Ozone predictabilities due to meteorological uncertainties in the Mexico City basin using ensemble forecasts

The purpose of the present study is to investigate the sensitivity of ozone (O-3) predictions in the Mexico City Metropolitan Area (MCMA) to meteorological initial uncertainties and planetary boundary layer (PBL) parameterization schemes using state-of-the-art meteorological and photochemical prediction models through ensemble forecasts. The simulated periods (3, 9, 15 and 29 March 2006) represent four typical meteorological episodes (South-Venting, O-3-North, O-3-South and Convection-North, respectively) in the Mexico City basin during the MCMA-2006/MILAGRO campaign. Our results demonstrate that the uncertainties in meteorological initial conditions have significant impacts on O-3 predictions, including peak time O-3 concentrations ([O-3]), horizontal and vertical O-3 distributions, and temporal variations. The ensemble spread of the simulated peak [O-3] averaged over the city's ambient monitoring sites can reach up to 10 ppb. The increasing uncertainties in meteorological fields during peak O-3 period contribute to the largest unpredictability in O-3 simulations, while the impacts of wind speeds and PBL height on [O-3] are more straightforward and important. The magnitude of the ensemble spreads varies with different PBL schemes and meteorological episodes. The uncertainties in O-3 predictions caused by PBL schemes mainly come from their ability to represent the mixing layer height; but overall, these uncertainties are smaller than those from the uncertainties in meteorological initial conditions.