Abstract
ABSTRACTThe impact of soil initialization is investigated through perturbation simulations with the regional climate model Consortium for Small‐scale Modelling (COSMO) and the Climate Limited‐area Modelling (COSMO–CLM). The focus of the investigation is to assess the sensitivity of simulated extreme periods, dry and wet, to soil moisture initialization in different climatic regions over Europe and to establish the necessary spin‐up time within the framework of decadal predictions for these regions. Sensitivity experiments consisted of a reference simulation from 1968 to 1999 and five simulations from 1972 to 1983. The Effective Drought Index (EDI) is used to select and quantify drought status in the reference run to establish the simulation time period for the sensitivity experiments. Different soil initialization procedures are investigated. The sensitivity of the decadal predictions to soil moisture initial conditions is investigated through the analysis of water cycle components' (WCC) variability. In an episodic timescale, the local effects of soil moisture on the boundary layer and the propagated effects on the large‐scale dynamics are analysed. The results show: (1) COSMO–CLM reproduces the observed features of the drought index. (2) Soil moisture initialization exerts a relevant impact on WCC, e.g. precipitation distribution and intensity. (3) Regional characteristics strongly impact the response of the WCC. Precipitation and evapotranspiration deviations are larger for humid regions. (4) The initial soil conditions (wet/dry), the regional characteristics (humid/dry) and the annual period (wet/dry) play a key role in the time that soil needs to restore quasi‐equilibrium and the impact on the atmospheric conditions. Humid areas, and for all regions, a humid initialization, exhibit shorter spin‐up times, also soil reacts more sensitive when initialized during dry periods. (5) The initial soil perturbation may markedly modify atmospheric pressure field, wind circulation systems and atmospheric water vapour distribution affecting atmospheric stability conditions, thus modifying precipitation intensity and distribution even several years after the initialization.
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