bluuebunny/crossref_metadata_2025 huggingface dataset:
An analysis of errors in the location, current, and velocity of lightning
Authors: Hager William W. , Wang Dongxing
Published: February 2004

Recently, Le Vine and Willett obtained a correction to the transmission line model for computing radiation from lightning. In this paper we examine procedures, based on the formula of Le Vine and Willett, for obtaining the location, velocity, and current of lightning discharges from multiple station measurements such as those being implemented by Thomson. In the appendix we apply the algorithms of this paper to recently obtained data for an observed in‐cloud pulse. We show that the propagation velocity of the pulse is 0.61 c, while the channel length is 118.7±9.2 m. In numerical simulations of lightning discharges, we observed in an earlier paper that before each flash, there was a channel formation phase where electric breakdown occurred along short channels in the cloud with length on the order of 100 m. Hence the length of the channel associated with this pulse is consistent with the length of the breakdown regions during the channel formation phase observed in the numerical simulations.
Ozone and aerosol distributions and air mass characteristics over the South Atlantic Basin during the burning season
Authors: Browell E. V. , Fenn M. A. , Butler C. F. , Grant W. B. , Clayton M. B. , Fishman J. , Bachmeier A. S. , Anderson B. E. , Gregory G. L. , Fuelberg H. E. , Bradshaw J. D. , Sandholm S. T. , Blake D. R. , Heikes B. G. , Sachse G. W. , Singh H. B. , Talbot R. W.
Published: February 2004

In situ and laser remote measurements of gases and aerosols were made with airborne instrumentation to investigate the sources and sinks of tropospheric gases and aerosols over the tropical South Atlantic during the NASA Global Tropospheric Experiment (GTE)/Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) field experiment conducted in September–October 1992. Gases from extensive fires in Brazil were transported by convective storms into the upper troposphere where tropospheric ozone (O3) was photochemically produced and advected eastward over the South Atlantic. In central Africa, the fires were widespread, and in the absence of deep convection, the fire plumes were advected at low altitudes (below ∼6 km) over the Atlantic. There was a positive correlation between O3 and aerosols found in the plumes that were not involved in convection. High O3 (>75 parts per billion by volume (ppbv)) was observed in the low‐altitude plumes, and also in the upper troposphere where O3 often exceeded 100 ppbv with low aerosol loading. The average tropospheric O3 distributions were determined for the following: Brazil and western South Atlantic, eastern and central South Atlantic, central and east coast of Africa, and the entire South Atlantic Basin. The tropopause heights and O3 columns across the troposphere were calculated for individual flights and for the average O3 distributions in the above regions. A maximum tropospheric O3 column of 56 Dobson units (DU) was found over the biomass burning region in Zambia and in the subsidence region over the central South Atlantic. The high O3 region over the South Atlantic from 4° to 18°S corresponded with the latitudinal extent of the fires in Africa. In situ and laser remote measurements were used to determine the frequency of observation and chemical composition of nine major air mass types. Biomass burning emissions contributed to most of the air masses observed over the South Atlantic Basin, and biomass burning was found to contribute up to half (28 DU) of the O3 column across this region.
Ozone fluxes over a patchy cultivated surface
Authors: Mahrt L. , Lenschow D. H. , Sun Jielun , Weil J. C. , MacPherson J. I. , Desjardins R. L.
Published: February 2004

This study examines the spatial variability of ozone fluxes over flat heterogeneous terrain consisting of a patchwork of irrigated and nonirrigated surfaces. Fluxes of ozone and other quantities are computed from eight sequential flight legs of the Canadian Twin Otter research aircraft over the same track at 33 m above the surface for each of 2 days. The fluxes are composited over the eight runs to reduce the random flux error. The fluxes of heat, moisture, and carbon dioxide are closely related to spatial variations of surface vegetation. However, the ozone flux is affected by additional factors including reaction with NO released from point sources. This effect is illustrated here with two examples of irrigation pumping stations driven by diesel engines. We conclude that the ozone deposition to the surface cannot be estimated from the measured ozone flux without correction for the NO sources.
Low‐level wind maxima and the transport of pyrogenic products over southern Africa
Authors: Zunckel M. , Held G. , Preston‐Whyte R. A. , Joubert A.
Published: February 2004

Low‐level jets (LLJ) over various regions of southern Africa are shown to occur mostly as a result of a thermal gradient established over gently sloping terrain and have been observed to overlie the nocturnal temperature inversions. Strong low‐level winds associated with plain‐mountain and mountain‐plain circulations have also been observed over the central Namib desert and Natal interior. With the exception of the Namib and Natal, where no temperature profiles accompanied wind profile observations, the LLJ is seen to increase in strength as the nocturnal surface temperature inversion intensifies through the night. The height of the LLJ above the surface also increases as the inversion deepens. Wind speeds in all regions exceed 10 m s−1 in the jet core, which typically is located between 200 and 300 m above ground level (agl). Over the central Namib, the summertime plain‐mountain wind reaches similar wind speeds in a jetlike core located about 200 to 600 m agl. The Natal mountain‐plain winds may exceed 10 m s−1 in a layer of up to 1000 m deep. It is suggested that all these regional‐scale wind systems transport pyrogenic products in excess of 400 km during a single night. Subsequent daytime convective lifting of these ozone precursors, coupled with synoptic‐scale anticyclonic circulation, provides a transport mechanism for pyrogenic products over southern Africa toward the Atlantic and Indian Oceans.
Evaluation of a sectional representation of size distributions for calculating aerosol optical properties
Authors: Wu X. A. , Seigneur C. , Bergstrom R. W.
Published: February 2004

A sectional method for representing the atmospheric aerosol size distribution is evaluated for calculating the optical properties used in atmospheric radiative transfer calculations. Many current atmospheric aerosol models and measurement analysis methods are based on a lognormal representation of the aerosol size distribution. The sectional method offers the advantage over the lognormal size distribution that as the aerosol size distribution evolves with time, for example, as in the case of the Mount Pinatubo stratospheric aerosol, regional haze aerosol, or stack plume particles, the optical properties are computed only once for each size section and do not need to be recomputed as the aerosol size distribution changes. Thus the computational costs are considerably reduced. The sectional approach was evaluated by comparing the aerosol optical properties calculated with a continuous distribution to those calculated with a sectional distribution. The sectional approach was shown to be most accurate when the optical properties were integrated assuming a constant surface area distribution within each section. For a sectional size distribution with nine sections between 0.01 and 10 μm in diameter, the results show a maximum error of 6% for the scattering coefficient, 10% for the phase function, and 4% in the sky radiance. The results indicate that the sectional approach offers an excellent compromise between accuracy and computational efficiency.
Aerosol optical depth over Canada and the link with synoptic air mass types
Authors: Smirnov A. , O'Neill N. T. , Royer A. , Tarussov A. , McArthur B.
Published: February 2004

Aerosol optical depth measurements acquired through the Canadian sunphotometer network were statistically analyzed for the 1987–1992 period in order to investigate spatial and temporal commonalities between the member stations. Four stations were chosen to yield a spatially representative sampling of atmospheric optical conditions across Canada (East Coast, eastern continental, western continental, and West Coast). All the stations were located in rural, local pollution free areas. The results of aerosol optical depth measurements showed significant differences between eastern and western Canadian stations. The effect of the Pinatubo volcanic eruption was clearly seen in the measurements acquired at Sable Island, Nova Scotia. Air mass relationships for the four stations sampled demonstrated the relevance of applying air mass classification criteria to the analysis and discrimination of atmospheric optical depth. Knowledge of the seasonal trend combined with information concerning air mass type enables a coarse a priori estimation of aerosol optical depth in the absence of traditional optical data. Synoptical air mass analysis facilitates the understanding of the mechanisms involved in the seasonal variations of aerosol optical depth and yields useful information about the atmospheric optical state. The relevance of the synoptical air mass approach was demonstrated in one particular case: a seasonal aerosol optical depth trend for Arctic air masses was observed for the three sunphotometer stations which regularly experience this type of air mass.
Validation of UARS particle environment monitor electron energy deposition
Authors: Sharber J. R. , Link R. , Frahm R. A. , Winningham J. D. , Lummerzheim D. , Rees M. H. , Chenette D. L. , Gaines E. E.
Published: February 2004

One of the primary goals of the particle environment monitor (PEM) on the Upper Atmosphere Research Satellite is the determination of the solar and magnetospheric particle energy inputs to the upper and middle atmosphere. This is accomplished by measuring the incident particle spectra and computing energy deposition and ionization rates as functions of altitude. The particle spectral measurements are made by detectors covering the energy range 5 eV to 5 MeV for electrons and 5 eV to 150 MeV for protons. An X ray camera provides global‐scale images and energy spectra of ∼3 to 100 keV bremsstrahlung X rays produced by electrons incident on the atmosphere. Thus PEM measures particles which lose their energy in an atmospheric region extending from the thermosphere down to ∼50 km, with the altitude of associated bremsstrahlung X rays reaching ∼25‐km levels. PEM validation documented in this report has involved an assessment of the data quality of each PEM component instrument, intercalibration of individual instruments, and validation of the computations of the rates of electron energy deposition into the atmosphere. The energy deposition rate calculations are validated against existing energy deposition codes over the complete energy range of PEM measurements.
Simple water balance model for estimating runoff at different spatial and temporal scales
Authors: Schaake John C. , Koren Victor I. , Duan Qing‐Yun , Mitchell Kenneth , Chen Fei
Published: February 2004

A parametric water balance model was developed based on statistical averaging of the main hydrological processes. The model has a two‐layer structure with both a physical and statistical basis for the model parameters. It was developed to fill a need for models with a small number of parameters and of intermediate complexity between a one‐parameter simple bucket and more complex hydrologically oriented models with many parameters such as the Sacramento model. The focus was to improve the representation of runoff relative to the simple bucket without introducing the full complexity of the Sacramento model. The model was designed to operate over a range of time steps to facilitate coupling to an atmospheric model. The model can be used for catchment scale simulations in hydrological applications and for simple representation of runoff in coupled atmospheric/hydrological models. An important role for the simple water balance (SWB) model is to assist in understanding how much complexity in representing land surface processes is needed and can be supported with available data to estimate model parameters. The model is tested using rainfall, runoff, and surface meteorological data for three catchments from different climate regimes. Model performance is compared to performance of a simple bucket model, the Sacramento model, and the Oregon State University land surface model. Finally, a series of tests were conducted to evaluate the sensitivity of SWB performance when it is operated at time steps different from the time step for which it was calibrated.
Impact of analysis uncertainty upon regional atmospheric moisture flux
Authors: Wang Muyin , Paegle Jan
Published: February 2004

Horizontal fluxes of atmospheric water vapor are studied for summer months during 1989 and 1992 over North and South America and are based on analyses from European Centre for Medium Range Weather Forecasts, U.S. National Meteorological Center, and United Kingdom Meteorological Office. The calculations are performed over 20° × 20° box‐shaped midlatitude domains located to the east of the Rocky Mountains in North America and to the east of the Andes Mountains in South America. The fluxes are determined from operational center gridded analyses of wind and moisture. Differences in the monthly mean moisture flux divergence determined from these analyses are as large as 7 cm/month precipitable water equivalent over South America and 3 cm/month over North America. Gridded analyses at higher spatial and temporal resolution exhibit better agreement in the moisture budget study. However, significant discrepancies of the moisture flux divergence computed from different gridded analyses still exist. The conclusion is more pessimistic than Rasmusson's [1968] estimate based on station data. Further analysis reveals that the most significant sources of error result from model surface elevation fields, gaps in the data archive, and uncertainties in the wind and specific humidity analyses. Uncertainties in the wind analyses are the most important problem. The low‐level jets, in particular, are substantially different in the different data archives. Part of the reason for this may be due to the way the different analysis models parameterize physical processes affecting low‐level jets. The results support the inference that the noise/signal ratio of the moisture budget may be improved more rapidly by providing better wind observations and analyses than by providing better moisture data.
Total ozone trends from quality‐controlled ground‐based data (1964–1994)
Authors: Bojkov Rumen D. , Bishop Lane , Fioletov Vitali E.
Published: February 2004

Seasonal and year‐round trend analyses of ground‐based total ozone data that have been carefully quality controlled are presented for 46 Dobson stations, for four regions of the former USSR and for the two polar regions. The trend model incorporated the 10.7‐cm solar flux and equatorial stratospheric wind as indicators for removal of the solar and quasi‐biennial components in ozone variations. The trends were calculated for two main time intervals: from January 1964 through March 1994, with slope since 1970, and from January 1979 through March 1994. The analyses show continuous year‐round ozone decline in middle latitudes (35°–60°) where the estimated values are −4.3 and −4.1% per decade for the period since January 1979 in the northern and southern hemispheres respectively. The northern hemisphere data permit estimation of the trends over vast regions such as North America −4.1±1.5%, Europe −5.1±2.0%, Siberia and Far East −5.6±1.8% (values and two sigma intervals are given in percent per decade for year‐round trend). The ozone decline over the Arctic is about −5.6±2.0% per decade for year‐round trend and −7.5±3.8% for the winter‐spring season. The most dramatic ozone decline occurs in spring over Antarctica, −22±7.2% per decade. The trends estimated for the 1979–1994 period are about 1.5% per decade stronger in northern and southern middle and high latitudes than the trends with slope since 1970, which shows a substantial acceleration of the rates of ozone decline. The sensitivity of the ozone trends to the extremely low ozone during the 1992–1993 period is discussed. Most of the ozone data and calibrations in this paper were the base for the Ozone Trends section of the World Meteorological Organization/United Nations Environment Program Ozone Assessment 1994. The results presented here in general concur with the assessment and provide further estimates of the ozone changes in the northern middle latitudes in view of another extremely low winter‐spring season of 1994–1995.
A “tropical pipe” model of stratospheric transport
Authors: Plumb R. Alan
Published: February 2004

A conceptual model of global stratospheric transport is described, based on the assumption of rapid isentropic mixing within midlatitude “surf zones” but weak mixing into the tropics. Thus the tropical region is isolated from middle latitudes, and trace species budgets there are balances between mean upwelling and local chemical sources and sinks. In middle latitudes, where long‐lived species are assumed to be in “slope equilibrium,” the budgets are more complex, being influenced by isentropic mixing, mean downwelling, entrainment from the tropics, and local chemistry. The one‐dimensional vertical flux‐gradient relation obtained in previous studies in which mixing was assumed to be global is lost in this model. Tracer correlations are compact separately in each region, with substantial differences between tropical and midlatitude relationships. The result discussed by Plumb and Ko linking the slope of the correlation diagram to net global fluxes (and thus to lifetimes) is valid in this model only at the midlatitude tropopause.
A century of accumulation and temperature changes in Dronning Maud Land, Antarctica
Authors: Isaksson Elisabeth , Karlén Wibjörn , Gundestrup Niels , Mayewski Paul , Whitlow Sallie , Twickler Mark
Published: February 2004

A mass balance program was initiated in the Vestfjella‐Heimefrontfjella area of western Dronning Maud Land during the austral summer of 1988–1989. As a part of this program, spatial and temporal variations in snow accumulation and temperature/stable isotopes are measured using shallow firn cores. In this paper we present surface accumulation data and discuss the climatic implications of the stable isotope records from two shallow firn cores. One 30‐m‐deep core, obtained about 200 km from the coast at 700 m asl (73°36′S, 12°26′W), covers the period 1932–1991. The other core was drilled at about 500 km from the coast at 2900 m asl (75°00′S, 2°00′E) and covers the period 1865–1991. The recent accumulation increase that has been reported from several areas of the Antarctic continent is not present in either of these records. Instead, our coastal record suggests a significant decrease in accumulation, with the strongest trend from about 1975. There is a positive trend in the oxygen isotope signal in both cores, which in the coastal core corresponds to a temperature increase of about 1.8°C since the early 1930s, and in the high‐altitude core to about 0.8°C since 1865. However, it is likely that part of this increase in δ18O is due to a change of moisture source.
Mesoscale meteorology of the New England coast, Gulf of Maine, and Nova Scotia: Overview
Authors: Angevine Wayne M. , Trainer Michael , McKeen Stuart A. , Berkowitz Carl M.
Published: February 2004

The North Atlantic Regional Experiment (NARE) domain includes the coastal and near‐coastal areas of New England and Atlantic Canada, and the intervening Gulf of Maine. This area has a complex coastline on all scales. The meteorology of the region is affected by the contrasting properties of the land and ocean, especially the temperature contrast. This paper reviews the knowledge of coastal meteorology on scales from tens of meters to 100 km, emphasizing those processes that most strongly affect pollutant transport in the NARE area in summertime. Examples from the NARE 1993 Summer Intensive measurements and modeling studies are used to illustrate these processes. The understanding and interpretation of regional and global air quality measurements depend on meteorological information about such issues as the amount and location of vertical mixing, the three‐dimensional trajectories of air parcels, the prevalence and scale of stratification, the speed of horizontal transport, and the scale of horizontal homogeneity or inhomogeneity. These issues are among those not fully understood in the complex coastal environment. From what is known, and from the NARE measurements, we can draw some conclusions. Polluted air from the U.S. East Coast transported over the Gulf of Maine becomes stably stratified, and vertical mixing is limited. The scale of vertical layering may be a few tens of meters or less. Layers separated by a few hundred meters in the vertical over Nova Scotia may have crossed the U.S. coast at points hundreds of kilometers apart. Wind speeds and directions, and therefore transport times, may differ substantially between the layers. Air at the surface may be isolated from, and have drastically different chemical contents from, air at only a few hundred meters above the surface.
Simultaneous observations of mesospheric gravity waves with the MU radar and a sodium lidar
Authors: Namboothiri S. P. , Tsuda T. , Tsutsumi M. , Nakamura T. , Nagasawa C. , Abo M.
Published: February 2004

Simultaneous observations of mesospheric gravity waves have been carried out using meteor wind measurements with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (34.9°N, 136.1°E), and density perturbations of the sodium lidar at Hachioji, Tokyo, Japan (35.6°N, 139.4°E). The study utilizes 7 hours of data collected on the night of December 15–16, 1993, during a time period when a fairly monochromatic gravity wave was dominant. Using hodograph analysis, the dominant gravity wave was found to exhibit a vertical wavelength of 16 km, an intrinsic period of 9.1 hours, and a horizontal wavelength of about 1900 km. The horizontal propagation direction of the gravity wave was determined from the phase relations between the horizontal wind components and the temperature perturbations at Shigaraki. The wave propagated southward, being almost orthogonal to the baseline between Shigaraki and Hachioji. Employing the dispersion and polarization relations for linear gravity waves, the wave‐induced neutral density perturbations from the MU radar observations were estimated. A comparison with the corresponding density perturbations derived from the sodium density measurements showed good agreement. The amplitudes of the neutral density perturbations observed at both locations, which are separated by 310 km, were similar, with a maximum perturbation of ∼7% and a good correlation of phase. Time variations of the hourly variance of the density perturbations also agreed quite well between the two independent determinations, which again supports the view that the radar and the lidar detected the same gravity wave.
Comparison of U.K. Meteorological Office and U.S. National Meteorological Center stratospheric analyses during northern and southern winter
Authors: Manney G. L. , Swinbank R. , Massie S. T. , Gelman M. E. , Miller A. J. , Nagatani R. , O'Neill A. , Zurek R. W.
Published: February 2004

Meteorological data from the United Kingdom Meteorological Office (UKMO), produced using a data assimilation system, and the U.S. National Meteorological Center (NMC), produced using an objective analysis procedure, are compared for dynamically active periods during the Arctic and Antarctic winters of 1992. The differences seen during these periods are generally similar to those seen during other winter periods. Both UKMO and NMC analyses capture the large‐scale evolution of the stratospheric circulation during northern hemisphere (NH) and southern hemisphere (SH) winters. Stronger vertical and horizontal temperature gradients develop in the UKMO than in the NMC data during stratospheric warmings; comparison with satellite measurements with better vertical resolution suggests that the stronger vertical temperature gradients are more realistic. The NH polar vortex is slightly stronger in the UKMO analyses than in the NMC in the middle and upper stratosphere, and midstratospheric temperatures are slightly lower. The SH polar vortex as represented in the UKMO analyses is stronger and colder in the midstratosphere than its representation in the NMC analyses. The UKMO analyses on occasion exhibit some difficulties in representing cross‐polar flow or changes in curvature of the wind field at very high latitudes. In addition to the above study of two wintertime periods, a more detailed comparison of lower‐stratospheric temperatures is done for all Arctic and Antarctic winter periods since the launch of the Upper Atmosphere Research Satellite. In the NH lower stratosphere during winter, NMC temperatures are consistently lower than UKMO temperatures and closer to radiosonde temperatures than are UKMO temperatures. Conversely, in the SH lower stratosphere during winter, UKMO temperatures are typically lower than NMC and are closer to radiosonde temperature observations.
Validation of UARS Microwave Limb Sounder ClO measurements
Authors: Waters J. W. , Read W. G. , Froidevaux L. , Lungu T. A. , Perun V. S. , Stachnik R. A. , Jarnot R. F. , Cofield R. E. , Fishbein E. F. , Flower D. A. , Burke J. R. , Hardy J. C. , Nakamura L. L. , Ridenoure B. P. , Shippony Z. , Thurstans R. P. , Avallone L. M. , Toohey D. W. , deZafra R. L. , Shindell D. T.
Published: February 2004

Validation of stratospheric ClO measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) is described. Credibility of the measurements is established by (1) the consistency of the measured ClO spectral emission line with the retrieved ClO profiles and (2) comparisons of ClO from MLS with that from correlative measurements by balloon‐based, ground‐based, and aircraft‐based instruments. Values of “noise” (random), “scaling” (multiplicative), and “bias” (additive) uncertainties are determined for the Version 3 data, the first version publicly released, and known artifacts in these data are identified. Comparisons with correlative measurements indicate agreement to within the combined uncertainties expected for MLS and the other measurements being compared. It is concluded that MLS Version 3 ClO data, with proper consideration of the uncertainties and “quality” parameters produced with these data, can be used for scientific analyses at retrieval surfaces between 46 and 1 hPa (approximately 20–50 km in height). Future work is planned to correct known problems in the data and improve their quality.
The role of aerosol variations in anthropogenic ozone depletion at northern midlatitudes
Authors: Solomon S. , Portmann R. W. , Garcia R. R. , Thomason L. W. , Poole L. R. , McCormick M. P.
Published: February 2004

Aerosol surface area distributions inferred from satelliteborne 1‐μm extinction measurements are used as input to a two‐dimensional model to study the effects of heterogeneous chemistry upon anthropogenic ozone depletion at northern midlatitudes. It is shown that short‐term (interannual) and longer‐term (decadal) changes in aerosols very likely played a substantial role along with trends in anthropogenic chlorine and bromine in both triggering the ozone losses observed at northern midlatitudes in the early 1980s and increasing the averaged long‐term ozone depletions of the past decade or so. The use of observed aerosol distributions enhances the calculated ozone depletion due to halogen chemistry below about 25 km over much of the past decade, including many periods not generally thought to be affected by volcanic activity. Direct observations (especially the relationships of NOx/NOy and ClO/Cly ratios to aerosol content) confirm the key aspects of the model chemistry that is responsible for this behavior and demonstrate that aerosol changes alone are not a mechanism for ozone losses in the absence of anthropogenic halogen inputs to the stratosphere. It is also suggested that aerosol‐induced ozone changes could be confused with 11‐year solar cycle effects in some statistical analyses, resulting in an overestimate of the trends ascribed to solar activity. While the timing of the observed ozone changes over about the past 15 years is in remarkable agreement with the model predictions that explicitly include observed aerosol changes, their magnitude is about 50% larger than calculated. Possible chemical and dynamical causes of this discrepancy are explored. On the basis of this work, it is shown that the timing and magnitude of future ozone losses at midlatitudes in the northern hemisphere are likely to be strongly dependent upon volcanic aerosol variations as well as on future chlorine and bromine loading.
Chlorine monoxide in the Antarctic spring vortex: 2. A comparison of measured and modeled diurnal cycling over McMurdo Station, 1993
Authors: Shindell D. T. , de Zafra R. L.
Published: February 2004

We have derived chlorine monoxide (ClO) mixing ratio profiles within the Antarctic vortex on an hourly basis from ground‐based measurements of pressure‐broadened emission line spectra. This data set has provided the first opportunity for a detailed comparison between the output of a photochemical model and the measured in situ diurnal behavior of ClO in the Antarctic spring stratosphere. We stress the importance of the diurnal behavior in furnishing a short‐term, crucial test of the catalytic chlorine chemistry which determines longer‐term ozone depletion. We obtain excellent agreement between our measured and modeled diurnal change using the rate constants recommended in the 1994 Jet Propulsion Laboratory (JPL) evaluation, giving support to current understanding of perturbed chlorine chemistry in the Antarctic spring vortex. We have furthermore found that we can use our data to narrow the listed 1994 JPL uncertainty range for the ClO dimer formation rate constant and the equilibrium constant between ClO dimer formation and thermal dissociation. We show that the new limits we set on the dimer formation rate constant reduce the uncertainty in the daily rate of chlorine catalyzed ozone loss calculated from observed ClO concentrations by ∼40% at 186–196 K. We find that a modeled total ozone loss rate including both chemistry and vertical transport, based on our measurements, agrees rather well with the amount and the linear trend of ozone loss seen throughout September in coincident balloon measurements.
Atmospheric sulfur cycling in the tropical Pacific marine boundary layer (12°S, 135°W): A comparison of field data and model results: 2. Sulfur dioxide
Authors: Yvon S. A. , Saltzman E. S.
Published: February 2004

The atmospheric chemistry of sulfur dioxide over the tropical South Pacific Ocean is investigated by using results from field measurements and numerical models. Simultaneous real time measurements of sulfur dioxide and its biogenic precursor dimethylsulfide were made at 12°S, 135°W for a 6‐day period from March 3 through March 9, 1992. The mean SO2 and DMS mole fractions were 71 ± 56 pmol mol−1 (1σ) and 453 ± 93 pmol mol−1 (1σ) respectively. These concentrations are compared to those predicted by a time‐dependent photochemical box model of the marine boundary layer. Model estimates of the yield of SO2 from DMS oxidation range from 27% to 54%. Even with low yields, DMS is the dominant source of SO2 in this region. Estimates of vertical entrainment velocities based on the tropospheric ozone budget suggest that vertical entrainment is a minor source of SO2. The relative rates of various loss mechanisms for SO2 are dry deposition to the sea surface (58%), in‐cloud oxidation (9%), OH oxidation (5%), and uptake by sea‐salt aerosols (28%).
Three‐dimensional radon 222 calculations using assimilated meteorological data and a convective mixing algorithm
Authors: Allen Dale J. , Rood Richard B. , Thompson Anne M. , Hudson Robert D.
Published: February 2004

The distribution of 222Rn is simulated using a three‐dimensional chemistry and transport model driven by assimilated data. The multiyear calculation is the first to use meteorological data from the Goddard Earth Observing System data assimilation system (GEOS‐1 DAS). In addition, this calculation is the first to use moist convection and boundary layer parameters directly from a DAS to calculate mixing via moist convection and turbulence. Previously, these quantities have been derived after the assimilation procedure. Model output and data are compared at sites selected to evaluate model performance in a range of dynamic environments. Simulated afternoon boundary layer concentrations are within 30% of observed concentrations. Simulated nighttime concentrations are ≈50% of measured values although the bias is greatly reduced (and even disappears at Socorro, New Mexico) when nocturnal sub‐grid‐scale turbulent mixing is suppressed. Continental profiles are “C‐like”; a result that is consistent with the moist convective algorithm's tendency to move material from the planetary boundary layer (PBL) to the upper troposphere directly. CTM‐calculated 222Rn at Bermuda closely matches observations even during periods of frontal passage showing that the atmospheric circulation is accurate and that ventilation of the continental PBL is realistic. Model‐calculated 222Rn concentrations in the marine upper troposphere are consistent with observations in the eastern Pacific and near Darwin.
Modal character of atmospheric black carbon size distributions
Authors: Berner A. , Sidla S. , Galambos Z. , Kruisz C. , Hitzenberger R. , ten Brink H. M. , Kos G. P. A.
Published: February 2004

Samples of atmospheric aerosols, collected with cascade impactors in the urban area of Vienna (Austria) and at a coastal site on the North Sea, were investigated for black carbon (BC) as the main component of absorbing material and for mass. The size distributions are structured. The BC distributions of these samples show a predominant mode, the accumulation aerosol, in the upper submicron size range, a less distinct finer mode attributable to fresh emissions from combustion sources, and a distinct coarse mode of unclear origin. It is important to note that some parameters of the accumulation aerosol are related statistically, indicating the evolution of the atmospheric accumulation aerosol.
New high‐resolution ground‐based studies of sprites
Authors: Winckler J. R. , Lyons W. A. , Nelson T. E. , Nemzek R. J.
Published: February 2004

New observations of sprites (cloud‐ionosphere luminous discharges above thunderstorms) were made from the Yucca Ridge Field Station 20 km northeast of Fort Collins, Colorado, on the night of July 11–12, 1994, as part of a summer 1994 observing campaign. The sprites appeared above a moderate mesoscale convective complex mostly over Kansas at a range of about 270 km. The sprites were observed with both wide‐field and telescopic image‐intensified CCD TV cameras, a telescopic photometer system, and a 1‐ to 50‐kHz band VLF sferics receiver. This paper is based on five 1‐s data intervals containing bright sprites, smaller sprites, and cloud and sky flashes. Telescopic TV images of bright sprites had a fan‐shaped upper plume with very fine features not well resolved by the TV, but dendritic (upward forked) and vertically striated forms adjacent to these plumes and bright points of luminosity around the plume‐shaped regions. Many sprites consisted entirely of groups of vertically aligned striations which sometimes appeared to diverge from a common point of origin at cloud tops. All sprites in the present data sample were preceded by a cloud to ground (CG) stroke with a coincident sferic and sky flash. All CG strokes associated with sprites were positive, and most were 100 kA or more inferred peak current. From the photometer, the duration of the CG‐induced sky flashes was about 3 ms and the additional sprite total light curve was also about 3 ms. The puzzling feature that the total duration of TV images of sprites was often longer than the photometric values is discussed and an explanation given. The sprites were attributed to strong negative charging, following the positive CG stroke, of a localized cloud top region which produced an intense electric field and a luminous discharge in the cloud‐ionosphere region. The concept of “break‐even” electric fields suggested by McCarthy and Parks may explain discharge initiation with moderate field strengths.
Chemical compounds in the remote Pacific troposphere: Comparison between MLOPEX measurements and chemical transport model calculations
Authors: Brasseur G. P. , Hauglustaine D. A. , Walters S.
Published: February 2004

A global three‐dimensional chemical transport model, called MOZART (Model of OZone And Related species in the Troposphere), is used to compare calculated abundances of chemical species and their seasonal evolution in the remote Pacific troposphere near Hawaii with values observed during the Mauna Loa Observatory Photochemistry Experiments (MLOPEX 1 and 2). MOZART is a fully diurnal model which calculates the time evolution of about 30 chemical species from the surface to the upper stratosphere. It accounts for surface emissions of source gases, wet and dry depositions, photochemical transformations and transport processes. The dynamical variables are provided by the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM2) at T42 resolution (2.8° × 2.8°) and 18 levels in the vertical. Simulated abundances of 222Rn reveal an underestimate of the transport of continental emissions to the remote Pacific troposphere, more particularly during winter and summer. Calculated concentrations of chemical species are generally in fair agreement with observations. However, the abundances of soluble species are overestimated, leading, for example, to concentrations of nitric acid (HNO3) and hydrogen peroxide (H2O2) which are overpredicted by a factor of 3–8, depending on the season. This feature is attributed to insufficient washout by clouds and precipitation in the model. MOZART succesfully reproduces the development of high‐NOx episodes at Mauna Loa Observatory (MLO) associated with anticyclonic conditions to the north of Hawaii and breakdown of the polar jet which tends to deflect to the central Pacific the flow of NOx transported from eastern Asia (China, Japan). During high NOx episodes, the calculated NOx mixing ratio in the vicinity of the MLO increases by about a factor of 3 over its background level (reaching 90–100 pptv) within 3–5 days.
Comparison of cryogenic limb array etalon spectrometer (CLAES) ozone observations with correlative measurements
Authors: Bailey P. L. , Edwards D. P. , Gille J. C. , Lyjak L. V. , Massie S. T. , Roche A. E. , Kumer J. B. , Mergenthaler J. L. , Connor B. J. , Gunson M. R. , Margitan J. J. , McDermid I. S. , McGee T. J.
Published: February 2004

Ozone measurements made by the Cryogenic Limb Array Etalon Spectrometer (CLAES) aboard the NASA Upper Atmosphere Research Satellite (UARS) are compared to nearly coincident correlative measurements taken in 1992 and 1993 and to mean ozone distributions observed by other satellite instruments during past missions. This paper describes the CLAES measurement characteristics, uncertainties, predicted, and observed precisions and compares the observations with independent measurements both qualitatively and statistically. Satellite‐ and ground‐based remote sensing as well as balloon‐borne in situ measurements are represented in the correlative data set. The CLAES data are shown to be within ±20% of all correlative measurements between 0.5 and 30 mbar. Differences at lower altitudes may be related to effects of the Pinatubo aerosol on certain of the correlative measurements and the CLAES retrieval. Comparisons with historical data from the LIMS, SAGE II, and SBUV instruments indicate good agreement with the spatial and seasonal ozone distributions seen by CLAES.
Chemical characteristics of continental outflow over the tropical South Atlantic Ocean from Brazil and Africa
Authors: Talbot R. W. , Bradshaw J. D. , Sandholm S. T. , Smyth S. , Blake D. R. , Blake N. R. , Sachse G. W. , Collins J. E. , Heikes B. G. , Anderson B. E. , Gregory G. L. , Singh H. B. , Lefer B. L. , Bachmeier A. S.
Published: February 2004

The chemical characteristics of air parcels over the tropical South Atlantic during September – October 1992 are summarized by analysis of aged marine and continental outflow classifications. Positive correlations between CO and CH3Cl and minimal enhancements of C2Cl4and various chlorofluorocarbon (CFC) species in air parcels recently advected over the South Atlantic basin strongly suggest an impact on tropospheric chemistry from biomass burning on adjacent continental areas of Brazil and Africa. Comparison of the composition of aged Pacific air with aged marine air over the South Atlantic basin from 0.3 to 12.5 km altitude indicates potential accumulation of long‐lived species during the local dry season. This may amount to enhancements of up to two‐fold for C2H6, 30% for CO, and 10% for CH3Cl. Nitric oxide and NOxwere significantly enhanced (up to ∼1 part per billion by volume (ppbv)) above 10 km altitude and poorly correlated with CO and CH3Cl. In addition, median mixing ratios of NO and NOxwere essentially identical in aged marine and continental outflow air masses. It appears that in addition to biomass burning, lightning or recycled reactive nitrogen may be an important source of NOxto the upper troposphere. Methane exhibited a monotonic increase with altitude from ∼1690 to 1720 ppbv in both aged marine and continental outflow air masses. The largest mixing ratios in the upper troposphere were often anticorrelated with CO, CH3Cl, and CO2, suggesting CH4contributions from natural sources. We also argue, based on CH4/CO ratios and relationships with various hydrocarbon and CFC species, that inputs from biomass burning and the northern hemisphere are unlikely to be the dominant sources of CO, CH4, and C2H6in aged marine air. Emissions from urban areas would seem to be necessary to account for the distribution of at least CH4and C2H6. Over the African and South American continents an efficient mechanism of convective vertical transport coupled with large‐scale circulations conveys biomass burning, urban, and natural emissions to the upper troposphere over the South Atlantic basin. Slow subsidence over the eastern South Atlantic basin may play an important role in establishing and maintaining the rather uniform vertical distribution of long‐lived species over this region. The common occurrence of values greater than 1 for the ratio CH3OOH/H2O2in the upper troposphere suggests that precipitation scavenging effectively removed highly water soluble gases (H2O2, HNO3, HCOOH, and CH3COOH) and aerosols during vertical convective transport over the continents. However, horizontal injection of biomass burning products over the South Atlantic, particularly water soluble species and aerosol particles, was frequent below 6 km altitude.
Raindrop charge, precipitation, and Maxwell currents under tropical storms and showers
Authors: Despiau S. , Houngninou E.
Published: February 2004

Measurements of electric charges of raindrops, electric field, and Maxwell current were made at Abidjan (5°N, 4°W) from October 1988 to June 1990. The examination of the data obtained allows us to differentiate between storms and showers. The former are characterized by high global precipitation charge density (about 8 μC m−2) and by raindrops, 90% of which carry a positive charge of average value 86 pC, while the average negative charge is −22 pC only. The majority of positive raindrops are generally associated with the active phase of the storm and lead to strong precipitation currents of the order of −4.3 nA m−2, on average. These values, compared with those of the displacement current (JD), calculated from electric field records, and with those of the experimental Maxwell current (JM), show that the approximation of JM by the displacement current alone is not valid. In those cases, the precipitation current (JP) must be included. The showers are characterized by weaker global precipitation charge density (about 1.6 μC m−2) and weaker charges (about 25 pC), equally distributed between positive and negative ones. The precipitation currents associated with them are consequently weak, and in those cases the Maxwell current may be approximated by the displacement current alone.
Three‐dimensional ozone transport during the ozone hole breakup in December 1987
Authors: Atkinson Roger J. , Plumb R. Alan
Published: February 2004

Since the onset of springtime ozone depletion in the Antarctic lower stratosphere, the question has arisen as to the extent to which transport of ozone hole material out into the surrounding regions might influence ozone levels at midlatitudes through a so‐called ozone hole dilution effect. One such event was previously identified which followed the vortex breakup during early December 1987, but the extent to which it was attributable to the presence of an Antarctic ozone deficit, rather than being the result of ozone transports which would have occurred anyway in the absence of an ozone hole, was not quantified at the time. Here we describe the results of a more detailed study of the December 1987 event, in which we have addressed this issue. A quasi‐conservative coordinate transformation technique is used on ozone data from the second stratospheric aerosol and gas experiment (SAGE II) to obtain a three‐dimensional description of the hemispheric ozone distribution immediately prior to the event. A contour advection technique is used to describe the stratospheric material evolution during the period, and this provides a detailed depiction of the quasi‐horizontal ozone transports which occurred at the time. The calculated dynamically induced total ozone changes during the period are then separated into contributions arising from “vertical” and “horizontal” advection. The potential vorticity tendency form of the quasi‐geostrophic omega equation is solved to provide insight into the horizontal scales and vertical domain of the dynamical “forcing” primarily responsible for the vertical advection component. Finally, by imposing a “no ozone hole” ozone distribution during the period, and comparing the resulting ozone distribution with that obtained with the unmodified reconstruction, we then isolate a significant component of the observed midlatitude total ozone changes which was attributable solely to the presence of Antarctic ozone depletion.
The partitioning of hydrogen species in the Arctic winter stratosphere: Implications for microphysical parameters
Authors: Schiller C. , Engel A. , Schmidt U. , Borchers R. , Ovarlez J.
Published: February 2004

Vertical profiles of H2O and CH4 were measured in situ in the Arctic winter during the European Arctic Stratospheric Ozone Experiment (EASOE) 1991/1992. The mixing ratio of 2·CH4 + H2O was constant in the altitude region between the hygropause and 30 km without a significant difference for measurements made inside and outside of the polar vortex. During winter, the partitioning between the hydrogen compounds changes inside the polar vortex. In late November, typical prewinter conditions exist with H2O mixing ratios of 5 ppmv at the 550 K level. By late winter, mixing ratios for the same potential temperature have increased to 6 ppmv inside the vortex. Owing to the increase of the water vapor mixing ratio, the minimum temperature at which either nitric acid trihydrate or ice particles are stable will be approximately 1 K higher at the end of the polar winter inside the vortex when compared to prevortex conditions.