bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical movements and material transport during hotspot activity: Seismic reflection profiling offshore La Réunion
Authors: de Voogd Béatrice , Palomé Salvador Pou , Hirn Alfred , Charvis Philippe , Gallart Josep , Rousset Dominique , Dañobeitia Juanjo , Perroud Hervé
Published: September 2002

The structure of the submerged part of the La Réunion hotspot island is determined by a grid of multichannel seismic reflection profiles. The submarine part of the edifice appears as a poorly stratified wedge of material lying above a significant thickness of preexisting sediments and the oceanic basement. The dense data coverage has allowed us to derive contour maps of the top of the basement and of the base of the volcaniclastic edifice, further constrained by coincident wide‐angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observation along a single seismic line since the geometry of these horizons varies significantly from one radial profile to the next. Both maps show a large degree of heterogeneity in the topography, with no axial nor cylindrical symmetry, indicating that plate flexure is not dominant. A slight depression toward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano‐sedimentary pile that covers the oceanic basement firmly establishes the lack of significant vertical movement and flexure. The base of the edifice is roughly domed, centered on the island, with several topographic highs or lows superimposed. The submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported material are observed on the seismic data south of the island, not only to the east of the active Piton de la Fournaise volcano. Oceanic sediments trapped beneath the apron seem undeformed.
Observed surface oceanic and atmospheric variability in the tropical Pacific at seasonal and ENSO timescales: A tentative overview
Authors: Delcroix Thierry
Published: September 2002

Seasonal and El Niño‐Southern Oscillation (ENSO)‐related variations of sea surface temperature (SST) and salinity (SSS), 0/450‐dbar dynamic height anomalies (η, an alias for sea level), zonal (τx) and meridional (τy) wind stress, wind stress curl (curl (τ)), and precipitation (P) are examined in the tropical Pacific during 1961–1995. In the equatorial band the El Niño (La Niña) events are chiefly concerned (1) in the east and center, with warmer (colder) than average SST and a η increase (decrease), and (2) in the west, with fresher (saltier) than average SSS, westerly (easterly) wind anomalies, above (below) average P limited to the east of about 150°E, and a η decrease (increase); Much smaller ENSO changes occur away from the equatorial band except in the convergence zones for SSS, P, arid τy changes and in two patches centered around 7°N and 7°S in the west for curl (τ). The ENSO‐related η changes are schematically concerned with a zonal “seesaw” in phase with the Southern Oscillation Index (SOI) in the equatorial band and a meridional seesaw between the regions situated north and south of about 5°N, which lags by about 1 year behind the SOI. The double seesaws result in a longitudinal mean η rise (drop) within about 5°N–20°S up to the mature phase of El Niño (La Niña), and not just until its beginning, partly compensated by a longitudinal mean η drop (rise) within about 5°–20°N. Aside from its intrinsic substance, this paper offers a novel and concise observational basis for testing theoretical studies and model simulations.
Sea surface height variations in the Yellow and East China Seas: 2. SSH variability in the weekly and semiweekly bands
Authors: Jacobs G. A. , Teague W. J. , Riedlinger S. K. , Preller R. H. , Blaha J. P.
Published: September 2002

Two types of models are used to examine the sea surface height (SSH) variability at the principle short time frequency bands in the Yellow and East China Seas. The models are a primitive equation numerical model and a statistical model based on TOPEX/POSEIDON altimeter data. In situ pressure gauge measurements at two mooring locations are first used to evaluate each model. In addition, a comparison of the numerical and statistical models is made through duplicate analysis of the response to the main extended empirical orthogonal function wind stress modes. In the northern Yellow Sea the wind stress, pressure gauge data, numerical model, and statistical model all indicate local spectral maxima in bands centered on the weekly and semiweekly periods. The coherence of the two models with the in situ data is significant down to 3‐day periods at the northern mooring and down to 5‐day periods at the southern mooring. The meridional wind stress variability in the semiweekly band is higher than the weekly band energy in the northern Yellow Sea and Bohai Bay and lower than the weekly band energy across the East China Sea. The SSH variability contained in the weekly and semiweekly bands is largest in the Bohai Bay and northern Yellow Sea with a region of high energy along the Chinese coast. The spatial structure of the weekly and semiweekly SSH variability is similar to the wind stress variability at these bands with semiweekly variability higher in the Bohai Bay and northern Yellow Sea and weekly variability dominant in the East China Sea.
Variability in particle attenuation and chlorophyll fluorescence in the tropical Pacific: Scales, patterns, and biogeochemical implications
Authors: Claustre Hervé , Morel André , Babin Marcel , Cailliau Caroline , Marie Dominique , Marty Jean‐Claude , Tailliez Dominique , Vaulot Daniel
Published: September 2002

The variability in particle attenuation (cp) and in chlorophyll in situ fluorescence (Fis) was examined in November 1994 along 150°W in the Pacific Ocean. Two main sources of variation in cp and Fis profiles are identified by analyzing data from a 16°S–1°N transect, and from two 5 day stations (5°S and 16°S). The first source reflects changes in the trophic status resulting from prevailing hydrodynamical regimes at large scales. By using flow cytometric data and some assumptions about the size distribution of the different biological stocks, a decomposition of cp into its vegetal (cveg) and nonvegetal (cnveg) components is attempted. Within the euphotic layer, cveg accounts for 43% of the total cp signal at the equator and for only 20% in the South Pacific gyre. The nonvegetal component is then subdivided into heterotrophic organisms and detritus contributions. The detrital material is an important contributor with 43% of cp at 5°S and 55% at 16°S. A further decomposition of Fis and cveg into the three dominant phytoplanktonic groups (Prochlorococcus, Synechococcus, and picoeucaryotes) confirms that picoeucaryotes are important contributors of the vegetal biomass, especially within and below the deep chlorophyll maximum (DCM) (>50% of the algal stock) at 16°S. The second, and essentially local, source of variation is related to specific rhythms in biological and physiological processes. The prominent signals detected during the time series occur at the daily scale: besides the pronounced fluorescence depression at noon in upper layers, particle attenuation in all the layers examined and fluorescence in the DCM display conspicuous daily oscillations. They result from the balance between daytime accumulation and night removal of particles, of algal cells in particular. Finally, the estimation of cp‐based growth rates points out the surprisingly rapid turnover time of the whole particulate matter stock in oligotrophic waters (16°S), not only in the euphotic zone (0.63 d−1) but also within the dimly lit layers of the DCM (0.36 d−1). The corresponding growth rate at 5°S, within a quasi‐mesotrophic regime, is 0.47 d−1 within the euphotic zone.
A climatology of the shelfbreak front in the Middle Atlantic Bight
Authors: Linder Christopher A. , Gawarkiewicz Glen
Published: September 2002

Description of the shelfbreak front in the Middle Atlantic Bight is hampered by the extreme variability of the front. In order to gain insight into both the seasonal variability and regional variations in the mean frontal structure and associated baroclinic jet, historical data are used to produce two‐dimensional climatological fields of temperature and salinity for the region south of Nantucket shoals, along the south flank of Georges Bank, and off the coast of New Jersey. Associated cross‐shelf fields of density and geostrophic velocity are also computed. The climatological temperature and salinity are consistent with previous descriptions of the frontal hydrography. The temperature contrast across the front varies seasonally between 2° and 6°C. The salinity contrast is 1.5–2, with little seasonal variation. The near‐surface density gradients are strongest during the winter and weakest during the summer, when the seasonal thermocline is established. The cross‐frontal density gradients are strongest near the foot of the front. Despite the inherent smearing of frontal gradients incurred by averaging over large temporal and spatial scales, the geostrophic velocity field south of Nantucket shows a strong (0.2–0.3 m s−1) baroclinic jet associated with the frontal density gradients. The core of the jet, having a width of 15–20 km, is located near the 150‐m isobath. Transport calculations for the flow over the outer shelf and slope are in the range of 0.2–0.3 Sverdrups (Sv) to the west. This is comparable to the estimated transport (0.4 Sv) shoreward of the 100‐m isobath.
Comparison of SIR‐C SAR wavenumber spectra with WAM model predictions
Authors: Monaldo Frank M. , Beal Robert C.
Published: September 2002

During April and October of 1994, the Space Radar Laboratory (SRL) flew on the space shuttle Endeavour at the relatively low altitude of 215 km. Using horizontal polarization, C‐band signal from the spaceborne imaging radar (SIR‐C), an onboard processor, designed and fabricated by the Johns Hopkins University Applied Physics Laboratory, formed synthetic aperture radar (SAR) images and computed over 100,000 corresponding image spectra. The low altitude, small look angle (23°–25°) and the use of horizontal polarization minimized both the loss of azimuth resolution caused by ocean surface motion and the relative contribution of the hydrodynamic component of the modulation transfer function. As a result, the SIR‐C SAR was able to image azimuth‐traveling waves with minimal distortion. After using linear inversions to convert image spectra to wave height‐variance spectra, the distributions of wavenumber and propagation direction from the processor‐derived spectra were consistent with wave model (WAM) predictions. Although the SAR wave height‐variance spectra underestimated significant wave height (SWH) at the higher SWHs, this error is compensated for by a simple linear correction. We collected 57 pairs of crossover spectra where the ground track pairs were nearly orthogonal. The crossovers were separated by 6 hours. Crossover comparisons show that the retrieved spectral parameters are independent of wave propagation direction. At this altitude and configuration, the SAR range and azimuth responses are nearly equal. The real‐time processing of spaceborne SAR data to produce accurate estimates of wavenumber, propagation direction, and SWH is clearly feasible with the orbital and instrument geometry of SIR‐C.
On the relative importance of the remote and local wind effects to the subtidal variability in a coastal plain estuary
Authors: Wong Kuo‐Chuin , Moses‐Hall Joy E.
Published: September 2002

A set of month‐long sea level and current measurements is used to examine the subtidal variability in the Delaware estuary and assess the relative importance of remote and local wind effects on the observed subtidal variability. The evidence indicates that the remote wind effect, through the impingement of coastal sea level at the mouth of the estuary, is more important than the local wind effect in producing the subtidal sea level fluctuations in the interior of the estuary. On the other hand, the local wind effect dominates over the remote wind effect in producing the observed subtidal current fluctuations at the mooring site. It appears that the remote wind effect is important to the sectionally averaged subtidal transport into or out of the estuary. However, the local wind effect may be more important than the remote wind effect in producing the subtidal currents at any given point along the estuary's cross section. The spatial structure associated with the local wind‐induced circulation is very important to the long term transport and distribution of waterborne material.
On the penetration of anthropogenic CO2 into the North Atlantic Ocean
Authors: Körtzinger Arne , Mintrop Ludger , Duinker Jan C.
Published: September 2002

The penetration of anthropogenic or “excess” CO2 into the North Atlantic Ocean was studied along WOCE‐WHP section A2 from 49°N/11°W to 43°N/49°W using hydrographic data obtained during the METEOR cruise 30–2 in October/November 1994. A backcalculation technique based on measurements of temperature, salinity, oxygen, alkalinity, and total dissolved inorganic carbon was applied to identify the excess CO2. Everywhere along the transect surface water contained almost its full component of anthropogenic CO2 ( ∼62 μmol kg−1). Furthermore, anthropogenic CO2 has penetrated through the entire water column in the western basin of the North Atlantic Ocean. Even in the deepest waters (5000 m) of the western basin a mean value of 10.4 μmol kg−1 excess CO2 was calculated. The maximum penetration depth of excess CO2 in the eastern basin of the North Atlantic Ocean was ∼3500 m with values falling below 5 μmol kg−1 in greater depths. These results compare well with distributions of carbontetrachloride. They are also in agreement with the current understanding of the role of the “global ocean conveyor belt” for the uptake of anthropogenic CO2 into the deep ocean.
Moisture uptake in the boundary layer above the Agulhas Current: A case study
Authors: Lee‐Thorp A. M. , Rouault M. , Lutjeharms J. R. E.
Published: September 2002

Shipboard meteorological measurements and radiosonde ascents are used to describe the modification of the marine atmospheric boundary layer as air is advected across the warm Agulhas Current toward the coast. The data were collected on April 28, 1995, during the Agulhas Current Air‐Sea Exchange Experiment south of Port Alfred, South Africa, in a region characterized by a strong sea surface temperature gradient. The surface latent heat flux increased from approximately 160 W m−2 over the seaward border to 270 W m−2 over the current (approximately 80–90 km wide) and decreased dramatically to approximately 40 W m−2 over the cool shelf. This spatial heat flux gradient was reflected in the overlying atmosphere by a transition in atmospheric stability from the warm current to the cool continental shelf. A convective boundary layer over the current was replaced by a stable boundary layer over the cool shelf, where a deep residual moisture layer indicated the former vertical extent of convective eddies. A progressive accumulation of moisture occurred within the boundary layer as the air mass was advected over the current. Mean specific humidity and precipitable water vapor content in the boundary layer over the shelf were 20–25% higher than over the seaward border of the current. The results give credence to the assertion by many investigators that moisture uptake above the Agulhas Current may contribute significantly to moisture convergence and rainfall over the interior of South Africa.
Physical characteristics of summer sea ice across the Arctic Ocean
Authors: Tucker W. B. , Gow A. J. , Meese D. A. , Bosworth H. W. , Reimnitz E.
Published: September 2002

Sea ice characteristics were investigated during July and August on the 1994 transect across the Arctic Ocean. Properties examined from ice cores included salinity, temperature, and ice structure. Salinities measured near zero at the surface, increasing to 3–4‰ at the ice‐water interface. Ice crystal texture was dominated by columnar ice, comprising 90% of the ice sampled. Surface albedos of various ice types, measured with radiometers, showed integrated shortwave albedos of 0.1 to 0.3 for melt ponds, 0.5 for bare, discolored ice, and 0.6 to 0.8 for a deteriorated surface or snow‐covered ice. Aerial photography was utilized to document the distribution of open melt ponds, which decreased from 12% coverage of the ice surface in late July at 76°N to almost none in mid‐August at 88°N. Most melt ponds were shallow, and depth bore no relationship to size. Sediment was pervasive from the southern Chukchi Sea to the north pole, occurring in bands or patches. It was absent in the Eurasian Arctic, where it had been observed on earlier expeditions. Calculations of reverse trajectories of the sediment‐bearing floes suggest that the southernmost sediment was entrained during ice formation in the Beaufort Sea while more northerly samples probably originated in the East Siberian Sea, some as far west as the New Siberian Islands.
Influence of biomass combustion emissions on the distribution of acidic trace gases over the southern Pacific basin during austral springtime
Authors: Talbot R. W. , Dibb J. E. , Scheuer E. M. , Blake D. R. , Blake N. J. , Gregory G. L. , Sachse G. W. , Bradshaw J. D. , Sandholm S. T. , Singh H. B.
Published: September 2002

This paper describes the large‐scale distributions of HNO3, HCOOH, and CH3COOH over the central and South Pacific basins during the Pacific Exploratory Mission‐Tropics (PEM‐Tropics) in austral springtime. Because of the remoteness of this region from continental areas, low part per trillion by volume (pptv) mixing ratios of acidic gases were anticipated to be pervasive over the South Pacific basin. However, at altitudes of 2–12 km over the South Pacific, air parcels were encountered frequently with significantly enhanced mixing ratios (up to 1200 pptv) of acidic gases. Most of these air parcels were centered in the 3–7 km altitude range and occurred within the 15°−65°S latitudinal band. The acidic gases exhibited an overall general correlation with CH3Cl, PAN, and O3, suggestive of photochemical and biomass burning sources. There was no correlation or trend of acidic gases with common industrial tracer compounds (e.g., C2Cl4or CH3CCl3). The combustion emissions sampled over the South Pacific basin were relatively aged exhibiting C2H2/CO ratios in the range of 0.2–2.2 pptv/ppbv. The relationships between acidic gases and this ratio were similar to what was observed in aged air parcels (i.e., >3–5 days since they were over a continental area) over the western North Pacific during the Pacific Exploratory Mission‐West Phases A and B (PEM‐West A and B). In the South Pacific marine boundary layer a median C2H2/CO ratio of 0.6 suggested that this region was generally not influenced by direct inputs of biomass combustion emissions. Here we observed the lowest mixing ratios of acidic gases, with median values of 14 pptv for HNO3, 19 pptv for HCOOH, and 18 pptv for CH3COOH. These values were coincident with low mixing ratios of NOx(<10 pptv), CO (≈50 parts per billion by volume (ppbv)), O3(< 20 ppbv), and long‐lived hydrocarbons (e.g., C2H6<300 pptv). Overall, the PEM‐Tropics data suggest an important influence of aged biomass combustion emissions on the distributions of acidic gases over the South Pacific basin in austral springtime.
Methane measurements in central New England: An assessment of regional transport from surrounding sources
Authors: Shipham Mark C. , Crill Patrick M. , Bartlett Karen B. , Goldstein Allen H. , Czepiel Peter M. , Harriss Robert C. , Blaha Denise
Published: September 2002

The Harvard Forest research site located in central New England is influenced by numerous anthropogenic methane sources on a year‐round basis. Methane is strongly correlated to other chemical species that have an anthropogenic component, including acetylene, propane, ethane, hexane, and additional short‐lived nonmethane hydrocarbons. The correlation between methane and acetylene is due to the colocation of landfills and cities. The correlation between methane and other short‐lived species implies that emissions from local and regional rather than distant sources are the primary cause of elevated events. Wind roses of chemical species are examined for annual and seasonal time periods with enhancements in anthropogenic species corresponding to the location of large cities and landfills. The southwest quadrant is subjected to the most severe pollution events and is impacted by outflow from nearby cities in that sector, including Northampton and Springfield, Massachusetts. Emissions from cities in other quadrants, including Boston and Worcester, Massachusetts, Providence, Rhode Island, and the close‐by town of Petersham, Massachusetts, also affect the site, but to a lesser degree. Case studies are used to identify atmospheric conditions that lead to high concentrations of methane and other species. The co‐occurrence of a persistent wind direction, light wind speed, and stable atmospheric conditions is the ideal scenario in which emissions from nearby cities and landfills are advected to the site. Emissions from local and regional, rather than distant sources, are the primary cause of elevated events.
Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode
Authors: Banta Robert M. , Senff Christoph J. , White Allen B. , Trainer Michael , McNider Richard T. , Valente Ralph J. , Mayor Shane D. , Alvarez Raul J. , Hardesty R. Michael , Parrish David , Fehsenfeld Fred C.
Published: September 2002

A 3‐day period of strong, synoptic‐scale stagnation, in which daytime boundary‐layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5–10 m s−1as a result of nocturnal decoupling from surface friction, which produced inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (O3). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled O3in the boundary layer as the airplane flew along. Vertical cross sections showed that O3concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the O3hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3‐D spatial distribution of O3and the interaction between urban and rural pollution: (1) daytime buildup of O3over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak O3concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light‐wind conditions were intra‐regional differences in daytime mixed‐layer depth over distances of 50 km or less, and indications of an urban heat‐island circulation.
Tests of precipitation parameterizations available in latest version of NCAR regional climate model (RegCM) over continental United States
Authors: Giorgi Filippo , Shields Christine
Published: September 2002

This paper presents a series of tests of resolvable scale precipitation and cumulus convection schemes available in the latest version of the NCAR regional climate model (RegCM). For resolvable scale precipitation we use a new simplified explicit moisture (SIMEX) scheme, while for cumulus convection, we test the mass‐flux scheme of Grell [1993], the Kuo‐type scheme of Anthes [1977], and the newly implemented scheme of Zhang and McFarlane [1995]. The period of simulation is 3 years long, from March 1993 to February 1996, and the domain covers the continental United States at 60 km resolution. When using the SIMEX and Grell schemes, the model shows the best performance, with generally good simulation of precipitation, surface air temperature, and large‐scale circulations throughout the United States. Both averages and anomalies associated with individual seasons are well reproduced at the regional scale. Main model deficiences are an underestimate of cold season precipitation over the eastern United States and a displacement of the summer precipitation maximum over the central plains. The Kuo scheme has a performance more similar to that of the Grell scheme than that of the Zhang and McFarlane (ZMF) scheme. Use of the ZMF scheme yields a reasonably good simulation of regionally averaged precipitation, but deteriorates some aspect of the simulation such as the intensity of the summer low‐level jet over the central plains. We also present sensitivity tests to various parameters in the schemes which can be of guidance to the model user for optimizing the model performance over different regions.
Photolysis frequency measurements using actinic flux spectroradiometry during the PEM‐Tropics mission: Instrumentation description and some results
Authors: Shetter Richard E. , Müller Martin
Published: September 2002

The in situ photolysis frequencies for 11 molecules were determined using new actinic flux spectroradiometer systems mounted on the NASA DC‐8 research aircraft during the Pacific Exploratory Mission (PEM)‐Tropics mission. Photolysis frequencies for O3, NO2, HONO, CH2O, H2O2, CH3OOH, HNO3, PAN, CH3NO3, CH3CH2NO3, and acetone were calculated from the 30 s averaged actinic flux measurements. The accuracy of the actinic flux measurements was approximately ±11.5% in the UV‐B range and 8% in the UV‐A range. Uncertainties of the reported photolysis frequencies vary between ±15% and ±20% dependent on the quality of the molecular absorption cross section and quantum yield data. Approximately 139 hours of data were taken during 17 flights over the Pacific Ocean, and photolysis frequencies have been reported to the mission archive. During the mission, latitudes range from 45°N to 72.5°S, the longitude ranges from 10°W to 173°E, and the altitude ranges from sea level to 11.9 km. The geographical extents covered, combined with local times from sunrise to sunset, encompass solar zenith angles between 1° and >90° resulting in a broad range of photolysis frequencies. Persistent scattered clouds created photolysis frequency enhancements of approximately a factor of 2 over clear‐sky values and reductions of greater than 90% of clear‐sky values for portions of the mission.
A multilayer model for inferring dry deposition using standard meteorological measurements
Authors: Meyers Tilden P. , Finkelstein Peter , Clarke John , Ellestad Thomas G. , Sims Pamela F.
Published: September 2002

In this paper, we describe the latest version of the dry deposition inferential model, which is used to estimate the deposition velocities (Vd) for SO2, O3, HNO3, and particles with diameters less than 2 μm. The dry deposition networks operated by the National Oceanic and Atmospheric Administration (NOAA) and the Environmental Protection Agency (EPA) use this model to estimate dry deposition on a weekly basis. This model uses a multilayer approach, discretizing the vegetated canopy into 20 layers. The use of canopy radiative transfer and simple wind profile models allows for estimates of stomatal (rs) and leaf boundary layer (rb) resistances to be determined at each layer in the plant canopy for both sunlit and shaded leaves. The effect of temperature, water stress, and vapor pressure deficits on the stomatal resistance (rs) have been included. Comparisons of modeled deposition velocities are made with extensive direct measurements performed at three different locations with different crops. The field experiment is discussed in some detail. Overall, modeled O3 deposition velocities are in good agreement with measured values with the average mean bias for all surfaces of the order of 0.01 cm/s or less. For SO2, mean biases range from −0.05 for corn to 0.15 cm/s for soybeans, while for HNO3, they range from 0.09 for corn to 0.47 cm/s for pasture.
Measurements of peroxides and related species during the 1995 summer intensive of the Southern Oxidants Study in Nashville, Tennessee
Authors: Weinstein‐Lloyd J. B. , Lee J. H. , Daum P. H. , Kleinman L. I. , Nunnermacker L. J. , Springston S. R. , Newman L.
Published: September 2002

Hydroperoxide measurements are presented for 12 flights of the U.S. Department of Energy G‐1 aircraft during the summer 1995 intensive of the Southern Oxidants/Middle Tennessee study. A three‐channel analyzer, utilizing both peroxidase/p‐hydroxy phenylacetic acid (pOHPAA) and ferrous sulfate/benzoic acid (FeBA) reagents permitted continuous measurements of hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH or MHP), and hydroxymethyl hydroperoxide (HOCH2OOH or HMHP). The median concentration of total hydroperoxide was 5.2 ppbv, with median concentrations of 2.4, 1.7, and 0.97 ppbv for H2O2, MHP, and HMHP respectively. In the free troposphere, H2O2 concentration correlated with the concentration of its precursors, ozone, and water vapor. H2O2 profiles do not show elevated concentrations in the boundary layer, in contrast to vertical profiles of primary pollutants. The combination of high NOx concentrations that inhibited peroxide formation, and peroxide loss through deposition and reaction with OH, led to markedly lower peroxide concentrations within urban and power plant plumes. Photochemical calculations performed to estimate the magnitude of in‐plume peroxide depression showed reasonable agreement with observations.
Interhemispheric transport and the role of convective parameterizations
Authors: Gilliland A. B. , Hartley D. E.
Published: September 2002

An F11 tracer simulation is performed here using the most recent Community Climate Model, the CCM3, of the National Center for Atmospheric Research (NCAR). Another F11 simulation performed with NCAR's CCM2 [Hartley et al, 1994] is also presented. Comparing the F11 mixing ratio in the models to Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment (ALE/GAGE) F11 observations shows that the CCM3 overpredicts F11 in the northern midlatitudes, while the CCM2 has simulated F11 distributions similar to those observed. Calculating the interhemispheric exchange rate (τex) shows that the CCM3 τex is more than 35% slower than the CCM2. To locate why the discrepancy exists between the two models' rate of transport, tropical deep convection, the primary mechanism of interhemispheric transport, is evaluated. The CCM3 convective precipitation and convective mass flux are significantly less than in the CCM2 during the summer, coinciding with differences in the models' monthly interhemispheric transport rate. However, only the convective mass flux shows significant differences in the annual averages between the CCM2 and CCM3. It appears that the CCM3 interhemispheric transport may be slower because the CCM3 has less convective activity than the CCM2, which is most evident in the convective mass flux. The vertical velocity associated with the tropical convective area is also significantly weaker in the CCM3 during the same period when the CCM3 has less convective activity. A past study including a cumulus momentum transport parameterization with the Zhang‐McFarlane convective scheme [Zhang and McFarlane, 1995b] suggests that a key issue for the CCM3 τex may involve the parameterization's vertical resolution and lack of accounting for the momentum flux. Further research into both the subgrid scale convective mass flux and the large‐scale circulation effects of the convective parameterization would be helpful in determining what is needed to improve the CCM3 interhemispheric transport rate.
Sulfate and nitrate firn concentrations on the Greenland ice sheet: 2. Temporal anthropogenic deposition changes
Authors: Fischer Hubertus , Wagenbach Dietmar , Kipfstuhl Joseph
Published: September 2002

Intercomparison of three new chemical ice core records from northern Greenland (covering the time span from approximately 1500 A.D. to present) with previously published records for southern and central Greenland reveals a uniform timing of anthropogenic changes in sulfate and nitrate firn concentrations over the entire ice sheet. The anthropogenic sulfate increase started around 1890, was interrupted by a transient decrease in the 1930s, and has resumed a major increase since 1950. Since the late 1970s though, a significant 30% decline in Greenland sulfate firn levels can be documented. The maximum anthropogenic increase in northern Greenland sulfate firn concentrations (up to 200–230 ppb) is 2–3 times larger than in southern and central Greenland. Nitrate records show an essentially steady increase since 1950 and, documented for the first time, a slight reduction during most recent years. Maximum nitrate firn levels of 100–130 ppb exceed the preindustrial background by 100% all over the Greenland ice sheet. Comparison with anthropogenic SO2 and NOx emission records indicates that the major increase in sulfate firn concentrations since 1950 can be attributed to Eurasian sources, while firn levels during the first half of this century appear to be dominated by North American emissions. A stronger North American source contribution is indicated over the entire 20th century in the case of nitrate. Applying a macroscopic deposition model separate time series for wet and dry deposition were derived which revealed a close correspondence of wet deposited sulfate with the timing of U.S. emissions, while the temporal evolution of Eurasian emissions is mainly reflected in the dry sulfate deposition record. During this century wet sulfate deposition increased by a factor of two while the total dry sulfate deposition flux increased by more than 500%. Wet and dry nitrate deposition both increased by 100% during the same period.
Comparison of observed and modeled direct aerosol forcing during TARFOX
Authors: Hignett Phillip , Taylor Jonathan P. , Francis Peter N. , Glew Martin D.
Published: September 2002

Aircraft measurements have been made of the downward and upward solar irradiance under cloud‐free conditions over a range of aerosol loadings in the summer haze plume off the East Coast of the United States during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Optical properties calculated from measured aerosol microphysical and chemical properties have been used as input to a shortwave radiative transfer model. This model was used to calculate the diurnally averaged direct aerosol forcing and to compare with values deduced from the aircraft radiative measurements. The modeled and observed forcings agree well when the aerosol has a significant absorbing component.
Evaluation of land‐surface interaction in ECMWF and NCEP/NCAR reanalysis models over grassland (FIFE) and boreal forest (BOREAS)
Authors: Betts Alan K. , Viterbo Pedro , Beljaars Anton , Pan Hua‐Lu , Hong Song‐You , Goulden Mike , Wofsy Steve
Published: September 2002

The National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium‐Range Weather Forecasts (ECMWF) reanalysis models are compared with First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) grassland data from Kansas in 1987 and Boreal Ecosystem‐Atmosphere Study (BOREAS) data from an old black spruce site in 1996 near Thompson, Manitoba. Some aspects of the comparison are similar for the two ecosystems. Over grassland and after snowmelt in the boreal forest, both models represent the seasonal cycle of near‐surface temperature well. The two models have quite different soil hydrology components. The ECMWF model includes soil water nudging based on low level humidity errors. While this works quite well for the FIFE grassland, it appears to give too high evaporation over the boreal forest. The NCEP/NCAR model constrains long‐term drifts by nudging deep soil water toward climatology. Over the FIFE site, this seems to give too low evaporation in midsummer, while at the BOREAS site, evaporation in this model is high. Both models have some difficulty representing the surface diurnal cycle of humidity. In the NCEP/NCAR reanalysis this leads to errors primarily in June, when the surface boundary layer stays saturated and too much precipitation occurs. In the ECMWF reanalysis there is a morning peak of mixing ratio, which an earlier work showed resulted from too shallow a boundary layer in the morning. Over the northern boreal forest there are important physical processes, which are not represented in either reanalysis model. In particular very high model albedos in spring, when there is snow under the forest canopy, lead to a very low daytime net radiation. This in turn leads to a large underestimate of the daytime surface fluxes, particularly the sensible heat flux, and to daytime model surface temperatures that are as much as 15 K low. In addition, the models do not account for the reduction in evaporation associated with frozen soil, and they generally have too large evapotranspiration in June and July, probably because they do not model the tight stomatal control of the coniferous forest.
Parameterization of the convective transport in a two‐dimensional chemistry‐transport model and its validation with radon 222 and other tracer simulations
Authors: Dvortsov Victor L. , Geller M. A. , Yudin V. A. , Smyshlyaev S. P.
Published: September 2002

Deep cumulus convection is essential for understanding and modeling of the atmospheric composition, providing a mechanism by which short‐lived species can be quickly transported from the planetary boundary layer to the upper troposphere. Because it is a small scale process, highly irregular in time and space, so far most of the scientific community has been skeptical about the legitimacy of zonally averaged representations of tracer convective transport in two‐dimensional (2‐D) models. To justify and validate such an approach, we derive our parameterization of zonally averaged convective transport from a 3‐D model and then validate our 2‐D results against the results from that 3‐D model using radon 222 and other tracer simulations. We show, both analytically and by comparing 2‐D and 3‐D model results, that straightforward substitution of zonally averaged parameters into the equations of tracer convective transport produces large errors because the intensity of the convection and the tracer concentration in the updraft and in the environment can be strongly correlated. If such correlations are taken into account in the zonally averaged equations for convective transport, the agreement between 2‐D and 3‐D results can be significantly improved. We show that our 2‐D model with monthly mean convective mass fluxes, properly zonally averaged, is capable of reproducing, with reasonable accuracy, the time series of 222Rn and other tracers obtained from the 3‐D simulations. Our convective transport algorithm and input data are available via anonymous ftp.
Chemical and microphysical properties of marine stratiform cloud in the North Atlantic
Authors: Borys R. D. , Lowenthal D. H. , Wetzel M. A. , Herrera F. , Gonzalez A. , Harris J.
Published: September 2002

The chemical and microphysical properties of marine stratiform cloud were measured at a ridgetop elevation of 992 m above mean sea level (AMSL) on Tenerife in the Canary Islands in the eastern North Atlantic during the summers of 1995 and 1996. The results show an inverse relationship between hourly‐averaged cloud droplet diameter and droplet number concentration, which ranged from 116 to 1355 cm−3. Strong relationships were observed between droplet number and equivalent clear air concentrations of non‐sea‐salt sulfate, nitrate, and elemental carbon in the droplets. Droplet sizes inferred from radiances measured by satellite for clouds offshore and upwind agreed with droplet sizes derived for clouds over the mountain sampling site, and also with those measured in cloud 4–5 hours later. Estimated cloud short‐wave radiative forcing was enhanced by 8% in radiative model studies of polluted versus clean clouds with droplet concentrations of 786 and 127 cm−3 and droplet effective radii of 6 and 10 μm, respectively.
Model of electric charges in thunderstorms and associated lightning
Authors: Mazur Vladislav , Ruhnke Lothar H.
Published: September 2002

An axisymmetrical electrostatic model of charges in a thunderstorm cloud at the mature stage is used to initiate a bidirectional vertical leader that develops into either a cloud‐to‐ground or an intracloud flash from the regions of maximum electric field. The principal result of this study is the determination of the physical and quantitative relationships among cloud charges, potentials, and electric fields, and the induced charges, currents, and electric field changes of the lightning channel from the numerical solution of the Poisson equation for a cloud charge model with lightning. An important consequence of the model is that the upper part of the cloud‐to‐ground leader and the lower part of the intracloud leader terminate inside the cloud.
Modeling ground heat flux in land surface parameterization schemes
Authors: Liang Xu , Wood Eric F. , Lettenmaier Dennis P.
Published: September 2002

A new ground heat flux parameterization for land surface schemes, such as those used in climate and numerical weather prediction models, is described. Compared with other approaches that lump the canopy layer and ground surface, or empirically based approaches that consider the effect of radiation attenuation through the canopy layer, the new parameterization has several advantages. First, the reduction of radiation available for conducting soil surface exchange under vegetated areas is represented in a manner that assures that heat is conserved in the long term. Second, problems in representing properly the phase of the ground heat flux are alleviated. Finally, the approach is relatively simple and is computationally efficient, requiring only two soil thermal layers. Comparison of the method with analytical solutions for special cases shows that the new method approximates the analytical solution very well for different conditions, and that the new method is superior to the force‐restore and the Crank‐Nicholson method. Model‐derived ground heat heat fluxes for the French HAPEX‐MOBILHY (Hydrology‐Atmosphere Pilot Experiment ‐ Modelisation de Bilan Hydrique) site and the Brazilian ABRACOS (Anglo‐Brazilian Amazonian Climate Observation Study) cleared ranch land site are shown to be in close agreement with observations. Sensitivity analyses show that if the attenuation of radiation under vegetation and soil heat storage are ignored, the daytime peak and nighttime minima of ground heat flux, latent and sensible heat fluxes, and surface temperature can be significantly in error. In particular, neglecting the radiation attenuation through the canopy layer can result in significant overestimation (underestimation) of daytime (nighttime) ground heat flux, while neglecting soil heat storage can result in significant phase errors.
Simulation of potential impacts of man‐made land use changes on U.S. summer climate under various synoptic regimes
Authors: Pan Zaitao , Takle Eugene , Segal Moti , Arritt Raymond
Published: September 2002

This study evaluates impacts of land use changes due to human settlement on regional summer climate over the central and western United States by performing 30‐day simulations during normal, drought, and flood years. Under current land use the simulated evapotranspiration increased noticeably over the central United States where grassland has been replaced by crops. Simulated evapotranspiration decreased slightly in the western United States. These changes produced wetter and cooler surface air over the central United States and slightly drier and warmer air over the western United States. Responses of surface fluxes and thus screen height variables to land use changes were consistent from year to year, whereas rainfall showed strong interannual variations because of the combination of various dynamic processes involved in precipitation. For normal year conditions, average evapotranspiration and rainfall under current land use increased by 18% and 8%, respectively, over the central United States, whereas they slightly decreased in the western United States. In both flood and drought years, current land use exhibited a rainfall increase in the western United States and a decrease over the central United States. The decrease of rainfall with increased evapotranspiration in the central United States was likely associated with weakening of the dynamic forcing needed to produce precipitation.
Simulating the water balance of the Aral Sea with a coupled regional climate‐lake model
Authors: Small Eric E. , Sloan Lisa Cirbus , Hostetler Steven , Giorgi Filippo
Published: September 2002

Before coupled atmosphere‐lake models can be used to study the response of large lake systems to climatic forcings, we must first evaluate how well they simulate the water balance and associated lake atmosphere interactions under present‐day conditions. We evaluate the hydrology simulated by a lake model coupled to NCAR's regional climate model (RegCM2) in a study of the Aral Sea. The meteorological variables that are input to the lake model are simulated well by RegCM2. Simulated surface air temperatures closely match observed values, except during spring and fall when the simulated temperatures are too cold. The magnitude of precipitation is too high in the region surrounding the Aral Sea during summer and fall. On a yearly basis, RegCM2 produces a reasonable amount of runoff throughout the drainage basin. The lake model coupled to RegCM2 accurately simulates Aral Sea surface temperatures (SSTs). The lake model also simulates observed mid‐winter ice fraction well, although the onset of ice growth occurs too late in the year and the ice melts too rapidly in the spring. The simulated annual evaporation from the Aral Sea is consistent with observed estimates; however, the simulated evaporation is greater than observed during summer and less than observed during winter. In a “stand‐alone” lake model simulation, the simulated Aral Sea hydrology does not match observations as closely as in the coupled model experiment. These results suggest that a stand‐alone lake model would not accurately simulate the hydrologic response of the Aral Sea to various forcings.
Possible self‐organizing dynamics for land‐atmosphere interaction
Authors: Rodríguez‐Iturbe Ignacio , D'Odorico Paolo , Rinaldo Andrea
Published: September 2002

Land‐atmosphere interactions play a central role in the mesoscale evolution of hydrologic fields like rainfall and soil moisture. A water balance model, capturing the essence of such interaction and driven by mesoscale and synoptic processes, was found to self‐organize into states showing global scaling features both in space and in time, which we also observed experimentally. Although driving synoptic mechanisms are crucial, a key role was found for the local dynamical rules through which the system self‐organizes. The results suggest the possibility of a dynamic origin for the space‐time scaling in precipitation from purely local dynamics and may have implications for modeling efforts which involve land‐atmosphere interaction.