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Seasonal dynamics of a suburban energy balance in Phoenix, Arizona

Authors

  • Winston T. L. Chow,

    Corresponding author
    1. Department of Geography, National University of Singapore, Singapore
    2. Department of Engineering, Arizona State University, Mesa, AZ, USA
    • Correspondence to: W. T. L. Chow, Department of Geography, National University of Singapore, 1 Arts Link, AS2, #04-32, Kent Ridge, Singapore 119260, Singapore. E-mail: winstonchow@nus.edu.sg

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  • Thomas J. Volo,

    1. School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA
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  • Enrique R. Vivoni,

    1. School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, AZ, USA
    2. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
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  • G. Darrel Jenerette,

    1. Department of Botany & Plant Sciences, University of California – Riverside, CA, USA
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  • Benjamin L. Ruddell

    1. Department of Engineering, Arizona State University, Mesa, AZ, USA
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ABSTRACT

Observations of local-scale urban surface energy balance (SEB), which include fluxes of net all-wave radiation (Q*), and eddy covariance measurements of sensible (QH) and latent heat (QE) were collected in an arid Phoenix, AZ suburb from January to December 2012. We studied diurnal variations in SEB partitioning over four distinct seasons: winter, equinoxes, and summer; the latter period is further subdivided into (1) months prior to and (2) months occurring during the North American Monsoon. Largest flux densities were observed in summer, with most available energy partitioned into QH. Much less energy is partitioned into QE, but this term is strongly affected by monsoonal precipitation, where greater-than-average QE can be discerned for several days after storm events. The presence of a positive daily flux residual (RES) [i.e. Q* − (QH + QE)] for most of the summer indicates that anthropogenic heat (QF) from residential cooling is likely a significant factor influencing SEB. Analysis of hourly ensemble SEB fluxes during all seasons also indicates that RES is largest in the morning, but QH dominates in the afternoon. Results of SEB trends and magnitudes from Phoenix were also compared with other urban sites, especially in (sub)tropical cities. When normalized with net radiation terms, a consistent diurnal hysteresis between ensemble QH and RES occurs, suggesting a robust parameterization of this relationship for model development during clear-sky conditions. SEB dynamics also appear to be affected by local surface characteristics, with regular nocturnal negative QH associated with a high urban sky-view factor. Measured QE fluxes during dry seasons were larger than expected based on the small proportion of irrigated plan area vegetated surfaces. A probable explanation could be an enhanced micro-scale advective forcing of evapotranspiration arising from leading-edge effects over patchy residential lawns, which has possible implications for modelling evapotranspiration in hot arid cities.

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