The Influence of Thermally-Induced Mesoscale Circulations on Turbulence Statistics Over an Idealized Urban Area Under a Zero Background Wind
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The influence of mesoscale circulations induced by urban-rural differential surface sensible heat flux and roughness on convective boundary-layer (CBL) flow statistics over an isolated urban area has been examined using large-eddy simulation (LES). Results are analyzed when the circulations influence the entire urban area under a zero background wind. For comparison, the CBL flow over an infinite urban area with identical urban surface characteristics under the same background meteorological conditions is generated as a control case (without circulations). The turbulent flow over the isolated urban area exhibits a mix of streaky structure and cellular pattern, while the cellular pattern dominates in the control case. The mixed-layer height varies significantly over the isolated urban area, and can be lower near the edge of the urban area than over the rural area. The vertical profiles of turbulence statistics over the isolated urban area vary horizontally and are dramatically different from the control case. The turbulent kinetic energy (TKE) sources include wind shear, convergence, and buoyancy productions, compared to only buoyancy production in the control case. The normalized vertical velocity variance is reduced compared to the control case except in the central urban area where it is little affected. The low-level flow convergence is mainly responsible for the enhanced horizontal velocity variance in the central urban area, while wind shear is responsible for the additional local maximum of the horizontal velocity variance near the middle of the CBL outside the central area. Parameterizations in the prognostic equation for TKE used in mesoscale models are evaluated against the LES results over the isolated urban area. We also discuss conditions under which the urban-induced circulations occur and when they may affect the entire urban area. Given that urban-induced circulations can influence the entire urban area within hours for an urban area of a realistic size, it is inappropriate to directly apply empirical relations of turbulence statistics derived under horizontally-homogenous flow conditions to an urban area.
KeywordsIsolated urban area Large-eddy simulation Mixed layer Thermally-induced mesoscale circulation Urban boundary layer
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- Arakawa A, Lamb V (1977) Computational design of the basic dynamical processes of the UCLA general circulation model. In: Chang J (ed) Methods in computational physics, vol 17. Academic Press, pp 173–265Google Scholar
- Brown MJ, Williams M (1998) An urban canopy parameterization for mesoscale meteorological models. In: Proceedings of second symposium on the urban environment, Albuquerque, NM1998, American Meteorological Society, pp 144–147Google Scholar
- Gopalakrishnan SG, Roy SB, Avissar R (2000) An evaluation of the scale at which topographical features affect the convective boundary layer using large eddy simulations. J Atmos Sci 57: 336–351Google Scholar
- Patton EG (1997) Large-eddy simulation of turbulent flow above and within a plant canopy. PhD dissertation, University of California, DavisGoogle Scholar
- Wang W (2009) The influence of topography on single-tower-based carbon flux measurements under unstable conditions: a modeling perspective. Theor Appl Climatol doi:10.1007/s00704-009-0130-0