Approach for the vertical wind speed profile implemented in the UTCI basics blocks UTCI applications at the urban pedestrian level

Abstract

The applicability of the Universal Thermal Climate Index (UTCI) at the urban pedestrian level is analysed in relation to the approach for the vertical wind speed profile (VWSP) used in the UTCI basics. As the UTCI refers to the human-biometeorological reference height (z_h−b) of 1.1 m above ground level (agl), all meteorological input variables for UTCI calculations should also be originated from z_h−b. Since the development of the UTCI considered meteorological conventions for stations routinely operated by national weather services, the wind speed (v) as one of four meteorological input variables is initially required at 10 m agl (v_{10 m}). In order to calculate v at z_h−b from v_{10 m}, the logarithmic law for the VWSP is implemented in the UTCI basics, but only for neutral atmospheric stability and a roughness length (z₀) for short-cut grassland (z₀ = 0.01 m). This methodological approach cannot be changed in the UTCI basics so far. The UTCI developers also recommend the application of the same logarithmic law, if v values at lower heights have to be extrapolated to 10 m agl. However, the use of this VWSP approach disregards basic air flow findings and principles in the urban canopy layer (UCL). If the UTCI is nevertheless applied at the urban pedestrian level, inaccuracies in the UTCI values will occur. With reference to z₀ = 0.80 m, which is more typical for the UCL, they can be up to 7 K, as shown in the example of three different climate zones.