Studying ozone climatology with a regional climate model 2. Climatology

The Canadian Regional Climate Model with oxidant chemistry (CRCM) has been used to study the climatology and transport of ground-level oxone in easter n North America. The climatology is derived from simulations of five consec utive Julys from 1990 to 1994. Biogenic emissions are calculated each time step, while anthropogenic emissions are taken to be identical for each simu lated July. Comparisons of daily ozone maximum for each individual month su ggest that the model is simulating satisfactorily the month-long periods an d usually correctly reproduces the high and low ozone regimes associated wi th the different parts of the domain and/or the various synoptic conditions , the Ohio region being the exception. The climatological number of exceede nces above 80 ppbv, calculated as the monthly average for the 5 years, yiel ds better agreement with the number of exceedences calculated from the obse rved climatological O-3 field than those for the individual months. This be havior suggests that a large fraction of the errors present in each simulat ion are randomly distributed. Thus it should be possible to obtain fields t hat are more representative of reality, for applications such as emission r eduction scenario studies, by averaging over several long simulations.