bluuebunny/crossref_metadata_2025 huggingface dataset:
North American weather‐type frequency and teleconnection indices
Authors: Sheridan Scott C.
Published: January 2003

The impact of teleconnections upon the surface climate has largely been examined via a response in monthly mean temperature or total precipitation. In this paper, a different approach is undertaken, by examining the response of synoptic weather‐type frequencies to different teleconnection phases.

For over 330 stations in the USA and Canada, the Spatial Synoptic Classification scheme has classified each day in each station's period of record into one of seven weather‐type categories, based on thermal, moisture, and other characteristics. The differences in how frequently these different weather types occur in different phases of the North Atlantic Oscillation (NAO) and Pacific–North American (PNA) teleconnection patterns is assessed, for Canadian stations from 1953 to 1993, and for US stations from 1950 to 1999.

For PNA, a significant shift in the transitional frequency is observed, suggesting changes in storm track. Concomitantly, a large shift in Dry Polar and Moist Tropical frequencies is observed across the continent. Across the West, in +PNA wintertime months far fewer Dry Polar days are observed. Across the eastern USA, these polar intrusions are more common, and Moist Tropical is diminished significantly.

The frequency of the transitional situation is also correlated with NAO phase, with differences as large as a factor of two across much of Canada and the northern USA. In northeastern Canada, there is a large replacement of Moist Polar conditions with Dry Polar conditions during +NAO. Farther south, however, across the eastern USA, both polar weather types occur much less often with +NAO. Although previous research has discovered eastern North American connections to the NAO, this research has shown that the connections often extend into the interior West during much of the year. Particularly strong in the spring, Dry Tropical conditions are much more common with +NAO throughout much of the continent, as far west as the Great Basin. Copyright © 2003 Royal Meteorological Society.
El Niño–southern oscillation influences on the Mahaweli streamflow in Sri Lanka
Authors: Zubair Lareef
Published: January 2003

Despite advances over the last two decades in the capacity to predict the evolution of the El Niño–southern oscillation (ENSO) phenomenon and advances in understanding of the relationship between ENSO and climate, there has been little use of climate predictions for water resources management in the tropics. As part of an effort to develop such a prediction scheme, the ENSO influences on streamflow and rainfall in the upper catchment of the Mahaweli river in Sri Lanka were investigated with correlation analysis, composite analysis and contingency tables. El Niño conditions were often associated with decreased annual flows and La Niña with increased flows. The relationship of streamflow and rainfall with the ENSO index of NINO3 contrasted between January to September and October to December. During El Niño episodes the streamflow declines from January to September, but from October to December there is no clear relationship. On the other hand, rainfall shows a clear increase from October to December and declines during January, February, March, July and August. The simultaneous correlations of NINO3 with the aggregate January to September streamflow (r = −0.50), with January to September rainfall (r = −0.44) and with October to December rainfall (r = 0.48) are all significant at the 99% level. The correlation between one‐season‐in‐advance NINO3 with both January to September streamflow and October to December rainfall remained significant at the 99% level.

This study demonstrates the potential of using ENSO‐based predictors for a seasonal hydro‐climatic prediction scheme in the Mahaweli basin. It shows the significant contrasts in ENSO influence on rainfall and streamflow due to various hydrological processes. It has demonstrated that the potential for prediction is improved by investigating ENSO influences for the appropriate season for the given river catchment. Copyright © 2003 Royal Meteorological Society.
AGE OF STRATOSPHERIC AIR: THEORY, OBSERVATIONS, AND MODELS
Authors: Waugh Darryn , Hall Timothy
Published: January 2003

We review the relationship between tracer distributions and transport timescales in the stratosphere and discuss the use of timescales to evaluate and constrain theories and numerical models of the stratosphere. The “age spectrum,” the distribution of transit times since stratospheric air last made tropospheric contact, provides a way to understand the transport information of tracers, how sensitive different tracers are to various transport processes, and how to use tracers in combination to constrain transport rates. Trace gas observations can be used to infer aspects of the age spectrum, most commonly the “mean age,” but also the shape of the spectrum. Observational inferences of transport timescales provide stringent tests of numerical models independent of photochemistry, and comparisons of these observations with chemical transport models have highlighted certain problems with transport in the models. Age simulations and comparisons with data can now be considered standard tests of stratospheric models.
Present‐day secular variations in the low‐degree harmonics of the geopotential: Sensitivity analysis on spherically symmetric Earth models
Authors: Tamisiea M. E. , Mitrovica J. X. , Tromp J. , Milne G. A.
Published: January 2003

We predict present‐day rates of change of the long‐wavelength components of the Earth's geopotential due to the late Pleistocene glacial cycles (hereinafter referred to as glacial isostatic adjustment, or GIA). These predictions are generated using spherically symmetric, self‐gravitating, (Maxwell) viscoelastic Earth models. Previous studies have generally considered only zonal (i.e., azimuthally independent) harmonics, or the so‐called coefficients. We extend these efforts to focus on the nonzonal harmonics and explore the sensitivity of our predictions to changes in a variety of model inputs. As an example, we examine the influence on the GIA predictions of ignoring rotational perturbations and assuming an elastically incompressible model and find that these assumptions can have a significant impact on the degree‐2, order‐1 coefficients, while the other harmonics are generally affected by <10%. Though the GIA predictions are generally insensitive to variations in lithospheric thickness and upper mantle viscosity, reasonable variations in either lower mantle viscosity or the late Pleistocene ice history will have a more significant effect. Mindful of upcoming dedicated gravity missions, we show that predictions of the GIA‐induced changes of the geoid can vary by at least 0.5 mm yr−1 over a plausible range of Earth and ice models.
Joint inversion of normal mode and body wave data for inner core anisotropy 1. Laterally homogeneous anisotropy
Authors: Ishii Miaki , Tromp Jeroen , Dziewoński Adam M. , Ekström Göran
Published: January 2003

We use mantle and inner core sensitive normal modes to invert for laterally varying mantle models and a transversely isotropic inner core. To obtain an inner core model that is compatible with long‐period mode and short‐period body wave data, we incorporate mantle‐corrected absolute travel time and differential travel time data in our inversions. Using this diverse data set, we investigate the dependence of inner core models upon mantle parameterization and possible radial structure within the inner core. Inner core anisotropy obtained from a simultaneous inversion for mantle and inner core structure has a weak dependence upon the parameterization within the mantle, that is, the results for the inner core are insensitive to whether the mantle is inverted only for shear velocity structure or for shear and compressional velocity structure as well as density. An inner core model with constant anisotropy is capable of fitting all types of data, but mode and absolute travel time data are more compatible with one another than with differential travel time data. Allowing for radial variations within the inner core reduces this incompatibility, and it is possible to obtain a radially dependent model that fits normal mode, absolute travel time, and differential travel time data with a variance reduction greater than 75%.
Shear wave splitting across the Iceland hot spot: Results from the ICEMELT experiment
Authors: Bjarnason Ingi T. , Silver Paul G. , Rümpker Georg , Solomon Sean C.
Published: January 2003

We report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves (SKS, SKKS, and PKS) recorded by the ICEMELT broadband seismometer network in Iceland. In a ridge‐centered hot spot locale, mantle anisotropy may be generated by flow‐induced lattice‐preferred orientation of olivine grains or the anisotropic distribution of magma. Splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and/or the distribution of retained melt associated with the Iceland mantle plume. In eastern Iceland, fast polarization directions lie between N10°W and N45°W and average N24°W; delay times between the fast and slow shear waves are generally 0.7–1.35 s. In western Iceland, in contrast, the fast polarization directions, while less well constrained, yield an average value of N23°E and delay times are smaller (0.2–0.95 s). We propose that splitting in eastern Iceland is caused by a 100‐ to 200‐km‐thick anisotropic layer in the upper mantle. The observed fast directions in eastern Iceland, however, do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the Iceland hot spot. We suggest that the relatively uniform direction and magnitude of splitting in eastern Iceland, situated on the Eurasian plate, may therefore reflect the large‐scale flow field of the North Atlantic upper mantle. We hypothesize that the different pattern of anisotropy beneath western Iceland, part of the North American plate, is due to the different absolute motions of the two plates. By this view, splitting in eastern and western Iceland is the consequence of shear by North American and Eurasian plate motion relative to the background mantle flow. From absolute plate motion models, in which the Eurasian plate is approximately stationary and the North American plate is moving approximately westward, the splitting observations in both eastern and western Iceland can be satisfied by a background upper mantle flow in the direction N34°W and a velocity of 3 cm/yr in a hot spot reference frame. This inference can be used to test mantle flow models. In particular, it is inconsistent with kinematic flow models, which predict southward flow, or models where flow is dominated by subduction‐related sources of mantle buoyancy, which predict westward flow. Our observations are more compatible with the flow field predicted from global seismic tomography models, which in particular include the influence of the large‐scale lower mantle upwelling beneath southern Africa. While the hypothesized association between our observations and this upwelling is presently speculative, it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the Atlantic basin.
Surface temperature and salinity variations between Tasmania and Antarctica, 1993–1999
Authors: Chaigneau Alexis , Morrow Rosemary
Published: January 2003

Continuous surface temperature and salinity measurements have been collected onboard a supply ship between Tasmania and Dumont D'Urville, Antarctica, as part of the SURVOSTRAL program (Surveillance de l'Océan Austral). The ship makes 6–10 repeat sections per year from October to March for the period 1993–1999, and these measurements form the longest time series of spring and summer variations available in the Southern Ocean. The surface fronts are more clearly indicated with the salinity data than the temperature data. The Levitus climatological and Reynolds satellite monthly mean sea surface temperature data compare well with our surface temperature data: All data sets are dominated by the large‐scale seasonal heating cycle with similar amplitude and phase. The SURVOSTRAL monthly mean sea surface salinity data show weak seasonal variations, except in the Antarctic Continental Zone where seasonal sea‐ice variations introduce large surface freshwater fluctuations. The Levitus climatological surface salinity has too much seasonal variability and fails to reproduce the sharp fronts, the near‐constant salinity in the Antarctic Zone, and the seasonal freshening near Antarctica. This has important consequences for projects that use Levitus climatology data to establish the surface salinity structure and water mass characteristics.
Subannual variability of the ocean circulation in the Kuroshio region
Authors: Schmeits Maurice J. , Dijkstra Henk A.
Published: January 2003

Using altimeter data of the Kuroshio region, dominant patterns of variability are determined by applying multichannel singular spectrum analysis. A statistically significant propagating mode of variability with a timescale close to 7 months is found. In addition, output from a high‐resolution simulation of the Parallel Ocean Climate Model (POCM) is analyzed, which also displays variability on a timescale of 7 months, although not statistically significant at the 95% confidence level. To explain the physics of the apparent preference for variability at this timescale within a strongly eddying flow, we investigate the stability of barotropic North Pacific flows. Within a shallow‐water model, there appear to be three different steady flow paths of the Kuroshio that all become unstable to oscillatory modes. One of these modes has a timescale of 7 months for reasonable values of the parameters and has other features in common with the 7‐month statistical modes from the POCM output and from the observations. The connection between results from the stability analysis and from the analysis of the observations and POCM output leads to the conjecture that the preference for the 7‐month variability is due to the existence of a large‐scale barotropic instability of the mean Kuroshio path.
Barrier layer formation during westerly wind bursts
Authors: Cronin Meghan F. , McPhaden Michael J.
Published: January 2003

Barrier layers between the base of a shallow halocline and the top of the thermocline are a common feature of the western Pacific warm pool. In this paper, we investigate barrier layer formation and erosion processes associated with two westerly wind bursts (WWB). WWBs are typically associated with increased rainfall, but increased wind stirring and convective mixing from surface cooling can cause the fresh water to mix down to the top of the thermocline and thereby erode preexisting or newly formed barrier layers. However, not all WWBs lead to barrier layer erosion. In this study, we show that a WWB in November 1989 was associated with the formation of an extremely (∼100 m) thick barrier layer. During this event, zonal and meridional advection of surface fresh water from the west and north were the dominant processes responsible for the thick barrier layer, while rainfall was a secondary process. Likewise, during the Tropical Oceans Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment, a relatively thick barrier layer formed near 0°, 160°E and 0°, 165°E following the October 1992 WWB. Zonal convergence associated with this WWB caused a zonal salinity gradient to intensify. The surface‐intensified wind‐driven jet then tilted the zonal salinity gradient into the vertical, thus generating a shallow halocline above the top of the thermocline. In both events, feedbacks appear to have occurred between formation of stratification and formation of vertical shear. Shear formed through a depth‐dependent pressure gradient associated with the salinity gradient and through trapping of wind‐forced momentum above the developing stratification. The increased sheared flow then led to further surface‐intensified fresh water advection and stratification. These examples illustrate how, in the presence of zonal and meridional salinity gradients, the equatorial ocean's response to WWBs can include the formation of thick, long‐lived barrier layers.
Geographic and temporal variations in fire history in boreal ecosystems of Alaska
Authors: Lynch Jason A. , Clark James S. , Bigelow Nancy H. , Edwards Mary E. , Finney Bruce P.
Published: January 2003

Charcoal and pollen analyses were used to determine geographic and temporal patterns of fire importance in boreal forests of the Kenai Peninsula and interior Alaska. Sieved, large charcoal particles were measured in continuously sampled cores of Rock, Portage, and Arrow Lakes (Kenai Peninsula) and Dune and Deuce Lakes (interior Alaska) to estimate regional fire importance and fire occurrence. Charcoal accumulation rates have been low for the past 1000 years in both regions with slightly higher values in interior Alaska than on the Kenai Peninsula. An exception to this general pattern was the period of post‐European settlement on the Kenai Peninsula, where charcoal accumulation rates increased by 10‐fold. This increase most likely reflected increased fire occurrence due to human ignition. The Holocene charcoal and pollen records from Dune Lake indicate low fire occurrence during the early (9000 to 5500 calibrated year before present (yr BP)) birch‐white spruce‐alder (Betula‐Picea glauca‐Alnus) communities and high fire occurrence as black spruce (Picea mariana) became established after 5500 yr BP. Increased fires probably resulted from a change to fire‐prone black spruce forests. For the past 5500 yr BP, two distinct fire regimes occurred. Frequent fires, with an average fire return interval of 98 years, characterized the period from 5500‐2400 yr BP. Fewer fires, with an average fire interval of 198 years, characterized the period after 2400 yr BP. Fuel accumulation, stand structure, and vegetation species contributed to the natural variability in fire regimes during past changes in climate.
Validation and data characteristics of water vapor profiles observed by the Improved Limb Atmospheric Spectrometer (ILAS) and processed with the version 5.20 algorithm
Authors: Kanzawa H. , Schiller C. , Ovarlez J. , Camy‐Peyret C. , Payan S. , Jeseck P. , Oelhaf H. , Stowasser M. , Traub W. A. , Jucks K. W. , Johnson D. G. , Toon G. C. , Sen B. , Blavier J.‐F. , Park J. H. , Bodeker G. E. , Pan L. L. , Sugita T. , Nakajima H. , Yokota T. , Suzuki M. , Shiotani M. , Sasano Y.
Published: January 2003

Vertical profiles of water vapor concentration at high latitudes (57–72°N; 64–89°S) were observed by the Improved Limb Atmospheric Spectrometer (ILAS) solar occultation sensor aboard the Advanced Earth Observing Satellite (ADEOS). These measurements were made continuously from November 1996 through June 1997 with some additional periods in September to October 1996. A validation study of the water vapor data processed with the version 5.20 ILAS retrieval algorithm is presented in this paper. Uncertainty and general characteristics of the ILAS water vapor measurements are briefly reviewed. Comparisons are made with data obtained by (1) the ILAS validation balloon campaigns at Kiruna, Sweden and at Fairbanks, Alaska; (2) the aircraft measurements under the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign; and (3) available satellite measurements of the version 19 Halogen Occultation Experiment (HALOE) and the version 6 Stratospheric Aerosol and Gas Experiment II (SAGE II). The agreement between ILAS water vapor and all independent reliable correlative measurements in the altitude region of 15–60 km is better than 10% for the majority of cases and better than 20% for all comparisons, with the exception of some isolated cases detailed in this paper. Climatological comparisons of ILAS data with Upper Atmosphere Research Satellite (UARS) climatology and HALOE data show the overall consistency of ILAS water vapor data considering the known features of atmospheric circulation. The characteristics of ILAS measurements, i.e., high sampling frequency in polar latitudes with high vertical resolution along with the good data quality, make the ILAS water vapor data set valuable for various polar stratospheric research applications.
Impact of doubled CO2 on global‐scale leaf area index and evapotranspiration: Conflicting stomatal conductance and LAI responses
Authors: Kergoat Laurent , Lafont Sébastien , Douville Hervé , Berthelot Béatrice , Dedieu Gérard , Planton Serge , Royer Jean‐François
Published: January 2003

Current increase in atmospheric CO2 is expected to modify both climate and plant function, thereby impacting plant structure and gas exchange. We investigate the effects of doubled CO2 on leaf area index (LAI) and evapotranspiration (ETR) using a global vegetation model for present‐day and doubled‐CO2 conditions. The model assumes that adaptation of plants to the local climate leads to an equilibrium LAI, which depends on resource availability (minimizing water stress, canopy carbon cost and self‐shading). The model compares reasonably well with remote sensing estimates of LAI. Four doubled‐CO2 simulations are designed to investigate the role of climate, CO2‐induced stomatal closure, enhanced photosynthesis, and a combination of these effects. These simulations show that plant physiological responses to doubled CO2 are potentially more important than climate changes for LAI, and equally important for ETR. In addition, even the sign of the simulated changes in LAI and ETR varies with the assumptions on plant responsiveness to CO2. A reduction of stomatal conductance moderates or cancels the water losses caused by a warmer and drier climate, but photosynthesis stimulation counteracts this stomatal effect, especially in the mid‐to‐high latitudes, because of enhanced LAI. Experimental evidence of LAI and ETR response to CO2 has been reviewed and compared to the different simulations. On the basis of this confrontation we argue that plant response to CO2 doubling may have a relatively small net impact on global ETR and may cause a moderate increase of LAI. Tree stomata may be less responsive to CO2 than was previously assumed, and stimulated plant growth partly cancels the water savings caused by stomatal closure. Understanding the responses of plant canopies to CO2 is therefore critical for land surface hydrology in a CO2 rich world.
On retrieving the microphysical properties of cirrus clouds using the moments of the millimeter‐wavelength Doppler spectrum
Authors: Mace Gerald G. , Heymsfield Andrew J. , Poellot Michael R.
Published: January 2003

Techniques that use only measurements provided by millimeter‐wavelength Doppler radar for retrieving the properties of clouds provide certain practical advantages over techniques that require measurements from multiple instruments. We concentrate on the capacity of the moments of the Doppler spectrum to provide information suitable for retrieving the microphysical properties of cirrus clouds. Specifically, we present an algorithm that uses the zeroth and first moments of the Doppler spectrum (radar reflectivity and mean Doppler velocity) to retrieve the cloud particle size distribution. We also discuss a methodology for evaluating the second moment of the Doppler spectrum for information related to the width of the particle size distribution. The properties of observed cirrus particle spectra are first examined to ascertain what degree of complexity is required of assumed functional representations of particle spectra in cirrus volumes. We find that although particle spectra with bimodal tendencies are common, single‐mode functions are able to capture the essential characteristics of these spectra to within observational uncertainty for most cirrus. Furthermore, two‐parameter functions such as the exponential function are shown to be adequate representations of cirrus particle spectra in situations where the radar reflectivity factor is less than about −5 dBZe and the temperatures are colder than about 253 K. A technique for separating air motion contributions from the Doppler velocity is examined, and an approach for similarly processing the Doppler spectral width is introduced. In comparison to aircraft data, we find uncertainty in the derived quiet air velocity and spectrum width to be on the order of 20% and 40%, respectively. Using recent parameterizations of ice crystal terminal velocity, mass, and backscatter cross section, we present a straightforward algorithm for retrieving cirrus cloud microphysical properties from observations of radar reflectivity and Doppler velocity. A simulation experiment with variable particle habits and realistic observational error shows that the retrieval uncertainties are on the order of 60% and 40% for ice water content and mass median particle size, respectively. These uncertainty magnitudes are supported by comparison to aircraft data.
Chaco low‐level jet events characterization during the austral summer season
Authors: Salio P. , Nicolini M. , Saulo A. C.
Published: January 2003

Previous studies showed a relationship between convection maxima and the convergence of vapor flux over southeastern South America. Nicolini and Saulo [2000] explored the hypothesis of an intensification of this mutual dependency during the Chaco Jet events. They defined these events as extreme cases of low‐level jets east of the Andes that penetrate southernmost to 25°S using ETA operative products during the 1997–1998 austral warm season. The interest of the present paper centers on the climatic characterization of the Chaco Jet events (CJEs) and the possibility of researching this hypothesis using the European Centre for Medium‐Range Weather Forecasts reanalyses (ERA), which cover a 15‐year period (1979–1993). The CJEs represent a subensemble of low‐level jet events east of the Andes that are infrequent in the ERA data set. Their duration varies from 1 to 10 days, more frequently extending from 1 to 5 days. The outstanding features of the circulation and the thermodynamic field that represent this ensemble are a maximum contrast of air masses in a latitude close to 39°S, the presence of a trough centered on 70°W within a baroclinic wave train penetrating from the Pacific Ocean, and a maximum of heat and moisture over northen Argentina and Paraguay. During the CJEs, there is an important flux of moisture and convergence at low and mid levels that is about 10 times more intense than the summer mean. The intensity found in the water vapor flux anomaly reinforces the importance of studying these episodes for the purpose of determining the water balance over southeastern South America. The statement that the CJEs represent an important characteristic of the southeastern South American climate is founded on the fact that, although the CJEs only represent 17% of the austral summer days, they account for a significant fraction of the precipitation (a maximun of 55%) over northeastern Argentina. However, it is indispensable to contrast these results with future field experiments to test the hypothesis addressed in this paper.
Results from an informal intercomparison of ammonia measurement techniques
Authors: Fehsenfeld F. C. , Huey L. G. , Leibrock E. , Dissly R. , Williams E. , Ryerson T. B. , Norton R. , Sueper D. T. , Hartsell B.
Published: January 2003

NH3 measurements made using a chemical ionization mass spectrometry (CIMS) method and a molybdenum‐oxide, converter‐difference (MoOx‐CD) approach were compared with those made using the citric acid denuder method. The citric acid denuder, which has fared well in a variety of other evaluations, was used as the benchmark method in the current intercomparison. A very high degree of correlation, r2 > 0.9, between the three ammonia measurement techniques was found. For ammonia mixing ratios greater than approximately 100 pptv, the denuder method and the CIMS technique were found to agree to within about 20%. However, the MoOx‐CD measurement results were larger by approximately 70% than the denuder measurements. For our measurements, we conclude that the MoOx‐CD method measured the sum of gas‐phase NH3 collected by the denuder tube and aerosol ammonium (NH4+) that was simultaneously collected on Teflon filters. The methods and the comparison along with the results of the ancillary measurements are described.
A semisimultaneous inversion algorithm for SAGE III
Authors: Ward Dale M.
Published: January 2003

The Stratospheric Aerosol and Gas Experiment (SAGE) III instrument was successfully launched into orbit on 10 December 2001. The planned operational species separation inversion algorithm will utilize a stepwise retrieval strategy. This paper presents an alternative, semisimultaneous species separation inversion that simultaneously retrieves all species over user‐specified vertical intervals or blocks. By overlapping these vertical blocks, retrieved species profiles over the entire vertical range of the measurements are obtained. The semisimultaneous retrieval approach provides a more straightforward method for evaluating the error coupling that occurs among the retrieved profiles due to various types of input uncertainty. Simulation results are presented to show how the semisimultaneous inversion can enhance understanding of the SAGE III retrieval process. In the future, the semisimultaneous inversion algorithm will be used to help evaluate the results and performance of the operational inversion. Compared to SAGE II, SAGE III will provide expanded and more precise spectral measurements. This alone is shown to significantly reduce the uncertainties in the retrieved ozone, nitrogen dioxide, and aerosol extinction profiles for SAGE III. Additionally, the well‐documented concern that SAGE II retrievals are biased by the level of volcanic aerosol is greatly alleviated for SAGE III.
Impact‐mobilized dust in the Martian atmosphere
Authors: Nemtchinov I. V. , Shuvalov V. V. , Greeley R.
Published: January 2003

We consider dust production and entrainment into the atmosphere of Mars by impacts. Numerical simulations based on the multidimensional multimaterial hydrocode were conducted for impactors 1 to 100 m in size and velocities 11 and 20 km/s. The size distribution of particles was based on experiments using TNT explosions. Dust can be mobilized even when the impactor does not reach the ground through the release of energy in the atmosphere. We found that the blast‐produced winds entrained dust by a mechanism similar to boundary layer winds as determined from the wind‐tunnel tests. For a 1‐m‐radius stony asteroid releasing its energy in the atmosphere the lifted mass of dust is larger than that in a typical dust devil and could trigger local dust storms. For a 100‐m‐radius meteoroid the amount of injected dust is comparable with the total mass of a global dust storm.
A probabilistic approach for channel initiation
Authors: Istanbulluoglu Erkan , Tarboton David G. , Pack Robert T. , Luce Charles
Published: January 2003

The channel head represents an important transition point from hillslope to fluvial processes. There is a nonlinear threshold transition across the channel head with sediment transport much larger in channels than on hillslopes. Deterministic specific catchment area, a, thresholds for channel initiation, sometimes dependent on slope, S, in the form of aSα ≥ C, have been suggested. In this paper the channel initiation problem is viewed probabilistically with a spatially variable probability of channel initiation that depends on slope, specific catchment area, and the probability distributions of median grain size, surface roughness, and excess rainfall rate. The channel initiation threshold C is cast as a random variable to characterize the variability of aSα at channel heads. Using field measurements from the Idaho Batholith, we show that median grain size measurements at each channel head explain a significant part of the observed variability of aSα. We then characterize the variability of model inputs (median grain size, roughness, and excess rainfall) using probability distributions and show that the probability distribution of area‐slope threshold derived from these inputs matches the probability distribution of area‐slope thresholds measured at field channel head locations. A gamma probability distribution provides a reasonable match to the distributions of area‐slope threshold measured and modeled at channel heads in our study area and in other published channel head data.
Steady shape analysis of tomographic pumping tests for characterization of aquifer heterogeneities
Authors: Bohling Geoffrey C. , Zhan Xiaoyong , Butler James J. , Zheng Li
Published: January 2003

Hydraulic tomography, a procedure involving the performance of a suite of pumping tests in a tomographic format, provides information about variations in hydraulic conductivity at a level of detail not obtainable with traditional well tests. However, analysis of transient data from such a suite of pumping tests represents a substantial computational burden. Although steady state responses can be analyzed to reduce this computational burden significantly, the time required to reach steady state will often be too long for practical applications of the tomography concept. In addition, uncertainty regarding the mechanisms driving the system to steady state can propagate to adversely impact the resulting hydraulic conductivity estimates. These disadvantages of a steady state analysis can be overcome by exploiting the simplifications possible under the steady shape flow regime. At steady shape conditions, drawdown varies with time but the hydraulic gradient does not. Thus transient data can be analyzed with the computational efficiency of a steady state model. In this study, we demonstrate the value of the steady shape concept for inversion of hydraulic tomography data and investigate its robustness with respect to improperly specified boundary conditions.
Intercode comparisons for simulating water balance of surficial sediments in semiarid regions
Authors: Scanlon Bridget R. , Christman Marty , Reedy Robert C. , Porro Indrek , Simunek Jirka , Flerchinger Gerald N.
Published: January 2003

Near‐surface water balance modeling is often used to evaluate land‐atmosphere interactions, deep drainage, and groundwater recharge. The purpose of this study was to compare water balance simulation results from seven different codes, HELP, HYDRUS‐1D, SHAW, SoilCover, SWIM, UNSAT‐H, and VS2DTI, using 1–3 year water balance monitoring data from nonvegetated engineered covers (3 m deep) in warm (Texas) and cold (Idaho) desert regions. Simulation results from most codes were similar and reasonably approximated measured water balance components. Simulation of infiltration‐excess runoff was a problem for all codes. Annual drainage was estimated to within ±64% by most codes. Outliers result from the modeling approach (storage routing versus Richards' equation), upper boundary condition during precipitation, lower boundary condition (seepage face versus unit gradient), and water retention function (van Genuchten versus Brooks and Corey). A unique aspect of the code comparison study was the ability to explain the outliers by incorporating the simulation approaches (boundary conditions or hydraulic parameters) used in the outlying codes in a single code and comparing the results of the modified and unmodified code. This approach overcomes the criticism that valid code comparisons are infeasible because of large numbers of differences among codes. The code comparison study identified important factors for simulating the near‐surface water balance.
In situ calcium carbonate dissolution in the Pacific Ocean
Authors: Feely R. A. , Sabine C. L. , Lee K. , Millero F. J. , Lamb M. F. , Greeley D. , Bullister J. L. , Key R. M. , Peng T.‐H. , Kozyr A. , Ono T. , Wong C. S.
Published: January 2003

Over the past several years researchers have been working to synthesize the WOCE/JGOFS global CO2 survey data to better understand carbon cycling processes in the oceans. The Pacific Ocean data set has over 35,000 sample locations with at least two carbon parameters, oxygen, nutrients, CFC tracers, and hydrographic parameters. In this paper we estimate the in situ CaCO3 dissolution rates in the Pacific Ocean water column. Calcium carbonate dissolution rates ranging from 0.01–1.1 μmol kg−1 yr−1 are observed in intermediate and deepwater beginning near the aragonite saturation horizon. In the North Pacific Intermediate Water between 400 and 800 m, CaCO3 dissolution rates are more than 7 times faster than observed in middle and deep water depths (average = 0.051 μmol kg−1 yr−1). The total amount of CaCO3 that is dissolved within the Pacific is determined by integrating excess alkalinity throughout the water column. The total inventory of CaCO3 added by particle dissolution in the Pacific Ocean, north of 40°S, is 157 Pg C. This amounts to an average dissolution rate of approximately 0.31 Pg C yr−1. This estimate is approximately 74% of the export production of CaCO3 estimated for the Pacific Ocean. These estimates should be considered to be upper limits for in situ carbonate dissolution in the Pacific Ocean, since a portion of the alkalinity increase results from inputs from sediments.
Oriented inclusions of magnetite in clinopyroxene: Source of stable remanent magnetization in gabbros of the Messum Complex, Namibia
Authors: Renne Paul R. , Scott Gary R. , Glen Jonathan M. G. , Feinberg Joshua M.
Published: January 2003

Crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the Early Cretaceous Messum Complex, Namibia. Rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single‐domain magnetite, consistent with the observed sizes and shape anisotropy of the magnetite inclusions. As in previous studies of similar inclusions, these are inferred to have formed by exsolution. Two arrays of inclusions are regularly present in the Messum clinopyroxenes, inclined at about 74°, consistent with formation at about 800°C deduced from optimization of phase boundary orientations. Virtual geomagnetic poles from these rocks are consistent with reference data, confirming that the magnetization is of thermoremanent origin. Bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals, suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal.
An alternative approach using integrated gamma‐ray and geochemical data to estimate the inputs to subduction zones from ODP Leg 185, Site 801
Authors: Révillon S. , Barr S. R. , Brewer T. S. , Harvey P. K. , Tarney J.
Published: January 2003

The goal of the subduction factory project is to obtain a better understanding of processes occurring at convergent margins and performs mass balance calculations for tracer elements. As part of this project, a major objective of ODP Leg 185 is to determine the chemical composition of the inputs to the West Pacific Subduction Factory for use in calculations of elemental mass balance across the subduction zone. To understand such elemental budgets, it is critical to know the lithological diversity and chemical characteristics of the down going oceanic plate. Hole 801C, located ocean‐ward of the Mariana island arc system, is the most complete section of old oceanic crust sampled to date but, core recovery in basement holes is often poor (e.g., <30%) resulting in a bias such that the less altered material is preferentially recovered. Integration of drilled cores and downhole wire line logging data was performed to reconstruct a complete section of the drilled basement. Furthermore we integrate gamma ray logging data with the geochemical analyses of recovered cores to estimate the average composition, in terms of potassium and uranium, of the volcanic section in Hole 801C. We used FMS (Formation MicroScanner) images to relocate the samples with respect to the logging depth and to directly compare the logging data and the geochemical data. We demonstrate that the logging tool is well calibrated for K and U in the basement and that differences observed between K2O and U contents measured by the tool and by geochemistry are strongly dependent on the nature of the samples. The average K2O concentration of Hole 801C basement section is 0.34 wt.% using core‐derived proportions, 0.51 wt.% using log‐derived proportions and 0.47 wt.% using the gamma ray data. In the same manner the average U concentrations are respectively 0.34 ppm, 0.39 ppm and 0.42–0.5 ppm. This implies that K and U budgets recycled in the Mariana subduction zone are 20–50% higher than previously reported. These results demonstrate that a careful core‐log‐geochemical integration is essential to accurately estimate the composition of a drilled section and in order to better constraint chemical fluxes into subduction zones.
Merging of coronal and heliospheric numerical two‐dimensional MHD models
Authors: Odstrcil D. , Linker J. A. , Lionello R. , Mikic Z. , Riley P. , Pizzo V. J. , Luhmann J. G.
Published: January 2003

Space weather research requires investigation of a complex chain of coupled dynamic phenomena occurring simultaneously on various spatial and temporal scales between the Sun and Earth. Specialized physically based numerical models have been developed to address particular aspects of the entire system. However, an integrated modeling approach is necessary to provide a complete picture suitable for interpretation of various remote and in situ observations and for development of forecasting capabilities. In this paper we demonstrate merging of coronal and heliospheric MHD models for a two‐dimensional hypothetical case involving a magnetic cloud, shock, streamer belt, and current sheet. The disruption of a sheared helmet streamer launches a coronal mass ejection (CME) (simulated by the coronal model), which evolves during its propagation through interplanetary space (simulated by the heliospheric model). These models employ different physical approximations and numerical grids to simulate physical phenomena over their respective spatial and temporal domains. The merging of the models enables accurate tracking of a CME from its origin in the solar atmosphere to its arrival at Earth.
Relationship between synoptic‐scale atmospheric circulation and ozone concentrations over Israel
Authors: Dayan Uri , Levy Ilan
Published: January 2003

“High‐ozone days,” defined as days featuring ozone concentrations in excess of 80 ppbv for at least 2 hours over broad geographical regions, were identified for the years 1997 to 2000. The 103 days so obtained were classified into key synoptic‐scale types prevailing over the eastern Mediterranean. Analyses of the temporal behavior of ozone mixing ratios during the high‐ozone days show that the peak values are more frequent during the transitional (spring and autumn) seasons (65% of 103 days) than during the summer season (35%). These transition season episodes are associated with hot and dry air masses originating east of Israel, where ozone precursor emissions are negligible, indicating that high ozone levels are more dependent on air mass characteristics than on upwind precursor emissions. A comparison of the spatial distribution of background concentrations (defined as those measured at 0600 LST) for “high‐ozone days” and for unstable winter days revealed little spatial variation for the winter cases, whereas a significant and consistent spatial variation existed for the high‐ozone cases regardless of circulation type. This finding suggests that ozone background concentrations are more affected by seasonal variation than by spatial variation. In addition, distinctive patterns of diurnal variations were found to be related to synoptic‐scale circulation type and geographical region.
Intercomparison of stratospheric HNO3 measurements over Antarctica: Ground‐based millimeter‐wave versus UARS/MLS Version 5 retrievals
Authors: Muscari Giovanni , Santee Michelle L. , de Zafra Robert L.
Published: January 2003

We present the first intercomparison between the two most comprehensive records of gas‐phase HNO3 profiles in the Antarctic stratosphere, covering the greater part of 1993 and 1995. We compare measurements by the Stony Brook Ground‐Based Millimeter‐wave Spectrometer (GBMS) at the South Pole with Version 5 HNO3 data from the Microwave Limb Sounder (MLS) aboard the Upper Atmospheric Research Satellite. Trajectory tracing was used to select MLS measurements in the 70°–80°S latitude band that sampled air observed by the GBMS during passage over the Pole. When temperatures were near the HNO3 condensation range, additional screening was performed to select MLS measurements that sampled air parcels within 1.5 K of the temperature they experienced over the Pole. Quantitative comparisons are given at 7 different potential temperature levels spanning the range ∼19–30 km. Agreement between the data sets is quite good between 465 and 655 K (∼20–25 km) during a large fraction of the year. Agreement is best during winter and spring, when seasonally averaged differences are generally within 1 ppbv below ∼25 km. At higher altitudes, and during summer and fall, the agreement becomes worse, and GBMS measurements can exceed MLS values by more than 3 ppbv. We provide evidence that differences occurring in the lower stratosphere during fall are due to lack of colocation between the two data sets during a period of strong poleward gradients in HNO3. Remaining discrepancies between GBMS and MLS V5 HNO3 measurements are thought to be due to instrumental or retrieval biases.
Insights into corona formation through statistical analyses
Authors: Glaze L. S. , Stofan E. R. , Smrekar S. E. , Baloga S. M.
Published: January 2003

Statistical analysis of an expanded database of coronae on Venus indicates that the populations of Type 1 (with fracture annuli) and 2 (without fracture annuli) corona diameters are statistically indistinguishable, and therefore we have no basis for assuming different formation mechanisms. Analysis of the topography and diameters of coronae shows that coronae that are depressions, rimmed depressions, and domes tend to be significantly smaller than those that are plateaus, rimmed plateaus, or domes with surrounding rims. This is consistent with the model of Smrekar and Stofan [1997] and inconsistent with predictions of the spreading drop model of Koch and Manga [1996]. The diameter range for domes, the initial stage of corona formation, provides a broad constraint on the buoyancy of corona‐forming plumes. Coronae are only slightly more likely to be topographically raised than depressions, with Type 1 coronae most frequently occurring as rimmed depressions and Type 2 coronae most frequently occurring with flat interiors and raised rims. Most Type 1 coronae are located along chasmata systems or fracture belts, while Type 2 coronae are found predominantly as isolated features in the plains. Coronae at hot spot rises tend to be significantly larger than coronae in other settings, consistent with a hotter upper mantle at hot spot rises and their active state.
A high‐resolution radiolarian‐derived paleotemperature record for the Late Pleistocene‐Holocene in the Norwegian Sea
Authors: Dolven Jane K. , Cortese Giuseppe , Bjørklund Kjell R.
Published: January 2003

Polycystine radiolarians are used to reconstruct summer sea surface temperatures (SSSTs) for the Late Pleistocene‐Holocene (600–13,400 14C years BP) in the Norwegian Sea. At 13,200 14C years BP, the SSST was close to the average Holocene SSST (∼12°C). It then gradually dropped to 7.1°C in the Younger Dryas. Near the Younger Dryas‐Holocene transition (∼10,000 14C years BP), the SSST increased 5°C in about 530 years. Four abrupt cooling events, with temperature drops of up to 2.1°C, are recognized during the Holocene: at 9340, 7100 (“8200 calendar years event”), 6400 and 1650 14C years BP. Radiolarian SSSTs and the isotopic signal from the GISP2 ice core are strongly coupled, stressing the importance of the Norwegian Sea as a mediator of heat/precipitation exchange between the North Atlantic, the atmosphere, and the Greenland ice sheet. Radiolarian and diatom‐derived SSSTs display similarities, with the former not showing the recently reported Holocene cooling trend.