Details for Stefan Gollvik and Patrick Samuelsson: A tiled land-surface scheme for HIRLAM

Name:Stefan Gollvik and Patrick Samuelsson: A tiled land-surface scheme for HIRLAM


This report describes the surface scheme, developed for HIRLAM, and called ”newsnow” during
the development time. It is physically based on the land surface scheme used at the Rossby Centre
(Samuelsson et al., 2006), with some exceptions. The code structure and data assimilation has its
origin in the earlier HIRLAM surface scheme (Rodriguez et al., 2003).
It is a tiled scheme with 7 tiles: Sea, ice, bare soil, low vegetation, forest, snowcovered bare soil and
low vegetation, and finally forest with snow on the forest floor. The basic difference from the earlier
scheme is the introduction of the snow scheme, and a completely new forest formulation. The other
tiles are in principle the same as the old scheme, but we have replaced the force-restore formulation
within the soil, with more layers and heat conduction.
Thus we have two different snow packs, with separate energy balances. For each tile we compute the
latent and sensible heat fluxes and momentum fluxes, and they are weighted together, according to
their grid fractions, to provide a lower boundary condition for the vertical diffusion. The forest has a
separate canopy temperature, common for the tiles with and without snow on the forest floor, so they
are treated together within the scheme. At present no snow on sea ice is incorporated, but we have
one temperature for the uppermost ice layer and the snow.
The water storages treated in the model are, except for soil moisture, intercepted water on low vegetation
and on the forest, snow water equivalent on the ground and forest floor. The snow packs also
treat liquid water within the snow, which can refreeze. We also treat intercepted snow on the forest
canopy, in a simplified way, without a separate temperature.
The main differences from the climate model of the Rossby Centre, is that we have three thermal
layers in the soil and a relaxation to a deep climatological temperature instead of a no flux lower
boundary condition , and that we have kept the force-restore formulation of soil moisture from the
earlier HIRLAM scheme.
We have also a simpler estimation of the snow fraction. The deviations from the climate version, has
mainly to do with the adaptation to the surface data assimilation.

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