bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanisms for discrete auroral arc breakup by nonlinear Alfvén wave interaction
Authors: Kinney R. M. , Coroniti F. V. , McWilliams J. C. , Pritchett P. L.
Published: August 2003

The breakup of discrete auroral arcs on closed field lines is modeled by numerical calculations of nonlinear magnetohydrodynamics in a cavity. The boundary conditions are shown to be crucially important in determining the available mechanisms for nonlinear evolution and breakup of the arc. Nonlinear evolution is inhibited if the sign of the electric field is reversed in a reflecting Alfvén wave. The weaker the shear time of the arc compared with the Alfvén wave travel time across the cavity, the more strongly the nonlinear evolution is inhibited. Because different transverse length scales have different reflection properties related to their ability to pass through the auroral acceleration region, arcs with small horizontal scales are subject to the Kelvin‐Helmholtz shear flow instability while large horizontal scales are not. Parallels are drawn to the observed breakup phenomenology of small‐scale and large‐scale arcs.
Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F
Authors: Fejer B. G. , Scherliess L. , de Paula E. R.
Published: August 2003

We use radar observations from the Jicamarca Observatory from 1968 to 1992 to study the effects of the F region vertical plasma drift velocity on the generation and evolution of equatorial spread F. The dependence of these irregularities on season, solar cycle, and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts. In the early night sector, the bottomside of the F layer is almost always unstable. The evolution of the unstable layer is controlled by the history of the vertical drift velocity. When the drift velocities are large enough, the necessary seeding mechanisms for the generation of strong spread F always appear to be present. The threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux. The geomagnetic control on the generation of spread F is season, solar cycle, and longitude dependent. These effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields. The occurrence of early night spread F decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset. The generation of late night spread F requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour. These irregularities occur most often at ∼0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes. The occurrence of late night spread F is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux.
Two‐dimensional time‐domain volume integral equations for scattering of inhomogeneous objects
Authors: Wang Jianguo , Fan Ruyu
Published: August 2003

This paper proposes a time‐domain volume integral equation based method for analyzing the transient scattering from a two‐dimensional inhomogeneous cylinder by invoking the volume equivalence principle for both the transverse magnetic and electric cases. The cylinder is discretized into triangular cells, and the electric flux is chosen as the unknown. For the transverse magnetic case, the electric flux is defined on the surfaces of the triangles. For the transverse electric case, because of the electric charges induced inside and on the surface of the cylinder, the electric flux is defined on the edges of the triangles, and expanded in space in terms of two‐dimensional surface roof‐top basis functions. The time‐domain volume integral equation is solved by using a marching‐on‐in‐time scheme. Numerical results obtained using this method are in excellent agreement with the data obtained using the finite‐difference time‐domain method.
Radiation efficiency analysis of submillimeter‐wave receivers based on a modified spectral domain integration technique
Authors: Vaupel Thomas , Hansen Volkert , Schäfer Frank
Published: August 2003

Reflector backed submillimeter‐wave slot antennas on substrate lenses are promising candidates for novel astronomical applications in integrated receivers for the far infrared region. Using an adjustable reflector, considerable resonance frequency tuning can be achieved, but with the risk of exciting parasitic parallel plate wave modes. Employing a surface/volume integral equation method and advanced spectral domain integration techniques, a complete radiation efficiency analysis of such kind of receivers is performed. Convenient integration path deformations in complex wavenumber planes allow a very fast evaluation of the spectral domain integrals in combination with efficient database and asymptotic extraction techniques. Saddle point and residue calculation techniques are employed to accurately determine the power of each parallel plate mode and the space wave contributions. The simulations reveal an appropriate set of parameters reducing the radiation loss to less than 10 percent for doubleslot and quadratic slot geometries preserving high tuning capability and optimized radiation patterns as well.
Evolution of amphibolite‐facies structural features and boundary conditions for deformation during exhumation of high‐ and ultrahigh‐pressure rocks, Nordøyane, Western Gneiss Region, Norway
Authors: Terry Michael P. , Robinson Peter
Published: August 2003

Geologic mapping and structural analysis of Baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high‐pressure rocks, where strain partitioning has preserved evidence for interpreting the evolution of late structural features. The earliest of these were extensional detachments juxtaposing eclogite‐facies rocks against overlying amphibolite‐facies rocks that show no evidence for eclogite‐facies metamorphism. These early detachments are strongly overprinted and complexly folded, and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels. Younger more localized mylonite zones formed synchronously with tubular, sheath, isoclinal, tight and open folding that shows a progression from WNW to ENE trends. The earliest mylonite zones, interpreted as originally subhorizontal, range in strike through a 20° angle from 110° to 90°. Later steeply dipping mylonite zones, formed under lower amphibolite‐facies conditions, strike 75° and locally truncate earlier structures. The youngest mylonite zones, formed under lowest amphibolite conditions, strike 50° and truncate all earlier structures. Folds developed during this progression show the range in orientation from WNW to ENE reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing. These changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation. L > S fabrics, absence of axial planar foliation, and chaotic orientations of axial surfaces of granulite to amphibolite‐facies folds indicate formation in a constrictional strain field. This is also supported by estimates of the finite strain accumulated at ∼780°C and 45 km and similar observations by previous workers. Assuming a simple monoclinic deformation for transtension, strain estimates and structural measurements indicate apparent transtensional angles of 9–11° that increased to greater than 20° and then decreased to less than 20°. These changes in transtensional angle agree with changes in the orientation of the X‐Y plane of strain during exhumation from 45 km to less than ∼20 km depth and appear to reflect changes in the boundary conditions of deformation. This upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of HP and UHP metamorphic rocks.
Slab‐derived boron isotope signatures in arc volcanic rocks from the Central Andes and evidence for boron isotope fractionation during progressive slab dehydration
Authors: Rosner Martin , Erzinger Joerg , Franz Gerhard , Trumbull Robert B.
Published: August 2003

Late Miocene to Quaternary volcanic rocks from the frontal arc to the back‐arc region of the Central Volcanic Zone in the Andes show a wide range of δ11B values (+4 to −7 ‰) and boron concentrations (6 to 60 ppm). Positive δ11B values of samples from the volcanic front indicate involvement of a 11B‐enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust‐mantle origin for these lavas. The δ11B values and boron concentrations in the lavas decrease with increasing depth of the Wadati‐Benioff Zone. This across‐arc variation in δ11B values and decreasing B/Nb ratios from the arc to the back‐arc samples are attributed to the combined effects of boron‐isotope fractionation during progressive dehydration in the slab and a steady decrease in slab‐fluid flux toward the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Three‐component mixing calculations for slab‐derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab‐fluid component dominates the boron composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30% crustal material. Modeling of fluid‐mineral boron‐isotope fractionation as a function of temperature shows that dehydration reactions liberate continuously changing fluid compositions from the slab during progressive subduction. A combination of a boron‐isotope fractionation model and a temperature model for the Central Andean subduction zone fits the across‐arc variation in δ11B and we conclude that the boron‐isotope composition of arc volcanic rocks, especially in island arcs, is dominated by changing δ11B‐composition of boron‐rich slab‐fluids during progressive dehydration. Owing to the decrease in slab‐derived fluid flux crustal contamination becomes more important toward the back‐arc. Because of the boron‐isotope fractionation effect, across‐arc variations in δ11B need not necessarily reflect different mixing proportions between boron derived from the slab‐fluid and the mantle wedge.
Constraints on deformation conditions and the origin of oceanic detachments: The Mid‐Atlantic Ridge core complex at 15°45′N
Authors: Escartín J. , Mével C. , MacLeod C. J. , McCaig A. M.
Published: August 2003

Deformed rocks sampled from a corrugated detachment fault surface near the Mid‐Atlantic Ridge (15°45′N) constrain the conditions of deformation and strain localization. Samples recovered in situ record deformation restricted to the cold (shallow) lithosphere (greenschist facies), with no evidence for significant high‐temperature deformation either at the fault zone or in the footwall near it. High‐temperature deformation (∼720–750°C) is observed only at two sites, and cannot be directly linked to the detachment. Detachment faulting was coeval with dyke intrusions that cross cut it, as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones, and by the presence of chill margins in diabase against fault rock. Basalts are very scarce and restricted to clasts in breccias, with no evidence of pillows or extrusive structures. Gabbros crop out along mass‐wasted and fault scarps structurally below the detachment. Footwall rocks show little or no deformation, due to strain localization along a narrow shear zone (<200 m) with fluid flow, as required to form talc‐ and amphibole schists after an ultramafic protolith. We speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time. Our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold (shallow) lithosphere is applicable to other corrugated surfaces along slow‐ and intermediate‐spreading ridges. These observations preclude detachment models rooting in melt‐rich zones (i.e., Atlantis Bank, Southwest Indian Ridge) or recording high‐temperature deformation. We infer that oceanic detachment faults (1) localize strain at T < 500–300°C, (2) persist during active magmatism, and (3) root at shallow rheological boundaries, such as a melt‐rich zone or magma chamber (“hot” detachments) or an alteration front (“cold” detachments).
Hf isotope constraints on mantle sources and shallow‐level contaminants during Kerguelen hot spot activity since ∼120 Ma
Authors: Ingle Stephanie , Weis Dominique , Doucet Sonia , Mattielli Nadine
Published: August 2003

We report new Hf isotopic data for basalts from the Kerguelen large igneous province (LIP) obtained using high‐precision multicollector inductively coupled mass spectrometry (MC‐ICP‐MS) analysis. All drill sites from the southern Indian Ocean Kerguelen Plateau–Broken Ridge, in addition to two volcanic suites from the Kerguelen Archipelago, are represented. These new data are integrated with other recently reported geochemical data for this LIP. We examine the geochemical signatures of the mantle sources and shallow‐level contaminants present during the past ∼120 Ma history of Kerguelen hot spot activity. Our results highlight the contribution of distinct mantle source compositions during Cretaceous (Kerguelen Plateau and Broken Ridge) and Cenozoic (northernmost Kerguelen Plateau and Kerguelen Archipelago) magmatism arising from melting of the Kerguelen plume head and plume tail, respectively. The Cretaceous Kerguelen plume basalts have primitive mantle‐like Pb, Sr, and Nd isotopic compositions and moderately depleted Hf isotopic compositions and are different from the Cenozoic plume basalts, which extend to more radiogenic Pb isotopic compositions. Neodymium and Hf isotopes are decoupled in Kerguelen plume‐derived rocks, and this, in combination with their Pb isotopic compositions, implies that the Kerguelen plume contains small amounts of ancient pelagic sediment mixed with old, recycled enriched oceanic crust. Different contributions from pelagic sediments relative to oceanic crust are able to account for the distinction between the isotopic compositions of the Cretaceous and Cenozoic mantle plume sources. Assimilation of shallow‐level continental crust, left stranded in the Indian Ocean during Gondwana breakup, by plume‐derived magmas was the dominant process recorded in Cretaceous Kerguelen Plateau basalts. During the Cenozoic, magmas from the Kerguelen plume mixed to varying degrees with local, Indian Ocean depleted upper mantle and assimilated the overlying Cretaceous Kerguelen Plateau lithosphere. Despite the geochemical heterogeneity of the Kerguelen LIP, we find evidence for a finite number of components involved in the genesis of Kerguelen Plateau–Broken Ridge and Kerguelen Archipelago rocks.
High‐temperature seawater circulation throughout crust of oceanic ridges: A model derived from the Oman ophiolites
Authors: Nicolas Adolphe , Mainprice David , Boudier Françoise
Published: August 2003

We describe here the high‐temperature (HT ∼ 700°C) and very high temperature (VHT ∼ 1000 °C) recharge and discharge systems which are responsible for HT‐VHT hydrothermal alterations at the oceanic ridge of origin in the crust of Oman ophiolites. The recharge system is a regular network of parallel microcracks, a fraction of a millimeter wide, which carries seawater to the walls of the magma chamber where hydrous melting is triggered. The discharge system is a network of clinopyroxene, pargasite gabbroic dikes issued from the hydrous melting; upsection, these HT‐VHT gabbroic dikes grade into the well‐documented low‐temperature (LT) (400–500°C) hydrothermal veins. Microcracks, gabbroic dikes, and veins are preferentially parallel to the diabase‐sheeted dikes and normal to magmatic lineations in enclosing gabbros. Thus the fractures of both the recharge and discharge systems are parallel to the paleoridge plane. In the brittle field, fracturing is induced by tensional stress related to seafloor spreading. In the ductile field and up to temperatures of ∼900°C, opening of microcracks is ascribed to the anisotropy of thermal contraction during gabbros cooling. Gabbro fabrics and anisotropy are such that the induced microcracks are also parallel to the ridge plane. Thermal contraction between 1200°C (temperature of magma chamber) and ∼700°C (brittle‐ductile limit) produces a differential strain of 0.1% comparable to that deduced from width and spacing of microcracks in the gabbros. Simple estimates on activity time, fluid velocity, and fluid flux in microcracks account for all available data and suggest that in microcracks, activity was short lived and episodic.
Atmospheric and oceanic excitation of the Earth's wobbles during 1980–2000
Authors: Gross Richard S. , Fukumori Ichiro , Menemenlis Dimitris
Published: August 2003

Because of the action of various geophysical excitation mechanisms, the Earth does not rotate about its figure axis, so it wobbles as it rotates. Here, the effectiveness of atmospheric and oceanic processes in exciting the Earth's wobbles during 1980–2000 is evaluated using estimates of atmospheric angular momentum from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project and estimates of oceanic angular momentum from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium's simulation of the general circulation of the oceans. On intraseasonal timescales, atmospheric surface pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining about twice as much of the observed variance as do either atmospheric wind or ocean bottom pressure changes and nearly 4 times as much of the observed variance as do oceanic currents. However, on interannual timescales, ocean bottom pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining more than 5 times as much of the observed variance as do atmospheric wind and pressure changes combined, and more than twice as much of the observed variance as do oceanic currents. Within the Chandler band it is found that during 1980–2000 atmospheric and oceanic processes have enough power to excite the Chandler wobble and are significantly coherent with it. The single most important mechanism exciting the Chandler wobble is found to be ocean bottom pressure variations. Atmospheric and oceanic processes do not appear to have enough power to excite the Earth's wobbles to their observed levels on pentadal and longer timescales, although series longer than the 21‐yearlong series used here need to be studied in order to obtain greater statistical significance of this result.
Location of seismic events and eruptive fissures on the Piton de la Fournaise volcano using seismic amplitudes
Authors: Battaglia Jean , Aki Keiiti
Published: August 2003

We present a method for locating the source of seismic events on Piton de la Fournaise. The method is based on seismic amplitudes corrected for station site effects using coda site amplification factors. Once corrected, the spatial distribution of amplitudes shows smooth and simple contours for many types of events, including rockfalls, long‐period events and eruption tremor. On the basis of the simplicity of these distributions we develop inversion methods for locating their origins. To achieve this, the decrease of the amplitude as a function of the distance to the source is approximated by the decay either of surface or body waves in a homogeneous medium. The method is effective for locating rockfalls, long‐period events, and eruption tremor sources. The sources of eruption tremor are usually found to be located at shallow depth and close to the eruptive fissures. Because of this, our method is a useful tool for locating fissures at the beginning of eruptions.
Nutrient status of Baltic Sea ice: Evidence for control by snow‐ice formation, ice permeability, and ice algae
Authors: Granskog M. A. , Kaartokallio H. , Shirasawa K.
Published: August 2003

Samples of land‐fast sea ice collected along the Finnish coast of the Baltic Sea, between latitudes 60.2°N and 65.7°N, in January to April 2000 were analyzed for physical, biological, and chemical parameters. Both spatial and temporal variability were investigated. Snow‐ice contributed in average a third of the total ice thickness, while the snow fraction (by mass) of the ice was 20% on average. Snow‐ice formation increased the nitrogen concentrations substantially, mainly in the upper parts of the ice cover. Phosphorus on the other hand was controlled by biological uptake, with distinct maxima in the bottommost parts of the ice cover. The chlorophyll‐a concentrations were dependent on the physical properties of the ice to some extent. In more saline waters the chlorophyll‐a concentrations in the ice were variable (1–17 μg l−1). However, in the less saline waters of the Bothnian Bay the concentrations were generally considerably lower (<1 μg l−1) than elsewhere. This is presumably caused by formation of ice of low salinity, due to the low ambient salinity in the area and the under‐ice flow of river waters, and formation of ice that has no habitable space for ice algae. Atmospheric nutrients possibly enhance the magnitude of the ice algae bloom, through downward flushing of surface deposited nutrients during periods when the ice was permeable. We surmise that atmospheric supply of nutrients plays an important role in biological productivity within the Baltic Sea ice sheet and potentially also in under‐ice waters.
Intersite comparison of interannual nearshore bar behavior
Authors: Ruessink B. G. , Wijnberg K. M. , Holman R. A. , Kuriyama Y. , van Enckevort I. M. J.
Published: August 2003

Long‐term (>years) bathymetric data sets collected in six multiple near‐shore sandbar systems were analyzed with complex empirical orthogonal function analysis to quantify intersite differences and similarities in cyclic offshore progressive bar behavior. The observations came from a 37‐year annually sampled data set of four regions along the Dutch coast (spanning 70 km of coastline), an 18‐year fortnightly to monthly sampled data set at Duck, North Carolina (alongshore extent ∼1 km), and a 7‐year daily sampled data set of a single cross‐shore profile at the Hasaki coast of Japan. The first complex mode, typically representing 50–70% of the total depth variance, described the long‐term offshore progressive behavior and allowed for an objective separation of the barred part of the profile from the shoreward‐ and seaward‐located nonbarred parts by considering a threshold bar amplitude below which the spatial results from the first mode were not considered reliable. The sandbars at the six examined sites share common lengths and nondimensional amplitude characteristics, which can be described by a negatively skewed Gaussian function. The absolute amplitude dimensions and the cycle return intervals differ, however, considerably between the sites. The key geometric parameters that steer this intersite variation are the time‐averaged mean depths at the shoreward and seaward side of the bar zone (shoreandsea, respectively) as well as their differencebz. The degree to which intersite differences inshore,sea, andbzare related linearly to intersite differences in bulk statistics of external forcings (wave, tide, sediment, and bed profile characteristics) is inconclusive.
Near‐surface circulation in the northern North Atlantic as inferred from Lagrangian drifters: Variability from the mesoscale to interannual
Authors: Jakobsen Philip K. , Ribergaard Mads H. , Quadfasel Detlef , Schmith Torben , Hughes Christopher W.
Published: August 2003

The near‐surface circulation of the Nordic Seas is basically cyclonic and consists of jets and recirculation cells, which are tightly linked to the bottom topography. Variable forcing by the large‐scale rotation of the wind leads to a modulation in the strength of the gyres and their interconnecting jets. This is seen in drifter and altimeter data. Currents are stronger during winter and during phases of high North Atlantic Oscillation Index. The exchanges between the North Atlantic and the Nordic Seas do not seem to be directly affected by this variable forcing. The narrow boundary currents and the intergyre jets are subject to instability, causing mesoscale current fluctuations, which contribute to the stirring and mixing of Polar and Atlantic water masses.
Analytical asymptotic solutions to determine interactions between the planetary boundary layer and the Earth's surface
Authors: Tsuang Ben‐Jei
Published: August 2003

This study derives an asymptotic analytical solution to calculate land skin temperature, planetary boundary layer (PBL) temperature, and turbulent heat fluxes over arid and wet ground surfaces. Applying the analytical solution to field data, the turbulent heat fluxes and the daytime canopy resistance (which are difficult to measure directly) can be easily determined on the basis of solar radiation and atmospheric radiation measurements, as well as other commonly available meteorological data. The results can be compared with a Lagrangian soil‐plant‐atmosphere model. Using the derived canopy resistance as input for the Lagrangian model, the simulated PBL temperature has a higher correlation coefficient with the real data than two other canopy schemes. The analytical solution proves that the mean skin temperature is slightly higher than the mean PBL temperature, and its amplitude is always larger than that of PBL temperature in general. An equation for determining the half‐life of either PBL temperature or skin temperature is derived. The half‐life is the time needed for the initial temperatures of the PBL or a land surface to decay to 50%. Note that the temperatures will alter in response to variations in the meteorological conditions such as changes in solar radiation. Over bare ground, vegetation, snow or ice, the half‐lives are ∼8 hours. For a body of water, it is 1.4 months with a thermocline depth of 100 m. Although the algorithm of determining canopy resistance is verified in an urban area in Taiwan, it is likely to be valid worldwide.
Saharan dust transport to the Caribbean during PRIDE: 2. Transport, vertical profiles, and deposition in simulations of in situ and remote sensing observations
Authors: Colarco P. R. , Toon O. B. , Reid J. S. , Livingston J. M. , Russell P. B. , Redemann J. , Schmid B. , Maring H. B. , Savoie D. , Welton E. J. , Campbell J. R. , Holben B. N. , Levy R.
Published: August 2003

We simulate Saharan dust transport during the Puerto Rico Dust Experiment (June–July 2000) with a three‐dimensional aerosol transport model driven by assimilated meteorology. The model does a reasonable job of locating the dust plume as it emerges from Africa but transports it somewhat farther south in the western North Atlantic Ocean than is seen in satellite imagery. The model is able to simulate low‐level, uniformly mixed, and elevated vertical dust layer profiles over Puerto Rico similar to observations made in PRIDE. We determine that the variability in the dust vertical profile across the North Atlantic Ocean is most strongly associated with descent of the dust by sedimentation and downward vertical winds during transit rather than low‐level transport directly from source regions. Wet removal plays a key role in modulating this process. Assuming our dust is 3.5% iron by mass, we estimate July 2000 iron deposition into the North Atlantic Ocean to be between 0.71 and 0.88 Tg, which is consistent with estimates derived from observed surface dust mass concentrations. We estimate that if annual dust deposition remains constant at five times our July 2000 estimates, there is an accumulation of 1 m of sediment from Saharan dust over the Florida peninsula every one million years.
Aerosol activation in marine stratocumulus clouds: 2. Köhler and parcel theory closure studies
Authors: Snider Jefferson R. , Guibert Sarah , Brenguier Jean‐Louis , Putaud J.‐P.
Published: August 2003

Aerosol properties and vertical velocities relevant to cloud droplet concentrations in marine stratocumulus clouds are presented. Data were collected at a coastal surface site, and on an aircraft that flew north (upwind) of the surface site during the CLOUDYCOLUMN portion of the second Aerosol Characterization Experiment (ACE‐2). In a closure study we compare observations, from five study days, and predictions based on Köhler and parcel theory. Measured and predicted cloud condensation nuclei (CCN) do agree after accounting for differences between the mobility equivalent (Dme) and sphere equivalent (Dse) diameters, but only when considering results from two study days unaffected by continental pollution. After applying the Dme − Dse correction, cloud droplet concentration closure was also achieved, or nearly achieved, on three study days. Discrepancies between predicted and measured CCN, or between predicted and measured droplet concentration, were always in the direction of predicted values being larger than that measured. Although theoretical studies do specify scenarios where droplet concentration and CCN overestimation can occur, the observed disparities may also be due to multiple and poorly characterized experimental biases.
Sea fog off the California coast: Viewed in the context of transient weather systems
Authors: Lewis J. , Koracin D. , Rabin R. , Businger J.
Published: August 2003

Widespread sea fog off the California coast occurred in conjunction with a transient weather disturbance in mid‐April 1999. In this paper we examine the larger‐scale influences that have bearing on the fog formation and its dissipation. Furthermore, this event is viewed in the context of the monthly changes of weather along the California coast. We compare and contrast the fog event with the precedent synoptic event that was fog‐free in an effort to understand the sensitivity of fog formation to large‐scale parameters. The principal results of the study are (1) the sea fog is characterized by long over‐water trajectories in persistent northwesterly flow. The sea surface temperatures (SSTs) are greater than the air temperature along these trajectories and gradual lowering of the stratus in the presence of strong subsidence eventually produces fog. (2) The synoptic‐scale subsidence associated with the anticyclone in these transient weather systems is a critically important factor in sea fog formation: a persistent, large‐magnitude subsidence favors fog by decreasing the depth of the marine layer while increasing the strength of the inversion. (3) Trajectories obtained by back‐tracking foggy and fog‐free air in the fog event of mid‐April 1999 make it clear that air with a path history over land is less likely to be associated with sea fog. In short, the demarcation between foggy and fog‐free zones is related to the differences in path history. (4) The occurrence of sea fog depends on the frequency of the transient weather systems, where an extended period of air mass transformation favors fog formation.
Regional chemical weather forecasting system CFORS: Model descriptions and analysis of surface observations at Japanese island stations during the ACE‐Asia experiment
Authors: Uno I. , Carmichael G. R. , Streets D. G. , Tang Y. , Yienger J. J. , Satake S. , Wang Z. , Woo Jung‐Hun , Guttikunda S. , Uematsu M. , Matsumoto K. , Tanimoto H. , Yoshioka K. , Iida T.
Published: August 2003

The Chemical Weather Forecast System (CFORS) is designed to aid in the design of field experiments and in the interpretation/postanalysis of observed data. The system integrates a regional chemical transport model with a multitracer, online system built within the Regional Atmospheric Modeling System (RAMS) mesoscale model. CFORS was deployed in forecast and postanalysis modes during the NASA Global Tropospheric Experiment (GTE)‐Transport and Chemical Evolution over the Pacific (TRACE‐P), International Global Atmospheric Chemistry project (IGAC)‐International Geosphere‐Biosphere Programme (IGBP) Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia), and National Oceanic and Atmospheric Administration Intercontinental Transport and Chemical Transformation of Anthropogenic Pollution 2002 (ITCT 2K2) field studies. A description of the CFORS model system is presented. The model is used to help interpret the Variability of Maritime Aerosol Properties (VMAP) surface observation data. The CFORS model results help to explain the time variation of both anthropogenic pollutants (sulfate, black carbon, and CO) and natural constituents including radon and mineral dust. Time series and time‐height cross‐section analysis of gases and aerosols are presented to help identify key processes. Synoptic‐scale weather changes are found to play an important role in the continental‐scale transport of pollution in the springtime in East Asia. The complex vertical and horizontal structure of pollutants in these outflow events is also presented and discussed.
Carbon and hydrogen isotopic compositions of stratospheric methane: 1. High‐precision observations from the NASA ER‐2 aircraft
Authors: Rice A. L. , Tyler S. C. , McCarthy M. C. , Boering K. A. , Atlas E.
Published: August 2003

Measurements of δ13C and δD of atmospheric CH4 from whole air samples collected in the upper troposphere and lower stratosphere aboard the NASA ER‐2 aircraft during the SOLVE (2000), POLARIS (1997), and STRAT (1996) campaigns are reported. Samples cover latitudes from 1°S to 89°N and altitudes from 11 to 21 km, providing CH4 mixing ratios that range from 1744 to 716 ppbv. Measurements of isotope ratios were made by continuous‐flow gas chromatography isotope ratio mass spectrometry which provides high‐precision analyses on 60 ml aliquots of air. These measurements comprise the first upper atmosphere isotopic CH4 data set to date using this technique and the most extensive with respect to latitude and season in any case. Values of δ13C‐CH4 on the V‐PDB scale range from −47.28‰ near the tropical tropopause to −34.05‰ in the high northern latitude stratosphere. Values of δD on the V‐SMOW scale range from −90.9‰ to +26.4‰. Correlations of isotope ratios with CH4 mixing ratios show enrichment in the heavy isotopes as CH4 mixing ratios decrease due to kinetic isotope effects associated with oxidation by reaction with OH, Cl, and O(1D). Empirical fractionation factors are found to be highly dependent on the range of CH4 mixing ratio considered, increasing with decreasing mixing ratio. Systematic nonlinearity in a Rayleigh fractionation model suggests a range of stratospheric fractionation factors, αCstrat = 1.0108 ± 0.0004 to 1.0204 ± 0.0004 (2σ) and αHstrat = 1.115 ± 0.008 to 1.198 ± 0.008 (2σ), from high to low CH4 mixing ratio, respectively. The variation in α over the range in mixing ratios reflect changes in partitioning between CH4 sink reactions in different regions of the stratosphere. In Part 1, these new high‐precision observations are discussed and compared with other stratospheric and tropospheric isotope measurements. In Part 2 [McCarthy et al., 2003] the observations are compared with 2‐D model results, and implications for the kinetic isotope effects for reactions with OH, Cl, and O(1D) are discussed.
Indonesian wildfires of 1997: Impact on tropospheric chemistry
Authors: Duncan B. N. , Bey I. , Chin M. , Mickley L. J. , Fairlie T. D. , Martin R. V. , Matsueda H.
Published: August 2003

The Indonesian wildfires of 1997 released large amounts of trace gases and aerosols (e.g., ∼130 Tg of carbon monoxide (CO)) from September to November. Using the GEOS‐CHEM model of tropospheric chemistry and transport, we conducted a study of this burning event, including sensitivity simulations, to estimate the impacts of the trace gas and aerosol emissions on tropospheric chemistry. The emissions from the fires were estimated using the Total Ozone Mapping Spectrometer (TOMS) Aerosol Index (AI) data product and fire‐count data from the Along Track Scanning Radiometer (ATSR) World Fire Atlas as surrogates for biomass burning. The model captures most of the daily variations of CO and ozone (O3) observed in the region affected by the pollution. Export of the pollution from the Indonesian region was primarily in the prevailing easterlies in the free troposphere to the tropical Indian Ocean where the bulk of the pollution lay between 20°N to 20°S from September to November. The model's tropospheric CO and O3columns were elevated by more than 50% and 10%, respectively, over the tropical Indian Ocean in a simulation with emissions from the fires relative to a simulation without the emissions. Another important export pathway was to the tropical and subtropical South Pacific Ocean in the southern subtropical jet. A more episodic pathway occurred to the tropics and subtropics of the North Pacific Ocean. By December, the tropospheric CO column from the fires had mixed zonally and somewhat symmetrically about the equator impacting both the Northern and Southern Hemisphere similarly. The model's CO column was elevated by 10–20% from 30°N to 45°S in December, by 5–10% poleward of 45°S, and by less than 5% poleward of 45°N. The relative impact of the fires was lower in the Northern Hemisphere, as the background CO column is typically higher there. The fires decreased the concentration of the hydroxyl radical (OH) in the model by more than 20% over much of the tropical Indian Ocean through consumption by CO, heterogeneous loss of odd‐oxygen radicals (HOx) on black (BC) and organic carbon (OC) aerosols, and reduction of UV light by the aerosols. The net direct, shortwave radiative forcing at the top of the atmosphere of OC and BC aerosols from the fires was relatively small, as their forcings were similar, but of opposite signs. The net forcing at the surface, however, was large, about −10 W m−2over most of the tropical Indian Ocean and as low as −150 W m−2over the burning regions in Indonesia, indicating that aerosols from the fires significantly perturbed the tropical radiative budget. The calculated forcing of O3was minor relative to those of BC and OC aerosols.
Aircraft‐based aerosol size and composition measurements during ACE‐Asia using an Aerodyne aerosol mass spectrometer
Authors: Bahreini Roya , Jimenez Jose L. , Wang Jian , Flagan Richard C. , Seinfeld John H. , Jayne John T. , Worsnop Douglas R.
Published: August 2003

An Aerodyne aerosol mass spectrometer (AMS) was deployed during the Aerosol Characterization Experiment‐Asia (ACE‐Asia) field campaign on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft to measure the size‐resolved chemical composition of submicron aerosols in the outflow from eastern Asia. Research flights were carried out from 31 March to 1 May 2001 in an area that covered 127°E–135°E and 32°N–38°N. Valid data from the AMS were obtained during 15 out of a total of 19 research flights. During the mission the AMS measured distinct layers (from the boundary layer to ∼3700 m) of submicron aerosols composed of sulfate, ammonium, and organics as the major nonrefractory components, separated by layers with much lower aerosol concentrations. Sulfate and organics mass concentrations of up to 10 μg m−3 and 13 μg m−3, respectively, were measured in some pollution layers. Back‐trajectory analysis shows that the polluted layers originated in urban and industrial areas of China and Korea. The mass‐weighed size distribution of the submicron sulfate was relatively constant from day to day and layer to layer, with an aerodynamic diameter mode of 400–500 nm and a width (full width half maximum) of about 450 nm in most of the layers. On the days with low influence of dust in the aerosol outflow, as indicated by other instruments aboard the Twin Otter, the total mass of nonrefractory aerosols estimated by the AMS correlated well with total volume of aerosols measured by a differential mobility analyzer.
Representation of a nonspherical ice particle by a collection of independent spheres for scattering and absorption of radiation: 2. Hexagonal columns and plates
Authors: Neshyba Steven P. , Grenfell Thomas C. , Warren Stephen G.
Published: August 2003

A cloud of nonspherical ice particles may be represented in radiation models by a collection of spheres, in which the model cloud contains the same total volume of ice and the same total surface area as the real cloud but not the same number of particles. The spheres then have the same volume‐to‐area (V/A) ratio as the nonspherical particle. In previous work this approach was shown to work well to represent randomly oriented infinitely long circular cylinders for computation of hemispherical reflectance, transmittance, and absorptance. In this paper the results have been extended to hexagonal columns and plates using a geometric optics technique for large particles and finite‐difference‐time‐domain theory (FDTD) for small particles. The extinction efficiency and single‐scattering coalbedo for these prisms are closely approximated by the values for equal‐V/A spheres across the ultraviolet, visible, and infrared from 0.2 to 25 μm wavelength. Errors in the asymmetry factor can be significant where ice absorptance is weak, at visible wavelengths for example. These errors are greatest for prisms with aspect ratios close to 1. Errors in hemispheric reflectance, absorptance, and transmittance are calculated for horizontally homogeneous clouds with ice water paths from 0.4 to 200,000 g m−2 and crystal thicknesses of 1 to 400 μm, to cover the range of crystal sizes and optical depths from polar stratospheric clouds (PSCs) through cirrus clouds to surface snow. The errors are less than 0.05 over most of these ranges at all wavelengths but can be larger at visible wavelengths because of the ideal shapes of the prisms. The method was not tested for, and is not expected to be accurate for, angle‐dependent radiances.
Sublimation of Mars's southern seasonal CO2 ice cap and the formation of spiders
Authors: Piqueux Sylvain , Byrne Shane , Richardson Mark I.
Published: August 2003

In this paper we define and describe morphological features that have colloquially been termed “spiders” and map their distribution in the south polar region of Mars. We show that these features go through a distinct seasonal evolution, exhibiting dark plumes and associated fan‐shaped deposits during the local defrosting of the seasonal cap. We have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain. These observations are consistent with a geyser‐like model for spider formation. Association with the transparent (cryptic) portion of the seasonal cap is consistent with basal sublimation and the resulting venting of CO2 gas. Also consistent with such venting is the observation of dark fan‐shaped deposits apparently emanating from spider centers. Spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface.
Martian impact crater ejecta morphologies as indicators of the distribution of subsurface volatiles
Authors: Barlow Nadine G. , Perez Carola B.
Published: August 2003

Fresh Martian impact craters display a variety of ejecta blanket morphologies. The fluidized appearance of most fresh ejecta types is commonly ascribed to heating and vaporization of subsurface volatiles during crater formation. We have conducted a study of the distribution of the three dominant fluidized ejecta morphologies (single layer ejecta (SLE), double layer ejecta (DLE), and multiple layer ejecta (MLE)) within the ±60° latitude zone on Mars. We have subdivided this region into 5° × 5° latitude‐longitude boxes and have computed the following for each box: (1) percentage of craters showing any ejecta morphology as a function of total number of craters, (2) percentage of SLE craters as a function of craters with an ejecta morphology, (3) percentage of DLE craters as a function of craters with an ejecta morphology, and (4) percentage of MLE craters as a function of craters with an ejecta morphology. We confirm previous reports that the SLE morphology is the most common ejecta type within the study area, constituting >70% of all ejecta morphologies over most of the study area. The DLE and MLE morphologies are much less common, but these morphologies are concentrated in localized regions of the planet. Using these results, we discuss how subsurface volatile reservoirs may be distributed across the planet. The regional variations found in this study generally correlate with the proposed locations of near‐surface H2O reservoirs detected by Mars Odyssey.
An S‐band frequency‐modulated continuous‐wave boundary layer profiler: Description and initial results
Authors: İnce Türker , Frasier Stephen J. , Muschinski Andreas , Pazmany Andrew L.
Published: August 2003

Operating principles of frequency‐modulated continuous‐wave (FMCW) radar are reviewed, and their measurement limitations for atmospheric targets are discussed. In particular, we outline misregistration errors due to Doppler velocities and target coherence limitations on range resolution. The latter is of importance to volumetric scattering from atmospheric targets. Parallax errors and near‐field operation are also considered. A high‐resolution S‐band FMCW radar developed at the University of Massachusetts is then described, and observations obtained by this system during the 1999 Cooperative Atmosphere‐Surface Exchange Study are used to illustrate system performance. In the convective boundary layer, Rayleigh scatterers appear to dominate the observed vertical profile of mean reflectivity at S‐band.
FUJI Dome: A large detachment fault near 64°E on the very slow‐spreading southwest Indian Ridge
Authors: Searle R. C. , Cannat M. , Fujioka K. , Mével C. , Fujimoto H. , Bralee A. , Parson L.
Published: August 2003

A continuous, domed detachment surface (FUJI Dome) has been imaged on the very slow‐spreading southwest Indian Ridge using deep‐towed side‐scan sonar, and has been investigated by manned submersible and sea‐surface geophysics. The Dome is morphologically similar to other oceanic detachments, core complexes or mega‐mullions. In addition to bathymetric mullions observed in ship‐borne bathymetry, finer scale spreading‐parallel striations were imaged with the side scan. On the detachment surface, metabasalt crops out near the termination, probably as part of a thin fault sliver. Gabbro and troctolite probably crop out near the summit of the dome. The rest of the detachment surface is covered with sediment and rubble which is basaltic except for a single sample of serpentinite. Most of the detachment surface dips toward the ridge axis at 10°–20°, but near the breakaway it is strongly rotated outward, and dips away from the axis at up to 40°. Normal, undeformed volcanic seafloor crops out adjacent to the detachment. Modeling of sea surface magnetic data suggest the detachment was active from 1.95 Ma for about 1 Ma during a period of reduced and asymmetric magmatic accretion. Modeling of sea surface and seafloor gravity requires laterally fairly uniform but high density material under the Dome, and precludes steeply dipping contacts between bodies with large density contrasts at shallow levels under the Dome.