bluuebunny/crossref_metadata_2025 huggingface dataset:
An airborne, real aperture radar study of the Chesapeake Bay outflow plume
Authors: Sletten Mark A. , Marmorino George O. , Donato Tim F. , McLaughlin David J. , Twarog Elizabeth
Published: September 2002

An airborne, real aperture radar (RAR) has been used to study the fronts associated with the Chesapeake Bay outflow plume during spring outflow conditions. The RAR produced images of the ocean surface with a range resolution of 10 m, an azimuthal resolution of approximately 30 m, and an image size of 2.5 km × 24 km. Two sampling strategies were utilized: one to synoptically map the entire mouth of the Chesapeake Bay at roughly hourly intervals; and a second to capture the rapid evolution of particular features. In addition, flight times were chosen such that over the course of the entire experiment, data were collected over all phases of the semidiurnal tidal cycle. Three distinct frontal signatures were observed in the imagery. A primary front extended from inside the estuary along the Chesapeake Channel to an anticyclonic turning region east of Cape Henry, and then extended southward along the coast toward Cape Hatteras. This is the classic expression of the plume front, inertial turning region, and coastal jet. A second front with a north‐south orientation was observed approximately 20 km east of the bay mouth. This secondary front appears to mark the residual offshore density gradient. A third front was identified east and south of Cape Henry, within 2 km of the coast. This front appears to mark the inshore edge of the plume and has not been documented previously. Time sequences of the imagery indicate that when moving in a clockwise sense around the primary front, the frontal translation speed varies systematically from 20 cm/s in the northern section to 50 cm/s in the south. The position of the primary front and the locations and trajectories of small‐scale frontal cusps suggest that bathymetry may be both a significant determinant of the front location as well as a source of along‐front variability. These observations are possible due to the airborne RAR's ability to collect high‐frame rate image sequences, a capability that is not shared by present space‐based radar systems.
Observations of high‐frequency internal waves in the Coastal Ocean Dynamics Region
Authors: Pringle James M.
Published: September 2002

Current meter data from the second Coastal Ocean Dynamics Experiment (CODE II) for July 1982 are analyzed for internal waves in the 6 to 40 cycles per day (cpd) frequency band. It is found that the wave field is anisotropic and that the current ellipses are oriented in approximately the cross‐isobath direction. The squares of the ratio of the major to minor axes of the current ellipses (the “ellipticity”) are consistent with a continuum of internal waves propagating onshore but are not consistent with a single wave propagating onshore. The reduction of internal wave energy across the shelf is consistent with propagation from the deep ocean or shelf break, as is the correlation between vertical velocities and velocities parallel to the minor axis. However, there is evidence for the generation of additional internal wave energy on the shelf in the evolution of the current ellipses across the shelf and in the bluing of the internal wave spectra across the shelf. Internal wave energy levels are elevated by a factor of 1.5 to 5 above Garrett and Munk [1972] levels at the moorings on the 130 and 365 m isobaths. The first vertical mode dominates at the 130 m isobath, the only mooring for which the vertical modal analysis was done.
Methyl iodide distribution in the ocean and fluxes to the atmosphere
Authors: Moore Robert M. , Groszko Wayne
Published: September 2002

Methyl iodide concentrations have been measured in air samples, surface, and subsurface waters of the NW Atlantic, NE Atlantic, and Pacific Oceans. They are shown to be substantially oversaturated in all surface waters. Our best estimate of methyl iodide flux from the ocean to the atmosphere is 0.9 − 2.5 × 109 mol yr1, but further substantial and unquantifiable uncertainities exist because of the limited spatial and temporal data on which this flux estimate is based. Pronounced subsurface maxima in methyl iodide concentrations seen in the Pacific Ocean and the Sargasso Sea are thought to result from production and accumulation of the gas in the relatively poorly ventilated waters beneath the surface mixed layer. Our calculations suggest that latitudinal variations in methyl iodide concentrations in the Pacific Ocean are partly controlled by temperature‐dependent chemical loss.
Anomalous warming in the Indian Ocean coincident with El Niño
Authors: Chambers D. P. , Tapley B. D. , Stewart R. H.
Published: September 2002

The TOPEX/POSEIDON altimeter has provided further evidence that interannual warming occurs in the Indian Ocean with a frequency similar to that of El Niño in the Pacific and has yielded important clues to the dynamics driving the warming. The signal is especially strong during the 1997 El Niño. The altimeter observes long waves which move westward from the southeastern Indian Ocean at about the same time as westwardly wind anomalies appear in the east‐central portion of the basin. The sea level peaks in the southwestern Indian Ocean and causes a sea level variation signal that is a near mirror image of El Niño in the eastern Pacific. Sea surface temperature data also show a similar correlation. An analysis of the altimeter data indicates significant variability in the Indian Ocean during the 1994 and 1997 El Niño events at the first and second baroclinic Rossby wave modes. Sea surface temperature and wind data suggest that the Indian Ocean warming has occurred during several previous El Niño events, particularly during the large events of 1982 and 1987. Based on these observations, it is suggested that the warming begins with wind‐forced Rossby waves in the southeastern Indian Ocean associated with the Southern Oscillation, similar to the forcing of Kelvin waves which precede El Niño in the Pacific.
Lagrangian particle transport in the Indian Ocean: A model study
Authors: Joseph Binson , Swathi P. S.
Published: September 2002

Lagrangian transport of “passive” particles in the Indian Ocean is diagnosed using the global Modular Ocean Model 2 of the Geophysical Fluid Dynamics Laboratory. First, the model is forced by time‐varying monthly mean climatological wind stresses, and a particle cloud released in the Indonesian throughflow (ITF) region is tracked for one year. In the next experiment, two separate clouds, one released in the ITF region and the other in the Arabian Sea region, are followed for three years by forcing the model with weekly varying NMC9195 wind stress data. In both the experiments, short‐term Liapounov exponents indicate that particle trajectories are chaotic, with typical e‐folding stretching time about 125 days. The zonal dispersion of the ITF cloud is faster than the shear‐flow‐dominated transport process in the first experiment, as well as for the first two years in the second experiment. However, at later times the zonal dispersion rate is subquadratic in the latter experiment. For the Arabian Sea cloud, zonal dispersion is diffusive when the whole time range of three years is considered. However, for the initial six months, it is shear‐flow‐dominated and thereafter becomes even subdiffusive. The meridional dispersion rate is roughly superdiffusive, in both experiments, when the whole time range is taken into account. Our estimates of correlation dimension, a lower bound on the fractal dimension, suggest that none of the particle clouds reach the area‐filling limit in the time range of the model integration. We also compare the present results with those from earlier studies of chaotic stirring in the oceans.
NSCAT normalized radar backscattering coefficient biases using homogenous land targets
Authors: Zec Josko , Long David G. , Jones W. Linwood
Published: September 2002

The NASA scatterometer (NSCAT) is a spaceborne radar sensor designed to measure the normalized radar backscattering coefficient σ0 of the Earth's surface. Over the ocean, backscatter measurements are used to infer surface wind vectors. Wind retrieval is based on a statistical relationship between short‐ocean wave roughness (that causes the backscatter) and the surface wind speed and direction. For NSCAT geometry, multiple antennas are used to provide backscatter measurements at several azimuth directions to resolve wind direction ambiguities. To achieve the desired wind vector accuracy, these antenna beams must be calibrated within a few tenths of a decibel. A simple relative‐calibration method is applied to the NASA scatterometer backscatter from homogenous, isotropic, large‐area targets. These targets exhibit both azimuth and time invariant radar response. A simple polynomial model for incidence angle dependence of σ0 is used, and the mean radar response from all antenna beams is taken as the reference. Corrections (σ0 biases) are calculated as differences (in log space) between measurements from particular beam and the reference. This simple model is applied to data from the Amazon rain forest and the Siberian plain. These areas are tested for temporal stability within the calibration period (several weeks). High‐resolution masks are applied to extract suitable calibration data sets. Calculated corrections for each antenna beam are added to NSCAT σ0 measurements as a function of incidence angle. The magnitudes of corrections show the necessity of on‐orbit calibration.
Effects of viscosity and elasticity on the nonlinear resonance of internal waves
Authors: Hill D. F. , Foda M. A.
Published: September 2002

It is well established that waves propagating through a viscoelastic medium experience both attenuation and frequency modulation. For the case of infinitesimal waves, linear theory may be utilized to solve the boundary value problem for either a complex wavenumber or a complex frequency, the imaginary components corresponding to exponential decay in space and time, respectively. Recent contributions to the body of literature on weakly nonlinear resonant interactions have demonstrated that, in an inviscid two‐layer system, internal waves can be parametrically excited by surface waves. Exponential growth, rather than decay, of the internal waves has been predicted and conclusively verified in the laboratory. The two mechanisms are considered together in the current paper. By considering a two‐layer system possessing both weak nonlinearity and viscoelasticity, the competition between the two effects is demonstrated. It is found that viscoelasticity reduces the exponential growth rate of the internal waves. Sufficiently large viscoelasticity is found to completely suppress the destabilizing effects of the nonlinearity. General results as well as results for conditions characteristic of an estuarine environment are presented.
Arctic sea ice drift from wavelet analysis of NSCAT and special sensor microwave imager data
Authors: Liu Antony K. , Zhao Yunhe , Wu Sunny Y.
Published: September 2002

Wavelet analysis of NASA scatterometer (NSCAT) backscatter and DMSP special sensor microwave imager (SSM/I) radiance data can be used to obtain daily sea ice drift information for the Arctic region. This technique provides improved spatial coverage over the existing array of Arctic Ocean buoys and better temporal resolution over techniques utilizing data from satellite synthetic aperture radars. Comparisons with ice motion derived from ocean buoys give good quantitative agreement. Both comparison results from NSCAT and SSM/I are compatible, and the results from NSCAT can definitely complement that from SSM/I when there are cloud or surface effects. Then, three sea ice drift daily results from NSCAT, SSM/I, and buoy data can be merged as a composite map by some data fusion techniques. The ice flow streamlines are highly correlated with surface air pressure contours. Examples of derived ice drift maps in December 1996 illustrate large‐scale circulation reversals over a period of 4 days. These calibrated/validated results indicate that NSCAT, SSM/I merged daily ice motions are suitably accurate to identify and closely locate sea ice processes and to improve our current knowledge of sea ice drift and related processes through the data assimilation of ocean‐ice numerical model.
Mesoscale and synoptic‐scale effects on the validation of NSCAT winds by means of data buoy reports
Authors: Austin Shermane , Pierson Willard J.
Published: September 2002

Consecutive 10‐min reports by National Data Buoy Center buoys are analyzed so as to show the behavior of the anemometer for light and variable low wind speeds and for the mesoscale effects that remain in the 10‐min averages for higher wind speeds. It is shown that the major contribution to the scatter in the comparison of NASA Scatterometer winds to data buoy winds is the result of the properties of the data buoy measurements. Calms, light winds and winds over 20 m s−1 still need additional study and further efforts at validation.
Satellite scatterometer studies of ocean surface stress and drag coefficients using a direct model
Authors: Weissman David E. , Graber Hans C.
Published: September 2002

This study evaluates the accuracy of estimates of sea surface friction velocity derived using a new model function that operates directly on the NASA scatterometer normalized radar cross‐section (NRCS) measurements. These NRCS data are first collocated with numerous National Data Busy Center buoys (within ±0.5° latitude and longitude) and are then processed using the new Weissman et al. [1994] Frontal Air‐Sea Interaction Experiment model to produce 25‐km by 25‐km spatial averages of the friction velocity for several contiguous areas within this region. One method of validation of these estimates involves comparisons with friction velocity estimates derived from data collected by the buoys using bulk methods. This was conducted at numerous locations; the Atlantic and Pacific coastlines and within the Gulf of Mexico. It is found that there is excellent agreement between these two different measurements. In addition, this approach enabled the derivation of estimates of the surface drag coefficient, with the use of the buoy winds, neutral stability winds at 10‐m elevation, U10N, and the scatterometer surface friction velocity u*. This permitted the study of the dependence of this drag coefficient CD on wind speed in these different regions on spatial scales, which had not previously been possible. Comparisons are made to previously published models of CD that only depend on wind speed, with generally good agreement at winds over 6 m−1 s, but with significant regional differences at winds less than 6 m−1 s.
On the influence of submarine ridges on translation and stability of Agulhas rings
Authors: Beismann Jens‐Olaf , Käse Rolf H. , Lutjeharms Johann R. E.
Published: September 2002

A series of experiments with a quasi‐geostrophic model have been carried out to investigate the influence of topographic obstacles on the translatory movement of Agulhas rings. The rings were initialized as Gaussian‐shaped anomalies in the stream function field of a two‐layer ocean at rest. Bottom topography consisted of a meridional ridge of constant height in the middle of the quadratic model domain. The vertical ring structure, the initial ring position, and the height of the ridge were varied. The general northwestward movement of the model eddies has been shown to be modified toward a more equatorward direction by encountering the upslope of the ridge. Sufficient topographic heights and strong slopes can even block the eddies and force them toward a pure meridional movement. During their translation the eddies lose their vertical coherence. After about 150 days the eddy can only be detected by the surface signal, while the lower layer eddy is dispersed by the radiation of Rossby waves. The passage of “young” (regarding the time between their initialization and their contact with the ridge) and energetic eddies is accompanied by the observation of along‐slope currents of significant strength. These may be due to the rectification of radiated Rossby waves at the topographic slope. Only eddies with a significant dynamic signal in the lower layer are influenced by the bottom topography. Strong, shallow eddies over deep lower layers can cross the ridge without strong modification of their translatory movement.
A case study of gas‐to‐particle conversion in an eastern Canadian forest
Authors: Leaitch W. R. , Bottenheim J. W. , Biesenthal T. A. , Li S.‐M. , Liu P. S. K. , Asalian K. , Dryfhout‐Clark H. , Hopper F. , Brechtel F.
Published: September 2002

Aerosol and trace gas measurements were made at Kejimkujik National Park, Nova Scotia, Canada, during the summer of 1996. A case study from July 7–8 provides evidence of nucleation and condensation of products related to the oxidation of different biogenic emissions. Particles from 5 nm to 50 nm in diameter evolved during the afternoon and early evening associated with variations in isoprene. Late in the evening the α‐ and β‐pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a sharp increase in RO2H/H2O2 that persisted until about 0100 LT. The initial increases in the pinenes and aerosols were strong and influenced by changes in winds. After 2200 LT, and into the early morning, the winds were relatively steady, and the α‐ and β‐pinene mixing ratios continually decreased. The decay of α‐pinene is explained through reaction with O3. However, the addition of OH radicals from the reaction of terpenes with O3 is necessary to explain the observed rate of decay of β‐pinene. During the same time, the aerosol volume increased with the decrease in α‐ and β‐pinene. The volume increase was distributed 40∶60 between particles in a mode centered at 80–90 nm and particles > 150 nm. The fine particle mass concentrations of the measured inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, and calcium) and organic ions (oxalate, formate, acetate, pyruvate, propionate) account for 25–30% of the total aerosol volume during the period (2.7 μm3 cm−3) indicating that the aerosol volume increase was due to unidentified species. Assuming that the increase in the aerosol was the result of products from the oxidation of α‐ and β‐pinene, an aerosol mass yield of 13% is estimated. The concentrations of cloud condensation nuclei active at 0.2% supersaturation were enhanced by the appearance of the 80–90 nm mode pointing to at least some of these forest‐generated particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.
Mesospheric Na layer at 40°N: Modeling and observations
Authors: Plane John M. C. , Gardner Chester S. , Yu Jirong , She C. Y. , Garcia Rolando R. , Pumphrey Hugh C.
Published: September 2002

A complete monthly record of the annual variation of Na and temperature in the upper mesosphere has been obtained from 3 years of nighttime lidar observations at two midlatitude sites, Urbana‐Champaign, Illinois (40°N), and Fort Collins, Colorado (41°N). The Na density exhibits a strong annual variation at all heights between 81 and 107 km, with the column abundance of the layer peaking in early winter and then decreasing by nearly a factor of 4 to a midsummer minimum. There are also significant semiannual components to the variations in the centroid height and thickness of the layer. The nighttime temperature profile between 81 and 105 km exhibits a high winter mesopause at about 101 km and a summer mesopause at about 85 km. During spring and autumn, the mesopause oscillates apparently randomly between these states. A seasonal model of the Na layer was then constructed incorporating recent laboratory studies of the pertinent neutral and ionic reactions of the metal. The background atmospheric composition was provided from three off‐line models, as well as from UARS/Microwave Limb Sounder satellite measurements of H2O. With a small number of permitted adjustable parameters, the model is able to reproduce many observed features of the Na layer remarkably well, including the monthly variation in column abundance and layer shape. The biggest discrepancy is during midsummer, when the modeled layer is displaced 2–3 km above that observed, although a factor contributing to this is that the lidar observing period during summer was relatively short and the effect of the diurnal tide could have been incompletely sampled. Both the observations and the model show that Na density and temperature are highly correlated below 96 km (correlation coefficient equal to 0.8–0.95), mostly as a result of the influence of odd oxygen/hydrogen chemistry on the partitioning of sodium between atomic Na and its principal reservoir species, NaHCO3. Above 96 km, a weak negative correlation (−0.2) is explained by the dominance of ion‐molecule chemistry. Finally, it was shown that if the eddy diffusion coefficient in the middle mesosphere is significantly smaller or if the global meteoric influx is much larger than the values used in the present model, then processes for permanently removing gas‐phase Na species in the mesosphere, such as polymerisation and deposition onto dust particles, will need to be included.
Nitric acid and ammonia at a rural northeastern U.S. site
Authors: Lefer B. L. , Talbot R. W. , Munger J. W.
Published: September 2002

Hourly mixing ratios of HNO3, NH3 and various other trace gas and aerosol species were determined at Harvard Forest in central Massachusetts between 1991 and 1995 in order (1) to ascertain the representativeness of this composite data set, (2) to identify general seasonal and diurnal trends in the mixing ratios of HNO3 and NH3 at this site, (3) to establish how these mixing ratios depend on characteristic meteorological parameters such as wind direction and temperature, and (4) to examine HNO3 relative to the other reactive nitrogen species measured. Harvard Forest receives air masses from both urban and rural source regions resulting in mean midday HNO3 mixing ratios 4 times higher when surface winds were from the SW (∼ 2000 parts per trillion by volume (pptv)) as opposed to the NW (∼ 500 pptv) wind sector. The HNO3 diel cycle provides evidence of entrainment of HNO3 from aloft as the nocturnal inversion breaks down. Gaseous NH3 mixing ratios are typically 200–300 pptv and on average exhibit little diel variability. High levels of atmospheric sulfate consistently suppress NH3 concentrations below the predicted NH3 compensation point of the canopy, even during periods when the sum of NH3 and NH4+ (NHX) mixing ratios are quite high. The aerosol SO42− regulation of NHX partitioning changes as a function of temperature. At the same SO42− mixing ratio, colder ambient temperatures result in lower NH3/NHX. On average HNO3 makes up about 20% of NOY at midday. The sum of the measured NOY species (NO, NO2, HNO3, and particulate NO3−) typically account for 60–80% of NOY, suggesting that peroxyacetylnitrate and other organic nitrates are a significant fraction of NOY at this predominantly oak forested site.
On the NO2 + soot reaction in the atmosphere
Authors: Aumont B. , Madronich S. , Ammann M. , Kalberer M. , Baltensperger U. , Hauglustaine D. , Brocheton F.
Published: September 2002

Recent modeling studies have suggested that soot is a key component of tropospheric chemistry in remote regions, acting to reduce HNO3 to NO2 and possibly NO2 to NO. It may be expected then that soot also affects the chemistry of rural and urban areas, where soot concentrations are typically several orders of magnitude higher than in the remote troposphere. In order to test this assumption, a modeling study was conducted for typical urban and rural areas, with the same HNO3/NO2/soot chemistry proposed in the previous modeling studies of the remote troposphere. Unreasonable results were found (e.g., nearly total suppression of urban ozone, in contradiction to common observations), suggesting that the NO2/soot reaction was considerably overestimated in previous modeling studies. Therefore the NO2/soot chemistry was reconsidered. A new preliminary mechanism is suggested, based on recent laboratory studies of this reaction. Results show that the NO2/soot reaction does not notably affect the Ox‐NOx‐HOx chemistry of the lower continental troposphere, except maybe during nighttime in urban environments. A potential contribution of the NO2/soot interaction to HONO production is noted.
Global nitrogen and sulfur inventories for oceangoing ships
Authors: Corbett James J. , Fischbeck Paul S. , Pandis Spyros N.
Published: September 2002

We present geographically resolved global inventories of nitrogen and sulfur emissions from international maritime transport for use in global atmospheric models. Current inventories of globally resolved sources of natural and anthropogenic emissions do not include the significant contribution of SO2 or NOx from oceangoing ships [Benkovitz et al., 1996]. We estimate the global inventory of ship emissions, using current emission test data for ships [Carlton et al., 1995] and a fuel‐based approach similar to that used for automobile inventories [Singer and Harley, 1996]. This study estimates the 1993 global annual NOx and SO2 emissions from ships to be 3.08 teragrams (Tg, or 1012 g) as N and 4.24 Tg S, respectively. Nitrogen emissions from ships are shown to account for more than 14% of all nitrogen emissions from fossil fuel combustion, and sulfur emissions exceed 5% of sulfur emitted by all fuel combustion sources including coal. Ship sulfur emissions correspond to about 20% of biogenic dimethylsulfide (DMS) emissions. In regions of the Northern Hemisphere, annual sulfur emissions from ships can be of the same order of magnitude as estimates of the annual flux of DMS [Chin et al., 1996]. Monthly inventories of ship sulfur and nitrogen emissions presented in this paper are geographically characterized on a 2° × 2° resolution. Temporal and spatial characteristics of the inventory are presented. Uncertainty in inventory estimates is assessed: the fifth and ninety‐fifth percentile values for global nitrogen emissions are 2.66 Tg N and 4.00 Tg N, respectively; the fifth and ninety‐fifth percentile values for sulfur emissions are 3.29 Tg S and 5.61 Tg S, respectively. We suggest that these inventories, available via the Ship Emissions Assessment (SEA) web site, be used in models along with the Global Emissions Inventory Activity (GEIA) inventories for land‐based anthropogenic emissions and modeled with ocean‐biogenic inventories for DMS.
Assimilation of total ozone satellite measurements in a three‐dimensional tracer transport model
Authors: Jeuken A. B. M. , Eskes H. J. , van Velthoven P. F. J. , Kelder H. M. , Hólm E. V.
Published: September 2002

The optimal‐interpolation data‐assimilation technique is used to combine TIROS Operational Vertical Sounder total‐ozone columns with the three‐dimensional tracer transport model TM3. An ozone chemistry parametrization and dry deposition of ozone are added to the model to give it the ability to simulate realistic ozone profiles. Starting from a separable form of the forecast covariance matrix, the optimal interpolation equation can be rewritten into a horizontal and vertical analysis step. First, the two‐dimensional ozone column measurements are analyzed using estimates for the horizontal error covariances. In the second step of the assimilation procedure the analysis increment for the column is distributed over the vertical model layers. This step depends only on a normalized vertical weight function which is equal to the vertical covariance. Three different estimates for this weight function are introduced, using either the ozone error covariance, or the ozone time variance, or the actual ozone mass to distribute column corrections in the vertical. A comparison with independent Total Ozone Mapping Spectrometer ozone observations shows a considerable improvement of the total ozone field due to assimilation. The model error growth is small, making it suitable for assimilating sparse measurements. Ozone profiles from the assimilation appear realistic and close to the ones observed by sondes. They are capable to describe dynamical features in the lower stratosphere. However, due to the absence of vertical information, the assimilation of ozone columns has only little impact on the shape of the vertical ozone profile, which is mainly determined by the transport.
Global chlorine emissions from biomass burning: Reactive Chlorine Emissions Inventory
Authors: Lobert Jürgen M. , Keene William C. , Logan Jennifer A. , Yevich Rosemarie
Published: September 2002

Emissions of reactive chlorine‐containing compounds from nine discrete classes of biomass burning were estimated on a 1° latitude by 1° longitude grid based on a biomass burning inventory for carbon emissions. Variations on approaches incorporating both emission ratios relative to CO and CO2and the chlorine content of biomass burning fuels were used to estimate fluxes and associated uncertainties. Estimated, global emissions are 640 Gg Cl yr−1for CH3Cl; 49 Gg Cl yr−1for CH2Cl2; 1.8 Gg Cl yr−1for CHCl3; 13 Gg Cl yr−1for CH3CCl3; and 6350 Gg Cl yr−1for the sum of volatile‐inorganic and particulate chlorine. Biomass burning appears to be the single largest source of atmospheric CH3Cl and a significant source of CH2Cl2; contributions of CHCl3and CH3CCl3are less than 2% of known sources.
Airborne Polar Experiment‐Polar Ozone, Leewaves, Chemistry, and Transport (APE‐POLECAT): Rationale, road map and summary of measurements
Authors: Stefanutti L. , MacKenzie A. R. , Balestri S. , Khattatov V. , Fiocco G. , Kyrö E. , Peter T.
Published: September 2002

The Airborne Polar Experiment‐Polar Ozone, Leewaves, Chemistry and Transport (APE‐POLECAT) mission took place between December 19, 1996, and January 16, 1997. APE‐POLECAT comprised the inaugural mission of the high‐altitude research aircraft, the M‐55 Geophysica, flights by the DLR Falcon, measurements from a number of Arctic ground stations, and atmospheric modeling. Both aircraft flew out of Rovaniemi in Finland. The Geophysica was equipped with a payload designed to probe the chemistry and microphysics of polar stratospheric clouds (PSCs) at, or above, the aircraft altitude (up to 20 km geometric altitude). The Deutsches Zentrum fur Luft und Raumfahrt (DLR) Falcon was fitted with an aerosol lidar called OLEX, which looked upward. Ground‐based measurements included aerosol lidar, meteorological sondes, and ozone sondes, from both sides of the Scandinavian Mountains, and from the southern Arctic Ocean. The original primary aim of the mission, to study PSC processes in situ, was modified in the light of unfavorable meteorological conditions. Flights concentrated on studies of transport and chemistry around the polar vortex, and on remote sensing of very high, mountain‐wave‐induced, PSCs. Here we report the objectives and rationale of the mission, provide basic descriptions of the conditions of the stratosphere at the time of each flight, and give a summary of the measurements made.
Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory
Authors: Keene William. C. , Khalil M. Aslam K. , Erickson David. J. , McCulloch Archie , Graedel Thomas E. , Lobert Jürgen M. , Aucott Michael L. , Gong Sun Ling , Harper David B. , Kleiman Gary , Midgley Pauline , Moore Robert M. , Seuzaret Christophe , Sturges William T. , Benkovitz Carmen M. , Koropalov Valentin , Barrie Leonard A. , Li Yi Fan
Published: September 2002

Emission inventories for major reactive tropospheric Cl species (particulate Cl, HCl, ClNO2, CH3Cl, CHCl3, CH3CCl3, C2Cl4, C2HCl3, CH2Cl2, and CHClF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea‐salt production and dechlorination, biomass burning, industrial emissions, fossil‐fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid‐displacement reactions involving S(IV) + O3, (IV) + O3 H2O2, and H2SO4 and HNO3 scavenging account for minor fractions of sea‐salt dechlorination globally. Other important chemical pathways involving sea‐salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3Cl from known sources account for about half of the modeled sink, suggesting fluxes from known sources were underestimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3Cl emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2Cl2 (25%), C2HCl3 (10%), and C2Cl4 (5%) are emitted from the surface ocean; the combined contributions of C2Cl4 and C2HCl3 from all natural sources may be substantially higher than the estimated oceanic flux.
Characteristics of dimethylsulfide, ozone, aerosols, and cloud condensation nuclei in air masses over the northwestern Pacific Ocean
Authors: Nagao Ippei , Matsumoto Kiyoshi , Tanaka Hiroshi
Published: September 2002

Long‐term measurements of several trace gases and aerosols were carried out from December 1994 to October 1996 at Ogasawara Hahajima Island over the northwestern Pacific Ocean. The continental impact on the concentrations of sulfur compounds, ozone (O3), and cloud condensation nuclei (CCN) was estimated on the basis of the classification of air mass into seven types by isentropic trajectory analysis. From May to October, the air mass originating from the central North Pacific Ocean is predominant and regarded as the clean marine air for the concentrations of sulfur compounds and CCN. From the results of the molar ratio of methane sulfonic acid to non‐sea‐salt sulfate (NSS) and the positive correlation between dimethylsulfide (DMS) and CCN in this air mass it can be concluded that DMS largely contributes to the production of NSS and CCN. On the other hand, continental and anthropogenic substances are preferably transported to the northwestern Pacific Ocean by the predominant continental air mass from November to March. The enhancement of concentrations by the outflow from the Asian continent are estimated by a factor of 2.8 for O3, 3.9 for SO2, 3.5 for CCN activated at 0.5% supersaturation (0.5% CCN), 4.7 for 1.0% CCN, and 5.5 for NSS. Moreover, the CCN supersaturation spectra are also affected by the continental substances resulting in factor 2 of enhancement of cloud droplet number concentration. The diurnal variations of DMS and O3 for each air mass show a pattern of daytime minimum and nighttime maximum, which are typically found in remote ocean, even though those amplitudes are different for each air mass. Consequently, it can be concluded that the influence of nitric oxides (NOx) for the daytime O3 production and nitrate (NO3) radical for the nighttime oxidation of DMS are small even in the continental air mass.
Aerosol‐induced radiative flux changes off the United States mid‐Atlantic coast: Comparison of values calculated from sunphotometer and in situ data with those measured by airborne pyranometer
Authors: Russell P. B. , Livingston J. M. , Hignett P. , Kinne S. , Wong J. , Chien A. , Bergstrom R. , Durkee P. , Hobbs P. V.
Published: September 2002

The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) measured a variety of aerosol radiative effects (including flux changes) while simultaneously measuring the chemical, physical, and optical properties of the responsible aerosol particles. Here we use TARFOX‐determined aerosol and surface properties to compute shortwave radiative flux changes for a variety of aerosol situations, with midvisible optical depths ranging from 0.06 to 0.55. We calculate flux changes by several techniques with varying degrees of sophistication, in part to investigate the sensitivity of results to computational approach. We then compare computed flux changes to those determined from aircraft measurements. Calculations using several approaches yield downward and upward flux changes that agree with measurements. The agreement demonstrates closure (i.e., consistency) among the TARFOX‐derived aerosol properties, modeling techniques, and radiative flux measurements. Agreement between calculated and measured downward flux changes is best when the aerosols are modeled as moderately absorbing (midvisible single‐scattering albedos between about 0.89 and 0.93), in accord with independent measurements of the TARFOX aerosol. The calculated values for instantaneous daytime upwelling flux changes are in the range +14 to +48 W m−2 for midvisible optical depths between 0.2 and 0.55. These values are about 30 to 100 times the global‐average direct forcing expected for the global‐average sulfate aerosol optical depth of 0.04. The reasons for the larger flux changes in TARFOX include the relatively large optical depths and the focus on cloud‐free, daytime conditions over the dark ocean surface. These are the conditions that produce major aerosol radiative forcing events and contribute to any global‐average climate effect.
Aerosol properties and radiative effects in the United States East Coast haze plume: An overview of the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX)
Authors: Russell Philip B. , Hobbs Peter V. , Stowe Larry L.
Published: September 2002

Aerosol effects on atmospheric radiation are a leading source of uncertainty in predicting climate change. The Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) was designed to reduce this uncertainty by measuring and analyzing aerosol properties and effects on the United States eastern seaboard, where one of the world's major plumes of urban/industrial haze moves from the continent over the Atlantic Ocean. The TARFOX intensive field campaign was conducted July 10–31, 1996. It included coordinated measurements from four satellites (GOES‐8, NOAA‐14, ERS‐2, Landsat), four aircraft (ER‐2, C‐130, C‐131A, and a modified Cessna), land sites, and ships. A variety of aerosol conditions was sampled, ranging from relatively clean, behind frontal passages, to moderately polluted, with aerosol optical depths exceeding 0.5 at midvisible wavelengths. Gradients of aerosol optical thickness were sampled to aid in separating aerosol effects from other radiative effects and to more tightly constrain closure tests, including those of satellite retrievals. Early results from TARFOX include demonstration of the unexpected importance of carbonaceous compounds and water condensed on aerosol in the United States East Coast haze plume, chemical apportionment of the aerosol optical depth, measurements of aerosol‐induced changes in upwelling and downwelling shortwave radiative fluxes, and generally good agreement between measured flux changes and those calculated from measured aerosol properties. This overview presents the TARFOX objectives, rationale, overall experimental approach, and key initial findings as a guide to the more complete results reported in this special section and elsewhere.
Interannual variation of ambient aerosol characteristics on the east coast of the United States
Authors: Remer Lorraine A. , Kaufman Yoram J. , Holben Brent N.
Published: September 2002

We investigate the interannual variability of the seasonally recurring aerosol haze of the United States East Coast. The data consist of observations collected by five Sun/sky radiometer stations. We compare the data collected in 1996 in conjunction with the TARFOX experiment to similar data collected at the same locations in 1993. The regional mean optical thickness remains essentially constant in the two years. The Mount Pinatubo stratospheric mode decreases spectral dependence in 1993, especially for optical thickness below 0.20. The volume size distributions inverted from the sky measurements show an increase in accumulation mode particle size with increasing aerosol optical thickness in both years. At moderate to high optical thickness (τ670 > 0.20) the particles in the fine particle mode, 0.1–0.6 μm radius range, from both years have the same lognormal parameters (rm = 0.21, σ = 0.45–0.50). The volume of these particles increases linearly with τ. These size particles dominate the optical properties of the aerosol in the visible spectrum and cause remarkable interannual consistency in the aerosol optical characteristics. The 1993 and 1996 phase functions agree to within 7–10% for optical thicknesses greater than 0.2.
A distinct class of isolated intracloud lightning discharges and their associated radio emissions
Authors: Smith D. A. , Shao X. M. , Holden D. N. , Rhodes C. T. , Brook M. , Krehbiel P. R. , Stanley M. , Rison W. , Thomas R. J.
Published: September 2002

Observations of radio emissions from thunderstorms were made during the summer of 1996 using two arrays of sensors located in northern New Mexico. The first array consisted of three fast electric field change meters separated by distances of 30 to 230 km. The second array consisted of three broadband (3 to 30 MHz) HF data acquisition systems separated by distances of 6 to 13 km. Differences in signal times of arrival at multiple stations were used to locate the sources of received signals. Relative times of arrival of signal reflections from the ionosphere and Earth were used to determine source heights. A distinct class of short‐duration electric field change emissions was identified and characterized. The emissions have previously been termed narrow positive bipolar pulses (NPBPs). NPBPs were emitted from singular intracloud discharges that occurred in the most active regions of three thunderstorms located in New Mexico and west Texas. The discharges occurred at altitudes between 8 and 11 km above mean sea level. NEXRAD radar images show that the NPBP sources were located in close proximity to high reflectivity storm cores where reflectivity values were in excess of 40 dBZ. NPBP electric field change waveforms were isolated, bipolar, initially positive pulses with peak amplitudes comparable to those of return stroke field change waveforms. The mean FWHM (full width at half maximum) of initial NPBP field change pulses was 4.7 μs. The HF emissions associated with NPBPs were broadband noise‐like radiation bursts with a mean duration of 2.8 μs and amplitudes 10 times larger than emissions from typical intracloud and cloud‐to‐ground lightning processes. Calculations indicate that the events represent a distinct class of singular, isolated lightning discharges that have limited spatial extents of 300 to 1000 m and occur in high electric field regions. The unique radio emissions produced by these discharges, in combination with their unprecedented physical characteristics, clearly distinguish the events from other types of previously observed thunderstorm electrical processes.
Application of the NCAR regional climate model to eastern Africa: 1. Simulation of the short rains of 1988
Authors: Sun Liqiang , Semazzi Fredrick H. M. , Giorgi Filippo , Ogallo Laban
Published: September 2002

The National Center for Atmospheric Research (NCAR) Regional Climate Model (RegCM2) is employed to investigate the physical mechanisms that govern the October‐December rains over eastern Africa. The model employs the Mercator conformai projection, with a domain of 5580 km × 5040 km centered at 31°E, 4°S, and a horizontal grid point spacing of 60 km. The simulation period is October‐December 1988, and the model initial and lateral boundary conditions are taken from ECMWF reanalysis. A number of month‐long simulations have been conducted to optimize various parameterizations of the model which include the following factors: cumulus convection, moisture parameterization, radiative transfer formulation, surface processes, boundary layer physics, and the lateral boundary conditions. The model was successfully customized over eastern Africa. The model simulates the large‐scale circulation characteristics over the region as well as local features such as the dominant precipitation maxima, the Turkana low‐level jet, and the diurnal reversal in the lake/land breeze circulation over Lake Victoria. Several model deficiencies are also identified. They include a negative rainfall bias over the western portions of the domain and the Kenya Highlands and a temperature bias over the tropical forest regions. Systematic analysis of surface water budget reveals that evapotranspiration is a major sink in the water budget over the regions where precipitation is moderate or small, while the role of runoff and drainage becomes important over the regions where precipitation is abundant. The model simulations also suggest that during the short‐rains season, the large‐scale circulation anomalies play the most important role in shaping the precipitation anomalies.
Spatial variability of climate and past atmospheric circulation patterns from central West Antarctic glaciochemistry
Authors: Reusch David B. , Mayewski Paul A. , Whitlow Sallie I. , Pittalwala Iqbal I. , Twickler Mark S.
Published: September 2002

Atmospheric circulation patterns and the spatial variability of atmospheric chemistry and moisture transport in central West Antarctica are investigated using new 40 year long (1954–1994 A.D.) glaciochemical and accumulation rate records developed from four firn cores from this region. The core sites lie on a 200 km traverse from 82° 22′ S, 119° 17′ W to 81° 22′ S, 107° 17′ W. The glaciochemical records represent the major ionic species present in Antarctic snow: Na+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−. High spatial variability appears in comparisons of full record averages and poor intersite linear correlation results. Accumulation rates show 50–100% changes over distances of 50–100 km and sea‐salt concentrations drop by 50% between the middle two sites. One likely contributor to the high variability seen at this spatial scale is variability in synoptic‐ and finer‐scale meteorology. Empirical orthogonal function (EOF) analysis shows that 80% or more of the variance in site chemistry can be attributed to two types of air masses: winter season air (50–70% of site variance) with a strong marine signature (heavy loading of sea‐salt species) and summer season air (21% of the variance), marked by marine biogenic non‐sea‐salt SO4 plus NO3. This pattern of winter and summer regimes appears at other West Antarctic sites suggesting it may apply to the entire region. We show that a general picture of the patterns of variability in West Antarctica can best be drawn by using an analysis technique that fully exploits high resolution, multiparameter, multisite data sets.
Changes in measured lightning flash count and return stroke peak current after the 1994 U.S. National Lightning Detection Network upgrade: 2. Theory
Authors: Wacker Robert S. , Orville Richard E.
Published: September 2002

A model of return stroke detection by the U.S. National Lightning Detection Network (NLDN) magnetic direction finder (MDF) sensors is used to approximate the pulse width criterion modification made to the sensors during the 1994 upgrade. Decreasing the pulse width detection criterion used by the MDF sensors increases their effective detection range, which increases their sensitivity to weak flashes (because of NLDN network geometry, increasing sensitivity has little effect on detection of strong flashes). Consequently, we observe an increase in the weak flash counts. The increased detection of weak flashes accounts, in part, for the decrease in mean peak currents observed in subsequent years to 1994. In addition to decreasing the mean peak current of detected positive and negative flashes, the NLDN upgrade has apparently had the unwanted effect of increasing the contamination of the positive CG flash data with cloud flashes.