bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations in creep rate along the Hayward Fault, California, interpreted as changes in depth of creep
Authors: Simpson R. W. , Lienkaemper J. J. , Galehouse J. S.
Published: February 2004

Variations in surface creep rate along the Hayward fault are modeled as changes in locking depth using 3D boundary elements. Model creep is driven by screw dislocations at 12 km depth under the Hayward and other regional faults. Inferred depth to locking varies along strike from 4–12 km. (12 km implies no locking.) Our models require locked patches under the central Hayward fault, consistent with a M6.8 earthquake in 1868, but the geometry and extent of locking under the north and south ends depend critically on assumptions regarding continuity and creep behavior of the fault at its ends. For the northern onshore part of the fault, our models contain 1.4–1.7 times more stored moment than the model of Bürgmann et al. [2000]; 45–57% of this stored moment resides in creeping areas. It is important for seismic hazard estimation to know how much of this moment is released coseismically or as aseismic afterslip.
Voltage changes induced by stick‐slip of granites
Authors: Takeuchi Akihiro , Nagahama Hiroyuki
Published: February 2004

Tri‐axial compression tests of dry granites with a pre‐cut shear surface were conducted with an electrode to detect electrical potential. The samples were shaped 19.5 mm in diameter and 60 mm in length, and deformed at a strain rate of 10−3 s−1 and confining pressure of 78.5 MPa. Pulse‐like electrode voltage changes were detected several times just before (0.5–5 ms) stick‐slip motion during compression. Magnitudes of stick‐slips (e.g. normal stress drop of 5–160 MPa) and intensities of electrode voltage changes (0.1–10 V) increased whenever stick‐slip occurred. Here we propose some scaling laws on the number of stick‐slips and relate those statistics to the intensity of the electrical phenomena. Maximum electrode voltage change is proportional to normal stress drop that depends on real contact area of the slip plane. Local charge distributions, fractoemission or both may due to failure of asperities just before stick‐slip.
Decadal change in the south tropical Pacific in a Global Assimilation Analysis
Authors: Chang Ping , Giese Benjamin S. , Ji Link , Seidel Howard F. , Wang Fan
Published: February 2004

Using an ocean general circulation model that incorporates data assimilation, we show that the northward flowing portion of the subtropical gyre in the South Pacific undergoes a well defined decadal variation. The maximum variability of ocean temperature is located at around 10° S and is largest in the western side of the basin with a vertical extension of up to 1000 m. The anomalous temperature shows a rapid warm‐to‐cold transition in the late 1970s with a peak‐to‐peak value of more than 1° C. Coupled with this change is a strengthened circulation in the northern portion of the subtropical gyre. Further analysis indicates that the decadal change in the south tropical Pacific is linked to increased Ekman suction induced by enhanced cyclonic wind‐stress curl in the region.
X‐ray emission from scattering and fluorescence of solar X‐rays at Venus and Mars
Authors: Cravens Thomas E. , Maurellis Ahilleas N.
Published: February 2004

X‐ray emission is modeled and x‐ray intensities are predicted for Venus and Mars. Two emission mechanisms are studied: (1) elastic scattering of solar x‐rays by atmospheric neutrals, (2) absorption of solar x‐rays in either the carbon or oxygen K‐shells followed by fluorescent emission of x‐rays. The carbon and oxygen atoms reside in the carbon dioxide molecules. We predict total soft x‐ray intensities of approximately 0.67 R for Venus and 0.15 R for Mars, corresponding to total soft x‐ray luminosities of about 35 MW and 2.5 MW, respectively. These luminosities exceed the x‐ray luminosities expected from the solar wind charge exchange mechanism.
Highly anisotropic distributions of energetic electrons and triggered VLF emissions
Authors: Trakhtengerts V. Y. , Demekhov A. G. , Pasmanik D. L. , Titova E. E. , Kozelov B. V. , Nunn D. , Rycroft M. J.
Published: February 2004

Bell et al. [2000] have reported an exciting set of new observations, made near the equatorial plane of the plasmasphere on the L=3.4 flux tube, of hiss emissions between 4.0 and 5.6 kHz, highly anisotropic energetic electrons, and unusual VLF triggering effects at 10.2 kHz. Here we show that these data as a whole can be considered as the first experimental evidence of the existence of a special type of distribution function of energetic electrons with a step‐like feature in the velocity component parallel to the magnetic field. Such a shape of velocity distribution can be crucial for explanation of triggered VLF emissions and chorus. Assuming the distribution to be step‐like, we can readily explain the spatial distribution of energetic electrons along the field line observed by Bell et al. [2000], and also strong wave amplification in the triggering process. We discuss how the step in the velocity distribution is maintained by the hiss emission. The frequency gap between the hiss band and the triggered signal is connected with the excitation of the quasi‐electrostatic mode near the upper frequency edge of the hiss band.
The Southern Ocean: A ventilation contributor with multiple sources
Authors: Hellmer Hartmut H. , Beckmann Aike
Published: February 2004

Based on water mass analysis, the Weddell Sea in the Atlantic sector was identified as the major source for Southern Ocean bottom water. Recent observations, and tracer analysis and modeling indicate that the Indian‐Pacific sector might be the location of additional bottom water sources similar in magnitude as their Atlantic counterpart. Numerical model results presented here suggest that the Atlantic and Indian‐Pacific contributions to Southern Ocean bottom water are roughly equal but for waters of different density. The observationally derived formation rate of dense Antarctic Bottom Water of the order of 10 Sv (1 Sv=106 m³ s−1) is confirmed by the model results but doubles if the lighter component of the Indian‐Pacific sector is included. This result suggests that southern and northern hemisphere sources are equal contributors to the ventilation of the deep world ocean.
Kilometer‐scale layered structures inside spread‐F
Authors: Mathews J. D. , González S. , Sulzer M. P. , Zhou Q.‐H. , Urbina J. , Kudeki E. , Franke S.
Published: February 2004

We report the first incoherent scatter radar observations—made at Arecibo Observatory—of remarkable bottom‐side (<280 km) F‐region sheet‐like plasma structures with horizontal/vertical‐scales of ≲1 km. These structures are an apparent manifestation of spread‐F, appear to give rise to VHF coherent (FAI) scattering, and are clearly electrodynamic in origin involving a form of the Perkins spread‐F instability process in the presence of relatively large E‐fields and horizontal plasma gradients. The net instability process appears to be associated with downwelling features characteristic of the nighttime F‐region. These structures—that may be self‐similar on scales of ∼ 1–100 km—indicate considerable electrodynamic activity in the mid‐latitude F‐region on scales that have profound space‐weather implications especially regarding deep scintillations on earth‐space communications links.
Raindrop sorting induced by vertical drafts in convective clouds
Authors: Kollias Pavlos , Albrecht B. A. , Marks F. D.
Published: February 2004

Evidence of raindrop sorting induced by a convective updraft is presented. Using a vertically pointing 94‐GHz Doppler radar (λ=3.2 mm) and capitalizing on the resonant nature of the backscattering cross‐section as a function of the raindrop size (Mie scattering), the vertical air motions to an accuracy of 0.1 ms−1, and the shape of the raindrop size distribution are retrieved from the Doppler spectra. The interaction of vertical drafts and raindrops is documented for the first time by high resolution radar data. The updraft structure clearly causes horizontal and vertical sorting of the raindrops. In the updraft core, small raindrops (D ≤ 1.7 mm) that have terminal velocities less than the updraft velocities (6–7 ms−1) and a clear absence of drops ≥3 mm are observed. Towards the updraft periphery, a gradual increase in the raindrop sizes is documented where large raindrops (D ≥ 3 mm) are observed. The observations demonstrate the importance of updrafts in distributing the raindrops in space.
The influence of the Sun's magnetic field on energetic particles at high heliospheric latitudes
Authors: Sanderson T. R. , Marsden R. G. , Tranquille C. , Balogh A. , Forsyth R. J. , Goldstein B. E. , Gosling J. T. , Harvey K. L.
Published: February 2004

The first passage of the Ulysses spacecraft to high latitudes was during the declining phase of solar cycle 22. The recent second passage to high latitudes was close to the maximum of solar cycle 23. The axis of the dipolar component of the Sun's magnetic field was close to 90° from the spin axis, and the coronal neutral line extended up to high latitudes. A variable but generally slow solar wind was observed all the way up to the highest latitudes reached by Ulysses, as was the sector structure of the magnetic field. The high‐latitude heliosphere was populated with intensities of energetic particles with energies around 1 MeV several orders of magnitude above background. We show how the changes in the Sun's magnetic field, the coronal holes, and the configuration of the heliosphere could be responsible for the differences between particle observations in the two orbits.
Horizontal transport and the dehydration of the stratosphere
Authors: Holton James R. , Gettelman Andrew
Published: February 2004

The coldest tropopause temperatures occur over the equatorial West Pacific during Northern Hemisphere winter. Horizontal transport through this “cold trap” region causes air parcels that reach the tropopause at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean tropopause temperature. Horizontal transport of water vapor can also explain how a persistent layer of subvisible cirrus can exist near the tropopause in the cold trap even though observations suggest that there is diabatic cooling and subsidence, rather than diabatic heating and rising through the tropopause in this region. Thus, horizontal transport in the tropical transition layer in the vicinity of the tropopause plays an important role in the water balance of the stratosphere.
Seismic wave velocity changes and stress build‐up in the crust of the Kanto‐Tokai Region
Authors: Furumoto Muneyoshi , Ichimori Yuko , Hayashi Nobuhiko , Hiramatsu Yoshihiro , Satoh Takashi
Published: February 2004

We show there are large temporal P‐wave velocity changes in the crust of the Kanto‐Tokai region in Japan from repeated observations of explosions. We find that the variation is the response of the crustal rocks to two components of stress change, the tidal stress and the secular stress accumulation. The inferred annual rate of fractional velocity change (Δν/ν/year) caused by the stress accumulation in the upper crust is of the order of 10−3/year. The results indicate that the build‐up of stress in the crust can be monitored by precise observations of seismic wave velocities.
Velocity field across the Southern Caribbean Plate Boundary and estimates of Caribbean/South‐American Plate Motion using GPS Geodesy 1994–2000
Authors: Pérez Omar J. , Bilham Roger , Bendick Rebecca , Velandia José R. , Hernández Napoleón , Moncayo Carlos , Hoyer Melvin , Kozuch Mike
Published: February 2004

Global Positioning System (GPS) observations between 1994 and 2000 at twenty‐two sites in the Lesser Antilles and northern South‐America indicate that the Caribbean plate, along its southern boundary, slips at a rate of 20.5±2 mm/a with an azimuth of N 84°±2°E at 65°W, relative to the South‐American plate. East of 68° W, 80% of the dextral slip is contained within a 80‐km wide shear zone centered on the El Pilar‐San Sebastián fault system. West of 68° W the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending Boconó fault (9–11 mm/a) in western Venezuelan, and an offshore system near the northern coast.
Probability of a reduction in the formation rate of the subsurface water in the North Pacific during the 1980s and 1990s
Authors: Watanabe Y. W. , Ono T. , Shimamoto A. , Sugimoto T. , Wakita M. , Watanabe S.
Published: February 2004

Comparing the apparent oxygen utilization (AOU) and apparent CFC tracer ages (τ) between extensive decadal reobservation data along 47°N (’85–’99) and 165°E (’87–’00) lines, we found that both AOU and τ markedly increased over the North Pacific between 26.4–27.4 σθ. The observed AOU increase was almost consistent with the AOU increase calculated from observed change of τ. Based on a linear trend of increasing AOU over 30 years (’68–’98) in the subpolar region [Ono et al., 2001], we concluded that the formation rate of the subsurface water in the North Pacific has continuously reduced at least during the last fifteen years. In the North Pacific, the recent uptake rate of oceanic anthropogenic carbon was also estimated as reduced by as much as 10% from the efficiency of anthropogenic carbon absorption in the middle of 1980s.
Eastward propagation of the intraseasonal variability of sea ice and the atmospheric field in the marginal ice zone in the Antarctic
Authors: Baba Kenji , Wakatsuchi Masaaki
Published: February 2004

We investigate the intraseasonal variability of the sea‐ice concentration in the Antarctic marginal ice zone(MIZ), using daily sea‐ice and atmospheric data for eight winters (1990–97). From 5–31 day band‐pass filtered sea‐ice concentration data for the whole sea‐ice region, it is clearly shown that the lower‐latitude MIZ has the maximum intraseasonal variability, taking the form of a narrow, circumpolar belt with alternately increasing and decreasing areas. This coherent pattern propagates eastward as a wave. A similar wave also appeared for the atmospheric field around the MIZ. The eastward propagating waves for sea‐ice and the atmosphere have almost same properties, a wavenumber of 2–4, a period of 10–15 days, and a phase velocity of 10–18 degrees/day. The common propagating signatures suggest a close relationship between the intraseasonal variability of sea‐ice and atmospheric field in the Antarctic MIZ in winter.
Evidence for tides in the subglacial Lake Vostok, Antarctica
Authors: Dietrich R. , Shibuya K. , Pötzsch A. , Ozawa T.
Published: February 2004

The phenomenon of lake tides is investigated for the subglacial Lake Vostok, East Antarctica. A model computation of equilibrium lake tides leads to a maximum lake level variation of about 18mm. To verify the hypothesis of really existing lake tides data from two different observational techniques are analysed. The re‐evaluation of old tidal gravity data from Vostok Station as well as satellite synthetic aperture radar interferometry (InSAR) reveal specific signals which are explained as lake tides. The results of gravimetric tidal observations at Vostok Station from 1969 [Schneider, 1971; Schneider and Simon, 1974] show systematic deviations from the expected solid earth tide signal which correspond to a vertical tidal displacement of 12 and 2 mm amplitude for the waves K1 and M2, respectively. The InSAR data show the areal pattern of a vertical motion at the southern lake tip which is analogous to the grounding zone of ice shelves.
Coseismic interferograms of two MS=6.6 earthquakes in the South Iceland Seismic Zone, June 2000
Authors: Pedersen Rikke , Sigmundsson Freysteinn , Feigl Kurt L. , Árnadóttir Thóra
Published: February 2004

We present InSAR observations of deformation due to two MS=6.6 earthquakes in the South Iceland Seismic Zone in June 2000. Coseismic deformation predominates in the ERS interferograms. Range change, due to mainly right‐lateral strike‐slip on N‐S striking faults, amounting to more than 15 cm is observed, although displacement is mainly perpendicular to the satellite look direction. Using elastic dislocation models in a trial‐and‐error scheme, we find a best‐fitting model that agrees well with the aftershock locations and moment magnitudes estimated from seismograms. The June 17 model has a fault patch 16 km long, 10 km deep, striking N05°E, dipping 86°, with a slip maximum of 2.40 m. The June 21 model has a vertical patch 15 km long, 9 km deep, striking N01°W, with a slip maximum of 2.15 m.
Correlation between surface damage and the coseismic displacement and stress relaxation of the 1999 Chi‐Chi, Taiwan Earthquake
Authors: Shieh Chiou‐Fen , Sheu Shyh‐Yang , Shih Ruey‐Chyuan
Published: February 2004

Various types of damage caused by the devastating 1999 Chi‐Chi, Taiwan earthquake were not only experienced in the hanging wall and footwall but also in the southern and northern parts of the Chelungpu fault. In this paper, we determine the coseismic displacements and determine stress relaxation using a rupture model and compare the values with surface damage. The patterns of damage can be clearly correlated with the calculated coseismic displacement and stress relaxation caused by the event. It is found that the surface damage was predominately caused by coseismic deformation and that large‐scale landslides were mainly a result of steep coseismic displacements.
Quiescent hydrogen sulfide and carbon dioxide degassing from Mount Baker, Washington
Authors: McGee Kenneth A. , Doukas Michael P. , Gerlach Terrence M.
Published: February 2004

Volcanic H2S emission rate data are scant despite their importance in understanding magma degassing. We present results from direct airborne plume measurements of H2S and CO2 on a 21‐orbit survey at eleven different altitudes around Mount Baker volcano in September 2000 utilizing instrumentation mounted in a light aircraft. Measured emission rates of H2S and CO2 were 5.5 td−1 and 187 td−1 respectively. Maximum concentrations of H2S and CO2 encountered within the 4‐km‐wide plume were 75 ppb and 2 ppm respectively. Utilizing the H2S signal as a marker for the plume allows the corresponding CO2 signal to be more easily and accurately distinguished from ambient CO2 background. This technique is sensitive enough for monitoring weakly degassing volcanoes in a pre‐eruptive condition when scrubbing by hydrothermal fluid or aquifers might mask the presence of more acid magmatic gases such as SO2.
A new insight on the cause of magnetic storms
Authors: Lui A. T. Y. , McEntire R. W. , Baker K. B.
Published: February 2004

A long outstanding problem in magnetospheric physics is the cause of magnetic storms. Traditionally, the ring current buildup is attributed to the accumulated effects of energetic magnetospheric particles injected by frequent occurrence of magnetospheric substorms. Recent studies, however, suggest enhanced magnetospheric convection as the sole cause. We employ for the first time the relatively new remote‐sensing technique based on global monitoring of energetic neutral atom (ENA) emissions to address this controversy. The ENA observations were combined with the ground‐based magnetic indices for substorm activity and ring‐current development as well as with the global monitor of magnetospheric convection in the ionosphere by the network of SuperDARN radars. The ring current is shown to intensify during enhanced convection without substorm occurrence and during a substorm with a reduction in convection. Therefore, both substorms and enhanced convection contribute to the buildup of the storm‐time ring current.
Contamination‐induced particle production during balloon flights: Origin for unexpected ice particle observations in the Arctic?
Authors: Schiller Cornelius , Deshler Terry , Peter Thomas
Published: February 2004

Stratospheric air in the vicinity of a balloon payload have been found to be contaminated with H2O mixing ratios of 10–80 ppmv during ascent, float, and slow descent at altitudes above 20 km. The contamination appeared in brief events often coincident with the occurrence of micron‐sized particles and with increased temperatures. The observed H2O mixing ratios may result in ice particle production within 10–60 s in the lower stratosphere, even at temperatures around 200 K, i.e. well above the frost point of the ambient air. Thus, plumes of H2O‐contaminated air must be seriously considered as an explanation for balloon‐borne observations in the past, which have been interpreted as apparent thin layers of large ice particles above the frost point. These possible artifacts reduce the weight of evidence for NAT‐coated ice particles in the stratosphere at temperatures above the frost point.
Impact of improved near infrared water vapor line data on absorption of solar radiation in GCMs
Authors: Bennartz Ralf , Lohmann Ulrike
Published: February 2004

Recently, two new water vapor line absorption datasets have become available. First, the HITRAN 2000 dataset provides updated information on several water vapor absorption lines in the near infrared. Secondly, an independent study initiated by the European Space Agency (ESA) created a new line dataset for the spectral range between 8592cm−1 and 20000cm−1. We provide the necessary coefficients to update the parameterization of near infrared absorption in the ECHAM4 global climate model based on these new data and investigate its impact on the absorption of solar radiation in ECHAM4. We find that compared to the original parameterization, which is based on HITRAN‐92, the water vapor absorption is increased by an amount of 0.2 % to 2.0 % depending on absorber mass and the dataset used for calculating the coefficients. The new parameterizations lead to a globally and annually averaged increase in absorption of solar radiation in the GCM of 3.4 W/m² and 3.8 W/m², respectively.
Water vapor control at the tropopause by equatorial Kelvin waves observed over the Galápagos
Authors: Fujiwara M. , Hasebe F. , Shiotani M. , Nishi N. , Vömel H. , Oltmans S. J.
Published: February 2004

Soundings of frost‐point hygrometers, ozonesondes, and radiosondes at San Cristóbal Island (0.9°S, 89.6°W) in September 1998 provide an observational evidence that equatorial Kelvin waves around the tropopause act as a dehydration pump for the stratosphere. During the downward‐displacement phase of a Kelvin wave, dry and ozone‐rich stratospheric air is transported into the upper troposphere. During the upward‐displacement phase, on the other hand, higher specific‐humidity air moves up in the tropopause region, but at the same time, this upward motion causes cooling of the air that limits the water vapor amount entering the stratosphere. Also, wave breaking contributes to the irreversible transport of ozone across the tropopause. Considering their omnipresence at the equatorial tropopause, we suggest that Kelvin waves may be one of the important agents for maintaining the dryness of the tropical lower stratosphere.
Simulation of tropospheric ozone changes during 1997–1998 El Niño: Meteorological impact on tropospheric photochemistry
Authors: Sudo Kengo , Takahashi Masaaki
Published: February 2004

The impact of the 1997–1998 El Niño event on the tropospheric ozone distribution and related processes is assessed by a simulation using a global chemical model. The changes in tropospheric ozone observed during the 1997–1998 El Niño are considered to have been caused by large‐scale processes due to the shift in the tropical convection pattern associated with sea surface temperature (SST) changes and by large emissions of ozone precursors due to pronounced biomass burning in the Indonesian region (mainly in Sumatra and Borneo). This simulation study is focused on the effects of meteorological changes caused by the El Niño, aside from the effects of the Indonesian biomass burning. Our simulation results show that about half of the ozone increase observed over the equatorial western Pacific and Indonesia (6–12 Dobson units in tropospheric column ozone) can be explained by the meteorological effects of the El Niño with the remainder attributed to the Indonesian biomass burning, whereas the ozone decrease over the eastern Pacific (4–8 Dobson units) can be attributed to the meteorological effects alone. Moreover, our study suggests that a change in photochemical processes (e.g., water vapor concentration controlling the lifetimes of ozone and other related species, and wet scavenging associated with convective precipitation) also plays an important role in the tropical ozone distribution during the course of the 1997–1998 El Niño event.
Longwave heating of the tropical lower stratosphere
Authors: Norton W. A.
Published: February 2004

The distribution of diabatic heating is diagnosed from ECMWF analyses. In the upper troposphere, there is heating in regions of deep convection particularly in the Western Pacific. However at 70 hPa in the lower stratosphere, heating occurs in the subtropics and in the Eastern Pacific while cooling (or only weak heating) occurs in the Western Pacific. Comparison is made with the distribution of outward longwave radiation and this shows strong similarity to the diabatic heating in the lower stratosphere. This suggests that longwave heating from absorption by ozone of upward radiation in the 9.6 µm band is important in determining the distribution of heating rates in the tropical lower stratosphere. The distribution of upward longwave radiation is strongly modulated by the distribution of convectively generated high‐level cirrus clouds. This produces anticorrelation between heating rates in the tropical troposphere and lower stratosphere.
Sequence of strong intraplate earthquakes in the Kodiak Island Region, Alaska in 1999–2001
Authors: Ratchkovski Natalia A. , Hansen Roger A.
Published: February 2004

A sequence of strong earthquakes was registered in 1999–2001 in the Kodiak Island region of the Alaska‐Aleutian subduction zone. Two Mw 7 earthquakes occurred in December, 1999 and January, 2001 and an Mw 6.5 event occurred in July, 2000. These events and their aftershocks recorded by the regional seismograph network were relocated using a Joint Hypocenter Determination (JHD) method. Regional broadband data were used to obtain seismic moment tensors for the main shocks and their largest aftershocks. Relocation and moment tensor inversion results indicate that these events originated inside the subducting Pacific plate. The focal mechanisms indicate down‐dip tension with the fault planes being nearly vertical and parallel to the strike direction of the subducting plate. The 1999 and 2000 events were located down‐dip of the locked portion of the megathrust, while the 2001 event was located directly beneath it.
Snowpack photochemical production of HONO: A major source of OH in the Arctic boundary layer in springtime
Authors: Zhou Xianliang , Beine Harald J. , Honrath Richard E. , Fuentes Jose D. , Simpson William , Shepson Paul B. , Bottenheim Jan W.
Published: February 2004

Both snow manipulation experiments and ambient measurements during the Polar Sunrise Experiment 2000 at Alert (Alert2000) indicate intensive photochemical production of nitrous acid (HONO) in the snowpack. This process constitutes a major HONO source for the overlying atmospheric boundary layer in the Arctic during the springtime, and sustained concentrations of HONO high enough that upon photolysis they became the dominant hydroxyl radical (OH) source. This implies a much greater role for OH radicals in Arctic polar sunrise chemistry than previously believed. Although the observations were made in the high Arctic, this finding has a significant implication for the boundary layer atmospheric chemistry in Antarctica during sunlit seasons and in the mid to high latitudes of the Northern Hemisphere during the winter and spring seasons when approximately 50% of the land mass may be covered by snow.
Spectral reflectance of oceanic whitecaps in the visible and near infrared: Aircraft measurements over open ocean
Authors: Nicolas Jean‐Marc , Deschamps Pierre‐Yves , Frouin Robert
Published: February 2004

The spectral reflectance of oceanic whitecaps in the visible and near infrared was derived from aircraft push‐broom radiometer observations over the open ocean near Toulon, France. Inter‐band calibration corrections were performed using spectrally white sun glint reflectance. Whitecap reflectance was found to decrease substantially with wavelength in the near infrared, in agreement with other findings in the coastal zone and the open ocean. The study supports previous conclusions regarding the consequences of neglecting the spectral dependence of whitecap reflectance in ocean color algorithms.
Annular modes in global daily surface pressure
Authors: Baldwin Mark P.
Published: February 2004

Annular modes are patterns characterized by synchronous fluctuations in surface pressure of one sign over the polar caps and the opposite sign at lower latitudes. The Southern Annular Mode (SAM) and Northern Annular Mode (NAM, also called the Arctic Oscillation) patterns are the leading empirical orthogonal functions (EOFs) of slowly‐varying, hemispheric, cold‐season, sea‐level pressure anomalies (deviations from climatology). Daily indices of the SAM and NAM are a measure of the similarity between surface pressure anomaly patterns and the annular modes. Here it is shown that the first two EOF time series of daily, global, year‐round, zonally‐averaged surface pressure are nearly identical to the SAM and NAM indices. Together they account for more than 57% of the daily variance of zonally‐averaged surface pressure. The SAM and NAM patterns extend through the tropics, well into the opposite hemispheres. Fluctuations of the SAM and NAM indices are accompanied by interhemispheric transfer of mass.
Vertical distributions of iron(III) hydroxide solubility and dissolved iron in the northwestern North Pacific Ocean
Authors: Nakabayashi Shigeto , Kusakabe Masashi , Kuma Kenshi , Kudo Isao
Published: February 2004

Detailed vertical distributions of Fe(III) hydroxide solubilities and dissolved Fe concentrations, which are strongly related to the concentration and affinity of natural organic Fe(III) chelators in seawater, were measured at three typical stations in the northwestern North Pacific Ocean. Iron(III) hydroxide solubility in the surface mixed layer was generally high and variable (0.3‐2.4 nM), corresponding with the depth of high chlorophyll a concentrations, and the solubility minima (0.2‐0.4 nM) occurred at 75‐125 m depth. The vertical profiles of Fe(III) hydroxide solubility in mid‐depth and deep waters are characterized by mid‐depth maxima (0.6‐0.7 nM) and, subsequently, a slight decrease to 0.4‐0.5 nM with depth, which is markedly similar to nutrient and dissolved Fe depth profiles. The solubility profiles reveal that dissolved Fe concentrations in deep ocean waters are controlled primarily by the Fe(III) complexation with natural organic ligands, which were released through the oxidative decomposition and transformation of biogenic organic matter in mid‐depth and deep waters.
Homogeneous vs heterogeneous subduction zone models: Coseismic and postseismic deformation
Authors: Masterlark T. , DeMets C. , Wang H. F. , Sánchez O. , Stock J.
Published: February 2004

A finite‐element model (FEM) incorporating geologic properties characteristic of a subduction zone is compared with FEMs approximating homogeneous elastic half‐spaces (HEHS)s to investigate the effect of heterogeneity on coseismic and postseismic deformation predictions for the 1995 Colima‐Jalisco Mw =8.0 earthquake. The FEMs are used to compute a coefficient matrix relating displacements at observation points due to unit dislocations of contact‐node pairs on the fault surface. The Green's function responses are used to solve the inverse problem of estimating dislocation distributions from coseismic GPS displacements. Predictions from the FEM with heterogeneous material properties, loaded with either of the HEHS dislocation distributions, significantly overestimate coseismic displacements. Postseismic deformation predictions are also sensitive to the coseismic dislocation distribution, which drives poroelastic and viscoelastic relaxation. FEM‐generated Green's functions, which allow for spatial variations in material properties, are thus preferable to those that assume a simple HEHS because the latter leads to dislocation distributions unsuitable for predicting the postseismic response.
Observation of low‐depolarization contrails at Florence (Italy) using a 532–1064 nm polarization LIDAR
Authors: Del Guasta Massimo , Niranjan Kandula
Published: February 2004

Contrail backscatter and polarization measurements made using a 532–1064 nm LIDAR in Florence (Italy) indicated that, in most of the cases, the contrail depolarisation and color ratio resemble the usual cirrus. In 20% of the cases, a depolarisation below 15% was observed, both in isolated contrails and in cirrus‐embedded contrails. In a few cases, a very low depolarisation, down to 3%, was observed at temperatures in the −40° to −65° C range. In two measurements, a low depolarization was coupled with a high color ratio in the 2–4 range. The low‐depolarisation, high color ratio contrail observations are discussed in the light of the results of recent T‐matrix scattering simulations. The two events lasted at least 10–20 minutes in the LIDAR data, suggesting a very slow growth or no growth at all of the initial micron‐sized contrail particles in the large ice crystals typical of cirrus.
Dynamic instability in tropospheric photochemistry: An excitability threshold
Authors: Tinsley Mark R. , Field Richard J.
Published: February 2004

Dynamic equations describing photoxidation of tropospheric chemical pollutants are nonlinear, containing complex feedback loops. Such nonlinearity is known to give rise to various dynamical instabilities including multiple steady states, oscillation, and even chaos. A related type of instability, excitability, is demonstrated here using a two‐variable (reduced from six) model of CH4photoxidation in which perturbation of a stable but excitable steady state beyond a threshold is dramatically amplified before the steady state is reapproached. Such switching/amplification responses may have important implications for atmospheric/climatic modeling. A phase‐plane analysis describes the origin of this excitability and suggests that it may be a relatively common phenomenon in environmental models.
Diffuse emission of carbon dioxide from Cerro Negro Volcano, Nicaragua, Central America
Authors: Salazar José M. L. , Hernández Pedro A. , Pérez Nemesio M. , Melián Gladys , Álvarez Julio , Segura Fabio , Notsu Kenji
Published: February 2004

We report the first detailed soil CO2 efflux survey carried out at Cerro Negro volcano, Nicaragua, Central America. Soil CO2 efflux values ranged from 0.5 to 35,000 g m−2 d−1. Spatial distribution of CO2 efflux shows that volcanic gas is mainly released from the summit crater and the fissures opened during the August 1999 eruption. The total CO2 output of Cerro Negro is conservatively estimated to be 2,800 t d−1, based on a study area of 0.58 Km². Contour maps of soil CO2 efflux, ground temperature, and δ13C(CO2) distributions support an important magmatic contribution for the total output of CO2 at Cerro Negro volcano.
Crustal heat production and the thermal evolution of Mars
Authors: McLennan Scott M.
Published: February 2004

The chemical composition of soils and rocks from the Pathfinder site and Phobos‐2 orbital gamma‐ray spectroscopy indicate that the Martian crust has a bulk composition equivalent to large‐ion lithophile (LIL) and heat‐producing element (K, Th, U) enriched basalt, with a potassium content of about 0.5%. A variety of radiogenic isotope data also suggest that separation of LIL‐enriched crust and depleted mantle reservoirs took place very early in Martian history (>4.0 Ga). Accordingly, if the enriched Martian crust is ≥30 km thick it is likely that a large fraction (up to at least 50%) of the heat‐producing elements in Mars was transferred into the crust very early in the planet's history. This would greatly diminish the possibility of early widespread melting of the Martian mantle.
Southern Antarctic Circumpolar Current Front to the northeast of South Georgia: Horizontal advection of krill and its role in the ecosystem
Authors: Murphy E. J. , Watkins J. L. , Meredith M. P. , Ward P. , Trathan P. N. , Thorpe S. E.
Published: February 2004

During December 2000 and January 2001 we conducted a high‐resolution hydrographic and bioacoustic transect (RRS James Clark Ross cruise 57) that extended across the South Georgia shelf from close to Cumberland Bay, across the shelf break and slope and into the deep waters of the Georgia Basin beyond. We observed a high biomass of zooplankton between 53.8° and 53.4°S associated with the inshore, northwestward flow of the Southern Antarctic Circumpolar Current Front (SACCF) that occurred in around 2500 m of water close to the base of the slope. There was very little zooplankton biomass present in the more offshore, eastward flowing waters where a second manifestation of the SACCF was also present on the section. The region of enhanced zooplankton biomass was over 50 km in horizontal extent with the highest densities (>10 g m−3) in the area of strongest flow (>35 cm s−1). The majority of the zooplankton present on the section was Antarctic krill and most of it occurred in the upper 100 m. The rate of physically mediated transport of Antarctic krill across the off‐shelf sections (∼10 km) of the transect showed marked variation, with highest rates (>106 g s−1) associated with the northwestward flow of the SACCF. Farther offshore, where the krill biomass and flow rates were much reduced, the flux of krill was very low. The integrated horizontal flux of krill across the offshore sections was large (192 × 103 t d−1) and to the northwest. A second occupation of the transect showed that the krill flux is highly variable, and we discuss the various physical and biological factors that will generate such variability. We show that horizontal flux of krill in ocean currents is a major factor in determining the abundance of krill around South Georgia.
Radiative forcing ‐ measured at Earth's surface ‐ corroborate the increasing greenhouse effect
Authors: Philipona Rolf , Dürr Bruno , Marty Christoph , Ohmura Atsumu , Wild Martin
Published: February 2004

The Intergovernmental Panel of Climate Change (IPCC) confirmed concentrations of atmospheric greenhouse gases and radiative forcing to increase as a result of human activities. Nevertheless, changes in radiative forcing related to increasing greenhouse gas concentrations could not be experimentally detected at Earth's surface so far. Here we show that atmospheric longwave downward radiation significantly increased (+5.2(2.2) Wm−2) partly due to increased cloud amount (+1.0(2.8) Wm−2) over eight years of measurements at eight radiation stations distributed over the central Alps. Model calculations show the cloud‐free longwave flux increase (+4.2(1.9) Wm−2) to be in due proportion with temperature (+0.82(0.41) °C) and absolute humidity (+0.21(0.10) g m−3) increases, but three times larger than expected from anthropogenic greenhouse gases. However, after subtracting for two thirds of temperature and humidity rises, the increase of cloud‐free longwave downward radiation (+1.8(0.8) Wm−2) remains statistically significant and demonstrates radiative forcing due to an enhanced greenhouse effect.
Resonances of a volcanic conduit triggered by repetitive injections of an ash‐laden gas
Authors: Molina Indira , Kumagai Hiroyuki , Yepes Hugo
Published: February 2004

A swarm of long‐period (LP) seismic events was recorded in December 2001 before heightened eruptive activity of Tungurahua Volcano, an andesitic stratovolcano in Ecuador. The LP events showed simple decaying harmonic oscillations with frequencies ranging from 2 to 3.5 Hz and quality factors (Q) significantly larger than 100. Our spectral analysis of the LP events identified systematic temporal variations in both frequency and Q of the LP events. Assuming a crack resonator at the source of LP events, the temporal variations can be consistently explained by increasing the ash content within the resonator, which may be caused by repetitive injections of an ash‐laden gas into a pre‐existing crack in the conduit as a preparatory process for eruptions.
Subsurface heat flow in an urban environment
Authors: Ferguson Grant , Woodbury Allan D.
Published: February 2004

The subsurface temperature field beneath Winnipeg, Canada, is significantly different from that of the surrounding rural areas. Downward heat flow to depths as great as 130 m has been noted in some areas beneath the city and groundwater temperatures in a regional aquifer have risen by as much as 5°C in some areas. Numerical simulation of heat transport supports the conjecture that these temperature changes can be largely attributed to heat loss from buildings and the temperature at any given point is sensitive to the distance from and the age of any buildings. The effect is most noticable when buildings are closely spaced, which is typical of urban areas. Temperature measurements in areas more than a few hundred meters away from any heated structure were only a few tenths of a degree Celsius greater than those observed outside the city, suggesting that other reasons for increases in subsurface temperature, such as changes in surface cover or climate change, may be responsible for some of the some of the observed increase in temperatures. These sources of additional heat to the subsurface make it difficult to resolve information on past climates from temperatures measured in boreholes and monitoring wells. In some areas, the temperature increases may also have an impact on geothermal energy resources. This impact might be in the form of an increase in heat pump efficiency or in the case of the Winnipeg area, a decrease in the efficiency of direct use of groundwater for cooling.
Role of land surface processes in monsoon development: East Asia and West Africa
Authors: Xue Yongkang , Juang H.‐M. H. , Li W.‐P. , Prince S. , DeFries R. , Jiao Y. , Vasic R.
Published: February 2004

Evidence is presented that exchanges of water and energy between the vegetation and the atmosphere play an important role in east Asian and West African monsoon development and are among the most important mechanisms governing the development of the monsoon. The results were obtained by conducting simulations for five months of 1987 using a general circulation model (GCM) coupled with two different land surface parameterizations, with and without explicit vegetation representations, referred to as the GCM/vegetation and the GCM/soil, respectively. The two land surface models produced similar results at the planetary scale but substantial differences at regional scales, especially in the monsoon regions and some of the large continental areas. In the simulation with GCM/soil, the east Asian summer monsoon moisture transport and precipitation were too strong in the premonsoon season, and an important east Asian monsoon feature, the abrupt monsoon northward jump, was unclear. In the GCM/vegetation simulation, the abrupt northward jump and other monsoon evolution processes were simulated, such as the large‐scale turning of the low‐level airflow during the early monsoon stage in both regions. With improved initial soil moisture and vegetation maps, the intensity and spatial distribution of the summer precipitation were also improved. The two land surface representations produced different longitudinal and latitudinal sensible heat gradients at the surface that, in turn, influenced the low‐level temperature and pressure gradients, wind flow (through geostrophic balance), and moisture transport. It is suggested that the great east‐west thermal gradient may contribute to the abrupt northward jump and the latitudinal heating gradient may contribute to the clockwise and counterclockwise turning of the low‐level wind. The results showed that under unstable atmospheric conditions, not only low‐frequency mean forcings from the land surface, such as monthly mean albedo, but also the perturbation processes of vegetation were important to the monsoon evolution, affecting its intensity, the spatial distribution of precipitation, and associated circulation at the continental scale.
An optimal control least squares method for solving coupled flow‐transport systems
Authors: Zeitoun David G. , Pinder George F.
Published: February 2004

Modeling the transport of groundwater contamination leads generally to problems wherein the continuity equation is coupled with the contaminant transport equation and Darcy's law. Such a system of equations is called the flow‐transport system and is difficult to solve numerically. An optimal control, H1, least squares formulation, associated with a conjugate gradient algorithm, is proposed for solving the coupled flow and transport problem. At each time step, each discretized differential equation is transformed into an optimal control problem. This problem is then stated as an equivalent minimization problem. The objective function associated with the transport problem is chosen in such a way that it accurately represents the advective behavior of the equation for high values of the Peclet number. The Nazareth version of the conjugate gradient method is used for solving the problem iteratively. At each iteration, only the right‐hand side of the equation is changed. Thus, the approach is attractive when large‐scale problems must be solved. Analysis of the preconditioning of the coefficient matrix provides insight into the behavior of this approach. Validation runs for a one‐dimensional example show accurate results for a wide range of Peclet and Courant numbers.
Transient flow of water to a well in an unconfined aquifer: Applicability of some conceptual models
Authors: Narasimhan T. N , Zhu Ming
Published: February 2004

Currently available methods for interpreting data from unconfined aquifers are based on analytical solutions which restrict attention to flow in the saturated zone and account for drainage of water from the unsaturated zone through a source term. The parameter, specific yield (Sy), is assumed to dictate the total quantity of water that is derivable through the desaturation process. It is commonly assumed that flow in the unsaturated zone has little effect on flow in the saturated zone and therefore that all the drainable water is instantly delivered at the water table as it declines in response to pumping. In a model proposed in 1954, Boulton empirically assumed that the drainable water associated with Sy is released gradually at the water table as an exponential function of time. This concept of Boulton is similar to the notion of first‐order kinetics frequently used to handle chemical transformations. Boulton's model is considered by some to have no physical basis. Numerical experiments performed on sand columns with reasonable properties suggest that vertical drainage of water at the water table due to a falling water table is a time‐dependent process which is mathematically more complex than a simple exponential relation. Although Boulton did not provide a rational physical explanation, his exponential assumption has some merit in that it implicitly provides for a time dependent drainage process which seems to occur in the unsaturated zone. However, the simple exponential approximation is not adequate to account for what is seemingly a complex process. It appears that a physically comprehensive model of radial flow in an unconfined aquifer will combine time‐dependent drainage from above the water table with vertical components of flow in the saturated zone. An additional assumption frequently made is that the well can be treated as a line source. Results from numerical experiments suggest that caution is in order before neglecting effects of well‐bore storage in interpreting data from tests on unconfined aquifers.