Offline validation of the ERA40 surface scheme

A new surface scheme is introduced in the upcoming ecmwf
global reanalysis covering a 40-year period 1957-1997 (known as
era-40). The new scheme is characterized by a tiled
treatment of subgrid fractions of evaporating surfaces, the use of a
global vegetation database plus an associated set of model parameters,
and changes into the parameterization of snow
processes, the coupling of the skin layer to the soil, and the vertical
soil moisture transport.

Parallel to the evaluation of the global impact in the ecmwf 3D
model, this tiled scheme was tested in offline mode with
data from 7 long term surface
field campaigns. The datasets cover a wide
range of climate regions and surface land use and vegetation types,
ranging from boreal (Boreas), tropical (Arme) and
temperate (Garderen) forest, via savannah (Sebex) and prairie (Fife) to crop (Hapex-Mobilhy) and grassland (Cabauw).

The offline evaluation was primarily used for validation purposes, but
gave also rise to adjusting parameter settings and/or
parameterization approaches. After the combined calibration/validation
exercise we can conclude that (a) the description of snow processes and
surface fluxes in the Boreal areas has improved significantly, (b) the
tiling approach does not give rise to major changes in the model
correspondence to data in case of grass- and cropland, and (c) model
performance becomes increasingly dependent on a proper choice of
vegetation dependent model
parameters, which gives rise to an increased number of degrees of
freedom.