408
W. Gong et al.
65.1 Introduction
A previous study using a fully coupled version of the Environment and Climate
Change Canada’s (ECCC) air quality prediction model, GEM-MACH, has shown
an increase in modelled cloud amount and liquid water content due to increased
cloud droplet number concentration, a decrease in cloud droplet size and a reduction
in warm precipitation, downwind of an urban-industrial area. The increased cloud
amount due to the aerosol indirect effect reduced the downward shortwave radiative
flux and air temperature at the surface, leading to a decrease in ozone over the region
of enhanced cloud and an increase in particle sulphate from an increased capacity
for aqueous-phase production [1]. In the meantime, observational evidence of urbaninduced or urban-modified convective phenomena have been reported and linked to
several possible attributing factors: urban roughness, urban heat island, and urban
aerosols. Study of idealised cases suggests that enhanced urban aerosols can lead
to a delay and suppression of precipitation from low-level clouds associated with
urban induced convection but an invigoration of deep convection and enhancement
of precipitation further downwind from the urban centre [2].
In this study, model simulations were carried out, using an updated version of
the fully coupled GEM-MACH, for the 2015 Pan America Games period when a
dense surface network for meteorological and air quality monitoring was set up. The
model domain is centred over the Greater Toronto and Hamilton area in southern
Ontario. Simulations were conducted with and without the aerosol indirect effect
in combination with an urban module (Town Energy Balance) turned on or off, to
investigate the impact of aerosol-cloud interaction on the lake-/land-breeze circulation and associated cloud and precipitation development and subsequent impact on
modelled air quality in this urban environment.
65.2 Model and Simulation Setup
Version 2 of the Global Environmental Multiscale—Modelling Air-quality and
CHemistry (GEM-MACH) model was used for this study. While a 2-bin configuration for aerosol size representation has been used for operational air quality forecast
at ECCC [8], a 12-bin configuration was used here. This study also makes use of the
interactive capability introduced previously within GEM-MACH to allow feedbacks
between on-line aerosols and meteorology through aerosol radiative (direct) effect
and aerosol microphysics (indirect) effect (see [1, 4] for a detailed description of the
implementation). In addition, a parameterization for representing urban landscape,
the heat and momentum exchange between urban surfaces and the atmosphere, based
on a Town-Energy-Balance (TEB) model [3, 5], was also employed.
Simulations were carried out in a cascade fashion with model runs on a 10 km
resolution North American domain which provide initial and boundary conditions
for the runs on a 2.5 km resolution domain focused on southern Ontario. Figure 65.1
W. Gong et al.
65.1 Introduction
A previous study using a fully coupled version of the Environment and Climate
Change Canada’s (ECCC) air quality prediction model, GEM-MACH, has shown
an increase in modelled cloud amount and liquid water content due to increased
cloud droplet number concentration, a decrease in cloud droplet size and a reduction
in warm precipitation, downwind of an urban-industrial area. The increased cloud
amount due to the aerosol indirect effect reduced the downward shortwave radiative
flux and air temperature at the surface, leading to a decrease in ozone over the region
of enhanced cloud and an increase in particle sulphate from an increased capacity
for aqueous-phase production [1]. In the meantime, observational evidence of urbaninduced or urban-modified convective phenomena have been reported and linked to
several possible attributing factors: urban roughness, urban heat island, and urban
aerosols. Study of idealised cases suggests that enhanced urban aerosols can lead
to a delay and suppression of precipitation from low-level clouds associated with
urban induced convection but an invigoration of deep convection and enhancement
of precipitation further downwind from the urban centre [2].
In this study, model simulations were carried out, using an updated version of
the fully coupled GEM-MACH, for the 2015 Pan America Games period when a
dense surface network for meteorological and air quality monitoring was set up. The
model domain is centred over the Greater Toronto and Hamilton area in southern
Ontario. Simulations were conducted with and without the aerosol indirect effect
in combination with an urban module (Town Energy Balance) turned on or off, to
investigate the impact of aerosol-cloud interaction on the lake-/land-breeze circulation and associated cloud and precipitation development and subsequent impact on
modelled air quality in this urban environment.
65.2 Model and Simulation Setup
Version 2 of the Global Environmental Multiscale—Modelling Air-quality and
CHemistry (GEM-MACH) model was used for this study. While a 2-bin configuration for aerosol size representation has been used for operational air quality forecast
at ECCC [8], a 12-bin configuration was used here. This study also makes use of the
interactive capability introduced previously within GEM-MACH to allow feedbacks
between on-line aerosols and meteorology through aerosol radiative (direct) effect
and aerosol microphysics (indirect) effect (see [1, 4] for a detailed description of the
implementation). In addition, a parameterization for representing urban landscape,
the heat and momentum exchange between urban surfaces and the atmosphere, based
on a Town-Energy-Balance (TEB) model [3, 5], was also employed.
Simulations were carried out in a cascade fashion with model runs on a 10 km
resolution North American domain which provide initial and boundary conditions
for the runs on a 2.5 km resolution domain focused on southern Ontario. Figure 65.1