Neighborhood scale air quality modeling in Houston using urban canopy parameters in MM5 and CMAQ with improved characterization of mesoscale lake-land breeze circulation

Advanced capability of air quality simulation models towards accurate performance at finer scales will be needed for such models to serve as tools for performing exposure and risk assessments in urban areas. It is recognized that the impact of urban features such as street and tree canopies will become more pronounced as the grid sizes decreases. This paper will focus on methods to introduce urban features into a predictive model to provide accurate and representative temporal and spatial resolve meteorological fields for running the Community Multiscale Air Quality (CMAQ) modeling system at neighborhood scales (order 1 km grid resolution).

A set of urban canopy parameters (UCP) have been derived for 1 km grid mesh for a modeling domain encompassing Harris County and surrounding areas. This set of gridded UCPs was specifically developed for implementation of the DA-SM2-U/MM5 system (Dupont, et al., 2004); specifically, a canopy drag approach was developed for incorporation into an advanced urbanized surface layer (soil-atmosphere) model (SM2-U) which was further implemented into the NCAR-Penn State Mesoscale Meteorological Model, Version 5 (MM5). A total of 20 UCP (combination of vertical profiles and surface values) were derived from a database consisting of buildings and vegetation features from airborne lidar measurements, ancillary data from satellites, high altitude photography, as well as detailed residential, commercial and industrial maps.

This paper examines results from two sets of CMAQ simulations each driven by MM5 at 1 km grid size, one with this specialized set of UCPs and the other using the standard version. Preparatory to this, it is recognized that the pollutant transport in the Houston area will be strongly affected by the sea and Galveston Bay breezes (Gilliam et al., 2004); thus, care is taken to ensure the accuracy of the modeled circulation induced by Galveston Bay. Introducing gridded inputs (showing relatively large diurnal amplitude) of the sea surface temperature (SST) of Galveston Bay) from observations taken from polar orbiting satellite platforms resulted in greatly improved accuracy of the MM5 land-bay breeze circulation simulations at 4 km grid resolution. It will be demonstrated that the 1 km grid size simulations are sensitive to the degree of accuracy of the input boundary condition of the flow field from the coarser 4 km grid nest. The resulting implications for the finer scale simulations with UCP on the flow and the subsequent air quality fields will be presented and discussed.