Canterbury Plains Windfield Analysis

The study employed the Colorado State University meso-scale model to simulate daytime flow patterns in Canterbury, New Zealand. While the model successfully predicted key features of the wind regime—including the dynamically-induced north-easterly regime, sea breeze effects, and convergent flow patterns inland from Banks Peninsula—its application was limited by computational and financial resources. Despite these constraints, the model indicated that thermal forcing associated with the regional terrain could augment low-level north-easterly flow by 4-5 m/s across the plains under ideal conditions. This highlights the importance of considering both model limitations and the influence of terrain in regional windfield analysis.

The research is grounded in the increasing recognition of the impact of local airflow patterns on various aspects of life and the environment. Over the past two to three decades, understanding regional windfields has become crucial for air quality, land-use management, weather forecasting, recreational activities, and wind energy resource development. Local circulations like land-sea breezes and mountain-valley winds, driven by thermal and topographic forces, are significant components of these windfields. This study builds on a substantial body of existing research, referencing comprehensive reviews by Defant (1951), Flohn (1969), and Munn (1966), as well as more recent work by Oke (1978) and Atkinson (1981), emphasizing the need for continued investigation into these complex interactions.

Recent advancements in computer technology have led to increased use of three-dimensional numerical models in regional windfield analysis. However, the study stresses the importance of integrating complementary empirical analysis to evaluate the models' performance across diverse environments. The South Island of New Zealand, with its significant north-south oriented mountain barrier, offers a valuable case study due to its isolated topographic characteristics within an oceanic mid-latitude setting. This combined approach—numerical modeling complemented by empirical data—is critical for validating the accuracy and applicability of the models in predicting real-world wind conditions. The study underscores the synergistic value of both modeling and empirical analysis in advancing understanding.

The study defines key meteorological scales: synoptic scale (referencing Atkinson, 1981) which describes features like anticyclones and fronts; and local scale (referencing Oke, 1978), encompassing phenomena like lake breezes and slope winds. Theoretical understanding of these scales has a long history, citing Jeffreys (1922) who classified winds based on their dynamics (geostrophic, Eulerian, and antitriptic winds), highlighting the balance of forces in various wind types. This section lays the groundwork for understanding the different scales of atmospheric motion relevant to the study, explaining the theoretical underpinnings of wind behavior and providing context for the subsequent analysis of Canterbury's complex wind patterns. The interplay of these scales is crucial to interpreting observed wind characteristics.

The study explores the significant impact of orographic effects (mountain barriers) on regional wind patterns. Airflow over these barriers results in pressure distortions, creating hydrostatically-induced high pressure on the windward side and low pressure in the lee (foehn nose). Smith (1982) provides examples of this phenomenon, including a severe downslope windstorm in the Southern Alps of New Zealand. These pressure distortions significantly influence wind direction and strength, as demonstrated by the creation of lee troughs and their relationship to foehn winds. The research highlights how the Southern Alps' shape the overall wind regime and create specific wind phenomena such as the ‘nor’wester’, drawing on the work of Beer (1975) and Smith (1979).

The primary focus of this chapter is a detailed description of the spatial and temporal variations in Canterbury's surface wind regime. While some research exists on specific aspects of Canterbury's wind patterns, a comprehensive analysis of both wind speed and direction has been lacking. Previous studies often provide only qualitative descriptions of these variations. For instance, de Lisle (1969) notes the well-developed coastal north-easterly wind near Christchurch, sometimes extending inland beyond Ashburton. Other quantitative studies, like Lamb's (1970, 1974) work on the nor'wester and its effect on the surface heat budget, lack a holistic approach. The study aims to fill this gap by providing a detailed quantitative analysis of wind speed and direction.

The analysis employs a kinematic approach to study spatial and temporal variations in the wind regime. Following Sturman and Tyson (1981), contoured frequencies of wind direction are used to describe temporal variations at each site. Windrose mapping (Tyson and Seely, 1980) further details spatial and temporal variations in both wind direction and speed, particularly useful for non-synchronous data from multiple sites. For synchronous observations (January 1982 – August 1983), a simplified crosstabulation of simultaneous wind direction observations (10-degree intervals) at Christchurch Airport with other sites is employed (Cehak and Pichler, 1968; Kamst et al., 1980). This approach facilitates identification of airflow type associations within a temporal and spatial context.

Analysis reveals generally similar wind directions across all sites, especially for surface winds between 180 and 360 degrees at Christchurch Airport. These are typically associated with synoptic-scale airflow, with exceptions for light katabatic north-westerlies. The greatest variations occur with the north-easterly circulation. A notable diurnal pattern at Christchurch Airport shows daytime backing of south-westerly winds in phase with the north-easterly maximum, attributed to sea breeze forcing by Sturman and Tyson (1981). Similar effects are observed at Winchmore and other locations, with the north-easterly showing a diurnal rhythm. The study delves into the spatial variations in the onset and strength of the north-easterly wind.

The most significant gradient wind direction during the onset of surface north-easterlies is south-westerly (28.8% of events), often linked to anticyclonic isobaric curvature and the approach of an anticyclone from the central Tasman Sea. North-easterlies are also associated with easterly or slack gradients with anticyclonic curvature, frequently occurring when an anticyclone is positioned over or near the South Island. Spatial variations in north-easterly onset are analyzed, considering potential instrument sensitivity differences and utilizing pilot balloon observations to support findings. The study notes the complex interplay between synoptic conditions and the development of localized wind patterns.

The study examines the interaction of katabatic winds with the north-easterly flow, highlighting the complex nature of their interaction and the varying spatial patterns of north-easterly onset across the plains. The research suggests that slope heating in the western foothills might eliminate the supply of cold air for katabatic flow, encouraging upslope flow and leading to coastal katabatic winds being stronger and longer-lasting. Differences in instrument sensitivity and detailed observational data (including a transect of pilot balloon observations) are considered to explain the spatial variations in north-easterly onset. The study emphasizes the significance of understanding this interaction for a more comprehensive understanding of the Canterbury Plains wind regime.

This chapter aims to determine the extent to which local thermal forcing contributes to the diurnal periodicity of onshore winds on the Canterbury Plains. An empirical approach is used to examine the relationship between onshore airflow and several factors influencing thermal forcing: land-sea temperature contrasts, synoptic situations, upper-level airflow, feedback effects, and stability influences. Large-scale thermal effects attributable to the Southern Alps are excluded from this analysis. The study uses an observational approach to connect observed wind patterns with various meteorological factors in order to quantify the influence of local thermal processes on the wind regime.

The earth's surface is characterized by variations in thermal properties, creating horizontal temperature gradients when different surfaces are juxtaposed. These gradients cause pressure variations that drive air movement at various scales. At the meso-scale, thermal forcing produces phenomena such as sea and land breezes, slope and valley winds, and summer thermal lows (Mathews, 1982). While general climatological summaries mention thermal forcing in Canterbury, specific studies are limited. The Canterbury Project (D.S.I.R., 1951) is cited as the most intensive study of the Canterbury sea breeze, focusing on advection ducts and radio-wave propagation. This section establishes the context for understanding how land-sea temperature differences and other factors contribute to thermal forcing and wind generation in Canterbury.

The primary driver of sea breeze generation is the magnitude of land-sea thermal contrasts. However, other important factors include the direction and strength of gradient airflow, low-level stability, and coastal upwelling. The diurnal cycle of the planetary boundary layer also contributes to the diurnal periodicity of onshore surface winds, especially in conjunction with downslope flows. Data analysis focuses on examining the relationship between these factors and the occurrence of onshore winds. The analysis focuses on the strength and frequency of the sea breeze, and considers the spatial scale and influence of other forcing mechanisms on the observed wind patterns. The chapter explains the data used and the methodology applied to understand the thermal forcing mechanisms at play.

Analysis of potential sea breeze events reveals that most occur in late morning or afternoon, often persisting into the evening or early morning. Seasonal variations are significant, with higher frequencies in December-February and few events in May-August. This seasonal pattern strongly suggests the influence of thermal forcing. However, daily solar radiation totals exceed monthly mean values on only 58% of occasions. Approximately two-thirds of events are recorded at Greendale (50km inland) and Winchmore, suggesting that the sea breezes are part of larger thermally forced systems, extending further inland than typical mid-latitude sea breezes (Atkinson, 1981). This finding indicates that other forcing mechanisms contribute significantly to the observed wind patterns.

The study investigates the effects of the north-easterly wind on the temperature profile, noting complications from simultaneous temperature increases due to diurnal heating. A simplified relationship between air temperature (T2) and sea surface temperature (SST) is derived (T2 = SST + 0.21), showing a seasonal component. Warmer months have onset temperatures exceeding SST, likely due to greater solar heating and a transition from warm north-west advection. Cooler months show onset temperatures lower than SST, indicating less temperature modification upon inland passage. This section highlights the role of seasonal variations and boundary layer stability, underscoring the complexity of the thermal processes influencing the Canterbury Plains wind regime. The interaction between the north-easterly wind and the thermal regime is further investigated by considering its influence on diurnal and nocturnal temperature fluctuations.

This chapter shifts focus to synoptic-scale processes influencing local airflow in Canterbury. Interactions between synoptic-scale circulation (Trenberth, 1977; Revell, 1972), the Southern Alps mountain barrier, and regional thermal effects are significant. A major consequence of these interactions is the distortion of the pressure field and airflow around the mountain barriers (Smith, 1982), a result of dynamic and thermal effects. Understanding these larger-scale atmospheric patterns is essential for comprehending the local wind regime, particularly given the significant influence of the Southern Alps on Canterbury's wind climate. The chapter builds upon prior chapters by investigating the larger atmospheric context that shapes the local wind patterns.

The Southern Alps significantly influence Canterbury's wind climate in several key ways. Firstly, airflow over the mountains generates the warm, gusty foehn wind, or 'nor'wester' (Kidson, 1932; de Lisle, 1969; Lamb, 1970). Secondly, the mountains deform the surface pressure field, particularly during strong north-westerly conditions. Airflow funneled through Cook Strait recurves into the Canterbury coast north of Banks Peninsula due to a pressure gradient directed towards the lee of the Alps (de Lisle, 1969). Trenberth (1977) suggests that the prevailing north-easterly wind in Canterbury is an enhanced meso-scale sea breeze, a result of the orographic distortion of the gradient wind flow and diurnal thermal forcing. Thirdly, the Southern Alps modify synoptic-scale circulation features, retarding the movement of cold fronts (Watts, 1947; Garnier, 1958; Sevele, 1969) and influencing the passage of other synoptic-scale systems (Watts, 1947; Sevele, 1969; Browne, 1975).

The intensive study period (January 1982 – August 1983) coincided with an extreme phase of the Southern Oscillation, a large-scale pressure anomaly affecting the New Zealand region. This anomaly increased the frequency of south-westerly flow, significantly impacting rainfall patterns across the South Island. Rainfall increased in the south and west, while decreasing in the north and east. This anomalous pattern, linked to the low-index phase of the Southern Oscillation, resulted in high frequencies of south-westerly flows, particularly evident from the winter of 1982 onward (Collen, 1983). The study acknowledges that while the gradient airflow was somewhat anomalous during this period, the impact on the thermal regime was not as marked.

Despite the anomalous gradient airflow, temperature data from Christchurch Airport show significant monthly variations from normal, exceeding 10°C in several months. Warmer-than-usual temperatures in January-March 1982 were associated with frequent north-westerly winds, while April 1982 was cooler due to easterly winds. August 1982 saw warmer temperatures under anticyclonic conditions, while October 1982 experienced unusually cold spells. Sea surface temperature (SST) variations are described, noting two distinct periods: January-November 1982 showing SSTs similar to previous years, and December 1982-July 1983 exhibiting considerably lower maximum and minimum SSTs, likely due to the Southern Oscillation's impact on wind frequency and direction. Cold advection and coastal upwelling are identified as contributing factors to the lower SSTs. The study also briefly acknowledges the various potential sources of error when using pilot balloon soundings for wind measurements.