Application of the urbanized MM5 to the Houston-Galveston TX region

Fine-resolution mesoscale meteorological modeling is critical for more accurate and detailed characterization of urban canopy-layer meteorological fields and processes. It is also important for a better assessment of potential impacts from surface-characteristic modifications, land-use changes, pollutant and heat emissions, and changes in vegetative cover. Such improved modeling also has ramifications in terms of regulatory air-quality modeling.

Thus far, mesoscale modeling extensions to urban scales have mostly relied on ‘conventional’ formulation of PBL models, e.g., roughness length approach. This formulation “does not see” the complex geometry of and phenomena in urban areas and thus does not account for the myriad of fluxes and interactions that occur in the canopy layer. To attempt at addressing this shortcoming, the US EPA has recently undertaken to modify the PSU/NCAR MM5 by inclusion of a drag-force approach sub-grid scale parameterization and soil model (DA-SM2-U) in conjunction with the Gayno-Seaman TKE-based PBL scheme.

In this study, we further improve upon the DA-SM2-U model towards an urbanized version of the MM5 and apply it to southern Texas, with an emphasis on the Houston-Galveston area. The region is simulated using a five-nest configuration, with the innermost 1-km grid using the urbanized code. The urbanized version is an improvement over the conventional model in that it captures local meteorological features and urban climate signatures of the heat island and flow fields. We explore the sensitivity of the model in simulating various canopy-layer meteorological fields, especially as related to urban forest trends and scenarios.