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| Titel |
Physically Accurate Soil Freeze-Thaw Processes in a Global Land Surface Scheme |
| VerfasserIn |
Matthias Cuntz, Vanessa Haverd |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250087750
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| Publikation (Nr.) |
 EGU/EGU2014-1808.pdf |
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| Zusammenfassung |
| Transfer of energy and moisture in frozen soil, and hence the active layer depth, are strongly
influenced by the soil freezing curve which specifies liquid moisture content as a function of
temperature. However, the curve is typically not represented in global land surface models,
with less physically-based approximations being used instead. In this work, we develop a
physically accurate model of soil freeze-thaw processes, suitable for use in a global land
surface scheme.
We incorporated soil freeze-thaw processes into an existing detailed model for the
transfer of heat, liquid water and water vapor in soils, including isotope diagnostics –
Soil-Litter-Iso (SLI, Haverd & Cuntz 2010), which has been used successfully
for water and carbon balances of the Australian continent (Haverd et al. 2013). A
unique feature of SLI is that fluxes of energy and moisture are coupled using a single
system of linear equations. The extension to include freeze-thaw processes and snow
maintains this elegant coupling, requiring only coefficients in the linear equations to be
modified. No impedance factor for hydraulic conductivity is needed because of
the formulation by matric flux potential rather than pressure head. Iterations are
avoided which results in the same computational speed as without freezing. The
extended model is evaluated extensively in stand-alone mode (against theoretical
predictions, lab experiments and field data) and as part of the CABLE global land surface
scheme.
SLI accurately solves the classical Stefan problem of a homogeneous medium undergoing
a phase change. The model also accurately reproduces the freezing front, which is observed
in laboratory experiments (Hansson et al. 2004). SLI was further tested against observations
at a permafrost site in Tibet (Weismüller et al. 2011). It reproduces seasonal thawing and
freezing of the active layer to within 3 K of the observed soil temperature and to
within 10% of the observed volumetric liquid soil moisture. Model-data fusion
suggests that model performance is improved when the relatively high thermal
conductivity of the ice phase is accounted for. However, the permafrost site is very
gravelly so that the model equations for thermal conductivity are at the edge of
applicability. The freezing-soil formulation is tested in the presence of snow, using
measurements at an orchard site in Idaho. The model reproduces well observed
snow-water equivalents and soil temperatures. However, it is highly sensitive to
snow emissivity and maximum liquid content of the snow, leading both to modified
refreezing of melted water. It is possible that the model would benefit from 1-2
more snow layers to permit simulation of density and temperature gradients in the
snow-pack.
SLI was run globally on 1°x1° grid as the soil part of the land surface scheme CABLE.
We could therefore demonstrate that this detailed and physically-realistic formulation is
fast enough to be a feasible alternative to the much simpler default soil-scheme in
CABLE.
References
Hansson et al. (2004) Vadose Zone J 3, 693ff
Haverd & Cuntz (2010) J Hydro 388, 434ff
Haverd et al. (2013) Biogeosci 10, 2011ff
Weismüller et al. (2011) The Cryosphere 5, 741ff |
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