bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional modeling of tidal currents and mixing quantities over the Newfoundland Shelf
Authors: Han Guoqi
Published: September 2002

Major semidiurnal (M2, S2, and N2) and diurnal (K1 and O1) barotropic tides over the Newfoundland Shelf are computed using a three‐dimensional nonlinear primitive equation model, with the vertical eddy viscosity calculated from a level 2.5 turbulence closure scheme. Computed elevation cotidal charts for the five constituents are generally consistent with previous knowledge for this region. Comparisons based on a statistical analysis of the differences between the computed elevations and currents and in situ observations indicate good agreement. While M2 tidal currents (up to 20–30 cm/s) are dominant, there are locally intensified diurnal currents (up to 5–10 cm/s) in some outer shelf locations. The diurnal current intensification is attributed to first‐mode continental shelf waves. An examination is carried out for the vertical structure of the computed M2 current and for the temporal and spatial variability of model turbulent kinetic energy, mixing length scale, vertical eddy viscosity, and bottom friction velocity. The examination indicates that large vertical eddy viscosity magnitudes and bottom friction velocities are associated with strong currents in shallow regions, where strong vertical shears produce large turbulent kinetic energy. Solutions with both specified and evolving vertical stratification indicate that the stratification has a significant influence on the vertical profile of tidal mixing parameters and currents in shallow areas. Tidally induced turbulence is substantially reduced in the bottom boundary layer and completely suppressed above it. Tidal currents decrease in the log layer, increase significantly in the rest of the bottom boundary layer, and decrease in the upper and middle water column.
ENSO‐like interdecadal variability in the Pacific Ocean as simulated in a coupled general circulation model
Authors: Yukimoto Seiji , Endoh Masahiro , Kitamura Yoshiteru , Kitoh Akio , Motoi Tatsuo , Noda Akira
Published: September 2002

Spatial and temporal structures of interdecadal variability in the Pacific Ocean are investigated using results from an atmosphere‐ocean coupled general circulation model (AOGCM). The model shows a basin‐wide spatial pattern of the principal sea surface temperature (SST) variability similar to the observed one. Both interdecadal and interannual temporal structures of the SST variability agree well between the observation and the AOGCM. On the other hand, a slab ocean model coupled to the same atmospheric model as the AOGCM fails to simulate the observed temporal structure. Therefore the timescale of the coupled variability is associated with dynamical processes in the ocean. A distinct interdecadal mode of the coupled atmosphere‐upper ocean temperature variability is found in the AOGCM, with a spatiotemporal structure coherent with the SST variability. The mode accompanies an El Niño‐Southern Oscillation (ENSO)‐like spatial pattern of SST and the surface wind and behaves like a delayed oscillator in ENSO. A wedge‐shaped anomaly pattern of the upper thermocline temperature is formed in the eastern Pacific, and its northern subtropical signal propagates westward, enhanced by a subtropical wind forcing at the central basin. Arrival of the subtropical signal at the western Pacific around 20°N switches the anomaly of subsurface temperature in the equatorial region through anomalous oceanic heat transport along the western boundary. The travel time of the trans‐Pacific signal in the subtropics appears to be responsible for the timescale of this mode. The AOGCM successfully simulated the second mode of SST with a major variation in the midlatitude North Pacific as in the observed SST. In the upper ocean heat content we found another distinct mode, which is characterized by a midlatitude‐subtropics dipole pattern migrating around the North Pacific subtropical gyre. However, the associated SST variation of this mode shows a poor correspondence in the dominant interdecadal modes for the observed SST.
Three‐dimensional aspects of the seasonal heat balance in the Gulf of California
Authors: Beron‐Vera F. J. , Ripa P.
Published: September 2002

Previously unresolved three‐dimensional aspects of the time mean, as well as the annual and semiannual cycles of the heat transport, are studied in the Gulf of California using a minimal box model suggested by the hydrographic data. At the very least three horizontal domains are needed for the model: a shallow region north of the big islands, a deep region identifiable with the Ballenas Channel, and another deep region spanning the rest of the gulf south of the archipelago. Only one vertical layer is considered in the northern region, whereas two layers are taken into account in the other two regions. Out of all of the possible heat exchanges among the various boxes of the model, the least‐flux‐variance solution is considered. The annual mean heat entering the surface in the northern region is exported in almost equal amounts to the Ballenas Channel and the southern region. Ballenas Channel, in turn, exports heat to the southern region mainly through the lower layer, producing a transversal asymmetry of the mouthward flux into the upper layer. At the mouth the heat is exported to the Pacific Ocean largely through the upper layer. Both the annual and semiannual amplitude of the exchanges between all regions are as important as the annual average, except at the mouth of the gulf, where the semiannual component is not statistically relevant. The Pacific Ocean fluxes estimates at the annual frequency agree well with those produced by a baroclinic wave, which has been shown to explain a big deal of the thermodynamics of the gulf. The annual mean vertical flux per unit horizontal area in the Ballenas Channel is much larger than that in the southern region. This reinforces previous speculations that mixing processes in this region are very important. A depth‐independent eddy diffusivity parameterization suggests that diffusion might not be invoked to explain the annual mean vertical flux in the Ballenas Channel.
Ozone production in the New York City urban plume
Authors: Kleinman Lawrence I. , Daum Peter H. , Imre Dan G. , Lee Jai H. , Lee Yin‐Nan , Nunnermacker Linda J. , Springston Stephen R. , Weinstein‐Lloyd Judith , Newman Leonard
Published: September 2002

In the summer of 1996 the Department of Energy G‐1 aircraft was deployed in the New York City metropolitan area as part of the North American Research Strategy for Tropospheric Ozone‐Northeast effort to determine the causes of elevated O3 levels in the northeastern United States. Measurements of O3, O3 precursors, and other photochemically active trace gases were made upwind and downwind of New York City with the objective of characterizing the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). Four flights are discussed in detail. On two of these flights, winds were from the W‐SW, which is the typical direction for an O3 episode. On the other two flights, winds were from the NW, which puts a cleaner area upwind of the city. The data presented include plume and background values of O3, CO, NOx, and NOy concentration and VOC reactivity. On the W‐SW flow days O3 reached 110 ppb. According to surface observations the G‐1 intercepted the plume close to the region where maximum O3 occurred. At this point the ratio NOx/NOy was 20–30%, indicating an aged plume. Plume values of CO/NOy agree to within 20% with emission estimates from the core of the New York City metropolitan area. Steady state photochemical calculations were performed using observed or estimated trace gas concentrations as constraints. According to these calculations the local rate of O3 production P(O3) in all four plumes is VOC sensitive, sometimes strongly so. The local sensitivity calculations show that a specified fractional decrease in VOC concentration yields a similar magnitude fractional decrease in P(O3). Imposing a decrease in NOx, however, causes P(O3) to increase. The question of primary interest from a regulatory point of view is the sensitivity of O3 concentration to changes in emissions of NOx and VOCs. A qualitative argument is given that suggests that the total O3 formed in the plume, which depends on the entire time evolution of the plume, is also VOC sensitive. Indicator ratios O3/NOz and H2O2/NOz mainly support the conclusion that plume O3 is VOC sensitive.
Lidar observations of seasonal variation of diurnal mean temperature in the mesopause region over Fort Collins, Colorado (41°N, 105°W)
Authors: Chen Songsheng , Hu Zhilin , White M. A. , Chen H. , Krueger D. A. , She C. Y.
Published: September 2002

Two campaigns from February 1997 to January 1999 have resulted in 18 sets of continuous temperature measurements in the mesopause region over Fort Collins, Colorado (41°N, 105°W), each covering a complete diurnal cycle, reasonably well distributed throughout the year. It is shown that the winter‐summer transitions, based on diurnal means, are abrupt and clear, further strengthening the concept of two‐level mesopause, and demonstrate the robustness of this picture. Similar to data taken at Urbana, Illinois, our data at Fort Collins also demonstrate the need to observe over a complete diurnal cycle to deduce the uncontaminated mean temperature structure and seasonal variations in the mesopause region. The difference between annual diurnal, nighttime, and daytime means may be ascribed to a temperature wave propagating with a downward phase speed of ∼0.8 km/h. Our data show that the nighttime annual mean is colder than the diurnal annual mean by no more than 2 K below 88 km and warmer by no more than 3 K above 88 km where the three means are nearly the same. As in the Urbana data, we also observed the altitude of a secondary temperature minimum in the summer to be ∼96 km, which without strong dynamical cooling in the summer would have been the summer mesopause altitude.
Estimates of atmospheric lifetimes and ozone depletion potentials for the bromopentafluoropropane isomers
Authors: Patten Kenneth O. , Li Zhuangjie , Wuebbles Donald J.
Published: September 2002

Bromopentafluoropropane (C3H2BrF5) has come under consideration as a potential replacement compound for ozone‐depleting chlorofluorocarbons and Halons. No rate constants for the reaction of the isomers of C3H2BrF5 with hydroxyl radical in the atmosphere have yet been measured. The existing ozone depletion potential recommendations, which are based on estimated reaction rate constants for the C3H2BrF5 isomers, are not in agreement. In this study, we seek to clarify the ozone depletion potential of the C3H2BrF5 isomers. First, we apply two hydroxyl reaction rate constant estimation methods to a series of bromofluorocarbons likely to have similar reactivity to C3H2BrF5 in order to select one estimation method to apply to C3H2BrF5. We then predict hydroxyl reaction rate constants using that estimation method for the C3H2BrF5 isomers and calculate both atmospheric lifetimes and ozone depletion potentials through semiempirical methods. This study obtains C3H2BrF5 atmospheric lifetimes ranging from 1.4 to 59 years and ozone depletion potentials ranging from 0.30 to 12, which reflects the strong dependence of hydroxyl reactivity on the isomeric structure of the potential replacement compound.
Identification of clear skies from broadband pyranometer measurements and calculation of downwelling shortwave cloud effects
Authors: Long Charles N. , Ackerman Thomas P.
Published: September 2002

We present an automated method to identify periods of clear skies for a 160° field of view using only 1‐min measurements of surface downwelling total and diffuse shortwave irradiance. The clear‐sky detection method is verified using Whole Sky Imager and lidar data, observer reports, and model comparisons. Identified clear‐sky irradiance measurements are then used to empirically fit clear‐sky irradiance functions using the cosine of the solar zenith angle as the independent variable. These fitted functions produce continuous estimates of clear‐sky total, diffuse, and direct component shortwave irradiances. While this method ignores diurnal changes in such variables as column water vapor and aerosol amounts and changes between clear‐sky days, it is shown that the resultant clear‐sky irradiance estimates have RMS uncertainty comparable to the uncertainty of the measuring instruments themselves. The estimated clear‐sky irradiances are used to estimate the effect of clouds on the downwelling shortwave irradiance as a difference between the measured and clear‐sky amounts. We show that the cloud effect calculations from this method appear to decrease the uncertainty due to systematic pyranometer offsets and cosine response errors. Thus any data set that includes downwelling diffuse and total shortwave measurements can be processed to identify clear‐sky periods and produce estimates of clear‐sky irradiance and cloud effects.
A comparison of cloud and boundary layer variables in the ECMWF forecast model with observations at Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp
Authors: Beesley J. A. , Bretherton C. S. , Jakob C. , Andreas E. L. , Intrieri J. M. , Uttal T. A.
Published: September 2002

Cloud and boundary layer variables from the European Centre for Medium‐Range Weather Forecasts (ECMWF) forecast model were compared with measurements made from surface instruments and from upward looking 8 mm wavelength radar and lidar at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp during November and December of 1997. The precipitation accumulation, near‐surface winds, and surface downward longwave irradiance predicted by the model were in good agreement with SHEBA observations during this period. However, surface downward longwave irradiance was underestimated by 10 W m−2 on average when low clouds were present in the model and observations. The model demonstrated considerable skill in predicting the occurrence and vertical extent of cloudiness over SHEBA, with some tendency to overestimate the frequency of clouds below 1 km. A synthetic radar reflectivity estimated from the ECMWF model variables was compared with 8 mm wavelength radar measurements. The two were broadly consistent only if the assumed snowflake size distribution over SHEBA had a smaller proportion of large flakes than was found in previous studies at lower latitudes. The ECMWF model assumes a temperature‐dependent partitioning of cloud condensate between water and ice. Lidar depolarization measurements at SHEBA indicate that both liquid and ice phase clouds occurred over a wide range of temperatures throughout the winter season, with liquid occurring at temperatures as low as 239 K. A much larger fraction of liquid water clouds was observed than the ECMWF model predicted. The largest discrepancies between the ECMWF model and the observations were in surface temperature (up to 15 K) and turbulent sensible heat fluxes (up to 60 W m−2). These appear to be due at least partially to the ECMWF sea ice model, which did not allow surface temperatures to respond nearly as rapidly to changing atmospheric conditions as was observed.
A mechanistic model study of slowly propagating coupled stratosphere‐troposphere variability
Authors: Kodera Kunihiko , Kuroda Yuhji
Published: September 2002

Observational studies reveal the oscillatory nature of coupled stratosphere‐troposphere variability, which is characterized by a slowly downward propagating anomalous zonal‐mean zonal wind during the winter. The term “slowly” is employed compared to the rapid variation associated with stratospheric sudden warmings. In fact, strong stratospheric warming events are embedded in this slowly propagating variability as is shown in a companion paper [Kodera et al., this issue]. In the present study, the mechanism producing such variability at all levels of the atmosphere is investigated by developing a simple model that extends from the Earth's surface to 80 km altitude. The model is a quasi‐geostrophic channel model similar to that used by Holton and Mass [1976], but it includes two meridional Fourier components and latitudinal variation in the gradient of the Coriolis parameter. By this extension, the present model can incorporate both the subtropical and polar night jet and also treat the horizontal propagation of planetary waves as well as vertical propagation. The results of the simulation reproduce qualitatively well the main observed features: Interaction between the planetary waves and zonal‐mean flow produces oscillations in the winter stratosphere, and zonal wind anomalies created in the subtropics of the stratopause propagate downward into the polar region of the troposphere, which provoke changes in the meridional propagation of tropospheric planetary waves.
A comparison of UV intensities calculated by spherical‐atmosphere radiation transfer codes: Application to the aerosol corrections
Authors: Petropavlovskikh I. , Loughman R. , DeLuisi J. , Herman B.
Published: September 2002

Various spherical radiative transfer (SRT) computer codes that model ultraviolet (UV) sky intensities incident at the Earth's surface are compared for clear‐sky (Rayleigh scattering and ozone absorption) and hazy‐sky (stratospheric volcanic aerosol, Rayleigh scattering, and ozone absorption) atmospheric conditions. Calculated sky intensities using Dave vector (pseudospherical), Dave scalar (pseudospherical), Herman's vector (full spherical), and Herman's scalar (full spherical) codes are compared for various parameters including solar zenith angle, direction of view, and wavelength. The differences in the calculated downward UV intensities are believed to arise from the differences in the code geometries and the neglect of polarization effects. The difference between downward UV intensities is within ±15% for clear‐sky conditions and between −30 and 18% for hazy‐sky conditions. The results of comparisons suggest that an experiment should be conducted to measure UV sky intensities for clear‐sky conditions, with minimal aerosol, to test the quality of the radiative transfer codes with actual observations. The Dave scalar radiative transfer code has been used in the past to calculate aerosol error corrections to Umkehr measurements. To evaluate the accuracy of these calculations, we performed a set of comparisons with results of various spherical radiative transfer codes. Ground level sky intensities were calculated for various solar zenith angle directions, for a vertically inhomogeneous Rayleigh atmosphere with ozone absorption, including and excluding stratospheric aerosols. For 0.11 stratospheric aerosol optical thickness, the method for calculating stratospheric aerosol errors to retrieved Umkehr ozone profile predicts either −22% or −32% error in layer 8 depending on whether vector or a scalar radiative transfer code had been used. The calculations were also used to study the effect of full spherical and pseudospherical forward model differences on Umkehr ozone profile retrievals. The difference in retrieved ozone profiles was found to be within ±4% for clear‐sky conditions and up to 13% in layer 8 for hazy‐sky conditions. The results of these comparisons suggest that further improvements to the profile retrieval and stratospheric aerosol error calculations could be made using a fully spherical RT code that accounts for polarization.
Stochastic model of the width function
Authors: Veneziano Daniele , Moglen Glenn E. , Furcolo Pierluigi , Iacobellis Vito
Published: September 2002

A new class of probabilistic models of the width function, based on so‐called iterated random pulse (IRP) processes, is proposed. IRP processes reproduce the main characteristics of empirical width functions (nonnegativity, nonstationarity, and power law decay of the spectrum) and require few and easily accessible parameters. IRP models are based on a simple conceptualization of the geometrical structure of river basins and exploit in a natural way the self‐similarity of natural channel networks. A result that is derived from the IRP representation is that the exponent α of Hack's law, L ∼ Aα, and the exponent β of the power spectral density of the width function, S(ω) ∼ |ω|−β, are related as α = 1/β. Empirical values of β are typically in the range 1.8–2.0 and are consistent with this theoretical result and the usual range of α.
Estimation of spatially variable residual nonaqueous phase liquid saturations in nonuniform flow fields using partitioning tracer data
Authors: James Andrew I. , Graham Wendy D. , Hatfield Kirk , Rao P. S. C. , Annable Michael D.
Published: September 2002

Estimates of spatially variable residual NAPL saturations SN are obtained in heterogeneous porous media using first temporal moments of breakthrough curves (BTCs) obtained from multilevel samplers during in situ partitioning tracer tests. An approach is adopted in which the distribution of the log NAPL/water volumetric ratio (Y = ln [SN/(1 − SN∥]) and log hydraulic conductivity (F = ln K) are treated as spatially correlated random fields. A nonlinear Gauss‐Newton search technique is used to identify the spatial distribution of Y that minimizes the weighted sum of the deviation of the temporal moment predictions from their measured values and the deviation of the estimate of Y from its prior estimate obtained from the temporal moments of extraction well BTCs. Sensitivities required for the algorithm are obtained using a coupled flow and transport adjoint sensitivity method. In addition to obtaining optimal estimates for the spatial distribution of Y, the method also provides the estimation error covariance. The estimation error covariance can be used to evaluate the information that may be obtained from alternate pumping and monitoring configurations for tracer tests designed to detect NAPL in the subsurface. To this end, we tested the method using two different NAPL distributions (one with a random spatially correlated field and a second that was a block of NAPL) and three different pumping configurations (a double five‐spot pattern, an inverted double five‐spot pattern, and a line‐drive pattern). The results show that measured temporal moments are more sensitive to Y in the double five‐spot and inverted double five‐spot patterns, and estimates produced in these configurations are slightly superior to those produced in the line‐drive pattern.
Effective water vapor diffusion coefficient of snow under a temperature gradient
Authors: Sokratov Sergey A. , Maeno Norikazu
Published: September 2002

The wave‐like temperature and density distributions in snow under an applied temperature gradient, found by Sokratov and Maeno [1998], were used to estimate the effective water vapor diffusion coefficient in snow. Values obtained were (0.6−2.0)×10−5m2 s−1, lower than the water vapor diffusion coefficient in air and much smaller than previous results found in the literature. The difference was discussed with the use of results of CO2 diffusion measurements in snow under an isothermal condition and the introduction of a new parameter, the gradient enhancement factor (ƒ). It was concluded that the effective water vapor diffusion coefficient of snow is dependent on the porosity, tortuosity (т), and gradient enhancement factor. The mean values of т and ƒ were 1.73 and 2.3, respectively, for the snow studied.
A generalized mathematical framework for stochastic simulation and forecast of hydrologic time series
Authors: Koutsoyiannis Demetris
Published: September 2002

A generalized framework for single‐variate and multivariate simulation and forecasting problems in stochastic hydrology is proposed. It is appropriate for short‐term or long‐term memory processes and preserves the Hurst coefficient even in multivariate processes with a different Hurst coefficient in each location. Simultaneously, it explicitly preserves the coefficients of skewness of the processes. The proposed framework incorporates short‐memory (autoregressive moving average) and long‐memory (fractional Gaussian noise) models, considering them as special instances of a parametrically defined generalized autocovariance function, more comprehensive than those used in these classes of models. The generalized autocovariance function is then implemented in a generalized moving average generating scheme that yields a new time‐symmetric (backward‐forward) representation, whose advantages are studied. Fast algorithms for computation of internal parameters of the generating scheme are developed, appropriate for problems including even thousands of such parameters. The proposed generating scheme is also adapted through a generalized methodology to perform in forecast mode, in addition to simulation mode. Finally, a specific form of the model for problems where the autocorrelation function can be defined only for a certain finite number of lags is also studied. Several illustrations are included to clarify the features and the performance of the components of the proposed framework.
The role of magma mixing in triggering the current eruption at the Soufriere Hills Volcano, Montserrat, West Indies
Authors: Murphy M. D. , Sparks R. S. J. , Barclay J. , Carroll M. R. , Lejeune A.‐M. , Brewer T. S. , Macdonald R. , Black S. , Young S.
Published: September 2002

The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non‐uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.
Polar stratospheric clouds: A high latitude warming mechanism in an ancient greenhouse world
Authors: Sloan L. Cirbus , Pollard D.
Published: September 2002

The presence of water vapor clouds in the stratosphere produces warming in excess of tropospheric greenhouse warming, via radiative warming in the lower stratosphere. The stratospheric clouds form only in regions of very low temperature and so the warming produced by the clouds is concentrated in polar winter regions. Results from a paleoclimate modeling study that includes idealized, prescribed polar stratospheric clouds (PSCs) show that the clouds cause up to 20°C of warming at high latitude surfaces of the winter hemisphere, with greatest impact in oceanic regions where sea ice is reduced. The modeled temperature response suggests that PSCs may have been a significant climate forcing factor for past time intervals associated with high concentrations of atmospheric methane. The clouds and associated warming may help to explain long‐standing discrepancies between model‐produced paleotemperatures and geologic proxy temperature interpretations at high latitudes, a persistent problem in studies of ancient greenhouse climates.
Evidence of honeycomb weathering on Mars
Authors: Rodriguez‐Navarro Carlos
Published: September 2002

Features similar to honeycomb weathering are observed in some Martian rocks at the Pathfinder landing site. Honeycomb weathering on Earth develops in a range of environments, including extremely arid and cold areas in Antarctica, and on many types of rocks, e.g. basalts and andesites, subjected to physical and/or chemical weathering. Salt weathering, in combination with frost shattering and chemical weathering, provides a viable explanation for the development of honeycomb features on Mars under current environmental conditions. Data supporting this theory are presented and discussed. These findings are important in understanding the present climate, geology, and, in particular, the geomorphology of Mars. They also suggest new paths for research and experimentation in future missions to Mars.
Crustal strains in the Japanese Islands as deduced from dense GPS array
Authors: Kato T. , El‐Fiky G. S. , Oware E. N. , Miyazaki S.
Published: September 2002

Data from the nationwide continuous GPS network, operated by the Geographical Survey Institute of Japan, were used to estimate crustal strains in the Japanese islands. We used horizontal velocity vectors at 610 sites for the period April, 1996 to March, 1997, and employed the Least Squares Prediction method to interpolate the velocities to a uniform 7km × 7km grid over Japan. The gridded velocities were then differentiated in space to calculate principal components of strains. Results clearly portray tectonic strainings of the Japanese islands in one year; (1) Dilatational strains show that the Japanese islands are under a compressive strain regime induced by the subducting oceanic plates, (2) Distribution of high maximum shear strains coincides with the regions of crustal activities, and (3) Principal axes of the strains indicate influences of E‐W and NW‐SE compression due to the converging oceanic plates. North‐south extension in Kyushu is also visible.
Modeling of compaction driven flow in poro‐viscoelastic medium using adaptive wavelet collocation method
Authors: Vasilyev Oleg V. , Podladchikov Yuri Yu. , Yuen David A.
Published: September 2002

Different regimes of compaction driven flow have been studied within the framework of a poro‐viscoelastic medium. A single dimensionless parameter, the Deborah number De, has been identified, which enables the portrayal of the solution from the purely viscous matrix limit (De ≪ 1) to the poro‐elastic (De ≫ 1) matrix limit. In viscous limit the evolution of a porosity disturbance (porosity wave) is governed by nonlinear convection‐diffusion equation, while in the poro‐elastic limit it evolves according to a Burgers‐like non‐linear advection equation. In both regimes porosity waves of higher amplitude propagate faster. However in the viscous limit porosity waves go through each other in soliton‐like fashion, while in poro‐elastic limit they coalesce and thus enhance melt segregation. The introduction of other variables, such as chemistry, would induce different responses in the flow for low and high De, allowing for diverse feedback situations.
Small‐scale (∼3 m) E region irregularities at and off the magnetic equator
Authors: Murthy B. V. Krishna , Ravindran Sudha , Viswanathan K. S. , Subbarao K. S. V. , Patra A. K. , Rao P. B.
Published: September 2002

VHF backscatter radar observations at and off the magnetic equator (Trivandrum and Gadanki, respectively) have been analyzed to study the small‐scale (∼3 m) irregularities at E region altitudes. The Doppler spectra observed at Gadanki invariably resemble type 2 (gradient drift instability) Doppler spectra in contrast to the spectra at Trivandrum where both type 1 and type 2 occur. The type 2 irregularities (at Gadanki) are attributed to the presence of steep (vertical) electron density gradients, which are necessary for the growth of gradient drift instabilities at the off magnetic equatorial latitude of Gadanki. The observed irregularity drifts at lower altitudes (<97 km) are attributed mainly to the meridional neutral winds and those at higher altitudes (>101 km) to magnetic field line‐linked F region electric fields at the magnetic equator.
ULF pulsations driven by a randomly varying magnetopause displacement
Authors: Smith J. M. , Wright A. N. , Rickard G. J.
Published: September 2002

A magnetospheric cavity with a two‐dimensional profile in Alfvén speed has been driven with a nonmonochromatic source. Detailed numerical results show that the magnetospheric cavity filters the random driving signal and excites preferentially the fast modes whose eigenfrequencies lie within the driving spectrum. These fast modes may also couple to the Alfvén mode, provided ky ≠ 0 and their eigenfrequencies lie within the Alfvén continuum. The resulting Alfvén modes and the position of the resonant field lines can be predicted to high accuracy by calculating the natural fast and Alfvén frequencies of the undriven system. A preliminary investigation into the seismology of the magnetosphere has also been undertaken. The ratio of the energy density in the two resonant field lines depends only on the equilibrium of the cavity and not on the nature of the driving source (e.g., initial condition, impulsive excitation, or random forcing). Good agreement is found with the ratio predicted by an approximate analytical treatment based upon the eigenfunctions of the equilibrium magnetosphere. The new seismological technique may prove to be a useful diagnostic tool in future studies.
Noncanonical Hamiltonian methods for particle motion in magnetospheric hydromagnetic waves
Authors: Chan Anthony A.
Published: September 2002

Hamiltonian equations of motion for a nonrelativistic charged particle in magnetospheric hydromagnetic perturbations are derived. The equations are gyroaveraged, allowing much larger time steps in numerical solutions of the equations of motion compared to integrating the full Lorentz equations of motion, but they contain finite‐gyroradius effects to all orders in k⊥ρ, where k⊥. is the perpendicular wave number and ρ is the particle gyroradius. The finite‐gyroradius effects are essential for the important class of particles which undergo magnetic drift‐bounce resonances with the waves. The equations are derived by finding a Lie transform of the perturbed guiding center phase‐space Lagrangian to a new Lagrangian which is independent of the gyrophase angle. The resulting Euler‐Lagrange equations contain nonlinear terms which automatically preserve the Hamiltonian properties of the original Lorentz system, such as conservation of energy (for static systems) and conservation of phase‐space volume. The Hamiltonian conservation properties are useful for checking the accuracy of numerical integration schemes and they are essential for the use of Poincaré surface‐of‐section plots. Compared to more traditional canonical Hamiltonian methods, the phase‐space Lagrangian Lie transform methods allow general, noncanonical phase‐space coordinates and transformations. This results in more power and flexibility in finding convenient forms for the final equations of motion. The results are given in coordinate‐free form and in terms of magnetic field coordinates. Applications of these results to calculations of hydromagnetic wave‐induced particle motion in the inner magnetosphere are discussed.
Duration of deep earthquakes determined by stacking of Global Seismograph Network seismograms
Authors: Bos A. G. , Nolet G. , Rubin A. , Houston H. , Vidale J. E.
Published: September 2002

The duration of each subevent of 48 earthquakes with magnitude larger than 5.5 and depth greater than 100 km was determined from stacked traces of broadband records of Global Seismograph Network stations. We fitted the source time function by one or more triangles convolved with attenuation. We found that global stacks of displacement seismograms yield reliable estimates of the rupture duration. The durations, scaled to a moment of 1019 N m, of both the subevents and the entire earthquake show a slight decrease with depth from 9 s for events at 100 km depth to about 7 s for events at 600 km depth. Assuming that the rupture velocity is a constant fraction of the shear wave speed, this decrease can be completely explained by the increase in shear velocity of 20%. In this sense, deep earthquakes are comparable to intermediate ones. For some intermediate‐depth events, Vidale and Houston [1993] found durations up to twice as long. We find that almost all of their slow events have been recorded at large epicentral distances. At these distances, we conjecture that the end of the P wave train may be extended by the arrival of reflections from the D″ layer.
Paleomagnetism of the Miocene intrusive suite of Kidd Creek: Timing of deformation in the Cascade Arc, southern Washington
Authors: Hagstrum Jonathan T. , Swanson Donald A. , Snee Lawrence W.
Published: September 2002

Paleomagnetic study of the intrusive suite of Kidd Creek in the southern Washington Cascades (23 sites in dikes and sills) was undertaken to help determine if these rocks are comagmatic and whether they postdate regional folding of the volcanic arc. Fission track and 40Ar‐39Ar age determinations indicate an age of ∼12.7 Ma (middle Miocene) for these rocks. The similarity of normal‐polarity characteristic directions for most samples corroborate the available geochemical data indicating that these rocks are most likely comagmatic. Reversed‐polarity directions for samples from four sites, however, show that emplacement of Kidd Creek intrusions spanned at least one reversal of the geomagnetic field. The paleomagnetic directions for the dikes and sills fail a fold test at the 99% confidence level indicating that the Kidd Creek rocks postdate regional folding. The mean in situ direction also indicates that the Kidd Creek and older rocks have been rotated 22°±6° clockwise about a vertical or near‐vertical axis from the expected Miocene direction. Compression and regional folding of the Cascade arc in southern Washington therefore had ended by ∼12 Ma prior to the onset of deformation resulting in rotation of these rocks.
Streaming potential in heterogeneous networks
Authors: Bernabé Yves
Published: September 2002

It is well‐known that when in contact with an electrolyte, the surfaces of minerals like quartz become charged. In porous rocks, this causes coupling of fluid flow and electrical conduction. In this paper, the theory of electrokinetic coupling in a single capillary is first reviewed. The rest of the paper is concerned with the upscaling of the microscopic (i.e., pore scale) equations and parameters into their macroscopic counterparts. Upscaling was explicitly performed here in a large number of arbitrarily heterogeneous, two‐dimensional networks. As a result, the macroscopic equations were established in a new form using C, the macroscopic coupling coefficient, and δk, a dimensionless correction to permeability. In particular, it was shown that Onsager's [1931] reciprocity holds at macroscopic scales and that the scale invariant Helmholtz‐Smoluchowski formula is not always valid. Accordingly, the macroscopic coupling coefficient C may not always be independent of permeability. In the networks considered here, the coupling coefficient appeared to be controlled by Johnson and Schwartz's [1989] weighted average length scale Λ. Another goal of this paper was to examine the spatial and statistical distributions of electrical currents and potential gradients in networks with varying heterogeneity levels. Similar to local fluid fluxes and pressure gradients, local electrical currents and potential gradients become localized in highly heterogeneous networks. The current field can equivalently be described in terms of current loops, and their spatial distribution can be analyzed. It was found that the current loop field becomes increasingly structured as the level of heterogeneity augments. One important consequence of the spatial distribution of currents and current loops is that at high levels of heterogeneity, the possibility exists to generate a magnetic field of significant intensity over wide areas of the networks. It was also observed that the magnitude of the local electric field appears normally distributed at low variance of the pore radii distribution and becomes increasingly non‐Gaussian as the heterogeneity is increased. Owing to the fact that the total current is zero, the local currents have an unusual distribution, i.e., uniform with a high‐end, −1 power, tail.
Geologic constraints on bedrock river incision using the stream power law
Authors: Stock Jonathan D. , Montgomery David R.
Published: September 2002

Denudation rate in unextended terranes is limited by the rate of bedrock channel incision, often modeled as work rate on the channel bed by water and sediment, or stream power. The latter can be generalized as KAmSn, where K represents the channel bed's resistance to lowering (whose variation with lithology is unknown), A is drainage area (a surrogate for discharge), S is local slope, and m and n are exponents whose values are debated. We address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method. We vary m, n, and K to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles. We find at least two end‐member incision laws, KA0.3–0.5S1 for Australian rivers with stable base levels and KfA0.1–0.2Sn for rivers in Kauai subject to abrupt base level change. The long‐term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion, characterized by Kf. Incision patterns from Japan and California could follow either expression. If they follow the first expression with m = 0.4, K varies from 10−7–10−6 m0.2/yr for granite and metamorphic rocks to 10−5–10−4 m0.2/yr for volcaniclastic rocks and 10−4–10−2 m0.2/yr for mudstones. This potentially large variation in K with lithology could drive strong variability in the rate of long‐term landscape change, including denudation rate and sediment yield.
Gas bubble dynamics model for shallow volcanic tremor at Stromboli
Authors: Ripepe Maurizio , Gordeev Evgenii
Published: September 2002

Volcanic tremor at Stromboli (Aeolian islands, Italy) is correlated to small infrasonic transients [Ripepe et al., 1996] which repeat almost rhythmically in time in a range between 0.8 and 1.2 s. We demonstrate that infrasonic transients are associated to small gas bubble (∼0.5 m) burstings which produces no transients in the seismic signal. Tremor ground displacement attenuates with the inverse of the distance from the craters indicating that the source is shallow. Short‐term energy release shows that infrasonic and seismic signals are linked to the same dynamical process, while at the long‐term scale it is evident that the two signals are controlled by two distinctive mechanisms. We suggest that the possible physical model acts in two steps: first, gas coalescence and, then, gas bursting. In our model, the seismic signal is related to the coalescence of a gas bubble from a layer of small bubbles, while the infrasonic signal is linked to the bursting of the bubble when it reaches the magma surface. Gas bubbles could form by free coalescence in magma or could be forced to coalesce by a structural barrier. We calculate that forced coalescence induces in magma a pressure change (∼104 Pa) 2 orders of magnitude higher than free coalescence, and it explains best the tremor ground displacement (10−5 m) recorded at Stromboli. Moreover, forced coalescence evidences the role of a structural barrier, such as a dike, in volcanic tremor source dynamics. In this gas dynamic process, the delay time of 1–2 s between infrasonic pulses could reflect the gas nucleation interval of basaltic magma [Thomas et al., 1993; Manga, 1996]. We propose that the source function for the shallow volcanic tremor at Stromboli could be the viscoelastic reaction of the magma to the pressure decrease induced by gas bubble growth rate under constant depressurization. The spectrum of our source function is controlled by the time duration of the pressure pulse, which represents the viscoelastic relaxation time of the magma and gas bubble growth rate. The predicted asymptotic decay of the frequency contents fits the spectral behavior of the volcanic tremor ground displacement recorded at Stromboli. We show that the same spectral behavior can be found in ground displacement spectra of volcanic tremor recorded on different volcanoes.
Effects of low‐frequency current variability on near‐inertial submesoscale vortices
Authors: Shay Lynn K. , Lee Tom N. , Williams Elizabeth J. , Graber Hans C. , Rooth Claes G. H.
Published: September 2002

From September 30 to October 14, 1993, and from May 18 to June 13, 1994, ocean surface currents along the inshore boundary of the Florida Current were measured using a land‐based ocean surface current radar (OSCR) deployed along the Florida Keys. A dual‐station OSCR mapped the current fields at 20‐min intervals with a horizontal resolution of 1.2 km over a 30×44 km domain using the high‐frequency (HF, 25.4 MHz) mode. An upward looking, narrowband acoustic Doppler current profiler (ADCP) was moored in 150 m of water seaward of the shelf break concurrently sampling the three‐dimensional current vector between 15 and 130 m at 30‐min intervals. Regression analyses between surface and subsurface currents at 15 m depth from the ADCP indicated biases of 3–12 cm s−1 and slopes of O(0.8–1). Root‐mean‐square differences were about 18 cm s−1 because of a 15‐m‐depth separation between the two measurements, and bulk current shears were O(10−3 s−1) in the Florida Current where maximum velocities exceeded 150 cm s−1 between 40 and 70 m. Surface tidal currents were 2–10 cm s−1 in the diurnal and semidiurnal bands and revealed fine‐scale variability in the tidal ellipses due to topographical variations. Differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s−1 in these constituents and explained more of the current variance with depth. During the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s−1. This feature was embedded in the near‐inertial wave band where the local inertial period was ≈29 hours that may have been forced by an abrupt change in the wind direction. This transient feature was trapped and advected along the inshore edge of the Florida Current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f. These energetic near‐inertial currents of 15–20 cm s−1 had horizontal and vertical wavelengths of 40 km and 50–100 m, respectively.
Thermohaline effects on upper layer circulation of the North Pacific
Authors: Tsujino Hiroyuki , Suginohara Nobuo
Published: September 2002

We investigate thermohaline effects on the upper layer circulation of the North Pacific by carrying out numerical experiments. The Pacific circulation north of 30°S is driven by wind forcing, thermohaline forcing at the sea surface, and Newtonian body forcing at the southern end of the model domain. The three driving agents for the circulation are incorporated step by step into the model to evaluate the role of each agent. The thermohaline forcing at the sea surface enhances the subsurface southward flow, which is the upper part of the layered deep Pacific meridional circulation. This flow suffers distortion but does not disappear, even when wind forcing is imposed. It follows the eastern and the southern perimeter of the subtropical gyre, becoming the southward western boundary undercurrent at low latitudes. The cross‐gyre southward western boundary undercurrent to the tropics, which is indicated in the observed distribution of tracers, is associated with the thermohaline circulation. Thus it is found that at the upper thermocline depths in the subtropical and tropical regions the flows in the poleward and westward region are wind driven, and those in the equatorward and eastward region are thermohaline. The inflow into the tropical South Pacific and Atlantic of the Antarctic Intermediate Water may be accounted for by this mechanism.