Comparison of summer- and winter-time suburban energy fluxes in Christchurch, New Zealand

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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 QH, and latent heat QE), and the heat storage flux ΔQS 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, QH is the dominant heat sink followed by ΔQS and QE. However, during daytime ΔQS can be considerable and may approach the magnitude of QH. 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 ΔQS, with some QE and a small QH 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 QH 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 QH 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

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