bluuebunny/crossref_metadata_2025 huggingface dataset:
Mass balance of higher‐elevation parts of the Greenland ice sheet
Authors: Thomas R. , Csatho B. , Davis C. , Kim C. , Krabill W. , Manizade S. , McConnell J. , Sonntag J.
Published: February 2004

Satellite radar and aircraft laser altimeter data and a volume budget comparison of total snow accumulation with total ice discharge give three independent estimates of the recent mass balance of parts of the Greenland ice sheet above ∼2000 m elevation. Results show the entire region, on average, to be in balance to within 10 mm yr−1, with very low rates of regional thickness change (dH/dt) in the northeast but high rates with large spatial variability in the south. Only the volume budget estimates show significant thinning in the northwest, but thinning in this area is also inferred from local measurements of ice vertical velocities down boreholes. South of latitude 68°N, there has been rapid thickening west of the ice divide, with equally rapid thinning in the southeast, but with large differences between the three estimates of dH/dt throughout this region. The radar data apply to the period 1978–1988, the laser data are for 1993/1994–1998/1999, and the volume budget calculations represent conditions over at least the last few decades. Consequently, many of the differences between results could be caused by temporal changes, particularly in snow accumulation rates, that occurred since the 1970s. However, taken with other information, our results suggest long‐term thickening in the southwest and possibly quite recent onset of rapid thinning in the southeast.
Airglow imaging of gravity waves: 2. Critical layer filtering
Authors: Stockwell R. G. , Lowe R. P.
Published: February 2004

Images of the variations of OH intensities have been recorded by a three‐station network of scanning radiometers in southwestern Ontario during the period from May until September 1998. The ensemble of gravity waves seen in all three instruments is analyzed and observed to have a strong dependence in their propagation direction with the majority of waves seen at each of the three sites having a horizontal wave vector in the northeast direction. In order to understand these results a model of the transmission of gravity waves through the atmosphere was employed. The gravity waves observed by the OH Scanning Radiometers (OHSCRs) are assumed to originate in the troposphere with an isotropic spectrum, meaning that the average wave amplitudes are equal in all directions. The gravity waves propagate upward where they will undergo a selection process owing to the effect of the “background winds.” Using the horizontal winds from the Horizontal Wind Model (HWM‐93), the transmission of gravity waves from a tropospheric source is modeled and shown to be consistent with the observations. The strong zonal winds account for the summer eastward and winter westward propagation, but it is the tidal components of the meridional wind that account for the northward tendency of wave propagation (and the corresponding relative absence of southward propagating waves). In order to further validate the transmission, model winter measurements from the OHSCR data set are examined and shown to be consistent.
Aircraft observations of the solar ultraviolet irradiance throughout the troposphere
Authors: Varotsos C. , Alexandris D. , Chronopoulos G. , Tzanis C.
Published: February 2004

Observations of the biological effective solar ultraviolet irradiance, made by instrumentation flown on a Falcon aircraft over the entire Greek area, from the sea up to the tropopause level, obtained in the frame of the Radiation Field in the Troposphere‐Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe project, are presented to discuss among others its altitude dependence. The experimental data of the biological effective solar UV irradiance as a function of height, throughout the troposphere, showed an average increase of about 7.2±1.2% km−1, which is in close agreement with the theoretically derived one, by using an appropriate algorithm, providing the biological effective solar UV irradiance at various altitudes from the Earth's surface. This increase rate has also been compared with the burden ozone content at each height level as it was derived from ozone concentration measurements obtained by both balloon ascents and the aircraft's instrumentation. This comparison showed a strong anticorrelation (correlation coefficient r = −0.98) between the biological effective solar UV irradiance and the integrated ozone content above each height level corresponding to 0.25% increase in UV per 1 Dobson Unit (1DU = 0.001 atm cm) decrease in total ozone column, approximately.
A new ice thickness and bed data set for the Greenland ice sheet: 2. Relationship between dynamics and basal topography
Authors: Layberry R. L. , Bamber J. L.
Published: February 2004

New data sets of bed and surface elevation have been combined with balance velocities, bed topography, and radar reflectivity to investigate the relationship between ice flow and bed conditions. It has been found that the bed topography affects the local driving stresses and initiates the onset of streaming flow in specific areas where there are large undulations. This onset region only appears significantly inland in three areas: the northeast Greenland ice stream, Kangerdlugssuaq Gletscher, and a region in the southwest associated with abnormally low driving stresses and an apparently unnamed but active glacier. Bed reflectance values below the northeast ice stream suggest that basal melting could be occurring along its length, and it seems likely therefore that the enhanced flow observed inland from the onset of streaming may be due, in part, to basal sliding.
Mesoscale flow modification induced by land‐lake surface temperature and roughness differences
Authors: Samuelsson P. , Tjernström M.
Published: February 2004

Numerical model simulations and airborne measurements have been used to study the mesoscale flow modification of the boundary layer due to the presence of a lake (34 km2) located in the southern boreal zone. The mesoscale model used is a three‐dimensional hydrostatic model with a “level 2.5” turbulence closure. The results from this study show that in general, differences in surface roughness are more important than differences in surface temperature for the flow modification. The dynamical effect due to the roughness difference between the lake and the surrounding land creates divergence over most of the lake because of the accelerating flow, while a convergence zone appears at the downwind shore because of the deceleration. This pattern is found during nighttime for all simulated cases with wind speeds from 4 to 17 m s−1. This pattern persists during daytime for moderate to high wind speeds but is reduced in strength owing to stable stratification over the lake. For wind speeds ≤6 m s−1 a lake breeze circulation develops during the afternoon. Complicated flow patterns occur when the forces due to friction and thermal pressure gradient are comparable in size. Available measurements qualitatively support the model results.
Contribution to the glaciology of northern Greenland from satellite radar interferometry
Authors: Rignot Eric , Gogineni Sivaprasad , Joughin Ian , Krabill William
Published: February 2004

Interferometric synthetic aperture radar (InSAR) data from the ERS‐1 and ERS‐2 satellites are used to measure the surface velocity, topography, and grounding line position of the major outlet glaciers in the northern sector of the Greenland ice sheet. The mass output of the glaciers at and above the grounding line is determined and compared with the mass input. We find that the grounding line output is approximately in balance with the input, except for the three largest glaciers for which the mass loss is 4±3 km3 ice year−1 or 11±8% of the mass input. Along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with InSAR, which implies that the outlet glaciers are thinning. The inferred coastal thinning is too large to be explained by a few warm summers. Glacier thinning must be of dynamic origin, that is, caused by spatial and temporal changes in ice velocity. Iceberg production from the glaciers is uncharacteristically low. It accounts for only 8% of the ice discharge to the ocean. About 55% of the ice is lost through basal melting (5–8 m ice year−1 on average) from the underside of the floating glacier tongues that are in contact with warm ocean waters. Mass losses are highest in the first 10 km of floating ice, where ice reaches the greatest depths and basal melting is 3 times larger than on average. Only a small increase in basal melting would suffice to disintegrate the floating glacier tongues.
Assessment of Greenland albedo variability from the advanced very high resolution radiometer Polar Pathfinder data set
Authors: Stroeve Julienne
Published: February 2004

The advanced very high resolution radiometer Polar Pathfinder (APP) data set is used to examine the variability of the surface albedo over Greenland. Analysis of the APP albedo record from 1981 to 1998 show anomalously low albedo during 1995 and 1998 over most of the ice sheet as compared with the other years. The low albedo encountered during these years suggests that the ice sheet experienced considerable melt in 1995 and 1998, particularly near the western margin of the ice sheet. Conversely, anomalously high albedos were found in 1992 as a result of colder temperatures and hence less melt following the eruption of Mount Pinatubo. The relationship between the annual North Atlantic Oscillation (NAO) index and the mean summer albedo from all the stations reveals a positive correlation of 0.44 and a positive correlation of 0.55 for the southern part of the ice sheet. Therefore variations in the mean summer albedo over Greenland can, in part, be explained by variations in the NAO such that during periods of intensification of the normal mode of the NAO the mean summer albedo is above normal. Trend analysis reveals an overall downward trend in surface albedo from 1981 to 1998, which agrees with recent trends in melt and precipitation. However, the trend was found not to be statistically significant but rather influenced by the low albedo in recent years.
Marine boundary layer peroxy radical chemistry during the AEROSOLS99 campaign: Measurements and analysis
Authors: Hernández Maria Dolores Andrés , Burkert Jörn , Reichert Lars , Stöbener Dirk , Meyer‐Arnek Julian , Burrows John P. , Dickerson R. R. , Doddridge B. G.
Published: February 2004

During the AEROSOLS99 cruise over the Atlantic and Southern Indian Oceans, peroxy radicals (RO2* = HO2 + ΣRO2, where R is an organic group) were continuously measured. Regular diurnal variations were observed with maximum mixing ratios of 30 to 55 parts per trillion by volume around noon. The latitudinal distribution of RO2* over the ocean seems to remain uniform under similar conditions of solar insolation. The composition of the marine boundary layer reflects long‐range transport of air from the African continent. The influence of biomass burning over remote areas of the marine boundary layer is confirmed, although the atmosphere over the Southern Hemisphere was often pristine, with rapid destruction of ozone observed during the daylight hours. Observed RO2* concentrations are in reasonable agreement with box model calculations, but cannot account for the highest diurnal variations of ozone observed. Possible explanations include halogen chemistry and nocturnal entrainment of O3 from the layers above, but available data cannot readily determine their relative contributions.
Balance mass flux and ice velocity across the equilibrium line in drainage systems of Greenland
Authors: Zwally H. Jay , Giovinetto Mario B.
Published: February 2004

Estimates of balance mass flux and depth‐averaged ice velocity through the cross section aligned with the equilibrium line are produced for each of six drainage systems in Greenland. The estimates are based on a model equilibrium line fitted to field data and on a revised distribution of surface mass balance for the conterminous ice sheet. Ice drainage divides and six major drainage systems are delineated using surface topography from ERS radar altimeter data. Ice thicknesses at the equilibrium line and throughout each drainage system are based on the latest compilation of airborne radar sounding data described elsewhere. The net accumulation rate in the area bounded by the equilibrium line is 399 Gt a−1, and net ablation rate in the remaining area is 231 Gt a−1. Excluding an east central coastal ridge reduces the net accumulation rate to 397 Gt a−1, with a range from 42 to 121 Gt a−1 for the individual drainage systems. The mean balance mass flux and depth‐averaged ice velocity at the cross‐section aligned with the modeled equilibrium line are 0.1011 Gt km−2 a−1 and 0.111 km a−1, respectively, with little variation in these values from system to system. In contrast, the mean mass discharge per unit length along the equilibrium line ranges from one half to double the overall mean rate of 0.0468 Gt km−1 a−1. The ratio of the ice mass in the area bounded by the equilibrium line to the rate of mass output implies an effective exchange time of approximately 6 ka for total mass exchange. The range of exchange times, from a low of 3 ka in the SE drainage system to 14 ka in the NE, suggests a rank as to which regions of the ice sheet may respond more rapidly to climate fluctuations.
A further study of the method for estimation of SAGE II opaque cloud occurrence
Authors: Wang Pi‐Huan , Veiga Robert E. , Vann Lelia B. , Minnis Patrick , Kent Geoffrey S.
Published: February 2004

Information on vertical cloud distribution is important to atmospheric radiative calculation, general circulation modeling, and climate study. The method used for estimating the vertical structure of opaque cloud occurrence from the solar occultation observations obtained by the Stratospheric Aerosol and Gas Experiment (SAGE) II has been reviewed for further understanding of the nature of the derived cloud statistics. Most importantly, based on the SAGE II tropical observations (1985–1998), the present study illustrates that the derived opaque cloud occurrence at a given altitude is generally independent of the cloud occurrence at other altitudes, except for some anticorrelation between high‐level (12.5 km) and low‐level (1–3 km) clouds. This feature of the layer cloud frequency independence is also evident when regional data over the Pacific warm pool and the eastern Pacific are examined. The independent information of the layer cloud frequency is significant and makes it possible to use the derived vertical distribution of cloud occurrence to estimate the probability of multilayer clouds. The limitation is that it is difficult to determine how frequently the multilayer clouds are actually overlapping or how frequently thick cloud (>1 km) really occurs based on the SAGE II observations alone. A discussion of the SAGE II tropical opaque cloud occurrence in relation to the cloud climatology based on visual observations from surface stations and ships, the International Satellite Cloud Climatology Project data, and the cloud statistics using rawinsonde records is also provided.
Aerosol properties around marine tropical cumulus clouds
Authors: Kaneyasu Naoki , Hobbs Peter V. , Ishizaka Yutaka , Qian Gong‐Wang
Published: February 2004

Airborne measurements of aerosols around convective clouds over the tropical Pacific Ocean show enhanced concentrations of small (Aitken) particles in the vicinity of some of the clouds. In the same regions there were increases in the concentrations of internally mixed particles with diameters around 0.3 μm that contained acidic sulfate. The acidic sulfate is attributed to heterogeneous oxidation of SO2 in cloud droplets. There was also some evidence of nitrate in the internally mixed particles. The enhancement in the concentrations of ∼0.3 μm diameter particles produced smoother (i.e., more Junge‐type) particle size spectra.
Deterministic chaos in atmospheric radon dynamics
Authors: Cuculeanu Vasile , Lupu Alexandru
Published: February 2004

The correlation dimension and Lyapunov exponents have been calculated for two time series of atmospheric radon daughter concentrations obtained from four daily measurements during the period 1993–1996. A number of about 6000 activity concentration values of 222Rn and 220Rn daughters have been used. The measuring method is based on aerosol collection on filters. In order to determine the filter activity, a low background gross beta measuring device with Geiger‐Müller counter tubes in anticoincidence was used. The small noninteger value of the correlation dimension (≃2.2) and the existence of a positive Lyapunov exponent prove that deterministic chaos is present in the time series of atmospheric 220Rn daughters. This shows that a simple diffusion equation with a parameterized turbulent diffusion coefficient is insufficient for describing the dynamics in the near‐ground layer where turbulence is not fully developed and coherent structures dominate. The analysis of 222Rn series confirms that the dynamics of the boundary layer cannot be described by a system of ordinary differential equations with a low number of independent variables.
A meteorological overview of the second Pacific Exploratory Mission in the Tropics
Authors: Fuelberg Henry E. , Newell Reginald E. , Westberg David J. , Maloney Joseph C. , Hannan John R. , Martin Brian D. , Avery Melody A. , Zhu Yong
Published: February 2004

Meteorological conditions over the central Pacific Basin are summarized during NASA's second Pacific Exploratory Mission in the Tropics (PEM‐B) which was conducted during February‐April 1999. Mean flow patterns during PEM‐B are described. Important features near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), and the Intertropical Convergence Zone (ITCZ). The ITCZ is found to exhibit a double structure, with branches at ∼5°N and ∼5°S. Both the ITCZ and SPCZ are areas of widespread cloudiness and convection. Extensive lightning occurs over the land masses surrounding the Pacific Basin and over the central South Pacific Ocean itself. PEM‐B occurs during a La Nina period of relatively cold sea surface temperatures in the tropical Pacific. Compared to climatology, the PEM‐B period exhibits deep convection located west of its typical position, stronger than normal easterly trade winds, a relatively strong (weak) northern (southern) hemispheric jet stream, the SPCZ located west of its normal position, and an upper tropospheric cyclonic wind couplet that straddles the equator. Circulation patterns during PEM‐B are compared with those of PEM‐A which occurred during August‐September 1996. PEM‐B is found to exhibit a less organized ITCZ, a comparatively weak jet stream in the Southern Hemisphere, a relatively strong jet stream in the Northern Hemisphere, and enhanced convection over the central Pacific. Finally, meteorological conditions for selected flights are discussed utilizing streamlines, 10‐day backward trajectories, thermodynamic soundings, and satellite imagery. Air parcels sampled by the aircraft are found to originate or pass over diverse regions, including Asia, South America, southern Africa, and Australia. Some parcels remain over the Pacific Ocean during the preceding 10‐day period.
Nonequilibrium coexistence of solid and liquid particles in Arctic stratospheric clouds
Authors: Biele J. , Tsias A. , Luo B. P. , Carslaw K. S. , Neuber R. , Beyerle G. , Peter T.
Published: February 2004

Observations of polar stratospheric clouds (PSCs) from Ny Ålesund, Spitsbergen, have been examined to quantify the occurrence of solid particles. The polarized backscatter ratio was found to be a more sensitive indicator of the presence of non‐spherical (solid) particles in a PSC than the aerosol depolarization, which approaches zero for clouds containing a large volume of liquid droplets. The analysis corroborates our previous finding that type Ia PSCs cannot be composed of nitric acid trihydrate particles in equilibrium with the gas phase, which would lead to a much too high backscatter ratio. Conversely, type Ib PSCs, previously thought to be composed solely of liquid droplets, often contain a very small fraction of solid particles. These clouds develop a high parallel backscatter, arising from Mie scattering by droplets, and a nonvanishing perpendicular signal due to the few solid particles. Other type Ib PSCs appear to contain only liquid particles. The mixed liquid/solid clouds are observed on the smallest spatial and temporal scale resolvable by the lidar instrument (30 m, l min), implying that their properties are not a result of spatial averaging of different cloud types. Comparison of the observations with optical calculations shows that such nonequilibrium particle distributions are to be expected, as temperature changes are sufficiently rapid to prevent the particles from assuming equilibrium sizes. The observed optical characteristics of type Ia and type Ib clouds can be reproduced in a model by assuming that a very small fraction of the particles are composed of nitric acid hydrate, with the majority being binary H2SO4/H2O or ternary HNO3/H2SO4/H2O droplets.
Evidence of stratosphere‐to‐troposphere transport within a mesoscale model and Total Ozone Mapping Spectrometer total ozone
Authors: Olsen Mark A. , Stanford John L.
Published: February 2004

We present evidence for stratospheric mass transport into, and remaining in, the troposphere in an intense midlatitude cyclone. Mesoscale forecast model analysis fields from the Mesoscale Analysis and Prediction System were compared with total ozone observations from the Total Ozone Measurement Spectrometer. Coupled with parcel back trajectory calculations, the analyses suggest that two mechanisms contributed to the mass exchange: (1) A region of dynamically induced exchange occurred on the cyclone's southern edge. Parcels originally in the stratosphere crossed the jet core and experienced dilution by turbulent mixing with tropospheric air. (2) Diabatic effects reduced parcel potential vorticity (PV) for trajectories traversing precipitation regions, resulting in a “PV hole” signature in the cyclone center. Air with values of ozone and water vapor typical of the lower stratosphere was left in the troposphere. The strength of the latter process may be atypical. These results, combined with other research, suggest that precipitation‐induced diabatic effects can significantly modify (either decreasing or increasing) parcel potential vorticity, depending on parcel trajectory configuration with respect to jet core and maximum heating regions. The diabatic heating effect on stratosphere‐troposphere exchange (STE) is more important to tropopause erosion than to altering parcel trajectories. In addition, these results underscore the importance of using not only PV but also chemical constituents for diagnoses of STE.
Contribution of road traffic emissions to the atmospheric black carbon burden in the mid‐1990s
Authors: Köhler Ines , Dameris Martin , Ackermann Ingmar , Hass Heinz
Published: February 2004

We developed a global inventory for black carbon (BC) emissions from road traffic for 1993. Global emissions of 2.4 TgC were found in this year. The inventory was implemented in the atmosphere general circulation model ECHAM4.L39(DLR), together with inventories for BC emissions from fossil fuel combustion, biomass burning, and air traffic. The transport of BC particles and the concentration changes due to these emissions were investigated. Particularly, we focused on the contributions of road traffic emissions from North America, Europe, and Asia to the atmospheric BC burden. The sink of BC was parameterized as an exponential decay process with a half‐lifetime depending on altitude. Because of the simplified BC cycle, the current investigation should be regarded as a pilot study. However, it is the first study dealing specifically with global road traffic BC emissions. The model results indicate that less than 20% of the BC mass concentrations predicted to be in the planetary boundary layer of the northern midlatitudes arise from road traffic. The road traffic fraction of BC concentrations over North America, Europe, and Asia originated mostly from emissions in the respective region. However, road traffic BC particles from Asia also significantly influence the tropopause region. In July their relative contribution is about 10%. A very rough estimate of the globally averaged direct radiative forcing of BC particles from global and Asian road traffic is given. As road traffic and its emissions in Asia are expected to rise in the near future, their relative importance will also grow and may, eventually, become a significant factor in anthropogenic climate change, unless the specific emissions are drastically reduced.
Sublimation on the Greenland Ice Sheet from automated weather station observations
Authors: Box Jason E. , Steffen Konrad
Published: February 2004

Greenland Climate Network (GC‐Net) surface meteorological observations are used to estimate net surface water vapor flux at ice sheet sites. Results from aerodynamic profile methods are compared with eddy correlation and evaporation pan measurements. Two profile method types are applied to hourly data sets spanning 1995.4 to 2000.4. One method type is shown to accurately gauge sublimation using two humidity and wind speed measurement levels. The other “bulk” method type is shown to underestimate condensation, as it assumes surface saturation. General climate models employ bulk methods and, consequently, underestimate deposition. Loss of water vapor by the surface predominates in summer at lower elevations, where bulk methods agree better with two‐level methods. Annual net water vapor flux from the two‐level method is as great as −87±27 mm at 960 m elevation and −74±23 mm at equilibrium line altitude in western Greenland. At an undulation trough site, net deposition is observed (+40 mm ±12). At the adjacent crest site 6 km away and at 50 m higher elevation, net sublimation predominates. At high‐elevation sites, the annual water vapor flux is positive, up to +32±9 mm at the North Greenland Ice core Project (NGRIP) and +6±2 mm at Summit. Sublimation is mapped using trend surface fits to calculated sublimation in terms of elevation and latitude. The resulting ice sheet total sublimation is −0.62 ± 0.25 × 1014 kg yr−1 for the two‐level profile method and −1.2 ± 0.65 × 1014 kg yr−1 for the one‐level method, indicating 12% or 23% precipitation loss, respectively. Monthly, seasonal, and annual sublimation grids and the mapping functions are available on the internet at http://cires.colorado.edu/steffen.
Global observations of the carbon budget: 1. Expected satellite capabilities for emission spectroscopy in the EOS and NPOESS eras
Authors: Engelen Richard J. , Denning A. Scott , Gurney Kevin R. , Stephens Graeme L.
Published: February 2004

This paper investigates the expected capabilities of the new generation of infrared satellite sounders for detecting CO2. A general circulation model is used to simulate realistic CO2 fields and to define the needed accuracy of CO2 observations in order to be useful in constraining surface sources and sinks of CO2, which will be described in more detail in a future paper. Optimal estimation retrieval theory is then used to determine the possible accuracy of the satellite measurements and to define the retrieval characteristics. A discussion of several factors that affect the retrievals is also included. We conclude that tropospheric column retrievals of CO2 are possible with an accuracy of better than 1 ppmv on a monthly mean basis. Several factors, like thin cirrus clouds and radiative transfer modeling errors, will degrade these results if not carefully accounted for. The possibility of extensive time and spatial averaging of the satellite observations will overcome some of these problems.
Climate change in recurrent regimes and modes of northern hemisphere atmospheric variability
Authors: Hsu C. Juno , Zwiers Francis
Published: February 2004

We identify recurrent regimes of atmospheric states in observed and model‐simulated data with and without anomalous greenhouse gas and aerosol forcing by testing against the null hypothesis that the atmospheric states are produced by a Gaussian autoregressive process of the order of 1 in a reduced phase space. In one approach the phase space is described by the leading Northern Hemisphere Empirical Orthogonal Functions (EOFs), while in another, it is described by sectorial EOFs of the northern Atlantic and the Pacific regions. The time series of model output are sufficiently long to show clearly, in the sectorial analysis, that the increased greenhouse gas forcing leads to a change in the frequencies of occurrence of the recurrent regimes. Such evidence is not obtained from the hemispheric analysis of model output. The observational time series is found to be too short to reliably identify changes in recurrent flow regimes with the exception that there appear to have been significant changes in the flow patterns corresponding to the Cold Ocean Warm Land pattern.
Dependence of aerosol light absorption and single‐scattering albedo on ambient relative humidity for sulfate aerosols with black carbon cores
Authors: Redemann Jens , Russell Philip B. , Hamill Patrick
Published: February 2004

Atmospheric aerosols frequently contain hygroscopic sulfate species and black carbon (soot) inclusions. In this paper we report results of a modeling study to determine the change in aerosol absorption due to increases in ambient relative humidity (RH), for three common sulfate species, assuming that the soot mass fraction is present as a single concentric core within each particle. Because of the lack of detailed knowledge about various input parameters to models describing internally mixed aerosol particle optics, we focus on results that were aimed at determining the maximum effect that particle humidification may have on aerosol light absorption. In the wavelength range from 450 to 700 nm, maximum absorption humidification factors (ratio of wet to “dry = 30% RH” absorption) for single aerosol particles are found to be as large as 1.75 when the RH changes from 30 to 99.5%. Upon lesser humidification from 30 to 80% RH, absorption humidification for single particles is only as much as 1.2, even for the most favorable combination of initial (“dry”) soot mass fraction and particle size. Integrated over monomodal lognormal particle size distributions, maximum absorption humidification factors range between 1.07 and 1.15 for humidification from 30 to 80% and between 1.1 and 1.35 for humidification from 30 to 95% RH for all species considered. The largest humidification factors at a wavelength of 450 nm are obtained for “dry” particle size distributions that peak at a radius of 0.05 μm, while the absorption humidification factors at 700 nm are largest for “dry” size distributions that are dominated by particles in the radius range of 0.06 to 0.08 μm. Single‐scattering albedo estimates at ambient conditions are often based on absorption measurements at low RH (∼30%) and the assumption that aerosol absorption does not change upon humidification (i.e., absorption humidification equal to unity). Our modeling study suggests that this assumption alone can introduce absolute errors in estimates of the midvisible single‐scattering albedo of up to 0.05 for realistic dry particle size distributions. Our study also indicates that this error increases with increasing wavelength. The potential errors in aerosol single‐scattering albedo derived here are comparable in magnitude and in addition to uncertainties in single‐scattering albedo estimates that are based on measurements of aerosol light absorption and scattering.
Interpretation of interannual winter temperature variations over southwestern Europe
Authors: Sáenz Jon , Rodríguez‐Puebla Conceptión , Fernández Jesús , Zubillaga Juan
Published: February 2004

A great part of winter temperature variability over southwestern Europe (SWE) shows a very low correlation with the North Atlantic or Arctic Oscillation indices. The patterns of winter mean surface air temperature over SWE were obtained from gridded and instrumental data. The first mode is highly correlated with the East Atlantic (EA) pattern and explains temperature variations of the same sign over the whole domain. The second mode, however, is correlated with the North Atlantic Oscillation (NAO) and shows a dipolar structure with significant negative values in northern Africa and the southern Iberian Peninsula and positive values toward northern Europe. Thus the NAO only influences the meridional gradient of temperature over the area, and the second mode explains a significantly lower fraction of variance than the first mode. Both modes of temperature variability depend on the stationary component of the sensible heat fluxes but cannot be explained through the eddy sensible heat fluxes. It is well known that precipitation depends on the existence of baroclinic systems over the area. According to the results in this paper, this does not hold for temperature. This explains the different sensitivities of SWE precipitation and temperature to the North Atlantic Oscillation.
Nucleation in the equatorial Pacific during PEM‐Tropics B: Enhanced boundary layer H2SO4 with no particle production
Authors: Weber R. J. , Moore K. , Kapustin V. , Clarke A. , Mauldin R. L. , Kosciuch E. , Cantrell C. , Eisele F. , Anderson B. , Thornhill L.
Published: February 2004

During the second phase of the NASA Pacific Exploratory Mission to the Pacific Tropics (PEM‐Tropics B), regions of unusually high concentrations of sulfuric acid vapor ranging from 107 to 108 molecules cm−3 were detected near the ocean surface in equatorial regions between Hawaii and Tahiti. No 3–4 nm diameter nanoparticles were observed near the ocean surface where acid concentrations were highest; however, 3–4 nm particles were detected at higher elevations in regions near clouds. Calculations show that in some regions of high acid concentrations newly formed particles would be readily detected by our instruments and thus the lack of nanoparticles suggests that there was no nucleation. In contrast, in the previous PEM‐Tropics A mission Clarke et al. [1998a] observed a large nucleation event in the equatorial marine boundary layer under similar temperatures, relative humidity, and sulfuric acid concentrations. Comparison between these two studies further demonstrates that some additional species or unknown process is necessary to significantly enhance nucleation in the remote marine troposphere and that this component is not always present at levels sufficient to sustain nucleation throughout the region, even at a low continuous rate. We speculate that if ammonia is one example of a critical nucleation precursor, tropospheric ternary nucleation (sulfuric acid/ammonia/water) under some conditions requires ammonia concentrations to be greater than typical background concentrations for particle production via this mechanism.
Modeled precipitation variability over the Greenland Ice Sheet
Authors: Bromwich David H. , Chen Qui‐Shi , Bai Le‐Sheng , Cassano Elizabeth N. , Li Yufang
Published: February 2004

On the basis of the evaluation of recent Greenland precipitation studies, some of the deficiencies in the modeled precipitation are probably related to the topographic data employed in modeling. In this paper the modern digital elevation data of Ekholm [1996] is used. If the horizontal pressure gradient force in σ coordinates is separated into its irrotational and rotational parts, which are expressed by the equivalent geopotential and geo‐stream‐function, respectively, the topographic effect on the precipitation can be accurately modeled. The equivalent geopotential and geo‐stream‐function are implemented in a fully consistent manner in the generalized ω‐equation in this paper. A simplified large‐scale condensation without evaporation of condensate is also used. These improvements are combined to yield an improved dynamic method. Two aspects of the precipitation distribution are refined by the improved dynamic method. One is the 10 cm yr−1 contour near Summit, Greenland, and the other is a relative large precipitation area centered near the point (70°N, 49°W). Extensive comparisons are made between the retrieved precipitation and the observed annual accumulation time series from 11 ice core sites on the ice sheet. The modeled precipitation from the original method must use sealers to have a high degree of interannual correspondence between the measured accumulation and the retrieved precipitation, but the retrieved precipitation from the improved method increases at all ice core sites and a good correspondence is obtained without any sealer being required. The spatial average of multiyear mean error (
) is 11.5 cm yr−1 for the modeled precipitation from the improved method, while that for P from ERA‐15 is 14.5 cm yr−1. The total mean error (εM) is 3.0 cm yr−1 for the improved method, while εM for the P from ERA‐15 is 4.0 cm yr−1. These two errors show that the precipitation modeled by the improved method is better than the P from ERA‐15. Thus the distribution of precipitation over the 11 sites retrieved by the improved dynamic method is considerably refined. Large downward trends in annual amounts are present in the precipitation retrieved by the improved dynamic method for all of Greenland and its southern and central west coastal regions. The modeled precipitation from the improved dynamic method and observed accumulation from ice cores are all in agreement with the Thomas et al. [1999] result that the southern Greenland ice sheet above 2000 m is approximately in balance. It also shows that local thickening and thinning areas of the ice sheet derived by airborne laser altimetry from 1993 to 1999 over the entire Greenland above 2000 m [Krabill et al., 2000] are approximately consistent with precipitation change retrieved by the improved dynamic method.
Structure and evolution of the Magnitogorsk forearc basin: Identifying upper crustal processes during arc‐continent collision in the southern Urals
Authors: Brown D. , Alvarez‐Marrón J. , Pérez‐Estaún A. , Puchkov V. , Gorozhanina Y. , Ayarza P.
Published: February 2004

The southern Urals of Russia contain a well‐preserved example of a Paleozoic arc‐continent collision in which the intraoceanic Magnitogorsk volcanic arc and its forearc basin sediments accreted to the East European Craton during the Devonian. The Magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc, and by comparing its surface geological features with those in active arc‐continent collision settings it is possible to identify upper crustal processes that were active in the southern Urals. The arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized. The late Lower to Middle Devonian Aktau Formation represents a remnant of the intraoceanic to collisional forearc basin to the Irendyk volcanic front, whereas the Middle Devonian to Lower Carboniferous Ulutau, Koltubanian, and Zilair Formations were deposited in a suture forearc basin to the east Magnitogorsk volcanic front. It was not until the Late Devonian that these two basins were joined. Structural mapping, combined with reflection seismic profiling, shows these basins to be affected by open, nonlinear, volcanic basement‐cored synsedimentary folds. The Karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin. The forearc region is affected by minor thrusting that involves the volcanic basement, although it is not clear if these thrusts reactivate preexisting trench‐parallel faults. Synsedimentary deformation, slumping, and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the Late Devonian, when the full thickness of the continental crust is interpreted as having arrived at the subduction zone.
40Ar/39Ar muscovite ages from the Penninic‐Austroalpine plate boundary, Eastern Alps
Authors: Liu Yongjiang , Genser Johann , Handler Robert , Friedl Gertrude , Neubauer Franz
Published: February 2004

The Radstadt Mountains, Eastern Alps, expose the tectonic boundary between the base of the Austroalpine continental plate (hanging wall) and the Penninic oceanic units (foot wall). Parts of the Austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the Permian (290–250 Ma). Austroalpine Permo‐Mesozoic cover rocks were deformed during the Cretaceous (∼80 Ma) and the Paleogene (55–50 Ma). These ages are interpreted as representing two distinct events of nappe stacking. The younger ages represent the collision of Austroalpine and Penninic tectonic units. The Penninic nappe complex displays a successive decrease of ages from ∼37 to 25 Ma from high to deep tectonic levels. A second age group of ∼22 Ma was found both in low‐temperature release steps and as plateau ages close to the Penninic‐Austroalpine boundary. It is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge.
Synlithification deformation processes of the Cretaceous sediments of the Ivory Coast‐Ghana transform margin: A way to detail the margin history
Authors: Huguen Caroline , Guiraud Michel , Benkhelil Jean , Mascle Jean
Published: February 2004

The Ivory Coast‐Ghana margin is a typical transform margin along which large‐scale tectonism has been well documented. The Cretaceous sediments of the Ivory Coast‐Ghana transform margin were subjected to numerous tectonic, gravitational, and hydrothermal forces during lithification. In this paper, we infer the order of occurrence of deformation regimes from characteristic deformations observed in samples of Ocean Drilling Program (ODP) Leg 159. Three main types of minor scale deformation are identified and characterized in terms of geometry, rheology, and kinematics: (1) extensional structures such as normal faults and irregular veins with angular, indented walls that are widely recorded at all ODP sites; (2) compressional structures that are confined to the marginal ridge of the Ivory Coast‐Ghana transform margin and include reverse faults, folds, and vertical crenulation cleavage and (3) various types of shear zone at low angles with respect to bedding, which are continuous throughout the Cretaceous succession of the ridge. Structural analysis combined with thin section observation shows that most of these deformations occurred in water‐rich sediments. The extensional, minor‐scale structures are related to the Early Cretaceous rifting stage of the margin as characterized at regional scale by half‐grabens and tilted blocks. The compressional structures observed in the samples are attributed to Turonian‐Santonian positive inversion tectonics and related to the occurrence of large‐scale flower structures and inverted extensional blocks. In contrast, the shear zones at low angles with respect to bedding observed throughout the Cretaceous series are linked to local gravity‐induced tectonics.
Paleostress magnitudes associated with development of mountain belts: Insights from tectonic analyses of calcite twins in the Taiwan Foothills
Authors: Lacombe Olivier
Published: February 2004

Determinations of stress magnitudes based on inversion of calcite twin data from fault‐related folds in the Taiwan Foothills show a decrease of differential stresses during synfolding erosion and exhumation, suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks. Regional frictional conditions linked to shallow and/or deep décollement tectonics probably also influenced stress magnitudes. Combination of the twin inversion technique with fracture analysis and rock mechanics data provides first‐order estimates of principal stress values related to the major Pleistocene shortening event in Taiwan. These stress estimates are compared to previous stress estimates in fold‐thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens. At the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland. This decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation. Differential stresses estimated from natural deformation consequently provide an independent support to the depth‐dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes.
Development of channel organization and roughness following sediment pulses in single‐thread, gravel bed rivers
Authors: Madej Mary Ann
Published: February 2004

Large, episodic inputs of coarse sediment (sediment pulses) in forested, mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness. A conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels. Channels exhibited self‐organizing behavior to various degrees based on channel gradient, presence of large in‐channel wood or other forcing elements, the size of the sediment pulse, and the number of bed‐mobilizing flows since disturbance. Typical channel changes following a sediment pulse were initial decreases in water depth, in variability of bed elevations, and in the regularity of bed form spacing. Trajectories of change subsequently showed increased average water depth, more variable and complex bed topography, and increased uniformity of bed form spacing. Bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale (two to five channel widths) than in streams without such forcing mechanisms (five to 10 channel widths). Channel roughness increased as bed forms developed.
Predictive modeling of flow and transport in a two‐dimensional intermediate‐scale, heterogeneous porous medium
Authors: Barth Gilbert R. , Hill Mary C. , Illangasekare Tissa H. , Rajaram Harihar
Published: February 2004

As a first step toward understanding the role of sedimentary structures in flow and transport through porous media, this work deterministically examines how small‐scale laboratory‐measured values of hydraulic conductivity relate to in situ values of simple, artificial structures in an intermediate‐scale (10 m long), two‐dimensional, heterogeneous, laboratory experiment. Results were judged based on how well simulations using measured values of hydraulic conductivities matched measured hydraulic heads, net flow, and transport through the tank. Discrepancies were investigated using sensitivity analysis and nonlinear regression estimates of the in situ hydraulic conductivity that produce the best fit to measured hydraulic heads and net flow. Permeameter and column experiments produced laboratory measurements of hydraulic conductivity for each of the sands used in the intermediate‐scale experiments. Despite explicit numerical representation of the heterogeneity the laboratory‐measured values underestimated net flow by 12–14% and were distinctly smaller than the regression‐estimated values. The significance of differences in measured hydraulic conductivity values was investigated by comparing variability of transport predictions using the different measurement methods to that produced by different realizations of the heterogeneous distribution. Results indicate that the variations in measured hydraulic conductivity were more important to transport than variations between realizations of the heterogeneous distribution of hydraulic conductivity.