bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical profiles, aerosol microphysics, and optical closure during the Atlantic Stratocumulus Transition Experiment: Measured and modeled column optical properties
Authors: Clarke A. D. , Porter J. N. , Valero F. P. J. , Pilewskie P.
Published: February 2004

During the Atlantic Stratocumulus Transition Experiment (ASTEX) in June 1992, two descents in cloud‐free regions allowed comparison of the change in aerosol optical depth as determined by an onboard total‐direct‐diffuse radiometer (TDDR) to the change calculated from measured size‐resolved aerosol microphysics and chemistry. Both profiles included a pollution haze layer from Europe, but the second also included the effect of a Saharan dust layer above the haze. The separate contributions of supermicrometer (coarse) and submicrometer (fine) aerosol were determined, and thermal analysis of the pollution haze indicated that the fine aerosol was composed primarily of a sulfate/water mixture with a refractory sootlike core. The soot core increased the calculated extinction by about 10% in the most polluted drier layer relative to a pure sulfate aerosol but had significantly less effect at higher humidities. A 3‐km descent through a boundary layer air mass dominated by pollutant aerosol with relative humidities (RH) 10–77% yielded a close agreement between the measured and calculated aerosol optical depths (550 nm) of 0.160 (±0.07) and 0.157 (±0.034), respectively. During descent the aerosol mass scattering coefficient per unit sulfate mass (inferred) varied from about 5 to 16 m2 g−1 and was primarily dependent upon ambient RH. However, the total scattering coefficient per total fine mass was far less variable at about 4 ± 0.7 m2 g−1. A subsequent descent through a Saharan dust layer located above the pollution aerosol layer revealed that both layers contributed similarly to aerosol optical depth. The scattering per unit mass of the coarse aged dust was estimated at 1.1 ± 0.2 m2 g−1. The large difference (50%) in measured and calculated optical depth for the dust layer exceeded estimated measurement uncertainty (12%). This is attributed to inadequate data on the spatial variability of the aerosol field within the descent region, a critical factor in any validation of this type. Both cases demonstrate that surface measurements may be a poor indicator of the characteristics and concentration of the aerosol column.
Validation of the Earth radiation budget as simulated by the Max Planck Institute for Meteorology general circulation model ECHAM4 using satellite observations of the Earth Radiation Budget Experiment
Authors: Chen C.‐T. , Roeckner Erich
Published: February 2004

In this study, the Earth radiation budget as simulated by the latest version of the ECHAM general circulation model (ECHAM4) is documented. The Earth radiation budget obtained from the model is evaluated through comparison with Earth Radiation Budget Experiment (ERBE) data. The model simulations generally agree with the satellite‐observed spatial distribution and seasonal variation of the radiation budget. The interannual variabilities of the simulated radiative quantities in the tropics are also comparable to the observed data. There are, however, biases in the details. The longitudinal structure of the radiation fields in the tropics is not exactly reproduced. The slight differences in the geographical location of the various radiation quantities are related to the errors in the simulation of the tropical east‐west longitudinal circulation and associated cloud fields. The most significant bias in the radiation budget is the simulated shortwave radiative effect from cloud. Consistent underestimation in the shortwave cloud radiative forcing is found over the midlatitude oceans in summer. This bias is due to an underestimation in the total cloud amount. In the tropics the simulated shortwave cloud radiative forcing is persistently larger than that derived from ERBE data. The systematic overestimation in the shortwave cloud radiative forcing at various values of the longwave cloud radiative forcing suggests that this problem is related to the neglect of sub‐grid‐scale cloud water content variability and its effect on the grid‐averaged shortwave radiative flux. Although the tropical interannual variability of cloud radiative forcing is reasonably well reproduced by the model, the major contribution for the simulated anomalies is from the cloud water anomalies, while observations suggest the dominant factor is the total cloud cover anomalies. Lack of observations for various cloud parameters limits the attempt of further validation.
Processes controlling the composition of precipitation at a remote southern hemispheric location: Torres del Paine National Park, Chile
Authors: Galloway James N. , Keene William C. , Likens Gene E.
Published: February 2004

Precipitation composition measured at Torres del Paine National Park, Chile (TdP) (51°10′S, 71°58′W), between 1984 and 1993 was acidic (volume‐weighted‐average pH of 4.96) with a dilute seawater component. H+ was the dominant non‐sea‐salt (nss) cation; in decreasing order of abundance, nss anions were HCOO−, Cl−, SO4=, CH3COO−, and NO3−. Relative to lower latitude, remote locations, concentrations and per‐event depositions of nss SO4=, NO3−, and NH4+ at TdP were lower; those of HCOOt (HCOO− + HCOOH) and CH3COOHt, (CH3COO− and CH3COOH) were similar; and those of CH3SO3− were higher. Concentrations and deposition fluxes of HCOOt, CH3COOt, nss SO4=, CH3SO3−, and NH4+ varied seasonally with summer maxima and winter minima. Carboxylic acids probably originated from both direct terrestrial emissions and oxidation of hydrocarbons emitted by marine and terrestrial biota. Nss SO4= and CH3SO3− originated primarily from oxidation of biogenic (CH3)2S emitted from the southern Pacific Ocean. Direct emissions of NH3 from upwind terrestrial and marine ecosystems probably accounted for most observed NH4+. NO3− concentrations and depositions were highest during the latter part of austral winter and spring suggesting abiotic controls. Transport of precursors from lightning, biomass burning, and fossil‐fuel combustion at lower latitudes and possibly transport of reactive N from the stratosphere apparently contributed oxidized N to the southern Patagonian troposphere. Although the ionic strength of precipitation at TdP is currently among the world's lowest, future changes are likely because of increases in local and regional population and energy and food production.
Synoptic patterns associated with the flux of excess ozone to the western North Atlantic
Authors: Berkowitz Carl M. , Daum Peter H. , Spicer Chester W. , Busness Ken M.
Published: February 2004

Observations of O3, CO, and aerosols taken in the vicinity of Halifax, Nova Scotia, during the summers of 1992 and 1993 indicate long‐range transport of O3 that was photochemically generated within urban areas of North America. We identify eastward moving cyclonic systems as the main synoptic‐scale transport mechanism into this region with the consequence that transport events are highly episodic. The potential for further well‐defined long‐range transport was found to be slight as a result of subsequent mixing by these cyclonic systems. The ozone flux associated with these cyclones into the lowest 1.5 km of this region is estimated to be on the order of 1 Gmol per 24‐hour cyclonic event. We conclude that the circulation patterns associated with these fronts are a major pathway for the export of pollutants from North America.
Measurement and speciation of gas and particulate phase organic acidity in an urban environment: 1. Analytical
Authors: Lawrence Joy , Koutrakis Petros
Published: February 2004

Organic acids are important contributors to ambient acidity, in both gas and particulate phase. Particulate phase organic acids represent an important fraction of organic particulate matter. This paper presents the results of a field study conducted in Philadelphia, Pennsylvania, during the summer of 1992. The purpose of this study was to measure the concentrations of gas and particulate phase organic acids. Formic acid was found to be the most abundant gas phase organic acid, with acetic and propionic acids detected at lower concentrations. Dicarboxylic acids and even‐carbon monocarboxylic acids were found to be present at relatively high concentrations in the particulate phase; odd‐carbon monocarboxylic acids were present at much smaller concentrations. This paper discusses the measurement and analytical techniques used in this study. A subsequent paper will address the data analysis and discuss the chemistry and origins of organic acids.
Estimates of stratosphere‐troposphere exchange: Sensitivity to model formulation and horizontal resolution
Authors: van Velthoven P. F. J. , Kelder H.
Published: February 2004

The vertical transport between the stratosphere and the troposphere is studied by evaluating a matrix that gives the amount of air transferred between the different model layers of a three‐dimensional off‐line transport model coupled to European Centre for Medium‐Range Weather Forecasts (ECMWF) analyses. The latitudinal structure of vertical exchange reflects the Hadley circulation with upward transport in the tropics and with downward transport at mid‐latitudes. Maxima in downward transport occur near the subtropical and polar jet streams. The seasonal cycle of downward transport and the asymmetry between the two hemispheres are at least qualitatively represented by the model. However, it is found that the evaluated total amount of air transferred from the stratosphere to the troposphere per month is sensitive to the chosen horizontal model resolution. Refinement of this resolution from 8° × 10° to 4° × 5° leads to almost a doubling of the computed downward transport. Also, the vertical transport is found to depend strongly on changes in the formulation of the parent model (ECMWF). It is therefore important to use internally consistent meteorological input data when performing multiyear simulations with off‐line chemistry‐transport models.
Black carbon: Atmospheric concentrations and cloud water content measurements over southern Nova Scotia
Authors: Chýlek Petr , Banic C. M. , Johnson Bruce , Damiano P. A. , Isaac G. A. , Leaitch W. R. , Liu P. S. K. , Boudala F. S. , Winter B. , Ngo D.
Published: February 2004

Atmospheric black carbon concentrations have been measured in air and cloud water samples collected over southern Nova Scotia, Canada, during the North Atlantic Regional Experiment. The average black carbon atmospheric concentration found in continental air masses is 0.23±0.04 μg/m3 compared with 0.03±0.01 μg/m3 for maritime air. The concentrations found in the 1‐ to 3‐km layer are higher than those below 1 km, suggesting that most of the black carbon is not of local origin. About 9% of black carbon was in the form of an internal mixture inside cloud droplets. The average black carbon content of cloud water was about 40 μg/kg. The average black carbon to non‐sea‐salt SO4 mass mixing ratio was found to be between 0.02 and 0.04.
Retrieving atmospheric temperature parameters from DMSP SSM/T‐1 data with a neural network
Authors: Butler Charles T. , v. Z. Meredith R. , Stogryn A. P.
Published: February 2004

We show that back propagation neural networks yield excellent results in retrieving air temperature profiles from the 1000‐ to the 10‐mbar pressure level together with tropopause temperature and pressure estimates using operational brightness temperature data from the special sensor microwave imager (SSM/T‐1) microwave radiometer. Networks trained and tested with matched SSM/T‐1 measurements and conventional soundings collected during a 30‐day period in northern hemisphere winter demonstrated rms retrieval errors substantially less than 2K from 500 to 30 mbar, significantly outperforming an operational linear‐regression algorithm using the same data. Tropopause temperature and pressure retrievals showed rms errors of 2.15 K and 19.8 mbar. Retrieval accuracy of the system exceeds that of any previously published method using DMSP data and equals or exceeds that of published studies using data from other satellite‐borne instruments. Retrieval accuracy under possible failure modes of the SSM/T‐1 instrument are also considered, as are ways to recover from single‐channel loss. The method retrieves profiles and tropopause parameters with acceptable accuracy either if the brightness‐temperature of any one channel is offset 1.5 K or more or if uniform random noise with a peak value in the range (−2, 2) K is added in one channel. The performance is only slightly more impaired if all channels are simultaneously offset up to 1.5 K or if random noise in the range (−1, 1) K is simultaneously added to all channels. Under single‐channel loss the retrieval error can be made small at virtually every level by retrieving with a network trained without that channel.
Simulation of lidar measurements of gravity waves in the mesosphere
Authors: Gibson‐Wilde D. E. , Reid I. M. , Eckermann S. D. , Vincent R. A.
Published: February 2004

Lidar measurements of mesospheric gravity wave characteristics are simulated using a simple numerical model. Simple theory is used to superimpose gravity wave fluctuations upon typical background densities and sodium abundances in the height range 80–100 km, which we then assume are perfectly observed by a Rayleigh and Na lidar, respectively. These lidar “data” are then analyzed for their gravity wave content using the standard reduction and analysis methods which are routinely applied to these data sets. Our goal is to assess whether this analysis of lidar data faithfully recovers the underlying wave field. In both quasi‐monochromatic and spectral wave studies, the height range over which Na number density can be measured and the shape of the background Na profile impose a limit on the gravity wave information which can be extracted from Na lidar data. The limitations due to background layer shape do not exist for Rayleigh lidar measurements. The simulations reveal that quasi‐monochromatic gravity waves with vertical wavelengths larger than approximately 10 km may not be reliably retrieved from Na lidar data. It is also shown that there are limitations on the accurate extraction of spectral parameters from Na lidar data due to the limited vertical extent of the mesospheric Na layer.
Wind velocity and temperature fluctuations due to a 2‐day wave observed with radio meteor echoes
Authors: Tsutsumi Masaki , Tsuda Toshitaka , Nakamura Takuji , Fukao Shoichiro
Published: February 2004

We study in this paper characteristics of both wind velocity and temperature fluctuations due to a 2‐day wave near the mesopause, by means of meteor echo measurements with the MU (middle and upper atmosphere) radar in Shigaraki, Japan (35°N, 136°E) and a meteor wind radar, newly constructed near Jakarta, Indonesia (6°S, 107°E). A 2‐day wave was conspicuously enhanced on July 12–19, 1993, at both Shigaraki and Jakarta. Fluctuations of temperature and northward wind were inphase in Shigaraki, and they became antiphase in Jakarta, consistent with a prediction assuming a Rossby‐gravity wave. The phase lag of the fluctuations between the two sites suggested that this wave propagated westward with a zonal wavenumber of 3. Seasonal variations of the 2‐day wave in Jakarta were examined by using a frequency spectrum and complex demodulation technique for temperature and wind fields, finding clear enhancements during periods corresponding to summer months in each hemisphere, especially in January 1993. The dominant wave period was fairly stable and was confined in a narrow range centered around 47 and 49 hours during summer months of the southern and northern hemispheres, respectively. The phase variations of temperature preceded those for the northward wind by 180°–225° when an enhanced 2‐day wave was seen in the northward component, indicating their antiphase relation in the southern hemisphere. These results strongly support the interpretation of the 2‐day wave near the mesopause as the (3,0) mode of a Rossby‐gravity wave with zonal wavenumber of 3.
Superposition model for multiple plumes and jets predicting end effects
Authors: Yannopoulos Panayotis C.
Published: February 2004

On the basis of integral forms of momentum, tracer, and kinetic energy equations for the mean flow, a superposition model is developed, which is able to predict end effects of multiple jet or multiple plume discharges. The general cases of inclined, single, or multiple buoyant jets in a row normal to a uniform ambient current or wind, and of equal or lower density than the ambient fluid, are being examined and the conditions under which the model can be applied are pointed out. The pertinent quantities describing the complicated field of interacting multiple plumes/jets are detected by comparing the integral equations produced for multiple discharges with the corresponding equations for single discharges. Provided that the single buoyant jet behavior is known, the model has been applied in problems of buoyant jet interaction, yielding analytical solutions, and it has been verified using literature data. The behavior of the limiting cases is also discussed, along with the pertinent explanations. Findings can be used in the design of disposal systems, that is, stacks, multiple cell cooling tower discharges, and multiport diffusers.
Trajectories and related variations in the chemical composition of air for the Mauna Loa Observatory during 1991 and 1992
Authors: Hess Peter G. , Srimani Nanda , Flocke Siri J.
Published: February 2004

Isentropic 10‐day back trajectories are computed each day for the fall 1991 and winter, spring, and summer 1992 intensives of the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). The trajectories are computed on an isentropic surface at the approximate height of the Mauna Loa Observatory (MLO). Trajectory variations on the daily, as well as annual timescale, are documented and compared to trajectory climatologies for Hawaii. During MLOPEX 2 the Pacific anticyclone was consistently farther west than normal, with important consequences for the computed trajectories. The concentrations of ozone, total reactive oxidized nitrogen, water, methyl nitrate, carbon monoxide, acetylene, radon, and peroxyacetyl nitrate (PAN) measured at MLO are examined in relation to trajectory characteristics on the daily and annual timescales. During the fall intensive and the summer intensive, changes in trajectory characteristics are related, in a broad sense, to changes in the measured concentrations of the above species; during the winter and especially the spring intensives, the trajectories prove to be poor indicators of the measured concentrations. For most species the seasonal variation in the computed trajectories does not satisfactorily explain the annual cycle in their measured concentrations.
Observed and model‐calculated photostationary state at Mauna Loa Observatory during MLOPEX 2
Authors: Hauglustaine D. A. , Madronich S. , Ridley B. A. , Walega J. G. , Cantrell C. A. , Shetter R. E. , Hübler G.
Published: February 2004

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), simultaneous clear sky measurements of [NO2], [NO], [O3], and NO2 photodissociation rate were collected during autumn 1991, winter, spring, and summer 1992 intensive measurement periods at Mauna Loa Observatory (MLO), Hawaii. These measurements were used to assess the O3‐NO‐NO2 photostationary state, and to infer peroxy radical concentration, ozone photochemical production and destruction rates, as well as the relationship between the deviation of the photostationary state parameter, ϕ = j2[NO2]/k1[O3][NO], from unity and environmental variables. In addition, a photochemical box model was used for the purpose of comparison with observations and interpretation of the results. A systematic deviation of ϕ from unity was determined from measurements, this parameter reached values of 1.5–2 for high‐sun conditions depending on the season. The inferred total peroxy radical concentrations were generally larger (25–70 pptv for high‐sun conditions depending on the season) than those directly measured (20–25 pptv during noontime). Model results seem generally to underestimate the value of ϕ and to overpredict the hydroxyl and total peroxy radical concentrations, suggesting loss processes not accounted for in the model for these species. Analysis of the contributions to the net ozone photochemical production indicates total production and destruction terms reaching 10–15 ppbv/day which tend to nearly cancel on a daily average basis. High‐sun values of ϕ − 1 were shown to be independent of NOx (= NO + NO2) for NOx concentrations above about 50 pptv. However, below this level, observations show that ϕ − 1 generally decreases with increasing nitrogen oxide concentrations. Furthermore, the measured ϕ − 1 correlation with
suggests the existence of NO to NO2 oxidants not accounted for in a simplified HOx chemistry.
Peroxy radical concentrations measured and calculated from trace gas measurements in the Mauna Loa Observatory Photochemistry Experiment 2
Authors: Cantrell C. A. , Shetter R. E. , Gilpin T. M. , Calvert J. G. , Eisele F. L. , Tanner D. J.
Published: February 2004

Measurements of peroxy radical concentrations ([HO2] + [RO2]) were made by the “chemical amplifier” technique during the four intensives of the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) at the Mauna Loa Observatory in 1991–1992. In this study these data are compared with the theoretical values of the peroxy radical concentrations obtained from steady state analysis of the complete suite of trace gas measurements and other relevant parameters also measured during the experiment. The data from 33 days of the study contain time overlap of the concentration and physical data which allow a meaningful theoretical treatment. The experimental results for [HO2] + [RO2] agree well with theory for many of the days, but are significantly suppressed from the theoretical expectations on other days. Two hypotheses are presented and tested to explain the observed suppression. The first involves the reaction of the peroxy radicals at aerosol surfaces. The second proposes the loss of [OH] through its reaction with unknown and undetected species to develop peroxy radicals subsequently to which the “chemical amplifier” is insensitive. Evidence at hand does not allow a clear choice between these or possible alternative explanations. The data suggest that the net rate of O3 generation in the free troposphere is about −1.5 parts per billion by volume per day (ppbv d−1; 24‐hour average).
Millimeter wave spectroscopic measurements over the South Pole: 3. The behavior of stratospheric nitric acid through polar fall, winter, and spring
Authors: de Zafra R. L. , Chan V. , Crewell S. , Trimble C. , Reeves J. M.
Published: February 2004

We present data from a 9‐month series of ground‐based measurements of stratospheric nitric acid, made over the South Pole from mid‐April 1993 to mid‐January 1994. Observations were typically made at 3‐ to 6‐day intervals. Both profiles and column densities have been retrieved from pressure‐broadened millimeter‐wave emission spectra. These measurements provide the first quasi‐continuous series of vertical mixing ratio profiles for this species in the heart of the south polar votex. Conversion of NOx to nitric acid by heterogeneous reactions, and its incorporation into polar stratospheric cloud (PSC) particles, along with subsequent gravitational settling, is considered to be the main denitrifying mechanism in the Antarctic stratosphere, setting up conditions for ozone destruction at the end of winter. In our observations, a small increase in HNO3 was seen between April and the end of May, after which a rapid loss took place below 25 km. Column density above ∼15 km decreased to ≤1/4 its maximum within 30 days, and depletion continued until middle to late July, by which time the nitric acid column above 15 km had diminished by more than a factor of 10. The initial depletion was coincident with the onset of a rapid increase in lidar backscatter from polar stratospheric cloud formation at the same altitude range. Gas‐phase depletion was tracked as a function of altitude and temperature and found to be consistent with the temperature and partial pressure relationship for formation of ternary mixtures of HNO3, H2SO4, and H2O. Depletion occurred ∼3 weeks earlier in 1993 than was seen in 1992 column density measurements by Van Allen et al. [1995]. In late June a new layer of HNO3 was generated in the vicinity of 40‐km altitude and, subsequently, appeared to be carried downward with general vertical transport of air within the vortex. In spring, as temperatures increased, no rapid increase of gas‐phase HNO3 was seen, indicating that gravitational settling had carried PSC‐accreted nitric acid to low altitudes. By the end of observations in January 1994, mixing ratios and column densities above ∼15 km had not yet reached more than about half the values seen the previous April, indicating that a rather large increase in stratospheric HNO3 occurs in the early austral fall over the south polar region.
Carbonaceous particles and stone damage in a laboratory exposure system
Authors: Sabbioni C. , Zappia G. , Gobbi G.
Published: February 2004

The interaction between carbonaceous particles and stones used in historic buildings and monuments was investigated in a laboratory exposure system. Simulation experiments were carried out in a flow chamber where temperature, relative humidity, and SO2 concentration were controlled. Samples of carbonate stones (Carrara marble, Travertine, and Trani stone) were exposed for 150 days in air with 3 ppm of SO2 concentration at 25°C and 95% relative humidity. The stone specimens were coated with three types of carbonaceous particles (P1, P2, and P3) collected at the emission points of three oil‐fueled combustion sources: one centralized domestic heating plant and two electricity generating stations. For comparison, particles of activated carbon and graphite were also deposited on the stone samples. After exposure, samples were analyzed by X ray diffraction and infrared spectroscopy to identify the main chemical species, by ion chromatography to quantify SO4= and SO3= concentrations, and also by scanning electron microscope. The results show that the amount of SO4= formed increases in the presence of carbonaceous particles and is related to their heavy metal content.
Statistical characteristics of aerosol in Baltic Sea region
Authors: Kikas Ü. , Mirme A. , Tamm E. , Raunemaa T.
Published: February 2004

The article presents the results of the aerosol measurements carried out on the eastern coast of the Baltic Sea during 1986 – 1993. Aerosol size distributions in the diameter range of 0.01–10 μm have been measured with the electric aerosol spectrometer (EAS). A short description of EAS is given. The majority of number distributions obtained by measurements were of monomodal shape. Modal diameter varied at 0.01–0.1 μm. Seven percent of all number distributions were of bimodal shape with modes at 0.010 – 0.032 μm and at 0.032 – 0.1 μm, separated with a minimum at 0.018 – 0.056 μm. The majority of aerosol mass distributions were of bimodal shape and consisted of an accumulation mode varying at 0.18 – 1.0 μm and of a coarse particle mode at 1.8 – 10 μm separated by a minimum varying at 0.56 – 3.2 μm. Parameters of the Junge distribution and the 4‐parameter KL distribution have been obtained. The variability of aerosol concentrations and the stability of aerosol at a fixed site have been studied analyzing the time sequences of aerosol number distributions. The variability of concentrations depended on particle size. The variability was about 0.7 at 0.1 – 0.3 μm and rose toward both the smaller and larger diameters up to values 1.4 at 0.01 μm and 1.7 at 10 μm.
Boundary layer isentropic and kinematic trajectories during the August 1993 North Atlantic Regional Experiment Intensive
Authors: Draxler Roland R.
Published: February 2004

Four per day backward trajectories were computed from Chebogue Point, Nova Scotia, for the month of August 1993 using two different three‐dimensional approaches: diagnostic vertical velocity fields (kinematic) and an isentropic assumption. Essentially, 90% of all the kinematic‐isentropic trajectory pairs were displaced within ±75 hPa of each other and differed in the horizontal by about 10% of the travel distance. The greatest vertical displacement differences occurred with flow from the northwest and only three time periods were associated with the greatest displacements; a cyclonic system was to the north of the trajectory starting location in each case. Precipitation was associated with two of these events. A moist isentropic departure was computed for both kinematic and isentropic cases and departures were typically about 5°K. Comparison of CO measurements and upwind trajectories suggested by temporal groupings of trajectories were consistent with transport from major urban areas. However, the peak measured CO values seemed to occur more frequently with flow transition periods, when trajectory uncertainty is the greatest.
Response of a stably stratified atmosphere to large‐scale diabatic forcing with applications to wind patters in Brazil and the Sahel
Authors: Wang T.‐A. , Lin Y.‐L. , Semazzi H. F. M. , Janowitz G. S.
Published: February 2004

The response of a stably stratified atmospheric flow to large‐scale elevated and low‐level thermal forcing on a β plane at several latitudes are investigated using a simple nonlinear numerical model. The formation of a westerly jet at the equator, a pair of low‐pressure and cyclonic circulation regions, and the decoupling of the westerly jet and the heating center are explained by the β effect. It is found that the vertical motion in the case with heating centered at the equator is much larger than that with heating centered at higher latitudes. This is explained by the trapping of more wave energy in equatorial regions when the heating is centered at the equator. On the other hand, the pressure perturbation is much stronger for cases with heating located at a higher latitude due to vortex stretching. It is found that there exists a vertical phase tilt which is explained by the upward propagating Kelvin waves near the equator and mixed Rossby gravity waves at higher latitudes. The vertical wavelength of the disturbance increases as the heating center is located at a higher latitude. In addition, we find that this type of flow is not sensitive to the existence of the stratosphere. One interesting feature is that the maximum vertical velocity for the case with heating centered at the equator is significantly larger than that with heating centered at 30°N. Comparing both linear and nonlinear cases with surface heating or cooling, we find (1) the nonlinearity suppresses (enhances) the disturbance induced by cooling (heating), (2) the differences between the nonlinear and linear responses are larger when the forcing is located at lower latitudes, and (3) the disturbed areas are much wider in the zonal direction for cooling cases than for cases with heating. The nonlinear effects are studied by inspecting individual terms. To explore the relationships between Atlantic sea surface temperature anomaly (SSTA) and rainfall anomalies in the Sahel and northeastern Brazil, the SSTA is represented by low‐level diabatic forcing in the numerical model. With a coupled warm SSTA in the South Atlantic and cold SSTA in the North Atlantic, model results indicate a combination of offshore flow and downward motion in the Sahel, while there exists an upward motion and an offshore flow in northeastern Brazil. When the SSTA pattern is reversed, then there exists an onshore flow and an upward motion in the western Sahel, while there exists a strong onshore flow and a downward motion in northeastern Brazil.
The total hydrogen budget in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment
Authors: Engel Andreas , Schiller Cornelius , Schmidt Ulrich , Borchers Reinhard , Ovarlez Henry , Ovarlez Joelle
Published: February 2004

Vertical profiles of H2O and CH4 were measured using in situ techniques in the Arctic winter stratosphere during the European Arctic Stratospheric Ozone Experiment (EASOE) 1991/1992 in the pressure range between 12 and 80 hPa. The measurements were grouped according to the meteorological situation, i.e., their position relative to the polar vortex. The mixing ratio of the quantity 2·CH4 + H2O was determined in the altitude region between the hygropause and 30 km to be quasi constant at a mean mixing ratio of 6.91 ± 0.41 ppmv. No significant difference of 2·CH4 + H2O was observed inside and outside the vortex. From the correlation between CH4 and H2O ΔH2O/ΔCH4, i.e., the water vapor yield of the methane oxidation, was calculated to be −1.82 ± 0.21. The globally averaged H2O mixing ratio of air entering the stratosphere was found to be 3.61 ± 0.28 ppmv. The data are compared with studies on the total hydrogen budget over the past 20 years. Despite the variety of instrumental techniques used in these studies, an increase of total hydrogen is apparent over this period.
An aircraft measurement technique for formaldehyde and soluble carbonyl compounds
Authors: Lee Yin‐Nan , Zhou Xianliang , Leaitch W. Richard , Banic Catharine M.
Published: February 2004

An aircraft technique was developed for measuring ambient concentrations of formaldehyde and a number of soluble carbonyl compounds, including glycolaldehyde, glyoxal, methylglyoxal, glyoxylic acid, and pyruvic acid. Sampling was achieved by liquid scrubbing using a glass coil scrubber in conjunction with an autosampler which collected 5‐min integrated liquid samples in septum‐sealed vials. Analysis was performed on the ground after flight using high‐performance liquid chromatography following derivatization of the carbonyl analytes with 2,4‐dinitrophenylhydrazine; the limit of detection was 0.01 to 0.02 parts per billion by volume (ppbv) in the gas phase. Although lacking a real‐time capability, this technique offers the advantage of simultaneously measuring six carbonyl compounds, savings in space and power on the aircraft, and a dependable ground‐based analysis. This technique was deployed on the Canadian National Research Council DHC‐6 Twin Otter during the 1993 summer intensive of the North Atlantic Regional Experiment. The data obtained on August 28, 1993, during a pollutant transport episode are presented as an example of the performance and capability of this technique.
Large‐scale cloud features during January 1993 in the North Atlantic Ocean as determined from SSM/I and SSM/T2 observations
Authors: Liu Guosheng , Curry Judith A.
Published: February 2004

Satellite microwave (special sensor microwave/imager (SSM/I) and special sensor microwave water vapor sounder (SSM/T2)) data are used to study the large‐scale cloud and precipitation features during January 1993 in the North Atlantic (30°–80°N, 80°W–30°E). Modification and validation of existing retrieval algorithms are made so that these algorithms are more suitable for high‐latitude situations. A new algorithm for atmospheric ice/snow retrieval using SSM/T2 data is developed. Although no direct validation has been conducted, the retrieved large‐scale ice water path distribution compares well with a snowfall frequency climatology derived from shipboard present‐weather reports. Monthly mean distributions of water vapor path, liquid water path, ice water path, and rainfall rate are investigated in relation to sea surface temperature. The results show that the magnitude of precipitable water is highly correlated with the absolute value of sea surface temperature. The largest values of liquid water path and rainfall are found over the Gulf Stream. Snowfall is greatest off the east coast of Canada, which is thought to correspond to the shallow convection associated with cold air outbreaks from the continent. Fresh water flux (precipitation minus evaporation) retrieved from satellite data show that rainfall and evaporation are almost in balance in the middle of North Atlantic (40°–60°N, 50°–10°W), while rainfall is larger than evaporation north of this region, and evaporation is larger than rainfall to the south. Also, high values of fresh water fluxes are found along the Gulf Stream. Strong correlation was found between northward water vapor transport across 60°N parallel and the cloud and precipitation properties in the Greenland Sea, implying that horizontal water vapor transport from lower latitudes plays an important role in the cloud development in this region.
Self‐affine (fractal) topography: Surface parameterization and radar scattering
Authors: Shepard Michael K. , Brackett Robert A. , Arvidson Raymond E.
Published: February 2004

Starting with the assumption that planetary surfaces are self‐affine (fractal) over the scales applicable to radar scattering, we derive various surface parameters, e.g., rms slopes and autocorrelation functions, and examine the implications for radar scattering models. The results of this work provide several new insights of interest to planetary geologists and others using radar to study surface features. First, the unidirectional slope histograms of self‐affine surfaces are Gaussian, and the adirectional slope histograms are Rayleigh. Normalization of the adirectional histogram by solid angle results in a Gaussian adirectional slope density function and therefore a Gaussian quasi‐specular angular scattering function. Next, the wavelength dependent behavior of surface roughness inferred from lunar radar observations is consistent with self‐affine topography. Finally, surface rms height measurements are functions of profile length. Therefore, when determining the applicability of the small perturbation model to a surface based on those measurements, it is necessary to consider the length of the profile with respect to the sampling wavelength.
Galileo multispectral imaging of Earth
Authors: Geissler Paul , Thompson W. Reid , Greenberg Richard , Moersch Jeff , McEwen Alfred , Sagan Carl
Published: February 2004

Nearly 6000 multispectral images of Earth were acquired by the Galileo spacecraft during its two flybys. The Galileo images offer a unique perspective on our home planet through the spectral capability made possible by four narrowband near‐infrared filters, intended for observations of methane in Jupiter's atmosphere, which are not incorporated in any of the currently operating Earth orbital remote sensing systems. Spectral variations due to mineralogy, vegetative cover, and condensed water are effectively mapped by the visible and near‐infrared multispectral imagery, showing a wide variety of biological, meteorological, and geological phenomena. Global tectonic and volcanic processes are clearly illustrated by these images, providing a useful basis for comparative planetary geology, Differences between plant species are detected through the narrowband IR filters on Galileo, allowing regional measurements of variation in the “red edge” of chlorophyll and the depth of the 1‐μm water band, which is diagnostic of leaf moisture content. Although evidence of life is widespread in the Galileo data set, only a single image (at ∼2 km/pixel) shows geometrization plausibly attributable to our technical civilization. Water vapor can be uniquely imaged in the Galileo 0.73‐μm band, permitting spectral discrimination of moist and dry clouds with otherwise similar albedo. Surface snow and ice can be readily distinguished from cloud cover by narrowband imaging within the sensitivity range of Galileo's silicon CCD camera. Ice grain size variations can be mapped using the weak H2O absorption at 1 μm, a technique which may find important applications in the exploration of the moons of Jupiter. The Galileo images have the potential to make unique contributions to Earth science in the areas of geological, meteorological and biological remote sensing, due to the inclusion of previously untried narrowband IR filters. The vast scale and near global coverage of the Galileo data set complements the higher‐resolution data from Earth orbiting systems and may provide a valuable reference point for future studies of global change.
Galilean satellite observation plans for the near‐infrared mapping spectrometer experiment on the Galileo spacecraft
Authors: Smythe W. D. , Lopes‐Gautier R. , Ocampo A. , Hui J. , Segura M. , Soderblom L. A. , Matson D. L. , Kieffer H. H. , McCord T. B. , Fanale F. P. , Calvin W. M. , Sunshine J. , Barbinis E. , Carlson R. W. , Weissman P. R.
Published: February 2004

On December 7, 1995, the Galileo spacecraft will begin observations of the Jovian system with orbit insertion and a satellite tour of 10 orbits. The Galilean satellites will be observed with four remote sensing instruments spanning the ultraviolet, visible, near infrared, and thermal infrared regions of the spectrum. The Galileo near‐infrared mapping spectrometer will observe the satellites in the wavelength range 0.7–5.2 μm, a region particularly well suited for analyzing volatile components. Planned observations include mapping most available longitudes at about 100‐km resolution per pixel at full wavelength resolution, together with observing limited regions at high spatial resolution. The opportunity to affect the choice and design of observations for the Galileo tour extends until June 1996.
Hoods, mists, frosts, and ice caps at the poles of Mars
Authors: Dollfus Audouin , Ebisawa Shiro , Crussaire Daniel
Published: February 2004

The sequence of seasonal events that occurs each Martian year around the two poles of Mars is more precisely described due to the input of new telescopic surveys and polarimetric measurements, combined with available orbital photographs, Viking lander images, and atmospheric pressure recordings. The interplays between vaporization and condensation of both carbon dioxide and water products, seasonally at work around both poles, produce clouds and mists in the atmosphere and frosts and ices at the surface. The succession of occurrences of prepolar hood frost depositions, polar hood formations and evolutions, and surface cap depositions and vaporizations is described for both poles as a function of the seasonal evolution parameter Ls (planetocentric longitude of the Sun).
Correction to “Processes involved in the formation of magnesian‐suite plutonic rocks from the highlands of the Earth's Moon” by Gregory A. Snyder, Clive R. Neal, and Lawrence A. Taylor
Authors: Snyder Gregory A. , Neal Clive R. , Taylor Lawrence A.
Published: February 2004

Models of coupled crustal deformation and mantle convection on Venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine. A previous model which assumed a relatively weak basalt flow law, more appropriate for hydrated crust, is compared to a similar model that assumes a strong basalt flow law, more appropriate for dry crust. A principal difference is the timescale for significant crustal thickening, of the order of 108 years for the weak basalt model and 109 years for the strong basalt model. These results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near‐surface weakness, of the type associated with plate margins on Earth, the lithospheric overturn time, which sets the timescale required for convective thickening of crust, scales dominantIy with the Rayleigh number defined by the average viscosity of the lithosphere. Theoretical scalings, as well as numerical models, suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle Rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103. Such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present‐day Earth. The large values inferred for the thickness of the mechanical lithosphere on Venus, if correct, would thus tend to favor timescales of the order of 109 years, if one assumes relatively low convective mantle heat loss over an equivalent time. This, in turn, would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows. If, instead, one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years, then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust.
Source location of Saturn's kilometric radiation: The Kelvin‐Helmholtz instability hypothesis
Authors: Galopeau Patrick H. M. , Zarka Philippe , Le Quéau Dominique
Published: February 2004

A new determination of the location of Saturn's kilometric radiation (SKR) sources is deduced from a limited set of hypotheses: assuming that the radio sources are fixed in latitude and local time, and that SKR is emitted on the X mode near its local cutoff frequency, we obtain source location footprints for northern and southern radio sources. The sources in both hemispheres appear magnetically conjugate and are consistent over the whole SKR frequency range. They are centered about 1300 local time (LT), at magnetic latitudes ≥80°, extending toward lower latitudes on the morning side (down to 60° at 0800–0900 LT) and on the evening side (down to 75° at 1900 LT). We interpret these source locations in terms of electron precipitations possibly caused by Kelvin‐Helmholtz instability arising on the flanks of the Saturnian magnetopause (mainly the morning one). Constraints on the average SKR beaming are derived.
Regional gravity fields on Venus from tracking of Magellan cycles 5 and 6
Authors: Kaula William M.
Published: February 2004

From August 1993 to October 1994, Magellan was at heights 180 to 550 km. Products of the Deep Space Network (DSN) X band tracking are line‐of‐sight (LOS) Doppler frequencies and accelerations residual to a 40th degree gravity field. The intrinsic accuracy and abundance of these data may be good enough to push the resolution (half wavelength) to less than spacecraft height. The observation equation to infer gravity from LOS accelerations is simple. However, noise and non uniform geometry necessitate a singular value analysis cutoff or an a priori weighting; the latter was chosen for computational economy. Further, the non‐Gaussian character of the noise necessitates a reject limit. Choices made were: (1) surface element size 100 km; (2) region size 1600 km; (3) buffer zone width 600 km; (4) reject limit 1 mGal (10−5 m/s) and (5) criterion for a priori weighting recovering a known solution; i.e., a field transferred from Earth to Venus (probably the most significant technical advance). This criterion was optimized by a priori increments to the normal main diagonal about 10% of the minimum main diagonal element, averaging about 0.5% of the maximum. Best resolutions, defined by spectral coherence of 0.7 for the known field, obtained were 110 km near the equator (from 51,285 points over Eisila) and 180 km at high latitudes (from 120,231 points over Maxwell and 162,000 over Akna/Freyja). The limitation on resolution near the equator is the cubic polynomial fitting over 335 km lengths to determine accelerations. However, environmental effects were the limiters elsewhere. Root‐mean‐square (rms) residuals to solutions had negative correlations with spacecraft altitude and the angle at the Earth between Venus and the Sun, and positive correlations with the Earth‐Venus distance and latitude on Venus, indicating Venus's influence on its electromagnetic surroundings out to several 100 km, especially on its side toward the Sun. Correlations with DSN zenith angles and the Kp index of solar activity were positive, but slight. The main characteristic of the enhanced resolution of regional gravity fields is greater correlation with the topography at shorter wavelengths. The main improvement yet to be made is to generate a full a priori matrix of the regional gravity field.