bluuebunny/crossref_metadata_2025 huggingface dataset:
Moisture budget in the tropics and the Walker circulation
Authors: Rao V. Brahmananda , Chapa S. R. , Cavalcanti I. F. A.
Published: February 2004

Moisture budget in the tropics is studied for two contrasting years, 1987 (El Niño) and 1988 (La Niña), and for two seasons, December, January, and February (DJF) and June, July, and August (JJA). To evaluate the moisture budget, data from the National Centers for Environmental Prediction (NCEP) reanalysis are used. First, the mean rainfall and evaporation characteristics of the NCEP reanalysis for January, April, July, and October are compared with other independent data. The general precipitation zones associated with the large‐scale features such as Intertropical Convergence Zone (ITCZ), South Pacific Convergence Zone (SPCZ), and South Atlantic Convergence Zone (SACZ) seem to be captured by the NCEP reanalysis. However, the seasonal variations, as seen in other data, are not reproduced well. Characteristics of ITCZ in the eastern Pacific is not well reproduced in the NCEP reanalysis data. The overall characteristics of latent heat flux (evaporation) in the NCEP reanalysis seem to be in qualitative agreement with other independent data. However, there are quantitative differences. The differences in rainfall over the western Pacific between DJF 1986/1987 (El Niño) and DJF 1988/1989 (La Niña) are associated with the differences in the position and intensity of SPCZ. The SPCZ is more intense in the El Niño year and is displaced northeastward. Northeast Brazil experienced higher rainfall in DJF 1988/1989 than in DJF 1986/1987. During the boreal summer (monsoon) season (JJA), rainfall was higher in 1988 over India than in 1987. The NCEP rainfall data are compared with the rainfall data from Schemm et al. and Huffman et al. The NCEP reanalysis seems to underestimate rainfall over the equatorial eastern Pacific. Higher precipitable water is found over the regions of intense convection such as SACZ, SPCZ, ITCZ, and the monsoon trough in JJA. The NCEP reanalysis seems to capture the general characteristics of vertically integrated moisture transport such as the general westward transport in the tropics, cross‐equatorial moisture transport over the Indian Ocean with high values near the Somali coast in JJA. Over the western Pacific the differences between the two periods are associated with differences in moisture convergence but not evaporation. The increment term (Dc) shows large values over the mountainous regions. The evaluation of moisture budget of the Walker circulation shows that over the western Pacific the large differences in precipitation between the two contrasting years are accounted mainly by the differences in moisture flux convergence. This verifies the important role of moisture convergence in this region as inferred indirectly by Cornejo‐Garrido and Stone. However, over the Amazon region, evapotranspiration seems to play an important role in the local precipitation.
Physicochemistry of aircraft‐generated liquid aerosols, soot, and ice particles: 1. Model description
Authors: Kärcher B.
Published: February 2004

A coupled microphysical‐chemical‐dynamical trajectory box model simulating the emission, formation, and evolution of particles in dispersing aircraft wakes and contrails is presented. The mutual interaction pathways between the various liquid and solid aerosols are elucidated, including a discussion of the involved microphysical processes. Free model parameters are constrained using data from in situ observations. Volatile aerosol formation by binary nucleation, nonequilibrium heteromolecular condensation, sulfur‐induced soot activation, and ice nucleation in wakes behind subsonic and supersonic aircraft are illustrated in case studies.
Physicochemistry of aircraft‐generated liquid aerosols, soot, and ice particles: 2. Comparison with observations and sensitivity studies
Authors: Kärcher B. , Busen R. , Petzold A. , Schröder F. P. , Schumann U. , Jensen E. J.
Published: February 2004

Results from a coupled microphysical‐chemical‐dynamical trajectory box model have been compared to recent in situ observations of particles generated in the wake of aircraft. Sulfur emissions mainly cause the formation of ultrafine volatile particles in young aircraft plumes (mean number radius ∼5 nm). Volatile particle numbers range between 1016 and 1017 per kg fuel burnt for average to high fuel sulfur levels, exceeding typical soot emission indices by a factor of 10–100. Model results come into closer agreement with observations when chemi‐ions from fuel combustion are included in the aerosol dynamics. Ice particles (mean number radius <1 μm) in young contrails mainly nucleate on water‐activated exhaust aerosols. Homogeneous freezing and soot‐induced heterogeneous freezing are competitive processes leading to ice formation, depending on the temperature and level of oxidized sulfur species. There is evidence that soot triggers freezing even for low fuel sulfur contents, suggesting a sulfur‐independent water activation pathway. Metal particles emitted by jet engines and entrained ambient aerosols may contribute to the formation of larger (>1 μm.) crystals. Contrails with larger crystals would also form without soot and sulfur emissions. The lifecycle of cirrus clouds can be modified by exhaust aerosols.
Denitrification observed inside the Arctic vortex in February 1995
Authors: Sugita T. , Kondo Y. , Nakajima H. , Schmidt U. , Engel A. , Oelhaf H. , Wetzel G. , Koike M. , Newman P. A.
Published: February 2004

Balloon‐borne in situ measurements of total reactive nitrogen (NOy) and nitrous oxide (N2O) were made from Kiruna (68°N, 21°E), Sweden on February 11, 1995. Ten hours later, N2O was again measured by an infrared spectrometer flown on another balloon launched from Kiruna. Both observations were made inside the polar vortex between 380 K (∼14 km) and at least 675 K (∼26 km). In the winter of 1994–1995, temperatures at 475 K (∼19 km) inside the vortex were extremely low, sometimes lower than ice frost point, especially from mid‐December to mid‐January. The NOy profiles obtained during both the ascent and descent revealed layered structures between 15 and 20 km with mixing ratios ranging from 2.7 to 9.3 parts per billion by volume (ppbv). The observed N2O profiles indicate significant downward transport of air due to diabatic cooling in the winter. To quantify the degree of irreversible removal of NOy (denitrification) between 12 and 28 km, the unperturbed values of NOy (i.e., NOy*) were estimated from the observed N2O values using the NOy ‐ N2O relationship obtained at midlatitudes by the atmospheric trace molecule spectroscopy ATLAS mission and in situ aircraft and balloon‐borne measurements. The largest denitrification was observed at 19±0.5 km, where the NOy values were lower than the
values by ∼10 ppbv, corresponding to a 70% removal of NOy. In spite of the large uncertainty in
the NOy values generally agreed well with the
values at ∼14 km as well as between 23 and 28 km. The relationship between NOy and N2O measured between 23 and 28 km agreed with that measured above 40 km at northern midlatitudes in fall, indicating that the air masses sampled at 23–28 km over Kiruna were transported from the midlatitude upper stratosphere followed by the descent inside the vortex.
On the use of hydrocarbons for the determination of tropospheric OH concentrations
Authors: Ehhalt D. H. , Rohrer F. , Wahner A. , Prather M. J. , Blake D. R.
Published: February 2004

This paper explores a new approach to estimating atmospheric hydroxyl radical concentrations from regional measurements of a suite of hydrocarbons. The approach is guided by the study of a suite of synthetic tracers, i, with uniform continental sources and constant but different lifetimes of 1, 2, 5, 20, 50, and 100 days, whose global distributions are calculated from a three‐dimensional chemical tracer model. With the help of the model we show that in a grid box the standard deviation σi divided by the average concentration
is a unique function of the chemical lifetime τi. In favorable cases, for instance, in surface air within a specific region sampled by the Pacific Exploratory Mission (PEM) West B campaign, that function takes a simple form:
, with α = 0.48, very close to 1/2. An analogous relation is found for the alkanes, ethane through n‐hexane, measured during the PEM West B campaign in the same domain, with their reaction rate constant with OH, kOH. That relation has the form
, with α′ = 0.49. Using the alkenes, for example propene, which also react with O3, the dependence on kOH,i can be related to a dependence on τi. This allows us to estimate the OH concentration, 6×105cm−3, with an error of roughly a factor of 2 for this region (boundary layer, 30°N‐40°N latitude, and 135°E‐155°E longitude in March). This estimate is essentially based on empirical relations only and the assumption that the considered hydrocarbons have the same source distribution. As a by‐product, we show that the α defined above is related to the slope in the (logarithmic) correlation plot between the mixing ratios of two trace gases with different lifetimes. We also show that the global distribution of a appears to be a useful tool to diagnose fast regional transport.
Remote sensing of smoke from MODIS airborne simulator during the SCAR‐B experiment
Authors: Chu D. A. , Kaufman Y. J. , Remer L. A. , Holben B. N.
Published: February 2004

MODIS (moderate resolution imaging spectroradiometer) airborne simulator (MAS) data acquired during the SCAR‐B (Smoke, Clouds, and Radiation‐Brazil) experiment provide a test bed to evaluate aerosol retrievals for MODIS measurements over land. The SCAR‐B MAS data covered forest and cerrado regions with varying smoke concentration as a result of fires in August–September, 1995. Excellent agreement is obtained by comparing the retrieved aerosol optical thickness from the MAS data with AERONET (Aerosol Robotic Network) ground‐based Sun photometer observations. The evaluation of MODIS aerosol algorithm was performed for a range of smoke optical thickness from 0.1 to 2 at 0.66 μm wavelength over cerrado and forest sites. All the retrieved values of optical thickness are found within the anticipated retrieval errors of Δτ= ±0.05±0.2τ. This confirms for smoke aerosol the validity of the MODIS algorithms and the use of the new dynamic aerosol models in this algorithm (the algorithm was already validated previously for urban/industrial aerosol). The retrieved optical thickness is found to be very sensitive to the assumed value of the single‐scattering albedo but not sensitive to the aerosol refractive index (real part). The single‐scattering albedo of 0.90 suggested in the aerosol model yields the best results. Test of the MODIS use of a grid box of 10×10 km in the aerosol algorithm shows that the standard deviation of the retrieved aerosol should generally not be more than 10% of the average value for a large range of optical thickness values from 0.2 to 2.0, close and far from fires.
Case studies of tropospheric ozone events observed at the summit of Mount Fuji
Authors: Tsutsumi Yukitomo , Igarashi Yasuhito , Zaizen Yuji , Makino Yukio
Published: February 2004

Tropospheric ozone events observed at the summit of Mount Fuji (3776 m above sea level) were analyzed as case studies. The ozone, intruding from the lower stratosphere by the cutoff low or the tropopause fold over the Asian continent, is transported to the middle troposphere over Japan even in summer. The subtropical jet, which is intensified over Japan, also contributes to the summer intrusion from the stratosphere. A long stratospheric streamer, as described previously by Appenzeller et al [1996], brings about a persistent ozone enhancement at the summit of Mount Fuji. The large variation of summer ozone over Japan is attributable to the alternate overspreading of these ozone‐rich stratospheic air masses and the ozone‐depleted subtropical maritime air mass. In contrast, winter ozone variation is relatively small at the summit of Mount Fuji. The steep potential temperature gradient between 500 hPa and 300 hPa in the winter cutoff low, which restrains vertical mixing in the upper troposphere, possibly causes less influence of the stratospheric air mass on the middle troposphere in winter, since mixing processes around a cutoff low play an important role in air mass exchange between the stratosphere and the troposphere.
NOAA 11 Solar Backscattered Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 1. Observations and long‐term calibration
Authors: Cebula Richard P. , DeLand Matthew T. , Hilsenrath Ernest
Published: February 2004

Measurements of solar ultraviolet (UV) irradiance are a valuable diagnostic of physical processes in the solar atmosphere and a key component in characterizing the external forcing of the Earth's atmosphere. However, the deleterious effects of solar UV radiation on satellite instrument components have complicated efforts to determine the magnitude of long‐term solar variations for almost 30 years. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument, primarily designed to measure stratospheric ozone, also made daily spectral scan measurements of solar UV irradiance in the 160–405 nm region from February 1989 to October 1994. An onboard calibration system and comparisons with coincident Shuttle SBUV (SSBUV) measurements were used to correct for long‐term NOAA 11 instrument sensitivity changes. Time series of the NOAA 11 solar irradiance data indicate a long‐term accuracy of approximately ±0.9–2.3% (2σ) over the 5.5‐year data record. Long‐term solar irradiance changes of approximately −3% are observed between 215 and 250 nm, increasing in magnitude to approximately −7% at 200–205 nm. Additional discussion of solar variations observed by NOAA 11 SBUV/2 are presented by DeLand and Cebula [this issue].
Atmospheric aerosol over Alaska: 1. Spatial and seasonal variability
Authors: Polissar Alexandr V. , Hopke Philip K. , Malm William C. , Sisler James F.
Published: February 2004

Aerosol chemical composition data for seven National Park Service locations in Alaska for the period from 1986 to 1995 were analyzed. Data for elements, black carbon, and total mass were obtained for samples of atmospheric particles less than 2.5 μm in diameter. Assuming that all elemental aerosol S was from SO4=, concentrations of excess (nonmarine) sulfate (XSO4=) were calculated by subtraction of the sea salt SO4= mass fraction to Na and observed Na concentrations. Mass concentration of XSO4= ranged from 0.01 to 3.9 μg m−3, and mass concentration of black carbon (BC) ranged from 0.01 to 5 μg m−3. Seasonal variations with maxima during the winter/spring season and minima in the summer for most of the sites in Alaska were observed for the XSO4= but not for the BC and the fine mass concentration. The seasonal variations of XSO4= concentration were always higher in northwestern Alaska. BC concentration in central Alaska peaked during the summertime because of the high BC emission from forest fires. The Pb concentration in central Alaska shows a negative trend over the timescale of measurements. It is concluded that all of the sites experience aerosol pollution from long distance, regional, and local sources. Seasonal variations of the XSO4= concentration are related to the seasonal variations in long‐range transport of midlatitudinal S emissions, to the varying rate of photochemical SO4= fine aerosol production and to fine aerosol removal processes in the troposphere. The spatial distribution of XSO4= shows a strong northwest to southeast negative gradient that can be associated with the long‐range transport of anthropogenic aerosol from pollution sources located to the north or northwest of Alaska.
Global lightning: Total, cloud and ground flash estimates
Authors: Mackerras David , Darveniza Mat , Orville Richard E. , Williams Earle R. , Goodman Steven J.
Published: February 2004

A worldwide survey of the ratio of cloud flashes to ground flashes in thunderstorms using CGR3 (Cloud‐Ground Ratio #3) instruments at sites covering a latitude range from 59.9°N to 27.3°S between 1986 and 1991 inclusive has provided information on the latitudinal variation of total flash density and the ratio of cloud flashes to total flashes. Two surveys of global lightning occurrence using satellites, the Defense Meteorological Satellite Program (DMSP) survey using optical sensors to detect lightning at near local midnight and the Ionospheric Sounding Satellite‐b (ISS‐b) survey using radio‐frequency lightning detection equipment, were used in conjunction with the CGR3 observations to obtain the latitudinal variation of total flash density over each major land mass and each major ocean. These three surveys and other sources provided data on the annual and diurnal variation in lightning occurrence at various latitudes. We have combined the above information in a computational model of global lightning occurrence and have separated total flashes into cloud flash and ground flash components. Using this model, we estimate the global annual number of total flashes to be 2.05×109 yr−1, corresponding to a mean rate of 65 s−1. This estimate is probably low because of inadequate detection of weak cloud flashes. Depending on the assumption made concerning the latitudinal variation of the ratio of cloud flashes to total flashes, the mean rates of occurrence are between about 51 s−1 and 55 s−1 for cloud flashes and between about 10 s−1 and 14 s−1 for ground flashes. The uncertainty in these estimates is a factor of 2 in either direction. The calculated global diurnal variation has a peak rate of occurrence of about 1.6 times the mean rate at about 1400 universal time (UT) in the northern hemisphere spring and has its lowest rate of about 0.5 times the mean rate at about 0300 UT in the northern hemisphere summer. About 54% of all lightning occurs in the northern hemisphere. The mean global land total flash density is about 8.3 km−2 yr−1. This is about 3.4 times the mean global total flash density over oceans.
A study of regional aerosol radiative properties and effects on ultraviolet‐B radiation
Authors: Wenny B. N. , Schafer J. S. , DeLuisi J. J. , Saxena V. K. , Barnard W. F. , Petropavlovskikh I. V. , Vergamini A. J.
Published: February 2004

A field experiment was conducted in western North Carolina to investigate the relationship between aerosol optical properties and atmospheric transmission. Two research measurement sites in close horizontal proximity but at different altitudes were established to measure the transmission of UV radiation through a slab of atmosphere. An identical set of radiation sensing instruments, including a broadband UV‐B radiometer, a direct Sun pyrheliometer, a shadowband radiometer, and a spectral photometer, was placed at both sites, a mountaintop site (Mount Gibbes 35.78°N, 82.29°W, 2004 m elevation) and a valley site (Black Mountain, North Carolina 35.66°N, 82.38°N, 951 m elevation). Aerosol size distribution sampling equipment was located at the valley site. Broadband solar pseudo‐optical depth and aerosol optical depths at 415 nm, 500 nm, and 673 nm were measured for the lowest 1‐km layer of the troposphere. The measurements exhibited variations based on an air mass source region as determined by back trajectory analysis. Broadband UV‐B transmission through the layer also displayed variations relating to air mass source region. Spectral UV transmission revealed a dependence upon wavelength, with decreased transmission in the UV‐B region (300–320 nm) versus UV‐A region (320–363.5 nm). UV‐B transmission was found to be negatively correlated with aerosol optical depth. Empirical relations were developed to allow prediction of solar noon UV‐B transmission if aerosol optical depth at two visible wavelengths (415 and 500 nm) is known. A new method was developed for determining aerosol optical properties from the radiation and aerosol size distribution measurements. The aerosol albedo of single scatter was found to range from 0.75 to 0.93 and the asymmetry factor ranged from 0.63 to 0.76 at 312 nm, which is close to the peak response of human skin to UV radiation.
Wet deposition in a global size‐dependent aerosol transport model: 2. Influence of the scavenging scheme on 210Pb vertical profiles, surface concentrations, and deposition
Authors: Guelle W. , Balkanski Y. J. , Dibb J. E. , Schulz M. , Dulac F.
Published: February 2004

The main atmospheric sink for submicron aerosols is wet removal. Lead 210, the radioactive decay product of 222Rn, attaches immediately after being formed to submicron particles. Here we compare the effects of three different wet‐scavenging schemes used in global aerosol simulations on the 210Pb aerosol distribution using an off‐line, size‐resolved, global atmospheric transport model. We highlight the merits and shortcomings of each scavenging scheme at reproducing available measurements, which include concentrations in surface air and deposition, as well as vertical profiles observed over North America and western and central North Pacific. We show that model‐measurement comparison of total deposition does not allow to distinguish between scavenging schemes because compensation effects can hide the differences in their respective scavenging efficiencies. Differences in scavenging parameterization affect the aerosol vertical distribution to a much greater extent than the surface concentration. Zonally averaged concentrations at different altitudes derived from the model vary by more than a factor of 3 according to the scavenging formulation, and only one scheme enables us to reproduce reliably the individual profiles observed. This study shows that ground measurements alone are insufficient to validate a global aerosol transport model.
Earth‐average temperature: A time delay approach
Authors: Bar‐Eli K. , Field Richard J.
Published: February 2004

Earth‐average temperature is investigated using a simple energy balance model of the Budyko‐Sellers‐Ghil type expressed as a delay‐differential equation describing the absorption and reflection of incoming solar radiation and surface‐reflected infrared radiation. Cloud albedos for both radiations and the surface albedo as affected by ice coverage are assumed to depend upon the temperature that existed some time ago, introducing a time delay into the model. A variety of steady and oscillatory states are possible for Earth‐average temperature, assuming plausible functions for the albedos, blackbody cooling, and suitable delay times. Multiple true steady states and multiple transient pseudo steady states as well as periodic, chaotic, and square‐wave relaxation‐oscillations result, The bifurcation structure relating the appearance of these phenomena to system parameters is explored. This very simple climate model exhibits complex behavior in global temperature without solar variation or environmental perturbation. Bifurcations exist at which small changes in parameters lead to destabilization of steady states and dramatic changes in qualitative behavior.
Oxidation of elemental mercury by aqueous chlorine (HOCl/OCl−): Implications for tropospheric mercury chemistry
Authors: Lin Che‐Jen , Pehkonen Simo O.
Published: February 2004

The stoichiometry and kinetics of elemental mercury (Hg0) oxidation by aqueous chlorine (HOCl/OCl−) have been investigated. The stoichiometric ratio of Hg0 to HOCl/OCl− is found to be 1:1, the same as the electron transfer ratio. The rate constants of the oxidation are measured in a novel fashion by using chloramine (NH2Cl) as the free chlorine reservoir. The rate constants at room temperature (23°∼ 25°C) for the Hg0‐HOCl and Hg0‐OCl− are measured to be (2.09±0.06)×106 and (1.99±0.05)×106 M−1 s−1, respectively. Based on the solubility data of chlorine, the intrinsic Henry's law constant of chlorine is calculated to be 7.61×10−2 M atm−1 at 25°C. Model study using the kinetic data in this investigation shows that the oxidation of Hg0 by aqueous chlorine is an important pathway contributing dissolved divalent mercury (Hg(II)) in atmospheric water, especially at higher cloud water pH when solubility of chlorine is greatly increased and before sunrise when chlorine reaches its peak concentrations in the marine troposphere.
The formation and evolution of aerosols in stratospheric aircraft plumes: Numerical simulations and comparisons with observations
Authors: Yu Fangqun , Turco Richard P.
Published: February 2004

The formation and evolution of aerosols in jet aircraft plumes at high altitude are simulated using a detailed aerosol microphysics model that explicitly resolves particle size and composition. Two approaches are used to simulate the microphysics: a standard, or “classical,” approach in which new particle formation initially occurs via homogeneous binary nucleation, followed by condensation and coagulation; and a “kinetic” approach in which the entire course of particle evolution is calculated as a unified collisional mechanism. In the latter approach chemiions generated in the engine combustors can affect molecular cluster growth and ultrafine particle aggregation. Simulations with both approaches reveal that large numbers of volatile ultrafine sulfuric acid particles are generated in the near field behind the engines. The concentrations and subsequent evolution of these “volatile” particles are sensitive to the initial sulfuric acid vapor concentration, the plume dilution rate, and microphysical factors, especially the chemiion abundance, which is brought out through a sensitivity analysis. By contrasting predictions against available field data, it is demonstrated that the kinetic model provides a more realistic representation of particle formation and growth and hence their observable properties than the classical model. Moreover, the effects of chemiions during the early evolution of the plume are found to be crucial to the evolution of the largest volatile aerosols, which may play a role as cloud condensation nuclei. Major sources of uncertainty in the plume aerosol models are noted.
Estimation of vegetation canopy leaf area index and fraction of absorbed photosynthetically active radiation from atmosphere‐corrected MISR data
Authors: Knyazikhin Y. , Martonchik J. V. , Diner D. J. , Myneni R. B. , Verstraete M. , Pinty B. , Gobron N.
Published: February 2004

The multiangle imaging spectroradiometer (MISR) instrument is designed to provide global imagery at nine discrete viewing angles and four visible/near‐infrared spectral bands. This paper describes an algorithm for the retrieval of leaf area index (LAI) and fraction of photosynthetically active radiation absorbed by vegetation (FPAR) from atmospherically corrected MISR data. The proposed algorithm is designed to utilize all the information provided by this instrument, using a two‐step process. The first step involves a comparison of the retrieved spectral hemispherically integrated reflectances with those determined from the model which depend on biome type, canopy structure, and soil/understory reflectances. The biome/canopy/soil/understory models that pass this comparison test are subject to the second step, which is a comparison of their directional reflectances at the MISR angles to the retrieved spectral directional reflectances. This procedure, however, can produce multiple acceptable solutions. The measure theory is used to specify the most probable values of LAI and FPAR using the set of all acceptable solutions. Optimization of the retrieval technique for efficient global processing is discussed. This paper is the second of a two‐part set describing a synergistic algorithm for producing global LAI and FPAR fields from canopy reflectance data provided by the MODIS (moderate resolution imaging spectroradiometer) and MISR instruments.
Some statistical methods for analyzing the TOMS data
Authors: Fang Dongping , Stein Michael L.
Published: February 2004

This paper investigates variations in both space and time of total column ozone through the Nimbus 7 Version 7 Total Ozone Mapping Spectrometer (TOMS) data. While the gridded daily TOMS dataset has high spatial and temporal resolution, the asynoptic nature of the data and artifacts in the observations make it difficult to use at the available level of resolution. In the past, only subsamples or averages over both space and/or time of the TOMS data have been subjected to statistical analysis. This work discusses three ways of modeling the space‐time structure of the TOMS data. We conclude that a representation in which the datapoints at a fixed latitude are treated as if they were on a line in the longitude‐time plane is the best. At each grid point, the total column ozone field is partitioned into a 31 day moving average and a remainder or residual ozone field. For a fixed latitude and a fixed season, the residual field can be reasonably modeled as a stationary random process with mean zero. The sample correlation functions are calculated for 52 seasons over 13 years and 7 latitudes. Comparing these correlation functions and their associated periodograms across seasons, years, and latitudes provides a useful means for discerning patterns in the TOMS data. For example, the correlation functions show that at latitudes around the equator, the apparent movement of ozone is from east to west, and at higher latitudes, the movement is from west to east. In addition, the movement is more strongly from west to east in the southern hemisphere than in the northern hemisphere. Periodograms for latitudes near the equator show clear evidence of a scan‐angle effect noted by others. However, our analysis demonstrates that the scan‐angle effect shows up at frequencies other than the orbital frequencies and its harmonics. We suggest a simple filtering method for removing most of this artifactual effect.
Accuracy of absolute precipitable water vapor estimates from GPS observations
Authors: Tregoning Paul , Boers Reinout , O'Brien Denis , Hendy Martin
Published: February 2004

We present GPS, radiosonde and microwave radiometer (MWR) estimates of precipitable water vapor (PW) at Cape Grim, Tasmania, during November and December 1995. The rms differences between GPS and radiosonde, MWR and radiosonde and GPS and MWR estimates of PW were 1.5 mm, 1.3 mm and 1.4 mm, respectively, whilst the biases between the three systems were ∼0.2 mm. However, there are occasions when the amount of PW was underestimated by GPS whilst at other times was over‐estimated by MWR. The average overlap error of the GPS estimates of PW between adjacent daily solutions is related to the orbit overlap error and we removed a 2 mm bias introduced using International GPS Service orbits by estimating more accurate global orbits. The discrepancies of up to 3–4 mm between the MWR and GPS systems are not caused by rain, waveguide losses, varying waveguide temperature, detector non‐linearity or inaccurate estimates of the mean radiating temperature of the atmosphere. However, small differences between mapping functions at low elevations can produce biases comparable with the bias between the two systems. Consequently, we suspect that the biases arise because the mapping functions do not represent the localized atmospheric conditions at Cape Grim. The most accurate GPS estimates are achieved when the GPS analysis contains station separations of more than 2000 km, an elevation cutoff angle of 12° is used and the CFA2.2 wet mapping function is used to map the wet delay at any angle to the delay in the zenith.
Characteristics of small tropical cumulus clouds and their impact on the environment
Authors: Benner Timothy C. , Curry Judith A.
Published: February 2004

This study uses a number of data sets (Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator data, space shuttle photography, Radiation Measurement System data, aircraft data, and shipboard soundings) to investigate the characteristics of small tropical cumulus clouds and their impact on their environment. The goal is to uncover useful information with application to radiative transfer simulation and satellite remote sensing. In fields of small cumulus clouds, size distributions are found to decrease in number with increasing diameter according to a double power law relation, often with a clear break diameter. Fractal dimensions corresponding to the horizontal area and perimeter of the clouds are greater for the larger clouds than for the smaller clouds, with the same break diameter as the size distributions, meaning that the larger clouds have more ragged perimeters. These two results suggest a characteristic horizontal length scale dividing larger and smaller boundary layer cumuli. Spatial distributions show a clear tendency toward clustering. Smaller cumuli appear to grow upward more quickly with increasing horizontal size than do larger cumuli. Albedo is found to increase with greater cloud fraction and higher solar zenith angle. Even sparse fields of small cumulus cause significant shortwave forcing at the ocean surface. Simulation suggests that small cumulus may introduce significant errors into sea surface temperature retrievals and that such clouds can be difficult to remove with operational cloud‐filtering schemes. Clouds smaller than about 1 km in diameter are not seen to precipitate.
South Pole electric field responses to overhead ionospheric convection
Authors: Tinsley B. A. , Liu Weiping , Rohrbaugh R. P. , Kirkland M. W.
Published: February 2004

Measurements of the vertical atmospheric electric field (Ez) at the South Pole between 1982 and 1986 have been analyzed, and the results reveal an important component due to magnetospheric‐ionospheric coupling processes. The results are consistent with the theoretical expectation that the ionospheric convection potential patterns in the polar caps, as measured by satellites, are superimposed on the otherwise nearly uniform (but varying in time) global ionospheric potential due to low‐latitude thunderstorm sources. The analysis technique was to calculate the ionospheric convection potential from a satellite empirical model, using as input measured solar wind parameters, except that when solar wind data were not available we used as inputs measured ground based magnetic disturbance parameters. We fitted the average low‐latitude Ez variation (the Carnegie curve) to the individually measured South Pole Ez diurnal curves. We then subtracted the fitted Carnegie curves from the measured curves and performed a regression analysis of the residual on the calculated ionospheric convection potential values, averaging the data in three hour intervals. Positive correlations with correlation coefficients of about 0.5 were found. Most of the remaining variability in Ez can be attributed to local meteorologically induced noise (especially in winter) and the day‐to‐day variability of the low‐latitude thunderstorm generators.
Mueller matrix for atmospheric aerosols at CO2 laser wavelengths from polarized backscattering lidar measurements
Authors: Ben‐David Avishai
Published: February 2004

The Mueller matrix
for atmospheric aerosol backscattering in Utah (a desert environment) is derived from lidar‐polarized backscattering measurements at CO2 wavelengths for a horizontal path a few meters above the ground. The Mueller matrix is nearly a diagonal matrix, with m11 = S11, m22 = (S22 − S33)/2, and m33 ≅ −m22, as is expected for randomly oriented aerosols with axial symmetry. The ratio m22/m11 is approximately 0.75, for which the linear depolarization ratio is approximately 0.15. In computing the Mueller matrix from lidar‐polarized backscattering measurements of various combinations of transmit/receive polarizations (xy), two steps are employed: (1) a deconvolution process to produce a lidar signal pxy(r) from the polarized lidar measurements by deconvolving the long CO2 laser pulse and the lidar system impulse‐response function (i.e., removing their smearing effect from the measured lidar signal), and (2) a constrained slope method to produce an aerosol backscattering coefficient βxy, for each polarization configuration xy, from which the Mueller matrix is computed.
Visible absorption cross sections and integrated absorption intensities of molecular oxygen (O2 and O4)
Authors: Newnham David A. , Ballard John
Published: February 2004

Absorption spectra of gas‐phase molecular oxygen and zero air at temperatures of 223 and 283 K have been measured in the laboratory using a coolable multipass‐optics gas cell and Fourier transform spectroscopy in the wavelength range 455 to 830 nm (12,000–22,000 cm−1). Net absorption cross sections of the O2A−, B−, and γ‐bands at <0.002 nm spectral resolution, and pressures of 100 and 1000 hPa zero air have been determined. Binary absorption cross sections of the collision‐induced O4 bands at <0.18 nm spectral resolution and a pressure of 1000 hPa pure oxygen have been determined, with corrections for the O2 γ‐band absorption. Calculated integrated absorption intensities and, for the O2A− and B−bands, “effective” Einstein A‐coefficients are compared with previous literature values.
Non‐local thermodynamic equilibrium in H2O 6.9 μm emission as measured by the improved stratospheric and mesospheric sounder
Authors: Zaragoza G. , López‐Puertas M. , Lambert A. , Remedios J. J. , Taylor F. W.
Published: February 2004

A detailed analysis of the H2O 6.9 μm limb radiance measured by the improved stratospheric and mesospheric sounder on board the Upper Atmosphere Research Satellite has been conducted using a non‐local thermodynamic equilibrium LTE) radiative transfer model. There are important non‐LTE effects in the measured radiance, both in the pressure modulator (sensitive only to the lines of the ν2 fundamental band of H2O) and the wideband signal (similar to that of a conventional radiometer) above about 50–55 km. They show no variability with the solar zenith angle and a strong dependence on mesospheric temperatures. We have found that these effects are due to the departure from LTE of the (010) and (020) levels of H2O in the mesosphere. A value between 1.0 and 3.0×10−12 cm3 s−1, close to the lower limit of the measurements, has been deduced for the rate coefficient of the vibrational exchange between the ν2 vibrational mode of H2O and O2(1). Also, evidence of the excitation of O2(1), with a quantum yield between 2 and 6, after photolysis of O3 in the Hartley band has been found in the measurements.
The nature and origin of Venusian canali
Authors: Williams‐Jones Glyn , Williams‐Jones Anthony E. , Stix John
Published: February 2004

Venusian canali have many characteristics of terrestrial rivers, notably cutoff meanders, braiding, point bars, and deltas, which required both erosion and sediment transport. This implies that the canali were not formed by construction but rather by thermal or mechanical erosion. We have evaluated the relative importance of these latter two mechanisms, assuming a basaltic substrate and a surface temperature similar to that currently prevailing, ∼470°C. In order to have thermally eroded the canali, the liquid must have been turbulent and at a temperature above that of the basalt solidus. The most plausible candidates for this liquid are basalt and komatiite lavas. However, at realistic flow rates and extrusion temperatures, flow of basaltic lava is laminar, and therefore basaltic lavas could not have thermally eroded the canali. Although komatiite flow is initially turbulent, the lava will cool in hours to its solidus temperature, whereas it will take months to thermally erode canali. By elimination, only mechanical erosion can adequately explain canali formation. Based on incision and lateral migration rates for terrestrial rivers, it could take from >5 years (in unconsolidated regolith) to 8×105 years (in solid basalt) to mechanically erode a typical Venusian canale. These estimates require that the eroding agent had a solidus temperature close to the Venusian surface temperature and that viscosities remained low until solidification. Only halogen‐rich, alkali carbonatite and sulfur lavas meet these criteria, and only the former could have been present in sufficient volumes to form the canali. We propose that the canali were mechanically eroded by such carbonatite lavas and that the latter originated from the fusion of anhydrous recycled crust, which had been altered by interaction with a CO2‐, SO2‐, and halogen‐rich atmosphere.
Electrostatic charging properties of Apollo 17 lunar dust
Authors: Horányi Mihály , Walch Bob , Robertson Scott , Alexander David
Published: February 2004

We report on our experimental studies of the electrostatic charging properties of an Apollo 17 soil sample and two lunar simulants, Minnesota Lunar Simulant (MLS‐I) and Johnson Space Center (JSC‐1). We have measured their charge after exposing individual grains to a beam of fast electrons with energies in the range of 20≤E≤90 eV. Our measurements indicate that the secondary electron emission yield of the Apollo 17 sample is intermediate between MLS‐1 and JSC‐1, closer to that of MLS‐1.
Characteristics of lunar mare deposits in Smythii and Marginis basins: Implications for magma transport mechanisms
Authors: Yingst R. Aileen , Head James W.
Published: February 2004

An analysis of 34 lunar lava flows and ponds in the eastern limb Smythii and Marginis basins was undertaken to examine and model the first stages of secondary crustal formation and assess processes involved in magma transport and eruption. In order to isolate the characteristics of single eruptive episodes, we focused on discrete mare ponds adjacent to the major maria. Mean values for areas and volumes of deposits estimated to be good candidates for single eruptive phases are large, approximately 950–1000 km2 and ∼200 km3 respectively. These eruptive volumes are commensurate with the largest known terrestrial eruptions (flood basalts). The lack of geomorphological structures indicative of shallow magma reservoirs indicates that deep, probably subcrustal source regions are prevalent. With respect to crustal thickness relationships, the magnitude and frequency of eruptive events are observed to be greatest in areas of thinnest crust. Specifically, regions of major maria (Mare Smythii, Mare Marginis) occur in areas of thinnest crust, while isolated ponds occur where the crust is relatively thicker. This is consistent with the correlation observed globally between crustal thickness and the magnitude and frequency of eruptive events. Ages of volcanic flows and pyroclastic events range from Early Imbrian to Imbrian‐Eratosthenian but are concentrated most heavily between 3.80–3.60 Ga, suggesting a period of peak volcanism beginning around 3.85 Ga and lasting approximately 200 Ma. However, the existence of dark‐halo crater clusters in non‐mare units within the region suggests the presence of cryptomaria, which would indicate an earlier onset of volcanism and a volume of mare material potentially greater than that currently exposed on the surface. Typical nearest‐neighbor distances suggest deposits derive from reservoirs <∼100 km in diameter. The observations made here are consistent with a magma transport model in which plumes rising diapirically stall at a density boundary under the lunar crust and propagate dikes to the surface through overpressurization.
The temperature‐dependent near‐infrared absorption spectrum of hexagonal H2O ice
Authors: Grundy W. M. , Schmitt B.
Published: February 2004

Transmission spectra were measured between 1.0 and 2.7 μm for monocrystalline samples of hexagonal water ice at temperatures between 20 and 270 K. Samples were crystallized from liquid water within closed cells, with thicknesses ranging from 100 μm to 1.0 cm. The absorption spectrum of ice changes with temperature in several ways. With higher temperature, the shapes of absorption bands become more smoothed, the strengths of some absorption bands decrease, the absorption in continuum wavelengths increases, and the band centers of some absorption bands shift to shorter wavelengths. In this paper we present the new absorption coefficient spectra along with an examination of the different temperature effects. These data should prove extremely valuable for analysis of near‐infrared reflectance spectra of low‐temperature icy surfaces, such as those of outer solar system satellites, Kuiper Belt objects, Pluto and Charon, comet nuclei, the polar caps of Mars, and terrestrial snow‐and ice‐covered regions. The data may also be of value in simulating radiative transfer in clouds of ice particles in the atmospheres of planets.
Measurements of radio frequency signals from lightning in Jupiter's atmosphere
Authors: Rinnert K. , Lanzerotti L. J. , Uman M. A. , Dehmel G. , Gliem F. O. , Krider E. P. , Bach J.
Published: February 2004

During the descent of the Galileo probe through Jupiter's atmosphere, the lightning and radio emissions detector (LRD) instrument measured radio frequency signals presumably from electrical discharges in the planet's atmosphere. The LRD was the only instrument that provided remote sensing, as well as in situ, measurements of atmospheric characteristics. The LRD measurements are presented here and some estimates are given on the energetics and frequency of occurrence of lightning in Jovian clouds. Propagation calculations of RF discharges in the Jovian atmosphere system and the statistics data obtained by the LRD, together with one very distinct lightning waveform, permit a unified and consistent interpretation of the data. We conclude that at the time of probe entry, Jovian discharges occur with a rate about one hundredth that of the global yearly average on Earth (Earth value is about 6 flashes km−2 yr−1) within about 15,000 km radius of the probe and that the average radiated power is of the order of 5 × 1011 W. The change in the electric dipole moment in Jovian lightning is about 107 coulomb in, roughly 100 times that of a typical terrestrial discharge.
Early Mars Climate Models
Authors: Haberle Robert M.
Published: February 2004

It is often stated that Mars and Earth had similar environmental conditions early in their history and that life might therefore have originated on Mars as well as on Earth. However, the atmospheric conditions required to produce and sustain a warm, wet climate on early Mars remain uncertain. State‐of‐the‐art greenhouse models predict global mean surface temperatures early in Mars' history that differ little from today's, unless special conditions are invoked. The greatest difficulty the models have is coping with a faint early Sun.