In the framework of the Global Soil Wetness Project (GSWP). the ISBA land-s
urface scheme of the ARPEGE atmospheric general circulation model has been
forced with meteorological observations and analyses in order to produce a
two-year (1987-1988) soil moisture climatology at a 1 degrees x 1 degrees h
orizontal resolution. This climatology is model dependent, but it is the cl
imatology that the ARPEGE model would produce if its precipitation and radi
ative fluxes were perfectly simulated. In the present study, ensembles of s
easonal simulations (March ch to September) have been performed for 1987 an
d 1988, in which the total soil water content simulated by ARPEGE is relaxe
d towards the GSWP climatology. The results indicate that the relaxation ha
s a positive impact on both the model's climatology and the simulated inter
annual variability, thereby confirming the utility of the GSWP soil moistur
e data for prescribing initial or boundary conditions in comprehensive clim
ate and numerical weather prediction models. They also demonstrated the rel
evance of soil moisture for achieving realistic simulations of the Northern
hemisphere summer climate. in order to get closer to the framework of seas
onal predictions, additional experiments have been performed in which GSWP
is only used for initialising soil moisture at the beginning of the summer
season (the relaxation towards GSWP is removed on 1st June). The results sh
ow a limited improvement of the interannual variability, compared to the si
mulations initialised from the ARPEGE climatology. However, some regional p
atterns of the precipitation differences between 1987 and 1988 are better c
aptured, suggesting that seasonal predictions can benefit from a better ini
tialisation of soil moisture.