bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal‐to‐interannual effects of the Indonesian throughflow on the tropical Indo‐Pacific Basin
Authors: Murtugudde Ragu , Busalacchi Antonio J. , Beauchamp James
Published: February 2004

The effects of the Indonesian throughflow (ITF) are studied in a reduced gravity, primitive equation, sigma coordinate model. The model domain includes the flow south of Australia. Unlike previous studies, this model includes a completely interactive upper ocean hydrology and the surface heat fluxes are provided by coupling the ocean general circulation model (GCM) to an advective atmospheric mixed layer model. It is shown that model simulation of the ITF on seasonal and interannual (1980–1995) timescales is in very good agreement with available estimates and other model studies. Effects of increased mixing in the Indonesian seas are also presented. There is an El Niño‐Southern Oscillation (ENSO) related signal in the ITF, but the correlation with the Southern Oscillation Index (SOI) is only −0.31. When the winds over the Indian Ocean are held to climatology, this correlation jumps to −0.65 indicating that the non‐ENSO signal in the ITF is caused by the downstream winds. On interannual timescales the ITF can be explained in terms of sea level differences between the western Pacific and eastern Indian Oceans when appropriate representative locations are chosen as demonstrated in both model and TOPEX data. It is shown that the main climatological effect of the ITF is to warm the Indian Ocean and to cool the Pacific. Spreading of the thermocline due to ITF in the Indian Ocean leads to reduced cooling of SST due to upwelling along the coasts of Java, western Australia, and Somalia. The seasonal cycle of sea surface temperature (SST) are shifted in the eastern equatorial Pacific and in the Leeuwin Current region and large‐scale dynamic effects of the ITF are also seen. Over the period 1980–1995, the effect of the ITF has interannual variability. While the main ENSO indices, NINO3 and NINO4, are almost identical with and without the ITF, the total SSTs show significant ENSO dependence. SSTs in the central Indian Ocean linked previously to rainfall deficit in the western Australian winter may be dependent on the ENSO related variability of the ITF.
Is the Arctic Ocean warming?
Authors: Grotefendt Kerstin , Logemann Kai , Quadfasel Detlef , Ronski Stephanie
Published: February 2004

Recent high‐quality hydrographic measurements in the Arctic Ocean have revealed a warming of up to 1 K of the Atlantic Layer when compared with Russian climatologies of the 1940s to 1970s. About half of this warming can be attributed to the different methods by which the two data sets were obtained: the climatologies are based on discrete bottle data in the vertical and necessarily involve smoothing in space and time, whereas the modern, quasi‐synoptic data are from continuously recording conductivity‐temperature‐depth sondes and give much better resolution in the vertical. Modern surveys also focused more on boundary current regimes where narrow warm bands of Atlantic Water are present. The remainder of the warming detected can be explained with the following physical arguments: (1) an increased inflow of Atlantic Water in the early 1990s and (2) a higher temperature of this inflowing water. Temperature time series in the Barents Sea since the beginning of this century suggest that the warming of the early 1990s is not a long‐term climate signal but, rather, is related to the inherent natural variability of the system with timescales of decades.
An occluded coastal oceanic front
Authors: Marmorino G. O. , Shen C. Y. , Allan N. , Askari F. , Trizna D. B. , Trump C. L. , Shay L. K.
Published: February 2004

Field observations, including hydrographic, microwave imaging radar, and HF radar measurements, reveal the evolution of a complicated frontal interaction between three water masses on the continental shelf near Cape Hatteras, North Carolina, during a period of incursion of water from the Gulf Stream. The water masses were found to be separated by intersecting frontal lines configured in a manner analogous to an occluded atmospheric front. The densest water lay between inshore and offshore fronts that gradually merged or occluded in the generally downstream direction, leaving a single surface front. The overall frontal structure appeared as a distinct Y‐shaped feature in the radar imagery, similar to historical imagery of the study area. The interpretation of the observations is aided by the use of a two‐dimensional numerical model. The model is initialized with two fronts idealized from the ocean measurements. The model fronts quickly sharpen and begin to move together, eventually occluding into a single surface front. As a result of the occlusion, the water mass having intermediate density subducts and intrudes under the most buoyant water, carrying with it strong horizontal and vertical shears, and a frontal band of diverging currents is created in the densest water mass. The model thus suggests that in the ocean there will be an increase in hydrographic and velocity fine structure downstream of the frontal occlusion point.
Inhomogeneous rodons
Authors: Ochoa José , Sheinbaum Julio , Pavía Edgar G.
Published: February 2004

A reduced gravity model which includes variable density is investigated analytically and numerically. A procedure is established to generate families of solutions with the same flow structure of any nondivergent solution of the classical reduced gravity model. Inhomogeneous rodons are then constructed as explicit examples of exact solutions on the f plane. A formal stability [Holm et al., 1985] theorem, valid for the case of circular inhomogeneous rodons, shows that for the vortex to be unstable the density must increase outward somewhere. A particle‐in‐cell numerical method is used to simulate the behavior of inhomogeneous rodons and circular vortices with realistic decay of velocity toward the edge of the buoyancy front. The results provide information on the long‐term evolution of unstable vortices. The unstable vortices show that the limited resolution available is never enough to resolve the convolution of isopycnals due to adiabatic stirring. This result is seen in the numerical model and interpreted physically as a homogenization process of the vortex interior.
Ocean color chlorophyll algorithms for SeaWiFS
Authors: O'Reilly John E. , Maritorena Stéphane , Mitchell B. Greg , Siegel David A. , Carder Kendall L. , Garver Sara A. , Kahru Mati , McClain Charles
Published: February 2004

A large data set containing coincident in situ chlorophyll and remote sensing reflectance measurements was used to evaluate the accuracy, precision, and suitability of a wide variety of ocean color chlorophyll algorithms for use by SeaWiFS (Sea‐viewing Wide Field‐of‐view Sensor). The radiance‐chlorophyll data were assembled from various sources during the SeaWiFS Bio‐optical Algorithm Mini‐Workshop (SeaBAM) and is composed of 919 stations encompassing chlorophyll concentrations between 0.019 and 32.79 μg L−1. Most of the observations are from Case I nonpolar waters, and ∼20 observations are from more turbid coastal waters. A variety of statistical and graphical criteria were used to evaluate the performances of 2 semianalytic and 15 empirical chlorophyll/pigment algorithms subjected to the SeaBAM data. The empirical algorithms generally performed better than the semianalytic. Cubic polynomial formulations were generally superior to other kinds of equations. Empirical algorithms with increasing complexity (number of coefficients and wavebands), were calibrated to the SeaBAM data, and evaluated to illustrate the relative merits of different formulations. The ocean chlorophyll 2 algorithm (OC2), a modified cubic polynomial (MCP) function which uses Rrs490/Rrs555, well simulates the sigmoidal pattern evident between log‐transformed radiance ratios and chlorophyll, and has been chosen as the at‐launch SeaWiFS operational chlorophyll a algorithm. Improved performance was obtained using the ocean chlorophyll 4 algorithm (OC4), a four‐band (443, 490, 510, 555 nm), maximum band ratio formulation. This maximum band ratio (MBR) is a new approach in empirical ocean color algorithms and has the potential advantage of maintaining the highest possible satellite sensor signal: noise ratio over a 3‐orders‐of‐magnitude range in chlorophyll concentration.
Statistical aspects of surface drifter observations of circulation in the Santa Barbara Channel
Authors: Dever E. P. , Hendershott M. C. , Winant C. D.
Published: February 2004

Argos‐tracked drifters are used to study the near‐surface circulation in the Santa Barbara Channel. The mean consists of a cyclonic cell in the western Santa Barbara Channel with weaker flow in the eastern Channel. Drifter mean velocities agree well with record means from near‐surface current meters. At the eastern entrance to the channel, drifter velocities are biased toward outflow (eastward velocity) conditions. Drifter variability at synoptic and seasonal scales shows a tendency for upwelling and eastward flow in spring, a strong cyclonic circulation in summer, poleward relaxation in fall, and weak, variable circulation in winter. Drifter estimates of eddy stress divergence indicate advective terms play a secondary role in the mean surface momentum balance. Lagrangian time and space scales are about 1 day and under 10 km, respectively. The mismatch between Lagrangian and Eulerian timescales indicates advective terms are important to the fluctuating circulation.
On the representation of the Southern Ocean water masses in an ocean climate model
Authors: Kim Seong‐Joong , Stössel Achim
Published: February 2004

In global ocean general circulation models used for climate studies, the simulated global deep ocean temperature and salinity generally appear to be lower than that observed. This may be due to an insufficient representation of the cryosphere in such models because the deep ocean thermohaline properties are strongly determined by high‐latitude surface conditions. In this study, it is our aim to arrive at an improved simulation of polar water mass characteristics and distribution in the framework of an ocean climate model. As a step toward identifying the critical factors influencing the high‐latitude water masses, in particular those of the Southern Ocean, we have proceeded a series of sensitivity experiments with a global sea ice‐ocean model. Forcing Southern Ocean sea ice with daily instead of monthly fluctuating wind enhances brine release due to new‐ice formation, which leads to a more pronounced core of Lower Circumpolar Deep Water (LCDW) in the Antarctic oceanic regime. Wider Antarctic shelf topography improves the distribution and thickness of sea ice, simultaneously providing a brine reservoir on the shelf, which is a crucial ingredient in the formation process of Antarctic Bottom Water (AABW). As a consequence, the salinity on the Antarctic shelf is increased with depth as observed. Intense southeasterly katabatic winds play a significant role in increasing the salinity, mainly off east Antarctica. It is suggested that modeled Southern Ocean water mass properties are highly sensitive to the large scale and regional forcing of Antarctic sea ice as well as the model representation of the shelf topography, the width of which is crucial to reproducing the first‐order features of near‐boundary convection.
Vertical water mass mixing and plankton blooms recorded in skeletal stable carbon isotopes of a Red Sea coral
Authors: Felis Thomas , Pätzold Jürgen , Loya Yossi , Wefer Gerold
Published: February 2004

The environmental interpretation of the 13C/12C variations in the skeletons of massive corals is still a matter of debate. A 19‐year seasonal skeletal 13C/12C record of a shallow‐water Porites coral from the northern Red Sea (Gulf of Aqaba) documents interannual events of extraordinarily large plankton blooms, indicated by anomalous 13C depletions in the coral skeleton. These blooms are caused by deep vertical water mass mixing, convectively driven in colder winters, which results in increased supplies of nutrients to the surface waters. The deep vertical mixings can sometimes be driven by the cooling occurring throughout the Middle East after large tropical volcanic eruptions. We therefore have evidence in our coral skeletal 13C/12C record for an indirect volcanic signal of the eruptions of El Chichón (1982) and Mount Pinatubo (1991). Deep mixing induced 13C/12C variations of the dissolved inorganic carbon in the surface waters can be neglected at this location. We therefore suggest that the 13C skeletal depletions can be best explained by changes in the coral's autotrophy‐heterotrophy diet, through increased heterotrophic feeding on Zooplankton during the blooms. Increased feeding on 13C‐depleted Zooplankton or increased heterotrophy at the expense of autotrophy can both result in a 13C‐depleted coral skeleton. However, this suggestion requires more testing. If our conclusions are substantiated, seasonal skeletal 13C/12C records of corals which change from autotrophy under normal conditions to increased heterotrophy during bloom events may be used as indicators of ocean paleoproductivity at interannual resolution, available from no other source.
Effect of nonlinear drag on the settling velocity of particles in homogeneous isotropic turbulence
Authors: Fung J. C. H.
Published: February 2004

We study the average settling velocity of small spherical particles under gravity through a Gaussian random velocity field generated by Fourier modes with a von Kárman energy spectrum. The particles are subjected to the effects of a linear (Stokes) or nonlinear drag force, inertia and gravity. It is shown that the effect of drag nonlinearity is a function of the particle to fluid density ratio ρp/ρf and a function of the ratio τp/τk of the particle time constant to the Kolmogorov timescale of the fluid. Simulations show that as ρp/ρf decreases from 877 to 2.65 or as τp/τk increases from 1 to 2.74, the drag nonlinearity increases as a result of the increase in particle Reynolds numbers. Hence the settling velocity changes from larger to smaller as compared with the still fluid settling velocity, showing that one of the major mechanisms governing the fall velocity reduction in a turbulent flow is the drag nonlinearity. The maximum increase in settling rate occurs at VT/υk ≈ 2 for ρp/ρf ≥ 87 (where VT is the terminal velocity of the particle and υk is the Kolmogorov velocity); this is consistent with the results of Wang and Maxey [1993]. The maximum decrease in settling rate occurs at VT/VK ≈ 1 for ρp/ρf ≈ 2.65, consistent with the results of Fung [1993]. In addition, the role of the spatial and temporal variations of the flow field on the settling rate is investigated. Finally, the Gaussian velocity field is also simulated with an exponential energy spectrum, and similar results are observed.
On the surface layer similarity at a complex forest site
Authors: Rannik Üllar
Published: February 2004

Eddy covariance (EC) and profile measurements from April until October 1996 have been used to analyze the variance and gradient similarity above a Scots pine forest in Hyytiälä, southern Finland. The practical motivation for the study is to test the quality of the EC measurements by the similarity‐predicted scaling law of variances and the potential of applying the gradient method to flux estimation. The correct stability dependence of the standard deviations (s.d.) of turbulent quantities scaled with relevant flux parameters indicates proper functioning of the EC system under unstable to moderately stable conditions. Modest variation of the nondimensional s.d. of vertical wind speed with wind direction is observed. Under strongly stable conditions the normalized standard deviations of wind speed components, but not of scalars, are substantially increased. The ratios of the nondimensional wind and temperature gradients to their corresponding values obtained from the ideal similarity formulas vary significantly with wind direction, indicating the influence of topography. No stability dependence of the ratios was observed. The profile method, applied to the measured wind and temperature profiles with the wind direction dependent corrections to flux‐profile relationships, reproduced the EC‐measured heat fluxes with high correlation. This indicates the possible applicability of the profile method at nonideal sites, if the modification of the profiles is studied beforehand by using independently determined fluxes.
Interactions between sulfur and soot emissions from aircraft and their role in contrail formation: 2. Development
Authors: Andronache C. , Chameides W. L.
Published: February 2004

A numerical box model of the time evolution of key gaseous and aerosol species in an expanding plume of a subsonic aircraft is developed to investigate the effects of aircraft emissions on the characteristics of contrails. The model simulates the formation and growth of new particles from H2SO4 and H2O and the growth of activated soot particles. Coagulation, dilution, and interaction with gaseous species are included. The evidence of contrail formation is determined by the condensed water concentration, using a visibility threshold of 10−5 kg m−3. The model simulates formation of large concentrations of H2SO4‐H2O particles with radii in the range ∼1–10 nm. The number concentration and the rate of growth of these particles is dependent on the sulfur emissions. We find that for a given initial concentration of soot particles and ambient conditions the sensitivity of contrail properties to fuel sulfur content is small for typical emission indices (0.1–1 g kg−1). For larger sulfur emissions and favorable conversion of sulfur into H2SO4 the calculations show a slight impact of sulfur on contrail properties, such as lifetime, onset, and threshold temperature. The magnitude of this influence is generally smaller than the variations caused by uncertainties in the mass accommodation coefficient of sulfur species on soot surface, entrainment rate of ambient air into the plume, soot emissions, and atmosphere parameters at flight altitude.
An empirical model study of the tropospheric meridional circulation based on SAGE II observations
Authors: Wang Pi‐Huan , Rind David , Trepte Charles R. , Kent Geoffrey S. , Yue Glenn K. , Skeens Kristi M.
Published: February 2004

This study investigates the tropospheric mean meridional circulation important to the development of opaque clouds and the measurement opportunity of the 1.02‐μm channel of the Stratospheric Aerosol and Gas Experiment (SAGE) II in the troposphere. A simple empirical model is formulated to derive the mean meridional circulation from the 6‐year (1985–1990) statistics of the SAGE II tropospheric measurement frequency. The vertical circulation of the model is assumed to be related to the departure field of the zonally averaged SAGE II measurement frequency from the corresponding global mean in a linear fashion. The proportional constant is calibrated with the observed upwelling circulation statistics in the tropics. The obtained model vertical circulation is then used to determine the distribution of meridional velocity according to the continuity equation. The derived model mean circulation features the influence from both the diabatic circulation and the eddy quasi‐isentropic transport, with a distinct pattern of material advection into the upper troposphere from both the lower troposphere and the stratosphere. Most significantly, the model circulation is shown to be highly consistent with the observed free tropospheric aerosol and ozone distributions, particularly with their seasonal variations given the aerosol and ozone source regions. This high degree of consistency illustrates the intimate relationship between the large‐scale circulation, cloudiness, and the SAGE II tropospheric measurement frequency, and the robust nature of the empirical model despite the model's simplicity. The discussion in relating the model circulation to the conventional Eulerian circulation and the Lagrangian transport, based on isentropic consideration, is also provided.
Retrieval of aerosol single‐scattering albedo from ground‐based measurements: Application to observational data
Authors: Devaux C. , Vermeulen A. , Deuzé J. L. , Dubuisson P. , Herman M. , Santer R. , Verbrugghe M.
Published: February 2004

The single‐scattering albedo ω0of atmospheric aerosols is a key parameter concerning the effect of the particles on the Earth's radiative budget. Retrieval of this parameter from in situ measurements is a difficult task. We address the possibility to derive ω0from ground‐based measurements of solar transmission and diffuse skylight, independent of the knowledge of the aerosol properties or the absorption mechanism. The proposed method is based on the normalization of the aerosol phase function that is derived from scannings of the skylight. The method has been applied to a series of observations conducted during a few years over the french site of La Crau, near the industrial area of Fos sur Mer in the southeast of France. The aerosol albedo, retrieved near the 870 nm wavelength, ranges from 0.60 to 0.95. The estimated error is Δω0≈ ±0.05. To test their reliability, the retrieved ω0have been used to calculate the diffuse/total solar fluxes at ground level. These estimates are consistent within about 10% with independent direct/diffuse flux measurements performed simultaneously.
Differential ablation of cosmic dust and implications for the relative abundances of atmospheric metals
Authors: McNeil William J. , Lai Shu T. , Murad Edmond
Published: February 2004

Although it is generally accepted that extraterrestrial material is the source of metals in the upper atmosphere, the relative abundances of mesospheric metal atoms and ions present us with a conundrum. Lidar observations have consistently shown that the abundances of neutral metals in the atmosphere and the abundances of these metals in the meteoric material that falls to Earth are significantly disproportionate. The column density of neutral sodium is perhaps 2 orders of magnitude larger than that of calcium, while the abundances in meteorites are approximately equal. By contrast, ion mass spectroscopy has shown that the abundances of the meteoric ions match reasonably well those in the meteorites. We present here a model that attempts to address these discrepancies. At the heart of the model is the concept of differential ablation, which suggests that more volatile metals sublimate earlier in the descent of a cosmic dust particle than do the less volatile components. We model three different meteoric metals: sodium, magnesium, and calcium. Results suggest that sodium ablates to a greater extent than does calcium and that it ablates at a substantially higher altitude. Deposition at lower altitudes leads to more rapid conversion of the atomic calcium into complexes through three‐body reactions. Thus the depletion of calcium arises from both a decrease in deposition and an increase in the rate of removal of that which is deposited. We examine the behavior of the model in several respects, comparing predicted results with measurements and finding reasonable agreement. We argue that the success of this model indicates that differential ablation is a key factor in the determination of the relative abundances of meteoric metals in the mesosphere.
Humidification factors of aerosols from biomass burning in Brazil
Authors: Kotchenruther Robert A. , Hobbs Peter V.
Published: February 2004

The light‐scattering coefficient of aerosols from biomass burning was measured as a function of relative humidity (RH) and wavelength in four regions of Brazil extending from the cerrado to the primary forests of the Amazon Basin. At a wavelength of 550 nm the ratio of the light‐scattering coefficient at a RH of 80% to that at an RH of 30% ranged from 1.01 to 1.51, with an average value of 1.16. The aerosol backscatter ratio as a function of RH decreased by 10 to 20% as the RH was increased from 30 to 80%. Measurements of the geometric growth of the aerosol with RH were consistent with the measured changes in light scattering. Use of these new measurements yields an estimate for the globally averaged direct radiative forcing by biomass burning aerosol of about 0.25 W m−2.
Relationships between ozone and carbon monoxide at surface sites in the North Atlantic region
Authors: Parrish D. D. , Trainer M. , Holloway J. S. , Yee J. E. , Warshawsky M. S. , Fehsenfeld F. C. , Forbes G. L. , Moody J. L.
Published: February 2004

As part of the North Atlantic Regional Experiment (NARE), measurements of O3 and CO at five surface sites were made from July 1991 to January 1995. The investigation of the variabilities and correlation of O3 and CO presented here indicates that the seasonal cycles of the medians and the means of O3 and CO are qualitatively similar to the cycles observed at other northern midlatitude sites. The signature of O3 produced from anthropogenic precursors is clearest in the spring at the Azores and in the summer at Sable Island. The influence of the natural stratospheric O3 source is apparent at Sable Island, particularly in the spring. At all sites the variability of CO throughout the year is dominated by episodes of pollution transport. The slopes of the monthly O3‐CO correlations in the summer in Atlantic Canada and the spring in the Azores are quite uniform at 0.3 to 0.4. However, individual pollution transport events often have larger (≤1.0) slopes, which indicate significantly different net O3 production efficiencies between episodes. The average slope of O3 versus CO at Sable Island in the winter for moderate pollution transport events (CO ≤180 ppbv) is −0.28, which indicates that the titration of ambient O3 by emitted NO with little if any photochemical O3 production dominates the O3 chemistry over eastern North America in winter. Diurnal cycles driven by photochemistry are observed in the summer for both O3 and CO at the Azores (net loss) and Sable Island (net production.) These observations are consistent with the work of Duderstadt et al. [this issue] who find positive net photochemical O3 production at Sable Island, and with the modeling of Atherton et al. [1996] who find a region dominated by photochemical loss of O3 and CO in the central Atlantic.
Photochemical production and loss rates of ozone at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive
Authors: Duderstadt K. A. , Carroll M. A. , Sillman S. , Wang T. , Albercook G. M. , Feng L. , Parrish D. D. , Holloway J. S. , Fehsenfeld F. C. , Blake D. R. , Blake N. J. , Forbes G.
Published: February 2004

Three weeks of summertime surface‐based chemical and meteorological observations at Sable Island, Nova Scotia during the North Atlantic Regional Experiment (NARE) 1993 summer intensive are used to study instantaneous photochemical production and loss rates of ozone by means of a numerical photochemical model. Results are most sensitive to the averaging scheme of data used to constrain the model and the ambient variability of the measurements. Model simulations driven by a time series of 5 min averaged data, most representative of the chemistry at the site, yield an average net photochemical ozone production of 3.6 ppbv/d. Estimates of net ozone production designed to filter out local sources, by using 1000–1400 LT median values of observations to drive the model and by excluding short‐lived hydrocarbons, give values ranging from 1 to 4 ppbv/d. These positive values of net ozone production within the marine boundary layer over Sable Island demonstrate the impact of polluted continental plumes on the background photochemistry of the region during the intensive. The dominant ambient variables controlling photochemical production and loss rates of ozone at the site during the measurement campaign appear to be levels of nitrogen oxides, ozone, nonmethane hydrocarbons, and solar intensity determined by cloud cover. The model partitioning of nitrogen oxides agrees for the most part with measurements, lending credence to calculated photochemical production and loss rates of ozone as well as inferred levels of peroxy radicals not measured at the site. Discrepancies, however, often occur during episodes of intermittent cloud cover, fog, and rain, suggesting the influence of cloud processes on air masses reaching the site.
Observations of atmospheric methane and carbon monoxide in Brazil: SCAR‐B mission
Authors: Alvala Plinio C. , Kirchhoff Volker W. J. H.
Published: February 2004

Methane (CH4) concentrations have been measured in the lower atmosphere in Brazil on board a Brazilian research aircraft. Air samples were obtained at different heights by grab sampling, and taken for analysis by flame ionization detector (FID) gas chromatography at INPE (the Brazilian Institute for Space Research). The sampling region is the cerrado area of central Brazil, around the cities of Brasilia, Cuiaba, Alta Floresta, and Porto Nacional. Instead of concentrating on plume characteristics, an effort was made to describe the average mixed atmosphere, keeping a distance from direct burning sources. The measurements of atmospheric CH4during the dry season have shown larger mixing ratios compared to the wet season. This finding has consistently been observed for different dry seasons, but especially for the Smoke, Clouds, and Radiation: Brazil (SCAR‐B) field mission. For comparison, measurements near the ground in a region not directly affected by biomass burning (Natal, Brazil) show concentrations that are 39 parts per billion by volume (ppbv) less than the biomass burning results, which is 4 times the size of the seasonal amplitude at oceanic stations within about 10° of the equator, thus making the biomass burning values comparable to those observed at northern hemisphere midlatitudes. The vertical distribution for these regions seems to be well mixed, showing differences of about 40 ppbv between the different sampling areas of the SCAR‐B experiment. The CH4mixing ratio was in general well correlated with the CO mixing ratio.
Enhancements of hydroperoxides and formaldehyde in biomass burning impacted air and their effect on atmospheric oxidant cycles
Authors: Lee Meehye , Heikes Brian G. , Jacob Daniel J.
Published: February 2004

Hydrogen peroxide, organic hydroperoxide species, and formaldehyde were measured in the troposphere over the South Atlantic, Brazil, and southern Africa during the NASA Global Tropospheric Experiment (GTE) Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE‐A) experiment, an investigation into the cause of an observed seasonal ozone maximum in this region. H2O2and CH2O concentrations were extremely high, up to 14 and 5 ppbv, respectively, and are the result of direct production in fire plumes as well as secondary photochemical production in the atmosphere. Enhanced hydroperoxide and formaldehyde concentrations have a significant effect in both odd‐hydrogen and odd‐oxygen cycles in the troposphere. The odd‐hydrogen radical production from hydroperoxide and formaldehyde photolysis exceeded that from ozone photolysis at high altitudes. At low altitudes where air was impacted by biomass burning the contribution of these species to odd‐hydrogen radical production was comparable to that of O(1D). The fraction of radical production due to H2O2and CH2O photolysis linearly increased with their concentrations. Odd‐hydrogen radical production from hydroperoxides is ∼30% of the total. These species also react with OH and cycle odd‐hydrogen from OH to HO2and CH3O2radicals. In and near biomass burning plumes, more hydroperoxide and formaldehyde reacted with OH than was photolyzed, and HO2radical production from H2O2and CH2O oxidation was also important. Hydroperoxide and formaldehyde from fires together with nitric oxide initiate odd‐oxygen production and contribute to new O3formation. The greatest destruction of odd‐oxygen in continental outflows leads to additional photochemical enhancement of the hydroperoxide. Therefore enhanced hydroperoxide and formaldehyde from biomass burning increases the oxidizing capacity of the atmosphere near fires and their high concentrations downwind reflect the stimulated chemistry.
Atmospheric circulation variability associated with shallow‐core seasonal isotopic extremes near Summit, Greenland
Authors: Rogers Jeffrey C. , Bolzan John F. , Pohjola Veijo A.
Published: February 2004

Seasonal back‐diffused δ18O time series, reconstructed for nine shallow firn cores from the Greenland Summit for 1959–1987, are evaluated in terms of (1) their spatial and temporal variability and (2) their association to the atmospheric circulation. Rotated principal component analysis evaluates spatial and temporal patterns of organization in the back‐diffused δ18O ice core time series. Mean sea level pressure and 500‐hPa height fields are obtained for winter and summer seasons that fall in the isotopically warmest and coldest thirds of the seasonal data. The winter back‐diffused δ18O data indicate that some degree of longitudinal separation takes place between isotopic records from western and eastern Greenland in agreement with coastal station climatic data. Temporal variability is linked with statistical significance to the variability of the atmospheric circulation over the Denmark Strait, as well as over the Norwegian and Barents Seas. The North Atlantic Oscillation (NAO) and the mean Icelandic low in the Denmark Strait affect isotopic variability over the cores west of the ice divide, just as they have been shown to prominently affect west coast air temperature records. Isotopic variability at some ice cores east of the divide is influenced by pressure variability over the Norwegian and Barents Seas, although statistical significance is lower. The NAO appears insignificant in summer isotopic variability, as the circulation in that season is very much localized and dominated by either ridging (warm cases) or troughs (cold cases) over eastern Greenland and the Arctic Ocean. Results suggest that long‐term winter δ18O variability can potentially be linked to climate variation over Europe, especially from the Mediterranean basin northward to about 55°N and as far east as central Siberia.
Effects of the quasi‐biennial oscillation on the zonally averaged transport of tracers
Authors: Jones Dylan B. A. , Schneider Hans R. , McElroy Michael B.
Published: February 2004

The influence of the quasi‐biennial oscillation (QBO) on the transport of long‐lived tracers out of the tropics and the mechanism responsible for the QBO in subtropical ozone and its dependence on the seasonal cycle are examined with a two‐dimensional model. The modeled QBO induces a meridional circulation which modulates transport of long‐lived tracers out of the tropics. The induced circulation also produces a QBO in ozone in the subtropics by advection of ozone out of the tropics and down from higher altitudes. In agreement with observations, the subtropical anomalies in ozone are greatest in the winter season. This seasonal synchronization of the subtropical anomalies occurs because the induced circulation is stronger always in the winter hemisphere as a result of nonlinear momentum advection in the tropics and subtropics. Meridional transport in the model is enhanced by the QBO through an “upper” and a “lower” transport regime, in agreement with the analysis by Hitchman et al. [1994]. When there are descending westerly winds in the tropics in the model, transport out of the tropics is enhanced in the lower stratosphere. When there are descending easterlies, transport out of the tropics is enhanced in the middle stratosphere. This modulation of transport out of the tropics significantly influences the stratospheric distribution of long‐lived tracers. Depending on the phase of the QBO, mixing ratio surfaces of long‐lived tracers (such as N2O) in the extratropics can be displaced poleward by more than 10°.
Stratosphere over Dumont d'Urville, Antarctica, in winter 1992
Authors: Ricaud P. , Monnier E. , Goutail F. , Pommereau J.‐P. , David C. , Godin S. , Froidevaux L. , Waters J. W. , Mergenthaler J. , Roche A. E. , Pumphrey H. , Chipperfield M. P.
Published: February 2004

We present an analysis of the temporal evolution of stratospheric constituents above the station of Dumont d'Urville in Antarctica (67°S, 140°E) from August 14 to September 20, 1992. Data sets include temperature profiles and H2O, ClO, O3, NO2, ClONO2, HNO3, N2O, and CH4 mixing ratios and aerosol extinction coefficients from 46 to 1 hPa measured by the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) instruments aboard the Upper Atmosphere Research Satellite (UARS). At the station, aerosol extinction coefficients and O3 profiles are obtained by a lidar together with O3 profiles provided by sondes. Integrated O3 and NO2 column amounts are given by a Système d'Analyse par Observation Zénithale (SAOZ) spectrometer located at the station. Column O3 is also provided by the Total Ozone Mapping Spectrometer (TOMS) instrument aboard the NIMBUS 7 satellite, complemented with potential vorticity derived from the U.K. Meteorological Office assimilated data set and temperature fields provided by the European Centre for Medium‐Range Weather Forecasts. Time evolution of these measurements is interpreted by comparison with results from the SLIMCAT three‐dimensional chemical transport model. We show that the site is near the vortex edge on average and is alternately inside the vortex or just outside in the region referred to as the “collar” region. There are no observations of polar stratospheric clouds (PSCs) over the station above 46 hPa (∼18 km). In fact, PSCs mainly appear over the Palmer Peninsula area at 46 hPa. The rates of change of chemical species are evaluated at 46 hPa when the station is conservatively inside the vortex collar region. The ozone loss rate is 0.04 ppmv d−1 (∼1.3% d−1), which is consistent with other analyses of southern vortex ozone loss rates; chlorine monoxide tends to decrease by 0.03 ppbv d−1, while chlorine nitrate increases by 0.025 ppbv d−1. These negative ClO and positive ClONO2 trends are only observed in the collar region of the vortex where O3 amounts are far from near zero, and little denitrification is observed. Loss and production rates as measured by UARS are more pronounced than the ones deduced from the SLIMCAT model, probably because of the moderate model horizontal resolution (3.75° × 3.75°), which is not high enough to resolve the vortex crossings above Dumont d'Urville and which leads to a larger extent of denitrified air than indicated by the UARS data. The analysis also shows activated ClO inside the vortex at 46 hPa, a dehydrated vortex at 46 hPa, and rehydrated above, with no trace of denitrification in the lower stratosphere. Good agreement between coincident measurements of O3 profiles by UARS/MLS, lidar, and sondes is also observed. Finally, the agreement between UARS and SLIMCAT data sets is much better in the middle stratosphere (4.6 hPa) than in the lower stratosphere (46 hPa).
Atmospheric trend and lifetime of chlorodifluoromethane (HCFC‐22) and the global tropospheric OH concentration
Authors: Miller B. R. , Huang J. , Weiss R. F. , Prinn R. G. , Fraser P. J.
Published: February 2004

Concentrations of CHClF2 (HCFC‐22) in clean background air collected at Cape Grim, Tasmania, over the period 1978–1996, and at La Jolla, California, over the period 1992–1997, have been measured by oxygen‐doped electron capture detection gas chromatography. The mid‐1996 dry‐air mole fractions and trends were 116.7 parts per trillion (ppt) and 6.0 ppt yr−1 in Cape Grim and 132.4 ppt and 5.5 ppt yr−1 in California, respectively. These observations, together with estimates of industrial emissions, have been fitted to a two‐dimensional global atmospheric model by an optimal estimation inversion technique to yield estimated tropospheric and total atmospheric lifetimes for chemical destruction of CHClF2 of 9.1−2.8+4.4 years and 10.0−3.0+4.4 years, respectively. These lifetimes correspond to a temperature− and density‐weighed global tropospheric OH abundance of 11.0−3.6+5.0 × 105 radical cm−3, which is in statistical agreement with our recent more accurate estimate of OH abundance based on measurements of CH3CCl3. Our analysis suggests that, compared to current industrial estimates, southern hemisphere emissions are higher, global emissions are larger in earlier years and smaller in later years, and, finally, production by nonreporting companies is less.
Matrix parameterization of the 15 μm CO2 band cooling in the middle and upper atmosphere for variable CO2 concentration
Authors: Fomichev V. I. , Blanchet J.‐P. , Turner D. S.
Published: February 2004

A matrix parameterization of the 15 μm CO2 band radiative cooling in the middle and upper terrestrial atmosphere for both local thermodynamic equilibrium (LTE) and non‐LTE (NLTE) layers is proposed. For the atmospheric region between ∼13 and 85 km, matrix coefficients for CO2 concentrations of 150, 360, 540, and 720 ppm have been precalculated using a line‐by‐line technique. A method for interpolation of the matrix coefficients for arbitrary CO2 concentration is suggested. In the layer between 85 and 110 km, where strong NLTE effects start to occur, a recurrence formula relating cooling rate values at two neighboring altitude levels is used. To calculate the cooling rate above 110 km, a simple approach accounting for both the absorption of the radiative flux formed below 110 km and the cooling‐to‐space terms is suggested. The accuracy of the parameterization is examined for different temperature and CO2 profiles as well as for different atomic oxygen concentrations in the NLTE layer.
Microlab‐1 experiment: Multipath effects in the lower troposphere
Authors: Gorbunov Mikhail E. , Gurvich Alexander S.
Published: February 2004

Interpretation of the Doppler‐shifted radio frequencies from the proof‐of‐concept GPS/MET radio occultation experiment is impeded by the effects of multipath propagation caused by strong variations of atmospheric refractivity. In this paper, we describe and validate a method of deciphering the ray structure of the measured electromagnetic field in regions of multipath propagation. The method is based on diffraction theory and the principles of the synthesized aperture, which allows for the refinement of the vertical resolution beyond the Fresnel scale as well as for the improvement of the signal/noise ratio. The validation of the method is based on artificial occultation data simulated using global fields from a NCEP analysis. An example of the application of the diffraction correction algorithm for the processing of real GPS/MET data is given. The comparison of the inversion results with and without diffraction correction indicates a very significant role of multipath propagation in the lower troposphere, which suggests that the diffraction correction can significantly improve the quality of the data processing.
Methane sulfonic acid to non‐sea‐salt sulfate ratio in coastal Antarctic aerosol and surface snow
Authors: Legrand Michel , Pasteur Elizabeth Claire
Published: February 2004

Multiple‐year time series of the weight ratio R of methane sulfonic acid (MSA) to non‐sea‐salt sulfate (nss SO4) in aerosols collected at three coastal Antarctic stations Neumayer (70°39′S, 8°15′W), Halley (75°35′S, 26°19′W), and Dumont D'Urville (66°40′S, 140°1′E) are presented here. Discarding the November 1991 to October 1992 time period, during which the Antarctic atmosphere is thought to have been contaminated by volcanic plumes (Pinatubo and Cerro Hudson), the multiple‐year data sets indicate mean annual R values of 17.9 ± 10.7% at Neumayer (1983–1995) and 11.7 ± 6.7% at Dumont D'Urville (1991–1996). On the basis of a more limited time series (February 1991/February 1992), which partly overlaps the volcanic time period, the Halley data had R values of 25.6 ± 11.9% from October to April. The R seasonal variations are broadly consistent at the three sites, with maxima in mid to late summer (from December to April) ranging from 18.2 ± 4.6% at Dumont d'Urville to 29.2 ± 6.5% at Neumayer and 32.2 ± 6.9% at Halley. Minima are lower than 10% during winter (from May to September) at Neumayer (8.5 ± 2.5%) and Dumont d'Urville (6.0 ± 3.0%). The interpretation of these R values in terms of relative abundance of MSA and nss SO4 produced by the oxidation of dimethylsulfide (DMS) at high southern latitudes is not straighforward. In winter, nss sulfate present at coastal Antarctic sites is likely originating in long‐range transported by‐products from marine DMS emissions taking place at 50°S and non‐DMS sulfate from the continental free troposphere. Midwinter R values show a mean ratio of up to 20% for the pure marine input. From September to November, both MSA and nss sulfate concentrations increase in relation to enhanced marine DMS emissions from 50° to 60°S. R values are only weakly enhanced. In summer, the further increase of MSA and nss sulfate concentrations is related to increased biogenic activity south of 60°S and is characterized by R values in the range of 25 to 35%. The mean ratio for DMS marine emissions south of 60°S is close to 62% at Neumayer and 32 to 38% at Dumont d'Urville. These baseline data obtained at coastal Antarctic sites allow us to investigate the role of various parameters that modulate seasonal variations of R at high southern latitudes including the oxidative capacity of the atmosphere, the temperature of the atmospheric oxidation of DMS, and the aging of marine air masses during their transport from source regions toward coastal Antarctic regions.
Dynamic aerosol model: Urban/industrial aerosol
Authors: Remer Lorraine A. , Kaufman Yoram J.
Published: February 2004

Aerosol properties vary with the aerosol type, loading, or atmospheric conditions. We found that a dynamical aerosol model that describes the variability of aerosol properties as a function of aerosol optical thickness can address this issue. Ambient urban/industrial aerosol is measured in the mid‐Atlantic region of the United States during the summer of 1993 by automatic Sun/sky radiometers. The volume size distributions inverted from the measured sky radiance represent the entire column of ambient particles. These data depict an aerosol regime in which accumulation mode particle size and volume increases as optical thickness increases. A dynamical aerosol model that is a function of optical thickness is constructed by fitting lognormal functions to the volume size distributions. It accounts for 60% of the variance in the aerosol accumulation mode size distributions. The accumulation mode is represented as a linear combination of two modes: one with volume modal radius 0.11 μm and the other with modal radius 0.21 μm. The model is shown to recreate the correct independently measured sky radiance in the backscattering direction and to predict the phase function within model uncertainties of data collected in subsequent years. The model is shown to be unbiased with respect to the time of day and to reproduce the wavelength dependence measured by the independent Sun measurements. The dynamic model has substantially different properties than a “static” continental model except for the limited optical thickness range of 0.0–0.25.