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

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.

Estimation of urban sensible heat flux using a dense wireless network of observations

“…In fact, the values obtained by the scintillometer which are used to calibrate k B −1 are definitely typical of measurements over this type of surface in winter and early spring. Spronken-Smith [44], in a study of energy fluxes over a suburban area in New Zealand, reports ensemble means of sensible heat flux peaking at 50-60 W/m 2 between 12:00 and 14:00 for the middle of winter. Grimmond [17] reports slightly higher values over a suburb of Vancouver for measurements taken in January and February.…”

Influence of seasonality and vegetation type on suburban microclimates

“…In urban and suburban areas, vegetation cover has been shown to be important for explaining spatial differences in urban and suburban evapotranspiration rates (Grimmond and Oke 1999;Spronken-Smith 2002), net CO 2 exchange (Soegaard and Møller-Jensen 2003;Moriwaki and Kanda 2004), and microclimate characteristics (Bonan 2000;Byrne et al 2008;Huang et al 2008). For example, Spronken-Smith (2002) found that residential neighborhoods in Christchurch, New Zealand with high vegetation cover had greater evapotranspiration rates than did neighborhoods with less vegetation cover.…”

“…For example, Spronken-Smith (2002) found that residential neighborhoods in Christchurch, New Zealand with high vegetation cover had greater evapotranspiration rates than did neighborhoods with less vegetation cover. Grimmond and Oke (1999) showed that the variability among North American cities in evapotranspiration rates was related to differences in the percent cover of vegetated surfaces, in addition to differences in precipitation and irrigation inputs.…”

Surface Energy Balance Measurements Above an Exurban Residential Neighbourhood of Kansas City, Missouri

“…Recent urban energy balance observations have also been conducted in other parts of the world, including Basel in Switzerland (Rotach et al 2005), Lódź in Poland (Offerle et al 2005a), Edinburgh in Scotland (Nemitz et al 2002), Christchurch in New Zealand (Spronken-Smith 2002), Tokyo in Japan (Moriwaki and Kanda 2004), Marseille in France (Mestayer et al 2004;Grimmond et al 2004b) and the west African Sahel: Ouagadougou in Burkina Faso (Offerle et al 2005b). Together, these studies document the temporal and spatial variability of energy flux partitioning in more densely built-up, old suburban residential locations with mature, tall tree canopies that are generally higher than the buildings and in downtown areas or central city sites (densely built-up area, low vegetation cover, tall buildings with massive walls and deep street canyons).…”