bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of surface heating by convective cloud systems in the central equatorial Pacific from surface and satellite measurements
Authors: Collins W. D. , Bucholtz A. , Flatau P. , Lubin D. , Valero F. P. J. , Weaver C. P. , Pilewski P.
Published: September 2002

The heating of the ocean surface by longwave radiation from convective clouds has been estimated using measurements from the Central Equatorial Pacific Experiment (CEPEX). The ratio of the surface longwave cloud forcing to the cloud radiative forcing on the total atmospheric column is parameterized by the ƒ factor. The ƒ factor is a measure of the partitioning of the cloud radiative effect between the surface and the troposphere. Estimates of the ƒ factor have been obtained by combining simultaneous observations from ship, aircraft, and satellite instruments. The cloud forcing near the ocean surface is determined from radiometers on board the National Oceanic and Atmospheric Administration P‐3 aircraft and the R/V John Vickers. The longwave cloud forcing at the top of the atmosphere has been estimated from data obtained from the Japanese Geostationary Meteorological Satellite GMS 4. A new method for estimating longwave fluxes from satellite narrowband radiances is described. The method is based upon calibrating the satellite radiances against narrowband and broadband infrared measurements from the high‐altitude NASA ER‐2 aircraft. The average value of ƒ derived from the surface and satellite observations of convective clouds is 0.15±0.02. The area‐mean top‐of‐atmosphere longwave forcing by convective clouds in the region 10°S–10°N, 160°E–160°W is 40 W/m2 during CEPEX. These results indicate that the surface longwave forcing by convective clouds was approximately 5 W/m2 in the central equatorial Pacific and that this forcing is the smallest radiative component of the surface energy budget.
Comparison of radar/radiometer retrievals of stratus cloud liquid‐water content profiles with in situ measurements by aircraft
Authors: Frisch A. Shelby , Martner Brooks E. , Djalalova Irina , Poellot Michael R.
Published: September 2002

In situ sampling of cloud droplets by aircraft in Oklahoma in 1997 is used to evaluate a ground‐based remote sensing technique for retrieving profiles of cloud liquid‐water content. The technique uses vertically pointing measurements from a high‐sensitivity millimeter‐wavelength radar and a collocated dual‐frequency microwave radiometer to obtain height‐resolved estimates of the liquid content of stratiform clouds. Comparisons with the aircraft measurements are made for 16 overpasses through thin cloud layers within a 1.5‐km radius of the remote sensor site. Over a range of liquid‐water contents from 0.04 to 0.57 g m−3 the mean difference between the aircraft and the radar/radiometer values was 0.02 g m−3, and the maximum difference was 0.09 g m−3. Although the number of comparisons is limited, these results suggest that the ground‐based estimates may be sufficiently accurate for many scientific purposes.
An analysis of processes determining the number‐to‐volume relationship for submicron aerosol in the eastern Atlantic
Authors: Hegg D. A. , Russell L. M.
Published: September 2002

Airborne data acquired with the Office of Naval Research Pelican aircraft during the second Aerosol Characterization Experiment are analyzed to address the issue of the relationship between aerosol number and volume concentration in the marine atmosphere. In agreement with past studies, the mean value of the number‐to‐volume ratio is found to be 168±67 μm−3 for the size range 0.06 ≤ r ≤ 1.8 μm. The data were further explored with respect to the factors influencing both the magnitude of the ratio and the linearity of the relationship between the number and volume concentrations. For the experimental venue examined here, a number of factors appear to modulate the magnitude of the number‐to‐volume ratio, and while the data are not definitive, the linearity of the number‐volume relationship appears to arise naturally owing to the dominant particle growth mechanisms in the boundary layer.
A near‐infrared optimized DOAS method for the fast global retrieval of atmospheric CH4, CO, CO2, H2O, and N2O total column amounts from SCIAMACHY Envisat‐1 nadir radiances
Authors: Buchwitz Michael , Rozanov Vladimir V. , Burrows John P.
Published: September 2002

A new method for the fast and accurate retrieval of atmospheric trace gas total column amounts from near‐infrared nadir radiances, to be measured by the scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) spectrometer on board the European Space Agency Envisat‐1 satellite, has been investigated. It can be characterized as a weighting function modified differential optical absorption spectroscopy approach (WFM‐DOAS). The reference spectra of the linear fit include the trace gas total column weighting functions, a weighting function for a temperature profile shift, and a low‐order polynomial. Systematic errors due to uncertainties in atmospheric and surface parameters which cannot be retrieved, such as the trace gas vertical profile shapes, pressure and temperature profiles, aerosols, and the surface reflectivity, are quantified. The total column precisions of all trace gases to be retrieved from the SCIAMACHY near‐infrared channels 7 (1940–2040 nm) and 8 (2265–2385 nm) have been estimated to be better than 1% for H2O, CO2, and CH4, and better than 10% for N2O and CO. The potential of SCIAMACHY to measure regional CO2 and CH4 emission fluxes has been investigated. Both are important greenhouse gases to be monitored and reduced according to the Kyoto Protocol of the U.N. Framework Convention on Climate Change.
Cambrian arc evolution along the SE Gondwana active margin: A synthesis from Tasmania‐New Zealand‐Australia‐Antarctica correlations
Authors: Münker Carsten , Crawford Anthony J.
Published: September 2002

Belts of Cambrian rocks with arc affinities in eastern Australia, Tasmania, New Zealand, and Antarctica are part of a single convergent margin, active over 30–40 Ma from the latest Early Cambrian to the Late Cambrian. Two of the most complete sequences are exposed in western Tasmania and the northern South Island of New Zealand (Takaka Terrane). Throughout the Middle Cambrian, magmatism in these two regions, in the Lachlan Fold Belt (SE Australia) and the Bowers Terrane (Antarctica) is represented by intra‐oceanic arc and back arc sequences. In the mid‐Middle Cambrian, collision of these arc segments with the proto‐Gondwana continent is recorded by obducted boninite‐bearing ophiolites in Tasmania and SE Australia. Postcollisional magmatism of latest Middle to early Late Cambrian age (e.g., Mount Read Volcanics of Tasmania and Stavely Volcanic Complex of Victoria) terminates convergent tectonics in SE Australia and Tasmania. In contrast, no postcollisional volcanism is known from the Bowers Terrane in Antarctica and from New Zealand. In Antarctica, Cambrian igneous activity in the Wilson Terrane and Transantarctic Mountains formed in an active continental margin setting and lasted through the Middle and Late Cambrian. This suggests that the Wilson Terrane is the lateral continuation on continental basement of the Bowers and Takaka Terrane arcs. The differences between the Australian ‐ Tasmanian and New Zealand ‐ Antarctic arc segments may result from a change in subduction polarity along the arc chain, suggested by structural features, sedimentation patterns, and isotope systematics. In the SE Australian and Tasmanian arc segments the proto‐Gondwana plate subducted beneath the Pacific plate whereas subduction in the Antarctic and New Zealand segments was of opposite polarity. Common to most Cambrian fragments in SE proto Gondwana is the tectonic overprint by the Ross‐Delamerian orogeny from the Middle Cambrian to Early Ordovician, thus paleogeographically linking all the fragments by the end of the Cambrian. The presence of Early Cambrian intra‐oceanic rocks at the SE proto‐Gondwana margin suggests separation of Laurentia from Gondwana prior to the Early Cambrian.
Experimental measurement of air‐water interfacial area during gravity drainage and secondary imbibition in porous media
Authors: Schaefer C. E. , DiCarlo D. A. , Blunt M. J.
Published: September 2002

A new experimental method was developed to determine air‐water interfacial area as a function of capillary pressure and water saturation in unsaturated porous media. The surfactant sodium dodecyl benzene sulfonate (SDBS) was used in equilibrium column adsorption experiments to estimate air‐water interfacial area for water saturations (milliliter water per milliliter void) ranging from 0.05 to 1.0 and pressures ranging from 0 to 20 cm of water. A comparison was made between columns which were equilibrated under gravity drainage versus columns equilibrated under secondary imbibition. Gravity drainage experiments showed the air‐water interfacial area decreased linearly with saturation, while imbibition experiments showed a more complex nonmonotonic relation to the saturation. The interfacial area data are then compared with existing network models.
Evaluation of laboratory dolomite core sample size using representative elementary volume concepts
Authors: Brown G. O. , Hsieh H. T. , Lucero D. A.
Published: September 2002

The adequacy for laboratory testing of four dolomite cores from the Culebra Dolomite of the Rustler Formation at the Waste Isolation Pilot Plant near Carlsbad, New Mexico, were evaluated using representative elementary volume (REV) theory. Gamma ray computerized tomography created three‐dimensional grids of bulk density and macropore index over volumes from 1.4 × 10−7 to 1.6 L. Three different methods for both volume averaging and REV analysis were applied and compared. Both density and macropore index converged to single values with increasing volume, which meets the most common qualitative definition of a REV. Statistical test results for the relatively homogeneous samples indicate that volumes larger than 1 to 7 mL have constant properties. Contrarily, a highly varied sample required 250 and 373 mL to achieve invariant density and macropore characteristics, respectively. Prismatic volume averaging was found to be better than slice averaging, while a qualitative test for the REV provided similar results as a rigorous statistical method. All cores were larger than the REV but were significantly different from one another. This implies that multiple cores are necessary to determine the entire range of transport properties within the rock.
GPS measurement of surface deformation around Soufriere Hills Volcano, Montserrat from October 1995 to July 1996
Authors: Mattioli Glen S. , Dixon Timothy H. , Farina Fredric , Howell Ellen S. , Jansma Pamela E. , Smith Alan L.
Published: September 2002

Global Positioning System geodesy was used to measure surface deformation on Soufriere Hills volcano, Montserrat from October 6, 1995 to July 1, 1997 during initial dome growth and gravitational collapse. Our data from this period show non‐axially symmetric horizontal displacements, and decreasing subsidence as a function of radial distance from the former topographic high of the volcanic edifice. Forward modeling suggests that surface deformation is caused by a shallow vertical dike (< 3 km), which expanded approximately 1 m, coupled with a deflating Mogi source at about 6 km depth. These inferred source parameters are in good agreement with independent observations of regional dike widths and preemption magma storage depth.
Raman study of phase D at various pressures and temperatures
Authors: Liu Lin‐Gun , Lin C ‐C. , Irifune T. , Mernagh T. P.
Published: September 2002

Samples of phase D were synthesized in high pressure and high temperature experiments with a multi‐anvil press. Phase D may have a chemical range of Mg/Si = 0.65 ± 0.05 and H2O in the range 6∼14 wt% on the basis of electron microprobe analyses. The nature of phase D was characterized by both powder X‐ray diffraction and Raman spectroscopy. The resolution of the Raman spectra of phase D is in general very poor, though the X‐ray diffraction data suggest that it is well crystalline. No clear OH Raman band of phase D was demonstrated, however. Variations of Raman spectra of phase D were investigated up to 18 GPa at room temperature and in the range 81–373 K at atmospheric pressure. Lower temperatures and/or high hydrostatic pressures did not improve the quality of the spectra. Phase D is extremely unstable when it was irradiated by laser and/or heated above 373 K at atmospheric pressure. Among the silicate vibrations, the pressure variation has been studied for only two bands and the temperature variation for four bands. The Raman frequencies for these bands of phase D increase with increasing pressure but decrease with increasing temperature, which are the same as the other dense hydrous magnesium silicates so far studied.
The explosive eruption of Soufriere Hills Volcano, Montserrat, West Indies, 17 September, 1996
Authors: Robertson R. , Cole P. , Sparks R. S. J. , Harford C. , Lejeune A. M. , McGuire W. J. , Miller A. D. , Murphy M. D. , Norton G. , Stevens N. F. , Young S. R.
Published: September 2002

On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7 × 106 m³ (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5–5%; magma discharge rates from 2300 to 4300 m³/s; ejecta volume of 3.2 (± 0.9) × 106 m³ (DRE). Ejecta consists of pumice (ρ =1160 kg/m³), higher density vesiculated ejecta (ρ =1300 to 2000 kg/m³), dense glassy clasts (ρ =2600 kg/m³), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse.
Ground deformation studies at Soufriere Hills Volcano, Montserrat I: Electronic distance meter studies
Authors: Jackson Paul , Shepherd John B. , Robertson Richard E. A. , Skerritt George
Published: September 2002

Frequent occupation of a network of instrument and target sites using an Electronic Distance Meter (EDM) over 21 months has led to the recognition of two distinct patterns of ground deformation. Corrected slant distances to reflector targets involve either continuous shortening or an oscillatory pattern of shortening and recovery. These two patterns are considered to reflect the response of the volcanic edifice at near‐field and far‐field positions, respectively. At near‐field locations non‐elastic deformation is predominant over elastic deformation and rebound associated with changes in the magma conduit pressure are not detectable. At far‐field locations elastic deformation is dominant allowing recognition of relaxation in the conduit magma pressure. Recognition of the two patterns assists monitoring of crater stability and may aid long‐range forecasting of future activity at the volcano.
Thermoremanence in monoclinic pyrrhotite particles containing few domains
Authors: Menyeh A. , O'Reilly W.
Published: September 2002

The intensity and stability of thermoremanent magnetization (TRM) acquired by synthetic Fe7S8 particles in the size range 1‐29 µm has been determined. The intensity falls with increasing particle size. The unblocking temperature increases with increasing particle size. The intensity‐particle size dependence can be fitted to a theory, in which a fraction of the particles fails to nucleate a domain wall on cooling, only if the TRM of the resultant monodomain fraction is much smaller than that expected of crystals of this size. This could arise if the particles were made of “nanocrystals” of much smaller size. The data may be given a much simpler theoretical description by adopting a particle size dependent effective demagnetizing factor with a fully formed domain structure with highly mobile walls above the blocking temperature.
Potential for improved ATSR dual‐view SST retrieval
Authors: Murray M. J. , Allen M. R. , Merchant C. J. , Harris A. R.
Published: September 2002

Recent validation studies have confirmed that the first along‐track scanning radiometer (ATSR) can retrieve sea surface temperature (SST) to an accuracy of 0.3K even in the presence of heavy atmospheric aerosol. However, using the standard (pre‐launch) retrieval, this accuracy is achieved only when data from all three thermal channels (3.7, 11 and 12 µm) are available; in the absence of 3.7 µm data, retrieved SSTs are subject to significant cold bias. As 3.7 µm data are useful only for nighttime observations, and ATSR's 3.7 µm channel failed in May 1992, only 11 and 12 µm data informed SST derivation for most of the 1991–1996 mission. We demonstrate the potential for improvement in this retrieval, based on comparison of observed brightness temperatures with precise SSTs derived using 3.7 µm data. A reduction in global‐mean cold bias from >0.6K to <0.1K is achieved, with standard deviation approximately halved. We also examine the treatment of optical pathlength variation around the ATSR swath.
Arc stresses determined by slabs: Implications for mechanisms of back‐arc spreading
Authors: Seno Tetsuzo , Yamanaka Yoshiko
Published: September 2002

There is an anti‐correlation in stress field between the back‐arc area and the shallow portion of the slab. Generally slabs showing down‐dip compression have tensional back‐arcs, and the reverse. We explain this anti‐correlation by a force balance in the fore‐arc wedge between the slab pull, collision force in the upper plate, and the ridge push. Back‐arc spreading can occur as a passive response to the down‐dip compressional slab. The Mariana, Kyushu and Aegean arcs are the exceptions to the anti‐correlation; in these arcs, the back‐arc is tensional although the slab is in down‐dip tension. This may be because the fore‐arc is driven by the mantle drag toward the trench, resulting in compression balanced with the slab pull. The flow in the mantle would cause back‐arc spreading in this case.
Evidence for the seasonal variation of photochemical activity of tropospheric ozone: Continuous observation of ozone and CO at Happo, Japan
Authors: Kajii Yoshizumi , Someno Kazuaki , Tanimoto Hiroshi , Hirokawa Jun , Akimoto Hajime , Katsuno Takao , Kawara Junichi
Published: September 2002

Ground based measurements of ozone as well as carbon monoxide have been carried out at Happo station. Annual cycles of ozone and CO concentration were obtained. Monthly means of ozone show maximum of ca. 65 ppbv in April or May and minimum of ca. 45 ppbv in December or January. The annual behaviour of CO is somewhat similar to that of ozone showing maximum of 225 ppbv in April and minimum of 151 ppbv in December. A positive correlation was obtained between ozone and CO during spring and summer. This fact indicates that photochemical ozone production in the troposphere is active in spring and summer. It should be noted that the highest correlation was obtained in April and May which corresponds to the month giving the ozone maximum concentration. The comparison between ozone and humidity in late spring revealed that although many events showing dry air parcels reaching the site were observed, simultaneous increase of ozone was not observed at all. This fact clearly indicates that the sporadic high concentration of ozone in spring is caused by the photochemical ozone formation in the troposphere rather than the intrusion of stratospheric air.
Enhanced ELF wave generation efficiency using ‘O’ Mode HF heating of the ionosphere: An instrumental explanation
Authors: Barr Richard , Stubbe P. , Rietveld M. T. , Nielsen E.
Published: September 2002

In [Barr et al. 1997] we presented ELF signals received near the Tromsø ionospheric heater facility when the heater radiated HF signals amplitude modulated by a 505Hz square wave. Square wave modulation enabled simultaneous multiple frequency measurements to be made of the signals radiated from the ionosphere as the heater was switched to radiate both ‘X’ and ‘O’ mode HF waves. We found that whereas the first 3 odd harmonics of the square wave (≥ 1515 Hz) all showed the accepted behaviour of reduced wave excitation on switching from ‘X’ to ‘O’ mode HF heating, the fundamental frequency showed enhanced excitation of more than 10dB with ‘O’ mode heating. These observations were given an ionospheric explanation in terms of a change from Hall to Pedersen current modulation with the change from ‘X’ to ‘O’ mode HF heating. We present here an alternative analysis, which has both instrumental and ionospheric components and is also able to explain the observations.
Sea ice transport: A highly variable link between Arctic and North Atlantic
Authors: Hilmer Michael , Harder Markus , Lemke Peter
Published: September 2002

The variability of the ice volume flux into the northeast Atlantic is investigated with an optimized dynamic‐thermodynamic sea ice model using 40 years (1958–1997) of atmospheric forcing fields derived from the NCEP/NCAR reanalysis project. The simulated sea ice export from the Arctic exhibits considerable interannual to decadal variability and is primarily a linear response to sea level pressure anomalies over Greenland and over the Barents and Kara Seas. Our model results suggest that ice export anomalies such as in 1968 which supposedly caused the so‐called “Great Salinity Anomaly” in the northern North Atlantic are not unique but rather frequent events as part of the variability of the Arctic climate system.
Southern hemisphere observations of a long‐term decrease in F region altitude and thermospheric wind providing possible evidence for global thermospheric cooling
Authors: Jarvis M. J. , Jenkins B. , Rodgers G. A.
Published: September 2002

F region peak heights, derived from ionospheric scaled parameters through 38‐year data series from both Argentine Islands (65°S, 64°W) and Port Stanley (52°S, 58°W) have been analyzed for signatures of secular change. Long‐term changes in altitude, which vary with month and time of day, were found at both sites. The results can be interpreted either as a constant decrease in altitude combined with a decreasing thermospheric wind effect or as a constant decrease in altitude which is altitude‐dependent. Both interpretations leave inconsistencies when the results from the two sites are compared. The estimated long‐term decrease in altitude is of a similar order of magnitude to that which has been predicted to result in the thermosphere from anthropogenic change related to greenhouse gases. Other possibilities should not, however, be ruled out.
Characteristics of interplanetary magnetic clouds in relation to their solar association
Authors: Bravo S. , Blanco‐Cano X. , López C.
Published: September 2002

Interplanetary magnetic clouds (MCs) are always associated with a coronal mass ejection. In some particular cases, they also have been related to the occurrence of near‐surface solar events such as filament eruptions and flares. In this paper we use a sample of 67 MCs observed between 1967 and 1982 at 1 AU to obtain better statistics of their near‐surface solar association, considering Hα flares, filament eruptions, X‐ray events, and coronal holes. We also study possible differences in the characteristics of the clouds depending on their solar association. We find that 57% of the MCs observed at 1 AU were associated with flares (F), 21% were associated with filament eruptions (P), 7% were associated with both (FP), and 15% were not associated with any reported near‐surface solar event. Most of the associated flares occurred at the west of the solar central meridian, and most of the associated filament eruptions occurred at the east. In 95% of the cases for which the coronal hole information was available, a nonpolar coronal hole was near the region of the solar event. In the cases where the cloud was not associated with a flare or filament eruption, a coronal hole was always near the solar central meridian. This suggests that coronal holes may be playing a role in the formation of interplanetary magnetic clouds. We consider the spatial profiles of magnetic field and plasma parameters inside MCs, observed as temporal variations as clouds pass detectors at 1 AU. The average magnitudes of these parameters inside the cloud were also considered. The profiles observed were the following: flat (A‐type), with a maximum (B‐type), descending (C‐type), ascending (D‐type), with a minimum (E‐type), fluctuating (F‐type), and with two minima (G‐type). We find that the most common profiles and the average values of the different parameters vary for different solar associations. We also find some correlations between the cloud parameters, which may appear different in clouds associated with F or P. However, as there is a fraction of MCs not associated with near‐surface solar events, it is possible that this kind of activity is not essential for the formation of an interplanetary magnetic cloud, but only adds particular characteristics to the clouds.
Laboratory experiments to simulate the hydrodynamics of supernova remnants and supernovae
Authors: Drake R. P.
Published: September 2002

Recent years have seen the emergence of a new approach to the laboratory simulation of astrophysical phenomena. Current experiments are designed to address, with good dimensionless scaling, specific physical issues that matter to the astrophysics. These include experiments to address the hydrodynamic evolution and instabilities in supernova remnants, experiments to address hydrodynamic instabilities in core‐collapse supernova explosions, and experiments to address the structure of thermonuclear supernova explosions. They are contrasted with somewhat older experiments that made first attempts in this direction, examining high‐velocity spherical expansions into unmagnetized plasma, and the deceleration of spherical expansions by a magnetic field.
P and S wave scattering from mantle plumes
Authors: Tilmann Frederik J. , McKenzie Dan , Priestley Keith F.
Published: September 2002

We discuss a new method for seismically imaging mantle plumes by modeling the plume as a vertical elastic cylinder and analyzing the effects of the cylinder on an incoming teleseismic plane wave. The incident plane wave decomposes into cylindrical waves for which the scattering coefficients can be computed by imposing continuity of stress and displacement at the cylinder boundary. We extended this development to cylinders with gradational boundaries by numerically integrating the elasto‐dynamic equations. Synthetic seismograms calculated using this technique for a cylinder with a range of parameters appropriate for a mid‐Pacific mantle plume (epicentral distance Δ = 50°–90°, ΔT = 200–400 K, boundary width = 0–60 km) have the following characteristics: (1) a delay of the P wave arrival for an observational point directly behind the plume, (2) the focusing of energy into the region directly behind the plume where the focal length is ∼2 times the plume diameter, and (3) the presence of secondary phases, and especially the excitation of a transverse component whose amplitude is up to 25% of the radial amplitude. Most of the phases arise from P‐to‐S conversions and reflections at the boundary of the cylinder. Increasing the epicentral distance reduces the focal length and strengthens the focusing effect for receivers near the plume. Increasing the thermal anomaly reduces both the focal length and the amplitude of the secondary phases. Using gradational boundaries does not affect the scattering of low‐frequency waves, but it does cause the reduction or disappearance of the secondary arrivals at high frequency because of the reduced efficiency of the conversions. Synthetic seismograms for an incident S wave arrival show a time delay more than twice as long as for the P wave and stronger focusing but only weak secondary phases. Considerable coupling takes place between SV and SH. On the basis of these findings we outline an observational program to image the Hawaiian plume: stations should preferably be installed close to but not directly on top of the most likely position of the plume axis.
An alternative model of the Lassen hydrothermal system, Lassen Volcanic National Park, California
Authors: Xu Wenyue , Lowell Robert P.
Published: September 2002

We describe an alternative two‐dimensional numerical model for the Lassen hydrothermal system. As in earlier models, the new model consists of a vertical upflow zone with a low permeability caprock near its top and a permeable horizontal channel that connects to the upflow zone at depth. The new model, however, also includes a recirculation region beneath the horizontal channel and a recharge zone. Magmatic heating is explicitly represented by specifying heat flows near the bottom of the upflow zone. Simulations show that a periodically appearing vapor‐dominated zone, which sometimes consists of pure‐steam, develops beneath the caprock and feeds steam‐type discharges at high elevations, while the horizontal channel feeds the hot‐spring discharge region at lower elevations. This model is able to reproduce the inferred ∼1:1 discharge ratio between water and steam and to match estimates on current heat output from the system. The development of the vapor‐dominated zone does not require dramatic changes in geohydrologic conditions. Our model also shows that both the vapor‐dominated zone and the whole system evolve quasiperiodically with a period of ∼103 years. These oscillations are related to the unstable two‐phase hydrothermal fluid flow within the vapor‐dominated zone. The observed high‐frequency oscillations near the vapor‐dominated zone superimposed on low frequency ones probably result from the oscillatory instability introduced by rapid changes in permeability of the low‐permeability caprock. The results indicate that the system is probably still in a transient state.
Mapping the rupture process of moderate earthquakes by inverting accelerograms
Authors: Hellweg Margaret , Boatwright John
Published: September 2002

We present a waveform inversion method that uses recordings of small events as Green's functions to map the rupture growth of moderate earthquakes. The method fits P and S waveforms from many stations simultaneously in an iterative procedure to estimate the subevent rupture time and amplitude relative to the Green's function event. We invert the accelerograms written by two moderate Parkfield earthquakes using smaller events as Green's functions. The first earthquake (M = 4.6) occurred on November 14, 1993, at a depth of 11 km under Middle Mountain, in the assumed preparation zone for the next Parkfield main shock. The second earthquake (M = 4.7) occurred on December 20, 1994, some 6 km to the southeast, at a depth of 9 km on a section of the San Andreas fault with no previous microseismicity and little inferred coseismic slip in the 1966 Parkfield earthquake. The inversion results are strikingly different for the two events. The average stress release in the 1993 event was 50 bars, distributed over a geometrically complex area of 0.9 km2. The average stress release in the 1994 event was only 6 bars, distributed over a roughly elliptical area of 20 km2. The ruptures of both events appear to grow spasmodically into relatively complex shapes: the inversion only constrains the ruptures to grow more slowly than the S wave velocity but does not use smoothness constraints.
Hydrate distribution off Vancouver Island from multifrequency single‐channel seismic reflection data
Authors: Fink C. R. , Spence G. D.
Published: September 2002

On the northern Cascadia margin, single channel seismic data clearly image the bottom‐simulating reflector (BSR) that marks the base of the hydrate stability zone. Airguns with three distinct source frequencies (30, 75, and 150 Hz) were recorded along several coincident lines, and at all frequencies the BSR appears as a single pulse. BSR reflection coefficients are slightly larger for the 30‐Hz data than for the 75‐ or 150‐Hz data. Multi‐frequency analysis indicates that the BSR is due to a single sharp interface at the base of the hydrate zone, whereas the velocity structure away from the interface must be gradationally‐varying so as to produce no separate reflection and no tuning effects at the 75‐ and 150‐Hz frequencies. In contrast, the seafloor reflection in some areas has significantly larger amplitudes for data recorded with the 75‐Hz source, relative to the 30‐Hz source. Synthetic seismogram analysis of the two data sets shows that the high amplitudes can be produced by a 2‐m‐thick high‐velocity layer at the seafloor, with velocities and densities corresponding to those of carbonate. Using the 75‐Hz source, seafloor and BSR amplitudes and reflection coefficients were measured over a tight grid of lines, nominally spaced at 200 m. Maximum BSR reflection coefficients of 0.15–0.18 were observed beneath topographic highs, while maximum seafloor reflection coefficients of up to 0.5–0.6 were found on the flanks of the topographic highs. This suggests that topography provides a major control on the flow of methane‐bearing fluids. Amplitude measurements, converted to values of velocity above the BSR, were used to estimate hydrate concentrations, which reaches a maximum of 15–18% of the total sediment volume with mean values near 5–10%. Over the 100 km2 survey area, the volume of methane gas in the hydrate reservoir is estimated as 6.4 × l010 m3, or 2.1 trillion cubic feet.
Plume‐ridge interactions of the Discovery and Shona mantle plumes with the southern Mid‐Atlantic Ridge (40°‐55°S)
Authors: Douglass Jill , Schilling Jean‐Guy , Fontignie Denis
Published: September 2002

We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the Mid‐Atlantic Ridge (MAR) from 40° to 55°S. The results strongly indicate interaction and mixing between the off‐ridge Discovery and ridge‐centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al [1987]. The Discovery and Shona plumes have enriched mantle and high‐μ(μ = 238U/204Pb) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning ∼13 Ma. The presence of a component with unusually low 206Pb/204Pb accompanied by high 87Sr/86Sr and low 208Pb/204Pb and 143Nd/144Nd in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three‐component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low‐μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the “C” component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.
Mass flux measurements at active lava lakes: Implications for magma recycling
Authors: Harris Andrew J. L. , Flynn Luke P. , Rothery David A. , Oppenheimer Clive , Sherman Sarah B.
Published: September 2002

Remotely sensed and field data can be used to estimate heat and mass fluxes at active lava lakes. Here we use a three thermal component pixel model with three bands of Landsat thematic mapper (TM) data to constrain the thermal structure of, and flux from, active lava lakes. Our approach considers that a subpixel lake is surrounded by ground at ambient temperatures and that the surface of the lake is composed of crusted and/or molten material. We then use TM band 6 (10.42–12.42 μm) with bands 3 (0.63–0.69 μm) or 4 (0.76–0.90 μm) and 5 (1.55–1.75 μm) or 7 (2.08–2.35 μm), along with field data (e.g., lava lake area), to place limits on the size and temperature of each thermal component. Previous attempts to achieve this have used two bands of TM data with a two‐component thermal model. Using our model results with further field data (e.g., petrological data) for lava lakes at Erebus, Erta 'Ale, and Pu'u 'O'o, we calculate combined radiative and convective fluxes of 11–20, 14–27 and 368–373 MW, respectively. These yield mass fluxes, of 30–76, 44–104 and 1553–2079 kg s−1, respectively. We also identify a hot volcanic feature at Nyiragongo during 1987 from which a combined radiative and convective flux of 0.2–0.6 MW implies a mass flux of 1–2 kg s−1. We use our mass flux estimates to constrain circulation rates in each reservoir‐conduit‐lake system and consider four models whereby circulation results in intrusion within or beneath the volcano (leading to endogenous or cryptic growth) and/or magma mixing in the reservoir (leading to recycling). We suggest that the presence of lava lakes does not necessarily imply endogenous or cryptic growth: lava lakes could be symptomatic of magma recycling in supraliquidus reservoirs.
Fault populations and their relationship to the scaling of surface roughness
Authors: Pickering G. , Bull J. M. , Sanderson D. J.
Published: September 2002

The relationship between the scaling properties of faulted geological surfaces and parameters describing the underlying fault population are investigated using simulations of a dip‐slip faulted surface. Analysis of multiple simulations of sections through the surface allowed the construction of a statistical relationship between the parameters defining the fault population and the fractal dimension of the surface. The results indicate a direct, if complex, relationship between the fault population and the scaling of the surface roughness. The main determining factor is the displacement distribution, with spacing and dip having only a minor contribution. This relationship is tested against examples from the Moray Firth, Scotland, and the central Indian Ocean.
S band Doppler radar measurements of bathymetry, wave energy fluxes, and dissipation across an offshore bar
Authors: McGregor John A. , Poulter E. Murray , Smith Murray J.
Published: September 2002

The use of a shore‐based microwave Doppler radar for the remote sensing of ocean wave propagation over an offshore sand bar was investigated. The radar yielded high spatial resolution measurements of several quantities of importance in the study of bar dynamics. The spatial variation in wave phase with distance along the radar beam direction was used to calculate bathymetry with sufficient accuracy and resolution to clearly reveal the presence of the bar and the tidal cycle variation of water depth. This real‐time bathymetry enabled the calculation of ocean wave energy fluxes from the radar velocity data. These energy fluxes showed good agreement with predictions from a numerical wave propagation model, except in regions of significant wave breaking, where the discrepancy between measured and modeled fluxes was found to be closely related to the modeled wave dissipation rate.
Age determination of mixed water masses using CFC and oxygen data
Authors: Karstensen Johannes , Tomczak Matthias
Published: September 2002

We present a new method based on a combination of optimum multiparameter analysis and CFC/oxygen mixing analysis to determine the ages of water masses in regions of mixing. It enables us to follow water mass movements in greater detail than with other methods, which give only the combined pseudoage of a water mass mixture. We define the age of a water mass as the time a water parcel needs to spread from its source region, where it received its individual tracer characteristics, to the point of observation. The age distribution allows us to determine pathways of water masses, which differ from simple advection trajectories because the age is determined by a combination of advective and diffusive processes. We apply the method to hydrographic data along World Ocean Circulation Experiment section 15 in the south east Indian Ocean. In the thermocline, Indian Central Water (ICW) and Subantarctic Mode Water (SAMW) meet and mix. These distinct water masses have different formation mechanisms but similar temperature/salinity characteristics. It is shown that the convective formation of SAMW is a major ventilation mechanism for the lower Indian thermocline. In the eastern part of the south Indian Ocean, SAMW dominates the oceanic thermocline and is found to be about 5 years old. Pure ICW is present only in the thermocline of the region 48°–55°E, with increasing age with depth, confirming the subduction theory. While most SAMW joins the equatorward gyre movement of the southeastern Indian Ocean, some of it propagates westward through turbulent diffusive mixing, reaching 55°E after 15–20 years. It takes ICW some 25–30 years to reach 110°E.
Introduction to special section: Studies of the ocean surface from the Spaceborne Imaging Radar‐C/X‐Band SAR experiments
Authors: Holt Benjamin
Published: September 2002

Nearly 20 years have now passed since Seasat briefly sampled the ocean surface with its suite of then extraordinarily advanced active microwave instruments. The tantalizing 3‐month snapshots from Seasat's altimeter and scatterometer were analyzed and reanalyzed and used to justify follow‐on missions that were finally implemented within the last decade, including the ERS missions, TOPEX/POSEIDON, NSCAT, and Geosat.

The third active microwave instrument carried on Seasat, the synthetic aperture radar (SAR), has had more of a mixed success in its contribution to physical oceanography, despite its being flown on four satellites since 1991, the European Space Agency's ERS‐I and ERS‐2, the Japanese Space Agency's JERS‐I, and Canada's RADARSAT. The recent special issue on oceanographic results from ERS‐I and ERS‐2 (special section on Advances in Oceanography and Sea Ice Research Using ERS Observations, Journal of Geophysical Research, 103(C4), 1998) has numerous papers that incorporate SAR for both ocean and polar sea ice research. The finescale twodimensional view of the ocean surface seen in SAR imagery provides unparalleled detail of the short wave field and its interactions with longer waves and currents. However, the detailed view comes at an expense. Using the imagery for oceanographic applications has required the unraveling of the data's often baffling signatures, arising not only from the complicated wave‐wave and wave‐current interactions but also from the varying local wind field. Once understood, these signatures provide new and unique oceanographic and atmospheric information at higher spatial resolution than is available in the other microwave instruments. One approach to untangle this challenging interaction of ocean and radar physics is to look at the ocean surface simultaneously with multiple radar frequencies and polarizations. Most commonly, this is done with aircraft and surface‐based instrumentation. This suite of multiple channels provides a more complete understanding of the radar scattering from the ocean surface, since it is scale‐dependent, which can then lead to improved understanding of the ocean physics.
The mean and annual cycle of upper layer temperature fields in relation to Sverdrup dynamics within the gyres of the Atlantic Ocean
Authors: Mayer Dennis A. , Molinari Robert L. , Festa John F.
Published: September 2002

Using 28 years of expendable bathythermograph data (1967–1994), we describe the mean and annual cycle of the upper ocean temperature fields in the Atlantic from 30°S to 50°N in the context of the basin‐scale wind‐driven gyres (Sverdrup stream function field), which provide a framework for describing the oceanographic measurements. We examine the circulation field implied by the temperature distributions, which are used as a proxy for the field of mass. Similarities between the temperature and stream function fields increase with depth. In the lower to subthermocline depths of the tropical and equatorial gyres the zonal currents form a closed circulation. A southeastward boundary current is suggested near and below 150 m that provides closure for the tropical gyre, and the equatorial gyre axis is southward of that suggested by the stream function field. Higher in the water column, the North Equatorial Countercurrent (NECC) may be a surface manifestation of the North Equatorial Undercurrent (NEUC), where the latter can be interpreted as the southern limb of the tropical gyre. Because there are large vertical shears in the tropics, the equatorial gyre is not clearly indicated in the vertically integrated temperature field but appears below about 200 m. Here, the South Equatorial Undercurrent (SEUC) can be interpreted as the eastward flowing northern limb of the equatorial gyre and is opposite in direction to the westward flowing South Equatorial Current above. Both the NEUC and SEUC are analagous to currents in the Pacific that are governed by non‐Sverdrup dynamics. Despite the shortcomings of the data, the mean annual cycle appears to be relatively stable, and we have discounted the possibility that in regions where it represents a significant percentage of the total variance, it is changing slowly over the 28 years of record. The wind‐forcing fields, which undergo large meridional movements (5°–6° of latitude) during their annual cycle, with some exceptions, have essentially no counterpart in gyre movements between their seasonal extremes. Most of the variability associated with the annual cycle is confined to the upper 300 m. Greatest variability, where ranges exceed 6°C, occurs in the northwestern Atlantic in late winter and early spring. During this time of year south of the Gulf Stream and below about 100 m, water temperatures exhibit a systematic phase lag with depth. The next largest area of variability, where ranges can also exceed 6°C, resides in the tropical western basin between the equator and 10°N just below 100 m. In the eastern basin, ranges decrease and shoal. Additionally, the phase fields are consistent with the intensification and relaxation of the tropical ridge‐trough system where the NECC disappears in March in the west, but the NECC/NEUC complex is strongest in September.
An assessment of NSCAT ambiguity removal
Authors: Gonzales Amy E. , Long David G.
Published: September 2002

The NASA scatterometer (NSCAT) estimates the wind speed and direction of near‐surface ocean wind. Several possible wind vectors (termed ambiguities) are estimated for each resolution element known as a wind vector cell (WVC). Typically, the speeds of the possible wind vectors are nearly the same, but the directions are very different. The correct wind must be distinguished in a step called ambiguity removal. Unfortunately, ambiguity removal algorithms are subject to error. In an attempt to evaluate the accuracy of the Jet Propulsion Laboratory NSCAT product, we use a new model‐based quality assurance algorithm that uses only NSCAT data. The algorithm segments the swath into overlapping 12 × 12 WVC regions and classifies each region according to estimated quality. The 9‐month NSCAT mission data set is analyzed. In 82% of the regions the ambiguity removal is over 99% effective, with the ambiguity errors correctable using a model‐based correction technique. In 5% of the regions, areas of significant ambiguity error are found. For remaining regions, all of which have root‐mean‐square (rms) wind speeds less than 4 m s−1, there is too much uncertainty in the wind field model or too much noise in the measurements to uniquely evaluate ambiguity selection with sufficient confidence. We thus conservatively conclude that for the set of regions with rms wind speed greater than 4 m s−1, NSCAT ambiguity removal is at least 95% effective.