bluuebunny/crossref_metadata_2025 huggingface dataset:
Glacio‐chemical study spanning the past 2 kyr on three ice cores from Dronning Maud Land, Antarctica: 2. Seasonally resolved chemical records
Authors: Sommer S. , Wagenbach D. , Mulvaney R. , Fischer H.
Published: February 2004

The ammonium, calcium, and sodium concentrations from three intermediate depth ice cores drilled in the area of Dronning Maud Land, East Antarctica, have been investigated. Since all measurements were performed by a high‐resolution Continuous Flow Analysis system, for the first time seasonal signals of chemical trace species could be obtained from the interior of central Antarctica over a period of approximately 2 millennia. Although the elevation as well as the accumulation rate differ between the drilling sites, similar values were obtained by comparing mean concentrations spanning the last 900 years. However, a distinct lack of intersite correlation was found on decadal timescales. Despite a noticeable accumulation change, apparent in one core, no significant concentration change of all three species has occurred. All the measured ions show clear seasonal signals over the whole records. While the sea‐salt‐related component sodium peaks simultaneously with calcium, the maximum ammonium concentration occurs in the snow with a time lag of 2 months after the sea‐salt peak. More than 60% of the calcium concentration can be attributed to an ocean source. Elevated sodium concentrations were found within this millennium compared to mean values of the whole records, but the spatially varying shape of the increase suggests that a possible climatic signal is biased by local deposition effects.
Ice nucleation in sulfuric acid and ammonium sulfate particles
Authors: Prenni Anthony J. , Wise Matthew E. , Brooks Sarah D. , Tolbert Margaret A.
Published: February 2004

Cirrus clouds are composed of ice particles and are expected to form in the upper troposphere when highly dilute sulfate aerosols cool and become supersaturated with respect to ice. In the laboratory we have used Fourier transform infrared spectroscopy to monitor ice nucleation from sulfate particles for relevant compositions of sulfuric acid/water and ammonium sulfate/water aerosols. Measured freezing temperatures are presented as a function of aerosol composition, and results are compared to existing aerosol data. We find that sulfuric acid solution aerosol exhibits greater supercooling than ammonium sulfate solution aerosol of similar weight percent. Ice saturation ratios based on these measurements are also reported. We find that ammonium sulfate solution aerosol exhibits a relatively constant ice saturation of S∼1.48 for ice nucleation from 232 to 222 K, while sulfuric acid solution aerosol shows an increase in ice saturation from S∼1.53 to S∼1.6 as temperature decreases from 220 K to 200 K. These high‐saturation ratios imply selective nucleation of ice from sulfate aerosols.
Monitoring of the stratospheric chlorine activation by Global Ozone Monitoring Experiment (GOME) OClO measurements in the austral and boreal winters 1995 through 1999
Authors: Wagner Thomas , Leue Carsten , Pfeilsticker Klaus , Platt Ulrich
Published: February 2004

Measurements of OClO total column amounts by means of the Global Ozone Monitoring Experiment (GOME) instrument conducted in the austral and boreal winter stratospheres from 1995 through 1999 are presented, GOME is a four‐channel UV/visible spectrometer (240–790 nm) deployed on the polar orbiting European ERS‐2 satellite since April 1995. Previous studies have shown that the observations of OClO, the symmetric chlorine dioxide formed in a side channel of the reaction of BrO + ClO, can serve as an indicator for a stratospheric chlorine activation. GOME's 3‐day coverage of the global atmosphere allows us to infer the first global data set of OClO, and to study continuous time series of its occurrence in both winter stratospheres. It is found that, while OClO regularly occurs over Antarctica in similar amounts and seasonal timing during the different winters, its occurrence is much more variable in the Arctic winter stratosphere, primarily because of the larger dynamic activity that result in warmer temperatures there. About 40% higher OClO column amounts are found in the Antarctic polar stratosphere than in its northern counterpart, a further indication for a significantly more efficient chlorine activation in the Antarctic than the Arctic late winter and spring stratosphere.
In situ observations of aerosol properties above ice saturation in the polar tropopause region
Authors: Petzold Andreas , Hoell Claudia , Kärcher Bernd , Beuermann Jürgen , Schiller Cornelius , Ziereis Helmut , Schlager Hans
Published: February 2004

In the polar winter tropopause region (T≅ 200 K, p = 200 hPa) over northern Europe, the occurrence of a haze‐mode aerosol in the diameter range 0.75 μm <D < 10 μm was frequently observed during airborne in situ measurements of aerosol properties. In ascending air parcels the haze mode was detected as soon as a water saturation ratio Swν = 0.5 (Sice ≅ 1.0) was exceeded. Larger cirrus particles with size D > 10 μm where found in regions with Swν > 0.57 (Sice ≥ 1.17). Hence, a range of Swν values existed between 0.5 and 0.57, where the haze mode was present outside of cirrus clouds. The ratio of number densities of haze‐mode particles to Aitken‐mode particles reached ∼1.25% inside and <0.3% outside of ice‐supersaturated regions. In descending air parcels a continuous decrease in the number density of haze‐mode particles was observed. This suggests a deliquescence transition in rising air parcels and evaporation of liquid haze droplets without recrystallization in sinking air parcels. Consideration of the possible chemical composition, phase, and growth timescales of the haze‐mode particles leads to the conclusion that they are likely to contain ammoniated sulfates in the liquid phase. On the basis of measured particle sizes and calculated extinction values, the observed haze mode may be a candidate for subvisible cirrus cloud particles.
Interhemispheric asymmetry in the seasonal variation of the zonal mean tropopause
Authors: Wong Sun , Wang Wei‐Chyung
Published: February 2004

The SUNYA Atmospheric Models Intercomparison Project 2 (AMIP 2) [Gates et al., 1999] simulation, which is based on NCAR/CCM3 with SUNYA ozone archive [Wang et al., 1995; Liang et al., 1997], is used to study the characteristics of the zonal and monthly mean tropopause. For comparison purposes, the NCEP/NCAR reanalysis data are also analyzed. Both data sets show strong interhemispheric asymmetry in the seasonal variation of tropopause. In addition, the physical and dynamical mechanisms driving the tropopause variation are studied by diagnosing the seasonal heating/cooling components affecting the temperature tendencies around the tropopause. Interhemispheric asymmetry in the seasonal variation of tropopause is related to the asymmetry in temperature, wave activity, and deep convective activity between the two hemispheres. In the Northern Hemispheric (NH) polar region during spring, the warming in the lower stratosphere ends earlier and thus yields a rising tropopause in the coming summer; however, the polar tropopause in the Southern Hemisphere (SH) sinks during SH summer. This asymmetry is attributed to the stronger thermal infrared cooling associated with warmer temperatures in the NH lower stratosphere. In the NH subpolar region, temperature tendencies are greater below the tropopause during spring and autumn; however, they are greater above the subpolar tropopause in the SFI during SH spring and autumn. As a result, the tropopause in the NH subpolar region varies in the direction opposite to that in the SH subpolar region in these seasons. This asymmetry is associated with a stronger dynamical warming in the NH during spring and a stronger thermal infrared cooling in the NH during autumn. In middle latitudes the variation of tropopause is symmetric between the two hemispheres during the respective seasons, but the NH tropopause varies with larger amplitudes. This asymmetry in amplitudes is related to the greater NH wave activity, which induces a greater warming above the tropopause in winter through the descending branch of the Brewer‐Dobson circulation, and the greater NH deep convective activity, which induces a greater warming below the tropopause in summer through latent heat release. The greater NH stratospheric wave activity also results in larger amplitudes of the tropopause variation in the SH subtropics, because of the greater cooling above the tropopause induced by the ascending branch of the Brewer‐Dobson circulation in NH winter. Compared to the situation in middle latitudes, latent heat release in the tropics plays a less direct role in driving the seasonal variation of the tropical zonal mean tropopause.
Homogeneous ice nucleation in aqueous ammonium sulfate aerosol particles
Authors: Chelf J. Holly , Martin Scot T.
Published: February 2004

The mechanisms involved in cloud formation affect the resulting cloud radiative properties. Therefore, accurate cloud formation models and related global warming models require freezing data for ammonium sulfate particles, a major constituent of upper tropospheric clouds. Homogeneous ice nucleation in aqueous ammonium sulfate particles has been studied by several laboratory groups; however, there is significant disagreement in the reported data. In the present work, the ice freezing temperatures are measured by sending (NH4)2SO4 particles, generated by vapor condensation, through a temperature‐controlled flow cell, where ice formation is detected with infrared spectroscopy. To determine possible explanations for disparate results among several laboratories, the experimental methods used in this study are closely examined. Experiments are performed to preclude artifacts, and experimental conditions are varied to develop a complete understanding of processes involved in particle freezing. All experiments produce consistent results. Mass transfer calculations for the aerosol flow tube technique employed in the current study suggest that one frozen particle in 104 to 106 may scavenge sufficient water vapor from surrounding supercooled aqueous particles such that a single ice particle accumulates enough mass to yield a positive freezing event in the infrared spectrum. This result appears to partially explain the confusing and conflicting results coming from several laboratories employing different experimental methods to observe homogeneous ice nucleation in ammonium sulfate. Six lower limit J (cm−3 s−1) values are determined as 1.1×106 (233±1 K, x = 0.013±0.001), 1.1×106 (231±1 K, x = 0.029±0.004), 1.7×106 (227±1 K, x = 0.066±0.015), 2.8×106 (226±1 K, x = 0.071±0.018), 8.4×106 (224±1 K, x = 0.097±0.034), and 8.4×108 (221±1 K, x = 0.153±0.076), where the associated temperatures and solute mole fraction aqueous compositions (x) are noted in parentheses.
Multiscale denoising of self‐similar processes
Authors: Vidakovic Brani D. , Katul Gabriel G. , Albertson John D.
Published: February 2004

A practical limitation to investigating self‐similarity in geophysical phenomena from their measured state variables is that measured signals are typically convolved with instrumentation noise at multiple scales. This study develops and tests a multiscale Bayesian model (BEFE) for separating a 1/f‐like signal from inherent instrumentation noise and contrasts its performance to the Wiener‐type (WAS) and Fourier amplitude (FAS) shrinkage methods. The novel feature in BEFE is that the separation is performed in the wavelet domain and involves the use of a Bayesian inference approach guided by existing theoretical power laws in the filtered signal energy spectrum. We contrast the performance of all three methods for synthetic fractional Brownian motion (fBm) signals and turbulent velocity time series collected in the atmospheric boundary layer. Differences between BEFE and WAS were minor except for the spectral properties at low signal‐to‐noise ratios and at the finest levels of details in which the filtered signal spectra by BEFE is more consistent with the spectra of the uncontaminated velocity signal.
Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO2]/[HCl] during POLARIS
Authors: Voss P. B. , Stimpfle R. M. , Cohen R. C. , Hanisco T. F. , Bonne G. P. , Perkins K. K. , Lanzendorf E. J. , Anderson J. G. , Salawitch R. J. , Webster C. R. , Scott D. C. , May R. D. , Wennberg P. O. , Newman P. A. , Lait L. R. , Elkins J. W. , Bui T. P.
Published: February 2004

We examine inorganic chlorine (Cly,) partitioning in the summer lower stratosphere using in situ ER‐2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [ClONO2]/[HCl] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO2], and an intercomparison with balloon observations, the most direct explanation for the model‐measurement discrepancy in Cly, partitioning is an error in the reactions, rate constants, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER‐2 ClONO2 measurement (too low). The high precision of our simulation (±15% 1σ for [ClONO2]/[HCl], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both the accuracy and precision of stratospheric photochemical models.
A physically based estimate of radiative forcing by anthropogenic sulfate aerosol
Authors: Ghan Steven J. , Easter Richard C. , Chapman Elaine G. , Abdul‐Razzak Hayder , Zhang Yang , Leung L. Ruby , Laulainen Nels S. , Saylor Rick D. , Zaveri Rahul A.
Published: February 2004

Estimates of direct and indirect radiative forcing by anthropogenic sulfate aerosols from an integrated global aerosol and climate modeling system are presented. A detailed global tropospheric chemistry and aerosol model that predicts concentrations of oxidants as well as aerosols and aerosol precursors, is coupled to a general circulation model that predicts both cloud water mass and cloud droplet number. Both number and mass of several externally mixed aerosol size modes are predicted, with internal mixing assumed for the different aerosol components within each mode. Predicted aerosol species include sulfate, organic and black carbon, soil dust, and sea salt. The models use physically based treatments of aerosol radiative properties (including dependence on relative humidity) and aerosol activation as cloud condensation nuclei. Parallel simulations with and without anthropogenic sulfate aerosol are performed for a global domain. The global and annual mean direct and indirect radiative forcing due to anthropogenic sulfate are estimated to be −0.3 to −0.5 and −1.5 to −3.0 W m−2, respectively. The radiative forcing is sensitive to the model's horizontal resolution, the use of predicted versus analyzed relative humidity, the prediction versus diagnosis of aerosol number and droplet number, and the parameterization of droplet collision/coalescence. About half of the indirect radiative forcing is due to changes in droplet radius and half to increased cloud liquid water.
Unresolved spatial variability and microphysical process rates in large‐scale models
Authors: Pincus Robert , Klein Stephen A.
Published: February 2004

Prognostic cloud schemes in large‐scale models are typically formulated in terms of grid‐cell average values of cloud condensate concentration q, although variability in q at spatial scales smaller than the grid cell is known to exist. Because the source and sink processes modifying q are nonlinear, the process rates computed using the mean value of q are biased relative to process rates which account for subgrid‐scale variability. A preliminary assessment shows that these biases can modify instantaneous process rates by as much as a factor of 2. Observations of q at a continental site suggest that the bias is avoided in current practice through the arbitrary tuning of model parameters. Models might be improved if subgrid‐scale variability in q were explicitly considered; several approaches to this goal are suggested.
UV‐B optical thickness observations of the atmosphere
Authors: Kirchhoff V. W. J. H. , Silva A. A. , Costa C. A. , Leme N. Paes , Pavão H. G. , Zaratti F.
Published: February 2004

The optical thickness of the atmosphere, τ, was deduced from measurements of narrowband direct solar UV‐B radiation. A measurement campaign was organized to obtain the radiation at three different sites, during the month of August 1999, using the same methods and instruments, in order to deduce the atmospheric optical thickness in the spectral UV‐B range (280–320 nm). The three observation sites were chosen to cover a wide range of measurement conditions; located near the tropical Atlantic Ocean (Natal, 5.8°S, 35.2°W), on the Andes mountains (La Paz, 16.5°S, 68.1°W), and in the biomass burning area of central Brazil (Campo Grande, 19.2°S, 54.3°W). The UV‐B measurements were made with a Brewer spectrophotometer at each site. Since the instrument measures atmospheric ozone and SO2 simultaneously, it is possible, from the total atmospheric optical thickness τ, to deduce the aerosol optical thickness τaerosol. The results have been combined in different ways to compare with satellite data, showing good performance. Time variations as short as about 10 min can be seen. On clear days the time variations are relatively small, as expected. On the other hand, for the biomass burning site, for conditions of mixed air masses (the instrument is not looking directly at plumes) one can see very large variations in τ in relatively short time intervals, for example, for one case, from 3.5 to 7.0 in about 30 min. Absolute values for τ at Natal and La Paz were near 2.0 and at Campo Grande, between 2.5 and 3.0, but with occasional highs of about 4.5. For τaerosol, Natal and La Paz had values between 0.0 and 0.4, whereas Campo Grande had most values near 0.4, with occasional highs near 1.0, 1.2, and 2.2.
Global direct radiative forcing due to multicomponent anthropogenic and natural aerosols
Authors: Jacobson Mark Z.
Published: February 2004

Global simulations of the composition of and direct forcing due to aerosols containing natural and/or anthropogenic sulfate, nitrate, chloride, carbonate, ammonium, sodium, calcium, magnesium, potassium, black carbon, organic matter, silica, ferrous oxide, and aluminum oxide were carried out. Chloride and natural sulfate were found to be the most important natural aerosol constituents in the atmosphere in terms of solar plus thermal‐infrared forcing. Sea spray was the most important natural aerosol type, indicating that it should be accounted for in weather and climate calculations. Ammonium was found to have a positive direct forcing, since it reduces water uptake in sulfate‐containing solutions; thus, anthropogenic ammonium contributes to global warming. The magnitudes of ammonium and nitrate forcing were smaller than those of chloride or sulfate forcing. When organics were divided into three groups with different assumed UV absorption characteristics, total aerosol direct forcing at the tropopause increased by about +0.03 to +0.05 W m−2 (direct forcing by organics remained negative), suggesting that UV absorption by organics is a nontrivial component of the global energy balance. Gypsum [CaSO4‐2H2O], sal ammoniac [NH4Cl], halite [NaCl], halite, and nitrum [KNO3] were estimated to be the most common sulfate‐, ammonium‐, sodium‐, chloride‐, and nitrate‐containing solid‐phase aerosol constituents, respectively, in the global atmosphere. Solid formation in aerosols was found to increase total‐aerosol direct forcing by +0.03 to +0.05 W m−2. Spatial and vertical forcing estimates, sensitivities of forcing to relative humidity and concentration, and estimates of global aerosol liquid water content are given. Modeled aerosol optical properties are compared with satellite and field measurements.
Sensitivity of washout on HNO3/NOx ratio in atmospheric chemistry transport models
Authors: Velders Guus J. M. , Granier Claire
Published: February 2004

Global chemistry transport models often overestimate the HNO3/NOx ratio in the free troposphere compared with observations. The sensitivity of the HNO3/NOx ratio to the parameterization of washout is studied using two global models, IMAGES and MOZART. The observed ratio of HNO3/NOx is about 4±3 in the free troposphere over remote areas. Calculations with the IMAGES model are in good agreement with these observations, but those with the MOZART model are too high (about 7–20). The good agreement found for IMAGES might be fortuitous, caused by the physically unrealistic parameterization of washout as a continuous removal rate. Applying different magnitudes of a continuous washout or different intermittent washout frequencies increases the HNO3/NOx ratio by about a factor of 2–3, worsening the agreement with observations. Smaller, but significant, changes of the order of 10–20% can be seen in NO, NO2, and ozone as a result of changes in the washout parameterization. The heterogeneous conversion of HNO3 on SO4 (used as a proxy for soot) yields only a small improvement in the HNO3/NOx ratio for most remote locations. Significant effects of this heterogeneous conversion can be expected over and downwind of polluted locations. Given the sensitivity of the HNO3/NOx ratio to the parameterization of washout, this ratio may be more suited to test the quality of the parameterization of the washout than the nitrogen chemistry in global chemistry transport models. Expressed differently, the HNO3/NOx ratio is little affected by and not very important for tropospheric chemistry in these models.
Interannual variability of ozone and UV‐B ultraviolet exposure
Authors: Herman J. R. , Piacentini R. D. , Ziemke J. , Celarier E. , Larko D.
Published: February 2004

Zonal averages of annual and seasonal averages of ozone amounts from Nimbus 7/TOMS (1979–1992) have been examined to estimate the systematic interannual variability of UV‐B (290–320 nm) exposure to solar radiation between ±60° latitude. As shown from statistical modeling, clear‐sky interannual UV‐B changes can be ascribed mainly to the quasi‐biennial oscillation (QBO) driven by stratospheric winds. The QBO oscillations can cause interannual changes in UV‐B exposure of ±15% at 300 nm and ±5% at 310 nm at the equator and at middle latitudes. In addition to QBO effects, there are larger interannual changes in ozone and UV‐B associated with dynamical effects at higher latitudes. When UV‐B attenuation from clouds is included, the general latitudinal structure of the interannual variability is maintained. At the equator the interannual variability of ozone amounts and UV exposure caused by the combination of the 2.3 year QBO and annual cycles implies that there is about a 5 year periodicity in UV‐B variability caused by dynamical effects. At higher latitudes the appearance of the interannual UV‐B maximum is predicted by the QBO but without the regular periodicity. The QBO effects on UV‐B irradiance are larger than the long‐term changes caused by the decrease in ozone amounts.
Estimating Olympic Peninsula precipitation from upper air wind and humidity
Authors: Rasmussen L. A. , Conway H. , Hayes P. S.
Published: February 2004

Daily precipitation at Forks, a lowland station on the west side of the Olympic Peninsula, correlates well over 1948–1996 with wind and moisture in twice‐daily upper air soundings at a nearby radiosonde station. Values at 850 mbar are taken as an index of the total moisture flux. The model estimates precipitation by using the component, raised to a power, of the wind in a particular critical direction scaled by the relative humidity. Thresholds are imposed for the wind component and the relative humidity to reduce the likelihood of estimating precipitation from weak onshore flow on dry days. The critical wind direction, about 238°, is where the long‐term mean of the relative humidity is maximum rather than where that of the absolute humidity is. A split‐sample analysis indicates that the model parameters are highly robust against sampling error. This simple moisture flux model estimates Forks precipitation well; the coefficient of determination for daily precipitation r2 = 0.50 improves to 0.84 for monthly values. Results from a mesoscale precipitation model were negligibly better. For 19 other stations around the periphery of the mountainous peninsula, the critical direction varies only from 210° to 257°. Precipitation on the peninsula is greatest where the moisture flux from the southwest encounters topographic upslope; elsewhere, it is lower but still occurs primarily when the 850‐mbar wind is from the southwest rather than in the local upslope direction. For stations on the north side of the peninsula the critical direction is more westerly, and for stations on the east side it is more southerly.
On satellite scatterometer model functions
Authors: Brown R. A.
Published: February 2004

Satellite scatterometer model functions are the empirical correlations between observed backscatter and geophysical parameters. One problem is the choice of “surface truth” for the latter. Currently, buoy data and general circulation model surface analyses have been used to establish 10 m neutrally stratified wind model functions. The correlation between backscatter and surface winds has been the foundation of the scatterometers. The backscatter is anisotropic with respect to wind direction, allowing wind direction information to be derived from multiple looks at the same patch of ocean. Here the evolution of a model function for the pressure gradient (hence surface geostrophic wind) is given. The European Center for Medium‐Range Weather Forecasts surface pressure analyses are used to establish the pressure‐based model function. There are limitations and advantages to both geophysical data sets.
A unified model for ultrafine aircraft particle emissions
Authors: Kärcher B. , Turco R. P. , Yu F. , Danilin M. Y. , Weisenstein D. K. , Miake‐Lye R. C. , Busen R.
Published: February 2004

To predict the environmental impacts of commercial aviation, intensive studies have been launched to measure the properties and effects of aircraft emissions. These observations have revealed an extremely wide variance with respect to the number and sizes of the particles produced in the exhaust plumes. An analytic parameterization is presented that explains most of the observational variance. It is shown that the observed scatter in emission indices of volatile particles is due mainly to variations of plume age, the detection threshold size of the particle counters, and condensable organic emissions. The principle trend of the volatile particle concentrations with fuel sulfur content can be explained with conversion fractions of sulfur into particulate sulfuric acid at emission within the range 0.5 to 5%. A novel assessment of the perturbation of the stratospheric aerosol layer by a future supersonic aircraft fleet confirms previous estimates and puts these simulations on a sounder physical basis.
An examination of thunderstorm‐charging mechanisms using a two‐dimensional storm electrification model
Authors: Helsdon John H. , Wojcik William A. , Farley Richard D.
Published: February 2004

The early, prelightning, electrification of a storm resulting from noninductive (NI) charging involving graupel, cloud ice/snow, and supercooled cloud water in a riming environment is studied using a comparative approach in a two‐dimensional storm electrification model. The primary schemes examined are NI charge transfers based on the laboratory work of Takahashi [1978] and Saunders et al. [1991]. The NI mechanism, based on Takahashi's work, develops a positive dipole (positive charge above negative) and electric fields approaching 185 kV m−1 as the cloud enters the dissipating stage. Charge transfers, based on the work of Saunders and colleagues, had to be reduced in magnitude to produce electrification that is consistent with the observations. In addition, the Saunders scheme produces an initially inverted dipole (negative charge above positive) which resolves to a positive dipole in the latter part of the simulation and produces electric fields approaching 250 kV m−1. Sensitivity tests show that the NI scheme, based on Takahashi's work, is sensitive to the number concentration of ice crystals, whereas the Saunders‐based scheme is much less sensitive to ice crystal numbers. The Saunders parameterization has strong positive charging of graupel at low effective liquid water content and low temperature. This positive charging can result in an unusual cloud‐top charge structure when used at full value but is benign when the charging is reduced in magnitude. The charge structure resulting from the Saunders scheme is quite sensitive to the calculation of the effective water content, which determines the level of charge reversal. Both of the NI schemes are capable of producing electrification that approaches thunderstorm levels.
Diagnosing warm season precipitation over the GCIP region from a GCM and reanalysis
Authors: Oglesby Robert J. , Marshall Susan , Roads John O. , Robertson Franklin R.
Published: February 2004

A 45‐year simulation using a global general circulation model (GCM), the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3), forced with observed sea surface temperatures (SST), and 39 years of global National Centers for Environmental Prediction (NCEP) reanalyses were analyzed to determine Mississippi River basin warm season (May, June, July or MJJ) wet and dry year composites in the water and energy budgets. Years that have increased MJJ soil moisture over the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP) region also have high precipitation, lower surface temperature, decreased Bowen ratio, and reduced 500‐hPa geopotential height (essentially reduced MJJ ridging). The reverse is true for years that have reduced MJJ soil moisture. Wet years are also accompanied by a general increase in moisture transport from the Gulf of Mexico into the central United States. There is some indication (though weaker) that soil moisture may then affect precipitation and other quantities and be affected in turn by 500‐hPa geopotential heights. The correlations are somewhat low, however, demonstrating the difficulty in providing definitive physical links between the remote and local effects. Analysis of two individual years with an extreme wet event (1993) and an extreme dry event (1988) yields the same general relationships as with the wet and dry composites. The composites from this study are currently serving as the basis for a series of experiments aimed at determining the predictability of the land surface and remote SST on the Mississippi River basin and other large‐scale river basins.
A new lightning return stroke model based on antenna theory
Authors: Moini Rouzbeh , Kordi Behzad , Rafi Gholamreza Z. , Rakov Vladimir A.
Published: February 2004

A new approach based on antenna theory is presented to describe the lightning returnstroke process. The lightning channel is approximated by a straight and vertical monopole antenna with distributed resistance (a so‐called lossy antenna) above a perfectly conducting ground. The antenna is fed at its lower end by a voltage source such that the antenna input current, which represents the lightning return‐stroke current at the lightning channel base, can be specified. An electric field integral equation (EFIE) in the time domain is employed to describe the electromagnetic behavior of this lossy monopole antenna. The numerical solution of EFIE by the method of moments (MOM) provides the time‐space distribution of the current and line charge density along the antenna. This new antenna‐theory (or electromagnetic) model with specified current at the channel base requires only two adjustable parameters: the return‐stroke propagation speed for a nonresistive channel and the channel resistance per unit length, each assumed to be constant (independent of time and height). The new model is compared to four of the most commonly used “engineering” return‐stroke models in terms of the temporal‐spatial distribution of channel current, the line charge density distribution, and the predicted electromagnetic fields at different distances. A reasonably good agreement is found with the modified transmission line model with linear current decay with height (MTLL) and with the Diendorfer‐Uman (DU) model.
Atmospheric regime of dust and salt through 75,000 years of Taylor Dome ice core: Refinement by measurement of major, minor, and trace metal suites
Authors: Hinkley Todd K. , Matsumoto Akikazu
Published: February 2004

Measurement of absolute and relative amounts of dust and salt deposited in the polar ice record is central to several fields of study, including nutrient delivery, atmospheric deposition of trace elements, past wind strengths, dust provenance, and other aspects of climate and geochemical history. We present a method intended to give a more accurate picture than has been possible before of the total amounts and relative proportions of the dust and salt deposited by the atmosphere into polar ice. It also permits us to distinguish different compositional types of dust in the ice. The method is based on precise measurement of a suite of several metals whose proportions contrast strongly between dust and salt and vary substantially between dust types. We apply the method to a small suite of ice samples from the Taylor Dome core in coastal West Antarctica. In full glacial times, when total impurities were high and dust dominated over salt, wind strength in the West Antarctic region was apparently high, and extensive sea‐ice cover prevented incorporation of salt into the atmospheric load. At the termination of the glacial period, increased salt in the dust‐salt mixture indicates that sea ice diminished, but wind strength continued high, and unchanged dust composition indicates unchanged source areas. At about 10,000–11,000 y.B.P., sea‐ice cover appears to have briefly returned to glacial conditions, but wind conditions remained in the milder postglacial condition. Soon after, sea ice retreated, and an abrupt change in dust composition indicates changed source materials or terranes. If extended by analysis of more samples from more sites, such information on salt and dust could provide firm constraints on past wind strengths, extent of sea‐ice cover, deposition fluxes of salt and dust, and changing continental source areas of dust, for both polar regions of the Earth.
Comparison of upper tropospheric humidity retrievals from TOVS and Meteosat
Authors: Escoffier Christelle , Bates John J. , Chédin Alain , Rossow William B. , Schmetz Johannes
Published: February 2004

Two different methods for retrieving upper tropospheric humidity (UTH) from the TIROS Operational Vertical Sounder (TOVS) instruments aboard NOAA polar orbiting satellites are presented and compared. The first one, from the Environmental Technology Laboratory, computed by J. J. Bates and D. L. Jackson, estimates UTH from a simplified radiative transfer analysis of the upper tropospheric infrared water vapor channel at 6.7 μm wavelength measured by High‐resolution Infrared Radiation Sounder (HIRS). The second one results from a neural network analysis of the TOVS (HIRS and Microwave Sounding Unit (MSU)) data developed at the Laboratoire de Météorologie Dynamique. Although the two methods give very similar retrievals in temperate regions (30°–60°N and S), the latter is larger by up to 16% in the tropics. The two data sets have also been compared with the UTH retrievals from infrared radiance measurements at 6.3 μm wavelength from the geostationary satellite Meteosat. These products are taken from the archive without any reprocessing that would take care of known biases. Since the Meteosat UTH in 1989 was confined to clear‐sky areas, it has a dry bias. The differences observed among the three data sets can be explained. UTH computation is sensitive to assumed air temperature and humidity profiles. Despite the biases the spatial and temporal correlations are very good. Overall, the comparison of the two TOVS retrievals provides an assessment of the UTH uncertainties, about 15–25% (relative). With regard to the Meteosat UTH it is concluded that the archived product performs well in depicting spatial and temporal changes. For future quantitative analyses, a reprocessing of the Meteosat UTH is suggested.
Validation of longwave atmospheric radiation models using Atmospheric Radiation Measurement data
Authors: Zhou Y. P. , Cess Robert D.
Published: February 2004

Data taken at the Atmospheric Radiation Measurement Program's central facility in Oklahoma and processed as part of the Clouds and the Earth's Radiant Energy System‐Atmospheric Radiation Measurement‐Global Energy and Water Cycle Experiment (CAGEX) project have been used to validate the top‐of‐the‐atmosphere and surface longwave radiative fluxes for two widely used radiation models: the Column Radiation Model from the National Center for Atmospheric Research Community Climate Model (CCM), and the Moderate Resolution Transmittance (MODTRAN3) radiation code. The results show that for clear skies the models slightly overestimate outgoing longwave radiation at the top of the atmosphere (OLR) and underestimate the surface downwelling longwave flux (SDLW). The accuracy of the radiation models is quite consistent with their respective levels of complexity. For MODTRAN3, for example, the OLR overestimate is 7.1 Wm−2 while the SDLW underestimate is 4.2 Wm−2. For cloudy skies it is emphasized that the cloud input parameters, as determined from measurements by various instruments, require careful examination and preprocessing. Spatial and temporal averaging could result in the parameters representing different volumes of the atmosphere. The discrepancy between model calculations and observations is shown to be significantly reduced through the proper choice of input parameters.
GATOR‐GCMM: A global‐ through urban‐scale air pollution and weather forecast model: 1. Model design and treatment of subgrid soil, vegetation, roads, rooftops, water, sea ice, and snow
Authors: Jacobson Mark Z.
Published: February 2004

A model that treats nesting of gas, size‐ and composition‐resolved aerosol, radiative, and meteorological parameters from the global through urban scales (<5‐km grid spacing) was developed. The model treats multiple one‐way‐nested layers and multiple air quality and meteorological domains in each layer between the global and the urban scales. This latter feature allows forecast of air pollution and weather at several urban or regional sites during the same simulation. Regardless of the number of domains used during a single continuous simulation, the central memory required never exceeds 1.5 times and 2.1 times that of the largest domain for gas and gas/aerosol simulations, respectively. A submodule was developed for all domains to treat ground temperatures, latent heat fluxes, and sensible heat fluxes over subgrid soil types (with and without vegetation), water, sea ice, and urban areas. Urban areas are divided into road surfaces, rooftops, vegetation, and bare soil. Snow is treated over all surface types. The global‐through‐urban model is applied in a companion paper to study elevated ozone, ozone in national parks, and weather during a field campaign in northern and central California.
A microphysics‐based investigation of the radiative effects of aerosol‐cloud interactions for two MAST Experiment case studies
Authors: Erlick Carynelisa , Russell Lynn M. , Ramaswamy V.
Published: February 2004

We use a size‐ and composition‐resolved externally mixed aerosol microphysical model and a delta‐Eddington exponential‐sum‐fit radiation algorithm to examine the interactions between aerosol particles and cloud drops, and their influence on solar radiation. Both the aerosol model and the radiation code are designed to explicitly handle external and internal aerosol particle and cloud drop mixtures. Using observations from the Monterey Area Ship Track (MAST) Experiment, we model changes in aerosol and cloud drop size distributions for a clean marine cloud and ship track and a continentally influenced marine cloud and ship track. Linking these results to the radiation algorithm with a Mie‐scattering subroutine, we investigate the corresponding changes in cloud albedo, cloud absorption, and transmission. The differences in 0.3–3.0 μm albedo and transmission between the clouds and ship tracks as a result of the changes in drop size distribution and composition are found to be substantial, and the composition of the cloud drops is found to be important particularly in the continentally influenced case. Both the clouds and ship tracks enhance atmospheric absorption with respect to a clear sky, with a cloud forcing ratio ranging from 1.15 to 1.37, where the clear sky is defined to be cloud‐ and aerosol‐free. Sensitivity studies are performed with respect to the updraft velocity, the updraft area fraction, dilution of the ship emissions, and the composition of supermicron continental particles. The radiation results are also compared with Meteorological Research Flight (MRF) C‐130 in situ aircraft measurements and with parameterizations of the “Twomey effect.”
Multivariate analysis of solar spectral irradiance measurements
Authors: Rabbette M. , Pilewskie P.
Published: February 2004

Principal component analysis is used to characterize approximately 7000 downwelling solar irradiance spectra retrieved at the Southern Great Plains site during an Atmospheric Radiation Measurement (ARM) shortwave intensive operating period. This analysis technique has proven to be very effective in reducing a large set of variables into a much smaller set of independent variables while retaining the information content. It is used to determine the minimum number of parameters necessary to characterize atmospheric spectral irradiance or the dimensionality of atmospheric variability. It was found that well over 99% of the spectral information was contained in the first six mutually orthogonal linear combinations of the observed variables (flux at various wavelengths). Rotation of the principal components was effective in separating various components by their independent physical influences. The majority of the variability in the downwelling solar irradiance (380–1000 nm) was explained by the following fundamental atmospheric parameters (in order of their importance): cloud scattering, water vapor absorption, molecular scattering, and ozone absorption. In contrast to what has been proposed as a resolution to a clear‐sky absorption anomaly, no unexpected gaseous absorption signature was found in any of the significant components.
Deuterium component of the global molecular hydrogen cycle
Authors: Gerst Steve , Quay Paul
Published: February 2004

The deuterium (D) component of the global molecular hydrogen (H2) cycle is discussed. Field measurements of the fractionation effect during soil uptake of atmospheric H2 (αsoil) were made using an in situ static flux chamber, and an αsoil of 0.943±0.024 (±1σ) was determined. The δD of H2 emitted during combustion of pine needles and pine branches was experimentally measured at −330±50‰ (versus Vienna SMOW) and was 230±60‰ lower than the δD of the plant matter combusted. This hydrogen isotope fractionation effect yielded an estimate of −290±60‰ for the δD of H2 emitted during global biomass burning. The δD of H2 emitted by fossil fuel combustion in the Seattle, Washington, urban area was −196±10‰. On the basis of a global H2 budget calculation, a δD of +130±70‰ is required for the photochemical H2 source in order for the available isotopic measurements to agree with the global H2 budget. A possible mechanism for a significant enrichment in the deuterium content of H2 produced by the photochemical destruction of methane is given. We suggest that the currently available isotopic measurements are not in disagreement with the current view of the global H2 budget.
Reactive uptake of ozone by proxies for organic aerosols: Surface‐bound and gas‐phase products
Authors: Thomas Elan R. , Frost Gregory J. , Rudich Yinon
Published: February 2004

Gas‐phase and surface‐bound products were determined for the reaction of ozone with self assembled monolayers of alkanes and terminal alkenes serving as proxies for atmospheric organic aerosols. The organic surfaces were characterized using infrared (IR) spectroscopy (direct absorption and attenuated total reflection) as well as contact angle measurements with water before and after the reaction with ozone. The contact angle of the organic surfaces was reduced by ∼20° owing to the reaction. Following the reaction, IR absorption due to the presence of carbonyls and carboxylic acids was observed on the surface. Gas‐phase products were determined using infrared spectroscopy immediately above the reaction surface. Under dry conditions, gas‐phase formaldehyde yields of 0.5±0.1 for organic monolayers of allyltrichlorosilane (C3=) and octenyltrichlorosilane (C8=) terminal alkenes were observed, in good agreement with the yields observed for gas phase ozonolysis of terminal alkenes. Surfaces of n‐octane (C8) as well as processed alkene surfaces were nonreactive toward ozone. The reaction mechanism of ozone with the surface alkenes is discussed. Finally, the possible implications for the chemistry of organic aerosols are discussed and studied using a box model and realistic atmospheric scenarios.
A parameterization of an aerosol physical chemistry model for the NH3/H2SO4/HNO3/H2O system at cold temperatures
Authors: Lin Jin‐Sheng , Tabazadeh Azadeh
Published: February 2004

Simple expressions are fitted to the results obtained from ion interaction thermodynamic models for calculating HNO3 and H2O vapor pressures over the NH3/H2SO4/HNO3/H2O system at cold temperatures. The vapor pressure expressions are incorporated into a mass conserving equilibrium solver for computing aerosol compositions in the lower stratosphere and upper troposphere. The compositions calculated from the aerosol physical chemistry model (APCM) are compared against previous parameterizations. The APCM compositions are in better agreement with the compositions obtained from ion interaction models than from other previous formulations of the NH3/H2SO4/HNO3/H2O system. The only advantage of the APCM over the ion interaction approach is that the numerical scheme used in the model is fast and efficient for incorporation into large‐scale models. The APCM is used to calculate HNO3 solubility in aminoniated aerosols as a function of HNO3, H2SO4 and NH3 mass loadings in the lower stratosphere and upper troposphere. While the uptake of HNO3 by ammoniated aerosols is strongly dependent upon the solution neutrality (or pH), we find that in both the lower stratosphere and upper troposphere a significant fraction of HNO3 will exist in aerosol solutions near and below the ice frost point irrespective of solution neutrality.