Estimation of turbulent sensible heat and momentum fluxes over a heterogeneous urban area using a large aperture scintillometer

Abstract

The accurate determination of surface-layer turbulent fluxes over urban areas is critical to understanding urban boundary layer (UBL) evolution. In this study, a remote-sensing technique using a large aperture scintillometer (LAS) was investigated to estimate surface-layer turbulent fluxes over a highly heterogeneous urban area. The LAS system, with an optical path length of 2.1 km, was deployed in an urban area characterized by a complicated land-use mix (residential houses, water body, bare ground, etc.). The turbulent sensible heat (Q H ) and momentum fluxes (τ) were estimated from the scintillation measurements obtained from the LAS system during the cold season. Three-dimensional LAS footprint modeling was introduced to identify the source areas ("footprint") of the estimated turbulent fluxes. The analysis results showed that the LAS-derived turbulent fluxes for the highly heterogeneous urban area revealed reasonable temporal variation during daytime on clear days, in comparison to the land-surface process-resolving numerical modeling. A series of sensitivity tests indicated that the overall uncertainty in the LAS-derived daytime Q H was within 20%-30% in terms of the influence of input parameters and the nondimensional similarity function for the temperature structure function parameter, while the estimation errors in τ were less sensitive to the factors of influence, except aerodynamic roughness length. The 3D LAS footprint modeling characterized the source areas of the LAS-derived turbulent fluxes in the heterogeneous urban area, revealing that the representative spatial scales of the LAS system deployed with the 2.1 km optical path distance ranged from 0.2 to 2 km 2 (a "micro-α scale"), depending on local meteorological conditions. Citation: Lee, S.-H., J.-H. Lee, and B.-Y. Kim, 2015: Estimation of turbulent sensible heat and momentum fluxes over a heterogeneous urban area using a large aperture scintillometer.

Scintillometry in urban and complex environments: a review

“…Recently, LASs have been deployed in Asian cities, for example at a highly heterogeneous site in Gongju, Korea (Lee et al, 2015) and on two short paths (< 500 m) at suburban and urban sites in China (Zhang and Zhang, 2015). In the latter study, QH_sc was often much larger than QH_EC.…”

“…Investigating the accuracy of the technique is very difficult in such complex urban environments. Lee et al (2015) used high resolution numerical modelling to assess LAS performance over an urban river. The Weather Research and Forecasting (WRF) model was run for two days providing QH at 200 m resolution, which was then averaged over the 2.1 km path.…”

Source area definition for local climate zones studies. A systematic review

“…Christchurch [180], Tokyo [96], Marseille [83,181], Essen [100], Houston (TX) [135], Basel [125,138], Helsinki [134], Swindon [117,182], Gongju [91], London [124], Black Rock City (NV) [110], Łódź [101], and Nanjin [142].…”

Spatial and temporal patterns of surface–atmosphere energy exchange in a dense urban environment using scintillometry

“…More extensively, large aperture scintillometers (LASs) have been used which allow path lengths of several kilometres. LAS measurements have been conducted in Marseille, France (Lagouarde et al , ); Łódź, Poland (Zieliński et al , ); Helsinki, Finland (Wood et al , ); Swindon, UK (Ward et al , ); Gongju, South Korea (Lee et al , ), and the Yangtze Delta, China (Zhang and Zhang, ). Finally, a dual‐wavelength approach to derive both sensible and latent heat fluxes was used in Swindon by Ward et al ().…”

“…In Gongju, Lee et al () compared scintillometer‐derived sensible heat and momentum fluxes to outputs from a high‐resolution (200 m) numerical model configured for their heterogeneous urban setting. Results showed good agreement between the model and scintillometer methods with estimated uncertainty in scintillometer Q H of 20–30%.…”