On the applicability of urban canopy parametrization in building grey zone

Abstract

AbstractWith increasing interest in urban meteorology and related services, the need to appropriately represent the urban environment in climate/weather models is rising. These regional weather/climate models typically use a km‐scale horizontal grid, which is insufficient to resolve the flow around buildings. Effects of the urban environment on the atmosphere above are represented through a bulk approach using the Urban Canopy Parametrization (UCP) schemes. Existing UCPs usually use the repeating canyon–roof representation that assumes homogeneous distribution of buildings within the grid box. It is commonly accepted that the assumption of homogeneity holds at km‐scale grid resolution but whether it also holds at sub‐km scale, where the regional models are increasingly approaching, is questionable. For this reason, among others, the resolution ranges from a few hundred metres to tens of metres (i.e. building‐resolving scales) is termed the building grey zone in the existing literature. This work shows that the assumption of homogeneity indeed does not hold at the building grey zone for the city‐state Singapore. To understand the possible influences of the use of UCPs at scales from the building grey zone to the conventional mesoscale, we use an urban‐grid method that allows us to estimate the parametrized fluxes from a typical UCP at varying resolutions over the urban landcover while keeping the same atmospheric model grid. Numerical results show that different urban‐grid resolutions yield variations in the near‐surface temperature and wind to a maximum of 0.5 K and 1 m·s−1. Their impact on the boundary‐layer parameters is found to be limited. Although these near‐surface variations are small, they are comparable to the near‐surface scaling variables and are thus physically significant.