coupled simulation, Tao had to define a minimum snow depth (1 cm ?) to avoid "crashes"; possible problem of energy conservation ?

Discussion

We discussed the potential impact of the non fully implicit "soil-snow-atm" scheme. Catherine mentionned that in ISBA, Aaron B. had similar issues and that the change to fully implicit did not impacted much the results.

However, we agreed that with very thin snow layers this could be a problem (see "trick" of Tao in coupled mode).

We proposed that a first improvement is to account for fractional snow cover for the energy budget (not only for the albedo). For that the equavalent "C" term passed to "Enerbil" should be a mix between the standard one of "Thermosoil" and the one from "ES". But this needs some revision of the equation and calls of the routine (to be writen explicitely first).

JP propose that we also split the fluxes from Thermosoil back to enerbil and ES with different values.. this would ensure also a full implicit scheme.

PROPOSED ACTIONS

• Snow Scheme:
  • Josefine with Tao will implement the updated version of ES (currently in MICT) into TRUNC
  • We then further add a fractional snow cover so that the problem of "very thin" snow layer (linked to the non fully implicit scheme) becomes most likely negligeable. FOR THAT, we need to start an email discussion about the requested changes (how does it change the call sequences to "thermosoil" and "ES" ?): Anyone is wellcome to launch the email discussion. Then, JLD propose that Fuxing helps with this implementation but other organizations are possible.
  • Few persons needs to gather to establish properly the equation-changes that are needed for a full implicit implementation between "enerbyl" "ES" and "Thermosoil": the list of the "few" needs to be decided ? (volunter ?)