An Urban Parameterization for a Global Climate Model. Part II: Sensitivity to Input Parameters and the Simulated Urban Heat Island in Offline Simulations

Abstract

In a companion paper, the authors presented a formulation and evaluation of an urban parameterization designed to represent the urban energy balance in the Community Land Model. Here the robustness of the model is tested through sensitivity studies and the model's ability to simulate urban heat islands in different environments is evaluated. Findings show that heat storage and sensible heat flux are most sensitive to uncertainties in the input parameters within the atmospheric and surface conditions considered here. The sensitivity studies suggest that attention should be paid not only to characterizing accurately the structure of the urban area (e.g., height-to-width ratio) but also to ensuring that the input data reflect the thermal admittance properties of each of the city surfaces. Simulations of the urban heat island show that the urban model is able to capture typical observed characteristics of urban climates qualitatively. In particular, the model produces a significant heat island that increases with height-to-width ratio. In urban areas, daily minimum temperatures increase more than daily maximum temperatures, resulting in a reduced diurnal temperature range relative to equivalent rural environments. The magnitude and timing of the heat island vary tremendously depending on the prevailing meteorological conditions and the characteristics of surrounding rural environments. The model also correctly increases the Bowen ratio and canopy air temperatures of urban systems as impervious fraction increases. In general, these findings are in agreement with those observed for real urban ecosystems. Thus, the model appears to be a useful tool for examining the nature of the urban climate within the framework of global climate models.

An Urban Parameterization for a Global Climate Model. Part I: Formulation and Evaluation for Two Cities

“…These must be provided at a time step that resolves the diurnal cycle. Over other land surfaces, they are determined by current parameterizations in CLM3 (Oleson et al 2004). An objective of this paper is to describe a set of parameterizations that determines the fluxes from an urban surface.…”

“…where sno ϭ 0.97 (Oleson et al 2004). As with solar radiation, the longwave interactions within the urban canyon are determined numerically by allowing for multiple reflections until a convergence criterion is met (the absorbed longwave radiation for a given reflection is less than 1 ϫ 10 Ϫ3 ).…”

“…The turbulent fluxes from the UCL to the atmosphere are derived from Monin-Obukhov similarity theory. The CLM3 formulations are used (Oleson et al 2004) but are modified where appropriate for urban surfaces. The flux formulations require estimates of roughness length and displacement height.…”

“…The aerodynamic resistances to sensible and latent heat transfer between the UCL air and the overlying atmosphere (r ah , r aw ) depend on stability as in CLM3 [see Eqs. (5.56) and (5.57) in Oleson et al 2004]. The specific humidities of the roof and impervious road surfaces, q g,roof and q g,imprvrd , are set to the saturated specific humidity evaluated at their respective surface temperatures.…”

“…This latter formulation is analogous to the treatment of the wetted fraction of the vegetated canopy in CLM3 (Oleson et al 2004). The approach used here to represent pervious surfaces is different from many urban schemes designed for use within mesoscale and global models.…”