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  • Author or Editor: C. S. B. Grimmond x
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M. J. Best
and
C. S. B. Grimmond

Abstract

Inclusion of vegetation is critical for urban land surface models (ULSM) to represent reasonably the turbulent sensible and latent heat flux densities in an urban environment. Here the Joint UK Land Environment Simulator (JULES), a ULSM, is used to simulate the Bowen ratio at a number of urban and rural sites with vegetation cover varying between 1% and 98%. The results show that JULES is able to represent the observed Bowen ratios, but only when the additional anthropogenic water supplied into the urban ecosystem is considered. The impact of the external water use (e.g., through irrigation or street cleaning) on the surface energy flux partitioning can be as substantial as that of the anthropogenic heat flux on the sensible and latent heat fluxes. The Bowen ratio varies from 1 to 2 when the plan area vegetation fraction is between 30% and 70%. However, when the vegetation fraction is less than 20%, the Bowen ratios increase substantially (2–10) and have greater sensitivity to assumptions about external water use. As there are few long-term observational sites with vegetation cover less than 30%, there is a clear need for more measurement studies in such environments.

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Xiangyu Ao
,
C. S. B. Grimmond
,
H. C. Ward
,
A. M. Gabey
,
Jianguo Tan
,
Xiu-Qun Yang
,
Dongwei Liu
,
Xing Zhi
,
Hongya Liu
, and
Ning Zhang

Abstract

The Surface Urban Energy and Water Balance Scheme (SUEWS) is used to investigate the impact of anthropogenic heat flux Q F and irrigation on surface energy balance partitioning in a central business district of Shanghai. Diurnal profiles of Q F are carefully derived based on city-specific hourly electricity consumption data, hourly traffic data, and dynamic population density. The Q F is estimated to be largest in summer (mean daily peak 236 W m−2). When Q F is omitted, the SUEWS sensible heat flux Q H reproduces the observed diurnal pattern generally well, but the magnitude is underestimated compared to observations for all seasons. When Q F is included, the Q H estimates are improved in spring, summer, and autumn but are poorer in winter, indicating winter Q F is overestimated. Inclusion of Q F has little influence on the simulated latent heat flux Q E but improves the storage heat flux estimates except in winter. Irrigation, both amount and frequency, has a large impact on Q E . When irrigation is not considered, the simulated Q E is underestimated for all seasons. The mean summer daytime Q E is largely overestimated compared to observations under continuous irrigation conditions. Model results are improved when irrigation occurs with a 3-day frequency, especially in summer. Results are consistent with observed monthly outdoor water use. This study highlights the importance of appropriately including Q F and irrigation in urban land surface models—terms not generally considered in many previous studies.

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