Impact of land surface stochastic physics in ALADIN‐LAEF

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AbstractTo deal with the land surface physics uncertainties, a stochastic scheme based on stochastic perturbation of physics tendencies is implemented and tested. The impact of land surface physics uncertainties and their relative importance to land surface initial uncertainties are investigated in the regional ensemble forecasting system ALADIN‐LAEF (Aire Limitée Adaptation Dynamique Développement InterNational – Limited Area Ensemble Forecasting). The land surface initial perturbation is generated by using an ensemble of land surface data assimilation; and the land surface physics uncertainties by applying the idea of stochastically perturbed parametrization tendencies (SPPT) scheme. Three experiments are conducted and compared with the reference ensemble over a 2‐month period. The results show the introduction of land surface stochastic physics increases the ensemble spread, reduces the ensemble bias, and keeps neutral in deterministic forecast skill of the ensemble, its impact strongly depending on the quality of ensemble initial conditions. The ensemble land surface data assimilation has stronger positive impact on the ALADIN‐LAEF than the land surface stochastic physics for screen‐level temperature and humidity. There is not much impact on 10 m wind and precipitation. Best results are obtained when both the ensemble land surface data assimilation and land surface stochastic physics are used simultaneously; it gives a more reliable and statistically consistent forecast, which is contributed mainly by ensemble land surface data assimilation in the first forecast hours and largely by land surface stochastic physics in the later forecast hours.