A numerical study of meteorological conditions during PACIFIC '93

An extensive field study (called PACIFIC '93), designed to obtain a comprehensive 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 to fill this gap by employing a mesoscale model, the Regional Atmospheric Modelling System (RAMS). In general, the model results agree well with observations, with the index of agreement for surface temperature and wind reaching 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 deteriorates overall performance. The modelled vertical structure of wind and temperature at a site close to the Coast Mountains agrees well with tethersonde observations, especially during the daytime. Modelled trajectories demonstrate that tracers released from a surface location near the root of the Coast Mountain follow trajectories determined by two wind systems: westerly sea breezes and southerly up-valley winds. Up-valley winds are shown to be a mechanism responsible for transport of tracers to tributary valleys in the Coast Mountains. This finding corroborates the results of McKendry et al. (1998) who investigated transport of pollutants up Pitt Lake Valley. The model results also suggest that convergent thermal plumes near mountain ridges might be mechanisms responsible for carrying pollutants to relatively high elevations in the stable layer. These pollutants, in the present case, are transported by the upper flows back to the LFV.