Abstract
Abstract. The performance of different urban surface parameterizations in the WRF (Weather Research and Forecasting) in simulating urban boundary layer (UBL) was investigated using extensive measurements during the Texas Air Quality Study 2006 field campaign. The extensive field measurements collected on surface (meteorological, wind profiler, energy balance flux) sites, a research aircraft, and a research vessel characterized 3-dimensional atmospheric boundary layer structures over the Houston-Galveston Bay area, providing a unique opportunity for the evaluation of the physical parameterizations. The model simulations were performed over the Houston metropolitan area for a summertime period (12–17 August) using a bulk urban parameterization in the Noah land surface model (original LSM), a modified LSM, and a single-layer urban canopy model (UCM). The UCM simulation compared quite well with the observations over the Houston urban areas, reducing the systematic model biases in the original LSM simulation by 1–2 °C in near-surface air temperature and by 200–400 m in UBL height, on average. A more realistic turbulent (sensible and latent heat) energy partitioning contributed to the improvements in the UCM simulation. The original LSM significantly overestimated the sensible heat flux (~200 W m−2) over the urban areas, resulting in warmer and higher UBL. The modified LSM slightly reduced warm and high biases in near-surface air temperature (0.5–1 °C) and UBL height (~100 m) as a result of the effects of urban vegetation. The relatively strong thermal contrast between the Houston area and the water bodies (Galveston Bay and the Gulf of Mexico) in the LSM simulations enhanced the sea/bay breezes, but the model performance in predicting local wind fields was similar among the simulations in terms of statistical evaluations. These results suggest that a proper surface representation (e.g. urban vegetation, surface morphology) and explicit parameterizations of urban physical processes are required for accurate urban atmospheric numerical modeling.
Highlights
Cities occupy less than 0.1% of the whole Earth’s surface, but about 50% of total population inhabits cities
The simulations were conducted over the Houston-Galveston area for a summertime period of 12–17 August using an original land surface model (LSM), a modified LSM, and a single-layer urban canopy model (UCM) in combination with the Yonsei University (YSU) scheme
In order to reproduce the effects of urban vegetation existing in the Houston urban areas, several physical parameters controlling hydrological processes were altered in the modified LSM
Summary
Cities occupy less than 0.1% of the whole Earth’s surface, but about 50% of total population inhabits cities. Lee et al.: Evaluation of urban surface parameterizations in the WRF model thermal, and hydraulic properties These characteristics in urban morphology can be involved in various urban physical processes such as in-canyon radiative transfer, turbulence exchanges of momentum, mass, and heat in and above the canyon, and thermal conduction at artificial surfaces. Masson, 2000; Kusaka et al, 2001; Martilli et al, 2002; Lee and Park, 2008; Oleson et al, 2008) In these models, the urban physical processes (e.g. in-canyon radiative transfer, turbulence momentum and heat exchanges) are explicitly calculated and interact. The extensive observations allow the urban surface parameterizations to be faithfully evaluated for the Houston metropolitan area, which will give clues for the improvement of the model performance in local weather forecasting and/or air-quality simulations.
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