Measurements and simulations of energy fluxes over a high-rise and compact urban area in Hong Kong

Abstract

Measuring energy fluxes in a dense and high-rise urban area is extremely challenging, thus our knowledge in such area remains limited. This study assessed the surface energy fluxes and investigated the energy balance closure (EBC) over such complex urban surface in Hong Kong. Net radiation (QN), sensible (QH) and latent (QE) heat fluxes were measured using an eddy covariance system from September 2018 to August 2019. Anthropogenic heat flux (QF) was simulated by a large-scale urban energy model (i.e., LUCY) and validated by an anthropogenic heat database (i.e., AHE_KL). Storage heat flux (QS) was estimated by an objective hysteresis model (OHM). Among five energy terms, QF showed the largest values of around 750 Wm−2 especially in the afternoon. Whereas, QE varying within 150 Wm−2 showed the smallest values. The variation range of net radiation, sensible heat flux and storage heat was respectively from −50 to 600, 0 to 450 and −30 to 300 W m−2. EBC generally showed a negative relationship with surface heterogeneity. Best EBC was observed in the direction with a relatively constant and large aspect ratio, and with the most occurrence of unstable stratifications. The uncertainties of QH and QE fluxes were respectively estimated to be approximately 8% and 7%. The result of LUCY was consistent with that of AHE_KL. EBC was not sensitive to different coefficients estimating QS. This study helped to fill a gap in our understanding of surface energy and turbulent fluxes in compact cities with high-rise buildings and shed insights into the future installation of eddy covariance tower in similar areas. The required height of the eddy covariance tower in such urban sites might not be as restrictive as that in other urban areas with low-rise buildings or with low building density, thus making it more feasible to set up such observation towers.