met%tk and met%tvrad confusion in ACCESS

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keyword_keepgit resolution_fixed type_ACCESS-CM2 | by inh599

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The JULES model evaluates the sensible heat flux as proportional to a difference (surface to atmosphere) in potential temperature; CABLE (broadly) evaluates the sensible heat flux as proportional to a difference in temperature. In an attempt to align CABLE with JULES within ACCESS, CABLE-within-ACCESS (UMv8.5) used two near surface air temperatures to force the model. i) The temperature from the UM (TL_1) is used directly within the radiation component of CABLE as %tvrad and ii) TL_1 is increment by an amount determined by the lowest model level as %tk elsewhere CABLE.

However the use of the different temperatures was incorrectly implemented leading to a ~2W/m2 energy imbalance in the surface as seen by the atmosphere (i.e. not within CABLE itself).

A review of the code base and underpinning literature identified three potential solutions

A) removal of the difference between %tvrad and %tk with no increment applied (option 2 in ppt)

B) removal of the difference between %tvrad and %tk retaining the increment in both(option 4 in ppt)

C) ensuring that use of %tvrad and %tk is correct throughout CABLE (option 3 in ppt)

Each of these options solves the underpinning energy balance issue with little change in short-run simulations.

After discussion, the approach identified for ACCESS-CM2 has been to i) retain the code changes (C) so that different temperatures could be used (in the future) but ii) to remove the temperature increment from both %tk and %tvrad (A).

The rationale for this decision is twofold: firstly, removal of the increment aligns the ACCESS version of CABLE closer to the offline model and is easier to document; secondly, the increment method does not address properly the underlying justification for the change and risks introducing other errors (even with a full and correct implementation). Retaining the code changes provides some security to future work and provides backwards compatibility to earlier versions of ACCESS (and possibly ACCESS-ESM1.5).

The underlying issue of the appropriate formulation of the sensible heat flux will require revisiting and impacts both offline and coupled CABLE.

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Issue migrated from trac:197 at 2023-11-27 11:26:21 +1100

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@mrd599@nci.org.au _uploaded file CABLE_ticket197.pptx (2419.5 KiB)_

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@mrd599@nci.org.au commented

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Attached presentation shows the surface energy budget problem in the ACCESS CM2 cable run (u-ax871) that prompted this analysis and various single column model experiments used to track it down and compare possible fixes.

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@jxs599@nci.org.au changed status from new to closed

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@jxs599@nci.org.au changed resolution from ` tofixed`

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@jxs599@nci.org.au changed milestone from ` to1. Closed`

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@ccc561@nci.org.au set keywords to maygit

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@ccc561@nci.org.au changed keywords from maygit to keepgit