engleski

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 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) method was applied to determine H over Zagreb, Croatia. Impact of urban areas on 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). ALADIN, a limited area numerical weather prediction (NWP) model for short-range 48-hour forecasts was used to provide input parameters. The values of H, the modelled and based on measurements are compared, 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 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 ease resulting in Ri_B becoming very large.

NWP model ALADIN ; radiosoundings ; mixing height ; bulk Richardson method

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