The critical bulk Richardson number in urban areas: verification and application in a numerical weather prediction model

Dispersion models require hourly values of the mixing height (H) that indicates the existence and vertical extent of turbulent mixing. Urban areas, which are usually industrial areas too, have H higher than rural areas, and commonly used methods for deriving H should not be applied under the same conditions as in homogeneous conditions. The bulk Richardson number (Ri B ) methodwas applied to determineH over Zagreb, Croatia. Impact of urban areas on the choice of critical values of bulk Richardson number (Ri Bc ) was explored, and different values were used for convective boundary layer (CBL) and for stable boundary layer (SBL). Aire Limitee Adaptation Dynamique development InterNational (ALADIN), a limited area numerical weather prediction (NWP) model for short-range 48-h forecasts, was used to provide one set of input parameters. Another input set comes from radio soundings. The values of H, modelled and based on compared measurements, and the correlation coefficient as well as standard deviation and bias were calculated on a large data set to determine Ri Bc ranges applicable in urban areas. It is shown that Ri B method can be used in urban areas, and that urban Ri Bc should have certain limitations despite of a wide spectrum of practical values used today. Significantly increased Ri Bc values in SBL were determined from the NWP and soundings data, which is the consequence of increased surface roughness in the urban area. The verification of ALADIN through the determination of H was also done. Decoupling from the surface in the very SBL was detected as a consequence of the flow cease resulting in Ri B becoming very large.