bluuebunny/crossref_metadata_2025 huggingface dataset:
Reconciliation of theories of synthetic aperture radar imagery of ocean waves
Authors: Plant William J.
Published: February 2008

Several theories of synthetic aperture radar (SAR) imagery of ocean waves, especially those dealing with the focusing issue, are examined. We show that the “time‐dependent” theories of Lyzenga (1988) and of Kasilingam and Shemdin (1988) and the “velocity‐bunching” theory of Brüning, Alpers, and Shroter (1991) are identical for short integration times but diverge on the focusing issue for long integration times. The reason for the divergence is simply that velocity bunching theory assumes that integration times are short compared with ocean wave periods. The inference is that the phase speed of the ocean wave plays a secondary, nearly negligible, role to that of orbital velocity and acceleration in determining the optimum focus setting for short integration times but becomes dominant as integration times are increased. For the case of zero focus adjustment, these focus‐dependent theories are shown to reduce to those of Plant and Keller (1983) and of Hasselmann et al. (1985). Thus several of the apparently differing theories of SAR imagery of the ocean are reconciled and shown to be a single consistent theory.
Nonlinear diffusion of the tidal signal in frictionally dominated embayments
Authors: Friedrichs Carl T. , Madsen Ole S.
Published: February 2008

The dynamics of many shallow tidal embayments may be usefully represented by a single “zero‐inertia” equation for tidal elevation which has the form of a nonlinear diffusion equation. The zero‐inertia equation clarifies the lowest order dynamics, namely, a balance between pressure gradient and friction. It also provides insight into the properties of higher‐order harmonic components via the identification of compact approximate solutions and governing nondimensional parameters. Approximate analytic solutions which assume a constant diffusion coefficient are governed by the nondimensional parameters x/L and ‖k0‖L, where L is the length of the embayment, and ‖k0‖−1 scales both the length of factional dissipation and the physical length of the diffusive waveform. As ‖k0‖L increases, the speed at which the tidal signal diffuses decreases, and the rate of decay of tidal amplitude with distance increases. The parameter ‖k0‖L increases as depth is reduced, friction is increased, forcing amplitude or frequency is increased, or total embayment width is increased relative to the width of the channel. Approximate analytic solutions which assume a time‐varying diffusion coefficient result in additional components at the zeroth, second, and third harmonic frequencies. The zeroth and second harmonics are governed by the parameter γ, as well as x/L and ‖k0‖L. Parameter γ measures the relative importance of time variations of channel depth (γ> 0) versus time variations in embayment width (γ< 0). If γ > 0, the diffusion coefficient is larger near the crest of the tidal waveform, causing the rising tide to be of shorter duration and mean elevation to be set up. If γ< 0, the diffusion coefficient is larger near the trough, causing the falling tide to be shorter and elevation to be set down. The third harmonic is produced by fluctuations in the diffusion coefficient associated with times of greatest surface gradient. The third harmonic is governed only by the parameters x/L and ‖k0‖L, which indicates the third harmonic is insensitive to time variations in cross‐sectional geometry. Comparisons to field observations and to numerical solutions of the full equations including inertia terms indicate that the zero‐inertia equation (1) reproduces the results of the more general one‐dimensional equations to within the accuracy predicted by scaling arguments and (2) reproduces the main features of the nonlinear tidal signal observed in many shallow tidal embayments.
Unforced decadal fluctuations in a coupled model of the atmosphere and ocean mixed layer
Authors: Barnett T. P. , Genio A. D. Del , Ruedy R. A.
Published: February 2008

Global average temperature in a 100‐year control run of a model used for greenhouse gas response simulations showed low‐frequency natural variability comparable in magnitude to that observed over the last 100 years. The model variability was found to be barotropic in the atmosphere, and located in the tropical strip with largest values near the equator in the Pacific. The model variations were traced to complex, low‐frequency interactions between the meridional sea surface temperature gradients in the eastern equatorial Pacific, clouds at both high and low levels, and features of the tropical atmospheric circulation. The variations in these and other model parameters appear to oscillate between two limiting climate states. The physical scenario accounting for the oscillations on decadal time scales is almost certainly not found in the real world on shorter time scales due to limited resolution and the omission of key physics (e.g., equatorial ocean dynamics) in the model. The real message is that models with dynamical limitations can still produce significant long‐term variability. Only a thorough physical diagnosis of such simulations and comparisons with decadal‐length data sets will allow one to decide if faith in the model results is, or is not, warranted.
The distribution of methane in the upper waters of the southern California Bight
Authors: Cynar Frank J. , Yayanos A. Aristides
Published: February 2008

Profiles of dissolved methane in the upper 300 m of a 190,000 km2 area of southern Californian waters were determined during November 1989 and March 1990. Methane in near‐surface waters ranged from 1 to 1416 nmol L−1. The lowest concentrations were at depths >200 m. The highest levels were found along the coast, especially in the region extending from Point Conception to Santa Barbara. Vertical profiles of methane are characterized by maxima in supersaturation within a narrow density band located within the pycnocline; this layer is continuous over a large geographic area. Offshore stations exhibiting elevated levels of methane are spatially coincident with mesoscale eddies of the California Current system. Interpretation of hydrographic and chemical data suggests that CH4‐rich waters are upwelled from relatively shallow areas that are characterized by numerous hydrocarbon seeps. The data are consistent with a nearshore source of methane, presumably submarine seeps. Subsurface maxima in offshore waters are likely maintained by lateral transport of allochthonous CH4 along density surfaces from coastal waters and autochthonous microbial production.
Biogenic volatile iodated hydrocarbons in the ocean
Authors: Klick Silke , Abrahamsson Katarina
Published: February 2008

Hitherto unnoticed biogenic volatile iodated hydrocarbons have been determined in open ocean and coastal surface waters. The results indicate that iodated substances are produced by planktonic organisms rather than by macroalgae. It is shown that the diversity of iodated organic compounds released in the seawater is larger than was previously suggested. In the global cycling of iodine it is necessary to consider organic compounds other than methyl iodide.
The response of the western equatorial Pacific Ocean to westerly wind bursts during November 1989 to January 1990
Authors: McPhaden M. J. , Bahr F. , Du Penhoat Y. , Firing E. , Hayes S. P. , Niiler P. P. , Richardson P. L. , Toole J. M.
Published: February 2008

Several 5 to 10 m s−1 westerly wind bursts of 10–15 days' duration occurred in the western equatorial Pacific during November 1989 to January 1990. The response to these wind bursts was characterized by a 400‐ to 600‐km‐wide eastward jet in the upper 100–150 m along the equator between 135°E and the date line. Flow in this jet accelerated to speeds of over 100 cm s−1 within 1 week after the onset of westerly winds in November 1989 in association with supertyphoon Irma. In addition, a 20 to 40 cm s−1 westward counterflow developed between 2°N and 2°S below the surface jet separating it from the eastward flow of the Equatorial Undercurrent in the thermocline. Changes in surface layer zonal volume transports in the western Pacific due to westerly wind bursts were 25–56 Sv based on comparison of three shipboard velocity transects in November and December 1989. Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer, the Equatorial Undercurrent decelerated to less than 20 cm s−1 (i.e., less than half its speed before the onset of westerlies) by early December 1989. Westerly winds also produced a strong surface convergence in both the meridional and zonal directions west of the date line. In late November 1989, this convergence was associated with downwelling of 2–3 m d−1 along the equator, and a 20‐ to 30‐m depression of the thermocline. In addition, sea level rose by 10 dyn cm from mid‐November to mid‐December between 1°S and 3°N, 165°E. Sea surface temperature dropped over a large region by 0.2°–0.5°C during episodes of high westerly winds, and some episodes of high winds were associated with 0.2°–0.3°C sea surface temperature inversions supported by salt‐stratified buoyancy gradients in the upper 100 m. The near‐surface salinity balance was dominated by lateral advection when relatively fresh water from north of a salinity front centered near 3°N was advected southward by Ekman drift in late November 1989. Displacement of this front past a current meter mooring at 0°, 165°E, appeared as a local drop of 1 psu in surface layer salinity in 10 days. These results are discussed in the context of a swing in the climate system towards El Niño‐Southern Oscillation‐like conditions in early 1990.
Modeling tidal exchange and dispersion in Boston Harbor
Authors: Signell Richard P. , Butman Bradford
Published: February 2008

Tidal dispersion and the horizontal exchange of water between Boston Harbor and the surrounding ocean are examined with a high‐resolution (200 m) depth‐averaged numerical model. The strongly varying bathymetry and coastline geometry of the harbor generate complex spatial patterns in the modeled tidal currents which are verified by shipboard acoustic Doppler surveys. Lagrangian exchange experiments demonstrate that tidal currents rapidly exchange and mix material near the inlets of the harbor due to asymmetry in the ebb/flood response. This tidal mixing zone extends roughly a tidal excursion from the inlets and plays an important role in the overall flushing of the harbor. Because the tides can only efficiently mix material in this limited region, however, harbor flushing must be considered a two step process: rapid exchange in the tidal mixing zone, followed by flushing of the tidal mixing zone by nontidal residual currents. Estimates of embayment flushing based on tidal calculations alone therefore can significantly overestimate the flushing time that would be expected under typical environmental conditions. Particle‐release simulations from point sources also demonstrate that while the tides efficiently exchange material in the vicinity of the inlets, the exact nature of dispersion from point sources is extremely sensitive to the timing and location of the release, and the distribution of particles is streaky and patchlike. This suggests that high‐resolution modeling of dispersion from point sources in these regions must be performed explicitly and cannot be parameterized as a plume with Gaussian‐spreading in a larger scale flow field.
Salinity profiles of Antarctic sea ice: Field data and model results
Authors: Eicken Hajo
Published: February 2008

In order to describe and compare sea ice salinity distributions, which are of interest in the study of physical and biological ice characteristics as well as ice‐ocean interaction, we have computed composite profiles and fitted third‐degree polynomials to salinity data of 129 cores from the Weddell Sea. Four characteristic profile shapes have been recognized and described through the polynomial coefficients (C‐, S‐, I‐ and ?‐type profiles). The field data have been compared with “ideal” salinity profiles generated by a simulation scheme based on thermodynamic growth under climatological conditions representative of the Weddell Sea. Composite salinity profiles agree well with simulations, irrespective of the growth mechanism (i.e., frazil or congelation), suggesting that ice properties which depend on salinity or porosity may evolve independent of ice growth conditions. Analysis of salinity and 18O data in conjunction with the simulations demonstrates that apart from meteorological conditions controlling growth velocity and temperature distribution within the ice, flooding and upward brine expulsion are important in raising top salinities as frequently observed. Low top salinities are typical of ice that has undergone retexturing and differential desalination. Salinity decreases towards the bottom of a floe are a result of high oceanic heat fluxes and snow‐cover effects. As a consequence of these processes the bulk salinity of Weddell Sea ice cannot be described by a simple linear age‐thickness relationship as characteristic of Arctic first‐year ice.
Offshore export of shelf production in the Cape Blanc (Mauritania) giant filament as derived from coastal zone color scanner imagery
Authors: Gabric Albert J. , Garcia Luis , Van Camp Lieve , Nykjaer Leo , Eifler Walter , Schrimpf Wolfram
Published: February 2008

Particulate organic carbon flux from the shelf to oceanic waters during upwelling between 19°N and 25°N off Mauritania has been analyzed using coastal zone color scanner (CZCS) imagery. Biomass in the giant filament has been estimated using surface pigment concentration registered by CZCS during time series in December 1983, March 1984, and October 1984. Evidence for active phytoplankton growth in offshore waters is presented. Formation of the giant filament seems to be favored by the combination of a number of factors perhaps unique to the Mauritanian shelf. The large‐scale circulation patterns promote offshore advection, and the typically long period of wind forcing can move the upwelling center to the shelf break area, thus causing maximal phytoplankton growth rate to be achieved in oceanic waters. The relatively high pigment concentrations registered by CZCS in oceanic waters suggest that nutrients have not been depleted on the shelf and that in situ growth is occurring in deep waters. We have used a baroclinic, hydrodynamic model to estimate cross‐slope flux of particulate organic carbon at the 200‐m isobath and derive a mean value of about 1 × 105 g C s−1 during each of the CZCS time series.
Relationship between sea ice freeboard and draft in the Arctic Basin, and implications for ice thickness monitoring
Authors: Wadhams P. , Tucker W. B. , Krabill W. B. , Swift R. N. , Comiso J. C. , Davis N. R.
Published: February 2008

We have confirmed our earlier finding that the probability density function (pdf) of ice freeboard in the Arctic Ocean can be converted to a pdf of ice draft by applying a simple coordinate transformation based on the measured mean draft and mean elevation. This applies in each of six 50‐km sections (north of Greenland) of joint airborne laser and submarine sonar profile obtained along nearly coincident tracks from the Arctic Basin north of Greenland and tested for this study. Detailed differences in the shape of the pdf can be explained on the basis of snow load and can, in principle, be compensated by the use of a more sophisticated freeboard‐dependent transformation. The measured “density ratio”R(actually mean draft/mean elevation ratio) for each section was found to be consistent over all sections tested, despite differences in the ice regime, indicating that a single value ofRmight be used for measurements done in this season of the year. The mean value 〈R〉 from all six sections is 7.89; on the assumption that all six values are drawn from the same population, the standard deviation is 0.55 for a single 50‐km section, and thus 0.22 for 300 km of track. In attempting to infer ice draft from laser‐measured freeboard, we would therefore expect an accuracy of about ±28 cm in 50 km of track (if mean draft is about 4 m) and about ±11 cm in 300 km of track; these accuracies are compatible with the resolution of predictions from numerical models. A simple model for the variability ofRwith season and with mean ice thickness gives results in reasonable agreement with observations. They show that although there is a large seasonal variability due to snow load, there is a stable period from November to April when the variability is chiefly dependent on the mean ice thickness alone. Thus, in principle,Rcan be mapped over the Arctic Ocean as a basis for interpreting survey data. Better field data are needed on the seasonal and spatial variability of three key quantities: area‐averaged snow load, mean density of first‐year and multiyear ice (including the effect of ridging with these two ice regimes), and density of near‐surface water.
Modeling the seasonal variability of a coupled Arctic ice‐ocean system
Authors: Häkkinen Sirpa , Mellor George L.
Published: February 2008

Results from modeling studies of the ice‐ocean system in the Arctic Basin and in the Norwegian‐Greenland‐Barents seas are presented. We used a three‐dimensional coupled ice‐ocean model developed at Princeton University. The ocean model applies the primitive equations and a second moment turbulence closure for turbulent mixing. The snow‐ice model uses a three‐level thermodynamic scheme which resembles Semtner's (1976a) model. Our conclusions based on the seasonal simulations are as follows. (1) Using monthly climatological surface heat flux and wind stress, the seasonal variability of the ice cover is quite realistic in that the thickest ice is located north of Greenland and the average ice thickness is about 3 m. The largest deviation between the simulated and observed ice cover is in the Greenland Sea where oceanic conditions determine the ice edge. Basically, the monthly climatological forcing does not result in strong enough mixing to bring sufficient heat from the deep ocean to keep the central Greenland Sea gyre ice free. The results improve for both the ice cover and ocean by invoking daily wind forcing for which we first chose year 1987. In the ocean model, the large mixing events associated with storm passages are resolved, and as a result, the overall oceanic structure in the Greenland Sea appears to be more realistic. However, no deep convection takes place in the model during 1987 which is likely the result of diminished storm activity in the northern part of the Greenland Sea. The ice thickness field appears to be very anomalous 1987, so an experiment with 1986 daily wind forcing was also done, which resulted in an ice thickness field similar to some reported from other ice models. (2) Both monthly and daily surface forcing result in a similar behavior of the Atlantic waters in the Arctic Basin. The Atlantic waters circulate at about the observed level, between 400 and 600 m. The survival of the Atlantic waters in the basin depends strongly on the heat loss through the ice cover, and it appears that too much heat is lost on the Eurasian side through the ice because the simulated Atlantic waters are too cool by about 0.2–0.5°C. (3) For the monthly climatology case, a large amount of cold and salty water enters the Eurasia Basin from the Kara and Laptev seas area and finds its way toward the Canada Basin. This water mass appears to result from ice formation in the Kara and Laptev seas. When applying the daily forcing, this deep salinity maximum disappears due to increased mixing on the shelves. Nevertheless, this suggests a mechanism within the Arctic Ocean as to why the deep Canada Basin is much saltier than the Eurasia Basin.
Cold surface ocean ventilation and its effect on atmospheric CO2
Authors: Keir Robin S.
Published: February 2008

The potential for greater air‐sea exchange rates in the northern Atlantic to decrease atmospheric CO2 during the ice ages is examined using a simple model of the solubility pump. This potential is shown to depend on whether the modern ocean's region of sinking cold water is reequilibrating with atmospheric CO2 at a rate comparable to the poleward transport of warm surface water. If this is the case, it is possible that greater ice age polar ventilation may have produced as much as 60% of the decrease observed in ice cores. In this scenario, the cold surface water CO2 partial pressure does not covary with the atmosphere but instead would remain approximately constant.
Photochemical studies of the eastern Caribbean: An introductory overview
Authors: Zika Rod G. , Milne Peter J. , Zafiriou Oliver C.
Published: February 2008

This special section of the Journal of Geophysical Research reports a multi‐investigator study of a number of sunlight‐initiated photoprocesses taking place in the varied biogeochemical and oceanographic environment found in the tropical Eastern Caribbean and Orinoco River delta in the spring and fall of 1988. Principal conceptual themes that were addressed by the program included (1) the characterization of the role of dissolved organic matter as the main chromophore initiating photoprocesses in surface seawater, (2) the determination of the fluxes and pathways of reactants and transient species involved in oxygen photoredox chemistry, and (3) the continuing development of chemical mapping strategies, including observing and modelling reactive phototransient distribution in terms of their sources, mixing, and fates. Ancillary supporting studies included observation of water mass tracers, dissolved trace gases, atmospheric components, nutrients and the geochemistry of estuarine mixing processes in an important continental margin. The observational and mechanistic investigations reported here feature a number of novel or improved methods allied with some advanced underway sampling, sensing and computing facilities that were implemented aboard the R/VColumbus Iselin. Results from the study showed large‐scale (∼1000 km) seasonal variations in surface water photoreactivity, optical and biooptical characteristics over much of the Caribbean basin. These changes resulted from seasonally varying riverine inputs of organic chromophores, nutrients and suspended material. Smaller scale (10–100 km) studies carried out in the Orinoco delta and the Gulf of Paria showed that estuarine mixing processes did not affect major net removal of dissolved organic matter, consistent with the hypothesis that riverine chromophore input plays a dominant role in open‐water photochemistry.
The photochemical decomposition of hydrogen peroxide in surface waters of the eastern Caribbean and Orinoco River
Authors: Moffett James W. , Zafiriou Oliver C.
Published: February 2008

The photochemical decomposition of hydrogen peroxide has been studied in surface waters collected on a transect from the Atlantic Ocean north of Puerto Rico through the eastern Caribbean Sea to the mouth of the Orinoco River. Absolute rates of photochemical decomposition were measured by adding 18O labelled H2O2 to the water samples and irradiating with a shipboard solar simulator. Photochemical decomposition was measurable at all station locations. Decomposition rates increased along the transect towards the Orinoco and were directly proportional to photochemical production rates, which also increased. Photodecomposition rates were also linearly related to the intensity of absorption of samples at 300 nm. Decomposition rates were generally ∼5% of formation rates, indicating that photochemical decomposition is probably a minor sink for H2O2 in the mixed layer compared to biological decomposition. In all seawater samples, photodecomposition led exclusively to 18O2, indicating that reaction with photochemically produced oxidants was the dominant reaction mechanism. The oxidizing species are not the best known photooxidants in seawater, singlet oxygen and OH and O2 radicals. Unidentified oxidants with formation rates ∼5% of the H2O2 formation flux are required. In contrast, H218O2 decomposition in unfiltered samples of Orinoco River water led solely to H218O2, indicating reaction with a photochemically produced reductant was the dominant mechanism. Further experiments indicated that the reductant was probably Fe(II) produced by photoreduction of particulate Fe(III), a known process. This pathway would produce an equivalent flux of OH radical: Fe(II) + HOOH → Fe(III) + OH + OH−.
On the distribution of hydrogen peroxide in the lower troposphere over the northeastern United States during late summer 1988
Authors: Tremmel H. G. , Junkermann W. , Slemr F. , Platt U.
Published: February 2008

H2O2 and NO, NO2, NOy, O3, SO2, and H2O were measured from an aircraft over the northeastern United States in August and September 1988. The data base consists of 13 flight missions; on 14 days during different meteorological conditions, 177 vertical profiles between about 100 and 3500 m were obtained. The mixing ratio of H2O2 varied from below the detection limit of 0.2 up to 5.9 parts per billion by volume (ppbv) and it was strongly dependent on altitude, indicating that processes controlling the H2O2 occurrence in the planetary boundary layer (PBL) were different from those in the free troposphere (FT). Average H2O2 mixing ratios varied between 0.20 and 1.30 ppbv in the PBL showing, in most cases, a decrease to the ground. High H2O2 values in the PBL were observed only when NO was below 1.5 ppbv. In more than 80% of the vertical profiles a pronounced maximum in the H2O2 mixing ratios (up to 2.3 ppbv) was observed at the top of the PBL. In the FT the H2O2 mixing ratio averaged over individual flights varied between 0.41 and 1.80 ppbv and was strongly correlated with the water vapor concentration, in agreement with a simple “low NOx case” photochemical model. The average SO2 mixing ratios were higher than the average H2O2 mixing ratios during all flights in the PBL and during most of the flights in the FT, indicating that the formation of sulfate in this region is oxidant limited for most of the time.
Atmospheric measurements of carbonyl sulfide, dimethyl sulfide,and carbon disulfide using the electron capture sulfur detector
Authors: Johnson James E. , Bates Timotny S.
Published: February 2008

Measurements of atmospheric dimethyl sulfide (DMS), carbonyl sulfide (COS), and carbon disulfide (CS2) were conducted over the Atlantic Ocean on board the NASA Electra aircraft during the Chemical Instrumentation Test and Evaluation (CITE 3) project using the electron capture sulfur detector (ECD‐S). The system employed cryogenic preconcentration of air samples, gas chromatographic separation, catalytic fluorination, and electron capture detection. Samples collected for DMS analysis were scrubbed of oxidants with NaOH impregnated glass fiber filters prior to preconcentration. The detection limits (DL) of the system for COS, DMS, and CS2 were 5, 5, and 2 ppt, respectively. COS concentrations ranged from 404 to 603 ppt with a mean of 489 ppt for measurements over the North Atlantic Ocean (31°N to 41°N), and from 395 to 437 ppt with a mean of 419 ppt for measurements over the Tropical Atlantic Ocean (11°S to 2°N). DMS concentrations in the lower marine boundary layer, below 600‐m altitude, ranged from below DL to 150 ppt from flights over the North Atlantic, and from 9 to 104 ppt over the Tropical Atlantic. CS2 concentrations ranged from below DL to 29 ppt over the North Atlantic. Almost all CS2 measurements over the Tropical Atlantic were below DL.
Aerosol in the arid southwestern United States: Measurements of mass loading, volatility, size distribution, absorption characteristics, black carbon content, and vertical structure to 7 km above sea level
Authors: Pinnick R. G. , Fernandez G. , Martinez‐Andazola E. , Hinds B. D. , Hansen A. D. A. , Fuller K.
Published: February 2008

Near‐surface and lower tropospheric aerosol characteristics have been determined at several remote sites near Orogrande, New Mexico, using a variety of methods and sensors including quartz fiber filter samplers, hi‐vol samplers, ground‐based and aircraft‐mounted light‐scattering aerosol counters, an aerosol counter equipped with a heated inlet, and an aethalometer (a device that measures aerosol absorption). The results of these measurements, which have been taken sporadically over the last 15 years but with a higher density of measurements during 1989–1991, suggest that regardless of season, aerosol consists of two modes: a submicron fraction composed primarily of ammonium/acid sulfates and elemental black carbon (BC) and a supermicron fraction composed mainly of quartz and clay minerals of soil origin. Aerosol mass concentrations determined from weekly or biweekly samples are dominated by wind‐derived aerosol of soil origin and typically range from ≈10 μg m−3 in the fall rainy season (August‐October) to ≈100 μg m−3 during the spring windy season (February‐June). The mass concentration frequency of occurrence of both total aerosol and BC are well approximated by lognormal probability distributions. Geometric mean masses of aerosol range from 16 to 32 μg m−3 and are smaller for mountain sites than for basin sites; geometric mean BC mass concentrations at a single basin site are ≈0.15 μg m−3. Aerosol loadings decrease slightly from 1975–1977 to 1990 measurement periods, whereas BC concentrations, within statistical error, remain unchanged. However, a statistically significant increase in BC did occur from 1990 to 1991. The fraction of (visible spectrum) aerosol absorption attributable to BC has a yearly mean value of ≈0.84, although during dusty conditions occurring in spring or early summer, non‐BC aerosol absorption contributes up to ≈30% of the total. Sulfate and BC aerosol concentrations are sometimes highly correlated, suggesting they have a common source and may be internally mixed. Limited aircraft measurements in the lowest few kilometers of the troposphere reveal a well‐mixed aerosol for a neutral atmospheric condition, and a significant decrease in aerosol concentration with altitude for a stable atmospheric condition.
Aerosol optical thickness and atmospheric path radiance
Authors: Kaufman Yoram J.
Published: February 2008

Spaceborne remote sensing of aerosol particles, evaluation of the climatic effects of aerosol, and atmospheric corrections of spaceborne imagery of the Earth's surface are based on an assumed relationship between the spectral aerosol optical thickness and the spectral path radiance. Path radiance is the radiance detected by a spaceborne sensor above a nonreflective surface and is the result of backscattering to space by particles and molecules in the atmosphere. In specific measurement conditions the path radiance can be measured also from the ground. Simultaneous measurements of the path radiance and the optical thickness from the ground are reported at over 30 locations spread all over the world. All the measurements are performed with a single eight‐channel portable sunphotometer/radiometer in the 0.44‐ to 1.03‐μm range. The observations are taken for constant solar and view directions, resulting in a constant scattering angle of 120°, which resembles space observations. One set of measurements is used to develop empirical relationships between the aerosol spectral optical thickness and the scattered spectral path radiance. A second independent set is used to test these relationships. It is shown that simple measurements performed from the ground can yield empirical relationships that can be used to check some of the common but not validated assumptions about the particle homogeneity, sphericity, composition, and size distribution used in remote sensing models and in estimates of the radiative effects of aerosol. Results are summarized in Table 4 and used to test concepts of atmospheric corrections and remote sensing of aerosol from space.
Measurement and speciation of gas phase peroxides in the atmosphere
Authors: Lee J. H. , Leahy D. F. , Tang I. N. , Newman L.
Published: February 2008

An improved gas phase sampler and a recently developed nonenzymatic measurement method for H2O2 and HOCH2OOH(HMHP) are described. The gas phase sampler is equipped with a surfaceless intake to alleviate heterogeneous loss of H2O2 in the sampling line. The nonenzymatic method makes use of the aqueous phase Fention reaction for H2O2 measurement and is capable of speciation between H2O2 and HMHP, using a dual channel technique. By adding a third channel employing the well‐established method of p‐hydroxyphenylacetic acid (POHPAA) and horseradish peroxidase for total peroxide measurement, it is possible to differentiate H2O2, HMHP, and other organic peroxides, if present. The measurement technique was successfully deployed in a field study at George L. Smith III State Park, Georgia (82°20′W, 32°30′N). Preliminary data analysis indicates that the measured total peroxides range from 0.4 to ∼6 ppbv, depending on meteorological conditions, and that organic peroxides, mostly HMHP, constitute between 20 and 80% of the total peroxides. The work reported here represents the first continuous and real‐time measurements for these peroxides in the atmosphere.
Atmospheric methyl bromide: Trends and global mass balance
Authors: Khalil M. A. K. , Rasmussen R. A. , Gunawardena R.
Published: February 2008

Atmospheric methyl bromide is of considerable environmental importance as the largest reservoir of gaseous bromine in the atmosphere. Bromine gases can catalytically destroy stratospheric ozone. Since agricultural activities, automobiles, biomass burning, and other human activities produce CH3Br, it is of interest to know its global mass balance and particularly the specific sources and sinks. In this paper we provide a decadal time series of global CH3Br concentrations in the Earth's atmosphere. The data show that average concentrations are about 10 pptv and during the last 4 years may be increasing at 0.3 ± 0.1 pptv/yr (3%/yr ± 1 %/yr). We estimate that the atmospheric lifetime of CH3Br that is due to reaction with OH, is about 2 years, resulting in a calculated global emission rate of about 100 Gg/yr. Ocean supersaturations of 140–180% are observed, and atmospheric concentrations over the open oceans are higher than at comparably located coastal sites. The ocean source is estimated to be about 35 Gg/yr. The remaining emissions must come from other natural sources and anthropogenic activities. Our results are based on some 2200 samples obtained over more than a decade. Mass balance calculations explain most aspects of the present data but other implications are not easily reconciled, leaving open the possibility of undiscovered sources and sinks.
Circulation mechanisms related to northeast Brazil rainfall anomalies
Authors: Hastenrath Stefan , Greischar Lawrence
Published: February 2008

This study explores the role of the upper ocean for circulation anomalies in the overlying atmosphere, with focus on the late boreal winter and extreme climatic events in northeast Brazil. Data sources comprise rainfall records in the Nordeste, surface ship observations of the sea surface temperature (SST), pressure, and wind fields (COADS, period 1948–1990), satellite observations of tropical convection (HRC, 1971–1988), and upper air analyses of the European Centre for Medium Range Weather Forecasts (ECMWF, 1980–1991), all for the tropical Atlantic sector. Interhemispheric SST gradients are most strongly associated with north‐south contrasts in pressure and the meridional wind component, whereas the inverse SST‐pressure relations in situ are less close. Enhanced northward temperature increase in the tropical Atlantic is accompanied by steeper meridional pressure gradient and accelerated southerly wind component, which is representative of a northward displaced Intertropical Convergence Zone (ITCZ), with the latter in turn leading to drought in northeast Brazil. Interhemispheric contrasts in the 850/1000 mbar layer mean temperature are similar to those of SST and account for most of the meridional surface pressure gradient. During wet as compared to dry years in northeast Brazil, the tropical North Atlantic is cooler and the South Atlantic warmer, and accordingly the lower tropospheric thickness is reduced to the north but inflated to the south of the equator, resulting in increased/reduced surface pressure over the North/South Atlantic, as well as reduced southerly surface wind, stronger subsidence over the outer tropics of the northern hemisphere and intensified ascending motion and convective activity over the Nordeste, related to a southward displaced near‐equatorial convection belt. The surface and upper air evidence thus indicates the ways in which the interhemispheric SST gradients exert a hydrostatic control on the lower tropospheric thickness pattern, and thus force the south‐north surface pressure gradients and the surface meridional wind component, and hence modulate the latitude position of the ITCZ and Nordeste rainfall.
The Upper Atmosphere Research Satellite (UARS) mission
Authors: Reber Carl A. , Trevathan Charles E. , McNeal Robert J. , Luther Michael R.
Published: February 2008

The Upper Atmosphere Research Satellite (UARS) is a NASA program aimed at improving our knowledge of the physical and chemical processes controlling the stratosphere, mesosphere, and lower thermosphere, emphasizing those levels that are known to be particularly susceptible to change by human activities. The spacecraft was launched by the Space Shuttle Discovery on September 12, 1991, into a near‐circular orbit at 585 km altitude and 57° inclination. Measurements include vertical profiles of temperature, many trace gases, and horizontal wind velocities, as well as solar energy inputs. Many of the limb‐scanning instruments can measure to as high as 80° latitude, providing near‐global coverage. The mission is supported by a large international correlative measurement program, yielding data both for validation of the UARS measurements and for complementary scientific studies. A dedicated data system provides rapid processing to geophysical quantities and makes these data available to UARS scientists.
Positive cloud‐to‐ground lightning discharges in Kathmandu thunderstorms
Authors: Baral K. N. , Mackerras D.
Published: February 2008

The occurrence of positive cloud‐to‐ground lightning discharges in thunderstorms in the vicinity of Kathmandu, Nepal (27.4°N, 85.2°E) has been recorded during the 21‐month period from March 1987 to November 1988 inclusive, using the daily registrations of a CGR3 lightning flash counter that provided records of the approximate numbers of positive ground flashes, negative ground flashes, and cloud flashes, with effective ranges of about 12 km for cloud flashes, 14 km for negative ground flashes, and 16 km for positive ground flashes. The detection of positive ground flashes was based on the presence of a relatively large negative‐going step in the electric field change caused by the positive return stroke in the discharge. It was found that the overall mean proportion of positive ground flashes to all ground flashes was 0.28, and that the overall mean positive ground flash density was 0.6 km−2 yr−1. The uncertainty in these values is about ±50%. The proportions of positive ground flashes in three of the climatic periods in the year were 0.26 in the premonsoon period (February to May inclusive), 0.34 in the monsoon period (June to August inclusive), and 0.38 in the postmonsoon period (September to November inclusive). This apparent increase in the proportion is accompanied by a seasonal decline in electrical activity, the premonsoon period being the most active, and the postmonsoon the least active. The increase also appears to be associated with a decrease in the proportion of cloud flashes to total flashes. The proportion of positive ground flashes is larger than is observed in most low‐altitude tropical and subtropical sites but is smaller than that observed in winter thunderstorms at some higher‐latitude sites. Possible reasons for the relatively high rate of occurrence of positive ground flashes (unsupported by concurrent atmospheric soundings) are the site altitude, thundercloud charge heights, vertical wind shear and the mountainous nature of the terrain.
Advection of sulfur dioxide over the western Atlantic Ocean during CITE 3
Authors: Thornton D. C. , Bandy A. R. , Beltz N. , Driedger A. R. , Ferek R.
Published: February 2008

During the NASA Chemical Instrumentation Test and Evaluation 3 sulfur intercomparison over the western Atlantic Ocean, five techniques for the determination of sulfur dioxide were evaluated. The response times of the techniques varied from 3 to 30 min. Based on the ensemble of measurements reported, it was clear that advection of SO2 from the North American continent occurred in the boundary layer (altitude <1 km) with only one exception. The vertical distribution of SO2 above the boundary layer for the northern and southern Atlantic Ocean was remarkably similar during this experiment.
Evidence for winter/spring denitrification of the stratosphere in the nitrate record of Antarctic firn cores
Authors: Mulvaney Robert , Wolff Eric W.
Published: February 2008

A firn core from within the polar vortex in the Weddell Sea sector of Antarctica shows nitrate peaks that occur in the spring or early summer. They are far less prominent in two other cores from sites that are further north. Although other data are needed, circumstantial evidence suggests that the peak may be due to sedimentation of polar stratospheric clouds from the stratosphere during winter and spring. No change in the form of the peak is seen over the last three decades. It may be possible to observe past stratospheric conditions over longer time scales from ice cores, but other sources may obliterate the signal in ice from the glacial periods.
Geographic variations in the thermal and diffusive stability of ground ice on Mars
Authors: Mellon Michael T. , Jakosky Bruce M.
Published: February 2008

To investigate the stability of ground ice within the top several meters of the Martian regolith, time‐dependent models of the thermal and diffusive behavior of the regolith have been developed. The geographic distribution of thermal inertia and albedo as well as the latitudinal variation in insolation have been included in calculations of surface and subsurface temperatures between ±60° latitude. Ground ice was found to be stable where the annual mean surface and subsurface temperatures were below the atmospheric frost point. This generally occurs poleward of the mid‐latitudes. The latitude poleward of which ground ice is stable varies by about 20° to 30° from one longitude to another. Geographic variations in thermal inertia and albedo are the primary factors controlling regional variations in ice stability. Calculations of temperatures at high and low obliquity suggest that ground ice would be stable globally at high obliquity and would not be stable between ±60° latitude at low obliquity. Thermally driven diffusion of atmospheric water vapor within the regolith was modeled accounting for both ordinary molecular and Knudsen transport and equilibrium between ice, vapor, and adsorbed phases. Atmospheric water vapor was found to be able to supply the top few meters of the regolith with ice in regions where the annual mean surface temperature was below the atmospheric frost point. Ice was found to begin condensing in as short as 1000 Martian years. Rapid accumulation of ice in the pore space of the upper layers of the regolith acted to choke transport to lower layers and slow the diffusion process. Even so, after 105 Martian years, in some cases, as much as 30% to 40% of the available pore space accumulated ice. The total amount of subsurface ice ranged from a few to more than 25 g/cm2 within the top few meters in regions of stability. Ground ice was found to form below a depth where the annual average vapor pressure over ice was equal to the annual average atmospheric vapor pressure near the surface. Atmospheric vapor was not found to accumulate as ice below a depth where the seasonal temperature oscillations gave way to the geothermal gradient. The time scales for condensation of ground ice were found to be comparable to that of orbital oscillations suggesting that the present geographic distribution of ground ice may depend on the orbital history of Mars.
On the density of the dust halo around Mars
Authors: Juhász Antal , Tátrallyay Mariella , Gévai Gábor , Horányi Mihály
Published: February 2008

This paper provides an estimate of the average number density of dust particles accumulated around Mars. As investigated earlier, submicron‐sized grains (a < 1 μm) are perturbed by electromagnetic forces and solar radiation pressure so that they do not stay close to the Keplerian orbit of the ejecting satellite. The lifetime of these particles in the vicinity of Mars ranges from a few days to a few hundred days. Their trajectories are contained in an approximately spherical volume. For larger grains (1 μm < a ≲ 7 μm), solar radiation pressure is the major loss mechanism and these particles fill a disklike volume. The corresponding lifetime due to this perturbation ranges from a few tens of days to half a Martian year. Dust grains with a ≳ 7 μm have relatively stable orbit forming a disk/torus around Mars, and their lifetime is determined by collisions with the satellite since they stay close to its orbit. Despite the short lifetime of submicron‐sized particles and the relatively large volume which contains their trajectories, their number density is comparable with that of particles whose size is between 7 and 50 μm. This is due to the fact that submicron‐sized grains are thought to dominate the number of escaping particles. We conclude that particles with sizes a < 0.5 μm and 7 μm ≲ a ≲ 50 μm will be the most abundant in the Martian dust environment.
Tektites and climate
Authors: Wasson John T. , Heins William A.
Published: February 2008

Only four tektite strewn fields have been produced during the past 40 m.y. even though at least 60 impact craters have been produced during the same period in continental lithologies having tektite‐like compositions. The apparent reason for this discrepancy is that the ejecta from most crater‐forming impacts was not completely melted. Laboratory and modeling studies show that projectiles as different as anorthosite and metal (densities from 2.8 to 8 g cm−3) produce similar amounts of melt, and this conclusion probably holds for comet‐like densities near 1.5 g cm−3. Our interpretation of the limited evidence indicates that the influence of projectile trajectory and velocity on melting efficiency is minor. The key factor affecting melt production seems to be the nature of the target, particularly its porosity; the fraction of projectile kinetic energy converted to heat may be an order of magnitude higher in a highly porous target than in a void‐free target. The grain size and water content of the target are also important. The ideal target is a porous, fine‐grained sediment such as loess, particularly if it is also dry. We suggest that the rate of impact production of fully molten crater ejecta is proportional to the fraction of the continental surface having thick (>10 m) blankets of dry loess and that tektites are mainly produced during cold, dry climate periods when such deposits are an order of magnitude more common than at present. This model is best tested at the Australasian site, since detailed climate records cannot be obtained for the Czechoslovakian and North American sites and have not yet been obtained for the Ivory Coast site. The fossil O isotopic records in cores containing the Australasian microtektites show that these were deposited during a period of cold climate near glacial stage 20.2. The chemical homogeneity and the high 10Be contents of Australasian tektites are well explained by melting a loess target.