Dynamic adjustment of climatological ozone boundary conditions for air-quality forecasts

Abstract

Abstract. Ten different approaches for applying lateral and top climatological boundary conditions for ozone have been evaluated using the off-line regional air-quality model AURAMS, driven with meteorology provided by the GEM weather-forecast model. All ten approaches employ the same climatological ozone profiles, but differ in the manner in which they are applied, via the inclusion or exclusion of (i) a dynamic adjustment of the climatological ozone profile in response to the model-predicted tropopause height, (ii) a sponge zone for ozone on the model top, (iii) upward extrapolation of the climatological ozone profile, and (iv) different mass consistency corrections. The model performance for each approach was evaluated against North American surface ozone and ozonesonde observations from the BAQSMet field study period in the summer of 2007. The original daily one-hour maximum surface ozone biases of about +15 ppbv were greatly reduced (halved) in some simulations using alternative methodologies. However, comparisons to ozonesonde observations showed that the reduction in surface ozone bias sometimes came at the cost of significant positive biases in ozone concentrations in the free troposphere and upper troposphere. The best overall performance throughout the troposphere was achieved using a methodology that included dynamic tropopause height adjustment, no sponge zone at the model top, extrapolation of ozone when required above the limit of the climatology, and no mass consistency corrections (global mass conservation was still Correspondence to: P. A. Makar (paul.makar@ec.gc.ca) enforced). The simulation using this model version had a one-hour daily maximum surface ozone bias of +8.6 ppbv, with small reductions in model correlation, and the best comparison to ozonesonde profiles. This recommended and original methodologies were compared for two further case studies: a high-resolution simulation of the BAQS-Met measurement intensive, and a study of the downwind region of the Canadian Rockies. Significant improvements were noted for the high resolution simulations during the BAQSMet measurement intensive period, both in formal statistical comparisons and time series comparisons of events at surface stations. The tests for the downwind-Rockies region showed that the coupling between vertical transport associated with troposphere/stratosphere exchange, and that associated with boundary layer turbulent mixing, may contribute to ozone positive biases. The results may be unique to the modelling setup employed, but the results also highlight the importance of evaluating boundary condition and mass consistency/correction algorithms against three-dimensional datasets.

A global ozone climatology from ozone soundings via trajectory mapping: a stratospheric perspective

“…Using an ozone climatology that includes a climatological tropopause with such models can produce, in effect, spurious stratospheric intrusions as air in the upper troposphere (when the actual tropopause is high) is assigned stratospheric values of ozone (e.g., Makar et al, 2010). The opposite effect can occur when the actual tropopause is low.…”

“…Previous studies have demonstrated the importance of upper boundary conditions to accurate modeling of tropospheric ozone in regional chemical forecast models Makar et al, 2010, and references therein). The Canadian Global Environmental Multiscale meteorology model (GEM, Côté et al, 1998) has been coupled with the Modelling Air quality and CHemistry (MACH) system (Talbot et al, 2009) to operationally predict tropospheric ozone in a global uniform 1 • × 1 • horizontal resolution.…”

“…For this experiment, rather than use the stratospheric and tropospheric climatologies separately, as suggested in Sect. 2.2, the complete stratosphere-troposphere climatology was used, with a dynamic adjustment of the ozone climatology to the model-predicted tropopause height, as employed by Makar et al (2010). The ozone profiles from the three GEM-MACH global simulations were averaged from 10 May to 20 June and compared with independent ozonesondes for the same period at six CalNex campaign stations (Fig.…”

Transport analysis of ozone enhancement in Southern Ontario during BAQS-Met

“…Above 100 hPa (the top of the Logan climatology), ozone values were extrapolated (where necessary) to 50 hPa, assuming a constant gradient of mixing ratio. This configuration (with no sponge layer at the model top, and no mass consistency corrections) was found to provide the best overall performance throughout the troposphere (Makar et al, 2010b). The 42-km domain supplies ozone boundary conditions for the 15-km domain, which in turn supplies them for the 2.5-km domain.…”

Projections of mid-century summer air-quality for North America: effects of changes in climate and precursor emissions

“…Short-lived chemical tracers have constant low concentration profiles on the boundaries. Ozone boundary conditions make use of monthly 3-D climatology with adjustments for model-predicted tropopause height (Makar et al, 2010b). Thus, meteorological downscaling was applied, but not chemical downscaling.…”