bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of the lower stratospheric age spectra derived from a general circulation model and two data assimilation systems
Authors: Schoeberl Mark R. , Douglass Anne R. , Zhu Zhengxin , Pawson Steven
Published: May 2003

We use kinematic and diabatic back trajectory calculations, driven by winds from a general circulation model (GCM) and two different data assimilation systems (DAS), to compute the age spectrum at three latitudes in the lower stratosphere. The age spectra are compared to chemical transport model (CTM) calculations, and the mean ages from all of these studies are compared to observations. The age spectra computed using the GCM winds show a reasonably isolated tropics, in good agreement with observations; however, the age spectra determined from the DAS differ from the GCM spectra. For the DAS diabatic trajectory calculations there is too much exchange between the tropics and midlatitudes. The age spectrum is thus too broad, and the tropical mean age is too old as a result of mixing older midlatitude air with tropical air. Likewise, the midlatitude mean age is too young because of the in‐mixing of tropical air. The DAS kinematic trajectory calculations show excessive vertical dispersion of parcels in addition to excessive exchange between the tropics and midlatitudes. Because air is moved rapidly to the troposphere from the vertical dispersion, the age spectrum is shifted toward the young side. The excessive vertical and meridional dispersion compensate in the kinematic case, giving a reasonable tropical mean age. The CTM calculation of the age spectrum using the DAS winds shows the same vertical and meridional dispersive characteristics of the kinematic trajectory calculation. These results suggest that the current DAS products will not give realistic trace gas distributions for long integrations; they also help explain why the extratropical mean ages determined in a number of previous DAS‐driven CTMs are too young compared with observations. Finally, we note that trajectory‐generated age spectra show significant age anomalies correlated with the seasonal cycles. These anomalies can be linked to year‐to‐year variations in the tropical heating rate. The anomalies are suppressed in the CTM spectra, suggesting that the CTM transport scheme is too diffusive.
Interaction of chemical heating and the diurnal tide in the mesosphere
Authors: Smith A. K. , Marsh D. R. , Szymczak A. C.
Published: May 2003

Much of the solar energy deposited into the upper part of the middle atmosphere is realized as heat as a consequence of exothermic chemical reactions. The heating varies with time primarily in response to variations in composition due to photochemistry and to vertical advection by tides and other processes. Diurnal variations in the heating can, in turn, cause modifications in the tidal amplitude and structure. Using a three‐dimensional chemical‐dynamical model, we show that the advection of atomic oxygen by tidal winds has a significant impact on the location, timing, and magnitude of the heating but that the feedback of this on the tidal amplitude is weak (less than 10%). The day/night cycle in chemical heating associated with photochemistry also contributes to the local amplification or damping of the tide. Tidal transport affects the overall composition and therefore also the total heating by solar energy. When tides are removed, the total global mean amount of solar heating (chemical heating plus direct solar heating) decreases by as much as 40% in the mesosphere.
Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite
Authors: Sioris Christopher E. , Haley Craig S. , McLinden Chris A. , von Savigny Christian , McDade Ian C. , McConnell Jack C. , Evans Wayne F. J. , Lloyd Nicholas D. , Llewellyn Edward J. , Chance Kelly V. , Kurosu Thomas P. , Murtagh Donal , Frisk Urban , Pfeilsticker Klaus , Bösch Hartmut , Weidner Frank , Strong Kimberly , Stegman Jacek , Mégie Gérard
Published: May 2003

Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo‐absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A “snapshot” meridional cross section between 40°N and 70°S is shown from observations during a fraction of an orbit.
Summary of the cloud chemistry modeling intercomparison: Photochemical box model simulation
Authors: Barth M. C. , Sillman S. , Hudman R. , Jacobson M. Z. , Kim C.‐H. , Monod A. , Liang J.
Published: May 2003

We report results of a cloud chemistry numerical modeling intercomparison, which shows good agreement among gas‐aqueous photochemistry box models that are being used in the community. For the case studied, cloud chemistry depleted concentrations of CH2O, CH3OOH, HNO3, and O3, while H2O2 (in the absence of sulfur chemistry), NO, and NO2 increased. Because parcels of air usually flow in and out of cloud in a matter of minutes rather than remain in cloud for an hour, an optional simulation was performed in which frequent brief cloud encounters were represented. Representing a cloud intermittently rather than continuously does not alter the total concentration of many of the species. However CH2O and HCOOH concentrations are decreased and increased, respectively, because of the timing of the CH2O production during clear‐sky intervals and its destruction during cloudy intervals. Further differences between a continuous cloud simulation and an intermittent cloud simulation are expected if pH is allowed to vary during the cloud periods. Simulating an intermittent cloud brought out the importance of using a chemistry time step that is a multiple of the cloud time step because deviations of results from a simulation in which the chemistry time step did not coincide with the appearance and disappearance of cloud were quite large. To better quantify the effect of cloud on HOx photochemistry, future investigations should include nonmethane hydrocarbon and sulfur chemistry. Future cloud chemistry modeling intercomparisons should bring in cloud physical and chemical measurements so that the models can be evaluated with observations.
Quantifying the magnitude of anomalous solar absorption
Authors: Ackerman Thomas P. , Flynn Donna M. , Marchand Roger T.
Published: May 2003

The data set from ARESE II, sponsored by the Atmospheric Radiation Measurement Program, provides a unique opportunity to understand solar absorption in the atmosphere because of the combination of three sets of broadband solar radiometers mounted on the Twin Otter aircraft and the ground based instruments at the Atmospheric Radiation Measurement (ARM) Southern Great Plains facility. In this study, we analyze the measurements taken on two clear‐sky days and three cloudy days and model the solar radiative transfer in each case with two different models. On the two clear days, the calculated and measured column absorptions agree to better than 10 W m−2, which is about 10% of the total column absorption. Because both the model fluxes and the individual radiometer measurements are accurate to no better than 10 W m−2, we conclude that the models and measurements are essentially in agreement. For the three cloudy days, the model calculations agree very well with each other and on 2 of the 3 days agree with the measurements to 20 W m−2 or less out of a total column absorption of more than 200 W m−2, which is again an agreement at better than 10%. On the third day, the model and measurements agree to either 8 or 14% depending on which value of surface albedo is used. Differences exceeding 10% represent a significant absorption difference between model and observations. In addition to the uncertainty in absorption due to surface albedo, we show that including aerosol with an optical depth similar to that found on clear days can reduce the difference between model and measurement by 5% or more. Thus we conclude that the ARESE II results are incompatible with previous studies reporting extreme anomalous absorption and can be modeled with our current understanding of radiative transfer.
Simultaneous observations of optical and electrical signals in altitude‐triggered negative lightning flashes
Authors: Chen Mingli , Watanabe Teiji , Takagi Nobuyuki , Du Yaping , Wang Daohong , Liu Xinsheng
Published: May 2003

This paper presents an analysis of the experimental data on five negative lightning flashes initiated using the altitude‐triggering technique in China. The data include highly time‐resolved optical images and electric fields measured 60 m and 1300 m from the lightning channel. The triggering technique involves the launching upward of a small rocket trailing a wire electrically floating. The data show that these 5 flashes have a similar chronological sequence of events, including a bidirectional leader system followed by a mini‐return stroke and a bidirectional discharge process. The bidirectional leader system consists of an upward positive leader initiated from the top of the wire and a downward negative stepped leader from the bottom, with the onset of the former prior to the latter by 3 to 8.3 ms. The downward negative stepped leader, having a step interval of 12–30 μs, appears to pause and resume several times while the upward positive leader extends forward continuously. With the downward negative stepped leader close to ground, a mini‐return stroke occurs between the ground and the bottom of the wire. The mini‐return stroke propagates upward with a speed of 1–2 × 108 m/s and emits intense light signals similar to a normal return stroke below the bottom of the wire. It becomes invisible after entering the bottom of the wire and appears again as a bright upward discharge from the top of the wire several microseconds later. This upper bright discharge ceases after propagating forward several hundred meters at a speed of 1.5–5.4 × 107 m/s. The cessation of the upper bright discharge is obviously associated with the disintegration of the wire at that moment. Right after the cessation of the upper bright discharge, a bidirectional discharge process starts from the bottom of the wire with its positively charged part having an upward speed of 3–10 × 105 m/s and its negatively charged part a downward speed of 2–2.6 × 105 m/s. Reflection of current waves at the bottom of the wire due to the explosion of the wire at that moment may be a major reason for the occurrence of this lower bidirectional discharge.
Maritime component in aerosol optical models derived from Aerosol Robotic Network data
Authors: Smirnov A. , Holben B. N. , Dubovik O. , Frouin R. , Eck T. F. , Slutsker I.
Published: May 2003

Aerosol optical properties above the oceans vary considerably, depending on contributions of major aerosol components, i.e., urban/industrial pollution, desert dust, biomass burning, and maritime. The optical characterization of these aerosols is fundamental to the parameterization of radiative forcing models as well as to the atmospheric correction of ocean color imagery. We present a model of the maritime aerosol component derived using Aerosol Robotic Network (AERONET) data from three island locations: Bermuda (Atlantic Ocean), Lanai, Hawaii (Pacific Ocean), and Kaashidhoo, Maldives (Indian Ocean). To retrieve the maritime component, we have considered the data set with aerosol optical depth at a wavelength 500 nm less than 0.15 and Angstrom parameter α less than 1. The inferred maritime component in the columnar size distribution, which was found to be very similar for the three study sites, is bimodal with a fine mode at an effective radius (reff) ∼ 0.11–0.14 μm and a coarse mode reff of ∼1.8–2.1 μm. The results are comparable with size distributions reported in the literature. The refractive index is spectrally independent and estimated to be 1.37‐0.001i (single‐scattering albedo is about 0.98), based on the single‐component homogenous particle composition assumption. Fractional contributions of the fine and coarse modes to the computed τa (500 nm) are within the range of τfine ∼ 0.03–0.05 and τcoarse ∼ 0.05–0.06 correspondingly. Angstrom parameters vary from ∼0.8 to 1.0 computed in the UV‐visible (340–670 nm) and from 0.4 to 0.5 estimated in the near IR (870–2130 nm) spectral ranges. Aerosol phase functions are very similar for all three sites considered. The maritime aerosol component presented in this paper can serve as a candidate model in atmospheric correction algorithms.
Mesospheric ozone response to changes in water vapor
Authors: Marsh Daniel , Smith Anne , Noble Erik
Published: May 2003

Observations made by the Upper Atmosphere Research Satellite (UARS) now extend in time for almost a decade and allow for an improved understanding of seasonal and interannual variations of middle atmospheric minor constituents. In particular, measurements of mesospheric ozone and water vapor made by the UARS Halogen Occultation Experiment (HALOE) reveal significant seasonal and decadal variability. At midlatitudes, the observed variation in water vapor is primarily an annual cycle that peaks in summertime driven by changes in the residual circulation. In contrast to earlier satellite observations, HALOE ozone sunset observations also have an annual cycle that is anticorrelated with the water vapor cycle. The anticorrelation is a consequence of water vapor photolysis producing hydrogen species that destroy ozone. However, HALOE sunrise observations of ozone show no seasonal cycle. Analysis of monthly anomalies between 1991 and 2001 show a long‐term water vapor increase of approximately 1%/year. The observed response of ozone depends on altitude and time of day and is strongly negative (up to −4%/year) for sunset observations around 80 km. Again, the response of ozone at sunrise is minimal. Sensitivity tests using a three‐dimensional global chemical transport model show good agreement with the observations and confirm that the observed decrease in sunset ozone is a response to increasing water vapor.
Antarctic polar stratospheric clouds under temperature perturbation by nonorographic inertia gravity waves observed by micropulse lidar at Syowa Station
Authors: Shibata Takashi , Sato Kaoru , Kobayashi Hiroshi , Yabuki Masanori , Shiobara Masataka
Published: May 2003

Type II polar stratospheric clouds (PSCs) were observed by micropulse lidar (MPL) at Syowa Station in the Antarctic on 30 June and on 1 July 2001. The vertical profiles of the PSCs had a wavy structure that was synchronized with the temperature fluctuations. A wave analysis using radiosonde data shows that the wavy fluctuations were associated with an inertia gravity wave that was not forced by ground topography, but probably by a spontaneous adjustment in association with synoptic‐scale wave‐breaking processes in the upper troposphere. It is suggested that the observed PSCs were generated under the low‐temperature conditions induced by these waves and that such gravity waves generated by spontaneous adjustment of large‐scale fields can be more important to the formation of PSC particles, in both the Antarctic and Arctic stratosphere, than topographically forced gravity waves, because the former are not fixed to the ground topography.
Annual cycle of global distributions of aerosol optical depth from integration of MODIS retrievals and GOCART model simulations
Authors: Yu Hongbin , Dickinson R. E. , Chin M. , Kaufman Y. J. , Holben B. N. , Geogdzhayev I. V. , Mishchenko M. I.
Published: May 2003

The Moderate‐Resolution Imaging Spectroradiometer (MODIS) instrument onboard the Earth Observing System (EOS) satellites provides an unprecedented opportunity to study aerosols from space with high accuracy and on a nearly global scale. However, difficulty with highly reflective arid and snow‐covered lands introduces significant gaps in global or regional coverage that must be filled by some other means. This study provides a complete global coverage of an annual cycle of aerosol optical depth by combining the MODIS retrievals and Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) simulations weighted with the uncertainties in each product. The assimilated aerosol optical depths over land are better correlated with the ground‐based Aerosol Robotic Network (AERONET) measurements than are either the MODIS retrievals or the GOCART simulations alone. The gaps in the MODIS retrievals are filled with values that are generally consistent with the AERONET aerosol climatology. The assimilated aerosol optical depths are in good agreement with the Advanced Very High Resolution Radiometer (AVHRR) aerosol climatology over the Atlantic and North Indian Oceans. In spring, large discrepancies between the MODIS retrievals in 2001 and the AVHRR climatology over the North Pacific are likely a result of extremely active transcontinental transport of Asian dust/pollutants to North America in the year 2001. Large model–satellite differences in the South Pacific and South Indian Oceans may be attributable to missing or underestimated sources in the model and/or cloud, whitecap, and glint contamination in satellite retrievals.
Effects of charge and electrostatic potential on lightning propagation
Authors: Coleman L. M. , Marshall T. C. , Stolzenburg M. , Hamlin T. , Krehbiel P. R. , Rison W. , Thomas R. J.
Published: May 2003

Three‐dimensional lightning mapping observations are compared to cloud charge structures and electric potential profiles inferred from balloon soundings of electric field in New Mexico mountain thunderstorms. For six individual intracloud and cloud‐to‐ground flashes and for a sequence of 36 flashes in one storm, the comparisons consistently show good agreement between the altitudes of horizontal lightning channels and the altitudes of electric potential extrema or wells. Lightning flashes appear to deposit charge of opposite polarity in relatively localized volumes within the preexisting lower positive, midlevel negative, and upper positive charge regions associated with the potential wells. The net effect of recurring lightning charge deposition at the approximate levels of potential extrema is to increase the complexity in the observed storm charge structure. The midlevel breakdown of both normal intracloud flashes and negative cloud‐to‐ground flashes is observed to be segregated by flash type into the upper and lower parts of the deep potential well associated with the midlevel negative charge. The segregation is consistent with perturbations observed in the bottom of the negative potential well due to embedded positive charge that was probably deposited by earlier flashes. It is also consistent with an expected tendency for vertical breakdown to begin branching horizontally before reaching the local potential minimum. The joint observations reconcile the apparent dichotomy between the complex charge structures often inferred from balloon soundings through storms and the simpler structures often inferred from lightning measurements.
Particle growth in urban and industrial plumes in Texas
Authors: Brock Charles A. , Trainer Michael , Ryerson Thomas B. , Neuman J. Andrew , Parrish David D. , Holloway John S. , Nicks Dennis K. , Frost Gregory J. , Hübler Gerhard , Fehsenfeld Fred C. , Wilson J. Charles , Reeves J. Michael , Lafleur Bernard G. , Hilbert Henrike , Atlas Elliot L. , Donnelly Stephen G. , Schauffler Sue M. , Stroud Verity R. , Wiedinmyer Christine
Published: May 2003

Particle size distributions and gas‐phase particle precursors and tracer species were measured aboard an aircraft in the plumes downwind from industrial and urban sources in the vicinity of Houston, TX during the daytime in late August and early September 2000. Plumes originating from the Parish gas‐fired and coal‐fired power plant, petrochemical industries along the Houston ship channel, the petrochemical facilities near the Gulf coast, and the urban center of Houston were studied. Most of the particle mass flux advected downwind of Houston came from the industries and electrical utilities at the periphery of the city rather than from sources in the urban core. In SO2‐rich plumes that did not contain elevated concentrations of volatile organic compounds (VOCs), particle volume increased with increasing plume oxidation (age) at a rate consistent with condensation and neutralization of the gas‐phase oxidation products of SO2. In plumes that were rich in both SO2 and VOCs, observed particle growth greatly exceeded that expected from SO2 oxidation, indicating the formation of organic particulate mass. In plumes that were enhanced in VOCs but not in SO2, and in the plume of the Houston urban center, no particle volume growth with increasing plume oxidation was detected. Since substantial particle volume growth was associated only with SO2‐rich plumes, these results suggest that photochemical oxidation of SO2 is the key process regulating particle mass growth in all the studied plumes in this region. However, uptake of organic matter probably contributes substantially to particle mass in petrochemical plumes rich in both SO2 and VOCs. Quantitative studies of particle formation and growth in photochemical systems containing nitrogen oxides (NOx = NO + NO2), VOCs, and SO2 are recommended to extend those previously made in NOx–VOC systems.
Efficiency of storm tracks an important climate parameter? The role of cloud radiative forcing in poleward heat transport
Authors: Weaver C. P.
Published: May 2003

Variability in poleward atmosphere‐ocean heat transport has been suggested as one driver of climate change on long timescales. The radiative impact of clouds, particularly extratropical clouds, is an important factor in determining the equilibrium value of this heat transport in the current climate. Therefore, significant changes in cloudiness and cloud radiative forcing could potentially force equally significant adjustments in total poleward heat transport, and thus in atmospheric and/or oceanic dynamics. At the same time, these extratropical clouds and their strong radiative impact are largely the result of the atmospheric eddies that make up the storm tracks. Coincidentally, these storm track eddies also accomplish most of the extratropical poleward heat transport. At high latitudes, the annual‐mean radiative cooling of these clouds is shown here to be of the same order as the atmospheric eddy heat transport convergence, and of opposite sign. Some potential implications of these points for climate change studies are discussed.
Global and seasonal variations of O3 and NO2 photodissociation rate coefficients
Authors: Mao Huiting , Wang Wei‐Chyung , Liang Xin‐Zhong , Talbot Robert W.
Published: May 2003

The global and seasonal variations in the photodissociation rate coefficients (J values) of the reactions of ozone forming O(1D) (J(O1D)) and nitrogen dioxide forming O(3P) (J(NO2)) were investigated systematically with a focus on the troposphere. The corresponding mechanisms were studied through sensitivity modeling experiments of temperature, surface albedo, aerosols, ozone column, and clouds. The one‐dimensional radiative transfer model PHODIS was applied with Total Ozone Mapping Spectrometer (TOMS) aerosol optical depth and surface reflectivity, observation‐based three‐dimensional ozone distributions, and other numerically simulated climate variables. Our results showed that under clear‐sky conditions, the zonal averaged J(O1D) and J(NO2) peaked in magnitude at 3 × 10−5 and 8 × 10−3 s−1, respectively, in the region of 30°S to 30°N with small latitudinal and vertical variations, and decreased precipitously at 30°/60° in the winter/summer hemisphere. J(O1D) exhibited a large meridional and vertical gradient as a result of high sensitivity to temperature and ozone. The predominant effect of surface albedo led to significant longitudinal variation in J(NO2) and a poleward increase beyond 60° of the summer hemisphere. Surface J(O1D) changes were regressed as a second‐degree polynomial function of changes in temperature (±20°K). The impact of low and middle clouds on J values was stronger than high clouds by a factor of 2 to 3 because of thick optical depth. In addition, the cloud‐induced changes in J values calculated by PHODIS differed by up to 20% from those parameterized in the Regional Acid Deposition Model (RADM) and Intermediate Model for the Global and Annual Evolution of Species (IMAGES). Regional photochemical model simulations showed that an arbitrary increase/decrease of 20% above/below the cloud base (750 m) in J(O1D) resulted in decreases up to 6% in the ozone concentration below 200 m at noon and late afternoon and increases ubiquitously elsewhere.
Scale invariance of regional wet and dry durations of rain fields: A diagnostic study
Authors: Pavlopoulos Harry , Gupta Vijay K.
Published: May 2003

Most of the recent work on rainfall data analyses and modeling has focused on either spatial or temporal variability. In this paper the structure of rainfall intermittence in space and time is investigated. Using a series of TOGA‐COARE radar scans converted to maps of pixel rain rate over a tropical oceanic region of size 240 × 240 km2, regionally scale‐invariant behavior of the probability distributions of wet and dry epoch durations is explored. Durations of wet and dry epochs are estimated by lengths of wet and dry spells, respectively, in time series of spatially averaged rain rate over sampled square subregions of spatial scales ranging from 120 km to 2 km. The investigation is based on sample quantiles and sample moments of the underlying marginal probability distributions, focusing on the behavior of their tails and their variation with respect to spatial scale. We find that sample tail quantiles and sample moments of wet durations exhibit power law multiscaling, while sample tail quantiles and sample moments of dry durations exhibit exponential multiscaling across the above range of scales. These findings provide new statistical diagnostic tools for validation of spatiotemporal models for rain fields.
Marine organohalogens in the atmosphere over the Atlantic and Southern Oceans
Authors: Carpenter L. J. , Liss P. S. , Penkett S. A.
Published: May 2003

Reactive halogen species (RHS) such as the halogen oxide radicals IO and BrO influence tropospheric oxidation processes in both polar and temperate regions. Oceanic photolabile halocarbons have been shown to be strong sources of RHS in midlatitudes. However, the global source strengths of these halocarbon precursors and the relative importance of the coastal and pelagic oceans on their concentrations are highly uncertain. Here we present atmospheric measurements of the reactive organic halogens CH3I, CH2I2, CH2ClI, CH2IBr, CHIBr2, CHBr3, CH2Br2, and CHBr2Cl made at Mace Head, Ireland, during the Particle Formation and Fate in the Coastal Environment campaign in September 1998 and at Cape Grim, Tasmania, during the Southern Ocean Atmospheric Photochemistry Experiment 2 campaign in January/February 1999. Mace Head is strongly influenced by local macroalgae, whereas Cape Grim, owing to its cliff‐top location, suffers much less impact from seaweeds. The very reactive halocarbons CH2I2, CH2IBr, and CHIBr2 observed at Mace Head were below detection limits at Cape Grim, although CH2ClI was detected at both locations. Mixing ratios of CH3I, CH2ClI, CHBr3, CHBr2Cl, and CH2Br2 at Cape Grim were on average 25–50% of those at Mace Head. Concentrations of the polybromomethanes correlated well at both sites. Using these correlations, we estimate molar source strengths of CHBr2Cl and CH2Br2 to be between 3 and 6% and between 15 and 25% of the global CHBr3 flux, respectively. These values fall within ranges estimated independently from concentration and lifetime data.
Downward migration of extratropical zonal wind anomalies
Authors: Plumb R. Alan , Semeniuk Kirill
Published: May 2003

We show (in confirmation of previous work) using one‐ and three‐dimensional models that extratropical zonal wind anomalies, produced by fluctuating Rossby wave forcing in the troposphere, appear first in the stratosphere, and migrate downward into the troposphere. By systematically eliminating wave reflection and “downward control” through an induced meridional circulation, it is shown that the downward migration is dependent on neither process. Rather, the mechanism appears to rely on local wave, mean‐flow interaction just as in the similar downward migration evident in the tropical quasi‐biennial oscillation. In particular, these results imply that the similar downward migration observed in the Arctic Oscillation should not be taken to indicate any controlling influence of the stratosphere on the troposphere.
Lidar measurements of tropospheric water vapor and aerosol profiles over southeastern Italy
Authors: De Tomasi F. , Perrone M. R.
Published: May 2003

A combined Rayleigh‐Raman lidar based on a XeF excimer laser (351 nm) has been used for the regular monitoring of tropospheric aerosols and water vapor profiles over southern Italy (40°20′N, 18°6′E) in the framework of the European Aerosol Research Lidar Network. Coincident measurements of vertical profiles of the aerosol backscattering ratio R(z), the aerosol extinction coefficient αλoaer(z), and the water vapor mixing ratio w(z) have been performed during several months to investigate the correlation of R(z) and αλoaer(z) with water vapor and to characterize the aerosol optical properties. A strong correlation has been found between the spatial and temporal evolution of R(z) and w(z) both in summer and in autumn regime. The experimental results have revealed that R(z) increases with w(z) and that the aerosol and water vapor burdens are lower in autumn than in the summer. The analysis of the data by using the 4‐day analytical backward trajectories has revealed that the dependence of αλoaer(z) and R(z) on w(z) is quite affected by the typical advection patterns over the lidar site. Aerosols providing a larger contribution to backscattering ratio and extinction coefficient as w(z) increases have been monitored when air masses from northern and eastern European countries were advected over Lecce. The findings in terms of the lidar ratio S have revealed that aerosols with average S values ranging from 50 to 63 sr have been monitored when air masses from northern and eastern Europe were advected over Lecce. On the contrary, average S values ranging from 48 to 74 have been retrieved for air masses advected from North Africa.
Trace gas and radical diurnal behavior in the marine boundary layer during INDOEX 1999
Authors: Burkert J. , Andrés‐Hernández M. D. , Reichert L. , Meyer‐Arnek J. , Doddridge B. , Dickerson R. R. , Mühle J. , Zahn A. , Carsey T. , Burrows J. P.
Published: May 2003

Selected trace gas mixing ratios (i.e., peroxy radicals (RO*2 = HO2 + ΣRO2), nonmethane hydrocarbons (NMHCs), O3, CO, HCHO, and NO) and photolysis rate coefficients of j(NO2) and j(O(1D)) were measured in the marine boundary layer (MBL) over the Indian Ocean. The measurements were performed during February, March, and April 1999 as a part of the Indian Ocean Experiment (INDOEX) on board the research vessel R/V Ronald H. Brown. During the campaign, air parcels having different origins and consequently variable compositions were encountered, but all air masses, including those heavily polluted with NMHCs and aerosols, were in the regime of rapid photochemical ozone destruction. The influence of aerosols on the photolysis frequencies was investigated by comparison of measurements and results from the radiative transfer model PHOTOST: the high optical depth (up to 0.6) and low single scattering albedo of the aerosol reduces the UV flux at the surface substantially downwind of India and Arabia causing, for instance, a reduction in j(O(1D)) by up to 40%. The diurnal behavior of the trace gases and parameters in the MBL has been investigated by using a time‐dependent zero‐dimensional chemical model. Significant differences between the diurnal behavior of RO*2 derived from the model and observed in measurements were identified. The measured HCHO concentrations differed from the model results and are best explained by some missing chemistry involving low amounts of Cl. Other possible processes describing these two effects are presented and discussed.
Retrieval of effective ice crystal size in the infrared: Sensitivity study and global measurements from TIROS‐N Operational Vertical Sounder
Authors: Rädel G. , Stubenrauch C. J. , Holz R. , Mitchell D. L.
Published: May 2003

The difference between cirrus emissivities at 8 and 11 μm is sensitive to the mean effective ice crystal size of the cirrus cloud, De. By using single scattering properties of ice crystals shaped as planar polycrystals, diameters of up to about 70 μm can be retrieved, instead of up to 45 μm assuming spheres or hexagonal columns. The method described in this article is used for a global determination of mean effective ice crystal sizes of cirrus clouds from TOVS satellite observations. A sensitivity study of the De retrieval to uncertainties in hypotheses on ice crystal shape, size distributions, and temperature profiles, as well as in vertical and horizontal cloud heterogeneities shows that uncertainties can be as large as 30%. However, the TOVS data set is one of few data sets which provides global and long‐term coverage. Having analyzed the years 1987–1991, it was found that measured effective ice crystal diameters De are stable from year to year. For 1990 a global median De of 53.5 μm was determined. Averages distinguishing ocean/land, season, and latitude lie between 23 μm in winter over Northern Hemisphere midlatitude land and 64 μm in the tropics. In general, larger Des are found in regions with higher atmospheric water vapor and for cirrus with a smaller effective emissivity.
POAM III ozone in the upper troposphere and lowermost stratosphere: Seasonal variability and comparisons to aircraft observations
Authors: Prados Ana I. , Nedoluha G. E. , Bevilacqua R. M. , Allen D. R. , Hoppel K. W. , Marenco A.
Published: May 2003

We present Polar Ozone and Aerosol Measurement (POAM) III observations in the upper troposphere and lower stratosphere (UT/LS) region. A statistical comparison is performed between POAM and coincident in situ ozone observations from the Measurement of Ozone and Water Vapor by Airbus In‐Service Aircraft (MOZAIC) program, with coincidences occurring over the 9 to 12 km altitude range. The comparison indicates that POAM ozone in the UT/LS is biased high by about 0.03–0.07 ppmv, in good agreement with the results of POAM/ozonesonde comparisons. We show that the seasonal variation of UT/LS ozone in the high latitude Northern Hemisphere, obtained from POAM data, agrees well with previous measurements and theoretical considerations. In the lowermost stratosphere, the ozone seasonal cycle is dominated by diabatic descent of ozone rich air from the midstratosphere during winter. In the tropopause region, changes in the ozone seasonal cycle reflect the transition from lower stratospheric to upper tropospheric air. At 350 K, POAM ozone and water vapor mixing ratios, and equivalent latitudes at POAM sampling locations indicate isentropic troposphere to stratosphere exchange occurs during summer and fall. Variations in ozone mixing ratios are also shown to be consistent with variations in isentropic mixing at 350 K.
Impact of leaf area index seasonality on the annual land surface evaporation in a global circulation model
Authors: van den Hurk Bart J. J. M. , Viterbo Pedro , Los Sietse O.
Published: May 2003

Numerical experiments with the global model of the European Centre for Medium‐Range Weather Forecasts (ECMWF) were devoted to the sensitivity of the modeled evaporation and precipitation to the vegetation leaf area index (LAI). The temporally static LAI distribution was replaced with a seasonally varying LAI, derived from normalized difference vegetation index (NDVI) archives. The seasonality of surface evaporation increased likewise, in combination with a response of increased precipitation seasonality over land. In a second set of experiments, the LAI estimates were perturbed by a noise term reflecting measurement accuracy and interannual variability. The resulting noise in evaporation and precipitation was compared to the noise intrinsically generated by the atmosphere. For periods and areas where evaporation forms a large term in the surface energy balance, the noise added to the LAI could be clearly discerned from the atmospheric noise, indicating that improved LAI estimation techniques can have a detectable impact on the surface evaporation calculated in the ECMWF global model.
Properties of polar stratospheric clouds observed by ILAS in early 1997
Authors: Lee K.‐M. , Park J. H. , Kim Y. , Choi W. , Cho H.‐K. , Massie S. T. , Sasano Y. , Yokota T.
Published: May 2003

The relative extinction coefficients for Arctic polar stratospheric clouds (PSCs) are determined using a transmittance‐ratio technique from spectral data observed in early 1997 by the Improved Limb Atmospheric Spectrometer (ILAS). The observed relative extinction coefficients are compared with theoretical coefficients calculated by the Mie theory for single mode lognormal size distributions of PSCs. The observed PSCs are then classified into three types: nitric acid droplets (HNO3/H2O), and nitric acid trihydrate (NAT) solid particles in the form of α‐NAT and β‐NAT. However, the definitive detection of α‐NAT is difficult due to uncertainties of atmospheric temperature measurements and laboratory determination of refractive indices.
Analysis of two independent methods for retrieving liquid water profiles in spring and summer Arctic boundary clouds
Authors: Löhnert U. , Feingold G. , Uttal T. , Frisch A. S. , Shupe M. D.
Published: May 2003

A large number of all‐liquid, nondrizzling stratus clouds (163 hours of measurements) were observed with a dual‐channel microwave radiometer and a colocated 35‐GHz cloud radar during the spring and summer months of the Surface Heat Budget of the Arctic Ocean (SHEBA) project. An algorithm developed by Frisch et al. [1995, 1998] to derive the liquid water content (LWC) is applied to these measurements assuming constant cloud drop number density and cloud drop size distribution breadth with height. A second algorithm developed by Löhnert et al. [2001] is specifically adapted for SHEBA clouds using a priori information from a large eddy simulation (LES) model initialized with summertime SHEBA radiosondes; about 50 soundings during nondrizzling, low‐level, all‐liquid water clouds are used. Using model‐derived drop size distributions, a relationship between simulated radar reflectivity (Z) and model LWC is derived as well as an a priori LWC profile. Once the theoretical error covariance matrix of the Z‐LWC relation is derived and the covariance matrix of the LWC profile is calculated, an optimal estimation method is applied to the SHEBA data. The Frisch et al. and Löhnert et al. methods are also applied to the LES model output, resulting in overall root‐mean‐square differences on the order of 30 to 60%. Both methods are sensitive to the assumed accuracies of the microwave‐radiometer‐derived LWP. When applied to LES model output, the Frisch et al. method shows a LWC overestimation in the lower parts of the cloud. These systematic errors are induced by the assumption of constant cloud number concentration with height.
Measurements of quantum yields of bromine atoms in the photolysis of bromoform from 266 to 324 nm
Authors: Bayes Kyle D. , Friedl Randall R. , Sander Stanley P. , Toohey Darin W.
Published: May 2003

The quantum yield for the formation of bromine atoms in the photolysis of bromoform, CHBr3, has been measured between 266 and 324 nm. For 303 to 306 nm the quantum yields are unity within the experimental uncertainty of the measurements. At longer wavelengths, where the bromoform cross sections decrease rapidly, an apparent trend to slightly lower quantum yields is probably the result of systematic and random errors or incorrect CHBr3 absorption cross sections. Support for a unit quantum yield for all wavelengths longer than 300 nm comes from the recent theoretical calculations of Peterson and Francisco. At 266 nm the bromine atom quantum yield is 0.76 (±0.03), indicating that at least one additional dissociation channel becomes important at shorter wavelengths. For modeling of the troposphere, it is recommended that a quantum yield of unity be used for wavelengths of 300 nm and longer.
Nitrogen oxides in the nocturnal boundary layer: Simultaneous in situ measurements of NO3, N2O5, NO2, NO, and O3
Authors: Brown Steven S. , Stark Harald , Ryerson Thomas B. , Williams Eric J. , Nicks Dennis K. , Trainer Michael , Fehsenfeld Frederick C. , Ravishankara A. R.
Published: May 2003

We report the first simultaneous in situ observation of a suite of compounds important in nocturnal nitrogen oxide chemistry. Measurements took place at a ground site near Boulder, Colorado, during the fall of 2001. Chemical measurements included NO3, N2O5, NO, NO2 and O3; meteorological data were also available. The concentrations of NO3 and N2O5 showed large dynamic ranges that were consistent with variations in NO2 and NO and with shifts in meteorological conditions at this site. The observed ratio of N2O5 to NO3 agreed with the ratio calculated from the measured NO2 concentration and the temperature‐dependent equilibrium constant. In addition, NO3 and N2O5 showed large short‐term variability that may indicate inhomogeneously mixed source and sink compounds and/or deposition at this ground‐based measurement site. Finally, N2O5 reached a peak concentration of nearly 3 ppbv under polluted conditions and accounted for an appreciable fraction of the total concentration of measured nitrogen oxide species.
Nonmethane Hydrocarbon Intercomparison Experiment (NOMHICE): Task 4, ambient air
Authors: Apel E. C. , Calvert J. G. , Gilpin T. M. , Fehsenfeld F. , Lonneman W. A.
Published: May 2003

The Nonmethane Hydrocarbon Intercomparison Experiment (NOMHICE) was designed to assess the accuracy and comparability of nonmethane hydrocarbon (NMHC) measurements from research groups around the globe. This is being accomplished by conducting a series of intercomparisons, called Tasks, of prepared mixtures or collected ambient air. This paper presents results for an ambient air challenge sample as part of the fourth NOMHICE installment (Task 4). Twenty‐three laboratories participated in Task 4, and 30 overall analytical results are compared. The air sample provided a wide dynamic range of mixing ratios (parts per trillion by volume (pptv) to parts per billion by volume (ppbv)) as well as large number of compounds (>100). Coelutions of NMHCs with other volatile organic compounds (VOCs) such as oxygenated VOCs are shown to make the analysis of whole air samples more challenging than for prepared standard mixtures. Individual canisters containing the air sample were prepared and analyzed by the NOMHICE group at the National Center for Atmospheric Research (NCAR‐NOMHICE), sent to participants for analysis, and reanalyzed upon return to NCAR‐NOMHICE. The mixing ratio of propylene increased by 10% (14 pptv) in the canisters with time, and some of the less volatile compounds decreased in the canisters with time; however, the majority of the compounds were stable throughout the experiment. Participants were asked to identify and quantify as many compounds as possible with their analytical techniques and to submit their results to NCAR‐NOMHICE scientists. Fifty‐four compounds were chosen for the intercomparison. Eight hundred eighty‐three measurements were compared overall; the average of the mean ratios of the participants' results to NCAR‐NOMHICE results was 1.03. The participants' results were combined and averaged for each individual NMHC measured and compared to the reference results; thirty‐three of the 54 compounds agreed to within ±20% of the reference results. Individual analyses from participant laboratories were compared and ranked with respect to agreement with the reference values; the ranking was reconciled with the analytical procedures employed for each analysis. From this, recommendations were derived for preferred analytical techniques and practice. Recommendations include but are not limited to the following: (1) National Institute of Standards and Technology (NIST) standards or NIST‐traceable standards should be used and, for mass spectrometric analyses, multicomponent NIST‐traceable standards should be used; (2) if solid adsorbents are used for preconcentrating NMHCs, extensive tests should be performed to test for artifact formation and compound losses; and (3) for whole air sampling in canisters, subsequent analyses should be performed as soon as is reasonably possible to avoid the potential for compositional changes.
Relationship of intracloud lightning radiofrequency power to lightning storm height, as observed by the FORTE satellite
Authors: Jacobson Abram R.
Published: May 2003

Prior studies have noted a strongly nonlinear enhancement of lightning flash rates with increasing cloud height. Here we report a related observation, of a tendency for increasing intracloud‐discharge radiofrequency‐emission power for increased height of the electrified cloud. The FORTE satellite's radio‐frequency‐receiver payload has performed extensive recordings of electromagnetic emissions of lightning discharges. The most commonly occurring such emission arises from intracloud electrical breakdown and is usually recognizable by a pulse followed by a delayed echo from the ground reflection. We have used other systems of lightning monitors to provide source locations for an extended data set of FORTE intracloud‐discharge signals. The interpulse separation within each pulse pair yields the discharge height above the reflective ground. The storm in which the pulse occurs usually provides many (at least 50) recorded events. From the pattern of these events' heights, we can usually infer a capping height, which serves as an upper bound on the lightning discharge heights for that storm. We find that there is a strong statistical increase of effective radiated power of intracloud discharges, for increasing capping height of the parent storm. Thus a future satellite‐based lightning monitor that triggers on only the most intense radiofrequency emissions will be strongly selective for electrified storms with very deep vertical development. Such storms are also indicated in severe convective weather.