bluuebunny/crossref_metadata_2025 huggingface dataset:
A circular median filter approach for resolving directional ambiguities in wind fields retrieved from spaceborne scatterometer data
Authors: Schultz Howard
Published: February 2008

A common problem associated with spaceborne scatterometry, including SASS, NSCAT and ERS‐1, is that retrieval algorithms often compute multiple wind vectors per retrieval cell. Only one wind vector per retrieval cell corresponds to the true solution while the others are aliases that arise from symmetries in the response of small scale surface waves to wind forcing. The NSCAT retrieval algorithm ranks each ambiguous wind vector within a retrieval cell according to the likelihood that the wind vector is the true solution. Although the most likely wind vectors are usually correct, in approximately 40% of the retrieval cells an alias is ranked first. Based on the mathematical properties of these likelihood assignment errors, an ambiguity removal algorithm is derived that utilizes a nonlinear circular median filter (CMF) to select the true solution in each retrieval cell. The CMF algorithm was tested by analyzing twelve simulated wind fields with an average likelihood assignment error rate of 389 per 1000 retrievals. After processing by the CMF algorithm, the error rate was significantly reduced to an average of only 36 per 1000. To better understand the performance of the CMF algorithm under operational conditions, the analysis was extended by adding spatially correlated likelihood assignment errors to one of the simulated wind fields. A re‐analysis of this wind field showed that the performance of the CMF algorithm may decrease significantly if the likelihood assignment errors are correlated over several retrieval cells. In addition to deriving an ambiguity removal algorithm, a wind field smoothing technique that utilizes a circular median filter is derived and tested on one of the resolved wind fields. The results showed that CMF smoothing technique increases the spatial coherency between the true and resolved wind fields. Based on these analyses, specific recommendations are made for monitoring the performance of the wind retrieval process and for improving the accuracy of the retrieved wind fields.
Mesoscale eddy diffusion, particle sinking, and the interpretation of sediment trap data
Authors: Siegel David A. , Granata Timothy C. , Michaels Anthony F. , Dickey Tommy D.
Published: February 2008

A Lagrangian analysis of a particle sinking through a random mesoscale eddy field is used to evaluate the effects of horizontal diffusion and particle sinking rates on paniculate fluxes sampled by an idealized sediment trap. The analysis indicates that the spatial region where collected particles are formed (Lx) is dependent upon the mean sinking rate of the collected particles and the mesoscale eddy advective field above the trap. This weighted spatial averaging creates difficulties in the interpretation of sediment trap data as fluxes of rapidly sinking particles (>200 m d−1) represent local processes (Lx < 20 km) while fluxes of slower sinking particles (<10 m d−1) may be averaged over much larger scales (Lx > 200 km) for a trap deployed at 1000 m. Several examples of the potential effects that this spatial averaging may have upon the ecological interpretation of the sediment trap collected particle fluxes are presented.
Unusually large 210Po deficiencies relative to 210Pb in the Kuroshio Current of the East China and Philippine seas
Authors: Nozaki Yoshiyuki , Ikuta Naoko , Yashima Mayumi
Published: February 2008

Three vertical water profiles of 210Pb and 210Po have been measured in the East China and Philippine seas. All the profiles show a large 210Po deficiency of ∼8 dpm cm−2 relative to 210Pb in the top ∼1000 m of the water column. Based on the 210Po deficiency, the steady state flux for 210Po removal from the surface water is estimated to be 14 dpm cm−2 yr−1. The 210Po/210Pb ratio of sinking particles will be too high unless an additional source of 210Pb into the surface is accounted for. Thus the large 210Po deficit is probably caused by the increased atmospheric input of 210Pb without any significant 210Po, which is focused in the Kuroshio region by isopycnal transport, and the preferential scavenging of 210Po relative to 210Pb by settling particles. The transient model calculations indicate that the model can account for the observed high excess 210Pb inventory and large 210Po deficiency in the water column if focusing of atmospherically derived 210Pb in the Kuroshio water is a factor of 2–3 more than the local input. Such a lateral redistribution by the western North Pacific gyre circulation is not inconsistent with the deeper penetration and the high water column inventories of anthropogenic substances observed in this region.
Hydrography and microstructure of an Arctic cyclonic eddy
Authors: Padman Laurie , Levine Murray , Dillon Thomas , Morison James , Pinkel Robert
Published: February 2008

Hydrographic and velocity profiles were made through a small baroclinic cyclonic eddy during the Arctic Internal Wave Experiment in the Canada Basin in April 1985. The maximum measured azimuthal velocity was 0.38 m s−1 at a depth of 115 m, with velocities decaying to near zero at 30 and 270 m. Maximum isopycnal displacements at the closest approach to the eddy's axis were about 40 m. The deduced radius of maximum velocity is r0 = 7±2 km, and the total radius is about 13 km. Kinetic energy dissipation rate, ε, was enhanced within 70 m of the surface throughout the transect, and for radius r < r0 near 60 m and 180m. The subsurface maxima in ε do not correspond to regions of low Richardson number, but are colocated with locally reduced shear, consistent with the observed dissipation rates being merely the remnants of recent, more energetic mixing events. The time scale for the decay of the eddy, based on its total energy and the measured dissipation rates, is O(10) years. Given this large dissipation time scale, critical layer absorption of vertically propagating internal wave momentum may be dynamically significant to the eddy's evolution.
Coupled sea ice‐mixed layer simulations for the Southern Ocean
Authors: Stössel Achim , Lemke Peter , Owens W. Brechner
Published: February 2008

A coupled sea ice‐mixed layer‐pycnocline model for the Weddell Sea (Lemke et al., this issue; Owens and Lemke, this issue) is extended to the entire sea ice area around the Antarctic continent. The monthly atmospheric forcing and the annual oceanic forcing are specified from climatology. Sensitivity runs were performed with a fixed mixed layer, without snow cover, with a different ice strength constant (P*) and a varying e‐folding constant for the closing of leads (h0), without dynamics, and without ocean currents. Since the atmospheric forcing fields, although derived from large, historical data sets, differ considerably depending on the analysis technique, alternative wind, temperature, and precipitation forcing has been applied. Finally, the model has been forced with mean monthly winds supplemented by stochastic variations representing short‐term variability. The model results are compared with analyses of satellite data as well as with single measurements, and with simulation results from earlier models.
Salt fingering in subsea permafrost: Some stability and energy considerations
Authors: Gosink J. P. , Baker G. C.
Published: February 2008

Salt fingering appears to be a viable mechanism for salt transport in the thawing layer (talik) above ice‐bonded permafrost near the seabed in the Arctic shelves [Baker and Osterkamp, 1988]. These saltwater infusions occur where concentrated brines overlie less concentrated or less dense brines. At least two mechanisms exist which cause this condition near the seabed: first, salts rejected during sea ice growth result in the formation of a concentrated brine layer on the seabed, and second, salts rejected during sediment freezing near the seabed can result in a concentrated brine layer forming within the deeper and yet unfrozen sediments. Unstable salt fingering in a vertical tube containing a porous media is known to develop whenever the density gradient exceeds the critical value: ∂ρ/∂z = 3.390 μk/gkb2 [Wooding, 1959]. This article presents a simple derivation and interpretation of this critical condition in terms of competing time scales. A convective time scale represents the time required for formation of a vertical wedge‐shaped flow due to density differences, and a diffusive time scale represents the time required to diffuse the density difference horizontally across the layer. The critical density gradient is determined from a ratio of these time scales. The stability criterion suggests downward salt fingering whenever the density gradient at Prudhoe Bay exceeds 6.2×10−5 g cm−4 in thawed subsea permafrost sediments. Maximum predicted velocity of the saltwater fingering is about 2 m d−1, and this is consistent with the direction and magnitude indicated by measurements of pressure gradients and numerical modeling in the thawing permafrost. The energy dissipated by viscous forces in the thawed layer balances the potential energy added by the salt fingers caused by concentrated brines on the seabed.
Experimental observations of baroclinic eddies on a sloping bottom
Authors: Whitehead John A. , Stern Melvin E. , Flierl Glenn R. , Klinger Barry A.
Published: February 2008

Baroclinic eddies in a rotating box with a sloping bottom were produced by squirting dense salt water up the sloping bottom and along the “eastern” wall. The jet stagnated in shallow water and was ejected normal to the wall. For certain parameters (volume flux of jet, etc.), a coherent lens of dense bottom water formed and propagated west with an overlying cyclonic vortex. The circulation in the bottom lens, on the other hand, was relatively weak. No such eddy forms when the depth of fresh water is relatively deep, and a regime diagram is given for the formation of the coherent eddies. Thus a relatively simple structure emerges despite the complexity of the generating process. The pressure field determined from density measurements is discussed in terms of an integral theorem for coherent eddies, and the westward propagation is also related to previous theories. Several other techniques for generating such eddies are discussed.
The behavior and stability of a lens in a strain field
Authors: Brickman David , Ruddick Barry
Published: February 2008

The behavior of an anticyclonic coherent eddy in a barotropic strain field is examined using a shallow water equation model. The “lens equation” model, a set of eight nonlinear ordinary differential equations, was first exploited by Cushman‐Roisin et al. [1985] and Ripa [1987] to examine the behavior and stability of isolated vortices. In this paper we expand the application to include the effect of external fields on the vortex, in particular, an external barotropic strain field. The equilibrium solutions to the equations are found to be elliptical lenses aligned along the high‐pressure axis of the strain field (up to a rate of strain of 0.1f), and an aphysical rain gutter aligned along the strain field outflow axes. The linear stability of the elliptical solutions is explored, and it is found that regardless of the rate of strain, vortices with ellipticities (E) greater than ∼2 are unstable to lens‐like and higher order perturbations, and that this value of E corresponds to a maximum in the conservative lens quantity Potential Vorticity (PV). Time dependent integrations of the model confirm the results of the instability analysis: lenses with Es or PVs greater than the stable maxima integrate toward the the rain gutter solution. The initial growth rates (O∼ f/20) correspond to the linear analysis, but once the perturbations are finite, the growth rate is superexponential. Numerical experiments with a time dependent external strain field show that lens PV can be used as a predictor of stability, and a general stability diagram for a lens in an external strain field is constructed. Laboratory experiments confirm some of the results of the theoretical model and also reveal its weaknesses.
Reflection and transmission characteristics at the edge of shore fast sea ice
Authors: Fox Colin , Squire Vernon A
Published: February 2008

The reflection and transmission of ocean waves at a sea ice boundary is reconsidered. The sea ice is modelled as a continuous, thin elastic plate of uniform thickness, floating on water of arbitrary constant depth. Unlike earlier solutions, matching of potentials between the free surface domain and the ice‐covered domain is done at all depths; previous solutions were incompletely matched, as the potentials in each domain were deficient. In the present solution a number of the infinity of evanescent modes, which were hitherto ignored, are included in the solution and allow matching to be carried out by minimization of an integrated error term from surface to seafloor. Reflection and transmission are found to be markedly influenced by the inclusion of these modes, suggesting that conclusions based on the incomplete potentials may be substantially in error.
A wind‐driven isopycnic coordinate model of the north and equatorial Atlantic Ocean 2. The Atlantic Basin Experiments
Authors: Smith Linda T. , Boudra Douglas B. , Bleck Rainer
Published: February 2008

A series of wind‐driven isopycnic coordinate model experiments are described and analyzed. In five preliminary experiments, a progression is established from the simple calculations of part 1 of this paper (Bleck and Smith, 1990) to multilayer simulations incorporating realistic forcing in a framework of coarse‐mesh Atlantic basin geometry and smoothed bottom topography. The primary experiment, a five‐layer seasonally forced model run, simulates known features of the annual mean wind‐driven circulation of the Atlantic, including a subtropical western boundary current separating from the coast near Cape Hatteras and a subpolar western boundary current flowing past the Grand Banks. The model reproduces (though with reduced amplitude) the annual cycle of Florida Current transport, seasonal variations in the equatorial region (including seasonal heat transport) and seasonal sea level cycles in the vicinity of strong currents.
Surface heat fluxes and marine boundary layer modification in the Agulhas Retroflection region
Authors: Mey Rosemary D. , Walker Nan D. , Jury Mark R.
Published: February 2008

The Subtropical Convergence Agulhas Retroflection Cruise, from February 12 to March 4, 1987, provided an opportunity for studying variability of surface heat fluxes and marine boundary layer modification within an ocean area where few atmospheric measurements have been made. From 3‐hourly surface observations it has been ascertained that surface heat flux processes are enhanced under cold air outbreak, postfrontal synoptic conditions and over warmer ocean areas. A maximum turbulent heat loss of 828 W/m2, 4 times the climatological value, was found. Over the cooler Subtropical Convergence waters and over the Agulhas Bank, heat fluxes were comparatively low. Two transect lines were performed across the Subtropical Convergence‐Agulhas SST front. In both cases, air temperatures, dew point temperatures, and particularly wind speeds increased over the warmer water owing to enhanced vertical mixing resulting from oceanic heat losses. The second transect revealed marked vertical shear of the zonal wind. The essential elements for cyclogenesis, namely, low static stability and strong low level baroclinicity, were found to be characteristic of the region.
Analysis of nonlinear internal waves observed by Landsat thematic mapper
Authors: Artale V. , Levi D. , Marullo S. , Santoleri R.
Published: February 2008

In this work we test the compatibility between the theoretical parameters of a nonlinear wave model and the quantitative information that one can deduce from satellite‐derived data. The theoretical parameters are obtained by applying an inverse problem to the solution of the Cauchy problem for the Korteweg‐de Vries equation. Our results are applied to the case of internal wave patterns elaborated from two different satellite sensors at the south of Messina (the thematic mapper) and at the north of Messina (the synthetic aperture radar).
Numerical experiments with a wind‐ and buoyancy‐driven two‐and‐a‐half‐layer upper ocean model
Authors: Cherniawsky J. Y. , Yuen C. W. , Lin C. A. , Mysak L. A.
Published: February 2008

We describe numerical experiments with a limited domain (15°–67°N, 65° west to east) coarse‐resolution two‐and‐a‐half‐layer upper ocean model. The model consists of two active variable density layers: a Niiler and Kraus (1977) type mixed layer and a pycnocline layer, which overlays a semipassive deep ocean. The mixed layer is forced with a cosine wind stress and Haney type heat and precipitation‐evaporation fluxes, which were derived from zonally averaged climatological (Levitus, 1982) surface temperatures and salinities for the North Atlantic. The second layer is forced from below with (1) Newtonian cooling to climatological temperatures and salinities at the lower boundary, (2) convective adjustment, which occurs whenever the density of the second layer is unstable with respect to climatology, and (3) mass entrainment in areas of strong upwelling, when the deep ocean ventilates through the bottom surface. The sensitivity of this model to changes in its internal (mixed layer) and external (e.g., a Newtonian coupling coefficient) parameters is investigated and compared to the results from a control experiment. We find that the model is not overly sensitive to changes in most of the parameters that were tested, albeit these results may depend to some extent on the choice of the control experiment.
The modulation of a radar signal from the ocean surface due to slope and hydrodynamic effects
Authors: West James C. , Hwang Paul A. , Moore Richard K. , Holtzman Julian C. , Shemdin Omar H.
Published: February 2008

The modulation of the signal level of a tower‐based radar scatterometer illuminating a small area of the ocean surface is predicted from direct measurements of the hydrodynamic conditions of the ocean surface and compared with X band radar measurements taken simultaneously. The radar backscatter is assumed to be due to Bragg resonance between the signal and the ripple waves. The effects of the slope of the ocean surface due to passing long waves are modeled by converting a measured wave height series into a slope time series. Laser‐slope‐gauge measurements of the spectrum of the ripple waves that ride upon the long ocean waves are used to predict the hydrodynamic effects. Comparisons with the measured radar signal show that the slope modulation alone gives a poor prediction of the modulation. Inclusion of the hydrodynamic effects in the simulation appears more promising, although it also is not entirely successful with this data set.
Cross‐frontal cold jets near Iceland: In‐water, satellite infrared, and Geosat altimeter data
Authors: Scott John C. , McDowall Anne L.
Published: February 2008

This paper reports detailed in‐water observations and satellite infrared images which are approximately coincident with a single Geosat altimeter track across the Iceland‐Faeroes Frontal Zone. The ARE thermistor chain covered the upper 300 m of the ocean along the track, and the first two of a long sequence of NOAA satellite infrared images were obtained, all within 24 hours of the Geosat overpass. The data are interpreted as showing cold cross‐frontal jets related to the formation of cold eddies south of the main frontal boundary. Implications for the use of altimetry for ocean monitoring are considered.
Application of a three‐dimensional turbulence energy model to the determination of tidal currents on the northwest European Shelf
Authors: Davies A. M. , Jones J. E.
Published: February 2008

A three‐dimensional turbulence energy tidal model, employing a sigma coordinate system through the vertical with a logarithmic type transformation to enhance near bed resolution, is used to examine M2 tidal currents on the European Continental Shelf. A no slip bottom boundary condition is applied in the model and vertical eddy viscosity is computed from a turbulence energy closure scheme. Comparisons of observed and computed current profiles, determined using the present model and an earlier model employing a spectral representation (Davies 1986) in the vertical, and a slip bottom boundary condition are made. In this spectral model a simple relationship between eddy viscosity and depth mean current was employed. These comparisons of current profile suggest that above the near bed layer the spectral model can reproduce tidal currents with a comparable accuracy to that of the present turbulence energy model. However, in the near bed region the turbulence energy model is significantly more accurate. Computed spatial distributions of current magnitude, turbulence energy intensity, and eddy viscosity show similar distributions over the model region, suggesting that a relationship between eddy viscosity and current intensity (the form used in the spectral model) is a valid alternative to using a turbulence energy model. Turbulence energy intensity, in particular, exhibits significant spatial variations both horizontally and vertically, with surface contours of turbulence corresponding to known positions of tidal fronts.
The Earth Radiation Budget derived from the NIMBUS 7 ERB Experiment
Authors: Jacobowitz Herbert , Tighe Richard J.
Published: February 2008

The earth radiation budget as determined from the ERB experiment aboard the NIMBUS 7 polar‐orbiting satellite is presented in the form of time‐latitude cross sections, hemispherically and globally averaged time plots, and annual global averages for the time period spanning November 1978 through October 1979. Comparisons are made between results derived from the fixed wide field‐of‐view (WFOV) radiometers and those derived from the scanning narrow field‐of‐view (NFOV) radiometers. While there is excellent agreement in regard to the spatial and temporal variations, the absolute magnitudes differ. The NFOV yields outgoing longwave fluxes and albedos that are about 4W/m2 and 2.5% respectively, greater than those derived from the WFOV sensors. Also, limited simultaneous comparisons are made between ERB results and those from the AVHRR on the NOAA 7 operational satellite.
Stratospheric ozone changes during the first year of SBUV observations
Authors: Nagatani R. M. , Miller A. J.
Published: February 2008

Synoptic analyses of solar backscattered ultraviolet (SBUV) ozone data have been performed on a daily basis for the first year of NIMBUS 7 operation, November 1978 through October 1979, at the standard pressure levels: 30, 10, 5, 2, 1, and 0.4 mbar. By combining these data with the daily NMC meteorological analyses of temperature and geostrophic wind at the same levels, we are able to examine elements of the ozone‐temperature relationship as well as the horizontal eddy transports. In particular, the existence of an upper stratospheric high‐latitude ozone minimum in winter is discussed, and the eddy fluxes are found to play a significant role in this feature, mainly associated with stratospheric warming pulses. At the same time, monthly average quantities are examined, and standing eddy components are related.
Sulfur dioxide and particles in quiescent volcanic plumes from Poás, Arenal, and Colima Volcanos, Costa Rica and Mexico
Authors: Casadevall Thomas J. , Rose William I. , Fuller William H. , Hunt William H. , Hart Mark A. , Moyers Jarvis L. , Woods David C. , Chuan Raymond L. , Friend James P.
Published: February 2008

Measurements of SO2 emission rates and concentrations and of particle distribution, size, shape, and composition were made in quiescent volcanic plumes emitted into the troposphere from Poás and Arenal volcanos, Costa Rica, and Colima volcano, Mexico. SO2 emission rates were 700±180 metric tons per day (t/d) for Poás, 210±30 t/d for Arenal, and 320±50 t/d for Colima. The concentrations of SO2 calculated from the COSPEC/lidar data were 5–380 ppb. Concentrations of SO2 measured directly by flame photometry were 10–250 ppb. Particles collected in the plumes with a quartz crystal microbalance impactor were mostly less than 3 μm in diameter and consisted of droplets of dilute sulfur‐bearing solutions and minor amounts of larger silicate particles coated with a sulfur‐bearing film or crust. Total particle concentrations were 4.7 μg/m3 for Poás and 18.8 μg/m3 for Colima. Comparison of concentrations of SO2 in the plumes with gas samples collected at fumaroles on the ground suggests that the plumes are diluted by the atmosphere by factors of up to 105.
Seasonal variation of the global water balance based on aerological data
Authors: Bryan Frank , Oort Abraham
Published: February 2008

The distribution of evaporation minus precipitation over the globe and its seasonal variations are estimated from global atmospheric circulation statistics for the period May 1963 to April 1973. Meridional profiles of evaporation minus precipitation over the Atlantic, Pacific, and Indian oceans, over all oceans combined, and over all continents combined, as well as the total evaporation minus precipitation over each oceanic and continental region, are shown. The Pacific Ocean is found to have an excess of precipitation and the Atlantic an excess of evaporation throughout the year. Over the Indian Ocean, precipitation exceeds evaporation during December‐February, while evaporation exceeds precipitation during the rest of the year and in the annual mean. The results are generally in qualitative agreement with previous estimates of the annual mean world water balance based on surface observations. There are large quantitative discrepancies, however, particularly in the subtropics. A comparison with the analysis of 2 months of data from the FGGE period suggests that the primary source of error in our results is associated with spatial sampling deficiencies in the general circulation statistics. It appears that in many regions the current operational rawinsonde network is inadequate to give reliable quantitative estimates of differentiated quantities such as are required in computing the atmospheric water balance.
13C/12C ratios in tree rings and the transfer of biospheric carbon to the atmosphere
Authors: Stuiver M. , Burk R. L. , Quay P. D.
Published: February 2008

The δ13C records of 11 Pacific coastal trees have been measured. Relationships between δ13C, ring area, and ring width were investigated. Three of the six trees with records longer than 200 years yield a significant (2σ level or better) correlation between δ13C and ring area. The correlation is most pronounced forSequoia giganteum, where the variability in ring area accounts for 60% of the δ13C variance. The environmental factors influencing the isotopic composition of trees are many. A major portion of the pre‐1850 variability in the δ13C record is most likely caused by variable isotope fractionation of the trees. Only about one quarter of the δ13C variance in the average record can be removed by normalizing on ring area. The remaining δ13C variability results in a standard deviation of 16 ppm in model‐calculated preindustrial CO2levels. This is an upper limit for atmospheric CO2variability induced by changes in the size of the biospheric reservoir. Average preindustrial CO2levels are estimated to be in the 260–290 ppm range. The cumulative amount of biospheric carbon released to the atmosphere between 1600 and 1975 is approximately 150±100 gigatons C.
Longwave radiation balance under a strong surface inversion in the Katabatic Wind Zone, Antarctica
Authors: Yamanouchi Takashi , Kawaguchi Sadao
Published: February 2008

Measurements and calculations of longwave radiation fluxes were made in order to clarify their characteristic features under a surface inversion and with katabatic winds. Measurements were made under the Japanese POLEX program (POLEX—South), 1979–1981, at Mizuho Station (70°42′S, 44°20′), where the katabatic wind was blowing continuously and a strong surface inversion existed. Direct measurements of the downward and upward longwave fluxes were extended through day and night using pyrgeometers (Eppley PIR) with a simple shading ring to cut off heating by direct solar radiation. Longwave radiation fluxes were also calculated using a simple wide band model, and the effect of the surface inversion was examined. The daily average of the downward flux varies greatly between 90 and 240 W/m2. The large day to day variation is due to clouds. Overcast skies give an increase of about 80 W/m2 in the downward longwave flux in all seasons. The effective emissivity of the atmosphere for clear sky is very small on account of the small amount of column water vapor. The drifting snow near the surface caused by the strong wind increases the downward longwave flux and suppresses the long‐wave cooling at the surface. The relation between the strength of the surface inversion and longwave fluxes is examined and approximated by a linear relation. When the surface inversion becomes strong, the net longwave flux decreases.
Photochemically produced ozone in the emission from large‐scale tropical vegetation fires
Authors: Delany A. C. , Haagensen P. , Walters S. , Wartburg A. F. , Crutzen P. J.
Published: February 2008

An aircraft measurement program was undertaken in the savanna regions of central South America in the dry season of 1980 to investigate the atmospheric effects of large‐scale biomass burning. The smoke from the fires was found to be largely confined within an approximately 3‐km‐deep boundary layer capped by a subsidence inversion or a stable layer. This condition typically persists for week‐long periods as a result of the synoptic subsidence occurring during the dry season. Photochemical production of ozone occurred in the polluted layer over the entire Cerrado region of central Brazil. The factors controlling the concentration of this ozone are examined, and an estimate of the amount of ozone produced is reported.
Tropospheric ozone: The role of transport
Authors: Levy H. , Mahlman J. D. , Moxim W. J. , Liu S. C.
Published: February 2008

The Geophysical Fluid Dynamics Laboratory general circulation/transport model, with photochemistry in the top level (middle stratosphere) only, is used to simulate global tropospheric ozone distributions for upper and lower limits of surface removal rates. We compared these simulations with available observations and find that large‐scale atmospheric transport plays a major role in the behavior of tropospheric ozone. Furthermore, we identify potential roles for tropospheric chemistry, discover defects in the model's simulations of transport, and gain a more global picture of tropospheric ozone. The transport model's mean cross‐tropopause flux is in the range of previous estimates, and the shapes of the simulated vertical profiles of mixing ratio and percent standard deviation are in good agreement with observations. South of 40°N, the simulated and observed latitude gradients are the same, the upper and lower limit calculations bracket measured values, and the seasonal cycles are well reproduced. While the transport model simulates a wide range of tropospheric ozone climatology, there are a significant number of disagreements. The need for additional ozone destruction in the maritime boundary layer suggests a role for chemical destruction, while in the continental boundary layer, it appears that chemical production, a seasonal cycle in surface deposition, and improved boundary layer transport are all required. The two major defects in the simulated “free troposphere” are (1) excess ozone at high latitudes of the northern hemisphere (NH) and (2) spring rather than summer maxima and fall rather than winter minima at NH mid‐ and high latitudes. While defect 1 has a number of possible causes, deficiencies in model transport play a major role. Although similar transport defects have not been ruled out for defect 2, tropospheric chemistry appears to be needed. Separate calculations of the net chemical production and loss demonstrate that this is a complex problem. The most likely solution involves the transport control of NOx which controls the ozone chemistry.
The effect of breaking gravity waves on the dynamics and chemical composition of the mesosphere and lower thermosphere
Authors: Garcia Rolando R. , Solomon Susan
Published: February 2008

The influence of breaking gravity waves on the dynamics and chemical composition of the 60‐ to 110‐km region has been investigated with a two‐dimensional dynamical/chemical model that includes a parameterization of gravity wave drag and diffusion. The momentum deposited by breaking waves at mesospheric altitudes reverses the zonal winds, drives a strong mean meridional circulation, and produces a very cold summer and warm winter mesopause, in general agreement with observations. The seasonal variations of the computed eddy diffusion coefficient are consistent with the behavior of mesospheric turbulence inferred from MST radar echoes. In particular, it is found that eddy diffusion is strong in summer and winter but much weaker at the equinoxes and that this seasonal behavior has important consequences for the distribution of chemical species. Comparison between computed atomic oxygen and ozone, and the abundances of these constituents inferred from the 557.7‐nm and 1.27‐μm airglow emissions, reveals excellent agreement. The consistency between model results and these diverse types of observations lends strong support to the hypothesis that gravity waves play a very important role in determining the zonally averaged structure of the mesosphere and lower thermosphere.
Sensitivities of the radiative forcing due to large loadings of smoke and dust aerosols
Authors: Ramaswamy V. , Kiehl J. T.
Published: February 2008

Sensitivities of the optical properties and of the radiative perturbations induced by the microphysical characteristics of smoke and dust aerosols in the aftermath of a nuclear war are investigated. All optical calculations assume homogeneous spherical particles, prescribed by lognormal size distributions. A change in the mode radius of the aerosol size distribution between 0.1 and 1.0 μm, for a constant smoke loading, results in a tenfold decrease in the visible optical depth. Over the same size range the long‐wave optical depth increases from 1/30 of the visible optical depth for a mode radius of 0.1 μm to 1/2 for a mode radius of 1.0 μm. The direct radiative forcing is studied using a spectrally dependent, one‐dimensional radiative transfer model. The radiative heating is strongly influenced by the vertical distribution of smoke and dust aerosols. When compared with a constant density profile, the maximum in the heating rate, owing to a 3‐km scale height profile of smoke aerosols (visible optical depth ∼3), is reduced from 16° to 4°K/d and is more widely distributed in the troposphere. Solar absorption in the troposphere is reduced by increases in dust loadings in the stratosphere (a loading of 0.5 g/m2 results in a 30% reduction), while the planetary albedo is enhanced over the smoke‐only case (up to 150% for 0.5 g/m2). Perturbations to the long‐wave fluxes due to aerosols are modulated by the column amount of water vapor. The increase in the long‐wave flux at the surface, however, even for an order of magnitude enhancement in water vapor, is less than the decrease in the solar flux. A time‐marching, one‐dimensional radiative convective model, which uses an eddy diffusion approach to represent convective processes, is employed to study the temporal development of the thermal structure. The relative altitude of the maximum solar absorption to long‐wave emission determines the strength and vertical extent of the tropospheric inversion and the surface temperature response. For a nominal long‐wave smoke optical depth (0.2) the higher the altitude of solar absorption, the greater the reduction in the surface temperature. Thus the assumption of a constant smoke density profile between 0 and 10 km results in a surface cooling of 32°K after 20 days, while the assumption of a constant scale height profile of 3 km causes a cooling of 22°K. For an arbitrarily large long‐wave smoke optical depth (2.0) arising owing to the presence of large (>2 μm) particles in high concentrations in relation to the submicron sizes (a rare event in the unperturbed atmosphere) or owing to large smoke loadings, the altitudes of maximum solar absorption and long‐wave emission become identical. If this altitude is located near the surface, the surface temperature response becomes weak. Thus for a 3‐km aerosol scale height profile, the surface cooling is 3°K, while an extreme assumption of a 1‐km scale height profile causes no change in the surface temperature.
Stratospheric and tropospheric components of 7Be in surface air
Authors: Dutkiewicz V. A. , Husain L.
Published: February 2008

7Be and 90Sr concentrations were measured simultaneously in samples from NASA's Global Atmospheric Sampling Program. The samples were collected from January 1978 through June 1979 at 9 to 12 km and between 11° and 60°N. 7Be concentrations ranged from 150 to 6900 fCi/m3, while 90Sr correspondingly varied from 10 to 320 fCi/m3. The average 7Be concentration in the samples collected in the lower stratosphere was 4500 fCi/m3. In the upper troposphere, 7Be concentrations were generally less than 1000 and often only a few hundred fCi/m3. In situ production of 7Be in the upper troposphere at ∼11 km was estimated at 340 fCi/m3. Since 90Sr is only present in the atmosphere because of nuclear testing, except immediately after such tests, 90Sr in the troposphere is due only to the subsidence of stratospheric air. Since atmospheric processes are not expected to alter 7Be/90Sr ratios during transport, surface 90Sr and stratospheric 7Be/90Sr ratios can be used to estimate 7Be transported from the stratosphere. Our analyses show that on an annual basis the stratosphere contributed ∼25% of the observed 7Be concentration. During spring and summer months, the stratospheric component can approach 40%, while on a daily basis this can be even larger.
Measurements of ion mobility to 30 km
Authors: Rosen J. M. , Hofmann D. J. , Gringel W.
Published: February 2008

Balloon‐borne measurements of atmospheric electrical parameters made during the last several years indicate that there has been a significant change in the average small‐ion mobility. In addition, on some soundings abrupt changes in the ion mobility profile have been noted. These variations provide an unusual opportunity to experimentally study the functional relationships between mobility, ion density, conductivity, and the recombination coefficient in the stratosphere. Within the uncertainties involved it is found that the measurements support the simple, predicted theoretical relationships between these various parameters. If fluctuations in ion mobility are in fact a usual occurrence, then this work indicates that variations frequently observed in the ion concentration are probably related to changes in ion mobility rather than aerosol variations, as is usually assumed. One somewhat surprising result predicted by the functional relationships between the electrical parameters indicates that the ambient conductivity may be relatively insensitive to mobility variations.
On solar‐terrestrial relationships in atmospheric electricity
Authors: Roble R. G.
Published: February 2008

There are many suggested solar‐terrestrial relationships in global atmospheric electricity. Of the various relationships, the downward mapping of ionospheric and magnetospheric electric fields, associated with the solar wind/magnetosphere and the ionosphere wind dynamos, is best understood theoretically and appears to be supported by the few available data. The solar cycle variations of ionospheric potential and air‐earth current appear to be related to variations in galactic cosmic rays and perhaps to their effect on the current output from thunderstorms. The solar flare and solar magnetic sector boundary variations are not well understood but may be related to Forbush decreases in cosmic ray flux and/or effects resulting from energetic particle precipitation. The available data on auroral effects on atmospheric electricity are confusing and not understood at all. There is a clear need for further research to better define the physical mechanisms responsible for all of these solar‐terrestrial relationships. The observed solar‐terrestrial variations and the need for current closure in the global circuit suggest that the function of the equalization layer in the “classical picture” of atmospheric electricity should be revised to be consistent with our current knowledge of upper‐atmospheric electrical processes.
Reflection and emission models for clouds derived from Nimbus 7 earth radiation budget scanner measurements
Authors: Staylor W. Frank
Published: February 2008

Statistical results from 2.8 million pairs of broadband shortwave and longwave radiance measurements were used to develop reflectance and emittance models for low, middle, and high water clouds and high ice clouds. The reflectance model was a function of the sum and product of the cosine of solar and viewing zenith angles, and the emittance model was related by a power law of the cosine of the viewing zenith angle.
Sferics rate in relation to thunderstorm dimensions
Authors: Cherna E. V. , Stansbury E. J.
Published: February 2008

On the basis of data from 3 days in 1978 this paper investigates the relation between thunderstorm dimensions and electrical activity to determine the relative importance of thunderstorm size and thunderstorm environment on the sferics rate. Sferics were recorded continuously by a wide‐band (100 ± 50 kHz) crossed‐loop radio direction finder located at the radar site; precipitation data during the corresponding period came from the radar volume scan. The number of sferics associated with specific storms and recorded in 5‐min intervals varied from a few tens to a few thousands, depending on the dimensions of the source storm and on its distance from the sferics receiver. For four thunderstorm situations this variation was reduced to a small scatter, with standard deviations corresponding to factors of 1.17–1.47, by fitting the data with the empirical relation: S ∝ Ax × 10yH × 10−zr, where S is the observed number of sferics per 5 min, A the cross‐sectional area, H the height, and r the range of the storm. For three of the four sets of data analyzed, x was just over unity, giving a nearly linear relation between the sferics rate and the storm's area; for a supercell storm the relation was higher than linear. For thunderstorms between 50 and 300 km distant the number of recorded sferics decreased exponentially, by a factor of about 2 every 45 km, or with z = 0.0068 ± 0.0011 per km. Thunderstorm height appeared the dominant parameter determining the rate of electrical activity; values of y for the 3 days were between 0.13 and 0.28 per km, the greater effect of thunderstorm height was found associated with a stronger wind shear in the 7.5‐ to 12‐km layer.
Methane emission during a cultivation period from an Italian rice paddy
Authors: Holzapfel‐Pschorn A. , Seiler W.
Published: February 2008

CH4 release rates from rice paddies were measured in Vercelli, Italy, in 1983 during a complete vegetation period, using a static box system. The rice paddies were found to be a source of atmospheric methane during the time of flooding. The CH4 release rates range between a few milligrams of CH4 per square meter per hour and 51 mg CH4 m−2 h−1 and show a seasonal variation with maximum emission rates between tillering and flowering. Averaged over the complete vegetation period, the CH4 release rate accounts for 16 mg m−2 h−1. Significant differences in the CH4 release rates of unfertilized field plots and plots fertilized with mineral fertilizer (CaCN2) were not observed. The CH4 release rates show strong diurnal variations, with highest values in the late afternoon and lowest values in the early morning, which coincides with the temperature variation in the upper soil layer (1–10 cm). These variations must be taken into account in estimating reliable global CH4 emission rates from rice paddies. CH4 is almost exclusively emitted into the atmosphere by gas bubbles during the first 6 weeks after flooding the rice paddies, that is, on fields without vegetation. Afterwards, 80% of the observed CH4 transport from the paddy soil into the atmosphere was due to diffusion through the stems of the rice plants. Transport by diffusion through the paddy water appeared to be of minor importance. The global annual CH4 emission from rice paddies is estimated to be of the order of 39–94 Tg yr−1 (Tg = 1012 g) for 1940 conditions and 70–170 Tg yr−1 for 1979 conditions, indicating a secular trend of the CH4 emission by rice paddies of about 1.6% per year during the last 35 years.
A statistical trend analysis of ozonesonde data
Authors: Tiao G. C. , Reinsel G. C. , Pedrick J. H. , Allenby G. M. , Mateer C. L. , Miller A. J. , DeLuisi J. J.
Published: February 2008

A detailed statistical analysis of monthly averages of ozonesonde readings is performed to assess trends in ozone in the troposphere and the lower to mid‐stratosphere. Regression time series models, which include seasonal and trend factors, are estimated for 13 stations located mainly in the mid‐latitudes of the northern hemisphere. At each station, trend estimates are calculated for 14 “fractional” Umkehr layers covering the altitude range from 0 to 33 km. For the 1970–1982 period, our main findings indicate an overall negative trend in ozonesonde data in the lower stratosphere (15–21 km) of about −0.5% per year, and some evidence of a positive trend in the troposphere (0–5 km) of about 0.8% per year. An in‐depth sensitivity study of the trend estimates is performed with respect to various correction procedures used to normalize ozonesonde readings to Dobson total ozone measurements. The main results indicate that the negative trend findings in the 15 to 21‐km altitude region are robust to the normalization procedures considered.
Transport of atmospheric tracers by planetary waves during a winter stratospheric warming event: A three‐dimensional model simulation
Authors: Kouker W. , Brasseur G.
Published: February 2008

In a simple three‐dimensional primitive equation model, a wave number 1 major stratospheric warming is simulated. With the aid of two idealized tracers it is shown that the transport during a major warming event is characterized by a small, well‐organized tongue of subtropical air flowing around the displaced winter vortex into the polar cap and by a wide area with strong quasi‐horizontal mixing (surf zone). The description of these dynamical processes requires a full three‐dimensional space resolution.
Ozone‐induced oxidation of SO2 in simulated clouds
Authors: Miller D. F. , Gertler A. W. , Whitbeck M. R. , Lamb D.
Published: February 2008

Cloud chamber experiments were conducted to measure the rate of conversion of gaseous SO2 to aqueous sulfate under conditions of warm‐cloud formation. The rate of SO2 conversion in cloudy air with 140 ppb ozone was approximately 1000 times faster than conversion in clouds without ozone. The rates of SO2 oxidation observed in these experiments with simulated clouds (pH 4.5–5.5) and ozone concentrations in the range of 25–430 ppb are consistent with previously published kinetics for SO2 in bulk‐water experiments with much higher concentrations of ozone.