Abstract. In the present study, a persistent heavy haze episode from 13 to 20 January 2014 in Beijing–Tianjin–Hebei (BTH) is simulated using the WRF-CHEM model through ensemble simulations to investigate impacts of meteorological uncertainties on the haze formation. Model results show that uncertainties in meteorological conditions substantially influence the aerosol constituent simulations at an observation site in Beijing, and the ratio of the ensemble spread to the ensemble mean (RESM) exceeds 50 %. The ensemble mean generally preforms well in reproducing the fine particles' (PM_2.5) temporal variations and spatial distributions against measurements in BTH. The meteorological uncertainties do not alter the PM_2.5 distribution pattern in BTH principally or dominate the haze formation and development, but remarkably affect the simulated PM_2.5 level, and the RESM for the simulated PM_2.5 concentrations can be up to 30 % at the regional scale. In addition, the rather large RESM in PM_2.5 simulations at the city scale also causes difficulties in evaluation of the control strategies. Therefore, our results suggest that the ensemble simulation is imperative to take into account the impact of the meteorological uncertainties on the haze prediction.