High resolution MC2 Modelling: Lake Breeze, Severe Weather and Air Pollution

One of the most pressing pollution problems facing much of North America and especially Ontario, is the photochemical smog associated with ground-level ozone and inhalable and respirable particulate matter (IP/RP). In Southern Ontario, the frequency and magnitude of exceedances of the ozone air quality objective are strongly affected by the presence of the Great Lakes. Ground-level ozone concentrations are observed to be higher within relatively narrow bands along the shorelines of the Lower Great Lakes. It has been argued that ozone and IP/RP pollution in Southern Ontario are very strongly linked to the influence of lake breezes. However, there are known deficiencies in the current state of modeling of lake breeze circulations, and of associated convective structures.

To develop and evaluate further the utility and accuracy of high resolution meteorological simulations for Southern Ontario, in the understanding of flow patterns responsible for the transport of ozone, particulates and other pollutants in a region of the Lower Great Lakes with complex topography, the Canadian Nonhydrostatic Mesoscale impressible Community (MC2) limited-area atmospheric model has been used to perform high resolution meteorological simulations for the Toronto and Hamilton areas. A one-way self-nesting procedure allows the MC2 model to make very high resolution simulations (1km) starting from a coarser resolution operational analysis (24 - 100km) as initial conditions. The MC2 model can run well at a wide range of resolutions with careful selection of appropriate convective and condensation schemes and boundary-layer schemes.

The MC2 model results show detailed meteorological fields in the Lower Great Lakes area, including the development and evolution of land and lake breezes with different flow patterns, the effect of varying roughness and heat/moisture sources within an urban environment, the development and evolution of thermal internal boundary layers and the geographic and temporal variation of the boundary layer height. Simulations of these phenomena are compared with climatology and theoretical expectations. The MC2 model can also perform air pollution transport simulations using tracers. Model results suggest that, under some conditions, the motions within the lake breeze circulation can dominate the local transport of pollutants and that the initial level at which the material resides is very important. The validity of the suggested pollutant flow has been assessed by consideration of known sources and available measurement data.