bluuebunny/crossref_metadata_2025 huggingface dataset:
Mapping the distribution of vesicular textures on silicic lavas using the thermal infrared multispectral scanner
Authors: Ondrusek Jaime , Christensen Philip R. , Fink Jonathan H.
Published: February 2008

The Thermal Infrared Multispectral Scanner (TIMS) is a potentially powerful tool for mapping detailed chemical variations in silicic lava flows which in turn could expand knowledge of pre‐eruption compositional gradients and mixing processes in silicic magma bodies. However, thermal infrared image data may be greatly influenced by the range of vesicular textures which occur on silicic flows. To investigate the effect of vesicularity on TIMS imagery independent of chemical variations, we studied Little Glass Mountain at the Medicine Lake Volcano of northern California, a large rhyolitic flow of uniform composition but textural heterogeneity. The imagery was recalibrated so that the digital number values for a lake in the scene matched a calculated ideal spectrum for water. TIMS spectra for the lava show useful differences in coarsely and finely vesicular pumice data, particularly in TIMS bands 3 and 4. Images generated by ratioing these bands accurately map out those areas known from field studies to be coarsely vesicular pumice. These texture‐related emissivity variations are probably due to the larger vesicles being relatively deeper and separated by smaller septa leaving less smooth glass available to give the characteristic emission of the lava. In studies of inaccessible lava flows (as on Mars) areas of coarsely vesicular pumice must be identified and avoided before chemical variations can be interpreted. Remotely determined distributions of vesicular and glassy textures can also be related to the volatile contents and potential hazards associated with the emplacement of silicic lava flows on Earth.
Overprint magnetizations in support of northward displacement of the Chugach‐Prince William Terrane, Alaska
Authors: Bol Alan J.
Published: February 2008

Widespread discrepancies between geologic and paleomagnetic estimates of northward displacement are ubiquitous in reconstructions of the North American Cordillera. In southern Alaska, this discrepancy persists into Paleogene time. Paleomagnetic data indicate that the Mesozoic arc complex of the Southern Alaska terrane was in place by Late Cretaceous‐early Tertiary time, while the outboard Mesozoic and Tertiary accretionary complex of the Chugach‐Prince William terrane still lay far to the south. The apparent Tertiary displacement of the Chugach‐Prince William terrane is difficult to reconcile with most apparent geologic constraints. I studied the paleomagnetism of early Tertiary mafic volcanic rocks from Prince William Sound, Alaska, in an attempt to corroborate the meager paleomagnetic database. I sampled at Knight Island and Glacier Island and found overprint magnetizations. At Knight Island, the paleomagnetic directions are too scattered for tectonic interpretation. At Glacier Island, I found three overprint magnetizations. The type 1 magnetization has a normal direction and is concordant with the expected direction for North America. The type 2N magnetization yields a characteristic direction which is too scattered for tectonic interpretation. The type 2R magnetization is reversed and implies 17° ± 10° of northward displacement with respect to North America assuming that the magnetization was acquired at 55 Ma and the ophiolite was not subsequently tilted. The type 2R magnetization agrees remarkably well with paleomagnetic results from the nearby Resurrection Peninsula in terms of inferred displacement, paleotrend of the sheeted dike complex, and reversed polarity of magnetization. The type 1 magnetization was subsequently acquired after the Chugach‐Prince William terrane arrived in southern Alaska.
The ambiguity in ray perturbation theory
Authors: Snieder Roel , Sambridge Malcolm
Published: February 2008

Ray perturbation theory is concerned with the change in ray paths and travel times due to changes in the slowness model or the end‐point conditions of rays. Several different formulations of ray perturbation theory have been developed. Even for the same physical problem different perturbation equations have been derived. The reason for this is that ray perturbation theory contains a fundamental ambiguity. One can move a point along a curve without changing the shape of the curve. This means that the mapping from a reference curve to a perturbed curve is not uniquely defined, because one may associate a point on the reference curve with different points on the perturbed curve. The mapping that is used is usually defined implicitly by the choice of the coordinate system or the independent parameter. In this paper, a formalism is developed where one can specify explicitly the mapping from the reference curve to the perturbed curve by choosing a stretch factor that relates increments in arc length along the reference curve and the perturbed curve. This is incorporated in a theory that is accurate to first order in the ray position and to second order in the travel time. The second order travel time perturbation describes the effect of changes in the position of the ray on the travel time. For first arrivals, rays are paths of minimum travel time. The travel time along a ray estimate is therefore necessarily longer than the travel time along the true ray. The resulting travel time bias is described to leading order by the second order perturbation of the travel time. This quantity may be of great importance in nonlinear travel time tomography. Several existing perturbation equations are shown to correspond to the general equation derived in this paper for specific choices of the stretch factor. Depending on the stretch factor, analytical solutions for the ray perturbation can be found for different models of the reference slowness. For two‐point ray tracing problems, the arc length (measured in terms of the independent parameter) may change with the ray perturbation. It is shown both numerically and analytically that for two‐point ray tracing problems, one must introduce a tuning parameter in the perturbation problem. Leaving out such a tuning parameter, as is done in current Hamiltonian ray perturbation theory, may lead to erroneous solutions. In the formulation of this paper, paraxial ray perturbations, slowness perturbations, and pure ray bending are treated in a uniform fashion. This may be very useful in nonlinear tomographic inversions which include earthquake relocation.
An analysis of errors in special sensor microwave imager evaporation estimates over the global oceans
Authors: Esbensen S. K. , Chelton D. B. , Vickers D. , Sun J.
Published: February 2008

Monthly averaged evaporation over the global oceans is estimated from 1 year of special sensor microwave imager (SSM/I) data using the method proposed by Liu (1984). Intercomparisons involving SSM/I and in situ data are made over a wide range of oceanic conditions during August 1987 and February 1988 to determine the source of errors in the evaporation estimates. The most significant spatially coherent evaporation errors are found to come from estimates of near‐surface specific humidity q. Systematic discrepancies of over 2 g kg−1 are found in the tropics, as well as in the middle and high latitudes. The q errors are partitioned into contributions from (1) the parameterization of q in terms of the columnar water vapor W (i.e., the Liu q:W relationship) and (2) the retrieval algorithm for W. It is found that the spatial pattern of discrepancies due to the q:W relationship can be interpreted readily in terms of dynamical and physical processes that maintain the vertical profile of atmospheric water vapor content. The effects of W retrieval errors are found to be smaller over most of the global oceans and due primarily to the implicitly assumed vertical structures of temperature and specific humidity on which the physically based SSM/I retrievals of W are based. To improve estimates of q, it will thus be necessary to have supplementary information on both the temperature and water vapor profiles that is able to distinguish the most important dynamical and physical regimes over the global oceans.
A stability dependent theory for air‐sea gas exchange
Authors: Erickson David J.
Published: February 2008

The influence of thermal stability at the air‐sea interface on computed values of the transfer velocities of trace gases is examined. The novel “whitecap” model for air‐sea gas exchange of Monahan and Spillane (1984), extended here to include thermal stability effects, is linked with an atmospheric general circulation model to compute global transfer velocity patterns of a climate reactive gas, CO2. The important terms in the model equations such as the whitecap coverage, friction velocity, neutral and local drag coefficients and the stability parameter ψm(Z/L) are discussed and analyzed. The atmospheric surface level air temperature, relative humidity, wind speed and sea surface temperature, obtained from the National Center for Atmospheric Research Community Climate Model 1 (CCM1) are used to drive algorithms describing the air‐sea transfer velocity of trace gases. The transfer velocity for CO2 (kCO2) is then computed for each 2.8° × 2.8° latitudinal‐longitudinal area every 24 hours for 5 years of the seasonal‐hydro runs of the CCM1. The new model results are compared to previously proposed formulations using the identical CCM1 forcing terms. Air‐sea thermal stability effects on the transfer velocity for CO2 are most important at mid‐high wind speeds. Where cold air from continental interiors is transported over relatively warm oceanic waters, the transfer velocities are enhanced over neutral stability values. The depression of computed kCO2 values when warm air resides over cold water is especially important, due to asymmetry in the stability dependence of the drag coefficient. The stability influence is 20% to 50% of kCO2 for modest air‐sea temperature differences and up to 100% for extreme cases of stability or instability. The stability dependent “whitecap” model, using the transfer velocity coefficients for whitecap and nonwhitecap areas suggested by Monahan and Spillane (1984), produces CO2 transfer velocities that range from 13 to 50 cm h−1 for a monthly mean. High‐latitude regions of both hemispheres experience winter season means of 40 to 50 cm h−1. The global area‐weighted mean CO2 transfer velocity is 19.2 cm h−1, in reasonable agreement with the 14C estimate of Broecker and Peng (1974). Although consistent with global 14C estimates, the initial version of the model predicts a factor of 2 to 3 higher CO2 transfer velocities over areas with low wind speeds relative to the parameterizations of Liss and Merlivat (1986) and Tans et al. (1990). New transfer velocity coefficients for whitecap and nonwhitecap areas are suggested that bring the low wind speed results into better agreement with observations and other models. The calculations described here suggests that oceanic gas exchange with the atmosphere is sensitive to thermal stability at the air‐sea interface. This specific, turbulence‐related geophysical forcing may account for a portion of the observed scatter in previously obtained experimental data that has been correlated with wind speed alone.
Observations and theoretical studies of microwave emission from thin saline ice
Authors: Wensnahan Mark R. , Grenfell Thomas C. , Winebrenner Dale P. , Maykut Gary A.
Published: February 2008

Observations of time‐dependent changes in microwave emission from thin (0–9 cm) saline ice were made at frequencies of 6.7, 10, 19, 37, and 90 GHz for both vertical and horizontal polarizations. Experiments were carried out on artificial sea ice in the laboratory and on natural ice in the Greenland Sea. Coincident data were also collected on ice thickness, temperature, and salinity together with a variety of basic meteorological data. Several phenomena were found to be common to all the laboratory measurements, with some indication that similar features were also present in the field data. These include (1) a sharp rise in surface temperature when the ice was between 1 and 2 cm thick, apparently unrelated to any environmental changes, (2) a decrease in brightness temperature (Tb) at 37 GHz during or just after the surface temperature rise, and (3) an initial increase in Tb with increasing ice thickness, followed by substantial decreases in Tb at the higher (19, 37, and 90 GHz) frequencies. The maximum Tb values observed were higher than those previously reported for young ice. Tb was also found to be much more sensitive to variations in ice properties at horizontal polarization than at vertical polarization. Numerical modeling results indicate that the high‐frequency decreases in Tb were not due to bulk changes in either ice temperature or salinity. The most likely explanation for the observed rise in surface temperature and decrease in Tb was the formation of a salinity‐enhanced ice or brine surface layer caused by the upward transport of brine as the ice grows.
Wave height spectrum from sun glint patterns: An inverse problem
Authors: Borrego Josué Alvarez
Published: February 2008

The problem of retrieving spatial information of sea surface heights from aerial images is considered. In this paper, some analytical and numerical results that relate the autocorrelation of the surface heights and those of the sun glint patterns are derived. Examples of these results are presented showing the nonlinear technique which can be applied to obtain the power spectrum of a real sea surface using aerial photographs. As a first step, two kinds of roughness spectra are presented: two roughness spectra which describe single‐scale surfaces, and the Pierson‐Moskowitz power spectrum which describe multiscale surfaces for a fully developed sea. Both simulations are presented in one dimension. However, a single‐scale bidimensional surface is analyzed considering a Gaussian roughness spectrum. Wave height spectra are obtained from the surface height autocorrelation via a Fourier transform. The results of the model compared with the theory and the data are in quite good agreement. Under favorable conditions, it is possible to invert the relation numerically and estimate the wave height spectrum from the sun glint data.
Eddy and wave dynamics in the South Atlantic as diagnosed from Geosat altimeter data
Authors: Forbes Cristina , Leaman Kevin , Olson Don , Brown Otis
Published: February 2008

Almost 3 years of Geosat data were analyzed to ascertain the characteristics of the sea surface height variability in the South Atlantic. The nature of the sea surface height field is shown to be nonstationary, heterogeneous, and anisotropic. In particular, the wavenumber spectral characteristics of the sea surface height field, such as spectral slopes, shape, breaks in spectral slopes, and energy density values, vary with latitude and longitude within the basin. The spectral slopes exhibit a spectral dependence approaching k−5 in the high energy areas and k−3 in the low energy areas. Along‐track characteristic wavelengths are found to decrease from north (500 km) to south (300 km) and from west to east. The (eddy available potential) energy exhibits maximum values in the highly energetic eddy shedding regions of the Malvinas‐Brazil confluence and the Agulhas leakage region. Mean frequency spectra, calculated from time series of sea surface height variability at crossover points, reveal evidence of annual and semiannual signatures with energy levels reaching 1000 cm2 in the high‐energy western and eastern systems. These values represent 20% of the energy found for periods of 100 days or less, which is the time scale of the mesoscale variability. Baroclinic Rossby waves with periods of 400–500 days are found to propagate away from the eddy‐shedding region of the Agulhas leakage into the Atlantic. The phase (energy) of these planetary waves propagates toward the west‐southwest (west‐northwest) from their source near the southern tip of Africa. Correlation functions in space (both in the zonal and along‐track directions) and time lag corroborate these findings. Zonal and meridional wavenumbers were calculated using a simple wavenumber projection technique which takes into a count the anisotropic property of the wave field. The resultant wavelengths (approximately 250 km) are consistent with those calculated from the dispersion relation for baroclinic Rossby waves. These waves propagate with a group speed of 0.4 cm s−1 and provide a net energy flux from the Agulhas leakage into the Atlantic toward the western boundary and the equator across 25°S.
Spatial variability of subducting water in the North Atlantic: A pilot study
Authors: Joyce Terrence M. , Jenkins William J.
Published: February 2008

Hydrographic and tritium‐3He data obtained on a small scale survey conducted in 1986 south of the Azores show strong horizontal and isopycnic gradients across the Azores Front. Examination of small‐scale spatial variability reveals a dominant scale for salinity, oxygen, and tritium‐3He age of the order of 160 km, comparable to estimates of sea surface height correlation scales from Geosat data. This spatial variability correlates strongly with large‐scale horizontal gradients. Two mesoscale resolving surveys separated by 300 km reveal spatial scales and amplitudes of thickness variability that decrease rapidly with age, becoming incoherent on scales greater than 18 km within 1 year, and approaching internal wave background values. There appears to be a substantial reduction in mode thickness between the two surveys, which may be the result of non conservation of potential vorticity, or the result of interannual changes in formation. Subduction rates computed from tritium‐3He age gradients are comparable to or larger than Ekman pumping estimates based on climatological data.
The joint distribution of heights and periods of shoaling waves
Authors: Doering John C. , Donelan Mark A.
Published: February 2008

The evolution of the joint distribution of wave heights and periods that results from shoaling is investigated using laboratory data obtained from experiments conducted on two (1:40 and 1:20) planar beach slopes. The data are compared to the theoretical joint distribution for wave heights and periods proposed by Longuet‐Higgins (1983). For unbroken waves with d/L >≈ 0.1 the shapes of the observed and predicted distributions agree reasonably well. However, the predicted joint distribution given by Longuet‐Higgins (1983) is displaced relative to the observed distribution. A parameterization for the observed displacement is offered. In addition, a parameterization for the variation of the spectral width parameter through the shoaling region is also given.
Sensitivity of satellite multichannel sea surface temperature retrievals to the air‐sea temperature difference
Authors: May Douglas A. , Holyer Ronald J.
Published: February 2008

The advanced very high resolution radiometer multichannel sea surface temperature (MCSST) retrieval technique provides global algorithm accuracy statistics generally showing a bias of less than 0.1°C and an rms error of less than 0.7°C when compared to colocated drifting buoy in situ data in the absence of aerosols. This remaining error is not always random but is shown to be correlated to the air‐sea temperature difference. The MCSST technique is modeled and then compared to in situ data to show this dependency. Atmospheric radiative transfer calculations are used to provide a simulation of satellite retrieval sensitivity to air‐sea temperature differences. Buoy sea surface temperature (SST) and air temperature observations are then presented as experimental verification of the simulation results. Retrieval errors depend both on the mean air‐sea temperature difference conditions present in the data set used to empirically derive the algorithm and on the changes in air‐sea temperature difference conditions relative to the derivation data set mean conditions. Retrieval error is found to respond linearly with air‐sea temperature difference changes. MCSST retrieval errors of 1.0°C can occur for air‐sea temperature difference changes of 7°–10°C from mean conditions when the dual‐window (channels 3 and 4) or triple‐window (channels 3, 4, and 5) algorithms are used. The split‐window (channels 4 and 5) MCSST algorithm is shown to be less sensitive to air‐sea temperature differences. Cross‐product SST (CPSST) and nonlinear SST (NLSST) algorithms are also examined. These algorithms generate results similar to the MCSST algorithm for the dual‐ and triple‐window equations. However, the CPSST and NLSST split‐window algorithms demonstrate greater sensitivity to air‐sea temperature difference changes than do the MCSST split‐window algorithm. Retrieval errors of 1°C can occur for air‐sea temperature difference changes of 10°–12°C from mean conditions. Users of satellite SST retrievals in regions that experience large fluctuations in air‐sea temperature difference should be aware of this possible error source.
A tritium budget for the North Atlantic
Authors: Doney Scott C. , Jenkins Wllliam J. , Östlund H. G.
Published: February 2008

We develop a new model bomb‐3H budget for the North Atlantic between 1950 and 1986. The model, which calculates the atmospheric and continental 3H delivery as well as the advective 3H transports from the South Atlantic and Arctic, agrees within about 10% with the 3H inventories computed from the 1972 GEOSECS and the 1981–1983 Transient Tracers in the Ocean (TTO) observations. The decay‐corrected 3H inventory for the North Atlantic increased by about 43% between the GEOSECS and TTO programs (1972 to 1981), and the inflow of high 3H polar water from the Arctic into the North Atlantic is found to be crucial for correctly simulating this increase. Key aspects of the model that differ from previous studies include the treatment of vapor/rain isotopic equilibrium, the continental vapor flux, and the downward flux of water vapor into the ocean. The sensitivity of the atmospheric 3H delivery to model parameters and to seasonal and interannual variability are explored.
Variability in the Gulf of Alaska from Geosat altimetry data
Authors: Bhaskaran Shyam , Lagerloef Gary S. E. , Born George H. , Emery W. J. , Leben Robert R.
Published: February 2008

Satellite altimetry was used to examine annual and interannual variability in the Gulf of Alaska region. Crossover data from the Geosat Geodetic Mission (GM) and collinear data from the Exact Repeat Mission (ERM) were processed separately to form sea surface height anomalies at grid points. The time series from the GM and ERM were then combined to produce a 3.75‐year data set. The time series from the ERM data set agreed fairly well with hydrographic dynamic heights at several locations, with an average correlation of 0.70 between the two data sets. The combined and ERM altimetric data sets were analyzed using empirical orthogonal functions (EOFs). These revealed variability that occurs primarily on annual and interannual time scales. A comparison with EOF analysis of the atmospheric pressure field during the same time periods showed that the annual variation in pressure seemed to be reflected in both the combined and ERM altimetric data sets. The amplitude time series of the first mode in the combined data set was very similar to the North Pacific pressure index during the 1985–1989 time frame. The maximum correlation was at a lag of 250 days. Finally, an interannual mode was present in all three data sets which was closely linked to the baroclinic variations measured by the hydrographic data.
Simulated seasonal circulation in the Indonesian Seas
Authors: Masumoto Yukio , Yamagata Toshio
Published: February 2008

Seasonal circulation in the Indonesian Seas is investigated in detail by use of a high‐resolution oceanic general circulation model forced by the monthly mean climatological wind stress. Although the net barotropic through flow between the Pacific and Indian oceans is not permitted, the model appears to be successful in predicting the observed transport through the Lombok Strait. The modeled transport varies from almost nothing in January to 5 Sv in September, with a local minimum in June; this seasonal variation is regulated by means of remote actions of the monsoonal winds in the tropical Indian Ocean as well as local and remote action of the monsoonal winds over the western Pacific. The drainage into the South China Sea through the Java Sea during the boreal summer may explain the apparent local minimum of the modeled Lombok through flow. The root cause of the semiannual aspect of the Lombok through flow, however, lies in the Wyrtki jets of the Indian Ocean, which reduce the positive pressure gradient across the Lombok Strait twice a year.
Surface drift effect on wind energy transfer to waves
Authors: Zhou Donghuo , Mendoza Cesar
Published: February 2008

The effect of surface drift velocity on the energy transfer from the wind to ocean waves and the growth rate of the dominant surface wave was studied analytically. The model analyzed considered first the airflow above the surface wave to be turbulent, the thin drift shear layer to be viscous, and the flow beneath the drift flow to be potential. Then an extension of the analysis was made for the case of a drift shear layer of considerable thickness. The analysis clearly demonstrated the important role that air turbulence and water surface drift play in the wind energy transfer to waves. It was also found that the exponential growth dominated the process of the wind energy transfer to waves. In general, the presence of surface drift increased the wave energy growth rate. The present analysis predicted the existence of a critical value of the normalized surface drift velocity, at which the increment of wave energy growth due to surface drift reached a maximum. When the surface drift exceeded the critical value, there was an adverse effect on the wave energy growth. In the extreme condition in which the surface drift matched the phase speed, it set up the upper limit of the wave energy growth rate. The numerical estimates computed with the theory were consistent with experimental measurements, and with the results obtained with more complex analyses based on numerical models.
The effect of wave‐current interaction on tidally forced estuarine circulation
Authors: Bender Leslie C. , Wong Kuo‐Chuin
Published: February 2008

Reductions in the velocity and surface elevation of tidal flows that have been observed in the Delaware Bay, a coastal plain estuary on the eastern coast of the United States, correlate well with local wind events. While there are several mechanisms that could account for this phenomenon, we consider hydrodynamic wave‐current interaction to be a reasonable explanation. In order to theoretically model the interaction between tides and surface gravity waves, and eliminate the need for a reference velocity required by the Grant and Madsen (1979) wave‐current interaction model, we propose a process‐oriented model that couples the mechanism of wave‐current interaction directly to the current dynamics of the tidal flow. The tidal model uses an eddy viscosity concept to parameterize the turbulent stresses and further characterizes the eddy viscosity in terms of a flow dependent shear velocity. Within the bottom boundary layer of the surface gravity wave, the turbulent stresses are also parameterized by an eddy viscosity that is coupled to the shear velocity of the tidal model. Results of the coupled model indicate, that as expected, wave‐current interaction increases the bottom friction felt by the tidal flow and usually reduces the volume flux. However, if the physical bottom roughness is sufficiently small (less than 30 cm) then it is possible to enhance the volume flux in a long, deep estuary, contrary to expectations. A simple one dimensional, linearized friction model shows this potentially dramatic effect can be attributed to an increase in the bottom friction that then tunes the estuary closer to its natural resonance. Obviously an estuary at resonance maximizes the volume flux that must pass through the estuary mouth. The effect of wave‐current interaction is to provide a possible means of increasing the bottom friction and enhancing the volume flux.
An objective mapping method for estimating geostrophic velocity from hydrographic sections including the equator
Authors: Cornuelle Bruce D. , Morris Michele Y. , Roemmich Dean H.
Published: February 2008

Objective mapping can remove the equatorial singularity from the problem of estimating geostrophic shear from noisy density measurements. The method uses the complete thermal wind relation, so it is valid uniformly on and off the equator. Errors in the thermal wind balance are due to neglected terms in the momentum balance, which are treated as noise in the inverse problem. The question of whether the geostrophic balance holds near the equator is restated as a need to estimate the size of the ageostrophic noise in the thermal wind equation. Objective mapping formalizes the assumptions about the magnitudes and scales of the geostrophic currents and about the magnitudes and scales of the ageostrophic terms and measurement errors. The uncertainty of the velocity estimates is calculated as part of the mapping and depends on the signal to noise ratio (geostrophic density signal to ageostrophic “noise”) in the data, as well as the station spacing and the scales assumed for the geostrophic velocities. The method is used to map zonal velocity from a mean Hawaii‐Tahiti Shuttle density section. These are compared with previous velocity estimates for the same dataset calculated using other techniques. By choosing appropriate scales, the objective map can duplicate previous results. New temperature data are presented from a repeating, high‐resolution expendable bathythermograph section crossing the equator at about 170° W with four cruises a year between 1987–1991. There appear to be significant differences between this mean temperature and the shuttle mean temperature. Temperature is converted to density with the aid of a mean T‐S relation and geostrophic velocity maps are calculated for the 4‐year mean. The mean geostrophic undercurrent obtained from our sections is weaker than in the shuttle estimate and is centered slightly north of the equator. Enforcing symmetry about the equator removes the offset of the current, giving a stronger, but narrow undercurrent. The density field apparently includes significant (O (0.5 kg m−3)) large‐scale ageostrophic variability which makes velocity estimates from single cruises poorly determined near the equator.
Numerical simulations of small‐scale oceanic fronts of river discharge type with the lattice gas automata method
Authors: Kougias Charilaos F.
Published: February 2008

For the first time, the lattice gas automata method is used to simulate small‐scale oceanic fronts of river discharge type. With this method, which is based on statistical physics, round‐off errors and computer code instabilities are absent, while mass and momentum conservation are easily implemented. We have developed a quasi‐stationary lattice gas automata model which describes the evolution process of such a front. This numerical model reproduces the characteristic convergence of the velocity field from both sides near the frontal interface. Furthermore, the overall Richardson number Rio is verified to be the governing dimensionless parameter. A typical density current is established only for Rio > 0.20 and in much shorter time than the total simulation time, which corresponds to the Brunt‐Väisälä time period. For longer simulation times the front remains stable, but its interface is blurred owing to diffusion. Moreover, the mean mass entrainment is found to be positive for Rio above 0.75, which favors the frontal stability. Our results are, in general, in qualitative and partly in quantitative agreement with observations in the field, tank experiments, and other numerical models.
A model to estimate the density, characteristic surface, and coverage of whitecaps using underwater sound
Authors: Dupuis Hélène , Weill Alain
Published: February 2008

A model of ambient noise is described which permits one to estimate the density of whitecaps at the sea surface as well as the scaling as a function of wind speed of whitecap coverage and characteristic surface. This model relates the statistical properties of underwater sound (first and second moments) to whitecap distribution at the sea surface and to underwater sound propagation. The sound propagation model accounts for seabed reflections. Simulations show that seabed reflection effects can be great in shallow water, implying that the listening radii for hydrophone measurements, as defined by Farmer and Vagle (1988), can often be very large compared to hydrophone depth.
Drifting buoy observations of a loop current anticyclonic eddy
Authors: Glenn Scott M. , Ebbesmeyer Curtis C.
Published: February 2008

The Loop Current penetrated deep into the Gulf of Mexico in early 1989. After several eddy formation and reattachment cycles, a southwestward propagating anticyclonic eddy was formed during the summer. The Loop Current and subsequent eddy produced strong currents over widespread areas on the Louisiana continental slope, prompting a series of current measurement programs. Because of the lack of satellite infrared coverage in the Gulf of Mexico during the summer, the trajectories of 53 ARGOS tracked drifting buoys deployed in the Gulf of Mexico in 1989 were assembled to determine the synoptic history of the Loop Current and anticyclonic eddy during this event. Ten of the most critical summertime buoy trajectories are discussed here. In addition, the trajectories of three of the buoys simultaneously deployed at different radii in the eddy were chosen for analysis with a kinematic feature model. The model assumes the looping buoy motion is generated by an elliptical orbit around a translating eddy center. The buoy trajectories were used to determine time series of the feature model parameters, including eddy center, shape and circulation characteristics. The time series results indicate that the eddy actually propagated in a series of short sprints separated by longer stalls. During the sprints, the eddy propagated as an elliptical but otherwise symmetric solid body. During the stalls, significant asymmetries developed. The asymmetries are consistent with attachment to the Loop Current during the first stall, the effects of topographic Rossby wave dispersion during the second, and the effects of planetary Rossby wave dispersion during the third. The results provide both a framework for the interpretation of in situ observations and a detailed evolutionary history for numerical modelers.
Ozone production in urban plumes transported over water: Photochemical model and case studies in the northeastern and midwestern United States
Authors: Sillman Sanford , Samson Perry J. , Masters Jeffrey M.
Published: February 2008

Abnormally high concentrations of O3 have been observed in rural locations on the shore of Lake Michigan and on the Atlantic coast in Maine, at a distance of 300 km or more from major anthropogenic sources. We hypothesize that this O3 is associated with transport from major urban centers and with the suppression of vertical mixing as urban plumes are transported over water. A dynamical/photochemical model is developed that represents formation of O3 in shoreline environments and is used to simulate case studies for Lake Michigan and the northeastern United States. Results suggest that a broad region with elevated O3, NOx and volatile organic carbon (VOC) forms as the Chicago plume travels over Lake Michigan, a pattern consistent with observed O3 at surface monitoring sites. Near‐total suppression of dry deposition of O3 and NOx over the lake is needed to produce high O3. Results for the east coast suggest that the observed peak O3 can only be reproduced by a model that includes suppressed vertical mixing and deposition over water, 2‐day transport of a plume from New York, and superposition of the New York and Boston plumes. An investigation of the sensitivity of O3 to emissions of NOx and VOC suggests that results vary greatly between cities, even when the composition of urban emissions is similar. An index for VOC versus NOx sensitivity is shown to correlate with total reactive nitrogen (NOy) at the time of peak O3.
Determination of molecular dissociation rates from measurements of scattered solar ultraviolet light
Authors: Montierth K. V. , Baker K. D. , Jensen L. L. , Megill L. R.
Published: February 2008

Scattered solar ultraviolet light in the spectral region from 250 to 320 nm was measured from a balloon platform at altitudes ranging from 14.5 to 38.8 km with a wavelength resolution of approximately 0.3 nm and an accuracy in determining the wavelength position of 0.5 nm. We used these scatter data to compare and validate a single scattering model developed for this purpose. Using these semi‐empirical results, we constructed the single scattered component of the ultraviolet irradiance as a function of height and wavelength. From these data, it is possible to determine the percentage dissociation rate of a variety of species due to scattered ultraviolet radiation. These results are presented as a ratio of intensities R = I1/(I0+I1), where I1 is the first order scattering and I0 is the direct attenuated radiation from the sun. The attenuated direct radiation from the sun can be multiplied by the factor 1/(1‐R), the Rayleigh amplification ratio used by Nicolet et al. (1982), to include the contribution to the irradiance from first order scattered light. The results are multiplied by a cross section of the species under consideration to obtain the dissociation rates. Ozone overburden and an approximate ozone particle density altitude profile are obtained when validating the single scatter model.
The vertical structure of diabatic heating associated with the Madden‐Julian oscillation simulated by the Goddard Laboratory for Atmospheres climate model
Authors: Chen Tsing‐Chang , Yen Ming‐Cheng , Pfaendtner James , Sud Y. C.
Published: February 2008

Previous studies have shown that numerical simulations of the Madden‐Julian oscillation (MJO) are very sensitive to the vertical distribution of diabatic heating. Since atmospheric diabatic heating is generally difficult to estimate, the vertical diabatic heating profile associated with the MJO is not well known. Judged by its propagation properties and spatial structure, the MJO is reasonably well simulated by the nine‐layer Goddard Laboratory for Atmospheres (GLA) general circulation model. Although only a simulation the model MJO may provide an indication of the vertical diabatic heating profile associated with the real oscillation. The diabatic heating structure of the model MJO is illustrated with composite charts made for those times when this low‐frequency mode reaches its maximum and minimum amplitudes. These composite charts compare the vertically integrated diabatic heating with potential functions, the vertical distribution of diabatic heating with the east‐west mass flux function in the tropics, and the vertical profiles of diabatic heating at the centers of maximum and minimum MJO amplitude. Three interesting features of the model MJO's diabatic heating are revealed: (1) the maximum heating rate of this low‐frequency mode is located over the Asian monsoon region and its amplitude is about a half of the maximum value of the seasonal mean heating rate in this region, (2) the vertical diabatic heating rate profile has a maximum at 500 mbar and resembles the seasonal mean total heating profile, and (3) the total diabatic heating is for the most part composed of the latent heat released by cumulus convection.
The 40‐ and 50‐day oscillations in atmospheric angular momentum at various latitudes
Authors: Magaña Victor
Published: February 2008

Intraseasonal oscillations in atmospheric angular momentum at various latitudes from 1979 to 1989 are analyzed and compared with fluctuations in tropical convective activity associated with the Madden‐Julian oscillation. Oscillations with periods of approximately 40 and 50 days are observed in both the globally integrated angular momentum M and tropical convective activity. M is maximum when the convective activity increases over the western/central Pacific and weakens over the Indian Ocean. Cross‐spectral analyses show that the main contributions to both 40‐ and 50‐day oscillations in M and in length‐of‐day (LOD) come from the tropics where fluctuations in regional atmospheric angular momentum with the same periodicities exist. In general, the signal of the oscillation propagates from the tropics to the subtropics. In the northern hemisphere (NH) mid‐latitudes, however, a small‐amplitude 40‐day oscillation in atmospheric angular momentum usually exists regardless of the presence or absence of fluctuations with the same periodicity in the tropics. In the southern hemisphere (SH) a dominant 50‐day oscillation exists independently from those in the tropics. However, the 40‐ and 50‐day oscillations in the NH and SH do not seem to drive oscillations with the same periodicity in LOD or M.
The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS)
Authors: Brueckner G. E. , Edlow K. L. , Floyd L. E. , Lean J. L. , VanHoosier M. E.
Published: February 2008

The state of solar ultraviolet irradiance measurements in 1978, when NASA requested proposals for a new generation of solar ultraviolet monitors to be flown on the Upper Atmosphere Research Satellite (UARS), is described. To overcome the radiometric uncertainties that plagued the measurements at this time, the solar ultraviolet spectral irradiance monitor (SUSIM) instrument design included in‐flight calibration light sources and multichannel photometers. Both are aimed at achieving a maximum precision of the SUSIM measurements over a long period of time, e.g., one solar cycle. The design of the SUSIM‐UARS instrument is compared with the original design specifications for the UARS instruments. Details including optical train, filters, detectors, and contamination precautions are described. Also discussed are the SUSIM‐UARS preflight calibration and characterization, as well as the results of the inflight performance of the instrument during the first 3 months of operation. Finally, flight operations, observation strategy, and data reduction schemes are outlined.
Electron scavenging of stratospheric chlorine to reduce ozone depletion: Will it work?
Authors: Prasad Sheo S.
Published: February 2008

A recently proposed scheme for reducing ozone depletion by electron scavenging of stratospheric chlorine may be much less efficient than originally thought by its proponents. The rapid photodetachment of the negative Cl− ion by sunlight is the main problem with the scheme. Photodetachment would prevent chlorine‐bearing negative ions from ever becoming the major ions, even when large numbers of electrons (of the order of 106 el− cm−3) were artificially injected into the atmosphere at an altitude of 40∼45 km. This conclusion is corroborated by the observed fact that chlorine bearing negative ions are not the major anions at these altitudes under normal conditions. The possible existence of negative chlorine ion chemistry based ozone depletion cycles is a potential hazard with this scavenging scheme. This hazard could make this scheme counterproductive if it is attempted in the altitude region where atomic oxygen is an important constituent.
Determination of lightning currents from far electromagnetic fields
Authors: Rachidi Farhad , Thottappillil Rajeev
Published: February 2008

Expressions relating far electromagnetic fields and return stroke channel base currents have been derived in the literature for various lightning return stroke models. The use of such relations permits the estimation of channel base currents of return strokes, testing of the individual models using sets of simultaneous current and field measurements, and also the determination of nondirectly measurable parameters of the models. In this paper we derive a relation between the far electromagnetic field and the channelbase current for the Diendorfer‐Uman (DU) and the modified Diendorfer‐Uman (MDU) models. Additionally, the current field relations for all models for which such an analytical relation can be derived are summarized. Finally, a numerical solution technique is proposed to solve the equations relating the far field and channel base current of the DU and the traveling current source (TCS) models.
Rhizospheric methane oxidation determined via the methyl fluoride inhibition technique
Authors: Epp Michelle A. , Chanton Jeffrey P.
Published: February 2008

Methane oxidation rates in the rhizosphere of aquatic macrophytes were quantified by development of a technique employing a recently described inhibitor of methane oxidation, methyl fluoride. Unlike other inhibitors, methyl fluoride appears to be nontoxic to the plants, allowing them to act as natural conduits, transporting the inhibitor from the headspace to the rhizosphere. Increases in methane emissions were recorded after closed chamber methyl fluoride incubations, primarily in greenhouse (Pontederia cordata and Sagittaria landfolia) experiments with some preliminary outdoor and field (Oryza sativa and Typha latifolia) data. Comparison of emissions before and after incubation indicated oxidation of 23 to 90% of the methane produced (defined as CH4 emission in the absence of oxidation) in greenhouse studies and 10 to 47% in field and outdoor studies. A comparison of 1.5 and 3.0% methyl fluoride chamber headspace incubations as well as initial dose response data indicated that the lower concentration was sufficient to obtain inhibition of methane oxidation in the greenhouse studies without significantly affecting methanogenesis. Inhibition was possible with one 16‐ to 18‐hour incubation period. Methyl fluoride within the rhizosphere disappeared after approximately 1 week due to plant ventilation and possible bacterial uptake.
Multiple equilibria in a zonal energy balance climate model: The thin ice cap instability
Authors: Matteucci Gianni
Published: February 2008

From an analysis of observational data and general circulation model (GCM) simulations a review is given of the empirical parameterizations which lead to the occurrence of multiple equilibria in previously formulated energy balance climate models (EBM) of the Budyko‐Sellers type. By drawing an analogy between the present‐day southern hemisphere high‐latitude conditions and the northern hemisphere (NH) high‐latitude conditions during glacial times, we have devised a new parameterization for the zonal planetary albedo. In particular, we hypothesize that changes in the temperature of the NH polar regions can cause the development of a high‐latitude planetary albedo anomaly (namely, the occurrence of continental ice sheets) and show that in this case a zonal mean EBM admits multiple equilibria. The zonal profiles of the climatic quantities for the warm and cold equilibrium states and their corresponding globally averaged values agree with the observational, geological, and paleoclimatic evidence and exhibit a poleward amplification of the temperature anomaly. The new parameterization of the ice albedo‐temperature feedback introduces what we have called the thin ice cap instability (TICI). The TICI is a more realistic version of the Small Ice Cap Instability discussed in earlier versions of EBMs. The physical meaning of the TICI and the sensitivity of the degree of nonlinearity it introduces to changes in the model parameters is then discussed. On the basis of modeling experience with a previously developed global EBM (to which the current EBM can be reduced by spatial integration), the stability of the equilibria is considered in terms of the Lyapunov pseudopotential. Finally, we perform time‐dependent experiments to evaluate the relaxation time scales of the system around its equilibria and to investigate the latitudinal dependence of the response to perturbations.