INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 22: 979–992 (2002) Published online 24 May 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.767 COMPARISON OF SUMMER- AND WINTER-TIME SUBURBAN ENERGY FLUXES IN CHRISTCHURCH, NEW ZEALAND RACHEL A. SPRONKEN-SMITH* Department of Geography, University of Canterbury, PB 4800 Christchurch, New Zealand Received 8 November 2000 Revised 7 December 2001 Accepted 16 December 2001 ABSTRACT Knowledge of the surface energy balance is fundamental to understanding the boundary layer meteorology and climatology of urban areas. This study reports some of the first direct measurements of energy fluxes over the city of Christchurch, New Zealand, during both summer and winter. Observations of the surface energy balance were made over two mainly residential suburbs: St Albans and Beckenham. Net all-wave radiation Q ∗ was measured with a net radiometer, the eddy covariance approach was used to measure the turbulent heat fluxes (sensible heat Q H , and latent heat Q E ), and the heat storage flux Q S was estimated as the energy balance residual. During the predominant northeasterlies and unstable conditions in summer, the fetch at St Albans includes a commercial warehouse as well as residential areas. In summer, on a daily basis, Q H is the dominant heat sink followed by Q S and Q E . However, during daytime Q S can be considerable and may approach the magnitude of Q H . Evaporation is low because the turbulent flux source areas are mainly centred over the commercial warehouse and yard, which have little greenspace. In winter the flux source areas are mainly residential for both sites, and the small daily surplus of Q ∗ is partitioned mostly into Q S , with some Q E and a small Q H that may be directed either towards or away from the surface depending largely on the synoptic conditions. Under strong inversion conditions, which occur frequently in Christchurch during winter, the turbulent heat fluxes are very small and Q H may be directed towards the surface for many hours overnight and early in the morning. During foehn events the energy partitioning is significantly altered, particularly in winter. Net radiation may be substantially decreased, evaporation is usually markedly increased and in winter Q H may be directed towards the surface for much of the event. The results highlight the importance of seasonal and synoptic controls in energy partitioning at this location, although difficulties with fetch complicate the analysis. Copyright  2002 Royal Meteorological Society. KEY WORDS: surface energy balance; heat fluxes; urban climate; foehn; Christchurch 1. INTRODUCTION Knowledge of the surface energy balance is fundamental to understanding the boundary layer meteorology and climatology of urban areas. The energy balance of an urban area can be expressed as: Q ∗ + Q F = Q H + Q E + Q S + Q A [W m −2 ] (1) where the flux densities are: Q ∗ net all-wave radiation, Q F anthropogenic heat, Q H turbulent sensible heat, Q E turbulent latent heat (or evaporation), Q S sensible heat storage, and Q A heat advection (representing the net gain or loss due to transport associated with the spatial heterogeneity of sources and sinks). This equation is typically applied to a volume that reaches from a measurement height well above the roughness elements (buildings and trees), down to ground level. Although several studies have examined surface energy exchanges of urban areas in the Northern Hemisphere (for a review see Grimmond and Oke (1995)), most focus on summertime energetics with few reports of wintertime observations. This study reports some of the first direct * Correspondence to: Rachel A. Spronken-Smith, Department of Geography, University of Canterbury, PB 4800 Christchurch, New Zealand; e-mail: r.spronken-smith@geog.canterbury.ac.nz Copyright  2002 Royal Meteorological Society