Premium
The momentum flux‐gradient relations derived from field measurements in the urban roughness sublayer in three cities in China
Author(s) -
Zou Jun,
Liu Gang,
Sun Jianning,
Zhang Hongsheng,
Yuan Renmin
Publication year - 2015
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2015jd023909
Subject(s) - turbulence , dimensionless quantity , momentum (technical analysis) , similarity (geometry) , mathematics , flux (metallurgy) , laminar sublayer , roughness length , richardson number , mathematical analysis , geometry , meteorology , physics , reynolds number , wind speed , mechanics , wind profile power law , materials science , finance , artificial intelligence , computer science , economics , image (mathematics) , metallurgy
Field measurements of the momentum flux and wind velocity gradient were carried out at three urban sites in the cities of Nanjing, Changzhou, and Suzhou, China. The observational data in the urban roughness sublayer are analyzed to derive the momentum flux‐gradient relations in terms of framework of the local similarity theory with a least squares fit, and the relations are then compared to the classical similarity relations of Businger et al. (1971). The results show that the momentum flux varies with height; thus, the constant‐flux assumption in the Monin‐Obukhov Similarity Theory is not met in the urban roughness sublayer. However, the dimensionless wind velocity gradient may be described by the local similarity theory, and the derived flux‐gradient relations have the same form as the classical similarity relations do. Under stable conditions, the coefficient β m in the fitted relations increases with height and gradually approaches the value of 4.7 in the classical similarity relations. The trend suggests that the turbulent flow may be described by the classical similarity relations once the height increases and reaches the top of the urban roughness sublayer, and thus, the height of the top of the urban roughness sublayer may be estimated by linear extrapolation of the derived relations in this study. The relation between the critical Richardson number Ri c and the coefficient β m is derived as Ri c = Pr t / β m , where Pr t is the turbulent Prandtl number. In the urban roughness sublayer, the value of Ri c is larger than 0.2.