bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of quasi‐linear diffusion coefficients for EMIC waves in a multispecies plasma
Authors: Albert J. M.
Published: June 2003

Quasi‐linear velocity‐space diffusion coefficients due to L‐mode electromagnetic ion cyclotron (EMIC) waves are considered in a multispecies plasma. It is shown, with slight approximations to exact cold plasma theory, that within EMIC pass bands the index of refraction is a monotonically increasing function of frequency. Analytical criteria are then derived which identify ranges of latitude, wavenormal angle, and resonance number consistent with resonance in a prescribed wave population. This leads to computational techniques which allow very efficient calculation of the diffusion coefficients, along the lines previously developed for whistler and ion cyclotron waves in an electron‐proton plasma. The techniques are applied to radiation belt electrons at L = 4, for EMIC waves in the hydrogen, helium, and oxygen bands representative of different phases of a magnetic storm. Finally, diffusion coefficients for recovery‐phase helium‐band EMIC waves are combined with those for typical whistler hiss, resulting in electron precipitation lifetimes substantially less than those due to hiss alone.
Distribution of precarious rocks at the Nevada Test Site: Comparison with ground motion predictions from nuclear tests
Authors: Brune James N. , von Seggern David , Anooshehpoor Abdolrasool
Published: June 2003

Precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years, thereby providing important constraints on seismic hazard estimates. Numerous precarious rocks exist in the region of large nuclear explosions at Nevada Test Site (NTS). Ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks. Very near large NTS explosions, there are no precarious rocks or semiprecarious rocks. Cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche: proof that the rockfalls have been caused by the explosions. As the distance from the explosions increases and the contours for predicted ground motion decrease, fewer and fewer rockfalls are observed, some semiprecarious rocks begin to appear, and finally, numerous precarious rocks are found with no evidence of recent rockfalls. To quantify the calibration of precarious rocks and ground motion we (1) do field tests on the rocks to determine the quasi‐static and dynamic toppling accelerations, (2) carry out computer tests using estimates of waveforms from NTS explosions, and (3) compare the results with ground motions predicted by empirical regressions for nuclear explosions. We conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations.
Strong ferrimagnetic resonance signal and magnetic susceptibility of the Nojima pseudotachylyte in Japan and their implication for coseismic electromagnetic changes
Authors: Fukuchi Tatsuro
Published: June 2003

Pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement. I examine the Nojima pseudotachylyte found along the Nojima fault, which caused the 1995 Kobe earthquake (M = 7.2) in Japan, using the electron spin resonance technique. The Nojima pseudotachylyte has a strong ferrimagnetic resonance (FMR) signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides. This FMR signal appears by heating the surrounding fault gouge that is the source material of the Nojima pseudotachylyte. Furthermore, the magnetic susceptibilities of the Nojima pseudotachylyte are 102–103 times larger than those of the surrounding fault gouge and are proportional to the FMR signal intensities. Here I reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the FMR signal. The results indicate that the instantaneous magnetization of the Nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction.
Large earthquake cycles and intermittent criticality on heterogeneous faults due to evolving stress and seismicity
Authors: Ben‐Zion Yehuda , Eneva Mariana , Liu Yunfeng
Published: June 2003

We analyze evolving stress and seismicity generated by three realizations of a discrete model of a strike‐slip fault in a three‐dimensional (3‐D) elastic half space using five functions of stress and five functions of seismicity. The first model (F) has realistic dynamic weakening (static minus dynamic frictions), the second (FC) has zero critical dynamic weakening, and the third (SYS) is constrained to produce only system size events. The results for model F show cyclical development, saturation, and destruction of fluctuations and long‐range correlations on the fault, punctuated by the system size events. The development stage involves evolution of stress and seismicity to distributions having broad ranges of scales, evolution of response functions toward scale‐invariant behavior, increasing seismicity rate and event sizes, and increasing hypocenter diffusion. Most functions reach asymptotically stable values around 2/3 of the cycle and then fluctuate until one event cascades to become the next large earthquake. In model FC the above evolution is replaced by scale‐invariant statistical fluctuations, while in model SYS the signals show simple cyclic behavior. The results suggest that large earthquake cycles on heterogeneous faults with realistic positive dynamic weakening are associated with intermittent criticality, produced by spontaneous evolution of stress heterogeneities toward a critical level of disorder having a broad range of scales. The stress evolution and development of large earthquake cycles may be tracked with seismicity functions.
Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models
Authors: van der Beek Peter , Bishop Paul
Published: June 2003

We have used early Miocene valley‐filling basalts to reconstruct fluvial long profiles in the Upper Lachlan catchment, SE Australia, in order to use these as well‐constrained initial conditions in a forward model of fluvial incision. Many different fluvial incision algorithms have been proposed, and it is not clear at present which one of these best captures the behavior of bedrock rivers. We test five different formulations; the ability of these models to reproduce the observed present‐day stream profiles and amounts of incision is assessed using a weighted‐mean misfit criterion as well as the structure of the misfit function. The results show that for all models, parameter combinations can be found that reproduce the amounts of incision reasonably well. However, for some models, these best fit parameter combinations do not seem to have a physical significance, whereas for some others, best fit parameter combinations are such that the models tend to mimic the behavior of other models. Overall, best fit model predictions are obtained for a detachment‐limited stream power model or an “undercapacity” model that includes a river width term that varies as a function of drainage area. The uncertainty in initial conditions does not have a strong impact on model outcomes. The model results suggest, however, that lithological variation may be responsible for variations in parameter values of a factor of 3–5.
Offsets in Global Positioning System time series
Authors: Williams Simon D. P.
Published: June 2003

Site velocities are a common product of continuous Global Positioning System (CGPS) networks. Estimation of site velocities from their time series can be biased if offsets are present. If left uncorrected, offsets can dominate velocity uncertainties. Artificial (nontectonic) discontinuities may arise from environmental and equipment changes or human intervention and error. Analysis of present GPS data sets reveals that on a component by component basis, one offset occurs every 9 years, although it could be as frequent as one offset every 2 years. The effect offset estimation has on rate uncertainty depends on the noise characteristics in the series. If the noise is white, then estimating a single offset in the center of the series doubles the uncertainty. If the noise is random walk, then neither the position nor the number of offsets in the time series seriously alters the rate uncertainty. Undetected offsets in the time series mimic random walk noise. If an offset detection algorithm is implemented, then there is threshold size below which the algorithm can select the wrong offset epoch. Offset position uncertainty increases as offset size decreases. However, small offset position uncertainties will still result in lower rate uncertainties with respect to undetected offsets. Despite the appeal of estimating offsets by detrending and averaging data either side of the discontinuity, such a strategy only serves to increase the rate and offset uncertainties. The best strategy is still to use the maximum likelihood estimator to simultaneously estimate the linear parameters and the noise components.
Tectonic implications of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China
Authors: Lin Wei , Chen Yan , Faure Michel , Wang Qingchen
Published: June 2003

A paleomagnetic study has been carried out in the east of the Tan‐Lu fault, in Liaoning Province, NE China, to understand the timing of Tan‐Lu fault activity. Samples aging from Early Paleozoic to Late Mesozoic from 51 sites have been analyzed. Paleozoic and Late Permian‐Early Triassic rocks are remagnetized by the recent geomagnetic field; however, Late Cretaceous (between 118 and 83 Ma) red tuffaceous sandstone passes a positive fold test, shows no Present Earth Field characteristic remanent magnetization carried by both magnetite and hematite, presents a solo normal polarity, and thus provides the only reliable paleomagnetic data in this study. The paleomagnetic pole calculated from these rocks (λp = 59.4°N, ϕp = 205.5°E, and A95 = 7.3°) is statistically undistinguishable from all available Cretaceous paleomagnetic data from Eastern Liaoning‐Korean Peninsula, indicating that these areas may belong to a single tectonic unit, here named the East Liaoning‐Korea (ELK) Block, at least since the Late Cretaceous. Conversely, a significant discrepancy between the ELK Block and Chinese Block (i.e., North and South China Blocks) characterized by a differential rotation (22.5° ± 10.2°) with a negligible latitudinal displacement (0.8° ± 6.1°) is demonstrated. This result indicates, first, that the left‐lateral displacement along the Tan‐Lu fault, if any, must have occurred before the Late Cretaceous and, second, that the Korean Block can not be considered as a rigid part of North China Block. Sedimentological and structural evidence show that the Meso‐Cenozoic triangle shaped plain, consisting of Songliao, Xialiaohe, Sanjiang, Zeya, and other smaller basins developing in northeast China and southeast Russia, south of the Mongol‐Okhotsk Belt, experienced a heterogeneous rifting from the Late Jurassic to Tertiary. The variable amount of extension, larger in the northeast (∼300 km) than in the southwest (∼80 km), is probably related to the clockwise rotation of the ELK Block centered at the south of the Bohai Bay Basin. This result shows that a significant segment of the East Eurasian margin (more than 1000 km long) experienced differential rotation in Cenozoic times.
Inversion of Earth's changing shape to weigh sea level in static equilibrium with surface mass redistribution
Authors: Blewitt Geoffrey , Clarke Peter
Published: June 2003

We develop a spectral inversion method for mass redistribution on the Earth's surface given geodetic measurements of the solid Earth's geometrical shape, using the elastic load Love numbers. First, spectral coefficients are geodetically estimated to some degree. Spatial inversion then finds the continental surface mass distribution that would force geographic variations in relative sea level such that it is self‐consistent with an equipotential top surface and the deformed ocean bottom surface and such that the total (ocean plus continental mass) load has the same estimated spectral coefficients. Applying this theory, we calculate the contribution of seasonal interhemispheric (degree 1) mass transfer to variation in global mean sea level and nonsteric static ocean topography, using published GPS results for seasonal degree‐1 surface loading from the global IGS network. Our inversion yields ocean‐continent mass exchange with annual amplitude (2.92 ± 0.14) × 1015 kg and maximum ocean mass on 25 August ±3 days. After correction for the annual variation in global mean vertical deformation of the ocean floor (0.4 mm amplitude), we find geocentric sea level has an amplitude of 7.6 ± 0.4 mm, consistent with TOPEX‐Poseidon results (minus steric effects). The seasonal variation in sea level at a point strongly depends on location ranging from 3 to 19 mm, the largest being around Antarctica in mid‐August. Seasonal gradients in static topography have amplitudes of up to 10 mm over 5000 km, which may be misinterpreted as dynamic topography. Peak continental loads occur at high latitudes in late winter at the water‐equivalent level of 100–200 mm.
On the parameterization of turbulent surface fluxes over heterogeneous sea ice surfaces
Authors: Schröder D. , Vihma T. , Kerber A. , Brümmer B.
Published: June 2003

Turbulent surface fluxes of momentum and sensible and latent heat as well as surface temperature, air temperature, air humidity, and wind speed were measured by the German Falcon research aircraft over the marginal ice zone (MIZ) of the northern Baltic Sea and the Fram Strait. Applying the bulk formulas and the stability functions to the measurements, the roughness lengths for momentum z0, sensible heat zT, and latent heat zq were calculated. As mean values over a wide range of sea ice conditions, we obtain z0 = 5 × 10−4 m, zT = 1 × 10−8 m, and zq = 1 × 10−7 m. These correspond to the following mean values (± standard deviations) of neutral transfer coefficients reduced to 10 m height, CDN10 = (1.9 ± 0.8) × 10−3, CHN10 = (0.9 ± 0.3) × 10−3, and CEN10 = (1.0 ± 0.2) × 10−3. An average ratio of z0/zT ≈ 104 was observed over the range of 10−6 m < z0 < 10−2 m and differs from previously published results over compact sea ice (10−1 < z0/zT < 103). Other observational results over heterogeneous sea ice do not exist. However, our z0/zT ratio approximately agrees with observations over heterogeneous land surfaces. Flux parameterizations based on commonly used roughness lengths ratios (z0 = zT = zq) overestimate the surface heat fluxes compared to our measurements by more than 100%.
Transport of North Pacific Intermediate Water across Japanese WOCE sections
Authors: You Yuzhu , Suginohara Nobuo , Fukasawa Masao , Yoritaka Hiroyuki , Mizuno Keisuke , Kashino Yuji , Hartoyo Djoko
Published: June 2003

Under the auspices of the World Ocean Circulation Experiment (WOCE) Hydrographic Program, Japan carried out a total of four sections in the North Pacific, P2 at 30°N, P13 at 165°E, P9 at 137°E, and P8 at 130°E. These Japanese and several other WOCE sections are used to estimate the transport of North Pacific Intermediate Water (NPIW) in the subtropical gyre. Previous studies speculate about 3 Sv (1 Sv = 106 m3 s−1) of NPIW transport, originating mainly in Okhotsk Sea, entering the subtropical gyre southward via the mixed water region in the east of Japan. The total subpolar input is 3.2 Sv when another source of NPIW in the Gulf of Alaska, estimated at 0.2 Sv in this study, is added. This transport from the Japanese WOCE sections (a mean of 2.8 ± 0.2 Sv) compares well with several other sections (a mean of 2.5 ± 0.2 Sv). From a total of five meridional and two zonal sections, a mean transport of 2.7 ± 0.2 Sv is found in the defining NPIW layer between σN = 26.5 and 27.4 neutral density surfaces. The transport shows a steady southward and westward increase. About 3 Sv transport is found mainly in the southern (south of 25°N) and western (west of the dateline) sections. Closer to the subarctic‐tropical frontal zone and in the eastern subtropical gyre, NPIW transport is less than 2 Sv. This implies that a lower subpolar input of about 2 Sv is sufficient for subtropical ventilation. Recirculation, entrainment with the upper and lower waters, and epineutral and dianeutral mixing likely add about 1 Sv to the NPIW transport. To interpret the transport distribution, a simple water mass mixing scheme is applied, which includes two formation sources, Okhotsk Intermediate Water (OIW) and Gulf of Alaska Intermediate Water (GAIW) in the subpolar regions, and a transformed aged NPIW (aNPIW) in the northwestern subtropical gyre southeast of Japan. The aNPIW mixing fraction increases southward and westward, while OIW and GAIW fractions decrease. This suggests that after NPIW source water parcels enter the subtropical gyre, they largely recirculate within the wind‐driven gyre and thus become aged. Therefore one usually sees very low oxygen and CFC values in the NPIW layer in the subtropical gyre.
Vertical profiles of light scattering, light absorption, and single scattering albedo during the dry, biomass burning season in southern Africa and comparisons of in situ and remote sensing measurements of aerosol optical depths
Authors: Magi Brian I. , Hobbs Peter V. , Schmid Beat , Redemann Jens
Published: June 2003

Airborne in situ measurements of vertical profiles of aerosol light scattering, light absorption, and single scattering albedo (ω0) are presented for a number of locations in southern Africa during the dry, biomass burning season. Features of the profiles include haze layers, clean air slots, and marked decreases in light scattering in passing from the boundary layer into the free troposphere. Frequency distributions of ω0 reflect the strong influence of smoke from biomass burning. For example, during a period when heavy smoke was advected into the region from the north, the mean value of ω0 in the boundary layer was 0.81 ± 0.02 compared to 0.89 ± 0.03 prior to this intrusion. Comparisons of layer aerosol optical depths derived from the in situ measurements with those measured by a Sun photometer aboard the aircraft show excellent agreement.
Mineral dust aerosol size distribution change during atmospheric transport
Authors: Maring H. , Savoie D. L. , Izaguirre M. A. , Custals L. , Reid J. S.
Published: June 2003

Airborne mineral dust can impact visibility, climate, biogeochemical processes, and possibly human health. The magnitude of the impact of dust depends on particle size. We measured the size distributions of airborne mineral dust over the Canary Islands during July 1995 and Puerto Rico during July 2000. Dust size distributions do not appear lognormal. Stokes gravitational settling overestimates losses of large dust particles during atmospheric transport from North Africa over the tropical North Atlantic and Caribbean. Normalized mineral dust size distributions of particles smaller than 7.3 μm over the Canary Islands and Puerto Rico were indistinguishable, indicating these particles were not preferentially removed during atmospheric transport. However, mineral dust aerosols larger than 7.3 μm were preferentially removed during atmospheric transport. Larger particles were more efficiently removed. A simple empirical model setting the vertical velocity of dust particles equal to the Stokes gravitational settling velocity minus an upward velocity of ∼0.33 cm s−1 accurately predicts changes in dust size distribution during atmospheric transport. Thus it appears some atmospheric process(es) partially counteracts gravitational settling.
Intercontinental transport, chemical transformations, and baroclinic systems
Authors: Hess P. G. , Vukicevic T.
Published: June 2003

The adjoint sensitivity technique is used to diagnose the transport pathway and chemical transformations of a pollutant plume from Asia to Hawaii in a three‐dimensional chemical transport model (HANK) driven by winds from the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Modeling System (MM5). The simulation takes place within the timeframe of the Mauna Loa Observatory (MLO) Photochemistry Experiment (MLOPEX) 2c (15 April to 15 May 1992). Unlike trajectories, the adjoint technique takes into account all model processes including diffusion and convection in accounting for the transport of the pollutant plume and clearly defines the footprint of the plume. The transport of the pollutant plume is primarily analyzed from an isentropic perspective. The plume is lofted upward out of the Asian boundary layer in a very narrow band immediately above the surface cold front in the warm conveyor belt of a developing synoptic storm. Once lofted into the free troposphere, the plume travels almost isentropically to Hawaii, with very little additional mixing. The chemical and physical transformations within the plume are consistent with measurements on Hawaii as part of MLOPEX 2c: in particular, the initial loss of water and HNO3, the conversion of NOy from PAN to NOx to HNO3, the slow mixing rate in the free troposphere and the small ratio of HNO3 to NOx are consistent with the measurements. The chemical signature of pollutant plumes at MLO is investigated. The plumes have much higher concentrations of hydrocarbons than average and low concentrations of HNO3. The concentrations of PAN, NOx, H2O, and H2O2 were not found to distinguish rapid transport from Asia.
Improved detection of airborne volcanic ash using multispectral infrared satellite data
Authors: Ellrod Gary P. , Connell Bernadette H. , Hillger Donald W.
Published: June 2003

A technique for improved detection of airborne volcanic ash has been developed that uses three infrared (IR) bands from meteorological satellites. The three IR bands are centered near 3.9, 10.7, and 12.0 μm wavelength. The technique is based on the sum of two brightness temperature differences (BTDs), scaled to maximize the brightness and contrast of volcanic ash in the output image. The physical effects attributed to the observed BTDs that help distinguish the volcanic ash from various meteorological cloud types are (1) differential absorption by volcanic ash or sulfur dioxide at 3.9 μm, 10.7 μm, and 12.0 μm and (2) strong solar reflectance by ash at 3.9 μm, which varies diurnally. On the basis of two examples using data from the Geostationary Operational Environmental Satellite (GOES) the three‐band IR technique is shown to provide better discrimination of volcanic ash from meteorological clouds than is possible using existing two‐band methods. This conclusion is supported by comparisons of brightness count profiles and estimation of false ash detection rate statistics. The best results from the three‐band IR technique are obtained during daylight hours over any surface, and at night when the ash cloud is over the ocean or other large body of water. The three‐band IR technique is one of the tools currently being employed operationally at the Washington Volcanic Ash Advisory Center.
Interannual variability in atmospheric mineral aerosols from a 22‐year model simulation and observational data
Authors: Mahowald Natalie , Luo Chao , del Corral John , Zender Charles S.
Published: June 2003

Mineral aerosols are important atmospheric constituents owing to their interactions with climate and biogeochemistry. The interannual variability in atmospheric mineral aerosols is evaluated using a model simulation of 1979–2000 and mineral aerosol observations. Overall, the variability in monthly means between different years is not as large as the variability within a month for column amount, surface concentration, and deposition fluxes. The magnitude of the variability predicted in the model varies spatially and appears similar to that seen in the available observations, although the model is not always able to simulate observed high‐ and low‐dust years. The area over which the interannual variability in the observing station data should be representative is estimated in the model simulation and is shown to be regional in extent. However, correlations between modeled surface concentrations at the stations and modeled deposition in the surrounding region is often low, suggesting that the observations of the variability of surface concentrations are difficult to extrapolate to variability in regional deposition fluxes. The correlations between modeled monthly mean optical depth and modeled deposition or mobilization are low to moderate (0.2–0.6) over much of the globe, indicating the difficulty of estimating mobilization or deposition fluxes from satellite retrievals of optical depth. In both the model and observations there are relationships between climate indices (e.g., North Atlantic Oscillation, El Niño, and Pacific Decadal Oscillation) and dust, although a 22‐year simulation is not long enough to well characterize this relationship. In this model, simulation of 1979–2000, dust concentration variability appears to be dominated by transport variability and/or transport and source covariance rather than source strength variability.
Moment‐based simulation of microphysical properties of sulfate aerosols in the eastern United States: Model description, evaluation, and regional analysis
Authors: Yu Shaocai , Kasibhatla Prasad S. , Wright Douglas L. , Schwartz Stephen E. , McGraw Robert , Deng Aijun
Published: June 2003

A six‐moment microphysics module for sulfate aerosols based on the quadrature method of moments has been incorporated in a host 3‐D regional model, the Multiscale Air Quality Simulation Platform. Model performance was examined and evaluated by comparison with in situ observations over the eastern United States for a 40‐day period from 19 July to 28 August 1995. The model generally reproduces the spatial patterns (sulfate mixing ratios and wet deposition) over the eastern United States and time series variations of sulfate mass concentrations. The model successfully captured the observed size distribution in the accumulation mode (radius 0.1–0.5 μm), in which the sulfate is predominately located, while underestimating the nucleation and coarse modes on the basis of the size distributions retrieved from the modeled six moments at the Great Smoky Mountains (GSM). This is consistent with better model performance on the effective radius (ratio of third to second moment, important for light scattering) than on number‐mean and mass‐mean radii. However, the model did not predict some of the moments well, especially the higher moments and during the dust events. Aerosol components other than sulfate such as dust and organics appear to have contributed substantially to the observed aerosol loading at GSM. The model underpredicted sulfate mixing ratios by 13% with about 50% of observations simulated to within a factor of 2. One of the reasons for this underestimation may be overprediction of sulfate wet deposition. Sulfate mass concentrations and number concentrations were high in the source‐rich Ohio River valley, but number concentrations were also high over the mid‐Atlantic coast (New Jersey area). Most (77%) sulfate amount was below 2.6 km, whereas most sulfate number (>52%) was above 2.6 km except over Ohio River valley (41%). These results demonstrate the accuracy, utility, practicality, and efficiency of moment‐based methods for representing aerosol microphysical processes in large‐scale chemical transport models.
Downward longwave irradiance uncertainty under arctic atmospheres: Measurements and modeling
Authors: Marty Christoph , Philipona Rolf , Delamere Jennifer , Dutton Ellsworth G. , Michalsky Joe , Stamnes Knut , Storvold Rune , Stoffel Tom , Clough Shepard A. , Mlawer Eli J.
Published: June 2003

Measurement and modeling of downward longwave irradiance are a special challenge in arctic winter due to its low water vapor content and the extreme meteorological conditions. There are questions about the representativeness of the instrument calibration, the consistency and unfcertainty of measurements and models in these environments. The Second International Pyrgeometer and Absolute Sky‐scanning Radiometer Comparison (IPASRC‐II), which was conducted at Atmospheric Radiation Measurement (ARM) program's North Slope of Alaska (NSA) site in Barrow provided a unique opportunity to compare high accuracy downward longwave irradiance measurements and radiative transfer model computations during arctic winter. Participants from 11 international institutions deployed 14 pyrgeometers, which were field‐calibrated against the Absolute Sky‐scanning Radiometer (ASR). Continuous measurements over a 10‐day period in early March 2001 with frequent clear‐sky conditions yielded downward longwave irradiances between 120 and 240 W m−2. The small average difference between ASR irradiances, pyrgeometer measurements, MODTRAN and LBLRTM radiative transfer computations indicates that the absolute uncertainty of measured downward longwave irradiance under arctic winter conditions is within ±2 W m−2.
Field test of spectral line intensity parameters for tropospheric water vapor
Authors: Sierk B. , Solomon S. , Daniel J. S. , Portmann R. W. , Gutman S. I. , Langford A. O. , Eubank C. S. , Holub K. H. , Florek S. V.
Published: June 2003

We report the results of a field experiment designed to study atmospheric water vapor absorption in the visible and near‐infrared spectral regions between 550 and 1000 nm. We carried out spectroscopic ground measurements of direct solar radiation under clear‐sky conditions in Boulder, Colorado. The data with a spectral resolution of approximately 1 nm were analyzed using the differential optical absorption spectroscopy (DOAS) technique in five different absorption bands of water vapor. We show that this technique can reveal the broadband effects of errors and inconsistencies in absorption spectra information for the water molecule. Retrievals of column tropospheric water vapor from the field spectra were compared to simultaneous independent estimates from Global Positioning System (GPS) data and radiosonde soundings. The data set is used to critically assess the widely used High‐Resolution Transmission Molecular Absorption Database (HITRAN) [Rothman et al., 1998]. The results indicate that line intensities in the 3ν + δ polyad centered at 820 nm are underestimated by 21% with respect to the strong 3ν polyad centered at 940 nm, while the 4ν polyad at 720 nm shows agreement within the measurement accuracy of 3%. Two weaker bands centered at 650 and 590 nm were found to be overestimated by about 8–10%. The effect of the proposed corrections on the absorption of incoming solar flux for a clear‐sky atmosphere is estimated to be 0.6 W/m2 for an overhead Sun.
The effects of the coastal environment on the atmospheric mercury cycle
Authors: Malcolm Elizabeth G. , Keeler Gerald J. , Landis Matthew S.
Published: June 2003

Atmospheric mercury (Hg) species were investigated on the east coast of Florida during June 2000. The site was impacted by air mass transport from the Atlantic Ocean and south Florida. Periods with atmospheric transport from the Atlantic were characterized by low concentrations of elemental gaseous Hg and inorganic divalent reactive gaseous mercury (RGM), demonstrating that the marine boundary layer was not a significant source of RGM to this coastal site as previous researchers have hypothesized. When anthropogenic impacts were observed at the site, indicated by elevated concentrations of gases including HNO3 and SO2, RGM concentrations had higher daytime maximums. Particulate phase Hg concentrations were higher than can be explained by sea spray alone, as determined by chemical analysis of the seawater, suggesting that gaseous Hg is diffusing into the sea‐salt aerosol. Although atmospheric Hg concentrations were not elevated, the observed scavenging of Hg gases by sea‐salt aerosols indicates that Hg may be rapidly cycled at the atmosphere‐ocean interface between gaseous, aerosol, and oceanic forms. Deposition of aerosol enriched in Hg via this process may constitute a significant global mercury flux to the oceans.
Sources of carbonaceous aerosols over the United States and implications for natural visibility
Authors: Park Rokjin J. , Jacob Daniel J. , Chin Mian , Martin Randall V.
Published: June 2003

We use a global three‐dimensional model (GEOS‐CHEM) to better quantify the sources of elemental carbon (EC) and organic carbon (OC) aerosols in the United States through simulation of year‐round observations for 1998 at a network of 45 sites (Interagency Monitoring of Protected Visual Environments (IMPROVE)). Simulation with our best a priori understanding of sources, including global satellite data to constrain fire emissions, captures most of the variance in the observations (R2 = 0.84 for EC, 0.67 for OC) with a low bias of 15% for EC and 26% for OC. Multiple linear regression to fit the IMPROVE data yields best estimates of 1998 U.S. sources of 0.60 Tg year−1 EC and 0.52 Tg year−1 OC from fossil fuel; 0.07 Tg year−1 EC and 0.89 Tg year−1 OC from biofuel; 0.08 Tg year−1 EC and 0.60 Tg year−1 OC from wildfires; and 1.10 Tg year−1 OC from vegetation. We find that fires in Mexico and Canada contributed 40–70% of annual mean natural EC in the United States for 1998 and 20–30% of annual mean natural OC. Transpacific transport from Asian pollution sources amounted to less than 10% of the natural EC and less than 2% of the natural OC; in contrast to ozone, we find that intercontinental transport of anthropogenic carbonaceous aerosols does not enhance significantly the natural background. IMPROVE observations and model simulations for the summer of 1995 show that Canadian fire emissions can produce large events of elevated EC and OC in the southeastern United States. Our best estimates of mean natural concentrations of EC and OC in the United States, using a model simulation with climatological monthly mean fire emissions, are 2–3 times higher than the default values recommended by the U.S. Environmental Protection Agency for visibility calculations, except for OC in the eastern United States (16% lower).
A conceptual model for the relationship between coronae and large‐scale mantle dynamics on Venus
Authors: Johnson Catherine L. , Richards Mark A.
Published: June 2003

Among the most enigmatic features on Venus are the more than 500 quasi‐circular structures collectively referred to as coronae. Associated volcanism and tectonism suggest that coronae result from small, transient mantle plumes, although only about 20% have clearly associated positive gravity anomalies indicative of current activity. Coronae are concentrated within a region defined by the dynamically active highland areas of Beta, Atla, and Themis Regiones (BAT). Coronae avoid the lowest or highest topography and geoid. Subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current BAT concentration. Recent laboratory experiments on thermal convection at high Rayleigh number, using fluids with temperature‐dependent viscosity, suggest a straightforward interpretation of these relationships. Small‐scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer‐wavelength downwelling structures from the cold upper boundary layer. The relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains. Coronae are largely excluded from compensated highlands, perhaps because the thickened crust is not easily disrupted in these regions. Laboratory experiments indicate that small, transient, boundary layer instabilities can coexist with, and be focused toward, large‐scale upwellings, possibly explaining the spatial concentration of coronae in the BAT region. Thus coronae provide important insights into interacting scales of convective flow in Venus's mantle and into the evolution of Venus's mantle dynamics. Direct tests of our model are possible through geological mapping.
Cross faults in extensional settings: Stress triggering, displacement localization, and implications for the origin of blunt troughs at Valles Marineris, Mars
Authors: Wilkins Scott J. , Schultz Richard A.
Published: June 2003

Motivated by conflicting interpretations concerning the origin of blunt terminations of troughs at Valles Marineris, Mars, we investigate the reactivation of preexisting cross faults in response to stress changes associated with slippage along a major, basin‐bounding normal fault (i.e., border fault). Coulomb stress changes indicate that cross fault reactivation is possible in both the footwalls and hanging walls of border faults, although this is dependent on the distance between the border and cross faults. Cross faults accommodate dip‐slip normal motion for most border fault geometries and conditions we tested, but strike‐slip motions are predicted when preexisting cross faults are vertical. Furthermore, although lateral extensions of the border fault (LEBFs) may nucleate within cross fault footwalls at all stages of border fault development, they are favored to develop when border faults and cross fault tip lines are proximal. Observations from the Valles Marineris extensional province, Mars, are consistent with (1) normal displacements along cross faults, (2) numerous examples of pit‐chains, interpreted to represent surface expressions of lateral extensions to the border fault (LEBF), (3) the lack of well‐developed cross faults in the footwall of border faults, and (4) the inference that tapered trough ends formed in areas that lacked cross faults prior to the main phase of extension at Valles Marineris. We suggest a new sequence of deformation that accounts for the formation of blunt‐trough terminations during the major phase of Valles Marineris extension: coeval and locally bidirectional extension, that results from local stress field changes associated with border fault growth in a dominantly unidirectional remote (extensional) strain field. By implication, many of the irregular closed troughs such as Hebes Chasma are better interpreted as grabens rather than collapse depressions.
Mars Orbiter Camera observation of linear and curvilinear features in the Hellas basin: Indications for multiple processes of formation
Authors: Ormö Jens , Komatsu Goro
Published: June 2003

The high resolution of the Mars Orbiter Camera images reveals many detailed landforms on Mars' surface, which include thin dark or bright linear/curvilinear features. The latter are generally only a few tens of meters wide but can be a couple of kilometers in length. Some of these features have previously been attributed to the action of dust devils. Our study of the linear/curvilinear features in the Hellas basin reveals a variety that can only be explained by the formation from many different processes. In order to evaluate different causes for their formation we created a mapping based on a classification scheme where the linear/curvilinear features were sorted into three groups and compared to terrestrial analogues. Features in group 1 are most frequent, and contain streaks that appear only to affect the ground surface. The best analogue is dust devil tracks. The second, very rare, group appears to penetrate the surface in a way expected of fractures. In the third, likewise rare, group the features have a topographic expression that makes them similar to grooves and/or trenches between narrow ridges. This group is probably formed by multiple processes. Possible terrestrial analogues are found in glacial and periglacial environments (e.g., glacial scouring, fluted moraine, cryoturbation patterns, and iceberg plow marks). The existence of such landforms in the Hellas basin is consistent with previously presented glacial models and has an important implication for the understanding of the past climatic evolution of Mars.
Viscous flow features on the surface of Mars: Observations from high‐resolution Mars Orbiter Camera (MOC) images
Authors: Milliken R. E. , Mustard J. F. , Goldsby D. L.
Published: June 2003

A unique morphology suggestive of viscous flow of a meters‐thick surface layer was identified in high‐resolution (<10 m/pixel) Mars Orbiter Camera (MOC) images. A global survey of 13,000 MOC images resulted in the identification of 146 images exhibiting these viscous flow features. Slope angles derived from Mars Orbiter Laser Altimeter (MOLA) data, along with an experimentally derived flow law for ice, were used to estimate the shear stress and assess the plausibility that a ∼10 m thick ice‐dust mixture could produce the observed viscous deformation on timescales in agreement with the estimated age (105–107 years) of the material. Our shear stress estimates of 10−1.5–10−2.5 MPa yield strain rates on the order of 10−11–10−16 s−1, which are within the superplastic flow regime of ice. Mean annual surface temperatures, age constraints, and strain estimates show that it is possible for a meters‐thick ice‐dust mixture to undergo viscous deformation under past or present surface conditions for ice grain sizes >10 μm. The meters‐thick layer in which the viscous flow features formed is morphologically similar to a degraded meters‐thick ice‐dust surface deposit (dissected mantle terrain). Locations of the viscous flow features, dissected mantle terrain, and recent gullies are concentrated in the midlatitude regions, and all three show identical distributions as a function of latitude, with the maximum frequency of occurrence at ∼40°N and S. The strong association between these small‐scale flow features and the dissected mantle terrain, large‐scale viscous flow features, and recent gullies imply that deposition, deformation, and removal of ice‐rich materials has played an important role in the modification of the surface in the midlatitudes of Mars during the Amazonian and possibly longer.
Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series
Authors: Anand Pallavi , Elderfield Henry , Conte Maureen H.
Published: June 2003

Paired Mg/Ca and δ18O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6‐year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of δ18O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre‐exponential constants in the sequence O. universa > G. aequilateralis ≈ G. bulloides > G. ruber ≈ G. sacculifer ≈ other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The ∼9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre‐exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species‐specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature.
Orbital suppression of wind‐driven upwelling in the California Current at 6 ka
Authors: Diffenbaugh Noah S. , Sloan Lisa C. , Snyder Mark A.
Published: June 2003

Marine records clearly show activity in the California Current (CC) system to have been altered during the mid‐Holocene. However, the specific nature of CC activity during this period remains ambiguous, as proxies that suggest cooler than present sea surface temperatures co‐occur in mid‐Holocene marine sediments with multiple proxies that suggest decreased coastal upwelling. To test the sensitivity of wind‐driven upwelling in the CC to mid‐Holocene orbital forcing, we have employed a high‐resolution regional climate model (RCM). The seasonality of modern CC wind‐driven coastal upwelling simulated by the RCM is in strong agreement with observational data. Further, we show that changes in the seasonality of solar insolation induced by mid‐Holocene Milankovitch forcing decrease early and peak season coastal upwelling in the CC, along with increasing late season coastal upwelling. These results are in agreement with several marine and terrestrial proxies, suggesting that, relative to present, CC activity during the mid‐Holocene was characterized by a longer and less vigorous upwelling season with decreased seasonal contrast.
Theory and applications of the Analytic Element Method
Authors: Strack O. D. L.
Published: June 2003

We present a review of the theory and applications of the Analytic Element Method as it exists today. The paper begins with a presentation of analytic elements used for modeling of divergence‐free and irrotational flow in both two and three dimensions, including a description of the superblock approach, which makes it possible to deal effectively with very large models both in terms of accuracy and speed. We briefly discuss a particular application of the Analytic Element Method to multiaquifer problems, present the theory of the Analytic Element Method for general vector fields, and discuss the inclusion of source terms (irrotational vector fields with nonzero divergence). We finish the paper with a brief discussion of three major applications on a regional scale.
Four centuries of geomagnetic data from historical records
Authors: Jonkers Art R. T. , Jackson Andrew , Murray Anne
Published: June 2003

We present a comprehensive review of historical geomagnetic data collected over the last 2 decades by many workers, culminating in the largest such compilation in the world. It spans over 4 centuries, from 1510 to 1930 inclusive, and consists of 151,560 declinations, 19,525 inclinations, and 16,219 intensities. The greater part of the new data set comprises unpublished observations recorded by mariners engaged in merchant and naval shipping across the world's oceans. Earlier published compilations and printed accounts of voyages have also been included, in particular when their original sources were inaccessible, missing, or no longer extant. The current effort has brought together an unprecedented number of early observations of the Earth's magnetic field, in standardized format, of potential use in many areas of geophysical research.