bluuebunny/crossref_metadata_2025 huggingface dataset:
Water isotope module of the ECHAM atmospheric general circulation model: A study on timescales from days to several years
Authors: Hoffmann G. , Werner M. , Heimann M.
Published: February 2004

Results are presented of a global simulation of the stable water isotopes H218O and HD16O as implemented in the hydrological cycle of the ECHAM atmospheric general circulation model. The ECHAM model was run under present‐day climate conditions at two spatial resolutions (T42,T21), and the simulation results are compared with observations. The high‐resolution model (T42) more realistically reproduced the observations, thus demonstrating that an improved representation of advection and orography is critical when modeling the global isotopic water cycle. The deuterium excess (d=δD−8*δ18O) in precipitation offers additional information on climate conditions (e.g., relative humidity and temperature) which prevailed at evaporative sites. Globally, the simulated deuterium excess agrees fairly well with observations showing maxima in the interior of Asia and minima in cold marine regions. However, over Greenland the model failed to show the observed seasonality of the excess and its phase relation to δD reflecting either unrealistic source areas modeled for Greenland precipitation or inadequate description of kinetics in the isotope module. When the coarse resolution model (T21) is forced with observed sea surface temperatures from the period 1979 to 1988, it reproduced the observed weak positive correlation between the isotopic signal and the temperature as well as the weak negative anticorrelation between the isotopic signal and the precipitation. This model simulation further demonstrates that the strongest interannual climate anomaly, the El Niño Southern Oscillation, imprints a strong signal on the water isotopes. In the central Pacific the anticorrelation between the anomalous precipitation and the isotope signal reaches a maximum value of−0.8.
Physical, chemical, and optical properties of regional hazes dominated by smoke in Brazil
Authors: Reid Jeffrey S. , Hobbs Peter V. , Ferek Ronald J. , Blake Donald R. , Martins J. Vanderlei , Dunlap Michael R. , Liousse Catherine
Published: February 2004

Gas and particle measurements are described for optically thick regional hazes, dominated by aged smoke from biomass burning, in the cerrado and rain forested regions of Brazil. The hazes tended to be evenly mixed from the surface to the trade wind inversion at 3–4 km in altitude. The properties of aged gases and particles in the regional hazes were significantly different from those of young smoke (<4 min old). As the smoke aged, the total amount of carbon in non‐methane hydrocarbon species (C<11) was depleted by about one third due to transformations into CO2, CO, and reactive molecules, and removed by dry deposition and/or by conversion to particulate matter. As the smoke particles aged, their sizes increased significantly due to coagulation and mass growth by secondary species (e.g., ammonium, organic acids and sulfate). During aging, condensation and gas‐to‐particle conversion of inorganic and organic vapors increased the aerosol mass by ∼20–40%. One third to one half of this mass growth likely occurred in the first few hours of aging due to the condensation of large organic molecules. The remaining mass growth was probably associated with photochemical and cloud‐processing mechanisms operating over several days. Changes in particle sizes and compositions during aging had a large impact on the optical properties of the aerosol. Over a 2 to 4 day period, the fine particle mass‐scattering efficiency and single‐scattering albedo increased by 1 m2g−1, and ∼0.06, respectively. Conversely, the Angstrom coefficient, backscatter ratio, and mass absorption efficiency decreased significantly with age.
Airborne remote sensing of NO2 in the Arctic winter of 1994–1995 and comparison with a three‐dimensional chemical transport model
Authors: Glatthor N. , Blom C. E. , von Clarmann T. , Fischer H. , Gulde T. , Piesch C. , Erle F. , Pfeilsticker K. , Chipperfield M. P. , Lee A. M. , Pyle J. A.
Published: February 2004

In the winter of 1994–1995, for the first time NO2 was observed by the Michelson Interferometer for Passive Atmospheric Sounding‐Flugzeug Transall (MIPAS‐FT). We present the NO2 data of MIPAS and of a Differential Optical Absorption Spectroscopy (DOAS) instrument, which was also operated on the Transall, and compare the measurements with the Single Layer Isentropic Model for Chemistry And Transport (SLIMCAT). The daytime midlatitude NO2 column amounts of MIPAS and DOAS increased from 1×1015 cm−2 in December to ∼3×1015 cm−2 in March. After sunset MIPAS observed strong NO2 increases, up to 3.2×1015 cm−2 on December 20, up to 7.2×1015 cm−2 on January 17 and up to 6.4×1015 cm−2 on March 27. These high nighttime values, showing considerable amounts of NOx in the midlatitude stratosphere, were qualitatively reproduced by SLIMCAT. The NO2 columns measured inside the Arctic vortex, ≤1×1015 cm−2 from December until mid‐January with no substantial increase after sunset, displayed strong denoxification. From the end of January the Arctic daytime NO2 columns grew to ∼2×1015 cm−2, and after sunset MIPAS observed similar strong increases as at midlatitudes, up to 3.4×1015 cm−2 on January 24 and up to 4.8×1015 cm−2 in March and April. The renoxification of the vortex was also shown by SLIMCAT. On April 5, MIPAS NO2 column amounts decreased by 1.5×1015 cm−2 after reaching the maximum, caused by N2O5 and ClONO2 formation and encounter of less renoxified air masses.
Global atmospheric impacts induced by year‐round open water adjacent to Antarctica
Authors: Bromwich David H. , Chen Biao , Hines Keith M.
Published: February 2004

A sensitivity study to evaluate the greatest impact that sea‐ice anomalies around Antarctica could have on the global atmospheric circulation is conducted with the National Center for Atmospheric Research Community Climate Model, Version 2. A 15‐year seasonal cycle simulation is performed in which all sea ice around Antarctica is removed and replaced by year‐round open water at −1.9°C. The results are compared to a standard 15‐year seasonal cycle run with boundary conditions set for the present climate. The comparison shows that substantial changes in pressure, vertical circulation, and precipitation are found in both hemispheres as a result of Antarctic sea‐ice removal. These anomalies are more significant in the southern hemisphere during May‐September, whereas the anomalies are more notable in the northern hemisphere during September‐November, a result which was not present in previous perpetual simulations. Convective precipitation increases and large‐scale precipitation decreases as the circumpolar trough moves closer to Antarctica in response to the sea‐ice removal. Positive and negative anomalies form a wave‐like train in the troposphere. The anomalies include the delayed onset of the winter monsoon over northern China during September. This result is in basic agreement with an observational study that found monsoon parameters are correlated with Antarctic sea‐ice characteristics. In addition, the mean meridional circulation and convective precipitation have a monthly modulation of roughly 5% in the tropics. Mechanisms for the global atmospheric propagation of the impact of the Southern Ocean surface heating anomalies are examined.
Aerosol light scattering properties at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1)
Authors: Carrico Christian M. , Rood Mark J. , Ogren John A.
Published: February 2004

Large uncertainties remain in understanding the effects of ambient aerosols on climate. As part of the First Aerosol Characterization Experiment (ACE 1, November 15 to December 15, 1995), the total light scattering and backscattering coefficients by particles (σsp and σbsp) were measured at Cape Grim, Tasmania. Such measurements were made to characterize aerosol radiative forcing relevant to climate change at a southern hemisphere site. A controlled relative humidity (RH) nephelometry system (humidograph) was used to measure the dependence of σsp and σbsp upon controlled RH, upper particle diameter (Dp), and wavelength of light (λ). Parameters derived from the measurements necessary to incorporate aerosols into climate models include the dependence of light scattering on aerosol hygroscopic growth (ƒ(RH)) and the hemispheric backscatter fraction (b), both as functions of wavelength of light. Differences in parameter values during clean marine and non‐clean‐marine time periods were greatest for σsp and σbsp for Dp ≤ 1 μm and were least for ƒ(RH) and b. During clean marine conditions, for Dp ≤ 10 μm, and at λ=550 nm, means and standard deviations of key parameters measured included ƒ(RH equal to 82%) = 1.98±0.08 for σsp, ƒ(RH equal to 82%) = 1.48±0.09 for σbsp, b = 0.16±0.01 at RH equal to 22%, and b = 0.12±0.03 at RH equal to 82%. These measurements contribute to characterizing aerosol optical properties at a remote marine site, serve as a basis of comparison to polluted sites, and may provide input to climate models.
Observations of aerosol volatility and elemental composition at Macquarie Island during the First Aerosol Characterization Experiment (ACE 1)
Authors: Kreidenweis S. M. , McInnes L. M. , Brechtel F. J.
Published: February 2004

A combined thermal/electron microscopy technique was used to analyze marine aerosol samples from Macquarie Island (54°30′S, 158°57′E), obtained during November and December 1995 as part of the First Aerosol Characterization Experiment (ACE 1). The analyses were performed for two size fractions, 80 < Dp < 200 nm and Dp > 200 nm. Sulfate particles were primarily observed in the 80 < Dp < 200 nm fraction and were deduced to be present also at smaller sizes from the volatile response of the aerosol. In two of the samples early in the experiment, sulfate particles appeared to be partially or fully neutralized, but the particles in later samples from clean marine air masses were more volatile, suggesting a sulfuric or methanesulfonic acid composition. Sea‐salt particles dominated the number concentrations of particles with Dp > 200 nm and also contributed significantly to the number concentrations in the 80 < Dp < 200 nm fraction. Evidence for increased sea‐salt number concentrations at higher wind speeds was found.
Winter and early spring CO2 efflux from tundra communities of northern Alaska
Authors: Fahnestock J. T. , Jones M. H. , Brooks P. D. , Walker D. A. , Welker J. M.
Published: February 2004

Carbon dioxide concentrations through snow were measured in different arctic tundra communities on the North Slope of Alaska during winter and early spring of 1996. Subnivean CO2 concentrations were always higher than atmospheric CO2. A steady state diffusion model was used to generate conservative estimates of CO2 flux to the atmosphere. The magnitude of CO2 efflux differed with tundra community type, and rates of carbon release increased from March to May. Winter CO2 efflux was highest in riparian and snow bed communities and lowest in dry heath, upland tussock, and wet sedge communities. Snow generally accrues earlier in winter and is deeper in riparian and snow bed communities compared with other tundra communities, which are typically windswept and do not accumulate much snow during the winter. These results support the hypothesis that early and deep snow accumulation may insulate microbial populations from very cold temperatures, allowing sites with earlier snow cover to sustain higher levels of activity throughout winter compared to communities that have later developing snow cover. Extrapolating our estimates of CO2 efflux to the entire snow‐covered season indicates that total carbon flux during winter in the Arctic is 13–109 kg CO2‐C ha−1, depending on the vegetation community type. Wintertime CO2 flux is a potentially important, yet largely overlooked, part of the annual carbon cycle of tundra, and carbon release during winter should be accounted for in estimates of annual carbon balance in arctic ecosystems.
Study of ozone formation and transatlantic transport from biomass burning emissions over West Africa during the airborne Tropospheric Ozone Campaigns TROPOZ I and TROPOZ II
Authors: Jonquières Isabelle , Marenco Alain , Maalej Ahmed , Rohrer Franz
Published: February 2004

Using the results obtained over the Ivory Coast during the airborne Tropospheric Ozone campaign (TROPOZ) I (December 1987) and over West Africa and South America during the meridional airborne campaign TROPOZ II (January 1991), a study is made of biomass burning emissions in the northern African tropics, a region which is much less well documented than that of southern Africa. The analysis of data for O3and its precursors (CO, CH4, NO, and NOy) confirms the existence of important ozone formations over northern Africa during the Boreal winter with photochemical O3formation rates of 15–35 ppbv O3d−1. The distributions obtained over the Ivory Coast show the presence of different layers associated with the Inter Tropical Convergence Zone (ITCZ), with an ozone‐ and precursor‐laden Harmattan layer embedded between the southerly depleted monsoon layer below and the free troposphere easterly circulation above. In front of the ITCZ discontinuity, air parcels of different ages are identified in biomass burning plumes resulting from the transfer to altitude of compounds emitted by fires followed by ascending motions of this polluted air above the monsoon layer. Using a chemical box model, the chemical evolution of concentrations in these air parcels is simulated, confirming the explanations already proposed from the study of the dynamic context and history of the air parcels along their trajectories. The ozone measurements performed during TROPOZ II on both sides of the central Atlantic (Brazil, West Africa) support the hypothesis that because of the prevailing meteorology, smoke‐laden air masses emanating from the burning regions in northern Africa can enter a meteorological regime which permits them to be transported out over the Atlantic where they can influence budgets over the south Atlantic Basin during the Austral summer.
Variations in the size distribution of non‐sea‐salt sulfate aerosol in the marine boundary layer at Barbados: Impact of African dust
Authors: Li‐Jones X. , Prospero J. M.
Published: February 2004

Four African mineral dust episodes occurred during a program of daily aerosol measurements at Barbados (13°15′N, 59°30′W) in April 1994. Non‐sea‐salt sulfate (nss SO4=) and dust were highly correlated (r2 = 0.93) and ranged from 0.5 to 4.2 μg m−3 and 0.9 to 257 μg m−3, respectively. Day‐to‐day variations in the size distributions of mineral dust and sea salt were relatively small. However, the coarse‐particle (aerodynamic diameter > 1 μm) fraction (CPF) of nss SO4= varied substantially, from 21% to 73%. The highest CPF SO4= values were associated with dust events; the lowest CPF was associated with the air mass from the central North Atlantic when the dust concentration was lowest, 0.9 μg m−3. We suggest that large CPF SO4= values are a consequence of SO2 in European pollutants that heterogeneously react with the suspended dust over North Africa. Nonetheless, the association of pollutants and dust does not always result in a large CPF; low CPF values suggest that SO2 may have been oxidized to SO4= prior to mixing with dust‐laden air. On those days when dust and pollution concentrations were low, the dominant source of nss SO4= was ascribed to oceanic dimethylsulfide (DMS) and the CPF remained close to 20%. Because such large variations can occur in the particle size distribution of nss SO4= in association with dust events, the role of mineral dust on nss SO4= size must be taken into account when estimating the impact of nss SO4= on radiation transfer in the atmosphere.
Aerosol impact on the surface UV radiation from the ground‐based measurements taken at Belsk, Poland, 1980–1996
Authors: Krzyścin Janusz W. , Puchalski Sylwester
Published: February 2004

The aerosol optical depth (AOD) at 550 nm, inferred from the solar radiation measurements taken at Belsk, Poland (52°N, 21°E), with a Sonntag pyrheliometer in the period 1980–1996, has been analyzed for detection of its long‐term characteristics. The overall mean AOD equals to about 0.27±0.14 (1σ). The AOD varies in the range 0.1–0.7, with a maximum (0.33) in April‐May and a minimum (0.21) in November‐December. The long‐term decrease of AOD (−7.4% per decade) yields an increase of the surface UV dose (erythemally weighted) of ∼1% per decade. The influence of different parameters on the transmission of UV through the atmosphere on a day‐to‐day timescale has been analyzed on the basis of the multiple regression model that relates the daily UV erythemal dose at the ground level with total ozone, global solar radiation (surrogate for cloud properties variations), and AOD in the period 1993–1996. It is found that a 10% increase (decrease) of AOD at 550 nm relative to its reference value manifests as ∼1.5% decrease (increase) of the UV erythemal daily dose at the ground level. The changes of UV radiation due to the AOD variations are, in the mean, comparable with those due to total ozone changes. The mean cloud signal in the UV data has been found to be a few times larger than the mean ozone signal. The statistical model suggests that the extreme values of AOD in the Belsk record (that is, 0.1 and 0.7) would be associated with 20–30% changes of the erythemally weighted UV daily dose. This estimate is corroborated by radiative transfer model calculations.
Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation: Theoretical basis
Authors: Torres O. , Bhartia P. K. , Herman J. R. , Ahmad Z. , Gleason J.
Published: February 2004

We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV‐absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near‐UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV‐absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look‐up table approach, a set of measurements given by the ratio of backscattered radiance at 340–380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single‐scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two methods.
The nucleation rate constants and freezing mechanism of nitric acid trihydrate aerosol under stratospheric conditions
Authors: Bertram A. K. , Sloan J. J.
Published: February 2004

The nucleation rates for submicron‐diameter nitric acid trihydrate (NAT) aerosol particles were measured under stratospheric conditions using a temperature‐programed flow tube and Fourier transform infrared extinction spectroscopy to detect the phase change from liquid to solid. The temperature range from 175 to 155 K was examined. The measured nucleation rate constants (in units of cm−3 s−1) are between 3.8±1.8 × 1010 and 9.7±6.3 × 1012 for the temperature range between 167.2 and 163.5 K. The experiments show directly that critical nuclei form rapidly in NAT droplets at these temperatures, but subsequent crystallization is very slow, so the nucleated droplets can persist as liquids for long times unless their temperature is raised to increase the crystal growth rate. In these experiments, complete crystallization of the nucleated droplets was achieved in the observation time (about 10 s) by raising their temperature to near 180 K. Although demonstrable in the laboratory, the freezing of NAT by homogeneous nucleation at low temperature and subsequent warming are unlikely to be important for type 1 polar stratospheric cloud formation because the temperatures required for nucleation are lower than those commonly found in the lower stratosphere.
Optical extinction properties of volcanic stratospheric aerosol derived from ground‐based lidar and Sun photometer measurements
Authors: Congeduti Fernando , DeLuisi John , DeFoor Thomas , Thomason Larry
Published: February 2004

Measurements of lidar‐integrated backscatter and Sun photometer optical thickness made at Mauna Loa, Hawaii (19.5°N), were used to compute the bulk columnar value of the extinction‐to‐backscatter ratio Sc of stratospheric aerosol during a period of major stratospheric aerosol contamination following the eruption of El Chichón (Mexico, 17.3°N, March‐April 1982). Values of Sc were also calculated using observations of integrated backscatter and aerosol optical thickness collected from various sources in the northern midlatitude region. The calculated values of Sc were found to range between 40 and 70 sr at a wavelength of 0.694 μm during periods when high and moderate stratospheric aerosol concentrations were observed. Variations of Sc with time of approximately 20–30% are noted in the results. Similar Sc results obtained by other investigators are compared with the results of this investigation. Overall, the agreement among the various results was found to be consistent within their uncertainties. Values of Sc were also calculated for the stratospheric aerosol from the eruption of Mount Pinatubo (Philippines, 15°N, June 1991). The Mount Pinatubo results were about 15% lower than the El Chichón results during their early stages. Differences between the El Chichón and the Pinatubo results are within their error bounds.
Factors affecting the detection of trends: Statistical considerations and applications to environmental data
Authors: Weatherhead Elizabeth C. , Reinsel Gregory C. , Tiao George C. , Meng Xiao‐Li , Choi Dongseok , Cheang Wai‐Kwong , Keller Teddie , DeLuisi John , Wuebbles Donald J. , Kerr James B. , Miller Alvin J. , Oltmans Samuel J. , Frederick John E.
Published: February 2004

Detection of long‐term, linear trends is affected by a number of factors, including the size of trend to be detected, the time span of available data, and the magnitude of variability and autocorrelation of the noise in the data. The number of years of data necessary to detect a trend is strongly dependent on, and increases with, the magnitude of variance (σN2) and autocorrelation coefficient (ϕ) of the noise. For a typical range of values of σN2 and ϕ the number of years of data needed to detect a trend of 5%/decade can vary from ∼10 to >20 years, implying that in choosing sites to detect trends some locations are likely to be more efficient and cost‐effective than others. Additionally, some environmental variables allow for an earlier detection of trends than other variables because of their low variability and autocorrelation. The detection of trends can be confounded when sudden changes occur in the data, such as when an instrument is changed or a volcano erupts. Sudden level shifts in data sets, whether due to artificial sources, such as changes in instrumentation or site location, or natural sources, such as volcanic eruptions or local changes to the environment, can strongly impact the number of years necessary to detect a given trend, increasing the number of years by as much as 50% or more. This paper provides formulae for estimating the number of years necessary to detect trends, along with the estimates of the impact of interventions on trend detection. The uncertainty associated with these estimates is also explored. The results presented are relevant for a variety of practical decisions in managing a monitoring station, such as whether to move an instrument, change monitoring protocols in the middle of a long‐term monitoring program, or try to reduce uncertainty in the measurements by improved calibration techniques. The results are also useful for establishing reasonable expectations for trend detection and can be helpful in selecting sites and environmental variables for the detection of trends. An important implication of these results is that it will take several decades of high‐quality data to detect the trends likely to occur in nature.
Methane emissions from a wetland region within the Hudson Bay Lowland: An atmospheric approach
Authors: Kuhlmann A. J. , Worthy D. E. J. , Trivett N. B. A. , Levin I.
Published: February 2004

Continuous observations of methane (CH4) mixing ratios in combination with the atmospheric 222Rn activity and stable isotope ratios of carbon and hydrogen in atmospheric CH4 at a remote midcontinental monitoring station at Fraserdale, Ontario (49°53′N, 81°34′W) are used to characterize and quantify the regional CH4 emissions from the Hudson Bay Lowland (HBL). Large diurnal variations are observed in both trace substances during the summer which are due to emissions from ground level sources accumulating during strong nocturnal inversions. Diurnal changes of the stable isotopic composition of atmospheric CH4 at Fraserdale were used to determine the isotopic signature of the regional wetland CH4 source to be δ13CVPDB = (−60.0±3.2)‰ and δDVSMOW = (−442±142)‰. These integrated values are regarded as representative of several hundred square kilometers. The atmospheric 222Rn daughter observations made during the months of September and October 1994–1996 were used to estimate total CH4 emission rates in the region of Fraserdale. The fluxes of CH4 for September and October were (11.5±1.2) mg CH4 m−2 d−1 and (4.7±2.7) mg CH4 m−2 d−1, respectively, with an estimated absolute uncertainty of ±30%. These values are well comparable with summer flux measurements performed during the 1990 NOWES campaign using static chambers [Roulet et al., 1994] as well as using eddy correlation measurements performed on a tall tower [Edwards et al., 1994].
Is a periglacial biota responsible for enhanced dielectric response in basal ice from the Greenland Ice Core Project ice core?
Authors: Tison Jean‐Louis , Souchez Roland , Wolff Eric W. , Moore John C. , Legrand Michel R. , de Angelis Martine
Published: February 2004

A detailed dielectric profiling (DEP) conductivity profile (σ∞) measured in the 6 m of the basal silty ice sequence from the Greenland Ice Core Project (GRIP) ice core (Summit, Central Greenland) is presented and compared to previous multi‐parametric studies. DEP conductivities span the whole glacial‐interglacial range observed higher up in the GRIP core (9–25 μS m−1). Values in the bottom meter of the sequence reach the level of some of the highest peaks from Holocene volcanic layers in the core (33 μS m−1). On a steady increase of the σ∞ values down the sequence are superimposed large fluctuations “inphase” with other variables measured in the core such as δ18O, debris content, or gas compositions in CO2 and CH4. Analysis of the type and strength of intercorrelations shows that the controlling variable for the DEP signal must be closely related to the gas content and composition of the ice. Plausible candidates for this causality link are investigated. Enhancing of the σ conductivity by CO2 and CH4 encaged in the ice lattice as gas hydrates is ruled out since these are nonpolar clathrates of structure I, known as having negligible impact on the orientational stability of the water molecules under ac currents. NH4+ is proposed as the best candidate since it has been shown to enhance DEP conductivities by introducing Bjerrum defects in the ice lattice and since it could have been initially present partly as gaseous NH3 in the ice. This proposition is supported by the NH4+ profile in the basal ice sequence. Using calibration curves from higher up in the core, it is shown that σ is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice. The origin of the NH4+ signal is discussed in the light of organic acid profiles (formate, acetate, and oxalate). It appears that the most likely source is local degradation of biological residues, which supports the hypothesis that part of the basal ice was formed locally, in the absence of the present‐day ice sheet.
Comparisons of two methods of removing anthropogenically related variability from the near‐surface observational temperature field
Authors: Jones P. D. , Hegerl G. C.
Published: February 2004

Assessments of the veracity of model‐generated variability are difficult because variability in observed climate data during the 20th century is composed of natural and anthropogenic (greenhouse gas and sulfate aerosol) factors in addition to internal climate fluctuations. Comparisons should be improved if some of these factors could either be extracted from real world data or additions be made to model‐generated variability. Both options involve several assumptions, the most important of which is that the climate system is linear to a first approximation. We discuss this and other assumptions and present results from two different methods of removing variability related to anthropogenic factors using energy balance models (EBMs) and atmosphere/ocean general circulation models (A/OGCMs). At a global scale, the pattern of the trend of surface temperature over the 1966–1995 period, after removing the anthropogenic effect, shows some strong similarities between the two methods, with the strongest residual warming evident over much of northern Asia and northern North America.
Ground measurements of the polarized bidirectional reflectance of snow in the near‐infrared spectral domain: Comparisons with model results
Authors: Leroux Catherine , Deuzé Jean‐Luc , Goloub Philippe , Sergent Claude , Fily Michel
Published: February 2004

Ground‐based measurements were carried out in the French Alps in March 1995 to study the bidirectional and polarized bidirectional reflectances of various snow covers. Atmospheric (optical thickness) and snow characteristics (grain size and type) were measured simultaneously. The ground reflectance data are compared with snow reflectance modeling results. A radiative transfer model based on the adding‐doubling method is used to compute the snow reflectance. The single‐scattering parameters are determined by a Mie or a ray‐tracing code depending on the particle shape introduced in the model (spherical or hexagonal ice particles). Special attention is paid to the comparisons in the near‐infrared spectral range (1.65 μm) because of the sensitivity of snow reflectance to the grain size and shape. In the visible and near‐infrared domains of the solar spectrum, the amplitude of the polarized signal is very low and independent of the snow grain size. The agreement between measurements and model results is good if hexagonal particles are introduced in the model instead of the spherical snow grains commonly used in reflectance studies.
Atmospheric aerosol over Alaska: 2. Elemental composition and sources
Authors: Polissar Alexandr V. , Hopke Philip K. , Paatero Pentti , Malm William C. , Sisler James F.
Published: February 2004

The fine particle (<2.5 μm) composition data from seven National Park Service locations in Alaska for the period from 1986 to 1995 was performed using a new type of factor analysis, positive matrix factorization (PMF). This method uses the estimates of the error in the data to provide optimum data point scaling and permits a better treatment of missing and below detection limit values. Eight source components were obtained for data sets from the Northwest Alaska Areas and the Bering Land Bridge sites. Five to seven components were obtained for the other Alaskan sites. The solutions were normalized by using aerosol fine mass concentration data. Squared correlation coefficients between the reconstructed mass obtained from aerosol composition data for the sites and the measured mass were in the range of 0.74–0.95. Two factors identified as soils were obtained for all of the sites. Concentrations for these factors for most of the sites have maxima in the summer and minima in the winter. A sea‐salt component was found at five locations. A factor with the highest concentrations of black carbon (BC), H+, and K identified as forest fire smoke was obtained for all data sets except at Katmai. Factors with high concentrations of S, BC‐Na‐S, and Zn‐Cu were obtained at all sites. At three sites, the solutions also contained a factor with high Pb and Br values. The factors with the high S, Pb, and BC‐Na‐S values at most sites show an annual cycle with maxima during the winter‐spring season and minima in the summer. The seasonal variations and elemental compositions of these factors suggest anthropogenic origins with the spatial pattern suggesting that the sources are distant from the receptor sites. The seasonal maxima/minima ratios of these factors were higher for more northerly locations. Four main sources contribute to the observed concentrations at these locations: long‐range transported anthropogenic aerosol (Arctic haze aerosol), sea‐salt aerosol, local soil dust, and aerosol with high BC concentrations from regional forest fires or local wood smoke. A northwest to southeast negative gradient suggesting long‐range transport of air masses from regions north or northwest of Alaska dominated the spatial distribution of the high S factor concentrations.
A comparison of Antarctic stratospheric radiative heating rates calculated from high‐resolution interferometer sounder and U.K. Meteorological Office data
Authors: Hicke Jeffrey , Tuck Adrian , Smith William
Published: February 2004

Radiative heating rates have been calculated in the region of the Antarctic polar vortex during 1994 using the National Center for Atmospheric Research Community Climate Model (CCM2) radiative transfer model. Temperature and tropospheric water vapor retrievals were used from the high‐resolution interferometer sounder (HIS) on board the ER‐2 in 1994 to compute clear sky heating rates in April, August, and October. The lower stratospheric heating rate is shown to be sensitive to the skin temperature due to the exchange of radiation within the ozone 9.6 μm band. This implies a coupling between the surface and the lower stratosphere, with the formation of sea ice decreasing the skin temperature and increasing lower stratospheric cooling rates. Computations show weak heating (strong cooling) rates for the fall flight, of the order of −0.7 K/day (potential temperature), with progressively greater heating (smaller cooling) rates associated with the late winter and spring flights and large variations in latitude. HIS temperatures are independent of numerical weather prediction analyses and so provide a useful comparison. Vertical mean (350–420 K) temperature differences between HIS retrievals and collocated U.K. Meteorological Office (UKMO) assimilations are generally within 3 K; HIS temperatures are greater for most flights around and poleward of the vortex edge. Calculated heating rates result in weaker HIS heating (stronger HIS cooling) for most flights resulting from the greater HIS temperatures. Heating rate differences are generally within 0.2 K/day for the 350–420 K vertical mean. The calculations show evidence of a minimum in the cooling rate in the region of the vortex edge for the August and October flights, also seen in the collocated UKMO results as well as in hemispheric calculations.
Radiative transfer in the 9.6 μm HIRS ozone channel using collocated SBUV‐determined ozone abundances
Authors: Joiner J. , Lee H.‐T. , Strow L. L. , Bhartia P. K. , Hannon S. , Miller A. J. , Rokke L.
Published: February 2004

We have carried out a detailed analysis of radiative transfer and observational errors for the high‐resolution infrared radiation sounder 2 (HIRS2)channel 9 centered in the 9.6 μm ozone absorption band. Several previous studies have shown significant differences between the total column ozone derived from HIRS2 and the ultraviolet (UV) radiometers. Here we use collocated ozone profiles derived from the solar backscatter ultraviolet (SBUV) spectrometer to isolate errors in HIRS2 channel 9. Radiative transfer in the 9.6 μm band is complicated because it is affected by atmospheric ozone, temperature, humidity, and the surface skin temperature and emissivity. We examine the accuracy of three fast radiative transfer algorithms. We validate current models for weak water vapor continuum absorption at 9.6 μm. In addition, we develop a method to correct for errors in older continuum models, in ozone transmittances, and in the channel 9 spectral response function. We also identify errors resulting from the spectral dependence of the surface emissivity and propose a correction method. Using UV‐derived reflectivity, we have detected instances where IR cloud‐detection and cloud‐clearing algorithms have apparently failed. Our results show that if appropriate corrections and quality control are applied, it is possible to compute HIRS2 channel 9 brightness temperatures with an root‐mean‐square (RMS) accuracy of better than ∼0.6 K and a bias less than 0.1 K in clear skies and RMS of better than ∼0.9 K and a bias less than 0.1 K in partially cloudy skies. We plan to use the methods described here to improve IR ozone retrieval algorithms.
Stratosphere‐troposphere exchange of ozone associated with the equatorial Kelvin wave as observed with ozonesondes and rawinsondes
Authors: Fujiwara Masatomo , Kita Kazuyuki , Ogawa Toshihiro
Published: February 2004

An intensive observation with ozonesondes and rawinsondes was conducted in Indonesia in May and June 1995 to investigate a phenomenon of ozone enhancement in the tropical upper troposphere. We obtained the characteristics of an enhancement that continued for about 20 days, concurring with a zonal wind oscillation associated with the equatorial Kelvin wave around the tropopause and the Madden‐Julian oscillation (MJO) in the troposphere. The isoline of ozone mixing ratio of 40 nmol/mol moved by 5.0 km downward from 17.8 km to 12.8 km, while the tropopause height was 16.2–17.8 km throughout the period. Moreover, the maximum ozone concentration of 300 nmol/mol at the tropopause was concurrent with the maximum eastward wind phase of the Kelvin wave. The detailed mechanism of the ozone transport is interpreted as follows: The downward motion associated with the Kelvin wave and the MJO transported the stratospheric ozone into the troposphere, and the air mixing due to the Kelvin wave breaking at the tropopause also caused stratosphere‐troposphere exchange. The upper limit of the net amount of ozone transported from the stratosphere was estimated to be 9.9 Dobson units with the zonal and meridional extents of the ozone‐increased region of more than 6.6×106 m and 1.8×106 m, respectively, to imply the potential to affect the photochemistry around the tropical tropopause.
Potential global fire monitoring from EOS‐MODIS
Authors: Kaufman Yoram J. , Justice Christopher O. , Flynn Luke P. , Kendall Jackie D. , Prins Elaine M. , Giglio Louis , Ward Darold E. , Menzel W. Paul , Setzer Alberto W.
Published: February 2004

The National Aeronautic and Space Administration (NASA) plans to launch the moderate resolution imaging spectroradiometer (MODIS) on the polarorbiting Earth Observation System (EOS) providing morning and evening global observations in 1999 and afternoon and night observations in 2000. These four MODIS daily fire observations will advance global fire monitoring with special 1 km resolution fire channels at 4 and 11 μm, with high saturation of about 450 and 400 K, respectively. MODIS data will also be used to monitor burn scars, vegetation type and condition, smoke aerosols, water vapor, and clouds for overall monitoring of the fire process and its effects on ecosystems, the atmosphere, and the climate. The MODIS fire science team is preparing algorithms that use the thermal signature to separate the fire signal from the background signal. A database of active fire products will be generated and archived at a 1 km resolution and summarized on a grid of 10 km and 0.5°, daily, 8 days, and monthly. It includes the fire occurrence and location, the rate of emission of thermal energy from the fire, and a rough estimate of the smoldering/flaming ratio. This information will be used in monitoring the spatial and temporal distribution of fires in different ecosystems, detecting changes in fire distribution and identifying new fire frontiers, wildfires, and changes in the frequency of the fires or their relative strength. We plan to combine the MODIS fire measurements with a detailed diurnal cycle of the fires from geostationary satellites. Sensitivity studies and analyses of aircraft and satellite data from the Yellowstone wildfire of 1988 and prescribed fires in the Smoke, Clouds, and Radiation (SCAR) aircraft field experiments are used to evaluate and validate the fire algorithms and to establish the relationship between the fire thermal properties, the rate of biomass consumption, and the emissions of aerosol and trace gases from fires.
Raman lidar measurements of aerosol extinction and backscattering: 1. Methods and comparisons
Authors: Ferrare R. A. , Melfi S. H. , Whiteman D. N. , Evans K. D. , Leifer R.
Published: February 2004

This paper examines the aerosol backscattering and extinction profiles measured at night by the NASA Goddard Space Flight Center Scanning Raman Lidar (SRL) during the remote cloud sensing (RCS) intensive operations period (IOP) at the Department of Energy Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) site in April 1994. These lidar data are used to derive aerosol profiles for altitudes between 0.015 and 5 km. Since this lidar detects Raman scattering from nitrogen and oxygen molecules as well as the elastic scattering from molecules and aerosols, it measures both aerosol backscattering and extinction simultaneously. The aerosol extinction/backscattering ratio varied between approximately 30 sr and 75 sr at 351 nm. Aerosol optical thicknesses derived by integrating the lidar profiles of aerosol extinction measured at night between 0.1 and 5 km are found to be about 10–40% lower than those measured by a Sun photometer during the day. This difference is attributed to the contribution by stratospheric aerosols not included in the lidar estimates as well as to diurnal differences in aerosol properties and concentrations. Aerosol profiles close to the surface were acquired by pointing the lidar nearly horizontally. Measurements of aerosol scattering from a tower‐mounted nephelometer are found to be 40% lower than lidar measurements of aerosol extinction over a wide range of relative humidities even after accounting for the difference in wavelengths. The reasons for this difference are not clear but may be due to the inability of the nephelometer to accurately measure scattering by large particles.
An isotopic study of atmospheric sulphur at three sites in Wales and at Mace Head, Eire
Authors: McArdle Nicola , Liss Peter , Dennis Paul
Published: February 2004

Sulphur isotope measurements made on aerosol and precipitation samples collected at three sites in Wales and Mace Head, Eire, between March 1993 and September 1994, had highest non‐sea‐salt sulphate δ34S values, up to 11.9‰ for aerosol and 9.7‰ for precipitation, during the summer months. Aerosol methanesulphonate shows a clear summer peak with values up to 3–4 nmol m−3. Assuming samples contained only sulphur from terrestrial/anthropogenic, sea salt, and marine biogenic (from the oxidation of dimethylsulphide) sources, we used the sulphur isotope values to determine the contribution of dimethylsulphide‐derived sulphate to total sulphate. In general the biogenic contribution was small, <10%, although in samples from marine air masses it was as high as 40%. A comparison of aerosol and precipitation δ34S values found no evidence for large isotopic fractionations during the oxidation of sulphur dioxide.
Charged precipitation and electric field in two thunderstorms
Authors: Stolzenburg Maribeth , Marshall Thomas C.
Published: February 2004

The net electric charge density on all particles inside a thunderstorm can be estimated from measurements of the electric vector along a mainly vertical path of a balloon‐borne sensor. In addition, the net charge density on precipitation particles such as raindrops, hail, and graupel can be deduced from measurements of the charge on each individual particle. This paper reports on two balloon soundings of electric field and precipitation charge in two different storms in central New Mexico. The two soundings are similar at lower heights and different at higher heights, possibly because one balloon ascended outside the main convective updraft core of the storm, while the other balloon ascended within the updraft. Six charge regions were inferred from the sounding outside the updraft core in the convective region. In the lowest four charge regions, including the lower positive, main negative, and two oppositely charged regions in between, the detected precipitation charge density (on particles with individual charges of at least 10 pC) was of the same polarity and as large or larger in magnitude than the total charge density. At the top of and 0.4 km above the main negative charge region a large number density (about 125 m−3) of negatively charged precipitation particles were detected; these particles had larger mean charge (−48.6 pC) than any other group of positive or negative particles detected elsewhere in the sounding. The second electric field sounding was typical of updraft soundings below 8.0 km, and the precipitation particles detected were all positively charged below 5.3 km and all negatively charged above. The detected precipitation charge density, although smaller in magnitude, was of the same polarity as the total charge density in the lower positive and main negative charge regions.
Long‐term monitoring of atmospheric aerosols in the Amazon Basin: Source identification and apportionment
Authors: Echalar Francisco , Artaxo Paulo , Martins José Vanderlei , Yamasoe Márcia , Gerab Fábio , Maenhaut Willy , Holben Brent
Published: February 2004

Continuous sampling of atmospheric aerosols was carried out at two different sites in the Amazon Basin: Cuiabá (16°S, 56°W), since July 1990, and Alta Floresta (09°S, 56°W), since August 1992. Aerosols were collected on polycarbonate filters mounted in stacked filter units (SFU). Particle‐induced X ray emission (PIXE) was used to measure concentrations of up to 26 elements (Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Zr, Au, and Pb). Black carbon (BC) and gravimetric mass analyses were also performed. Inhalable particle ground concentrations showed a marked seasonality, with maxima of more than 100 μg m−3in the dry season. The high aerosol optical thickness (AOT) values in this period (up to 4.0 at λ = 0.440 μm) indicate that the whole air column has a significant aerosol load. Three main types of aerosol sources were identified: biomass burning, natural biogenic emissions, and soil dust resuspension. During the dry season the fine mode aerosol originated predominantly from biomass burning emissions (they were responsible for about 73% of fine aerosol mass), while the coarse mode was dominated by soil dust particles (50 to 60% of the aerosol mass). Crustal elements exhibited a unimodal coarse mode size distribution, while the pyrogenic elements showed a clear submicrometer mode. Black carbon had a submicrometer mode centered at 0.175 μm. During the wet season, biogenic aerosols were the dominant particle type and also the main source of atmospheric P in the region.