bluuebunny/crossref_metadata_2025 huggingface dataset:
Empirical age spectra for the lower tropical stratosphere from in situ observations of CO2: Implications for stratospheric transport
Authors: Andrews A. E. , Boering K. A. , Daube B. C. , Wofsy S. C. , Hintsa E. J. , Weinstock E. M. , Bui T. P.
Published: September 2002

Empirical age spectra for the lower tropical stratosphere (from the tropopause to ∼19.5 km) have been derived from in situ measurements of CO2, using information provided by the vertical propagation of the tropospheric seasonal cycle and long‐term positive trend. Our method provides accurate and unambiguous mean ages for this region which are difficult to obtain by simple analysis of lag times from tracer measurements. We find that the air is 30–40% younger in northern spring than in autumn. For example, at 460 K the mean age (relative to the tropical tropopause) was 0.4 years in March and 0.6 years in September. The phase lag times and attenuation of CO2 seasonal extrema in the stratosphere are shown to depend on seasonal variations in transport rates and on the presence of harmonics in the CO2 boundary condition with frequencies higher than 2π/yr. Profiles of stratospheric water vapor, generated from the derived age spectra with a stratospheric boundary condition based on observed tropical tropopause temperatures, are consistent with in situ observations of H2O. Comparison of the predicted water vapor seasonal cycle with satellite observations suggests that satellite‐borne instruments underestimate the amplitude near the tropical tropopause. We relate the empirical age spectra to the analytic solution for the 1‐D advection‐diffusion tracer continuity equation to obtain seasonally resolved estimates of the ascent rate and the vertical diffusion coefficient. The derived age spectra provide a unique observation‐based diagnostic for evaluating the simulation of tracer transport in models.
Degree of linear polarization of light emerging from the cloudless atmosphere in the oxygen A band
Authors: Stam D. M. , De Haan J. F. , Hovenier J. W. , Stammes P.
Published: September 2002

We used radiative transfer calculations and model atmospheres for a theoretical investigation of the behavior of the degree of linear polarization, P, of light emerging from the top or the bottom of the cloudless atmosphere in the wavelength region of the O2 A absorption band, between 755 and 775 nm. Results of P are shown for four model atmospheres and for various albedos of the underlying Lambert surface. One of the model atmospheres contains only molecules, whereas the other atmospheres contain also aerosols. It is shown that when the molecular absorption optical thickness of the atmosphere is much smaller than one, which represents the continuum and the weak absorption lines in the band, the state of polarization of the emerging radiation is mainly determined by low‐order scattering in the lowest atmospheric layers and by reflection by the surface. In the strong absorption lines, where the molecular absorption optical thickness is much larger than one, we distinguish the three following situations: (1) P of the reflected light is mainly determined by single scattering by molecules and aerosol in the upper atmospheric layers, (2) for small solar zenith and/or viewing angles, P of the diffusely transmitted light is mainly determined by single scattering, and (3) for other geometries, P of the diffusely transmitted light is predominantly due to second‐order scattering, with the first scattering taking place in the upper atmospheric layers and the second in the lower layers. For a given surface albedo, the variation of P across an absorption line thus depends on the scattering properties of the atmospheric particles and on their vertical distribution. The surface albedo and P of light emerging from the atmosphere in the principal plane are shown to be related through a simple formula at each wavelength within the absorption band. It is concluded that high‐resolution spectropolarimetry in wavelength regions with strongly varying molecular absorption optical thickness can provide valuable information on aerosol at various altitudes in the atmosphere.
On moistening of the tropical troposphere by cirrus clouds
Authors: Sherwood Steven C.
Published: September 2002

Sublimation of ice crystals in cirrus clouds is often invoked as a moistening mechanism for the free troposphere, particularly in the tropics. An alternative mechanism is investigated here: “pumping” of water vapor into the cloudy atmospheric column from surrounding regions by diabatic transports resulting from the local radiative effects of the cloud. The effectiveness of this mechanism is computed for a variety of cirrus cloud types, using a simple model. For nonprecipitating cirrus clouds it is found that the pumping mechanism can import much more vapor mass into the cloudy column than the clouds contain as ice, especially as the clouds become optically thin. The advected vapor per unit ice mass (or “pumping ratio”) also varies with cloud height, is proportional to cloud lifetime, and (for thin clouds) is approximately inversely proportional to effective particle radius. Thin cirrus above 300 hPa can pump vapor rapidly enough to sustain itself against dessication. The overall results are supported by the limited information available from observations and indicate that at sufficient distances from active convection, direct moistening of the environment by the sublimation of ice has significantly less impact on vapor distributions than cloud‐radiative‐dynamic effects.
Radiative forcing by volcanic aerosols from 1850 to 1994
Authors: Andronova Natalia G. , Rozanov Eugene V. , Yang Fanglin , Schlesinger Michael E. , Stenchikov Georgiy L.
Published: September 2002

We use our detailed radiative transfer model and the latitude‐altitude distributions of zonal mean optical properties for the Pinatubo aerosol to calculate the time evolution of its instantaneous radiative forcing. We represent the zonal mean of this radiative forcing in terms of the zonal mean optical depth of the Pinatubo aerosol, together with the solar insolation, effective emitting temperature, daylight fraction of a day, and the planetary albedo in the absence of the aerosol. We use this representation, together with the volcano optical depths compiled by Sato et al. [1993], to calculate the instantaneous and adjusted radiative forcing by volcanic aerosols from 1850 to 1994.
NO signatures from lightning flashes
Authors: Stith J. , Dye J. , Ridley B. , Laroche P. , Defer E. , Baumann K. , Hübler G. , Zerr R. , Venticinque M.
Published: September 2002

In situ measurements of cloud properties, NO, and other trace gases were made in active thunderstorms by two research aircraft. Concurrent measurements from a three‐dimensional (3‐D) VHF interferometer and the 2‐D National Lightning Detection Network were used to determine lightning frequency and location. The CHILL Doppler radar and the NOAA‐WP‐3D Orion X band Doppler radar were also used to measure storm characteristics. Two case studies from the (STERAO) Stratosphere‐Troposphere Experiments: Radiation, Aerosols, and Ozone project in northeastern Colorado during the summer of 1996 are presented. Narrow spikes (0.11–0.96 km across), containing up to 19 ppbv of NO, were observed in the storms. Most were located in or downwind of electrically active regions where the NO produced by lightning would be expected. However, it was difficult to correlate individual flashes with NO spikes. A simple model of the plume of NO from lightning is used to estimate NO production from the mean mixing ratio measured in these spikes. The estimates range from 2.0×1020 to 1.0×1022 molecules of NO per meter of flash length.
A river flow routing scheme for general circulation models
Authors: Arora Vivek K. , Chiew Francis H. S. , Grayson Rodger B.
Published: September 2002

The routing of runoff estimates from General Circulation Models (GCMs) is important to model river flow from large river basins and to estimate freshwater inflow into the oceans. Present routing approaches use arbitrary constants and empirical equations to determine travel times between the GCM grid cells. A new river flow routing scheme is developed, which uses physical catchment and river channel information and river discharge data. The scheme uses surface runoff and deep percolation data from land surface parameterization schemes, and routing is performed via linear surface and groundwater reservoirs. Geomorphological relationships between mean annual river discharge and other physical variables are used to interpolate existing information to define the channel morphology for the digital river networks at the routing model resolution. Applications of the routing scheme to the Mississippi and Amazon River Basins indicate that it performs adequately. The scheme does not require the calibration of parameters and can thus be easily used in GCMs.
Observed leader and return‐stroke propagation characteristics in the bottom 400 m of a rocket‐triggered lightning channel
Authors: Wang D. , Takagi N. , Watanabe T. , Rakov V. A. , Uman M. A.
Published: September 2002

Using a high‐speed digital optical system, we determined the propagation characteristics of two leader/return‐stroke sequences in the bottom 400 m of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart‐stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dart‐stepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10–90% risetime ranging from 0.3 to 0.8 μs with a mean value of 0.5 μs and a half‐peak width ranging from 0.9 to 1.9 μs with a mean of 1.3 μs. The interpulse interval ranges from 1.7 to 7.2 μs with a mean value of 4.6 μs. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9×107 to 1.0×108 m/s with a mean value of 6.7×107 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8×107 m/s, very close to the return‐stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leader‐conditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return‐stroke speeds within the bottom 60 m or so of the channel are 1.3×108 and 1.5×108 m/s for the two events analyzed, with a potential error of less than 20%.
Weighting of geophysical data in Principal Component Analysis
Authors: Chung Chul , Nigam Sumant
Published: September 2002

The purpose of this paper is to encourage the use of field weighting over field interpolation in achieving grid‐area parity in principal component analysis of geophysical data. While field weighting has been shown to achieve exact parity in unrotated analysis, we show that it is highly effective in rotated analysis, too, by comparing the three leading modes of 200‐mbar geopotential height variability in the 1958–1998 NCEP reanalysis winters obtained from both methods.
Observations of the vertical and regional variability of aerosol optical properties over central and eastern North America
Authors: Sheridan Patrick J. , Ogren John A.
Published: September 2002

Aerosol optical properties were measured in situ from a research aircraft during three recent field experiments in the central and eastern parts of North America as well as over areas of the western Atlantic Ocean. Regional and vertical variability of the aerosol properties for boundary layer and free tropospheric air were determined. In general, the differences between distributions of aerosol properties measured at low (planetary boundary layer) and high (free troposphere) altitudes were small but statistically significant at the 3% level or better. However, most of the aerosol optical thickness of the layers studied (∼5 km down to ∼100 m) was encountered in the lowest 1 km of each layer. As a result, the near‐surface measurements of aerosol optical properties adequately represented the portion of the lower column that dominates the radiative effects. The estimated error encountered by using near‐surface aerosol measurements to calculate the layer forcing was typically <10%.
Tropospheric sulfur simulation and sulfate direct radiative forcing in the Goddard Institute for Space Studies general circulation model
Authors: Koch Dorothy , Jacob Daniel , Tegen Ina , Rind David , Chin Mian
Published: September 2002

Global simulations of tropospheric sulfur are performed in the Goddard Institute for Space Studies (GISS) general circulation model (GCM) and used to calculate anthropogenic sulfate direct radiative forcing. Prognostic species are in‐cloud oxidant H2O2, dimethylsulfide (DMS), methanesulfonic acid (MSA), SO2 and sulfate. Compared with most previous models (except others with prognostic H2O2), this model has relatively high anthropogenic SO2 and sulfate burden. We show that this is due partly to the depletion of the prognostic H2O2 and that moist convection delivers significant levels of SO2 to the free troposphere in polluted regions. Model agreement with surface observations is not remarkably different from previous studies. Following some previous studies, we propose that an additional in‐cloud or heterogeneous oxidant is likely to improve the simulation near the surface. Our DMS source is lower than sources in previous studies, and sulfur values in remote regions are generally lower than those observed. Because of the high flux of SO2 to the free troposphere and the relatively low natural source, our model indicates a larger global anthropogenic contribution to the sulfate burden (77%) than was estimated by previous global models. Additional high‐altitude observations of the sulfur species are needed for model validation and resolution of this issue. Direct radiative forcing calculations give an annual average anthropogenic sulfate forcing of −0.67 W/m2. We compare the radiative forcings due to online (hourly varying) versus offline (monthly average) sulfate and find little difference on a global average, but we do find differences as great as 10% in some regions. Thus, for example, over some polluted continental regions the forcing due to offline sulfate exceeds that of online sulfate, while over some oceanic regions the online sulfate forcing is larger. We show that these patterns are probably related to the correlation between clouds and sulfate, with positive correlations occuring over some polluted continental regions and negative correlations over high‐latitude oceanic regions.
UV‐B irradiance at Madrid during 1996, 1997, and 1998
Authors: Córdoba C. , Aguirre de Cárcer I. , Pérez A. , Sanz A. , Angulo C. , Vila P. , Monroy E. , Muñoz E. , Jaque F.
Published: September 2002

During 1996, 1997, and 1998, several total ozone depletion events were detected over Madrid (40°N, 3°W) in late spring and in winter. A broadband interferential filter instrument (UV‐300) centered at 298 nm was used to monitor the solar UV‐B irradiance on the ground, registering UV‐B levels as high as 17.1 μW/cm2 (May 2, 1997, solar zenith angle SZA = 35°, O3 = 268 Dobson units). The data obtained have been compared to that measured with two commercial biometers (PMA2102 and Solar Light 501A), showing that the evolution of UV‐B due to total ozone changes are better followed with broadband instruments with effective windows centered at lower wavelength than with instruments responding to the erythemal action spectra. The development of simple, portable, and reliable instruments for the analysis of solar UV‐B phenomena at the high‐energy side of that band is advocated.
On the relationship of HO2 + RO2 with j(O1D) during the Free Tropospheric Experiment (FREETEX '96) at the Jungfraujoch Observatory(3580 m above sea level) in the Swiss Alps
Authors: Zanis Prodromos , Monks Paul S. , Schuepbach Evi , Penkett Stuart A.
Published: September 2002

Continuous measurements of peroxy radicals (HO2+RO2), the photolysis rate coefficients j(NO2) and j(O1D), and a number of other trace gases including O3, NOx, and CO, were carried out at the alpine site at Jungfraujoch (3580 m asl), Switzerland, in April/May 1996 during the Free Tropospheric Experiment (FREETEX '96). In spring, the Jungfraujoch Observatory is very often located in the lower free troposphere. A mean midday concentration of 17.1±2.2 parts per trillion by volume (pptv) was calculated from continuous measurements of the sum of inorganic and organic peroxy radicals using the chemical amplification technique. The average diurnal cycles for peroxy radicals and the photolysis rate coefficient j)O1D) during FREETEX '96 showed a statistically more significant square root than first‐order relationship, indicative of a relatively clean atmosphere at the mountain site, where peroxy radical self‐ and cross‐reactions were the dominant peroxy radical loss processes. The average ozone diurnal cycle during FREETEX '96 displayed an increase of about 2 ppbv during the daytime despite the square root relationship. Similar results were derived from individual days during the campaign. A simple photochemical box model indicated that there is a regime in NO concentration from about 20 pptv to 100 pptv where a better square root (than first‐order) correlation between peroxy radicals and j(O1D) is accompanied by net ozone production. Taking into account that median daytime concentrations of NO at Jungfraujoch range from 51 pptv in March to 93 pptv in April, it is suggested that Jungfraujoch, during FREETEX '96, has been in the net ozone production regime in spite of the observed square root dependence of peroxy radicals on j(O1D)
Mesopause temperature variability above a midlatitude station in Europe
Authors: Bittner M. , Offermann D. , Graef H. H.
Published: September 2002

More than 2000 mean night temperatures (1987–1995) were derived from OH* near‐infrared emissions in the upper mesosphere (around 87 km) above Wuppertal (51°N, 7°E) from ground‐based measurements. Variations of 4–50 days' period were analyzed using maximum entropy and wavelet methods. A climatology showing the seasonal dependence of occurrence and long‐term evolution of the strength of these variations is presented. While longer period oscillations (greater than ∼10 days' period) are observed to be more frequent during local winter and almost absent during local summer. There is an opposite behavior for the shorter period oscillations. Temperature oscillation amplitudes are found to be modulated with a quasi 2 year oscillation. It is believed that the quasi‐biennal oscillation is responsible for these modulations. A long‐term trend showing an increase of about 16% per decade is found in the temperature variability. Furthermore, positive and negative long‐term trends are found in the temporal evolution of temperature oscillation amplitudes. A possible relationship to solar activity is investigated. Evidence is found that shorter period oscillations (smaller than ∼20–25 days) are anticorrelated while longer period oscillations are correlated with solar activity.
Identification of the bright band through the analysis of volumetric radar data
Authors: Sánchez‐Diezma Rafael , Zawadzki Isztar , Sempere‐Torres Daniel
Published: September 2002

The identification of the bright band by volumetric scanning radars is analyzed. An average vertical profile of reflectivity (VPR) containing bright band enhancement is used as an input to simulate the VPR registered by the radar as a function of range. It is found that the bright band intensity enhancement detected by the radar decreases cyclically with range. The cycle depends on the relative positions of the antenna elevation angles and the bright band. The accuracy of the bright band identification is analyzed from different viewpoints: the radar scan strategy, the number of available elevation angles, the altitude of the bright band, and the effect of areal integration of radar measurement. An algorithm for detecting the bright band is devised, and its application to a case study exhibits features similar to those resulting from the simulation. The findings show that high‐resolution volume scans and an optimized radar scan strategy are necessary for an adequate bright band identification. Moreover, the identification is clearly limited in range. We found the detection limit to be 70 km with a beam width of the order of 1°, although the bright band contamination appreciably affects the reflectivity measurements beyond this range.
Sprite observations in the Northern Territory of Australia
Authors: Hardman Simon F. , Dowden Richard L. , Brundell James B. , Bahr John L. , Kawasaki Zen‐ichiro , Rodger Craig J.
Published: September 2002

Sprites, a form of brief luminous discharge in the upper atmosphere above a thunderstorm, were observed and imaged on two video cameras in Australia's Northern Territory. These were the first such ground‐based observations made outside the United States. Sprite discharges typically took place between the altitudes of 50 km and 80 km and spanned an average width of 44 km. Many of the sprite events were of long duration, with an average of 145 ms. These spatial and temporal features were similar to those observed from the ground and the air in the United States. During the longer events, some luminous discharge elements were observed to decay as other new elements formed. As the new elements were often laterally displaced from the old, the sprites sometimes appeared to dance across the sky. This phenomenon has been observed in Colorado and named “dancing sprites.” The lateral progression of sprite elements observed in the Northern Territory was overwhelmingly in one direction and covered distances of up to 90 km.
Spatial patterns of variability in the global surface air temperature data set
Authors: Kelly P. M. , Jones P. D. , Jia Pengqun
Published: September 2002

We identify the major spatial patterns of variability in the global surface air temperature data set using empirical orthogonal analysis. We rotate the major components to simplify physical interpretation. Of the five patterns which account for the largest proportion of the total variance in the data set, three are global in extent and two are continental in scale. The pattern accounting for most of the variance in the data set indicates a global change in temperature, affecting most regions in the same sense but most marked in lower latitudes. The second most important pattern is a measure of the El Niño‐Southern Oscillation phenomenon. The third pattern represents a contrast in temperature between the northern and southern oceans and is shown to be related to Sahel rainfall variations, confirming, though with reservations, the results of previous work. The fourth pattern is an index of the temperature effects of the North Atlantic Oscillation. The fifth pattern is a measure of temperature variations over Siberia. It is shown that this component is related to an atmospheric circulation fluctuation affecting, among other things, the strength of the depression track over the Barents and Kara Seas and neighboring areas. We term this process the Euro‐Siberian Oscillation. The effects of changing data coverage are explored, and it is shown that patterns with a strong maritime component are particularly susceptible to this form of inhomogeneity. Finally, potential causal mechanisms, such as volcanic pollution of the atmosphere, are investigated, and the evolution over time of the spatial response to these forcing factors is defined using the empirical orthogonal functions. The analysis demonstrates that empirical orthogonal functions representing the spatial patterns of temperature change provide a concise means of monitoring climate trends and can assist identification of the causes of recent climate change as well as supporting climate model validation and assessment of the representativeness and reliability of climate reconstructions based on proxy indicators such as tree ring data.
Nitrous oxide emissions from intensive agricultural systems: Variations between crops and seasons, key driving variables, and mean emission factors
Authors: Dobbie K. E. , McTaggart I. P. , Smith K. A.
Published: September 2002

Emissions of nitrous oxide from intensively managed agricultural fields were measured over 3 years. Exponential increases in flux occurred with increasing soil water‐ filled pore space (WFPS) and temperature; increases in soil mineral N content due to fertilizer application also stimulated emissions. Fluxes were low when any of these variables was below a critical value. The largest fluxes occurred when WFPS values were very high (70–90%), indicating that denitrification was the major process responsible. The relationships with the driving variables showed strong similarities to those reported for very different environments: irrigated sugar cane crops, pastures, and forest in the tropics. Annual emissions varied widely (0.3–18.4 kg N2O‐N ha−1). These variations were principally due to the degree of coincidence of fertilizer application and major rainfall events. It is concluded therefore that several years' data are required from any agricultural ecosystem in a variable climate to obtain a robust estimate of mean N2O fluxes. The emissions from small‐grain cereals (winter wheat and spring barley) were consistently lower (0.2–0.7 kg N2O‐N per 100 kg N applied) than from cut grassland (0.3–5.8 kg N2O‐ N per 100 kg N). Crops such as broccoli and potatoes gave emissions of the same order as those from the grassland. Although these differences between crop types are not apparent in general data comparisons, there may well be distinct regional differences in the relative and absolute emissions from different crops, due to local factors relating to soil type, weather patterns, and agricultural management practices. This will only be determined by more detailed comparative studies.
Effects of aerosols on tropospheric photolysis rates in clear and cloudy atmospheres
Authors: Liao Hong , Yung Yuk L. , Seinfeld John H.
Published: September 2002

The effect of aerosols on 14 tropospheric photolysis reactions is examined under noncloudy and cloudy sky conditions by using a detailed one‐dimensional radiative transfer model. Pure (NH4)2SO4, pure soot, and internal and external mixtures of the two aerosols, as well as mineral dust aerosol, are considered. Nonabsorbing aerosol generally enhances photolysis rates above and in the upper part of the aerosol layer in both noncloudy and cloudy atmospheres, with the enhancement effect reduced in the presence of clouds. In contrast, soot aerosol reduces photolysis rates under both noncloudy and cloudy sky conditions, with the reduction accentuated by a cloud layer. Mixtures of absorbing and nonabsorbing aerosols may produce enhancement or reduction in photolysis rates under clear sky conditions, whereas they generally reduce rates when a cloud is present. In the absence of cloud, sulfate aerosol at urban levels enhances tropospheric average photolysis rates from 11 to 18% for the 14 reactions studied; soot aerosol decreases tropospheric average rates from 6 to 11%. In the presence of a 500‐m‐thick stratus cloud, sulfate aerosol enhances each of 14 tropospheric average photolysis rates by about 5%; soot aerosol decreases tropospheric average photolysis rates from 9 to 19%.
Sensitivity of surface UV radiation and ozone column retrieval to ozone and temperature profiles
Authors: Lapeta B. , Engelsen O. , Litynska Z. , Kois B. , Kylling A.
Published: September 2002

This paper discusses the influence of ozone and temperature profiles on surface UV radiation, and on total ozone column derived from global irradiance measurements. Measured ozone and temperature profiles from Legionowo, Poland, are used together with typical surface and cloudless atmosphere conditions. The effects of assuming a U.S. standard profile with scaled ozone column instead of actual profiles are analyzed. Variable temperature/ozone vertical distributions and different sets of ozone absorption cross section data may change erythemally weighted radiation by as much as 14% with respect to reference conditions. The mean and standard deviations of errors were generally below 2% but increased another 2–3% for large solar zenith angles. Uncertainties of up to 10% may be caused by using an inappropriate profile in total ozone column retrieval. We analyzed the underlying processes causing the uncertainties by selecting three ozone and temperature profile pairs characterized by the same unsealed total ozone amount but with different vertical distributions. Results obtained for cases with ozone redistribution from the stratosphere to the troposphere are consistent with earlier work. However, if the temperature profiles differ significantly in the stratosphere, an ozone redistribution may lead to a strong decrease in UV doses for high solar zenith angles. It is also shown that differences in ozone maximum height as well as in ozone concentration in the upper troposphere have a significant influence on surface UV radiation.
An inversion algorithm using neural networks to retrieve atmospheric CO total columns from high‐resolution nadir radiances
Authors: Hadji‐Lazaro Juliette , Clerbaux Cathy , Thiria Sylvie
Published: September 2002

Up to the first years of the next millennium, several observation programs of the troposphere are scheduled, including the Infrared Atmospheric Sounding Interferometer, which uses Fourier transform spectroscopy to record the radiance of the Earth‐atmosphere system with a nadir‐viewing geometry. The Interferometric Monitor for Greenhouse Gases (IMG), launched aboard the Advanced Earth Observing System in August 1996, was a precursor of these forthcoming missions. A new inversion algorithm based on neural network techniques is in development to retrieve trace gases from high‐resolution nadir radiances. Neural networks offer a technical alternative to classical methods and allow efficient inversion calculations as required to treat the huge volume of data which will be provided by continuous observation of the atmosphere from space. To develop a network to retrieve the carbon monoxide total column, realistic simulations of the IMG measurements were obtained by coupling a three‐dimensional chemical‐transport model with a high‐resolution line‐by‐line radiative transfer code adjusted to the instrumental features. The application of the algorithm on simulated data allowed the checking of its performance: for about 99% of the cases, the relative inversion error was less than 10%. This algorithm has been applied to the spectra recorded by the IMG instrument between June 16 and 19, 1997. Global‐scale distributions of CO total columns were obtained for the first time by using a neural network, and this technique proved its ability to achieve real‐time inversion of atmospheric CO.
Chemical ionization mass spectrometer technique for the measurement of HNO3 in air traffic corridors in the upper troposphere during the SONEX campaign
Authors: Miller Thomas M. , Ballenthin John O. , Meads Roger F. , Hunton Donald E. , Thorn Willard F. , Viggiano A. A. , Kondo Yutaka , Koike Makoto , Zhao Yongjing
Published: September 2002

A chemical ionization mass spectrometer (CIMS) was used for rapid detection of HNO3 in air traffic corridors, primarily over the North Atlantic region, during the NASA Subsonic Assessment Ozone and Nitrogen Oxide Experiment (SONEX) campaign in the fall of 1997. The sensitivity of the CIMS instrument approaches 1 ion count per second for each 106 molecules cm−3, under ideal conditions. During the SONEX mission the precision of the experiment was considerably lower due to inlet fluctuations. Ten‐second integration periods were used to obtain a precision of typically 10 parts per trillion by volume. A description is given of the instrument and the technique, including inflight calibration using a permeation tube. Comparisons are made with NOy data and with the University of New Hampshire HNO3 data obtained with a mist chamber method.
Carbonaceous aerosol (soot) measured in the lower stratosphere during POLARIS and its role in stratospheric photochemistry
Authors: Strawa A. W. , Drdla K. , Ferry G. V. , Verma S. , Pueschel R. F. , Yasuda M. , Salawitch R. J. , Gao R. S. , Howard S. D. , Bui P. T. , Loewenstein M. , Elkins J. W. , Perkins K. K. , Cohen R.
Published: September 2002

This paper describes recent measurements of carbonaceous aerosol made by wire impactors during the Photochemistry Ozone Loss in the Arctic Region in Summer (POLARIS) campaign and assesses their role in stratospheric photochemistry. Ninety‐five percent of the carbonaceous aerosol collected during this campaign was in the form of black carbon aerosol (BCA), or soot. A new method of analyzing impactor samples is described that accounts for particle bounce and models the BCA as fractal aggregates to modify the aerodynamic collection efficiency and determine particle surface area. Results are compared to previously used methods. The new method results in an increase in the measured BCA number density of 4 times, surface area density of ∼15 times, and an increase in mass loading of 6.15 times over one previously used approach. Average values of number, surface area, and mass densities are 0.06 no./cm3,0.03 μm2/ cm3, and 0.64 ng/m3, respectively. BCA number densities are ∼1% of total aerosol number density, and BCA surface area density is ∼10% of the measured sulfuric acid aerosol surface area. Including heterogeneous reactions on BCA in a photochemical model can affect photochemistry leading to renoxification and increased ozone depletion. However, these predicted effects are not supported by the POLARIS observations, in particular, the NOx/NOy ratios. The laboratory data is not conclusive enough to determine to what extent the heterogeneous reaction is catalytic or carbon consuming. Including catalytic reactions on BCA does not statistically improve the agreement between model and measurement in any of the several scenarios considered. Furthermore, if the reactions cause even partial carbon oxidation, the BCA would be consumed at a rate inconsistent with POLARIS observations. These inconsistencies lead us to conclude that the presence of BCA in the stratosphere did not affect stratospheric photochemistry during POLARIS.
Measurements of aerosol optical depth at Ispra: Analysis of the correlation with UV‐B, UV‐A, and total solar irradiance
Authors: Meleti C. , Cappellani F.
Published: September 2002

Aerosol optical depths measured with a Brewer spectrophotometer in the UV region and with a CIMEL Sun photometer in the visible and near infrared, both in operation at Ispra, Italy, are presented and analyzed. The analysis of the data acquired during 1 year includes a comparison of the spectral aerosol optical depth measured with the Brewer spectrophotometer at various wavelengths, as well as comparisons between the aerosol optical depth measured by the Brewer and the simultaneous measurements taken with the CIMEL Sun photometer. The strong correlation between the measurements of aerosol optical depth obtained from the two instruments demonstrates proper behavior of the instruments and of the methodologies used to retrieve the data. The variability of global UV‐B and UV‐A and total solar irradiance is investigated in relation to simultaneous spectral aerosol optical depth measured by the Brewer spectrophotometer and the CIMEL Sun photometer. The effect of the total ozone column on the correlation between the global UV‐B data and the aerosols is analyzed.
Intercomparison of total ozone observations at Fairbanks, Alaska, during POLARIS
Authors: Lloyd Steven , Swartz William H. , Kusterer Thomas , Anderson Donald , McElroy C. Thomas , Midwinter Clive , Hall Robert , Nassim Karen , Jaffe Daniel , Simpson William , Kelley Jennifer , Nicks Dennis , Griffin Dale , Johnson Bryan , Evans Robert , Quincy Dorothy , Oltmans Samuel , Newman Paul , McPeters Richard , Labow Gordon , Moy Leslie , Seftor Colin , Toon Geoffrey , Sen Bhaswar , Blavier Jean‐Francois
Published: September 2002

The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign in the spring and summer of 1997 is defined. Five independent data sets of total ozone observations at Fairbanks are presented, from the Earth Probe and ADEOS Total Ozone Mapping Spectrometer (TOMS) satellite instruments, balloon‐borne electrochemical concentration cell ozonesondes, and ground‐based (Brewer spectroradiometer, Dobson spectrophotometer, and the Jet Propulsion Laboratory MkIV infrared interferometer) instruments. The excellent agreement between different observational techniques lends confidence to the observed rate of summertime loss of total ozone at high latitudes. In addition, the small offsets between the data sets are well understood.
Determining the index of refraction of polar stratospheric clouds above Andoya (69°N) by combining size‐resolved concentration and optical scattering measurements
Authors: Deshler Terry , Nardi Bruno , Adriani Alberto , Cairo Francesco , Hansen Georg , Fierli Federico , Hauchecorne Alain , Pulvirenti Lucio
Published: September 2002

Observations within two polar stratospheric clouds (PSCs) of aerosol scattering and size‐resolved aerosol concentration were compared to infer the index of refraction of the PSC particles. The observations were completed in situ with balloon‐borne aerosol counters and a laser scatterometer (692, 830 nm) and remotely with an ozone (308, 353 nm) and Rayleigh (532, 1064 nm) lidar. A Monte Carlo analysis, accounting for the errors of the individual measurements, indicates the comparison method has a precision of ±0.03 for an index of refraction range of 1.30–1.60. Measurements from all instruments were obtained in one PSC with relatively little vertical or horizontal structure. The comparison suggested that the index of refraction of the PSC particles was near 1.47±0.01 in the nondepolarizing region of the cloud and 1.52–1.56±0.04 in the depolarizing region. These values were consistent for the observations at 308, 353, 692, and 830 nm. The comparisons with the Rayleigh lidar were not as consistent. Aerosol volumes inferred from the particle measurements agree closely with volumes expected for liquid ternary aerosol (LTA) at the base of the cloud, with nitric acid trihydrate (NAT) above 23 km, in the depolarizing region, and with both LTA and NAT in the bulk of the nondepolarizing portion of the cloud. A much more limited set of measurements was obtained in a second PSC with strong vertical structure, evident in the temperature and aerosol profiles. Comparisons in this cloud were difficult because of the inherent problems in comparing in situ and remote measurements in clouds with strong vertical and horizontal structure. In this PSC the comparisons of in situ aerosol size distribution and remote aerosol scattering did not converge to a clear index of refraction.
Threshold friction velocities for large pebble gravel beds
Authors: Batt R. G. , Peabody S. A.
Published: September 2002

Experiments have been performed in a high‐speed wind tunnel to determine threshold friction velocities for the onset of continuous particle motion (the fluid threshold velocity per Bagnold [1941]) for two types of gravel soil beds. The 50% finer‐by‐weight particle diameter
for the fine gravel sample was ∼6.35 mm and for the coarse gravel bed was 42 mm. Measured threshold friction velocities (u*t), as determined with rake‐mounted Pitot‐static probes, were 3.1 and 9.9 m/s for the fine and coarse gravel soils, respectively. Corresponding free‐stream threshold velocities (u∞t) were 41.8 and 94.3 m/s. Summarized results from the current measurements and other natural soil experiments [Gillette, 1978, 1988] vary approximately with the square root of particle diameter, in agreement with the prediction of Bagnold [1941] but exceed his results by a factor of 2–3.
Canopy fluxes of 2‐methyl‐3‐buten‐2‐ol over a ponderosa pine forest by relaxed eddy accumulation: Field data and model comparison
Authors: Baker Bradly , Guenther Alex , Greenberg James , Goldstein Allen , Fall Ray
Published: September 2002

Canopy level flux measurements of 2‐methyl‐3‐buten‐2‐ol (MBO) were made over a 7 year old ponderosa pine plantation in the Sierra Nevada range of California using the relaxed eddy accumulation method. Fluxes peaked in late afternoon (1600 LT), at an average rate of 1.11 mg C m−2 h−1. This corresponds to the time of peak temperatures. MBO fluxes were strongly correlated with both temperature and photosynthetically active radiation (PAR). Model calculations of MBO flux from this site using a model developed for isoprene provide an accurate simulation of the diurnal emissions pattern but overestimate the measured flux by a factor of 2. Discrepancies between the measurements and the model predictions are likely due to the uncertainties in choosing model inputs. These results indicate a significant flux of reactive carbon in the western United States where MBO‐emitting pines are the dominant forest vegetation.
Characterization of fire regimes in the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) Study Area
Authors: Nielsen T. T.
Published: September 2002

Temporal and spatial distribution patterns of detected fire affected pixels within the Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) window from 1994 to 1997 were analyzed, and key parameters for describing the fire regime for each season were determined. The parameters were three temporal parameters and five spatial parameters. The spatial parameters were shown to be highly correlated, while the temporal parameters only show low correlation coefficients. The spatial parameters were transformed into the first principal component which was used for subsequent analyses. The temporal parameters were used in a decision tree classification to classify fire regimes, based on the onset of the season, the steepness of a hypothetical logistical curve, and the length of the fire season. This classification resulted in three generic fire regime types; rapid increase regime, late fire regime, and prolonged fire regime. By a combination of temporal generic fire regime types with the first principal component, representing most of the variance in the spatial parameters, the fire regimes of the EXPRESSO window were classified into 12 different regimes. These fire regimes were found to be correlated to the general vegetation types, and it was found that, even though interannual fluctuations in the fire regime existed, the major part of the EXPRESSO window is affected by low and average intensity rapidly increasing fire regimes and that the late and prolonged fire regimes occur mainly in the southeastern parts of the EXPRESSO window. Furthermore, it is shown that the spatial extent of the fire regimes is controlled by vegetation zone characteristics.
Parameterization of the scattering and absorption properties of individual ice crystals
Authors: Yang Ping , Liou K. N. , Wyser Klaus , Mitchell David
Published: September 2002

We present parameterizations of the single‐scattering properties for individual ice crystals of various habits based on the results computed from the accurate light scattering calculations. The projected area, volume, and single‐scattering properties of ice crystals with various shapes and sizes are computed for 56 narrow spectral bands covering 0.2–5 μm. The ice crystal habits considered in this study are hexagonal plates, solid and hollow columns, planar and spatial bullet rosettes, and aggregates that are commonly observed in cirrus clouds. Using the observational relationships between the aspect ratios and the sizes of ice crystals, we can define the three‐dimensional structure of these ice crystal habits with respect to their maximum dimensions for light scattering calculations. The volume and projected area of ice crystals, expressed in terms of the diameters of the corresponding equivalent spheres, are first parameterized by employing the ice crystal maximum dimensions. Further, various analytical expressions as functions of the effective dimensions of ice crystals have been developed to parameterize the extinction and absorption efficiencies, asymmetry factor, and the truncation of the forward peak energy in the phase function. The present parameterization scheme provides an efficient approach to obtain the basic scattering and absorption properties of nonspherical ice crystals.
Identification of extratropical two‐way troposphere‐stratosphere mixing based on CARIBIC measurements of O3, CO, and ultrafine particles
Authors: Zahn A. , Brenninkmeijer C. A. M. , Maiss M. , Scharffe D. H. , Crutzen P. J. , Hermann M. , Heintzenberg J. , Wiedensohler A. , Güsten H. , Heinrich G. , Fischer H. , Cuijpers J. W. M. , van Velthoven P. F. J.
Published: September 2002

Simultaneous measurements of O3, CO, and ultrafine aerosol particles (UFP), conducted on board of a Boeing 767‐ER passenger aircraft flying from Sri Lanka to Germany (project CARIBIC), are used to study two‐way cross‐tropopause mixing near a subtropical tropopause fold. On the equatorward side of the fold, downward mixing of stratospheric air into the upper troposphere is identified by enhanced concentrations of O3 and 14CO. Very high UFP number concentrations of up to 1.5×104 cm−3 (STP) were encountered inside the poleward half of the fold. This accumulation of small particles is explained by recent extensive aerosol nucleation, most likely triggered by the mixing of stratospheric air with tropospheric air injected into the fold. Further, nine particle formation events were observed outside the fold which are attributed to isolated cells of deep convection and to rising air parcels under cyclonic conditions that mix with surrounding air. In the upper troposphere O3 and CO were found to be correlated with high ΔO3/ΔCO ratios of 0.6 to 1.5. In the fold the correlation was strongly negative with ΔO3/ΔCO; = −3.5; but the high CO mixing ratios of 100 ppb at O3 mixing ratios of 250 ppb point to earlier injection of tropospheric air, in agreement with the UFP measurements.