bluuebunny/crossref_metadata_2025 huggingface dataset:
The fate of river discharge on the continental shelf: 2. Transport of coastal low‐salinity waters under realistic wind and tidal forcing
Authors: Kourafalou Vassiliki H. , Lee Thomas N. , Oey Lie‐Yauw , Wang John D.
Published: February 2004

A three‐dimensional numerical simulation of shelf circulation is presented. We employ realistic forcing for the Southeast U.S. Continental Shelf during the spring season. We show that the strongest offshore transport of river‐induced, coastal, low‐salinity waters and associated materials occurs near the surface. The preferred mean pathway is in the northeastward direction, and it takes about 2 months to cross the entire shelf. Owing to the mean direction of surface transport and the topography of the South Atlantic Bight shelf, the preferred location for springtime removal is off Charleston, South Carolina, and presumably in the vicinity of the Charleston Bump. The transport and fate of the river‐induced, coastal, low‐salinity waters during the spring season are determined by (1) the stratification of nearshore waters, which is due to the high river runoff and causes the decoupling between “near‐surface” and “near‐bottom” layers; (2) the prevailing northeastward winds, which cause significant offshore transport within the shallow near‐surface Ekman layer; and (3) the tidally induced bottom stirring (M2 tides). Comparison of model and data time series of currents shows very good agreement. Standard deviations of the model and data‐computed empirical orthogonal functions are almost identical, while the respective variance‐conserving spectra agree both in amplitude and phase.
Origins and variability of the Benguela Current
Authors: Garzoli S. L. , Gordon A. L.
Published: February 2004

A subset of the Benguela Sources and Transports (BEST) 1992–1993 data is analyzed to study the magnitude and variability of the large scale transports in the area. The data consist of inverted echo sounder series and conductivity‐temperature‐depth stations. The mean 16 month transport values for the upper 1000 m indicate that, of the 13‐Sv northward transport within the Benguela Current, 50% is derived from the central Atlantic (which from geometry may be chiefly South Atlantic water), 25% comes from the Indian Ocean (which may be chiefly Agulhas water), and the remaining 25% may be a blend of Agulhas and tropical Atlantic water. A simple schematic of the transport pattern with a somewhat restricted corridor for Agulhas eddies translation is envisioned. To the west of the eddy corridor flows the South Atlantic source for the Benguela Current; to the east is the Agulhas (Indian Ocean) source. The corridor is breached by South Atlantic and Indian water as the transient eddy field stirs these water masses.
Structure and variability of Langmuir circulation during the Surface Waves Processes Program
Authors: Plueddemann Albert J. , Smith Jerome A. , Farmer David M. , Weller Robert A. , Crawford William R. , Pinkel Robert , Vagle Svein , Gnanadesikan Anand
Published: February 2004

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface‐scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity‐based estimates were only indirectly related to circulation strength. The near‐surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave‐current interaction theories of Langmuir cell generation.
Cadmium in the California Current System: Tracer of past and present upwelling
Authors: van Geen Alexander , Husby David M.
Published: February 2004

Over 100 samples were collected off the west coast of North America during 1991–1993 to determine the relation between wind‐driven upwelling and nearshore concentrations of dissolved silicate (Si), phosphate (P), and cadmium (Cd). Highly enriched in deep water offshore, these constituents are sensitive indicators of upwelling. Coastal water was sampled from the shore in January and June 1992 at 12 sites distributed between 36° and 48°N latitude. In January the composition of nearshore water along this transect was fairly uniform: 5–15 μmol/kg for Si, 0.5 to 1.0 μmol/kg for P, and 0.1–0.3 nmol/kg for Cd. In June, elevated concentrations of Si (30 μmol/kg), P (2.0 μmol/kg), and Cd (0.6 nmol/kg) revealed a region of intense upwelling between 38° and 40°N. The pattern is broadly consistent with meridional gradients in coastal upwelling calculated from the long‐term mean of alongshore winds compiled from ship reports. Nearshore water was also collected biweekly to monthly at two sites 3 km apart near San Francisco Bay (37.5°N) during 1991–1993. The variability seen in the time series suggests that the composition of nearshore water integrates the effect of alongshore winds over timescales of several weeks. Seasonal variations in Si (5–50 μmol/kg), P (0.5–2.5 μmol/kg), and Cd (0.1–0.8 nmol/kg) concentrations were consistent with upwelling during spring and summer. Maximum Si, P, and Cd concentrations reached in May 1991 were consistent with advection to the very nearshore region from a depth of about 300 m relative to a vertical profile at a distance of 200 km from the coast. Nearshore Si, P, and Cd concentrations were reduced relative to 1991 in 1992, and, to a lesser extent, in 1993 due to weaker upwelling linked to the warm phase of the El Niño‐Southern Oscillation. During periods of weaker upwelling or downwelling, variations in P, Si, and Cd concentrations became uncoupled. There is a good correlation between the coastal Cd time series near San Francisco Bay (37.5°N) and a second order polynomial function of the upwelling index of Bakun [1975] at 36°N, filtered with a 30‐day running mean (r2 = 0.71, n = 39). The index is a daily estimate of coastal upwelling calculated from 6‐hourly mean atmospheric pressure distributions at 36°N. From this function and a record of daily upwelling indices, we infer a range of annually averaged coastal Cd concentrations of at least 0.3–0.5 nmol/kg since 1967. Cd/Ca ratios in shells of foraminifera from San Francisco Bay suggest that average coastal Cd concentrations 3500–4500 years ago were at the upper end of this range.
Near‐inertial oscillations over the Texas‐Louisiana shelf
Authors: Chen Changsheng , Reid Robert O. , Nowlin Worth D.
Published: February 2004

Wind‐induced, near‐inertial oscillations over the Texas‐Louisiana shelf in spring and summer 1992 are described using the current and wind observations taken during the first year of the Texas‐Louisiana Shelf Circulation and Transport Processes Study (LATEX A). Rotary spectral analysis shows clockwise‐rotating energy peaks at near‐inertial frequencies for records from all current meter moorings after the suppression of principal tidal signals. The vertical structure of near‐inertial oscillations is characterized by a first baroclinic mode with a near 180° phase difference between the upper mixed layer and the lower stratified layer. The oscillations are intermittent with a modulation timescale of about 5–10 days. They are surface‐intensified and have maximum values near the shelf break, decaying gradually toward the coast but rapidly offshore. Near‐inertial oscillations appear to accompany a sudden change of the wind stress during frontal passages. Diagnostic analysis suggests that the large near‐inertial oscillations over the LATEX shelf are mainly generated by high‐frequency (near‐inertial) variation of the wind stress accompanying the passage of atmospheric fronts. When the downward transfer of the near‐inertial energy to the deep stratified layer is small, a simple mixed layer model forced by the observed wind stress provides a reasonable prediction of the near‐inertial currents in the mixed layer.
What controls the origin of the Indonesian throughflow?
Authors: Nof Doron
Published: February 2004

The question of which water enters the Indonesian passages is a subject of outstanding debate. This article represents another attempt to address this issue using a new nonlinear model. The new model illustrates that the origin and composition of the Indonesian throughflow are determined by the structure of the opposing and retroflecting currents situated to the east of the passages. The nonlinear “layer‐and‐a‐half model” is composed of an eastern and western basin (corresponding to the Pacific and Indian Oceans) connected via a channel representing the Indonesian passages. The channel cuts through a separating wall which runs from the northwest to the southeast; the retroflecting currents are taken to be zonal. Nonlinear solutions are constructed analytically by balancing the flow force associated with the boundary currents flowing along the wall in the western Pacific. It is found that, without retroflection (that is, the entire flow along the western wall in the Pacific enters the passage), the throughflow must originate in both hemispheres, with 70% of the transport coming from the north and the remainder coming from the south. It is impossible for the flow to originate only from one side of the channel's entrance because, under such conditions, the momentum imparted by this flow on the fluid near the entrance to the channel cannot be balanced. When retroflection is present (that is, only a fraction of the western boundary current system in the Pacific enters the channel) and the coastline is tilted, the above division of transports is dramatically altered. For some range of parameters the balance requires that fluid exit the channel rather than enter it. This is not the case, however, for the Indonesian throughflow, where about 1 Sv must enter from the south and 11 Sv (±5 Sv) must enter from the north. Since opposing retroflecting currents flowing next to a solid wall containing no channels or gaps can be stationary only if their transports satisfy a given ratio, it is suggested that the fact that the retroflection is situated immediately to the east of the Indonesian passages is not accidental. Namely, it is argued that the western boundary current system in the Pacific is stationary because it is situated next to the Indonesian passages. In this particular position, the portions of the transports which do not allow satisfaction of the stationarity condition leak out into the Indian Ocean.
Numerical modeling of suspended sediment due to deep‐sea mining
Authors: Jankowski J. A. , Malcherek A. , Zielke W.
Published: February 2004

A numerical model was developed in order to estimate the residence time of a sediment plume generated by potential deep‐sea mining activities, with special attention to discharges in the bottom boundary layer. The site of the Disturbance and Recolonization Experiment (DISCOL) in the Peru Basin in the southeast Pacific Ocean was chosen as a case study. The model includes the actual bathymetry, as well as the characteristic flow patterns of this region. Various aspects affecting the transport and sedimentation of the plume, such as stratification, flocculation in a sediment‐laden water column, and the hydrodynamics are discussed in conjunction with field data and studied with the overall aim of providing a reliable risk assessment of deep‐sea mining environmental impacts.
Geostrophic transport of the Pacific‐Indian Oceans throughflow
Authors: Fieux M. , Molcard R. , Ilahude A. G.
Published: February 2004

For the dynamic balance of the world ocean circulation, the flow through the Indonesian archipelago needs to be quantified. Part of the Java Australia Dynamic Experiment program was to occupy hydrographic sections during the two opposite seasons. Here we present and discuss the February 1992 geostrophic estimate of the throughflow. The appearance of the east‐flowing Java Current, flowing in the 80 km off the Indonesian coast, led to a net total transport of a few 106 m3 s−1 eastward, opposite to the expected sense of net transport. Uncertainty in this transport, estimated from the repeated casts, reaches 9 106 m3 s−1. This large error could be attributable to internal wave motions. Estimates of the transport from the conductivity‐temperature‐depth data and from the temperature profile associated to the Levitus [1982] data set have been compared. Comparison with August 1989 results shows that the main westward current, corresponding to the South Equatorial Current, is located north of the hydrological front in both seasons. The largest transport variation between the two cruises is located along the Indonesian coast, with the reversal of the Java Current. The mesoscale current variability is enhanced in the southern part of the section in February 1992. These transport evaluations, made in the two opposite seasons and in two different years, 18 ± 7 106 m3 s−1 westward in August 1989 and 2.6 ± 9 106 m3 s−1 eastward in February 1992, give an extreme range of the net throughflow.
Numerical analysis of the sea state bias for satellite altimetry
Authors: Glazman R. E. , Fabrikant A. , Srokosz M. A.
Published: February 2004

Theoretical understanding of the dependence of sea state bias (SSB) on wind wave conditions has been achieved only for the case of a unidirectional wind‐driven sea [Jackson, 1979; Rodriguez et al., 1992; Glazman and Srokosz, 1991]. Recent analysis of Geosat and TOPEX altimeter data showed that additional factors, such as swell, ocean currents, and complex directional properties of realistic wave fields, may influence SSB behavior. Here we investigate effects of two‐dimensional multimodal wave spectra using a numerical model of radar reflection from a random, non‐Gaussian surface. A recently proposed ocean wave spectrum is employed to describe sea surface statistics. The following findings appear to be of particular interest: (1) Sea swell has an appreciable effect in reducing the SSB coefficient compared with the pure wind sea case but has less effect on the actual SSB, owing to the corresponding increase in significant wave height. (2) Hidden multimodal structure (the two‐dimensional wavenumber spectrum contains separate peaks, for swell and wind seas, while the frequency spectrum looks unimodal) results in an appreciable change of SSB. (3) For unimodal, purely wind‐driven seas, the influence of the angular spectral width is relatively unimportant; that is, a unidirectional sea provides a good qualitative model for SSB if the swell is absent. (4) The pseudo wave age is generally much better for parametrizing the SSB coefficient than the actual wave age (which is ill‐defined for a multimodal sea) or wind speed. (5) SSB can be as high as 5% of the significant wave height, which is significantly greater than predicted by present empirical model functions tuned on global data sets. (6) Parameterization of SSB in terms of wind speed is likely to lead to errors due to the dependence on the (in practice, unknown) fetch.
Inertial currents in the Beaufort Sea: Observations of response to wind and shear
Authors: Merrifield Mark A. , Pinkel Robert
Published: February 2004

Doppler sonar observations from the Arctic Internal Waves Experiment and Arctic Leads Experiment in the Beaufort Sea are used to examine currents below the ice during spring conditions. Following two strong wind events, the dominant fine‐scale current consists of both downward and upward propagating near‐inertial internal waves. The downward propagating waves are most evident above 100 m depth, while the upward waves are most energetic below this depth. The appearance of these waves coincides in time and depth with a westward subinertial current following the winds with maximum speeds of 0.05 m s−1at 100 m depth. Simple ray simulations show that the observed subinertial shear can significantly refract near‐inertial waves with length scales similar to those observed. In particular, waves that propagate both in the direction of the current and toward the depth of maximum current are enhanced. These simulations suggest that the observed near‐inertial wave enhancement is due, in part, to wave‐shear effects rather than solely to surface forcing.
Edge wave trapping and amplification on barred beaches
Authors: Bryan K. R. , Bowen A. J.
Published: February 2004

Theoretical predictions show that edge waves may be trapped on bars, providing a possible mechanism for bar growth. Simple solutions to the linear shallow water equations reveal that bar trapping is controlled by the depth of the water over the bar hbar and the geometry of the inshore trough relative to the bar. For specific geometries, amplification over the bar approaches an order of magnitude or more as the phase speed approaches , where amplification is defined as the surface displacement over the bar relative to the surface displacement at the shoreline. The amplification increases with the cross‐shore length scales of the profiles. The amplification is somewhat frequency dependent, generating stronger currents at the location of the bar in specific frequency ranges. Previous work showed that particular edge wave modes have the correct length scales to generate beach features such as bars and beach cusps, but field observations, notably shoreline run‐up data, frequently show no evidence of edge wave energy being concentrated at the appropriate frequencies. However, for bar‐trapped modes, edge wave energy is concentrated at the location of the bar; therefore this edge wave energy will not necessarily be evident in shoreline measurements. Strong correlation exists between the energy distribution of the longshore component of the orbital velocity observed in data collected at Superduck, October 11, 1986, and the distribution expected for a white edge wave spectrum at the shoreline. This suggests that bar‐trapped edge waves exist, in particular, at incident wave frequencies.
Obliquely incident irregular waves in surf and swash zones
Authors: Kobayashi Nobuhisa , Karjadi Entin A.
Published: February 2004

A time‐dependent numerical model is developed for predicting the free surface elevation and depth‐averaged cross‐shore and alongshore velocities in the swash and surf zones under obliquely incident irregular waves. The assumption of small incident angles is made to simplify the finite‐amplitude, shallow‐water equations and allow the computation of the cross‐shore fluid motion using the existing one‐dimensional model for normally incident waves. The developed numerical model is compared with available field data. The time‐dependent model is shown to be capable of predicting the cross‐shore variations of the root‐mean‐square wave height and longshore current in the surf zone. The numerical model predicts large cross‐shore and alongshore velocities near the shoreline. The causes of these large velocities are examined but cannot be ascertained for lack of velocity data near the shoreline.
Heat flux through sea ice in the western Weddell Sea: Convective and conductive transfer processes
Authors: Lytle V. I. , Ackley S. F.
Published: February 2004

The heat flux through the snow and sea ice cover and at the ice/ocean interface were calculated at five sites in the western Weddell Sea during autumn and early winter 1992. The ocean heat flux averaged 7 ± 2 W/m2from late February to early June, and average ice/air heat flux in the second‐year floes depended on the depth of the snow cover and ranged from 9 to 17 (±0.8) W/m2. In late February, three of the five sites had an ice surface which was depressed below sea level, resulting, at two of the sites, in a partially flooded snow cover and a slush layer at the snow/ice interface. As this slush layer froze to form snow ice, the dense brine which was rejected flowed out through brine drainage channels and was replaced by lower‐salinity, nutrient‐rich seawater from the ocean upper layer. We estimate that about half of the second‐year ice in the region was covered with this slush layer early in the winter. As the slush layer froze, over a 2‐ to 3‐week period, the convection within the ice transported salt from the ice to the upper ocean and increased total heat flux through the overlying ice and snow cover. On an area‐wide basis, approximately 10 cm of snow ice growth occurred within second‐year pack ice, primarily during a 2‐ to 3‐week period in February and March. This ice growth, near the surface of the ice, provides a salt flux to the upper ocean equivalent to 5 cm of ice growth, despite the thick (about 1 m) ice cover, in addition to the ice growth in the small (area less than 5%), open water regions.
The influence of an Alaskan Stream eddy on flow through Amchitka Pass
Authors: Okkonen Stephen R.
Published: February 2004

Flow into the Bering Sea through Amchitka Pass in the central Aleutian Islands is shown to respond to the presence of a large meander/eddy in the Alaskan Stream south of the pass. Maximum flow into the Aleutian Basin of the Bering Sea occurs when the eddy center lies nearest to the axis of Amchitka Pass. A conceptual model of low‐frequency circulation variability within the Bering Sea, based upon results of empirical orthogonal function analyses of sea surface height anomalies measured by the Geosat radar altimeter, is presented.
On the nature of decadal anomalies in North Atlantic sea surface temperature
Authors: Hansen Donald V. , Bezdek Hugo F.
Published: February 2004

North Atlantic sea surface temperature data from the Comprehensive Ocean‐Atmosphere Data Set were used to investigate the behavior of temperature anomalies on multiple‐year timescales during the period 1948–1992. Monthly anomaly time series for each 2° square from the equator to 70°N were low‐pass filtered at 4 years and normalized by the local standard deviation. Attention is focused on the extreme events, the upper and lower deciles, of the anomaly time series. A 45‐year sequence of January maps shows the already familiar phenomena of generally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The years 1978 through 1982 were largely devoid of persistent strong anomalies, but moderate cold conditions returned during 1983–1986. Warm conditions dominated the North Atlantic from 1987 onward. Within these thermal epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20° of latitude or longitude, but they range from barely detectable to spanning the width of the basin, the latter especially in lower latitudes. Most of the anomalies move long distances along certain preferred paths. These paths generally follow the routes of the subarctic and subtropical gyres. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approximate route of the North Atlantic Current. Midlatitude anomalies originating at the eastern boundary tend to spread both northward and southward along the coast. The speed of these movements (1–3 km d−1) is generally less than the expected speed of the near‐surface ocean circulation. Simple ideas about the effects of beta dynamics and air‐sea heat exchanges are briefly considered but do not provide a satisfactory explanation for the movements of the anomalies. The long timescale of these extreme events and the continuity of their movements suggest a useful degree of predictability of sea surface temperature based on persistence and propagation of features.
Coupled energy‐balance/ice‐sheet model simulations of the glacial cycle: A possible connection between terminations and terrigenous dust
Authors: Peltier W. Richard , Marshall Shawn
Published: February 2004

We apply a coupled energy‐balance/ice‐sheet climate model in an investigation of northern hemisphere ice‐sheet advance and retreat over the last glacial cycle. When driven only by orbital insolation variations, the model predicts ice‐sheet advances over the continents of North America and Eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses. The orbital forcing alone, however, is unable to induce the observed rapid ice‐sheet retreat, and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting. In the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination. These include marine instability, thermohaline circulation effects, carbon dioxide variations, and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration. For the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records. Of these various influences, our analyses suggest that the albedo variations in the ice‐sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism. Using our parameterization of “dirty” snow in the ablation zone we find glacial retreat to be strongly accelerated, such that complete collapse of the otherwise stable Laurentide ice sheet ensues. The last glacial maximum configurations of the Laurentide and Fennoscandian complexes are also brought into much closer accord with the ICE‐3G reconstruction of Tushingham and Peltier (1991,1992) and the ICE‐4G reconstruction of Peltier (1994) when this effect is reasonably introduced.
Further observations of cloud‐ionosphere electrical discharges above thunderstorms
Authors: Winckler John R.
Published: February 2004

During the night of 9–10 August 1993 more than 150 luminous cloud‐ionosphere discharges (CIs) were observed above a thunderstorm complex moving SE across the state of Iowa. Images of the CIs were obtained through clear air by intensified CCD TV cameras at the O'Brien Observatory of the University of Minnesota located about 60 km NE of Minneapolis and 250–500 km from the storm center. The discharges consisted of bright vertical striations extending from 50–80 km altitude, often covering tens of kilometers laterally, with tendrils of decreasing intensity visible for the brighter events down to cloud tops below 20 km altitude. All the more intense CIs were coincident with a VLF sferic in the 300Hz–12kHz range, but small events often did not yield a detectable sferic. There is no unambiguous evidence that CIs were sources of sferics. Some of the CIs were observed to be coincident with a cloud brightening and with a cloud‐ground stroke recorded by the National Lightning Detection Network. The duration of the images was generally less than one TV field (< 16.7 ms). Many of these discharges have now been observed by the space shuttle, by aircraft‐borne TV cameras and a large number by a ground‐based camera observations in Colorado. The present results are compared with these observations and recent theoretical ideas related to the CI events are discussed. It is proposed that CIs arise from intense bursts of cloud electrification and may follow the preexisting paths of cloud‐to‐ionosphere thunderstorm currents.
Observation of local cloud and moisture feedbacks over high ocean and desert surface temperatures
Authors: Chahine Moustafa T.
Published: February 2004

New data on clouds and moisture, made possible by reanalysis of weather satellite observations, show that the atmosphere reacts to warm clusters of very high sea surface temperatures in the western Pacific Ocean with increased moisture, cloudiness, and convection, suggesting a negative feedback limiting the sea surface temperature rise. The reverse was observed over dry and hot deserts where both moisture and cloudiness decrease, suggesting a positive feedback perpetuating existing desert conditions. In addition, the observations show a common critical surface temperature for both oceans and land; the distribution of atmospheric moisture is observed to reach a maximum value when the daily surface temperatures approach 304±1 K. These observations reveal complex dynamic‐radiative interactions where multiple processes act simultaneously at the surface as well as in the atmosphere to regulate the feedback processes.
A model for the solar radiation budget of the northern hemisphere: Comparison with Earth Radiation Budget Experiment data
Authors: Vardavas Ilias Mihail , Koutoulaki Katerina
Published: February 2004

A simple atmospheric radiation transfer model was used with climatological data to predict the mean monthly‐zonal (10° latitudinal zones) solar radiation budget at the Earth's surface and at the top of the atmosphere for the northern hemisphere. The model also computes the amount of energy absorbed by the atmosphere. The model's outgoing fluxes agree within 5 W m−2 on average with the corresponding values determined from the Earth Radiation Budget Experiment (ERBE) (1985–1989) satellite data. Comprehensive measurements of the surface radiation budget for all zones and months are not available to validate the model predictions. However, the validation of the model for the top‐of‐atmosphere radiation budget engenders more confidence in the model's surface radiation budget than would be the case if the model used the ERBE outgoing fluxes as input data. The model gives a mean planetary albedo for the northern hemisphere of 0.306 compared with 0.297±0.007 from the 5 years of ERBE data and a flux‐mean surface albedo of 0.12 which agrees well with 0.13 from observations. The percentage of the hemispherical mean incoming solar radiation that is absorbed by the atmosphere is computed to be 25% while that absorbed at the surface is 44%, in agreement with the results of other models.
Observational study of the quasi‐biennial oscillation in ozone
Authors: Hollandsworth Stacey M. , Bowman Kenneth P. , McPeters Richard D.
Published: February 2004

The structures of the quasi‐biennial oscillations (QBOs) in zonal wind, temperature, and layer ozone amounts are investigated using 11.5 years (January 1979 to June 1990) of National Meteorological Center (NMC) global geopotential height data and global ozone data from the solar backscatter ultraviolet spectrometer (SBUV) on Nimbus 7. The QBO signals are isolated by computing lagged correlations between the deseasonalized, detrended variable fields and a reference signal representative of the equatorial QBO. Lagged correlations are calculated for the full time series and for each season separately to determine seasonal effects. The results depict an equatorial zonal wind QBO in good agreement with the observed QBO in ground‐based equatorial zonal wind measurements, although the amplitude of the derived QBO in the NMC data is ∼30% too weak. The vertical extent of the oscillation is significantly higher (2 mbar) than that previously reported. The temperature QBO is consistent with ground‐based observations in the lower stratosphere but weakens with height above ∼50 mbar. The ozone QBO is strong at all levels from 5 mbar into the lower stratosphere. Though the annual average total ozone QBO is quite symmetric about the equator, the oscillation is highly variable from layer to layer. The phase of the ozone QBO near the equator is consistent with that of the zonal wind and temperature in the middle and upper stratosphere, but the vertical resolution of the SBUV data in the lower stratosphere is too low to accurately represent the vertical phase of the ozone QBO in this region. Subtropical temperature and total ozone anomalies are found to be dependent on season.
Impact of averaged photolysis rates on stratospheric chemical models
Authors: Smith Anne K.
Published: February 2004

An evaluation is made of the effect of two approximations to the diurnal variation of photolysis rates on the simulation of stratospheric chemistry. The daylight average approximation uses photolysis rates that are given a value representative of the daylight average over that portion of the day that the Sun is shining, and zero otherwise. Although this distorts the diurnal cycle, the zonal average concentrations using this approximation are within about 5% of those from the fully resolved diurnal cycle for most of the important species in the stratosphere. Larger discrepancies occur in the partitioning of inorganic chlorine, although even this is in error by only about 10% for the dominant species. The 24‐hour average photolysis approximation gives perpetual daylight except in the polar night. The error associated with this is quite large, especially for those species such as NO3 and N2O5 that are produced during night. Over an integration time of a month, it can lead to differences in the simulated concentrations of major species such as ozone of the order of 20% or more.
Aerosol distributions in the North Atlantic marine boundary layer during Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange
Authors: Jensen Tara L. , Kreidenweis Sonia M. , Kim Young , Sievering Herman , Pszenny Alex
Published: February 2004

Marine boundary layer aerosol size distributions observed during June 1992 are described. Measurements were made from onboard the National Oceanic and Atmospheric Administration ship Malcolm Baldrige in support of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange field experiment in the area surrounding 32°N latitude and 25°W longitude. Measurements obtained from three instruments, the differential mobility particle sizer, the active scattering aerosol spectrometer probe, and the forward scattering spectrometer probe, were used to investigate the contributions of the different aerosol modes to total number, surface area, and volume concentrations. It was observed that aerosol populations in air masses that appear to have originated from remote marine areas have a bimodal number distribution with total number concentrations ranging from 200 to 800 particles cm−3. In air masses that have been affected by anthropogenic emissions, the aerosol size distributions become monomodal and total number concentrations increase to 500 to 1800 cm−3. The data also suggest that air mass boundaries can be very sharp and that there may be some mesoscale variability in the aerosol population within an air mass.
Effects of land cover type and greenness on advanced very high resolution radiometer bidirectional reflectances: Analysis and removal
Authors: Wu Aisheng , Li Zhanqing , Cihlar Josef
Published: February 2004

The objectives of the study are (1) to examine the effects of land cover type, green biomass, and solar zenith angle (SZA) on the bidirectional reflectance distributions (BRDs) of the visible and near‐infrared advanced very high resolution radiometer (AVHRR) data from NOAA 11 over terrestrial surfaces; and (2) to correct for these effects by developing BRD functions (BRDF). Four land cover types are examined: barren, grassland, forest, and cropland. The data used consist of 1‐km daily AVHRR measurements for three growing seasons collected over 19 homogeneous land sites (20×20 km2 for each site) in the conterminous United States and parts of Canada. BRD is found to be strongly land cover dependent. For the same cover type, BRD is altered significantly by the green biomass present (represented by the normalized difference vegetation index (NDVI)). The effects of SZA on BRDs are also observed over all surface types under investigation. Semiempirical BRDFs were developed to account for these effects that are the functions of the SZA, satellite viewing zenith angle, relative azimuth angle, and NDVI. Good agreements were found between the observed and the modeled bidirectional dependencies for wide ranges of NDVI and SZA. A single BRDF appears to be sufficient for bidirectional correction of the clear‐sky AVHRR measurements made over a specific land cover type throughout a season. Finally, the developed BRDFs were used to normalize AVHRR reflectance data to a common geometry and to infer the hemispherical albedos for monitoring the seasonal variations of land surfaces.
Aerosol absorption measurements at Barrow, Mauna Loa and the south pole
Authors: Bodhaine Barry A.
Published: February 2004

Aerosol absorption (σap) has been measured continuously using aethalometers at Barrow, Alaska (1986 to present); Mauna Loa, Hawaii (1990 to present); and south pole, Antarctica (1987–1990). These three stations are part of a network of baseline monitoring stations operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA). Condensation nucleus (CN) concentration and multiwavelength aerosol scattering (σsp) have also been measured continuously for many years at these stations. Aethalometer measurements are usually reported in terms of atmospheric black carbon aerosol (BC) concentration using the calibration suggested by the manufacturer. Here we deduce the in situ σap(550 nm) from aethalometer measurements by assuming that the aerosol absorption on the aethalometer filter is enhanced by a factor of 1.9 over that in the atmosphere. This is consistent with using 19 m2 g−1 for the specific absorption of BC on the aethalometer filter and 10 m2 g−1 for the in situ specific absorption of BC in the atmosphere (the ratio of the two specific absorptions is 1.9). Although these values of specific absorption may vary significantly for different environments, the ratio might be expected to be relatively constant. The single‐scattering albedo, defined by ω = σsp/(σsp + σap), has been estimated from the simultaneous measurements of σap and σsp. Furthermore, assuming a 1/λ dependence for σap in the 450 to 700‐nm wavelength region, multiwavelength σsp measurements allow the estimation of the wavelength dependence of ω. Each station shows a considerable annual cycle in σap, σsp, and ω. The maximum in the Barrow annual cycle is caused primarily by the springtime Arctic haze phenomenon; the maximum in the Mauna Loa annual cycle is caused by the springtime Asian dust transport; and the maximum in the south pole annual cycle is caused by late winter transport from southern midlatitudes. It was found that annual mean values are σap = 4.1 × 10−7 m−1 (≈41 ng m−3 BC) and ω = 0.96 for Barrow; σap = 5.8 × 10−8 m−1 (≈5.8 ng m−3 BC) and ω = 0.97 for Mauna Loa; and σap = 6.5 × 10 −9 m−1 (≈0.65 ng m−3 BC) and ω = 0.97 for south pole. It was also found that the wavelength dependence of ω may be important at Barrow and south pole, but not important at Mauna Loa.
Simultaneous measurement of lightning parameters for strokes to the Toronto Canadian National Tower
Authors: Hussein Ali M. , Janischewskyj W. , Chang J.‐S. , Shostak V. , Chisholm W. A. , Dzurevych P. , Kawasaki Z.‐I.
Published: February 2004

Successful simultaneous measurements of significant parameters for lightning strikes to the Canadian National (CN) Tower in Toronto have been performed since the summer of 1991. Three 10‐bit 10‐ns computer‐controlled double‐channel digitizers, with long segmented memory, have simultaneously captured the current derivative at the CN Tower and the corresponding electric and magnetic fields 2.0 km north of the tower. Lightning flashes to the tower were videotaped from two mutually perpendicular directions for the purpose of constructing a three‐dimensional image of the lightning path. Furthermore, the return stroke velocity, a parameter also needed for the analysis of lightning radiation models, has been measured by a computer‐controlled photodiode system. In this paper all relevant parameters for a CN Tower lightning stroke, observed on August 17, 1991, are shown and analyzed.
Global distribution of nitric oxide produced by lightning and its seasonal variation
Authors: Kumar P. Pradeep , Manohar G. K. , Kandalgaonkar S. S.
Published: February 2004

The global distribution of NO production due to lightning has been estimated using the lightning discharge frequency observed from the global distribution of thunderstorm activity obtained from the ISSS‐b satellite observations during June 1978 to May 1980 and issued by Radio Research Laboratories, Japan. The seasonal and latitudinal variation has been investigated. The annual production of NO by lightning from the present study is found to be 2 Tg N yr−1.
Temperature and accumulation at the Greenland Summit: Comparison of high‐resolution isotope profiles and satellite passive microwave brightness temperature trends
Authors: Shuman C. A. , Alley R. B. , Anandakrishnan S. , White J. W. C. , Grootes P. M. , Stearns C. R.
Published: February 2004

Long‐term satellite passive microwave brightness temperature trends, supported by short‐term automatic weather station (AWS) temperature data, show that the Greenland Summit area experiences secondary warm periods in the late fall and/or winter as well as primary midsummer warmth. High‐resolution isotope profiles from snow pits dug in 1989, 1990, and 1991 near the Greenland Ice Sheet Project II; (GISP2) site reveal that stable isotope ratios (δ18O and δD) preserve this distinctive temperature cycle. This indicates that snow accumulation occurs frequently through the year at the Greenland Summit and that the isotope record initially contains temperature information from many times of the year. Through an empirically derived emissivity model using AWS air temperature data and satellite microwave brightness temperatures, our approach allows isotope values preserved in the snow to be related to estimated near‐surface air temperatures. Density‐corrected profiles allow the amounts and timing of accumulation to be determined as well. Our results indicate that stable isotope ratios from the near‐surface snow at the Greenland Summit are currently a reliable, high‐resolution temperature proxy. This result increases confidence in the paleoclimatic interpretation of isotope signal variations in the GISP2 and Greenland Ice Core Project (GRIP) deep ice cores.
Pacific Exploratory Mission‐West A (PEM‐West A): September–October 1991
Authors: Hoell J. M. , Davis D. D. , Liu S. C. , Newell R. , Shipham M. , Akimoto H. , McNeal R. J. , Bendura R. J. , Drewry J. W.
Published: February 2004

The NASA Pacific Exploratory Mission‐West (PEM‐West) is a major component of the East Asia/North Pacific Regional Study (APARE), a project within the International Global Atmospheric Chemistry (IGAC) program. The broad objective of the PEM‐West/APARE initiative is to study chemical processes and long‐range transport over the northwestern Pacific Ocean and to estimate the magnitude of the human impact on the oceanic atmosphere over this region particularly for tropospheric ozone and its precursors as well as for sulfur species. The first phase of this mission, PEM‐West A, was conducted during September–October 1991. The PEM‐West A included intensive airborne measurements of trace gases from the NASA DC‐8 aircraft coordinated with measurements at PEM‐West A surface sites as well as with measurements obtained from collaborating APARE ground and airborne platforms. This paper reports the experimental design for PEM‐West A with a brief summary of the general content and focus of companion papers in this special issue.
Estimating the global ozone characteristics during the last 30 years
Authors: Bojkov Rumen D. , Fioletov Vitali E.
Published: February 2004

All available total ozone data from over 150 past and present Global Ozone Observing System (GO3OS) stations, after careful quality control and reevaluation, have been analyzed in order to deduce the basic global ozone characteristics both for pre‐ozone‐hole and during “ozone hole” time periods. Utilizing Total Ozone Mapping Spectrometer (TOMS) data, the longitudinal inhomogeneity of the total ozone distribution was estimated. That permitted the use of ground‐based data for establishing long‐term zonal as well as hemispheric and global ozone variations for the 1964–1994 period. The difference between the estimations of monthly zonal variations from ground‐based and TOMS data for the overlapping period of 1979–1993 is less than 1% in latitudes 40°S–60°N. The ozone changes are several times larger than possible errors of the estimated values; therefore the results are highly reliable. They show that the northern hemisphere average ozone was ∼312 and the southern average was ∼300 matm cm in the pre‐ozone‐hole decades (1964–1980) and that the global average for the 1984–1993 period was lower by ∼3% (from 306.4±1.0 down to 297.7±2.2 matm cm). The southern hemisphere contributed ∼64% of the overall ozone decline. The levels of annual ozone maximum have been reduced by 5.8% in the southern hemisphere and 3.2% in the northern hemisphere, and the levels of ozone minimum have been reduced by 2.1% and 1.2%, respectively. The ozone trends for midlatitudinal bands (35–65°) show a pronounced seasonal dependence varying from ∼3% to 8 % (and even more for the southern hemisphere) for the cumulative decline since 1970. The ozone decline calculated in percent per decade from 1980 is almost twice as large as the decline calculated from 1970. The cumulative year‐round global ozone decline is 4.8±0.6%; however, the cumulative year‐round decline over middle and polar latitudes is more than 7%. The advantages of establishing ozone “norms” for estimations of long‐term ozone variations from ground‐based data are emphasized.
Modeling the clear‐sky surface energy budget during FIFE 1987
Authors: Lee Tsengdar J. , Pielke Roger A. , Mielke Paul W.
Published: February 2004

A canopy level soil‐vegetation‐atmosphere transfer model (land ecosystem‐atmosphere feedback (LEAF) model is used to simulate components of the daily surface energy budget. The Sellers et al. (1992b) model is used to relate unstressed canopy conductance to the fraction of the incident photosynthetically active radiation absorbed by the canopy and the simple ratio vegetation index. The multivariate randomized block permutation (MRBP) procedure is used to obtain values for a number of parameters used in environmental stress functions and the aerodynamic resistances. It is shown that the MRBP procedure was able to improve the LEAF model predictions for three responses (i.e., net radiation, sensible, and latent heat fluxes). When comparing model predictions to the observations for the four intensive field campaigns of the First International Satellite Land Surface Climatology Project Field Experiment, the model performance is reasonably good. The difference in the parameterization of canopy conductance between the Sellers et al. (1992b) model and the LEAF model is discussed.