旋转流体物理实验中见到的大气环流特征

在合适的实验参数下(热力Rossby数R_(OT)=0.1,Taylor数T_a=2.2 ×10~7),在旋转斜压流中,大尺度地形强迫造成低频振荡以及大气环流中的“阻塞”流型。这是由于地形强迫造成的准静止波与行进波的相互作用及共振引起的。地形把波数单一的流动变成多波数的流动。地形强迫使波数减少。     

Use of GIS in Optimizing Timber Thinning Strategies in the Eastern Sierra Nevada*

This paper demonstrates the use of a Geographic Information System (GIS) to develop timber thinning strategies on the Kyburz Planning Area of the Tahoe National Forest in northeastern California. The primary criteria used in an assessment of selective thinning potential were forest health and fire hazard ratings. By eliminating environmentally sensitive, economically unfeasible, or low fire hazard areas from consideration, the use of GIS reduced the area that was considered appropriate for thinning by approximately 58%. GIS offers considerable potential for improving resource management strategies.     

A comparison of Piché atmometers with conventional estimates of the ventilation term of the Penman model

When shielded from the sun and exposed to the ambient wind regime, Piché atmometers accurately reproduce estimates of the ventilation term of the Penman model using conventional psychrometry and anemometry. Results show that 98% of the variability in conventional estimates are accounted for by the variability in the atmometers when a square-root dependence on wind speed is incorporated into calculations of the evaporative flux from the atmometers. The potential for using a fairly simple instrument for the spatial extension of latent heat flux estimates from freely evaporating surfaces is discussed.     

Accuracy analysis for the NAD 83 geodetic reference system

The North American Datum of 1983 (NAD 83) provides horizontal coordinates for more than 250,000 geodetic stations. These coordinates were derived by a least squares adjustment of existing terrestrial and space-based geodetic data. For pairs of first order stations with interstation distances between 10km and 100km, therms discrepancy between distances derived fromNAD 83 coordinates and distances derived from independentGPS data may be suitably approximated by the empirical rulee=0.008 K^0.7 where e denotes therms discrepancy in meters and K denotes interstation distance in kilometers. For the same station pairs, therms discrepancy in azimuth may be approximated by the empirical rule e=0.020 K^0.5. Similar formulas characterize therms discrepancies for pairs involving second and third order stations. Distance and orientation accuracies, moreover, are well within adopted standards. While these expressions indicate that the magnitudes of relative positional accuracies depend on station order, absolute positional accuracies are similar in magnitude for first, second, and third order stations. Adjustment residuals reveal a few local problems with theNAD 83 coordinates and with the weights assigned to certain classes of observations.     

Evaluation of magmatic processes for the products of the Nevado del Ruiz Volcano, Colombia from geochemical and petrological data

Trace-element concentrations of the eruptive products from the Nevado del Ruiz volcano indicate that at least for the last 1 Ma, the major controlling factor in the evolution of the magma has been a simple fractional crystallization.The Zr/Hf ratios suggest that the source material is mantle-like in origin and whilst subduction-related contamination of the source material has taken place, there is no evidence that assimilation of continental crust has affected the series.No convincing geochemical evidence for magma mixing was found, and data on banded pumices of Pleistocene, Historic and the 1985 eruptions strongly support a crystal fractionation model to explain the compositional variation of their glasses. Therefore, if mixing has taken place it can only be a mixing of layers within a stratified or partially stratified magma chamber.Modeling of fractional crystallization using a stepwise program has been undertaken and least-squares approximations with small residuals are commensurate with analyzed samples. Crystal fractionation models with plagioclase dominating two-pyroxenes and iron oxides give good fits for all calculations. We conclude that simple crystal fractionation is the process most consistent with major- and trace-element variations in the Ruiz series. This closed-system model requires at least 77% crystal fractionation of a basaltic parent to generate the observed compositions of dacites.     

Conservation status of wetlands in Zimbabwe: past and present

The conservation status of dambos, a particular type of wetland associated with the headwaters of rivers draining the central plateau area of Zimbabwe, is examined in terms of past and present utilisation and erosion problems. Pre- and post-1950 phases are recognized within both the commercial farming areas and the Communal Lands (or peasant farming areas). The pre-1950 phase on commercial farms was one of exploitation of dambos mainly for winter wheat production, such cultivation contributing towards locally serious erosion. Consequently legislation was passed to restrict disturbance of wetlands, especially for cultivation. The post-1950 phase on commercial farms was one of conservation, with minimal use of dambos for grazing and limited erosion. The pre-1950 phase within the Communal Lands, the former native reserves, was one involving progressive modification of the traditional farming system through, in particular, centralisation. Dambo cultivation, a well-established practice, continued throughout this place, but towards the end of this period there were localised signs of degradation associated, in part, with grazing. The post-1950 phase was one of increased pressure on land with a growing human and livestock population. Thus dambos were required to support more and more cattle throughout the year, whilst efforts to curtail dambo cultivation were not well-recieved by peasant farmers and may have compromised general conservation in the reserves.     

Volcanic geology and eruption frequency,São Miguel,Azores

Six volcanic zones comprise São Miguel, the largest island in the Azores. All are Quaternary in age except the last, which is partly Pliocene. From west to east the zones are (1) the trachyte stratovolcano of Sete Cidades, (2) a field of alkali-basalt cinder cones and lava flows with minor trachyte, (3) the trachyte stratovolcano of Agua de Pau, (4) a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite, (5) the trachyte stratovolcano of Furnas, and (6) the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite, and trachyte. New radiocarbon and K-Ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency. Average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for Sete Cidades, 145 for zone 2, 1150 for Agua de Pau, and 370 for Furnas. However, the average dormant interval at Sete Cidades increased from 400 to about 680 years before each of the past two eruptions, and the interval at Furnas decreased from 370 to about 195 years before each of the past four eruptions. Eruptions in zone 4 occurred about once every 1000 years during latest Pleistocene and early Holocene time; none has occurred for about 3000 years. The Povoação caldera truncates part of the Nordeste shield and probably formed during the middle to late Pleistocene. Calderas formed during latest Pleistocene time at the three younger stratovolcanoes in the sequence: outer Agua de Pau (between 46 and 26.5 ka), Sete Cidades (about 22 ka), inner Agua de Pau (15.2 ka), and Furnas (about 12 ka). Normal faults are common, but many are buried by Holocene trachyte pumice. Most faults trend northwest or west-northwest and are related to the Terceira rift, whose most active segment on São Miguel passes through Sete Cidades and zone 2. A major normal fault displaces Nordeste lavas 150–250 m and may mark the location of an ancestral Terceira rift. Recent seismicity (e.g., in the 1980s) generally has been scattered, but some small earthquake swarms have occurred beneath the north-eastern flank of Agua de Pau.     

Monitoring active volcanoes and mitigating volcanic hazards: the case for including simple approaches

Simple approaches to problems brought about eruptions and their ensuing hazardous effects should be advocated and used by volcanologists while awaiting more sophisticated remedies. The expedients we advocate have all or many of the following attributes: only locally available materials are required; no extensive training of operators or installation is necessary; they are affordable and do not require foreign aid or exports; they are often labor intensive and are sustainable without outside assistance. Where appropriate, the involvement of local residents is advocated. Examples of simple expedients which can be used in forecasting or mitigating the effects of crises emphasize the relative ease and the less elaborate requirements with which simple approaches can be activated. Emphasis is on visual observations often by untrained observers, simple meteorogical measurements, observations of water level in lakes, temperature and chemistry of springs and fumaroles, new springs and collapse areas and observations of volcanic plumes. Simple methods are suggested which can be applied to mitigating damage from mudflows, nuées ardentes, tephra falls and gas discharge. A review in hindsight at Ruiz includes the use of both chemical indicators and simple mudflow alarms. Simple expedients are sufficiently effective that any expert volcanologist called to aid in a crisis must include them in the package of advice offered. Simple approaches are a critical and logical complement to highly technical solutions to hazardous situations.