A Simple Method of Estimating Scalar Fluxes Over Forests

A simple aerodynamic-variance method is proposed to fill gaps in continuous CO₂ flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions (friction velocity greater than 0.1 ms^–1), and uses measurements of mean wind speed, and standard deviations of temperature and CO₂ concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible heat flux and CO₂ flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat and CO₂ flux estimates that are well correlated with the measured values.     

Modeling of inundation dynamics on Banda Aceh,Indonesia during the great Sumatra tsunamis December 26, 2004

The tsunami inundation flows on Banda Aceh, Indonesia reached 5 km inland during the December 26, 2004, event and devastated most of the houses, buildings, and infrastructure along the coast and killed more than 167,000 people. The overland flows from the northwest coast and the west coast collided at Lampisang village approximately 3.7 km from Ulee Lheue (northwest coast) and 6.8 km from Lhok Nga (west coast) as reported by survivors. Inundation modeling based on the nonlinear shallow-water wave equations reproduces the inundation pattern and demonstrates a colliding of the overland flows. The model suggests that wave characteristics on the northwest coast of Banda Aceh were different from those on the waves that impacted upon the west coast. The areas, which experienced higher inundation levels, did not always experience greatest overland flow speeds, and the damage areas mostly coincide with the flow speed distribution rather than the runup and inundation depth.     

Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour

This is the second paper describing a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. Two eddy correlation units were operated in the daytime to measure the fluxes of sensible heat and water vapour and other turbulence statistics at various heights within and above the stand. Net radiation was measured above the overstory using a stationary net radiometer and beneath the overstory using a tram system. Supplementary measurements included soil heat flux, humidity above and beneath the overstory, profiles of wind speed and air temperature, and the spatial variation of sensible heat flux near the forest floor.The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. On some days, energy budget closure was far better than on others. The average value of the Bowen ratio was 2.1 above the stand and 1.4 beneath the overstory. The mid-morning value of the canopy resistance was 150–450 s/m during the experiment and mid-day value of the Omega factor was about 0.20. The daytime mean canopy resistance showed a strong dependence on the mean saturation deficit during the two-week experimental period.The sum of sensible and latent heat fluxes beneath the overstory accounted for 74% of the available energy flux beneath the overstory. One of the reasons for this energy imbalance was that the small number of soil heat flux plates and the short pathway of the radiometer tram system was unable to account for the large horizontal heterogeneity in the available energy flux beneath the overstory. On the other hand, good agreement was obtained among the measurements of sensible heat flux made near the forest floor at four positions 15 m apart.There was a constant flux layer in the trunk space, a large flux divergence in the canopy layer, and a constant flux layer above the stand. Counter-gradient flux of sensible heat constantly occurred at the base of the canopy.The transfer of sensible heat and water vapour was dominated by intermittent cool downdraft and warm updraft events and dry downdraft and moist updraft events, respectively, at all levels. For sensible heat flux, the ratio of the contribution of cool downdrafts to that of warm updrafts was greater than one in the canopy layer and less than one above the stand and near the forest floor.     

Geology and geochronology of the Arunta complex north of Ormiston Gorge,Central Australia

A major west‐trending lineament marked by a wide belt of highly deformed rocks (the Redbank Zone), lies in the Arunta Complex, north of the Amadeus Basin. Along its southern margin the Zone has been progressively affected by, and is hence older than, migmatization and granite intrusion. The migmatization event yields a Rb‐Sr isochron age of 1076 ± 50 m.y. Within the migmatite complex, relicts of a pre‐migmatite metasedimentary sequence around the Chewings Range yield a Rb‐Sr isochron age of 1620 ± 70 m.y. The migmatites are unconformably overlain by the basal unit of the Amadeus Basin sequence, the Heavitree Quartzite. The 1076 ± 50 m.y. date thus provides a maximum age for the start of sedimentation along the northern margin of the Basin. The existence of a major zone of weakness in the basement probably exerted a strong control on basement and cover deformation during the Palaeozoic Alice Springs Orogeny.     

Radar imaging of Saturn's rings

We present delay-Doppler images of Saturn's rings based on radar observations made at Arecibo Observatory between 1999 and 2003, at a wavelength of 12.6 cm and at ring opening angles of 20.1°?|B|?26.7°. The average radar cross-section of the A ring is ∼77% relative to that of the B ring, while a stringent upper limit of 3% is placed on the cross-section of the C ring and 9% on that of the Cassini Division. These results are consistent with those obtained by Ostro et al. [1982, Icarus 49, 367-381] from radar observations at |B|=21.4°, but provide higher resolution maps of the rings' reflectivity profile. The average cross-section of the A and B rings, normalized by their projected unblocked area, is found to have decreased from 1.25±0.31 to 0.74±0.19 as the rings have opened up, while the circular polarization ratio has increased from 0.64±0.06 to 0.77±0.06. The steep decrease in cross-section is at variance with previous radar measurements [Ostro et al., 1980, Icarus 41, 381-388], and neither this nor the polarization variations are easily understood within the framework of either classical, many-particle-thick or monolayer ring models. One possible explanation involves vertical size segregation in the rings, whereby observations at larger elevation angles which see deeper into the rings preferentially see the larger particles concentrated near the rings' mid-plane. These larger particles may be less reflective and/or rougher and thus more depolarizing than the smaller ones. Images from all four years show a strong m=2 azimuthal asymmetry in the reflectivity of the A ring, with an amplitude of ±20% and minima at longitudes of 67±4° and 247±4° from the sub-Earth point. We attribute the asymmetry to the presence of gravitational wakes in the A ring as invoked by Colombo et al. [1976, Nature 264, 344-345] to explain the similar asymmetry long seen at optical wavelengths. A simple radiative transfer model suggests that the enhancement of the azimuthal asymmetry in the radar images compared with that seen at optical wavelengths is due to the forward-scattering behavior of icy ring particles at decimeter wavelengths. A much weaker azimuthal asymmetry with a similar orientation may be present in the B ring.     

U‐Pb zircon age for the Etheridge Group,Georgetown region,north Queensland: Implications for relationship with the Broken Hill and Mt Isa sequences*

The Einasleigh Metamorphics are the lowest exposed component of the Etheridge Group, part of the Etheridge Province, in the Georgetown region of north Queensland. Previous dating of granites has imposed a younger age limit of about 1550 Ma for the depositional age of the Etheridge Group. Based on SHRIMP U‐Pb analyses of zircon from mafic (1674.9 ±3.3 Ma and 1655.9 ±2.2 Ma) and felsic (1695.8 ± 1.5 Ma and 1684.2 ± 2.1 Ma) intrusive rocks in the Einasleigh Metamorphics, it can now be demonstrated that deposition commenced at about 1700 Ma, and continued for an estimated 100 million years. The stratabound, base‐metal deposits in the Mt Isa Inlier, Broken Hill Block and McArthur Basin formed in the early part of this time span, thereby suggesting potential for these styles of mineralisation in the Georgetown region.     

Importance of establishing sources of uncertainty for the derivation of reliable SHRIMP ages

The ion microprobe, as exemplified by SHRIMP, has long been an invaluable resource for the derivation of geological ages. The derivation of those ages is critically dependent on the identification and individual quantification of all sources of contributing uncertainty. In recent years, it has been proposed that the only component of uncertainty arising from the instrument itself is predictable from counting statistics. The adoption of that approach has led to several conclusions including: (i) that zircon U–Pb ages are relatively easily reset, which necessitates the enhanced editing of individual analyses before a grouped age can be obtained; and (ii) that other studies have overestimated analytical uncertainties and, as a consequence, have reported incorrect and/or overly imprecise ages. We present evidence for the presence of additional sources of instrument‐related uncertainty that necessitates a different (but not new) approach for the processing of SHRIMP data. Fortunately, this complication does not represent a serious problem, provided that a high‐quality zircon‐calibration standard has been used for Pb/U calibration. SHRIMP ages obtained some time ago from the Crudine Group of the Hill End Trough (New South Wales) have recently been placed at the centre of this controversy. A significant part of the problem is that most of those ages were based on a standard (SL 13) that is now known to be heterogeneous. The more reliable parts of the original data have been re‐reprocessed on the basis of the new evidence. They fail to detect a significant age difference between the bottom and the top of the Merrions Formation, a conclusion that is contrary to earlier expressed opinions.