| Summary: | The principal concern of this study was to examine those processes responsible for the low level windfield over the Canterbury Plains. A secondary concern was the evaluation of a three-dimensional numerical meso-scale model in a wind environment incorporating lee effects aassociated with an adjacent mountain barrier.
Three complementary approaches were adopted: detailed kinematic analysis of spatial and temporal variations in the wind regime, an empirical investigation of the relative roles of dynamic and thermal forcing in contributing to the regime, and application of the Colorado State University meso-scale model. Data collection was undertaken over an 18 month period during 1982 and 1983 and included continuous monitoring of thermal and airflow characteristics over the plains, and intensive discrete case studies.
Investigation of spatial and temporal variations in the surface wind regime showed that low level airflow lee of the Southern Alps is strongly diurnal, low in the spatially variable, and seldom reflects gradient airflow. During daytime, and in all seasons north-easterly flow (approximately 1000m in depth) predominates over the majority of the plains. Within this regime, localised south-easterly sea breezes may occur south of Banks Peninsula, while foehn north-westerlies frequently invade the western plains (and occasionally coastal regions) mostly during spring and summer. The nocturnal surface flow regime is dominated by katabatic westerly flow particularly in the western and south-western plains during between components of the winter. Notable interactions between components of the wind regime include convergence between the south-easterly sea breeze embedded in the larger scale north-easterly inland from Banks Peninsula, and initiation of low level north-easterly flow adjacent to the Alps on mornings when katabatic winds covering the entire plains are removed by mixing associated with daytime heating.
The influence of local thermal forcing was evident in the wind regime in all seasons. Particular effects include the localised sea breeze south of Banks Peninsula and the addition of a sea breeze component to both the gradient south-westerly and low level north-easterly flows. Rather than reflecting a local north-easterly sea breeze, the strong diurnal periodicity of the surface north-easterly regime could be attributed to decoupling of near surface flow from the north-easterly prevailing above the planetary boundary layer on 36.8% of occasions when surface north-easterly flow was observed on consecutive days.
The dynamic interaction between synoptic scale flow and the topography of the South Island was found to be the major tor determining the nature airflow over the plains. Analysis of the synoptic scale pressure distribution revealed that the direction of the lee-side pressure gradient parallel to the Alps is critical in determining the general direction of low level flow over the plains. The direction of this gradient, and hence airflow over the plains, appears to be modulated at two scales. Firstly, within the time scale of the passage of typical synoptic scale circulation features, the lee side pressure gradient is characterised by a single abrupt reversal associated with a transition from general westerly or south-westerly gradient flow to low level north-easterly flow over the plains. Secondly, on a diurnal basis lesser variations occur as a result of thermally related atmospheric tidal effects whereby pressures tend to fall over Canterbury relative to areas both north and west from early morning to midafternoon. This latter process contributes strongly to the diurnal periodicity of the northeasterly regime.
Although application of the Colorado State University meso-scale model was constrained by available computational and financial resources, two series of simulations of daytime flow patterns incorporating realistic terrain successfully predicted salient features of the wind regime. These included the dynamically-induced north-easterly regime, sea breeze effects, and convergent flow patterns inland from Banks Peninsula. Model results indicated that thermal forcing associated with the regional terrain may result in augmentation of low level north-easterly flow by 4 - 5 ms-1 over the entire plains under ideal conditions. Augmentation of north-easterly flow in the vicinity of the Canterbury coast by local sea breeze effects was also suggested.
Verification of model results suggest that it is a valid theoretical tool in such environments. However, application of the model in the Canterbury region highlighted the need to account for spatial variations in critical input parameters and the complex interactions associated with rapidly changing synoptic scale flows in mid-latitude mountainous regions.
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