bluuebunny/crossref_metadata_2025 huggingface dataset:
Entrainment results from the Flatland boundary layer experiments
Authors: Angevine Wayne M. , Grimsdell Alison W. , McKeen Stuart A. , Warnock J. M.
Published: February 2004

A primary objective of the 1995 and 1996 Flatland boundary layer experiments, known as Flatland95 and Flatland96, was to measure and characterize entrainment at the top of the convective boundary layer. The experiments took place in the area near the Flatland Atmospheric Observatory near Champaign‐Urbana, Illinois, in August‐September 1995 and June‐August 1996. The site is interesting because it is extraordinarily flat, has uniform land use, and is situated in a prime agricultural area. Measurements in the entrainment zone are difficult to make due to the time and space scales involved. We will present entrainment estimates derived from budget calculations with data from UHF wind profiling radars and from radiosondes. The results demonstrate that the remote sensing instruments produce results comparable to radiosondes and have significant advantages for boundary layer studies. Surface flux measurements are also used in the calculations. Direct heating by shortwave radiation absorbed by aerosols in the boundary layer is found to be an important component of the boundary layer heat budgets. The entrainment virtual temperature flux and the ratio of entrainment to surface flux found from the budget calculations are somewhat larger than expected. Advection of warm air, which is not accounted for in the budget calculations, is probably a factor in some periods but may not be significant in the full data set. For the full data set, the mean entrainment velocity found from the heat budget is 0.03±0.01 m s−1, slightly less than the mean rate of change of the boundary layer height. The mean entrainment ratio AR is 0.51±0.12 and the median is 0.43, comparable to results from some other studies in comparable conditions.
Sphericity and morphology of smoke particles from biomass burning in Brazil
Authors: Martins J. Vanderlei , Hobbs Peter V. , Weiss Ray E. , Artaxo Paulo
Published: February 2004

The degree of nonsphericity of smoke particles from biomass burning in Brazil was measured aboard the University of Washington C‐131A aircraft during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) project for several regions, types of fuel, and combustion. The nonsphericity (αo) of the particles was obtained from electrooptical light‐scattering measurements, using an aerosol asymmetry analyzer, and from scanning electron microscopy (SEM) images of the particles. The electrooptical measurements provide a measure of the nonsphericity based on the difference between the light scattering coefficient for aligned and randomly oriented particles. The SEM photographs provide information on the geometric shapes of the particles. The maximum value of αo obtained during the SCAR‐B for biomass burning in Brazil was below 13%. The degree of nonsphericity of the particles is shown to be related to the combustion efficiency, the mass absorption efficiency, and the fraction of black carbon to total particle mass. It is concluded after smoke particles from biomass burning in Brazil have been in the atmosphere for more than about 1 hour that the spherical approximation (and therefore Mie theory) is reasonably valid for estimating the physical and optical properties of the particles.
Multiphase chemistry of NO3 in the remote troposphere
Authors: Rudich Y. , Talukdar R. K. , Ravishankara A. R.
Published: February 2004

The effect of NO3 uptake at night into marine aerosol and cloud droplets on (1) sulfur oxidation, (2) NO3 ambient concentrations, and (3) NO3 atmospheric lifetime is examined using a simple chemical kinetics model. It is shown that the atmospheric lifetime toward multiphase removal of NO3 is < 10 min in presence of cloud droplets. We also show that at low mixing ratios of H2O2 (<150 pmol/mol), NO3 can enhance sulfur oxidation in clouds via a catalytic cycle with a chain length of ∼8. Finally, in presence of sea‐salt aerosol, the uptake of NO3 leads to low ambient concentrations of NO3, efficient S(IV) oxidation, and possibly, to release of halogens from the aerosol. The results of this study imply that the multiphase reaction of NO3 may be important in the remote marine boundary layer.
Solar wind influences on atmospheric electricity variables in polar regions
Authors: Michnowski Stanislaw
Published: February 2004

The measurement techniques applied in magnetospheric and ionospheric research enable detection of strong, intrinsic effects of solar wind on ionospheric electrical potential distribution and conductivity of the atmosphere. These manifestations of the solar wind interaction with the magnetosphere and ionosphere are especially evident at high latitudes. The possibility of observing there the response of the atmospheric electricity variables to solar wind has been questioned for a long time despite the fact that the atmospheric electric field and current variations at the ground are physically linked with electric potential of the ionosphere and conductivity of the lower atmosphere. The serious doubts were mainly due to the generally accepted opinion that the highly conducting ionosphere is an almost ideal equipotential electric screen that separates the weakly conductive lower atmosphere of the influence from space. This assumption could not be further upheld in view of the new findings. They have been provided for some time by ground‐based atmospheric electric field and current measurements (AEMs) with simultaneous upper atmosphere observations and by corresponding balloon measurements. Recent ground‐based AEMs in polar regions, i.e., in the near‐subauroral, auroral, and polar cap high‐latitude regions, have detected considerable influence of solar wind on the lower‐atmosphere electric variables. However, the use of atmospheric electric observations in studying solar‐terrestrial relations is still limited. The main reason is difficulty in separating various local meteorological effects, anthropogenic effects, and the effects of the global electric current circuit which affect simultaneously the measured quantities. Transmission of the electric signals through the lower atmosphere can also introduce troublesome disturbances. The paper outlines these problems and hints how the difficulties involved might be partly overcome in a feasible way. The needs and possible prospects of the polar AEMs are discussed. At selected favorable conditions, these measurements appear to be able to bring supplementary and complementary data on the temporal ionospheric potential variations over the measuring site and, in the case of suitable network of the AEM stations, also on its spatial variation. They are indispensable to providing a better understanding of current flow in atmospheric global electric current circuit. Some suggestions are given toward an extension of present models of this circuit into a model of geospace global electric current circuit.
Radon daughter disequilibria and lead systematics in the western North Atlantic
Authors: Hussain N. , Church T. M. , Véron Alain J. , Larson R. E.
Published: February 2004

Concentrations of 222Rn and 210Pb were measured in the North Atlantic troposphere in 1989 between April 12 and 28, during the Sulfide Experiment (SEX) Cruise I, and those of 222Rn, 210Pb and 210Po, between October 24 and November 9, during the SEX Cruise II. Concentrations of 210Pb and 210Po were also measured in the rain water, surface seawater, and marine microlayer collected during the SEX Cruise II Other data used and published previously include stable lead and its isotopes [Vèron et al., 1992, 1993] on parallel samples. Low 222Rn contents, of the order of 0.1 and 0.3 Bq m−3, were found in the marine air, while continental air showed nearly 10 times higher concentrations of 222Rn. These results corroborate with the air mass trajectory analyses and continental signatures of stable lead isotopes. Significant correlation is found between 222Rn and 210Pb on the aerosol, indicative of excess continental 222Rn supporting the ingrowth of 210Pb from the atmosphere, in spite of its first‐order removal by precipitation. Correlation between 210Pb and stable Pb on the aerosol and in the precipitation document the source of pollutant lead from the continental surface. Mean residence times of marine aerosol based on 210Pb is estimated to be 5.4±1.8 days during the April cruise and 19.7±1.9 days during the October cause. Corresponding deposition velocity for 210Pb is estimated to be 1.9±1.9 cm s−1, a value that suggests the dominant role of precipitation scavenging, or aerosol scavenging by larger host phases such as dust or sea salt. Excess 210Po activities are found on the aerosol relative to what would be expected based on 210Pb and the aerosol residence times. In surface seawaters, deficiencies of 210Po are observed. Mechanisms of 210Po enrichment in atmospheric aerosol may include enrichments from the organic components of marine microlayer, sea‐salt aerosol, dust, or air‐sea exchange of volatile organo‐polonium species.
Correction of aircraft pyranometer measurements for diffuse radiance and alignment errors
Authors: Boers R. , Mitchell R. M. , Krummel P. B.
Published: February 2004

A method is outlined whereby the orientation of upward pointing pyranometers on an aircraft is fully characterized. The incoming solar flux is corrected by means of a method that incorporates the fact that the flux is composed of partly diffuse and partly direct radiation. After the angle correction is found, a practical method is outlined to find the pyranometer normal with respect to the aircraft normal. It uses the heading of the aircraft and the position of the sun to adjust the pyranometer to the aircraft horizontal level. While the method is general enough to be used for any aircraft, it is applied here specifically for the National Center for Atmospheric Research C‐130 aircraft in the configuration flown during the First Aerosol Characterization Experiment (ACE 1). It was found that when the difference between the sun azimuth angle and the aircraft heading is near either 0° or 180°, special care has to be taken in interpreting the incoming flux, as it is enhanced by a few percent due to reflections off elements of the aircraft frame.
Synthetic spectra for terrestrial ultraviolet from discrete measurements
Authors: Min Qilong , Harrison Lee C.
Published: February 2004

We describe a rapid inversion algorithm for constructing synthetic terrestrial spectra from measurements made by instruments with multiple narrowband detectors. We show tests of algorithm performance over large dynamic ranges of ozone amount, solar zenith angle, surface albedo, and optical depths of aerosol and clouds. These tests demonstrate that the accuracy of synthetic spectra for total, diffuse, and direct irradiances will be limited by instrument accuracy in almost all cases. Consequently, we show uncertainty analyses for the effects of measurement and calibration errors (including filter wavelength shifts), and the impact of filter wavelength selection on the inverted spectra.
Radiation budgets and cloud radiative forcing in the Pacific warm pool during TOGA COARE
Authors: Chou Ming‐Dah , Zhao Wenzhong , Chou Shu‐Hsien
Published: February 2004

Empirical relationships between the surface radiative fluxes and satellite‐measured radiances derived for the Tropical Ocean and Global Atmosphere, Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) radiation sites are applied to compute the shortwave (solar) and longwave (thermal infrared) fluxes over the Pacific warm pool (narrowly defined as the region 10°S–10°N and 135°–175°E). Even averaged over the entire COARE Intensive Observation Period, November 1992 to February 1993, the net surface radiative heating has a large spatial variation. Regions of minimum shortwave heating, which implies large clouds and strong convection, do not necessarily coincide with regions of maximum sea surface temperature. This indicates that the large‐scale atmospheric dynamics, land‐sea distribution, and sea surface temperature all play an important role in forcing convection in the warm pool. The total radiative heating (shortwave heating minus longwave cooling) ranges from 125 to 175 W m−2. The large spatial variation of surface radiative heating in the warm pool, averaged over a period of 4 months, could have a significant impact on the sea surface temperature and oceanic and atmospheric circulations. Partitioning of the shortwave heating between the surface and the atmosphere has been estimated. For the entire warm pool during the COARE Intensive Observation Period, the mean insolation at the top of the atmosphere is 420 W m−2, and the planetary albedo is ∼30%. Two thirds (199 W m−2) of the total heating of the Earth‐atmosphere system (294 W m−2) is absorbed at the surface, and one third (95 W m−2) is absorbed in the atmosphere. The effect of clouds on the shortwave heating of the atmospheric column is only +9 W m−2, which is not much different from radiation model calculations. The Earth Radiation Budget Experiment fluxes at the top of the atmosphere in the Pacific warm pool for the period November 1986 to February 1987 are used as a proxy for the shortwave heating of the Earth‐atmosphere system.
Carbon monoxide in the upper troposphere over the western Pacific between 1993 and 1996
Authors: Matsueda Hidekazu , Inoue Hisayuki Y. , Sawa Yosuke , Tsutsumi Yukitomo , Ishii Masao
Published: February 2004

Air samples at 8.5–13 km were collected regularly using a commercial airliner between Australia and Japan, and they were measured for CO mixing ratios to obtain time series data from April 1993 to July 1996. When averaged over 12 latitudinal bands between 30°N and 30°S, two overall features emerge from these data. First, CO levels in the upper troposphere decreased in all latitudinal bands. Second, the seasonal cycle showed significant differences between the northern and southern hemispheres. In the southern hemisphere a strong maximum in the CO mixing ratio (up to around 90 ppb) was found in October‐November. The most likely source for this enhanced CO is tropical biomass burning. Methane oxidation and transport of industrial CO from the northern hemisphere were estimated as relatively minor sources during the austral spring. Air mass trajectories indicate that an extremely high CO level of ∼130 ppb observed in November 1994 between 10° and 20°S was due to enhanced biomass burning in Southeast Asia and/or northern Australia. On the other hand, air mass trajectories for the 20°‐30°S region indicate that CO‐rich air from biomass burnings over southern Africa or South America was transported across the South Indian Ocean within ∼1 week by the strong westerly winds around the subtropical jet. Thus it is concluded that a rapid horizontal transport coupled with deep convection plays an important role in the appearance of the CO spring peak in the upper troposphere over the western South Pacific.
Remote sensing of dust in deep ice at the South Pole
Authors: He Y. D. , Price P. B.
Published: February 2004

A three‐dimensional array of phototubes in deep ice at the South Pole called the Antarctic Muon and Neutrino Detector Array (AMANDA) is recording Cherenkov light pulses that serve as tracers of high‐energy neutrinos from throughout the Universe. The performance of this neutrino observatory will ultimately be constrained by the optical properties of the ice at near‐ultraviolet and visible wavelengths. At depths greater than ∼1.4 km, air bubbles are absent and light travels great distances, limited only by absorption and scattering by dust in the ice. In this paper, the Mie theory is used to predict the magnitude and wavelength dependence of the scattering and absorption coefficients and mean cosine of the scattering angle for deep South Pole ice. The results depend on the composition, size distribution, and depth profile of insoluble mineral grains, sea salt grains, liquid acid droplets, and soot particles. With most probable values for mineral grains, sea salt, acid, and soot, we fit optical data in the wavelength interval of 300–500 nm for depths of 1.6–1.83 km, taken with pulsed laser beams as light sources and with AMANDA phototubes as receivers. Our work provides quantitative evidence that aerosols deposited in snow and compacted into the ice account for the optical properties at wavelengths ∼ 300–500 nm. We finally predict optical properties of the South Pole ice at 2.5 km, a depth future AMANDA strings may reach. We expect that at 2.5 km the effective scattering, which is predominantly due to acid droplets, decreases by a factor of ∼1.5 relative to that at 1.7 km and that absorption, which is predominantly due to the mineral and soot, decreases by a factor of ∼3–5 relative to that at 1.7 km.
Open‐path, near‐infrared tunable diode laser spectrometer for atmospheric measurements of H2O
Authors: May Randy D.
Published: February 2004

A new instrument for in situ measurements of atmospheric water vapor from aircraft platforms has been developed based upon near‐infrared turnable diode laser sources operating near 1.37 μm. The spectrometer features a unique open‐path, multipass (Herriott) cell for true in situ monitoring of water vapor concentrations with precision levels exceeding those of existing Lyman α and frost point hygrometers. External sampling outside of the aircraft boundary layer minimizes ambiguities in the measured water vapor abundances. Variable spectrum acquisition rates up to 10 Hz provide fast temporal resolution free from sampling or flow rate limitations. In its current configuration the instrument operates from the right wing pod of the NASA ER‐2 research aircraft and is optimized for measurements in the upper troposphere and stratosphere (to 20 km). Measurement precision is ±0.05 ppmv in the stratosphere for a 2‐s measurement integration period. The flight‐ready instrument weight is 8.2 kg, and power consumption, exclusive of structural heaters, is 7.5 W.
Sensitivity of multiangle imaging to aerosol optical depth and to pure‐particle size distribution and composition over ocean
Authors: Kahn Ralph , Banerjee Pranab , McDonald Duncan , Diner David J.
Published: February 2004

Multiangle, multispectral remote sensing observations, such as those anticipated from the Earth Observing System (EOS) Multiangle Imaging Spectroradiometer (MISR), can significantly improve our ability to constrain aerosol properties from space. Simulations over cloud‐free, calm ocean conditions were studied for pure particles with natural ranges of optical depth, particle size, and indices of refraction. According to the theoretical simulations we can retrieve column optical depth from measurements over calm ocean for all but the darkest particles, with typical size distributions and compositions, to an uncertainty of at most 0.05 or 20%, whichever is larger, even if the particle properties are poorly known. For one common particle type, soot, constraints on the optical depth over dark ocean are very poor. The simulated measurements also allow us to distinguish spherical from nonspherical particles, to separate two to four compositional groups based on indices of refraction, and to identify three to four distinct size groups between 0.1 and 2.0 μm characteristic radius at most latitudes. The technique is most sensitive to particle microphysical properties in the “accumulation mode” sizes, where particle scattering undergoes the transition from Rayleigh to large‐particle regimes for the MISR wavelengths. On the basis of these results we expect to distinguish air masses containing different aerosol types, routinely and globally, with multiangle remote sensing data. Such data complement in situ and field data, which can provide detailed information about aerosol size and composition locally. An extension of this study to mixtures of pure particles is part of continuing work.
Transport and effective diffusion of aircraft emissions
Authors: Gerz Thomas , Dürbeck Tilman , Konopka Paul
Published: February 2004

The transport and effective diffusion of exhaust are analyzed in the wake flow of a large‐bodied aircraft which flies through a stably stratified, sheared, and turbulent atmosphere. The analysis is based on data sets from large‐eddy simulations of the wake in the atmosphere. Diffusion and dilution measures are obtained from a chemically inert species concentration. Most of the exhaust is concentrated and isolated in the two wing tip vortices (the “primary wake”). However, as the vortices sink through a stably stratified atmosphere, a baroclinic torque develops between the vortices and the surrounding flow and detrains about 10 to 30% of the exhaust mass from the vortices into the ambient air (the well mixed “secondary wake”). Consequently, the entrainment rates computed for the primary and secondary wakes differ by orders of magnitude. In the period between 1.5 and 3 min the vortices collapse into aircraft turbulence. The trapped emissions of the primary wake are now released and diffused by ambient turbulence and shear. After about 5 min the exhaust concentration has been diluted by 2 × 10−5 and 4 × 10−6 compared to the value at the nozzle exit for the primary and secondary wakes, respectively, and covers areas of about 5 × 104 m2 and 2 × 104 m2. Under flow conditions typically found at cruising heights the emissions are diluted to background concentrations within 2 and 12 hours for wind shear between 0.002 and 0.01 s−1. The spatial plume extension does not exceed the lower mesoscale range (20 km horizontally and 0.3 km vertically). Good to excellent agreement is achieved between the numerical results and in situ measured data.
Climate variability in a nonlinear atmosphere‐like dynamical system
Authors: Dethloff K. , Weisheimer A. , Rinke A. , Handorf D. , Kurgansky M. V. , Jansen W. , Maaß P. , Hupfer P.
Published: February 2004

A nonlinear atmosphere‐like dynamical system with sparse horizontal resolution including orographic and thermal forcing in a long wave and its nonlinear interaction with both a baroclinic wave and a zonal flow is constructed and has been integrated over 10.000 years. It reproduces variability in a broad range of timescales from intraseasonal to decadal and even centenary ones. Empirical orthogonal function analysis, including Fourier spectra of the principal components computations, as well as wavelet transform analysis, are applied to the results of 10.000 years' model runs with and without a seasonal cycle in an axis‐symmetric thermal forcing. The dominant mechanisms generating the long‐term climate variability are internally driven by shorttime scale instabilities and nonlinearities connected to large‐scale atmospheric processes related to orography. Mountains play an important role in triggering baroclinic instability processes. Interactions between the zonal flow, and both baroclinic and planetary waves generate a frequency spectrum with a large portion of the variance concentrated in the decadal and centenary timescale. We hypothesize that variations of the atmospheric climate might be consistent with those exhibited by our nonlinear dynamical system.
Global surface ultraviolet radiation climatology from TOMS and ERBE data
Authors: Lubin Dan , Jensen Elsa H. , Gies H. Peter
Published: February 2004

A global climatology of biologically active solar ultraviolet radiation (UVR) at the Earth's surface is derived using NASA total ozone mapping spectrometer (TOMS) measurements of column ozone abundance and NASA Earth Radiation Budget Experiment (ERBE) measurements of solar reflectance from the Earth‐atmosphere system. These two sources of satellite data are used as input to a delta‐Eddington radiative transfer model to estimate climatological cloud opacity and thereby demonstrate how surface UVR varies with geography and season. The surface UVR fluxes are spectrally resolved to enable weighted integration with any biological action spectrum. Solar elevation is shown to be more important than total column ozone abundance in governing the variability of surface UVR over large geographic areas, although some regions with pronounced local minima in ozone (30 Dobson units or more) will cause noticeable enhancements of integrated UV‐B (280–315 nm) flux relative to UV‐A (315–400 nm). The greatest variability in surface UVR within a given climate zone is induced by cloud cover. During summer, regions that show lower surface UVR fluxes relative to their surrounding regions include the eastern United States (versus the western United States), India, China (in the vicinity of the Yangtze River), and Japan (relative to the surrounding oceans). Cloud cover over tropical rainforest areas reduces the surface UVR flux relative to ocean areas at the same latitudes. The UVR cloud transmission derived from the TOMS and ERBE data correlates with an independent climatology of global cloud coverage. The UVR mapping method, based on the TOMS and ERBE data, allows a direct investigation of diurnal variability and a rigorous calculation of the biologically relevant integrated daily dose of UVR. However, it is shown that a UVR mapping method based on TOMS data alone, which is limited to only local noon satellite measurements, can make defensible estimates of the integrated daily UVR dose and the instantaneous local noon UVR surface flux.
Observations and modeling of the seasonal variation of surface ozone at Amsterdam Island: 1994–1996
Authors: Gros V. , Poisson N. , Martin D. , Kanakidou M. , Bonsang B.
Published: February 2004

Since January 1994, continuous surface O3 measurements have been carried out at Amsterdam Island (37°S, 77°E) in the southern Indian Ocean using a UV absorption analyzer. Mean O3 concentrations and standard deviations are recorded every 5 min. This paper shows and discusses the O3 time series for a 3‐year period (1994–1996). During this period, O3 shows a seasonal variation with maxima around 30 ppbv during winter (July‐September) and minima around 13 ppbv during summer (December‐February). O3 levels at Amsterdam Island are close to those observed at Cape Grim (Tasmania, 41°S, 145°E) for the period of 1991 to 1995. In order to improve the understanding of the mechanisms controlling the O3 seasonal variation at Amsterdam Island, our observations have been analyzed by using the global three‐dimensional climatological Model of the Global Universal Tracer Transport in the Atmosphere (MOGUNTIA). The model reproduces well the observed O3 mixing ratios and their seasonal cycle. The seasonal variation of stratospheric origin O3 calculated by MOGUNTIA shows a maximum of 18 ppbv in September and a minimum of 13 ppbv in April. The oxidation of continentally emitted O3 precursors during their transport to Amsterdam Island contributes to the photochemical production of O3 by up to 12 ppbv during austral summer. From this amount, 3–7 ppbv are directly linked to nonmethane hydrocarbon oxidation chemistry under relatively high NOx conditions. In particular, biomass burning emissions contribute up to 5.5 ppbv to the observed O3 levels in September. Both photochemical production by biomass burning and stratospheric influx of O3 lead to a maximum in O3 mixing ratios during late winter to early spring in agreement with the observations. Minimum O3 mixing ratios observed during austral summer are related to photochemical O3 depletion.
Global atmospheric distributions and source strengths of light hydrocarbons and tetrachloroethene
Authors: Gupta Mohan L. , Cicerone Ralph J. , Blake Donald R. , Rowland F. Sherwood , Isaksen Ivar S. A.
Published: February 2004

The atmospheric distributions of CH4, C2H6, C3H8, C2H2, and C2Cl4 and their annual chemical removal rates in steady state are determined versus latitude using a modified version of the Oslo two‐dimensional global tropospheric photochemical model. A photochemically calculated hydroxyl radical distribution, which has been validated with methylchloroform data, and seasonally varying surface measurements of the title species are used to compute their respective global annual surface source strengths and steady state lifetimes. Computed annual surface source strengths of CH4, C2H6, C3H8, C2H2, and C2Cl4 are 490, 10.4, 8.4, 3.1 Tg (1 Tg = 1012 g), and 432 kT (1 kT = 109 g), respectively. The calculated annual chemical removal rates of these compounds show distinct latitudinal distributions. Because their steady state global lifetimes are less than the model interhemispheric exchange time (about 1 year), the calculated north to south ratios of the deduced surface emission strengths of C2H6, C3H8, C2H2, and C2Cl4 probably reflect the locations of their sources. Within the limits of previously estimated industrial emissions of C2Cl4 (3–4 kT) for the southern hemisphere, our calculations indicate that about 47 kT of additional southern hemispheric source of C2Cl4 is required for 1989–1990 to attain steady state mass balance in this region. There are two possibilities for this needed source: either other industrial sources are missing, or there are unidentified natural sources of C2Cl4. So far, oceans have been suggested as a natural source. Normalization of monthly varying ratios of hemispherically averaged calculated surface mixing ratios of C2H6, C3H8, and C2H2 and their respective observed mixing ratios with respect to those for C2Cl4 indicates that the sources of these hydrocarbons are seasonal in nature. It is also shown that convective transport effectively redistributes these short‐lived species but their calculated surface source strengths are relatively independent of this transport process.
Variability of water vapor in the tropical upper troposphere as measured by the Microwave Limb Sounder on UARS
Authors: Clark H. L. , Harwood R. S. , Mote P. W. , Read W. G.
Published: February 2004

The Microwave Limb Sounder (MLS), an instrument on the Upper Atmosphere Research Satellite (UARS), measures water vapor in the upper troposphere, with best sensitivity at the standard UARS level at 215 hPa. In this paper, we analyze the MLS observations with a view to characterizing the temporal and zonal variations of upper tropospheric water vapor between 20°N and 20°S. Time series of water vapor throughout the tropics show a strong annual cycle with maximum amplitude at 20°N and 90°E. An intraseasonal cycle with a period of 30–85 days is evident over the Western Pacific at latitudes from 10°N to 20°S. The cycle is associated with eastward propagating disturbances of zonal wavenumbers 1–2, suggesting that this intraseasonal cycle is related to the Madden‐Julian oscillation.
Comparisons of airborne lidar measurements of ozone with airborne in situ measurements during the 1995 Southern Oxidants Study
Authors: Alvarez R. J. , Senff C. J. , Hardesty R. M. , Parrish D. D. , Luke W. T. , Watson T. B. , Daum P. H. , Gillani N.
Published: February 2004

As part of the 1995 Southern Oxidants Study (SOS), the National Oceanic and Atmospheric Administration airborne ozone lidar was deployed during June and July to provide detailed three‐dimensional lower tropospheric ozone distribution information in the Nashville, Tennessee, area. The airborne portion of the study included six aircraft instrumented with an assortment of in situ and remote sensors. Results of comparisons between the ozone values obtained using the lidar and the values obtained using in situ instrumentation aboard other aircraft are presented. High resolution (90 m vertical and 520 m horizontal corresponding to 8 s at a flight speed of 65 m/s) was maintained during the analysis of the lidar data which caused statistical variations in the calculated ozone concentrations from 3 ppbv (5% for typical ozone concentrations encountered during the study) at a range of 1500 m from the lidar to 11 ppbv (17%) at a range of 2500 m from the lidar. However, the lidar ozone concentration measurements compare favorably with the in situ values, with the average offset less than 10 ppbv (16%) for each flight, and the average magnitude of the offsets over all the flights at 4.3 ppbv (7%).
Atmospheric distributions of soot particles by current and future aircraft fleets and resulting radiative forcing on climate
Authors: Rahmes Timothy F. , Omar Ali H. , Wuebbles Donald J.
Published: February 2004

One of the least understood of the possible environmental effects from trace constituents emitted by aircraft is that of elemental carbon, soot. Distributions of soot in the troposphere and stratosphere based on emissions from current and projected fleets of aircraft are determined in this study; these distributions are then used in evaluating the resulting effects on radiative forcing on climate. The University of Illinois two‐dimensional chemical transport model of the troposphere and stratosphere is used in deriving the distributions of soot. Model results for 1992 aircraft emissions generally compare well with the upper tropospheric and lower stratospheric measurements for the same year. Estimates of atmospheric soot distributions due to aircraft are also derived for roughly the year 2015, when a fleet of high‐speed civil transports is assumed to be fully operational. Using the present and future distributions of atmospheric soot, direct radiative forcing estimates on climate were made with a modified version of the National Center for Atmospheric Research Community Climate Model 3 column radiation model. When compared with other forcings on climate, the determined soot distributions were found to have negligible impact.
The Galileo probe mission to Jupiter: Science overview
Authors: Young Richard E.
Published: February 2004

This paper is an introduction to and overview of the accompanying papers in this issue which give detailed results from the Galileo probe mission to Jupiter, including results from the Galileo orbiter and Earth‐based observations that are relevant for understanding the probe data and placing them in context. A summary of prior knowledge of Jupiter's atmosphere is also presented. All probe scientific investigations were successful. As anticipated, the Galileo probe results confirm some expectations about Jupiter, refute others, and raise important new questions. The Galileo probe defined the atmospheric thermal structure at the probe entry site from ∼1000 km above the 1 bar pressure level to a depth near 22 bars. During direct atmospheric sampling beginning near 0.4 bar, instruments on the probe measured composition, cloud properties, thermal structure, winds, radiative energy balance, and electrical properties of the Jovian atmosphere. Prior to reaching the Jovian atmosphere, probe instruments measured properties of the inner magnetosphere, observing regions not previously sampled by any spacecraft.
Thermal structure of Jupiter's atmosphere near the edge of a 5‐μm hot spot in the north equatorial belt
Authors: Seiff Alvin , Kirk Donn B. , Knight Tony C. D. , Young Richard E. , Mihalov John D. , Young Leslie A. , Milos Frank S. , Schubert Gerald , Blanchard Robert C. , Atkinson David
Published: February 2004

Thermal structure of the atmosphere of Jupiter was measured from 1029 km above to 133 km below the 1‐bar level during entry and descent of the Galileo probe. The data confirm the hot exosphere observed by Voyager (∼900 K at 1 nanobar). The deep atmosphere, which reached 429 K at 22 bars, was close to dry adiabatic from 6 to 16 bars within an uncertainty ∼0.1 K/km. The upper atmosphere was dominated by gravity waves from the tropopause to the exosphere. Shorter waves were fully absorbed below 300 km, while longer wave amplitudes first grew, then were damped at the higher altitudes. A remarkably deep isothermal layer was found in the stratosphere from 90 to 290 km with T ∼ 160 K. Just above the tropopause at 260 mbar, there was a second isothermal region ∼25 km deep with T ∼ 112 K. Between 10 and 1000 mbar, the data substantially agree with Voyager radio occultations. The Voyager 1 equatorial occultation was similar in detail to the present sounding through the tropopause region. The Voyager IRIS average thermal structure in the north equatorial belt (NEB) approximates a smoothed fit to the present data between 0.03 and 400 mbar. Differences are partly a result of large differences in vertical resolution but may also reflect differences between a hot spot and the average NEB. At 15 < p < 22 bars, where it was necessary to extrapolate the pressure calibration to sensor temperatures up to 118°C, the data indicate a stable layer in which stability increases with depth. Consistent with the indication of stability, regular fluctuations in probe vertical velocity imply gravity waves in this layer. At p > 4 bars, probe descent velocities derived from the data are consistently unsteady, suggesting the presence of large‐scale turbulence or gravity waves. However, there was no evidence of turbulent temperature fluctuations >0.12 K. A conspicuous pause in the rate of decrease of descent velocity between 1.1 and 1.35 bars, where a disturbance was also detected by the two radio Doppler experiments, implies strong vertical flow in the cloud seen by the probe nephelometer. At p < 0.6 bar, measured temperatures were ∼3 K warmer than the dry adiabat, possible evidence of radiative warming. This could be associated with a tenuous cloud detected by the probe nephelometer above the 0.51 bar level. For an ammonia cloud to form at this level, the required abundance is ∼0.20 × solar.
The solar reflected component in Jupiter's 5‐μm spectra from NIMS/Galileo observations
Authors: Drossart P. , Roos‐Serote M. , Encrenaz T. , Lellouch E. , Baines K. H. , Carlson R. W. , Kamp L. W. , Orton G. S. , Calcutt S. , Irwin P. , Taylor F. W. , Weir A.
Published: February 2004

A comparison between low‐flux dayside and nightside spectra of Jupiter recorded by the Galileo near‐infrared mapping spectrometer (NIMS) experiment gives the first accurate estimate of the solar reflected component at 5 μm, in the equatorial zone of Jupiter. A minimum flux level of about 0.6 μ W cm−2 sr−1/μm is found on the dayside, compared with 0.1 μ W cm−2 sr−1/μm on the nightside. These fluxes are 100–800 times lower respectively than the bright 5‐μm thermal emission in the north equatorial belt (NEB) hot spots. The day/night difference can be interpreted as a solar reflected component from a cloud, presumably the ammonia cloud, with an albedo of the order of 15%, located at a pressure level of 0.79 bar or at higher altitudes (corresponding to cloud temperature of 160 K or lower). Compared to the measurements in hot spots made at other wavelengths from ground‐based observations and from NIMS real time spectra, they imply a high cloud opacity in cold regions at atmospheric levels where the cloud optical depth in the hot spots is very low. The residual flux on the nightside arises from (1) a very small cloud transparency giving some access to deeper thermal emission or (2) as high‐resolution solid‐state imaging (SSI) images of Galileo suggest, to cloud inhomogeneities, with clearer regions of medium brightness temperatures, mixed with dark regions of much lower thermal emission. If the former have the same brightness as a typical hot spot, a filling factor of a few percent is sufficient to explain the observed flux level on the nightside cold regions.
Radio wave propagation below the Jovian ionosphere
Authors: Rinnert Klaus , Lanzerotti L. J.
Published: February 2004

The lightning and radio emission detector (LRD) instrument on board the Galileo probe measured the spectral intensity and other characteristics of radio frequency (RF) signals during descent. The measured spectra show a frequency dependent height profile with a maximum at the beginning of descent around the 1 bar level and a minimum around the 3–6 bar level [Rinnert et al., 1998]. The RF propagation is modeled assuming a source region in the water cloud level between 0 and −25 km altitude, an ionospheric reflector above 300 km, and a primarily absorbing interior below about −2000 km altitude (zero altitude is at 1 bar). This paper shows that the LRD observations can be described by an electrical discharge source at a distance of about 15,000 km; we conclude that no sources existed within the equatorial clouds inside this approximate radius around the probe. We also conclude from the propagation modeling that the measured spectral frequency dependence can be interpreted in terms of a source spectrum that is dominated by low frequencies.
The sequence of events surrounding Termination II and their implications for the cause of glacial‐interglacial CO2 changes
Authors: Broecker Wallace S. , Henderson Gideon M.
Published: February 2004

Events surrounding Termination II, as preserved in the Vostok ice core, provide a number of clues about the mechanisms controlling glacial to interglacial climate change. Antarctic temperature and the atmosphere's CO2 content increased together over a period of ∼8000 years. This increase is bounded by a drop in dust flux at its onset and by a drop in the δ18O of trapped air at its finish. A similar lag between dust flux and foraminiferal δ18O is seen in a Southern Ocean marine record, suggesting that the δ18O in air trapped in Vostok ice is a valid proxy for ice volume. The synchronous change of atmospheric CO2 and southern hemisphere temperature thus preceded the melting of the northern hemisphere ice sheets. This observation, coupled with the fact that nutrient reorganization in the North Atlantic occurs with or after the sea level rise, eliminates many scenarios proposed to explain the CO2 rise, including those which rely on sea level change, conveyor‐related nutrient redistribution, or North Atlantic cooling. Southern Ocean scenarios become the front runners, but the most popular mechanism, iron fertilization, has two problems in explaining the CO2 rise before Termination II. First, much of the dust demise occurs prior to the change in CO2, so if iron is the villain, a threshold value of its supply must be called upon above which productivity does not continue to increase. Second, the CO2 rise continues for some 4–5 kyr after the dust flux has fallen to close to zero. These problems may be solved if the increased iron supply in dust caused higher rates of nitrogen fixation during the glacial periods. In this case the residence time of oceanic nitrate of a few thousand years would enable decreasing productivity to be a global rather than a local phenomenon and would explain the slow rampup of atmospheric CO2.
Precessional forcing of productivity in the North Atlantic Ocean
Authors: Villanueva Joan , Grimalt Joan O. , Labeyrie Laurent D. , Cortijo Elsa , Vidal Laurence , Louis‐Turon Jean
Published: February 2004

The sedimentary C37alkenone fluxes in core SU90/08 situated in the North Atlantic (43°N) showed that export productivity related to some Haptophyta was driven by precession (1/23 kyr−1) in the last 260 kyr during the glacial periods. In the interglacial stages these alkenones were strongly depleted. This different behaviour is attributed to the displacement of Ekman pumping to lower latitudes and to thermocline stability decreases in the glacial periods. The alkenones were coherent and nearly phased with boreal winter insolation, suggesting a link between productivity and the strength of the westerlies. They were also coherent with the precessionally driven nutriocline minima and winter sea surface temperature in the equatorial Atlantic Ocean. The combination of the measurements at these different latitudes point to a strong coupling in the dynamics of the Hadley and Ferrel cells and to a dominant North Atlantic modulation of marine productivity to precession during the glacial stages.
Sedimentary regimes at the Macquarie Ridge Complex: Interaction of Southern Ocean circulation and plate boundary bathymetry
Authors: Schuur Catherine L. , Coffin Millard F. , Frohlich Cliff , Massell Christina G. , Karner Garry D. , Ramsay Doug , Caress David W.
Published: February 2004

Seafloor structure at the Macquarie Ridge Complex strongly influences the intensity and circulation pattern of ocean currents south of New Zealand. New marine geophysical data show heterogeneous sedimentary environments on Macquarie seafloor that reflect interaction of highly variable bathymetry with the Antarctic Circumpolar Current and north flowing Antarctic Bottom Water. Acoustic backscatter, bathymetry, and seismic reflection data collected aboard R/VRig Seismicin 1994 show five bathymetrically constrained sedimentary provinces flanking the ridge complex: (1) northwest Macquarie hemipelagic drifts, (2) current‐modified Solander submarine fan complex, (3) southwest Macquarie manganese nodule province, (4) Emerald Basin pelagic drift province, and (5) sediment‐free oceanic crust related to the 53.5°S passage in the Macquarie Ridge Complex. The late Miocene‐Pliocene opening of a 53.5°S passage in the ridge complex caused a major increase in the intensity of ocean current circulation, sediment reworking, and erosion in all sedimentary provinces.
Comparison of water mass changes in the deep tropical Atlantic derived from Cd/Ca and carbon isotope records: Implications for changing Ba composition of Deep Atlantic Water Masses
Authors: Martin Pamela A. , Lea David W.
Published: February 2004

Eastern tropical Atlantic benthic foraminiferal Ba/Ca and Cd/Ca data from core V30–49 (3093 m) reveal large inferred changes in nutrient concentrations of deep Atlantic waters during the last 250 kyr. Relative changes in North Atlantic Deep Water contribution to this site are estimated by scaling the V30–49 Cd/Ca record to values of modern end‐member water masses; these estimates agree well with the relative structure and timing of circulation changes in the eastern tropical Atlantic reconstructed from a δ13C record‐based mixing model [Raymo et al., 1997]. Temporal differences between V30–49 Cd/Ca and Ba/Ca records suggest that the Ba/Ca record reflects changes in circulation with an additional increase in the Ba composition of deep Atlantic water masses during glacial episodes, possibly resulting from increased productivity. Similarity between the δ13C and Ba/Ca records suggests that carbon isotopes in the deep glacial Atlantic also reflect productivity increases.
Topographic control of oceanic flows in deep passages and straits
Authors: Whitehead J. A.
Published: February 2004

Saddle points between neighboring deep ocean basins are the sites of unidirectional flow from one basin to the next, depending on the source of bottom water. Flow in these sites appears to be topographically controlled so the interface between the bottom water and the water above adjusts itself to permit bottom water flow from the basin that contains a source of bottom water into the next. Examples in the Atlantic include flow in the Romanche Fracture Zone, the Vema Channel, the Ceara Abyssal Plain, the Anegada‐Jungfern passage, and the Discovery Gap, but there are many more. Theoretical predictions of volume flux using a method that requires only conductivity‐temperature‐depth data archives and detailed knowledge of bathymetry near the saddle point are compared with volume flux estimates using current meters and/or geostrophic estimates for seven cases. The ratio of prediction to volume flux estimate ranges from 1.0 to 2.7. Some ocean straits that separate adjacent seas are also found to critically control bidirectional flows between basins. Theory of the influence of rotation on such critical flows is reviewed. Predictions of volume flux in eight cases are compared with ocean estimates of volume flux from traditional methods.