A Sensitivity Study of the Urban Effect on a Regional-Scale Model: An Idealized Case

Abstract

Urban infrastructure impacts the surface and atmospheric properties, such as wind, temperature, turbulence and radiation budgets. The well-recognized urban heat island phenomenon, characterized by the temperature contrast between the city and the surrounding rural area, is one such impact. Many field experiments have been conducted to study the urban heat island, which is typically most intense under clear sky and weak ambient wind conditions at night. In some cases, a cool island may even exist during the day.

To consider these urban effects in a numerical model with horizontal grid resolution on the order of kilometers, some sort of parameterization is required to account for the sub-grid building impacts on these effects. To this end, Brown and Williams (1998) have developed an urban parameterization by extending Yamada's (1982) forest canopy scheme to include drag, turbulent production, radiation balance, and anthropogenic and roof-top heating effects in a mesoscale model. In this study, we further modify this urban parameterization by adding the rooftop surface energy equation to eliminate a simplifying assumption used in Brown and Williams (1998). Results indicate that this modification leads to significant improvement on the evolution of heat island effect, and exhibits a clear relationship of this urban effect with the building fraction required in this urban parameterization.

In this paper, we are focused on studying the sensitivity of the modified urban parameterization to the grid scale features of a mesoscale model for an idealized case using atmospheric sounding profiles. The objective of this work is to assess the impact of individual process of the urban canopy parameterization on turbulent production, airflow and atmospheric stability of boundary layer, and to further the understanding of the cause and effect of the urban heat island phenomenon. Based on a series of sensitivity experiments using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), developed by the Naval Research Laboratory, we will present the details of results using the modified urban parameterization.