bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle scattering, backscattering, and absorption coefficients: An in situ closure and sensitivity study
Authors: Wex Heike , Neusüß Christian , Wendisch Manfred , Stratmann Frank , Koziar Christian , Keil Andreas , Wiedensohler Alfred , Ebert Martin
Published: October 2002

Comparisons between measured and calculated aerosol scattering, backscattering, and absorption coefficients were made based on in situ, ground‐based measurements during the Melpitz INTensive (MINT) and Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) field studies. Furthermore, airborne measurements made with the same type of instruments are reviewed and compared with the ground‐based measurements. Agreement between measured and calculated values is on the order of ±20% for scattering and backscattering coefficients. A sensitivity analysis showed a large influence on the calculated particle scattering and backscattering coefficients resulting from sizing uncertainties in the measured number size distributions. Measured absorption coefficients were significantly smaller than the corresponding calculated values. The largest uncertainty for the calculated absorption coefficients resulted from the size‐dependent fraction of elemental carbon (EC) of the aerosol. A correction for the measured fractions of EC could significantly improve the agreement between measured and calculated absorption coefficients. The overall uncertainty of the calculated values was investigated with a Monte Carlo method by simultaneously and randomly varying the input parameters of the calculations, where the variation of each parameter was bounded by its uncertainty. The measurements were mostly found to be within the range of uncertainties of the calculations, with uncertainties for the calculated scattering and backscattering coefficients of about ±20% and for the absorption coefficients of about ±30%. Thus, to increase the accuracy of calculated scattering, backscattering, and absorption coefficients, it is crucial to further reduce the error in particle number size distribution measurement techniques. In addition, further improvement of the techniques for measuring absorption coefficients and further investigation of the measurement of the fraction of EC of the aerosol is necessary.
Simulated three‐dimensional branched lightning in a numerical thunderstorm model
Authors: Mansell Edward R. , MacGorman Donald R. , Ziegler Conrad L. , Straka Jerry M.
Published: October 2002

Lightning discharges are simulated by using a stochastic dielectric breakdown model within a numerical thunderstorm model with extensive parameterizations of electrification mechanisms. The lightning model simulates the macroscopic bidirectional extension of discharges as a step‐by‐step stochastic process. Discharge channels are propagated on a uniform grid, and the direction of propagation (including diagonals) for a particular step is chosen randomly, with the probability for choosing a particular direction depending on the net electric field. After each propagation step the electric fields are recomputed via Poisson's equation to account for the effect of the conducting channel. The lightning parameterization produces realistic looking, three‐dimensional, branched lightning discharges. A variety of lightning types have been produced, including intracloud discharges, negative cloud‐to‐ground (CG) lightning, and positive CG lightning. The model simulations support the hypothesis that negative CG flashes occur only when a region of positive charge exists below the main negative charge region. Similarly, simulated positive CG flashes were found to occur only in regions of storms where the two significant charge layers closest to ground had roughly a “normal dipole” structure (i.e., positive charge above negative).
A statistical model for latitudinal correlations of satellite data
Authors: Choi Dongseok , Tiao George C. , Stein Michael L.
Published: October 2002

A new class of models is developed to represent the latitudinal correlation patterns of satellite‐based measurements of ozone and temperature. We employ the monthly average series for each 5‐degree latitudinal zone of the TOMS data from the Nimbus 7 satellite over 14 years from November 1978 to November 1992. For each latitudinal zone a temporal regression model that includes systematic seasonal variations, possible long‐term trends, solar radio flux effect, and time dependency is fitted separately by maximum likelihood. From the residuals at all latitudinal zones, we observe strong contemporaneous latitudinal correlations, which decay at different rates depending on latitudes and become negative at moderate distances. A three‐component model with two associated weight functions is developed that can represent the main features of the concurrent spatial correlations of the residuals. We use low‐order ARMA models as components. A similar analysis is done on the latitudinal correlations of the residuals from the monthly average series of Climate Prediction Center temperature data at 30 mbar from January 1979 to December 1993. The new class of models can be applied to other remotely sensed geophysical data sets whose covariance structures exhibit similar patterns.
Correlated time derivatives of current, electric field intensity, and magnetic flux density for triggered lightning at 15 m
Authors: Uman M. A. , Schoene J. , Rakov V. A. , Rambo K. J. , Schnetzer G. H.
Published: October 2002

We present measured current and its time derivative correlated with the corresponding electric field intensity and magnetic flux density and their time derivatives measured at 15 m for two lightning return strokes triggered in 1999 at Camp Blanding, Florida. Lightning was triggered to a vertical 2‐m rod mounted at the center of a 70 × 70 m buried metallic grid. The rocket‐launching system was located underground at the center of the grid. The experiment was designed to minimize any influence of either the strike object or a finite‐conducting Earth (ground surface arcing and propagation effects) on the fields and field derivatives. The measured current derivative waveform and the return stroke portion of the magnetic flux density derivative and electric field intensity derivative waveforms associated with the two strokes are observed to be essentially unipolar pulses that have similar waveshapes for the first 150 ns or so, including the initial rising portion, the peak, and about 50 ns after the peak. The current and magnetic field derivative waveshapes are very similar for their total duration, and both decay to near zero about 200 ns after the peak derivative is reached. The electric field derivative decays more slowly than the current derivative after about 150 ns, taking about 500 ns to decay to near zero. The transmission‐line model, the simplest available and most used return stroke model, is employed to calculate the return stroke field derivatives, given the measured current derivative as a model input, for return stroke speeds of 1 × 108 m s−1, 2 × 108 m s−1, and 3 × 108 m s−1 (the speed of light). A reasonable match between calculated and measured field derivative waveshapes is achieved for both strokes for a return stroke speed between 2 × 108 m s−1 and 3 × 108 m s−1. Although the measured field and current derivatives have similar waveshapes for about 150 ns, which might appear to be consistent with the hypothesis that the radiation field component of the total field derivative is dominant for that time, transmission line model calculations indicate that this is not the case. Further, electric field derivatives measured simultaneously at 15 m and at 30 m for many strokes are observed to have similar waveshapes, which also might appear to be consistent with the hypothesis that the derivatives are dominated by the radiation field component; however, according to transmission line model calculations, while the calculated total field derivative waveshapes are similar at 15 and 30 m, all field components significantly contribute to the waveforms at both distances, and the mix of field components at 15 m and at 30 m is quite different.
Biomass burning emissions of reactive gases estimated from satellite data analysis and ecosystem modeling for the Brazilian Amazon region
Authors: Potter Christopher , Brooks‐Genovese Vanessa , Klooster Steven , Torregrosa Alicia
Published: October 2002

To produce a new daily record of trace gas emissions from biomass burning events for the Brazilian Legal Amazon, we have combined satellite advanced very high resolution radiometer (AVHRR) data on fire counts together for the first time with vegetation greenness imagery as inputs to an ecosystem biomass model at 8 km spatial resolution. This analysis goes beyond previous estimates for reactive gas emissions from Amazon fires, owing to a more detailed geographic distribution estimate of vegetation biomass, coupled with daily fire activity for the region (original 1 km resolution), and inclusion of fire effects in extensive areas of the Legal Amazon (defined as the Brazilian states of Acre, Amapá, Amazonas, Maranhao, Mato Grosso, Pará, Rondônia, Roraima, and Tocantins) covered by open woodland, secondary forests, savanna, and pasture vegetation. Results from our emissions model indicate that annual emissions from Amazon deforestation and biomass burning in the early 1990s total to 102 Tg yr−1 carbon monoxide (CO) and 3.5 Tg yr−1 nitrogen oxides (NOx). Peak daily burning emissions, which occurred in early September 1992, were estimated at slightly more than 3 Tg d−1for CO and 0.1 Tg d−1for NOx flux to the atmosphere. Other burning source fluxes of gases with relatively high emission factors are reported, including methane (CH4), nonmethane hydrocarbons (NMHC), and sulfur dioxide (SO2), in addition to total particulate matter (TPM). We estimate the Brazilian Amazon region to be a source of between one fifth and one third for each of these global emission fluxes to the atmosphere. The regional distribution of burning emissions appears to be highest in the Brazilian states of Maranhao and Tocantins, mainly from burning outside of moist forest areas, and in Pará and Mato Grosso, where we identify important contributions from primary forest cutting and burning. These new daily emission estimates of reactive gases from biomass burning fluxes are designed to be used as detailed spatial and temporal inputs to computer models and data analysis of tropospheric chemistry over the tropical region.
Water balance of the Indus River Basin and moisture source in the Karakoram and western Himalayas: Implications from hydrogen and oxygen isotopes in river water
Authors: Karim Ajaz , Veizer Jan
Published: October 2002

Stable isotope measurements of hydrogen and oxygen for surface waters from the Indus River Basin (IRB), together with historical records for river discharge, annual precipitation, and groundwater levels, are used to assess water balance for the basin. The Indus River presently drains 53 km3 yr−1 or roughly one‐eighth of the 398‐km3 water that annually falls on the basin in the form of rain and snow, with the remainder returned to the atmosphere by evapotranspiration. Monthly samples for the Indus River close to its mouth, for the water year March 1994 to February 1995, show a tight correlation in δD and δ18O space. The slope of the linear regression is 7.5, which is not significantly different from the slopes of the Local Meteoric Water Lines (LMWL; 7.3 and 7.1). This observation argues against significant loss of water by direct evaporation from river surfaces or from soils in hydrologic continuum with surface waters. An upper limit for evaporation from poorly drained soils is calculated to be ∼10 km3 yr−1 or only 2.5% of the annual precipitation flux. Groundwater storage in the entire Canal Command Area received a maximum of 23 km3 yr−1 or 5.8% of the annual precipitation during the early stages of irrigation, but modern recharge is probably balanced by discharge to rivers and well exploitation. Transpiration by natural vegetation and crops annually returns 83% of the precipitation flux and constitutes the largest pathway for the loss of water from the basin. Deuterium excess (d‐excess) in the IRB ranges between 4‰ and 28‰, with values for 95% of the sample population exceeding 10‰. The Indus main channel close to its mouth varies in d‐excess between 12‰ and 20‰ during low and high water stands, respectively, with a discharge weighted average of 18‰. These values are distinctly higher than the long‐term average for the Indian monsoon (∼8‰) and reflect contributions from water vapor originating in the Mediterranean (22‰) or other inland seas. Using these end‐member compositions and the discharge weighted average d‐excess, isotope balance calculations require that up to 72% of the Indus discharge close to its mouth must be derived from the Mediterranean end‐member and implies that bulk of the Indus discharge is the result of delayed runoff from the Karakoram and the Himalayas.
A study of land‐atmosphere interactions during summertime rainfall using a mesoscale model
Authors: Barros Ana P. , Hwu Wenje
Published: October 2002

A coupled mesoscale model (e.g., MM5V3) was used to investigate the mechanisms of land‐atmosphere interaction during summertime rainfall in the southern Great Plains. Numerical experiments were conducted for two storms and for clear weather conditions using three distinct physical parameterizations of convective processes. Analysis of the dynamics of moisture and energy states in the boundary layer for all model grid cells revealed the existence of dual limit cycles in the model's phase space of Bowen ratio and relative humidity. The scaling behavior of these limit cycles is consistent with self‐similarity of the second kind (i.e., in the sense of intermediate asymptotics). Specifically, the dual limit cycles suggest two distinct regimes of land‐atmosphere interactions: (1) a divergent (splitting) trajectory of increasing diurnal variations of Bowen ratio and relative humidity for persistently dry weather conditions and (2) a convergent (merging) trajectory of rapidly decreasing Bowen ratio and increasing relative humidity for wet weather conditions. At the onset of rainfall the convergent cycle exhibits intermittent instability and collapses into a very limited region of high relative humidity and low Bowen ratio, the rainfall attractor. The position of the attractor in phase space is determined by surface controls of land‐atmosphere interactions (e.g., soil moisture availability in our applications): That is, land surface heterogeneity determines the dynamic range of land‐atmosphere interactions. Regionally, the footprint of these two regimes of land‐atmosphere interactions is apparent in the correlation between Bowen ratio and the vertical distribution of relative humidity in the troposphere. In dry weather the correlation varies periodically from positive at night to negative during the day and generally does not penetrate into the upper air. Under wet conditions, however, the correlation is strong and persistently positive throughout the troposphere. These results support the argument that increased surface evaporation during rainfall affects stability conditions in the boundary layer, and thus convective activity in the model, therefore establishing a positive feedback mechanism of land‐atmosphere interactions consistent with the rainfall attractor.
Effect of cold clouds on satellite measurements near 183 GHz
Authors: Greenwald Thomas J. , Christopher Sundar A.
Published: October 2002

Near‐global analysis of 183 GHz measurements from the NOAA‐15 advanced microwave sounding unit (AMSU) B was conducted to investigate the impact of cold (< 240 K at 11 μm) clouds on upper tropospheric humidity (UTH) observations and in assessing the potential for deriving cloud microphysical properties. Collocated advanced very high resolution radiometer (AVHRR) data aided in identifying clouds and isolating the ice‐cloud‐scattering effect. This effect was determined by subtracting the measured AMSU‐B brightness temperature (Tb) from a background Tb estimated using AVHRR‐derived cloud optical depth data. Results for December 1999 over land and ocean show that nonprecipitating cold clouds have a measurable impact on 183 GHz Tbs although the average effect is rather weak (1.4 K). Cold clouds associated with precipitation had a much larger average effect (7 K); therefore only for these types of clouds is there sufficient information for potential quantitative estimation of cloud/precipitation physical properties. Nonprecipitating cold clouds bias estimates of UTH, on average, by 5% but can reach 20% for optically thick clouds. Precipitating clouds produce an 18% average bias. On the basis of these results it is recommended that UTH retrievals undergo filtering for precipitation (using combined microwave and infrared window channels) as well as for optically thick nonprecipitating cold clouds that fill a sensor's field of view, which may be screened using infrared split window techniques.
Airborne CH2O measurements over the North Atlantic during the 1997 NARE campaign: Instrument comparisons and distributions
Authors: Fried Alan , Lee Yin‐Nan , Frost Greg , Wert Bryan , Henry Bruce , Drummond James R. , Hübler Gerd , Jobson Tom
Published: October 2002

Airborne CH2O measurements were acquired by tunable diode laser absorption spectroscopy (TDLAS) and coil/2,4‐dinitrophenylhydrazine (CDNPH) techniques over remote regions of the North Atlantic Ocean from the surface to 8 km during the North Atlantic Regional Experiment (NARE‐97) in September of 1997. There were eight aircraft flights when both instruments were simultaneously operating, producing 665 overlapping time intervals for comparisons. A number of approaches were used in the comparisons, and indicated that on average both instruments measured identical ambient CH2O concentrations to within 0.1 ppbv, and more typically within 0.08 ppbv, over the 0 to 0.8 ppbv‐concentration range. However, significant differences, larger than the combined 2σ total uncertainty estimates, were observed in 29% of the full time‐coincident data set. The two instruments produced very similar altitude trends. Under clean background conditions in the 35° to 55°N latitude band, the median TDLAS and CDNPH CH2O concentrations were 0.399 and 0.410 ppbv for 0–2 km, 0.250 and 0.355 ppbv for 2–4 km, and 0.217 and 0.280 ppbv for 4–8 km, respectively. Elevated CH2O concentrations were observed in this study at both high altitudes (4–8 km) and in the marine boundary layer by both instruments. Thus vertical transport of CH2O and/or its precursors may provide a greater contribution to upper tropospheric HOxthan previously thought. The results of this study, which are based upon instruments employing entirely different measurement principles, calibration, and sampling approaches, not only reinforce this conclusion but also provide a high‐quality database necessary to further explore CH2O measurement‐model relationships in the clean background atmosphere.
Relative contribution of submicron and supermicron particles to aerosol light scattering in the marine boundary layer
Authors: Kleefeld Christoph , O'Dowd Colin D. , O'Reilly Sarah , Jennings S. Gerard , Aalto Pasi , Becker Edo , Kunz Gerard , de Leeuw Gerrit
Published: October 2002

Measurements of the aerosol light scattering coefficient (σsp) at a wavelength of λ = 550 nm were conducted at a coastal atmospheric research station in the east Atlantic Ocean during June 1999. Size distribution measurements between diameters of 3 nm and 40 μm (at ambient humidity) were used to derive scattering coefficients from Mie theory. The calculated scattering coefficients were about a factor of 7.4 higher than the measured scattering coefficients. The discrepancy was explained by a reduced cutoff of the sampling system at particle diameters between 6 and 8 μm, dependent on wind speed. The calculated aerosol scattering was about 1 order of magnitude higher than previously reported measurements in the MBL and is attributed to supermicrometer particles at sizes d > 10 μm dominating aerosol scattering.
Modulation of convection in the southwestern Amazon basin by extratropical stationary fronts
Authors: Rickenbach Thomas M. , Ferreira Rosana Nieto , Halverson Jeffrey B. , Herdies Dirceu Luis , Silva Dias Maria A. F.
Published: October 2002

Regimes of lower tropospheric northwesterly wind observed in southwestern Amazônia during the Tropical Rainfall Measuring Mission Large‐Scale Biosphere Atmosphere (TRMM‐LBA) field campaign were shown to be local manifestations of stationary frontal systems that extended into the deep Tropics along the South Atlantic Convergence Zone (“SACZ” regime). Frontal systems were transient and were confined to the subtropics during the interim periods of easterly and weak westerly flow (“non‐SACZ” regime). Observations from surface‐based radar in TRMM‐LBA suggested that mesoscale convective systems in the SACZ regime were significantly larger in areal coverage, with weaker rainfall intensity and weaker vertical development of the convective cells. The diurnal variation of rain intensity and rain areal coverage generally showed afternoon maxima for both regimes, but with important differences suggesting explosive convective cell growth in the non‐SACZ regime and the dominance of nocturnal stratiform rain processes in the SACZ regime. These results had implications for the parameterization (in general circulation models) of subgrid‐scale convective processes in Amazônia in terms of the large‐scale flow and for improving precipitation and latent heating retrieval from spaceborne platforms such as the TRMM satellite.
A TRMM‐calibrated infrared rainfall algorithm applied over Brazil
Authors: Negri, A. J. , Xu L. , Adler R. F.
Published: October 2002

The development of a satellite infrared technique for estimating convective and stratiform rainfall and its application in studying the diurnal variability of rainfall in Amazonia are presented. The convective‐stratiform technique (CST), calibrated by coincident, physically retrieved rain rates from the TRMM microwave imager (TMI), is applied during January–April 1999 over northern South America. The diurnal cycle of rainfall, as well as the division between convective and stratiform rainfall, is presented. Results compare well (a 1‐hour lag) with the diurnal cycle derived from TOGA radar‐estimated rainfall in Rondonia. The satellite estimates reveal that the convective rain constitutes, in the mean, 24% of the rain area while accounting for 67% of the rain volume. Estimates of the diurnal cycle for the entire Amazon Basin are in agreement with those from the TRMM precipitation radar, despite the limited sampling of the latter. The effects of geography (rivers, lakes, coasts) and topography on the diurnal cycle of convection are examined. In particular, the Amazon River, downstream of Manaus, is shown to both enhance early morning rainfall and inhibit afternoon convection. Monthly estimates from this technique, dubbed CST/TMI, are verified over a dense rain gage network in the state of Ceara, in northeast Brazil. The CST/TMI showed a high bias equal to +33% of the gage mean, indicating that possibly the TMI estimates alone are also high. The root‐mean‐square difference (after removal of the bias) equaled 36.6% of the gage mean. The correlation coefficient was 0.77 based on 72 station months.
Concentrations and species composition of atmospheric volatile organic compounds (VOCs) as observed during the wet and dry season in Rondônia (Amazonia)
Authors: Kesselmeier J. , Kuhn U. , Rottenberger S. , Biesenthal T. , Wolf A. , Schebeske G. , Andreae M. O. , Ciccioli P. , Brancaleoni E. , Frattoni M. , Oliva S. T. , Botelho M. L. , Silva C. M. A. , Tavares T. M.
Published: October 2002

We measured atmospheric gas‐phase volatile organic compounds (VOCs) at the end of the wet and end of the dry season at a tropical rainforest site in Rondônia, Brazil, using various sampling techniques such as trapping on different adsorbents or cryogenic trapping combined with appropriate analysis techniques. The measuring sites were located inside the forest of a biological reserve near Ji‐Paraná. Sampling was performed from 3 May 1999 to 17 May 1999 and from 24 September 1999 to 2 November 1999 during the “wet‐to‐dry season transition” and “dry‐to‐wet season transition” periods in Rondônia, respectively. Samples were obtained at the canopy top close to the potential sources/sinks for these compounds as well as above the forest. We report the measured concentrations of a large number of different VOCs and their oxidation products, such as isoprenoids, organic acids, carbonyls, aromatics, and alcohols. The most prominent VOCs present in air over the last part of the wet season were isoprene, formaldehyde, and formic acid, with mixing ratios of each ranging up to several parts per billion (ppb). Methyl vinyl ketone as well as methacrolein, both oxidation products of isoprene, ranged around 1 ppb. The sum of the measured monoterpene concentrations was below 1 ppb. At the end of the dry season, the amount of C1–C2organic acids and C1–C2aldehydes increased significantly up to 17 and 25 ppb, respectively, which is thought to result significantly from vegetation fire emissions. High methanol concentrations also support this scenario. At the same time, however, atmospheric mixing ratios of biogenic compounds such as isoprene increased up to 30 ppb near the crown region and well above 10 ppb at 10–20 m over the forest, whereas monoterpene species seem to decrease. We discuss seasonal development of the vegetation and climatological factors to be responsible for such concentration pattern. The results give an impression about the variability and concentration of VOCs during the different seasons.
Development and characterization of a fast measurement system for gas‐phase nitric acid with a chemical ionization mass spectrometer in the marine boundary layer
Authors: Furutani Hiroshi , Akimoto Hajime
Published: October 2002

A chemical ionization mass spectrometer (CIMS) and an automated air sampling/background signal measurement system designed for fast, reliable, and continuous ground‐based measurement of gas‐phase nitric acid (HNO3) were developed and characterized in a remote marine boundary layer site, Rishiri Island Observatory in Japan, under various meteorological conditions. HNO3 transmission efficiency of air sampling line, interference of NO and NO2 by corona discharge ion source, time response, detection sensitivity, and detection limit of the system were determined under the ambient condition. Detection limit of the system, defined as 3 times the standard deviation of background signal, varied depending on the atmospheric HNO3 concentration, 3–5 parts per trillion by volume (pptv) for the clean condition (HNO3 < 100 pptv) and 15–20 pptv for the polluted condition (HNO3 1–2 ppbv) with 2‐s integration time. The determining factor of HNO3 transmission efficiency (HNO3 loss) and critical points for reliable and fast measurement of gas‐phase HNO3 in the marine boundary layer were identified on the basis of field and laboratory tests of the CIMS system.
Marine air intrusion into the Adelie Land sector of East Antarctica: A study using the regional climate model (MAR)
Authors: Naithani Jaya , Gallée Hubert , Schayes Guy
Published: October 2002

Marine air intrusion and subsequent cloud formation plays a dominant role in the energy budget and mass balance of the Antarctic. However, the intrusion is very difficult to understand using the ground‐based measurements alone. In this paper we present simulations of marine air intrusion into the Adelie Land, East Antarctica, using the Modèle Atmosphérique Régional (MAR), for July 1994 and January 1995. The model is nested into the European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. The simulations show a strong influence of large‐scale disturbances, over the ocean, which helped in the penetration of marine air into the interior and the formation of clouds. Each marine air intrusion episode resulted in cloud formation in July 1994. Blocking anticyclones have also been found to be responsible for much of the moisture transport far into the interior elevated locations. MAR simulations, as well as ECMWF analyses, show influence of cyclones in strengthening and prolonging the surface layer flow. The study also indicated that the influence of depressions on surface winds is pronounced during the period when the depression is approaching the Adelie Land coast.
The cloud albedo‐cloud droplet effective radius relationship for clean and polluted clouds from RACE and FIRE.ACE
Authors: Peng Yiran , Lohmann Ulrike , Leaitch Richard , Banic Catharine , Couture Mark
Published: October 2002

Twenty‐eight liquid water cloud cases selected from two field studies (the Canadian Radiation, Aerosol and Cloud Experiment (RACE) and the First ISCCP Regional Experiment‐Arctic Cloud Experiment (FIRE.ACE)) are analyzed with respect to the first and second indirect aerosol effects and the relationship between cloud droplet effective radius and cloud albedo for clean and polluted clouds. For the same liquid water path the polluted clouds have more and smaller cloud droplets and thus a higher cloud albedo and less drizzle size drops. The effective radius is positively correlated with cloud albedo for polluted clouds caused by the absence of drizzle size drops. Conversely effective radius is negatively correlated with cloud albedo for clean clouds.
New evidence of an organic layer on marine aerosols
Authors: Tervahattu Heikki , Hartonen Kari , Kerminen Veli‐Matti , Kupiainen Kaarle , Aarnio Päivi , Koskentalo Tarja , Tuck Adrian F. , Vaida Veronica
Published: October 2002

An extraordinary episode of fine particles (diameter mainly <2.5 μm) occurred in Helsinki, southern Finland, at the end of February 1998. The air masses came from the North Atlantic Ocean and passed over France, Germany, and southern Scandinavia. Particles were collected during the episode as well as before and after it. Uncoated particle samples were adhered to an indium substrate and were studied by a scanning electron microscope (SEM) coupled with an energy dispersive X‐ray microanalyzer (EDX). A great proportion of the particles behaved differently than aerosols previously studied by microscopic techniques. The particles (size mainly 0.5–1 μm) did not exhibit solid shape. They were “bubbling” or “pulsating” continually, enlarging in one part and shrinking in another. Some particles were broken down, especially when the beam of the electron microscope was focused on them. EDX analyses showed that the particles contained much carbon together with oxygen, sulfur, and sodium. Ion analyses by ion chromatography revealed high concentrations of sodium, sulfate, nitrate, and ammonium. The particles were identified as marine sea‐salt aerosols, which had accumulated anthropogenic emissions and lost chloride during their flow through continental polluted air. The shape fluctuations and the high carbon content observed by SEM/EDX led to the conclusion that the aerosols were enclosed by an organic membrane. Direct insertion probe/mass spectrometry investigations showed remarkable amounts of fragmented aliphatic hydrocarbons, which were considered as an evidence of a lipid membrane on the surface of the particles. The impact of the posited organic film on the properties of sea‐salt particles, as well as on Earth's climate, is discussed.
Seasonality in CO2 and H2O flux at an eastern Amazonian rain forest
Authors: Carswell F. E. , Costa A. L. , Palheta M. , Malhi Y. , Meir P. , Costa J. de P. R. , Ruivo M. de L. , Leal L. do S. M. , Costa J. M. N. , Clement R. J. , Grace J.
Published: October 2002

Previous studies of CO2 fluxes in Amazonia have suggested seasonal variation in net ecosystem exchange. We find little evidence of this seasonality at a new site in eastern Amazonia, despite the expectation that this site would be particularly sensitive to seasonal fluctuation of rainfall. The average rate of peak net ecosystem exchange was −19 ± 0.9 (1 S.E.) μmol CO2 m−2 s−1. Canopy conductance, evaporation, and vapour pressure deficit were all increased during the dry season, consistent with an increase in bulk temperature and solar radiation. The lack of a dry season decrease in photosynthesis was thought to be due to the observed increase in leaf area following dry season flushing. This was accompanied by an increase in solar radiation, and we suggest that the effect of “dryness” was merely to preclude optimality of photosynthetic response to this increase in radiation. The gross primary productivity of this site was estimated to be 36 t C ha−1 yr−1. This is similar to that reported for other Amazon forest stands. The year may have been a particularly productive one due to the lack of an El Niño event.
Seasonal variations of a mixing layer in the lowermost stratosphere as identified by the CO‐O3 correlation from in situ measurements
Authors: Hoor Peter , Fischer Horst , Lange Lutz , Lelieveld Jos , Brunner Dominik
Published: October 2002

Within the Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM) project, airborne measurements of various trace species such as CO, CO2, N2O, and O3 have been performed in several years and seasons. Here we focus on a summer campaign conducted from Timmins (Canada, 47°N, 80°W) in July 1998 and winter measurements carried out in March 1997 from Kiruna (Sweden, 68°N, 20°E). From correlations between CO and O3 we identify a mixing layer in the lowermost stratosphere of which the vertical extent shows a significant seasonal variability. Troposphere to stratosphere exchange appears to be more vigorous in summer, although CO mixing ratios of the order of 15 ppbv indicate the presence of aged stratospheric air. While a significant tropospheric influence was observed up to potential temperatures of Θ = 330 K in March, the mixing layer in July increased to Θ = 360 K. In addition, its elevation above the tropopause also showed an increase. The CO‐O3 correlation indicates that the higher elevation in July is most likely caused by a stronger contribution of subtropical tropospheric air, which was mixed into the lowermost stratosphere at the subtropical jet.
Sensitivity of the Northern Hemisphere blocking frequency to the detection index
Authors: Doblas‐Reyes F. J. , Casado M. J. , Pastor M. A.
Published: October 2002

The sensitivity of Northern Hemisphere blocking features to the use of two different blocking indices is investigated using European Centre for Medium‐Range Weather Forecasts (ECMWF) reanalyses (ERA) and data from a climate simulation carried out with the atmospheric general circulation model Action de Recherche, Petite Echelle, Grande Echelle ARPEGE. The seasonal cycle of blocking frequency shows a good agreement between both indices, for both analyses and model, when the seasonal cycle of the 500‐hPa geopotential height is previously removed from the data. Furthermore, the blocking frequency seasonal cycle agrees well with the 5‐ to 45‐day band‐pass‐ filtered standard deviation (a proxy indicator of blocking activity) seasonal cycle. In contrast to previous results, this demonstrates that a consensus estimate of blocking features may be obtained with two different blocking indices. Nevertheless, some differences in the results from the two indices have been detected, particularly for the Euro‐Atlantic blocking. Regional differences suggest that on some occasions, an index may detect blocking events that the other index misses, and vice versa. An important consequence is that uncertainty in blocking interannual variability estimates, which depend on the index used, is expected to be high. The study shows that the model underestimates the blocking frequency regardless of the index used. However, the model proves to be able to simulate realistic blocks when removing an estimate of the geopotential time‐mean bias. Blocking drawbacks in the model are interpreted as a consequence of a specific zonal wind forcing associated with a decrease in the frequency of large‐scale ridges over the main blocking regions. These results strengthen the interest of analyzing model blocking as a complex phenomenon with regional dynamical differences and tight links with other atmospheric phenomena.
Chemical, physical, and optical characterization of aerosols during PAUR II experiment
Authors: Kouvarakis G. , Doukelis Y. , Mihalopoulos N. , Rapsomanikis S. , Sciare J. , Blumthaler M.
Published: October 2002

During a 1‐month campaign conducted in May 1999 in the frame of Photochemical Activity and Solar Ultraviolet Radiation (PAUR II) project, determination of the ionic composition and physical and optical properties of the aerosol have been performed at a coastal site in Crete. Non‐sea‐salt sulfate (nss‐SO42−), nitrate (NO3−) non‐sea‐salt calcium (nss‐Ca2+), and ammonium (NH4+) have been identified as the main non‐sea‐salt ionic components of the aerosol. Air mass origin was found to be an important factor controlling the variation of the above ions as well as their particle size distribution. During the experiment, aerosol scattering coefficient ranges from 2 to 51 (M m−1) with no significant difference between dusty and nondusty periods. Significant correlations have been observed between (NH4)2SO4 mass measured during the campaign, aerosol scattering coefficient, and aerosol single‐scattering albedo indicating the key role of (NH4)2SO4 in determining the radiative forcing of the eastern Mediterranean area. Finally, non‐sea‐salt calcium was found to better reproduce the variation of optical depth, more than any other ionic species.
Global chemical model analysis of biomass burning and lightning influences over the South Pacific in austral spring
Authors: Staudt Amanda C. , Jacob Daniel J. , Logan Jennifer A. , Bachiochi David , Krishnamurti T. N. , Poisson Nathalie
Published: October 2002

A global three‐dimensional model of tropospheric chemistry driven by reanalyzed European Centre for Medium‐Range Weather Forecasts meteorological data is used to examine the sources of O3, CO, and nitrogen oxides (NOx = NO + NO2) in the South Pacific troposphere during the NASA Pacific Exploratory Mission to the Tropics (PEM‐Tropics A) in September–October 1996. Aircraft observations up to 12 km during that mission revealed considerable biomass burning influence on O3 and CO in terms of elevated pollution layers and regional enhancements. The model reproduces the long‐range transport of biomass burning effluents from southern Africa and South America in the westerly subtropical flow over the South Pacific. Meteorological conditions in 1996 were particularly favorable for this transport. Africa and South America make comparable contributions to the biomass burning pollution over the South Pacific; the contribution from Australia and Indonesia is much less. Biomass burning dominates the supply of NOx in the lower troposphere over the South Pacific (through long‐range transport and decomposition of peroxyacetylnitrate), but lightning dominates in the upper troposphere. Observations in PEM‐Tropics A and elsewhere indicate low HNO3/NOx concentration ratios and an imbalance in the chemical budget of NOx in the upper troposphere. We reproduce these observations in our model and show that they reflect the subsidence of primary NOx injected by lightning into the uppermost troposphere, rather than any fast chemistry recycling HNO3 to NOx. We find that biomass burning and lightning made similar contributions to O3 production over the South Pacific during PEM‐Tropics A. Biomass burning plumes sampled in PEM‐Tropics A contained little NOx, and the O3 enhancements observed in these plumes originated from production over the source continents rather than over the South Pacific.
Interannual variations in the atmospheric heat budget
Authors: Trenberth Kevin E. , Stepaniak David P. , Caron Julie M.
Published: October 2002

Interannual variability of the atmospheric heat budget is explored via a new data set of the computed vertically integrated energy transports to examine relationships with other fields. A case study reveals very large monthly divergences of these transports regionally with El Niño‐Southern Oscillation (ENSO) and the associated changes with the Pacific‐North American teleconnection pattern, and with the North Atlantic Oscillation. In the tropical Pacific during large El Niño events the anomalous divergence of the atmospheric energy transports exceeds 50 W m−2 over broad regions for several months. Examination of the corresponding top‐of‐the‐atmosphere net radiative fluxes shows that it is primarily the surface fluxes from the ocean to the atmosphere that feed the divergent atmospheric transports. A systematic investigation of the covariability of sea surface temperatures (SSTs) and the divergence of atmospheric energy transport, using singular value decomposition analysis of the temporal covariance, reveals ENSO as dominant in the first two modes, explaining 62% and 12% of the covariance in the Pacific domain and explaining 39.5% and 15.4% globally for the first and second modes, respectively. The first mode is well represented by the time series for the SST index for Niño 3.4 region (170°W–120°W, 5°N–5°S). Regression analysis allows a more complete view of how the SSTs, outgoing longwave radiation, precipitation, diabatic heating, and atmospheric circulation respond with ENSO. The second mode indicates aspects of the systematic evolution of ENSO with time, with strong lead and lag correlations. It primarily reflects differences in the evolution of ENSO across the tropical Pacific from about the dateline to coastal South America. High SSTs associated with warm ENSO events are damped through surface heat fluxes into the atmosphere, which transports the energy into higher latitudes and throughout the tropics, contributing to loss of heat by the ocean, while the cold ENSO events correspond to a recharge phase as heat enters the ocean. Diabatic processes are clearly important within ENSO evolution.
Evolution of El Niño–Southern Oscillation and global atmospheric surface temperatures
Authors: Trenberth Kevin E. , Caron Julie M. , Stepaniak David P. , Worley Steve
Published: October 2002

The origins of the delayed increases in global surface temperature accompanying El Niño events and the implications for the role of diabatic processes in El Niño–Southern Oscillation (ENSO) are explored. The evolution of global mean surface temperatures, zonal means and fields of sea surface temperatures, land surface temperatures, precipitation, outgoing longwave radiation, vertically integrated diabatic heating and divergence of atmospheric energy transports, and ocean heat content in the Pacific is documented using correlation and regression analysis. For 1950–1998, ENSO linearly accounts for 0.06°C of global surface temperature increase. Warming events peak 3 months after SSTs in the Niño 3.4 region, somewhat less than is found in previous studies. Warming at the surface progressively extends to about ±30° latitude with lags of several months. While the development of ocean heat content anomalies resembles that of the delayed oscillator paradigm, the damping of anomalies through heat fluxes into the atmosphere introduces a substantial diabatic component to the discharge and recharge of the ocean heat content. However, most of the delayed warming outside of the tropical Pacific comes from persistent changes in atmospheric circulation forced from the tropical Pacific. A major part of the ocean heat loss to the atmosphere is through evaporation and thus is realized in the atmosphere as latent heating in precipitation, which drives teleconnections. Reduced precipitation and increased solar radiation in Australia, Southeast Asia, parts of Africa, and northern South America contribute to surface warming that peaks several months after the El Niño event. Teleconnections contribute to the extensive warming over Alaska and western Canada through a deeper Aleutian low and stronger southerly flow into these regions 0–12 months later. The 1976/1977 climate shift and the effects of two major volcanic eruptions in the past 2 decades are reflected in different evolution of ENSO events. At the surface, for 1979–1998 the warming in the central equatorial Pacific develops from the west and progresses eastward, while for 1950–1978 the anomalous warming begins along the coast of South America and spreads westward. The eastern Pacific south of the equator warms 4–8 months later for 1979–1998 but cools from 1950 to 1978.
Exchange of short‐chain monocarboxylic acids by vegetation at a remote tropical forest site in Amazonia
Authors: Kuhn U. , Rottenberger S. , Biesenthal T. , Ammann C. , Wolf A. , Schebeske G. , Oliva S. T. , Tavares T. M. , Kesselmeier J.
Published: October 2002

As part of the project LBA‐EUSTACH (European Studies on Trace gases and Atmospheric Chemistry as a contribution to the Large‐Scale Biosphere‐Atmosphere experiment in Amazonia), the exchange of formic acid and acetic acid between vegetation and the atmosphere was investigated in the wet‐to‐dry season transition and the dry‐to‐wet season transition periods in 1999 in Rondônia, Brazil. Direct exchange measurements on the branch level mainly exhibited uptake of formic acid and acetic acid for all plant species in both seasons, although diel, seasonal, and interspecies variations were observed. Even though other physiological and physico‐chemical parameters may have contributed, the uptake of organic acids was found to be primarily a function of the ambient atmospheric mixing ratios. The linear dependence suggests a bidirectional exchange behavior of the plants and calculated deposition velocities (0.17–0.23 cm s−1), compensation point mixing ratios (0.16–0.30 ppb), and potential emission rates under purified air conditions (0.013–0.031 nmol m−2 s−1) are discussed. Vertical profile measurements in and above the primary forest canopy further strengthened the assumption that the forest is rather a sink than a source for organic acids. The generally lower mixing ratios observed within the canopy were indicative of an uptake by vegetation and/or the soil surface. Continuous measurements of the ambient atmospheric mixing ratios at the canopy top revealed strong diel variations in both seasons and a marked seasonality with higher mixing ratios during the dry season, both being mirrored in the variation of observed uptake rates of the plants. High atmospheric concentrations during the dry season were attributed to biomass burning. During the wet season, when biomass burning activity was low, indirect emission by the vegetation, i.e., photochemical oxidation of primarily emitted biogenic reactive hydrocarbons, was assumed to dominantly contribute to the atmospheric burden of the organic acids. The high degree of correlation between atmospheric formic acid and acetic acid indicated that similar atmospheric processes were affecting their mixing ratios.