On the influence of the urban roughness sublayer on turbulence and dispersion

Abstract

The concept of the urban roughness sublayer is discussed and this lowest atmospheric layer over a rough surface is shown to have a non-negligible vertical extension over typical urban surfaces. The existing knowledge on the turbulence and flow structure within an urban roughness sublayer is reviewed, focusing on the height dependence of turbulent fluxes and a scaling approach for turbulence statistics, such as velocity variances, in the above-roof part of the roughness sublayer. Finally, the implication of this turbulence and flow structure upon dispersion characteristics is investigated. The most prominent difference of explicitly taking into account the roughness sublayer in a dispersion simulation (as compared to assuming a `constant flux layer') is a clearly enhanced ground level concentration far downwind from the source. For the example of a tracer release experiment over a (sub) urban surface (Copenhagen) it is shown that introducing the roughness sublayer clearly improves the model performance.

Introduction

On a mesoscale perspective an urban settlement can be considered a rough, warm `spot’ within the surrounding rural surfaces. Consequently, an internal boundary layer develops at the rural–urban interface (e.g., Oke, 1988) that is both mechanical and thermal in origin. In this contribution we restrict the discussion to areas within the urban environment that are far enough from this transitional region so that the urban boundary layer (UBL) has replaced the former rural boundary layer. The lowest atmospheric layer of this UBL must be considered as a roughness sublayer (RS). This layer, that constitutes the rough-wall counterpart of the viscous sublayer over `smooth walls’ (Raupach et al., 1991) has, unlike the latter, a vertical extension of several tens of meters over typical urban settings and is therefore of importance when modelling flow or dispersion processes in urban environments.

Most of the present knowledge on roughness sublayers over rough surfaces stems from vegetated or artificial surfaces (see Raupach et al., 1991 for an excellent review). In contrast, relatively little is known about the flow and turbulence structure over real urban or suburban surfaces. Important differences between the two types of surfaces are the characteristic density of roughness elements and also their stiffness (or flexibility to react upon drag forces).

The RS can be regarded as the layer adjacent to the surface, wherein the flow and turbulence is influenced by individual roughness elements. Consequently, it has a fully three-dimensional structure. However, for many practical applications it is convenient to consider spatial averages of the variables of interest in order to retain the simplistic one-dimensional (vertical) description of flow and turbulence characteristics. Also, a detailed description of the effect of individual buildings can be avoided. On the other hand it is clear that from full scale observational programs it is almost impossible to truly determine spatially averaged properties. Here, the argumentation of Rotach (1993a) is followed by noting that an observation at a particular point in space represents a variety of upwind and downwind geometries due to changing wind direction. Thus spatial averages can be approximated by averaging over all wind directions of approaching flow.

Due to the lack of a comprehensive physically based theory for the (spatially) average vertical structure of the RS, observational data is often compared to the predictions of Monin–Obukhov similarity theory, which is, strictly speaking, only valid for the inertial sublayer (note that over smoother surfaces, where the RS has a negligible vertical extension, this layer is often called surface layer). For the upper part of the RS (i.e. for z>h, where z is the physical height and h denotes the average height of the canopy) this approach has become quite standard for flow over vegetated surfaces (e.g. Kaimal and Finnigan, 1994) and was also followed in the few studies of full scale urban surfaces (see Section 3). However, it will be pointed out in Section 3 that due to the non-constancy of turbulent fluxes within the RS, a particular form of local scaling is much more appropriate to describe the turbulence variables. Within the canopy (i.e., in the lower part of the RS) it is often convenient to construct (properly scaled) average vertical profiles for the variables of interest.

In this contribution the concept of a roughness sublayer is described in Section 2, where also its vertical extension for typical urban settings is discussed. In Section 3, the vertical turbulence and flow structure in the urban RS is investigated, making use of the results from an extensive field study within the Zurich Urban Climate Program. Wherever possible, the results from this program are compared to those from other (including artificial) rough surfaces. Finally, in Section 4, the effect of including the turbulence structure of the roughness sublayer upon dispersion of passive scalars is demonstrated.