bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric excitation of nonseasonal polar motion
Authors: Kuehne John , Johnson Stuart , Wilson Clark R.
Published: February 2008

Analysis of nonseasonal polar motion excitation and atmospheric mass equatorial angular momentum (EAM) over land for the period 1980–1989 reveals a clear pattern of high power and correlation during the northern hemisphere (NH) winter followed by low power and correlation during the NH summer. A special case of this pattern occurs for longer than 14 months (from January 1987 to March 1988) when the correlation throughout the NH summer remains statistically significant. During this epoch an average of 72% of the nonseasonal polar motion excitation power at frequencies between −30 and +12 cycles/yr is linearly related to atmospheric EAM over land. During the southern hemisphere winter there is significant correlation between the atmospheric EAM over midlatitude southern oceans and polar motion excitation indicating the existence of a dynamic atmosphere‐ocean excitation. The atmospheric excitation power is too small to explain the large correlation during the NH winter. The effects of winds probably account for the deficit in power. The implication of these results is that there are two main excitation sources each dominant at different seasons. Atmospheric mass redistribution over land forces polar motion during the NH winter, and a dynamic atmosphere‐ocean response is important during the SH winter.
Elastic thickness of the lithosphere in the Marquesas and Society Islands
Authors: Filmer Paul E. , McNutt Marcia K. , Wolfe Cecily J.
Published: February 2008

Analysis of sea surface gravity data along six profiles collected during the Crossgrain 2 and EW9103 expeditions to the Marquesas Islands demonstrates that the elastic plate thickness Te is remarkably uniform, with a value of 18±2 km over the entire chain. Given the 50 Ma age of the Marquesan lithosphere at the time of loading, this Te value agrees with the empirical relationship between elastic plate thickness and plate age for hotspot volcanoes in the North Pacific, Atlantic, and Indian oceans. Using more limited gravity data from the Society chain, we find that the best fitting elastic plate thickness there is resolvably higher, 23±2 km. In both cases, our values for Te are larger than those derived from analysis of satellite altimetry profiles. We attribute the low Te values derived from satellite altimetry to inaccuracies in available gridded bathymetric databases for this part of the world and the assumption of a high value for the density of the seafloor. Although our shipboard data sample only two French Polynesian island chains, the results suggest that the amount of elastic plate thinning over the South Pacific Superswell may be less than previously proposed, thus eliminating conflict between estimates of the thermal structure of the Superswell lithosphere as derived from elastic plate thickness and that derived from heat flow, depth anomalies, and Love wave dispersion.
Crustal thickness and the average depth and degree of melting in fractional melting models of passive flow beneath mid‐ocean ridges
Authors: Forsyth Donald W.
Published: February 2008

In simple models of passive flow beneath mid‐ocean ridges, fractional melting of the upwelling mantle produces unexpected relationships amongst crustal thickness, degree of partial melting, and the average depth of melting. If the degree of melting is estimated on the basis of basalt composition, then the average degree of melting is one‐third the maximum degree of melting, and the average depth of melting is about two‐thirds the maximum depth in the simplest models. The crustal thickness is about 1.5 times the product of the average fraction of melting and the height of the melting column. If the average degree of melting represented in mid‐ocean ridge basalts were 10% and melt production continued to the base of the crust, then only a 40‐km‐high residual melting column would be required to produce a 6‐km‐thick crust. In more realistic models, melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6‐km‐thick crust with average degree of melting of 6.67% or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6‐km‐thick crust with an average degree of melting of only about 5.4%. Models of composition produced by fractional melting must consider the rate of melt production and the degree of melting throughout the melting region at the points where the melt is actually produced.
Depth of the decoupling plate interface and thermal structure under arcs
Authors: Furukawa Yoshitsugu
Published: February 2008

Subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate. At depth below this mechanically decoupling plate interface, the subducting plate couples with the overlying mantle, and flow is induced in the mantle wedge. This induced flow is the main factor that controls thermal structure under arcs. Depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern Japan arc using a two‐dimensional numerical simulation of thermal structure. The estimated depth is about 70 km. The termination depth of thrust earthquakes agrees well with the depth of the coupling transition, which indicates that these earthquakes occur at the decoupling plate interface. The mechanism of this decoupling plate interface is discussed in this paper. It is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible. Petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2.0 GPa and have little temperature dependence. Termination depths of thrust earthquakes estimated from seismic studies are nearly the same (60 ∼ 80 km) for various arcs. This indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust. Across‐arc heat flow profiles observed in various subduction zones show similar features; heat flow is very low in the forearc region and increases steeply at the volcanic front. Considering that the depth of the slab is about 100 km at the volcanic front for various arcs, this heat flow feature is closely related to the depth of the slab. Thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust.
Experimental and analytical study of the dynamic response of low‐porosity brittle rocks
Authors: Nagy Geza , Murakami Hidenori , Hegemier Gilbert A. , Florence Alexander L.
Published: February 2008

Motivated by an inadequate understanding of large differences observed in particle velocities and displacements in in situ large‐scale tests in wet and dry granite, a combined experimental and analytical study was undertaken to investigate the influence of porosity, crack population, pore water content and pressure, and confining pressure on the characteristics of high‐amplitude stress wave propagation in a low‐porosity (0.8–1.0%) brittle rock under well‐controlled laboratory conditions. A unique porosity enhancement technique, using a gas fracturing process, was developed to provide a controlled, homogeneous, isotropic increase in porosity to 2.4–2.9% (a factor of 2.5–3.5) in Sierra White granite. Combined with a technique to independently control confining and pore pressures, the experiments provided measured radial particle velocities, calculated displacements, and peak velocity and displacement attenuations. In the experiments, key parametric effects due to fracture‐induced porosity increase combined with pore water content were observed. In particular, a significant porosity (and accompanying fracture) increase combined with fully saturated, undrained condition (no effective stress) resulted in a substantial decrease in peak velocities and increased pulse durations, leading to greatly increased particle displacements. Reducing the pore pressure in enhanced‐porosity rock produces substantially smaller displacements because of narrower pulses. It was concluded that in the region where deviatoric response is important, anisotropic damage evolution through compression‐induced fracture formation rather than the resulting porosity increase itself appears to dominate material response. A phenomenological plastic damage model allowing damage accumulation under compression‐induced (splitting) cracking was used to numerically simulate the key experimental observations.
The configuration of Asia prior to the collision of India: Cretaceous paleomagnetic constraints
Authors: Chen Yan , Courtillot Vincent , Cogné Jean‐Pascal , Besse Jean , Yang Zhenyu , Enkin Randy
Published: February 2008

Paleomagnetic data from Central Asia show that 1700±610 km of shortening of southern Asia since Cretaceous time have been absorbed by distributed deformation between southern Tibet and the Siberia craton. This result is based on a compilation of Cretaceous poles from the Junggar, Tarim, Tibet, Indochina, South China, North China, and Mongolia blocks, complementing the recent compilation of Enkin et al. (1992a). We propose a paleogeographic reconstruction of Asia in the Cretaceous, in which the position of Siberia is derived from the synthetic apparent polar wander path of Besse and Courtillot (1991). The resulting map, which likely represents Asia as it remained throughout the Cretaceous until the collision with India began, features an “unbent” Tibet, with an east‐west trending Andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from Sichuan to Tarim through Tibet. The map allows one to estimate continental shortening and rotations between the blocks, which are attributed to the collision. Despite large uncertainties, these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of Avouac (1991).

Appendix is available with entire paper on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, DC 20009. Document B93‐007; $2.50. Payment must accompany order.
In situ high P‐T X ray diffraction studies on three polymorphs (α, β, γ) of Mg2SiO4
Authors: Meng Y. , Weidner D. J. , Gwanmesia G. D. , Liebermann R. C. , Vaughan M. T. , Wang Y. , Leinenweber K. , Pacalo R. E. , Yeganeh‐Haeri A. , Zhao Y.
Published: February 2008

Unit cell volumes of the beta (β) and spinel (γ) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume‐temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the β phase from 724 to 872 K at 7.6 GPa to be 2.28(±0.45)×10−5/K and of the γ phase from 759 to 962 K at 9.8 GPa to be 1.71(±0.14)×10−5/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (KT) from the high‐pressure thermal expansion data: (∂KT/∂T)P is found to be −2.7(±0.5)×10−2 GPa/K for the β phase and −2.8(±0.3)×10−2 GPs/K for the γ phase. Unit cell volumes of the olivine (α) phase, back‐transformed from the β and γ phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (∂KT/∂T)P for the α phase to be −2.1(±0.2)×10−2 GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the β to γ phase transition near 520 km depth.
Tectonic sediment thickening, fluid expulsion, and the thermal regime of subduction zone accretionary prisms: The Cascadia Margin off Vancouver Island
Authors: Hyndman R. D. , Wang K. , Yuan T. , Spence G. D.
Published: February 2008

Tectonic sediment thickening, fluid expulsion, and the resulting thermal regime of the northern Cascadia subduction zone accretionary prism have been described by numerical models. Both the sediment thickening and the fluid expulsion are found to have important thermal effects. Constraints on the models are provided by (1) detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom‐simulating reflector (BSR), and (2) porosity changes across the accretion zone inferred from the landward variation in the P wave velocity‐depth profiles. The heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20% below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes. The porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea Cascadia basin. The model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically, generating a high porosity underconsolidated section. Reconsolidation and fluid expulsion in response to the tectonic thickening is slow, occurring mainly 20–30 km landward of the deformation front. The maximum predicted fluid expulsion rate is about 4 × 10−11 m s−1 (1.3 mm a−1). The pore pressure distribution across the deformation front region has been estimated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure; pore pressures are high, reaching 80% of lithostatic 15 to 20 km landward of the deformation front. An estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10−16 m2 near the top to about 10−18 m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data. The prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism.
Optimum multiparameter analysis of the water mass structure in the western North Atlantic Ocean
Authors: Hinrichsen Hans‐Harald , Tomczak Matthias
Published: February 2008

Hydrographic data of temperature, salinity, oxygen, nitrate, phosphate, and silicate at 81 stations with 435 samples on 3 sections between the Azores, the Grand Banks of Newfoundland, and the Bermuda Islands are used to determine the mixing of water masses by optimum multiparameter analysis over the depth range 100–1500 m. The method optimally utilizes all information from our hydrographic data set by solving an overdetermined set of linear mixing equations for all parameters using the method of least squares residuals. It is shown that the method gives quantitative information on the influence of the various water masses of the western North Atlantic. The Gulf Stream and the North Atlantic Current appear as broad bands transporting large amounts of Western North Atlantic Central Water at their warm flank. Western Subarctic Intermediate Water and Shelf Water supplied by the Labrador Current and containing significant amounts of Labrador Current Water are found on their inshore side. The area of the Azores front is found in the vicinity of the Comer Seamounts, where the uniform water mass distribution of the Sargasso Sea changes into a more complex structure that reflects the influence of water masses originating in the Labrador Sea. Small‐scale structures, like eddies or Gulf Stream rings, are also detectable by this analysis method. Comparison with dynamic height analysis supports the circulation pattern of the North Atlantic Current as a continuation of the Gulf Stream, and of the southeastward flowing Azores Current originating in the area of the Southeast Newfoundland Rise.
Seasonal variations in equatorial Atlantic Ocean zonal volume transport at 28°W
Authors: Tang T. Y. , Weisberg Robert H.
Published: February 2008

Zonal volume transport is examined over the upper 150 m of the water column at 28°W, using the Seasonal Response of the Equatorial Atlantic experiment moored current meter data at 0.75°S, 0.0°, and 0.75°N and a time domain empirical orthogonal function (EOF) analysis to improve spatial and temporal resolution. Gaussian distributions are fit to the mean volume transports per unit width and the fluctuations about these means that are found to be represented by a single EOF mode. The mean volume transport is distributed symmetrically both about the equator and about 75 m depth, with a meridional scale width of 101 km and a magnitude of 20.3 Sv. The standard deviation for the fluctuations is 4.7 Sv, and in contrast to the mean the fluctuations are confined to above 100 m depth, with a larger meridional scale width. The scale width for the mean is consistent with an inertial scale associated with conservation of potential vorticity, while the scale width for the fluctuations is consistent with an equatorial Rossby radius of deformation associated with equatorially trapped waves. These findings support a hypothesis that different physical processes are controlling the volume transport, as integrated from the surface across the Equatorial Undercurrent, over different time scales. On the time scale of the record length average, inertial dynamics appear to be controlling, while on the time scales of the seasonal fluctuations, linear equatorial long‐wave dynamics appear to be controlling. However, given the limited record length, there are insufficient degrees of freedom to test this hypothesis statistically.
Solution of the linear shallow water equations by the fourth‐order leapfrog scheme
Authors: Kowalik Z.
Published: February 2008

Numerical schemes of the first and second order of approximation introduce numerical distortion when the wave propagation over a long distance is investigated. To alleviate this problem, the fourth‐order leapfrog scheme is constructed. The standard leapfrog method is based on the truncated Taylor series expansion which depicts an error proportional to the second‐order terms. In the proposed method the numerical solution is corrected for these terms. The space and time corrections work well in diminishing numerical dispersion and dissipation.
Roughness of Weddell Sea ice and estimates of the air‐ice drag coefficient
Authors: Andreas Edgar L , Lange Manfred A. , Ackley Stephen F. , Wadhams Peter
Published: February 2008

The roughness of a sheet of sea ice encodes its deformational history and determines its aerodynamic coupling with the overlying air and underlying water. Here we report snow surface, ice surface, and ice underside roughness computed from 47 surface elevation profiles collected during a transect of the Weddell Sea. The roughness for each surface, parameterized as the standard deviation of the surface elevation, segregates according to whether or not a floe has been deformed: deformed ice has greater roughness than undeformed ice. Regardless of deformational history, the underside roughness is almost always greater than the snow surface and ice surface roughnesses, which are nearly equal. Roughness spectra for all three surfaces and for both deformed and undeformed ice roll off roughly as k−1 when the wavenumber k is between 0.1 and 3 rad m−1. The snow surface and underside spectra roll off somewhat faster than k−1, and the ice surface spectra roll off somewhat slower than k−1. Both top and underside Arctic ice roughness spectra, on the other hand, have been reported to roll off faster than k−2. We speculate that the excess spectral intensity at high wavenumbers in the Antarctic ice surface spectra results from the small‐scale roughness that the ice sheet had on consolidation. This excess high‐wavenumber spectral intensity persists in the ice surface spectra of second‐year ice. Evidently, once formed, the ice surface remains unchanged on the microscale until the entire ice sheet melts. With a remote measurement of roughness, we should be able to decide whether an ice floe is deformed or undeformed. Our spectral analysis hints that remote sensing may also be able to differentiate between first‐year and second‐year ice. From the snow surface spectra, we compute a roughness scale ξ that parameterizes the air‐ice momentum coupling and lets us estimate the neutral stability drag coefficient referenced to a height of 10 m, CDN10. Typical CDN10 values are 1.1–1.4 × 10−3 over undeformed ice and 1.3–1.8 × 10−3 over deformed ice.
An intermediate nepheloid layer associated with high microbial metabolic rates and denitrification in the northwest Indian Ocean
Authors: Naqvi S. W. A. , Kumar M. D. , Narvekar P. V. , De Sousa S. N. , George M. D. , D'Silva C.
Published: February 2008

Extensive optical, physical, chemical, and biochemical measurements made simultaneously in the northwest Indian Ocean reveal the occurrence of an intermediate nepheloid layer (INL) invariably associated with the secondary nitrite maximum. Maxima in particulate protein and the activity of the respiratory electron transport system (ETS) are also found within the INL. Since the INL persists long distances from the continental margin with an offshore intensification, it may not be related to the transport of material resuspended along the continental margin. An apparent correlation of the INL with the previously reported subsurface maximum in bacterial abundance suggests that a local increase in the abundance of bacteria may be responsible for the increased turbidity. Positive correlations of the beam attenuation anomaly with nitrite and nitrate deficit suggest that most of these bacteria may be denitrifiers. The organic carbon demand within the denitrifying layer, computed from the observed ETS activity, appears to be severalfold higher than the sinking carbon fluxes to the denitrifying layer, requiring additional modes of supply of the biodegradable organic matter. It is proposed that a bacterial maximum could be maintained with efficient utilization of the dissolved organic matter within the denitrifying waters.
Sea surface height fluctuations observed simultaneously with inverted echo sounders and Geosat
Authors: Hallock Zachariah R. , Teague William J.
Published: February 2008

Time series of sea surface height anomaly as measured by inverted echo sounders with pressure gauges (IES/PGs) and Geosat altimetry are compared at six locations in the northwest Atlantic near 40°N, 55°W. Three IES/PG sites lay along each of two adjacent Geosat‐Exact Repeat Mission ground tracks. Better agreement was found along the westernmost ground track; pressure sensor noise is the probable source of the larger errors along the easternmost ground track. The overall correlation is 0.78; along the westernmost ground track it is 0.91. Decorrelation scales are about 100 km along track and about 5 days. Uncorrelated errors dominate, but along the westernmost ground track, IES/PG‐derived sea surface height variability is systematically smaller than that derived from Geosat data; this factor, determined by regression analysis, is about 0.81. However, a consistent adjustment of IES calibration removes the systematic discrepancy.
Natural oil slicks in the Gulf of Mexico visible from space
Authors: Macdonald I. R. , Guinasso N. L. , Ackleson S. G. , Amos J. F. , Duckworth R. , Sassen R. , Brooks J. M.
Published: February 2008

Natural oil seepage in the Gulf of Mexico causes persistent surface slicks that are visible from space in predictable locations. A photograph of the sun glint pattern offshore from Louisiana taken from the space shuttle Atlantis on May 5, 1989, shows at least 124 slicks in an area of about 15,000 km2; a thematic mapper (TM) image collected by the Landsat orbiter on July 31, 1991, shows at least 66 slicks in a cloud‐free area of 8200 km2 that overlaps the area of the photograph. Samples and descriptions made from a surface ship, from aircraft, and from a submarine confirmed the presence of crude oil in floating slicks. The imagery data show surface slicks near eight locations where chemosynthetic communities dependent upon seeping hydrocarbons are known to occur on the seafloor. Additionally, a large surface slick above the location of an active mud volcano was evident in the TM image. In one location the combined set of observations confirmed the presence of a flourishing chemosynthetic community, active seafloor oil and gas seepage, crude oil on the sea surface, and slick features that were visible in both images. We derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate, downstream movement on the surface, half‐life of floating oil, and threshold thickness for detection. Applying this equation to the lengths of observed slicks suggested that the slicks in the Atlantis photograph and in the TM image represent seepage rates of 2.2–30 m3 1000 km−2 d−1 and 1.4–18 m3 1000 km−2 d−1, respectively. Generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20,000 m3 yr−1 (120,000 barrels yr−1).
Response of lead cycling in the surface Sargasso Sea to changes in tropospheric input
Authors: Veron Alain J. , Church Thomas M. , Flegal A. Russell , Patterson Clair C. , Erel Yigal
Published: February 2008

Lead and its stable isotopes have been analyzed in surface water samples (0–600 m) and trapped particles collected at the Bermuda Atlantic Time Series Station (U.S. Joint Global Ocean Flux Study) in April and November 1989. These results are compared with wet atmospheric lead deposition as determined from precipitation continuously collected in Bermuda since August 1988 as pan of the Atmosphere‐Ocean Chemistry Experiment program. Despite an expected seasonal variability, lead concentrations in surface waters have clearly decreased by 30 to 40% since 1979 in response to a corresponding decline by a factor of 5 to 8 of the tropospheric deposition. This result is corroborated by stable lead isotope measurements with 206Pb/207Pb ratios which are significantly less radiogenic (1.18–1.20) in the first 500 m of the 1989 profile than those measured in 1984 (1.20–1.21). This isotopic shift reflects changes of lead ore supply in the United States as well as a relative increase of the Eurafrican contribution to lead input in the northwest Atlantic that is likely due to the reduction of lead emissions from gasoline consumption in North America. This fast response of lead to interannual variations of the tropospheric input into surface waters is related to its efficient bioreactivity, as demonstrated with a sediment trap deployed at 150 m in April 1989. Sediment trap results show the rapid penetration of lead into the first 200 m associated with large particles during a period of high plankton activity. Retrospective isentropic air mass trajectories in three dimensions coupled with the precipitation events collected in Bermuda in 1988–1989 show that 30 to 40% of the annual lead deposition originates from the trade easterly meteorological regime. This input is clearly evidenced with lead isotopic signals observed in surface waters (0–100 m) in April and November 1989. We show that lead accumulated in the seasonal mixed layer (0–50 m) reflects this atmospheric input. Taking into account the isotopic signal measured in this mixed layer (1.178±0.001) as well as the respective contribution of the temperate westerlies and trade easterlies to atmospheric lead deposition to the Sargasso Sea, we calculate that the isotopic signature from the Eurafrican regions is 1.155 (±0.004). Based on these results, we calculate that the actual 206Pb/207Pb average ratio in surface waters of the Sargasso Sea is 1.188 (±0.004).
Excitation of seiche observed in a small harbor
Authors: Okihiro Michele , Guza R. T. , Seymour R. J.
Published: February 2008

Seiche measured within a small (0.6 by 0.6 km), shallow (12‐m depth) harbor is dominated by oscillations in several narrow infragravity frequency bands between approximately 10−3 and 10−2 Hz. Energy levels within the harbor are amplified, relative to just outside the harbor in 8.5‐m depth, by as much as a factor of 20 at the lowest (grave mode) resonant frequency (∼10−3 Hz) compared to amplifications of roughly 5 at higher resonant frequencies (∼10−2 Hz). At nonresonant frequencies, energy levels observed inside the harbor are lower than those outside. These amplifications are compared to predictions of a numerical model of seiche excited by linear, inviscid long waves impinging on a harbor of variable depth. The amplification of higher‐frequency (∼10−2‐Hz) seiches is predicted within a factor of about 2. However, at the grave mode (10−3 Hz), the observed amplification decreases with increasing swell and seiche energy levels, possibly owing to the sensitivity of this highly amplified mode to dissipation not included in the inviscid model. The energy levels of higher‐frequency seiche within the harbor were predicted from the offshore sea and swell spectra by the ad hoc coupling of the linear model for the amplification of harbor modes with a nonlinear model for the generation of bound infragravity waves outside the harbor. The predictions are qualitatively accurate only when the swell is energetic and bound waves are a significant fraction of the infragravity energy outside the harbor.
Production of spume drops by the wind tearing of wave crests: The search for quantification
Authors: Wu Jin
Published: February 2008

Much attention has been attracted recently to spume drops produced by the wind tearing of wave crests. Two models of their generation were proposed (Monahan et al., 1986; Andreas, 1992) on the basis of our general observations on spray, one in a wind‐wave tank (Wu, 1973) and the other in the field (Wu et al., 1984); these models are critically reviewed. All relevant measurements of spray droplets in tanks and the field are analyzed to deduce the production spectrum of spume drops. While the present generation function appears to adhere closely with physical conditions and experimental evidence of spume drop production, wide discrepancies existing among models demand direct observations of this production by tearing.
An estimate of the Geosat altimeter wind speed algorithm at high wind speeds
Authors: Young Ian R.
Published: February 2008

Existing wind speed algorithms for satellite altimeters have been developed from data in the range 0 to 20 m/s. At higher wind speeds the available algorithms diverge alarmingly. By comparing Geosat altimeter values of the radar cross section with model predictions of the surface winds during satellite overpasses of tropical cyclones, a wind speed algorithm valid for wind speeds between 20 and 40 m/s is developed. There is, however, considerable scatter in these data. An error analysis shows that this observed scatter is consistent with the accuracy to which the tropical cyclone wind fields can be inferred. An approximately linear dependence of the radar cross section on wind speed is, however, clearly evident within these data. At the lower limit of its applicability (U10 = 20 m/s), these data are consistent with the previously derived algorithms. At its upper limit (U10 = 40 m/s), mean squared slope values inferred from the radar cross section are consistent with the theoretical upper limit.
Climatic atmospheric outflow at the rim of the Antarctic continent
Authors: Dalu G. A. , Baldi M. , Moran M. D. , Nardone C. , Sbano L.
Published: February 2008

Using Gauss's divergence theorem, we examine the role of the net radiative cooling and of the entrainment of potentially warm air from the free atmosphere into the surface layer in determining the intensity of the cold‐air outflow at the rim of the Antarctic continent. The result shows that a large amount of cold air, significant at even a global climatological scale, is exported through this process from high‐ to low‐latitude regions. The theory gives also the average intensity of the wind at the coast. In the coastal regions of Antarctica the wind exhibits a complex behavior. Observations show that katabatic winds in the near‐coastal region can be periodic or continuous, or can stay a long time in a quiescent near‐critical state, then burst in short and strong gusts. We examine some aspects of these katabatic winds in terms of the mesoscale energy theory developed by Dalu and Green (1983). This complex behavior is described by a nonlinear system, where the relevant parameter is the Froude number of the inflow at a Rossby radius distance from the coast. The analytical theory developed agrees well with qualitative features observed.
Occurrence of ozone laminae near the boundary of the stratospheric polar vortex
Authors: Reid S. J. , Vaughan G. , Kyro E.
Published: February 2008

Analysis of ozonesonde data collected at high northern latitudes in winter and spring shows that laminae of enhanced and depleted ozone are associated with the polar vortex. In January and February, they are most common at all latitudes in the potential temperature range 370–430 K, but are abundant up to 500 K between 60° and 70°N. In March and April they occur most frequently northward of 75 N, and are abundant up to 520 K, whereas they are largely confined to the range 320–440 K at lower latitudes. Analysis of data gathered during the Airborne Arctic Stratospheric Expedition (AASE) in 1989 confirms that laminae are confined below 420 K in January and February when the 400K potential vorticity exceeds 15 μK kg−1 m2 s−1, i.e., the polar vortex boundary. Further analysis of ozone lidar data obtained during AASE depicts clearly the extrusion of laminae of enhanced ozone concentration from the polar regions in the altitude range 13–15 km. These extrusions form a class of laminae which transport ozone equatorward in the lowest levels of the stratosphere. Possible dynamical mechanisms for these laminae are discussed.
Implications of three‐dimensional tracer studies for two‐dimensional assessments of the impact of supersonic aircraft on stratospheric ozone
Authors: Douglass Anne R. , Rood Richard B. , Weaver Clark J. , Cerniglia Mark C. , Brueske Kurt F.
Published: February 2008

A two‐dimensional (2D) model, which uses a residual circulation and diffusion, and a three‐dimensional (3D) model, which uses winds from a stratospheric data assimilation system, have been used to estimate the transport and dispersion of aircraft exhaust (tracer) in the lower stratosphere. Four month calculations using the 2D model with tracer injected continuously between 40°–50° north and south latitudes are compared with similar 3D calculations for the same time period. The seasonal behavior of the tracer fields in the two models is similar. The zonal mean of the 3D tracer distribution resembles the 2D distribution when the 2D calculation uses a residual circulation derived from the assimilated wind fields, but the 3D distribution indicates more rapid vertical mixing. The similarity of the 2D and 3D tracer distributions suggests the similarity of the seasonal mean mass transport in both models. However, there is a significant difference in the placement of stratosphere/troposphere exchange in the two models. In the 2D model, tracer transport to the troposphere takes place mostly at high latitudes. In the 3D model, most tracer transport takes place at middle latitudes, and is clearly associated with synoptic scale events. This may be particularly important to assessment calculations, as the pollutant source is mostly in middle latitudes. The 3D model is also used to consider the buildup of tracer in oceanic flight corridors. North Atlantic (Boston‐London), north Pacific (Los Angeles ‐ Tokyo) and tropical (Los Angeles‐Sydney) corridors are considered. For northern hemisphere winter, the tracer distributions remain zonally asymmetric. The tracer from Boston‐London is largely excluded from the Aleutian anti‐cyclone, and the tracer from Los Angeles ‐ Tokyo is largely contained within the anticyclone. The tropical tracer distribution is also asymmetric; the time scale for zonal mixing is long compared to the time for meridional transport processes due to weak zonal winds. Although more of the tracer injected in the tropical corridor is transported to higher altitude than for the other corridors, the transfer of mass from the stratosphere to the troposphere is nearly the same for the three corridors. There are no systematic differences that suggest that one corridor is inherently more or less polluting than another.
Ozone observations and a model of marine boundary layer photochemistry during SAGA 3
Authors: Thompson A. M. , Johnson J. E. , Torres A. L. , Bates T. S. , Kelly K. C. , Atlas E. , Greenberg J. P. , Donahue N. M. , Yvon S. A. , Saltzman E. S. , Heikes B. G. , Mosher B. W. , Shashkov A. A. , Yegorov V. I.
Published: February 2008

A major purpose of the third joint Soviet‐American Gases and Aerosols (SAGA 3) oceanographic cruise was to examine remote tropical marine O3 and photochemical cycles in detail. On leg 1, which took place between Hilo, Hawaii, and Pago‐Pago, American Samoa, in February and March 1990, shipboard measurements were made of O3, CO, CH4, nonmethane hydrocarbons (NMHC), NO, dimethyl sulfide (DMS), H2S, H2O2, organic peroxides, and total column O3. Postcruise analysis was performed for alkyl nitrates and a second set of nonmethane hydrocarbons. A latitudinal gradient in O3 was observed on SAGA 3, with O3 north of the intertropical convergence zone (ITCZ) at 15–20 parts per billion by volume (ppbv) and less than 12 ppbv south of the ITCZ but never ≤3 ppbv as observed on some previous equatorial Pacific cruises (Piotrowicz et al., 1986; Johnson et al., 1990). Total column O3 (230–250 Dobson units (DU)) measured from the Akademik Korolev was within 8% of the corresponding total ozone mapping spectrometer (TOMS) satellite observations and confirmed the equatorial Pacific as a low O3 region. In terms of number of constituents measured, SAGA 3 may be the most photochemically complete at‐sea experiment to date. A one‐dimensional photochemical model gives a self‐consistent picture of O3‐NO‐CO‐hydrocarbon interactions taking place during SAGA 3. At typical equatorial conditions, mean O3 is 10 ppbv with a 10–15% diurnal variation and maximum near sunrise. Measurements of O3, CO, CH4, NMHC, and H2O constrain model‐calculated OH to 9 × 105 cm−3 for 10 ppbv O3 at the equator. For DMS (300–400 parts per trillion by volume (pptv)) this OH abundance requires a sea‐to‐air flux of 6–8 × 109 cm−2 s−1, which is within the uncertainty range of the flux deduced from SAGA 3 measurements of DMS in seawater (Bates et al., this issue). The concentrations of alkyl nitrates on SAGA 3 (5–15 pptv total alkyl nitrates) were up to 6 times higher than expected from currently accepted kinetics, suggesting a largely continental source for these species. However, maxima in isopropyl nitrate and bromoform near the equator (Atlas et al., this issue) as well as for nitric oxide (Torres and Thompson, this issue) may signify photochemical and biological sources of these species.
A physical retrieval of cloud liquid water over the global oceans using special sensor microwave/imager (SSM/I) observations
Authors: Greenwald Thomas J. , Stephens Graeme L. , Vonder Haar Thomas H. , Jackson Darren L.
Published: February 2008

A method of remotely sensing integrated cloud liquid water over the oceans using spaceborne passive measurements from the special sensor microwave/imager (SSM/I) is described. The technique is comprised of a simple physical model that uses the 19.35‐ and 37‐GHz channels of the SSM/I. The most comprehensive validation to date of cloud liquid water estimated from satellites is presented. This is accomplished through a comparison to independent ground‐based microwave radiometer measurements of liquid water on San Nicolas Island, over the North Sea, and on Kwajalein and Saipan Islands in the western Pacific. In areas of marine stratocumulus clouds off the coast of California a further comparison is made to liquid water inferred from advanced very high resolution radiometer (AVHRR) visible reflectance measurements. The results are also compared qualitatively with near‐coincident satellite imagery and with other existing microwave methods in selected regions. These comparisons indicate that the liquid water amounts derived from the simple scheme are consistent with the ground‐based measurements for nonprecipitating cloud systems in the subtropics and middle to high latitudes. The comparison in the tropics, however, was less conclusive. Nevertheless, the retrieval method appears to have general applicability over most areas of the global oceans. An observational measure of the minimum uncertainty in the retrievals is determined in a limited number of known cloud‐free areas, where the liquid water amounts are found to have a low variability of 0.016 kg m−2. A simple sensitivity and error analysis suggests that the liquid water estimates have a theoretical relative error typically ranging from about 25% to near 40% depending on the atmospheric/surface conditions and on the amount of liquid water present in the cloud. For the global oceans as a whole the average cloud liquid water is determined to be about 0.08 kg m−2. The major conclusion of this paper is that reasonably accurate amounts of cloud liquid water can be retrieved from SSM/I observations for nonprecipitating cloud systems, particularly in areas of persistent stratocumulus clouds, with less accurate retrievals in tropical regions.
Negatively charged precipitation in a New Mexico thunderstorm
Authors: Marshall Thomas C. , Marsh Stephen J.
Published: February 2008

Substantial quantities of negatively charged precipitation were found in the main negative charge center of a small New Mexico thunderstorm. The charge density of the negative precipitation was equal to or greater than the deduced total charge density of the region.
Multi‐isotope study of the O(1D) + CO2 exchange and stratospheric consequences
Authors: Wen J. , Thiemens Mark H.
Published: February 2008

The first multi‐isotope (δ17O and δ18O) study of an isotopic exchange between an electronically excited species O(1D) and CO2 has been performed. The O(1D), which was generated from O3 UV photolysis, undergoes an isotopic exchange with CO2 via a CO3* intermediate transition state. A non‐mass‐dependent isotopic effect is observed via the exchange, which has a direct application to recent stratospheric CO2 observations and analysis of water presumed to be derived from the Martian surface. The isotopic data are also important in assessing the physical‐chemical properties of the CO3* complex.
Common physical processes in natural and artificially triggered lightning
Authors: Mazur Vladislav , Ruhnke Lothar H.
Published: February 2008

Common physical processes are identified in various types of natural and artificially triggered lightning flashes, both in summer and winter storms. By applying an electrostatic model of bidirectional, uncharged and monopolar, charged leaders, the main physical principles are defined for interpretation of common lightning processes. These principles focus on lightning initiation, charges on the leader, the leader's electrical potential, the electrical break‐down at the leader tip, leader branching and current cutoff, the occurrence of recoil streamers, and the conclusion of lightning propagation. The bidirectional, uncharged leader model is compared, on both physical grounds and by analyzing the electric field changes, with the conventional model of a unidirectional, uniformly charged leader which originates from a space charge source. These two models are also tested against experimental data obtained with a VHF mapping system and in situ measurements of electric fields in storms on the heights of lightning origins in cloud‐to‐ground flashes.
Correlations of atmospheric dynamics with solar activity evidence for a connection via the solar wind, atmospheric electricity, and cloud microphysics
Authors: Tinsley Brian A. , Heelis Roderick A.
Published: February 2008

We respond to several criticisms of the view that there is a physical linkage between solar activity and the dynamics of the troposphere and lower stratosphere, and we provide further evidence in support of a mechanism for such a linkage involving atmospheric electricity and cloud microphysics. The main criticisms are (1) that the decadal time scale variations in stratified data result from aliasing introduced by the sampling process and are not responses to a decadal time scale physical input; (2) that the observed correlations are due to chance coincidence or an atmospheric periodicity that is not uniquely related to solar variability; and (3) that there are no plausible mechanisms that can amplify one of the weak solar‐varying inputs in the region where the correlations are found. We show that the aliasing criticism is inadequate because the real quasi‐biennial oscillation departs from an ideal sine wave in a way that reduces aliasing effects to insignificant levels. The nonuniqueness of identification of the 11‐year solar cycle as the period of the arctic forcing for the Arctic winter stratospheric temperatures is a problem only for the short 33‐year record of polar temperatures; in much longer time series of unstratified climate data the periods of 11 and 22 years are prominent. Highly unique signatures of solar wind forcing of tropospheric dynamics exist on the day‐to‐day time scale via two independent inputs to atmospheric electricity. These are (1) through changes in tropospheric ion production as a result of solar wind modulation of galactic cosmic rays and (2) through changes in the potential difference between the polar ionospheres and the surface, forced by the solar wind By component. The product of the cosmic ray flux and the ionospheric potential determines the vertical air‐earth electrical current. In the presence of clouds of large horizontal extent, this current determines the rate of polarization charging of the clouds via the accumulation of positive electrostatic charges on droplets near cloud tops. The observed correlations, and theoretical and laboratory results for the effects of electrostatic charges on droplets and aerosols on the rates of ice nucleation, are consistent with the postulate that for certain regions and seasons and atmospheric levels the large‐scale atmospheric electrical parameters have significant effects on the rates of initial ice nucleation. In such cases the chain of consequences includes changes in the rates of precipitation, net latent heat release, vertical motions, atmospheric vorticity, and ultimately in the general circulation. Much more work is required before the mechanism can be considered to have a secure basis in laboratory experiment and quantitative atmospheric modeling.
Trace gas exchange above the floor of a deciduous forest: 2. SO2 and O3 deposition
Authors: Meyers Tilden P. , Baldocchi Dennis D.
Published: February 2008

The pollutant uptake capacity of the surface beneath forest canopies is poorly understood. In this study we show that the eddy correlation method can be applied within a canopy. Direct measurements of SO2 and O3 deposition to the forest floor of two deciduous forests were made with the eddy correlation method. For dry periods the surface uptake resistance for SO2 was of the order of 700 s m−1. When the soil and litter were moist, the SO2 surface uptake resistance was small, resulting in deposition velocities at the forest floor that were about 2 times greater than during dry periods. SO2 deposition to the forest floor constitutes between 20 and 30% of total uptake to the forest ecosystem when the surface is moist. Soil surface uptake resistances for O3 were about 2000 s m−1 for both wet and dry periods.