bluuebunny/crossref_metadata_2025 huggingface dataset:
A neural network approach for modeling nonlinear transfer functions: Application for wind retrieval from spaceborne scatterometer data
Authors: Thiria S. , Mejia C. , Badran F. , Crepon M.
Published: February 2008

The present paper shows that a wide class of complex transfer functions encountered in geophysics can be efficiently modeled using neural networks. Neural networks can approximate numerical and nonnumerical transfer functions. They provide an optimum basis of nonlinear functions allowing a uniform approximation of any continuous function. Neural networks can also realize classification tasks. It is shown that the classifier mode is related to Bayes discriminant functions, which give the minimum error risk classification. This mode is useful for extracting information from an unknown process. These properties are applied to the ERS1 simulated scatterometer data. Compared to other methods, neural network solutions are the most skillful.
Gas transfer experiment on Georges Bank using two volatile deliberate tracers
Authors: Wanninkhof Rik , Asher William , Weppernig Ralf , Chen Hua , Schlosser Peter , Langdon Chris , Sambrotto Ray
Published: February 2008

A gas exchange experiment was performed on Georges Bank using deliberate tracers sulfur hexafluoride (SF6) and helium 3 (3He). The concentrations of the tracers were measured in the water column over a period of 10 days. During this time the patch grew from an 8‐km‐long injection streak to an area of about 500 km2. The gas transfer velocity was determined from the change in the ratio of the tracers over time scaled to the ratio of their Schmidt numbers. A near‐linear relationship between gas exchange and wind speed was observed based on four experimental points covering a wind speed range from 3 to 11 m/s. The results fall in the upper part of the range of gas transfer‐wind speed relationships developed to date. Wind speeds during the experiment obtained from anemometers on the ship, on a free floating drifter, and on a fixed mooring showed significant differences. With the ability to measure gas transfer velocities over the ocean on timescales of several days, accurate wind speed/stress measurements are imperative to obtain a robust relationship between gas transfer and wind speed.
A laboratory study on supercooling and frazil ice production processes in winter coastal polynyas
Authors: Ushio Shuki , Wakatsuchi Masaaki
Published: February 2008

The rates of frazil ice production and salt flux in wind‐generated open water were estimated on the basis of results from laboratory experiments that model coastal polynyas in a severe winter. The measurements were made under various conditions of wind speed, air temperature, and water salinity. The rate of ice production increased with increasing wind speed and water salinity, and with decreasing air temperature. The high rates of ice production are attributed to the vigorous production of underwater frazil crystals. Salt fluxes due to frazil production were also much greater than those of sheet ice growing vertically under calm atmospheric conditions. The important factor that governs such a process of high ice production lies in supercooling and underwater ice production. In the case of common seawater of salinities larger than 25‰, supercooled water that formed on the surface sinks into the underlying seawater at its freezing point by a density instability as well as by a wind‐forced convection. The sinking of the supercooled water and the resulting production of numerous frazil crystals under the water lead to the maintenance of open water for a long period. Thus large heat fluxes occur through the open water that is continuously exposed to cold air, producing a large amount of the underwater ice at high rates. This mechanism for ice production in open waters is driven by a strong, cold wind and is enhanced by increasing water salinity.
Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945–1990
Authors: Petrie Brian , Drinkwater Kenneth
Published: February 2008

Examination of temperature and salinity data from the Scotian Shelf, Gulf of Maine, Gulf of St. Lawrence, and the adjacent continental slope has shown that the dominant low‐frequency event over the last 45 years was a cooling and subsurface freshening of the water masses from 1952 to 1967, followed by a rapid reversal of these trends. The largest temperature and salinity changes (1952–1967) were 4.6°C and 0.7, respectively, and occurred at about 100 m over the slope. Exchanges with shelf waters and vertical mixing gave rise to the surface manifestation of this variability. The westward transport of the Labrador Current was found to have similar variability, increasing from about 1 × 106 m3 s−1 in the early 1950s to about 4 × 106 m3 s−1 in the mid‐1960s. A simple model that accounts for this variation of transport and has a constant entrainment of North Atlantic water indicates that changes of the westward flow of the Labrador Current could contribute significantly to the T‐S fluctuations.
How well can regional fluxes be derived from smaller‐scale estimates?
Authors: Moore Kathleen E. , Fitzjarrald David R. , Ritter John A.
Published: February 2008

Regional surface fluxes are essential lower boundary conditions for large‐scale numerical weather and climate models and are the elements of global budgets of important trace gases (Stewart et al., 1989). Surface properties affecting the exchange of heat, moisture, momentum and trace gases vary with length scales from 1 m to hundreds of kilometers. A classical difficulty is that fluxes have been measured directly only at points (towers) or along lines (from aircraft). The process of “scaling up” observations limited in space and/or time to represent larger areas has been done by assigning properties to surface types and combining estimated or calculated fluxes using an area‐weighted average. Because of nonlinear influences, such as the effect of internal boundary layers, it is not clear that a simple area‐weighted average is sufficient to produce the large scale from the small scale, nor is it known how important the uncertainty is to large‐scale model outcomes. Simultaneous aircraft and tower data obtained in the relatively simple terrain of the western Alaskan tundra were used to determine the extent to which surface type variation can be related to regional‐scale fluxes of heat, moisture, and other properties. Surface type was classified as lake or land with an aircraft‐borne infrared thermometer, and flight‐level heat and moisture fluxes were related to surface type. The magnitude and variety of sampling errors inherent in eddy correlation flux estimation place limits on how well any flux can be known even in simple geometries. Because of the presence of intrinsic and site‐specific uncertainties, regional‐scale flux of heat and moisture using aircraft observations in our study area can be reasonably verified to be estimated correctly from linear combinations of smaller‐scale estimates only to within a factor of 1.5. Flights at lower levels or in a more comprehensive or systematic pattern might be able to resolve the contributions from individual surface types better, but an experiment to test any scaling‐up hypothesis is difficult to devise.
Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 9. Observations of OClO from April to October 1991
Authors: Sanders R. W. , Solomon S. , Smith J. P. , Perliski L. , Miller H. L. , Mount G. H. , Keys J. G. , Schmeltekopf A. L.
Published: February 2008

The first spectroscopic measurements of chlorine dioxide throughout Antarctic fall, winter, and spring were carried out at McMurdo Station during April to October 1991. Two different observing modes were employed to extend the measurements over the broadest possible seasonal range: direct Moon measurements were used as well as observations of the scattered light from the sky at an 80° angle relative to the zenith in the direction toward the Sun. The latter observing scheme facilitates measurements during the late twilight, when the brightness in the more conventional zenith viewing direction is much weaker. The measurements made using both the scattered light and the lunar sources show that OClO was below detection limits in late April and May (twilight OClO vertical column <3×l012 cm−2), for a corresponding total ozone loss rate due to the coupling of chlorine and bromine of no more than about 0.25 DU/d. OClO was first observed in late June using lunar spectra, at which time high nighttime vertical column abundances of about 1.0×1014 cm−2 were measured, implying extensive conversion of inorganic chlorine to reactive forms in the lower stratosphere. The observation of OClO in midwinter implies that the air parcels had been exposed to sunlight, presumably via wave‐driven excursions of airflow to latitudes outside the polar night region. High levels of OClO were measured using moonlight near the full Moon phases in late July, August, and September. Further, these measurements of high OClO indicate that significant ozone loss should have taken place during midwinter. The winter ozone loss rate suggested by these data is likely to increase in the future as atmospheric loading of chlorine and bromine continues to rise. In addition, scattered light measurements in late July and August revealed very large twilight column abundances then, implying significant ozone loss rates. Somewhat smaller twilight column abundances were measured in September, and the OClO dropped below detection levels in October.
Insolation and Earth's orbital periods
Authors: Berger André , Loutre Marie‐France , Tricot Christian
Published: February 2008

Solar irradiance received on a horizontal surface depends on the solar output, the semimajor axis of the elliptical orbit of the Earth around the sun (a), the distance from the Earth to the sun (r), and the zenith distance (z). The spectrum of the distance, r, for a given value of the true longitude, λ, displays mainly the precessional periods and, with much less power, half precession periods, eccentricity periods, and some combination tones. The zenith distance or its equivalent, the elevation angle (E), is only a function of obliquity (ϵ) for a given latitude, ϕ, true longitude, and hour angle, H. Therefore the insolation at a given constant value of z is only a function of precession and eccentricity. On the other hand, the value of the hour angle, H, corresponding to this fixed value of z varies with ε, except for the equinoxes, where H corresponding to a constant z also remains constant through time. Three kinds of insolation have been computed both analytically and numerically: the instantaneous insolation (irradiance) at noon, the daily irradiation, and the irradiations received during particular time intervals of the day defined by two constant values of the zenith distance (diurnal irradiations). Mean irradiances (irradiations divided by the length of the time interval over which they are calculated) are also computed for different time intervals, like the interval between sunrise and sunset, in particular. Examples of these insolations are given in this paper for the equinoxes and the solstices. At the equinoxes, for each latitude, all insolations are only a function of precession (this invalidates the results obtained by Cerveny [1991]). At the solstices, both precession and obliquity are present, although precession dominates for most of the latitudes. Because the lengths of the astronomical seasons are secularly variable (in terms of precession only), a particular calendar day does not always correspond to the same position relative to the sun through geological time. Similarly, a given longitude of the Sun on its orbit does not correspond to the same calendar day. For example, 103 kyr ago, assuming arbitrarily that the spring equinox is always on March 21, autumn began on September 13, and 114 kyr ago, it began on September 27, the length of the summer season being 85 and 98 calendar days, respectively, at these remote times in the past.
WINDII, the wind imaging interferometer on the Upper Atmosphere Research Satellite
Authors: Shepherd G. G. , Thuillier G. , Gault W. A. , Solheim B. H. , Hersom C. , Alunni J. M. , Brun J.‐F. , Brune S. , Charlot P. , Cogger L. L. , Desaulniers D.‐L. , Evans W. F. J. , Gattinger R. L. , Girod F. , Harvie D. , Hum R. H. , Kendall D. J. W. , Llewellyn E. J. , Lowe R. P. , Ohrt J. , Pasternak F. , Peillet O. , Powell I. , Rochon Y. , Ward W. E. , Wiens R. H. , Wimperis J.
Published: February 2008

The WIND imaging interferometer (WINDII) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991. This joint project, sponsored by the Canadian Space Agency and the French Centre National d'Etudes Spatiales, in collaboration with NASA, has the responsibility of measuring the global wind pattern at the top of the altitude range covered by UARS. WINDII measures wind, temperature, and emission rate over the altitude range 80 to 300 km by using the visible region airglow emission from these altitudes as a target and employing optical Doppler interferometry to measure the small wavelength shifts of the narrow atomic and molecular airglow emission lines induced by the bulk velocity of the atmosphere carrying the emitting species. The instrument used is an all‐glass field‐widened achromatically and thermally compensated phase‐stepping Michelson interferometer, along with a bare CCD detector that images the airglow limb through the interferometer. A sequence of phase‐stepped images is processed to derive the wind velocity for two orthogonal view directions, yielding the vector horizontal wind. The process of data analysis, including the inversion of apparent quantities to vertical profiles, is described.
A detailed evaluation of the heating efficiency in the middle atmosphere
Authors: Mlynczak Martin G. , Solomon Susan
Published: February 2008

The conversion to heat of solar ultraviolet radiation absorbed by ozone and molecular oxygen in the terrestrial mesosphere and lower thermosphere occurs through a series of complex processes. Upon photolysis of the O3 or O2, significant amounts of chemical potential energy and atomic and molecular internal energy are generated. The disposition of the internal energy largely determines the rate at which the atmosphere is heated. In addition, the chemical energy is released subsequent to exothermic chemical reactions which may occur long after and far away from the location of photon deposition. Energy may be lost from the atmosphere by airglow from excited photolysis products or by chemiluminescent emission from product species of exothermic chemical reactions. In this paper we examine the role of airglow losses in reducing the efficiency of solar heating in the Hartley, Huggins, and Chappuis bands of ozone and in the Herzberg, Ly α, Schumann‐Runge continuum, and Schumann‐Runge bands of molecular oxygen. We also examine the role of heating due to seven chemical reactions and calculate the efficiencies for those reactions with significant chemiluminescent loss. Parameterizations of the heating efficiency that are readily applicable to numerical models are given for those processes with nonunit efficiencies. Results from the calculation of heating rates for individual processes indicate that the reaction of atomic hydrogen and ozone is potentially the largest single source of heat in the vicinity of the mesopause. However, significant improvement is still needed in the knowledge of the quenching and chemical reaction rates of vibrationally excited OH before the efficiency of this reaction can be confidently calculated. Our calculations also indicate that even under strong quenching, most of the OH is in vibrationally excited form above 85–90 km. Finally, the bulk heating efficiency due to the combination of solar and chemical heating is calculated. The calculated bulk efficiencies demonstrate that airglow and chemiluminescent emission significantly reduce the amount of energy available for heat throughout the mesosphere and lower thermosphere.
Concentrations and fluxes of dissolved biogenic gases (DMS, CH4, CO, CO2) in the equatorial Pacific during the SAGA 3 experiment
Authors: Bates Timothy S. , Kelly Kimberly C. , Johnson James E.
Published: February 2008

The equatorial Pacific Ocean is a source of both sulfur and carbon to the atmosphere. In February and March 1990, as part of the Soviet‐American Gases and Aerosols (SAGA 3) expedition, dimethysulfide (DMS), methane (CH4), carbon monoxide (CO), and carbon dioxide (CO2) partial pressures were determined in both surface seawater and the overlying atmosphere of the central equatorial Pacific Ocean (15°N to 10°S, 145°W to 165°W). The partial pressures were used to calculate the net flux of these gases from the ocean to the atmosphere. The average regional DMS and CO fluxes were similar, 7.1 and 4.2 μmol/m2/d, respectively. The mixing ratio of CH4 in surface seawater was close to equilibrium with the overlying atmosphere and hence the average flux was only 0.39 μmol/m2/d. The flux of CO2 clearly dominated the air‐sea carbon exchange with an average regional flux of 5.4 mmol/m2/d.
Isoprene and monoterpene emission rate variability: Model evaluations and sensitivity analyses
Authors: Guenther Alex B. , Zimmerman Patrick R. , Harley Peter C. , Monson Russell K. , Fall Ray
Published: February 2008

The emission of isoprene and monoterpenes from plants is influenced by light and leaf temperature, which account for almost all short‐term variations (minutes to days) and a large part of spatial and long‐term variations. The temperature dependence of monoterpene emission varies among monoterpenes, plant species, and other factors, but a simple exponential relationship between emission rate (E) and leaf temperature (T), E = Es [exp (β(T − Ts))], provides a good approximation. A review of reported measurements suggests a best estimate of β = 0.09 K−1 for all plants and monoterpenes. Isoprene emissions increase with photosynthetically active radiation up to a saturation point at 700–900 μmol m−2 s−1. An exponential increase in isoprene emission is observed at leaf temperatures of less than 30°C. Emissions continue to increase with higher temperatures until a maximum emission rate is reached at about 40°C, after which emissions rapidly decline. This temperature dependence can be described by an enzyme activation equation that includes denaturation at high temperature. Algorithms developed to simulate these light and temperature responses perform well for a variety of plant species under laboratory and field conditions. Evaluations with field measurements indicate that these algorithms perform significantly better than earlier models which have previously been used to simulate isoprene emission rate variation. These algorithms account for about 90% of observed diurnal variability and can predict diurnal variations in hourly averaged isoprene emissions to within 35%.
Sensing climate change using the global positioning system
Authors: Yuan Lester L. , Anthes Richard A. , Ware Randolph H. , Rocken Christian , Bonner William D. , Bevis Michael G. , Businger Steven
Published: February 2008

Using simulated atmospheric data from the National Center for Atmospheric Research (NCAR) community climate model (CCM), we test the hypothesis that the global positioning system (GPS) can be used to detect global and regional climate change. We examine how the fundamental GPS variables (wet and total delays and vertical profiles of refractivity) as well as precipitable water as estimated by ground‐based GPS receivers would change in a climate with 2 times the present concentration of carbon dioxide (CO2). Because of the higher water vapor content in the doubled CO2simulation the wet delay and the precipitable water show a significant increase in the tropics and middle latitudes. Refractivity also shows an increase in the lower troposphere. We also simulate the changes in the GPS signal delay in a doubled CO2climate as would be measured by a radio occultation technique using low Earth‐orbiting (LEO) satellites equipped with GPS receivers. Increases in temperature and water vapor in the lower troposphere of the model atmosphere produce opposite effects on the occultation delay. Increased temperature tends to decrease the delay, while increased water vapor increases the delay. Amplified by the long “lever arms” of the LEO‐atmosphere‐GPS link, a strong “greenhouse warming” signal is simulated, with increases in occultation delay of nearly 100 m using the globally averaged data. This increase indicates that globally the effect of increased water vapor dominates. However, significant regional differences are present in the occultation delay response. In the tropics, where the temperature increase is smallest and the water vapor increases are largest, increases in delay of about 300 m are simulated. In contrast, in the polar regions where the increased temperatures are greatest and the increases in water vapor are smallest, the temperature effect dominates and a decrease in occultation delay of nearly 70 m is simulated. When compared to expected errors in measuring the occultation delay of about 1 m, these results indicate that monitoring trends in occultation delays would be a practical way to detect global and regional climate change.
Scattered light and accuracy of the cross‐section measurements of weak absorptions: Gas and liquid phase UV absorption cross sections of CH3CFCl2
Authors: Fahr A. , Braun W. , Kurylo M. J.
Published: February 2008

Ultraviolet absorption cross sections of CH3CFCl2 (HCFC‐141b) were determined in the gas phase (190–260 nm) and liquid phase (230–260 mm) at 298 K. The liquid phase absorption cross sections were then converted into accurate gas phase values using a previously described procedure. It has been demonstrated that scattered light from the shorter‐wavelength region (as little as several parts per thousand) can seriously compromise the absorption cross‐section measurement, particularly at longer wavelengths where cross sections are low, and can be a source of discrepancies in the cross sections of weakly absorbing halocarbons reported in the literature. A modeling procedure was developed to assess the effect of scattered light on the measured absorption cross section in our experiments, thereby permitting appropriate corrections to be made on the experimental values. Modeled and experimental results were found to be in good agreement. Experimental results from this study were compared with other available determinations and provide accurate input for calculating the atmospheric lifetime of HCFC‐141b.
Seasonal variation of surface and atmospheric cloud radiative forcing over the globe derived from satellite data
Authors: Gupta Shashi K. , Staylor W. Frank , Darnell Wayne L. , Wilber Anne C. , Ritchey Nancy A.
Published: February 2008

Global distributions of surface and atmospheric cloud radiative forcing parameters have been derived using parameterized radiation models with satellite meteorological data from the International Satellite Cloud Climatology Project, and directly measured top‐of‐atmosphere radiative fluxes from the Earth Radiation Budget Experiment. Specifically, shortwave, longwave, and total cloud forcing at the surface, and column‐averaged values of longwave cloud forcing of the atmosphere were derived for the midseasonal months of April, July, and October 1985 and January 1986, covering a complete annual cycle. Seasonal variability is illustrated by comparing the results for July 1985 and January 1986, which represent the seasonal extremes. Surface shortwave cloud forcing is always negative, representing a cooling of the surface, with strongest cooling (−120 to −180 W m−2) occurring over midlatitude storm tracks of the summer hemisphere. Surface longwave cloud forcing is always positive, representing a warming of the surface, with strongest warming (60 to 75 W m−2) occurring over storm tracks of the winter hemisphere. Zonal averages show the entire summer hemisphere dominated by shortwave cooling, the middle and high latitudes of the winter hemisphere dominated by longwave warming, and a broad zone of transition in between. The globally averaged total cloud forcing amounts to a cooling throughout the year, ranging from a low of about −12 W m−2for July 1985 to a high of about −25 W m−2for January 1986. The longwave cloud forcing of the atmosphere shows a strong warming over deep convective regions in the tropics and a moderate cooling outside the tropics, amounting to a weak cooling (−2 to −5 W m−2) in the global average. Comparisons of the results with general circulation model simulations show broad qualitative agreement regarding the locations of prominent warming and cooling regions. Quantitative comparisons, on the other hand, show significant differences between the magnitudes of warming and cooling in these regions. Most of the larger differences can be attributed to known deficiences of the general circulation model simulations. Comparisons with satellite‐derived results of other investigators show much better agreement.
A three‐dimensional numerical model of cloud dynamics, microphysics, and chemistry: 4. Cloud chemistry and precipitation chemistry
Authors: Wang Chien , Chang Julius S.
Published: February 2008

A study of the cloud chemistry and precipitation chemistry of the three‐dimensional cloud chemical modeling of a severe local storm case (Wang and Chang, this issue) is discussed in this paper. In our simulation the microphysical processes, especially those related to the ice phase, were found to have significant effects on influencing both the rhythm of time evolution and the amount of quantitative variation of pollutants. Most chemical species in the ice phase particles were transformed from liquid phase droplets by the riming process. The ratio of the transfer of chemical species from aqueous phase to ice phase induced by riming to that induced by freezing was about 55 to 1 for N(V) and 8 to 1 for S(VI). The average equivalent pH value had a linear correlation with the logarithm of total water content and with the logarithm of [S(VI)] + [N(V)] in the cloud. The average equivalent pH value for the whole cloud changed from 5.1 at 28 min to 4.4 at 55 min. Most low pH values appeared in the less convective region. For this particular case study, aerosol scavenging was the major source of S(VI) production, which provided a total amount 50 times that provided by sulfur oxidation in the cloud. The major contributor of sulfur oxidation changed during the lifetime of the modeled storm. The peaks of acid deposition did not correspond to the peaks of precipitation. The sub cloud dissolution and microphysics‐related transformation played an important role in the acid production and deposition. The nitric acid production total in the sub cloud area doubled that in the cloud. Most of the S(VI) produced by the aqueous phase SO2 oxidation was also found to be due to sub cloud processes.
Clustering, randomness, and regularity in cloud fields: 3. The nature and distribution of clusters
Authors: Weger Ronald C. , Lee Jonathan , Welch Ronald M.
Published: February 2008

This study applies the K‐nearest‐neighbor (KNN) statistic as well as a morphological filtering approach for the identification and analysis of cloud clusters. These approaches first are evaluated by application to synthetically generated normally distributed clusters within a random background. They are found to provide conservative estimates of cluster size while accurately estimating the percentage of clouds in clusters. In Landsat scenes, typical clusters are found to contain four to five clouds and typically two to three clouds per cluster in advanced very high resolution radiometer (AVHRR) data. In the Landsat imagery the KNN analysis finds that about 15% to 50% of the total cloud field participates in clusters. These clusters are found to be concentrated in less than 2% of the total cloud field area. In AVHRR scenes the KNN method yields from 1.5% to 16% of the cloud field participating in clusters, concentrated within 1% of the total cloud field area. Mean cluster radii are typically 300 m to 600 m in the Landsat scenes and less than 3 km in the AVHRR images. Both the cloud cluster centers and the non‐participating background clouds are randomly distributed within the cloud field. Less than 10% of the cloud clusters form superclusters (i.e., clusters of clusters). When clouds are segregated into classes of uniform size, the clustering signal weakens with increasing cloud size. When the scene as a whole is divided into 30‐km subregions, the clustering signal is relatively homogeneous throughout the scene in spite of significant fluctuations in cloud number density within the various subregions. These results show that the physical mechanism underlying this clustering signal, although producing clusters on scales less than 3 km, is operative over the entire field. A nearest‐neighbor angle statistic is applied to search for regularity at the subkilometer scale. The angle statistic results are consistent with normally distributed point processes and inconsistent with any form of regularity at this scale. Presently, no method is presented which is effective at probing the cloud field structure at scales intermediate to 2 km and 30 km. Therefore cloud field regularity cannot be ruled out at these intermediate scales.
A reexamination of the peak current calibration of the National Lightning Detection Network
Authors: Idone Vincent P. , Saljoughy Arsalan B. , Henderson Ronald W. , Moore Paul K. , Pyle Richard B.
Published: February 2008

The peak current calibration of the National Lightning Detection Network (NLDN) recently reported by Orville (1991) has been reexamined with a roughly threefold larger data set of 57 directly measured stroke peak currents,Ipeak(kiloamperes), and their corresponding NLDN mean normalized magnetic signal strengths,(LLP units). These 57 lightning strokes originated in 36 triggered lightning flashes initiated at the Kennedy Space Center (KSC), Florida, during the research campaigns of 1985–1991. Identification of correspondingIpeakandmeasurements was verified through accurate coincidence in absolute time of the two independent data sets. Thedata (with one point excluded as an outlier) is apparently linearly related with a correlation coefficient of 0.881, consistent with that predicted by application of the transmission line model of the lightning return stroke. The regression equation for prediction ofIpeakfrom NLDNmeasurements iswhere the slope is expressed in kiloampere/(LLP units). Examination of the overalldata set for the possible influence of two different models of signal strength attenuation with distance,D, (power law,Dβ, and exponential, [expαDD]−1) indicates negligible sensitivity to the proposed variations; other larger error sources likely mask the true attenuation effect. Twelve flashes were detected with four or more direction finders; a power law fit to the direction finder signal strength variation with distance of these individual flashes yields a mean β value of −1.09. Examination of the overalldata set for the possible effect of a nonlinear relation between the source stroke peak current and return stroke propagation speed indicates no obvious influence. The 95% confidence bounds for peak current prediction in the mean suggest a percent uncertainty of 10–15% for peak currents between 15 and 60 kA. Similar accuracy is expected for peak currents greater than 60 kA provided care is taken to minimize possible nonlinear amplification effects hi the NLDN data. Below 15 kA, the percent uncertainty rapidly increases suggesting that the inferred distribution of peak current values less than this limit may be quite unreliable.
A theoretical study of the effect of relative humidity on light scattering by tropospheric aerosols
Authors: Hegg Dean , Larson Timothy , Yuen Po‐Fat
Published: February 2008

A theoretical analysis of the factors which contribute to the hygroscopic growth of atmospheric aerosols is presented within the context of the climatic impact of these aerosols. It is found that the factor which most influences the growth factor is the position of the initial dry aerosol size distribution relative to the efficient light scattering particle size range. When this factor is taken into consideration, rural aerosols have much larger growth factors than those currently utilized in assessment of the climatic impact of atmospheric aerosols.
Coupled surface‐atmosphere reflectance (CSAR) model: 2. Semiempirical surface model usable with NOAA advanced very high resolution radiometer data
Authors: Rahman Hafizur , Pinty Bernard , Verstraete Michel M.
Published: February 2008

A new semiempirical model to describe the bidirectional reflectance of arbitrary natural surfaces using only three parameters has been developed. This model successfully accounts for the observed variability of reflectance measurements in laboratory and field conditions, ranging from bare soil to full canopy cover, in both the visible and the near‐infrared bands. Coupled with a simple atmospheric radiation transfer model, this model has been inverted against actual NOAA/advanced very high resolution radiometer (AVHRR) data from several desert sites in northern Africa. This procedure allows the retrieval of surface properties and average amounts of atmospheric constituents (aerosol optical thickness and water vapor) for the duration of the measurement period. Further work is required to expand the usability of the coupled model to other locations and shorter periods of time, but the paper demonstrates the feasibility of inverting a coupled surface‐atmosphere model against existing AVHRR data and documents the current limits of this approach.
Effect of long‐term solar variability in a two‐dimensional interactive model of the middle atmosphere
Authors: Huang Theresa Y. W. , Brasseur Guy P.
Published: February 2008

An assessment of the influence of long‐term solar variability on the climatology of the middle atmosphere is made using a two‐dimensional model incorporating interactive chemistry, radiation, and dynamics. The primary motivation behind this work is the more updated measurements of the solar flux variation and the more comprehensive treatment of the chemical and physical processes in the two‐dimensional model. Despite various feedback mechanisms inherent to the model, imposing a solar flux variation in the two‐dimensional model does not create any substantial response in the stratosphere. Since the largest changes in the solar flux occur in wavelengths shorter than 200 nm, the most obvious effect of solar variability is above the altitude of 60 km. In general, the response of the middle atmosphere to long‐term solar variability predicted by the two‐dimensional model is substantially weaker than those predicted by previous model studies, which is primarily a result of the smaller solar flux variation adopted in this study. Compared with recent data analyses, the calculated response of ozone and temperature near the tropical stratopause and the calculated zonal‐wind response at midlatitude upper stratosphere are much weaker. In addition, the anticorrelated temperature and ozone response at 70 km seen in observational analyses for short‐term solar variability cannot be simulated by the model, albeit for long‐term solar variability simulations. These suggest that the physical and chemical processes and their feedback effects in the middle atmosphere are not yet fully understood.
Degradation of selected terrestrial and Martian impact craters
Authors: Grant John A. , Schultz Peter H.
Published: February 2008

Degraded craters on Mars record the cumulative effects of the complex interplay between erosion, transport and deposition by competing intermittent processes. Fortunately, impact craters are instantaneous landforms common to both the Earth and Mars that are characterized by similar topography and local lithofacies. Deconvolving the signatures of individual degradation processes around young craters on Earth (e.g., drainage scale and density, changing wall slope, deflation features) allows calibrating degradation signatures around craters on Mars and places first‐order limits on controlling processes and perhaps past climates. Such comparisons are valid despite differences in vegetation cover and time scales over which activity occurs on the two planets. Analysis of degradation at Meteor Crater, Lonar Crater, and Talemzane Crater reveals a terrestrial sequence of advancing degradation by fluvial, eolian, and mass wasting processes. At Meteor Crater, fluvial processes currently dominate primary erosion; however, combined deflation from the ejecta and both alluvium and colluvium make eolian processes most important in overall ejecta degradation. Lonar and Talemzane reveal that advancing fluvial degradation is characterized by larger drainages and decreasing wall slopes. At Talemzane, interior drainages breach the rim‐crest and pirate headward regions of exterior drainage basins. When comparable degradation signatures resolvable at Viking resolutions are used in selected areas on Mars, important differences from the terrestrial sequence are revealed. All morphologies of craters in southern Ismenius Lacus (SIL) are typically incised by small valleys with low drainage densities, display rare alluvial deposits, possess walls sloped at the angle of repose, and are sometimes partially covered by airfall deposit remnants. Analogy with the terrestrial craters indicates signatures associated with advancing fluvial degradation should be readily detected on Mars. Together with other differences between degraded craters on the two planets, the paucity of fluvial signatures in SIL implies that mass wasting and eolian activity predominated degradation of martian craters preserved since the Hesperian. Crater statistics for SIL indicate most craters were degraded during geologically brief intervals in the Noachian and mid‐Hesperian. Therefore, fluvial signatures preserved in SIL imply any runoff since the Hesperian was short lived, locally debouched, and unrelated to rainfall.
Source characteristics of Jovian narrow‐band kilometric radio emissions
Authors: Reiner M. J. , Fainberg J. , Stone R. G. , Kaiser M. L. , Desch M. D. , Manning R. , Zarka P. , Pedersen B.‐M.
Published: February 2008

New observations of Jovian narrow‐band kilometric (nKOM) radio emissions were made by the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses‐Jupiter encounter in early February 1992. These observations have demonstrated the unique capability of the URAP instrument for determining both the direction and polarization of nKOM radio sources. An important result is the discovery that nKOM radio emission originates from a number of distinct sources located at different Jovian longitudes and at the inner and outermost regions of the Io plasma torus. These sources have been tracked for several Jovian rotations, yielding their corotational lags, their spatial and temporal evolution, and their radiation characteristics at both low latitudes far from Jupiter and at high latitudes near the planet. Both right‐hand and left‐hand circularly polarized nKOM sources were observed. The polarizations observed for sources in the outermost regions of the torus seem to favor extraordinary mode emission.
Compressional tectonism on Mars
Authors: Watters Thomas R.
Published: February 2008

Contractional features on Mars were identified on the basis of photogeologic evidence of crustal shortening and comparison with terrestrial and planetary analogs. Three classes of structures, wrinkle ridges, lobate scarps and high‐relief ridges, were mapped and their spatial and temporal distribution assessed. Wrinkle ridges account for over 80% of the total cumulative length of the mapped contractional features and occur in smooth plains material interpreted to be volcanic in origin. Lobate scarps, not wrinkle ridges, are the dominant contractional feature in Martian highland material. The pattern of contractional features in the western hemisphere reflects the hemispheric‐scale influence of the Tharsis rise. Although no comparable hemispheric‐scale pattern is observed in the eastern hemisphere, prominent regional‐scale patterns exist, the most notable of which occurs in Hesperia Planum. Contractional features that locally parallel the trend of the crustal dichotomy boundary in the eastern hemisphere suggest the influence of stresses related to the evolution of the dichotomy. Compressional deformation apparently peaked during the Early Hesperia, if the tectonic features are roughly the same age as the units in which they occur. This peak in compressional deformation corresponds with Early Hesperian volcanic resurfacing of a large portion of the planet. Thermal history models for Mars, based on an initially hot planet, are inconsistent with estimates of the timing of peak compressional tectonism and the rate of volcanism. A pulse of global volcanism during the Early Hesperian may have resulted in a punctuated episode of rapid cooling and global contraction that contributed to compressional tectonism. Although global contraction may have contributed a significant component of the total stress that resulted in compressional deformation on Mars, nonhydrostatic horizontal stresses derived from local and regional‐scale sources are necessary to account for the uniform orientations of the tectonic features.
Crustal diversity of the moon: Compositional analyses of Galileo solid state imaging data
Authors: Pieters C. M. , Head J. W. , Sunshine J. M. , Fischer E. M. , Murchie S. L. , Belton M. , McEwen A. , Gaddis L. , Greeley R. , Neukum G. , Jaumann R. , Hoffmann H.
Published: February 2008

The multispectral images of the lunar limb and farside obtained by the solid state imaging (SSI) system on board the Galileo spacecraft provide the first new pulse of compositional data of the Moon by a spacecraft in well over a decade. The wavelength range covered by SSI filters (0.4–1.0 μm) is particularly sensitive to the composition of mare basalts, the abundance of mafic (ferrous) minerals, and the maturity of the regolith. To a first order, the limb and farside material is consistent with previous characterization of nearside lunar spectral types for mare and highland soils and craters. Most basalts are of an intermediate TiO2 composition and most of the highland crust is feldspathic with local variations in mafic content identified principally at impact craters. Dark mantling material on the farside can be interpreted in terms of known properties of lunar pyroclastic glass. Regions of cryptomare are shown to have spectral properties intermediate between those of highland and mare soils, as would be expected from a mixture of the two. There are several important exceptions and surprises, however. Unlike the basalt types identified on the nearside, limb and farside basalts exhibit an exceptionally weak 1 μm ferrous absorption band. This may indicate a compositionally distinct lunar basalt group that, for example, is more Mg‐rich than most basalts of the nearside. Some of the most notable compositional anomalies are associated with South Pole‐Aitken Basin. This large region has a much lower albedo than surrounding highlands. The inner, darkest, portion of the basin exhibits optical properties indistinguishable from low‐Ti basalts. Deposits to the south exhibit unique properties with a strong and broad ferrous 1 μm absorption, most consistent with abundant olivine. The unusual compositions associated with South Pole‐Aitken and their spatial extent suggests the impact creating this huge lunar basin excavated mafic‐rich lower crust or perhaps mantle material.
Lunar impact basins: New data for the western limb and far side (Orientale and South Pole‐Aitken Basins) from the first Galileo flyby
Authors: Head James W. , Murchie Scott , Mustard John F. , Pieters Carle M. , Neukum Gerhard , McEwen Alfred , Greeley Ronald , Nagel Engelbert , Belton Michael J. S.
Published: February 2008

Compositional aspects of impact basin materials can be analyzed using multispectral image data acquired by the Galileo solid state imaging (SSI) experiment during the December 1990 lunar encounter. These data provide important information on the spectral properties of the western lunar limb and parts of the far side. The SSI images cover the wavelength range 0.4–1.0 μm, allowing measurement of spectral slope and estimation of the strength of the 1 μm absorption due to iron in the mafic minerals olivine and pyroxene. Among deposits of the 930‐km‐diameter Orientale basin, exterior ejecta comprising the Hevelius Formation is relatively homogeneous and spectrally similar to mature Apollo 16 soils, suggesting an upper crustal source. The centrally located Maunder Formation is distinct from the younger mare basalts but comparable to the Hevelius Formation in its spectral reflectance properties, supporting an interpretation as basin impact melt. The Montes Rook Formation, located in an annulus between the Maunder and the Hevelius, shows a slightly stronger mafic absorption and may be the deepest crustal material excavated. The distal Orientale deposits show local mafic enhancements (in the Schiller‐Schickard and Mendel‐Rydberg regions) interpreted to represent pre‐Orientale mare deposits, or cryptomaria, intermixed with overlying basin ejecta. In this case, maria of sizes comparable to those presently observed were widespread in this region before the Orientale impact. Mixing‐model analyses are consistent with the ballistic erosion and sedimentation model for ejecta emplacement in the distal regions beyond the continuous ejecta deposit. On the southern lunar farside, a huge area with an enhanced mafic absorption corresponds to the interior and rim of the pre‐Nectarian South Pole‐Aitken impact basin, 2000–2500 km in diameter. The anomaly is interpreted to be due to several factors, including excavation into the more mafic lower crust, and the presence of extensive early volcanic fill (cryptomare), similar to that seen in ancient basins such as Smythii and Australe. These results show that although basin‐forming events are an important factor in producing lateral heterogeneities in crustal composition, and in modifying preexisting deposits (such as cryptomaria), the majority of material in even the largest basins was excavated from crustal levels. Our results suggest a gradational vertical crustal stratigraphy consisting of an uppermost mixed crustal layer of anorthosite, basin ejecta, and cryptomaria deposits (generally corresponding to the megaregolith), an upper crustal layer of anorthosite, and a lower more noritic layer. Many of the basic questions remaining from this study could be addressed by global high‐resolution geochemical and mineralogical data obtained by polar orbiting spacecraft.