bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of rock friction constitutive parameters using an iterative least squares inversion method
Authors: Reinen Linda A. , Weeks John D.
Published: February 2008

In order to understand the behavior of slip in fault zones that can lead to earthquakes, a detailed description of the constitutive behavior of the slipping rocks is needed. Rate‐ and state‐variable constitutive laws have been very successful in describing results of laboratory studies of rock friction, but the actual determination of constitutive law parameter values has been limited to forward trial‐and‐error methods only. This paper presents a method of inverting rock friction experimental data to determine the parameters in the Dieterich‐Ruina friction constitutive model. We use an iterative least squares method to solve the inverse problem, and we describe the solutions to several difficulties that arose owing to the nonlinearity of both the model and the iterated solution. These solutions include (1) using a finite differences method to estimate the derivatives of the constitutive model, (2) incorporating a vector to weight the relative “importance” of the friction observations, (3) using singular‐value decomposition to solve the inverse problem, and (4) creating a damped version of the inverse routine to enhance the ability of the program to converge on a final solution. We explore the effects of machine stiffness and added noise on the covariances and correlations between model parameters. We find that increasing stiffness reduces parameter variances, covariances, and the strong correlations between some model parameters; increasing noise increases parameter variances and covariances without affecting the correlation between model parameters.
A procedure for obtaining the complete horizontal motions within zones of distributed deformation from the inversion of strain rate data
Authors: Haines A. J. , Holt W. E.
Published: February 2008

We present a new method for obtaining relative horizontal motions on the surface of a sphere from strain rate data. Strain rates can be obtained from the summation of earthquake moment tensors or from estimates of Quaternary rates of deformation on major faults. The method is particularly useful for determining the kinematics within zones of distributed continental deformation, or any region where there is distributed strain. All relative motions, including rotation rates about the vertical axis, are uniquely determined when the three rates of horizontal strain
are everywhere defined within the region of interest. The forward problem is set up such that all relative velocities
on the surface of the sphere are defined by
, where
is the three‐dimensional rotation vector that describes the velocities uθ and uϕ at all points on the surface of Earth of radius r with position unit radial vectors
. The three‐dimensional rotation vector
is expressed as an infinite power series expansion, truncated at finite order N‐1. Coefficients of this polynomial are sought in a damped least squares inversion such that the strain rates

, which define all of the coefficients in the power series expansion of
, are optimally matched by the smooth polynomial function. Formal uncertainties are introduced that take into account observational error as well as the inability of the polynomial function to accommodate the more rapid spatial variations of the rate‐of‐strain field. We demonstrate the method on a deforming part of Asia. Strain rates for the region were obtained from the summation of moment tensor elements of moderate and large‐sized earthquakes in this century. Solutions, both velocity fields and rotation rates, are investigated as a function of polynomial smoothing and polynomial order. We demonstrate that the velocity field, obtained by the polynomial fitting of the regions where strain rates were averaged, is by nature extremely robust and is almost independent of the amount of polynomial smoothing. The rotation rate field on the other hand shows the same order of smoothing as the polynomials used in the fitting procedure.
Buoyancy effects on mantle flow under Mid‐Ocean Ridges
Authors: Su Wusi , Buck W. Roger
Published: February 2008

Results of a series of two‐dimensional numerical experiments of mantle flow, melting, and melt migration under a spreading center are reported. The model predicts the distribution of melt in the subridge mantle, the width over which most melt is delivered to the crust, and the thickness of crust. The sources of buoyancy considered are thermal expansion, compositional variation caused by melt extraction, and the phase change of solid to melt. We infer that the steady state average viscosity of the mantle below a ridge cannot be much less than about 1019 Pa s. For a lower average viscosity, thermal convection causes rapid cooling of a large region under a ridge, raising the viscosity. Results imply that transient increases in mantle temperature should lead to larger increases in the oceanic crustal thickness for slow spreading ridges than for fast spreading ridges. We assume that the viscosity is proportional to exp (‐ϕ/ϕ0), where ϕ is melt fraction. We parameterize the permeability in terms of the reference velocity for percolation of the melt vr, where the relative velocity of melt to solid is vr ϕ. Analytic approximations are used to extrapolate the model results to large values of permeability and small values of ϕ0. If vr is less than 1 m/yr, then more melt would be retained in the subridge mantle than is estimated from analysis of topography and gravity data at fast spreading centers. For vr greater than about 100 m/yr, so little melt would be retained in the mantle that it is difficult to explain the gravity data and the low shear wave velocity structure close to the East Pacific Rise estimated from seismic surface waves. Buoyancy effects can lead to a region of mantle upwelling and melting that is as narrow as the observed zone of oceanic crustal accretion. For most melt to be added to the crust within a few kilometers of a fast spreading center requires that ϕ0 be less than 0.015 if vr equals 1 m/yr and less than 0.003 if vr is 100 m/yr.
Role of size distributions on physical properties: Real size renormalization group
Authors: Madden T. , Williams E.
Published: February 2008

This study looks at the role of particle size distributions in modifying the physical properties of inhomogeneous materials and develops an efficient technique for modeling these effects. This technique, which we call the real size renormalization group (RZRG), is a discretized version of the differential equivalent medium method, but it keeps track of the local densities of the smaller particles that can greatly influence the average properties. Using this method, we demonstrate the conditions under which the symmetric equivalent medium method gives very good results. We also apply the method to the electrical properties of sandstones. When only grain size distributions are given, the modeling is nonunique, as different pore size distributions can also be present. The RZRG models that fit the electrical data appear very similar to views of the rock samples and involve some inefficiency in the pore space distribution. The RZRG modeling is also able to determine the volume density and size distributions of sulphide particles in a sandstonelike environment using induced polarization data.
Two forms of volcanism: Implications for mantle flow and off‐axis crustal production on the west flank of the southern East Pacific Rise
Authors: Shen Yang , Forsyth Donald W. , Scheirer Daniel S. , Macdonald Ken C.
Published: February 2008

SeaMarcII side‐scan sonar and bathymetric data on the west flank of the East Pacific Rise (16°–18°S) reveal a large number of seamounts, organized primarily in chains. The easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows. In addition, areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis. The flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m. In other cases, the scarps appear to be buried by flows. The disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image. Under this criterion, lava flows and volcanic debris surrounding the seamounts cover 40–50% of the survey area of about 18,000 km2. With an estimated average thickness of about 100 m, the volume of lava flows and volcanic debris is about 0.8±0.3% of that of the total crust. The volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic, median‐filtering technique. The volume of seamount edifices is 1.05±0.05% of the volume of the crust. Thus the total volume of volcanic material extruded off‐axis is about 1.5 to 2.2% of the volume of the crust. The decreasing volcanic activity away from the ridge axis suggests that near‐axis seamounts may be associated with the upwelling system beneath the ridge, perhaps by preferential melting of embedded, passive heterogeneities. However, the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma. Since no apparent east‐west faults and no simultaneous volcanic activity along the length of a seamount chain are observed, north‐south extension and ridge‐perpendicular convection are unlikely to be major causes of seamount formation in the study area. The existence of closely spaced linear chains and the tendency for fresh flows to be found at near‐ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods (over 1.8 m.y.). Passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent, nearly stationary melting anomalies. A variety of mechanisms may be responsible for the off‐axis volcanism, but the single hypothesis most consistent with all observations is a mini‐hotspot origin. Although fresh lavas 80 km off‐axis are consistent with broad mantle upwelling beneath the ridge, the possibility of narrow upwelling cannot be ruled out, because the seamounts could be caused by independent buoyant upwelling of mini‐plumes that can penetrate thin lithosphere more easily near the axis.
Corrections and further discussion of the primary magmas of mid‐ocean ridge basalts, 1 and 2
Authors: Kinzler Rosamond J. , Grove Timothy L.
Published: February 2008

An error has been detected in the methodology for estimating batch melt compositions from plagioclase and spinel Iherzolites at mantle conditions presented by Kinzler and Grove (1992a,b) (hereafter referred to as K&G). In a mass balance calculation employed by K&G the melt fraction was not properly transformed from weight units to molar units, resulting in an overestimate of the effective melt fraction employed in the calculation of the Mg # (Mg/[Mg+Fe2+]) of the melt, at melt fractions greater than 0.01. In the batch melts estimated by K&G, the abundance of FeO increases then decreases in the melt with increasing melt fraction from 0.01 to 0.20. Using the corrected methodology described here, the abundance of FeO in melts produced via batch melting increases as melt fraction increases from 0.01 to 0.20 (up to the point of exhaustion of clinopyroxene). The correction has essentially no effect on the compositions of melts produced via the near‐fractional melting processes described in K&G because melt fractions employed in the near‐fractional melting calculations never exceeded 0.01. In addition we apply the methodology of K&G to estimate the mantle temperature, average pressure of melting and average extent of melting beneath a mid‐ocean ridge segment from the FeO and Na2O content of a mid‐ocean ridge basalt from that segment.
Self‐lubrication for Long Runout Landslides: Examination by computer simulation
Authors: Cleary Paul W. , Campbell Charles S.
Published: February 2008

This paper describes the use of a discrete particle computer simulation to test whether the apparent low friction exhibited by long runout landslides could be explained in terms of simple granular mechanics. The flow structure consists of an active basal shear zone supporting the majority of the landslide, which travels as a relatively solid plug. The observed runout appears to depend on the total energy dissipation which is a trade‐off between (1) the inelasticity of the particles, which governs the energy dissipated in each event, and, (2), the size of the dissipating shear zone, which governs the number of particles that are actively dissipating energy. Very inelastic particles quickly dissipate the work performed at boundaries and result in thin dissipative zones. As a result, these two factors balance and the runout is nearly independent of particle inelasticity. Along the same lines, increasing the boundary roughness was found to increase the size of the dissipative shear zone and markedly decrease the runout. Also, a distribution of particle sizes destroyed ordered structures within the body which, once again, increases the dissipation and decreases the runout. Unfortunately, the simulations also indicate that the runout should be independent of the depth of the flow and thus cannot account for the observed volumetric effect on runout.
Modeling interannual variability in the Indian Ocean using momentum fluxes from the operational weather analyses of the United Kingdom Meteorological Office and European Centre for Medium Range Weather Forecasts
Authors: Anderson David L. T. , Carrington David J.
Published: February 2008

A 16‐level general circulation model of the Indian Ocean is used to reproduce interannual variability in ocean circulation from 1987 to 1990 using wind stresses from the United Kingdom Meteorological Office (UKMO) operational forecast model analyses. The results of this integration are compared with those obtained using the Hellerman and Rosenstein [1983] (HR) climatological forcing. The differences in the two wind products are large, as are the resulting surface and subsurface currents. Model results are compared with observations at 55°E and 73°E on the equator. The undercurrent using UKMO stresses is more strongly eastward than that using HR, and in the four years studied, the model always simulates an undercurrent in February‐March at Gan (73°E), contrasting with the inconstancy of its appearance in the years sampled by the observations. Interannual differences in the surface currents can be large, as much as 100 cm s−1 in May on the equator. The years 1987 and 1988 are worthy of special consideration, since 1987 was a drought year over India and 1988 had above average rainfall. Therefore, the period 1987–1988 is repeated using stresses from the European Centre for Medium Range Weather Forecasts (ECMWF). There are large differences between the ECMWF and the UKMO stresses and, consequently, in the model currents, where differences can be as much as 80 cm s−1 over a large area of the equatorial zone and in the Somali current region. At undercurrent depth, the response at Gan (73°E) is very similar in the first half of the year in all three integrations, but differs significantly in the second half with the UKMO and ECMWF stresses inducing a semiannual response, whereas HR leads to an annual cycle. Although interannual differences are large on a month to month basis, they are not long lasting. Interannual variations in model sea surface temperatures (SST) resulting from variations in the surface stress are poorly related to observed SST anomalies. There are no direct measurements of currents in the 1987–1990 period with which to validate the model simulations. There are, however, satellite measurements of the distortion of the sea surface and so comparison is also made between the simulations of dynamic topography from the model and GEOSAT altimetry data. While there is broad‐scale similarity in pattern between the three model simulations and the GEOSAT observations, detailed agreement between the fields is poor. The model results are also compared to the dynamic topography derived from Levitus data.
Synthetic aperture radar interferometry applied to ship‐generated internal waves in the 1989 Loch Linnhe experiment
Authors: Thompson D. R. , Jensen J. R.
Published: February 2008

Interferometer synthetic aperture radar images collected during the 1989 Loch Linnhe experiment showed mean Doppler variations across the phase of ship‐generated internal waves that corresponded to “velocity” variations of the order of 50 to 100 cm/s. The in situ current data, however, showed surface currents associated with the internal wave features of the order of 5 to 10 cm/s and virtually ruled out the existence of surface currents as large as the interferometer‐inferred values. In this paper we show how the pixel‐to‐pixel phase difference measured by the Jet Propulsion Laboratory interferometer is related to the mean Doppler frequency of the backscattered field. Model calculations are used to show how this frequency can sometimes change by a large amount, even when rather small surface currents are present. In particular, for winds blowing roughly across the internal wave features, as was the case for the interferometer runs in Loch Linnhe, computations based on our wave‐current interaction and time dependent scattering models show that changes in the mean Doppler frequency corresponding to large velocities can, in fact, be produced from the much smaller measured surface currents. We show that the larger interferometer velocity estimates are essentially due to the different modulation strengths of the surface Bragg waves advancing toward and receding from the radar. Thus for these crosswind conditions, care must be taken in converting the phase differences measured by the interferometer to a surface current image. When the wind is aligned more nearly along the internal wave propagation direction, the mean Doppler shifts (and the phase differences) are dominated mostly by advection, and interferometer current estimates are more accurate. C band computations predict that if the antenna spacing is small enough so that the fields from the two antennas remain correlated, then the C band interferometer current estimates will be better than those at L band.
Characteristics of an eddy over a continental shelf: Shelikof Strait, Alaska
Authors: Schumacher J. D. , Stabeno P. J. , Bograd S. J.
Published: February 2008

During May 1990, satellite‐tracked buoy, water property, and ichthyoplankton observations were collected over the Shelikof sea valley in the western Gulf of Alaska. These observations revealed the presence of an anticyclonic mesoscale eddy (radius 10–12 km). Characteristics of the eddy included a relatively warm, low‐salinity core and high concentrations of walleye pollock larvae. The eddy remained nearly stationary for several weeks before moving westward into shoaler waters. During translation, the eddy maintained its characteristics. Speeds from geopotential topography and direct measurements agreed. Current speeds generally increased with distance from the center of the eddy; maximum speeds (>30 cm s−1) existed near the perimeter. Analysis of water properties indicates little or no exchange of mass occurred with adjacent waters. The lack of dispersion permits us to estimate larval mortality (≤4.7% d−1) based only on biological factors. Eddy formation occurred through baroclinic instability upstream within Shelikof Strait proper.
Distributions of pigments and primary production in a Gulf Stream meander
Authors: Lohrenz Steven E. , Cullen John J. , Phinney David A. , Olson Donald B. , Yentsch Charles S.
Published: February 2008

An investigation was made of physical effects of Gulf Stream meandering on the vertical and horizontal distributions of photosynthetic pigments and primary production. Cruises were conducted in the vicinity of a meander east of 73°W and north of 37°N from September 21 to October 5 (leg 1) and October 12–21, 1988 (leg 2), on the R/V Cape Hatteras. Relationships of photosynthesis (normalized to chlorophyll) to irradiance (P‐I) did not show large horizontal variation, and water column composite P‐I curves from leg 1 and leg 2 were similar. Therefore a single P‐I curve derived from pooled data was used to model distributions of primary production. Distributions of photosynthetic pigments were characterized on the basis of in vivo fluorescence profiles and empirical relationships with extracted pigment concentrations. Subsurface irradiance was described using a spectral irradiance model. Cross sections of the Gulf Stream revealed consistently higher pigment concentrations and primary production on the slope water side. Along‐stream variations in pigment distributions and primary production were apparently related to density structure influenced by meander circulation. Such variations were less pronounced during leg 2, which came after a transition from a well‐defined meander interacting with a warm‐core ring (leg 1) to a more linear stream (leg 2). Higher water‐column‐integrated primary production during leg 2 was attributed to mixing‐induced nutrient injection and redistribution of chlorophyll in the photic zone.
Estimators of primary production for interpretation of remotely sensed data on ocean color
Authors: Platt Trevor , Sathyendranath Shubha
Published: February 2008

The theoretical basis is explained for some commonly used estimators of daily primary production in a vertically uniform water column. These models are recast into a canonical form, with dimensionless arguments, to facilitate comparison with each other and with an analytic solution. The limitations of each model are examined. The values of the photoadaptation parameter Ik observed in the ocean arc analyzed, and Ik is used as a scale to normalize the surface irradiance. The range of this scaled irradiance is presented. An equation is given for estimation of Ik from recent light history. It is shown how the models for water column production can be adapted for estimation of the production in finite layers. The distinctions between model formulation, model implementation and model evaluation are discussed. Recommendations are given on the choice of algorithm for computation of daily production according to the degree of approximation acceptable in the result.
Ocean acoustic tomography at 1000‐km range using wavefronts measured with a large‐aperture vertical array
Authors: Cornuelle Bruce D. , Worcester Peter F. , Hildebrand John A. , Hodgkiss William S. , Duda Timothy F. , Boyd Janice , Howe Bruce M. , Mercer James A. , Spindel Robert C.
Published: February 2008

Broadband acoustic signals transmitted from a moored 250‐Hz source to a 3‐km‐long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source‐receiver pair. A range‐independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range‐dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross‐validation, since the typical cast spacing of 20–25 km gives a Nyquist wavelength of 40–50 km. The small travel time signals associated with high‐wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high‐wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two‐dimensional tomographic array. The high‐ocean‐wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only under restricted conditions.
Observations of net heat flux into the surface mixed layer of the western equatorial Pacific Ocean
Authors: Bradley E. F. , Godfrey J. S. , Coppin P. A. , Butt J. A.
Published: February 2008

For a 10‐day period during September 1990 the R/V Franklin worked around a drifting buoy drogued at 20‐m depth in the Bismarck Sea near 4°S, 149°E. Continuous measurements were made of the air‐sea fluxes of radiation and sensible and latent heat, and a conductivity/temperature/depth cast to 400 m was made about every 6 hours. The aim was to close the heat budget of a sample volume of the surface mixed layer to within 10 W m−2, in preparation for our participation in the 1992–1993 Tropical Ocean and Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE). Temperature and salinity between the surface and 30‐m depth were quite uniform, but below 30 m, variability was observed which suggested the possible intrusion of horizontal and vertical advection of heat. Heat content was analyzed for depths of 40 m and 20 m; bulk Richardson numbers generally greater than 0.8 and 0.4, respectively, in the two cases indicated that diapycnal mixing through the bottom of the 40‐m volume could be neglected at 40 m and possibly at 20 m (Peters et al., 1988; Godfrey and Lindstrom, 1989). An eddy diffusivity for salt at 20 m was obtained to account for the steady decrease of observed freshwater content in the top 20 m over that expected from the surface flux. Using this diffusivity, the turbulent heat flux through 20 m was of order 6 W m−2, supporting the view that vertical mixing of heat was small even at this depth. Then, neglecting advection and vertical mixing, the heat budget closure to 40‐m depth was satisfied to about 25 W m−2 on average over the period, but both integrated heat and freshwater time series were “noisy” because of variability below 30 m. Limited to 20‐m depth, the average difference between incident energy and heat content was reduced to about 12 W m−2, with close agreement over the diurnal cycle. The model for air‐sea exchange of sensible and latent heat by Liu et al. (1979) is verified at low wind speeds, although it may overestimate slightly for winds over the range 3–6 m s−1. This study has identified several possible sources of measurement, parameterization, and sampling error in determining the net heat flux into the ocean; however, with good sampling for the advective components, errors in heat budgets should be within the 10 W m−2 accuracy aimed for during COARE.
Observations of nonlinear interactions in directionally spread shoaling surface gravity waves
Authors: Elgar Steve , Guza R. T. , Freilich M. H.
Published: February 2008

Shoaling wave fields generated in laboratory experiments were analyzed to determine the sensitivity of nonlinear interactions to the directional distributions of incident waves. Peaks in the directional spectra observed in shallow water were consistent with near‐resonant, quadratic interactions between two primary waves transferring energy to a third wave with the sum frequency and vector sum wavenumber of the primary waves. Directionally colinear waves forced a higher‐frequency wave propagating in the same direction as the primary waves, while directionally spread (i.e., noncolinear) primary waves forced a higher‐frequency wave that propagated in a direction between those of the interacting primary waves. Deepwater wave fields with similar frequency spectra but different directional spectra evolved to different shallow‐water directional spectra, yet their shallow‐water frequency spectra were remarkably similar. This result suggests that the shape of the directional spectrum of the incident wave field has only a small effect on the magnitudes of nonlinear energy transfers during shoaling. The principal effect of directionality in the incident wave field is on the directions, not the amplitudes, of the nonlinearly generated waves. The laboratory data demonstrate clearly the importance of triad interactions between noncolinear and colinear shoaling waves.
On western boundary current separation in an upper ocean general circulation model of the north Pacific
Authors: Cherniawsky Josef , Holloway Greg
Published: February 2008

An upper ocean numerical model of the North Pacific is used to investigate the influence on western boundary current separation of different wind stress climatologies, variation of net surface buoyancy forcing, and flow exchange with the Sea of Japan. It was found previously (Cherniawsky and Holloway, 1991) that the modeled Kuroshio splits into two branches near 35°N. The stronger (Kuroshio‐like) flow separates east, while a weaker (East Japan) coastal current continues to about 42°N. The latter is opposite to observed flow. The present study explores possible reasons for this discrepancy. When we use the Harrison (1989) instead of the Hellerman and Rosenstein (1983) monthly wind stress data, the model Oyashio front moves south by about 200 km. This frontal shift is not sufficient to reverse the direction of the East Japan current. Increasing buoyancy loss at high latitudes also enhances a southward intrusion of the Oyashio front, which is consistent with thermocline theories. However, this effect is inhibited by a small subpolar gyre and stability of upper layers in the North Pacific. Separation latitude of the model western boundary current was found to be sensitive to the opening of the Sea of Japan. The latter results in a Tsugaru Strait outflow driving a southward buoyant jet along the east coast of Japan and Kuroshio separation near 36°N, in better agreement with observations.
Vertical structure of the undertow outside the surf zone
Authors: Putrevu Uday , Svendsen Ib A.
Published: February 2008

The vertical structure of the undertow in the shoaling region outside the surf zone is quite different from that inside the surf zone. Inside the surf zone the undertow typically has a large seaward directed velocity near the bottom and either a shoreward directed or a small seaward directed velocity at trough level. Measurements show that outside the surf zone the undertow has a small seaward directed velocity near the bed and a large seaward directed velocity at trough level. In this paper we develop theoretical expressions for the undertow outside the surf zone and show that the steady streaming from the bottom boundary layer which was earlier found to have a negligible effect in the surf zone (Svendsen et al., 1987) does have a significant influence on the vertical structure of the undertow in that region.
Twenty years of balloon‐borne tropospheric aerosol measurements at Laramie, Wyoming
Authors: Hofmann David J.
Published: February 2008

Over the period 1971 to 1990, 265 high‐altitude balloons with optical particle counters were launched at Laramie, Wyoming, in a long‐term study of the stratospheric sulfate aerosol layer. The tropospheric aerosol record obtained at this background continental site is examined here. All aerosol particle size ranges display pronounced seasonal variations, with the condensation nuclei concentration and the optically active (r ≥ 0.15μm) component showing a summer maximum throughout the troposphere. The dominant source of this aerosol is believed to be photochemically driven gas to particle conversion. Larger particles (r> 1 μm) peak in spring and are probably the result of long range transport of Asian desert dust although there is evidence that sulfate aerosol is also present in these air masses. Mass estimates, assuming spherical sulfate particles, indicate an average column mass between altitudes of 2.5 km (33 m above the surface) and 10 km of about 4 and 16 mg m−2 in winter and summer, respectively. The annual average of about 10 mg m−2 is somewhat larger than estimated with model calculations for sulfate aerosol in this region but is within the uncertainties of model and measurements. Calculated optical depths vary between 0.01 and 0.04 from winter to summer, the estimated mass scattering cross section is about 3 m2 g−1 throughout the troposphere. A distinct anticorrelation exists between the optically active and the condensation nuclei components, resulting in a maximum in the mixing ratio of the latter just below the tropopause where the larger particles generally show a minimum. This relation is due to coagulation of the small, newly nucleated particles with the existing larger particles and to the competition for available condensable vapors presented by the larger particles, resulting in an effective new particle source and reservoir region occurring in the upper troposphere. There is evidence for a decreasing trend of 1.6–1.8% per year in the optically active tropospheric aerosol over the past 20 years which may be related to a similar reduction in SO2 emissions in the United States over this period.
Dimethylsulfide/cloud condensation nuclei/climate system: Relevant size‐resolved measurements of the chemical and physical properties of atmospheric aerosol particles
Authors: Quinn P. K. , Covert D. S. , Bates T. S. , Kapustin V. N. , Ramsey‐Bell D. C. , McInnes L. M.
Published: February 2008

Atmospheric aerosol particles resulting from the oxidation of dimethylsulfide (DMS) may have an impact on global climate if they result in an enhancement in the cloud condensation nuclei (CCN) number concentration and shortwave cloud albedo. To characterize and quantify relationships within the DMS/CCN/climate system, simultaneous measurements were made over the northeastern Pacific Ocean in April and May 1991 of particulate non‐sea‐salt sulfate, methanesulfonate, and ammonium mass size distributions, number size distributions of particles having diameters between 0.02 and 9.6 μm, CCN concentrations at 0.3% supersaturation, relative humidity, and temperature. Comparisons between particle mass and surface area indicate that non‐sea‐salt sulfate, methanesulfonate, and ammonium were not involved in new particle production on the 12‐ to 24‐hour time scale of the measurements. Instead, high levels of available particulate surface area resulted in the condensation of the gas phase precursors onto existing aerosol. A doubling of non‐sea‐salt sulfate, methanesulfonate, and ammonium mass corresponded to a 40 to 50% increase in number in the accumulation mode size range. Likewise, a doubling of the non‐sea‐salt sulfate mass corresponded to a 40% increase in the CCN number concentration. As methanesulfonate made up a very small fraction of the soluble particulate mass, no correlations were found between methanesulfonate mass and CCN number. In a separate experiment, measurements were made of particulate non‐sea‐salt sulfate, methanesulfonate, and ammonium mass size distributions over the central Pacific Ocean during February 1991. The percent of methanesulfonate in the supermicrometer particle size fraction was greater in these samples than in those collected over coastal waters of the northeastern Pacific. In both regions the non‐sea‐salt sulfate mass size distributions were bimodal, while ammonium was found to be concentrated in larger accumulation mode particles.
The diurnal cycle of tropical convection
Authors: Hendon Harry H. , Woodberry Karen
Published: February 2008

The diurnal cycle of tropical convection is investigated with global cloud imagery constructed from 11μm radiance measurements taken aboard six satellites. Four harmonics of the diurnal cycle are resolved in the cloud imagery with about 50 km horizontal resolution. To isolate deep convective activity from other processes which cause diurnal fluctuations in longwave radiance, an index of deep convective activity is constructed by thresholding to brightness temperatures less than 230 K. Significant diurnal amplitude of deep convection is found only over tropical landmasses. Over the tropical oceans the diurnal cycle is weak and is barely discernible from the background red spectrum of convective variance. Nonetheless, oceanic convection exhibits a systematic diurnal fluctuation with maximum intensity in the early morning. Nocturnal subsidence along the cloud‐free equator is postulated to play a role in forcing diurnal variation in the intertropical convergence zones. Other mechanisms are also implied to contribute as a similar early morning maximum in deep convection is seen even where no adjoining cloud‐free regions occur. The diurnal cycle of deep convection is found to be organized on planetary scales predominantly in nonmigrating modes (i.e., modes with phase speeds not equal to that of the Sun). The nonmigrating modes are postulated to be produced by the nonuniform distribution of convective centers. The nonmigrating, zonally symmetric component is found to be especially large. While the role of this mode in forcing diurnal variations of the tropospheric circulation (e.g., the Hadley cell) is questionable, its role in modulating the ionospheric electric potential is well established. Discrepancies with the traditional notion that cloud‐to‐ground lightning is the dominant mechanism by which the Earth is negatively charged are discussed. In particular, the probable role of nonlightning‐producing clouds (e.g., the stratiform portion of convective complexes) in diurnally modulating the negative charge on Earth is discussed.
Atmospheric nuclei in the Pacific midtroposphere: Their nature, concentration, and evolution
Authors: Clarke Antony D.
Published: February 2008

An extensive flight series was carried out during May‐June 1990 in the remote North and South Pacific free troposphere aboard the NASA DC‐8. Condensation nuclei counters and optical particle counters provided information on aerosol particles with diameters between 0.003 and 7.0 μm. Vertical profiles revealed aerosol layers to be a common feature of the free troposphere. Regions with highest aerosol mass tended to have the highest concentrations of surface‐derived nuclei but the lowest concentrations of total nuclei. Regions with lowest aerosol mass tended to have the highest concentrations of the smaller “ultrafine” condensation nuclei with diameters below 0.02 μm. Horizontal transects totaling over 35,000 km at about 9 to 10‐km altitude exhibited variability of approximately 3 orders of magnitude in both aerosol mass and number concentrations over spatial scales ranging from 1 to 1000 km. At these altitudes an approximate inverse relationship between ultrafine concentrations and the surface area of the larger aerosol was evident. Regions having lowest aerosol mass were characterized by aerosol thermal volatility, indicative of a predominately sulfuric acid composition, and by very high concentrations of ultrafine nuclei, indicative of recent homogeneous nucleation. These conditions were frequently observed but were conspicuously evident above cloud over the intertropical convergence zone. The clean, free troposphere appears to be a significant source region for new tropospheric nuclei. A simplified model of the lifetime, coagulation, and cycling of these nuclei suggests that they constitute a source of cloud condensation nuclei in the lower troposphere.
Dimethylsulfide, aerosols, and condensation nuclei over the tropical northeastern Atlantic Ocean
Authors: Putaud J.‐P. , Belviso S. , Nguyen B. C. , Mihalopoulos N.
Published: February 2008

Concentration of dimethylsuifide (DMS) in seawater, concentrations of DMS and sulfur dioxide (SO2) in the atmosphere, concentrations of methanesulfonate (MSA) and non‐sea‐salt sulfate (nss‐SO42−) in size‐segregated aerosols, and number concentration of condensation nuclei (CN) were measured during September and October 1991 in the northern tropical Atlantic Ocean in order to assess the role of DMS in CN production over this oceanic area. Radon‐222 activity and aerosol ionic composition were used to distinguish air masses with oceanic, continental, and/or polluted characters. No obvious covariation appeared between DMS and its oxidation products (SO2, H2SO4, and MSA) over the whole period of the experiment. However, the division of data into subsets according to continental tracer information allowed us to show that SO2 and nss‐SO4 concentrations correlated with DMS concentration in unpolluted air masses. MSA and nss‐SO42− were found to be mainly concentrated in particles with diameters < of 0.6 μm. Daily mean nss‐SO42− in the <0.6‐μm‐diameter range and CN concentration were correlated (R = 0.91, n = 17, P < 0.001), which suggests that H2SO4 is an important CN precursor. Atmospheric DMS and CN number daily mean concentrations also correlated (R = 0.82, n = 21, P < 0.001). However, the CN population was strongly influenced by continental inputs less than 500 km downwind of Africa, whereas DMS seemed to be able to affect the CN number concentration at about 1500 km from this continent.
Upper tropospheric relative humidity from the GOES 6.7 μm channel: Method and climatology for July 1987
Authors: Soden Brian J. , Bretherton Francis P.
Published: February 2008

This paper presents an analysis of upper tropospheric relative humidity and clouds determined from geostationary satellite observations of the upwelling infrared radiation. The 6.7‐μm channel is located near the center of a strong water vapor absorption band and under clear sky conditions is primarily sensitive to the relative humidity averaged over a depth of atmosphere extending from 200 to 500 mbar. Estimates of the clear sky radiance at 6.7‐μm are obtained by utilizing the local spatial structure of the radiance field at 11‐μm and the correlation between 11‐μm and 6.7‐μm, to discriminate between clear and cloudy pixels. This approach is demonstrated to be more reliable than cloud clearance based solely upon the local spatial structure of the 6.7‐μm channel alone and provides estimates of the clear sky 6.7‐μm brightness temperature for areas 2° × 2° of latitude and longitude which are repeatable from successive images 30 min apart to within approximately 1 K. To facilitate the interpretation of the clear sky brightness temperatures, a simplified model of the radiative transfer at 6.7‐μm is presented. This model, based upon a set of irregularly spaced, strongly absorbing, pressure‐broadened lines, demonstrates that accurate to within approximately 1 K or 10% of the actual relative humidity, the brightness temperature at 6.7‐μm is proportional to the natural logarithm of the appropriate vertical average of the relative humidity divided by the cosine of the viewing zenith angle. Estimates of upper tropospheric water vapor inferred in this way from GOES E observations are presented for July 1987. The geographic distribution reflects many well‐known features of the large‐scale atmospheric circulation. A clear dependence of the greenhouse effect of upper tropospheric water vapor upon the large‐scale dynamics is also demonstrated. Finally, the observed relationship between the upper tropospheric relative humidity and the occurrence of upper tropospheric cloud cover is presented and its implications for the parameterization of clouds in general circulation models are discussed.
One year of cloud lidar data from Dumont d'Urville (Antarctica): 1. General overview of geometrical and optical properties
Authors: Del Guasta Massimo , Morandi Marco , Stefanutti Leopoldo , Brechet Jacques , Piquad Jacques
Published: February 2008

Tropospheric clouds play a major role in climate regulation but, so far, only a few long‐term ground‐based observations, devoted to the study of the cloud optical and radiative properties, have been carried out. In this work the statistics of 1 year of coastal Antarctic cloud lidar measurements (532 nm) are shown. Cloud macrophysical and optical parameters have been retrieved from lidar returns; radiosonde data allowed us to build statistics of such quantities in terms of cloud temperature. Information about the physical phase of water and ice crystal habit has been obtained from the depolarization and extinction backscattering ratio; a change in crystal habit (and/or size) around −30°C is evidenced by the depolarization versus temperature behavior. The extinction/backscattering ratio shows a negative trend with temperature, reflecting microphysical changes. Visible extinction shows wide data dispersion, but with a marked negative trend with temperature. The relative derivatives of extinction and optical depth with temperature have been computed, such quantities result quite constant with temperature.
Modeling trace gas budgets in the troposphere: 1. Ozone and odd nitrogen
Authors: Law K. S. , Pyle J. A.
Published: February 2008

A two‐dimensional photochemical model of the atmosphere has been used to calculate dynamical and chemical budgets in the troposphere. Model distributions of O3 and NOy species are compared with observations. The seasonal variations of these species can be explained in terms of the variation of the different components of their budgets. Regional variations are apparent. In the northern hemisphere, where results represent a “mean” polluted troposphere, the O3 budget is essentially a balance between net photochemical production and loss via dry deposition in the summer. The flux from the stratosphere plays an important role in the winter and early spring. In the southern hemisphere the flux of O3 from the stratosphere again makes a significant contribution in the winter and spring. The chemical production is much reduced compared with the northern hemisphere. The NOy budget is a balance between NOx emissions and loss via wet and dry deposition of HNO3.
Effect of marine stratocumulus on TOMS ozone
Authors: Thompson A. M. , McNamara D. P. , Pickering K. E. , McPeters R. D.
Published: February 2008

The algorithm used to correct total O3 from the total ozone mapping spectrometer (TOMS) for cloud effects is based on the measured reflectivity, a climatological cloud top height, and an assumed tropospheric O3 column amount below clouds. In regions of persistent subtropical marine stratocumulus it is assumed that this introduces a positive error into total O3 because these clouds are lower than the assumed mean cloud height used in the algorithm. This appears to be confirmed by high correlation between Nimbus 7 TOMS total O3 and reflectivity data for typical regimes of persistent stratus, as identified by the international satellite cloud climatology project (ISCCP) observations. The TOMS total O3 overestimate has been computed using Nimbus 7/solar backscattered ultraviolet total O3 derived using temperature humidity infrared radiometer (THIR) data for years 1979–1984. A functional relationship between the THIR/non‐THIR total O3 difference and reflectivity is used with TOMS reflectivity to modify Nimbus 7 TOMS O3 data for selected regions and periods. The correction diminishes or eliminates a number of apparent O3 maxima, with reductions of up to 20 Dobson units (DU) in total O3 on daily maps and ∼5 DU on monthly mean O3 maps. Significant correlation between corrected TOMS O3 and reflectivity data remains because low‐altitude O3 is retrieved more efficiently over a high‐albedo surface. It is also possible that dynamical influences leading to stratocumulus formation bring O3‐enriched air into the area. These results imply that although good arguments can be made for the use of TOMS total O3 as a proxy for tropospheric O3 in the tropics, caution must be exercised in the use of daily and even monthly O3 maps in the vicinity of clouds. Further research into the TOMS algorithm in cloudy regions is required to derive reliable estimates of tropospheric O3.