Uncertainty Analysis of the Eddy-Covariance Turbulent Fluxes Measured over a Heterogeneous Urban Area: A Coordinate Tilt Impact

Abstract

An accurate determination of turbulent fluxes over an urban area is a challenging task due to its morphological diversity and associated flow complexity. In this study, an eddy covariance (EC) method is applied over a highly heterogeneous urban area in a small city (Gongju), South Korea to investigate the quantitative influence of 'coordinate tilt' in determining the turbulent fluxes of sensible heat, latent heat, momentum, and carbon dioxide mass. Two widely-used coordinate transform methods are adopted and applied to eight directional sections centered on the site to analyze a 1-year period EC measurement obtained from the urban site: double rotation (DR) and planar fit (PF) transform. The results show that mean streamline planes determined by the PF method are distinguished from the sections, representing morphological heterogeneity of the site. The sectional pitch angles determined by the DR method also compare well with those in the PF method. Both the PF and DR methods show large variabilities in the determined streamline planes at each directional section, implying that flow patterns may form in a complicate way due to the surface heterogeneity. Resulting relative differences of the turbulent fluxes, defined by (F DR -F PF )/F DR , are found on average +13% in sensible heat flux, +21% in latent heat flux, +37% in momentum flux, and +26% in carbon dioxide mass flux, which are larger values than those reported previously for fairly homogeneous natural sites. The fractional differences depend significantly on wind direction, showing larger differences in northerly winds at the measurement site. It is also found that the relative fractional differences are negatively correlated with the mean wind speed at both stable/unstable atmospheric conditions. These results imply that EC turbulent fluxes determined over heterogeneous urban areas should be carefully interpreted with considering the uncertainty due to 'coordinate tilt' effect in their applications.

The Microscale Urban Surface Energy (MUSE) Model for Real Urban Application

“…The study area consists of several buildings of apartments, campus buildings, and low residential houses. A micrometeorological tower has been installed at the rooftop of a campus building (36.4717 • N, 127.1426 • E), from which various meteorological and radiative variables have been obtained such as winds, air temperature, specific humidity, shortwave, and longwave radiative fluxes, and turbulent sensible and latent fluxes [69]. The surface temperatures were measured at six points on the building surfaces, with two horizontal surfaces (a roof and a road) and four vertical surfaces at different directions (east, west, south, and north) using a dual-point laser infrared thermometer (SK-8140, Sato Keiryoki Mfg.…”

“…u * , T * , and L are the friction velocity, the surface-layer temperature scale, and the Obukhov length, which were measured directly from the flux tower. The aerodynamic roughness length is set to 1.06 m following the simple formulation of Grimmond and Oke [72] from the analysis of the site measurement [69], and the integrated stability functions for momentum (ψ m ) and heat (ψ h ) are given in Högström's work ( [73] and [74], respectively).…”

Comparison of Actual Evapotranspiration Amounts Estimated Using Water Balance and Eddy-Covariance Methods: Applications to Seolmacheon and Cheongmicheon Basins

“…관련 연구는 에디공분산방법으로 관측한 증발산량의 검정문제 (Sur et al, 2012;Lee et al, 2013;Kang et al, 2014)에서부터 불확실성 또는 관측오차문제 (Lee et al, 2016;Lim et al, 2016) 등 연구의 폭도 다양하다. 논 (Lee et al, 2005), 산림 (Yuan et al, 2007;Gwak et al, 2013;Kang et al, 2014;Moon and Lim, 2016), 도시지역 (Lee et al, 2016) 등 다양한 지역에서의 측정도 시도되었다. 광릉수목원 (Ryoo and Kim, 2000;Kang et al, 2009), 설마천유역 (Kwon et al, 2009), 용담시험유역 (Kim et al, 2012).…”