bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of wind direction, wind waves, and density stratification upon sediment transport in shelf edge regions: The Iberian shelf
Authors: Xing Jiuxing , Davies Alan M.
Published: October 2002

A prognostic baroclinic model including a suspended sediment transport module is used in cross‐sectional form to examine processes, namely, internal tide, wind, and wind waves, which lead to cross‐shelf sediment movement. Although the main focus is a process study to quantify the role of various mechanisms giving rise to ocean‐shelf sediment exchange, the calculations are set in the context of the Iberian shelf. Eddy viscosity and diffusivity are computed from a turbulence energy submodel. Calculations with upwelling and downwelling favorable winds showed that friction velocity and turbulent mixing were a maximum in near‐coastal regions. In these areas, suspended sediment concentration was a maximum. For upwelling favorable winds in homogeneous water, near‐bed sediment was moved toward the coast where upwelling occurred. In this area, sediment was transported into the near‐surface layer and advected offshore by the wind‐driven flow. For the parameters considered here, advection in the near‐surface layer exceeded vertical settling, and maximum sediment concentration occurred at the surface. For a downwelling favorable wind the strong surface currents are onshore, and hence any sediment in the surface layer is advected toward the coast. Although the bottom current is off‐shelf, it is weak, and hence the off‐shelf export of sediment in this layer is less than that found in the surface layer with an upwelling favorable wind. In stratified conditions, with an upwelling favorable wind the intensity of the stratification at the shelf edge increases. Associated with this, there is an increase in the vertical sediment concentration gradient. With downwelling favorable winds, rapid mixing occurs on the shelf, and a shelf front is formed. Sediment concentration on the shelf and at the shelf edge shows a similar distribution to that found under homogeneous conditions. When tidal forcing is included an internal tide is produced that interacts with the wind‐driven flow in a nonlinear manner. Increased bed stress due to the internal tide and vertical circulation cells lead to enhanced sediment erosion above that found with wind forcing only. For an upwelling wind this leads to enhanced off‐shelf sediment movement in the surface layer. For a downwelling wind a circulation develops that tends to inhibit off‐shelf sediment transport. The effect of wind waves is to increase significantly sediment suspension on the shelf. Although the concentration of the sediment is influenced by the wind waves, its spatial distribution is determined by the wind and tidal flows. A detailed analysis of the terms in the sediment transport equation is used to quantify the effects of each process upon the sediment distribution in the water column.
Wind‐ and buoyancy‐forced upper ocean circulation in two‐strait marginal seas with application to the Japan/East Sea
Authors: Spall Michael A.
Published: October 2002

The wind‐ and buoyancy‐forced upper ocean circulation in a marginal sea connected to the open ocean through two straits is investigated using idealized numerical and analytical models. The study is motivated by the Japan/East Sea (JES) and other marginal seas found along the western North Pacific. It is shown that for anticyclonic wind stress curl and atmospheric cooling in the marginal sea, the inflow transport branches into eastern and western boundary currents, in qualitative agreement with observed branching of the Tsushima Current in the southern JES. The eastern boundary current arises because wind forcing in the open ocean, and a circulation integral around the island that separates the marginal sea from the open ocean, maintains the temperature on the island to be warmer than that found in the interior of the marginal sea. Buoyancy forcing in the marginal sea plays a key role in maintaining the eastern boundary current. The dynamics that control the water mass transformation and downwelling are described and related to the model parameters. The largest heat loss to the atmosphere is found in the eastern boundary current. The exchange rates with the open ocean, downwelling within the marginal sea, and current structure within the marginal sea predicted by a linear analytic theory compare closely with results from a shallow water numerical model.
Observations of wave‐generated vortex ripples on the North Carolina continental shelf
Authors: Ardhuin Fabrice , Drake T. G. , Herbers T. H. C.
Published: October 2002

Sand ripples with wavelengths between 0.5 and 3 m were observed on the bottom across the U.S. east coast continental shelf off North Carolina during three side‐scan sonar surveys in September and December 1999. Ripples were present in about 75% of the survey images, in particular, in regions with coarser sediments. Analysis of surficial sediment samples shows that median grain diameters range from 0.1 to 4.7 mm with large variations on the inner shelf over distances <1 km. The observed ripple properties are consistent with wave‐generated vortex ripples. Analysis of concurrent wave observations indicates that the ripple crests were aligned perpendicular to the average direction of near‐bottom wave‐induced motions during preceding events that were sufficiently energetic to mobilize surficial sediments. Furthermore, the ripple wavelengths proportionality to near‐bottom wave orbital excursions is consistent with wave‐formed vortex ripples. These findings support the hypothesis that the observed strong attenuation of waves across the shelf resulted from form drag over large vortex ripples.
Oceanographic observations in Chilean coastal waters between Valdivia and Concepción
Authors: Atkinson Larry P. , Valle‐Levinson Arnoldo , Figueroa Dante , De Pol‐Holz Ricardo , Gallardo Victor A. , Schneider Wolfgang , Blanco Jose L. , Schmidt Mike
Published: October 2002

The physical oceanography of the biologically productive coastal waters of central Chile (36° to 40°S) is relatively unknown. In December 1998 we made a short exploratory cruise between Valdivia (40°S) and Concepción (37.8°S) taking temperature, salinity, oxygen, and current velocity profiles. Coincident sea surface temperature and color measurements were obtained by satellite. The results showed an area dominated by wind‐induced coastal upwelling, river runoff, intrusion of offshore eddies, mixing, and heating. Upwelling centers were found over the shelf at three locations: inshore of Mocha Island, off Valdivia, and off Lavapie Point. At these centers, equatorial subsurface water (ESSW) intrudes into the coastal waters, sometimes affecting the surface waters. Since ESSW has characteristically low‐oxygen and high‐salinity values, it is easily detected. Off Valdivia, runoff imparts stratification, while farther north, solar heating and reduced mixing may facilitate stratification. In some areas, even strong winds would not destroy the stratification imparted by the advection of buoyancy that occurs during the upwelling process. Strong equatorward currents (>1 m s−1) in the form of an upwelling jet were found off Lavapie Point. This is also the location of an intruding anticyclone. Elsewhere, currents were mainly northward but highly variable because of intrusions from offshore eddies. The sea surface temperature and ocean color images show a complex field of onshore and offshore intrusions combined with the effects of mixing on chlorophyll concentrations. The residence time of upwelled water on the shelf is estimated to be less than 1 week.
Deterministic modeling of driving and dissipation for ocean surface gravity waves in two horizontal dimensions
Authors: Willemsen Jorge F.
Published: October 2002

Previous work introducing deterministic modeling of driving and dissipation to nonlinear surface gravity wave dynamics is extended to two horizontal spatial dimensions. It is shown that the wave spectrum rapidly develops into a form with two important features. First the spectral peak location is determined by the wind speed and shifts toward lower wave numbers as a function of time. This is due in part to nonlinear interactions, but it is also due to the functional form of the wind‐forcing term. In addition, the spectrum rapidly develops an asymptotic power law tail in the downwind direction. The spectral exponent governing the asymptotics is sensitively dependent on the precise form of the dissipation term, and it can be “tuned” by adjusting that term in a quantitatively established manner. The angular dependence of the wave spectrum is also obtained. The strength and the role of the nonlinear interactions in the development of the spectral shape are studied in detail. The question of whether the wave amplitude statistics approach a Gaussian form is investigated. We find that a low‐order odd moment is nonvanishing.
Linear modeling of infragravity waves during Delilah
Authors: Reniers A. J. H. M. , van Dongeren A. R. , Battjes J. A. , Thornton E. B.
Published: October 2002

The generation of infragravity waves by directionally spread shortwaves incident on an alongshore uniform beach is investigated. Two mechanisms responsible for the generation of infragravity waves are considered: the release of the bound infragravity waves associated with changes in the spatial variation of the incident shortwave energy and the forcing of trapped waves by obliquely incident directionally spread shortwaves. The infragravity wave response is examined using linear shallow water equations, taking into account the presence of bottom friction, setup of the mean water level, rollers, and longshore current. Summing all infragravity contributions due to difference interactions between pairs of shortwave spectral components with the same frequency difference results in infragravity energy density spectra. Calculated spectra are compared with measured spectra obtained during the Delilah field experiment for 11 consecutive days, including calm, mild, and storm conditions. In the comparison, attention is focused on the frequency distribution of the infragravity energy density throughout the surf zone and the transformation of the root mean square infragravity wave height. The measured infragravity response in the frequency range from 0.01 to 0.06 Hz is reproduced by the computations, with relatively broad spectra offshore and a clear nodal structure closer to shore. Typically 80% of the infragravity wave height variability is explained by the model of which 30% or less is due to bound infragravity waves. Differences occur mainly during storm events in which the infragravity energy is underpredicted in the outer surf zone and overpredicted in the inner surf zone.
Variability and predictability of the North Atlantic wave climate
Authors: Woolf D. K. , Challenor P. G. , Cotton P. D.
Published: October 2002

Wave climate across the ocean basins can be described using satellite altimetry; here, we concentrate on the North Atlantic region. Waves in the North Atlantic are strongly seasonal and peak in the winter season. The northeastern sector of the Atlantic and adjoining shelf seas also exhibit exceptionally high interannual variability in the winter, with monthly average significant wave height varying by up to a factor of 2 from one year to the next. The strength and geographical distribution of variability is broadly consistent throughout the winter months (December–March). A large fraction of these wave height anomalies is associated with a single pattern of pressure anomalies that resembles the North Atlantic Oscillation (NAO). A predictor based on NAO dependence is “trained” from relatively recent satellite data and then tested against earlier satellite and in situ data. The predictor is successful in large areas of the North Atlantic, confirming a robust relationship between wave height anomalies and the NAO over the last few decades. A substantial rise (up to 0.6 m) in monthly mean wave heights on the northeastern Atlantic during the latter part of the twentieth century is attributable to changes in the NAO. Substantial residual anomalies in wave heights exist after the influence of the NAO has been subtracted; these are partly explained by a second pair of North Atlantic patterns in wave height anomalies and sea level pressure anomalies. This “East Atlantic” pattern is particularly influential in midwinter and affects the southern part of the northeastern sector (including the region of Seven Stones Light Vessel).
Application of an internal tide generation model to baroclinic spring‐neap cycles
Authors: Gerkema T.
Published: October 2002

A numerical internal tide generation model is used to study the behavior of baroclinic spring‐neap cycles in open basins and channels. The motivation for this study comes from an observation on tidal currents in Faeroe‐Shetland Channel, which is briefly described; a surprisingly large baroclinic semidiurnal tidal signal is found in the lower part of the water column above the slope on the Shetland side, concurring with barotropic neap tides. The numerical model results indeed show that the baroclinic spring‐neap cycle may have a phase shift with respect to the barotropic spring‐neap cycle and that the phase of the baroclinic cycle may vary strongly within short distances. It is also shown that even small changes in the background conditions (e.g., stratification) can provoke a large phase shift in the baroclinic cycle; a possible connection to “intermittency” is discussed.
Transport of mineral and anthropogenic aerosols during a Kosa event over East Asia
Authors: Uematsu Mitsuo , Yoshikawa Akifumi , Muraki Hiroaki , Arao Kimio , Uno Itsushi
Published: October 2002

A quantitative appraisal of the formation processes and distribution of mineral aerosols internally mixed with anthropogenic substances is necessary for estimating the aerosol radiative forcing, especially over the North Pacific region. A large‐scale Kosa (“Yellow Dust”) phenomenon associated with the movement of a low‐pressure system was observed at Nagasaki, about 1000 km east of the Asian continent in the spring of 1996. During the Kosa period (April 16–17), fine sulfur particles appeared first, and after 12 hours, a major plume of large mineral particles reached Nagasaki, which was detected and measured by the single‐particle analysis with sampling intervals of 2 hours. The results demonstrate that Asian dust storms are occasionally capable of transporting mineral and anthropogenic particles in separate air masses from the source regions via the marginal seas to the western North Pacific. We note that it takes a certain time to admix the different air masses containing various types of aerosols along their transport paths from the Asian continent to remote marine atmosphere. The interactions of mineral particles with anthropogenic substances predominantly take place within the marine boundary layer toward the east over the North Pacific.
Water‐soluble organic compounds in biomass burning aerosols over Amazonia 1. Characterization by NMR and GC‐MS
Authors: Graham Bim , Mayol‐Bracero Olga L. , Guyon Pascal , Roberts Gregory C. , Decesari Stefano , Facchini M. Cristina , Artaxo Paulo , Maenhaut Willy , Köll Peter , Andreae Meinrat O.
Published: October 2002

As part of the European contribution to the Large‐Scale Atmosphere‐Biosphere Experiment in Amazonia (LBA‐EUSTACH), aerosols were sampled at representative pasture and primary rainforest sites in Rondônia, Brazil, during the 1999 “burning season” and dry‐to‐wet season transition (September‐October). Water‐soluble organic compounds (WSOCs) within the samples were characterized using a combination of1H Nuclear Magnetic Resonance (NMR) spectroscopy for chemical functional group analysis, and Gas Chromatography‐Mass Spectrometry (GC‐MS) for identification and quantification of individual low‐molecular‐weight compounds. The1H NMR analysis indicates that WSOCs are predominantly aliphatic or oxygenated aliphatic compounds (alcohols, carboxylic acids, etc.), with a minor content of aromatic rings carrying carboxylic and phenolic groups. Levoglucosan (1,6‐anhydro‐β‐D‐glucose), a well‐known cellulose combustion product, was the most abundant individual compound identified by GC‐MS (0.04–6.90 μg m−3), accounting for 1–6% of the total carbon (TC) and 2–8% of the water‐soluble organic carbon (WSOC). Other anhydrosugars, produced by hemicellulose breakdown, were detected in much smaller amounts, in addition to series of acids, hydroxyacids, oxoacids, and polyalcohols (altogether 2–5% of TC, 3–6% of WSOC). Most correlated well with organic carbon, black carbon, and potassium, indicating biomass burning to be the major source. A series of sugar alcohols (mannitol, arabitol, erythritol) and sugars (glucose, fructose, mannose, galactose, sucrose, trehalose) were identified as part of the natural background aerosol and are probably derived from airborne microbes and other biogenic material. The bulk of the WSOCs (86–91% WSOC) eluded analysis by GC‐MS and may be predominantly high‐molecular weight in nature.
Diurnal march of the convection observed during TRMM‐WETAMC/LBA
Authors: Machado Luiz A. T. , Laurent Henri , Lima Alexandra A.
Published: October 2002

Radiosonde, satellite data, Tropical Ocean–Global Atmosphere (TOGA) radar 2 km constant altitude plan position indicator (CAPPI), and rainfall collected from the TRMM‐Wet Season Atmospheric Mesoscale Campaign (WETAMC)/Large‐Scale Biosphere‐Atmosphere (LBA) Experiment in Amazonia have been used to investigate the diurnal cycle of the tropical convection. Geostationary Operational Environmental Satellite (GOES 8) images were used to describe the diurnal modulation of the total/high/convective cloud fraction and the diurnal evolution of the size spectrum and initiation/dissipation of the convective systems. Radar 2 km CAPPI were used to describe the diurnal cycle of the rain fraction for different thresholds and the diurnal evolution of the size spectrum and initiation/dissipation of the rain cells. An average over the four rain gauge networks was applied to describe the average hourly rainfall. The upper air network data set was used to compute the thermodynamic variables: equivalent potential temperature (θe), convective available potential energy (CAPE), thickness of positive buoyancy, instability, and convective inhibition. High and convective cloud area fractions reach their maximum some hours after the maximum rainfall detected by rain gauge and radar 2 km CAPPI. The minimum cloud cover occurs only a few hours before the maximum precipitation and the maximum cloud cover occurs during the night. The maximum rainfall takes place at the time of the maximum initiation of the convective systems observed by satellite and rain cells. At the time of maximum precipitation the majority of the convective systems and rain cells are small sized and present the maximum increasing area fraction rate. The diurnal evolution of θealso presents a very clear diurnal variation, with maximum occurring in the early afternoon. The CAPE is well related to θe. When θeis high CAPE is high; the atmosphere is unstable and has a deep layer of positive buoyancy and small convective inhibition. These results suggest the following mechanism controlling the diurnal of convection: In the morning, cloud cover decreases as the solar flux reaching the surface increases and consequently increases θe. In the early afternoon, convection rapidly develops, high and convective cloud fractions increase rapidly, and the maximum precipitation and initiation is observed. After convection is developed the atmosphere profile is modified, reaching a nearly saturated state; the water vapor flux decreases in the boundary layer which becomes very stable, thereby inhibiting surface fluxes and consequently extinguishing the convection.
Biogenic NO emissions from forest and pasture soils: Relating laboratory studies to field measurements
Authors: van Dijk S. M. , Gut A. , Kirkman G. A. , Gomes B. M. , Meixner F. X. , Andreae M. O.
Published: October 2002

During September and October 1999, dynamic chamber measurements were carried out to determine nitric oxide (NO) fluxes from a primary forest soil and an old pasture in the Brazilian Amazon basin as part of the project “European Studies of Trace Gases and Atmospheric Chemistry as a Contribution to the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia” (LBA‐EUSTACH). In addition, soil samples were collected from these two sites, and laboratory experiments were conducted to determine the NO production and consumption rate constants as functions of soil temperature and soil moisture. These laboratory results were converted into NO fluxes using a simple algorithm, which required additional information on the gas diffusion in soil, the soil bulk density, and the field conditions (soil temperature and soil moisture). Over the entire measurement period, the calculated and measured NO fluxes agreed well both for the forest (6.9 ± 2.9 and 5.0 ± 4.6 ng m−2 s−1, respectively) and for the pasture (0.67 ± 0.09 and 0.65 ± 0.37 ng m−2 s−1, respectively). Forest to pasture conversion decreased NO production and gas diffusion and resulted in smaller NO fluxes from pasture than forest soil.
On the surface and atmospheric temperature changes following the 1991 Pinatubo volcanic eruption: A GCM study
Authors: Yang Fanglin , Schlesinger Michael E.
Published: October 2002

Ensemble numerical simulations are performed using an atmospheric general circulation model to study the responses of the atmosphere to volcanic aerosol forcing and sea surface temperature (SST) forcing during the two years following the Pinatubo eruption. The simulated surface air temperature (SAT) anomalies, forced respectively and jointly by the Pinatubo aerosol and the observed SST anomalies, are compared to those derived from observations using a singular value decomposition (SVD) model. The simulated land SAT anomalies, forced only by the observed SST anomalies, match well in magnitude and time the leading mode of SAT anomalies from the SVD analysis, which represents the ENSO‐forced SAT variations. When forced by both the Pinatubo aerosol and the observed SST anomalies, the model captures the observed surface cooling over land in June–July–August and September–October–November 1992 but misses the surface warming over northern Eurasia in December–January–February (DJF) 1991–1992 and DJF 1992–1993. In the atmosphere the simulated temperature anomalies in the troposphere (stratosphere) forced by the Pinatubo aerosol are sensitive (insensitive) to the initial conditions and the type of prescribed SST. The signal of SST forcing is stronger in the troposphere and near the surface than in the stratosphere, while the signal of the Pinatubo aerosol forcing is strongest in the lower stratosphere. The simulated global‐mean stratospheric temperature anomalies induced by the Pinatubo aerosol are generally 1°C to 1.5°C larger than observed. Empirical data analyses and numerical model simulations showed that this discrepancy is explained in part by the influences of the quasi‐biennial oscillation and the observed ozone depletion, which the ensemble numerical simulations did not resolve.
A case study of convective organization into precipitating lines in the Southwest Amazon during the WETAMC and TRMM‐LBA
Authors: Silva Dias M. A. F. , Petersen W. , Silva Dias P. L. , Cifelli R. , Betts A. K. , Longo M. , Gomes A. M. , Fisch G. F. , Lima M. A. , Antonio M. A. , Albrecht R. I.
Published: October 2002

A case study of convective development in the Southwest Amazon region during the Wet Season Atmospheric Mesoscale Campaign (WETAMC) and Tropical Rainfall Measuring Mission (TRMM)/Large‐Scale Biosphere‐Atmosphere (LBA) Experiment in Amazonia is presented. The convective development during 7 February 1999 is shown to occur during a period of very weak large‐scale forcing in the presence of topography and deforestation. The available data include dual Doppler radar analysis, radiosonde launches, and surface and boundary layer observations. The observational analysis is complemented with a series of model simulations using the RAMS with 2‐km resolution over a 300 km × 300 km area forced by a morning radiosonde profile. A comparison of the observed and simulated thermodynamic transformation of the boundary layer and of the formation of convective lines, and of their kinematic and microphysical properties is presented. It is shown that only a few very deep and intense convective cells are necessary to explain the overall precipitating line formation and that discrete propagation and coupling with upper atmosphere circulations may explain the appearance of several lines. The numerical simulation indicates that topography may be the cause of initial convective development, although later on the convective line is parallel to the midlevel shear. There are indications that small‐scale deforestation may have an effect on increasing rainfall in the wet season when the large‐scale forcing is very weak.
Contrasting convective regimes over the Amazon: Implications for cloud electrification
Authors: Williams E. , Rosenfeld D. , Madden N. , Gerlach J. , Gears N. , Atkinson L. , Dunnemann N. , Frostrom G. , Antonio M. , Biazon B. , Camargo R. , Franca H. , Gomes A. , Lima M. , Machado R. , Manhaes S. , Nachtigall L. , Piva H. , Quintiliano W. , Machado L. , Artaxo P. , Roberts G. , Renno N. , Blakeslee R. , Bailey J. , Boccippio D. , Betts A. , Wolff D. , Roy B. , Halverson J. , Rickenbach T. , Fuentes J. , Avelino E.
Published: October 2002

Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol‐rich October and aerosol‐poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the “green ocean” westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.
Near‐surface variations and surface fluxes over the northern Bay of Bengal during the 1999 Indian summer monsoon
Authors: Bhat G. S.
Published: October 2002

Detailed observations of the atmosphere and the ocean were carried out in the Bay of Bengal during the peak monsoon months of July and August 1999 under the Bay of Bengal Monsoon Experiment (BOBMEX). The emphasis in BOBMEX was on time series observations of the ocean and the atmosphere from fixed locations in the bay. Indian research vessels INS Sagardhwani and ORV Sagar Kanya were deployed at 13°N, 87°E and 17.5°N, 89°E in the southern and northern Bay of Bengal, respectively. This paper describes the near‐surface characteristics during phase I (27 July to 6 August) and phase II (13–24 August) of BOBMEX observed at the northern location. During phase I, average surface pressure was 999 hPa, average winds were 10 m s−1, and three monsoon systems developed. Sea surface temperature (SST) remained around 28.5°C. Phase I also saw the arrival of low‐saline water plume, and the surface salinity decreased from 33 practical salinity units (psu) to less than 29 psu in a span of just 4 days. During phase II of BOBMEX, average surface pressure was 1006 hPa, average wind speed was lower at 7.5 m s−1, and no monsoon system developed. There was a weak phase of convection characterized by low winds and clear‐sky conditions, during which period SST increased from around 28°C to more than 29.5°C in 5 days. Convective available potential energy of the surface air decreased by 2–3 kJ kg−1 following convection and recovered in a time period of 1 or 2 days. SST predicted by using observed surface fluxes showed that horizontal advection is important in the northern bay and that a one‐dimensional heat balance model works sometimes, but not always. It is also shown that near‐surface characteristics over the northern Bay of Bengal are different from those observed over the west Pacific warm pool.
A Coupled Land‐Atmosphere Simulation Program (CLASP): Calibration and validation
Authors: Gutowski William J. , Vörösmarty Charles J. , Person Mark , Ötles Zekai , Fekete Balazs , York Jennifer
Published: October 2002

We present a model and application designed to study the coupled land‐atmosphere hydrologic cycle, following water from its inflow into a region by horizontal atmospheric transport through surface‐atmosphere exchange processes and aquifer recharge to outflow as runoff and river discharge. The model includes a two‐way water flow among its major reservoirs (atmosphere, vadose zone, groundwater, surface water, river). A unique feature of the model is that phreatophytic interactions are included when the water table intersects the root zone. The model emulates a uniform grid box of an atmospheric general circulation model, but with finer horizontal resolution for the land processes, and forms a test bed for developing continental‐scale simulation of the hydrologic cycle. The model is calibrated using the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations for 1987 and validated using FIFE observations for 1988 and 1989. Four physical factors emerge as important for simulating the FIFE water cycle: effective relative humidity for initiating stable (large scale) condensation, length of the growing season, amount of available soil water, and cloud cover parameterization. Further evaluation uses water table and river discharge measurements for years up to 1993. The model simulates multiyear behavior in the hydrologic cycle reasonably well. Average differences between FIFE observations and simulated fluxes during the calibration period are only a few percent, including fluxes not specifically calibrated. Model‐observation differences in surface sensible and latent heat fluxes are larger during the 1988 drought but recover to relatively small values for 1989, suggesting some difficulty in simulating hydrologic extremes occurring outside the calibration conditions. A model sensitivity study using statistical disaggregation to allow precipitation to fall on only a portion of the landscape indicates that spatial disaggregation of precipitation can have strong impact on groundwater storage and surface discharge, potentially improving agreement between observed and simulated streamflow. Water redistributed through the model's aquifer‐river network can at times raise the water table high enough for water to seep back to the vegetation root zone and increase evapotranspiration. During relatively dry periods, up to 33% of monthly evapotranspiration was derived from groundwater‐supported evapotranspiration, emphasizing the need to quantify better aquifer‐atmosphere interaction. The work also demonstrates the feasibility and utility of fully coupled water budgeting schemes.
Inversion algorithms for recovering minor species densities from limb scatter measurements at UV‐visible wavelengths
Authors: Auvinen Harri , Oikarinen Liisa , Kyrölä Erkki
Published: October 2002

Stratospheric ozone can be measured with good global coverage and good vertical resolution by continuous scanning of the limb of the sunlit atmosphere. In the near future there will be several satellite instruments exploiting this limb‐scanning method using the UV‐visible region of the spectrum, e.g., the Optical Spectrograph and Infrared Imaging System (OSIRIS) launched on the Odin satellite in February 2001, and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography launched on Envisat in March 2002. Envisat also carries the Global Ozone Monitoring by Occultation of Stars instrument, which will measure limb‐scattered sunlight under bright limb occultation conditions. In this paper we present an inversion method to retrieve vertical ozone profiles from limb scatter measurements. The method uses a modified onion‐peeling approach. Multiple scattering is taken into account by a precalculated total to single‐scattering radiance ratios tabulated as a function of wavelength, tangent altitude, and several other relevant parameters. The sensitivity of the retrieval method is studied using the OSIRIS instrument as an example. Constituent retrieval errors are estimated by applying the method to simulated OSIRIS measurements.
Contrails in a comprehensive global climate model: Parameterization and radiative forcing results
Authors: Ponater Michael , Marquart Susanne , Sausen Robert
Published: October 2002

A parameterization of contrails for use in comprehensive global climate models is introduced. It is based on the thermodynamic theory of contrail formation, which has been applied in a consistent way with the cloud parameterization scheme of the version 4 European Center/Hamburg General Circulation Model. Both the coverage and the optical properties of contrails are calculated as functions of instantaneous values of atmospheric variables as they are provided by the climate model. The resulting spatial distributions of contrail parameters prove to be useful for explaining observed differences between contrails in different geographical regions. The time mean properties of the simulated contrails are in fair agreement with observations, though the values of ice water path and optical depth tend to be somewhat lower than those reported from in situ measurements. The radiative forcing of contrails resulting from the climate model experiments is substantially lower than estimated in a previous study, where mean parameters for contrails and the ambient atmosphere were prescribed in a radiative transfer model. One contribution to this disagreement arises from the smaller mean ice water content in the climate model simulations. However, the largest part must be related to a different treatment of the interference of contrails with natural high clouds. The sensitivity of the contrail radiative forcing to systematic errors in simulated ambient atmospheric variables (like temperature, humidity, and natural clouds) as well as to the parameterization of cloud and contrail overlap needs to be investigated further.
Impact of polar stratospheric cloud particle composition, number density, and lifetime on denitrification
Authors: Jensen Eric J. , Toon Owen B. , Tabazadeh Azadeh , Drdla K.
Published: October 2002

One‐dimensional simulations of polar stratospheric clouds (PSCs) composed of nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) particles are used to investigate vertical redistribution of nitric acid by clouds with these compositions. We find that for typical Arctic PSC lifetimes of a few days or less the net depletion of HNO3 is well below 50%, regardless of the NAT or NAD particle number density. With cloud lifetimes of a week or longer, greater than 50% denitrification is possible even with relatively low NAT particle number densities (as low as 1 × 10−3 cm−3). Denitrification is most efficient with NAT particle number densities of about 3 × 10−3 cm−3 to 10−2 cm−3. For a given temperature history the degree of denitrification by NAD clouds is much less severe than that by NAT clouds. The clouds also cause a local maximum in HNO3 mixing ratios at the cloud base where the cloud particles sublimate. Our simulations suggest that repeated short‐lived PSC events do not build up the denitrification beyond about 50%; however, the depth of the denitrified layer increases with each successive PSC event.
Effect of Mount Pinatubo H2SO4/H2O aerosol on ice nucleation in the upper troposphere using a global chemistry and transport model
Authors: Liu Xiaohong , Penner Joyce E.
Published: October 2002

A 2‐year simulation of Mount Pinatubo volcanic aerosol is performed using a global chemistry and transport model. The model is driven by meteorological fields from the NASA Goddard Data Assimilation Office (DAO) general circulation model (GCM) with a horizontal resolution of 2° latitude by 2.5° longitude and 46 vertical levels. The model reproduces the equatorial aerosol reservoir bounded between 20°S and 30°N several months after the eruption and the detrainment of sulfate aerosol from the equatorial reservoir occurring in connection with anticyclonic circulation systems. Our model results are generally consistent with the dispersal characteristics of sulfate aerosol derived from the Stratospheric Aerosol and Gas Experiment II (SAGE II) aerosol extinction observations. On the basis of global distribution of Pinatubo sulfate, the homogeneous ice nucleation rate of H2SO4/H2O aerosol (number of aerosol particles freezing per cm3 of air mass and per second) in the lower stratospheric and upper tropospheric regions is calculated and compared with those of natural and anthropogenic sulfate from sources injected near Earth's surface. Large ice nucleation rates from Pinatubo H2SO4/H2O aerosol are found in the region near the equatorial tropopause and generally in the regions along the tropopause in the bottom of the main Pinatubo aerosol layer during the first year following the eruption. Ice nucleation rates of Mount Pinatubo H2SO4/H2O aerosols are much larger than those of sulfate aerosols from the surface sources in many regions of the upper troposphere during the first year and could still be comparable to rates from these background sources during the second year after the eruption. Our results suggest that the Pinatubo sulfate aerosol could have influenced cirrus formation and evolution globally through homogeneous ice nucleation, although the exact impact also requires a more complete calculation that includes cirrus cloud dynamics and microphysics. Available satellite observations support the above assertion.
Climatic and monsoon isotopic signals (δD, δ13C) of northeastern China tree rings
Authors: Aucour A.‐M. , Tao F.‐X. , Sheppard S. M. F. , Huang N.‐W. , Liu C. Q.
Published: October 2002

The δD and δ13C values of cellulose nitrate in tree rings (δDtree and δ13C, respectively) record annual climatic variations. However, very few data exist on the coherence of isotopic signals between trees and on the calibration of these signals versus climatic variations in monsoon areas. The δDtree and δ13C of annual wood increments were analyzed for the period 1958–1988 in two korean pines from a single site in northeast China. The correlation between isotopic variations in the two trees is better for δDtree than for δ13C. Correlation analysis was performed between the isotopic data and (1) local temperature, precipitation amount, and relative humidity and (2) the East Asian summer monsoon index (MI). No relationship is found with the summer precipitation amount. The best correlation is obtained between δDtree and temperature in the preceding summer. This stresses the need to include climatic variables from the preceding year in calibration studies as well as the potential of δDtree as a proxy for summer temperature in northeast China, where precipitation falls primarily in summer. The East Asian summer monsoon index also presents a significant relationship with δDtree, which can be explained by the relationship between MI and summer temperature, which is observed at most weather stations in China.
POAM III observations of arctic ozone loss for the 1999/2000 winter
Authors: Hoppel Karl , Bevilacqua Richard , Nedoluha Gerald , Deniel Carole , Lefèvre Franck , Lumpe Jerry , Fromm Mike , Randall Cora , Rosenfield Joan , Rex Markus
Published: October 2002

During the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO) campaign, Polar Ozone and Aerosol Measurement (POAM) III sampled in the vortex core, on the vortex edge, and outside the vortex on a near‐daily basis from December 1999 through mid‐March 2000. During this period, POAM observed a substantial amount of ozone decline. For example, ozone mixing ratios in the core of the vortex dropped from about 3.5 ppmv in mid‐January to about 2 ppmv by mid‐March at 500 K. The ozone chemical loss indicated by these measurements is assessed using two methodologies. First, the POAM data is used to construct vortex‐averaged ozone profiles, which are advected downward using vortex average descent rates. The maximum ozone loss (1 January to 15 March) is found to be about 1.8 ppmv. In a second approach, the REPROBUS 3‐D CTM is used to specify the passive ozone distribution throughout the winter. The chemical loss in the vortex is estimated by performing a point‐by‐point subtraction of the POAM measurements inside the vortex from the model passive ozone evaluated at the time and location of the POAM measurements. Both ozone loss estimates are in general agreement and they agree well with published loss estimates from ER2 and ozonesonde measurements.
Comparison of empirically derived ozone losses in the Arctic vortex
Authors: Harris N. R. P. , Rex M. , Goutail F. , Knudsen B. M. , Manney G. L. , Müller R. , von der Gathen P.
Published: October 2002

A number of studies have reported empirical estimates of ozone loss in the Arctic vortex. They have used satellite and in situ measurements and have principally covered the Arctic winters in the 1990s. While there is qualitative consistency between the patterns of ozone loss, a quantitative comparison of the published values shows apparent disagreements. In this paper we examine these disagreements in more detail. We choose to concentrate on the five main techniques (Match, Système d'Analyse par Observation Zénithale (SAOZ)/REPROBUS, Microwave Limb Sounder (MLS), vortex average descent, and the Halogen Occultation Experiment (HALOE) ozone tracer approach). Estimates of the ozone losses in three winters (1994/1995, 1995/1996 and 1996/1997) are recalculated so that the same time periods, altitude ranges, and definitions of the Arctic vortex are used. This recalculation reveals a remarkably good agreement between the various estimates. For example, a superficial comparison of results from Match and from MLS indicates a big discrepancy (2.0 ± 0.3 and 0.85 ppmv, respectively, for air ending at ∼460 K in March 1995). However, the more precise comparisons presented here reveal good agreement for the individual MLS periods (0.5 ± 0.1 versus 0.5 ppmv; 0.4 ± 0.2 versus 0.3–0.4 ppmv; and 0.16 ± 0.09 ppmv versus no significant loss). Initial comparisons of the column losses derived for 1999/2000 also show good agreement with four techniques, giving 105 DU (SAOZ/REPROBUS), 80 DU (380–700 K partial column from Polar Ozone and Aerosol Monitoring (POAM)/REPROBUS), 85 ± 10 DU (HALOE ozone tracer), and 88 ± 13 (400–580 partial column from Match). There are some remaining discrepancies with ozone losses calculated using HALOE ozone tracer relations; it is important to ensure that the initial relation is truly representative of the vortex prior to the period of ozone loss.
Analysis of black carbon and carbon monoxide observed over the Indian Ocean: Implications for emissions and photochemistry
Authors: Dickerson R. R. , Andreae M. O. , Campos T. , Mayol‐Bracero O. L. , Neusuess C. , Streets D. G.
Published: October 2002

Air from South Asia carries heavy loadings of organic and light‐absorbing aerosol but low concentrations of ozone. We investigate ambient pollutant concentrations measured during the Indian Ocean Experiment (INDOEX), and we estimate emissions to determine the origin, magnitude, and impacts of air pollution from South Asia and to understand better the uncertainty in emission inventories. In India, the preponderance of motorcycles with small, two‐stroke engines and the practice of adulterating gasoline with kerosene lead to high CO emission factors; for 1999, we estimate release of 15 Tg yr−1 from fossil fuel use and 40 Tg yr−1 from biomass burning. With the addition of isoprene oxidation, the total CO emissions were 67 Tg yr−1 from India and 87 Tg(CO) yr−1 from all of South Asia. These values indicate a somewhat larger contribution from fossil fuels but slightly lower overall emissions when compared to prior emission inventories. Two‐stroke engines also exhibit high emission factors for volatile organic compounds (VOC) and particulate organic matter but produce only modest amounts of NOx. Near sources in India, VOC to NOx ratios appear too high for efficient O3 formation, although other factors probably contribute to observed low O3 mixing ratios. An inventory based on source characteristics and known emission factors for black carbon (BC) from South Asia yields 0.7 Tg yr−1 (upper limit of about 1.0 Tg yr−1) with biomass burning as the dominant source. We can test this inventory with measurements of ambient CO and BC—ship, island, and aircraft observations of air from South Asia all show a positive correlation between CO and BC (r2 = 0.71–0.81). Such strong correlations have also been observed over North America and Europe, but with a lower BC/CO slope. Ambient concentrations indicate high BC emission from South Asia: 2–3 Tg(BC) yr−1. This disagreement with emission inventories demonstrates the need for direct measurements from sources in India.
Surface exchange of nitric oxide, nitrogen dioxide, and ozone at a cattle pasture in Rondônia, Brazil
Authors: Kirkman G. A. , Gut A. , Ammann C. , Gatti L. V. , Cordova A. M. , Moura M. A. L. , Andreae M. O. , Meixner F. X.
Published: October 2002

Measurements of NO–NO2–O3 trace gas exchange were performed for two transition season periods during the La Niña year 1999 (30 April to 17 May, “wet–dry,” and 24 September to 27 October, “dry–wet”) over a cattle pasture in Rondônia. A dynamic chamber system (applied during the dry–wet season) was used to directly measure emission fluxes of nitric oxide (NO) and surface resistances for nitrogen dioxide (NO2) and ozone (O3) deposition. A companion study was simultaneously performed in an old‐growth forest. In order to determine ecosystem‐representative NO2 and O3 deposition fluxes for both measurement periods, an inferential method (multiresistance model) was applied to measure ambient NO2 and O3 concentrations using observed quantities of turbulent transport. Supplementary measurements included soil NO diffusivity and soil nutrient analysis. The observed NO soil emission fluxes were nine times lower than old‐growth rain forest emissions under similar soil moisture and temperature conditions and were attributed to the combination of a reduced soil N cycle and lower effective soil NO diffusion at the pasture. Canopy resistances (Rc) of both gases controlled the deposition processes during the day for both measurement periods. Day and night NO2 canopy resistances were significantly similar (α = 0.05) during the dry–wet period. Ozone canopy resistances revealed significantly higher daytime resistances of 106 s m−1 versus 65 s m−1 at night because of plant, soil, and wet skin uptake processes, enhanced by stomatal activity at night and aqueous phase chemistry on vegetative and soil surfaces. The surface of the pasture was a net NOx sink during 1999, removing seven times more NO2 from the atmosphere than was emitted as NO.