bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane
Authors: Saueressig Gerd , Crowley John N. , Bergamaschi Peter , Brühl Christoph , Brenninkmeijer Carl A. M. , Fischer Horst
Published: February 2004

Measurements of the 13C and D kinetic isotope effects (KIE) in methane, 13CKIE = k(12CH4)/k(13CH4) and DKIE = k(12CH4)/k(12CH3D), in the reactions of these atmospherically important methane isotopomers with O(1D) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, 13CKIEOH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For DKIEOH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(1D) 13CKIEO(1D), was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by DKIEO(1D) = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE‐1) in the reaction of O(1D) with 12CH4 versus CH4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13CKIE and DKIE for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two‐dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the 13CH4/12CH4 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.
An improved method for determining snowmelt onset dates over Arctic sea ice using scanning multichannel microwave radiometer and Special Sensor Microwave/Imager data
Authors: Drobot Sheldon D. , Anderson Mark R.
Published: February 2004

Ablation of snow over sea ice is an important physical process affecting the Arctic surface energy balance. An improved understanding of the spatial and temporal variations in snowmelt onset could be utilized to improve climate simulations in the Arctic, as well as monitor the Arctic for signs of climate change. Utilizing an updated approach for monitoring snowmelt onset over Arctic sea ice, spatial variability in passive microwave derived snowmelt onset dates is examined from 1979 through 1998. The improved technique, termed the advanced horizontal range algorithm (AHRA), utilizes temporal variations in 18/19 GHz and 37 GHz passive microwave horizontal brightness temperatures obtained from the scanning multichannel microwave radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) to identify snowmelt onset. A qualitative assessment of spatial variability in snowmelt onset discusses the 1979 through 1998 mean snowmelt onset pattern, and it also illustrates that there are significant variations in snowmelt onset on an annual basis. Principal component analysis of the snowmelt onset dates suggests snowmelt onset variability is dominated by a zone of abnormally early (late) snowmelt onset near the Siberian coast and another zone of abnormally late (early) snowmelt onset near Baffin Bay. Statistical analysis between the first principal component and March‐June monthly averaged Arctic Oscillation values implies that variations in snowmelt onset are related to alterations in the phase of the spring Arctic Oscillation.
A modeling study of global mixed aerosol fields
Authors: Wilson Julian , Cuvelier Cornelis , Raes Frank
Published: February 2004

A mixed aerosol dynamical model which simulates the growth of mixed aerosol populations of sulfate, black carbon, organic carbon, and sea salt is described, and results from the implementation of this model in the Tracer Model 2 (TM2) off‐line chemical transport model are presented. We represent the aerosol size distribution by eight modes: nucleation mode sulfate, Aitken mode sulfate, accumulation mode sulfate, pure fossil fuel black carbon, a mixed mode of fossil fuel black carbon, organic carbon, and sulfate, pure biomass burning black carbon, a mixed mode of biomass burning black carbon, organic carbon, and sulfate, and a sea‐salt mode. The model reproduces both observed zonal average marine aerosol number concentrations and observed sulfate mass/accumulation mode number concentration ratios from the North Atlantic but does less well at reproducing number concentrations at individual sites and consistently overpredicts nucleation and Aitken mode concentrations in the free troposphere. A comprehensive validation of the model is not possible with the available data, but qualitatively, it is consistent with current understanding. The model shows that the accumulation mode at the surface is dominated by the mixed modes over the fossil fuel and biomass source regions, the pure sulfate mode in peripheral continental and marine areas and sea salt in the southern oceans. A preindustrial study showed that there is not a consistent positive linear relationship between the anthropogenic increase in aerosol mass burden and the corresponding increase in aerosol number burden, and regionally there may be an inverse relationship.
Evaluation of a regional climate model for paleoclimate applications in the Arctic
Authors: Felzer Benjamin , Thompson Starley L.
Published: February 2004

The Paleoclimates From Arctic Lakes and Estuaries (PALE) project has been investigating methods of doing high‐resolution model‐data comparisons for the Arctic. As a prelude to a paleosimulation of the North Atlantic region, a modern simulation using observationally driven reanalysis data has been completed. The ARCSyM mesoscale model has been configured for the North Atlantic region, including Labrador, Ungava, Baffin Island, Ellesmere Island, Greenland, and Iceland, with a resolution of 70 km. This high resolution is necessary to predict sub‐GCM grid‐scale climate processes, such as precipitation and storm patterns that depend upon the detailed topography and coastlines of the region. Experiments were performed for the time period of September 1987 to March 1990, driven by observational analyses. The model accurately captures the major summer and winter circulation systems in the North Atlantic region. Comparisons with meteorological station data show high correlations for winter and summer surface temperatures, with a cold bias in winter and a warm bias in summer. Winter precipitation is well simulated by the model because it is driven by the large‐scale circulation. The orographically driven summer precipitation is overrepresented and does not correlate well with observations, although the overall pattern is correct. These results show that the model is capable of capturing the correct temperature and precipitation patterns, although grid‐to‐grid comparisons are not possible. The mesoscale model is therefore useful for regionally based data‐model comparisons, but should not be used to compare individual cores with specific model grids.
Atmospheric particle number size distribution in central Europe: Statistical relations to air masses and meteorology
Authors: Birmili Wolfram , Wiedensohler Alfred , Heintzenberg Jost , Lehmann Katrin
Published: February 2004

Atmospheric particle number size distributions determined over 1.5 years at a central European site were statistically analyzed in terms of their relation to time of day, season, meteorology, and synoptic‐scale air masses. All size distributions were decomposed into lognormal particle modes corresponding to the accumulation, Aitken, aged nucleation, and nucleation modes. The concentration of nucleation mode particles (<30 nm) behaved in a strongly diurnal fashion as a result of both anthropogenic source influence and secondary new particle formation events. The concentrations of Aitken and accumulation mode particles (>30 nm) lacked such diurnal behavior, and proved to be indicative of different synoptic‐scale air mass types. Over 70% of the time, air masses of Atlantic origin and maritime character prevailed, showing obvious signs of anthropogenic influence most of the time (accumulation mode: 500 cm−3; Aitken mode: 2300 cm−3). During a limited period of time (10%), however, continentally aged air with significantly enhanced concentrations of aerosol was observed (accumulation mode: 1200 cm−3; Aitken mode: 3300 cm−3). These air masses were advected from source regions in Russia, and eastern, southeastern, and central Europe, mainly under anticyclonic and high‐pressure influence. The analysis provides a refined picture of the behavior of the particle number size distribution and provides parameterizations that are representative for a variety of air masses in Europe and thus suitable for future climate modeling applications.
Zonal wind vacillation and its interaction with the ocean: Implications for interannual variability and predictability
Authors: Watterson I. G.
Published: February 2004

The influence of air‐sea interaction on the vacillation of the southern midlatitude zonal mean zonal winds is explored by comparing a 1000‐year simulation of a coupled atmosphere‐ocean general circulation model with 500‐year simulations of the atmospheric model with either a simple mixed layer ocean or specified sea surface temperatures (SSTs). In the interactive models the zonally averaged SST responds, in a dipole pattern, to the surface heat fluxes perturbed by the vacillation, with ocean dynamics modifying the pattern, particularly around 60°S, The feedback of the ocean on the atmosphere is most prominent in the lower tropospheric temperatures, but it is also significant on the interannual and longer timescales in the tropospheric zonal winds. Interdecadal variability of the 500‐hPa wind index is increased 50% by the ocean. However, the structure of the wind anomaly differs only a little as the timescale increases. A simple stochastic model of the system is able to reproduce the index statistics over timescales from months to decades. Strong relationships exist between the winds, SSTs, and precipitation anomalies in both annual and 4‐year means. However, the predictability of temperatures and rainfall, using either wind or SST indices, is limited by the small persistence of the wind anomalies and the weak influence of the SSTs. Relationships between time‐filtered indices and the following annual means are substantial, comparable to results recently found for the Antarctic Circumpolar Wave. However, these are not realizable as predictions, prompting caution with regard to predictability based on Southern Ocean temperatures.
Holocene variations in the global hydrological cycle quantified by objective gridding of lake level databases
Authors: Viau A. E. , Gajewski K.
Published: February 2004

Lake level fluctuations provide evidence about past variations in the global hydrological balance. The geostatistical approach is here used to more objectively identify global patterns using an ensemble of lake level databases by examining spatial autocorrelation between sites. The spatial structures of the lake level data are then modeled and grids produced for the last 12,000 years at 3000‐year intervals using ordinary and indicator kriging techniques. The two gridding techniques produced almost identical estimated regional lake status patterns, thus suggesting a robust estimation. The resulting lake‐status grids are in general agreement with previous paleoclimatic reconstructions using only site‐by‐site lake status point maps; however, the reduction of local fine‐scale variability resulted in more coherent regional spatial patterns in areas of high local variability. The 6 ka lake‐status grids were compared to simulations of four atmospheric general circulation models to illustrate their usefulness in validating broad‐scale climate model outputs.
A one‐dimensional simulation of the water vapor isotope HDO in the tropical stratosphere
Authors: Ridal Martin , Jonsson Andreas , Werner Martin , Murtagh Donal P.
Published: February 2004

An existing one‐dimensional chemical model has been extended with dynamics and isotopic chemistry to simulate chemical production, vertical ascent, and diffusion of H216O and HDO in the tropical stratosphere. Less abundant isotopes of water vapor and methane have been added to the model's original chemical scheme. This has led to 11 additional compounds being considered, and the methane oxidation chain has been extended by 47 new reactions. The dynamical model includes vertical diffusion and a vertical ascent rate that varies throughout the year. The results of the model show values of the isotopic ratio that are expected from theoretical calculations. The δD values range from around −550‰ at the tropopause to about −300‰ at 1 hPa. The model simulations are also in agreement with the few existing measurements of δD in the stratosphere. An annual variation of the isotopic ratio at the tropopause will cause a wave pattern in the vertical profile similar to the “tape recorder effect” for water vapor. The size and shape of this annual variation is not clear, but simulations show that the effect it has on the resulting δD profile is significant.
A climatology of isentropic cross‐tropopause exchange
Authors: Seo Kyong‐Hwan , Bowman Kenneth P.
Published: February 2004

We present a climatology of cross‐tropopause air mass exchange on isentropic surfaces in the middle world based on a Lagrangian framework. One‐month isentropic trajectories are computed for 8 years from 1992 to 1999 using the United Kingdom Meteorological Office assimilated data set. This study provides seasonal and interannual variations of isentropic mass transport between the lowermost stratosphere and the troposphere. The spatial distribution of the exchange is investigated to infer the primary mechanism for the exchange. The climatology of the stratosphere‐troposphere exchange exhibits a prominent annual cycle of two‐way exchange. The largest downward flux of stratospheric air to the troposphere occurs in late spring to summer in both hemispheres. The smallest downward flux of stratospheric air into the troposphere occurs during the fall season in both hemispheres. For the upward flux of tropospheric air to the stratosphere, the greatest transport appears in fall and winter, and the smallest appears in summer, in both hemispheres. The preferred routes for downward transport in both hemispheres correspond to the regions with the highest frequency of Rossby wave‐breaking events and with the storm tracks along the jets, implying that tropopause folding and baroclinic eddy activity are responsible for the gross horizontal distribution. A notable interannual variation is the existence of much stronger flux of stratosphere‐to‐troposphere transport in the northern winter during the period 1996–1999 than during the period 1992–1995. Increased medium‐scale wave activity is the likely explanation for this interannual variability. An observed increase in the northern midlatitude total column ozone in 1998 winter/spring may arise partly from the decreased upward transport of ozone‐poor tropospheric air.
Surface energy budget over the central Arctic Ocean during late summer and early freeze‐up
Authors: Nilsson E. Douglas , Rannik Üllar , Håkansson Måns
Published: February 2004

During the Arctic Ocean Expedition 1996 we measured radiative, turbulent, and conductive heat fluxes from late summer to early freeze‐up, over the Norwegian Sea into the Arctic Ocean to latitude 88°N, mainly in air that had spent several days over the pack ice. In summer the net radiation was positive, on average +26 W m−2, where the leads contributed more than the ice floes. Clouds had a cooling net effect. During the freeze‐up period the net radiation was dominated by the ice floes, on average −8 W m−2, with a warming cloud net effect. The turbulent fluxes were on average small and unable to balance the net radiation, except during cyclone activity, when the total turbulent heat fluxes averaged to +11 W m−2. During the freeze‐up period the conductive heat flux averaged to −4 W m−2. In summer, melting of ice must largely have caused the average positive residual (+21±5 W m−2). During the freeze‐up period the average negative residual (−7±3 W m−2) must result from formation of new ice and oceanic turbulent/convective fluxes in the leads. There was a diurnal cycle in net radiation both in summer (17 W m−2 average amplitude) and the freeze‐up period (8 W m−2). The turbulent sensible heat flux (average amplitude ∼5 W m−2), as well as the turbulent latent heat flux (1 W m−2 in summer) and conductive flux in the ice (2.5 W m−2 during the freeze‐up) responded to this. These results emphasize the high sensitivity of the heat and radiation budget balances to changes in cyclone activity, leads, and clouds.
Effective diffusivity as a diagnostic of atmospheric transport: 1. Stratosphere
Authors: Haynes Peter , Shuckburgh Emily
Published: February 2004

The transport and mixing properties of the isentropic flow in the lower and middle stratosphere are analyzed by using observed winds to advect a tracer on isentropic surfaces in the range 400–850 K. The effective diffusivity diagnostic introduced by Nakamura and collaborators is applied to the tracer field in order to identify barriers to transport and mixing regions, and to follow their seasonal evolution. Large effective diffusivity corresponds to strong mixing, and small effective diffusivity corresponds to weak mixing, i.e., to barriers. The effective diffusivity shows, in the winter stratosphere of each hemisphere, the evolution of the vortex‐edge barrier and the midlatitude surf zone, and also the extent of any mixing within the vortex. At low latitudes in the stratosphere there is a region of low effective diffusivity whose latitudinal width varies with height, broadening substantially from 400 K to 550 K. The low values of effective diffusivity in this “tropical‐reservoir” region imply little isentropic transport into or out of it. There is a strong seasonal cycle to the reservoir, which has different forms at 400 K, 450–600 K, and above 650 K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves. Comparison of effective diffusivity between the Northern Hemisphere winters 1996/1997 and 1997/1998 shows strong differences at low latitudes according to the phase of the quasi‐biennial oscillation (QBO). When there are QBO easterlies, there is a broad region of very low effective diffusivity at low latitudes. When there are QBO westerlies, there are very low values of effective diffusivity at low latitudes within the westerlies themselves but larger values at their edges.
Surface albedo retrieval from Meteosat: 1. Theory
Authors: Pinty Bernard , Roveda Fausto , Verstraete Michel M. , Gobron Nadine , Govaerts Yves , Martonchik John V. , Diner David J. , Kahn Ralph A.
Published: February 2004

Land surface albedo constitutes a critical climatic variable, since it largely controls the actual amount of solar energy available to the Earth system. The purpose of this paper is to establish a theory for the exploitation of space observations to solve the atmosphere/surface radiation transfer problem on an operational basis and to generate surface albedo, aerosol load, and possibly land cover change products. Surface albedo is rather variable in space and time and depends both on the structure and on the radiative characteristics of the surface, as well as on the angular and spectral distribution of radiation at the bottom of the atmosphere. Weather and climate models often use preset distributions or simple parameterizations of this environment variable, even though such approaches do not accurately account for the actual effect of the underlying surface. From a mathematical point of view, the determination of the surface albedo corresponds to the estimation of a boundary condition for the radiation transfer problem in the coupled surface‐atmosphere system. A relatively large database of 10 years or more of Meteosat data has been accumulated by EUMETSAT. These data, collected at half‐hour intervals over the entire Earth disk visible from longitude 0°, constitute a unique resource to describe the anisotropy of the coupled surface‐atmosphere system and provide the opportunity to document changes in surface albedo which may have occurred in these regions over that period. In addition, since the coupled surface‐atmosphere radiation transfer problem must be solved, the proposed procedure also yields an estimate of the spatial and temporal distribution of aerosols. The proposed inversion procedure yields a characterization of surface radiative properties that may also be used to document and monitor land surface dynamics over the portion of the globe observed by Meteosat. Results from preliminary applications and an error budget analysis are discussed in a companion paper [Pinty et al., this issue].
Analysis and comparison of two linear regularization methods for passive atmospheric observations
Authors: Eriksson Patrick
Published: February 2004

Linear regularization using statistical information or solution simplicity as stabilizing functional has been examined for inversions of atmospheric radiometric measurements. Statistical regularization is commonly known as the optimal estimation method (OEM), while the latter approach is often denoted as Tikhonov regularization (TR). The study focuses on considerations and parameters important for the methods in order to obtain a close to optimal inversion accuracy, here defined as the lowest possible total retrieval error. Two criteria to determine a suitable value for the Tikhonov regularization parameter, the discrepancy principle and the L curve, have been extended to handle ensemble retrievals. The natural variance implied by the solution measure selected for TR, and the importance for OEM to consider uncertainties of the assumed mean profile, are discussed. Detailed simulations of ground‐based ozone measurements around 110.8 GHz have been used to derive more specific results. For such observations, both methods can give a close to optimal performance, but the problems to address differ. TR needs fewer basic considerations but more circumstantial calculations, while OEM uses statistical information of natural variability, which is a drawback when this quantity is poorly known. The assumed vertical correlation is shown to be a critical retrieval parameter for both methods.
Dimethyl sulfide in the Southern Ocean: Seasonality and flux
Authors: Curran Mark A. J. , Jones Graham B.
Published: February 2004

The first flux estimate of dimethyl sulfide (DMS) from the Australian sector of the Southern Ocean (63°E to 162°E) has been calculated from seven voyages, which span spring and summer seasons from 1991 to 1995. Increases in seawater DMS and its precursor, dimethyl sulfoniopropionate (DMSP) generally occurred in Southern Ocean surface waters during the transition from spring to summer. DMS flux from the Subantarctic Zone (SAZ), Antarctic Zone (AZ), and Seasonal Ice Zone (SIZ) ranged from 1.7 to 49 μmol/m2/d with a mean value of 9.4 μmol/m2d. These flux calculations are believed to be underestimates, and do not include potential contributions from sea ice. Very high levels of DMSP in sea ice suggest that the SIZ may be a source of DMS to the atmosphere. The different types of vertical DMSP profiles found in sea ice possibly reflect the type of algal assemblage present and the age of the sea ice. Without considering contributions of DMS from sea ice, the overall Southern Ocean DMS emission estimate from this work was 139 Gmol S/yr. The emission estimate for the Antarctic region alone (AZ and SIZ) was 85 Gmol S/yr. This represents 17% of the global emission estimate, from 6% of the ocean surface area. This emission estimate is almost double that of an earlier estimate by Berresheim [1987] of 48 Gmol S/yr, and is likely to be higher when the amount released from the sea ice surrounding Antarctica is more accurately characterized.
Gravity waves in the tropical lower stratosphere: A model study of seasonal and interannual variability
Authors: Alexander M. Joan , Vincent Robert A.
Published: February 2004

A model study is presented to clarify the relationship between gravity‐wave properties observed in the stratosphere and the sources for the waves, presumed to be in the troposphere. The observations are balloon‐borne radiosondes launched from Cocos Island in the tropical Indian Ocean (12°S, 97°E), and the analysis of these data is described in a companion paper [Vincent and Alexander, this issue]. The dominant time variations in the observed gravity wave activity are annual and quasi‐biennial patterns in the zonal momentum flux and kinetic energy density. The background zonal winds at this site vary with the same periods, and these are known to be capable of causing dramatic variations in the observable properties of the waves even if the sources for the waves are constant in time. The results presented here clarify (1) the nature of the sources for the gravity waves observed in the stratosphere, (2) the limitations of the observations for observing the full range of gravity wave perturbations potentially present in the atmosphere, and (3) the role the observed waves can play in forcing the quasibiennial oscillation (QBO) in the zonal winds at this latitude. The stratospheric waves appear to originate near the height of the tropopause, so the source is apparently related to deep convection. No seasonal or interannual variations in the convection need be assumed to understand the observations. The waves at the tropopause appear to have a phase speed distribution that is narrowly confined near zero phase speed relative to the ground. The source is likely related to slowly propagating tropospheric convection and the wind near the tropopause. Variations observed in the stratospheric data are caused by both the wind shear in the stratosphere and the ability of waves with these characteristics to propagate vertically without severe dissipation. Higher phase speed waves may be present and could carry significant momentum flux vertically into the stratosphere and mesosphere but would be extremely difficult to see in these radiosonde data. The observed waves can contribute substantially to the descent of the eastward shear zones characteristic of the “westerly” phase of the QBO in the lower stratosphere zonal winds.
A study of the morphology of midstratospheric ozone minima using SAGE II data
Authors: Lehmann P.
Published: February 2004

Analysis of the 1984–1996 SAGE II ozone mixing ratio data revealed that the occurrence of minima was strongly stratified in altitude, both globally and seasonally, when analyzed against the curvature of minima. Four dominant stratified regions of ozone mixing ratio minima were found to exist in the 20–45 km altitude range, and studies of their frequency of occurrence, altitude distribution, vertical extent, depth of ozone reduction, and minima profiles are included. Dominant minima near 30 km were found to be more frequent closer to the subtropics during the winter, and they displayed a semiannual variation in frequency and depth in the tropics. Time variation in the mean depth of minima near 30 km did not reveal significant change over the period 1979–1996. Significant positive departures in the ozone mixing ratio were found to occur immediately above and below minima, and the difference between these upper and the lower departures was observed to depend on the vertical distance of minima from the mean altitudes of stratification. A comparison between minima analyses of SAGE and ozonesonde profiles from the South Australian region, for the two periods December‐March and May‐August, indicated consistency in the vertical distribution, in the minima depths, and in the positive departures in mixing ratio above and below the altitude of the minima.
Fine‐scale comparison of TOMS total ozone data with model analysis of an intense Midwestern cyclone
Authors: Olsen Mark A. , Gallus William A. , Stanford John L. , Brown John M.
Published: February 2004

High‐resolution (∼40 km) along‐track total column ozone data from the Total Ozone Mapping Spectrometer (TOMS) instrument are compared with a high‐resolution mesoscale numerical model analysis of an intense cyclone in the Midwestern United States. Total ozone increased by ∼100 DU (nearly 38%) as the TOMS instrument passed over the associated tropopause fold region. Complex structure is seen in the meteorological fields and compares well with the total ozone observations. Ozone data support the meteorological analysis showing that stratospheric descent was confined to levels above ∼600 hPa; significant positive potential vorticity at lower levels is attributable to diabatic processes. Likewise, meteorological fields show that two pronounced ozone streamers extending north and northeastward into Canada at high levels are not bands of stratospheric air feeding into the cyclone; one is a channel of exhaust downstream from the system, and the other apparently previously connected the main cyclonic circulation to a southward intrusion of polar stratospheric air and advected eastward as the cut‐off cyclone evolved. Good agreement between small‐scale features in the model output and total ozone data underscores the latter's potential usefulness in diagnosing upper‐tropospheric/lower‐stratospheric dynamics and kinematics.
Transport and scavenging of soluble gases in a deep convective cloud
Authors: Mari Céline , Jacob Daniel J. , Bechtold Peter
Published: February 2004

A one‐dimensional entraining/detraining plume model is used to examine the transport and scavenging of soluble gases in tropical deep convection. The model is applied to a continental system observed over Brazil during the Trace and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE‐A) TRACE‐A aircraft campaign with outflows extending from 7 to 16 km altitude. Six gases are simulated: CO (inert tracer), CH3OOH, CH2O, H2O2, HNO3, and SO2. Observed (simulated) convective enhancement factors (CEF) at 7–12 km altitude, representing the ratios of postconvective to preconvective mixing ratios, are 2.4 (1.9) for CO, 11 (9.5) for CH3OOH, 2.9 (3.1) for CH2O, 1.9 (1.2) for H2O2, and 0.8 (0.4) for HNO3. Simulated scavenging efficiencies in the convective column are 5% for CH3OOH, 23% for CH2O, 66% for H2O2, 77% for HNO3, and 28% for SO2. The large CEF for CH3OOH reflects its low solubility and its boundary layer enrichment relative to the upper troposphere. The Henry's law constant for CH2O puts it at the threshold for efficient scavenging. Scavenging of SO2 is limited by the rate of aqueous phase reaction with H2O2, as H2O2 is itself efficiently scavenged by Henry's law equilibrium; efficient scavenging of SO2 requires unusually high cloud water pH (pH>6) to enable fast aqueous phase oxidation by O3. Both HNO3 and H2O2 are efficiently scavenged in the lower (warm) part of the cloud, but H2O2 is released as the cloud freezes due to low retention efficiency during riming. Significant scavenging of H2O2 still takes place by cocondensation with ice in the glaciated cloud but is less efficient than in the warm cloud. Inefficient scavenging of H2O2 in glaciated clouds may explain the observation, in TRACE‐A and elsewhere, that H2O2 is enhanced in deep convective outflows while HNO3 is depleted. Model results indicate little direct transfer of air from the boundary layer to the cloud anvil in the convective plume, because of low‐level detrainment in the warm cloud and high‐level entrainment in the glaciated cloud. We find instead a convective ladder effect where midlevel outflow during the growing phase of the storm is reentrained into the convective plume as the storm matures.
Electric field at the ground in a large tornado
Authors: Winn W. P. , Hunyady S. J. , Aulich G. D.
Published: February 2004

A number of observers have reported lightning, diffuse luminosity, or other manifestations of electrical activity in tornadoes. To try to quantify these observations, eight instruments with sensors for electric field and other parameters were placed in front of a large tornado that passed by Allison, Texas, on June 8, 1995. The edge of the tornado vortex passed over two of the instruments and near other instruments. When the two instruments were in the low‐pressure region near the edge of the vortex, they indicated electric field amplitudes less than about 3 kV/m, which is low compared with amplitudes of 10 kV/m or greater that are often present below thunderclouds. The thunderstorm produced frequent lightning, but there is no evidence from the measurements or from visual observations of lightning in the vortex. However, there was one interesting electrical effect associated with the tornado: the electric field at the two instruments in the vortex relaxed to zero quickly after lightning flashes, whereas the electric field at nearby instruments outside the vortex did not relax quickly after the same lightning flashes. The most likely cause of the rapid relaxation is shielding of the electric field at the ground by charge induced on soil, leaves, grass, and other debris lofted by the strong winds.
Hygroscopic growth of ultrafine ammonium sulphate aerosol measured using an ultrafine tandem differential mobility analyzer
Authors: Hämeri Kaarle , Väkevä Minna , Hansson Hans‐Christen , Laaksonen Ari
Published: February 2004

An ultrafine tandem differential mobility analyzer has been developed for measurements of the hygroscopicity of ultrafine aerosol particles, between 8 and 50 nm in mobility diameter. In this paper, the main operation features of the device are presented along with a detailed evaluation for the limits of its operation. The instrument is suitable for both laboratory‐generated and atmospheric aerosol measurements. Hygroscopic growth data are presented for ammonium sulphate particles in the ultrafine size range, and comparisons are made with both experimental literature data and with theory. The data include determination of hygroscopic growth curves, deliquescence behavior, and hysteresis. These data will find applications in studies of the formation and growth of atmospheric aerosols.
Data composites of airborne observations of tropospheric ozone and its precursors
Authors: Emmons Louisa K. , Hauglustaine Didier A. , Müller Jean‐François , Carroll Mary Anne , Brasseur Guy P. , Brunner Dominik , Staehelin Johannes , Thouret Valerie , Marenco Alain
Published: February 2004

Tropospheric data from a number of aircraft campaigns have been gridded onto global maps, forming “data composites” of chemical species important in ozone photochemistry. Although these are not climatologies in the sense of a long temporal average, these data summaries are useful for providing a picture of the global distributions of these species and are a start to creating observations‐based climatologies. Using aircraft measurements from a number of campaigns, we have averaged observations of O3, CO, NO, NOx, HNO3, PAN, H2O2, CH3OOH, HCHO, CH3COCH3, C2H6, and C3H8 onto a 5° latitude by 5° longitude horizontal grid with a 1‐km vertical resolution. These maps provide information about the distributions at various altitudes, but also clearly show that direct observations of the global troposphere are still very limited. A set of regions with 10°–20° horizontal extent has also been chosen wherein there is sufficient data to study vertical profiles. These profiles are particularly valuable for comparison with model results, especially when a full suite of chemical species can be compared simultaneously. The O3 and NO climatologies generated from measurements obtained during commercial aircraft flights associated writh the MOZAIC and NOXAR programs are incorporated with the data composites at 10–11 km. As an example of the utility of these data composites, observations are compared to results from two global chemical transport models, MOZART and IMAGES, to help identify incorrect emission sources, incorrect strength of convection, and missing chemistry in the models. These comparisons suggest that in MOZART the NOx biomass burning emissions may be too low and convection too weak and that the transport of ozone from the stratosphere in IMAGES is too great. The ozone profiles from the data composites are compared with ozonesonde climatologies and show that in some cases the aircraft data agree with the long‐term averages, but in others, such as in the western Pacific during PEM‐Tropics‐A, agreement is lacking. Finally, the data composites provide temporal and spatial information, which can help identify the locations and seasons where new measurements would be most valuable. All of the data composites presented here are available via the Internet (http://aoss.engin.umich.edu/SASSarchive/).
Observations and retrievals of cirrus cloud parameters using multichannel millimeter‐wave radiometric measurements
Authors: Wang J. R. , Liu G. , Spinhirne J. D. , Racette P. , Hart W. D.
Published: February 2004

The May 26, 1998, measurements by millimeter‐wave imaging radiometer (MIR) onboard the NASA ER‐2 aircraft over the arctic region north of Alaska are analyzed to study the characteristics of cirrus clouds. The brightness from the 1.88 and 10.4 μm channels of the MODIS airborne simulator (MAS) and the 1.064 μm backscatter from the cloud lidar system (CLS) are used to identify these clouds. The brightness temperature depressions derived from the 340 and 183.3±7 GHz measurements at 43° incidence are compared with the results of radiative transfer calculations to arrive at an estimation of ice water path (IWP) and median equivalent mass sphere diameter (Dme). The measurements at a high incidence angle of 43° are used to minimize the effect of surface emission. Two different particle size distributions are assumed in these calculations. The estimated average Dme values are ∼270 and ∼241 μm, and the average IWPs are ∼174 and ∼86 g/m2 for the two distributions. The large difference in the estimated IWP values occurs in the domain of radiative transfer calculations at Dme ≤ 100 μm. There are also a number of pairs of 340 and 183.3±7 GHz brightness temperature depressions that occur in the same domain, and the estimation of IWP and Dme cannot be made. These difficulties are probably caused by the low sensitivity to cirrus cloud detection at 183.3±7 GHz.
Global three‐dimensional simulation of aerosol optical thickness distribution of various origins
Authors: Takemura Toshihiko , Okamoto Hajime , Maruyama Yoshihiro , Numaguti Atusi , Higurashi Akiko , Nakajima Teruyuki
Published: February 2004

A global three‐dimensional model that can treat transportation of various species of aerosols in the atmosphere is developed using a framework of an atmospheric general circulation model (AGCM). Main aerosols in the troposphere, i.e., soil dust, carbonaceous (organic and black carbon), sulfate, and sea‐salt aerosols, are introduced into this model. Prior to the model calculations the meteorological parameters are calculated by the AGCM with the nudging technique using reanalysis data. To evaluate aerosol effects on the climate system and to compare simulated results with observations, the optical thickness and Ångström exponent are also calculated taking into account the size distribution and composition. The model results are validated by both measured surface aerosol concentrations and retrieved aerosol optical parameters from National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer. A general agreement is found between the simulated result and the observation globally and seasonally. One of the significant results is that the simulated relative contribution of anthropogenic carbonaceous aerosols to the total optical thickness is comparable to that of sulfate aerosols at midlatitudes of the Northern Hemisphere, which agrees with recent observations. This result leads to a conclusion that the radiative effect evaluation of aerosols on the climate system is necessary to be modified because optical properties of carbonaceous aerosols are different from those of sulfate aerosols. The other finding is that the seasonal shift off the west coast of North Africa observed by satellites, i.e., the latitude of the maximum optical thickness moves seasonally, is also reproduced in consideration of a mixed state of soil dust and carbonaceous aerosols.
A comparison of northern hemisphere winds using SuperDARN meteor trail and MF radar wind measurements
Authors: Hussey G. C. , Meek C. E. , André D. , Manson A. H. , Sofko G. J. , Hall C. M.
Published: February 2004

The main purpose of the Super Dual Auroral Radar Network (Super‐DARN) is to use paired radars to deduce the F‐region convection from Doppler measurements of backscatter seen at large ranges, typically beyond ∼900 km. Nearer to each HF radar, the nearest ranges at ∼165–400 km are dominated by meteor trail echoes. Once formed, the motion of these meteor trails is normally controlled by neutral winds in the 80–110 km altitude range. By combining the line‐of‐sight velocities from all 16 receiver beams (∼52° in azimuth) of a given SuperDARN radar, it is possible to determine the full horizontal wind vector field over the meteor trail height range. Elevation angles are also measured using an interferometer mode and as such height information can, in principle, be obtained from the combined range and elevation angle data. A comparison with neutral wind measurements from a colocated (Saskatoon, Canada) MF wind radar indicates good agreement between the two radar systems at heights of ∼95 km. Based on these detailed comparisons, a simple common method for determining two‐dimensional winds for all SuperDARN radars, which have extensive longitudinal coverage, was developed. Comparisons with other systems used for dynamical studies of tides and planetary waves are desirable and prove to be essential to obtain a good SuperDARN neutral wind motion analysis. The MF radars at Saskatoon and Tromsø, Norway, are located near the western and eastern ends of the Northern Hemisphere network of six SuperDARN radars. Comparisons between the two types of radars for two seasonal intervals (September and December) show that the SuperDARN radars provide good longitudinal coverage of tides in support of the more detailed MF radar data. The two systems complement each other effectively.
Trends in tropospheric aerosol loads and corresponding impact on direct radiative forcing between 1950 and 1990: A model study
Authors: Tegen Ina , Koch Dorothy , Lacis Andrew A. , Sato Makiko
Published: February 2004

Global aerosol optical thicknesses and radiative properties need to be known for the study of decadal temperature change. Aerosol distributions have been developed from global transport models for a mixture of sulfate and carbonaceous aerosols from fossil fuel burning, including also contributions from other major aerosol types such as soil dust and sea salt. Between the years 1950 and 1990 the aerosol distributions change due to changes in emissions of SO2 and carbon particles from fossil fuel burning. The optical thickness of fossil fuel derived aerosols increased by nearly a factor of 3 during this period, with particularly strong increase in eastern Asia. In countries where environmental laws came into effect since the early 1980s (e.g., United States and western Europe), emissions and consequently aerosol optical thicknesses did not increase considerably after 1980, resulting in a shift in the global distribution pattern. In addition to the optical thickness, aerosol single scattering albedos may have changed during this period due to different trends in absorbing black carbon and reflecting sulfate aerosols. However, due to uncertainties in the emission trends, which are especially large in the case of carbonaceous aerosols, such change cannot be determined with any confidence. Radiative forcing of this aerosol distribution is calculated for several scenarios. Uncertainties in the contribution of the strongly absorbing black carbon aerosol leads to a range in top‐of‐atmosphere forcings of ≈ −0.5 to +0.1 Wm−2.
Reconstruction of lightning currents and return stroke model parameters using remote electromagnetic fields
Authors: Popov M. , He S. , Thottappillil R.
Published: February 2004

Estimating parameters of a lightning return stroke model and the current at the base of the lightning channel from measurements of remote electromagnetic fields is an important practical problem. In the present paper we apply a genetic algorithm (a global optimization technique) to the above problem. Firstly, we use the genetic algorithm to reconstruct the return stroke model parameters using the channel‐base current and the remote electromagnetic field measured simultaneously. The channel‐base current is used to calculate the electric field at the measurement point. The difference between the calculated and the measured fields is minimized using the genetic algorithm, which finds the optimal solution for the return stroke parameters. The transmission line and Diendorfer‐Uman return stroke models are used in the illustrations. Secondly, we develop a method to reconstruct the return stroke model parameters and channel‐base current from electric or magnetic fields measured at two different distances, one of which is a far‐field and the other is a near field. The Diendorfer‐Uman model is used in the reconstruction. Thirdly, we develop a method to reconstruct the return stroke model parameters and channel‐base current from two magnetic fields at two different distances, one of which is an intermediate field and the other is a near field. For doing the above with the Diendorfer‐Uman model we derive an explicit inversion formula using both the induction and the radiation terms of the magnetic field.
Day‐to‐day temperature variability trends in 160‐ to 275‐year‐long European instrumental records
Authors: Moberg A. , Jones P. D. , Barriendos M. , Bergström H. , Camuffo D. , Cocheo C. , Davies T. D. , Demarée G. , Martin‐Vide J. , Maugeri M. , Rodriguez R. , Verhoeve T.
Published: February 2004

Day‐to‐day temperature variability is investigated in eight European series of daily mean temperatures beginning between 1722 and 1833. Eight statistical measures of day‐to‐day temperature variability are compared. The intramonthly standard deviation of daily temperature anomalies is found to be a good measure. The absolute change in temperature anomaly from one day to the next is sensitive to changes in observational procedures and is suggested as a diagnostic tool for identification of inhomogeneities in instrumental temperature series. Because many changes in observational procedures have taken place, quantitative estimates of trends in day‐to‐day variability, based on all series, could only be calculated for 1880–1998. A trend analysis over this period indicates an increase by 5% in southwest Europe, 0 to −5% change in the northwest, and a decrease by 5 to 10% in northeast Europe. On a longer time perspective, day‐to‐day temperature variability in winter, spring, and autumn in northern Europe has decreased over the last 200–250 years. The frequency of extremely cold winter days in northern Europe was lower in the twentieth century than in the eighteenth and nineteenth centuries. Day‐to‐day temperature variability in winter in northern Europe was negatively correlated with a North Atlantic Oscillation index in the period 1826–1997, but some other factor must also have contributed to the long‐term variability decrease. More long daily temperature series, and development of homogenization methods for such data, are needed for an improved knowledge of long‐term changes in day‐to‐day temperature variability.