|Title||The European summer of 2003: sensitivity to soil water initial conditions.|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Ferranti, L, Viterbo, P|
|Place Published||Shinfield Park, Reading|
The European summer 2003 is used as a case study to analyse the land surface role in augmenting the local temperature anomalies. Using ECMWF analysis and ERA-40 climate it is shown that, in the months preceding the extreme summer events, positive anomalies in the surface shortwave radiation and a large precipitation deficit indicated an impending dry summer in early June. The use of soil water analysis values as possible predictors for drought is currently limited by the systematic attenuation of its seasonal cycle. Several numerical simulations with the ECMWF atmospheric model have been used to explore the atmospheric model sensitivity to the soil water initial conditions. The atmospheric response to large initial perturbations in the root zone extends up to month 2 and is non-linear, larger for drier regimes. Perturbations to the whole soil depth increase the amplitude of the atmospheric anomaly and extend its duration up to 3 months. The response of large initial dry soil anomalies greatly exceeds the impact of the ocean boundary forcing. Results from numerical simulations indicate the possible benefit of using perturbations in the soil water initial conditions, commensurate with soil moisture uncertainties, in the generation of the seasonal forecast ensembles.