Simulations of multi-scale circulations and boundary layer structure and their effect on particulate layering over and downwind of the central Mexican plateau

Much has been learned about the boundary layer structure and circulations associated with the complex terrain surrounding Mexico City from field experiments and modeling studies. Several types of thermally-driven circulations, including slope flows, gap flows, and propagating density currents, form in the region that interact with the larger-scale ambient flow. Thermally-driven circulations often produce strong convergence over the city that enhances vertical mixing and ventilation of pollutants. Mesoscale models, however, often have difficulty in simulating these complex circulation patterns and their effect on air quality. While extensive chemistry and particulate measurements have been collected at the surface to characterize air quality over the city, relatively few measurements have been made aloft and downwind of the city to characterize the vertical variations of anthropogenic trace gases and aerosols emitted from Mexico City and their impact on the local and regional environment. This knowledge gap has been addressed by the recent field campaigns conducted in Mexico during March 2006, as part of the Megacities Initiative: Local and Global Research Observations (MILAGRO).

This study will employ MILAGRO data obtained from a range of meteorological instrumentation (surface networks, radar wind profilers, radiosondes) as well as surface and aircraft-based lidars to examine the evolution of regional circulations and boundary layer depth over the central plateau of Mexico. The extensive aircraft lidar measurements provide information on the vertical extent of mixing of particulates over the valleys and ridges as well as multiple layers of particulates in the middle troposphere transported over the Gulf of Mexico. In contrast to most urban areas located near sea-level, Mexico City's altitude at ~2.2 MSL permit pollutants to be injected directly into the free troposphere as convective boundary layers collapse. The altitude of the layers likely depends on the terrain elevation of particulate sources and strength of local boundary layer mixing. We use the meteorological and lidar measurements to determine the performance of the WRF-chem model in simulating the local and regional wind and boundary layer patterns and how the meteorological processes contribute to the layering of particulates. The performance of the model is compared to previous studies that have employed older boundary layer parameterizations to determine whether the skill of mesoscale models has improved over the past ten years.