Rümker hills: a lunar volcanic dome complex

The Rümker Hills, a volcanic dome-flow complex in the northern Oceanus Procellarum, is characterized by overlapping plains-forming units with lobate scarps, volcanic domes, a 60 km ring, and a scarp which separates the plateau from surrounding mare materials. Plains-forming units are interpreted as fluid volcanic flows, and domes as viscous extrusions. One dome may be a stratovolcano. The ring system is discordant with regional structural trends and probably has a local origin. The Rümker Hills is the closest lunar analog to the large martian shield structures revealed on the Mariner 9 photographs of Mars.     

The circumstellar imager: Direct detection of extra-solar planetary systems

The Astrometric Imaging Telescope (AIT) is designed to probe the circumstellar environment by both direct imaging and indirect astrometric measurements. The Circumstellar Imager (CI) is a coronagraphic camera and is the direct imaging component of the AIT. The CI is designed to obtain high-sensitivity images of the circumstellar region. It provides crucial non-inferential information relating to the frequency, origin, and evolution of planetary systems and all forms of circumstellar matter. Such imaging is usually limited by the scattered and diffracted light halos of the star itself, which are greatly suppressed in the CI by mating a novel high-efficiency coronagraph with a phasecompensated optical system. For faint point sources in the circumstellar region, the CI will have a sensitivity in excess of 5 magnitudes fainter than the as-designed Hubble Space Telescope (HST). Laboratory data are shown for the coronagraph, which, in a diffraction-limited environment, is capable of suppressing the stellar diffraction sidelobes by several orders of magnitude without significant sacrifice of field of view. In order to realize the high rejection levels inherent in the coronagraph design, it is necessary to limit scatter in the optical systems, imposing a mid-spatial frequency figure error requirement an order of magnitude smaller than that of the HST. Experimental data directed toward meeting this requirement are also shown.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Probability distributions of concentration fluctuations of a weakly diffusive passive plume in a turbulent boundary layer

Results are presented from an experimental investigation of turbulent dispersion of a saline plume of large Schmidt number (Sc=830) in a turbulent boundary-layer shear flow simulated in a laboratory water channel. The dispersion measurements are obtained in a neutrally buoyant plume from an elevated point source over a range of downstream distances, where both plume meandering and fine-structure variations in the instantaneous plume are important. High-resolution measurements of the scalar fluctuations in the plume are made with a rake of conductivity probes from which probability distributions of concentration at various points throught the plume are extracted from the time series.Seven candidate probability distributions were tested, namely, the exponential, lognormal, clipped normal, gamma, Weibull, conjugate beta, andK-distributions. Using the measured values of the conditional mean concentration, , and the conditional fluctuation intensity,i _p , the Weibull distribution provided the best match to the skewness and kurtosis over all downstream fetches. The skewness and kurtosis were always overpredicted by the lognormal probability density function (pdf), and underpredicted by the gamma pdf. The conjugate beta distribution for which the model parameters are determined using a method of moments based on the fluctuation intensity,i _p , and skewness,S _p , was capable of modeling the distribution of scalar concentration over a wide range of positions in the plume.     

Multiscaling properties of concentration fluctuations in dispersing plumes revealed using an orthonormal wavelet decomposition

The dynamical characteristics of concentration fluctuations in a dispersing plume over the energetic and inertial-convective range of scales of turbulent motion are studied using a multiscale analysis technique that is based on an orthonormal wavelet representation. It is shown that the Haar wavelet concentration spectrum is similar to the Fourier concentration spectrum in that both spectra exhibit an extensive inertial-convective subrange spanning about two decades in frequency, with a scaling exponent of -5/3. Analysis of the statistical properties (e.g., fluctuation intensity, skewness, and kurtosis) of the concentration wavelet coefficients (i.e., the concentration discrete detailed signal) suggests that the small scales are always more intermittent than the large scales. The degree of intermittency increases monotonically with decreasing scale within the inertial-convective subrange, reaching a plateau at the very small scales associated with the beginning of the near-dissipation subrange. The probability density function (pdf) of the concentration discrete detailed signal displays stretched exponential tails with an intermittency exponent (tail slope) q that increases as ^a , where is the scale or dilation and a is a power-law exponent that is dependent on downwind distance, plume height, and stratification strength with typical values in the range from about 0.25 to 0.35. It is shown that the concentration variance cascade process requires a phase coherency of eddies between different scales at the small-scale end of the inertial-convective subrange.The variation of the concentration wavelet statistics with height above the ground is investigated. The increased mean shear near the ground smooths the fine-scale plume structure for scales within the inertial-convective subrange, producing a weaker spatiotemporal intermittency in the concentration field compared to that measured higher up in the plume. The pdf of the concentration detailed signal at a fixed scale possesses less elongated tails with decreasing height z. The intermittency exponent q is found to decrease roughly linearly with increasing z.Finally, the results of the wavelet decomposition are combined to provide a conceptual model of the turbulent transport, stirring, and mixing regimes in a dispersing plume. The implications of the results for contaminant texture in a plume are discussed.     

A Spatial-Temporal/3-D Model for Volcanic Hazard Assessment: Application to the Yucca Mountain Region, Nevada

We present a 3-D Poisson model that permits identification and quantification of volcanic phenomena distributed through space and evolving in time (i.e., spatiotemporal data). Specifically, the model: (1) is volcanologically informative in solving problems of volcanic risk/hazard which depends on the location and time of future events; (2) contains model fitting computation algorithms that are efficient; and (3) is flexible enough to handle a large class of volcanic risk/hazard studies. Furthermore, we apply the model fitting techniques developed in this paper to the volcanic data from the Yucca Mountain project to demonstrate a unified volcanic hazard analysis. This study also evaluates the sensitivity of the statistical models developed by experts who have addressed the volcanic hazard/risk assessment problem near the Yucca Mountain region.     

A magnetic dynamo in the moon?

The existence of fossil lunar magnetism has caused speculation that the Moon had, at one time, an internally produced dynamo magnetic field. Quantitative analysis of this idea, constrained by the largest iron lunar core compatible with observations, implies that the Moon would have had to rotate faster than its breakup angular velocity in order to support a dynamo magnetic field.A paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973.     

A retrospective analysis of trace metals, C, N and diatom remnants in sediments from the Mississippi River delta shelf

The development of oil and gas recovery offshore of the Mississippi River delta began in shallow water in the 1950s, expanded into deeper waters, and peaked in the 1990s. This area of the outer continental shelf (OCS) is the historical and present location of >90% of all US OCS oil and gas production and reserves. The juxtaposition of its 4000 producing platforms, recovering $10 billion yr(-1) of oil, gas and produced water in the same area where about 28% of the US fisheries catch (by weight) is made and near 40% of the US coastal wetlands, makes this an area worth monitoring for regional pollutant loading. This loading may come from several sources, including sources related to OCS development, but also from the Mississippi River watershed. In this context, any contaminant loading on this shelf may be neither detectable nor significant against a background of climatic or biological variability. We examined the sedimentary record for indicators of industrial byproducts from OCS oil and gas development and of industrial products entering via the Mississippi River, primarily using vanadium (V) and barium (Ba) concentrations normalized for aluminum (Al). Barium is primarily used in drilling muds in the form of barite, whereas V is an important strengthening component of metal alloys, including steel. The fluctuations in the accumulation of Ba, but not V, were coincidental with the presumed use of barite. The fluctuations in V concentration in the sediments were coincidental with the national consumption of V. Copper (Cu), cadmium (Cd) and zinc (Zn) concentrations in sediments fluctuate coincidentally with V, not Ba, thus indicating that the dominant source of these trace metals in offshore sediments were derived from riverine sources, and were not primarily from in situ industrial processes releasing them on the shelf. This is not to suggest that local site-specific contamination is not a significant management or health concern. The low oxygen (hypoxia; < or = 2 mg l(-1)) zone that consistently covers much of this continental shelf's bottom layer in summer is attributed to nitrate loading from the Mississippi River. Increased nitrogen loading from river to shelf stimulates diatom production whose loading to the bottom layer and subsequent metabolism results in oxygen being depleted faster than it is replaced. In the last two decades there has been an increased accumulation of organic matter in sediments near the mouth of the Mississippi River. This coupling between river water, surface water and bottom water has recently expanded westward of the Atchafalaya River delta towards the Texas coast. The accumulation of biogenic silica (BSi) and carbon in dated sediments is coincidental with variations in riverine nitrate flux, but not with either V or Ba accumulation rates. These analyses indicate that both OCS development and riverine sources exert strong influences on the sediment constituents offshore, and that these influences may be independent of one another.     

A convenient method for the determination of carbon in marine “net zooplankton”

Measurements of carbon in marine “net zooplankton” were made by use of a Leco 70 Second TC-12 Automatic Carbon Determinator. The instrument employs high-temperature (ca. 1600°C), dry-combustion and thermal-conductivity detection of the purified carbon-dioxide combustion product and oxygen carrier gas mixtures. The methodology developed in this study is convenient and rapid (ca. 70 sec per freeze-dried sample), with a mean error ($\text{±2σ/}\text{x}\text{·100}$) of ±4.4% at the 3 mg-C level and ±7.4% at the 10 mg-C level when benzoic acid is used as a standard, and ±9.2% at the 5 mg-C level and ±6.0% at the 10 mg-C level when casein is used as the standard. The technique was applied to the “splash zone” marine copepod Tigriopus californicus to obtain an average value of 38.6% C by weight. Concentrated samples of “net zooplankton” of varying size fractions were collected in the Monterey upwelling region over an eight-month period. Values up to 2000 mg-C/m² ocean surface (0–200 m) were observed in a seasonal cycle doubly peaked for some size fractions but not for others.     

Improving Wind-Ramp Forecasts in the Stable Boundary Layer

The viability of wind-energy generation is dependent on highly accurate numerical wind forecasts, which are impeded by inaccuracies in model representation of boundary-layer processes. This study revisits the basic theory of the Mellor, Yamada, Nakanishi, and Niino (MYNN) planetary boundary-layer parametrization scheme, focusing on the onset of wind-ramp events related to nocturnal low-level jets. Modifications to the MYNN scheme include: (1) calculation of new closure parameters that determine the relative effects of turbulent energy production, dissipation, and redistribution; (2) enhanced mixing in the stable boundary layer when the mean wind speed exceeds a specified threshold; (3) explicit accounting of turbulent potential energy in the energy budget. A mesoscale model is used to generate short-term (24 h) wind forecasts for a set of 15 cases from both the U.S.A. and Germany. Results show that the new set of closure parameters provides a marked forecast improvement only when used in conjunction with the new mixing length formulation and only for cases that are originally under- or over-forecast (10 of the 15 cases). For these cases, the mean absolute error (MAE) of wind forecasts at turbine-hub height is reduced on average by 17%. A reduction in MAE values on average by 26% is realized for these same cases when accounting for the turbulent potential energy together with the new mixing length. This last method results in an average reduction by at least 13% in MAE values across all 15 cases.