bluuebunny/crossref_metadata_2025 huggingface dataset:
A three‐dimensional representation of geomagnetic reversal records
Authors: Mary Claire , Courtillot Vincent
Published: February 2008

Representations of the reversing geomagnetic field are generally based on paleomagnetic directions or virtual geomagnetic poles, i.e., one coordinate related to field intensity is missing. Interpretations based on these representations have led to conflicting models of reversals. We have assembled records of reversals for which an intensity determination is available, either absolute (volcanic sequences) or relative (sedimentary sequences), and we have chosen to represent them in terms of directions in local Cartesian coordinates: this representation is identical to the Zijderveld representation which is traditional in paleomagnetism. The 38 selected records correspond to Matuyama‐Brunhes, Jaramillo, Olduvai and some older reversals up to 5 m.y. old. A total of 22% are from volcanic sequences and the rest from sedimentary ones. Vector behavior falls into one of five categories: A, the reversing field follows the local trend of the geocentric axial dipole (GAD) passing through the origin, with noise added; B, same as A, but offset by a constant component from the origin; C, the reversing field follows a linear trend going through the origin in Cartesian coordinates but the direction is different from that of the GAD; D, same as C but offset from the origin by a constant component; E, large noise, no linear or other simple bahavior. Over half (55%) of the records fall in either category A or B, 18% in category E. The C‐type data often consist of dipole‐parallel segments separated by jumps, the significance of which is as yet unclear. The number of cases with a linear behavior is remarkable (82%). Large directional swings in the traditional representations often correspond to far less conspicuous changes on the vector plots at points close to the origin; behavior there is dominated by “noise”, one source of which may simply be secular variation of the nondipole field. The constant offsets sometimes observed (34% of the cases) are as yet unexplained but may be due to rock magnetic problems and therefore may have no global significance. We doubt whether the present data allow us to extract resolvable terms such as g20 or h31 which have been inferred from the directional representation.
Trace elements in migrating high‐temperature fluids: Effects of diffusive exchange with the adjoining solid
Authors: Kenyon Patricia M.
Published: February 2008

Trace element concentrations and isotopic ratios are frequently used to study the behavior of high‐temperature fluids in both metamorphic and igneous systems. Many theoretical formulations of the effects of fluid migration on trace elements have assumed instantaneous reequilibration between the migrating fluid and the solid material through which it is passing. This paper investigates the additional effects which arise when equilibration is not instantaneous due to a limited rate of diffusion in the solid, using an analytical steady state solution to a set of partial differential equations describing the exchange of trace elements between the fluid and the solid during the migration of the fluid. It is found that the rate of transport of the trace element variations depends on the spatial and temporal scale of the initial variation in the trace element concentration in the fluid, as well as porosity, distribution coefficient, diffusion coefficient in the solid, and channel size. In addition, the amplitude of any initial trace element variations decreases with increasing distance of fluid migration at a rate which depends on the scale of the initial trace element variation, the parameters listed above, and the velocity of fluid migration. The effect of varying the system parameters controlling transport rate and damping is investigated for the case of basaltic magma migrating through a lherzolite matrix. The operation of diffusion‐limited reequilibration during fluid migration can be recognized in nature from studies of the altered regions surrounding veins in peridotite and from time stratigraphic studies of erupted lavas. Consideration of one geochemical study for the East Pacific Rise generates reasonable estimates of migration velocity and suggests that the magma has migrated primarily through macroscopic channels rather than along grain boundaries.

Appendices are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N. W.,Washington, DC 20009. Document B93‐006; $2.50. Payment must accompany order.
Calculations of pressure‐induced phase transitions in silica
Authors: Lacks Daniel J. , Gordon Roy G.
Published: February 2008

The crystal structures and energies of the quartz, stishovite and CaCl2 phases of silica were calculated as a function of pressure with the polarization‐included electron gas (PEG) model. The calculated structures are in good agreement with experiment throughout the pressure range in which experimental results exist. The calculated pressure of the quartz to stishovite phase transition is about 3.5 GPa, as compared to an experimental value of 5.5 GPa. The calculated pressure of the stishovite to CaCl2 structure phase transition is slightly less than 200 GPa, higher than the value suggested by experiments of about 100 GPa. The compressibility of quartz, the quartz to stishovite transition pressure, and the stishovite to CaCl2 structure transition pressure are all calculated more accurately with the PEG model, which allows the ions to distort from spherical symmetry, than with the modified electron gas model, which constrains the ions to be spherical.
A new automated method of cloud masking for advanced very high resolution radiometer full‐resolution data over the ocean
Authors: Gallegos Sonia C. , Hawkins Jeffrey D. , Cheng Chiu Fu
Published: February 2008

This paper reports on a new algorithm to remove cloud‐contaminated pixels from daytime and nighttime 1‐km advanced very high resolution radiometer (AVHRR) data. The technique was developed in response to Navy needs to efficiently and accurately eliminate cloud contaminated pixels from real‐time satellite digital images. The remaining “cloud free” sea surface temperature (SST) pixels would be available for analysts to utilize in tracking ocean mesoscale fronts and eddies as well as input to SST and ocean thermal analysis. Initial poor results with existing cloud‐masking techniques led to this effort. The method uses a series of approaches to locate cloud‐contaminated pixels which include (1) use of multiple bands to detect signatures not readily available from single‐channel data, (2) extraction of cloud edge information through local segmentation of the image using the cluster shade texture measure based on the gray level cooccurrence (GLC) matrix and, (3) discrimination of cloud‐free from cloud‐contaminated regions with an area labeling procedure. This technique is evaluated on identical data sets utilizing other experimental and operational cloud algorithms and cloud masks produced through human interpretation. This method, tested over a wide range of conditions and geographical locations, produces accurate and efficient daytime and nighttime SST data sets.
Characteristics of the summer 1987 flow field in the Ionian Sea
Authors: Nittis Kostas , Pinardi Nadia , Lascaratos Alex
Published: February 2008

We present an extensive analysis of the first complete data set in the northern Ionian Sea collected during the Physical Oceanography of the Eastern Mediterranean (POEM) general circulation survey of September 1987. A four water mass structure of the basin is found to be represented by a first internal baroclinic Rossby radius of deformation of 11.8 km. The horizontal correlation scales decrease with depth, and the subsurface flow is dominated by anticyclonic gyres. Large‐scale circulation trends of the temperature and salinity covariance matrices are compensated below 200 m, and only the gyre scales (∼100 km) persist at intermediate and deep levels. The empirical orthogonal functions of the data set show that an horizontal scale separation exists between the first and higher vertical modes of the dynamic height field.
Sea level variability and semiannual Rossby waves in the South Atlantic subtropical gyre
Authors: Le Traon P.‐Y. , Minster J.‐F.
Published: February 2008

We derived the sea level variability in the South Atlantic subtropical gyre between 15° and 35°S from 2 years of the Geosat exact repeat mission altimeter data. On the mesoscale, along‐track wavenumber spectra are those of nonlinear dynamics with a steeper slope and stronger energy along the eastern boundary. Frequency/wavenumber spectra reveal a significant semiannual signal at a wavelength of about 500 km. West of the Walvis ridge (5°E), this signal appears to correspond to semiannual Rossby waves of 2‐ to 3‐cm amplitude, propagating westward at 3 cm s−1, and showing latitude refraction effects. The pattern is very similar to that of annual Rossby waves as modeled by Reason et al. (1987). These waves are probably related to the semiannual wind component. However, understanding the exact mechanism for their excitation requires dynamical modeling. The large‐scale variability also shows a significant semiannual component (about 6‐cm amplitude and 15% of the yearly variance) in phase with climatological winds.
Linear instability of a mixed‐layer front
Authors: Samelson R. M.
Published: February 2008

The linear instabilities of a geostrophically balanced frontal jet are computed numerically in an effort to understand the origin of intense small‐scale cold‐core features observed in the North Atlantic subtropical convergence zones during the Frontal Air‐Sea Interaction Experiment (FASINEX). A simple analytic continuously‐stratified basic state is used to represent the observed frontal structure. The most unstable linear modes have e‐folding timescales of 2–3 days, alongfront wavelengths near 70 km, and significant asymmetry across the front. The results provide indirect evidence that baroclinic instability of the frontal jet is a possible source of the cold‐core features observed during FASINEX.
Ice drift in the Weddell Sea in 1990–1991 as tracked by a satellite buoy
Authors: Vihma Timo , Launiainen Jouko
Published: February 2008

Ice drift in the Weddell Sea has been studied by using a satellite buoy deployed on an ice floe. The buoy survived for a 20‐month period, indicating a drift trajectory of 10,000 km and yielding 13 months of marine meteorological data. The drift of the ice floe was studied with respect to the winds measured by the buoy. In the central Weddell Sea, the mean drift speed of the ice floe was 0.15 m/s, which was about 3% of the wind speed. More specifically, the drift ratio was 3.4% in the marginal ice zone and 2.4% in the inner pack ice field. On average, the drift was directed 36° left of the wind direction, but the turning angle was larger during the austral summer and smaller during the winter. On time scales of days the drift was primarily wind‐dependent, except for cases during winter periods of high ice concentration and internal ice resistance. For time scales of several months, purely wind‐based simulations of the drift resulted in a discrepancy between the observed and simulated trajectories, but the inclusion of a slow (0.02 m/s) residual current made the simulations significantly better. The geostrophic wind based on European Centre for Medium‐Range Weather Forecasts pressure analyses was estimated for a 1‐month period, and the ice floe was found to drift almost parallel to the geostrophic wind with a speed of 2% of the geostrophic wind speed. Inertial‐type motion superimposed on the wind‐induced drift was found to be a characteristic feature in the marginal ice zone during the austral summer, but it could not be found from the drift in winter when kinetic energy was transferred to larger scales of motion and dissipated into the ice field.
A primitive equation ocean circulation model using a general vertical coordinate transformation: 1. Description and testing of the model
Authors: Gerdes Rüdiger
Published: February 2008

The ocean general circulation model of the Geophysical Fluid Dynamics Laboratory has been modified to accommodate a generalized vertical coordinate transformation. The model with a slightly modified σ transformation (σ = z/H) is applied to three simple test cases. A clear improvement of barotropic and baroclinic topographic Rossby waves has been achieved compared with the z‐coordinate model. A somewhat more typical time‐independent problem for the overflow of dense water masses over a zonal ridge exhibits the potential benefits as well as the flaws of the new model. Vertically integrated mass transport is very sensitive to changes in bottom slope in regions where planetary and topographic β are of comparable magnitudes. This translates into large errors in the z model due to the crude approximation to the actual topography. The deep flow in the z model does not follow the bottom exactly. Horizontal overshoots of dense water lead to convective mixing on the southern side of the ridge. This can be a serious problem for modeling more realistic overflow situations with the z model. Increased diapycnal diffusion has been identified as the most serious problem with the terrain‐following coordinate system for this specific application.
Influence of density stratification and bottom depth on vertical mode structure functions in the tropical Pacific
Authors: Picaut Joël , Sombardier Laurence
Published: February 2008

Vertical mode structure functions are computed over the entire tropical Pacific Ocean (30°N to 30°S) by using the Levitus temperature‐salinity climatological file and a National Oceanic and Atmospheric Administration bathymetry file and over a few selected areas of the Pacific by using a more accurate conductivity‐salinity temperature depth (CTD‐STD) file. At every 1° grid point of the tropical Pacific, phase speeds of the first five modes are estimated from the Levitus density profiles and the actual bottom depth. They range from 305 to 195 cm s−1 for mode 1 to 65 to 40 cm s−1 for mode 5. Because of the coarse vertical resolution in the Levitus profiles, the corresponding phase speeds are underestimated by about 8% compared to phase speeds calculated with CTD‐STD profiles. In order to estimate the relative importance of density stratification and bottom depth on vertical mode structure functions, two sets of modes are calculated. The first set is calculated with the Levitus density profiles truncated or extended to a mean basin bottom depth (3570 m) at every grid point. The resulting modes are representative of the influence of density stratification on modal calculations. The second set is computed with a mean basin density profile and the actual bottom depth at every grid point. This set represents the influence of bottom depth. Phase speed comparison between these two sets of modes and the original modes indicates that for the first two modes, the bottom depth contribution is an order of magnitude less than the density contribution. For modes 3 and 4, the bottom depth contribution increases, and for mode 5 it is almost equal to the density contribution. The relative importance of deep density stratification and upper layer stratification on vertical mode structure functions is evaluated by using the more accurate CTD‐STD file. This study is restricted to a few small areas with enough deep profiles, taken within a few days, to ensure proper statistical results. Vertical modes are calculated first with complete vertical density profiles and then with the same profiles truncated at a certain depth and replaced below by a mean density profile (obtained from 159 CTD‐STD deep profiles in the tropical Pacific). Statistical comparison reveals that meaningful computation of the first five vertical modes can be obtained in the tropics by using precise density information in the first 600 m and mean density information below.
Airborne synthetic aperture radar observations and simulations for waves in ice
Authors: Vachon Paris W. , Olsen Richard B. , Krogstad Harald E. , Liu Antony K.
Published: February 2008

The Canada Centre for Remote Sensing CV‐580 aircraft collected C‐band synthetic aperture radar (SAR) data over the marginal ice zone off the east coast of Newfoundland during the Labrador Ice Margin Experiment (LIMEX) in March 1989. One component of the LIMEX'89 program was the study of ocean waves penetrating the marginal ice zone. In this paper, we consider nearly coincidental observations of waves in ice by airborne SAR and wave‐induced ice motion measurements. We explain the wave patterns observed in the SAR imagery, and the corresponding SAR image spectra, in terms of SAR wave imaging models. These include the well‐known tilt cross‐section modulation, linear, quasi‐linear, and nonlinear velocity bunching forward mapping models (FMMs), and the assertion that the concept of coherence time limitation applies differently to the cases of waves in ice and open water. We modify the concept of the scene coherence time to include two parts: first, a decorrelation time deduced from the inherent azimuth cutoff in the nonlinear velocity bunching FMM; and second, the intrinsic scene coherence time which is a measure of the time scale over which an open water Bragg scattering patch retains its phase structure. Either of these coherence time scales could dominate the SAR image formation process, depending upon the environmental conditions (the wave spectrum and the wind speed, for example). These two coherence time scales are independently estimated based upon a quasi‐linear velocity bunching FMM applied to some of the LIMEX'89 observations. Observed SAR image spectra and forward mapped ice motion package spectra are favorably compared.
Shorerise and bar‐berm profiles on ocean beaches
Authors: Inman Douglas L. , Elwany M. Hany S. , Jenkins Scott A.
Published: February 2008

A beach equilibrium model is developed that treats the outer (shorerise) portion of the profile independently from that of the inner (bar‐berm) portion. The two portions are matched at the breakpoint‐bar. The partitioning of the profile in this way is consistent with the different forcing modes on either side of the breakpoint. This formulation utilizes beach profile data not previously available. It is shown that both portions of the profile are well fitted by curves of the formh=Axm, wherehis positive downward andxis the positive offshore coordinate. Surprisingly, the value ofm≈ 0.4 is nearly the same for shorerise and bar‐berm and does not change significantly with seasonal beach changes (summer/winter). The principal difference between seasonal profiles is that in winter (higher waves) the breakpoint‐bar is deeper and farther offshore while the berm crest is displaced landward. Thus the changes in seasonal equilibria are manifested by simple, self‐similar displacements of the bar‐berm and shorerise curves as a consequence of changes in surf zone width andO(1) variations in the factorA.
Subtidal circulation over the northern California shelf
Authors: Largier J. L. , Magnell B. A. , Winant C. D.
Published: February 2008

The circulation over the shelf and upper slope off northern California, between 38°N and 42°N, was observed using moored current and temperature recorders deployed as part of the Northern California Coastal Circulation Study (NCCCS), from March 1987 through October 1989. The results of this study provide a larger‐scale view of the wind‐driven circulation than that described through the 1981–1982 Coastal Ocean Dynamics Experiment (CODE), particularly with regard to alongshore and temporal variations. From an improved description of the frequency structure of wind and current, a very low frequency (VLF) signal is identified in the observed currents at frequencies below those typical of wind forcing. The majority of this VLF variance appears to be accounted for by persistent flow events associated with the presence of mesoscale circulation in the adjacent ocean. The longer duration of the NCCCS also allows an improved description of the seasonality of flow regimes off northern California. Three oceanic seasons are identified: an upwelling season (April‐July), a relaxation season (August‐November), and a storm season (December‐March). Alongshore variations in the strength of upwelling, in the strength of the alongshore flow, both near‐surface and undercurrent, and in water temperature not only are a function of latitude, as is the wind, but they also correspond to location relative to promontories, notably Cape Mendocino. Immediately south of Cape Mendocino, the near‐surface flow exhibits an equatorward minimum and a temperature minimum, whereas the undercurrent exhibits a poleward maximum. Conversely, at the moorings immediately north of the cape, temperatures are a maximum and the undercurrent exhibits a minimum. The maximum in near‐surface temperature relates to a minimum in upwelling; no significant correlation was found between local wind and current immediately north of Cape Mendocino. This upwelling minimum and the upwelling maximum south of the cape were also observed as persistent sea surface temperature patterns in satellite imagery.
Effects of westerly wind bursts upon the western equatorial Pacific Ocean, February–April 1991
Authors: Delcroix Thierry , Eldin Gérard , McPhaden Michael , Morlière Alain
Published: February 2008

In February‐April 1991, episodes of 2 to 8 m s−1 westerly winds of 3 to 11 days' duration occurred in the western Pacific warm pool. Resulting modifications of the upper ocean in current and hydrology are quantified using data from an equatorial mooring at 165°E and from three cruises within 30 days of one another along 165°E. During westerly wind bursts (WWB) stronger than 4 m s−1, the upper 50 m becomes isothermal to within 0.1°C and sea surface temperature (SST) drops by 0.3–0.4°C between 5°S and 2.5°N. Conversely, SST starts warming and the upper 50 m restratifies in 4–5 days after the end of WWB. In contrast to previous observations, salinity between 0 and 50 m appears almost unaffected by WWB; it freshens by 0.4 practical salinity unit in March within an area of 1°–2° of latitude around the equator but not necessarily in direct response to WWB. As for zonal circulation, surface equatorial flow accelerates eastward 2–3 days after the beginning of westerlies. Then, after less than 2 weeks, eastward and westward jets both develop from 2°N to 2°S in the upper and lower halves of the temperature mixed layer, respectively. Changes in zonal mass transport in this layer were as much as 30 Sv between 2.5°S and 2.5°N from one cruise to the next.
Diurnal tides in the Arctic Ocean
Authors: Kowalik Z. , Proshutinsky A. Y.
Published: February 2008

A two‐dimensional numerical model with a space grid of about 14 km is applied to calculate diurnal tidal constituents K1 and O1 in the Arctic Ocean. Calculated corange and cotidal charts show that along the continental slope, local regions of increased sea level amplitude, highly variable phase and enhanced currents occur. It is shown that in these local regions, shelf waves (topographic waves) of tidal origin are generated. In the Arctic Ocean and Northern Atlantic Ocean more than 30 regions of enhanced currents are identified. To prove the near‐resonant interaction of the diurnal tides with the local bottom topography, the natural periods of oscillations for all regions have been calculated. The flux of energy averaged over the tidal period depicts the gyres of semitrapped energy, suggesting that the shelf waves are partially trapped over the irregularities of the bottom topography. It is shown that the occurrence of near‐resonance phenomenon changes the energy flow in the tidal waves. First, the flux of energy from the astronomical sources is amplified in the shelf wave regions, and afterwards the tidal energy is strongly dissipated in the same regions.
Longshore current and wave height modulation at tidal frequency inside the surf zone
Authors: Thornton Edward B. , Kim Chang S.
Published: February 2008

Data were acquired continuously during the 19‐day DELILAH nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies. It is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations, and since radiation stress is a function of the wave height, longshore currents are forced at the tidal frequency inside the surf zone. The measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions, indicating that the tide is a dominant mechanism associated with longshore current variability. Simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by Thornton and Guza (1986) are used to explain observations. The measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model, verifying the hypothesis.
Lead and aluminum in Atlantic surface waters (50°N to 50°S) reflecting anthropogenic and natural sources in the Eolian transport
Authors: Helmers Eckard , Rutgers van der Loeff Michiel M.
Published: February 2008

Lead and aluminum were measured with a 40–100 km resolution in surface water on two transects across the Atlantic Ocean, one in May 1990 from Cape Town to the North Sea, the other in November 1990 from the North Sea to the Strait of Magellan. Samples were drawn 14 m below surface at normal speed from a 2‐m‐long snorkel system mounted on the bottom of the ship directly into a clean‐room area. In the tropics, both Pb and Al show maximum concentrations in the Intertropical Convergence Zone (ITCZ) correlated with each other and with minimum salinities, indicating wet deposition as their common source. Even in this area characterized by large inputs of mineral aerosols, the Pb/Al ratio shows that the major source of soluble lead (>95%) is anthropogenic. At higher latitudes, Al is low throughout (10–20 nmol/kg), whereas enhanced Pb values show the anthropogenic inputs off south Africa, northern Argentina and especially western Europe. Very low Pb and especially Al concentrations in the upwelling areas associated with the Canary and Benguela currents show that the enhanced biogenic particle fluxes cause an efficient scavenging of both lithogenic particles known to arrive here by dry deposition, and of the adhering reactive trace metals.
A generation‐dispersion model of ambient and transient bubbles in the close vicinity of breaking waves
Authors: Baldy Serge
Published: February 2008

Bubble models are necessary to ascertain bubble contribution to ocean‐atmosphere fluxes of gas, aerosols, humidity, and latent heat. Previous theories flatten the wave breaking layer to a theoretical boundary from which bubbles are dispersed by turbulence working against buoyancy lift. As a consequence, bubble population characteristics next to the surface are not derived from these models but depend on empirical or semiempirical assumptions made at this boundary. By considering bubble injection with puffs of intense turbulence, specifying how bubbles are first created by a small‐scale similarity reasoning, and using a wavy interface, the present bubble theory expands this layer to a more physical breaking layer. Bubble concentration density as a function of bubble diameter, depth, and sea state parameters is obtained through explicit integrals. The model is found to be consistent with the previous bubble theories: the back flattening of the model breaking layer indeed results in equations compatible with these theories. The model variations in bubble concentration density with different parameters is coherent with experimental laws: the dominant bubble concentration is found to vary as about d−4 with bubble diameter and u*3 with wind friction velocity, but because of breaking patches, a d−2 bubble distribution is obtained very close to the surface. The concordance of the model with experimental data in the recent and classic bubble literature is quite good.
Low‐frequency passive‐microwave observations of sea ice in the Weddell Sea
Authors: Menashi James D. , St. Germain Karen M. , Swift Calvin T. , Comiso Josefino C. , Lohanick Alan W.
Published: February 2008

The microwave emission properties of first‐year sea ice were investigated from the R/V Polarstern during the Antarctic Winter Weddell Gyre Project in 1989. Radiometer measurements were made at 611 MHz and 10 GHz and were accompanied by video and visual observations. Using the theory of radiometric emission from a layered medium, a method for deriving sea ice thickness from radiometer data is developed and tested. The model is based on an incoherent reflection process and predicts that the emissivity of saline ice increases monotonically with increasing ice thickness until saturation occurs.
Observations of topographic planetary waves in the Bering Slope Current using the Geosat altimeter
Authors: Okkonen Stephen R.
Published: February 2008

Topographic planetary waves associated with the downstream leg of the Bering Slope Current are observed in measurements of sea surface height anomalies acquired by the Geosat altimeter during 1987 and 1988. The waves are most prominent from May to August 1987, during which time the estimated wave period and wavelength are about 72 days and 103 km, respectively. The direction of propagation is roughly north‐northeast.
Effects of precipitation on the thermodynamic structure of the trade wind boundary layer
Authors: Albrecht Bruce A.
Published: February 2008

A model of the thermodynamic structure of the trade wind boundary layer is formulated to include the parameterization of precipitation in relatively shallow clouds. Although the area‐averaged simulated precipitation rates are relatively small (less than 1 mm/day), the inclusion of precipitation has an appreciable effect on the predicted thermodynamic structure. The cloud layer structure simulated with precipitation is warmer, drier, and more stable than that simulated without precipitation. The simulated inversion height is lowered by as much as 60 mbar when precipitation is included.
Aircraft measurement of electric field: Self‐calibration
Authors: Winn W. P.
Published: February 2008

The electric field of a thunderstorm is distorted by the metallic surfaces of airplanes and rockets. The amplitude of the local, distorted electric field can be measured at selected locations on the surface of the vehicle with electric field meters. Measuring the thunderstorm field entails finding the relations between the local fields at the meters and the ambient, undistorted electric field that would exist in the absence of the vehicle. Calibration can be performed in the following steps: (1)Find linear combinations F0, F1, …, of local field amplitudes that are independent of the charge on the vehicle. (2) While the ambient electric field is constant in the Earth coordinate system, rotate the vehicle (roll and turn or roll and pitch) and record the local field amplitudes and the roll, pitch, and heading angles. Find the direction of the electric vector and the ratios between all of the calibration coefficients that best fit the linear combinations of local field amplitudes. (3) Find the magnitude of one of the coefficients by comparison with a calibrated instrument; from this magnitude and all of the ratios, find all of the magnitudes. With this method of calibration, finding out how the linear combinations F0, F1, … depend on the ambient electric field does not require a symmetric placement of the electric field meters, nor does it require any knowledge of the locations of the field meters. However, inverting the expressions to find the ambient field components as functions of the F𝒾 does place constraints on the locations of the field meters. The results of the above calibration can be used only when the signal at each field meter is a linear function of four parameters: the charge on the airplane and the three components of a uniform electric field from distant charge. When there are five or more electric field meters, the presence of nearby charge can be detected because it is possible to find linear combinations of the signals from five field meters whose value is not zero only when there is nearby charge. This paper presents one method of finding the linear combinations.
Recent changes in the North American Arctic boundary layer in winter
Authors: Bradley Raymond S. , Keimig Frank T. , Diaz Henry F.
Published: February 2008

Analysis of significant level radiosonde data from a network of Arctic stations reveals a systematic reduction in midwinter surface‐based inversion depths over the past few decades, accompanied by a rise in surface temperature. Similar trends are observed over a wide sector, from 62°W to 162°W and from 70°N to 83°N. Possible causes for these changes include increases in warm air advection, cloud cover, ice crystals, aerosols, and greenhouse gases, but the specific reasons are difficult to identify, due to strong interactions between many potentially important factors. Nevertheless, the changes are significant for studies of Arctic haze, since the midwinter stable boundary layer has been decreasing in depth over time.
Variations in snow on sea ice: A mechanism for producing climate variations
Authors: Ledley Tamara Shapiro
Published: February 2008

Studies of the impact of increases in greenhouse gases on the climate system indicate that in addition to increases in temperature there will be changes in the precipitation rate, possibly both in the mean and in the extremes. While the magnitudes of the changes in the precipitation rate are not well known, it is important to access the impact of the possible changes in precipitation rate and to identify the mechanisms by which these changes affect climate. In this study the impact of various precipitation rates, and thus snowfall rates, on sea ice thickness, its subsequent effect on climate, and how this effect occurs is examined. The results show that the general effect of a thin layer of snow on the sea ice surface is to thin the sea ice. When the snow is thick enough in the poleward most zone to survive the summer season, the sea ice thickens. In addition, snow cools the climate system, and this cooling increases as the snow thickens. The cooling is caused by two factors. The first is the increase in the surface albedo for an increasingly snow‐covered surface, which causes a decrease in the absorbed solar radiation by the entire system. The second is the decrease in the turbulent energy transfer from the ocean to the atmosphere, which is the result of the increased insulation of the surface from the relatively warm ocean as the snow thickens and sea ice becomes more extensive through the year. Thus as the snow thickens, the climate cools.
The meteorological environment of the tropospheric ozone maximum over the tropical South Atlantic Ocean
Authors: Krishnamurti T. N. , Fuelberg H. E. , Sinha M. C. , Oosterhof D. , Bensman E. L. , Kumar V. B.
Published: February 2008

Atmospheric flow patterns are examined over the South Atlantic Ocean where a maximum of tropospheric ozone has been observed just west of southern Africa. We investigate the flow climatology during October and perform a case study for 6 days during October 1989. Analyses from the European Center for Medium‐Range Weather Forecasting are employed, and a high‐resolution global spectral model is used to prepare forecasts during the period. Horizontal and vertical motions are examined and used to prepare three‐dimensional backward trajectories from the region of greatest ozone. An initially zonally symmetric distribution of ozone is treated as a passive tracer and advected by three‐dimensional flows forecast by the global model. Results from the passive tracer simulation indicate that three‐dimensional advection alone can produce a maximum of tropospheric ozone in the observed location. In addition, the trajectories suggest that by‐products of biomass burning could be transported to the area of maximum ozone. Low‐level flow from commonly observed regions of burning in Africa streams westward to the area of interest. Over Brazil, if the burning by‐products are carried into the upper troposphere by convective process, they then could be transported eastward to the ozone feature in approximately 5 days. There is considerable subsidence over the tropical southern Atlantic, such that stratospheric influences also are a factor in producing the ozone maximum. Both planetary‐scale and transient synoptic‐scale circulation features play major roles in the various transport processes that influence the region. In summary, the observed tropospheric ozone maximum appears to be caused by a complex set of horizontal and vertical advections, transport from regions of biomass burning, and stratospheric influences.
Composite Mg II solar activity index for solar cycles 21 and 22
Authors: DeLand Matthew T. , Cebula Richard P.
Published: February 2008

The Mg II core‐to‐wing index was first developed for the Nimbus 7 solar backscatter ultraviolet spectrometer (SBUV) instrument as an indicator of solar middle ultraviolet activity that is independent of most instrument artifacts. This index is defined as the ratio of the irradiance in the core of the unresolved Mg II doublet at 280 nm to the nearby continuum irradiance and measures solar variability on both rotational and solar cycle time scales. Mg II index data sets have also been derived for the NOAA 9 and NOAA 11 SBUV/2 instruments. The combined Mg II index data record from the Nimbus 7, NOAA 9, and NOAA 11 instruments presented in this paper extends from November 1978 to January 1992. Differences in the absolute value of the Mg II index and long‐term response to solar variations due to differences in wavelength scale and band pass among the three instruments require the use of linear regression fits to create a single composite Mg II index data set which includes more than 13 years of data. This paper documents version 1.0 of the composite Mg II index data set, which has been widely distributed on CD‐ROM. Using this composite data set, the change in 27‐day running average of the Mg II index from solar maximum to solar minimum is approximately 8% for solar cycle 21 and approximately 9% for solar cycle 22 through January 1992. This difference is not statistically significant when the errors in the linear regression fits used to construct the composite Mg II index are considered. Scaling factors based on the short‐term variations in the Mg II index and solar irradiance data sets are developed for each instrument to estimate solar variability at mid‐ultraviolet and near‐ultraviolet wavelengths. A set of composite scale factors are derived for use with the composite Mg II index presented here. Near 205 nm, where solar irradiance variations are important for stratospheric photochemistry, the estimated change in irradiance during solar cycle 22 is approximately 10(±1)% using the composite Mg II index (version 1.0) and scale factors. However, the actual magnitude of ΔF205 is probably closer to 9% due to unconnected SBUV/2 wavelength scale drift in the current composite Mg II index data set.
On the evaluation of air mass factors for atmospheric near‐ultraviolet and visible absorption spectroscopy
Authors: Perliski Lori M. , Solomon Susan
Published: February 2008

The interpretation of UV‐visible twilight absorption measurements of atmospheric chemical constituents is dependent on how well the optical path, or air mass factor, of light collected by the spectrometer is understood. A simple single scattering model and a Monte Carlo radiative transfer scheme have been developed to study the effects of multiple scattering, aerosol scattering, surface albedo and refraction on air mass factors for scattered light observations. At fairly short visible wavelengths (less than about 450 nm), stratospheric air mass factors are found to be relatively insensitive to multiple scattering, surface albedo and refraction, as well as aerosol scattering by background aerosols. Longer wavelengths display greater sensitivity to refraction and aerosol scattering. Tropospheric air mass factors are found to be highly dependent on aerosol scattering, surface albedo and, at long visible wavelengths (about 650 nm), refraction. Absorption measurements of NO2 and O4 are shown to support these conclusions.
Fourier transform infrared studies of the interaction of HCl with model polar stratospheric cloud films
Authors: Koehler Birgit G. , McNeill Laurie S. , Middlebrook Ann M. , Tolbert Margaret A.
Published: February 2008

Heterogeneous reactions involving hydrochloric acid adsorbed on the surfaces of polar stratospheric clouds (PSCs) are postulated to contribute to polar ozone loss. Using Fourier transform infrared (FTIR) spectroscopy to probe the condensed phase, we have examined the interaction of HCl with ice and nitric acid trihydrate (NAT) films representative of types II and I PSCs, respectively. For HCl pressures in the range of 10−7 to 10−5 Torr our FTIR studies show that a small amount of crystalline HCl·6H2O formed on or in ice at 155 K. However, for higher HCl pressures we observed that the entire film of ice rapidly converted into an amorphous 4:1 H2O:HCl mixture. From HCl‐uptake experiments with PHCl = 8 × 10−7 Torr, we estimate roughly that the diffusion coefficient of HCl in ice is around 2 × 10−12 cm2/s at 158 K. For higher temperatures more closely approximating those found in the stratosphere, we were unable to detect bulk HCl uptake by ice. Our experiments also detected no bulk uptake of HCl by α‐NAT or β‐NAT under various temperature and pressure conditions. Indirect evidence suggests that HCl adsorption onto the surface of model PSC films inhibited the evaporation of both ice and NAT by 3–5 K.
The onset of disintegration and corona in water drops falling at terminal velocity in horizontal electric fields
Authors: Kamra A. K. , Bhalwankar Rohini V. , Sathe A. B.
Published: February 2008

The onset of disintegration and corona in water drops falling at their terminal velocity in a vertical wind tunnel and exposed to horizontal electric fields has been investigated. Contrary to previous observations, the drops elongate in horizontal direction and distort into the shape of a concavo‐convex lens with a convex bottom and a sharp‐edged rim facing upward. Drops of diameter < 6.6 mm which do not break up in absence of electric field in this wind tunnel, break up in presence of the horizontal electric field. The values of horizontal electric field required for instability of the drops are much lower than those either predicted by Taylor's criterion of instability or observed in previous experimental studies. The criterion for instability of water drops freely suspended in presence of horizontal electric fields can be expressed as FH (ro/σ)½ = 0.98 ± 0.03 where FH is the horizontal electric field in esu, ro is the drop radius in centimeters and σ is the surface tension in dynes per centimeter. Most of the drops produce corona just before their breakup. Among various drops that are freely suspended in the wind tunnel, one by one, the number of drops that produce corona and/or breakup increases with increase in the electric field and/or drop size. While all drops of diameter ≥ 7.1 mm produce corona in a horizontal electric field of 500 kV/m, only a small fraction of very large drops of 8.0 mm diameter produce corona when the electric field is equal to 200 kV/m. Comparatively, very low values of instability field observed in our experiment are qualitatively explained because of long exposure of the freely suspended drops to the horizontal electric fields. The drops become unstable and produce corona when the drop's oscillation amplitude overshoots its equilibrium value and the plane of the drop oscillation coincides with the direction of electric field. From the results, it seems likely that horizontal electric fields in the bases of thunderclouds may cause disintegration of large raindrops and the occurrence of corona from their surfaces may trigger a lightning discharge.
Stable isotopes as tracers of methane dynamics in Everglades marshes with and without active populations of methane oxidizing bacteria
Authors: Happell James D. , Chanton Jeffrey P. , Whiting Gary J. , Showers William J.
Published: February 2008

Methane flux fromCladium jamaicensevaried from 0.2 to 15 mmol m−2d−1and was 1.4 to 26 (avg = 5.64 ± 8.57, n = 13, error is ± 1 standard deviation throughout) times greater than the flux from the flood water. The lack of diurnal variations in both the rate of CH4emission and its stable carbon isotopic composition suggests that CH4flux fromCladiumwas independent of stomatal aperture and that gases were transported through the plant mainly via passive diffusion and/or effusion as opposed to active pressurized ventilation. Rhizospheric CH4oxidation did not cause13C‐enriched CH4to be emitted to the atmosphere byCladium jamaicense. Previous workers have shown that Everglades soil types differ in that CH4oxidizing bacteria are active in peat soils and inactive in marl soils (King et al., 1990; Gerard, 1992), however a comparison of the stable isotopic composition of emitted and sedimentary CH4fromCladiummarshes within marl and peat soils provided no evidence that rhizospheric CH4oxidizing bacteria were consuming significant quantities of CH4in situ within peat soils. Either CH4oxidation in the rhizosphere was insignificant due to O2limitation or it occurred quantitatively in discrete zones within the sediment, thereby imparting no isotopic signal to sedimentary CH4. Linear relationships between CH4flux and live abovegroundCladiumbiomass in marl and peat soils were identical and offered no evidence for rhizospheric CH4oxidation in peat soils. In contrast core incubation experiments indicated that CH4oxidizing bacteria at the sediment‐water interface in peat soils intercepted and oxidized from 41 to 93 % (avg = 71 ± 20 %, n = 9) of the CH4diffusing from the sediments toward the overlying flood water. Furthermore, we were able to detect sediment‐water interface oxidation with stable isotopes as CH4emitted from the flood water (δ13C = 57.3 ± 3.6 ‰, n = 5) after plants were clipped below the water surface was enriched in13C by over 10 ‰ relative to CH4emitted from vegetated plots (δ13C = −68.1 ± 2.5 ‰, n = 10). Methane within flood water (before clipping) at peat sites was also13C enriched (δ13C = −57.6 ± 4.3 ‰, n = 7). Lowering of the water table below the sediment surface caused an Everglades sawgrass marsh to shift from CH4emission to the consumption of atmospheric CH4at a rate of 55 ± 41 μmol m−2d−1.
A model for heterogeneous chemical processes on the surfaces of ice and nitric acid trihydrate particles
Authors: Tabazadeh Azadeh , Turco Richard P.
Published: February 2008

A model is developed that incorporates the physics and physical chemistry of ice surfaces relevant to polar stratospheric clouds. The Langmuir and Brunauer, Emmett, and Teller (BET) adsorption isotherms are used to compute surface concentrations of H2O, HCl, HOCl, ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) crystals. Assuming pseudo‐first‐order kinetics with respect to adsorbed HOCl, ClONO2 and N2O5, surface reaction rates and reaction probabilities (sticking coefficients) are determined. The model parameters (surface morphology and energies) are extracted from measured uptake coefficients and reaction probabilities. For gas pressures of about 10−7 torr and temperatures in the range of 180–200 K, HCl completely coats ice and water‐rich NAT surfaces, while HOCl, ClONO2 and N2O5 may cover 0.01–1% of these surfaces. The model is applied to analyze laboratory data, leading to estimates of adsorption free energies, enthalpies and entropies for HCl, HOCl, ClONO2 and N2O5 on ice and NAT surfaces, and activation energies for the heterogeneous reactions of HCl and H2O with HOCl, ClONO2 and N2O5 on these surfaces. The energy parameters are used to calculate surface parameters such as adsorption and desorption constants, surface coverages, reaction rate coefficients, surface diffusion coefficients and reaction probabilities for varies species and chemical interactions on ice and NAT surfaces. Implications for chemical processing on polar stratospheric clouds are discussed.