bluuebunny/crossref_metadata_2025 huggingface dataset:
Isoprene fluxes measured by enclosure, relaxed eddy accumulation, surface layer gradient, mixed layer gradient, and mixed layer mass balance techniques
Authors: Guenther Alex , Baugh William , Davis Ken , Hampton Gary , Harley Peter , Klinger Lee , Vierling Lee , Zimmerman Patrick , Allwine Eugene , Dilts Steve , Lamb Brian , Westberg Hal , Baldocchi Dennis , Geron Chris , Pierce Thomas
Published: February 2004

Isoprene fluxes were estimated using eight different measurement techniques at a forested site near Oak Ridge, Tennessee, during July and August 1992. Fluxes from individual leaves and entire branches were estimated with four enclosure systems, including one system that controls leaf temperature and light. Variations in isoprene emission with changes in light, temperature, and canopy depth were investigated with leaf enclosure measurements. Representative emission rates for the dominant vegetation in the region were determined with branch enclosure measurements. Species from six tree genera had negligible isoprene emissions, while significant emissions were observed for Quercus, Liquidambar, and Nyssa species. Above‐canopy isoprene fluxes were estimated with surface layer gradients and relaxed eddy accumulation measurements from a 44‐m tower. Midday net emission fluxes from the canopy were typically 3 to 5 mg C m−2 h−1, although net isoprene deposition fluxes of −0.2 to −2 mg C m−2 h−1 were occasionally observed in early morning and late afternoon. Above‐canopy CO2 fluxes estimated by eddy correlation using either an open path sensor or a closed path sensor agreed within ±5%. Relaxed eddy accumulation estimates of CO2 fluxes were within 15% of the eddy correlation estimates. Daytime isoprene mixing ratios in the mixed layer were investigated with a tethered balloon sampling system and ranged from 0.2 to 5 ppbv, averaging 0.8 ppbv. The isoprene mixing ratios in the mixed layer above the forested landscape were used to estimate isoprene fluxes of 2 to 8 mg C m−2 h−1 with mixed layer gradient and mixed layer mass balance techniques. Total foliar density and dominant tree species composition for an approximately 8100 km2 region were estimated using high‐resolution (30 m) satellite data with classifications supervised by ground measurements. A biogenic isoprene emission model used to compare flux measurements, ranging from leaf scale (10 cm2) to landscape scale (102 km2), indicated agreement to within ±25%, the uncertainty associated with these measurement techniques. Existing biogenic emission models use isoprene emission rate capacities that range from 14.7 to 70 μg C g−1 h−1 (leaf temperature of 30°C and photosynthetically active radiation of 1000 μmol m−2 s−1) for oak foliage. An isoprene emission rate capacity of 100 μg C g−1 h−1 for oaks in this region is more realistic and is recommended, based on these measurements.
Observations of the quasi‐two‐day wave in the middle and lower atmosphere over Christmas Island
Authors: Palo S. E. , Avery S. K.
Published: February 2004

We present analyses of data collected at Christmas Island in the mesosphere and lower thermosphere using a meteor echo detection and collection system (MEDAC) in addition to data collected in the troposphere and lower stratosphere using a clear air stratosphere/troposphere (ST) radar. These data, which provide 5 years of semicontinuous horizontal wind velocity estimates, are analyzed to determine the temporal structure of the quasi‐two‐day wave in the middle and lower atmosphere. Our results show the presence of strong wave activity in the middle atmosphere during late January and early February as well as a secondary maxima in June/August for each of the years we have data. However, the magnitude and duration of the wave events observed in late January and early February exhibit interannual variability.
Cloud feedback in atmospheric general circulation models: An update
Authors: Cess R. D. , Zhang M. H. , Ingram W. J. , Potter G. L. , Alekseev V. , Barker H. W. , Cohen‐Solal E. , Colman R. A. , Dazlich D. A. , Del Genio A. D. , Dix M. R. , Dymnikov V. , Esch M. , Fowler L. D. , Fraser J. R. , Galin V. , Gates W. L. , Hack J. J. , Kiehl J. T. , Le Treut H. , Lo K. K.‐W. , McAvaney B. J. , Meleshko V. P. , Morcrette J.‐J. , Randall D. A. , Roeckner E. , Royer J.‐F. , Schlesinger M. E. , Sporyshev P. V. , Timbal B. , Volodin E. M. , Taylor K. E. , Wang W. , Wetherald R. T.
Published: February 2004

Six years ago, we compared the climate sensitivity of 19 atmospheric general circulation models and found a roughly threefold variation among the models; most of this variation was attributed to differences in the models' depictions of cloud feedback. In an update of this comparison, current models showed considerably smaller differences in net cloud feedback, with most producing modest values. There are, however, substantial differences in the feedback components, indicating that the models still have physical disagreements.
A one‐dimensional lightning parameterization
Authors: Solomon Robert , Baker Marcia
Published: February 2004

In an extension of earlier work [Helsdon et al., 1992; Mazur, 1989], we have developed a one‐dimensional, electrostatic representation of the electric field change and charge redistribution in a cloud subsequent to a lightning flash. In this parameterization, these effects are determined by the properties of the vertical electric field. We assume lightning is initiated when the electric field magnitude reaches a prescribed breakdown value at some point. The lightning channel is represented by a prolate spheroidal conductor of a specified radius, whose length is determined by a crude energy criterion. Charge is induced on the channel surface and this charge modifies the surrounding electric field. We have applied this parameterization to the vertical electric field resulting from an idealized charge sounding to explore the features of the charge profile that determine whether lightning is likely to be intracloud or cloud to ground. We discuss the results of this analysis in terms of lightning patterns observed climatologically and the evolution of lightning patterns in a thunderstorm during its life cycle.
Vertical transport of tropospheric aerosols as indicated by 7Be and 210Pb in a chemical tracer model
Authors: Koch Dorothy M. , Jacob Daniel J. , Graustein William C.
Published: February 2004

We use the natural radionuclides 7Be and 210Pb as aerosol tracers in a three‐dimensional chemical tracer model (based on the Goddard Institute for Space Studies general circulation model (GCM) 2) in order to study aerosol transport and removal in the troposphere. Beryllium 7, produced in the upper troposphere and stratosphere by cosmic rays, and 210Pb, a decay product of soil‐derived 222Rn, are tracers of upper and lower tropospheric aerosols, respectively. Their source regions make them particularly suitable for the study of vertical transport processes. Both tracers are removed from the troposphere primarily by precipitation and are useful for testing scavenging parameterizations. In particular, model convection must properly transport and scavenge both ascending 210Pb and descending 7Be. The ratio 7Be/210Pb cancels most model errors associated with precipitation and serves as an indicator of vertical transport. We show that over land the annual average 7Be/210Pb ratio for surface concentrations and deposition fluxes vary little globally. In contrast, the seasonal variability of the 7Be/210Pb concentration ratio over continents is quite large; the ratio peaks in summer when convective activity is maximum. The model overestimates 7Be in the tropics, a problem which we relate to flaws in the GCM parameterization of wet convection (excessive convective mass fluxes and no allowance for entrainment). The residence time of tropospheric 7Be calculated by the model is 23 days, in contrast with a value of about 9 days calculated for 210Pb, reflecting the high‐altitude versus low‐altitude source regions of these two tracers.
Airborne measurements of the photolysis frequency of NO2
Authors: Volz‐Thomas Andreas , Lerner Ansgar , Pätz Hans‐Werner , Schultz Martin , McKenna Daniel S. , Schmitt Rainer , Madronich Sasha , Röth Ernst Peter
Published: February 2004

A set of photoelectric detectors for airborne measurements of the photolysis frequency of NO2, i.e., JNO2, was developed and integrated aboard the research aircraft Hercules C‐130 operated by the U.K. Meteorological Office. The instrument consists of two separate sensors, each of which provides an isotropic response over a solid angle of 2π steradian (sr). The sensors are mounted on top and below the aircraft, respectively, to obtain a field of view of 4π sr, and permit the discrimination of the upwelling and downwelling components of the actinic flux. From experimental tests and model calculations it is demonstrated that small differences between the spectral sensitivity of the sensors and the spectral response of JNO2 can lead to significant errors in the determination of JNO2, especially under cloudy conditions. We present correction factors for clear sky conditions and suggest the use of a new filter combination in the sensors which requires only small corrections and provides acceptable accuracy, even under cloudy conditions. A climatology of JNO2 values is presented from a series of flights made in 1993 at latitudes of 36°–59°N. For clear sky conditions and solar zenith angles of 33°–35°, JNO2 was 8.3 × 10−3 s−1 at sea level and increased with altitude to values of 13 × 10−3 s−1 at 7.5 km altitude. Above clouds, JNO2 reached maximum values of 24 × 10−3 s−1, and peak values of 29 × 10−3 s−1 were observed for very short periods in the uppermost layers of clouds. Enhancement of the actinic flux due to light scattered from clouds was also observed at altitudes below 0.5 km. Comparison of the clear sky data with predictions from different radiative transfer models reveals the best agreement for models of higher angular resolution. The Delta Eddington method underpredicts the measurements significantly, whereas the JNO2 values predicted by the discrete ordinate method and multidirectional model are only about 5% smaller than our measurements, a difference that is within the experimental uncertainties.
Climatic‐scale space‐time variability of tropical precipitation
Authors: Gershunov Alexander , Michaelsen Joel
Published: February 2004

More than 15 years of monthly microwave sounding unit rainfall data over the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5° × 2.5° grid square. Regions of highest rainfall variability tend to be characterized by a powerful annual cycle. The semiannual cycle is generally a trivial component of the seasonal cycle, except in some regions where either the mean climatological precipitation is low or where the total seasonal cycle is weak. An interesting exception, in this respect, is a band of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highest climatological mean rainfall are characterized by weak seasonality but strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via principal component analysis in the time domain. The two dominant modes together describe precipitation variability associated with the El Niño‐Southern Oscillation: they outline the evolution of warm‐ and cold‐event precipitation anomalies and contrast the intense 1982–1983 warm event with the moderate events of 1986–1987 and 1992–1993. These two modes display oscillations with predominantly quasi‐biennial and ∼5‐year periods. Another coherent mode summarizes intraseasonal variability which, although inadequately resolved by the monthly average rainfall data, displays typical signs of the 40‐ to 50‐day oscillation. All coherent modes, despite having much of their energy concentrated around rather different frequencies, show signs of interaction.
Reconstruction and frequency estimates of extreme daily areal precipitation
Authors: Skaugen Thomas , Creutin Jean‐Dominique , Gottschalk Lars
Published: February 2004

The analysis and modeling of extreme areal precipitation is carried out focusing the attention on extreme areal precipitation events rather than extreme point precipitation. The data is daily precipitation values from southern Norway. By investigating the spatial coverage of the fractional area (fraction of the considered catchment) where it rains over a certain intensity threshold, it was found, for a range of rain intensities, that the ratio between two fractional areas of consecutive thresholds is independent of the actual value of the rain intensity and the size of the considered catchment. The mean areal precipitation is seen as the sum of the fractional thresholded rainy areas for all intensity thresholds. By the properties of independence of the consecutive area ratios, it is possible to simulate fractional thresholded rainy areas, and thus compute the mean areal rainfall. The mean and the standard deviation of simulated series of extreme areal precipitation for five catchments of different size (from 48000 to 468 km2) are in very good agreement with those of the observed. This is also the case for the observed and simulated values of the fractional thresholded rainy areas. The mean values of the spatial coverage of thresholded areas for mesoscale convective complexes in the United States are found to be well modeled by the proposed method.
The storm of October 21–22, 1994, over Greece: Observations and model results
Authors: Lagouvardos K. , Kotroni V. , Dobricic S. , Nickovic S. , Kallos G.
Published: February 2004

During October 21, 1994, a cold front passed over Greece. This frontal passage provoked catastrophic floods and there were many casualties. Eleven deaths were reported during this event, nine of them inside the Greater Athens Area. Significant damages occurred in transportation telecommunication and energy supply networks, especially in the eastern part of the country. This paper reports on the simulations of the observed storm conducted by two numerical models: the Colorado State University‐Regional Atmospheric Modelling System (CSU‐RAMS) and the η‐eta/National Meteorological Center (ETA/NMC) model. The intercomparison of results between a regional research‐oriented model (RAMS) with an operational model (ETA/NMC) permitted to explore the capabilities and limitations of each one of them. RAMS was operated in a nonhydrostatic mode using explicit microphysics and grid nesting (two nests with 40‐ and 10‐km horizontal grid interval) and provided results which compare favorably with observations, suggesting that the model can adequately represent the mesoscale structure of the system. ETA/NMC is a hydrostatic limited‐area model using parameterization of large‐scale and convective precipitation. It was operated with 25‐km horizontal resolution and it forecasted successfully the major characteristics of the system but failed in reproducing quantitatively the precipitation pattern at the mesoscale.
Atmospheric NO3: 5. Off‐axis measurements at sunrise: Estimates of tropospheric NO3 at 40°N
Authors: Weaver A. , Solomon S. , Sanders R. W. , Arpag K. , Miller H. L.
Published: February 2004

The sunrise column abundance of NO3 has been measured at Fritz Peak, Colorado (40°N, 105°W), using visible absorption spectroscopy with scattered sky light as the light source. Measurements were made in the “off‐axis” configuration, viewing the sky near the horizon. This is a convenient alternative to direct Moon measurements, and the tropospheric component of the total NO3 column can be measured with comparable signal‐to‐noise levels to the direct Moon data. The average tropospheric NO3 vertical column measured during August–October 1993 was about 4 × 1013 molecule cm−2. Assuming a layer 1 km thick in the lowest part of the troposphere, this corresponds to 4 × 108 molecule cm−3, or about 20 pptv. Under these conditions, the nighttime oxidation of isoprene by NO3 is comparable to the daytime oxidation by OH.
Infrared spectroscopic signatures of (NH4)2SO4 aerosols
Authors: Weis David D. , Ewing George E.
Published: February 2004

Ammonium sulfate particles in air with average diameters ranging from 0.1 to 0.5‐μm have been generated by atomizing aqueous solutions of (NH4)2SO4 of various concentrations at ambient temperatures and pressures. The infrared spectra from 4000 to 600 cm−1 of the resulting aerosols have been investigated. This spectral region has allowed us to study the four infrared‐active vibrational modes of this salt: ν3(NH4+), ν4(NH4+), ν3(SO42−), and ν4(SO42−). The frequencies of these modes are similar to published results obtained from infrared studies of the single crystal but are displaced to higher wavenumbers. Depending on relative humidity, the aerosol particles are crystalline or supersaturated aqueous droplets. These phase identifications are possible because liquid water absorption features are found in the droplets but not in the crystals. Extensive Mie theory calculations have been performed for spheres of diameters ranging from 0.1‐μm to 2.0‐μm to explore frequency shifts and the relative contributions to extinction of scattering and absorption with particle size. We show that, for the smaller particles, the molecular cross section in the ν3(SO42−) region can be used to determine the number of (NH4)2SO4 molecules in an aerosol sample. The (small) frequency shifts in this region provide information on the aerosol particle size. A Mie theory calculation of extinction for a model polydisperse aerosol, believed to approximate that of an experimental aerosol, gives reasonable agreement with the observed spectrum. While calculated band centers of the four modes are within 1% of those observed, values of extinction can differ by as much as 50%. We discuss possible reasons for the discrepancies. Spectroscopic changes observed for an aerosol as the particles settle are discussed in terms of kinetic models and Mie theory. We discuss the potential of spectroscopic signatures of tropospheric (NH4)2SO4 aerosols for the characterization of their size, morphology, phase, and composition. Finally, we propose a field experiment to measure sulfate aerosol in the arctic troposphere.
Effects of a polydisperse cloud on tropospheric chemistry
Authors: Audiffren N. , Chaumerliac N. , Renard M.
Published: February 2004

Effects of a polydisperse cloud on tropospheric chemistry have been studied in the framework of a two‐dimensional model where dynamical, microphysical, and chemical processes are fully interactive. The chemical module describes the tropospheric photochemistry of ozone precursors in both gaseous and aqueous phases for a remote atmosphere. Impacts of the cloud polydisperse feature have been obtained by comparing the results in the case of a monodisperse cloud created under the same meteorological conditions. The [NO]/[NO2] ratio decreases more sharply in the case of the polydisperse cloud. The partitioning of the most soluble species does not follow the Henry's law equilibrium except in the middle of the cloud. This result has implications for airborne measurements made within clouds. Deviations from Henry's law found in samples are usually explained only by the effect of variations of the liquid water content with time, assuming that no real deviations exist in the real cloud. Here, it is shown that deviations from Henry's law equilibrium may exist even for clouds consisting of small droplets.
Are stratospheric aerosols the missing link between tropospheric vorticity and Earth transits of the heliospheric current sheet?
Authors: Kirkland Matt W. , Tinsley Brian A. , Hoeksema J. Todd
Published: February 2004

Evidence has accumulated for the past two decades demonstrating a correlation between Earth transits of the heliospheric current sheet (HCS) and changes in winter tropospheric vorticity. These correlations persisted for a few years following the Agung and El Chichón volcanic eruptions, but were significantly weaker at other times. This suggests that the missing link in a physical mechanism explaining the correlation may involve volcanic aerosols and their effect on cloud microphysics, via atmospheric electricity. An analysis of 500‐mbar northern hemispheric vorticity for the 1991–1994 winter periods following the Pinatubo eruption shows a similar correlation between tropospheric vorticity and Earth transits of the HCS, supporting the previous interpretation.
HO2/OH and RO2/HO2 ratios during the Tropospheric OH Photochemistry Experiment: Measurement and theory
Authors: Stevens P. S. , Mather J. H. , Brune W. H. , Eisele F. , Tanner D. , Jefferson A. , Cantrell C. , Shetter R. , Sewall S. , Fried A. , Henry B. , Williams E. , Baumann K. , Goldan P. , Kuster W.
Published: February 2004

Ambient concentrations of the hydroxyl (OH), hydroperoxyl (HO2), and total peroxy (ΣRO2) radicals were measured as part of the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, during August and September of 1993. OH radicals were measured using ion‐assisted mass spectroscopy and low‐pressure laser‐induced fluorescence (LIF) detection techniques. HO2 was measured using chemical conversion and LIF detection of OH. ΣRO2 radicals were measured using a chemical amplifier technique. The simultaneous measurements of these key species provide an opportunity to test our present understanding of the fast photochemistry of the troposphere. Measured HO2/OH ratios were typically between 15 and 80, and agreed well with predictions under conditions where NO mixing ratios were greater than 100 pptv. However, under clean conditions the measured ratio was a factor of 3–4 lower than predicted. The RO2/HO2 ratio was typically a factor of 4–15 larger than predicted by present theories of tropospheric chemistry. A steady state model was used in an attempt to analyze the discrepancies between the measured HO2/OH and RO2/HO2 ratios with present theories of hydrocarbon oxidation in the troposphere.
Ground‐based comparison of NO2, H2O, and O3 measured by long‐path and in situ techniques during the 1993 Tropospheric OH Photochemistry Experiment
Authors: Harder J. W. , Williams E. J. , Baumann K. , Fehsenfeld F. C.
Published: February 2004

One of the major objectives of the 1993 Tropospheric OH Photochemistry Experiment (August 8 to October 4, 1993) was to compare atmospheric measurements of the important trace gas species NO2, O3, and H2O by in situ techniques and by long‐path differential absorption spectroscopy. These compounds were measured over a 20.6 km folded optical path from Fritz Peak Observatory to Caribou Mine where a retroreflector array is located. In situ measurements were made at Idaho Hill, 0.7 km northwest of Caribou Mine and 25 km west of Boulder, Colorado. NO2 was determined at Idaho Hill by photolysis followed by NO chemiluminescence. O3 and H2O were measured at this site using calibrated commercial instruments. Using fractional differences ([long path ‐ Idaho Hull]/Idaho Hill) as a measure of agreement, overall agreement for ozone and water are +3±7% and −4±17%, respectively. For NO2 the comparison is complicated by the influence of strong sources and large ambient variability during the measurements. During times of clean westerly flow, fractional differences of +30±110% were observed. Further analysis of the data to evaluate these biases show that for mixing ratios greater than 300 pptv (well above the 1σ detection limit of 30–45 pptv) the two methods differ by no more than 10%, a value well within the combined uncertainties of the measurement techniques.
Interannual variations in Antarctic precipitation related to El Niño‐Southern Oscillation
Authors: Cullather Richard I. , Bromwich David H. , Van Woert Michael L.
Published: February 2004

The accurate estimation of Antarctic precipitation variability is an essential component in understanding global sea level fluctuations; direct measurement techniques, however, are replete with practical difficulties. In this study, net precipitation (precipitation minus sublimation) for the Antarctic continent is computed for 1980–1994 using operational numerical analyses obtained from the ECMWF (European Centre for Medium‐Range Weather Forecasts). The resulting estimations reveal a strong interannual variability for the Antarctic continent, implying a ±1.2 − 1.5 mm yr−1 maximum range in the Antarctic eustatic change contribution. In particular, variability for the South Pacific sector (120°W–180°W) is shown to be correlated with the El Niño‐Southern Oscillation (ENSO) phenomenon for 1980–1990. The relation becomes anticorrelated after 1990, associated with a strong East Antarctic ridging pattern that coincides with the start of the prolonged series of warm events of the early 1990s. This result is relevant to other studies relating ENSO variability to high southern latitudes, and a more elaborate picture of this teleconnection pattern is presented. Comparisons of sea level pressure values using available ship observations show good agreement and offer a confirmation of the analyses in this data‐sparse region. Additionally, a comparison of results with values obtained from the precipitation fields of the NCEP/NCAR (NCEP: National Centers for Environmental Prediction; NCAR: National Center for Atmospheric Research) reanalysis project are discussed.
The Southern Tropical Atlantic Region Experiment (STARE): Transport and Atmospheric Chemistry near the Equator‐Atlantic (TRACE A) and Southern African Fire‐Atmosphere Research Initiative (SAFARI): An introduction
Authors: Andreae Meinrat O. , Fishman Jack , Lindesay Janette
Published: February 2004

In November 1988 some 50 atmospheric scientists met at Dookie College, a small campus in the agricultural lands of Victoria, Australia, to map out the scientific goals of the International Global Atmospheric Chemistry (IGAC) Program, which was to become one of the first operational Core Projects of the International Geosphere‐Biosphere Program (IGBP). They identified the tropical regions as one of the priority areas for future international, coordinated research in atmospheric chemistry because of the vast biological activity in the tropics, with a correspondingly large potential for biogenic emissions, and the rapidly growing human populations and resulting land use change in these regions. In view of the prominent role that biomass burning plays in the tropics as a source of atmospheric pollutants and of the important ecological functions of vegetation fires in the tropics, the scientists at Dookie created the Biomass Burning Experiment (BIBEX) with the goals of characterizing the fluxes of gases and aerosols from biomass burning to the global atmosphere and assessing the consequences of pyrogenic emissions on chemical and physical climate.

The southern tropical Atlantic region, defined here as the region containing the Amazon basin, the tropical South Atlantic, and southern Africa, was the obvious first focus of research for this project. Large tropical forest and savanna fires had been known to occur here every year. In addition, observations from satellites and from the space shuttle had shown high levels of tropospheric ozone and carbon monoxide to be present over this region every year in the August‐to‐October period. Results from previous campaigns (ABLE 2A, CITE 3, DECAFE 88) also suggested a widespread impact of vegetation fires on both continents on the trace gas and aerosol content of the troposphere in this region.
Multifractal analysis and modeling of rainfall and river flows and scaling, causal transfer functions
Authors: Tessier Yves , Lovejoy Shaun , Hubert Pierre , Schertzer Daniel , Pecknold Sean
Published: February 2004

River flows have been known to be scaling for over 40 years and scaling notions have developed rapidly since the 1980s. Using the framework of universal multifractals and time series of rainfall and river runoff for 30 French catchments (basin sizes of 40 km2 to 200 km2) from 1 day to 30 years, we quantify types and extent of the scaling regimes. For both flow and rain series, we observed a scale break at roughly 16 days, which we associate with the “synoptic maximum”; the time scale of structures of planetary spatial extent. For the two scaling regimes in both series, we estimate the universal multifractal parameters as well as the critical exponents associated with multifractal phase transitions. Using these exponents, we perform (causal) multifractal time series simulations and show how a simple (linear) scaling transfer function can be used to relate the low‐frequency rainfall series to the corresponding river flow series. The high‐frequency regime requires nonlinear transforms.
Apparent solar cycle variations of upper stratospheric ozone and temperature: Latitude and seasonal dependences
Authors: McCormack J. P. , Hood L. L.
Published: February 2004

Although only 15 years of continuous global satellite data are available, existing measurements are consistent with a significant, in‐phase solar cycle variation of upper stratospheric ozone and temperature. Here we investigate the latitude and seasonal dependences of this variation using 15 years (1979–1993) of combined solar backscattered ultraviolet (SBUV) and SBUV/2 ozone profile measurements and 14 years (1980–1995) of National Meteorological Center (NMC) temperature analyses. These dependences are estimated by applying a multiple regression statistical model to monthly zonal mean time series extending from 60°S to 60°N latitude and approximately 25‐ to 50‐km altitude. Solar variability is represented in the statistical model by the Mg II index, a close proxy for solar UV variations at wavelengths that affect the photochemical production of ozone. In agreement with earlier studies, although an apparent solar cycle variation of both ozone and temperature is present in the upper stratosphere, no detectable solar cycle variation is present in the middle stratosphere (30‐ to 35‐km altitude). Ozone increases of 4 – 6% from solar minimum to solar maximum are found near 2 mbar at middle latitudes, with comparable amplitudes in both summer and winter hemispheres. While the presence of a large positive ozone response in the northern hemisphere during winter appears to be qualitatively consistent with theoretical predictions, the amplitude of the observed ozone variation is nearly twice as large as estimates based on current two‐dimensional models of the middle atmosphere. Temperature increases of 2.5 K are found near 1 mbar at low latitudes throughout the year, in addition to a seasonally varying temperature response of 2.5 – 3 K near 5 mbar at middle and high latitudes in the summer hemisphere. A one‐dimensional radiative model is used to calculate the expected change in equilibrium temperature associated with the observed solar cycle variability in the ozone profile under the assumption of fixed dynamical heating (FDH). Near the equatorial stratopause, the temperature response calculated with the FDH model is within 0.5 K of the observed response. At higher latitudes near 4 mbar, the FDH model cannot account for the large amplitudes and strong latitude dependences of the NMC‐derived temperature variation. This would suggest that changes in stratospheric dynamics over the 11‐year solar cycle may also be important for understanding the observed temperature and ozone variations.
Reactive uptake of NO3 on pure water and ionic solutions
Authors: Rudich Yinon , Talukdar Ranajit K. , Ravishankara A. R. , Fox R. W.
Published: February 2004

The reactive uptake coefficients (γ) of NO3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO2 were measured using a wetted‐wall flow‐tube setup combined with a long‐path absorption cell for the detection of NO3. The measured γ values were in the range 1.5 × 10−4 – 6 × 10−3, depending on the salt concentration in the water. By measuring γ as a function of salt concentration, HD𝓁0.5 for NO3 in water was determined to be (1.9 ± 0.4) × 10−3 M atm−1 cm s−0.5 at 273 K, assuming that the rate coefficient for the reaction of NO3 with Cl− is 2.76 × 106 M−1 s−1 at 273 K. The Henry's law coefficient for NO3 in water is estimated to be 0.6 ± 0.3 M atm−1, assuming that the diffusion coefficient of NO3 in water is D𝓁 = (1.0 ± 0.5) × 10−5 cm2 s−1. Uptake of NO3 on pure water is interpreted as due to reaction of NO3(aq) with H2O(𝓁) to produce HNO3 and OH in the liquid phase. Implications of these findings to the chemistry of NO3 in the troposphere are also discussed.
Activation of chlorine in sulfate aerosol as inferred from aircraft observations
Authors: Kawa S. R. , Newman P. A. , Lait L. R. , Schoeberl M. R. , Stimpfle R. M. , Kohn D. W. , Webster C. R. , May R. D. , Baumgardner D. , Dye J. E. , Wilson J. C. , Chan K. R. , Loewenstein M.
Published: February 2004

The abundance of reactive chlorine in the lower stratosphere is observed to increase sharply with exposure to temperatures below about 195 K, a temperature which is near the nitric acid trihydrate (NAT) equilibrium condensation point. Measurements from the NASA ER‐2 aircraft and a model of chemistry along back trajectories are used to examine the mechanism for this apparent temperature threshold in chlorine activation. The flight of July 28, 1994, from the Airborne Southern Hemisphere Ozone Experiment/Measurement s for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign in the southern hemisphere is studied because it provides measurements in an ongoing activation episode. Isentropic back trajectories from the aircraft sampling points indicate that the sampled air was cooling at the rate of 20 to 30 K d−1 to temperatures below the NAT condensation point and had not been below the NAT condensation point prior to that for at least 10 days. Hence the observed amount of active chlorine should be kinetically limited by the recent parcel temperatures. The measurements show enhanced ClO and decreased HCl at temperatures below 195 K even in the absence of significant polar stratospheric cloud particle surface area. The model of chemistry along back trajectories, constrained by the ER‐2 chemical and microphysical measurements, indicates that an initial inorganic chlorine (Cly) partitioning of approximately half HCl and half C1ONO2 is consistent with the observations. At this initial Cly, partitioning, the model using heterogeneous reactions on liquid sulfate and ternary solutions with the most recent sticking coefficient evaluations closely reproduces the latitude gradient and temperature threshold of chlorine activation observed in the data. The sudden increase in activation in the model is a result of the steep exponential temperature dependence of the sticking coefficient for HCl+ ClONO2 on liquid aqueous solutions.
An updated review of polar mesosphere summer echoes: Observation, theory, and their relationship to noctilucent clouds and subvisible aerosols
Authors: Cho John Y. N. , Röttger Jürgen
Published: February 2004

Peculiar atmospheric radar echoes from the high‐latitude summer mesosphere have spurred much research in recent years. The radar data (taken on frequency bands ranging from 2 to 1290 MHz) have been supplemented by measurements from an increasing arsenal of in situ (rocket borne) and remote sensing (satellites and lidars) instruments. Theories to explain these polar mesosphere summer echoes (PMSEs) have also proliferated. Although each theory is distinct and fundamentally different, they all share the feature of being dependent on the existence of electrically charged aerosols. It is therefore natural to assume that PMSEs are intimately linked to the other fascinating phenomenon of the cold summer mesopause, noctilucent clouds (NLCs), which are simply ice aerosols that are large enough to be seen by the naked eye. In this paper we critically examine both the data collected and the theories proposed, with a special focus on the relationship between PMSEs and NLCs.
Evaluation of global numerical weather prediction analyses and forecasts using DMSP special sensor microwave imager retrievals: 1. Satellite retrieval algorithm intercomparison study
Authors: Deblonde Godelieve , Wagneur Nicolas
Published: February 2004

Special sensor microwave/imager (SMM/I) satellite retrieval schemes covering a wide range of retrieval techniques were compared for environmental parameters that play a key role in the atmospheric hydrological cycle. These were column integrated water vapor (IWV), near‐surface oceanic wind speed (SWS), liquid water path (LWP), and surface rain rate (SRR). The objective was to evaluate the accuracy and limitations of the retrieval schemes on a monthly time‐scale (October 1993). The retrieval algorithms that were found to be the most accurate were used for the evaluation of an analysis/forecast system whose results are presented in part 2. The agreement between IWV retrieval algorithms was found to be good, in concordance with prior studies. However, depending on the range of IWV values considered, considerable biases were shown to exist. The choice of precipitation screen for which IWV retrievals were rejected had a non‐negligible impact on the monthly statistics. Saturation, which is often present in IWV retrieval algorithms, also influenced these statistics. The agreement between different SWS retrieval algorithms was not as good as for IWV. The largest differences between the Wentz [1994] and the Goodberlet et al. [1989] retrieval algorithms were found in the intertropical convergence zone (ITCZ) (largest difference) and in the midlatitudes. The impact of using different precipitation screens was also analyzed. SWS retrieved with the Goodberlet et al. algorithm was sensitive (in particular in the ITCZ) to the amount of data rejected. Among the LWP retrieval algorithms considered, two algorithms gave acceptable and similar results except for a constant factor (1.75). It is argued that the discrepancy of a constant factor was in part due to the lack of in situ measurements for calibration and the fact that the beam‐filling problem was taken into account empirically. For SRR retrievals, considerable disagreement was demonstrated between different retrieval algorithms. However, several algorithms gave retrievals that were in reasonable agreement with long‐term climatological data sets. This agreement was best in the tropics and decreased considerably in the midlatitudes where the retrieval capability of SRR for the SSM/I is not optimal.
Ground‐based search for X rays generated by thunderstorms and lightning
Authors: Suszcynsky David M. , Roussel‐Dupre Robert , Shaw Glenn
Published: February 2004

A series of ground‐based experiments have been performed to investigate gamma ray/X ray count rate increases that are associated with thunderstorm and lightning activity. NaI scintillation detectors were configured in a long‐timescale channel to monitor long‐term count rate variations (on timescales of 1–104 s) as a function of time, pressure, temperature, humidity, electric field, rainfall, and general thunderstorm activity and in a short‐timescale channel that was triggered by an electric field change meter to monitor correlations of individual counts (on timescales of 10−7–10−2 s) with particular phases of a lightning flash. Long‐timescale results typically show count rate increases of a few percent to as much as 100% above background levels during thunderstorm activity and are likely due primarily to gamma ray emissions from radon daughter‐ion decay as the daughter ions are precipitated to the ground by rainfall. The production of bremsstrahlung X rays by a thunderstorm runaway electron mechanism cannot be ruled out by this data, but the data indicate that if the mechanism is in operation, the bremsstrahlung flux at the ground is at best intermittent and/or barely discernible above background levels. Short‐timescale results do not show any evidence for the production of X rays by individual lightning flashes. However, these results are inconclusive since only 10 cases of lightning flashes within a 1 km distance were recorded. The results of the overall study are compared to previous studies which claim positive correlations of count rate increases with thunderstorm and lightning activity. They are also discussed in terms of runaway electron acceleration in thunderstorm electric fields and in terms of the runaway air breakdown mechanism for lightning initiation. A review of the physics of and previous studies of X ray emissions from thunderstorms and lightning is presented in the introduction.
Deducing droplet concentration and supersaturation in marine boundary layer clouds from surface aerosol measurements
Authors: Hoppel W. A. , Frick G. M. , Fitzgerald J. W.
Published: February 2004

Air parcels in the marine boundary layer (MBL) are mixed up through nonprecipitating clouds at the top of the MBL many times (on average) before they can be removed by precipitation scavenging. The equivalent dry size of the particles (cloud condensation nuclei, CCN) upon which droplets are formed increases because of liquid phase oxidation of soluble trace gases during the cloud processing. The observed separation of the submicron size distribution into an interstitial mode and cloud droplet residue mode makes it possible to infer the effective MBL cloud supersaturation and cloud droplet concentrations from surface measurements of the aerosol size distribution during periods when nonprecipitating MBL clouds are present in the back trajectory and the MBL is well mixed. The effect of particle composition on the accuracy of the inferred cloud supersaturations is evaluated. A large database of hundreds of size distributions taken on an Atlantic and a Pacific cruise and an airship flight off the Oregon coast are used to calculate the range of effective MBL cloud supersaturations and droplet concentrations encountered during these expeditions. The inferred droplet concentrations on the Pacific cruise were mostly in the 25 to 150 cm−3 range, whereas they were mostly in the 50 to 500 cm−3 range for the Atlantic cruise. The inferred effective supersaturation in the tropical MBL clouds was typically in the 0.15% to 0.25% range. Recent work of Tang and Munkelwitz [1994] would indicate that particles consisting of mixtures of ammonium sulfate and sulfuric acid would not have recrystalized in the differential mobility analyzer (DMA) within the range of relative humidities (45% to 60%) at which the DMA was operated. At these humidities the hydrated size can be as much as 20% greater than the dry size. Corrections for the hydrated size within the DMA at the time of measurement are included and are also used to correct previous measurements of the relationship between dry size and critical supersaturation made using the Naval Research Laboratory (NRL) DMA and NRL thermal gradient CCN counters.
Tidal generation of the phase‐locked 2‐day wave in the southern hemisphere summer by wave‐wave interactions
Authors: Walterscheid R. L. , Vincent R. A.
Published: February 2004

The quasi‐2‐day wave is a prominent dynamical feature of the mesopause region. The onset of the large‐amplitude summertime 2‐day wave in the southern hemisphere is characterized by a sudden increase in amplitude and by phase locking so that the wind maximum occurs at nearly the same local time from cycle to cycle. The phase locking suggests a linkage to a diurnally repeatable phenomenon. We suggest that the phase‐locked 2‐day wave is a nearly resonant wave forced parametrically by the zonally symmetric diurnal tides. Recent observations and modeling support significant tidal forcing. It may also be possible for the migrating tides to generate the 2‐day wave nonlinearly through a set of mutually reinforcing interactions. The process involves the self‐excitation of the 2‐day wave by an interaction with a diurnal zonal wavenumber 6 oscillation, itself generated nonlinearly in a two‐step process initiated by the interaction of the 2‐day wave and the migrating tides. Sudden onset, large amplitude, and phase locking are all consistent with the instability of a wave that has become resonantly tuned to a subharmonic frequency of the tides by the background wind system.
A three‐dimensional synthesis study of δ18O in atmospheric CO2: 1. Surface fluxes
Authors: Ciais Philippe , Denning A. Scott , Tans Pieter P. , Berry Joseph A. , Randall David A. , Collatz G. James , Sellers Piers J. , White James W. C. , Trolier Michael , Meijer Harro A. J. , Francey Roger J. , Monfray Patrick , Heimann Martin
Published: February 2004

The isotope 18O in CO2 is of particular interest in studying the global carbon cycle because it is sensitive to the processes by which the global land biosphere absorbs and respires CO2. Carbon dioxide and water exchange isotopically both in leaves and in soils, and the 18O character of atmospheric CO2 is strongly influenced by the land biota, which should constrain the gross primary productivity and total respiration of land ecosystems. In this study we calculate the global surface fluxes of 18O for vegetation and soils using the SiB2 biosphere model coupled with the Colorado State University general circulation model. This approach makes it possible to use physiological variables that are consistently weighted by the carbon assimilation rate and integrated through the vegetation canopy. We also calculate the air‐sea exchange of 18O and the isotopic character of fossil emissions and biomass burning. Global mean values of the isotopic exchange with each reservoir are used to close the global budget of 18O in CO2. Our results confirm the fact that the land biota exert a dominant control on the δ18O of the atmospheric reservoir. At the global scale, exchange with the canopy produces an isotopic enrichment of CO2, whereas exchange with soils has the opposite effect.
A reconsideration of the initial conditions used for stable water isotope models
Authors: Jouzel Jean , Koster Randal D.
Published: February 2004

Many of the “simple models” used to study and explain the global distributions of stable water isotopes assume that the initial isotope content of an air mass can be estimated from local conditions at the oceanic source through the application of a global‐scale closure equation. Using a general circulation model fitted with isotope tracer diagnostics, we show that the closure equation is generally invalid at the local scale, leading to a systematic bias in the modeled isotope contents. The quantitative results obtained in earlier simple model studies may need some adjustment in light of this bias, particularly if deuterium excess was used to characterize conditions prevailing at the evaporative source.