bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow characteristics in the interfacial shear layer between a fluid and a granular bed
Authors: Giménez‐Curto Luis A. , Corniero Miguel A.
Published: October 2002

The shear layer that forms at the interface between a fluid in motion and a granular bed is shown to be tractable in the most general case, including the occurrence of flow separation from the grains, which appears to be the most common situation. The phenomenon of separation produces fundamental leading order effects, especially in relation to the mean flow between grains and also in the generation of a disturbed ensemble‐averaged flow, which previous formulations were unable to detect. In this respect, we argue that some known results pertaining to the jet regime, recently introduced by the authors, can be directly extended outside that regime: in particular, the mean force that bed grains exert on the fluid, which appears to be generally applicable provided only that separation occurs. Furthermore, the maximum pressure disturbances, as well as the velocity disturbances at the outer edge of the shear layer, can be evaluated just as in the jet regime. This extension permits very general estimates of the main forces and the thickness of the shear layer, above and beneath the bed surface. In particular, the forces on the grains are shown to exhibit a spectrum. We present very simple expressions for the mean force as well as for the maximum force, which are compared with existing observations on sediment motion initiation under oscillatory flow, showing an excellent agreement.
Repeated CFC sections at the Greenwich Meridian in the Weddell Sea
Authors: Klatt Olaf , Roether Wolfgang , Hoppema Mario , Bulsiewicz Klaus , Fleischmann Uli , Rodehacke Christian , Fahrbach Eberhard , Weiss Ray F. , Bullister John L.
Published: October 2002

Repeated observations of the tracer chlorofluorocarbon‐11 (CFC‐11) for a section along the Greenwich Meridian from Antarctica (70°S) to about 50°S are presented for the period 1984–1998. The CFC sections display a highly persistent pattern. A middepth CFC minimum in the central Weddell Sea is bounded laterally by elevated levels of dissolved CFCs at the southern margin of the Weddell Basin and by a column of elevated CFC concentrations around 55°S near to the northern margin. Part of the latter column covers waters of the Antarctic Circumpolar Current, which indicates that a moderate portion of these waters was ventilated in the Weddell Sea. Deep CFC maxima adjoining the southern and northern margins of the Weddell Basin indicate advective cores of recently ventilated waters. The southern core supports previous notions of deep water import into the Weddell Sea from the east. For all deep and bottom waters, the portions ventilated on the CFC timescale (∼50 years) are small. Effective initial CFC saturations for these portions are estimated to be between 60 and 70%, using in part new data from off the Filchner‐Ronne Ice Shelf. For various CFC features along the section (mostly advective cores), ventilated fractions and mean ages of these fractions were obtained (with error limits). The procedure was to fit an age distribution of a prescribed form to CFC‐11 time series for these features, constructed from the various realizations of the CFC section. The ages are between 3 and 19 years, and the ventilated fractions range between 6 and 23%, indicating a rather limited ventilation of the interior Weddell Sea subsurface layer waters on the CFC timescale. It is shown that the concurrent CFC‐12 data provide little additional information. The work demonstrates a high information content of repeated tracer observations and encourages similar approaches also in other ocean regions.
Simulations of ice‐ocean dynamics in the Weddell Sea 1. Model configuration and validation
Authors: Timmermann R. , Beckmann A. , Hellmer H. H.
Published: October 2002

In the framework of the Bremerhaven Regional Ice Ocean Simulations a coupled sea ice‐ocean model, based on a dynamic‐thermodynamic sea ice model and the S‐coordinate Primitive Equation Model, has been developed. The model is run on a circumpolar grid that is focused on the Weddell Sea. It includes the major Antarctic ice shelves; ice shelf‐ocean interaction is described by the sea ice model's thermodynamic component. Numerical simulations are forced with 6 hourly data of the European Centre for Medium‐Range Weather Forecasts reanalysis of the period 1985–1993 and validated against observations of hydrography as well as sea ice extent, thickness, and drift. Qualitatively and quantitatively, a good agreement with observations is achieved. The basin‐wide circulation features a double‐cell structure of the Weddell Gyre with maximum transports close to recent observations. Summer sea ice coverage is underestimated, but winter ice extent, as well as sea ice thickness and drift, is well reproduced.
Inflow of oceanic water into Tokyo Bay and generation of a subsurface hypoxic water mass
Authors: Fujiwara Tateki , Yamada Yoshiaki
Published: October 2002

Temporal and spatial variations in the inflow of oceanic water into Tokyo Bay and the accompanying changes in circulation within the bay have been studied by repeated measurement of the longitudinal distributions of hydrographic parameters and oxygen concentration from the bay head to off the bay mouth. Data collection took place in summer 1998 with the particular aim of tracking the movement of a hypoxic water mass. Density changes in the upper ocean induce inflow and outflow of oceanic water in the bay so that when water density increased in the upper ocean, heavy saline water intruded into the lower layer of the bay. This inflow forced a “bottom hypoxic water mass” to the bay head and, finally, lifted it up to form a “subsurface hypoxic water mass,” which then moved out as a thin layer just under the pycnocline, reaching beyond the bay mouth. When the intruding heavier deep water retreated and an intrusion of oceanic water at intermediate depth occurred, the hypoxic water mass was displaced from the center to the mouth of the bay. The bottom hypoxic water mass dominates in the northern half of the bay, while the subsurface oxygen minimum appears frequently in the southern half.
Video‐imaged surf zone wave and roller structures and flow fields
Authors: Govender K. , Mocke G. P. , Alport M. J.
Published: October 2002

The measurement of mean water levels, roller geometries, and phase ensemble‐averaged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two‐dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble‐averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time‐averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone.
On the eddy‐Kuroshio interaction: Evolution of the mesoscale eddy
Authors: Waseda Takuji , Mitsudera Humio , Taguchi Bunmei , Yoshikawa Yashushi
Published: October 2002

Presented are results from a numerical study of the interaction of a mesoscale eddy and the Kuroshio using a high‐resolution regional general circulation model (GCM). The distinct evolution of the mesoscale eddies is not new in the literature, but most of the previous studies were conducted with simpler model configurations. To our knowledge the present study is the first to use a high‐resolution GCM, proven to replicate a realistic ocean circulation. An anticyclonic eddy was injected to the south of Kuroshio (140°E, 30°N) by means of sequential data assimilation of TOPEX/Poseidon altimeter data of October 1992 for 30 days. The strength of the assimilation was varied to produce five eddies (Rossby number ϵ ∼ 0.012–0.014) comparable in scale to those observed in this region. The two results (ϵ = 0.0131, 0.0141) resemble the observed onset of short‐term meandering events in 1993 and 1998. The westward propagating anticyclonic eddy collides with the Kuroshio southeast of Kyushu, propagates downstream, and triggers the short‐term Kuroshio meander (occurring between the Kii Peninsula and Izu ridge with duration of half a year). The sequence resembles the scenario hypothesized on the basis of altimeter observations and other in situ measurements during Tokyo Ogasawara Line Experiment. The description of these cases is important, but we focus here on the differentiation between the strong eddy cases (ϵ = 0.0131, 0.0141) and the weak eddy cases (ϵ = 0.0118, 0.0124, 0.0125) classified on the basis of their subsequent evolutions: the strong geddy (meandering case) propagates west as it elongates zonally and barotropic tripolar vortices form, and the weak eddy (nonmeandering case) makes an abrupt southward migration while the eddy core splits as a result of advection by lower layer geostrophic motions. Such distinct evolutions are conjectured a result of a competition of the effects of nonlinearity, dispersion, and barotropicity of which their relative importance varied among the simulated eddies. Because the scale variation of the injected eddies are natural, we expect that even the weak‐eddy case exists in nature; in fact, such a case was observed in 1994.
A modeling study of salinity variability and its effects in the tropical Pacific Ocean during the 1993–1999 period
Authors: Vialard Jérôme , Delecluse Pascale , Menkes Christophe
Published: October 2002

This paper investigates the mechanisms of near‐surface salinity variability in the tropical Pacific during the 1993–1999 period, including the 1997–1998 El Niño. The role of salinity in the oceanic mean state and variability is also investigated. To this end, a general circulation model is forced by European Remote Sensing (ERS) + Tropical Atmosphere–Ocean (TAO) wind stresses and simple parameterizations of the heat and freshwater fluxes interannual variability. The model correctly reproduces interannual variability of sea surface temperature (SST) and sea surface salinity (SSS). At the peak of the 1997–1998 El Niño, the model and observations show that water fresher than 34.8 psu (usually confined to the western Pacific and close to the South American coast) has invaded the whole equatorial Pacific. This is the combined result of an eastward displacement of the western Pacific fresh pool to 150°W by zonal advection and increased precipitation in the eastern part of the basin. Sensitivity experiments allow us to evaluate the impact of salinity in the equatorial band. The 1993–1996 oceanic mean state is only weakly modified (by less than 0.2°C in SST and 0.05 m s−1 in surface currents) in experiments with homogeneous salinity, but salinity has a larger impact on variability (standard deviation of 0.4°C in SST and 0.15 m s−1 in surface currents). During the 1997–1998 El Niño, salinity effects are the strongest in the vicinity of the eastern edge of the warm and fresh pool. In this region, through its effect on both vertical mixing and horizontal pressure gradients, salinity increases SST by up to 0.8°C and surface currents by up to 0.2 m s−1. Based on these results, it seems advisable to include salinity effects in quantitative studies or forecasts of the El Niño phenomenon.
Coherent Doppler Profiler measurements of near‐bed suspended sediment fluxes and the influence of bed forms
Authors: Smyth C. , Hay Alex E. , Zedel L.
Published: October 2002

This paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3–4 m water depth. The measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0.7 cm vertical resolution. Normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states (irregular ripples, cross ripples, linear transition ripples, and flat bed) indicate a small but significant peak at incident wave frequencies but are otherwise rather flat, with weak redness. Estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling. Two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples. The contribution from the high‐frequency turbulent component is small. Wave phase averages for low‐energy bed states exhibit near‐bed peaks in the suspended sediment flux following wave phase reversal. Wave phase averages for the high‐energy cases do not exhibit a diffusive signature. Observed vertical profiles of turbulence intensity for different bed states reveal that the near‐bed turbulence levels are relatively independent of bed state. Friction velocity predictions from presently available models, including a bed stress model and a sediment eddy diffusion model, are compared to measured values of near‐bed turbulence intensity. Reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors fromTolman[1994]are used.
A modeling study of shelf circulation off northern California in the region of the Coastal Ocean Dynamics Experiment: Response to relaxation of upwelling winds
Authors: Gan Jianping , Allen J. S.
Published: October 2002

A two‐part modeling study of the wind‐forced flow on the continental shelf off northern California in the region (37°–40°N) of the Coastal Ocean Dynamics Experiment (CODE) is pursued. This paper involves a process‐oriented study with idealized wind stress forcing. Gan and Allen [2002] involves forcing with observed winds and heat flux for April–May 1982 and comparison of model results with CODE observations. A characteristic, but previously unexplained, response observed during CODE following the weakening, or relaxation, of southward upwelling favorable winds is the time‐dependent development of northward currents over the inner shelf next to the coast. The presence of northward winds is not necessary for this occurrence. The objective in this paper is to investigate the dynamics of the shelf flow response to upwelling wind relaxation events under idealized conditions. In the basic case experiment a spatially uniform upwelling favorable southward wind stress of 0.1 Pa is applied to the ocean initially at rest. The stress is held constant for 10 days and then decreased linearly to zero over 3 days. In response to the southward wind stress, southward alongshore currents develop on the shelf accompanied by upwelling of cold, dense water near the coast. Considerable spatial variability in the shelf flow, clearly related to the alongshore variations in coastline and bottom topography, is found. The alongshore currents tend to separate from the coast south of capes, and the coldest surface water is found at those locations. As the winds decrease, northward currents, similar to those observed, develop on the inner shelf next to the coast at many alongshore locations. Examination of the alongshore momentum balances shows that the northward currents are forced by a northward pressure gradient force associated with negative alongshore pressure gradients. These pressure gradients are set up by the interaction of the wind‐forced flow with the alongshore variations in shelf topography. In general, negative alongshore pressure gradients, intensified off Pt. Reyes and Pt. Arena by the gradient wind balance, are found south of capes. The negative pressure gradients geostrophically balance onshore flow at depth, and upwelling is strengthened in these locations. North of capes, positive pressure gradients that are primarily in balance with nonlinear advective effects are found. After the winds cease the forced across‐shelf circulation weakens, and the resulting unbalanced negative pressure gradients south of the capes accelerate the alongshore currents northward. Processes with similar dynamics are found embedded in the more complex coastal ocean response to observed time varying winds by Gan and Allen [2002].
Seasonal salinity balance in the Gulf of California
Authors: Beron‐Vera F. J. , Ripa P.
Published: October 2002

Historical data in various domains within the Gulf of California are used to describe the seasonal balance of the average salinity, 〈S〉. The difference of evaporation minus precipitation, E − P, is positive along the course of a year. This produces a positive salinity anomaly, S′, which is exported to the Pacific Ocean through the mouth of the gulf. Even though E − P has strong annual and semiannual cycles, it is not enough to explain the seasonal variation of the transport of S′. A linear one‐dimensional nondiffusive inhomogeneous two‐layer model suggests that the seasonal balance of 〈S〉 is largely controlled by the Pacific Ocean, which excites a baroclinic wave at the mouth of the gulf. Advection due to this wave is the main carrier of S′ within the gulf, and the associated water rearrangement produces a considerable change in 〈S〉. The Pacific Ocean has been previously shown to maintain the seasonal heat balance through a similar mechanism. This paper thus adds importance to the Pacific Ocean influence on the determination of the gulf's seasonal dynamics and thermodynamics.
Chemical components of single particles measured with Particle Analysis by Laser Mass Spectrometry (PALMS) during the Atlanta SuperSite Project: Focus on organic/sulfate, lead, soot, and mineral particles
Authors: Lee Shan‐Hu , Murphy Daniel M. , Thomson David S. , Middlebrook Ann M.
Published: October 2002

Using a Particle Analysis by Laser Mass Spectrometery (PALMS) instrument, we have measured the chemical components of 0.35–2.5 μm in aerodynamic diameter aerosols during the Atlanta SuperSite Project in August 1999. During the whole campaign period, over a half million single‐particle mass spectra were obtained, and about 380,000 spectra were selected for our data analysis. This paper describes the results for organic/sulfate, lead, soot, and mineral dust particles. The most common feature of the Atlanta single particles was the internal mixture of organic compounds and sulfate: 70–99% of the total spectra contained ions characteristic of organic species, and 90–98% contained sulfate ions. These organic/sulfate particles often contained other chemical components such as nitrate, ammonium, halogens, metals, soot/hydrocarbons, and aluminosilicates. Various metals and metal oxides were also observed in Atlanta particles, including Li, B, Na, Mg, K, Ca, Fe, Cu, Zn, V, Sn, and Pb. In particular, small amounts of lead were observed in nearly half of the mass spectra with high signal‐to‐noise ratio. Many soot/hydrocarbon particles in Atlanta had water‐soluble species such as sulfate, nitrate, and oxidized organic components in addition to metals. Mineral particles were detected as large aluminosilicates that also often contained sulfate and nitrate. More than 50 different elements were observed in single particles in Atlanta, which might be an issue for human health effects. Many aerosol components were not strongly dependent on wind direction, indicating that these aerosols were characteristic of the metropolitan area. The mass spectra observed in Atlanta were further compared with our previous results from Idaho Hill, Colorado (relatively remote continental air). These combined results show that the internal mixture of organic components and sulfate can be a general characteristic of tropospheric aerosols in the continental region.
Spectral aerosol radiative forcing at the surface during the Indian Ocean Experiment (INDOEX)
Authors: Bush Brett C. , Valero Francisco P. J.
Published: October 2002

We describe a radiation experiment performed at the Kaashidhoo Climate Observatory (KCO), Republic of Maldives, during the Indian Ocean Experiment (INDOEX) field campaign in February through March 1999. Both the total solar broadband (0.3 to 3.81 μm) and visible spectral narrowband (seven spectral channels in the region from 0.4 to 0.7 μm) quantities were measured for the total (hemispherical), direct solar, and diffuse (including forward scattering) components of the radiation field. The aerosol optical depth at 500 nm, obtained from one of the narrrowband spectral channels, ranged from approximately 0.2 to 0.7 during the period encompassing the intensive field phase (IFP) of INDOEX. The diurnally averaged atmospheric forcing efficiencies determined for the total solar broadband and visible spectrum are −72.2 ± 5.5 W m−2 and −38.5 ± 4.0 W m−2, respectively. Model simulations driven by in situ measurements and realistic aerosol optical properties infer a single scattering albedo of 0.874 ± 0.028 at 500 nm.
Influence of anthropogenic aerosols on the total and spectral irradiance at the sea surface during the Indian Ocean Experiment (INDOEX) 1999
Authors: Meywerk Jens , Ramanathan V.
Published: October 2002

Unique measurements of the spectral signature of the aerosol forcing at the surface between 350 and 1050 nm wavelength during the 1999 haze event over the tropical Indian Ocean are presented and discussed. The aerosol visible optical depths reached values as high as 0.7 in the northern Indian Ocean and decreased to about 0.1 south of the Intertropical Convergence Zone (ITCZ) in the Southern Hemisphere (SH) tropical oceans. Measurements of aerosol optical depth and global (direct plus diffuse) irradiance have been taken with a grating spectroradiometer with a resolution of 3 nm onboard R/V Ronald H. Brown. The radiometer was calibrated onboard in real time. We inverted the direct solar spectra to obtain aerosol optical depth and to compare these with three independent sunphotometer optical depth at selected spectral intervals to assess the validity of the retrieved optical depth. We took the difference between aerosol optical depth between polluted and pristine air mass spectra to obtain the spectral signature. The difference in optical depth between the polluted regime north of the ITCZ and the pristine air masses south of the ITCZ were between 0.5 at 350 nm wavelength, 0.35 at 500 nm wavelength, and 0.1 at 1050 nm wavelength. Next, we obtained the aerosol forcing by correlating optical depth variations (for the duration of the cruise) with corresponding global irradiance variations. We found the Southern Asian aerosol reduces the solar irradiance (for a unit increase in optical depth at 500 nm wavelength) by about 25% at 350 nm decreasing to about 10% in the near‐infrared. The reduction in the direct solar was a factor of 2 to 3 larger. The spectral data shown here will provide a critical test for aerosol‐radiation models used for aerosol‐forcing estimates.
Electrical energy constraints on lightning
Authors: Marshall Thomas C. , Stolzenburg Maribeth
Published: October 2002

We use a one‐dimensional model to calculate the electrostatic energy of several horizontally extensive, electrified clouds. The model calculation is initiated with known charge distributions. By making an assumption of the way in which a hypothetical lightning flash changes the charge distribution, we use the model to determine if this flash is energetically allowed. The model shows that the charge structure in the cloud can constrain the types of lightning flashes that occur. A single‐charge cloud‐to‐ground flash of one polarity can be absolutely prohibited, while the opposite polarity flash is allowed. Discharging a screening charge layer may require another charge region of the opposite polarity to be involved in the flash. Small reductions in the charge density of a region often can provide a relatively large amount of energy to drive a flash. Certain intracloud flashes are always allowed. Model calculations suggest that positive cloud‐to‐ground flashes that produce Q‐bursts have energies of about 1 × 1010 J and discharge an area of about 40 km × 40 km. Model estimates of the total electrostatic energy stored in two stratiform clouds of mesoscale convective systems were 5 × 1011 J and 2 × 1012 J. These energies are sufficient to support hundreds or thousands of typical lightning flashes, but only 10–100 of the energetic positive cloud‐to‐ground flashes with associated Q‐bursts.
Estimation of natural and anthropogenic contributions to twentieth century temperature change
Authors: Tett Simon F. B. , Jones Gareth S. , Stott Peter A. , Hill David C. , Mitchell John F. B. , Allen Myles R. , Ingram William J. , Johns Tim C. , Johnson Colin E. , Jones Andy , Roberts David L. , Sexton David M. H. , Woodage Margaret J.
Published: October 2002

Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well‐mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well‐mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well‐mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well‐mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near‐surface temperature from well‐mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near‐surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.
Simulation of aerosol distributions and radiative forcing for INDOEX: Regional climate impacts
Authors: Collins William D. , Rasch Phillip J. , Eaton Brian E. , Fillmore David W. , Kiehl Jeffrey T. , Beck C. Trevor , Zender Charles S.
Published: October 2002

The direct radiative forcing by aerosols over the Indian Ocean region is simulated for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during Spring 1999. The forcing is calculated for the top‐of‐atmosphere (TOA), surface, and atmosphere by differencing shortwave fluxes computed with and without aerosols. The calculation includes the effects of sea‐salt, sulfate, carbonaceous, and soil‐dust aerosols. The aerosol distributions are obtained from a global aerosol simulation including assimilation of satellite retrievals of aerosol optical thickness (AOT). The time‐dependent, three‐dimensional aerosol distributions are derived with a chemical transport model driven with meteorological analyses for this period. The surface albedos are obtained from a land‐surface model forced with an identical meteorological analysis and satellite‐derived rainfall and insolation. These calculations are consistent with in situ observations of the surface insolation over the central Indian Ocean and with satellite measurements of the reflected shortwave radiation. The calculations show that the surface insolation under clear skies is reduced by as much as 40 W/m2 over the Indian subcontinent by natural and anthropogenic aerosols. This reduction in insolation is accompanied by an increase in shortwave flux absorbed in the atmosphere by 25 W/m2. The inclusion of clouds in the calculations changes the direct effect by less than 2 W/m2 over the Indian subcontinent, although the reduction is much larger over China. The magnitude of the difference between all‐sky and clear‐sky forcing is quite sensitive to the three‐dimensional spatial relationship between the aerosol and cloud fields, and other estimates of the difference for the INDOEX Intensive Field Phase are as large as 5 W/m2.
Chemical characterization of submicron aerosol particles collected over the Indian Ocean
Authors: Gabriel R. , Mayol‐Bracero O. L. , Andreae M. O.
Published: October 2002

Submicron aerosol particles (Dp < 1 μm) were sampled with stacked filter units on the National Center for Atmospheric Research (NCAR) Hercules C‐130 aircraft during February–March 1999 as a contribution to the Indian Ocean Experiment (INDOEX). We determined the vertical and spatial distribution of the major aerosol components (NH4+, Na+, K+, Mg2+, Ca2+, methyl sulfonic acid, Cl−, NO3−, SO42−, oxalate, organic carbon, and black carbon) over the Indian Ocean to examine the role of pollution aerosols on indirect and direct radiative forcing. High pollution levels were observed over the entire northern Indian Ocean down to the Intertropical Convergence Zone (ITCZ) located between the equator and 10°S. In the northern part of the Indian Ocean (5°–15°N, 66°–73°E), high concentrations of carbonaceous aerosol and pollution‐derived inorganic species were found in a layer extending from the sea surface to about 3.5 km asl. In this layer, the average mass concentration of all aerosol species detected by our technique ranged between 7 and 34 μg m−3, comparable to pollution levels observed in industrialized regions. In the Southern Hemisphere (1°–9°S, 66°–73°E), the aerosol concentrations rapidly declined to remote background levels of about 2 μg m−3. The concentrations of non‐sea‐salt sulfate (the main light scattering component) ranged from maximum values of 12.7 μg m−3 in the Northern Hemisphere to 0.2 μg m−3 in the Southern Hemisphere. Carbonaceous aerosol contributes between 40% and 60% to the fine aerosol mass of all determined components. An unusually high fraction of black carbon (up to 16% in the polluted areas) is responsible for its high light absorption coefficient.
An intercomparison of aerosol light extinction and 180° backscatter as derived using in situ instruments and Raman lidar during the INDOEX field campaign
Authors: Masonis Sarah J. , Franke Kathleen , Ansmann Albert , Müller Detlef , Althausen Dietrich , Ogren John A. , Jefferson Anne , Sheridan Patrick J.
Published: October 2002

Aircraft in situ and Raman lidar profiles of aerosol light extinction (σep) and 180° backscattering (βp) are compared for 6 days during the Indian Ocean Experiment (INDOEX). The measurements of σep and βp were made from the National Center for Atmospheric Research C‐130 aircraft using two integrating nephelometers to measure light scattering and one Radiance Research Particle Soot Absorption Photometer to measure light absorption. Particulate 180° backscattering was measured in situ using a new instrument, the 180° backscatter nephelometer. The Institute for Tropospheric Research Raman lidar was located on the island of Hulule (4.18°N, 73.53°E), and all of the in situ profiles presented are from descents into the Hulule airport. Aerosol optical depth was also measured from Hulule using a Sun photometer, and these data are included in the intercomparison. On average, the lidar‐derived values of σep and βp are ∼30% larger than the in situ‐derived values to a 95% confidence interval. Possible reasons for the overall discrepancy are (1) a low bias in the in situ measurements because of losses in the C‐130 Community Aerosol Inlet; (2) underestimation of the humidification effect on light extinction in the in situ measurements; (3) overestimation of σep and βp in the lidar because of subvisible cloud contamination; (4) errors in data processing that could be biasing either measurement, though the lidar retrievals are especially sensitive to this type of error. Temporal and spatial variability also appear to be the source of at least some of the discrepancy in two of the six cases, none of which are well collocated.
Measurements of aerosol vertical profiles and optical properties during INDOEX 1999 using micropulse lidars
Authors: Welton, Ellsworth J. , Voss, Kenneth J. , Quinn, Patricia K. , Flatau, Piotr J. , Markowicz, Krzysztof , Campbell, James R. , Spinhirne, James D. , Gordon, Howard R. , Johnson James E.
Published: October 2002

Micropulse lidar (MPL) systems were used to measure aerosol properties during the Indian Ocean Experiment (INDOEX) 1999 field phase. Measurements were made from two platforms: the NOAA ship R/V Ronald H. Brown, and the Kaashidhoo Climate Observatory (KCO) in the Maldives. Sun photometers were used to provide aerosol optical depths (AOD) needed to calibrate the MPL. This study focuses on the height distribution and optical properties (at 523 nm) of aerosols observed during the campaign. The height of the highest aerosols (top height) was calculated and found to be below 4 km for most of the cruise. The marine boundary layer (MBL) top was calculated and found to be less than 1 km. MPL results were combined with air mass trajectories, radiosonde profiles of temperature and humidity, and aerosol concentration and optical measurements. Humidity varied from approximately 80% near the surface to 50% near the top height during the entire cruise. The average value and standard deviation of aerosol optical parameters were determined for characteristic air mass regimes. Marine aerosols in the absence of any continental influence were found to have an AOD of 0.05±0.03, an extinction‐to‐backscatter ratio (S ratio) of 33±6 sr, and peak extinction values around 0.05 km−1 (near the MBL top). The marine results are shown to be in agreement with previously measured and expected values. Polluted marine areas over the Indian Ocean, influenced by continental aerosols, had AOD values in excess of 0.2, S ratios well above 40 sr, and peak extinction values approximately 0.20 km−1 (near the MBL top). The polluted marine results are shown to be similar to previously published values for continental aerosols. Comparisons between MPL derived extinction near the ship (75 m) and extinction calculated at ship level using scattering measured by a nephelometer and absorption using a particle soot absorption photometer were conducted. The comparisons indicated that the MPL algorithm (using a constant S ratio throughout the lower troposphere) calculates extinction near the surface in agreement with the ship‐level measurements only when the MBL aerosols are well mixed with aerosols above. Finally, a review of the MPL extinction profiles showed that the model of aerosol vertical extinction developed during an earlier INDOEX field campaign (at the Maldives) did not correctly describe the true vertical distribution over the greater Indian Ocean region. Using the average extinction profile and AOD obtained during marine conditions, a new model of aerosol vertical extinction was determined for marine atmospheres over the Indian Ocean. A new model of aerosol vertical extinction for polluted marine atmospheres was also developed using the average extinction profile and AOD obtained during marine conditions influenced by continental aerosols.
A multisatellite analysis of deep convection and its moist environment over the Indian Ocean during the winter monsoon
Authors: Roca Rémy , Viollier Michel , Picon Laurence , Desbois Michel
Published: October 2002

The aim of this paper is to characterize the deep convective systems over the Indian Ocean during Indian Ocean Experiment (INDOEX) and their relationship to cloudiness and to the Upper Tropospheric Humidity (UTH) of their environment together with the relevant longwave radiation fields. Multisatellite analyses are performed (Meteosat, Scanner for Radiation Budget (ScaRaB), and Special Sensor Microwave Imager (SSM/I)) to measure these environmental parameters. The use of Meteosat water vapor (WV) channel appears very efficient not only for estimating UTH but also for separating high level cloudiness, including thin cirrus, from clear sky and low clouds. The Meteosat infrared (IR) and WV channels are also used for correlating Meteosat and ScaRaB measurements, allowing to retrieve continuously the longwave radiative flux. The longwave flux is used to compute the cloud radiative forcing as well as the clear‐sky greenhouse effect. Spatial relationships between upper level cloudiness and UTH are established. A strong positive linear relationship is found suggesting a local moistening of the upper troposphere by convection. The temporal analysis reveals that during the active phase of the intraseasonal oscillation, the longwave cloud radiative forcing reaches a mean value up to 40 W m−2 over a large region in the open ocean, while the average clear‐sky greenhouse effect is in excess of 180 W m−2. These radiative parameters are strongly correlated with the upper level cloudiness and upper level moisture, respectively. The temporal variability of UTH explains up to 80% of the greenhouse effect variability. The structure of the convective cloud systems is then studied. The observed population of systems spans a wide spectrum of area from 100 to 1,000,000 km2. The contribution to the high level cloudiness of the systems with a strong vertical development is dominant. These systems, with at least one convective cell reaching the highest levels (below 210 K), present indices of overshooting tops and are the most horizontally extended. The largest system exhibits an average longwave radiative forcing of around 100 W m−2. Their contribution to the cloud forcing over the Indian Ocean is overwhelming. The spatial and temporal variability of the systems is finally related to the UTH and to the clear‐sky greenhouse effect. Strong correlations are found indicating that these organized convective systems at mesoscale play a leading role in the Indian Ocean climate. The analysis suggests that deeper convection is associated with larger cloud desks with larger cloud radiative forcing. It is also associated with a moister upper troposphere and a larger clear‐sky greenhouse effect. These two effects would provide a positive feedback on the surface conditions.
Simulation of the circulation and related photochemical ozone dispersion in the Po plains (northern Italy): Comparison with the observations of a measuring campaign
Authors: Dosio, A. , Galmarini, S. , Graziani, G.
Published: October 2002

The atmospheric flow and pollutant dispersion over the Po plains in high‐pressure conditions and weak synoptic circulation is investigated by means of a model simulation. This study stems from the work conducted during the Pianura Padana Production of Ozone (PIPAPO) campaign conducted in 1998. The typical mesoscale flow that establishes in the Po plains under these conditions is responsible for the diffusion of atmospheric pollutants in the prealpine valleys, their recirculation over the plain, and in general for the condition of stagnation and pollution buildup often found in the region. The model simulation presented here reproduces correctly the meteorological circulation that occurred in the central part of the Po valley during the PIPAPO first intensive observational period. The model results are compared with the measurements collected at a large number of surface observational sites and the airborne instruments deployed during the campaign. Subsequently, the diffusion and transformation process of the atmospheric pollutants is analyzed. Direct comparison with the measurements collected during the campaign indicates the good quality of the model results and the relevance of the conclusions deduced from the model simulation. This paper represents one of the few attempts made so far to simulate the circulation and photochemical ozone production over the Po plains.
A new Chemical Lagrangian Model of the Stratosphere (CLaMS) 2. Formulation of chemistry scheme and initialization
Authors: McKenna Daniel S. , Grooß Jens‐Uwe , Günther Gebhard , Konopka Paul , Müller Rolf , Carver Glenn , Sasano Yasuhiro
Published: October 2002

The first simulations of stratospheric chemistry using the Chemical Lagrangian Model of the Stratosphere (CLaMS) are reported. A comprehensive chemical assimulation procedure is described that combines satellite, airborne, and balloon‐borne tracer observations with results from a two‐dimensional photochemical model simulation. This procedure uses tracer‐tracer and tracer‐potential vorticity mapping techniques. It correctly reproduces all basic features of the observed tracer distribution. This methodology is used to generate the initial composition fields that will be used for subsequent chemical simulations. Results from a 6‐day simulation starting on 20 February 1997 show that the simulated HNO3 distribution displays the correct morphology, although the extremes of the observed HNO3 distribution are underestimated. The simulated ClO distribution exhibits a similar morphology to the observed Microwave Limb Sounder ClO distribution. Because of unseasonally low temperatures in the arctic lower stratosphere during spring 1997, high levels of chlorine activation are maintained in the simulation, resulting in up to 1.8 ppmv of chemical ozone loss over a 5‐week period. Furthermore, simulations show strong spatially inhomogeneous chemical ozone depletion within the polar vortex and show that greatest ozone loss is confined to the vortex core. These results are confirmed by several Halogen Occultation Experiment and ozone sonde profiles, although the minimum ozone concentrations are overestimated. These studies demonstrate that CLaMS is capable of simulating vortex isolation, an essential feature of the polar vortex.
Aerosol direct radiative impact over the INDOEX area based on passive and active remote sensing
Authors: Léon J.‐F. , Chazette P. , Pelon J. , Dulac F. , Randriamiarisoa H.
Published: October 2002

This study is dedicated to the direct radiative impact assessment of the pollution aerosol particles during the Indian Ocean Experiment (INDOEX). We use here an instrumental synergy based on active and passive ground‐based (Goa University, India), airborne (Mystere 20 research aircraft), and spaceborne (Meteosat‐5) devices. An original method based on ground‐based active actinic flux measurements is developed to assess the atmospheric, columnar aerosol single scattering albedo. This parameter has been found between 0.88 and 0.93 (at 440 nm) with an absolute uncertainty of 0.04 during the operating period from 11 to 23 March 1999. We have also assessed this parameter off the West Indian coast by comparing simultaneous airborne lidar and flux measurements with radiative transfer simulations. The value retrieved for the atmospheric column is close to 0.83 ± 0.05 (at 523 nm) in agreement with the coastal value. The horizontal and vertical extent of the aerosol plume is investigated using airborne lidar and Meteosat‐5 satellite. Using the Meteosat‐5‐derived aerosol optical thickness, we have estimated the regional extent of the shortwave aerosol direct radiative forcing. The vertical profile of the aerosol extinction coefficient derived from lidar sounding is used to assess the atmospheric shortwave heating rate induced by the aerosol layer. For an aerosol optical thickness of 0.6 (±0.12), as it has been observed in the aerosol plume in late March, the top of atmosphere direct aerosol forcing is −17 (±5) W m−2. The surface forcing is between 2.5 and 4.5 higher than the top of atmosphere forcing. This difference leads to a significant heating of the three lowest kilometers of the atmosphere by the aerosols. For an aerosol optical thickness of 0.6, the atmospheric heating rate induced by aerosols is between 0.8 and 1.2 Kd−1 depending on the aerosol single scattering albedo value.
Further evidence of a two‐level mesopause and its variations from UARS high‐resolution Doppler imager temperature data
Authors: Thulasiraman S. , Nee J. B.
Published: October 2002

The temperature data from the High Resolution Doppler Imager on board the Upper Atmospheric Research Satellite are used to study mesospheric dynamics. Data spread over 7 years (1992–1998) in the height region of 75–105 km are analyzed to study the seasonal and long‐term changes in the mesopause height and temperatures. During December solstice (1992), the mesopause is at the high level of 100 km in the Northern Hemisphere with a temperature of 190 K, and around 40°S, the mesopause changes to a low level of 88 km with a cool temperature (150 K) of 40 K less than the high‐level mesopause. In June solstice (1993), the mirror image of this pattern is observed. This kind of two‐level mesopause structure is consistently seen during all solstices at the same height and the transition occurring at the same latitudes. The high‐level mesopause with temperatures of 180–190 K and the low‐level mesopause with temperatures of 140–150 K are observed. The two‐level mesopause is not observed in equinoxes, and a single‐level mesopause is seen around 100‐km altitude in both hemispheres. The annually averaged temperature data show 5 K warming of high‐level mesopause during 1992 and 1993, which may be due to Pinatubo volcanic aerosols. This warming is comparable but less than the 9 K warming at 100‐km height observed by She et al. [1998] at 41°N. The time series analysis of temperature at 99 km shows a decrease of 1.2 ± 0.15 K/yr over the equatorial region during 1992–1998. The lowest mesopause temperatures at northern high latitudes are observed between May and August, which is consistent with the period of polar mesosphere summer echo observed by using VHF radar at these latitudes.