Urban uMM5 land-use patterns and its effects on Houston UHI strength

An improved version of MM5 and the use of more sophisticated input LU/LC data sets have improved its performance in the simulation of Houston urban heat island (UHI) characteristics. In particular, the EPA version of MM5 (called uMM5 at SJSU): (a) better simulates canopy-layer meteorological fields by use of a drag-force approach in lieu of typical roughness length theory, (b) more accurately simulates at finer resolutions, e.g., 1 km, and (c) accepts available fine-resolution input parameters based on urban morphological and LU/LC data developed as part of an joint US EPA and University of Utah (UofU) effort.

This paper describes simulations that used uMM5, recent aerometric data from the Texas2000 field study, new EPA/UofU urban morphological data, and USFS developed urban-reforestation scenarios to simulate the changes in near-surface (UHI) intensities. Results from uMM5 versus the standard MM5 showed an increased (and more accurate) midday/maximum UHI magnitude, which increased 2.5 to 4.0 K, while urban reforestation decreased that new value by 0.9 K due to shadowing, increased soil moisture, and conversion from concrete/asphalt to soil. Resulting uMM5 met fields will used in future Houston-Galveston CMAQ SIP simulations, as EPA allows for emission reduction credits for UHI mitigation by urban reforestation, as cooler urban areas to decrease electricity use, photolysis rates, and biogenic HC emissions.