Andrew Sturman

Spatial Variability of Surface Radiation Fluxes in Mountainous Terrain

Abstract This paper investigates the magnitude and causes of spatial variability of surface radiative fluxes in a complex alpine landscape in the Southern Alps of New Zealand. Radiative flux components are simulated for the Tekapo watershed at 100-m resolution for clear-sky summer days, using a surface radiation budget model in conjunction with satellite imagery and topographic modeling to derive surface parameters. Overall, the model results agree well with observations made at a range of sites, with shortwave fluxes simulated more accurately than longwave fluxes. Sensitivity studies were conducted to isolate the role of spatial variability of surface characteristics in generating variance in the radiation budget. In order of most to least important, these characteristics were found to be slope aspect, slope angle, elevation, albedo, shading, sky view factor, and leaf area index. Spatial variability was greatest in midmorning and midafternoon, as a function of optical depth. The role of landscape complex...

Identifying regional dust transport pathways: ­application of kinematic trajectory modelling to a trans-Tasman case

PI kinematic trajectory model is used to investigate potential pathways of dust transport from Australia to New Zealand. Historically, these have been assumed to follow rather direct west-east trajectories spanning 2 to 3 days, often resulting in red snow events in the Southern Alps of New Zealand. However, results from the present study which examined the route taken by air parcels originating in southern Australia during dust storms on 24 and 25 May 1994, indicate that trans-Tasman dust transport trajectories are more diverse than previously thought, and display considerable variation during single events. These mon divergent pathways tie in more closely with aeolian dust sedimentation patterns identified by ocean coring in the Tasman Sea, and may account for the deposition of Australian dust on sub-Antarctic islands located well south of the Australian continent. Copyright 2000 John Wiley Sons, Ltd.

The New Zealand Southern Alps Experiment

The Southern Alps Experiment is being mounted to study the influence of New Zealands Southern Alps on local weather and climate. This paper describes these alpine influences and outlines proposed field and modeling experiments. Experiment goals include understanding and quantifying factors that govern the intensity and spatial distribution of heavy rainfall, the west to east distribution of precipitation across the mountains, and the intensity of lee wind storms and warming. Linked research will explore the use of deterministic rainfall models to predict river flows from mountain watersheds.

Aeolian dust transport and deposition by foehn winds in an alpine environment, Lake Tekapo, New Zealand

Abstract Observations from an automatic weather station and anemograph network are used in conjunction with recorded dust deposition rates to identify principle sources of wind blown dust within an alpine lake basin. Meteorological observations made throughout the study indicate that foehn windstorms provide ideal conditions for the initiation of aeolian processes, including dust storm genesis. A mean wind speed during foehn windstorms of 7.5 m s −1 at 2.65 m above the surface was observed to initiate entrainment of dust particles from geomorphically active areas within the lake basin, particularly from the dry braid channels of glacier fed rivers, exposed lacustrine deltas and degraded tussock grasslands. This threshold entrainment velocity is similar to others presented within the international literature for such surface types. A diurnal modulation of meteorological parameters monitored during foehn wind events suggests that aeolian processes, such as dust entrainment, are most likely to occur from late morning to early evening. However, where topography enhances wind speeds, for example by channelling airflow down river valleys, favourable conditions for dust entrainment may prevail throughout the night in the absence of precipitation. Potential dust roses were determined for two sites within the lake basin. The dust roses supported anecdotal and observational evidence of principle dust sources within the study area, and appear to be an effective tool for identifying the potential hazard to a particular location from blowing dust, once the physical trigger conditions for dust entrainment are known. This requires detailed and concurrent monitoring of meterological variables in association with dust transportation and deposition rates, especially in complex terrain where many micro-climates often exist.

A semi-empirical model for predicting the effect of changes in traffic flow patterns on carbon monoxide concentrations

A simple semi-empirical model for predicting the effect of changes in traffic flow patterns on carbon monoxide concentrations is presented. The traffic component of the model requires average vehicle emission rate estimates for a range of driving conditions, as well as traffic flow data for the site of interest. The dispersion component of the model is based on a modified empirically optimised box model requiring only wind speed and direction information. The model is evaluated at a suburban site in Hamilton, New Zealand. Despite the simplicity of the model, produces reliable concentration predictions when tested on days with significantly different traffic flow patterns from those days with which the optimum model parameters were evaluated.

Circulation changes and teleconnections between glacial advances on the west coast of New Zealand and extended spells of drought years in South Africa

ABSTRACTTwentieth century changes in the terminal position of the Franz Josef Glacier on the west coast of New Zealand are comparedwith an area-averaged mean annual rainfall series for the summer rainfall region of South Africa. Distinctive teleconnectionsare evident in an out-of-phase relationship between the two series, each of which exhibits an oscillation of 18–20 years.Periods of glacial advance are shown to coincide with extended dry spells in South Africa, when drought years are prevalent.Reconstructed pressure anomaly fields are presented for periods of advance and recession of the glacier and for the inter-decadal wet and dry spells in South Africa. In both cases major regional atmospheric circulation adjustments take place in aquasi-regular fashion over time to produce an enhancement in westerly airflow during the periods of drought in Africa andglacial advance on the west coast of New Zealand. Extended wet periods and glacial recession are likewise shown to beassociated with an enhancement of easterly components of the circulation. Changes in precipitation, temperature and moisturetransport are examined and associations with the Southern Oscillation Index are investigated.Franz Josef glacial advances are shown to occur 4–5 years after the onset of enhanced south-westerly airflow on to the westcoast of New Zealand. Advances occur on average 4 years after the onset of extended dry spells in South Africa. Similaratmospheric circulation anomalies in the respective sectors of the Southern Hemisphere and adjustments in the locations of thepositions of the ridges in standing wave three are responsible for this correspondence. # 1997 Royal Meteorological Society.Int. J. Climatol., Vol. 17: 1499–1512 (1997)(No. of Figures: 5. No. of Tables: 0. No. of References: 89)

A study of interacting multi-scale wind systems, Canterbury plains, New Zealand

SummaryThe wind regime of the Canterbury region, New Zealand, is composed of several interacting multi-scale wind systems all of which show strong diurnal periodicity. The dynamic orographic effect of the Southern Alps on the prevailing westerly flow results in perturbations to the pressure field and localized antitriptic airflow. Superimposed on this larger scale process are thermotopographic effects resulting from both regional and local land-sea thermal contrasts and slope heating. These processes act within an hierarchy of scales to produce a complex wind regime characterized by marked temporal variability, a layered vertical structure and the frequent occurrence of convergence lines and shear zones. The synergistic nature of the forcing mechanisms and the tendency for nocturnal decoupling of the boundary layer due to stability variations makes it difficult to differentiate and label discrete wind components.Attempts to simulate this regime using the Colorado State University mesoscale model showed that the model was unable to adequately resolve both the dynamic orographic effect and the local thermotopographic effect because of their differing scales of influence. These results suggest that a more holistic approach to both empirical and theoretical studies in such environments is required if more accurate wind field forecasting is to be achieved.ZusammenfassungDas Windregime des Gebiets von Canterbury, Neuseeland, setzt sich aus verschiedenen zusammenwirkenden Windsystemen verschiedener Größenordnungen zusammen, die alle einen starken Tagesgang aufweisen. Der dynamisch-orographische Effekt der neuseeländischen Alpen auf die vorherrschende Westströmung führt zu Störungen im Druckfeld und lokalen Luftbewegungen im Lee. Diesem großräumigen Prozeß sind thermisch-topographische Effekte überlagert, die sowohl durch regionale als auch lokale thermische Unterschiede zwischen Land und Meer und die Erwärmung der Hangregion hervorgerufen werden. Die Vorgänge spielen sich in einer Hierarchie von Größenordnungen ab. Sie erzeugen ein kompliziertes Windsystem, das durch hohe zeitliche Variabilität, eine schichtweise thermische Struktur und häufige Konvergenz- und Scherungszonen gekennzeichnet ist. Die synergetische Natur der Antriebe und die Tendenz zum nächtlichen Entkoppeln der planetaren Grenzschicht aufgrund von Stabilitätsschwankungen macht es schwer, die einzelnen Windkomponenten zu trennen und zuzuordnen.Die Versuche, dieses Regime mit Hilfe des Mesoscale-Modells der Colorado State University zu simulieren, zeigten, daß es aufgrund der verschiedenen Größenordnungen des Einflusses nicht geeignet war, gleichzeitig den dynamisch-orographischen und den thermo-topographischen Effekt zu reproduzieren. Diese Ergebnisse legen sowohl für empirische wie für theoretische Untersuchungen einen holistischeren Ansatz nahe, um eine genauere Prognose des Windfeldes zu ermöglichen.

A preliminary study of the transport of air from Africa and Australia to New Zealand

Kinematic trajectory modelling is used in a preliminary examination of inter-regiona l transport of air within the Southern Hemisphere In particular, the westerly zonal transport of air in the lower troposphere from southern Africa and Australia to New Zealand is determined for January and July, using both forward and backward trajectories originating or ending up in the 850-800 hPa layer respectively Mean transport plumes are derived from swarms of individual trajectories, and the results show that in winter around 22% of air originating over the central interior of South Africa reaches the central Tasman Sea south of New Zealand In summer the amount is insignificant owing to seasonal changes in the position of major circulation features In contrast, both summer and winter low-level air transport from southeastern and southwestern Australia and adjacent oceans to New Zealand is substantial, with 83% of all low-level air parcels from the Sydney area in summer passing over central New Zealand 5 days later In winter the transport plume passes over the northern part of the North Island within 3 days Back trajectories show that in some seasons two distinct paths are followed by air arriving at Christchurch and Auckland, from the west-northwest and southwest Analysis of the vertical structure of the transport plumes arriving in New Zealand shows that the westerly air reaching Auckland in the 850-800 hPa layer does so in a subsiding airstream throughout the year, whereas that arriving in the same layer over Christchurch experiences only minor subsidence en route from Australia and the ocean to the south This descending motion is related to the dominance of anticyclomc circulation, particularly over the northern section of the New Zealand region

Thermal influences on airflow in mountainous terrain

The effects of mountains on airflow have interested researchers for many years. Until the last decade, much work was concentrated on dynamic and thermodynamic effects operating at a range of scales from global to local. Of particular note has been the orographic influence on Rossby or planetary scale waves, cyclogenesis, foehn wind development, and lee waves. The extensive range of publications has been periodically reviewed, for example in Yoshino (1981), Barry ( 1981 ), and Smith (1979). The emphasis of this early research appears to have been dictated by aviation and general weather forecasting requirements. More recently, there has developed a need to understand the more localized airflow within mountainous terrain. The reasons for this include problems associated with air pollution, forest fire control, thunderstorms and precipitation, wind energy potential and aerial spraying and topdressing, where greater knowledge is required of airflow characteristics in the bound-

Cloud cover measured by satellite and from the ground: relationship to UV radiation at the surface

Clouds are important modulators of UV radiation, but quantification of their effects has been limited. This preliminary study assesses satellite-derived fractional cloud cover as an estimator of cloud effects on UV radiation measured at Lauder, New Zealand. Although there is a good correlation between measures of cloud cover from satellite and from the ground, their correlation with surface irradiance in either the visible or UV parts of the spectrum is found to be weaker. Measurement of fractional cloud cover alone is inadequate to parameterize cloud effects on incoming radiation. In particular, knowledge of whether or not the Sun is obscured is required, while estimates of cloud optical depth also appear to be important. The strong correlation between UV and visible radiation suggests that the use of ground-based pyranometer data to infer cloud effects on UV radiation is more accurate than the use of presently available satellite data for process studies of the relationship between cloud cover and insta...