bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of SAGE II observations using data assimilation by the SUNY‐SPB two‐dimensional model and comparison to TOMS data
Authors: Smyshlyaev Sergei P. , Geller Marvin A.
Published: February 2004

The two‐dimensional SUNY‐SPB chemical‐transport model is used to assimilate SAGE II 1985–1993 ozone, water vapor, and aerosol data. Combining the observations and model calculations allows one to fill in significant gaps in the SAGE II altitude‐latitude cross sections; to correct the SAGE II data using data assimilation quality control procedures and to compare SAGE II measurements to independent TOMS observations of column ozone. The model/data assimilation was used for the analysis of ozone behavior for the period 1980–1995. Diagnostics of the observed variability of the total ozone content for 1980–1995 have been carried out using this procedure. The obtained results demonstrate that the negative trend in total ozone, which has been observed during the last several years, was a result of ozone destruction by the chlorine and bromine catalytic cycles, mainly in two height regions: near 20 and 40 km. The percent changes of ozone concentration are approximately similar in both height regions (with a maximum loss of 8–12%). However, the total ozone variability was 1 order of magnitude larger near 20 km than near 40 km. These calculations strengthen the hypothesis that heterogeneous processes on aerosol surfaces have played the dominant role in the observed variability of total ozone during the 1980–1995 period. No solar cycle effects were included in the assimilation model for this study.
Comparison of measured and modeled stratus cloud infrared spectral signatures
Authors: Rizzi Rolando , di Pietro Pierluigi , Loffredo Gianluca , Smith Jonathan A.
Published: February 2004

The purpose of the paper is to evaluate our capability to model upwelling radiances at high resolution over broad spectral ranges in clear and cloudy conditions. Spectroradiometric measurements are taken from aircraft flying at low altitude over a clear scene and over a low‐level stratus cloud. It is the ideal case to capture the change in signal due to the presence of a cloud without the interfering effect of the atmosphere above, unavoidable in higher‐altitude measurements. The knowledge of the variability of the radiometric signal over the measurement periods and the availability of in situ measurement from the same aircraft of the meteorological parameters (temperature and humidity profile) and cloud parameters (depth and microphysical parameters of the cloud layer) allow a nearly complete control of the experiment. The results for the clear case show deviations between simulation and measurement generally smaller than 0.5%, while in the cloudy case, differences over most of the spectral range are less than the variability seen during the recording of the measured spectra. The cloud result is not significantly changed when tested with realistic changes in the measured cloud properties. Over cloud an absorbing medium alone is insufficient, and full scattering is required to achieve good agreement. This shows that care is required when assimilating infrared radiances from above such clouds, which can often extend over large areas. The adopted simulation methodology generates spectral radiances in very good agreement with the measurements both in clear air and above the stratus cloud layers.
Mean ages of stratospheric air derived from in situ observations of CO2, CH4, and N2O
Authors: Andrews A. E. , Boering K. A. , Daube B. C. , Wofsy S. C. , Loewenstein M. , Jost H. , Podolske J. R. , Webster C. R. , Herman R. L. , Scott D. C. , Flesch G. J. , Moyer E. J. , Elkins J. W. , Dutton G. S. , Hurst D. F. , Moore F. L. , Ray E. A. , Romashkin P. A. , Strahan S. E.
Published: February 2004

Accurate mean ages for stratospheric air have been derived from a spatially and temporally comprehensive set of in situ observations of CO2, CH4, and N2O obtained from 1992 to 1998 from the NASA ER‐2 aircraft and balloon flights. Errors associated with the tropospheric CO2 seasonal cycle and interannual variations in the CO2 growth rate are <0.5 year throughout the stratosphere and <0.3 year for air older than 2 years (N2O<275 ppbv), indicating that the age spectra are broad enough to attenuate these influences over the time period covered by these observations. The distribution of mean age with latitude and altitude provides detailed, quantitative information about the general circulation of the stratosphere. At 20 km, sharp meridional gradients in the mean age are observed across the subtropics. Between 20 and 30 km, the average difference in mean age between the tropics and midlatitudes is ∼2 years, with slightly smaller differences at higher and lower altitudes. The mean age in the midlatitude middle stratosphere (∼25–32 km) is relatively constant with respect to altitude at 5±0.5 years. Comparison with earlier balloon observations of CO2 dating back to the 1970s indicates that the mean age of air in this region has remained within ±1 year of its current value over the last 25 years. A climatology of mean age is derived from the observed compact relationship between mean age and N2O. These characteristics of the distribution of mean age in the stratosphere will serve as critically needed diagnostics for models of stratospheric transport.
Effects of aircraft wake dynamics on measured and simulated NOx and HOx wake chemistry
Authors: Lewellen D. C. , Lewellen W. S.
Published: February 2004

High‐resolution numerical large‐eddy simulations of the near wake of a B757 including simplified NOx and HOx chemistry were performed to explore the effects of dynamics on chemistry in wakes of ages from a few seconds to several minutes. Dilution plays an important basic role in the NOx ‐ O3 chemistry in the wake, while a more interesting interaction between the chemistry and dynamics occurs for the HOx species. These simulation results are compared with published measurements of OH and HO2 within a B757 wake under cruise conditions in the upper troposphere taken during the Subsonic Aircraft Contrail and Cloud Effects Special Study (SUCCESS) mission in May 1996. The simulation provides a much finer grained representation of the chemistry and dynamics of the early wake than is possible from the 1 s data samples taken in situ. The comparison suggests that the previously reported discrepancy of up to a factor of 20–50 between the SUCCESS measurements of the [HO2]/[OH] ratio and that predicted by simplified theoretical computations is due to the combined effects of large mixing rates around the wake plume edges and averaging over volumes containing large species fluctuations. The results demonstrate the feasibility of using three‐dimensional unsteady large‐eddy simulations with coupled chemistry to study such phenomena.
Production of stabilized Criegee intermediates and peroxides in the gas phase ozonolysis of alkenes: 2. Asymmetric and biogenic alkenes
Authors: Hasson Alam S. , Ho Andy W. , Kuwata Keith T. , Paulson Suzanne E.
Published: February 2004

Organic hydroperoxide, hydrogen peroxide, and >C1 carbonyl yields have been measured from the reaction of a set of structurally diverse and atmospherically significant terminal and exocyclic alkenes with ozone. Product yields were investigated for 1‐butene, 1‐pentene, 1‐octene, methylene cyclohexane, β‐pinene, camphene and isoprene for humidities from 0 to 80% using gas chromatography with flame ionization detection and high‐performance liquid chromatography with fluorescence detection. The yields of these products were used to estimate the following stabilized Criegee intermediate yields: 1‐butene (0.27), 1‐pentene (0.29), 1‐octene (0.36), methylene cyclohexane (0.18), β‐pinene (0.28), camphene (0.31), and isoprene (0.27). The reaction of stabilized Criegee intermediates with water produces primarily hydroxymethyl hydroperoxide from CH2OO, and H2O2 and a carbonyl compound for larger Criegee intermediates; acid formation is expected to be low. The exception is camphene, for which the large Criegee intermediate generates the corresponding hydroxyalkyl hydroperoxide in its reaction with water. These results were used to develop a structure activity relationship to estimate stabilized Criegee intermediate yields and to demonstrate that this model is consistent with literature values for OH yields from these ozone‐alkene reactions. The mechanisms of the formation of these products are discussed and a hypothesis for the decrease in OH formation with increasing chain length for terminal alkenes is provided. Finally, a parameterization of the reactions for incorporation into atmospheric models is developed.
Measurement of aerosol sulfuric acid: 2. Pronounced layering in the free troposphere during the second Aerosol Characterization Experiment (ACE 2)
Authors: Curtius J. , Sierau B. , Arnold F. , de Reus M. , Ström J. , Scheeren H. A. , Lelieveld J.
Published: February 2004

Measurements of aerosol sulfuric acid in the free troposphere were performed in the vicinity of Tenerife, Canary Islands (28°N, 16°W), in July 1997. These measurements were carried out on board a Dutch Cessna Citation II research aircraft within the framework of the second Aerosol Characterization Experiment (ACE 2). We used the Volatile Aerosol Component Analyzer for the detection of sulfuric acid (H2SO4). Vertical profiles between 2 and 13 km altitude were obtained. Typically, H2SO4 mixing ratios ranged between 10 and 120 pptv. Between 4 and 6 km altitude a distinct H2SO4 aerosol layer was observed repeatedly with H2SO4 mixing ratios of up to 550 pptv. The measurements are in agreement with total aerosol mass concentrations inferred from simultaneous measurements of aerosol size distributions using two condensation particle counters, a differential mobility analyzer, and an optical aerosol counter. At altitudes above 4 km the predominant aerosol component was sulfuric acid, frequently correlated with ozone, suggesting photochemical air pollution as a common source. Sulfur dioxide measured by chemical ionization mass spectrometry technique revealed typical mixing ratios between 10 and 60 pptv at altitudes above 6 km and up to 200 pptv in the lower troposphere.
Bayesian solution for nonlinear and non‐Gaussian inverse problems by Markov chain Monte Carlo method
Authors: Tamminen Johanna , Kyrölä Erkki
Published: February 2004

In this paper we apply the Bayesian approach for solving retrieval problems encountered in remote sensing measurements of the atmosphere. The approach gives as a solution the posterior probability distribution of the unknown parameters and allows a possibility to combine new measurements with prior knowledge. While the Bayesian solution can easily be computed in the case of linear, Gaussian inverse problems, the characterization of the solution in all other cases is difficult. Here we apply Markov chain Monte Carlo (MCMC) method for computing posterior distributions for inverse problems. The advantage of the MCMC technique is that it can easily be implemented in a great variety of inverse problems including nonlinear problems with various prior or noise structures. The MCMC algorithms are not yet effective enough for operational processing of large amounts of data, but they provide excellent tools for development and validation purposes. We have applied successfully the MCMC technique to the inverse problem arising from the Global Ozone Monitoring by Occultation of Stars instrument.
Particle formation and growth from ozonolysis of α‐pinene
Authors: Hoppel William , Fitzgerald James , Frick Glendon , Caffrey Peter , Pasternack Louise , Hegg Dean , Gao Song , Leaitch Richard , Shantz Nicole , Cantrell Christopher , Albrechcinski Thomas , Ambrusko John , Sullivan William
Published: February 2004

Observations of particle nucleation and growth during ozonolysis of α‐pinene were carried out in Calspan's 600 m3 environmental chamber utilizing relatively low concentrations of α‐pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas‐phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment by assuming only one condensable product produced with a molar yield of 5% to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. While only one component was required to simulate the data, more than one product may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace amounts of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the α‐pinene/ozone system in terms of classical nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest that the nucleation rate in the α‐pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).
Global stratospheric effects of the alumina emissions by solid‐fueled rocket motors
Authors: Danilin M. Y. , Shia R.‐L. , Ko M. K. W. , Weisenstein D. K. , Sze N. D. , Lamb J. J. , Smith T. W. , Lohn P. D. , Prather M. J.
Published: February 2004

We simulate accumulation of Al2O3 particles in the atmosphere produced by solid‐fueled rocket motors by using the Goddard Institute for Space Studies/University of California at Irvine three‐dimensional (3‐D) chemistry‐transport model (CTM). Our study differs from Jackman et al. (1998) by applying a 3‐D CTM, considering 13 size bins for the emitted particles from 0.025 to 10 μm and taking into account their washout, gravitational sedimentation, and coagulation with background sulfate aerosol. We assume an initial trimodal size distribution of Al2O3 particles (Beiting, 1997) with 2.8% by mass of the alumina emitted as particles with radius of less than 1 μm. Our test case adopts a stratospheric source of 1120 tons/yr equivalent to nine space Shuttle and four Titan IV launches annually. The calculated steady state surface area density (SAD) and mass density for the scenarios with sedimentation of alumina particles have maximum values in the lower stratosphere in the Northern Hemisphere of up to 7×10−4 μm2/cm3 and 0.09 ng/m3, respectively, or about 1000 times smaller than those of the background sulfate aerosol. Our results are sensitive to the emitted mass fractionation of alumina (EMFA) showing the values for the SAD or mass density higher or lower by an order of magnitude owing to a poorly known EMFA. Chemical implications of alumina particle accumulation for the ozone balance are estimated by using the Atmospheric and Environmental Research 2‐D model assuming chlorine activation on Al2O3 surfaces via the C1ONO2 + HCl → Cl2 + HNO3 reaction with a probability of 0.02 (Molina et al., 1997). Owing to the very small Al2O3 SAD, any additional ozone depletion due to Al2O3 emissions is also small (0.0028% on a global annually averaged basis for the scenario with sedimentation, or about 4 times smaller than the ozone response to chlorine emissions only). The ozone depletion potential of the alumina emissions is about 0.03–0.08 for the scenarios using the EMFA of Beiting (1997) and larger by an order of magnitude for the EMFA of Brady and Martin (1995).
Large‐scale advection of continental aerosols during INDOEX
Authors: Leon J.‐F. , Chazette P. , Dulac F. , Pelon J. , Flamant C. , Bonazzola M. , Foret G. , Alfaro S. C. , Cachier H. , Cautenet S. , Hamonou E. , Gaudichet A. , Gomes L. , Rajot J.‐L. , Lavenu F. , Inamdar S. R. , Sarode P. R. , Kadadevarmath J. S.
Published: February 2004

In this paper, we present passive and active remote sensing measurements of atmospheric aerosols over the North Indian Ocean (NIO) during the Intensive Field Phase (IFP, January to March 1999) of the Indian Ocean Experiment. The variability of the aerosol load over NIO is discussed based on three‐dimentional numerical simulations made at a local scale by use of Regional Atmospheric Modeling System (RAMS) and at a regional scale using the zoomed Laboratoire de Météorologie Dynamique global circulation model (LMD‐Z version 3.3). Ground‐based measurements of the columnar aerosol optical thickness (AOT) and of surface black carbon (BC) concentration were carried out at two different sites in India: Goa University on the NIO coast and Dharwar 150 km inland. Local‐scale investigations point out that the trend in BC concentration at the ground is not correlated with AOT. Lidar profiles obtained both from the surface at Goa and in the NIO from the Mystere‐20 research aircraft indicate that a significant contribution to the total AOT (more than 50%) is due to a turbid monsoon layer located between 1 and 3 km height. RAMS simulation shows that the advection of aerosols in the monsoon layer is due to the conjunction of land‐sea breeze and topography. We present the regional‐scale extent of the aerosol plume in terms of AOT derived from the visible channel of Meteosat‐5. During March, most of the Bay of Bengal is overcast by a haze with a monthly average AOT of 0.61±0.18, and a spatially well‐defined aerosol plume is spreading from the Indian west coast to the Intertropical Convergence Zone with an average AOT of 0.49±0.08. Those values are bigger than in February with AOT at 0.35±0.18 and 0.37±0.09 for the Bay of Bengal and the Arabian Sea, respectively. One of the principal findings of this paper is that a significant contribution to the aerosol load over the NIO is due to the advection of continental aerosols from India in a well‐identified monsoon layer above the marine boundary layer. Moreover, it is suggested that the increase in biomass burning plays a significant role in the increasing trend in AOT during the winter dry monsoon season.
Effect of sub‐grid‐scale variability of soil moisture and precipitation intensity on surface runoff and streamflow
Authors: Arora Vivek K. , Chiew Francis H. S. , Grayson Rodger B.
Published: February 2004

Land surface scheme simulations, which differ only in their parameterization of surface runoff, are performed for the Amazon and Mississippi River basins at ∼1.88°×1.90° resolution. In the first simulation, soil moisture and precipitation intensity are assumed to be uniformly distributed. In the second simulation, precipitation intensity is assumed to be exponentially distributed, and soil moisture subgrid variability is expressed in terms of the variable water‐holding capacity of the grid cell. The effect of subgrid variability of precipitation intensity and soil moisture is taken into account to model surface runoff more realistically. National Center for Environmental Prediction (NCEP) reanalysis data with its precipitation estimates adjusted to long‐term values are used to drive the land surface scheme, and flow routing is performed to obtain streamflow at three locations in each river basin. Comparisons with observations indicate that the inclusion of subgrid variability results in slightly improved streamflow simulations in terms of reduced root‐mean‐square errors and better correlation coefficients. Sub‐grid‐scale variability also results in significant changes in the magnitude, time, and frequency of surface runoff generation, partitioning of total runoff into surface runoff and drainage and slightly drier soil moisture conditions.
Atmospheric gravity wave spectra: A stochastic approach
Authors: Souprayen Claude , Vanneste Jacques , Hertzog Albert , Hauchecorne Alain
Published: February 2004

The evolution of the spectral distribution of atmospheric gravity wave energy, pseudomomentum, and action with altitude is calculated. We apply the eikonal, or ray, formalism to the description of the off‐resonant interaction between scale‐separated waves where the wave action is projected on elementary wave packets distributed in a position‐wave number phase space. We then integrate the ray equations in random vertical shears of horizontal winds. Using stochastic theory, we derive the universal form of the energy‐spectrum dependence on the vertical wave number which results from the vertical propagation away from a stationary source. This spectrum varies with altitude and possesses the main features of observed spectra or their related empirical descriptions. An asymptotic limit at large wave number (tail) is formed for arbitrary sources, scaling as N2m−3 (where N is the Brunt‐Väisälä frequency and m is the vertical wave number). A knee in the spectrum appears at a central wave number m* as the wave field propagates upward. Whereas the tail limit remains approximatively constant, m* decreases with altitude to account for the increase of the wave energy. If, as a proxy for dissipation, we impose a complete loss of action for waves whose wave numbers are larger than a given threshold mc, there is still a large spectral range where the asymptotic limit N2m−3 remains unchanged. With such a proxy, energy and pseudomomentum deposition can be estimated from the particle statistics. This provides the first steps toward a new parameterization of the wave‐induced drag on the mean flow that is required in general circulation models.
Cloud‐to‐ground lightning activity in the Iberian Peninsula: 1992–1994
Authors: Soriano Luis Rivas , de Pablo Fernando , Díez Eulogio García
Published: February 2004

For the first time, the temporal and spatial distribution of cloud‐to‐ground lightning activity in the Iberian Peninsula are being studied. The database covers the years 1992–1994, and it was recorded over 2.2×106 cloud‐to‐ground lightning flashes. The monthly variation shows a single peak in the warmer months (May to September) with a strong year‐to‐year variability. The diurnal cycle of lightning activity peaks at 1700 LT with a subsequent slow decrease toward the minimum in the morning hours. The percentage of positive flashes is 8%, although this percentage is higher in the winter than in the summer. The lightning flash polarity is found to be very similar throughout the day. The average multiplicity is found to be 2.0 for the negative flashes and 1.1 for the positive flashes and is very similar in the winter and summer seasons, while the percentage of single‐stroke flashes tends to increase in the winter. The median (mean) intensity is found to be 22.5 kA (32.4 kA) for the negative flashes and 52.3 kA (69.3 kA) for the positive flashes. The excessively high intensity for the positive flashes could be due to the detection system has low efficiency in detecting weak amplitude flashes. The distribution of amplitudes is very similar in the winter and summer seasons. The decay at large amplitudes is slower for the positive flashes. The median and mean amplitudes are higher in the winter for both polarities. The spatial cloud‐to‐ground lightning distribution confirms the well‐known preference for certain areas in the development of thunderstorms. The average maximum negative, positive, and combined flash densities are 3.1, 0.2, and 3.3 flashes km−2 yr−1, respectively. The comparison between individual years shows a considerable year‐to‐year variability. The average percentage of positive flashes ranges from 0% to 41%. The grid block median peak current amplitudes vary from 12.5 to 47.4 kA for the negative flashes and from 17.6 to 107.5 kA for the positive flashes. The results are discussed in the context of other measurements taken at different parts of the world.
Impact of BOREAS on the ECMWF forecast model
Authors: Betts Alan K. , Viterbo P. , Beljaars A. C. M. , van den Hurk B. J. J. M.
Published: February 2004

This paper reviews the impact of the Boreal Ecosystem‐Atmosphere Study (BOREAS) on the development of a new land‐surface parameterization in the European Centre for Medium‐Range Weather Forecasts (ECMWF) global forecast system and discusses briefly the improvements that resulted in the model climate at high latitudes. We show how a long time series at a representative site was used to validate the model parameterizations of observed physical processes on both diurnal and seasonal timescales. As a direct result of BOREAS, the representation of several surface processes was greatly improved over the boreal forest. The introduction of separate tiles for tall and short vegetation meant that the boreal forest could be represented with some realism. In winter the albedo with snow under the trees was greatly reduced, and the introduction of a prognostic snow model with its own energy balance under the canopy meant that evaporation of snow in winter and spring was similarly reduced. An improved separate handling of liquid and frozen soil water meant that evaporation of frozen soil water is shut off, and surface runoff occurs when snow melts on frozen ground. Trees in the model now have larger more realistic unstressed vegetative resistance, as well as a stress factor for vapor pressure deficit, in addition to one for low light levels; both of which reduce summer transpiration to the lower levels observed over the boreal forest. The model does not yet have a global soil distribution, which means that it does not represent the organic soils that are characteristic of the black spruce sites, which have a larger water storage.
Scale issues in verification of precipitation forecasts
Authors: Tustison Ben , Harris Daniel , Foufoula‐Georgiou Efi
Published: February 2004

Precipitation forecasts from numerical weather prediction models are often compared to rain gauge observations to make inferences as to model performance and the “best” resolution needed to accurately capture the structure of observed precipitation. A common approach to quantitative precipitation forecast (QPF) verification is to interpolate the model‐predicted areal averages (typically assigned to the center point of the model grid boxes) to the observation sites and compare observed and predicted point values using statistical scores such as bias and RMSE. In such an approach, the fact that the interpolated values and their uncertainty depend on the scale (model resolution) of the values from which the interpolation was done is typically ignored. This interpolation error, which comes from scale effects, is referred to here as the “representativeness error.” It is a nonzero scale‐dependent error even for the case of a perfect model and thus can be seen as independent of model performance. The scale dependency of the representativeness error can have a significant effect on model verification, especially when model performance is judged as a function of grid resolution. An alternative method is to upscale the gauge observations to areal averages and compare at the scale of the model output. Issues of scale arise here too, with a different scale dependency in the representativeness error. This paper examines the merits and limitations of both verification methods (area‐to‐point and point‐to‐area) in view of the pronounced spatial variability of precipitation fields and the inherent scale dependency of the representativeness error in each of the verification procedures. A composite method combining the two procedures is introduced and shown to diminish the scale dependency of the representativeness error.
Local to regional‐scale variability of annual net accumulation on the Greenland ice sheet from PARCA cores
Authors: Mosley‐Thompson E. , McConnell J. R. , Bales R. C. , Li Z. , Lin P.‐N. , Steffen K. , Thompson L. G. , Edwards R. , Bathke D.
Published: February 2004

A suite of spatially distributed cores collected under the Program for Arctic Regional Climate Assessment (PARCA) provides an unprecedented opportunity to assess local to regional variability of annual accumulation rates over the Greenland ice sheet. PARCA cores are unique in their broad spatial distribution and accurate dating of annual layers using multiple seasonally varying indicators. The core data provide (1) a more rigorous evaluation of spatial and temporal variations in accumulation rates, (2) critical input to ice sheet mass balance estimates, (3) ground truth measurements for satellite observations and climate model‐based precipitation estimates, and (4) important constraints on paleoclimatic interpretations from ice cores. Multiple closely spaced cores demonstrate that signals of high‐frequency (annual to possibly decadal scale) climate variability preserved in the ice sheet are partially masked by glaciological noise. Two 350‐year accumulation histories, one from northwest Greenland and one from the summit area, reveal significant multidecadal variability. The regional trends show long periods (60–90 years) of strong positive correlation and an equally long period of strong negative correlation. Since 1940 the trends have been decoupled. This spatial variability reflects the strong modulation of Greenland precipitation (and the climate information it contains) by changes in North Atlantic atmospheric circulation patterns. Proxy records from the PARCA cores document that climate reconstructions from a single core must be interpreted cautiously, with application of appropriate filters to reduce local noise and careful extrapolations from local to regional scales. Richer, more robust ice core‐derived data sets should result from combining multiple, more widely spaced cores to produce regional stacked records.
Mesospheric sprite current triangulation
Authors: Füllekrug Martin , Moudry Dana R. , Dawes Graham , Sentman Davis D.
Published: February 2004

A network of three time‐synchronized high‐precision induction coil magnetometers is installed in North America to measure sprite‐associated lightning flash waveforms in the frequency range 0.1–1000 Hz during the Energetics of Upper Atmosphere Excitation by Lightning, 1998, sprite campaign in July 1998. Simultaneous intensified video observations on board an aircraft are used to investigate 16 sprites with long time delays >33.33 ms relative to the parent lightning discharge reported by the National Lightning Detection Network. Three different long‐delayed sprite‐associated waveforms can be distinguished: 38% do not exhibit any significant magnetic intensity variation, 25% exhibit slow variations ∼100 ms, and 25% exhibit short pulses ∼4 ms. The source locations of the sprite‐associated short pulses are triangulated by use of arrival time difference analysis. One source location exhibits a substantial spatial displacement ∼60 km relative to the parent lightning discharge, in agreement with the azimuths of sprite luminosity edges determined from the corresponding background star field of the video observations on board the aircraft. It is concluded from the temporal and spatial coincidence of the secondary short pulse and the sprite luminosity that this particular sprite is associated with current in the mesosphere.
Continental sources, transoceanic transport, and interhemispheric exchange of carbon monoxide over the Pacific
Authors: Staudt A. C. , Jacob D. J. , Logan J. A. , Bachiochi D. , Krishnamurti T. N. , Sachse G. W.
Published: February 2004

The Pacific Exploratory Mission to the Tropics phase B (PEM‐Tropics B) aircraft campaign in March‐April 1999 surveyed the chemical composition of the Pacific atmosphere from 35°N to 35°S and up to 12 km altitude. We use these observations in combination with a global three‐dimensional model driven by assimilated meteorological observations to investigate the transport of northern hemispheric pollutants over the Pacific. We focus on carbon monoxide (CO) and tag it in the model by its region of origin. The model reproduces the observed large‐scale latitudinal, longitudinal, and vertical gradients of CO concentrations over the Pacific. Biomass burning in Southeast Asia, which was particularly intense in spring 1999, contributed most of the CO enhancements observed in the free troposphere over the northern tropical Pacific but played only a minor role in the boundary layer. Fossil fuel combustion in Europe and Asia contributed most of the observed CO enhancements in the boundary layer over the North Pacific; the European influence dominated over Asian influence north of 35°N. European influence over the Pacific is particularly strong in spring because of wintertime accumulation of CO at high latitudes. North American pollution made little contribution to CO anywhere over the Pacific. Circulation of Eurasian industrial pollution around the Pacific High and into the trade winds produced a tropical “river of pollution” flowing in the lower troposphere from the northeastern to the western equatorial Pacific and in the vicinity of the South Pacific Convergence Zone. This pathway, however, made little contribution to interhemispheric exchange. Elevated concentrations observed for CO and other northern hemispheric tracers in the upper troposphere over the southeastern Pacific provide evidence for efficient interhemispheric exchange through a narrow region of upper tropospheric westerlies in the eastern equatorial Pacific (the “westerly duct”). We find that this westerly duct was the most important pathway for global interhemispheric exchange during PEM‐Tropics B. It was particularly well developed because of the La Nina conditions.
Deuterium in water vapor evaporated from a coastal salt marsh
Authors: He Hui , Lee Xuhui , Smith Ronald B.
Published: February 2004

Atmospheric water vapor was sampled simultaneously at two heights in the atmospheric surface layer above a coastal salt marsh near New Haven, Connecticut, during June 11–27, 1997. The water vapor samples were analyzed for their mixing ratio, Q, and deuterium isotope ratio, δD. The observed δD varied in the range of −145 to −89 per mil and was positively correlated with Q, reflecting the influence of in‐cloud condensation and surface evapotranspiration processes at a regional scale. Both Q and δD were, in general, lower at the upper level, indicating an upward transport of H216O and HD16O. The Craig‐Gordon model reproduced reasonably well the combined effects of equilibrium and kinetic fractionations and atmospheric demand on the evaporation process. Transpiration of the marsh plants, Spartina patens (Ait.), was a minor component (11%) of the total evapotranspiration during the experimental period. We also suggest that the relationship between δD and salinity, S, of the marsh surface water can be used to infer the isotope flux ratio. More data, however, are needed to verify this mass balance approach.
Record of cosmogenic in situ produced 14C in Vostok and Taylor Dome ice samples: Implications for strong role of wind ventilation processes
Authors: Lal D. , Jull A. J. T. , Donahue D. J. , Burr G. S. , Deck B. , Jouzel J. , Steig E.
Published: February 2004

We present results of studies of in situ cosmogenic 14C in several ice samples from the Vostok and Taylor Dome cores, spanning the time intervals of 20 and 11 k.y. B.P., respectively. The results are in variance with our findings for the Greenland Ice Sheet Project 2 (GISP 2) samples, where near quantitative 14C retention was observed. The partitioning of 14C in the CO and CO2 phases is, however, quite similar in the GISP and two Antarctic ice samples. Noting that most of the in situ 14C is produced in the ice during its accumulation to thicknesses of up to ∼10 m, we interpret the observed 14C deficiencies in Antarctic ice samples as due to grain metamorphism (recrystallization and sublimation or evaporation caused by wind ventilation). Simplified models for wind ventilation exhibit an interesting feature of the firnification processes; they differently affect the concentrations of in situ 14C and of the cosmogenic nuclides 10Be and 36Cl scavenged from the atmosphere in the accumulating firn. By studying the concentrations of in situ cosmogenic 14C and the atmospheric cosmogenic nuclides in the same ice samples, one can hope to obtain fairly realistic models of the chemical impacts of firnification processes: specifically, relationships between precipitation and accumulation and the main processes contributing to modifications in the nuclide concentrations.
Greenland snow accumulation estimates from satellite radar scatterometer data
Authors: Drinkwater Mark R. , Long David G. , Bingham Andrew W.
Published: February 2004

Data collected by the C band ERS‐2 wind scatterometer (EScat), the Ku band ADEOS‐1 NASA scatterometer (NSCAT), and the Ku band SeaWinds on QuikScat (QSCAT) satellite instruments are used to illustrate spatiotemporal variability in snow accumulation on the Greenland ice sheet. Microwave radar backscatter images of Greenland are derived using the scatterometer image reconstruction (SIR) method at 3‐day intervals over the periods 1991–1998 and 1996–1997 for EScat and NSCAT, respectively. The backscatter coefficient σ° normalized to 40° incidence, A, and gradient in backscatter, B, in the range 20°–60° are compared with historical snow accumulation data and recent measurements made in the Program for Arctic Regional Climate Assessment (PARCA) shallow snow pits. Empirical relationships derived from these comparisons reveal different exponential relationships between C and Ku band A values and dry snow zone mean annual accumulation, Q. Frequency difference images between overlapping scatterometer images suggest that C band data are more sensitive to snow layering and buried inhomogeneities, whereas Ku band data are more sensitive to volume scattering from recently accumulated snow. Direct comparisons between NSCAT B values and in situ Q measurements show a linear relationship between ln (Q) and B, with a negative rank correlation of R = −0.8. The root‐mean‐square residual in fitting regression line equation ln (Q) = 3.08 − 17.83B to the data is 0.05‐m snow water equivalent. This simple Ku band empirical relationship is exploited to investigate decadal changes in dry snow zone accumulation between Seasat (1978) and NSCAT (1996). Additional comparisons between NSCAT and recent QSCAT (1999) data reveal significant upslope shifts in the dry snow line along the southwestern flank of the ice sheet. Recent acceleration in the increase in intensity of scattering is observed in the percolation zone, suggesting increased melting between 2000‐ and 3000‐m elevation in the southern half of the ice sheet.
Mapping Canadian boreal forest vegetation using pigment and water absorption features derived from the AVIRIS sensor
Authors: Fuentes David A. , Gamon John A. , Qiu Hong‐lie , Sims Daniel A. , Roberts Dar A.
Published: February 2004

Using imagery of the Canadian boreal forest, we explored the ability of the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) to map vegetation type by taking advantage of pigment and water absorption features. Two techniques were exploited. In the first classification routine, laboratory‐acquired leaf spectra representing different “pigment classes” were used in a spectral unmixing procedure to map the relative abundance of pigments in the landscape. The resulting images were then used in a maximum likelihood routine to map the distribution of vegetation cover types. Accuracies for this method range between 66.6–80.1%, when compared to a vegetation map prepared by the Saskatchewan Environment and Resource Management (SERM), Forestry Branch Inventory Unit (FBIU). In the second approach, seven indices of vegetation structure and physiological function were calculated from AVIRIS. Cover types were then derived using the index images as inputs in a maximum likelihood classification. Levels of accuracy for this method were between 56.6 and 73.3%, when compared to the same vegetation map. Both of these complementary techniques were able to differentiate important vegetation types such as fen, deciduous trees, and wet and dry conifers at accuracies superior to other well‐established classification methods for this area. This improved vegetation classification can now be used to evaluate regional surface‐atmosphere fluxes of carbon and water vapor.
Longwave radiation budget of the southern hemisphere using ISCCP C2 climatological data
Authors: Hatzianastassiou N. , Vardavas I.
Published: February 2004

The longwave radiation budget at the top of the atmosphere (TOA) and at the surface of the Southern Hemisphere was estimated for 10° latitudinal zones, using a radiative transfer model and long‐term mean monthly climatological data from the International Satellite Cloud Climatology Project (ISCCP) from 1983 to 1990. The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) scanner S4 satellite data (1984–1989) while at the surface they have been compared with other model results and available surface measurements. Large discrepancies were found between model and ERBE TOA fluxes over Antarctica that were resolved by increasing the winter polar cloudiness significantly. Generally, our model TOA fluxes agree within the standard deviation of the ERBE data. Important differences from previous works are also found in the polar regions, in terms of magnitudes, and seasonal/latitudinal trends in surface fluxes. The mean annual and hemispherical outgoing longwave radiation at TOA, upward longwave radiation, downwelling, and net upward longwave radiation at the surface, are computed to be 236, 387, 341, and 46 W m−2, respectively.
Comparison of airborne NO2 photolysis frequency measurements during PEM‐Tropics B
Authors: Lefer B. L. , Hall S. R. , Cinquini L. , Shetter R. E. , Barrick J. D. , Crawford J. H.
Published: February 2004

During the NASA Pacific Exploratory Mission ‐ Tropics B (PEM‐Tropics B) the NO2 photolysis frequency was simultaneously measured by colocated National Center for Atmospheric Research scanning actinic flux spectroradiometer (SAFS) and commercial filter radiometer (FR) instruments on the NASA DC‐8 and the NASA P‐3B research aircraft. The measurements were collected from early March to mid‐April 1999 over the Pacific Ocean, spanning the region between 35°N‐25°S latitude and 80°W‐170°E Longitude. Over the course of the mission the SAFS and FR instruments performed quite well with both systems reporting data for more than 85% of the PEM‐Tropics B flight hours. For both the SAFS and FR systems the reported totaljNO2 is the sum of the signals from independent zenith and nadir viewing instruments. The 1‐min average SAFS and FRjNO2 values were quite highly correlated (r2>0.98) for both the zenith and nadir systems in this complex radiation environment. During PEM‐Tropics B these instruments exhibited a difference ranging from 5 to 40% (depending on the instrument pair), with the jNO2 FR measurements always being higher. In contrast, a consistent 30% disagreement was observed for the SAFS and FR instruments during PEM‐Tropics A. Use of newer NO2 cross‐section data would increase the SAFS jNO2 by 4%, while the larger difference between one of the FR and SAFS instrument pairs could be related to a change in the FR instrument sensitivity.
HO cycle in 1997 and 1998 over the southern Indian Ocean derived from CO, radon, and hydrocarbon measurements made at Amsterdam Island
Authors: Williams J. , Gros V. , Bonsang B. , Kazan V.
Published: February 2004

A new empirical method for the derivation of average HO radical concentrations is presented. The method is based on estimation of CO lifetime through the relative variability of CO and Rn measurements, and hydrocarbon measurements are used to independently determine the relative effects of chemistry and dynamics. Data from Amsterdam Island (37°S, 77°E), a remote site in the southern Indian Ocean, are used to calculate annual and daily HO levels for 1997 and 1998. A seasonal variation in calculated daily HO, consistent with seasonally changing photolysis rates, with maxima in summer and minima in winter is also derived which is comparable but slightly lower than the most recent zonal mean HO estimates of Spivakovsky et al. [2000]. The calculated annual HO shows a decrease from 2.7×105 molecule cm−3 in 1997 to 0.8×105 molecule cm−3 in 1998, possibly as a result of El Niño related meteorological changes. The empirically calculated HO correlates with temperature and relative humidity measured at the island but anticorrelates strongly with CO and to a lesser extent with O3. The limitations and improvements to this method are discussed. The method has potential for long‐term monitoring of HO changes over areas upwind from remote sites.
Relationship between upper tropospheric humidity and deep convection
Authors: Sassi Fabrizio , Salby Murry , Read William G.
Published: February 2004

Satellite measurements of upper tropospheric humidity (UTH) are used together with global cloud imagery to study the relationship between deep convection and moisture in the upper troposphere. Throughout the tropics, regions of cold cloud coincide with regions of enhanced UTH, in time‐mean distributions, in daily synoptic maps, and even on the short scales that characterize individual convective systems. Cold‐cloud fraction and UTH have very similar instantaneous structure. The horizontal correlation scales are narrow for both cloud fraction and UTH at 315 hPa and 215 hPa. The correlation scale of UTH expands at 146 hPa, signaling an increased role of horizontal transport. Ventilation of the upper troposphere by convection is also evidenced by thermal structure: Inside regions of deep convection the vertical separation of θ surfaces is expanded, stability being weakened by convective mixing inside a deep layer centered near 200 hPa. Supported by a cloud detrainment, convection humidifies the upper troposphere, elevating mixing ratio surfaces over their surroundings. The reverse is observed at subtropical latitudes, where air is anomalously dry. Jointly, these features characterize vertical transport by the Hadley and Walker circulations.
Soil moisture tendencies into the next century for the conterminous United States
Authors: Georgakakos Konstantine P. , Smith Diane E.
Published: February 2004

A monthly snowpack and soil‐moisture‐accounting model is formulated for application to each of the climate divisions of the conterminous United States for use in climate impacts assessment studies. Statistical downscaling and bias adjustment components complement the model for the assimilation of large‐scale global climate model precipitation and temperature. The model produces monthly streamflow that is broadly consistent with observed streamflow from several drainage basins in the United States for the period 1931–1998. Simulated historical soil moisture fields reproduce several features of the available observed soil moisture in the Midwest. The simulations produce large‐scale coherent seasonal patterns of soil moisture field moments over the conterminous United States, with high soil moisture means over divisions in the Ohio Valley, the northeastern United States, and the Pacific Northwest, and with pronounced low means in most of the western U.S. climate divisions. Characteristically low field standard deviations are produced for the Ohio Valley and northeastern United States, the Pacific Northwest in winter, and the southwestern United States in summer. Differences in extreme standardized anomalies of soil moisture over the historical record possess high values (2.5–3) in the central United States, where the available water capacity of the soils is high. Application of the methodology for future periods using output from the Canadian coupled global climate model (CGCM1) shows that for at least the first few decades of the 21st century, somewhat drier‐than‐present soil conditions are projected, with highest drying trends found in the southeastern United States. The soil moisture deficits in most areas are of the same order of magnitude as the soil moisture field standard deviations arising from historical natural variability. Brumbelow and Georgakakos [this issue] study the implications for crop yield in the United States.