bluuebunny/crossref_metadata_2025 huggingface dataset:
The generalized eastern boundary conditions and the three‐dimensional structure of the ideal fluid thermocline
Authors: Huang Rui Xin
Published: February 2008

A generalized eastern boundary condition of no net zonal flux is applied to a continuously stratified model of the ideal fluid thermocline. Assuming that vertical exchange of water masses at the eastern boundary can be realized by the effect of mixing, water particles can move into or out of the eastern boundary in the form of vertical modes of no net zonal flux. For a purely wind‐driven model, a simple mode with the upper layer water moving into the eastern boundary and the lower layer water coming back into the interior is consistent with the assumption that vertical velocity is everywhere nonpositive within the subtropical interior. The stratification at the eastern boundary is an inseparable part of the global dynamics of the subtropical gyre and thus cannot be specified a priori. In fact, the eastern boundary stratification should be determined as part of a solution. One numerical example shows that relaxing the strict no‐zonal‐flux eastern boundary condition releases the nonphysical constraint over the upper surface density gradient near the intergyre boundary and gives rise to sloping isopycnals in the upper ocean and a vast zone of the eastern boundary ventilated thermocline in the southeastern basin. Therefore in contrast to the passive role played in the previous homogeneous ocean circulation models, the eastern boundary layer in a continuously stratified model plays an active role of setting up the global structure of the subtropical gyre. The dynamics of the eastern boundary layer which can transport the vertical mass exchange remains an issue for further study.
Similarity scaling of viscous and thermal dissipation in a convecting surface boundary layer
Authors: Lombardo C. P. , Gregg M. C.
Published: February 2008

By continuously deploying a turbulence profiler in the upper ocean, we observed the rate of viscous dissipation of turbulent kinetic energy, ε, and the rate of diffusive smoothing of turbulent temperature fluctuations, χ, during eleven diurnal cycles of the surface boundary layer (SBL). Even though we restricted our analysis to times when the ocean lost buoyancy at a nearly constant rate, we observed a wide range of conditions, including dominance of the turbulent production by the wind stress. Throughout, ε was normalized very well by the sum of the similarity scalings for turbulence produced by wind stress and by convection. Scaling of χ was less successful and applied only when turbulent production was dominated either by wind stress or by convection, and then only within part of the SBL.
Interpentadal variability of salinity in the upper 150 m of the North Atlantic Ocean, 1970–1974 versus 1955–1959
Authors: Levitus Sydney
Published: February 2008

We have composited and objectively analyzed historical hydrographic observations for the North Atlantic Ocean for two pentads, 1955–1959 and 1970–1974. Our purpose is the study of the temporal variability of salinity in the upper layers of the North Atlantic Ocean. Upper ocean (0–150 m) salinity differences between the two pentads exhibit statistically significant changes. The subarctic gyre east and north of Newfoundland freshened by as much as 0.5‰. The northern portion of the subtropical gyre increased in salinity by as much as 0.4‰, while the interior of this gyre decreased in salinity.
Response of South Carolina continental shelf waters to wind and Gulf Stream forcing during winter of 1986
Authors: Lee Thomas N. , Williams Elizabeth , Wang John , Evans Robert , Atkinson Larry
Published: February 2008

The South Carolina shelf water response to strong winter atmospheric forcing and Gulf Stream variations over the slope is discussed with the aid of a comprehensive set of atmospheric and oceanic observations made during project GALE (Genesis of Atlantic Lows Experiment). A mixed response is observed to wind and Gulf Stream forcing. Subtidal variability of inner and middle shelf waters is primarily a barotropic, forced response to coherent, synoptic scale, alongshore winds. This response is not trapped over the shelf, as has been found for other wide, non‐equatorial shelves, but is also significant at the shelf edge, possibly because of the comparable widths of the shelf and slope. The outer shelf also shows a significant Gulf Stream influence. The Gulf Stream appears to have two preferred position modes, either onshore, with the stream flowing over the Charleston bump and along the shelf break, or offshore, when the stream can be located up to 100 km east of the shelf break; transition between the two states is rapid. Subtidal variability in the outer shelf is dominated by weekly period frontal eddies and meanders when the stream is in an onshore mode, and by persistent southward flow when the stream is offshore. This southward flow is produced by enlarged cyclonic eddies with upwelled centers located over the slope between the offshore Stream and the shelf edge, sometimes referred to as the Charleston gyre. Positive mean alongshore sea level slopes at the shelf edge, due to the persistence of the cyclonic gyres, are geostrophically balanced by mean onshore flow, which transports upwelled waters onto the shelf, enhancing the biological productivity of the area relative to other regions of the South Atlantic Bight.
Sand transport by unbroken water waves under sheet flow conditions
Authors: Trowbridge John , Young Donald
Published: February 2008

A model of wave‐induced onshore sand transport and resulting topographical changes seaward of the break point on a sloping beach is presented, for the specific case in which the waves are random, normally incident, weakly nonlinear, and relatively long, the bottom boundary layer is fully turbulent, and sand transport occurs as sheet flow. The model consists of the following components: (1) a new empirical expression for sand transport as sheet flow, in which the instantaneous transport rate is directly proportional to the instantaneous bottom shear stress; (2) an expresssion for the mean bottom shear stress, based on a theoretical analysis of the bottom boundary layer; (3) a simple representation of the shoaling wave field, in which the local properties are described by linear long wave theory, the probability density function of the surface displacement is Gaussian, and the waves conserve energy flux as they shoal; and (4) the equation for conservation of sediment mass. Computations based on the model compare favorably with recently reported field measurements of the onshore motion of a long offshore bar.
Two‐dimensional instability of finite amplitude internal gravity wave packets near a critical level
Authors: Winters Kraig B. , D'Asaro Eric A.
Published: February 2008

A high‐resolution two‐dimensional numerical model is used to simulate the propagation of finite amplitude internal wave packets into a mean shear flow that varies slowly in space. For moderate packet amplitudes the interaction is well described by weakly nonlinear asymptotic theory. At higher amplitude, however, a region develops near the critical level in which nonlinearity dominates, the wave packet becomes unstable, and the wavelike motion breaks down into smaller scales. The form of the observed internal wave breakdown is unexpected in that convectively unstable density gradients persist for many buoyancy periods. The eventual rapid transition, from large‐amplitude wavelike motion to a more complicated flow, is triggered by an instability driven by the intensified wave shear and not by a convective breakdown of the unstable stratification. This illustrates that unstable stratification does not necessarily lead to convective instability in two dimensions.
Bed load transport of sand mixtures in estuaries: A review
Authors: Ludwick John C.
Published: February 2008

Real estuaries are fundamentally inhomogeneous. This is evident in their irregular boundaries, waters of varying density, fluid motions that arise from multiple unsteady forcings, and sediments that are mixtures of various grain sizes. In the paper, a review is given of the influence of one of these heterogeneities, namely, mixed particle size, on the transport of sand as bed load. Essential early studies and recent complete theories are discussed. Present‐day investigators commonly assume a reference transport function (RTF) which is any established formulation for the flux of bed load under steady unidirectional flow over a substrate of monosized particles. The aim of the modern work is to develop a procedure which yields tailored values of sheltering‐exposure coefficients for the different size fractions. These coefficients are correction factors which when applied to the acting bed shear permit the use of the RTF to compute fraction transports over a mixed bed. There are strong interactions among the various size fractions; for instance, minor admixture of a coarse‐end ingredient disproportionately reduces the overall mobility of a finer‐grained bed. However, coarse fractions are more mobile in a bed of mixed sizes than they are in a bed of the same uniform size. Recommendations are made for an estuarine field study utilizing a modified Helley‐Smith bed load yield sampler.
Sediment processes in estuaries: Future research requirements
Authors: Dyer K. R.
Published: February 2008

The gaps in current understanding of the erosion, transport and deposition of sediment in estuaries are briefly reviewed. It is concluded that future work should give priority to (1) the formation, movement and entrainment of high concentration near bed layers; (2) particle interactions, including flocculation, cycling processes, and chemical and biological interactions; (3) intertidal mudflat processes, sediment exchanges in shallow water and wave induced mud transport; (4) development of improved parameterization of exchange processes for inclusion in 3D mathematical models. (5) development and use of new instrumentation for field measurements, especially of intermittent events, and over the long term. This work should be carried out within interdisciplinary studies involving physicists, sediment dynamicists, biologists, and chemists.
Modeling the effect of suspended sediment stratification on bottom exchange processes
Authors: Sheng Y. Peter , Villaret Catherine
Published: February 2008

A vertical gradient of suspended sediment concentration often exists in estuaries, particularly within the bottom boundary layer where sediment erosion and deposition take place. This results in a vertical density gradient and hence modification of the flow. However, this important effect has often been ignored in past sediment studies. Because of this and because of other empirical assumptions, exisiting erosion models cannot be used as predictive tools. This paper employs a simplified second‐order closure model to simulate the effect of sediment‐induced stratification on bottom boundary layer dynamics, and particularly the erosion process. Numerical models, which employ the concept of Richardson number dependent eddy viscosity, have been developed in the past for stratified flows. These models require a large number of data for tuning parameters of the eddy viscosity formula. The simplified second‐order closure model used here consists of the dynamic equations of motion for mean variables (velocity, temperature, salinity, density, and suspended sediment concentration) and turbulent quantities of turbulent kinetic energy and turbulence macroscale. Model constants are thus invariant owing to the added physics. Model simulations of laboratory sediment‐laden boundary layers indicate that, as sediments are eroded or resuspended from the bottom, a vertical gradient of suspended sediment concentration can lead to a significant reduction of the turbulent shear stresses and a slowdown of the erosion‐resuspension process. Hence significant error may be contained in some empirical sediment erosion rate formulae, which were derived by ignoring flow‐sediment interactions. Simulations of a wave boundary layer and a thermally stratified boundary layer, obtained with the same numerical model, are also presented. There is good agreement between measured and simulated mean and turbulent quantities in both cases.
The role of sea ice and other fresh water in the Arctic circulation
Authors: Aagaard K. , Carmack E. C.
Published: February 2008

Salinity stratification is critical to the vertical circulation of the high‐latitude ocean. We here examine the control of the vertical circulation in the northern seas, and the potential for altering it, by considering the budgets and storage of fresh water in the Arctic Ocean and in the convective regions to the south. We find that the present‐day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection. Small variations in the fresh water supplied to the convective gyres from the Arctic Ocean via the East Greenland Current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past. The North Atlantic salinity anomaly of the 1960s and 1970s is a recent example; it must have had its origin in an increased fresh water discharge from the Arctic Ocean. Similarly, the freshening and cooling of the deep North Atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the Arctic Ocean into the convective gyres. Finally, we note that because of the temperature dependence of compressibility, a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean.
On the role of ice interaction due to floe collisions in marginal ice zone dynamics
Authors: Lu Qian‐ming , Larsen Jesper , Tryde Per
Published: February 2008

A constitutive relation based on floe collisions as introduced by Shen et al. (1986) is used in a sea ice model for calculating the internal ice force. The formulation is generalized to collisions of discs of two sizes and of multiple sizes and a comparison with existing constitutive models is presented. A coupling of the floe collision model with a viscous‐plastic model (Hibler, 1977) in the simulation of mesoscale sea ice processes in subpolar regions is suggested in order to account for the marginal ice zone dynamics.
Solitonlike solutions in loop current eddies
Authors: Nakamoto Shoichiro
Published: February 2008

The application of the nonlinear quasi‐geostrophic equations to an isolated eddy in the western continental slope region in the Gulf of Mexico is examined for a two‐layer ocean model with bottom topography. In the linear limit, solutions are topographic nondispersive waves. Form‐preserving solutions, or solitons, have been found. The solution is shown to be a limiting form for a nonlinear dispersive system propagating northward along the topographic waveguide in the western continental slope region in the Gulf of Mexico. Using satellite‐tracked drifter data, a linear relationship is found between the amplitude of the deduced stream function of the eddy and its observed translational velocity over the continental slope, which supports the hypothesis that some mesoscale eddies interacting with the continental slope behave as solitons.
Observations on electromagnetic bias in radar altimeter sea surface measurements
Authors: Walsh Edward J. , Jackson Frederick C. , Uliana Enzo A. , Swift Robert N.
Published: February 2008

Because the relative radar cross section of the sea surface increases below mean sea level and decreases above it, the range measurements of satellite radar altimeters are biased toward the wave troughs. Published and unpublished direct measurements of this electromagnetic (EM) bias are examined as well as the predictions of theoretical developments. The EM bias is predominantly a function of the radar frequency used, averaging 1.2% of the wave height of Ka band and 3.3% of the wave height at X band. The airborne measurements present a consistent picture of the variation of the relative radar cross section as a function of deviation from mean sea level. A technique to measure EM bias at the Ku and C band operating frequencies of the TOPEX satellite altimeter is described.
The role of breaking wavelets in air‐sea gas transfer
Authors: Csanady G. T.
Published: February 2008

Molecular diffusion sustains the flux of soluble gases on the water side of the air‐sea interface. The “handover” of this flux to more efficient eddy mixing begins with the smallest eddies, of size l;, which interact with the surface diffusion boundary layer (DBL), of thickness δ. Owing to the discrepancy of the scales, δ ≪ l, the flow field on the δ scale consists of horizontal motions of a velocity constant with depth and varying horizontally on the l scale. The vertical velocity is proportional to the divergence of the horizontal flow and increases linearly with depth. An exact solution of the advection‐diffusion equation for the simple model of divergent stagnation point flow shows the mass transfer coefficient (velocity) k to be proportional to (aD1/2) and DBL thickness δ to be proportional to (D/a1/2), where a is divergence, D diffusivity. Over a solid wall a similar model of Hiemenz flow yields a more complex relationship, also involving viscosity. These models reveal the mechanism by which the DBL is kept thin. The most intense surface divergences on a wind‐blown sea surface are associated with rollers on breaking wavelets. Vorticity and divergence in the rollers are both proportional to u*2/v;, where u* is friction velocity and v is viscosity. The mass transfer coefficient resulting from divergences of this magnitude is then given by k = const u* Sc−1/2, where Sc is Schmidt number. Exact solutions of the advection‐diffusion equation for model rollers reveal the details of the handover process. A thin DBL is maintained over divergences by the upward velocity. At convergences, narrow downward plumes convey DBL fluid into the turbulent interior. Flux lines (analogous to streamlines) are horizontal over divergences and dive down under convergences. Application to the sea surface requires a parameter quantifying the surface density of divergences. Laboratory data imply that a substantial fraction of the surface is covered by the divergences at higher wind speeds. However, in the open ocean straining by the large waves, and especially whitecapping, may significantly reduce the density of divergences and with it the area‐average gas transfer rate. On the other hand, bubble and droplet production in whitecaps may diminish this effect or even reverse it.
Circulation in the Algerian Basin during June 1986
Authors: Perkins Henry , Pistek Pavel
Published: February 2008

Hydrographic measurements, made in the Algerian Basin during June 1986 as part of the Western Mediterranean Circulation Experiment, together with satellite infrared images, are used to describe the flow of major water types in the region. Modified Atlantic Water flows eastward along the Algerian coast as the Algerian Current to 4°E, where it breaks into a series of eddies and spreads far offshore. It then flows partly northward into the Ligurian‐Provencal Basin and partly eastward through the Strait of Sardinia. The subsurface Levantine Intermediate Water turns sharply northward after flowing westward through the Strait of Sardinia and forms a narrow vein along the Sardinian Coast. Winter Intermediate Water flows southward around the Balearic Islands. Its eastern branch recirculates back to the north, whereas part of the western branch contributes to the Algerian Current by flowing eastward beneath the Modified Atlantic Water. Two large, long‐lived eddies are confirmed in the area. The easternmost of these, and other features with unresolved horizontal structure between Sardinia and Menorca, extend from the surface through the pycnocline.
Effects of sea maturity on satellite altimeter measurements
Authors: Glazman Roman E. , Pilorz Stuart H.
Published: February 2008

For equilibrium arid near‐equilibrium sea states, the wave slope variance is a function of wind speed U and of the sea maturity. The influence of both factors on the altimeter measurements of wind speed, wave height, and radar cross section σ0 is studied experimentally. The necessary theoretical background, both on wave statistics and on the near‐nadir radar backscatter, is outlined. It is based on a recently proposed view that the effective fractal dimension for a surface patch corresponding to the equilibrium range of wave spectra increases as the wave field matures. The experimental part is based on 1 year's worth of Geosat altimeter observations colocated with in situ wind and wave measurements by 20 NOAA buoys. Errors and biases in altimeter wind speed and wave height measurements are investigated. A geophysically significant error trend correlated with the sea maturity (i.e., with C0/U, where C0 is the phase velocity of waves corresponding to the spectral peak and U is the mean wind speed well above the surface) is found in wind speed measurements. This trend is explained by examining the effect of the generalized wind fetch,
, on the curves of the observed dependence σ0 = f(U, X). We conclude, in particular, that unambiguous measurements of wind speed by altimeter, in a wide range of sea states, are impossible without accounting for the actual degree of wave development. An approach to a more accurate retrieval of wind speeds is suggested as based on accounting for the actual degree of wave development, which can be achieved through processing the total radar cross section simultaneously with the significant wave height information contained in altimeter waveforms. The potential capability of a satellite altimeter to estimate wave age and similar measures of wind‐wave coupling is discussed. This capability expands the use of satellite altimeters for air‐sea interaction studies.
On the influence of local and north Atlantic wind forcing on the seasonal variation of sea level on the Newfoundland and Labrador Shelf
Authors: Greatbatch Richard J. , de Young Brad , Goulding Allan , Craig Joe
Published: February 2008

Sea level data from St. John's, Newfoundland, and from Nain, Labrador, are compared with results from a 1/4°×1/4° resolution numerical model of the Newfoundland and Labrador shelf and the neighboring Labrador Sea. The model is barotropic, employs dynamics linearized about a state of rest, and uses linear bottom friction. The model is driven by the seasonal part of the Hellerman and Rosenstein wind stress field and by inflow from the rest of the North Atlantic specified along the eastern boundary. The latter is taken from a 1°×1° version of the model applied to the North Atlantic between 10°S and 80°N. The results indicate that our simple model can account for that part of the annual cycle of sea level on the shelf directly attributable to wind forcing. In particular, the model can account for the difference between observed monthly mean sea level at St. John's, corrected for atmospheric pressure variations, and the seasonal steric height anomalies above 150 dbar on the neighboring shelf. We also compare our model results with the linear regression analysis carried out by Thompson et al. on monthly mean sea level data from Nain for the period 1964 to 1972. The model confirms the gains calculated for both the local wind driven signal and also that part of the signal due to wind forcing over the North Atlantic beyond the Labrador Sea. Comparing St. John's with Nain, we find that at St. John's, the model response is accounted for primarily by wind set‐up on the shelf, whereas at Nain, the influence of North Atlantic wind forcing is a larger part of the total signal. These results suggest that data from the tide gauge at Nain could be useful for verifying model‐predicted changes in the large‐scale ocean circulation. The results also seem to be confirming the presence of the offshore transport variations predicted by the model.
A simulation of the 1982–1983 El Niño
Authors: Jia Y. L. , Wells N. C. , Rowe M. A.
Published: February 2008

A general circulation model of the equatorial Pacific Ocean was used to simulate the 1982–1983 El Niño event. The wind data set used to force the model is that of Florida State University (FSU). The results are presented here and compared with those from other simulations. The model results show that the simulation is successful in reproducing the major features of the 1982–1983 El Niño such as the disappearance of the Equatorial Undercurrent, the pattern and magnitude of the sea surface temperature anomaly, and the variations of the thermocline. However, there is a delay of about 2–3 months in these events in the model compared with those observed. This is attributed to the wind stress forcing during 1982 in the eastern equatorial Pacific, which is more intense in the FSU wind data set than in the National Meteorological Center data set. Along the South American coast the model results show a persistent cooling before a slight warming in April 1983 during the event, which is not understood.
Thermohaline structure and zonal pressure gradient in the western equatorial Pacific
Authors: Mangum L. J. , Hayes S. P. , Toole J. M. , Wang Z. , Pu S. , Hu D.
Published: February 2008

Two recent sections of temperature and salinity along the equator in the western Pacific were examined in the context of the historical hydrographic data. Both sections occurred during the December–February period when climatological mean winds are northwesterly. The historical data indicate that the zonal sea surface slope responds to the seasonal change in wind stress. During the southeasterly monsoon the historical mean surface relative to 500 dbar dynamic height slopes upwards from east to west; during the northwesterly monsoon this slope reverses. Vertical profiles of the mean zonal pressure gradient relative to 500 dbar suggest that the wind influence may extend quite deep in the water column. The two quasi‐synoptic sections both indicated that the sea level sloped downward towards the west between 155°E and 140°E. The sign of this slope agrees with the historical seasonal mean; however, the zonally averaged sea level in this westernmost region changed by nearly 0.1 dyn m between sections. Upper ocean water properties and stratification confirmed the importance of salinity in determining mixed layer depth. Comparisons of repeat stations collected during a 16‐day period indicated relatively rapid changes in near surface properties and the importance of lateral advection.
Satellite ocean color studies of Antarctic ice edges in summer and autumn
Authors: Comiso J. C. , Maynard N. G. , Smith W. O. , Sullivan C. W.
Published: February 2008

Large areas of elevated phytoplankton pigment concentrations were observed using CZCS satellite data in the Weddell Sea marginal ice zone and adjacent regions during the austral summer‐autumn transition. The study was made in conjunction with in situ observations of pigment levels and ice data derived from satellite passive microwave observations. The area of satellite investigation includes the eastern and western parts of the Weddell Sea and the western part of the Antarctic Peninsula. Phytoplankton blooms, about 200 km wide and extending several hundred kilometers along the ice edge, were observed. Higher pigment values in the south than in the north indicate that irradiance is not the only factor controlling spatial distributions of phytoplankton pigment concentrations at this time. A time series in Marguerite Bay also shows the persistence of a bloom for at least 12 days and significant effects on spatial distribution of the phytoplankton due to ocean and sea ice movement. An unusual pattern of sea ice formation in the Weddell Sea was also observed in a series of CZCS images near the Greenwich meridian. Turbulent oceanic and atmospheric forcings are suggested during the early stages of development of sea ice in the region and may account for the low pigment concentrations observed adjacent to the area. This study shows that ice edge phytoplankton blooms are not simply a spring‐summer feature but extend into the austral autumn and that they may contribute significantly to regional productivity.
Theoretical estimates of light reflection and transmission by spatially complex and temporally varying sea ice covers
Authors: Perovich Donald K.
Published: February 2008

The reflection, absorption, and transmission of light at visible and near‐infrared wavelengths by snow and ice covers is important for a number of geophysical problems. The focus of this paper is on the reflection and transmission of light by spatially inhomogeneous and temporally varying sea ice covers. This is investigated using a two‐stream, multilayer radiative transfer model in the wavelength region from 400 to 1000 nm. The model is computationally simple and utilizes the available experimental data on the optical properties of sea ice. The ice cover is characterized as a layered medium composed of selections from nine distinct snow and ice types. Three case studies are presented illustrating values of spectral albedo, transmittance, and transmitted photosynthetically active radiation (PAR) for (1) a spatially inhomogeneous ice cover, (2) a uniform ice cover as it undergoes a melt cycle, and (3) a temporally changing spatially variable ice cover. Results indicate that small‐scale horizontal variations in snow depth and ice thickness can cause light transmission to change over 3 orders of magnitude. Dramatic changes in light reflection and transmission are predicted in the early part of the melt season as the ice cover evolves from an opaque, snow‐covered medium to translucent bare or ponded ice.
Two‐dimensional wave number spectra of small‐scale water surface waves
Authors: Jähne Bernd , Riemer Klaus S.
Published: February 2008

Two‐dimensional wave slope spectra have been measured in the large Delft wind‐wave facility using an imaging optical technique and digital image processing. The data cover wavelengths from 0.4 to 24cm and wind speeds (U10) from 2.7 to 17.2 ms−1. The spectral densities of small gravity waves at higher wind speeds are proportional to k−3.5 and u*. Capillary‐gravity and capillary waves show features which clearly manifest that the energy balance for these waves is much different from that for gravity waves. The degree of saturation is approximately constant at a given wind speed, but strongly increases with friction velocity (∝ u*2.5). A sharp cutoff, which is almost independent of the wind speed, occurs at a wavelength of about 7 mm.
Dynamic sea surface topography, gravity, and improved orbit accuracies from the direct evaluation of Seasat altimeter data
Authors: Marsh J. G. , Koblinsky C. J. , Lerch F. , Klosko S. M. , Robbins J. W. , Williamson R. G. , Patel G. B.
Published: February 2008

A gravitational model incorporating Seasat altimetry, surface gravimetry, and satellite tracking data has been determined in terms of global spherical harmonics complete to degree and order 50. This model, PGS‐3337, uses altimeter data as a dynamic observation of the satellite's height above the sea surface. A solution for the ocean's dynamic topography is recovered simultaneously with the orbit parameters, gravity, and ocean tidal terms. The recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 2000 km and is very similar to the mean upper ocean dynamic height derived from historical ship observations. The PGS‐3337 geoid has an uncertainty of 60 cm rms globally but only 25 cm rms over the ocean because of the altimeter measurements. Seasat orbits determined in this solution have an estimated accuracy for the radial position of 20 cm rms. The difference between the altimeter observed sea height and the geoid plus dynamic topography model is 30 cm rms. Contained in these residuals are the sea height variability, as well as errors from the geoid, orbits, tidal models, and altimeter range measurement. This performance level is 2 to 3 times better than that achieved with previous Goddard gravitational models.
A redistribution function applicable to a dynamic model of sea ice
Authors: Shinohara Yoshio
Published: February 2008

A redistribution function of compactness is obtained for a dynamic model of sea ice on the assumption that the work done by the internal stress of sea ice is used to pile up the sea ice and to reduce its compactness. Supposing that the yield curve has an elliptical form and the stress is a function of strain rate, the redistribution function obtained in the present study is written in the following form: Ψ(ε1, ε2) =F(A) * [(ε1+ ε2) − {(ε1+ ε2)2+ (ε1− ε2)2}1/2] whereF(A) is a function of compactnessA, and ε1and ε2are principal components of the strain rate tensor. When this redistribution function was applied to the model proposed by Hibler (1979), calculated ice velocity and ice growth increased up to 50%.
Motion and evolution of oceanic rings in a numerical model and in observations
Authors: Chassignet Eric P. , Olson Donald B. , Boudra Douglas B.
Published: February 2008

Observed properties of oceanic rings are compared to rings produced in a two‐gyre wind‐driven numerical ocean model and in a model of the South Atlantic/Indian Ocean. Their temporal evolution is discussed in terms of structure and translation rate. They exhibit substantial similarity in terms of thermocline depth, ring size, swirl velocities, and translations speeds. In both observations and numerical model results, the propagation speeds are 2 to 5 times faster than that of an equivalent isolated eddy (which is of the order of the long Rossby wave speed). This is attributed to advection by the mean flows. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom. There is strong evidence that this is also the case in real oceanic rings. One major difference between observed and modeled rings is in their decay rate. The temporal decay of the rings in the models and observations is therefore discussed in relation to the decay mechanisms at work and the frictional parameterization of the model.
Using the nutrient ratio NO/PO as a tracer of continental shelf waters in the central Arctic Ocean
Authors: Wilson Cara , Wallace Douglas W. R.
Published: February 2008

Historical nitrate, phosphate, and dissolved oxygen data from the central Arctic Ocean are examined with particular emphasis on the conservative parameters NO (9 * NO3 + O2) and PO (135 * PO4 + O2). The NO/PO ratio is shown to increase with depth in the Canada Basin, being ∼0.78 in Surface and Upper Halocline Waters and ∼1.0 in the Atlantic Layer and Deep Waters. Lower Halocline Water is marked by NO and PO minima and intermediate NO/PO. NO/PO ratios from the Arctic shelf seas are examined to determine possible source regions for the various water masses. The NO/PO ratio of Canada Basin Deep Water implies an upper bound of ∼11% shelf water contribution to this water mass. A slight oxygen maximum core in the Lower Halocline Water is identified at a salinity of S = 34.5 in the vicinity of the Alpha Ridge. This core appears to be diminished by diapycnal mixing and does not extend into the Beaufort Gyre.
Correlated observations of three triggered lightning flashes
Authors: Idone Vincent P. , Orville Richard E. , Hubert Pierre , Barret Louis , Eybert‐Berard Andre
Published: February 2008

A cooperative analysis of three triggered lightning flashes initiated during the Thunderstorm Research International Program (TRIP '81) at Langmuir Laboratory, New Mexico, has resulted in the determination of three‐dimensional return stroke propagation speeds and peak currents for 56 subsequent strokes. We find a nonlinear relationship between the return stroke propagation speed and the stroke peak current that is qualitatively similar to that proposed by both Lundholm and Wagner. A nonlinear correlation is also found between the return stroke propagation speed and the dart leader propagation speed for 32 strokes. Linear correlation coefficients are evaluated for the following quantities: dart leader propagation speed and ensuing return stroke peak current (32 strokes; r = 0.84); stroke peak current and interstroke interval (69 strokes; r = 0.57); return stroke propagation speed and interstroke interval (56 strokes; r = 0.49); dart leader propagation speed and interstroke interval (32 strokes; r = 0.43). The weak positive correlation found between the dart leader propagation speed and the interstroke interval contradicts previous corresponding natural lightning observations. The mean three‐dimensional return stroke propagation speed for 56 strokes is 12×107 m/s with a range of 6.7×107 m/s to 17×107 m/s; the mean three‐dimensional propagation speed found for 32 dart leaders is 20×106 m/s with a range of 9.5×106 m/s to 43×106 m/s. The mean dart length measured is 50 m with a range of 15–90 m. In two of the three flashes, the downward propagating dart leaders appear to meet upward propagating streamers at heights of approximately 20 and 30 m, respectively.
Aerosol characteristics from bistatic lidar observations
Authors: Parameswaran K. , Rose K. O. , Murthy B. V. Krishna
Published: February 2008

A method of obtaining the physical characteristics of aerosols has been developed using bistatic continuous wave lidar observations on atmospheric scattering. The results obtained using this method are also presented.
A model of oxygen isotope composition of precipitation: Implications for paleoclimate data
Authors: Covey Curt , Haagenson Philip L.
Published: February 2008

The ratio of oxygen isotopes in glacial ice is often used as an indicator of climatic conditions during the period of time in which the ice was deposited. We have developed a model which predicts the isotopic composition of snow, given the composition of the source water and the meteorological path between the formation of an air‐water parcel in the boundary layer and the final deposition of precipitation onto the ice sheets. After comparing the model results with isotope data from the present, we draw inferences about the past isotopic composition of continental ice sheets. Our results depend strongly on the details of the meteorological path. They are consistent with standard interpretations of isotope data, but in some cases they allow alternate interpretations.
The photochemistry of a remote marine stratiform cloud
Authors: Chameides William L.
Published: February 2008

The coupled gas‐ and aqueous‐phase photochemistry of a stratiform cloud in a remote region of the marine atmosphere is investigated with a time‐dependent box model. Both scavenging of ambient acidic aerosols and gases as well as aqueous‐phase chemical reactions within droplets are found to be important sources of acidity to cloud water and can lead to pH levels in cloud water in the remote marine atmosphere well below 5.6. The major sources of acidity via aqueous‐phase chemical reactions are the generation of sulfuric acid from dissolved SO2 and the generation of formic acid from dissolved formaldehyde. In both cases, aqueous‐phase free radicals can play a significant role either directly by oxidizing dissolved SO2 and HCHO or indirectly by producing the aqueous‐phase oxidant H2O2. The rate of SO2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO2, H2O2, HO2, and OH, the liquid water content, and the ambient levels of SO2, HNO3, and other acidic or basic gases. Because high levels of SO2 tend to deplete cloud water of H2O2, the possibility exists that the pH of precipitation in polluted regions will respond nonlinearly to reduced SO2 emissions.
Parameterization of IR cooling in a Middle Atmosphere Dynamics Model: 2. Non‐LTE radiative transfer and the globally averaged temperature of the mesosphere and lower thermosphere
Authors: Apruzese John P. , Strobel Darrell F. , Schoeberl Mark R.
Published: February 2008

An efficient technique for including non‐LTE radiative transfer effects has been added to our LTE IR cooling model. The physical processes of thermal and turbulent conduction, odd oxygen transport, gravity wave dissipation, and NO cooling are incorporated into the radiation model to calculate the globally averaged equilibrium temperature profile up to 120 km. The quantitative importance of each process is discussed. We find that gravity wave effects are crucial in cooling the lower thermosphere, which would otherwise be 15–100 K warmer. The magnitude of required gravity wave cooling provides a stringent upper limit on the globally averaged turbulent diffusion coefficient in the mesopause region of <106 cm2 s−1. In the 65‐ to 75‐km mesospheric region, IR cooling rates are uncertain primarily due to the lack of accurate CO2 hot band collisional activation rates. Without any gravity wave heating in this region, total IR cooling rates of 2 K d−1 are required for the computed equilibrium temperature profile to agree with observations, in contrast to the 3.5 K d−1 obtained with a two‐level‐atom non‐LTE source function. Gravity wave heating in this region is estimated at 0.5 K d−1.
NIMBUS‐Earth Radiation Budget Sensor characterization for improved data reduction fidelity
Authors: Maschhoff R. , Jalink A. , Hickey J. , Swedberg J.
Published: February 2008

Detailed characterizations of flight spare earth flux sensors from the NIMBUS Earth Radiation Budget (ERB) program have been performed which, when coupled with a more careful accounting of the orbital instrument environment, provide the potential for improved accuracy in the final data products. The characterizations included detailed field‐of‐view mappings, responses to transient long and short wavelength radiation, and response to sensor temperature changes. These sensor and environment characterizations, along with the outstanding low noise and stability properties of the ERB instrument signal processing system, promise improvement of the data accuracy to levels sufficient for long‐term budget and climatological purposes. The combined data sets from NIMBUS 6 and 7 are expected to span a period in excess of 10 years. The improvements in data accuracy are particularly significant over zonal latitude bands because the corrections are strongly latitude dependent.
Validation of temperature retrievals obtained by the Limb Infrared Monitor of the Stratosphere (LIMS) Experiment on NIMBUS 7
Authors: Gille John C. , Russell James M. , Bailey Paul L. , Gordley Larry L. , Remsberg Ellis E. , Lienesch James H. , Planet Walter G. , House Frederick B. , Lyjak Lawrence V. , Beck Sharon A.
Published: February 2008

The LIMS is a six‐channel limb scanning infrared radiometer flying on the NIMBUS 7 spacecraft. It measured radiances from October 25, 1978 to May 28, 1979, from which vertical profiles of temperature, ozone, water vapor, nitric acid and nitrogen dioxide were derived. The temperature is of major importance for studies of middle atmospheric dynamics, as well as for chemical and radiative studies. It is also needed by LIMS in order to derive the concentrations of the trace gases. This paper briefly outlines the temperature determination (as a function of pressure) from measurements in two channels covering portions of the 15‐μm band of CO2. The known sources of error, from the radiometer and data reduction, are used to estimate the systematic and random errors (accuracy and precision, respectively) expected of the results. Observational determinations of the complete end‐to‐end precision are obtained by computing the standard deviation of six sequential temperature retrievals in regions where the atmosphere is horizontally uniform. This yields values of 0.2 to 0.6 K, in reasonable agreement with the estimates. A correction for horizontal gradients in the atmosphere leads to a large reduction in the differences between the stratospheric temperatures determined on the ascending and descending portions of the orbit. The temperatures agree in the mean with radiosondes and rocketsondes to within 1–2 K in most regions below 1 mbar. This is approximately the accuracy of these in situ sensors in the stratosphere. Above 1 mbar, LIMS temperatures are cooler than rocketsondes by a few degrees at all latitudes, especially in mid‐latitude winter. However, comparisons with other results suggest that rocket temperatures are high, contributing at least part of the error. The temperature cross sections show several interesting features, such as cold regions in the mid‐latitude mesosphere and wavelike vertical variations in the tropics, which have not been seen before. These data provide a useful and in some ways unique basis for studies of the stratosphere and mesosphere.
Accuracy and precision of the nitric acid concentrations determined by the Limb Infrared Monitor of the Stratosphere Experiment on NIMBUS 7
Authors: Gille John C. , Russell James M. , Bailey Paul L. , Remsberg Ellis E. , Gordley Larry L. , Evans Wayne F. J. , Fischer Herbert , Gandrud Bruce W. , Girard Andre , Harries John E. , Beck Sharon A.
Published: February 2008

The LIMS is a six channel limb scanning IR radiometer flying on the NIMBUS 7 spacecraft. It measured radiances from October 24, 1978, to May 28, 1979, from which vertical profiles of temperature, ozone, water vapor, nitrogen dioxide, and nitric acid were determined. Nitric acid (HNO3) plays an important role in stratospheric chemistry, both as reservoir and as sink for the nitrogen compounds and because of its relationship to the hydroxyl radical. This paper describes the validation of the HNO3 results, beginning with an outline of the measurements and data reduction leading to the retrievals. The error sources due to instrumental effects and data reduction are described, and their effects on the results are calculated. The predicted random errors are shown to be somewhat larger than the observed values of ∼0.10 ppbv. The LIMS results are within 20% of a set of 15 correlative balloon‐borne measurements, obtained with several techniques over a range of season, latitude, and pressure between 100 and 20 mbar, the region of largest HNO3 mixing ratio. This is about the accuracy of the correlative measurements. The percent differences are larger at altitudes above 10 mbar. LIMS results agree with earlier measurements at all but the highest levels. Total overburdens above 200 mbar calculated from LIMS agree with those measured spectroscopically from aircraft. LIMS and other HNO3 measurements show similar agreement with model predictions. The consistency suggests that the major LIMS error sources are reasonably well understood. The HNO3/NO2 ratio also leads to reasonable OH concentrations. The global distribution of HNO3 in November shows strong latitudinal variation at the altitude of the maximum, with a large build up over the northern hemisphere pole at ∼40 mbar, but lower values over the southern pole. This reverses by early northern spring. Some characteristics and limits on the data are noted, but the LIMS HNO3 determinations are usually of comparable accuracy to other determinations. These results provide a valuable addition to our knowledge of the distributions of trace gases in the stratosphere. By themselves, but especially in conjunction with the LIMS measurements of temperature, ozone, water vapor, and nitrogen dioxide, they form the basis for a wide range of atmospheric studies.
Solar Mesosphere Explorer Ultraviolet Spectrometer: Measurements of ozone in the 1.0–0.1 mbar region
Authors: Rusch David W. , Mount George H. , Barth Charles A. , Thomas Ronald J. , Callan Michael T.
Published: February 2008

The ozone density of the earth's mesosphere in the 1.0–0.1 mbar (48 to 70 km) region has been measured at sunlit latitudes for the period from December 1981 until the present by an ultraviolet spectrometer on the Solar Mesosphere Explorer satellite. Results for 1982 are reported. The ozone mixing ratios are found to be highly variable in time and place, with maxima occurring in the winter hemispheres. The results show complex time variations at all pressure levels, with annual and semiannual variations apparent at most pressures and latitudes. A relative maximum occurs in July at the equator.