bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulated airborne particle size distributions over Greenland during Last Glacial Maximum
Authors: Unnerstad Lars , Hansson Margareta
Published: February 2004

Polar ice cores indicate that the deposition of dust from the atmosphere was strongly enhanced during Last Glacial Maximum (LGM). The concentration of dust in the ice sheets and in the overlaying atmosphere are not proportional to each other but are dependent, among other things, on the relative magnitudes of dry and wet deposition which change with climate. Observed dust particle size distributions in the Greenland ice sheet are shifted toward larger particles during LGM. By applying common theories for particle removal processes we show that the airborne particle size distributions over Greenland probably remained the same in the two different climates. This leads to the conclusion that the airborne dust concentration was even higher during LGM than indicated by the enhancement in deposition flux. We suggest a LGM/pre‐industrial current climate aerosol ratio (including the soluble fraction) over Greenland of about 90–125 by mass and 75–100 by number.
Physical properties of ammonia‐rich ice: Application to Titan
Authors: Lorenz Ralph D. , Shandera Sarah E.
Published: February 2004

We report simple measurements of the physical properties of frozen aqueous solutions of NH3 down to 80 K. These measurements are relevant for the geophysics of icy satellites and Titan in particular. We find that the thermal conductivity of NH3‐rich (∼10–30%) water ice is 1–2 Wm−1K−1, or 2–3 times lower than that of pure water ice. We find evidence for significant microwave absorptivity in NH3‐rich ice—even at 100K—and that the electrical properties are very strongly temperature‐dependent. At around 100K the Young's Modulus for NH3‐free and NH3‐rich ice appears to be about the same, while at 160K the NH3‐rich ice is 10x more compliant.
Generation of field‐aligned currents in the near‐Earth magnetotail
Authors: Lee Dong‐Hun , Lysak Robert L. , Song Yan
Published: February 2004

Sudden brightening of aurora occurs at an early phase of substorm onset. Such auroral enhancements are strongly associated with parallel currents or shear Alfven waves. With MHD wave equations, we investigate the generation of field‐aligned currents near the equatorial region of the inner magnetotail, at the boundary between cold and hot plasmas. Our results show that strong localized shear Alfven waves are excited near this boundary via mode conversion when compressional input is assumed. We suggest that this boundary region is strongly associated with the auroral location.
Correlation of aerosol and carbon monoxide at 45°S: Evidence of biomass burning emissions
Authors: Jones Nicholas B. , Rinsland Curtis P. , Liley J. Ben , Rosen Jim
Published: February 2004

Altitude profiles of Carbon Monoxide (CO) and aerosols have been compared from the Network for Stratospheric Change (NDSC) mid‐latitude southern hemisphere site at Lauder, New Zealand. The CO mixing ratio profile was derived from infrared spectra recorded with a very high resolution Fourier Transform interferometer using three lines of the (1–0) band between 2057 and 2160 cm−1. The aerosol surface area was derived from balloon‐borne backscatter radiation at 940 nm. Both datasets show significant enhancements occurring over the observation site in the austral spring. When displayed together their combined effect illustrates the close correlation between CO and aerosols. Peak concentrations are consistently recorded between September and October over a five year time frame (1994–1999), with the enhancements typically occurring at heights of between 3 to 8 km. The temporal and spatial correlation between the aerosol plumes and enhanced CO concentrations are interpreted in terms of the effect of long range transport of biomass burning plumes in combination with the El Nino‐Southern Oscillation (ENSO) cycles influence on southern hemisphere climate dynamics.
Springtime enhancement of upper tropospheric aerosol at 45°S
Authors: Ben Liley J. , Rosen James M. , Kjome Norman T. , Jones Nicholas B. , Rinsland Curtis P.
Published: February 2004

Monthly sonde data for Lauder in Central Otago, New Zealand show profiles of aerosol backscatter from the surface to over 30 km altitude. The tropospheric data vary by season, with greater aerosol backscatter throughout the free troposphere in springtime. Aerosol mixing ratios in layers in the upper troposphere at these times are often much higher than anywhere else above the boundary layer, suggesting that they arise from horizontal transport. Ozone measurements from the sonde show correspondence in vertical structure to the backscatter data and also seasonal enhancement. The latter correlates with aerosol, but competing causes of ozone enhancement make the correspondence indistinct. High concentrations of carbon monoxide are observed by Fourier transform spectroscopy in spring, and altitude profiles derived from line shape suggest that the peak in CO occurs in the same altitude range as the aerosol enhancement.
Allowing for solar forcing in the detection of human influence on tropospheric temperatures
Authors: Hill D. C. , Allen M. R. , Stott P. A.
Published: February 2004

Previous workers have used the radiosonde record of atmospheric vertical temperature structure to detect anthropogenic influence on climate [e.g. Santer et al., 1996a]. However, none of these studies explicitly considered the influence of natural forcing factors. Whilst decadal changes of radiative solar activity are small, they could be amplified by various mechanisms (e.g. Haigh, 1996), potentially accounting for a significant fraction of the observed signal. Gillett et al. [2000] show that the diagnostics used to detect anthropogenic influence on atmospheric temperature structure are strongly influenced by solar forcing. Here we repeat the analysis of Allen and Tett, [1999] (AT99) including a model‐simulated solar signal, to assess whether the observed tropospheric warming might be explained by solar forcing and the observed stratospheric cooling by the effects of ozone depletion. It transpires that previously reported results are robust to the inclusion of solar forcing since the greenhouse gas response is detectable even in the presence of solar forcing and stratospheric ozone depletion, allowing for an arbitrary amplitude of both solar and ozone signals.
Rapid changes of the aftershock P axes 3 years after the 1995 Hyogo‐ken Nanbu (Kobe) Earthquake
Authors: Yamada Takuji , Ando Masataka , Katao Hiroshi
Published: February 2004

Aftershock focal mechanisms of the 1995 Hyogo‐ken Nanbu earthquake (Mw=6.9) are analyzed to detect temporal variations in the stress field on the Nojima fault and the Rokko fault system, which ruptured during the mainshock. Following the mainshock, we observe temporal changes in the P axes of aftershocks. Around the Nojima fault, P‐axis orientations of aftershocks which occurred in 1996 and 1997 are almost normal to the fault. In contrast, aftershocks which occurred after 1998 have P axes consistent with the regional stress. Around the central part of the Rokko fault system, P‐axis directions of aftershocks which occurred in 1996 with focal depths less than 10 km are almost normal to the faults. In contrast, aftershocks which occurred after 1997 have P axes consistent with the regional stress. These results indicate that the Nojima fault and the Rokko fault system started to accumulate significant shear stress in mid‐1998, only 3 years after the mainshock.
Vitreous forsterite (Mg2SiO4): Synthesis, structure, and thermochemistry
Authors: Tangeman Jean A. , Phillips Brian L. , Navrotsky Alexandra , Weber J. K. Richard , Hixson April D. , Key Thomas S.
Published: February 2004

Here we report the first synthesis of a forsterite (Mg2SiO4) composition glass as an essentially phase‐pure bulk material. Under containerless conditions, with heterogeneous nucleation sites minimized, glass forms by cooling ca. 1 mm liquid Mg2SiO4 droplets in oxygen at 700 K/s. 29Si NMR spectroscopic data indicate that the SiO4 tetrahedra and MgO6 octahedra exist in a corner sharing arrangement in the glass, but upon crystallization the polyhedral units reorganize to form edge‐sharing linkages. Transposed temperature drop calorimetry shows that the glass is 61.4±1.3 kJ/mol higher in enthalpy than the crystal.
Wave driven non‐linear flow oscillator for the 22‐year solar cycle
Authors: Mayr Hans G. , Wolff Charles L. , Hartle Richard E.
Published: February 2004

In the Earth's atmosphere, a zonal flow oscillation is observed with periods between 20 and 32 months, the Quasi Biennial Oscillation. This oscillation does not require an external time dependent source but is maintained by non‐linear wave momentum forcing. We propose that such a mechanism also drives long‐period oscillations in planetary and stellar interiors, and we apply it here to generate a flow oscillation for the 22‐year solar cycle. The oscillation would occur just below the convective envelope—in a region where gravity waves can propagate. We present results from a simplified analytical model that incorporates Hines' gravity wave parameterization. Wave amplitudes <10 m/s can produce reversing zonal flows of 20 m/s that should be sufficient to generate a corresponding oscillation in the poloidal magnetic field. The low buoyancy frequency and associated increase in turbulence help produce the desired oscillation period of the flow and should help generate the currents for the reversing magnetic field.
A Low QScS Anomaly Near the South Pacific Superswell
Authors: Suetsugu Daisuke
Published: February 2004

We analyzed multiple ScS waves from Tonga‐Fiji deep events recorded at the IRIS and SPANET broadband stations to determine lateral variation in attenuation (Q) of the mantle in the South Pacific. We measured average Q values of ScS waves in the mantle between earthquakes and each station, and between two stations. The results of the measurement show that the average Q in the South Pacific is 140. We obtain a very low Q of 70–80 for multiple ScS waves traveling in the South Pacific Superswell, suggesting the existence of higher temperature anomalies in the entire mantle in hot spot regions.
Reconstruction of magnetic flux ropes in the solar wind
Authors: Hu Qiang , Sonnerup Bengt U. Ö.
Published: February 2004

We recover two‐dimensional magnetic structures of small‐scale flux ropes in the solar wind from WIND spacecraft data by solving the plane Grad‐Shafranov equation as a spatial initial value problem. The cross‐sections of two recovered flux ropes show nested non‐circular loops of transverse field lines rather than concentric circles. The expected helical structure is recovered, albeit with significant distortions from axial symmetry. The rope structures are self‐consistent, non‐force free, magnetohydrostatic equilibria. Their orientations are determined by a novel technique.
Inter‐calibration of CERES and ScaRaB Earth Radiation Budget datasets using temporally and spatially collocated radiance measurements
Authors: Haeffelin Martial , Wielicki Bruce , Duvel Jean Philippe , Priestley Kory , Viollier Michel
Published: February 2004

Comparisons of radiance measurements from overlapping independent Earth and cloud radiation budget (ERB) missions are an important contribution to the validation process of these missions and are essential to the construction of a consistent long‐term record of ERB observations. Measurements from two scanning radiometers of different design and calibration, the Clouds and the Earth's Radiant Energy System (CERES) and the Scanner for Radiation Budget (ScaRaB), are compared during simultaneous operation in January and March 1999. The instruments are found to be consistent to within 0.5% and 1.5% in the longwave and shortwave spectral domains, respectively.
Consideration of ocean tides in an OGCM and impacts on subseasonal to decadal polar motion excitation
Authors: Thomas Maik , Sündermann Jürgen , Maier‐Reimer Ernst
Published: February 2004

Ocean induced changes of Earth's rotation are attributed either to tides or to variations of the general circulation. Analogously, numerical world ocean models can still be divided into Ocean General Circulation and tidal models, although a neglect of nonlinear interactions in favor of a linear superimposition of both components of motion is questionable. By means of a simultaneous simulation of the ocean's circulation and tides we estimate the importance of two oceanic effects with respect to excitation of polar motion: nonlinearities between circulation and long‐period tides and the circulation induced potential due to loading and self‐attraction of a baroclinic ocean. Comparing the linear superimposition of separate model simulations with the simultaneous calculation of circulation and tides it turns out that these second‐order effects contribute about 8% to ocean induced changes in Earth's orientation.
Discrimination of cloud and rain liquid water path by groundbased polarized microwave radiometry
Authors: Czekala Harald , Crewell Susanne , Simmer Clemens , Thiele Ariane
Published: February 2004

We propose a new approach for groundbased remote sensing of liquid water path (LWP) in the presence of precipitating clouds. Dual polarized groundbased microwave radiometers are capable of detecting the unique scattering signature of nonspherical precipitation sized particles. This polarization signal is only produced by the precipitation particles for which the brightness temperature emission has a different sensitivity to LWP than the smaller cloud drops. By using the information that is contained in the polarization difference of the downwelling brightness temperature the cloud and rain liquid water fractions can be estimated independently. Future retrieval algorithms based on our proposed approach will enable the detection of small precipitation fractions in thick clouds and also allow for estimates of cloud and rain LWP in raining conditions.
Observations of currents on the West Florida Shelf Break
Authors: Meyers Steven D. , Siegel Eric M. , Weisberg Robert H.
Published: February 2004

Year‐long measurements by five acoustic Doppler current profilers moored across the central West Florida Shelf (WFS) and shelf break reveal new information on the local velocity structure and its temporal evolution. The moorings were between the 30 and 300 m isobaths. At depths greater than about 100 m the currents were sometimes directly affected by the deep ocean. During the measurement period the Loop Current (LC) directly impacted the central WFS three times, amounting to a total of about 13% of the year. For the remainder of the year the shelf break currents had equal likelihood of flowing northward or southward. Northward currents achieved over 70 cm/s and the southward currents (driven by the LC) achieved 120 cm/s. Neighboring instruments recorded strong (∼30 cm/s) currents flowing in opposite directions. Current reversals, in one case totally nearly 100 cm/s within a few days, were also found.
Origin of intraseasonal variability of circulation in the tropical central Indian Ocean
Authors: Sengupta Debasis , Senan Retish , Goswami B. N.
Published: February 2004

Observed upper ocean currents south of Sri Lanka exhibit large, irregular fluctuations with periods of days to weeks. An ocean model driven by daily surface winds is able to reproduce the observed fluctuations. We find from model experiments that low frequency (30–50 day) intraseasonal variability (ISV) arises when Rossby waves radiated from the eastern boundary are amplified by hydrodynamic instability in the eastern and central Indian Ocean. High frequency (10–15 day) ISV is forced directly by ISV of the wind field in the eastern Indian Ocean. In spite of the contribution from instability, the ocean circulation south of Sri Lanka is a deterministic response to wind forcing.
Fine mode participate sulphur in the tropopause region measured from intercontinental flights (CARIBIC)
Authors: Martinsson Bengt G. , Papaspiropoulos Giorgos , Heintzenberg Jost , Hermann Markus
Published: February 2004

Here the first systematic study of the concentration of particulate sulphur in the upper troposphere and lower stratosphere is presented. The measurements were undertaken in the CARIBIC programme during intercontinental flights from a commercial aircraft, which was equipped with an aerosol inlet and a research payload in the cargo bay. Aerosol samples were collected and analysed for elemental composition. The data set comprises 21 flights between Germany and Male/Colombo in the Indian Ocean. The average fine mode, particulate sulphur concentration was 14 ng/m³ STP, which is low compared to remote areas at the Earth's surface. A pronounced dependence with latitude with elevated concentrations occurring within the densely industrialised northern hemispheric mid latitudes was observed, thus suggesting anthropogenic influence on the climate from sulphate particles in the tropopause region.
High interseismic coupling of the Alaska Subduction Zone SW of Kodiak Island inferred from GPS data
Authors: Fletcher Hilary J. , Beavan John , Freymueller Jeffrey , Gilbert Lewis
Published: February 2004

We use Global Positioning System (GPS) measurements to make the first geodetic study of the Semidi segment of the Alaska‐Aleutian subduction zone. This segment, which sustained an MW 8.2 earthquake in 1938, lies between Kodiak Island where the subduction interface appears to presently be fully locked, and the Shumagin Islands segment where substantial aseismic slip occurs. We invert the GPS station velocity estimates using a nonlinear least squares algorithm to solve for the width of the locked zone, the dip, and the interseismic coupling of a model subduction interface. The data are consistent with a shallow plate interface dipping ∼6°, a locking depth of ∼23 km, and high interseismic coupling of ∼80%.
Comparing atmospheric [HO2]/[OH] to modeled [HO2]/[OH]: Identifying discrepancies with reaction rates
Authors: Lanzendorf E. J. , Hanisco T. F. , Wennberg P. O. , Cohen R. C. , Stimpfle R. M. , Anderson J. G.
Published: February 2004

Reactions that inter‐convert OH and HO2 are directly involved in the catalytic removal of O3 in the lower stratosphere and in the catalytic production of O3 in the upper troposphere. The agreement between the measured and modeled [HO2]/[OH] tests our current understanding of this important chemistry. Recent changes to the recommended rate constants for OH+O3 and HO2+O3 call into question how accurately the chemistry of the stratosphere is understood. [HO2]/[OH] calculated with the new recommendations is 48% higher than the observations throughout the lower stratosphere, exceeding the uncertainty limits of the observations (20%). The extensive atmospheric data set allows tests of the rates of the individual processes that couple these free radicals. This work shows that the discrepancy is largest when the ratio is controlled by the reactions of OH and HO2 with ozone.
On the skewness of vorticity in the upper ocean
Authors: Rudnick Daniel L.
Published: February 2004

Conservation of potential vorticity suggests that when the magnitude of relative vorticity approaches the planetary vorticity, cyclonic vorticity should be preferred. Small‐scale, 3‐km relative vorticity is examined using shipboard Acoustic Doppler Current Profiler (ADCP) velocity measurements and P‐code Global Positioning System (GPS) navigation from a cruise in the North Pacific. The skewness of relative vorticity in the mixed layer is positive, consistent with theory. The probability density function of vorticity reveals that positive skewness occurs because large vorticities are more likely to be cyclonic than anticyclonic. The skewness is shown to be significant relative to measurement and random errors. In contrast, skewness in the thermocline has no preferred sign. The skewness peaks at the surface and decays to zero at the base of the mixed layer.
A simulation of the Last Glacial Maximum with a coupled atmosphere‐ocean GCM
Authors: Kitoh Akio , Murakami Shigenori , Koide Hiroshi
Published: February 2004

A simulation of the Last Glacial Maximum (LGM) climate has been performed by a global coupled atmosphere‐ocean general circulation model (AOGCM). Two simulations are conducted for the LGM with different initial conditions: one with the present‐day initial condition and the other with a pre‐conditioning of fresh water flux over the North Atlantic. After more than 200 year integration both LGM simulations attained a similar quasi‐equilibrium state. The global mean surface air temperature dropped 3.9°C and precipitation decreased 11% compared to the present day simulation. The sea surface temperature dropped 1.7°C in the tropics, with larger decreases in the Atlantic than in the Indo‐Pacific. There is a region with warmer than present sea surface temperatures over the subtropical eastern Pacific. The thermohaline circulation in the North Atlantic in the LGM simulation is slightly stronger than in the present day simulation.
Scaling and persistence in observed and modeled surface temperature
Authors: Syroka Joanna , Toumi Ralf
Published: February 2004

We introduce a simple and effective method to test for temperature persistence on a range of time scales. Daily temperatures for three regions are examined: Central England, El‐Nino region and global. On time scales of less than a year, climate model simulations reproduce the observed high persistence in all regions. However, for longer time scales the climate model systematically underestimates the persistence of global mean temperatures. The anti‐persistence in the El‐Nino region is well simulated by the model. Scaling behaviour is seen for Central England temperatures but is not reproduced by the model. Some of these discrepancies may be accounted for by insufficient ocean‐atmosphere coupling.
Anthropogenic emission of mercury to the atmosphere in the northeast United States
Authors: Lee Xuhui , Bullock O. Russell , Andres Robert J.
Published: February 2004

The severity and spatial extent of the health impact of anthropogenic mercury (Hg) emission to the atmosphere depend on the emission rate and chemical form of the emitted species. The few measurements of combustion flue gas give highly variable results about how the emission is divided between the elemental (Hg°) and reactive forms and are difficult to extrapolate to a regional scale. Here we combine measurement of total gaseous mercury (TGM) and carbon dioxide (CO2) concentrations at a background site in the winter with carbon (C) emission inventory to show that at a regional (500 km) scale, the effective anthropogenic Hg° flux is 41(±2) g km−2yr−1 in the north‐east United States. This regional‐scale flux was higher under clear skies than under cloudy skies, suggesting some removal of Hg° by cloud water, but the physical mechanisms of the removal are yet to be identified.
On applications of photochemical models to the design of measurement strategies
Authors: Khattatov B. , Lyjak L. , Gille J.
Published: February 2004

Several research groups recently demonstrated [e.g., Fisher and Lary, 1995; Elbern et al., 1997; Khattatov et al., 1999] that concentrations of some non‐observed chemical species can be constrained to a certain extent by the observed ones through our knowledge of atmospheric photochemistry. By utilizing mathematical relationships between chemically coupled species, observations of a subset of chemicals can be used to determine the future state of the whole system. The goal of this manuscript is to provide a mathematical formalism for assessing uncertainties of such an analysis. Practical applications of this study include designing future instruments and measurement strategies optimized to deliver the maximum amount of information given limited resources. In addition, it opens possibilities for testing our knowledge of atmospheric photochemistry by comparing predicted and measured concentrations and their estimated uncertainties.
Dynamics of equatorial F region irregularities from spaced receiver scintillation observations
Authors: Bhattacharyya A. , Basu S. , Groves K. M. , Valladares C. E. , Sheehan R.
Published: February 2004

Spaced receiver observations of amplitude scintillations on a 244 MHz signal, at an equatorial station, have been used to study random temporal changes associated with the scintillation‐producing irregularities and the variability of their motion. The computed drift of the scintillation pattern shows the presence of velocity structures associated with equatorial bubbles in the early phase of their development. On magnetically quiet days, after 22:00 LT, the estimated drifts fall into a pattern which is close to that of the ambient plasma drift. There is considerable decorrelation between the two signals until 22:00 LT. The power spectra of the most highly correlated scintillations recorded by spaced receivers indicate that the associated irregularities are confined to a thin layer on the bottomside of the equatorial F region. This suggests that the convection pattern associated with bottomside irregularities is stable due to the dominance of ion‐neutral collisions over ion inertia.
Hot diamagnetic cavities upstream of the Martian bow shock
Authors: Øieroset Marit , Mitchell David L. , Phan Tai D. , Lin Robert P. , Acuña Mario H.
Published: February 2004

We present Mars Global Surveyor (MGS) observations of hot diamagnetic cavities upstream of the Martian bow shock. The events are characterized by a region of turbulent magnetic field, high electron temperature, and densities at or below the ambient solar wind density. The region of high temperature is flanked by layers of high magnetic field and electron density and is accompanied by a rotation in the interplanetary magnetic field (IMF). We suggest that the events are generated by interplanetary discontinuities interacting with the Martian bow shock, i.e., the events are the Martian counterpart to the hot flow anomalies observed upstream of the Earth's bow shock.
Application of stable sulfur isotopes to differentiate sources of size‐resolved Particulate sulfate in polluted marine air at Bermuda during spring
Authors: Turekian Vaughan C. , Macko Stephen A. , Keene William C.
Published: February 2004

Stable sulfur isotopic compositions of size‐segregated aerosols were measured at Bermuda during spring 1998 to differentiate contributions of marine biogenic and pollution sources of non‐sea‐salt SO42− (NSS). Marine biogenic sources contributed an average 0.41±0.33 nmol m−3 (15±7%) of supermicron NSS and 2.55±0.45 nmol m−3 (11±2%) of submicron NSS. Calculated concentrations of submicron NSS from marine biogenic sources are within the range of reported values for the remote southeastern Pacific. The average molar ratio of marine biogenic NSS/CH3SO3− for aerosol with geometric mean radius (GMR) of 0.6 µm (10.1±1.9) was significantly greater than that for aerosols with GMR 0.34 µm (3.9±1.4), suggesting differential atmospheric processing. Supermicron radius aerosol accounted for 75% of the NSS dry‐deposition flux.
Airborne observations of DMSO, DMS, and OH at marine tropical latitudes
Authors: Nowak J. B. , Davis D. D. , Chen G. , Eisele F. L. , Mauldin R. L. , Tanner D. J. , Cantrell C. , Kosciuch E. , Bandy A. , Thornton D. , Clarke A.
Published: February 2004

This paper reports the first simultaneous fast‐time resolution measurements of dimethylsulfide (DMS), dimethylsulfoxide (DMSO), and the hydroxyl radical (OH) at marine tropical latitudes. These observations were recorded during the PEM‐Tropics B field program on NASA's P‐3B aircraft. The observations of DMSO, using a Selected Ion Chemical Ionization Mass Spectrometry (SICIMS) technique, are of particular significance. They have revealed two unique findings: 1) average midday‐tropical levels of DMSO are significantly higher than those predicted from current models when constrained by observed DMS and OH levels (e.g., 10 pptv versus 1 to 3 pptv); and 2) DMSO concentration profiles are significantly out‐of‐phase with model predictions, maximum values being seen under near dark conditions and minimum values being observed at midday. Although no simple explanation has yet been found for these unusual results, the fact that no evidence points to a problem in the measurements suggests that others may exist. Clearly, if the observations are correct, they indicate that at least for tropical upwelling regions the atmospheric sulfur budget may need to be adjusted to accommodate additional sources of DMSO.
Charge transfer during Crystal‐Graupel Collisions for two different cloud droplet size distributions
Authors: Avila Eldo A. , Pereyra Rodolfo G.
Published: February 2004

Laboratory experiments of graupel charging during ice crystal collisions reveal a charge sign dependence on temperature, liquid water concentration as well as the size distribution of the cloud droplets used for riming. The measurements were performed with an impact velocity of 8.5 m s−1, the ambient temperature was varied in the range −5 to −30°C and the effective water content up to 4 g m−3. Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature (Ta) and the effective liquid water content (EW) for each of the two cloud droplet spectra used are presented. The results indicate that broadening the droplet spectrum leads to negative graupel charging and the implications of these findings to thunderstorm electrification are discussed.
Factors controlling the diamagnetic pressure in the polar cusp
Authors: Zhou X. W. , Russell C. T. , Le G. , Fuselier S. A. , Scudder J. D.
Published: February 2004

The solar wind has direct access to the polar cusps although the plasma may be processed first by reconnection. The energy density of the polar cusp plasma is expected to be controlled by the component of the unshocked solar wind dynamic pressure along the normal to the cusp/magnetosheath interface. The factors controlling this angle are the local time of the observation and the dipole tilt angle. We use Polar observations of the magnetic field and plasma to identify the polar cusp, and the change in magnetic pressure in the cusp to calculate the peak diamagnetic pressure of the plasma. As expected this pressure is directly proportional to the solar wind dynamic pressure in quantitative agreement with expectations. When the dipole tilts more toward the Sun, the angle between the solar wind flow and the magnetopause normal is smaller, and the pressure is higher, and when it tilts away from the Sun, the pressure is less. For large tilts away from the Sun, for local times far away from noon and for low solar wind dynamic pressure the diamagnetic depression marking the polar cusp is too weak to be resolved by the magnetometer.
Arctic sea ice thickness remained constant during the 1990s
Authors: Winsor P.
Published: February 2004

The ice cover of the Arctic Ocean is considered to be a sensitive indicator of global climate change. Recent research, using submarine‐based observations, suggests that the Arctic ice cover was thinner in the 1990s compared to an earlier period (1958–1979), and that it continued to decrease in thickness in the 1990s. Here I analyze subsurface ice thickness (draft) of Arctic sea ice from six submarine cruises from 1991 to 1997. This extensive data set shows that there was no trend towards a thinning ice cover during the 1990s. Data from the North Pole shows a slight increase in mean ice thickness, whereas the Beaufort Sea shows a small decrease, none of which are significant. Transects between the two areas from 76°N to 90°N also show near constant ice thicknesses, with a general spatial decrease from the Pole towards the Beaufort Sea. Combining the present results with those of an earlier study, I conclude that the mean ice thickness has remained on a near‐constant level around the North Pole from 1986 to 1997.
Crustal structure of the Indian Shield: New constraints from teleseismic receiver functions
Authors: Ravi Kumar M. , Saul Joachim , Sarkar Dipankar , Kind Rainer , Shukla Anshu K.
Published: February 2004

Teleseismic earthquake waveform data from 10 broadband stations spread over the Indian shield and in operation since 1997, were analyzed to infer the crustal structure, using the receiver function technique. The South Indian shield is characterized by a 33–39 km thick, and remarkably simple crust, with an average Poisson's ratio close to 0.25. The Archaean crust is devoid of any prominent intra‐crustal discontinuities. The velocity contrast at the well developed Moho is large, resulting in very clear P‐to‐S conversions as well as first‐order multiples. In contrast, the predominantly Proterozoic crust in the northern part of the shield exhibits a complex character, due to the presence of additional seismic discontinuities. Moho conversions, which are considerably weaker compared to the Archaean terrains, indicate crustal thicknesses of more than 40 km.
East Asian Winter Monsoon and Arctic Oscillation
Authors: Gong Dao‐Yi , Wang Shao‐Wu , Zhu Jin‐Hong
Published: February 2004

In this study, the connection between Arctic Oscillation (AO) and variability of East Asian winter monsoon is investigated. Two indices are chosen to describe the winter monsoon. One is the intensity of the Siberian High, defined as the average SLP over the center region, and the other is the temperature of eastern China, averaged over 76 surface stations. These are two tightly related components, correlate at −0.62 for period 1951–99. Temperature drops by 0.64 degrees Celsius in association with a one standard deviation increase in Siberian High intensity. It is found that there are significant out‐of‐phase relationships between the AO and the East Asian winter monsoon. The correlation coefficient between the AO and the Siberian High intensity index is −0.48 for period 1958–98. AO is also significantly correlated with the temperature of eastern China at 0.34. However, when the linear trend is removed, the correlation between AO and temperature is no longer significant. But the strong connection between the AO and Siberian High, and between the Siberian High and temperature are still significant. These results reveal that the AO influences the East Asian winter monsoon through the impact on the Siberian High. Negative phase of the AO is concurrent with a stronger East Asian Trough and an anomalous anticyclonic flow over Urals at the middle troposphere (500hPa). Both the AO and the Eurasian pattern play important roles in changes of the Siberian High and/or East Asian winter monsoon. They account for 13.0% and 36.0% of the variance in the Siberian High respectively.
Evidence for earthquake interaction in central Chile: the July 1997–September 1998 Sequence
Authors: Lemoine Anne , Madariaga Raùl , Campos Jaime
Published: February 2004

Seven Mw > 6 events occurred between July 1997 and January 1998 along the shallow dipping subduction zone of Central Chile. We used body waveform modeling and master event relocation to study them. During July 1997, typical shallow interplate thrust events located close from each other followed a southward migration path and were located close from each other. On 15 October, the largest shock of the series (Mw=7.1) occured at 68 km depth. From the directivity, we found that its rupture plane was almost vertical with a downward rupture. It was a down‐dip compressional mechanism which is rare in Chile. Then, several thrust events occurred above it, along the plate interface. The rupture zones of the July 1997 events followed a cascade pattern with strong stress interaction. The slab push event of 15 October does not seem to have been triggered by static stress transfer from the July swarm.
Zonal Wave‐1 structure in TOMS tropical stratospheric ozone
Authors: Newchurch M. J. , Sun D. , Kim Jae H.
Published: February 2004

Using measurements of the stratospheric ozone column observed by NIMBUS‐7 TOMS above high‐altitude clouds observed by the collocated Temperature Humidity InfraRed (THIR) sensor, we analyze the stratospheric ozone column zonal wave structure in the tropics. A zonal wave‐1 structure occurs roughly longitudinally coincident with the total‐ozone wave with its peak near the prime meridian and trough near the central Pacific Ocean. The monthly average stratospheric ozone exhibits a wave‐1 pattern with amplitude of ∼4 DU and a maximum peak‐to‐trough difference of ∼13 DU in the 1979–1984 time period. This stratospheric amplitude represents about one half of the ∼7 DU amplitude in total column ozone. This partitioning of some of the total ozone column wave into the stratosphere affects the tropospheric ozone component of derivations that assume stratospheric zonal invariance.
High latitude biospheric activity during the Last Glacial Cycle revealed by ammonium variations in Greenland Ice Cores
Authors: Hansson Margareta , Holmén Kim
Published: February 2004

The short (325 m) Renland ice core from East Greenland was the first ice core to provide a continuous record of ammonium (NH4+) through the whole glacial period. The deep ice cores GRIP and GISP2 from Summit have confirmed that it is representative of the NH4+deposition on Greenland. We argue that high levels of NH4+in the ice during periods of warming (at the end of the last glaciation, during Dansgaard/Oeschger events, in MIS 5a and 5c, and during the Eem interglacial) provide evidence for rapid response by high latitude ecosystems and soils since the atmospheric turn‐over time of ammonia (including its existence as NH4+) is short. Sources which emit ammonia usually also emit methane. High Greenland NH4+levels coincide with high global methane concentrations. Northern high latitude biogenic sources are here suggested to contribute significantly to global methane concentrations during periods of warming during the last glacial period.
Trends in evaporation and surface cooling in the Mississippi River Basin
Authors: Milly P. C. D. , Dunne K. A.
Published: February 2004

A synthesis of available data for the Mississippi River basin (area 3 × 106 km²) reveals an upward trend in evaporation during recent decades, driven primarily by increases in precipitation and secondarily by human water use. A cloud‐related decrease in surface net radiation appears to have accompanied the precipitation trend. Resultant evaporative and radiative cooling of the land and lower atmosphere quantitatively explains downward trends in observed pan evaporation. These cooling tendencies also reconcile the observed regional atmospheric cooling with the anticipated regional ‘greenhouse warming.’ If recent high levels of precipitation (which correlate with the North Atlantic Oscillation) are mainly caused by an internal climatic fluctuation, an eventual return to normal precipitation could reveal heretofore‐unrealized warming in the basin. If, instead, they are caused by some unidentified forcing that will continue to grow in the future, then continued intensification of water cycling and suppression of warming in the basin could result.
Identification of the Kuroshio Extension, its bifurcation and northern branch from altimetry and hydrographic data during October 1992–August 1999: Spatial and temporal variability
Authors: Sainz‐Trapaga Susana M. , Goni Gustavo J. , Sugimoto Takashige
Published: February 2004

A methodology is proposed using altimter‐derived upper layer thickness and baroclinic transport to identify the Kuroshio Extension, the Bifurcation Point and the Northern Branch, by combining TOPEX/POSEIDON altimeter and climatological data within a two‐layer reduced gravity model. Results obtained from the Japanese coast to 175°W show that the location of the Bifurcation Point presents interannual variability that is related with upstream conditions. The longitude of the Bifurcation Point ranged from 147 to 160°E. Estimates of baroclinic transport at the Kuroshio Extension and its Northern Branch decrease steadily to the east trough the region of study from 35 to 11 and from 10 to 3 Sv, respectively.
Deep circulation in the equatorial Atlantic Ocean
Authors: Gouriou Y. , Andrié C. , Bourlès B. , Freudenthal S. , Arnault S. , Aman A. , Eldin G. , du Penhoat Y. , Baurand F. , Gallois F. , Chuchla R.
Published: February 2004

In the Atlantic Ocean, the northward export of warm surface water is compensated by a southward flow of cold North Atlantic Deep Water (NADW). The NADW is transported southward along the American continental margin within the Deep Western Boundary Current (DWBC). Some tracer and float observations have shown that part of the DWBC water flows eastward along the equator. Here we present three meridional velocity sections which give an instantaneous image of the top‐to‐bottom zonal circulation along the equatorial Atlantic. They reveal the presence of Equatorial Deep Jets (EDJs) between 1°30′N and 1°30′S, alternating eastward‐westward currents with short vertical scale, surrounded by columns of eastward currents (the Extra Equatorial Jets or EEJs) at 2°N and 2°S. In addition to direct velocity measurements, tracer distributions give indications of water‐mass feeding of the EDJs and EEJs by the DWBC.
Titan, Mars and Earth : Entropy production by latitudinal heat transport
Authors: Lorenz Ralph D. , Lunine Jonathan I. , Withers Paul G. , McKay Christopher P.
Published: February 2004

Temperature contrasts between warm tropics and cool high latitudes depends on how efficiently heat is transported by the atmosphere (and oceans) from the tropics. This heat transport is generally assumed to be proportional to atmospheric pressure, but we show with a simple model that this prediction fails by orders of magnitude for Mars and Titan. However, a basic principle, previously postulated for the Earth, does successfully predict the heat flows and zonal temperatures of Mars and Titan. The circulation predicted by this Maximum Entropy Production (MEP) principle is consistent with Titan's observed zonal structure and the winds and CO2 frost cycle on Mars. The principle makes powerful predictions where detailed information is lacking, such as on the early Earth and on possibly habitable extrasolar planets.
Global observations of proton and electron auroras in a substorm
Authors: Mende S. B. , Frey H. U. , Lampton M. , Gerard J.‐C. , Hubert B. , Fuselier S. , Spann J. , Gladstone R. , Burch J. L.
Published: February 2004

This is the first report of a substorm observed by the IMAGE FUV instruments permitting global observations of electron and proton produced auroras. On the 28th of June 2000 at 1956 UT in the pre‐substorm phase at early evening local time the proton aurora was equatorward of the electron precipitation and near midnight they were collocated. There was bright electron and proton aurora in the post midday afternoon side. The sudden brightening of the aurora at substorm onset near midnight is seen in the electrons only although there are protons present at this location. During the expansive phase both the electrons and protons expand poleward. The electron aurora forms a bright surge at the poleward boundary while the protons just show diffuse spreading. The peak intensity of the protons did not change substantially during the entire event. The proton aurora is brighter on the dusk while the electron aurora on the dawn side. As the electron surge expands poleward it leaves the protons behind. The electrons form a discrete auroral feature near the aurora‐polar cap boundary, which is devoid of substantial energetic (>1 keV) proton precipitation. The presence of precipitating protons at the point where the initial brightening is seen shows that substorms are initiated on closed field lines.
Cloud filter for CO retrieval from IMG infrared spectra using ECMWF temperatures and POLDER cloud data
Authors: Hadji‐Lazaro Juliette , Clerbaux Cathy , Couvert Pierre , Chazette Patrick , Boonne Cathy
Published: February 2004

Infrared atmospheric measurements provided by space nadir looking remote sensors are affected by the presence of clouds. To obtain cloud‐free CO global distributions from the radiance spectra measured by the IMG instrument, we have developed a simple method of spectra selection using skin temperatures provided by the ECMWF model. Information on the cloud cover was provided by the cloud data of the POLDER instrument which flew aboard the same platform as IMG. We show that a filter based on skin temperature, with thresholds of 8 K over sea and 15.3 K over land, allows to remove most of the cloudy cases.
Slip correlations on a creeping fault
Authors: Perfettini Hugo , Schmittbuhl Jean , Vilotte Jean‐Pierre
Published: February 2004

Using a quasi‐static three dimensional fault model which accounts for long range elastic interactions, we examine the influence of spatial heterogeneities of frictional strength on the slip distribution along a creeping fault. Slip fluctuates spatially because of pinning on local asperities. We show that three regimes of slip correlations exist. The first regime results in a uniform slip as in an homogeneous medium. On the contrary, slip in the second regime highly fluctuates and is controlled by heterogeneities of frictional strength. The third regime is intermediate and develops areas of high slip that are much bigger than the local asperity size (self‐affine properties of the slip distribution). This particular regime illustrates the possible misinterpretation of low frequency slip data (e.g. interferometric and GPS data) in terms of structural or compositional properties along the fault.
Mid‐latitude (30°–60° N) climatic warming inferred by combining borehole temperatures with surface air temperatures
Authors: Harris Robert N. , Chapman David S.
Published: February 2004

We construct a mid‐latitude (30°–60° N) reduced temperature‐depth profile from a global borehole temperature database compiled for climate reconstruction. This reduced temperature profile is interpreted in terms of past surface ground temperature change and indicates warming on the order of 1°C over the past 100 to 200 years. The combination of an initial temperature (the primary free parameter) with the last 140 years of gridded surface air temperature (SAT) data yields a synthetic temperature profile that is an excellent fit to observations, accounting for 99% of the observed variance and a RMS misfit of only 12 mK. The good correlation suggests that this reduced temperature profile shares much information with the mean SAT record over large areas and long time‐scales. Our analysis indicates 0.7°±0.1°C of ground warming between pre‐industrial time and the 1961–1990 mean SAT.
Do tropical cells ventilate the Indo‐Pacific Equatorial Thermocline?
Authors: Hazeleger Wilco , de Vries Pedro , van Oldenborgh Geert Jan
Published: February 2004

Source waters of the Indo‐Pacific equatorial thermocline are studied with a high‐resolution ocean model. In the annual mean fields, tropical and subtropical overturning cells are found that upwell at the equator and downwell at 5 degrees and 20 degrees poleward of the equator respectively. Tropical cells are common in ocean models, but their role in ventilating the equatorial thermocline is obscure because the downwelled water is too warm to match the subsurface equatorial waters. The tropical cells are much weaker when the overturning is considered in density coordinates. When high‐frequency mass fluxes are included tropical cells are compensated by an eddy‐induced overturning. Seasonal variations and tropical instability waves are responsible for the compensation. It follows that only subtropical cells transfer surface water to the equatorial thermocline. Strong tropical cells are shown to be an artefact.