Linking urban water balance and energy balance models to analyse urban design options

Using a water balance modelling framework, this paper analyses the effects of urban design on the water balance, with a focus on evapotranspiration and storm water. First, two quite different urban water balance models are compared: Aquacycle which has been calibrated for a suburban catchment in Canberra, Australia, and the single-source urban evapotranspiration-interception scheme (SUES), an energy-based approach with a biophysically advanced representation of interception and evapotranspiration. A fair agreement between the two modelled estimates of evapotranspiration was significantly improved by allowing the vegetation cover (leaf area index, LAI) to vary seasonally, demonstrating the potential of SUES to quantify the links between water sensitive urban design and microclimates and the advantage of comparing the two modelling approaches. The comparison also revealed where improvements to SUES are needed, chiefly through improved estimates of vegetation cover dynamics as input to SUES, and more rigorous parameterization of the surface resistance equations using local-scale suburban flux measurements. Second, Aquacycle is used to identify the impact of an array of water sensitive urban design features on the water balance terms. This analysis confirms the potential to passively control urban microclimate by suburban design features that maximize evapotranspiration, such as vegetated roofs. The subsequent effects on daily maximum air temperatures are estimated using an atmospheric boundary layer budget. Potential energy savings of about 2% in summer cooling are estimated from this analysis. This is a clear ‘return on investment’ of using water to maintain urban greenspace, whether as parks distributed throughout an urban area or individual gardens or vegetated roofs.

The influence of Antarctica on the momentum budget of the southern extratropics

The antarctic plateau acts as a strong heat sink for the global climate, cooling the atmosphere and radiating energy to space. A cold dense atmospheric boundary layer is formed. Strong surface winds are formed as the boundary layer drains off the plateau. These drainage winds and the eddy fluxes necessary to maintain them are analysed in a general circulation model (GCM). The drainage flow is well represented in the GCM. The associated mean meridional circulation is analysed in isentropic coordinates. The momentum budget over Antarctica reveals a balance between the Eliassen-Palm flux convergence and the Coriolis torque exerted by the mean meridional mass flux. Both vertical and horizontal components of the Eliassen-Palm flux contribute, the vertical component being the greater.