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UBC Theses and Dissertations

Development and evaluation of a suburban evaporation model : |b a study of surface and atmospheric controls on the suburban evaporation regime Cleugh, Helen Adair


This research focusses on observing and modelling the suburban surface energy balance. The initial objective is to use measurements to elucidate the controls on the size and temporal variability of the latent heat flux. This is achieved by synchronous observations of suburban and rural energy balances. On the basis of this comparison it is proposed that the day-to-day variability of the partitioning of the suburban turbulent fluxes is linked both to larger-scale atmospheric influences and variations in the energy and moisture availability within the suburban 'canopy'. This hypothesis is examined through measurement and modelling. Further observations of the suburban energy balance components reveal that the size of the Bowen ratio is linked to the surface moisture availability. This is comprised of soil moisture variations in unirrigated greenspace areas and also the anthropogenic influence of lawn irrigation. However, in addition to this, the day-to-day variability of the Bowen ratio is a function of an advective influence upon the saturation deficit in the surface and mixed-layers. The mechanisms which determine this relationship are identified as meso-scale advective effects resulting from differing land-uses. This influences the nature of the mixed-layer and hence surface fluxes. In light of this interaction of scales and atmospheric processes, a model is developed that couples advectively-dominated mixed-layer dynamics with surface-layer exchanges of heat and mass. The acronym for the model is SCABLE, Suburban Canopy and Boundary Layer Evaporation model). It predicts the diurnal evolution of the mixed-layer depth, temperature and humidity. The saturation deficit of the mixed-layer is an input to the surface evaporation model. In turn this enables the surface sensible heat flux to be calculated from the surface energy balance (using measurements of the available energy). This modelled surface sensible heat flux drives the growth of this mixed-layer and thus the rate of entrainment from the capping inversion. The temperature and moisture structure of the mixed-layer is determined by both inputs from the surface-layer, and from the "free" atmosphere. The suburban canopy evaporation sub-model is based on the 'big leaf' Combination model, with a parameterisation scheme for the surface and aerodynamic resistances based upon the approaches taken by Shuttleworth (1976, 1978). The model performs adequately for simulating the day-to-day variability of the saturation deficit and surface evaporation. Its performance on an hourly basis indicates that the model weaknesses lie in the simulation of the diurnal behaviour of the surface resistance and potential temperature of the mixed-layer. It is concluded in the thesis that such an approach is necessary and valid for predicting and understanding the evaporation regime in areas the size of suburbia. This is especially true where there is likely to be a combination of factors determining the surface evaporation rate.

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