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1. Introduction
The 2015 Pan and Parapan American Games from 10 July to 15 August provided Environment and Climate Change Canada (ECCC) with a unique opportunity to undertake an extensive observation campaign in the Greater Toronto Area (GTA), including a mesoscale network specifically designed to detect and track lake breezes and, in particular, the lake-breeze front (Joe et al. 2018). Additionally, two Doppler lidars (hereinafter referred to as lidars) provided real-time observations of winds. The Canadian Global Environmental Multiscale (GEM) numerical model was run at the horizontal grid spacings of 2.5, 1, and 0.25 km to study its ability to predict lake breezes and urban meteorological conditions (Leroyer et al. 2018).
Lake breezes develop because of the temperature contrast between air over cool lake water and air over the warm land surface (Atkinson 1981; Pielke 1984). The thermal contrast produces a pressure difference between the lake and land that forces cooler air inland off the lake. Figure 2 of Sills et al. (2011) shows an idealized lake breeze circulation. The lake-breeze front develops at the leading edge of the inflow layer. The surface convergence and updraft at the lake-breeze front can generate a narrow band of convective clouds (Lyons 1972). The depth of the inflow layer typically ranges from 100 to 1000 m (Lyons 1972; Keen and Lyons 1978; Curry et al. 2017; Mariani et al. 2018); however, the return flow above the inflow layer can be twice as deep (Lyons 1972).
The GTA is often affected by lake breezes because of its proximity to Lake Ontario. Estoque et al. (1976) investigated the structure and diurnal variations of lake breezes over the southern part of Lake Ontario using both observations and numerical simulations. The passage of the lake-breeze front was marked by a sharp shift in wind direction, decrease in temperature, and increase in relative humidity. Estoque et al. (1976) also showed that the lake-breeze front depth can reach 250 m and the lake breeze can penetrate as far as 30 km inland. Comer and McKendry (1993) extended the work of Estoque et al. (1976) by investigating a wider range of data. They used the lake-breeze index developed by Biggs and Graves (1962) to identify lake breezes. They found that lake breezes developed on over...