bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of the slow solar wind in a coronal streamer
Authors: Einaudi Giorgio , Boncinelli Paolo , Dahlburg Russell B. , Karpen Judith T.
Published: September 2002

We have investigated a magnetohydrodynamic mechanism that accounts for several fundamental properties of the slow solar wind, in particular its variability, latitudinal extent, and bulk acceleration. In view of the well‐established association between the streamer belt and the slow wind, our model begins with a simplified representation of a streamer beyond the underlying coronal helmet: a neutral sheet embedded in a plane fluid wake. This wake‐neutral sheet configuration is characterized by two parameters that vary with distance from the Sun: the ratio of the cross‐stream velocity scale to the neutral sheet width, and the ratio of the typical Alfvén velocity to the typical flow speed far from the neutral sheet. Depending on the values of these parameters, our linear theory predicts that three kinds of instability can develop when this system is perturbed: a tearing instability and two ideal fluid instabilities with different cross‐stream symmetries (varicose and sinuous). In the innermost, magnetically dominated region beyond the helmet cusp, we find that the streamer is resistively and ideally unstable, evolving from tearing‐type reconnection in the linear regime to a nonlinear varicose fluid instability. Traveling magnetic islands are formed which are similar to features recently revealed by the large‐angle spectroscopic coronagraph on the joint European Space Agency/NASA Solar and Heliospheric Observatory (SOHO) [Brueckner et al., 1995]. During this process, the center of the wake is accelerated and broadened slightly. Past the Alfvén point, where the kinetic energy exceeds the magnetic energy, the tearing mode is suppressed, but an ideal sinuous fluid mode can develop, producing additional acceleration up to typical slow wind speeds and substantial broadening of the wake. Farther from the Sun, the system becomes highly turbulent as a result of the development of ideal secondary instabilities, thus halting the acceleration and producing strong filamentation throughout the core of the wake. We discuss the implications of this model for the origin and evolution of the slow solar wind, and compare the predicted properties with current observations from SOHO.
Statistical characteristics of electromagnetic energy transfer between the magnetosphere, the ionosphere, and the thermosphere
Authors: Fujii Ryoichi , Nozawa Satonori , Buchert Stephan C. , Brekke Asgeir
Published: September 2002

We have determined, based on 28 days of European Incoherent Scatter Common Program 1 mode I data obtained between 1989 and 1991, statistical characteristics of the energy‐coupling processes between the lower thermosphere, ionosphere, and magnetosphere through an analysis of the electromagnetic energy transfer rate J·E, the Joule heating rate J·E′, and the mechanical energy transfer rate U·(J×B) at altitudes of 125, 117, 109, and 101 km. At all altitudes the input electromagnetic energy is distributed to both Joule heating and mechanical energy. The energy distributed to Joule heating is larger than that to mechanical energy, but the latter is generally not negligible. All three rates respectively have two maxima, not in the midnight region but in the dawn and dusk. The enhancements of these rates have positive correlations with the increase of geomagnetic activity represented by the Kp index. The electromagnetic energy transfer rate is greatest at 117 km, becoming smaller with decreasing altitude. It is mostly positive but can be negative. At 117 km the mechanical energy transfer rate is considerably smaller than the electromagnetic energy transfer rate, suggesting that most of the electromagnetic energy at this altitude is converted to Joule heating and a small portion of the electromagnetic energy goes to mechanical energy. At 125 km the mechanical energy transfer rate is larger than that at 117 km. On average, 65% of the input electromagnetic energy is converted to Joule heating and 35% is converted to neutral mechanical energy. At 109 and 101 km altitude the mechanical energy transfer rate becomes negative, hence the Joule heating rate is greater than the electromagnetic energy transfer rate, suggesting that not only electromagnetic energy but also mechanical energy contribute to Joule heating.
Finding gravity wave source positions using the Super Dual Auroral Radar Network
Authors: Hall G. E. , MacDougall J. W. , Cecile J.‐F. , Moorcroft D. R. , St.‐Maurice J. P.
Published: September 2002

Through the groundscatter process the Super Dual Auroral Radar (Super‐DARN) has become a powerful tool for studying F region gravity waves. However, the measurement of the gravity wave position is not direct and relies on an assumption relating ground scatter distance to reflection distance. In previous studies it has been assumed that the tilting of the ionospheric reflecting layer was negligible. Hence the gravity wave distance has been calculated as if the reflecting layer was strictly horizontal. Using virtual height data from an ionosonde and ray tracing, we show that this assumption leads to a systematic error of about 16% in the positioning of the ionospheric reflection point, with the error more than 30% on occasion. Using ray tracing, we obtained an improved relation between ionospheric reflection and ground scatter distances. With this improved distance calculation, we have found the direction and velocity for a number of gravity waves. These waves were found to be traveling equatorward, usually, with velocities between 50 and 280 m/s, in agreement with previous gravity wave observations and with the notion of filtering by the thermospheric wind. In some cases the source locations were determined by using gravity wave dispersion. These locations were found to be on the poleward side of the auroral oval during periods of weak, but observable, magnetic disturbance. Our ray‐tracing studies found that the strongest features were due to gravity waves of 3–20 km amplitude.
Computer‐aided interpretation of side‐looking sonar images from the eastern intersection of the Mid‐Atlantic Ridge with the Kane Transform
Authors: Gao Dengliang , Hurst Stephen D. , Karson Jeffrey A. , Delaney John R. , Spiess Fred N.
Published: September 2002

Approximately 600 km2of Deep‐Tow side‐looking sonar data were collected at the eastern intersection of the Mid‐Atlantic Ridge with the Kane Transform. The merged digital image mosaic provides a synoptic view of the oceanic crust exposed across major escarpments at this ridge‐transform intersection (RTI). We characterized this large acoustic data set using textural attributes extracted from the backscatter image by means of a gray level co occurrence matrix method and Fourier fractal analysis. False‐color texture attribute maps created from these methods aid in the interpretation and discrimination of surficial deposits and bedrock units on the seafloor. Principal components analysis performed on the textural attributes reduced the dimensionality of the textural feature vector and optimized imago texture discrimination. The textural attributes were calibrated to ground truth geologic data and extrapolated to regions with no ground data. The resultant texture and classification maps are consistent with the results of submersible studies but reveal more details of the seafloor geology than the conventional visual interpretation of the backscatter image and ground geologic observations. The inferred distribution of gabbros, basalts, surficial sediments, and rubble deposits shown in the classification map provides new constraints on seafloor geology at the eastern Kane RTI. Over 60% of the southern transform valley wall is covered by pelagic sediments and talus or rubble deposits, with intermittent exposures of coarse‐grained gabbroic rocks and basaltic volcanic rocks. The western rift valley wall is classified as mainly massive outcrops of gabbroic rocks. The contact between the gabbroic outcrops on the western rift valley wall and the pillow basalt terrane on the rift valley floor can be traced for several kilometers along the base of the median valley wall. This new perspective supports the interpretation of the “inside‐corner massif” as an “oceanic core complex” which is being degraded by mass wasting along tectonically active escarpments of the bounding median valley and transform valley walls.
The formation and evolution of layered structures in porous media
Authors: Schoofs Stan , Trompert Ron A. , Hansen Ulrich
Published: September 2002

Horizontally layered structures can develop in porous or partially molten environments, such as magma chambers, the early Earth's mantle, and hydrothermal systems. We have studied the generation and evolution of these horizontally layered structures in a rigid porous medium by heating a compositionally stably stratified fluid from below. Growth of a convective layer through entrainment, the formation of a vertical density interface on top of this layer and destabilization of the next layer are closely coupled. It is shown that the growth of the first convective layer stops once the thermal equilibrium is reached. Since heat (solute) transfer across the thin density interfaces becomes purely diffusive (dispersive), the separately convecting layers can persist on a compositionally dispersive timescale. While the number of layers that develop is determined by the magnitude of the thermal Rayleigh number, the final height of a newly formed layer is inversely proportional to the buoyancy ratio. Two dynamical mechanisms which lead to sudden interface disappearance determine the vertical layer scales, rather than the initial evolution toward the thermal equilibrium. Finally, the results are discussed with respect to the above mentioned geophysical scenarios.
Stress field associated with the rupture of the 1992 Landers, California, earthquake and its implications concerning the fault strength at the onset of the earthquake
Authors: Bouchon Michel , Campillo Michel , Cotton Fabrice
Published: September 2002

We investigate the space and time history of the shear stress produced on the fault during the 1992 Landers earthquake. The stress is directly calculated from the tomographic image of slip on the fault derived from near‐source strong motion data. The results obtained shed some light on why the earthquake rupture cascaded along a series of previously distinct fault segments to produce the largest earthquake in California in over 40 years. Rupture on the 30 km long northernmost segment of the fault was triggered by a large dynamic increase of the stress field, of the order of 20 to 30 MPa, produced by the rupturing of the adjacent fault segments. Such a large increase was necessary to overcome the static friction on this strand of the fault, unfavorably oriented in today's tectonic stress field. This misorientation eventually led to the arrest of rupture. The same mechanism explains why rupture broke only a small portion of the Johnson Valley fault on which the earthquake originally started, before jumping to an adjacent fault more favorably oriented. We conclude from these results that the dynamic stress field could not sustain and drive the rupture along the strongly misoriented NW‐SE strands of the preexisting fault system. Instead, the dynamic stress field produced new fractures favorably oriented in a N‐S direction and connecting parts of the old fault system.
Deformation across the rupture zone of the 1964 Alaska earthquake, 1993–1997
Authors: Savage J. C. , Svarc J. L. , Prescott W. H. , Gross W. K.
Published: September 2002

A linear array of 15 geodetic monuments was installed in 1993 across the rupture zone of the 1964 Alaska earthquake (Mw = 9.2). The array extends from Middleton Island (at the edge of the continental shelf and 80 km from the Alaska‐Aleutian trench) to north of Palmer, Alaska (380 km from the trench), in the approximate direction of Pacific‐North American plate convergence (N15.5°W). The array was surveyed in June 1993, May 1995, and June 1997. The changes between surveys are a measure of the deformation of the continental margin across the subduction zone in southern Alaska. Measured relative to the interior of the North American plate, the horizontal velocities on the outer plate margin are parallel to the direction of plate convergence (N15.5°W ) and reach a maximum (58 mm yr−1) about 150 km from the trench. Beyond about 300 km from the trench the observed horizontal velocities are small. A narrow (halfwidth 50 km) zone of significant uplift (10 mm yr−1 maximum) is observed about 300 km from the trench, coinciding roughly with the locus of maximum coseismic subsidence associated with the 1964 Alaska earthquake. Although the deformation is roughly described by the conventional model of deformation at a subduction zone (deformation due to virtual back slip on the main thrust zone at the 55 mm yr−1 plate convergence rate), a better fit is given with a 65 mm yr−1 virtual back (normal) slip rate. This higher rate is attributed to continued postseismic relaxation. The model does not explain the relatively high uplift rate and low N15.5°W velocity observed at Middleton Island. That anomalous motion is attributed to continued thrusting on postulated upward trending splays from the subduction zone beneath the island.
Wind‐forced upwelling and internal Kelvin wave generation in Mackenzie Canyon, Beaufort Sea
Authors: Carmack Eddy C. , Kulikov Evgueni A.
Published: September 2002

Low‐frequency current, temperature, and salinity variations over the shelf break in the southeastern Beaufort Sea (Mackenzie Shelf) are examined to determine the response of seasonally ice‐free waters to wind forcing and the role of submarine canyons in upwelling events and shelf circulation. Upwelling events were found to be statistically correlated to northeasterly winds, which transport surface waters offshore and draw up water from deeper layers. The response to alongshore wind stress is significantly amplified in Mackenzie Canyon, where isopycnal displacements of over 400 m are observed. Such displacements are then observed to propagate eastward at a phase velocity near 0.6 m s−1. The canyon is thus a site of intense upwelling, while the collapse of displaced isopycnals subsequent to cessation of wind forcing creates a disturbance propagating northeast along shelf as a free internal Kelvin wave.
Imaging of ocean waves on both sides of an atmospheric front by the SIR‐C/X‐SAR multifrequency synthetic aperture radar
Authors: Melsheimer Christian , Bao Mingquan , Alpers Werner
Published: September 2002

Radar images of an ocean scene containing an atmospheric front and almost range‐propagating ocean waves which were acquired by the multifrequency/multipolarization synthetic aperture radar (SAR) aboard the space shuttle Endeavour during the spaceborne imaging radar‐C/X‐band synthetic aperture radar (SIR‐C/X‐SAR) mission over the North Atlantic in 1994 are analyzed. The L‐band SAR image spectra calculated from two areas located on opposite sides of the atmospheric front are quite similar, whereas the corresponding X‐ and C‐band SAR image spectra differ significantly. It is shown that this is a consequence of the SAR imaging mechanism; at L band the SAR imaging mechanism depends weakly on the local wind field, and at X and C band it depends strongly on the local wind field. This is in agreement with earlier results obtained from the analysis of airborne multifrequency/multipolarization SAR images acquired over the North Sea during the SAR and X‐Band Ocean Nonlinearities ‐ Forschungsplattform Nordsee (SAXON‐FPN) experiment. In this investigation it was found that at X and C band the phase of the real aperture radar modulation transfer function (RAR MTF) changes by almost 90°, when in the reference system moving with the group velocity of the dominant wave, the component of the wind velocity in the direction of the wave propagation changes sign. However, at L band such a change in local wind direction affects the phase of the RAR MTF only slightly. Using this phase behavior of the RAR MTF in simulations of the SIR‐C/X‐SAR image spectra, we show that the observed differences of the X‐ and C‐band SAR image spectra measured on both sides of the atmospheric front are consistent with a change in wind speed and direction across the front. From this we conclude that for inverting X‐ or C‐band SAR image spectra into ocean wave spectra that contain ocean waves propagating near the range direction, it is quite important to have a good knowledge of the local wind field, whereas for L‐band SAR image spectra this is of minor importance.
Airborne remote sensing applications to coastal wave research
Authors: Hwang Paul A. , Walsh Edward J. , Krabill William B. , Swift Robert N. , Manizade Serdar S. , Scott John F. , Earle Marshall D.
Published: September 2002

Airborne sensors provide effective coverage of a broad region and are suitable for large‐scale experiments. In this paper, two scanning sensors that use the direct ranging technique to measure surface wave displacement are described. On a NASA P‐3 aircraft the sensors can complete one run across a 100‐km continental shelf in 17 min. A case study is presented using radar‐measured, two‐dimensional surface topography to derive wave damping due to bottom friction. The results are in good agreement with an analytical model based on a quadratic formulation of bottom shear stress. This study demonstrates that remote sensing measurements can be used for rapid characterization of surface waves on the continental shelf and in coastal regions. Examples illustrated in this paper include the derivation of wavenumber spectra and estimation of the dissipation rate of shoaling ocean swell.
A high‐resolution three‐dimensional numerical study of intermediate water formation in the Levantine Sea
Authors: Lascaratos A. , Nittis K.
Published: September 2002

We use a three‐dimensional primitive equation ocean model to study the formation of Levantine Intermediate Water (LIW), the characteristic intermediate water mass of the Mediterranean Sea. The model is forced by atmospheric monthly climatological values and incorporates a realistic air‐sea interaction scheme. The cyclonic Rhodes gyre, a permanent general circulation feature of the Levantine basin, is found to be the unique formation site under these mean climatological conditions. The convection event has a duration of 2 months (February–March), and the estimated annual mean formation rate is 1.2 Sv. Using two different horizontal resolutions, an eddy‐resolving (5.5 km) and a non‐eddy‐resolving (11 km) grid, we are able to make comparative experiments on the influence of eddy dynamics on the convection process. The results indicate that baroclinic eddies formed at the periphery of the cyclonic convection area control the formation process through horizontal advection of buoyant water from the periphery toward the center of the gyre. This mechanism reduces the extent and duration of the LIW formation event. The large number of small‐scale and mesoscale baroclinic eddies that dominate the flow field at intermediate depths, together with the Asia Minor Current, are responsible for the spreading of the newly formed LIW, mainly in zonal direction.
Numerical studies of the 4‐day oscillation in Lake Champlain
Authors: Hunkins Kenneth , Manley Thomas O. , Manley Patricia , Saylor James
Published: September 2002

The summer thermocline of Lake Champlain, which is found at depths of 20–30 m, oscillates with typical vertical amplitudes of 20–40 m and periods of ∼4 days. Fluctuations at the ends of the lake are opposite in phase and accompanied in the central lake by strong shears across the thermocline. These are basin‐wide baroclinic disturbances which are forced by wind. A numerical, one‐dimensional, two‐layer, shallow‐water model incorporating nonlinear and frictional effects in a rectangular basin forced by wind was first tested with idealized wind impulses. The results do not resemble the observed thermocline motion. However, when this simple model is forced with wind data from a nearby shore site, there is reasonable agreement between the model results and observed long‐period thermocline motions in Lake Champlain. Dispersion effects appear to be negligible here. This contrasts with other long, narrow lakes where dispersion effects are important and internal surges are followed by wave trains resembling the soliton solutions of the Korteweg‐deVries equation. A possible explanation for the different regime in Lake Champlain may be found in its unique bathymetry with sloping bottom at the ends and numerous embayments on the sides that provide traps to collect wind‐driven warm water and then release it slowly during recovery of equilibrium, preventing the formation of steep fronts and soliton wave trains.
Effect of the topographic stress on the tide‐ and wind‐induced residual currents in the Gulf of California
Authors: Marinone S. G.
Published: September 2002

Because eddy‐topography interactions (topographic stress) are difficult to resolve by numerical models, a parameterization has been proposed by Holloway (1992) based on the statistical mechanical equilibria tendencies (Neptune effect). A vertically integrated numerical model is used to study the influence of Neptune effect upon the tide‐ and wind‐induced residual circulation of the Gulf of California. The model is forced at the mouth of the gulf by the tides and at the sea surface by the wind. It is found that the parameterization induces mean currents in areas where the residuals produced by tides and winds are small. Tide‐ and wind‐induced residuals are large along the mainland coast and from the archipelago to the north, where the Neptune effect parameterization is less important. Observations give some support to this parameterization.
Effects of subsurface ocean dynamics on instability waves in the tropical Pacific
Authors: Lawrence Sean P. , Allen Myles R. , Anderson David L. T. , Llewellyn‐Jones David T.
Published: September 2002

Tropical instability waves in a primitive equation model of the tropical Pacific Ocean, forced with analyzed wind stresses updated daily, show unexpectedly close phase correspondence with observation through the latter half of 1992. This suggests that these waves are not pure instabilities developing from infinitesimal disturbances, but that their phases and phase speeds are at least partially determined by the wind stress forcing. To quantify and explain this observation, we perfomed several numerical experiments, which indicate that remotely forced Rossby waves can influence both the phase and phase speed of tropical instability waves. We suggest that a remote wind forcing determines the high model/observation phase correspondence of tropical instability waves through a relatively realistic simulation of equatorial Kelvin and Rossby wave activity.
Relationships between strong internal waves in the coastal zone and their radar and radiometric signatures
Authors: Kropfli R. A. , Ostrovski L. A. , Stanton T. P. , Skirta E. A. , Keane A. N. , Irisov V.
Published: September 2002

Strong internal waves (IW) in the form of soliton groups were observed off the Oregon coast with in situ and remote sensors, including shore‐based X band and Kaband Doppler radars and an airborne microwave radiometer operating at 37 GHz. Here we analyze the relationships between oceanic isotherm vertical displacements, internal currents, and radar backscatter cross sections, along with Doppler velocity signals at horizontal (HH) and vertical (VV) polarizations, and 37‐GHz brightness temperatures measured at intermediate incidence angles from an airship. Analysis of these observations shows that (1) the horizontal spatial structure of the IW field depends on whether it is forced by a strong (spring) tide or weak (neap) tide; (2) while both HH and VV are strongly modulated by IWs, the modulation depth of HH always exceeds that of VV, and they both increase with the IW amplitude; (3) 37‐GHz brightness temperature modulations were in phase with radar signal modulations; and (4) the phase of radar signal modulation with respect to the IW is such that the minimum radar signal intensity and lowest microwave brightness temperature lie close to the maximum of the IW thermocline depression or, equivalently, the horizontal current excursion near the surface. This last observation conflicts with the expectation that the highest backscattered signal would be found near the region of greatest surface strain rate (the surface current gradient). Existing theoretical models are briefly reviewed for interactions between the IW field and the intensity of short gravity‐capillary waves that might be responsible for this behavior.
Trace gas concentrations and emissions in downtown Nashville during the 1995 Southern Oxidants Study/Nashville intensive
Authors: Kleinman Lawrence I. , Daum Peter H. , Imre Dan G. , Cardelino Carlos , Olszyna Kenneth J. , Zika Rod G.
Published: September 2002

Continuous measurements of O3, CO, NOy, and SO2 and twice a day samples of hydrocarbons were made over a 40 day period in June and July 1995 at the downtown Nashville Southern Oxidants Study (SOS) site located on the rooftop of the Polk office building, 106 m above ground level, in the urban center. This site experiences elevated concentrations of VOCs, CO, and NOy due to local emissions from the Nashville urban area. High O3 concentrations were also observed and are discussed with reference to violations of the existing air quality standard and the much harder task of meeting the proposed 8 hour average standard. A comparison between the VOC measurements made from the Department of Energy G‐1 aircraft with those measured downtown indicates a preferential loss of reactive ingredients in the samples taken outside of the city. The concentration ratios, VOC/CO (ppbC/ppb), VOC/NOx (ppbC/ppb), and CO/NOx (ppb/ppb) were calculated to be 0.32, 2.9, and 9.3, respectively, from morning samples. These ratios agree to within 35% with estimates from Nashville emissions.
Tropical tropospheric ozone from total ozone mapping spectrometer by a modified residual method
Authors: Hudson Robert D. , Thompson Anne M.
Published: September 2002

A modified residual method for deriving time‐averaged tropical tropospheric ozone (TTO) from total ozone mapping spectrometer (TOMS) has been refined [cf Kim et al., 1996] and applied to a 2‐year period, 1991–1992. This period was selected because of the availability of a unique sonde record at tropical Atlantic and near‐Atlantic sites: Natal, Brazil; Ascension Island; and Brazzaville. The essentials of the modified‐residual method are (1) Fourier analysis to identify the latitude range for use of the method; (2) use of the wave‐like pattern to estimate stratospheric ozone column and a background tropospheric ozone column, which is defined as the amount of ozone expected in the absence of chemical pollution; (3) assumption that the wave‐one feature is tropospheric; and (4) the stratospheric and background tropospheric ozone amounts consistent with the constraint of the three station ozonesonde measurements for total tropospheric ozone. An excess ozone (pollution) signal is also derived from TOMS total ozone. Parameters derived from the modified residual method are evaluated with independent data to verify the quality of the TTO. Derived tropical stratospheric column ozone for 1991–1992 agrees very well with the corresponding UARS Microwave Limb Sounder and Stratospheric Aerosol and Gas Experiment records (to within 5 Dobson units (DU) for the latter). Background tropospheric ozone, which exhibits a seasonally in 1991–1992, has a 24±4.8 DU mean value at wave maximum (∼0° longitude) and a 15.2±6.0 DU mean value at 180° longitude. The Atlantic value is remarkably similar to the climatological 26 DU previously inferred from subtropical sondes [Kim et al., 1996; Komhyr et al., 1989]. The ∼5 DU standard deviation is the accuracy of the modified‐residual method and is the best that can be achieved given the 15‐day averaging of the sondes (1σ = 5.3 DU). The wave‐like pattern in 1991–1992 has a semiannual periodicity with zero wave amplitude in June and December.
Internal consistency tests for evaluation of measurements of anthropogenic hydrocarbons in the troposphere
Authors: Parrish D. D. , Trainer M. , Young V. , Goldan P. D. , Kuster W. C. , Jobson B. T. , Fehsenfeld F. C. , Lonneman W. A. , Zika R. D. , Farmer C. T. , Riemer D. D. , Rodgers M. O.
Published: September 2002

Measurements of tropospheric nonmethane hydrocarbons (NMHCs) made in continental North America should exhibit a common pattern determined by photochemical removal and dilution acting upon the typical North American urban emissions. We analyze 11 data sets collected in the United States in the context of this hypothesis, in most cases by analyzing the geometric mean and standard deviations of ratios of selected NMHCs. In the analysis we attribute deviations from the common pattern to plausible systematic and random experimental errors. In some cases the errors have been independently verified and the specific causes identified. Thus this common pattern provides a check for internal consistency in NMHC data sets. Specific tests are presented which should provide useful diagnostics for all data sets of anthropogenic NMHC measurements collected in the United States. Similar tests, based upon the perhaps different emission patterns of other regions, presumably could be developed. The specific tests include (1) a lower limit for ethane concentrations, (2) specific NMHCs that should be detected if any are, (3) the relatively constant mean ratios of the longer‐lived NMHCs with similar atmospheric lifetimes, (4) the constant relative patterns of families of NMHCs, and (5) limits on the ambient variability of the NMHC ratios. Many experimental problems are identified in the literature and the Southern Oxidant Study data sets. The most important conclusion of this paper is that a rigorous field intercomparison of simultaneous measurements of ambient NMHCs by different techniques and researchers is of crucial importance to the field of atmospheric chemistry. The tests presented here are suggestive of errors but are not definitive; only a field intercomparison can resolve the uncertainties.
Generation and validation of oxygenated volatile organic carbon standards for the 1995 Southern Oxidants Study Nashville Intensive
Authors: Apel E. C. , Calvert J. G. , Greenberg J. P. , Riemer D. , Zika R. , Kleindienst T. E. , Lonneman W. A. , Fung K. , Fujita E.
Published: September 2002

Two volatile organic compound (VOC) mixtures were made available and utilized for the calibration of instruments and intercomparison exercises at the Youth, Inc. (YI) site during the Southern Oxidants Study Nashville Intensive. Cylinder 1, made by Scott‐Marrin, Inc., contained 14 components (3 nonmethane hydrocarbons (NMHCs) and 11 oxygen‐containing VOCs (OVOCs)) and is referred to as OVOCl. Cylinder 2, made at the National Center for Atmospheric Research (NCAR), contained 4 components (one NMHC and three OVOCs) and is referred to as OVOC2. The mixtures were not prepared by either laboratory as primary standards but significant effort was applied to validate their concentrations by several different laboratories using several different techniques. The mixtures were prepared in high‐pressure cylinders in the parts per million by volume (ppmv) range using calibrated syringe methods. Gas Chromatographic (GC) and 2,4‐dinitrophenylhydrazine (DNPH) cartridge/high‐pressure liquid Chromatographic (HPLC) systems were utilized in the calibration of the cylinders. GC detectors included the flame ionization detector (FID) and atomic emission detector (AED). Results obtained with the GC/FID systems for OVOCs were adjusted in terms of the effective carbon number (ECN), obtained from literature values, to correct for the reduced response of the FID for compounds containing oxygen, relative to compounds containing only carbon and hydrogen. Cylinder concentrations were derived and compared based on the ECN‐adjusted FID results, the AED results, and the DNPH cartridge results. The various methods employed agreed to within approximately 15%. Both cylinders were stable (±4%) over a period of 2 years.
A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory
Authors: Erickson David J. , Seuzaret Christophe , Keene William C. , Gong Sun Ling
Published: September 2002

As part of the Reactive Chlorine Emissions Inventory, a global model of chemical processes in the marine boundary layer (MBL), Marine Aerosol and Gas Phase Interactions (MAGPI), was developed to calculate direct monthly production of HCl and ClNO2 from sea salt dechlorination on a 2.8 × 2.8 latitude‐longitude grid. Sea salt mass and size distributions and associated surface exchange fluxes were calculated using the Canadian General Circulation Model; integrated annual production of sea salt Cl− was 1785 Tg Cl yr−1. Corresponding distributions of gas‐phase HNO3, SO2, N2O5, H2O2, O3, H2SO4 and NH3 were calculated using different global chemical transport models in which sea salt reactions were not considered. A chemical scheme was developed to estimate the monthly mean steady‐state phase partitioning of product and reactant species at each grid point. Average annual gridded fluxes of HCl and ClNO2 varied spatially from 1 to 300 mg Cl m−2 yr−1 and from 1 to 8 mg Cl m−2 yr−1, respectively. Maxima occurred in polluted coastal regions of the North Atlantic, the western North Pacific and the Mediterranean where up to 20% of the total Cl and 80% of the sub‐micron Cl volatilized. In remote oceanic regions, available acidity was insufficient to titrate all sea salt alkalinity, thus, significant HCl was not produced via acid displacement. However, in these regions virtually all HNO3 was scavenged by sea salt. The integrated annual global fluxes for HCl and ClNO2 were 7.6 Tg Cl yr−1 and 0.06 Tg Cl yr−1, respectively; virtually all in the Northern Hemisphere. Largest HCl and ClNO2 fluxes occur in northern hemisphere winter due to high sea salt loading and elevated HNO3, SO2 and N2O5 concentrations. 70% of the HCl dechlorination occurs on particles between 0.75 μm and 4 μm radius; ClNO2 volatilized from slightly larger particles. The aerosol pH of each particle size bin equilibrates towards the same value once the alkalinity has been titrated.
Comparison between DC‐8 and ER‐2 species measurements in the tropical middle troposphere: NO, NOy, O3, CO2, CH4, and N2O
Authors: Weinheimer A. J. , Montzka D. D. , Campos T. L. , Walega J. G. , Ridley B. A. , Donnelly S. G. , Keim E. R. , Del Negro L. A. , Proffitt M. H. , Margitan J. J. , Boering K. A. , Andrews A. E. , Daube B. C. , Wofsy S. C. , Anderson B. E. , Collins J. E. , Sachse G. W. , Vay S. A. , Elkins J. W. , Wamsley P. R. , Atlas E. L. , Flocke F. , Schauffler S. , Webster C. R. , May R. D. , Loewenstein M. , Podolske J. R. , Bui T. P. , Chan K. R. , Bowen S. W. , Schoeberl M. R. , Lait L. R. , Newman P. A.
Published: September 2002

We compare measurements of six species taken aboard NASA DC‐8 and ER‐2 aircraft during two flight legs in the tropical middle troposphere near Hawaii. NO, NOy, O3, CH4, and N2O measurements agree to within the limits set by the known systematic errors. For CO2, which can be measured with better relative precision than the other five species, differences in measured values from the two platforms are slightly larger than expected if the air masses sampled by the two aircraft were indeed similar in CO2 composition to better than 0.08%.
Measurements of PAN, PPN, and MPAN made during the 1994 and 1995 Nashville Intensives of the Southern Oxidant Study: Implications for regional ozone production from biogenic hydrocarbons
Authors: Roberts James M. , Williams Jonathan , Baumann Karsten , Buhr Martin P. , Goldan Paul D. , Holloway John , Hübler Gerhard , Kuster William C. , McKeen Stuart A. , Ryerson Thomas B. , Trainer Michael , Williams Eric J. , Fehsenfeld Fredrick C. , Bertman Steven B. , Nouaime George , Seaver Craig , Grodzinsky Gil , Rodgers Michael , Young Valerie L.
Published: September 2002

Isoprene and a variety of other reactive hydrocarbons are released in large quantities by vegetation in forested regions and are thought to participate in the NOx‐catalyzed production of ozone, a serious air quality problem in North America and Europe [National Research Council, 1991]. The determination of the fraction of O3 formed from anthropogenic NOx and biogenic hydrocarbons (BHC) is a crucial step in the formulation of effective control strategies. Peroxymethacrylic nitric anhydride (MPAN, CH2C(CH3)C(O)OONO2) is formed almost entirely from the atmospheric oxidation of isoprene in the presence of NOx and is an excellent indicator of recent ozone production from isoprene and therefore biogenic hydrocarbons. Measurements are presented here of MPAN, peroxyacetic nitric anhydride (PAN, CH3C(O)OONO2), peroxypropionic nitric anhydride (PPN, CH3CH2C(O)OONO2) and ozone from separate data sets acquired during the 1994 and 1995 Nashville intensive studies of the Southern Oxidant Study. It was found that PAN, a general product of HC‐NOx photochemistry, could be well represented as a simple linear combination of contributions from BHC and anthropogenic hydrocarbon (AHC) chemistries as indicated by MPAN and PPN, respectively. The PAN:MPAN ratios found to be characteristic of BHC‐dominated chemistry ranged from 6 to 10. The PAN:PPN ratios found to be characteristic of AHC‐dominated chemistry ranged from 5.8 to 7.4. These BHC and AHC attributions were used to estimate the contributions of anthropogenic and biogenic hydrocarbons to regional tropospheric ozone production, and substantial BHC‐O3 (50–60 ppbv) was estimated in cases where high NOx from power plants was present in areas of high BHC emission. This estimation method provides direct evidence of significant photochemical ozone production from the oxidation of biogenic hydrocarbons in the presence of NOx.
An overview of the airborne activities during the Southern Oxidants Study (SOS) 1995 Nashville/Middle Tennessee Ozone Study
Authors: Hübler G. , Alvarez R. , Daum P. , Dennis R. , Gillani N. , Kleinman L. , Luke W. , Meagher J. , Rider D. , Trainer M. , Valente R.
Published: September 2002

The cause and extent of elevated ozone levels which are often found during summer in the southeastern United States were the focus of the intensive Southern Oxidants Study (SOS) 1995 Nashville/Middle Tennessee Ozone Study. Six aircraft were extensively instrumented and were deployed in concert during the summer of 1995 from the Nashville Metropolitan Airport. This overview describes the capabilities of the deployed assets, and how their use was managed. Results from the measurements on individual aircraft and their interpretation are contained in the individual papers that follow.
Relationships between regional ozone pollution and emissions of nitrogen oxides in the eastern United States
Authors: Kasibhatla P. , Chameides W. L. , Saylor R. D. , Olerud D.
Published: September 2002

This study examines the relationships between regional ozone (O3) pollution and emissions of nitrogen oxides (NOx) in the eastern United States during summer. Using measurements from rural sites during the summer of 1995, three 4‐day time periods are identified during which significant enhancements of surface O3 occurred on spatial scales ranging from 0.5 to 1.8 million km2. Each of these episodes was characterized by relatively stagnant meteorological conditions conducive to the photochemical formation and accumulation of O3 in the boundary layer. The surface ozone accumulation efficiency (SOAE), a parameter which relates the O3 accumulation to the NOx emission density in a given region, is estimated to range from 1 to 2 ppbv O3 kg−1 N km−2 in the eastern United States during summer. This result is discussed in the context of regional NOx‐based O3 control strategies. In addition, the net ozone production efficiency (OPE) is estimated to range from 2 to 3 ppbv O3 ppbv−1 NOx in this region.
Sulfate and nitrate firn concentrations on the Greenland ice sheet: 1. Large‐scale geographical deposition changes
Authors: Fischer Hubertus , Wagenbach Dietmar , Kipfstuhl Joseph
Published: September 2002

Eleven shallow firn cores and three deeper ice cores drilled along a meridional line over the Greenland ice sheet from 71° to 80°N have been specifically evaluated for their nitrate and sulfate concentrations. Recent average annual sulfate concentrations strongly increase from around 100 ppb in central Greenland to 175 ppb in the northeastern part of the ice sheet, while nitrate levels remain essentially constant at around 150 ppb. Recent sulfate firn concentrations exhibit a distinct seasonal cycle with maximum concentrations in spring. The seasonal amplitude is 5 times higher over the northern ice sheet compared to the central Greenland region. The corresponding nitrate cycle is only weakly defined with a tendency toward higher summer concentrations. Application of a macroscopic deposition model, developed for irreversibly deposited sulfate as well as reversibly deposited nitrate, shows that geographical variations in sulfate and nitrate firn concentrations can be explained largely by the spatial change in snow accumulation rate over the entire interior of the Greenland ice sheet both for recent and for preindustrial conditions. The recent wet sulfate deposition flux has increased by a factor of 2 since preindustrial times, while the dry deposition flux has risen by a factor of 5. In the case of nitrate, depositon fluxes have increased by a factor of 2 both for wet and for dry deposition. On the basis of the empirically determined model parameters, up to 50% of the initially deposited nitrate appears to be remobilized from the firn column.
Relationship between sea surface temperature, vertical dynamics, and the vertical distribution of atmospheric water vapor inferred from TOVS observations
Authors: Chaboureau Jean‐Pierre , Chédin Alain , Scott Noëlle A.
Published: September 2002

With the aim of better understanding the respective role of sea surface temperature (SST) and vertical dynamics on the vertical distribution of atmospheric water vapor, particularly in the tropics, global scale observations from NOAA 10, covering a 31−month period, have been processed using the improved initialization inversion ((3I) [Chédin and Scott, 1984]) retrieval method and interpreted in terms of tropospheric layered water vapor contents. The method of analysis uses the power law, which expresses the specific humidity q at pressure p as a function of their values at the surface, q0 and p0; q = q0(p/p0)λ. This description is applied independently to three layers giving three values of λ: λ1 for surface‐700 hPa, λ2 for 700–500 hPa, and λ3 for 500–300 hPa. It is shown that λ2 is a good indicator of the large‐scale vertical dynamics and gives results equivalent to those obtained using the vertical velocity at 500 hPa issued from a model. Consequently, the role of enhanced upward motion with increased SST for the “super greenhouse effect” situations is confirmed as well as the contribution of externally forced subsidence on the suppression of the deep convection for cases where SSTs exceed about 303 K. In addition, the influence of SST on the vertical distribution of water vapor is analyzed together with the large‐scale vertical dynamics contribution. The results show that the rate of change of water vapor content in the 700‐ to 500‐hPa and 500‐ to 300‐hPa layers with respect to SST increases with decreasing rate of change of λ2 with respect to SST, that is, with increasing rate of change of upward vertical dynamics with respect to SST.
An analysis of the global water cycle of present and doubled CO2 climates simulated by the CSIRO general circulation model
Authors: Watterson I. G.
Published: September 2002

The processes controlling the global distribution of precipitation and atmospheric water vapor are studied using the Commonwealth Scientific and Industrial Research Organisation Mark 2 general circulation model (GCM), which includes a semi‐Lagrangian transport of water vapor. The precipitation rate, P, water column, W, and horizontal moisture fluxes, averaged over the months of January and July from a 30‐year present climate simulation compare well with observations. Influences on these quantities are discussed. The convergence of the moisture flux is described here as a dynamic influence on both P and W. The (monthly) mean‐flow component of the flux converges on the tropical maxima of both fields. The transient‐flow flux is important in the middle and high latitudes. A two‐dimensional eddy‐flux theory is proposed for this component, in which the flux is influenced by the gradient of W and by the variance of the winds at 800 hPa. Possible changes in the hydrological cycle due to climate change are explored by examining the equilibrium doubled CO2 climate simulated by the GCM. The hydrological cycle is enhanced by the thermodynamic effect, with the annual mean precipitation and evaporation increased by 10% and the mean water column increased by 29%. Dynamic effects are also important. Changes in both the mean winds and the water column are important to the mean‐flow flux convergence. Midlatitude transient fluxes typically increase, but the effect of the increased gradients of W is partially countered by decreases in wind variance. It is concluded that the causes of any future precipitation changes are likely to be complex.
Modeling of rainfall time series using two‐state renewal processes and multifractals
Authors: Schmitt F. , Vannitsem S. , Barbosa A.
Published: September 2002

The high variability of rainfall fields comes from (1) the occurrence of wet and dry events and (2) from the intermittency of precipitation intensities. To model these two aspects for spatial variability,Over and Gupta[1996] have proposed a lognormal cascade model with an atom at zero, which corresponds to combine in one model two independent cascade models, the β and the lognormal multifractal models. In the present work, we test this approach for time variability, using a high‐resolution rainfall time series. We built a continuous version of the discrete β model and investigate some of its dynamical properties. We show that the β model cannot fit the probability density for the duration of the wet state. In order to model the succession of wet and dry periods we therefore use a two‐state (or alternate) renewal process based on appropriate fits of the empirical densities. The synthetic series obtained this way reproduces the scaling of the original support. The intensity of the rainfall events is then modeled using the universal multifractal model, generalizing the lognormal model. We show that the fractal support of the rainfall events must be taken into account to retrieve the parameters of this model. This combination of two different models allows to closely reproduce the high variability at all scales and long‐range correlations of precipitation time series, as well as the dynamical properties of the succession of wet and dry events. Simulations of the high‐resolution rainfall field are then performed displaying the salient features of the original time series.