bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of off‐equatorial subsurface anomalies in initiating the 1991–1992 El Niño as revealed by the National Centers for Environmental Prediction ocean reanalysis data
Authors: Zhang Rong‐Hua , Rothstein Lewis M.
Published: September 2002

The National Centers for Environmental Prediction ocean reanalysis data for 1980–1995 have been analyzed along mean constant density surfaces (mean isopycnals) in order to describe better and understand the three‐dimensional space‐time evolution of the 1991–1992 El Niño event in the tropical Pacific. The major finding is of a well‐defined, shallow pathway of off‐equatorial temperature anomalies along the Pacific North Equatorial Countercurrent (NECC) associated with the onset phase of the event. This pathway originates from the western boundary off the equator in the Northern Hemisphere along 6°–10°N and then basically follows the mean circulation eastward and upward along the NECC path toward the central basin. Along this pathway, temperature anomalies show coherent phase relationships off and on the equator and in surface and subsurface layers, respectively. A sequence of events is described that lead to E1 Niño conditions in the tropical Pacific Ocean. Beginning in early 1989, a positive temperature anomaly progressed coherently eastward along the NECC path off the equator toward the central basin, finally making its way equatorward with the expected mean circulation in late 1990. As the thermocline shoals eastward and upward along the NECC, in due course the progressing subsurface anomaly outcropped near the date line off the equator and initiated and sustained a midbasin warm sea surface temperature (SST) anomaly during the period from late 1990 through early 1991. This SST anomaly induced westerly wind anomalies over the western tropical Pacific, favoring Ekman convergence onto the equator. These anomalous surface currents advected these initially subsurface‐produced SST anomalies equatorward. Subsequently, these SST and wind anomalies were coupled, which resulted in a large‐scale relaxation of the trade winds over the western and central tropical basin, generating eastward currents that transported water mass eastward along the equator. These results are markedly different from the delayed oscillator physics in that the major role can be played by advection and outcropping of off‐equatorial subsurface thermal anomalies along the shallow NECC pathway, not necessarily involving the western boundary reflection of equatorial Rossby waves.
Structure of the mesospheric Na layer at 40°N latitude: Seasonal and diurnal variations
Authors: States Robert J. , Gardner Chester S.
Published: September 2002

Lidar observations obtained throughout the diurnal and annual cycles at Urbana, Illinois (40°N, 88°W), are used to examine the structure of mesospheric Na between 76 and 108 km. The Na layer is strongly influenced by seasonal and diurnal variations in solar illumination and by seasonal variations of mesospheric temperatures. Although there is little evidence of direct tidal perturbations in Na density, 24 hour oscillations dominate the diurnal variations. Na abundance and densities above 90 km are controlled by photoionization and charge exchange reactions during the day and recombination at night. The abundance is maximum at sunrise, and the peak‐to‐peak diurnal variation averages more than 30%. Below 85 km, density variations are controlled by photochemistry which interrupts the conversion of Na to the bicarbonate reservoir, and by warmer temperatures which enhance the liberation of Na from the reservoir. In this region the peak‐to‐peak diurnal Na variations can exceed 200%. The combined effects of photoionization above 90 km and photochemistry below 85 km induce a strong 24 hour oscillation in the layer centroid height. The peak‐to‐peak variation exceeds 1 km. The lowest centroid height is at local noon when Na densities below 85 km are maximum and photoionization above 90 km is strongest. The seasonal variations in Na densities and abundance are influenced primarily by changes in mesopause region temperatures which are coldest during midsummer. Below 95 km, reactions leading to the bicarbonate sink and the liberation of Na from this reservoir dominate Na chemistry. These reactions and their temperature dependencies are responsible for the large annual variation in Na abundance (2.3 × 109 to 5.3 × 109 cm−2). The measured correlation between Na abundance and the layer weighted temperature is +0.89. The ratio of the abundance to temperature variations is 1.9 × 108 cm−2K−1. The seasonal Na density and temperature perturbations are well‐correlated (+0.6 to +0.9) below 98 km and negatively correlated above (−0.2 to −0.5).
NOy‐N2O correlation observed inside the Arctic vortex in February 1997: Dynamical and chemical effects
Authors: Kondo Y. , Koike M. , Engel A. , Schmidt U. , Mueller M. , Sugita T. , Kanzawa H. , Nakazawa T. , Aoki S. , Irie H. , Toriyama N. , Suzuki T. , Sasano Y.
Published: September 2002

Simultaneous balloon‐borne in situ measurements of total reactive nitrogen (NOy) and nitrous oxide (N2O) were made up to 29 km over Kiruna, Sweden (68°N, 21°) on February 10 and 25, 1997. Kiruna was located at the edge or inside of the Arctic vortex at potential temperatures between 475 (∼19 km) and 675 K (∼26 km). Below 500 K (∼21 km) the N2O values were >120 ppbv on both days, and the observed NOy mixing ratios agreed well with those calculated using the NOy‐N2O correlation previously obtained at northern midlatitudes. An exception was a sharp dip in NOy at 445 K (18.4 km) observed on February 25. Back trajectory analyses indicate that this layer had experienced cold temperatures close to ice saturation, i.e., favorable conditions for denitrification. Between 500 and 600 K (∼24 km) the N2O values were <120 ppbv, and the observed NOy values were some 4–6 ppbv lower than those calculated using the midlatitude NOy‐N2O correlation, which includes the NOy reduction due to the N + NO reaction. The temperatures in the Arctic winter above 550 K were too high to cause extensive denitrification. The combined processes of (1) diabatic descent and (2) quasi‐horizontal mixing of vortex air are likely causes of the anomalous NOy‐N2O correlation. The CH4‐N2O correlation obtained inside the Arctic vortex in February 1997 also supports this hypothesis. A similar anomalous NOy‐N2O correlation was observed from the ER‐2 measurements and from the atmospheric trace molecule spectroscopy ATLAS 2 measurements made inside the vortex in the winters of 1991–1992 and 1992–1993.
Global emissions of hydrogen chloride and chloromethane from coal combustion, incineration and industrial activities: Reactive Chlorine Emissions Inventory
Authors: McCulloch Archie , Aucott Michael L. , Benkovitz Carmen M. , Graedel Thomas E. , Kleiman Gary , Midgley Pauline M. , Li Yi‐Fan
Published: September 2002

Much if not all of the chlorine present in fossil fuels is released into the atmosphere as hydrogen chloride (HCl) and chloromethane (CH3Cl, methyl chloride). The chlorine content of oil‐based fuels is so low that these sources can be neglected, but coal combustion provides significant releases. On the basis of national statistics for the quantity and quality of coal burned during 1990 in power and heat generation, industrial conversion and residential and commercial heating, coupled with information on the chlorine contents of coals, a global inventory of national HCl emissions from this source has been constructed. This was combined with an estimate of the national emissions of HCl from waste combustion (both large‐scale incineration and trash burning) which was based on an estimate of the global quantity released from this source expressed per head of population. Account was taken of reduced emissions where flue gases were processed, for example to remove sulphur dioxide. The HCl emitted in 1990, comprising 4.6 ± 4.3 Tg Cl from fossil fuel and 2 ± 1.9 Tg Cl from waste burning, was spatially distributed using available information on point sources such as power generation utilities and population density by default. Also associated with these combustion sources are chloromethane emissions, calculated to be 0.075 ± 0.07 Tg as Cl (equivalent) from fossil fuels and 0.032 ± 0.023 Tg Cl (equivalent) from waste combustion. These were distributed spatially exactly as the HCl emissions, and a further 0.007 Tg Cl in chloromethane from industrial process activity was distributed by point sources.
Tropical cyclone Marlene and stratosphere‐troposphere exchange
Authors: Baray Jean‐Luc , Ancellet Gérard , Randriambelo Tantely , Baldy Serge
Published: September 2002

Convective tropical cyclones initiate exchanges of mass and energy between the troposphere and the stratosphere. Recent analyses of ozone and water vapor measurements from Measurements of Ozone by Airbus In‐Service Aircraft (MOZAIC) data [Suhre et al., 1997] suggest that stratospheric ozone inputs to the troposphere can occur in the tropics near zones of deep convection. In the present paper, we analyze a case study of spectacular stratosphere‐troposphere exchange directly linked to the strong tropical cyclone Marlene, that occurred near Mauritius and Reunion Island on April 1995. The amplitude of the ozone peak (300 ppbv at 300 hPa) is comparable to those observed by Suhre et al. In this case, ageostrophic phenomena dominate advective phenomena. Therefore convective effects can be at the origin of mesoscale tropospheric ozone contaminations which affect the entire tropical free troposphere.
The 214Bi to 214Pb ratio in lower boundary layer aerosols and aerosol residence times at New Haven, Connecticut
Authors: Turekian Vaughan C. , Graustein William C. , Turekian Karl K.
Published: September 2002

The activities of 214Pb (half‐life 26.8 min) and 214Bi (half‐life 19.7 min), decay products of 222Rn, are nearly equal in 187 aerosol samples collected over 1 year at an altitude of 12 m in New Haven, Connecticut. These observations do not agree with the large variations in (214Bi)/(214Pb) reported by others for southern California. We derive a “reference ratio” as a technique for relating the measured activities of 214Bi and 214Pb accumulated on filters over sampling times that are longer than their half‐lives to the (214Bi)/(214Pb) in the atmosphere and derive simple models relating the reference ratio to aerosol residence time. The observed (214Bi)/(214Pb) ratios imply a mean residence time of 16 hours (with the 2 standard error limits of +72 and −7 of this mean) in a steady state box model and mean age of 2.8 hours (with the 2 standard error limits of +0.9 and −0.4 of this mean) in an isolated parcel model. The variance of the data indicates that the conditions of sampling are closer to the assumptions of the box model than the isolated parcel model.
Simulating the evolution of the Asian and African monsoons during the past 30 Myr using an atmospheric general circulation model
Authors: Fluteau Frédéric , Ramstein G. , Besse Jean
Published: September 2002

At geologic timescales, many proxy data suggest a contrasting evolution of Asian and African monsoons since the Oligocene. The Asian summer monsoon increases drastically around 8 Ma, whereas the African summer monsoon gradually weakens during the Miocene. Using an atmospheric general circulation model, we simulate most of the spatial evolutions of both monsoons only accounting for the changes of paleogeography, including continental drift, orogeny, and sea level change. The paleogeographic changes modify drastically the climate over central and southern Asia between the Oligocene and the present. The retreat of an epicontinental sea warms central Eurasia in summer. The heating of this area and the uplifts of the Tibetan plateau and of the Himalayas deepen the Asian low‐pressure cell and displace it northwest. This then shifts precipitation from Indochina toward the southern flank of the Himalayas. This is in good agreement with proxy data. Therefore our modeling studies support a shift and a strengthening of the Asian monsoon during the late Tertiary rather than a real “onset”. We suggest that the increase in seasonal precipitation and the strengthening of the number of days with heavy rainfall over the Himalayas from 30 Ma to the present may be of critical importance to explain the long‐term evolution of physical erosion of this area. We also investigate the respective impact of the Paratethys shrinkage and of the Tibetan plateau uplift through sensitivity experiments and prove that the Paratethys retreat plays an important role in monsoon evolution. The northward drift of the African continent confines summer monsoon precipitation to a thin belt which favors the stretching of the subtropical desert, in good agreement with data. We finally show that during the Oligocene, the African and Asian monsoon systems are clearly separated by the Tethys seaway. The closure of this seaway and the evolution of the Asian monsoon induce a connection between both monsoon systems in the low and middle troposphere.
Continuous fields of vegetation characteristics at the global scale at 1‐km resolution
Authors: DeFries R. S. , Townshend J. R. G. , Hansen M. C.
Published: September 2002

The geographic distribution of vegetation over the Earth's land surface is traditionally described using classification schemes with discrete numbers of vegetation types. When such land cover data sets are used as boundary conditions in Earth system models, abrupt boundaries and unrealistic homogeneity are introduced into parameter estimates. This paper proposes an alternative approach to describe global land cover with continuous fields of vegetation characteristics. A linear mixture model is applied to 1‐km advanced very high resolution radiometer data to estimate proportional cover for three important vegetation characteristics: life form (percent woody vegetation, percent herbaceous vegetation, and percent bare ground), leaf type (percent needleleaf and percent broadleaf), and leaf duration (percent evergreen and percent deciduous). Linear discriminants for input into the mixture model are derived from 30 metrics representing the annual phenological cycle. Through comparison with training data derived from a global network of Landsat multispectral scanner scenes, we conclude that the linear assumption implicit in the linear mixture model is not severely violated. The linear relationships between percent cover as determined from the training data and the linear discriminants are used to estimate end‐member values, and the mixture model is applied to derive the seven layers of global continuous fields. The availability of Moderate Resolution Imaging Spectroradiometer data in the future holds promise for refining the simple technique used in this paper to derive improved global continuous fields.
Outgoing longwave radiation from the TOVS Pathfinder Path A data set
Authors: Mehta Amita , Susskind Joel
Published: September 2002

A new outgoing longwave radiation (OLR) and clear‐sky outgoing longwave radiation (COLR) data set derived from the Goddard TIROS operational vertical sounder (TOVS) Pathfinder Path A retrievals is introduced. Comparisons of OLR and COLR from TOVS Path A and the Earth Radiation Budget Experiment (ERBE) have been carried out for the period February 1985 to May 1989. In the OLR comparison, advanced very high resolution radiometer (AVHRR) OLR has also been included. The mean OLR differences and standard deviations of the differences between TOVS and ERBE are, in general, within 5 W m−2. The differences AVHRR minus ERBE, and AVHRR minus TOVS OLR are close to −4 and −7 W m−2, respectively. However, over oceanic subtropical regions, AVHRR OLR differs by −8 to −12 W m−2 compared with the other two data sets. The differences of TOVS and ERBE COLR over most regions are within ±4 W m−2, with standard deviations of differences between 4 and 12 W m−2. The OLR and COLR data sets have larger discrepancies over snow/ice‐covered polar regions. Interannual variability of the OLR and COLR are compared for 1986–1988 in terms of their monthly mean anomalies. The global mean monthly ERBE OLR and COLR anomalies have somewhat larger magnitudes (−3.0 to 1.5 and −2.0 to 1.0 W m−2, respectively) compared with the other data sets (−1.0 to 1.0 and −0.7 to 0.7 W m−2, respectively). Both sets of ERBE anomalies appear to be influenced by the change of satellite from NOAA 9 to NOAA 10. The results of this study indicate that TOVS Path A and ERBE OLR and COLR are of comparable accuracy. The TOVS Path A data set covers a substantially longer time period (1985–1998) than ERBE and would be very useful in climate research studies.
Distribution of UV radiation at the Earth's surface from TOMS‐measured UV‐backscattered radiances
Authors: Herman J. R. , Krotkov N. , Celarier E. , Larko D. , Labow G.
Published: September 2002

Daily global maps of monthly integrated UV‐erythemal irradiance (290–400 nm) at the Earth's surface are estimated using the ozone amount, cloud transmittance, aerosol amounts, and surface reflectivity from the solar UV radiation backscattered from the Earth's atmosphere as measured by the total ozone mapping spectrometer (TOMS) and independently measured values of the extraterrestrial solar irradiance. The daily irradiance values at a given location show that short‐term variability (daily to annual) in the amount of UV radiation, 290–400 nm, reaching the Earth's surface is caused by (1) partially reflecting cloud cover, (2) haze and absorbing aerosols (dust and smoke), and (3) ozone. The reductions of UV irradiance estimated from TOMS data can exceed 50 ± 12% underneath the absorbing aerosol plumes in Africa and South America (desert dust and smoke from biomass burning) and exceeded 70 ± 12% during the Indonesian fires in September 1997 and again during March 1998. Recent biomass burning in Mexico and Guatemala have caused large smoke plumes extending into Canada with UV reductions of 50% in Mexico and 20% in Florida, Louisiana, and Texas. Where available, ground‐based Sun photometer data show similar UV irradiance reductions caused by absorbing aerosol plumes of dust and smoke. Even though terrain height is a major factor in increasing the amount of UV exposure compared to sea level, the presence of prolonged clear‐sky conditions can lead to UV exposures at sea level rivaling those at cloudier higher altitudes. In the equatorial regions, ±20°, the UV exposures during the March equinox are larger than during the September equinox because of increased cloudiness during September. Extended land areas with the largest erythemal exposure are in Australia and South Africa where there is a larger proportion of clear‐sky days. The large short‐term variations in ozone amount which occur at high latitudes in the range ±65° cause changes in UV irradiance comparable to clouds and aerosols for wavelengths between 280 nm and 300 nm that are strongly absorbed by ozone. The absolute accuracy of the TOMS monthly erythemal exposure estimates over a TOMS field of view is within ±6%, except under UV‐absorbing aerosol plumes (dust and smoke) where the accuracy is within ±12%. The error caused by aerosols can be reduced if the height of the aerosol plume is more accurately known. The TOMS estimated irradiances are compared with ground‐based Brewer spectroradiometer data obtained at Toronto, Canada. The Brewer irradiances are systematically 20% smaller than TOMS irradiance estimates during the summer months. An accounting of systematic errors brings the Brewer and TOMS irradiances into approximate agreement within the estimated instrumental uncertainties for both instruments.
Influence of southern hemispheric biomass burning on midtropospheric distributions of nonmethane hydrocarbons and selected halocarbons over the remote South Pacific.
Authors: Blake Nicola J. , Blake Donald R. , Wingenter Oliver W. , Sive Barkley C. , McKenzie Lisa M. , Lopez Jimena P. , Simpson Isobel J. , Fuelberg Henry E. , Sachse Glen W. , Anderson Bruce E. , Gregory Gerald L. , Carroll Mary Anne , Albercook George M. , Rowland F. Sherwood
Published: September 2002

Aircraft measurements of nonmethane hydrocarbons (NMHCs) and halocarbons were made over the remote South Pacific Ocean during late August‐early October 1996 for NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A). This paper discusses the large‐scale spatial distributions of selected trace gases encountered during PEM‐Tropics A. The PEM‐Tropics A observations are compared to measurements made over the southwestern pacific in early November 1995 as part of Aerosol Characterization Experiment (ACE 1). Continental pollution in the form of layers containing elevated levels of O3 was observed during a majority of PEM‐Tropics flights, as well as during several ACE 1 flights. The chemical composition of these air masses indicates that they were not fresh and were derived from nonurban combustion sources. The substantial impact of biomass burning on the vertical structure of the South Pacific troposphere is discussed.
Horizontal wavenumber spectra of winds, temperature, and trace gases during the Pacific Exploratory Missions: 2. Gravity waves, quasi‐two‐dimensional turbulence, and vortical modes
Authors: Cho John Y. N. , Newell Reginald E. , Barrick John D.
Published: September 2002

We examine the horizontal wavenumber spectra of horizontal velocity and potential temperature collected by aircraft above the Pacific Ocean to determine whether gravity waves, quasi‐two‐dimensional (Q‐2‐D) turbulence, or vortical modes dominate atmospheric fluctuations at scale sizes of 1–100 km and altitudes of 2–12 km. We conclude from the study of Doppler‐shifting effects that Q‐2‐D turbulence and/or vortical modes are more prevalent than gravity waves over the ocean, except in the equatorial zone. The results are consistent with recent numerical simulations of Q‐2‐D turbulence, which show that the characteristic inverse cascade of energy is greatly facilitated by the presence of background rotation. Furthermore, a Stokes‐parameter analysis reveals the general paucity of coherent wavelike motions, although specific cases of gravity‐wave propagation are observed. Finally, a case study of a long flight segment displays a k−3 horizontal velocity variance spectrum at scales longer than about 100 km. A Stokes‐parameter analysis indicates that these large‐scale fluctuations were likely due to vortical modes rather than inertio‐gravity waves.
Long‐term measurements of the transport of African mineral dust to the southeastern United States: Implications for regional air quality
Authors: Prospero Joseph M.
Published: September 2002

Continuous daily aerosol sampling carried out at a coastal site in Miami, Florida, for the past 23 years shows that large quantities of African mineral dust are periodically carried into Florida every summer, yielding daily concentrations in the range of about 10 μg m−3 to 100 μg m−3. Dust events typically last several days or longer. The maximum dust concentration occurs in July (monthly mean, 16.3 μg m−3), but relatively high concentrations are also observed in June (8.4 μg m−3) and August (9.8 μg m−3). There is considerable year‐to‐year variability that is apparently linked to various meteorological factors including climate conditions in North Africa as manifested by drought. Satellite data show that African dust incursions are synoptic‐scale events; consequently, they will impact a large region of the southern and eastern United States. The incursion of dust events over this large region, coupled with inputs from local emissions, could have important implications regarding regional air quality.
Controlling variables for the uptake of atmospheric carbonyl sulfide by soil
Authors: Kesselmeier J. , Teusch N. , Kuhn U.
Published: September 2002

Soil samples from arable land were investigated for their exchange of carbonyl sulfide (COS) with the atmosphere under controlled conditions using dynamic cuvettes in a climate chamber. The investigated soil type acted as a significant sink for the trace gas COS. Atmospheric COS mixing ratios, temperature, and soil water content were found to be the physicochemical parameters controlling the uptake. Emission was never observed under conditions representative of a natural environment. The observed compensation point (i.e., an ambient concentration where the consumption and production balance each other and the net flux is zero) for the uptake was about 53 parts per trillion. Uptake rates ranged between 1.5 and 10.3 pmol m−2 s−1. The consumption of COS by the soil sample depended on the physiological activity of the microorganisms in the soil, as indicated by a clear optimum temperature and by a drastic inhibition in the presence of the enzyme inhibitor 6‐ethoxy‐2‐benzothiazole‐2‐sulfonamide (EZ), a specific inhibitor for carbonic anhydrase.
The potential of hyperspectral bidirectional reflectance distribution function data for grass canopy characterization
Authors: Sandmeier Stefan R. , Middleton Elizabeth M. , Deering Donald W. , Qin Wenhan
Published: September 2002

Hyperspectral bidirectional reflectance distribution function (BRDF) data of Konza prairie grassland acquired in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) on the ground with two SE‐590 instruments and remotely with the airborne advanced solid‐state array spectroradiometer (ASAS) are analyzed and compared to BRDF data of dense ryegrass obtained in the laboratory and field with the European goniometric facility (EGO) and the Swiss field‐goniometer system (FIGOS). The soil underlying the relatively sparse Konza prairie grass disturbed the spectral BRDF effects of the vegetation components. After a correction of the soil influence based on the bidirectional canopy gap probability, the Konza data from SE‐590 and ASAS sensors showed a consistently strong dependence of spectral BRDF effects from nadir reflectance as was observed in the EGO and FIGOS data. BRDF effects were inversely related to reflectance intensities, low reflectances being associated with pronounced BRDF effects and high reflectances with low BRDF effects. This relationship is due to multiple scattering effects and is influenced by the canopy optical properties and architecture parameters such as leaf area index (LAI), leaf angle distribution (LAD), and the gap fraction. The BRDF data of the Konza prairie grass from both ground and aircraft measurements showed a strong relationship between LAI and spectral BRDF variability. Reflectance data with high spectral resolution in the red edge range from 675 to about 900 nm wavelength, acquired from the two viewing directions with maximum and minimum reflectance intensities proved to be useful for deriving vegetation canopy architecture characteristics from hyperspectral BRDF data. BRDF data with high spectral resolution from the airborne ASAS sensor and from planned commercial remote sensing satellites are therefore an ideal testbed for a further exploration of this promising approach.
Stratospheric tides and data assimilation
Authors: Swinbank Richard , Orris Rebecca L. , Wu Dong L.
Published: September 2002

In the upper stratosphere the atmosphere exhibits significant diurnal and semidiurnal tidal variations with typical amplitude of about 2 K in midlatitudes. In this paper we examine how well the tidal variations in temperature are represented by the GEOS‐2 data assimilation system. It is shown that the GEOS‐2 atmospheric model is quite successful at simulating the tidal variations. However, the assimilation of satellite temperature soundings significantly damps the simulated tides. The reason is that the tides are not well represented by the satellite retrievals used by the assimilation system (which have a typical tidal amplitude of about 1 K). As a result of this study, we suggest improvements that should be made to the treatment of satellite sounding data by the assimilation system.
In situ observations of contrail microphysics and implications for their radiative impact
Authors: Poellot Michael R. , Arnott W. Patrick , Hallett John
Published: September 2002

In this study we present the microphysical characteristics of 21 jet contrail clouds sampled in situ and examine the possible effects of exhaust on natural cirrus and radiative effects of contrails. Microphysical samples were obtained with Particle Measuring Systems (PMS) 2D‐C, 1D‐C, and FSSP probes. About one half of the study contrails were generated by the sampling aircraft, a Cessna Citation, primarily at times of 3–15 min after generation; the source and age of the others is unknown. On average, the contrails contained particles of mean diameter of the order of 10 μm in concentrations exceeding 10,000 L−1. Contrails embedded in natural cirrus appeared to have little effect on the natural cloud microphysics. Anomalous diffraction theory was used to model radiative properties of sampled contrails. The contrail cirrus showed considerably more spectral variation in extinction and absorption efficiencies than natural cirrus because of the large numbers of small crystals in contrails. Embedded contrails also displayed greater emissivity and emission than natural cirrus and a greater spectral variation in transmission.
On the generation and evolution of aeroelectric structures in the surface layer
Authors: Anisimov S. V. , Mareev E. A. , Bakastov S. S.
Published: September 2002

Ultralow frequency pulsations of electric field in the surface atmospheric layer were investigated under fair weather conditions. A new method of structural‐temporal analysis has been applied to the study of spatiotemporal structures of the electric field described previously by [Anisimov et al., 1994]. The method is based on exploration of the structural function by averaging the remote sensing data over respective temporal spans. This analysis allows quantitative estimations of spatial scales L ≃ 500 ‐ 103 m and temporal scales not less than τ = 10 min for the structural elements of the planetary boundary layer electricity; we call these recently examined elements “aeroelectric structures” (AES). Quasiperiodic sequences and high‐amplitude solitary AES have been recognized. Three‐dimensional structural‐temporal patterns are presented which directly characterize the level of electric energy perturbations connected with AES formation during night‐day evolution. A model of AES formation has been developed, taking into account the occurence of convective cells with respective turbulent air and space charge density distributions that are transferred by the wind over the ground and cause the electric field fluctuations at the points of observation. Therefore formation of such submesoscale structures can be explained by the redistribution of space charge within the surface layer, with the structures of the smallest scales coupled to the turbulent mixing of the ions and aerosols. In addition to the advection and turbulent mixing of space charge, we also consider the cooperative electroaerodynamic effects which might occur in a system of bipolar ion and aerosol particles under the influence of a terrestrial electric field. We have proposed an advanced model treating the AES formation as the result of instability arising in such a system, taking into account the dependence of the effective ion‐aerosol attachment coefficient on the external electric field strength.
Ultraviolet spectral irradiance in the French Alps: Results of two campaigns
Authors: Pachart Eric , Lenoble Jacqueline , Brogniez Colette , Masserot Dominique , Bocquet Jean Louis
Published: September 2002

The ultraviolet spectral irradiance has been measured in the Alps in Briançon (altitude 1300 m asl) in July 1996 and March 1997, with the spectroradiometer of the University of Lille. The erythemal dose rate has been measured simultaneously with a broadband instrument. The aerosol optical depth, necessary as input for modeling, was measured with a Sun photometer. On cloudy days, irradiance is highly variable, with values exceeding those of clear days when the direct solar beam is not obstructed by clouds. For clear days, measured values are compared with the results of a radiative transfer model, and the influence of altitude and ground surface reflectance is analyzed. For the same solar zenith angle, irradiance in UVA is 8–10% higher in Briançon than in Brussels, half of the difference being due to the higher altitude of Briançon. Comparison of winter and summer values shows an amplification of ∼10–15% in winter owing to the snow reflectance; it can be explained by an effective reflectance of 0.3–0.4.
Photochemical modeling of OH levels during the First Aerosol Characterization Experiment (ACE 1)
Authors: Frost G. J. , Trainer M. , Mauldin R. L. , Eisele F. L. , Prevot A. S. H. , Flocke S. J. , Madronich S. , Kok G. , Schillawski R. D. , Baumgardner D. , Bradshaw J.
Published: September 2002

Comparisons are made between a steady state photochemical model and the first airborne measurements of hydroxyl radical concentrations, [OH], in the lower free troposphere and marine boundary layer, taken during the First Aerosol Characterization Experiment (ACE 1). This paper describes in more detail the modeling results and the model‐measurement comparisons which were presented by Mauldin et al. [1997, 1998]. Modeled and observed [OH] agree to within the combined uncertainties, with a median model overestimate of 32%. The model quantifies the sources and sinks of OH for low levels of nitric oxide (NO). Predicted concentrations of the peroxy radicals HO2 and CH3O2 and total radical production and loss rates from these model calculations are presented. Sensitivities to model input parameters and the uncertainty each contributes to the calculated [OH] are discussed. Model overpredictions of observed [OH] within clouds can be corrected by including a loss of gas phase OH to cloud droplets. The low [NO] conditions preclude HO2 uptake alone from explaining observed depletions of OH within clouds. We examined the dependence of gas phase [OH] on its assumed mass accommodation coefficient, which the measurements suggest is 0.1 or greater for the cloud droplets encountered in ACE 1.
Intercomparison of satellite and aircraft observations of ozone, CFC‐11, and NOy using trajectory mapping
Authors: Bacmeister Julio T. , Kuell Volker , Offermann Dirk , Riese Martin , Elkins James W.
Published: September 2002

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) satellite measured global distributions of over 15 trace constituents at high horizontal resolution (<500 km). The CRISTA mission began shortly before the end of the Airborne Southern Hemisphere Ozone Experiment and Measurements to Assess the Effects of Stratospheric Aircraft (ASHOE/MAESA) airborne measurement campaign. CRISTA measurements of ozone, CFC‐11, and HNO3 are available below 20 km, in the altitude range sampled by the ER‐2 aircraft during ASHOE/MAESA. However, the time separation between the two missions makes a direct comparison of the data impossible. In this study, trajectory techniques are used to compare CRISTA measurements of ozone, CFC‐11, and HNO3 with ER‐2 measurements taken during the last two flights of ASHOE/MAESA on November 2 and 4, 1994. Visual comparison of CRISTA and ER‐2 data from the November 4 flight is generally good. Mean tracer gradients across scales of ∼1000 km, as well as absolute mixing ratios agree well with those measured from the ER‐2. Correlation coefficients for coincident pairs of ER‐2 and CRISTA measurements are between 0.68 and 0.80. The comparison for the November 2 flight is poor. We argue that the poor agreement for this flight maybe due to both smaller CRISTA measurement density as well as rapid stretching in the air mass sampled by the ER‐2 on November 2.
In situ measurements of the NOx distribution and variability over the eastern North Atlantic
Authors: Ziereis H. , Schlager H. , Schulte P. , Köhler I. , Marquardt R. , Feigl C.
Published: September 2002

Between 1994 and 1996 the research aircraft Falcon of the Deutsches Zentrum für Luft‐ und Raumfahrt was used to probe the upper troposphere and lowermost stratosphere over the eastern North Atlantic. In situ measurements of NO, NO2, and O3 were performed during 32 flights. The measurements were carried out during five aircraft campaigns in summer and late autumn, respectively, based from Shannon, Ireland, and Prestwick, Scotland. Most of the flights were conducted in the region of the North Atlantic flight corridor. Main objectives of these measurements included the study of the large‐scale distribution of NO and NOx and the development of reliable monthly mean values for different seasons in a region of the atmosphere that is strongly affected by aircraft emissions. Substantial variability of NO and NOx volume mixing ratios was observed in the upper troposphere and lowermost stratosphere. In spite of this variability a significant seasonal dependence was found. NO mean values (averages over all measurements made during one campaign) at altitudes between 10,500 and 11,500 m, where most of the data have been obtained, ranged between about 0.1 and 0.14 ppbv in summer and 0.03 and 0.10 ppbv in late autumn. NO and NOx did not show a significant gradient across the tropopause. The correlation between NOx and O3 in the upper troposphere and lowermost stratosphere was only very weak. The present measurements represent a suitable data set for comparison with predictions of the NOx distribution in the upper troposphere over the eastern North Atlantic by three‐dimensional models. A comparison with the NOx fields simulated with the European Center for Medium‐Range Weather Forecasts, Hamburg Version 3 (ECHAM 3) climate model extended by a simplified NOx chemistry reveals good agreement for summer and autumn conditions.
Climatic controls on interannual variability of precipitation δ18O: Simulated influence of temperature, precipitation amount, and vapor source region
Authors: Cole Julia E. , Rind David , Webb Robert S. , Jouzel Jean , Healy Richard
Published: September 2002

We use an atmospheric GCM that incorporates stable isotopes and regional vapor source tracers in the hydrologic cycle to explore the relationship between interannual variability in climate and precipitation δ18O globally. On the basis of a 12‐year simulation forced by observed sea surface temperatures (SSTs), we identify changes in the amount of precipitation and in the contributions of local and nearby vapor sources as the most important determinants of simulated interannual isotopic changes. The model simulates weak positive correlation between temperature and isotopic variability only in certain continental regions, mostly in the extratropics. Comparison with long observed records of isotopes and climate indicates that the model simulates realistic patterns of temperature‐isotope correlation but may overestimate the isotopic influence of precipitation amount. Perturbations in circulation patterns that alter the transport and mixing of air masses at a site also change the relative contributions of vapor from different source regions. Simulated changes in vapor source regions are large, reaching ±10–15% of the total precipitation, and cause significant isotopic variability in nearly all grid cells. Our results suggest that shifts among vapor sources may provide an important control on the interannual isotopic variability observed in modern precipitation and paleoclimatic records. The isotopic variability simulated in this experiment results from the interaction of several aspects of climate. Interannual temperature variability generally involves circulation changes that alter air mass transport, vapor source regions, and condensation history; this advective mechanism may explain the relative weakness of temperature‐isotope correlations in both the model and the observations.
In situ measurements of microphysical properties and trace gases in two cumulonimbus anvils over western Europe
Authors: Ström Johan , Fischer Horst , Lelieveld Jos , Schröder Franz
Published: September 2002

This study presents in situ measurements performed in two cumulonimbus anvils over western Europe during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) in July 1994. Sampling of cloud particles between 5 and 60 μm aerodynamic diameter was performed using a counterflow virtual impactor. Peak values of cloud water content and crystal number density for this size‐range exceeded 2 g m−3 and 200 cm−3 (normalized to standard temperature and pressure), respectively. Concurrent measurements of aerosol particles, water vapor, CO, and O3 support the idea that much of the air in the anvil has been rapidly transported directly from the boundary layer while experiencing limited dilution.
Nitric oxide emissions from forest soils
Authors: van Dijk Saskia M. , Duyzer Jan H.
Published: September 2002

Nitric oxide (NO) fluxes to the atmosphere were determined from different soil types in coniferous forest and deciduous forest in the Netherlands. Dynamic chambers were used to determine the emission every week throughout a year. Forest type and soil texture are important parameters leading to large differences in fluxes between sites. High fluxes of up to 178 ng m−2 s−1 were observed in summer in a Douglas fir forest on a sandy soil. In a nearby beech forest, on the same soil, fluxes of up to 63 ng m−2 s−1 were observed. Emissions from clayey soils in a beech forest were much lower, ranging from 0 to 14 ng m−2 s−1. Besides the important effect of the forest community type and the soil texture on the NO flux, a strong positive correlation with soil temperature was observed (accounting for up to 70% of the variance). Soil moisture could explain up to 60% of the variance of the NO flux. It seems that an optimum range of intermediate moisture content exists, ranging from 30 to 45% dry weight or from 55 to 85% water‐filled pore space, approximately, where high temperatures lead to high emissions.
The cold winters of the middle 1990s in the northern lower stratosphere
Authors: Pawson Steven , Naujokat Barbara
Published: September 2002

Lower stratospheric temperatures in the northern winters of 1994/1995, 1995/1996, and 1996/1997 were low enough to support polar stratospheric cloud (PSC) formation for prolonged periods. While the seasonal evolution of each winter was quite different, there are some common characteristics: notably, the occurrence of extremely cold periods of long duration and the coldness of the late winter in each year. Comparison with observations over more than three decades indicate the stratosphere was atypically cold in these three years, with the largest anomalies occurring in the late winter and spring. In January and February the coldness seems to be determined by the interannual variability of the circulation, while in March the persistence of the polar vortex dominated the circulation in these three years. This may be related to the lack of major midwinter warmings in those years. Comparison with other winters shows that although the persistence of the polar vortex well into the spring is not unprecedented, this did not occur frequently in the previous two decades. Further, there is a general temperature decrease in the northern lower stratosphere which contributed to the coldness of the three winters. Comparison of the late winter and spring of 1997 with 1967, both of which were forced only weakly by dynamics, supports the idea that this is due to a change in the radiative balance (with equilibrium at a lower temperature), although there are many caveats to this conclusion.
Total ozone influence on the surface UV‐B radiation in the late spring‐summer 1963–1997: An analysis of multiple timescales
Authors: Krzyścin Janusz W.
Published: September 2002

Monthly means and minima of total ozone for the late springs and summers (May–August) of 1963–1997 have been examined for the European Dobson stations (Arosa, Belsk, Hohenpeissenberg, Hradec Kralove, Uccle). It is shown that long‐term tendencies in total ozone means were almost similar to those in the total ozone minima. Analyses of the late spring/summer means of UV daily doses, total ozone, and global solar radiation (proxy for the overall atmospheric transparency), measured at Belsk (52°N, 21°E) for the period 1976–1996, show that an importance of the total ozone changes for the UV‐B level increases with the timescale. Decadal variations in total ozone are the main source of the UV trend at Belsk. Frequency of appearance of extreme daily total ozone values in the selected late spring/summer season seems to be important for analyses of the ozone forcing in the interannual timescale. Regional and temporal differences in the number of days with extreme low ozone values are discussed using the total ozone extrema taken at Arosa, Belsk, and Hradec Kralove in the 1963–1997 period. A statistical model is developed for diagnosis of the next day value of the UV‐B level. The changes in the overall atmospheric transparency are essential for the UV‐B level when the day‐to‐day variations in the UV forcing factors are examined.
Determination of an effective spectral surface albedo from ground‐based global and direct UV irradiance measurements
Authors: Kylling A. , Persen T. , Mayer B. , Svenøe T.
Published: September 2002

Ultraviolet radiation may be enhanced by the presence of snow cover. For areas with snow the surface is often highly inhomogeneous owing to the presence of low‐albedo surfaces together with snow‐covered surfaces. In order to better understand the coupling between snow cover, surface albedo, and UV radiation, an estimate of an effective albedo for such highly inhomogeneous surfaces is required. In this paper a method to derive an effective wavelength dependent Lambertian surface albedo in the UV and visible part of the spectrum is described. The retrieval method is used to analyze direct and global irradiance measurements performed in the wavelength region 320–500 nm during the winter‐spring‐summer seasons of 1997 in Tromsø, Norway. The derived albedo reached a maximum of 0.57 and 0.78 at 320 and 450 nm, respectively, when the snow depth was at maximum. The corresponding values for completely snow‐free conditions were 0.08 and 0.16. The absolute uncertainty in these values is estimated to be 0.1 at the shortest wavelengths and 0.2 at the higher wavelengths. Calculations with a three‐dimensional Monte Carlo model indicate that the open waters surrounding Tromsø all the year round decrease the effective albedo by approximately 0.2 compared to a completely snow‐covered surface.
Inverse modeling of annual atmospheric CO2sources and sinks: 1. Method and control inversion
Authors: Bousquet P. , Ciais P. , Peylin P. , Ramonet M. , Monfray P.
Published: September 2002

A primary goal of developing the CO2atmospheric measurement network is to better characterize the sources and sinks of atmospheric CO2. Atmospheric transport models can be used to interpret atmospheric measurements in terms of surface fluxes using inverse methodology. In this paper we present a three‐dimensional (3‐D) inversion of CO2measurements in order to infer annual sources and sinks of CO2at a continental scale (continents and ocean basins) for a climatological year representing the 1985–1995 period. Solving this inverse problem requires (1) a data space representing monthly CO2measurements, here at 77 sites (surface, ships, planes), (2) a flux space describing a priori fluxes between carbon reservoirs, and (3) a 3‐D transport model linking the flux space to the data space. Knowledge of these three elements, together with their associated errors, allows one to reduce the uncertainties of the CO2sources and sinks. In the 1985–1995 period, for our control inversion, the global continental sink is found to be 2.7±1.5 Gt C yr1for an optimized deforestation source of 1.4±0.6 Gt C yr1, yielding a net land uptake of 1.3±1.6 Gt C yr1(fossil fuel removed). The continental partition of this budget is (in units of Gt C yr1): Arctic +0.2±0.3, North America −0.5±0.6, Europe −0.3±0.8, north Asia −1.5±0.7, tropics (except Asia) +0.3±0.9, tropical Asia +0.8±0.4, and Southern Hemisphere −0.1±0.3. The inferred partition for the controversial Northern Hemisphere CO2sink reveals that a major sink is located over the north Asia continent. For oceans we find a net global sink of 1.5±0.5 Gt C yr1with the following partition (in units of Gt C yr1): North Pacific −0.3±0.2, North Atlantic −0.8±0.3, equator +0.6±0.2, 20°S–50°S oceans −0.9±0.3, and austral ocean −0.1±0.1.