Chemical Analysis of Surface-Level Ozone Exceedances during the 2015 Pan American Games

Surface-level ozone (O3) continues to be a significant health risk in the Greater Toronto Hamilton Area (GTHA) of Canada even though precursor emissions in the area have decreased significantly over the past two decades. In July 2015, Environment and Climate Change Canada (ECCC) led an intensive field study coincident with Toronto hosting the 2015 Pan American Games. During the field study, the daily 1-h maximum O3 standard (80 ppbv) was exceeded twice at a measurement site in North Toronto, once on July 12 and again on July 28. In this study, ECCC’s 2.5-km configuration of the Global Environmental Multi-scale (GEM) meteorological model was combined with the Modelling Air-quality and CHemistry (MACH) on-line atmospheric chemistry model and the Town Energy Balance (TEB) urban surface parameterization to create a new urban air quality modelling system. In general, the model results showed that the nested 2.5-km grid-spaced urban air quality model performed better in statistical scores compared to the piloting 10-km grid-spaced GEM-MACH model without TEB. Model analyses were performed with GEM-MACH-TEB for the two exceedance periods. The local meteorology for both cases consisted of light winds with the highest O3 predictions situated along lake-breeze fronts. For the July 28 case, O3 production sensitivity analysis along the trajectory of the lake-breeze circulation showed that the region of most efficient O3 production occurred in the updraft region of the lake-breeze front, as the precursors to O3 formation underwent vertical mixing. In this updraft region, the ozone production switches from volatile organic compound (VOC)-sensitive to NOx-sensitive, and the local net O3 production rate reaches a maximum. This transition in the chemical regime is a previously unidentified factor for why O3 surface-level mixing ratios maximize along the lake-breeze front. For the July 12 case, differences between the model and observed Lake Ontario water temperature and the strength of lake-breeze opposing wind flow play a role in differences in the timing of the lake-breeze, which impacts the predicted location of the O3 maximum north of Toronto.

Theoretical Studies of Convectively Forced Mesoscale Flows in Three Dimensions. Part I: Uniform Basic-State Flow

Convectively forced mesoscale flows in three dimensions are theoretically investigated by examining the transient response of a stably stratified atmosphere to convective heating. Solutions for the equations governing small-amplitude perturbations in a uniform basic-state wind with specified convective heating are analytically obtained using the Green function method. In the surface pulse heating case, it is explicitly shown that the vertical displacement at the center of the 3D steady heating decreases as fast as t−1 for large t. Hence, unlike in two dimensions, the steady state is approached in three dimensions. In the finite-depth steady heating case, the perturbation vertical velocity field in the stationary mode shows a main updraft region extending over the heating layer and V-shaped upward and downward motions above and below the heating layer. Including the third dimension results in a stronger updraft at an early stage, a weaker compensating downward motion, and a weaker stationary gravity wave field in a quasi-steady state than in the case of two dimensions. An examination of flow response fields for various vertical structures of convective heating indicates that stationary gravity waves above the main updraft region become strong in intensity as the height of the maximum convective heating increases. In response to the transient heating, a main updraft region extending over the heating layer no longer appears at a dissipation stage of deep convection. Instead, alternating regions of upward and downward motion with an upstream phase tilt appear.

CO2 Condensation in Baroclinic Eddies on Early Mars

The efficacy of a cloud-radiative feedback on early Mars is reexamined in the context of the theory of baroclinic waves in midlatitudes. The feedback has been proposed to explain fluvial features on the surface of Mars. The radiative–convective models used to calculate the magnitude of the feedback require knowledge of the cloud depth, thickness, and areal coverage. Using a quasigeostrophic model and primitive equation simulations, it seems that less than 50% areal cloud cover is likely for early Mars, at least in midlatitudes. In accordance with previous studies for Earth, when condensational heating is included, the updraft region is narrower than in dry eddies. This results in reduced cloud cover and the feedback being weaker than previously thought. The simulations also show that condensation in eddies may have been important on a thick but cold early Mars but that its effects are much weaker for warmer climates.

The Estimation of Hydrometeor Profiles from Wideband Microwave Observations

Profiles of the microphysical properties of clouds and rain cells are essential in many areas of atmospheric research and operational meteorology. To enhance the understanding of the nonlinear and underconstrained relationships between cloud and hydrometeor microphysical profiles and passive microwave brightness temperatures, estimations of cloud profiles for an anvil region, a convective region, and an updraft region of an oceanic squall were performed. The estimations relied on comparisons between radiative transfer calculations of incrementally estimated microphysical profiles and concurrent dual-altitude wideband brightness temperatures from the 22 February 1993 flight during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. The wideband observations (10–220 GHz) are necessary for estimating cloud profiles reaching up to 20 km. The low frequencies enhance the rain and cloud water profiles, and the high frequencies are required to detail the higher-altitude ice microphysics. A microphysical profile was estimated for each of the three regions of the storm. Each of the three estimated profiles produced calculated brightness temperatures within ∼10 K of the observations. A majority of the total iterative adjustments were to the estimated profile’s frozen hydrometeor characteristics and were necessary to match the high-frequency calculations with the observations. This requirement indicates a need to validate cloud-resolving models using high frequencies. Some difficulties matching the 37-GHz observation channels on the DC-8 and ER-2 aircraft with the calculations simulated at the two aircraft heights (∼11 km and 20 km, respectively) were noted, and potential causes were presented.