Observed turbulence characteristics in unstable conditions over the city of Tehran based on similarity theory

Autor: Nafiseh Pegahfar, Peyman Zawar-Reza
Rok vydání: 2016
Předmět:
Zdroj: Meteorology and Atmospheric Physics. 129:479-494
ISSN: 1436-5065
0177-7971
DOI: 10.1007/s00703-016-0482-9
Popis: In a previous study, applicability of Monin–Obukhov similarity theory (MOST) was established for stable and relatively neutral conditions for Tehran, based on local scaling approach. This paper extends the application of MOST to unstable conditions for Tehran using SODAR data and five 2-D sonic anemometers. Normalized standard deviations of three components of wind velocity ( $$\sigma_{u, v, w} /u_{*}$$ , $$u_{*}$$ is friction velocity) as functions of $$\zeta = (z - z_{d} )/\varLambda$$ stability parameter (where $$\Lambda$$ is the local Obukhov length, $$z$$ is the height of measurement and $$z_d$$ is the zero-plane displacement height) have been computed and analyzed at various levels. Only the parameters measured at 105 m height provided sufficient accuracy for MOST evaluation and also reliable universal similarity relations, while those at the lower heights (45 and 75 m) did not match MOST. Near zero dependence of $$\sigma_{ w} /u_{*}$$ on $$\zeta$$ motivated further investigation using convective velocity scale and also another stability parameter (defined using mixed-layer height). Applying the second scaling parameters leads to more suitable similarity functions. For the present research, we demonstrate that under unstable conditions, normalized values of $$\sigma_{u, v, w}$$ could be collapsed into the similarity expressions, but are highly dependent on proper selection of scale and vertical position. So for the case of Tehran, transformation from one kind of similarity scale to another one is possible, as suggested by Hu and Zhang (Sci Atmos Sin 17:10–20, 1993), with the provision that it only works for certain focused variables and vertical position. In addition, we show significant differences in empirical constants between Tehran and the values reported for the other cities in the literature (from the other surface types). This is most likely due to the local effects of the underlying surface (both urban area and topographical features) in Tehran.
Databáze: OpenAIRE
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