Abstract

This dissertation investigates the effects of increasing surface reflectivity (ISR) on urban climate, air quality, and heat-related mortality and some of the details of simulations and modelling. Meteorological and photochemical models are applied to assess the benefits of albedo enhancement in the Greater Montreal Area (GMA, Quebec) in Canada and Sacramento (California), Houston (Texas) and Chicago (Illinois) in the United State.
Mesoscale models are comprised of physical parameterizations (cumulus, microphysics, planetary boundary layer, radiation, and land-surface) that need to be carefully selected to predict weather conditions. A proper simulation platform is essential to have a better understanding of the effects of UHI and its mitigation strategy on urban climate and air quality for environmental policymakers. The sensitivity of near surface air temperature, wind speed, relative humidity and precipitation to different physical models was evaluated by applying the WRF for Greater Montreal Area, Canada for the period 9–11 August 2009. A combination of WDM6 as microphysics estimation, Grell 3D for cumulus scheme, MYJ as planetary boundary layer and RRTMG as radiation scheme, resulted in the least error compared to the measurements. Thus, this combination is suggested as an appropriate platform for urban climate simulations and heat island mitigation strategy in Greater Montreal Area. Increasing the surface albedo of roofs, walls, and pavements from 0.2 to 0.65, 0.60, and 0.45, respectively, resulted in a decrease in 2-m air temperature by 0.2oC in a rainy day and by 0.7 in a sunny day, a slight increase in 10-m wind speed, a decrease in relative humidity by 3%, and a decrease in precipitation by 0.2 mm/day across the domain.
The proper physical parameterizations for Montreal were applied to investigate the effects of increasing surface reflectivity on meteorological parameters (air temperature, wind speed, relative humidity, and dew point temperature), heat stress indices (National Weather Service – Heat Index, apparent temperature, Canadian Humid Index, and Discomfort Index), and heat-related deaths. The simulation domain was the Greater Montreal Area. The simulations were conducted during the 2005 and 2011 heat wave periods. Heat-related mortality correlations were developed for Montreal. The beneficial contributions of albedo enhancement were a decrease in temperature by 0.8oC, an increase in relative humidity by 2%, an increase in dew point temperature by 0.4oC, a slight increase in wind speed, and a decrease in heat-related mortality by 3.2%. Increasing surface reflectivity could save seven lives and improve the level of comfort for urban dwellers.
To assess the effects of increasing surface reflectivity on mitigating urban heat islands and improving air quality, simulations were carried out over a larger geographical area (North America with horizontal resolution of 12km) within nested domains as urban areas (Sacramento in California, Houston in Texas, and Chicago in Illinois with horizontal resolution of 2.4km) in a two-way nested approach by online coupling of chemistry package with the solver of WRF (WRF-Chem). The 2-way nested approach provided an integrated simulation setup to capture the full impacts of meteorological and photochemical reactions and decrease the uncertainties associated with scale separation and grid resolution. The Lin, Goddard, Rapid Radiative Transfer Model, Mellor-Yamada-Janjic and Grell-Devenyi ensemble schemes are respectively selected for microphysics, shortwave radiation, longwave radiation, planetary boundary layer and cumulus parameterization. For anthropogenic and biogenic emission estimations, the models of the United States National Emission Inventory for 2011 (US-NEI11) and Model of Emissions of Gases and Aerosols from Nature (MEGAN) are respectively simulated for the inner domains. The Modal Aerosol Dynamics Model for Europe and Regional Atmospheric Chemistry Mechanism (RACM) are applied to estimate the effects of aerosols on radiation processes and hydrological cycles in the atmosphere and to estimate the gas-phase reactions. Photolysis frequencies are calculated by the Fast_J model scheme. Increasing surface albedo resulted a decrease in air temperature by 2-3oC in urban areas of these three cities. Albedo enhancement resulted in a slight increase in wind speed; an increase in relative humidity (3%) and dew point temperature (0.3oC) during simulation period. Increasing urban reflectivity led to a decrease in PM2.5 and O3 concentrations by 2-4μg/m3 and 4-8 ppb in urban areas of these three cities based on their locations. Sacramento showed a larger reduction in ozone concentration as a result of larger decrease in air temperature because of the heat island mitigation strategy.
The two-way nested approach was employed to investigate the effects of albedo enhancement on aerosol-radiation-cloud (ARC) interactions over the Greater Montreal Area during the 2011 heat wave period. The third domain of simulation covers the GMA with the horizontal resolution of 800m. Four sets of simulations with and without aerosol estimations and convective parameterizations were carried out to explore the direct, semi-direct and indirect effects of aerosols. The physical and chemical parameterizations are modified to be coupled with the Model for Simulating Aerosol Interactions with Chemistry (MOSAIC) aerosol scheme and the Carbon Bond Mechanism (CBM-Z) gas phase chemistry scheme. The Morrison double-moment scheme and the Mellor-Yamada-Janjic scheme are selected as microphysics and planetary boundary layer options, respectively. The Grell-Devenyi ensemble scheme and the rapid radiative transfer model are respectively used for cumulus parameterization and shortwave and longwave radiations. The US-NEI11 and MEGAN are applied to calculate the anthropogenic and biogenic emission estimation, respectively. The Fast-J is used for the photolysis scheme in WRF-Chem. Aerosols cause a decrease in shortwave radiation reaching to the ground (20 Wm-2) and thus reduces the radiation budget (25 Wm-2). The albedo enhancement induced a decrease in air temperature by nearly 0.5oC in Montreal during heat wave period. The relative humidity and water mixing ratio also decreased by 0.5 g/kg and 3%, respectively. Increasing surface reflectivity led to a decrease of 8-h ozone concentrations by 2ppb across the GMA. Reducing temperature induced a reduction in planetary boundary layer height, which reduced the advection and diffusion of pollutants. Hence, reducing planetary boundary layer height increases the pollutant concentrations and assists the O3 and NO reaction rates to produce NO2. The fine particulate matter also decreased by nearly 3 µg/m3 in GMA during simulation period. An increase of albedo led to a net decrease of radiative flux into the ground and therefore a decrease of convective cloud formation.
The comparisons between simulated air temperature using WRF and WRF-Chem with measurements indicated that both models predict the temperature reasonably well. The modeling results indicated that each of these four cities (Montreal, Sacramento, Houston, Chicago) across North America can benefit from increasing surface reflectivity. But, the extent to which surface modification can improve urban climate and air quality effectively depends on meteorology, geography, scale, topography, morphology, land use patterns, the emission rates and mixture of biogenic and anthropogenic pollutants, baseline albedo fraction distribution, and the potential for surface modification in that specific city.