bluuebunny/crossref_metadata_2025 huggingface dataset:
Snow cover model for global climate simulations
Authors: Loth Bettina , Graf Hans‐F. , Oberhuber Josef M.
Published: February 2008

Because of the high sensitivity of the climate system to anomalies of the snow cover, appropriate models of the latter are required. In this paper a one‐dimensional multilayer model of a snow cover is presented combining relatively accurate model physics with minimal computer time. The model is based on the balance of mass and energy, including the important internal processes such as the diffusion of temperature and water vapor, melting and freezing, the extinction of short wave radiation, and the retention of liquid water. In order to keep the possibility of using the model on a global scale, no local parameterizations are employed. An efficient numerical scheme makes the model suitable for long‐term climate studies. Integrations for Potsdam (52°N, 13°E) with standard synoptic measurements over a period of six winters demonstrate good correspondence between the model results and the observed values. The most serious problem remaining is the choice of a satisfactory criterion to distinguish between snowfall and rain which strongly affects the simulation of snow depth and water equivalent.
A model for the separation of cloud and aerosol in SAGE II occultation data
Authors: Kent G. S. , Winker D. M. , Osborn M. T. , Skeens K. M.
Published: February 2008

The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite experiment measures the extinction due to aerosols and thin cloud, at wavelengths of 0.525 and 1.02 μm, down to an altitude of 6 km. The wavelength dependence of the extinction due to aerosols differs from that of the extinction due to cloud and is used as the basis of a model for separating these two components. The model is presented and its validation using airborne lidar data, obtained coincident with SAGE II observations, is described. This comparison shows that smaller SAGE II cloud extinction values correspond to the presence of subvisible cirrus cloud in the lidar record. Examples of aerosol and cloud data products obtained using this model to interpret SAGE II upper tropospheric and lower stratospheric data are also shown.
Infrared cross sections and global warming potentials of 10 alternative hydrohalocarbons
Authors: Clerbaux C. , Colin R. , Simon P. C. , Granier C.
Published: February 2008

Absorption cross sections have been obtained in the infrared atmospheric window, between 600 and 1500 cm−1, for 10 alternative hydrohalocarbons: HCFC22, HCFC123, HCFC124, HCFC141b, HCFC142b, HCFC225ca, HCFC225cb, HFC125, HFCl34a and HFCl52a. The measurements were made at three temperatures (287 K, 270 K and 253 K) with a Fourier transform spectrometer operating at 0.03 cm−1 apodized resolution. Integrated cross sections have been introduced into a two‐dimensional radiative‐chemical‐dynamical model in order to calculate the global warming potential (GWP) of each gas.
The UARS particle environment monitor
Authors: Winningham J. D. , Sharber J. R. , Frahm R. A. , Burch J. L. , Eaker N. , Black R. K. , Blevins V. A. , Andrews J. P. , Rudzki J. , Sablik M. J. , Chenette D. L. , Datlowe D. W. , Gaines E. E. , Imhof W. I. , Nightingale R. W. , Reagan J. B. , Robinson R. M. , Schumaker T. L. , Shelley E. G. , Vondrak R. R. , Voss H. D. , Bythrow P. F. , Anderson B. J. , Potemra T. A. , Zanetti L. J. , Holland D. B. , Rees M. H. , Lummerzheim D. , Reid G. C. , Roble R. G. , Clauer C. R. , Banks P. M.
Published: February 2008

The overall objective of the particle environment monitor (PEM) is to provide comprehensive measurements of both local and global energy inputs into the Earth's atmosphere by charged particles and Joule dissipation using a carefully integrated set of instruments. PEM consists of four instruments: the atmospheric X ray imaging spectrometer (AXIS), the high‐energy particle spectrometer (HEPS), the medium‐energy particle spectrometer (MEPS), and the vector magnetometer (VMAG). AXIS provides global scale images and energy spectra of 3‐ to 100‐keV bremsstrahlung X rays produced by electron precipitation into the atmosphere. HEPS and MEPS provide in situ measurements of precipitating electrons in the energy range from 1 eV to 5 MeV and protons in the energy range from 1 eV to 150 MeV. Particles in this energy range deposit their energy in the atmosphere at altitudes extending from several hundred kilometers down to as low as ∼30 km. VMAG provides the magnetic field direction needed to indicate and interpret the locations and intensities of ionospheric and field‐aligned currents as well as providing a reference for the particle measurements. This paper describes each instrument separately and also in the context of the PEM objectives which include the determination of energy deposition and ionization production rates as functions of altitude. Examples of data acquired early in the Upper Atmosphere Research Satellite (UARS) mission are presented.
Effective Bass‐Paur 1985 ozone absorption coefficients for use with Dobson ozone spectrophotometers
Authors: Komhyr W. D. , Mateer C. L. , Hudson R. D.
Published: February 2008

Details are presented concerning the derivation of new, standard, effective ozone absorption coefficients for use with Dobson ozone spectrophotometers. The work was performed under auspices of the World Meteorological Organization (WMO) Global Ozone Research and Monitoring Program, with the goal of improving and standardizing ozone measurements throughout the world. The new coefficients, based on the laboratory measurements of Bass and Paur (1985), were sanctioned for use worldwide, beginning January 1, 1992, by the WMO Executive Panel on Environmental Pollution and Atmospheric Chemistry. The new coefficients, together with use of slightly improved Rayleigh molecular scattering coefficients also adopted as standard, yield total ozone amounts 2.6% smaller than values obtained during July 1, 1957 to December 31, 1991, derived from the use of Vigroux (1953, 1967) ozone absorption coefficients. Information is provided also on correcting ozone data obtained in the past to the new ozone absorption coefficient scale.
The Halogen Occultation Experiment
Authors: Russell James M. , Gordley Larry L. , Park Jae H. , Drayson S. Roland , Hesketh W. Donald , Cicerone Ralph J. , Tuck Adrian F. , Frederick John E. , Harries John E. , Crutzen Paul J.
Published: February 2008

The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) spacecraft September 12, 1991, and after a period of outgassing, it began science observations October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure with an instantaneous vertical field of view of 1.6 km at the Earth limb. Latitudinal coverage is from 80°S to 80°N over the course of 1 year and includes extensive observations of the Antarctic region during spring. The altitude range of the measurements extends from about 15 km to ≈ 60–130 km, depending on channel. Experiment operations have been essentially flawless, and all performance criteria either meet or exceed specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere.
Atmospheric sulfur chemistry and cloud condensation nuclei (CCN) concentrations over the northeastern Pacific Coast
Authors: Berresheim H. , Eisele F. L. , Tanner D. J. , McInnes L. M. , Ramsey‐Bell D. C. , Covert D. S.
Published: February 2008

Correlated measurements of dimethylsulfide (DMS), gas phase dimethylsulfoxide (DMSO), methane‐sulfonic acid (MSA)(g), sulfuric acid (H2SO4), and cloud condensation nuclei (CCN) were conducted in April 1991 at a Pacific coastal site in northern Washington. Measurements of SO2, aerosol methanesulfonate (MSA)(p), and non‐sea‐salt sulfate (nss‐SO4) concentrations were also included. Maximum DMS concentrations between 100 and 240 pptv were observed when the measurement site (480 m above sea level) was embedded in clouds and air from the marine boundary layer was flowing upslope to the site. DMS levels measured in continental air and/or above the mixed layer were typically less than 20 pptv. The sulfur gases DMSO, H2SO4, and MSA(g) were measured in real time on a continuous basis (once every 60–150 s) using selected ion chemical ionization mass spectrometry. Corresponding concentrations ranged between <0.5–3.2 pptv, 0.001–1.19 pptv, and 0.002–0.19 pptv, respectively. All three sulfur gases showed significant diel variations mostly in phase with each other. Their corresponding lifetimes in the marine atmosphere are estimated to be of the order of a few hours. The results in connection with recent laboratory studies and model calculations suggest that dimethylsulfone (DMSO2) was the dominant end product of DMS oxidation under the present conditions. CCN concentrations measured in marine air ranged roughly between 10–200 cm−3 and 200–400 cm−3 at 0.3% and 0.9% supersaturation, respectively. A statistical analysis using only data obtained in predominantly marine air and during non‐fog/non‐precipitation periods showed significant correlations between individual sulfur species and between CCN (0.3% ss) and H2SO4, and CCN (0.3% ss) and nss‐SO4. The results indirectly support a relationship between DMS and CCN concentrations. However, other observations also suggest that at higher supersaturations (0.9%), compounds less soluble than sulfate may become important in marine CCN formation.
Convective cloud systems and warm‐pool sea surface temperatures: Coupled interactions and self‐regulation
Authors: Waliser Duane E. , Graham Nicholas E.
Published: February 2008

Questions regarding the upper limits on tropical sea surface temperatures and the processes determining those limits have recently come under renewed interest and debate. We present results from an analysis of the relationship between observed sea surface temperature (SST) and organized deep convection in the tropics that has produced new and important findings relevant to this issue. First, the analysis reveals that the highest observed tropical SSTs are generally associated with diminished convection. Second, the maximum convective activity occurs, on average, at an SST of about 29.5°C. Third, at SSTs of about 29°C and greater, intense deep convection is associated with ocean surface cooling of approximately 0.1°C per month, while suppressed deep convection is associated with a similar degree of ocean surface warming. These three findings, together with results from simplified model analyses, emphasize the importance of the cooling mechanisms associated with deep convection in determining the observed upper limits on tropical SST. Implications of the observed relationship between deep convection and SST on the temporal correlations between these fields is discussed, as is the convective cloud system's relative influence on the solar and evaporative heat flux components of the surface energy budget.
Upper tropospheric ozone production following mesoscale convection during STEP/EMEX
Authors: Pickering Kenneth E. , Thompson Anne M. , Tao Wei‐Kuo , Kucsera Tom L.
Published: February 2008

Aircraft data from the Stratosphere‐Troposphere Exchange Project (STEP) and the Equatorial Mesoscale Experiment (EMEX) flights conducted on February 2, 1987, off northern Australia are used in cumulus cloud and photochemical models to determine the effects of convection on upper tropospheric O3 production. Ozone production is calculated as the amount integrated over cloud outflow layers for the first 24 hours after convection. Ozone production with convection is compared to ozone formation in undisturbed conditions. Model simulations of the EMEX 9 convective system indicate lower tropospheric air relatively rich in CO and low in NOx exiting in cloud outflow, slightly depressing the rate of O3 formation in the middle and upper troposphere. Other convective complexes, 800–900 km upstream, caused even greater perturbations to measured profiles of CO, NOx, O3, and H2O and implied a 15–20% reduction in the rate of O3 production from 14.5 to 17 km. The greatest factor affecting O3 formation in the upper troposphere in the STEP/EMEX flight might have been lightning‐produced NOx. We estimate that O3 production from 12 to 17 km is 2–3 times more rapid than it would be with no lightning. This STEP/EMEX event adds to a climatology of half a dozen cases we have analyzed to determine the effects of convection on free tropospheric O3 production. The study region represents the “maritime continent” in contrast to continental regions studied previously. Relatively small quantities of species from the lower troposphere were transported to the upper troposphere because of the relatively weak vertical velocities in the storm and because chemical species gradients had been minimized by frequent convection prior to the February 2 event. Earlier in the convective season, the chemical consequences of a single episode might have been more substantial.
A two‐dimensional model of ion composition in the stratosphere: 1. Positive ions
Authors: Beig G. , Walters S. , Brasseur G.
Published: February 2008

Ion composition measurements have shown that the dominant ions are mainly proton hydrates (PH) (or H+ (H2O)n) in the upper stratosphere and mesosphere and nonproton hydrates (NPH) (or H+ X (H2O)m) in the lower stratosphere. Theoretical and experimental studies have indicated that the identity of X is almost certainly methyl cyanide (CH3CN). This paper reports a two‐dimensional model of ion composition from 15 km to the stratopause. The model accounts for the latitudinal variation in the ionization rate by galactic cosmic rays and in the concentration of neutral species which affect ion chemistry. The results obtained indicate several new and interesting features. The altitude of the crossover point of PH over NPH is found to vary with latitude. The PH ions start to dominate as low as 27 km near the poles, whereas at the equatorial latitude, NPH ions remain the dominant ions up to about 45 km. The modeled profiles at 45°N, the only latitude at which experimental data are available, conform very well to those data. The ion composition is found to be highly dependent on the reaction rate coefficient (K) for the conversion of NPH to PH, which is poorly known. The meridional distribution of ion composition and a comparison with the available observational data is presented. Finally, the need for future ion composition measurements at other latitudes is emphasized.
Rainforest burning and the global carbon budget: Biomass, combustion efficiency, and charcoal formation in the Brazilian Amazon
Authors: Fearnside Philip M. , Leal Niwton , Fernandes Fernando Moreira
Published: February 2008

Biomass present before and after burning was measured in forest cleared for pasture in a cattle ranch (Fazenda Dimona) near Manaus, Amazonas, Brazil. Aboveground dry weight biomass loading averaged 265 t ha−1(standard deviation (SD) = 110,n= 6 quadrats) at Fazenda Dimona, which corresponds to approximately 311 t ha−1total dry weight biomass. A five‐category visual classification at 200 points showed highly variable burn quality. Postburn aboveground biomass loading was evaluated by cutting and weighing of 100 m2quadrats and by line intersect sampling. Quadrats had a mean dry weight of 187 t ha−1(SD = 69,n= 10), a 29.3% reduction from the preburn mean in the same clearing. Line intersect estimates in 1.65 km of transects indicated that 265 m3ha−1(approximately 164 t ha−1of aboveground dry matter) survived burning. Using carbon contents measured for different biomass components (all ∼50% carbon) and assuming a carbon content of 74.8% for charcoal (from other studies near Manaus), the destructive measurements imply a 27.6% reduction of aboveground carbon pools. Charcoal composed 2.5% of the dry weight of the remains in the postburn destructive quadrats and 2.8% of the volume in the line intersect transects. Thus approximately 2.7% of the preburn aboveground carbon stock was converted to charcoal, substantially less than is generally assumed in global carbon models. The findings confirm high values for biomass in central Amazonia. High variability indicates the need for further studies in many localities and for making maximum use of less laborious indirect methods of biomass estimation. While indirect methods are essential for regional estimates of average biomass, only direct weighing such as that reported here can yield information on combustion efficiency and charcoal formation. Both high biomass and low percentage of charcoal formation suggest the significant potential contribution of forest burning to global climate changes from CO2and trace gases.
An investigation of the global solar radiative forcing due to changes in cloud liquid water path
Authors: Ramaswamy V. , Chen C.‐T.
Published: February 2008

The instantaneous solar radiative forcing of the surface‐atmosphere system associated with a change in the liquid water path (LWP) of low clouds has a significant space‐time dependence, owing to the spatial and temporal variations in insolation, solar zenith angle, and surface albedo. This feature is demonstrated by considering globally uniform LWPs and LWP changes. Keeping cloud amounts fixed in space and time, we find that an increase in LWP imparts a distinct meridional gradient to the solar forcing, while the difference between summer and winter forcings introduces a seasonal variation at any given latitude. Relative to the global, annual mean (GAM) value (a negative quantity for an increase in LWP) the forcing is more negative at low latitudes throughout the year and during summer at the high latitudes. In contrast, the forcing is more positive than the GAM value during the winter season at the higher latitudes (poleward of 40°). Thus even the simple assumption of a globally uniform LWP change does not yield a uniform forcing at all latitudes and/or times. However, because of the contrasts in the contributions from the low and high latitudes and over the different seasons the global and annual average of the radiative forcing turns out to be nearly identical to that computed using a global, annual mean atmospheric profile and mean insolation conditions. The annual mean meridional gradient of the forcing is sensitive both to the “control” LWP values and to the changes in those values. A factor that can introduce an additional nonuniformity in the solar forcing is the latitudinal variation in the cloud climatology. We also find that the zonal, annual mean pattern of the forcing due to the cloud LWP change is different from that for carbon dioxide doubling. Thus while a specific globally uniform LWP increase can yield a global, annual mean radiative forcing that is opposite to but has the same magnitude as that for carbon dioxide increases, such a compensation in the forcing cannot be expected to be uniform with latitude or month.
A global model of thunderstorm electricity
Authors: Stansbery E. K. , Few A. A. , Geis P. B.
Published: February 2008

An axisymmetric numerical model in an Earth‐centered spherical coordinate system is created to calculate the electric field distribution and current distribution from a thunderstorm source in the global electrical circuit. The model includes a hemisphere in which the thunderstorm is located, an atmosphere and ionosphere with anisotropic height‐variable conductivities, and a passive magnetic conjugate hemisphere. Both single‐cell thunderstorms and symmetric multicell thunderstorm complexes can be modeled. The current output from the thunderstorm spreads out in the ionosphere and flows along the magnetic field lines into the conjugate hemisphere. Approximately half of the current that reaches the ionosphere flows into the conjugate hemisphere, and the rest is redirected to the fair‐weather portion of the storm hemisphere. Two examples of this general model are discussed, a single‐cell severe storm and a multicell severe storm. Results of this study show that it is important to include a realistic model of the ionosphere to evaluate the spread of current in the ionosphere and the mechanism of thunderstorm charging of the global electric circuit.
A seasonal cycle in the southwest Pacific free tropospheric aerosol concentration
Authors: Kristament I. S. , Harvey M. J. , Liley J. B.
Published: February 2008

Measurements made of the concentration of aerosol particles in the diameter range 0.15 − 3μm in the free troposphere over the southwest Pacific show evidence of a seasonal cycle. The maximum concentration occurs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30°S where, in early spring, the mean aerosol concentration is 18 cm−3, more than 10 times the mean autumn value. In the vicinity of the equator the seasonality disappears and concentrations throughout the year apparently remain close to the autumn levels at other latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that the peak in early spring is due to biomass burning through the tropical southern hemisphere dry season. Continued high values through late spring and early summer are thought to be the result of the seasonal production of aerosol precursors, such as dimethyl sulfide (DMS), at middle and high latitudes.
Drop size modification by forest canopies: Measurements using a disdrometer
Authors: Hall Robin L. , Calder Ian R.
Published: February 2008

The modification of the drop size spectra of natural and simulated rainfall by the canopies of three tropical plantation tree species was measured using a disdrometer. Contrary to previously published results, large differences were found between species in the degrees of modification. The median‐volume drop diameters measured were 2.3, 2.8, and 4,2 mm for Pinus caribaea, Eucalyptus camaldulensis, and Tectona grandis, respectively. The characteristic drip spectra for the different species are equivalent to the corresponding drop size spectra for rainfall with approximate intensities of 50, 100, and 3000 mm h−1 (essentially infinite) for P. caribaea, E. camaldulensis, and T. grandis, respectively. These results have implications for the choice of the best tree species for areas susceptible to soil erosion.
Atmospheric NO3 observations over low‐latitude northern hemisphere during night
Authors: Lal M. , Sidhu J. S. , Das S. R. , Chakrabarty D. K.
Published: February 2008

Nighttime column abundance of NO3 has been measured in the low latitude region by visible absorption spectroscopy technique. For that, direct moonlight irradiance on the surface of the Earth has been measured near full Moon in the 655.0‐ to 667.0‐nm region. Errors due to solar Fraunhofer effect, atmospheric aerosol and Rayleigh scattering, atmospheric water vapor and ozone contents have been removed. High zenith angle direct solar spectrum and low zenith angle lunar spectrum have been used as background spectra. The vertical column abundance of NO3 was found to be about 10 times higher than that reported earlier by others for a low latitude region.
Comparison of lightning return‐stroke models
Authors: Thottappillil Rajeev , Uman Martin A.
Published: February 2008

Five return‐stroke models, each allowing the use of measured channel‐base current and return‐stroke speed as inputs for the computation of channel current distribution and remote electric field, are compared and evaluated using 18 sets of three simultaneously measured triggered lightning features: channel‐base current, return‐stroke speed, and electric field at a distance of about 5 km from the channel base. The experimental data were acquired during a triggered lightning experiment at the NASA Kennedy Space Center, Florida, in 1987 and were reported in part by Willett et al. (1989) and Leteinturier et al. (1991). The models compared are the transmission line (TL) model, the modified transmission line (MTL) model, the traveling current source (TCS) model, the Diendorfer‐Uman (DU) model, and the modified Diendorfer‐Uman (MDU) model. The TL, MTL, DU, and MDU models each predict the measured initial electric field peaks with an error whose mean absolute value is about 20%; the TCS model has a mean absolute error about twice that value. For the prediction of overall measured field wave shape, none of the models is clearly preferred, although for the model parameters assumed, the MDU model gave the best wave shape match. Most of the return strokes that exhibited very narrow sharp initial peaks in the measured electric field waveforms had a maximum rate of rise of channel‐base current closer to the peak of the measured channel‐base current waveform than did return strokes not exhibiting these sharp field peaks. The calculated fields from the TL and the MTL models do not have narrow sharp peaks similar to those found in many of the measured fields, while the fields calculated from the TCS, DU, and MDU models had somewhat similar peaks in most of the cases where those peaks were found in the measured fields. On the basis of the comparison of the five models, we recommend the TL model for calculating the peak channel‐base current from the measured initial peak electric field because the TL model provides a similar or better result from a simpler mathematical relation.
The three‐dimensional structure of cumulus clouds over the ocean: 1. Structural analysis
Authors: Kuo Kwo‐Sen , Welch Ronald M. , Weger Ronald C. , Engelstad Mark A. , Sengupta S. K.
Published: February 2008

Thermal channel (channel 6, 10.4–12.5 μm) images of five Landsat thematic mapper cumulus scenes over the ocean are examined. These images are thresholded using the standard International Satellite Cloud Climatology Project thermal threshold algorithm. The individual clouds in the cloud fields are segmented to obtain their structural statistics which include size distribution, orientation angle, horizontal aspect ratio, and perimeter‐to‐area (PtA) relationship. It is found that the cloud size distributions exhibit a double power law with the smaller clouds having a smaller absolute exponent. The cloud orientation angles, horizontal aspect ratios, and PtA exponents are found in good agreement with earlier studies. A technique also is developed to recognize individual cells within a cloud so that statistics of cloud cellular structure can be obtained. Cell structural statistics are computed for each cloud. Further examination reveals that unicellular clouds are generally smaller (≤ 1 km) and have smaller PtA exponents, while multicellular clouds are larger (≥ 1 km) and have larger PtA exponents. Cell structural statistics are similar to those of the smaller clouds. Each cell is approximated as a quadric surface using a linear least squares fit. Most cells are found to have the shape of a hyperboloid of one sheet. However, about 15% of the cells are best modeled by a hyperboloid of two sheets. Contrary to intuition, less than 1% of the clouds are found to be ellipsoidal. The number of cells in a cloud is found to increase slightly faster than linearly with increasing cloud size. The mean nearest neighbor distance between cells in a cloud, however, appears to increase linearly with increasing cloud size and to reach a maximum when the cloud effective diameter is about 10 km; then it decreases with increasing cloud size. Sensitivity studies of threshold and lapse rate show that neither has a significant impact upon the results. A goodness‐of‐fit ratio is used to provide a quantitative measure of the individual cloud results. Significantly improved results are obtained after applying a smoothing operator, suggesting that eliminating subresolution scale variations with higher spatial resolution may yield even better shape analyses.
Large‐scale isentropic mixing properties of the Antarctic polar vortex from analyzed winds
Authors: Bowman Kenneth P.
Published: February 2008

Winds derived from analyzed geopotential height fields are used to study quasi‐horizontal mixing by the large‐scale flow in the lower stratosphere during austral spring. This is the period when the Antarctic ozone hole appears and disappears. Trajectories are computed for large ensembles of particles initially inside and outside the main polar vortex. Mixing and transport are diagnosed through estimates of finite time Lyapunov exponents and Lagrangian dispersion statistics of the tracer trajectories. At 450 K and above prior to the vortex breakdown: Lyapunov exponents are a factor of 2 smaller inside the vortex than outside; diffusion coefficients are an order of magnitude smaller inside than outside the vortex; and the trajectories reveal little exchange of air across the vortex boundary. At lower levels (425 and 400 K) mixing is greater, and there is substantial exchange of air across the vortex boundary. In some years there are large wave events that expel small amounts of vortex air into the mid‐latitudes. At the end of the spring season during the vortex breakdown there is rapid mixing of air across the vortex boundary, which is evident in the mixing diagnostics and the tracer trajectories.
40Ar/39Ar age spectra of Apollo 15 impact melt rocks by laser step‐heating and their bearing on the history of lunar basin formation
Authors: Dalrymple G. Brent , Ryder Graham
Published: February 2008

We have obtained 26 high‐resolution (16–51 steps) 40Ar/39Ar age spectra using a continuous laser system on submilligram fragments of recrystallized melt and single‐crystal plagioclase clasts from 12 Apollo 15 impact melt rocks collected at the Apennine Front where the Imbrium and Serenitatis basins intersect. These melt rocks represent a wide range of compositions and at least half a dozen different impacts. Six of the melt rocks have reproducible, intermediate‐temperature plateaus over 40% or more of the 39Ar released; the plateaus are interpreted as crystallization (impact) ages and much of the non‐plateau behavior is attributable to recoil. Samples 15294,6,21,15304,7,69, 15314,26,156, 15357,15, and 15359,12 have mean 40Ar/39Ar plateau ages that are statistically indistinguishable and fall within the narrow range 3852 ± 14 (2σ) Ma to 3870 ±12 Ma with a weighted mean of 3865 ± 5 Ma. Sample 15356,9 has a mean plateau age of 3836 ± 11 Ma and may represent a distinctly younger impact. A seventh sample (15314,30,158) has a peculiar but reproducible double plateau; a low‐T one at 3873 ± 9 Ma, which we think records the crystallization age, and a high‐T one of 3831 ± 10 Ma, which we interpret as an experimental (39Ar recoil) artifact. Four of the remaining melt rocks (15308,9, 15414,2,37, 15436,2, 15445,253) have complex 40Ar/39Ar age spectra that indicate that they either formed in or were disturbed by impacts that occurred ≤3850 Ma but did not completely reset the K‐Ar isotopic system. Sample 15414,3,36 is different. Its spectrum may represent release mainly from clasts that were not well degassed in a melt event at 3870 Ma; the melt phase contains little potassium. Because most of the Apennine Front material must be coeval with or predate formation of the Imbrium Basin, it seems likely that the Imbrium impact is no older than 3870 Ma and probably no older than 3836 Ma. So far there is no convincing evidence in the lunar record for melt‐producing impacts, such as basin formation, older than about 3.9 Ga.
Quantitative analysis of the extensional tectonics of Tharsis Bulge, Mars: Geodynamic implications
Authors: Thomas Pierre G. , Allemand Pascal
Published: February 2008

About 10% of the Tharsis bulge surface is intensely faulted, with a fault density up to 1 fault/km. In these intensely faulted areas, 54 impact craters which exhibit faulted walls and floor are now elliptical in shape. These ellipticities indicate an average strain value of 10% inside these provinces. This strain value is 4 times greater than the strain deduced from normal fault morphometric analysis. This high value indicates that the strain is not only due to 60° dip normal faults, and that some additional tectonic processes exist such as boudinage. This important strain value corresponds to an elongation of about 50 km across Thaumasia and Tempe‐Mareotis provinces, and 20 km across Alba Patera Regio. The amount of elongation for the entire Tharsis Region is 2 orders of magnitude higher than the strains and motions related to standard models of bulge formation. These intensely faulted areas are restricted areas where faults are both radial to the bulge and perpendicular to the topographic slope. The stretching does not influence the regional topography and there are no topographic shoulders or important level differences between faulted and nonfaulted areas, which would indicate that the crust is not thinned under the stretched provinces. Outside these intensely faulted areas the graben width is remarkably constant all over the Tharsis bulge, indicating the existence of a brittle ductile transition or an incompetent level at about 1–2 km below the surface. A new origin for the Tharsis tectonic features is proposed as a preliminary hypothesis: the intensely faulted areas correspond to the uphill parts and to denudation zones of gravity slidings which move on a 2km‐deep incompetent level. The gliding of the slab on the very low dipping flanks of Tharsis needs three conditions: (1) the viscosity of the incompetent layer must be equal to the viscosity of the ice, (2) the brittle layer must have been previously faulted by the bulging itself, (3) these preexisting radial fault directions must be perpendicular to topographic slope. The ridged plain units in Lunae Palus and Coprates provinces thus correspond to a zone of blocking inside the gliding slab, due to an interruption of the decollement level.
Sounder stimulated Dn resonances in Jupiter's Io plasma torus
Authors: Osherovich V. A. , Benson R. F. , Fainberg J. , Stone R. G. , MacDowall R. J.
Published: February 2008

On February 8, 1992, the Ulysses spacecraft passed through Jupiter's Io plasma torus, where rich spectra of narrow‐band resonances were stimulated by the relaxation sounder of the Ulysses unified radio and plasma wave (URAP) instrument. Since the gyrofrequency ƒg is comparable to the plasma frequency ƒp in the Io torus, it was predicted that the general classification of stimulated ionospheric Dn resonances, developed for 1 ≤ ƒp/ ƒg ≤ 8 in the Earth's topside ionosphere, should apply in the Io torus as well as the Earth's magnetosphere (Osherovich, 1989). The URAP plasmagrams (sounder spectra) in the portions of the Io torus satisfying these plasma conditions are dominated by the Dn resonances for frequencies below ƒp. On most of these plasmagrams the ƒp resonance is also present, but it is seldom the dominant resonance. Neither upper hybrid nor nƒg resonances have been found on these plasmagrams. The identification of Dn resonances has allowed both the electron density and the magnetic field amplitude to be calculated. The derived densities on the outbound pass agree well with a Voyager model of Bagenal (1992). The derived magnetic field values are close to the Goddard Space Flight Center O6 magnetic field model.
Magellan observations of the Venera and Vega landing site regions
Authors: Weitz Catherine M. , Basilevsky Alexander T.
Published: February 2008

Geologic maps have been produced for each of the seven Venera/Vega landing sites on Venus. The dominant terrain identified at all the sites is plains. The sites contain minor amounts of complex ridge terrain (CRT), complex terrain (CT), and an older plains (Po) that are all embayed by younger plains (Py). Some of the sites (Venera 9 and Vega 1 and 2) are made up of vast regional plains units while the other sites (Venera 8, 10, 13, and 14) contain more localized plains. The Venera 8 and 13 sites have unusual volcanic features associated with them and a nontholeiitic composition was measured by the landers at these sites. At the other five sites where a tholeiitic composition was measured, lava flows with morphologies that suggest an apparent low viscosity have been identified. This indicates a good correspondence between the geochemistry measured by the landers and the Magellan imagery. A good correlation also exists between the TV panoramas taken by four of the landers and the Magellan imagery. At the Venera 9, 10, 14 and Vega 1, 2 sites, the most likely material sampled are plains composed of tholeiitic lavas. A steep‐sided dome, lamprophyre‐like lavas, or ash beds are the most likely material sampled by the Venera 8 lander. The Venera 13 lander may have sampled lava flows from a corona‐like feature or from a steep‐sided dome in the southeast of the landing circle. Radiophysical properties of the landing sites show a good correspondence with the local geology, with most variations in backscatter, emissivity, and rms slopes reflecting differences in surface roughness. The Venera/Vega sites are good examples of typical venusian plains, indicating that geochemical measurements and TV observations provide representative characteristics of this most abundant terrain type on Venus.
A mechanism for crustal recycling on Venus
Authors: Lenardic A. , Kaula W. M. , Bindschadler D. L.
Published: February 2008

Entrainment of lower crust by convective mantle downflows is proposed as a crustal recycling mechanism on Venus. The mechanism is characterized by thin sheets of crust being pulled into the mantle by viscous flow stresses. Finite element models of crust/mantle interaction are used to explore tectonic conditions under which crustal entrainment may occur. The recycling scenarios suggested by the numerical models are analogous to previously studied problems for which analytic and experimental relationships assessing entrainment rates have been derived. We use these relationships to estimate crustal recycling rates on Venus. Estimated rates are largely determined by (1) strain rate at the crust/mantle interface (higher strain rate leads to greater entrainment); and (2) effective viscosity of the lower crust (viscosity closer to that of mantle lithosphere leads to greater entrainment). Reasonable geologic strain rates and available crustal flow laws suggest entrainment can recycle ≈ 1 km3 of crust per year under favorable conditions.
Optical effects of space weathering: The role of the finest fraction
Authors: Pieters C. M. , Fischer E. M. , Rode O. , Basu A.
Published: February 2008

The optical properties of lunar soils are different than those of rocks from which they are derived. As a consequence of lunar space weathering, soils are darker and exhibit a distinctive red‐sloped continuum and weaker mineral absorption bands. The accumulation of dark glass‐welded aggregates (agglutinates) has been thought to account for these optical effects of space weathering on lunar soils. Spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar soils presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration. It is the finest fraction of lunar soils (<25 μm which constitute ∼25 wt %) that dominates the optical properties of the bulk soil. Unlike size fractions of most silicates for which the finest fraction is the brightest, the lunar soil size fractions all have comparable albedos in the short‐wavelength visible. In the near infrared, however, it is the finest fraction that exhibits the steep red continuum and weak absorption bands. The properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate‐rich soil particles. These results suggest space weathering on airless bodies is dominated by surface correlated processes (perhaps associated with the development of fine‐grained Fe° on or near the surface of grains), and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration.
Large‐scale electric field measurements on the Earth's surface: A review
Authors: Lanzerotti L. J. , Chave A. D. , Sayres C. H. , Medford L. V. , Maclennan C. G.
Published: February 2008

There exist only a few reported measurements of quasi‐stationary (near dc) electric potentials over very large spatial scales (hundreds of kilometers or more) on the Earth's surface. Such measurements have typically been made using unpowered submarine telecommunications cables. The measurements pose unique experimental challenges and require careful procedures to avoid data contamination by electrode contact potentials and local ground currents. In addition, there are possible interpretational problems from pervasive, poorly understood, low‐frequency electric fields induced by ocean water motion through the Earth's stationary magnetic field. Nevertheless, estimates of the magnitude of the electric field computed from large‐scale potential difference measurements, made principally to date in the Pacific Ocean, can be used to place a limit on the size of the toroidal magnetic field at the core‐mantle boundary under certain conditions on the Earth's electrical conductivity profile. Thus, large‐scale electric potential measurements can serve as an adjunct probe of the Earth's dynamo process in addition to measurements of the poloidal magnetic field and its secular changes made at and above the surface of the Earth. A review of all of these data suggests that the toroidal and poloidal magnetic fields at the top of the core are comparable in magnitude.
Impact crater outflows on Venus: Morphology and emplacement mechanisms
Authors: Chadwick D. John , Schaber Gerald G.
Published: February 2008

Many of the 932 impact craters discovered by the Magellan spacecraft at Venus are associated with lobate flows that originate at or near the crater rim. They extend for several to several hundred kilometers from the crater, and they commonly have a strong radar backscatter. A morphologic study of all identifiable crater outflows on Venus has revealed that many individual flows each consist of two areas, defined by distinct morphologic features. These two areas appear to represent two stages of deposition for each flow. The part of the flow that is generally deposited closest to the crater tends to be on the downrange side of the crater, flows in the downrange direction, and it is interpreted to be a late‐stage ejecta. In many cases, this proximal part of the flow is too thin to completely bury the large blocks in subjacent ejecta deposits. Dendritic channels, present in many proximal flows, appear to have drained liquid from the proximal part in the downhill direction, and they debouch to feed the outer part of the flows. This distal part flows downhill, fills small grabens, and is ponded by ridges, behavior that mimics that of volcanic lava flows. The meandering and dendritic channels and the relation of the distal flows to topography strongly suggest that the distal portion is the result of coalescence and slow drainage of impact melt from the proximal portion. Impact melt forms a lining to the transient crater and mixes turbulently with solid clasts, and part of this mixture may be ejected to form the proximal part of the flow during the excavation stage of crater development. A statistical study of the Venusian craters has revealed that, in general, large craters produced by impacts with relatively low incidence angles to the surface are more likely to produce flows than small craters produced by higher‐angle impacts. The greater flow production and downrange focusing of the proximal flows with decreasing incidence angle indicate a strong control of the flows by the impactor flight direction, and a high downrange velocity imparted to the proximal flow material in lower angle impacts. On the Moon, small flows interpreted to be composed of impact melt are observed atop the ejecta of large, fresh craters; on Earth, melt‐rich suevite deposits form the uppermost layer of ejecta of some fresh craters. These features, albeit much smaller, may be analogous to the flows on Venus. Numerical models have predicted that larger volumes of impact melt would be produced on Venus than on the cooler terrestrial bodies due to high atmospheric and target temperatures, perhaps 3 times the volume produced on the Moon for a given crater diameter.
A Quantitative Study of Lower Cretaceous Cyclic Sequences from the Atlantic Ocean and the Vocontian Basin (Se France)
Authors: Huang Z. , Ogg J. G. , Gradstein F. M.
Published: February 2008

Milankovitch‐type periodicity was identified within Lower Cretaceous cyclic sequences from the western Central Atlantic Ocean and the Vocontian Basin (southeast France) through application of the Walsh spectral method to detailed stratigraphic sections. The repetitious bedding in these sequences appears to be the product of cyclic climatic changes controlled by the orbital forcing. Obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation, whereas precession is less important. Upper Berriasian and Valanginian strata from the Atlantic Ocean contain indications of eccentricity‐induced periodicity and of a nonlinear response of the climatic system. Variations in orbitally tuned pelagic sedimentation rates in the Atlantic Ocean correlate with the changes in the character of cycles and with the occurrence of calcareous turbidites; therefore this suite of sedimentary features are responding to evolving paleoceanographic conditions and possible relative sea level fluctuations. The orbitally tuned durations of the Valanginian and Hauterivian stages in the Vocontian Basin are a minimum of 5.9 and 5.3 m.y., respectively.
Diatom Evidence for Late Holocene Climatic Events in Granite Harbor, Antarctica
Authors: Leventer Amy , Dunbar Robert B. , DeMaster David J.
Published: February 2008

Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high‐resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global‐scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long‐term changes in floral assemblages, as recorded by changes in the relative concentration of Nitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little ice Age. The sedimentary record reveals several distinct, short‐lived events, in which nearly monospecific blooms of Corethron criophilum and Chaetoceros spp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short‐lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor.