Seasonal dynamics of a suburban energy balance in Phoenix, Arizona

Observations of local-scale urban surface energy balance (SEB), which include fluxes of net all-wave radiation (Q*), and eddy covariance measurements of sensible (Q(H)) and latent heat (Q(E)) 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 Q(H). Much less energy is partitioned into Q(E), but this term is strongly affected by monsoonal precipitation, where greater-than-average Q(E) can be discerned for several days after storm events. The presence of a positive daily flux residual (RES) [i.e. Q* - (Q(H) + Q(E))] for most of the summer indicates that anthropogenic heat (Q(F)) 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 Q(H) 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 Q(H) 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 Q(H) associated with a high urban sky-view factor. Measured Q(E) 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.