A numerical study of meteorological conditions during PACIFIC '93

An extensive field study (called PACIFIC '93), designed to obtain a compreh ensive dataset for use in understanding photochemical episodes, was carried out in the Lower Fraser Valley (LFV), B.C., Canada in the summer of 1993. Although many observational studies on local chemistry and meteorology have been reported, there is no complete description of three-dimensional, time -varying meteorological conditions over the whole LFV This study attempts t o fill this gap by employing a mesoscale model, the Regional Atmospheric Mo delling System (RAMS). In general, the model results agree well with observ ations, with the index of agreement for surface temperature and wind reachi ng as high as 0.8 during the daytime, but dropping to 0.4 at night. A good performance is achieved by the model in this long integration for less than three days. Occurrence of local weather systems such as thunderstorms dete riorates overall performance The modelled vertical structure of wind and te mperature at a site close to the Coast Mountains agrees well with tetherson de observations, especially during the daytime. Modelled trajectories demon strate that tracers released from a surface location near the root of the C oast Mountain follow trajectories determined by two wind systems: westerly sea breezes and southerly rip-valley winds. Up-valley winds are shown to be a mechanism responsible for transport of tracers to tributary valleys in t he Coast Mountains. This finding corroborates the results of McKendry et al . (1998) who investigated transport of pollutants up Pitt Lake Valley The m odel results also suggest that convergent thermal plumes near mountain ridg es might be mechanisms responsible for carrying pollutants to relatively hi gh elevations in the stable layer These pollutants, in the present case, ar e trans ported by the upper flows back to the LFV.