WAVE HEIGHTS IN THE SURF ZONE

The advantages and disadvantages of measuring ocean wave heights in the surf zone by using terrestrial photogrammetry are outlined. Difficulties arising from a lack of control points can be overcome by using the apparent horizon to provide camera orientations about two axes, provided that the apparent horizon is the sea surface and that an unconventional orientation of the camera axis system is adopted. Following practical tests and error simulation studies, height differences were determined for 12 points in the breaking wave zone off a Newcastle (New South Wales) beach. It is hoped that the method can be developed as a convenient means of collecting wave data.     

The impact of dynamic sea-ice on the climatology and climate sensitivity of a GCM: a study of past, present, and future climates

 We assess two parametrisations of sea-ice in a coupled atmosphere–mixed layer ocean–sea-ice model. One parametrisation represents the thermodynamic properties of sea-ice formation alone (THERM), while the other also includes advection of the ice (DYN). The inclusion of some sea-ice dynamics improves the model's simulation of the present day sea-ice cover when compared to observations. Two climate change scenarios are used to investigate the effect of these different parametrisations on the model's climate sensitivity. The scenarios are the equilibrium response to a doubling of atmospheric CO₂ and the response to imposed glacial boundary conditions. DYN produces a smaller temperature response to a doubling of CO₂ than THERM. The temperature response of THERM is more similar to DYN in the glacial case than in the 2×CO₂ case which implies that the climate sensitivity of THERM and DYN varies with the nature of the forcing. The different responses can largely be explained by the different distribution of Southern Hemisphere sea-ice cover in the control simulations, with the inclusion of ice dynamics playing an important part in producing the differences. This emphasises the importance of realistically simulating the reference climatic state when attempting to simulate a climate change to a prescribed forcing. The simulated glacial sea-ice cover is consistent with the limited palaeodata in both THERM and DYN, but DYN simulates a more realistic present day sea-ice cover. We conclude that the inclusion of simple ice dynamics in our model increases our confidence in the simulation of the anomaly climate. Received: 24 May 2000 / Accepted: 25 October 2000     

Three-dimensional simulations of air flow and momentum transfer in partially harvested forests

In order to predict wind loading on trees (canopy height h) in partially harvested forests, it is necessary to characterize air flow and momentum transfer in progressively more complex patterns where groups of trees (or aggregates) are retained. In this study, we used large-eddy simulation to explore the effects of aggregate size, inter-aggregate spacing, and the ratio between the aggregate size and inter-aggregate spacing on air flow and momentum transfer. Forty-five grid points across an aggregate were needed to achieve an adequate level of turbulence. Using grid sizes of h/15 throughout was too viscous for the smaller aggregates. Vertical and horizontal flow deflection by the leading aggregates sheltered some of the downstream aggregates to varying degrees where turbulence increased for subsequent rows. The number of rows of protected aggregates decreased as aggregate dimensions and the space between aggregates increased. A theoretical treatment of time-dependent wind is presented for the lead aggregate and a simulation case is presented for the case of a gust of reduced wind passing through the aggregate pattern. The leading aggregate responded with decreasing moment for decreasing ambient wind speed as predicted by theory. However, downwind aggregates experienced substantial increases in bending moment. The overall results of the disruptive aspects of time dependence agrees with arguments regarding the role of irrotational (potential) flow to this problem. Our treatment of retention pattern design is only a first step and further research suggestions are presented.     

Titaniferous phlogopites from the leucite lamproites of the West Kimberley area,Western Australia

Phlogopites in the wolgidites, fitzroyites and cedricites (leucite lamproites) of the West Kimberley area range in composition from titaniferous phlogopite to titaniferous tetraferriphlogopite. Two distinct compositional trends are present. In wolgidites micas are characterized by strong total iron enrichment, moderate Al and Mg depletion and little enrichment in tetrahedral ferric iron. Micas in fitzroyites and cedricites are characterized by strong depletion in Al and strong enrichment in tetrahedral ferric iron at approximately constant Mg contents. Individual centers of fitzroyite intrusion are characterized by distinct mica TiO₂ contents. The phlogopite compositional trends at low pressures are very different to those of kimberlite micas. An origin of these lamproites involving differentiation of a kimberlitic magma is considered unlikely.     

Sea-surface topography around Australia

The sea-surface topographys, as represented by the separation between the ocean surface and a level surface, is viewed as a problem involving and concerning both geodesy and physical oceanography. The determination of this topography bygeodetic levelling processes, in conjunction with tide-gauge observation, is examined. Sources of error, difficulties, estimates of accuracies, and actual results are mainly related to the third-order Australian levelling net, which has indicated a sea-surface topography variation, with position, of 2 m, with a standard deviation estimated to be about 30 cm. The expectedoceanographic influences on the sea-level are described, the individual contributing factors being discussed separately. Around Australia, differences in water density can account for an estimated 60 cm of the above mentioned 200 cm sea-level variation, while the airpressure effect appears to account for another 10 cm only. The wind influence undoubtedly also contributes to the sea-surface topography but it is presently virtually impossible to provide a suitable figure. Some discussion is given to the apparent differences between the results from these separate sources, for this continent.     

KAr data for the age and evolution of Gettysburg Bank,North Atlantic Ocean

Gettysburg Bank forms the western end of the Gorringe Seamount which is situated in the North Atlantic 110 km west of the tip of the Iberian Peninsula, on the eastern end of the Azores/Gibraltar fracture zone.Gabbros dredged from the Gettysburg Bank record a complex history of events. K-Ar ages of separated mineral phases fall into three concordant groups (plus some discordant ages). The oldest ages are from three brown kaersutitic hornblendes and their mean age of135 ± 3Ma is taken to be that of their formation. Six plagioclase feldspars yielded concordant ages of105 ± 3Ma which is possibly a consequence of a thermal event occurring at that time. Ages from three deformed plagioclases are concordant with a mean of82 ± 3Ma which is believed to relate to a phase of shearing, perhaps occurring during transform motion at the plate boundary.     

Lg Coda Q and its Relation to the Geology and Tectonics of the Middle East

—Regional seismograms were collected to image the lateral variations of Lg coda Q at 1 Hz (Q ₀?) and its frequency dependence <(eta)> in the Middle East using a back-projection method. The data include 124 vertical-component traces recorded at 10 stations during the period 1986–1996. The resulting images reveal lateral variations in both Q ₀ and <eta>. In the Turkish and Iranian Plateaus, a highly deformed and tectonically active region, Q ₀ ranges between about 150 and 300, with the lowest values occurring in western Anatolia where extremely high heat flow has been measured. The low Q ₀ values found in this region agree with those found in other tectonically active regions of the world. Throughout most of the Arabian Peninsula, a relatively stable region, Q ₀ varies between 350 and 450, being highest in the shield area and lowest in the eastern basins. All values are considerably lower than those found in most other stable regions. Low Q values throughout the Middle East may be caused by interstitial fluids that have migrated to the crust from the upper mantle, where they were probably generated by hydrothermal reactions at elevated temperatures known to occur there. Low Q ₀ values (about 250) are also found in the Oman folded zone, a region with thick sedimentary deposits. <eta> varies inversely with Q ₀ throughout most of the Middle East, with lower values (0.4–0.5) in the Arabian Peninsula and higher values (0.6–0.8) in Iran and Turkey. Q ₀ and <eta> are both low in the Oman folded zone and western Anatolia.