bluuebunny/crossref_metadata_2025 huggingface dataset:
Ring shedding in the Agulhas Current System
Authors: Feron Raymond C. V. , De Ruijter Wilhelmus P. M. , Oskam Dick
Published: February 2008

After the Agulhas Current separates from the African coastline, it forms the Agulhas Retroflection and flows back into the Indian Ocean. If the current loop closes onto itself, a warm Agulhas ring is pinched off in the area south of Africa (between 15° and 20°E longitude). These rings can be monitored relatively easily with satellite altimetry when they migrate into the South Atlantic. However, the shedding process itself is difficult to extract from altimeter measurements. Geosat Exact Repeat Mission altimeter data of the period from November 1986 to September 1989 are used to analyze the ring‐shedding process in the Agulhas Current System. The data set has been processed using the collinear method over the area which extends from 30°W to 90°E, and from 15°S to 50°S. The original altimeter data are statistically interpolated to a regular 1° × 1° (spatial) grid using objective analysis. The obtained sea level anomaly maps are computed at regular time steps. Combination of harmonic analysis and principal component analysis enables extraction of typical Agulhas Current frequencies from the 3‐year altimetric data set. The analysis indicates that 11 or 12 dominant events took place over the 3‐year period. Variations occur from year to year. February, March, and April (summer‐autumn) are identified as anomalous months in which the sea level anomaly signal is not as well defined as in the other part of the year. The combined analyses showed that the first three modes of variability (explaining 25%, 20%, and 12% of the variance, respectively) are dominant. The structure of the spatial and temporal scales leads to the hypothesis that these modes can be associated with periodic Agulhas front movements, culminating in the formation of large Agulhas rings. Sharp changes (pulses) in the stability of the sea level pattern then determine the time of pinch off. This indicates a number of 18 (±2) pulses over the 3‐year period in which an anomaly in the average decorrelation time occured. If these pulses can be connected to the formation of Agulhas rings, they are of great importance for the large‐scale circulation because they contribute significantly to the energy and freshwater flux between the Indian Ocean and the South Atlantic.
A southwest monsoon hydrographic climatology for the northwestern Arabian Sea
Authors: Brock John C. , McClain Charles R. , Hay W. W.
Published: February 2008

This paper provides a detailed hydrographic climatology for the shallow northwestern Arabian Sea prior to and during the southwest monsoon, presented as multiple‐year composite vertical hydrographic sections based on National Oceanographic Data Center historical ocean station data. Temperature and salinity measurements are used to infer the water masses present in the upper 500 m. The hydrographic evolution depicted on bimonthly sections is inferred to result from wind‐driven physical processes. In the northwestern Arabian Sea the water mass in the upper 50 m is the Arabian Sea Surface Water. Waters from 50 to 500 m are formed by mixing of Arabian Sea Surface Water with Antarctic and Indonesian intermediate waters. The inflow of Persian Gulf Water does not significantly influence the hydrography of the northwestern Arabian Sea along the Omani coast. Nitrate has a high inverse correlation with temperature and oxygen in the premonsoon thermocline in the depth interval 50–150 m. During the southwest monsoon, coastal upwelling off Oman and adjacent offshore upward Ekman pumping alter the shallow hydrography.
On the response of the equilibrium thickness distribution of sea ice to ice export, mechanical deformation, and thermal forcing with application to the Arctic Ocean
Authors: Björk Göran
Published: February 2008

The areal ice distribution in a pack ice field is described using a number of classes, characterized by different ice thicknesses. Each class may have a snow layer on top and evolves according to the thermodynamic forcing (same for all classes) and is followed through time. Mechanical forcing leads to ice export and deformation. The ice export is quantified by the parameter e describing the areal export of ice per unit of time. The deformation is quantified by the parameter r describing the area of open water generated by deformation per unit of time. The fraction of summer heat absorbed in leads and utilized for ice melting is quantified by the parameter gop. The model is applied to the Arctic Ocean. It is run to a steady state for different combinations of e, r, and gop, and the response of the thickness distribution to these parameters may thereby be quantified. It is shown that the ice generally gets thinner for increasing export. The response to deformation is dependent on gop such that the mean thickness increases with r for small gop (i.e., no effect or just a small melting effect of the heat absorbed in leads) but decreases with r for large gop. The overall effect of gop is that the mean thickness decreases with increasing gop. For gop close to 1 and moderate values of e and r the ice sheet melts completely in the summer.
Ice growth and oceanic heat flux: Models and measurements
Authors: Omstedt A. , Wettlaufer J. S.
Published: February 2008

Heat fluxes at the ice‐ocean interface and ice thickness are investigated by comparing field data from the Coordinated Eastern Arctic Experiment (CEAREX) drift phase with model calculations. The calculations are based on two types of models. The first one is a one‐dimensional ice‐ocean model with high vertical resolution. This model is based on the conservation equations for heat, salt, and momentum and uses turbulence models to achieve closure. A discrete element approach is also introduced to explicitly parameterize the ice roughness. The second model is a simple one‐dimensional bulk heat transfer model. In this version, the interfacial salinity is modelled on the basis of salt conservation at the ice‐ocean interface. The bulk heat transfer model is then calibrated using the former model. The two models predict ocean heat fluxes that are quite variable in time owing to short‐term variations in the ice drift. Both models calculate realistic ice thicknesses. It is demonstrated that the observed time variation in ice thickness from eight different experimental sites with varying initial thicknesses and bottom topographies can be reproduced by applying bulk heat transfer coefficients in the range (2.8 ± 1) × 10−4. Horizontal variation of the thermal state within a single pack ice floe results in simultaneous freezing and melting over relatively small spatial scales. When modeling or averaging ice data in space these aspects need to be considered.
Dimethyl sulfide concentrations in the surface waters of the Australasian Antarctic and Subantarctic oceans during an austral summer
Authors: McTaggart Andrew Robert , Burton Harry
Published: February 2008

Dimethyl sulfide (DMS) was measured in surface seawater samples collected between Tasmania and Antarctica in the Australasian sector of the southern ocean during the 1988–1989 austral summer. The mean DMS concentration in subantarctic waters for January was 20.5 nmol L−1 decreasing to 4.0 nmol L−1 in the Antarctic Convergence. During December and January the Australasian Antarctic Divergence appears to be an area of high primary productivity as the concentration of DMS in these surface waters remained relatively constant at 18.5 nmol L−1. Samples collected on the shelf and in coastal waters of Prydz Bay, East Antarctica, show a distinct rise in DMS concentration on all voyages with a maximum concentration of 122 nmol L−1 recorded in January near Davis.
Vertical diffusivity and overturning length in stably stratified turbulence
Authors: Weinstock Jerome
Published: February 2008

Theoretical expressions are derived in a simple, but approximate, way for variations of vertical diffusivity and overturning length with Froude number, gradient Richardson number, and Reynolds number, in both sheared and unsheared stratified flow. These variations are then compared with observations in laboratories and oceans. For the case of strongly stratified turbulence (Froude number near unity) a previous value of 0.8 for the mixing efficiency is attributed to an incorrect assumption for the numerical value of Lm/LB, the ratio of overturning to buoyancy length, and the efficiency is recalculated by use of recent measurements of that length ratio.
Rotational modes for warm and cold eddies
Authors: Kawano T. , Nakamoto S.
Published: February 2008

Analytical solutions are found to describe a water parcel motion in both warm and cold eddies. An ƒ plane, reduced gravity model, with upper layer thickness and velocity fields respectively Gaussian and linear in space, was reduced to the eight‐dimensional system of nonlinear coupled ordinary differential equations which describe time evolution of differential kinematic parameters and eddy shape parameters. Analytical solutions for steady rotating modes for warm eddies and for cold eddies are discussed. For warm eddies the rigidly rotating ellipse mode solution corresponds to the “rodon” solution for lenslike warm eddies. For cold eddies the corresponding rigidly rotating mode solution exhibits triangular path with abrupt direction change. Elliptical paths for warm eddies and triangular paths for cold eddies were observed in Western Pacific Ocean Circulation Study III drifter tracking data, which may be interpreted to be a possible manifestation of the rotating mode solutions predicted theoretically.
Analysis of a general circulation model: 2. Distribution of kinetic energy in the South Atlantic and Kuroshio/Oyashio systems
Authors: Garraffo Zulema , Garzoli Silvia L. , Haxby William , Olson Donald
Published: February 2008

The energy of the model transient eddies at 37.5 m is compared with Geosat altimeter observations, for the South Atlantic Ocean and for the Kuroshio system. The model shows areas of transient motions overlapping the ones obtained from Geosat altimeter data. For the South Atlantic Ocean, the modeled eddy kinetic energy is smaller than the one observed with Geosat, by a factor of 3 for area average on the whole South Atlantic region, and by a factor of 4 for its western boundary. On the Agulhas system, transient eddy activity develops in the region where the Agulhas current retroflects. In the western South Atlantic, the modeled eddy activity is concentrated on the Confluence front; observed variability along a more extended region following the topography is not resolved in the model. For the Kuroshio region, the energy level of the modeled transient motions is comparable with Geosat observations, but the model eddy activity is more concentrated in the Kuroshio Current and not in the Kuroshio extension. The observations show the opposite. For the South Atlantic Ocean, a comparison is also done between model eddy kinetic energy (defined as including standing and transient eddy contributions) with values obtained from surface drifters. The analysis shows differences in the western boundary, and good agreement across the South Atlantic Ocean between 35°S and 45°S. In this formulation, the model mean energy level is smaller than the observed with drifters from the First GARP Global Experiment; differences might be due to an overestimation in the values obtained with the drifters.
Large‐scale electric and magnetic fields generated by the oceans
Authors: Stephenson David , Bryan Kirk
Published: February 2008

The magnetostatic equations are used to derive a consistent set of equations capable of describing the global‐scale, low‐frequency electric and magnetic fields induced by the motion of the ocean through the geomagnetic field. The equations are solved numerically with realistic 2° × 2° topography in a global domain with ocean flow simulated by a detailed ocean circulation model. Estimates of the annual mean and the first annual harmonic of the electric potential, the vertical component of the oceanic magnetic field, and the vertically integrated electric current density stream function are obtained. With the idea of using electric and magnetic fields to deduce large‐scale oceanic flow, emphasis is placed on the geographical location of interesting features. The fields are not found to be basin‐wide but rather are found to be localized and strongest in shallow regions. The magnetic fields generated by ocean currents are of the order of 1 nT, and while these can be measured by magnetometers, they would be difficult to detect owing to contamination from other sources of magnetic variation. In finding the electric field, electric currents cannot be neglected where ocean currents cut across isobaths. However, in regions where the ocean flow is aligned with the isobaths, measurement of electric fields is sufficient to find the ocean flow.
Variability of bio‐optical properties of the upper ocean associated with diel cycles in phytoplankton population
Authors: Stramska Malgorzata , Dickey Tommy D.
Published: February 2008

The variability of bio‐optical properties of seawater associated with planktonic responses to the daily light cycle was examined. We performed a spectral analysis of a 60‐day time series of open ocean mooring measurements of the scalar irradiance (photosynthetically available radiation, PAR), beam attenuation coefficient at 660 nm (c660), stimulated fluorescence, and dissolved oxygen concentration. The measurements were done from April through May 1989 in the North Atlantic south of Iceland, as a part of the Marine Light in the Mixed Layer program. We have shown that the statistical significance of the daily cycles of bio‐optical properties of the open ocean varies in time throughout the spring season. The diurnal periodicity of c660 and O2 was especially well pronounced during the development of the phytoplankton bloom in May. The fluorescence signal was dramatically affected by the ambient light intensity. The measurements at depths of 10 and 30 m showed fluorescence rhythms completely out of phase with each other. The comparison between the 10‐ and 30‐m beam attenuation signals suggests that the 30‐m signal was more sensitive to within day PAR variability. Further investigations are needed to determine how widespread the daily variations of bio‐optical properties in the ocean are, what conditions favor this cycling, and how this variability may impact procedures for estimating the phytoplankton biomass and production.
Gas transfer: A gas tension method for studying equilibration across a gas‐water interface
Authors: Anderson Meredith L. , Johnson Bruce D.
Published: February 2008

New strategies are required in the study of gas exchange under energetic and changeable conditions where current techniques are limited, especially in their ability to represent the contribution of injected bubbles. We have developed a gas tension device (GTD) that uses a solid‐state differential pressure sensor to make continuous in situ measurements of the difference between pressure of dissolved gas (gas tension, for succinctness) and pressure of gas in the gas phase above a water parcel. This information allows the determination of the rate of approach to a dynamic equilibrium and of the degree of water phase saturation at steady state. The method is demonstrated in the lab by following the invasion of CO2 into seawater with bubble injection. The GTD introduces a new and versatile measurement method for the study of gas transfer across air‐water interfaces, one which avoids many of the sampling and bias problems of traditional approaches.
Long‐period transport changes in the eastern North Atlantic and their simulation by propagating waves
Authors: Siedler Gerold , Finke Michael
Published: February 2008

An analysis is presented of geostrophic volume transport across a zonal line along 28°N in the eastern Atlantic. The data are from an array of five moorings with 200‐km spacing carrying temperature sensors and one current meter each for 1 or 2 years. Transport changes in the main thermocline relative to a fixed depth level are obtained by the use of temperature‐salinity relationships. The transport variability is simulated by two propagating waves with first‐order baroclinic mode structure. Solutions exist with annual and semi‐annual periods and zonal wavelengths of 100–200 km and 300 km, respectively. Assuming quasi‐geostrophic dynamics and using results on the Reynolds stress, the dominating waves of annual and semi‐annual period are found to propagate to the southwest, with 45°–60° and 25° to the south off the westward direction, respectively. Wave solutions with a 90‐day period and a zonal wavelength of about 300 km are interpreted as an effect of barotropic waves arising due to horizontal temperature inhomogeneity. The propagation is about ±25° off the westward direction. In general, good approximations are obtained with the propagating wave simulations in the western and central part of the array, while large differences occur between observation and simulation close to the Canary archipelago. Possible causes for these differences are discussed.
A note on the possible connection between the El Chichón eruption and ocean production in the northwest Arabian Sea during 1982
Authors: Strong Alan E.
Published: February 2008

The coastal zone color scanner (CZCS) has provided what may be the most convincing evidence to date of a large‐scale ocean response to a volcanic eruption. Brock and McClain's (1992) 4‐year time series of pigments over the Arabian Sea appears to reveal a slowdown in production over much of this basin. Apparently, the entire southwest monsoon relaxed beneath the tremendous pall of the El Chichón aerosol cloud that had overshadowed the region 4 months earlier, producing dramatically reduced concentrations of phytoplankton during summer and fall 1982.
Coastal zone color scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features
Authors: Comiso J. C. , McClain C. R. , Sullivan C. W. , Ryan J. P. , Leonard C. L.
Published: February 2008

The spatial and seasonal distributions of phytoplankton pigment concentration over the entire southern ocean have been studied for the first time using the coastal zone color scanner historical data set (from October 1978 through June 1986). Enhanced pigment concentrations are observed between 35°S and 55°S throughout the year, with such enhanced regions being more confined to the south in the austral summer and extending further north in the winter. North and south of the polar front, phytoplankton blooms (>1 mg/m3) are not uniformly distributed around the circumpolar region. Instead, blooms appear to be located in regions of ice retreat (or high melt areas) such as the Scotia Sea and the Ross Sea, in relatively shallow areas (e.g., the Patagonian and the New Zealand shelves), in some regions of Ekman upwelling like the Tasman Sea, and near areas of high eddy kinetic energy such as the Agulhas retroflection. Among all features examined by regression analysis, bathymetry appears to be the one most consistently correlated with pigments (correlation coefficient being about −0.3 for the entire region). The cause of negative correlation with bathymetry is unknown but is consistent with the observed abundance of iron in shallow areas in the Antarctic region. It is also consistent with resuspension of phytoplankton cells by wind‐induced mixing, especially in shallow waters. On the other hand, in the deep ocean (especially at latitudes <45°S where surface nutrients may be limiting), upwelling induced by topographic features may cause resupply of nutrients to the surface and shoaling of the subsurface chlorophyll maximum. Low pigment values are common at low latitudes and in regions of high wind stress, where deep mixing and net loss of surface pigment occur. Nutrients (phosphate, nitrate, and silicate) are found to correlate significantly with pigments when the entire southern ocean is considered, but south of 55°S the correlation is poor, probably because the Antarctic waters are not nutrient limited. Nutrients are also highly correlated with Ekman upwelling. Although cloud cover and normalized aerosol radiance are correlated to the patterns of pigment concentrations in some areas, the correlations are weak in other areas, suggesting that light and iron may not be the primary factors responsible for the spatial variability of pigment concentrations, especially during summer. Large interannual variability (>30%) in average pigment concentration over the entire region during different seasons indicates possible influence of time dependent parameters.
Time and space scales of significant wave heights
Authors: Tournadre Jean
Published: February 2008

This paper presents a study of the temporal and spatial variability of the significant wave height (Hs) based on stationarity and correlation (spectral) analyses. A sea state is defined as a stationary state of a piecewise stationary stochastic random wave process. Rupture detection, i.e., detection of abrupt changes in Hs, is used to identify the stationary components of the wave process. A sea state is characterized by its energy (i.e., Hs) and by its duration of stationarity (spatial analysis) and length of stationarity (temporal analysis). Intensive in situ measurements of Hs and Geosat radar altimeter data are used to study the temporal and spatial Hs scales at two locations, in the North Sea and in the equatorial Atlantic.The stationarity analysis leads to the following results; (1) duration and length show a great variability and are distributed according to exponential probability laws, (2) Hs is distributed according to a Gumbel probability law in both time and space, (3) energy and duration and energy and length can be considered as statistically independent, and (4) the duration and length distributions present very similar nondimensional statistical characteristics. Stationary state detection can also be used to filter the high‐frequency geophysical and/or instrumental noise from the Hs variations. A spectral analysis is performed on the raw Hs, the stationary states, and the residual. The salient features of the results are summarized as follows: (1) for both locations, the spectra of the filtered data are consistent with a power law dependence on the wavenumber or frequency, (2) the spectral dependence is nearly the same for time and space, which suggests a mean linear dispersion relation for Hs, (3) the slope of the spectra are close to the −5/3 turbulence cascade (−1.74 for the North Sea, −1.69 for the equatorial Atlantic), and (4) the residual spectrum is nearly a white noise spectrum indicating the quality of the stationary state detection filtering.
Effects of low and high Orinoco River flow on the underwater light field of the eastern Caribbean Basin
Authors: Farmer Charles T. , Moore Cynthia A. , Zika Rod G. , Sikorski Richard J.
Published: February 2008

In the spring and fall of 1988, two cruises were conducted in the Caribbean to examine the effects of the Orinoco River on the photochemistry of the eastern Caribbean. Detailed irradiance measurements were made at 29 stations using two scanning spectrophotometers; one measuring surface irradiance, the second measuring the underwater light field. The underwater light field data are presented and examined in this paper. The increased Orinoco River input in the rainy season produced significant changes in the underwater irradiance. This influence included a shift to longer wavelength in the light that penetrated deepest in the water column and an overall reduction in the total irradiance at any depth by approximately a factor of 2. Data will also be presented indicating the importance of accurate depth measurements collected in concert with the spectral irradiance data.
The heat budget in the North Atlantic subtropical frontal zone
Authors: Rudnick Daniel L. , Weller Robert A.
Published: February 2008

The heat budget in the North Atlantic subtropical frontal zone is examined using moored measurements of horizontal velocity, temperature, and surface heat flux obtained during the Frontal Air‐Sea Interaction Experiment (FASINEX). The three moorings used in this calculation defined a triangle with base of 19 km and height of 28 km, and were densely instrumented from the surface to 160‐m depth. Empirical orthogonal functions (EOFs) are employed to identify components of current and temperature variability with scales that are resolved by the observations. A least squares fit to the EOF‐filtered data is used to derive maps of velocity and temperature. The temperature field is dominated by the passage of fronts, with the highest variability at 160 m. Horizontal temperature gradients at 160 m peak at values approaching 1 × 10−4 °C m−1 and are indicative of a tilting of the seasonal thermocline. Velocities are directed primarily along isotherms and are swifter at the surface than at depth. The heat budget of the upper 160 m, for periods longer than 48 hours, is essentially a balance between‐rate of change of heat and horizontal advection; the correlation between these terms is 0.7. A simple explanation is the northward advection by a mean current of a series of fronts. The effect of surface heating becomes apparent in the upper 40 m, notably at the diurnal frequency and its first harmonic. An imbalance in the mean (102 days) heat budget can be rectified by a mean downwelling velocity of 5 × 10−5 m s−1 at 160 m.
Mesoscale pigment fields in the Gulf Stream: Observations in a meander crest and trough
Authors: Hitchcock Gary L. , Mariano Arthur J. , Rossby T.
Published: February 2008

In September‐October 1988 and April 1989 a series of hydrographic transects were completed across the Gulf Stream front in a meander crest and meander trough, respectively. One of the main experimental objectives was to relate the spatial distribution of chlorophyll to the physical fields of Gulf Stream meanders. Chlorophyll distributions are derived from conductivity‐temperature‐depth (CTD)/fluorescence profiles calibrated with discrete pigment samples collected from bottles at several depths at various stations. Objective analysis (OA) maps and vertical sections, in stream coordinates, of chlorophyll on density surfaces show the chlorophyll distribution was strongly related to the structure of the Gulf Stream front. In particular, chlorophyll concentrations greater than 0.4 mg m−3 were at, or inshore of, the Gulf Stream north wall. Characteristic length scales of chlorophyll distribution determined from the horizontal and temporal correlation function are the same order (50–100 km) as length scales of the physical variables. The maximum chlorophyll concentrations in the vertical were from the surface to 50‐m depth on the western flank of the meander crest and deepened to 75‐ to 100‐m depth on the eastern flank. This coincides with the deepening of the 24.4–25.7 σθ surfaces from the western to the eastern flank of the meander crest. Although in the spring cruise the maximum chlorophyll concentrations were also found at depths between the surface and 100 m, there were no clear distinctions in pigment distributions between the western and eastern transects of a relatively weak trough. Maximum chlorophyll concentrations, 1.0–1.5 mg m−3, from the 1989 spring bloom in slope waters were about double those observed in the 1988 fall data. The primary physical mechanisms influencing the mesoscale pigment distribution in Gulf Stream meanders observed in this study are (1) outcropping of nutrient‐bearing strata in the spring, (2) meander‐induced upwelling of nutrients along sloping isopycnals, and (3) Gulf Stream‐ring interactions.
Property fluxes at 30°S and their implications for the Pacific‐Indian throughflow and the global heat budget
Authors: Macdonald Alison M.
Published: February 2008

Six hydrographic basinwide sections, two in each of the three major ocean basins, are employed in a set of inverse calculations to determine the extent of exchange between the Pacific and Indian Oceans through the Indonesian Archipelago and the net global oceanic heat flux. All existing estimates of Indonesian Passage throughflow, including the largest (20 Sv), are consistent with the model constraints which combine data from the southern Pacific, Indian and Atlantic Oceans. The models and data are unable to limit the extent of the exchange, i.e., both smaller and larger throughflows produce physically reasonable circulation patterns. Seasonal and interannual variations, which have been found by other investigators and which are not resolved, suggest that in the long‐term mean an estimate of about 10 Sv for the throughflow is most reasonable. Globally, at 30°S, the estimated net oceanic heat flux is −0.7 ± 0.1 PW (1 PW = 1015 W), dominated by a large (> 1 PW), southward flux in the Indian Ocean. Large equatorward heat flux values, O(0.8 PW), in the South Atlantic Basin are not consistent with the data. Therefore although the data are consistent with some water following the “warm water” return path for North Atlantic Deep Water (NADW), the “cold water” path must play the dominant role in the maintenance of the global thermohaline cell associated with the formation process of NADW.
Estimates of atmospheric hydroxyl radical concentrations from the observed decay of many reactive hydrocarbons in well‐defined urban plumes
Authors: Blake N. J. , Penkett S. A. , Clemitshaw K. C. , Anwyl P. , Lightman P. , Marsh A. R. W. , Butcher G.
Published: February 2008

An analytical system has been developed which allows the confident identification and measurement of 35 hydrocarbons of different reactivities in air samples collected in many locations. This paper describes the application of the technique to follow the differential decay of hydrocarbons in urban plumes spreading from London during the summer. The data have been used to determine atmospheric hydroxyl radical concentrations, averaged over several hours, on the assumption that the decay of the hydrocarbons is entirely due to reaction with these radicals. Hydroxyl radical concentrations were derived from the measured decay of several alkenes which agreed with theoretical estimates. There are strong indications, however, that substituted aromatic molecules decay much faster than could be accounted for solely by reaction with hydroxyl radicals; this may indicate the presence of a further chemical removal mechanism.
Two‐dimensional velocity, optical risetime, and peak current estimates for natural positive lightning return strokes
Authors: Mach Douglas M. , Rust W. David
Published: February 2008

We report velocities, optical risetimes, and transmission line model peak currents for seven natural positive return strokes. The data were taken with our return stroke velocity device mounted on the National Severe Storms Laboratory mobile laboratory. The average two‐dimensional positive return stroke velocity for channel segments of <500 m in length starting near the base of the channel is 0.8 ± 0.3 × 108 m s−1, which is slower than our corresponding average velocity for natural negative first return strokes of 1.7 ± 0.7 × 108 m s−1. This implies that positive stroke peak currents in the literature, which assume the same velocity as negative strokes, are low by a factor of 2. The average two‐dimensional positive return stroke velocity for channel segments of >500 m starting near the base of the channel is 0.9 ± 0.4 × 108 m s−1. The corresponding average velocity for our natural negative first strokes is 1.2 ± 0.6 × 108 m s−1. We find no significant velocity change with height for positive return strokes. The average 10–90% optical risetime from channel segments with an average length of 3.2 ± 1.7 m for positive return strokes within 100 m of the base of the channel is 9.4 ± 3.0 μs; the corresponding measurement for negative strokes is 3.5 ± 1.7 μs. We detected no optical leaders from the positive flashes. This limits the relative brightness of positive stroke leaders to approximately 0.08 times the optical output of the return stroke. The logarithmic average peak current from the transmission line model for positive return strokes is 59.8 kA, approximately 5 times the logarithmic average peak current we calculated for natural negative first return strokes. All differences between positive and negative natural strokes noted above are statistically different at the 99% level.
Spectral actinic flux and its ratio to spectral irradiance by radiation transfer calculations
Authors: Ruggaber Ansgar , Forkel Renate , Dlugi Ralph
Published: February 2008

Spectral irradiance and spectral actinic flux in the troposphere are calculated by a matrix operator model (Nakajima and Tanaka, 1986), with respect to solar zenith angle, height, and wavelength. Different surface albedos, aerosol, and air density distributions are considered. The actinic flux and the irradiance are split into a diffuse and a direct part. The ratio of diffuse irradiance to diffuse actinic flux rd is calculated and discussed. The calculations have shown that the approximation diffuse ratio rd = 0.5 results in an error up to over 20% for calculated photolysis frequencies. However, no simple expression for the diffuse ratio can be given. The diffuse ratio is not constant with height and depends on the solar zenith angle and wavelength.
A comparison of Arctic and Antarctic mesoscale vortices
Authors: Turner John , Lachlan‐Cope Thomas A. , Thomas Jeremy P.
Published: February 2008

The mesoscale (less than 100km) vortices occurring in the two polar regions are considered in terms of their geographical and seasonal distribution, satellite cloud signatures and forcing mechanisms. Environmental conditions important in the development of the vortices are considered, including sensible heat flux, stability throughout the troposphere and synoptic factors, such as baroclinicity and upper air cold pools. A scheme to classify the observed vortices in the two polar regions in terms of the physical mechanisms behind their formation and development is proposed. The processes considered important are baroclinic instability, convection and vorticity generation through cyclonic vorticity advection and topographic forcing. The major difference between the systems observed in the two polar regions is the lack of deep convection in the southern hemisphere, which precludes the development of many of the vigorous types of system found in the north and the major role that topography plays in the Antarctic coastal region. The most common type of vortex found in the Antarctic occurs over the ice‐free ocean to the west of synoptic scale disturbances and is similar to the type of northern system know as a comma cloud.
Associations between pollutant emissions and precipitation chemistry: An empirical analysis
Authors: Chapman E. G. , Luecken D. J.
Published: February 2008

Empirical analyses were conducted of the associations between anthropogenic SO2 emissions and precipitation SO42− concentrations as well as between anthropogenic NOx emissions and precipitation NO3− concentrations across central North America. Both the 1982 Electric Power Research Institute (EPRI) National Air Pollutant Emissions Inventory and the 1985 National Acid Precipitation Assessment Program (NAPAP) Emissions Inventory, along with precipitation chemistry data from up to 130 monitoring sites, were used in the analyses. Comparisons of the EPRI and NAPAP emissions inventories in their geographic area of overlap indicate that SO2 emissions differ more than do the NOx emissions, on both absolute and fractional difference scales. Empirical analysis results indicate that the associations between anthropogenic SO2 emissions and precipitation SO42− are stronger than associations between anthropogenic NOx emissions and precipitation NO3− concentrations, with the associations equally well represented by either a power law or linear law function. Distinct regional and seasonal differences exist in the strength of the associations.
Water vapor and cloud water measurements over Darwin during the STEP 1987 tropical mission
Authors: Kelly K. K. , Proffitt M. H. , Chan K. R. , Loewenstein M. , Podolske J. R. , Strahan S. E. , Wilson J. C. , Kley D.
Published: February 2008

Measurements of stratospheric and upper tropospheric cloud water plus water vapor (total water) and water vapor were made with two Lyman α hygrometers as part of the STEP tropical experiment. The in situ measurements were made in the Darwin, Australia, area in January and February of 1987 on an ER‐2 aircraft. Average stratospheric water vapor at a potential temperature of 375 K (the average value of θ at the tropopause) was 2.4 parts per million by volume (ppmv). This water mixing ratio is below the 3.0 to 4.0 ppmv necessary to be consistent with the observed upper stratospheric dryness. Saturation with respect to ice and the potential for dehydration was observed up to θ = 402 K.
An intercomparison of artificial intelligence approaches for polar scene identification
Authors: Tovinkere V. R. , Penaloza M. , Logar A. , Lee J. , Weger R. C. , Berendes T. A. , Welch R. M.
Published: February 2008

Six advanced very high resolution radiometer local area coverage arctic scenes are classified into 10 classes. These include water, solid sea ice, broken sea ice, snow‐covered mountains, land, stratus over ice, stratus over water, cirrus over ice, cumulus over water, and multilayer cloudiness. Eight spectral and textural features are computed. The textural features are based upon the gray level difference vector method. Six different artificial intelligence classifiers are examined: (1) the feed forward back propagation neural network; (2) the probabilistic neural network; (3) the hybrid back propagation neural network; (4) the “don't care” perceptron network; (5) the “don't care” back propagation neural network; and (6) a fuzzy logic‐based expert system. Accuracies in excess of 95% are obtained for all but the hybrid neural network. The “don't care” back propagation neural network produces the highest accuracies and also has low CPU requirements. Thin fog/stratus over ice is the class consistently with the lowest accuracy, often misclassified as broken sea ice. Water, land, cirrus over ice, and snow‐covered mountains are all classified with high accuracy (≥98%). The high accuracy achieved in the present study can be traced to (1) accurate classifiers; (2) an excellent choice for the feature vector; and (3) accurate labeling. A sophisticated new interactive visual image classification system is used for the labeling.
Formate and acetate in monsoon rainwater of Agra, India
Authors: Kumar Nandini , Kulshrestha Umesh C. , Saxena Anita , Kumari K. M. , Srivastava S. S.
Published: February 2008

Formate and acetate concentrations were estimated using ion chromatography in 19 precipitation samples collected on an eventwise basis during the monsoon season (July through September), 1991, at Dayalbagh, Agra. Volume‐weighted average (VWA) concentrations for formate and acetate were 5.8 and 6.55 μmol L−1, respectively. The VWA hydrogen ion concentration was 0.084 μeq L−1 (pH 7.07) and the correlation coefficient between the two ions was 0.85. The average formate to acetate ratio was low (0.88), possibly due to an increase in acetate contribution from direct emissions associated with heavy vehicular traffic load and/or indirect acetate formation by alkaline hydrolysis of PAN. Widespread local use of biomass as a domestic fuel may also contribute acetate. In 4 of the 19 precipitation events studied, higher values of both species were recorded. Contributions from soil in addition to vegetation, were suspected in these samples. Inputs from soil and combustion activities were supported by correlations among formate, acetate and Ca2+ (terrigenous species), K+, SO42− and NO31−.
Influence of the mean wind field on the separability of atmospheric perturbation spectra
Authors: Gardner Chester S. , Hostetler Chris A. , Lintelman Scott
Published: February 2008

The effects of Doppler shifting caused by the mean wind field on the separability of gravity wave spectra are analyzed using the gravity wave models developed recently by Gardner et al. (1993). If the intrinsic joint vertical wave number (m) and temporal frequency (ω) spectrum is separable, Doppler effects associated with even small mean winds will destroy separability of the observed joint (m, ω0) spectrum, particularly at high vertical wave numbers. The effect can have a substantial impact on the measured m‐spectrum especially when the observations are not sensitive to the lowest or highest observed frequencies. Data averaging or low‐pass temporal filtering can result in substantial reduction of the spectrum magnitude at high m values while data prewhitening or high‐pass temporal filtering can result in significant distortion of the spectrum at low m values. These results have important implications for observational tests of the various gravity wave spectral models.
Height dependence of the Batchelor scale for positive‐ion‐traced neutral turbulence in the summer Arctic mesosphere
Authors: Hall Chris
Published: February 2008

Homogeneous isotropic three‐dimensional turbulence may be characterized by a kinetic energy‐wavenumber spectrum exhibiting distinct regimes. This spectrum is variously referred to as the Richardson cascade or the Kolmogorov spectrum; the former reflects the cascade of energy from large scale‐sizes to small where ultimately kinetic energy is dissipated into heat by viscous damping. In the upper atmosphere it is difficult to measure the form of the Richardson cascade directly for the neutral flow and experimentation often relies on measurement of density fluctuations of a passive tracer assumed to be representative of the neutral component. Here we shall address the form of the Richardson cascade for turbulence traced by positive ions in the height regime 70–90 km at high latitudes in summer. These ions may be much heavier than the neutral particles and indeed may well be the building blocks for multiply charged aerosol particles (macro ions) and we shall see how easily estimation of neutral turbulence parameters may become incorrect when using such charged particles as tracers assuming them to be passive and scalar. This problem manifests itself by the introduction of a viscous‐convective subrange in the Richardson cascade, parameterized by the Batchelor scale. We show how the Batchelor scale can become very small within a narrow layer situated a little below the temperature minimum at the mesopause. Within such a layer both in situ and remote observations may yield anomalous results.
Future methane emissions from animals
Authors: Anastasi C. , Simpson V. J.
Published: February 2008

The future global emission of CH4 from enteric fermentation in animals has been estimated for cattle, sheep, and buffalo, which together contribute approximately 91% of the total CH4 emitted from domesticated animals at present. A simple model has been used to relate livestock levels to the national human populations for each country involved in breeding the three species included in this analysis. United Nations population predictions to 2025 were then included in the model to estimate future CH4 emissions. A variational analysis was carried out to investigate the effect of future changes in both the land available for grazing and the nutritional content of feedstocks. Results suggest that the total emission of CH4 from enteric fermentation in domestic animals will increase from 84 Tg CH4 per year (Tg = 1012 g) in 1990 to 119 (±12) Tg CH4 yr−1 by 2025. These values correspond to an average rate of increase over the next 35 years of 1.0 Tg CH4 yr−1.
Mesoscale cyclogenesis dynamics over the southwestern Ross Sea, Antarctica
Authors: Carrasco Jorge F. , Bromwich David H.
Published: February 2008

Previous work has shown that frequent mesoscale cyclogenesis adjacent to Franklin Island is linked to the strong and persistent katabatic winds from East Antarctica which funnel into Terra Nova Bay and then blow out over the southwestern Ross Sea. Four mesoscale cyclones that formed near Terra Nova Bay between February 16 and 20, 1988 are examined to more clearly define the governing mechanisms. These events are investigated using all available observations, including automatic weather station data, high‐resolution satellite images, satellite soundings, and hemispheric synoptic analyses. The first two cyclones formed on low‐level baroclinic zones established by the synoptic scale advection of warm moist air toward the cold continental air blowing gently from East Antarctica. In the second case, baroclinic instability of this small‐scale cold front was apparently triggered by the enhanced upward vertical motion associated with the approach of a midtropospheric trough. The third mesocyclone formed shortly after on a baroclinic zone over the polar plateau; the second vortex completely disrupted the usual katabatic drainage over the plateau and forced warm moist air over the coastal slopes. All three cyclones moved to the north in the prevailing cyclonic flow, but the plateau vortex lasted for only 6 hours. The fourth mesoscale low formed in conjunction with an abrupt and intense surge of katabatic air from Terra Nova Bay which resharpened the coastal baroclinic zone. At the same time a transiting midtropospheric trough probably associated with lower tropospheric upward vertical motion apparently accelerated the katabatic winds and triggered the vortex formation. A similar katabatic wind‐forced mesocyclone formed near Byrd Glacier. The two vortices moved to the east‐southeast and northeast, respectively, apparently being steered by the generating katabatic airstreams, and merged just to the north of the Ross Ice Shelf. The combined vortex reintensified as another trough passed overhead and moved eastward to West Antarctica where it dissipated two days later.
Sensitivity of Asian and African climate to variations in seasonal insolation, glacial ice cover, sea surface temperature, and Asian orography
Authors: deMenocal Peter B. , Rind David
Published: February 2008

The sensitivity of Asian and African climate to prescribed changes in specific high‐latitude boundary conditions is examined using the GISS general circulation model (GCM) (8° × 10°). In agreement with previous GCM simulations, summer Asian monsoon winds and rainfall were considerably enhanced by the ∼8% increase in 11 kyr B.P. insolation; the African monsoon was less responsive to insolation forcing in this model. Increased high‐latitude glacial ice cover strengthened winter Asian trade winds and imposed cooler and drier conditions over Arabia and northeastern Africa; summer monsoon winds and associated precipitation were diminished over parts of south Asia. Increased high‐latitude ice cover alone had little effect on African climate. Cooler North Atlantic sea surface temperatures (SSTs) enhanced winter trade wind circulation over North Africa, promoted cooler and drier conditions, and reduced the summer monsoon circulation; southern Asian climate was relatively unaffected. Reducing Himalayan and Tibetan elevation to 500 m produced dramatic increases in winter Asian trade wind circulation; the summer monsoon circulation persisted but was weakened considerably. African climate was relatively insensitive to the reduced Asian orography. Many of the model results agree qualitatively with available paleoclimate data. These model results suggest that African and southern Asian climate respond to separate elements of high‐latitude climate variability. Specifically, Arabian and northeastern African climate is most sensitive to changes in Fennoscandian ice elevation and albedo, whereas northwestern African climate is most sensitive to changes in the North Atlantic SST field.