Abstract
Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin–Obukhov similarity theory (MOST) in numerical weather prediction models can be improved upon by using RSL corrections when modelling the vertical profiles of wind speed and friction velocity in the urban RSL using MOST. Using anisotropic building morphological information improves the agreement between observed and parametrized profiles of wind speed and momentum fluxes for selected methods. The largest improvement is found when using dynamically-varying aerodynamic roughness length and displacement height. Adding a RSL correction to MOST, however, does not improve the parametrization of the vertical profiles of temperature and heat fluxes. This is expected since sources and sinks of heat are assumed uniformly distributed through a simple flux boundary condition in all RSL formulations, yet are highly patchy and anisotropic in a real urban context. Our results can be used to inform the choice of surface-layer representations for air quality, dispersion, and numerical weather prediction applications in the urban environment.
Highlights
When modelling urban meteorological processes, it is crucial to represent the exchange of momentum and scalars such as temperature and humidity between the surface and overly-Freiburg, Freiburg, Germany 5 Present Address: Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands ing atmosphere
The wind speed and potential temperature profiles are based on the flux-gradient relations that follow from the adaptation of Monin–Obukhov similarity theory (MOST) to the roughness sublayer (RSL) (Garratt 1980), κ(z − zd) u∗
Where z is the height above ground level, zd is the zero-plane displacement, z∗ is the depth of the RSL, κ is the von Kármán constant (=0.4), u∗ is the friction velocity, θ∗ is the temperature scale, L is the Obukhov length, ΦM and ΦH are the MOST functions for momentum and heat respectively, and φM and φH are the RSL profile functions for momentum and heat, respectively
Summary
When modelling urban meteorological processes, it is crucial to represent the exchange of momentum and scalars such as temperature and humidity between the surface and overly-Freiburg, Freiburg, Germany 5 Present Address: Meteorology and Air Quality, Wageningen University, Wageningen, The Netherlands ing atmosphere. When modelling urban meteorological processes, it is crucial to represent the exchange of momentum and scalars such as temperature and humidity between the surface and overly-. Urban obstacles (e.g., buildings and trees) disturb the flow and in turn cause vertical divergence of turbulent fluxes, modifying the turbulent exchange (Rotach 1995; Christen et al 2007). The layer in which the flow and turbulent fluxes are disturbed by obstacles is one definition of the roughness sublayer (RSL); this layer can be two to five times the mean canopy height (Raupach et al 1991; Grimmond and Oke 1999; Kastner-Klein and Rotach 2004; Grimmond et al 2004). The RSL is the inertial sublayer (ISL), and for surfaces with small roughness elements the RSL is very shallow and the ISL exchanges of momentum and scalars can be modelled using Monin–Obukhov similarity theory (MOST). The RSL effects on measurements and model calculations are only significant above surfaces such as tall crops and forests, and above the urban canopy layer (UCL)
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