engleski

The possibility of urban mixing height estimations with the bulk Richardson method in stable boundary layer conditions

The bulk Richardson number (RiB) method has been widely used to calculate urban and rural mixing heights (H) from measured as well as from predicted data. Nevertheless, considerable uncertainty in the choice of appropriate critical bulk Richardson number (RiBc) is empirically stated. Based on statistical parameters obtained from a significant data set of one year radio soundings data in urban Zagreb area it is shown that urban RiBc does not vary much from its theoretical value 0.25, in convective boundary layer (CBL). In CBL transition from turbulent viscous flow to laminar viscous flow is significant and very well determined with RiBc=0.25. However in stable boundary layers (SBLs) it is shown that RiBc applied to urban conditions vary from its theoretical value having the highest correlation coefficient for 1.5 < RiBc ≤ 2. This shift to higher RiBc indicates histeresis effect and possible lack to determine H in urban SBL with ‘ classical’ RiB method. Further, RiB method was applied in numerical weather prediction (NWP) model ALADIN. In very SBL conditions when RiB→ ∞ , frictional decoupling (FD) effect, which can lead to numerical instabilities in the NWP model, is evidenced. The stationary limit of the urban SBL height (He) is determined from radio soundings with RiB method having RiBc=1.5 as a critical value. Also, this He is used in a prognostic relation for H of a relaxation type for the parameterization of the SBL height to calculate hourly mixing heights during the night (Hmeasured). These values are compared with the modelled mixing height values (Hmodelled) and it is obvious that NWP model ALADIN needs improvements in urban mixing height representations for SBL conditions. This also implies that a simple parameter, e.g., various forms of Ri based on similarity theory, seems insufficient for determining H in urban SBLs, which ought to be a multi-valued function of a couple additional parameters.

urban mixing height; bulk Richardson method; ALADIN model; stable conditions

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