bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of tropical OH seasonality using atmospheric 14CO measurements from Barbados
Authors: Mak John E. , Southon John R.
Published: February 2004

14CO can be used to assess the mean OH concentration if the atmospheric inventory is determined. However, atmospheric transport must be well represented. The dependence on transport has previously been predicted in model studies to be much smaller in the tropics than in the middle latitudes. To test this, we made weekly measurements of 14CO from a tropical station in the Atlantic (Barbados, West Indies, 13°10′N, 53°26′W) from July 1996 to July 1997. These data are compared with the 14CO seasonal cycle predicted from a zero dimensional chemistry model. Results show that the observed seasonal cycle correlates well with predicted 14CO from the chemistry model during the summer and fall, however significant deviation from the chemistry model becomes apparent in the wintertime, which is probably largely a result of 14CO being transported to the tropics from the middle latitudes, and is in contrast to earlier modeling studies. Transport has a stronger effect in the winter months because the latitudinal gradient in 14CO is large and the lifetime is long, since OH concentrations are at their seasonal low.
Stochastic regularization: Smoothness or similarity?
Authors: Maurer Hansruedi , Holliger Klaus , Boerner David E.
Published: February 2004

Inversions of geophysical data often involve solving large‐scale underdetermined systems of equations that require regularization, preferably through incorporation of a priori information. Since many natural phenomena exhibit complex random behavior, statistical properties offer important a priori constraints. Inversion constrained by model covariance functions, a form of stochastic regularization, is formally equivalent to imposing simultaneously the auxiliary constraints of (i) model correlation (smoothness) and (ii) similarity with a preferred model (damping). We show that a priori stochastic information defines uniquely the relative contributions of smoothing and damping, such that the higher the fractal dimension the greater the damping contribution. However, if the model discretization interval exceeds the characteristic scale length of the parameters to be resolved, stochastic regularization artificially reduces to only damping constraints.
A solitary wave packet in the atmosphere observed from space
Authors: Zheng Quanan , Yan Xiao‐Hai , Liu W. Timothy , Klemas Vic , Greger Dietmar , Wang Zongming
Published: February 2004

A group of parallel cloud lines was identified in seven successive space shuttle photographs taken over the Pakistani offshore zone in the northern Arabian Sea on April 29, 1993. There are a total of 19 lines aligned with the length of the leading line longer than 250 km and an average separation distance (wavelength) of 1 km. We suggest that the lines are generated by an atmospheric solitary wave packet characterized by decreases in amplitudes and wavelengths from the front to the rear. These features are comparable with the dnoidal soliton solution to the Korteweg‐de Vries (KdV) equation. The land breeze and katabatic flow are proposed as a possible generation mechanism.
An effect of neutral collisions on the excitation threshold of electrostatic ion‐cyclotron waves
Authors: Koepke M. E. , Zintl M. W. , Good T. N.
Published: February 2004

Laboratory experiments using a sodium plasma are reported that demonstrate a robustness of electrostatic ion‐cyclotron waves in the presence of elevated neutral‐helium pressure. It is found that the excitation threshold decreases with neutral pressure, and that, in the presence of a transverse localized electric field, the waves are able to tolerate much higher collisionality than previously believed. When the ion‐neutral collision frequency and ion gyrofrequency are comparable, the excitation threshold is significantly smaller than its collisionless value.
Small‐comet “atmospheric holes” are instrument noise
Authors: Mozer F. S. , McFadden J. P. , Sircar I. , Vernetti J.
Published: February 2004

Frank and Sigwarth claim that dark pixel clusters observed by the VIS Earth Camera are atmospheric holes created by small comets. We have examined their catalog, which contains about 700,000 of these “atmospheric holes”, for consistency with this small‐comet hypothesis and with instrument noise. A geometrical requirement of the small‐comet hypothesis is that the number of pixels in a typical cluster must vary by a factor > 100 with spacecraft altitude because of the inverse‐square law of the apparent cluster area versus distance. We find no systematic variation of duster size with spacecraft altitude. The Iowa catalog data are consistent with instrument noise because neither the size distribution nor the event rate of dark pixel clusters depend on altitude. At altitudes outside of the radiation belts during the one day of available raw data, more than 75% of the dark pixel dusters result from the process that Frank and Sigwarth employ to remove bright pixels caused by energetic particles. This data processing also causes additional meaningless dark pixel clusters to occur in the dark sky or over the dark Earth.
Effects of 1997–1998 El Niño on tropospheric ozone and water vapor
Authors: Chandra S. , Ziemke J. R. , Min W. , Read W. G.
Published: February 2004

This paper analyzes the impact of the 1997–1998 El Niño on tropospheric column ozone and tropospheric water vapor derived respectively from the Total Ozone Mapping Spectrometer (TOMS) on Earth Probe and the Microwave Limb Scanning instrument on the Upper Atmosphere Research Satellite. The 1997‐1998 El Niño, characterized by an anomalous increase in sea‐surface temperature (SST) across the eastern and central tropical Pacific Ocean, is one of the strongest El Niño Southern Oscillation (ENSO) events of the century, comparable in magnitude to the 1982–1983 episode. The major impact of the SST change has been the shift in the convection pattern from the western to the eastern Pacific affecting the response of rain‐producing cumulonimbus. As a result, there has been a significant increase in rainfall over the eastern Pacific and a decrease over the western Pacific and Indonesia. The dryness in the Indonesian region has contributed to large‐scale burning by uncontrolled wildfires in the tropical rainforests of Sumatra and Borneo. Our study shows that tropospheric column ozone decreased by 4–8 Dobson units (DU) in the eastern Pacific and increased by about 10–20 DU in the western Pacific largely as a result of the eastward shift of the tropical convective activity as inferred from National Oceanic and Atmospheric Administration (NOAA) outgoing longwave radiation (OLR) data. The effect of this shift is also evident in the upper tropospheric water vapor mixing ratio which varies inversely as ozone (O3). These conclusions are qualitatively consistent with the changes in atmospheric circulation derived from zonal and vertical wind data obtained from the Goddard Earth Observing System data assimilation analyses. The changes in tropospheric column O3 during the course of the 1997–1998 El Niño appear to be caused by a combination of large‐scale circulation processes associated with the shift in the tropical convection pattern and surface/boundary layer processes associated with forest fires in the Indonesian region.
Monitoring the TOPEX Microwave Radiometer with GPS: Stability of columnar water vapor measurements
Authors: Haines Bruce J. , Bar‐Sever Yoaz E.
Published: February 2004

Using data from terrestrial global positioning system (GPS) receivers, we describe an anomalous drift in columnar water vapor measurements from the TOPEX microwave radiometer (TMR). Based on their long‐occupation histories and proximity to open‐water TOPEX/POSEIDON (T/P) ground tracks, we identified four geodetic‐quality GPS stations that are suitable for calibrating the TMR. We constructed time series at each site based on the differences of instantaneous vertical wet tropospheric path delay (PDwet) derived independently from GPS and TMR data at T/P overflight times. The ensemble results span 1992–1997 and suggest that the TMR measurements of PDwet are gradually drifting lower. Our nominal estimate of the drift is −1.2 ± 0.4 mm yr−1 (one standard error). Accounting for this would increase the estimated rate of change in global mean sea level from T/P by the same amount.
Water and partial melting in mantle plumes: Inferences from the dissolved H2O concentrations of Hawaiian basaltic magmas
Authors: Wallace Paul J.
Published: February 2004

Knowledge of dissolved H2O in primary Kilauean magmas provides a constraint on H2O abundance in the deep mantle region that feeds the upwelling plume beneath Hawaii. Given an H2O/K2O mass ratio of ∼1.3 for basaltic glasses and melt inclusions from Kilauea, the mantle source is estimated to contain 450±190 ppm H2O. This value is ∼3 times greater than that estimated for the mantle source for mid‐ocean ridge basalts. Consideration of OH solubility in olivine suggests that water undersaturated melting of the upwelling Hawaiian plume probably begins at a depth of ∼250 km. Thus during plume ascent through most of the upper mantle, water is partitioned between nominally anhydrous silicates and a small mass fraction of entrained hydrous silicate melt. Because water strongly influences the viscosity of olivine aggregates, partial melting and melt extraction will have important effects on the rheology of the upwelling plume beneath Hawaii.
Plasma flow in the magnetosphere of Ganymede
Authors: Eviatar Aharon , Cheng Andrew F. , Paranicas Chris , Mauk Barry H. , McEntire Richard W. , Williams Donald J.
Published: February 2004

Galileo Energetic Particles Detector (EPD) measurements indicate that convective flow speeds near Ganymede decrease to less than one‐third of rigid corotation well before any significant increase in the magnetic field strength along the trajectory is observed. In an Alfvén wing model, such a deceleration implies that the Ganymede magnetosphere is a conducting object. We assess the magnitude of various conductivities and find that closure of the Alfvén wing current is attainable only through the body of Ganymede itself. We discuss briefly the planetological implications of this magnetospheric result.
Slow elastic dynamics in a resonant bar of rock
Authors: Guyer R. A. , McCall K. R. , Van Den Abeele K.
Published: February 2004

Recent resonant bar experiments on Berea sandstone show that nonlinear excitation of the sample excites a slow dynamics with a time scale many orders of magnitude longer than the excitation period, 2π/ω. That is, a nonlinear resonant frequency decays to the linear resonant frequency long after the high amplitude drive has been turned off. We postulate a phenomenological theory of slow nonlinear dynamics in the context of a resonant bar experiment. The normalized elastic modulus of the resonant bar is allowed to be nonlinear and time dependent. The nonlinear terms are derived from a model of elasticity in rocks that includes anharmonic and hysteretic contributions. We use this theory to explain the experimental results. We find an explanation for the slow relaxation of the experimental resonant frequency using an anharmonic contribution to the modulus that responds instantaneously to a disturbance, and a contribution derived from elastic hysteresis that displays slow dynamics. We suggest an acoustic NMR‐type experiment to explore slow nonlinear dynamics.
Stepwise changes in stratospheric temperature
Authors: Pawson Steven , Labitzke Karin , Leder Silke
Published: February 2004

Northern hemispheric temperature analyses for the time period July 1964 until June 1997 are used to investigate the long‐term changes of the lower and middle stratosphere. Use of annual, hemispheric means removes many of the dynamical effects and allows the thermal structure to be investigated with more certainty. There is a clear cooling over the 33 years considered. It is not well described by a linear trend because of the increased cooling since 1991. These changes are consistent with those derived from independent satellite‐based soundings. It is shown that the low temperatures in the 1990s are not well described by a simple statistical model including the linear trend plus the interannual variability for the period 1964/1965–1980/1981, when the stratosphere was relatively undisturbed by volcanic debris and the O3 changes were small. Possible links between the stratospheric changes reported here and apparent changes in the thermal structure in the 1990s (reported in other studies) are discussed.
Excessive transmission of solar radiation through the cloud‐free atmosphere in GCMs
Authors: Wild Martin , Liepert Beate
Published: February 2004

Large uncertainties exist in the distribution of solar radiation in the global climate system and its representation in General Circulation Models (GCMs). The present paper assesses the transmission of solar radiation through cloud‐free atmosphere in three GCMs using information from direct observations at the surface. Clear‐sky insolation climatologies have been constructed from hourly radiation data through composites of clear‐sky episodes for seven sites in Germany with ten years of accurate measurements. The results suggest that GCMs overestimate clear‐sky insolation at the surface, indicating that the cloud‐free atmosphere is overly transparent to solar radiation in the models. Quantitatively, the annual mean overestimation for the German sites is on the order of 10 Wm−2 in the models which include an aerosol climatology and 30 Wm−2 in the models without aerosol. The GCM‐predicted atmospheric water vapor content is abundant compared to the ECMWF Re‐Analysis and therefore not considered responsible for the missing absorption in the model atmosphere. Rather, understimated water vapor absorption in the GCM radiation codes is implied.
Observations of internal solitary waves near the Oregon coast with an inverted echo sounder
Authors: Trevorrow Mark V.
Published: February 2004

Observations of internal solitary waves (ISWs) were made near the Oregon coast with a moored, 200‐kHz inverted echo sounder (IES) and a 12‐element vertical array of thermistors over a 14‐day period during September 1995. Downward pycnocline displacements conforming to the classical Kortweg‐deVries, hyperbolic secant squared profile, with typical amplitudes of 5 to 14 m, propagation speeds of 0.4 to 0.6 m s−1, and duration of 100 to 200 s were observed with both the IES and thermistor array. Pycnocline displacements from 5 to 22 m depth were observed acoustically exploiting the backscatter from Zooplankton layers, which acted as tracers of water motions. The typical volumetric scattering strength of these Zooplankton layers was near −75 dB (referenced to 1 m−1). Within the top 10 m, plumes of microscopic air bubbles injected by breaking surface waves were advected downward by the ISWs, with volumetric scattering strengths in excess of −36 dB. Finally, near‐surface acoustic scattering zones suspected to exhibit backscatter from turbulent microstructure were observed, with typical volumetric scattering strength near −68 dB. These near‐surface zones were coincident with model predictions of large shear instabilities and local gradient Richardson numbers below the critical value for generation of turbulence.
WOCE section P1W in the Sea of Okhotsk: 1. Oceanographic data description
Authors: Freeland H. J. , Bychkov A. S. , Whitney F. , Taylor C. , Wong C. S. , Yurasov G. I.
Published: February 2004

We describe the results of a World Ocean Circulation Experiment (WOCE) survey (line P1W) from the open Pacific Ocean, through the Bussol' Strait and across the Sea of Okhotsk. Observations are presented of distributions of temperature and salinity, dissolved oxygen and nutrients, and some derived quantities. Discussion centers on the distribution of the Sea of Okhotsk component of the North Pacific Intermediate Water mass and its properties. The Sea of Okhotsk has been suggested as the only location where deep convection may occur within the North Pacific or its marginal seas. Our analysis suggests that the surface σθ = 26.85 has recently been ventilated, and arguments might be made to suggest that ventilation has occurred to σθ = 27.05. This is discussed here, but detailed evidence is covered in a companion paper analyzing CFC observations. The Okhotsk component of the North Pacific Intermediate Water extends to substantially greater depths and densities, and so some other process is needed to create the denser water mass.
Internal tides observed at 2°S–156°E by in situ and TOPEX/POSEIDON data during the Coupled Ocean‐Atmosphere Response Experiment (COARE)
Authors: Gourdeau L.
Published: February 2004

During the enhanced monitoring phase of the Coupled Ocean‐Atmosphere Experiment (COARE), semidiurnal baroclinic tides were observed. At 2°S–156°E (1739 m depth) the Tropical Atmosphere‐Ocean (TAO) mooring outfitted with additional sensors from surface to bottom provides an interesting tool for describing the internal tides. M2, S2, and N2 baroclinic components were detected, explaining 59% of the semidiurnal variance on surface dynamic height. At the semidiurnal frequency, vertical displacements of isopycnals, highly variable in time, can reach 30 m and are mostly a combination of the M2, S2, and N2 internal waves. The two first baroclinic modes explain 70% of the variance in vertical displacement. M2 and S2 internal waves have different vertical structure: M2 resembles the first baroclinic mode and S2 resembles the second baroclinic mode. The M2 and S2 internal waves projected on their respective baroclinic mode are highly related to the modeled barotropic tide. Two TOPEX/POSEIDON tracks cross over the mooring every 10 days, and the internal wave signature can be expected in sea level along the tracks despite the weakness of the signal. Sea level spectra along both ascending and descending tracks show the existence of energy around the 250 km and the 120 km wavelength, respectively, which is in accordance with a first baroclinic wave propagating northeastward, as suggested in previous studies. From wavelet transform, the baroclinic tide signatures on the ascending (descending) track are centered around 2.5°S (3.5°S) with a 1° (2°) latitudinal extension, respectively, limited to the Ontong Java Plateau. We confirm the northeastward propagation of the baroclinic tides, and their source could be a series of islands parallel to the Kilinailau Trench or the large sloping topography between the Kilinailau Trench and the Ontong Java Plateau.
Variability of tidal currents in a wide strait: A comparison between drifter observations and numerical simulations
Authors: Crawford W. R. , Cherniawsky J. Y. , Cummins P. F. , Foreman M. G. G.
Published: February 2004

The spatial variability of semidiurnal tidal currents throughout the waters of the northwest coast of Canada is investigated by examination of velocities of surface drifters recording half‐hourly Loran‐C positions. Time series are examined to remove any records with significant inertial currents. The remaining records are segmented into sequential 25‐hour blocks, detrended, and analyzed for semidiurnal energy. Tidal current ellipses are computed and compared with two different numerical models: a finite element barotropic simulation, and a finite difference baroclinic simulation that was initialized with horizontally uniform density surfaces. Models and observations show the dominance of negative rotation (clockwise rotary) with the amplification of currents shoreward of the continental slope. In shallow waters both models agree well with the observations. Along the edge of the continental shelf in waters deeper than 200 m the baroclinic model provides better agreement with observations because of its ability to simulate internal tides. However, observations also reveal that internal tidal currents at and seaward of the shelf break may be irregular because of the presence of plumes and jets, and a baroclinic model with horizontally uniform stratification cannot reproduce local features in these jets and plumes. Internal tidal currents in Dixon Entrance exceed those simulated by the baroclinic model by a significant degree. This is likely due to the constraints of the density field used to initialize the model and to terrain smoothing in its 5‐km grid.
Retreat of the cold halocline layer in the Arctic Ocean
Authors: Steele Michael , Boyd Timothy
Published: February 2004

We present a comparison of Arctic Ocean hydrographic data sets from the 1990s, with a focus on changes in the upper few hundred meters of the Eurasian Basin. The most recent observations discussed here were collected during the spring 1995 Scientific Ice Expedition (SCICEX'95), the second in a series of scientific cruises to the Arctic Ocean aboard U.S. Navy nuclear submarines. Although the 1990s have seen an abundance of synoptic cruises to the Arctic, this was the only one to take place in winter/spring conditions. Other data considered here were collected during the first SCICEX cruise in summer 1993 (SCICEX'93) and during an icebreaker cruise to the Eurasian Basin in summer 1991 (Oden'91). A new Russian‐American winter climatology is also used as a reference. These comparisons reveal that the Eurasian Basin “cold halocline layer” has retreated during the 1990s to cover significantly less area than in previous years. Specifically, we find a retreat from the Amundsen Basin back into the Makarov Basin; the latter is the only region with a true cold halocline layer during SCICEX'95. Changes are also seen in other halocline types and in the Atlantic Water layer heat content and depth. Since the cold halocline layer insulates the surface layer (and thus the overlying sea ice) from the heat contained in the Atlantic Water layer, this should have profound effects on the surface energy and mass balance of sea ice in this region. Using a simple mixing model, we calculate maximum ice‐ocean heat fluxes of 1–3 W m−2 in the Eurasian Basin, where during SCICEX'95 the surface layer lay in direct contact with the underlying Atlantic Water layer. The overall cause of water mass changes in the 1990s might have been a shift in the atmospheric wind forcing and resulting sea ice motion during the late 1980s, which we speculate influenced the location where fresh shelf waters flow into the deeper basins of the Arctic Ocean. Finally, we discuss two different mechanisms that have been proposed for cold halocline water formation, and we propose a compromise that best fits these data.
Wind‐forced pressure response of the Gulf of Maine
Authors: Brown Wendell S.
Published: February 2004

Bottom pressure measurements, coastal synthetic subsurface pressures, and wind stresses, derived from operational wind measurements throughout the Gulf of Maine, are used to explore the seasonal wind‐forced response of the gulf. Barotropic pressure fluctuations throughout the gulf are highly correlated all year (>0.75) and even more so in the winter (>0.85), with more than 90% of the variance explained by the relevant mode 1 empirical orthogonal functions (EOF). Both the pressures and the wind stress variability are concentrated in the 2‐ to 10‐day “weather band,” with typical winter wind stresses (0, 10 Pa) about 5 times more energetic than those in summer (0.05 Pa). Pressure fluctuations are highly correlated with 255°–75°T wind stress fluctuations. Typical gulf‐scale westward wind stress fluctuations force general gulf‐scale increases in the sea level pressure of about 10 eq cm in winter and about 4 eq cm in summer. The spatially averaged gulf pressure response to westward wind stress is 92±6 eq cm Pa−1 in winter and 83±8 eq cm Pa−1 in summer. The Gulf of Maine wind stress‐forced pressure variability was numerically modeled using the Dartmouth linear, homogeneous, three‐dimensional (3‐D), finite element diagnostic model (FUNDY 5). A series of 36‐hour‐averaged, spatially variable, optimally interpolated operational wind stress patterns was used to force model sea level pressure time series, which looked like observations and explained about 40% of the observed variance. The model provided detailed information on the linkage among the fluctuating near‐westward (eastward) wind stress‐forced surface Ekman transport into (out of) the gulf, the compensatory outflows (inflows) around Georges Bank, and the intensified flow along the western coast of the gulf.
Multiple deep gyres of the western North Pacific: A WOCE section along 149°E
Authors: Wijffels Susan E. , Hall Melinda M. , Joyce Terrence , Torres Daniel J. , Hacker Peter , Firing Eric
Published: February 2004

The top to bottom large‐scale ocean circulation in the northwest Pacific is described using a World Ocean Circulation Experiment (WOCE) onetime hydrographic section along 149°E between Papua New Guinea and Japan. The circulation is quantified using a combination of geostrophic and lowered acoustic Doppler current profiler velocity estimates. At the northern end of the section the flow regime is distinct in that the deep flow largely reflects that at the surface: the Kuroshio jet and its northern and southern recirculations have deep expressions. South of 25°N, the deep and bottom water flows do not mirror the surface flows, and the circulation assumes a highly baroclinic structure. Below the depth of local North Pacific ventilation the flow in the upper deep waters (800–2500 m) alternates in sign roughly every 10° of latitude revealing a set of deep clockwise gyres with significant transports of 40 Sv (1 Sv = 106m3s−1) for a tropical gyre (south of 6°N) and 20 Sv in a subtropical gyre (6°–24°N). These gyres provide a pathway for South Pacific influences to reach 22°N (the location of a strong water mass front) through exchange along the western boundary. Maps of properties on density surfaces suggest that the zonal extent of the upper deep water gyres found along 149°E is basin wide. Below 2500 m, flow across the section is isolated from the Philippine Sea by the Izu‐Ogasawara‐Mariana Ridge and the flow regime and property distribution reflect this: Lower Circumpolar Water flows west in a deep western boundary current near 10°N and coalesces at the Izu‐Ogasawara‐Mariana Ridge with a tongue of North Pacific Deep Water also flowing west near 15°N. About 4 Sv of a mixture of these waters flows east again near 25°N, associated with an abyssal water mass front. North of the front, the water properties are laterally homogeneous on density surfaces in the strongly recirculating gyres associated with the deep Kuroshio system.
Effect of salinity on density in the Leeuwin Current System
Authors: Batteen Mary L. , Huang Ming‐Jer
Published: February 2004

Climatological temperature and salinity fields are used to calculate the salinity contribution to density and dynamic height fields in the Leeuwin Current System (LCS). While the temperature gradient is primarily linear, with warmest water to the north, the salinity fields are spatially inhomogeneous. A comparison of density fields, calculated with constant and variable salinity, shows that, off Western Australia, the density field is primarily determined by temperature. Off southwestern Australia, the density field is dependent on warm and salty (subtropical) and fresh and cold (sub‐Antarctic) water masses. While the dynamic height fields, calculated with constant and variable salinity, show similar flow patterns off Western Australia, different flow patterns are found off southwestern Australia. In addition to the analysis of climatological fields, a primitive equation ocean model is used in a process‐oriented study to investigate the role of salinity in the formation of currents and eddies in the LCS. Two identical ocean models, one with a climatological salinity field and the other with no horizontal salinity gradients, are run and compared with each other. Despite the model runs being initialized with similar temperature distributions, there are relatively large temperature and density differences in the southwestern Australian region due to the advection of water masses by the Leeuwin Current. On the basis of the climatological analyses and the results of the model experiments, it is concluded that, descriptively and dynamically, both temperature and salinity are essential to accurately characterize the large‐scale circulation of the LCS.
Winter snow cover variability on East Antarctic sea ice
Authors: Massom R. A. , Lytle V. I. , Worby A. P. , Allison I.
Published: February 2004

Analysis of the first detailed data set of snow characteristics collected over East Antarctic sea ice in winter confirms that on small scales, snow on Antarctic sea ice is highly variable in both thickness and properties. High‐amplitude cyclical variability in atmospheric forcing related to the passage of storms is responsible for the high degree of textural heterogeneity observed. Changes in snow properties were examined over a 3‐week period, during which a largely icy snow cover, formed at near‐freezing temperatures, metamorphosed to snow in which facetted crystals and depth hoar dominated, as the air temperature plummeted. Even on flat ice, significant localized thickening of snow occurs in the form of barchan dunes. Although we observed great variability in snow thickness and properties on local scales, overall snow thickness distribution and the complex textural assemblage of snow types are similar from region to region. Similar observations were made by Sturm et al. [1998] in West Antarctica. Large‐scale similarities are also apparent in mean snow density, grain size, and bulk snow salinity, although high variability is again found across individual floes. Rapid depth hoar formation is a ubiquitous process that greatly affects the density, texture, grain size, and effective thermal conductivity of the snow cover. The observed heterogeneity results in varying snow effective thermal conductivities. The mean bulk effective thermal conductivity, computed from the proportion of observed snow types, is 0.164 W m−1 K−1, significantly lower than values typically used in large‐scale sea ice modeling but similar to that derived by Sturm et al. [1998] in a near‐simultaneous experiment in the Bellingshausen and Ross Seas. It varies from 0.097 to 0.383 W m−1 K−1 in different snow pits. The findings support those of Sturm et al. [1998] that periodic flooding and subsequent snow ice formation, which are also ubiquitous processes, effectively diminish the degree to which basal snow processes create inhomogeneities in the snow pack.
Monte Carlo simulations of the microwave emissivity of the sea surface
Authors: Liu Quanhua , Simmer Clemens , Ruprecht Eberhard
Published: February 2004

A Monte Carlo model is developed to calculate the microwave emissivity of the sea surface based on the Kirchhoff approximation combined with modified Fresnel coefficients. The modified Fresnel coefficient depends on the incident angle of the electromagnetic wave and the height variance of small‐scale roughness, which is an approximation to account partly for the scattering effect from small ripples. The advantage of the Monte Carlo model is its inherent capability to treat multiple scattering events. Using a two‐dimensional Gaussian distribution for the sea surface slope variability, the model is capable of simulating the azimuthal dependency of the microwave emission caused by the alignment of waves perpendicular to the wind direction. Good agreement between model calculations and measurements is obtained.
A mechanism for layer formation in stratified geophysical flows
Authors: Pelegrí J. L. , Sangrà P.
Published: February 2004

We discuss the possibility that steplike structures are formed in subcritical regions of vertically stratified shear flow. The mechanism we propose essentially consists of localized intense mixing in highly stratified and sheared flows, probably following frontogenesis. Its main assumption is that the vertical density flux increases monotonically with decreasing gradient Richardson numbers, which corresponds to enhanced stratification and/or diapycnal shear. This differs from Phillips' [1972] mechanism, which we argue may not apply to vertically stratified shear flow. An essential condition for the formation of constant density steps is the incorporation of a Langevin type equation which takes into account that turbulence must last for some finite characteristic time. We present numerical computations for the case of approximately constant diapycnal shear which lead to the formation of a staircase depth‐density structure.
Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific
Authors: Gilson John , Roemmich Dean , Cornuelle Bruce , Fu Lee‐Lueng
Published: February 2004

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy‐resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (λ<500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long‐wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (λ>500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin‐wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal‐to‐noise ratio is highest in the western quarter of the transect. Basin‐wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9×104 m2 s−1 RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8°C at 200 m to 0.3°C at 400 m. Geostrophic volume transport, 0–800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (106 m3 s−1) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.
Vortex dynamics beneath a surfactant‐contaminated ocean surface
Authors: Tsai Wu‐ting
Published: February 2004

Data of a vorticity field from direct numerical simulation of the subsurface turbulent shear flow near a surfactant‐contaminated ocean surface are analyzed. The aim of the study is to understand the coupled interaction processes among the surface renewal eddies, the surfactant, and the interface. It is found that as a subsurface coherent intense eddy approaches the interface, such as the identified rising horseshoe vortex, a strong counterrotating spanwise vortex is induced at the interface in the early stage of the interaction. This surface vortex acts as the vortex shed from a no‐slip boundary to repel the approaching eddy. The competition between the vortex stretching owing to the inviscid blocking effect of the interface and the viscous dissipation and diffusion causes the surface‐parallel portion of the approaching subsurface vortex to disconnect. In the latter stage of the interaction, the surviving surface‐normal portion of the underlying vortex continues to redistribute the surfactant and induces additional surface vortices. Viscous diffusion, which is the only production mechanism of vertical vorticity flux near a contaminated interface, eventually results in the disconnected underlying vortex reconnecting with the interface. Enstrophy transport between the surface‐connected vortices and the latterly induced surface vortices dominate the enstrophy budget in the immediate vicinity of the contaminated interface. The identified interaction scenario between the surfactant‐contaminated interface and the subsurface vortical flow can exist at neither a no‐slip nor a free‐slip surface.
Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions
Authors: Munger J. William , Fan Song‐Miao , Bakwin Peter S. , Goulden Mike L. , Goldstein Allen. H. , Colman Albert S. , Wofsy Steven C.
Published: February 2004

Measurements of nitrogen deposition and concentrations of NO, NO2, NOy (total oxidized N), and O3 have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990–1996 averaged 47 mmol m−2 yr−1 (126 μmol m−2 d−1, 6.4 kg N ha−1 yr−1), with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m−2 d−1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NOx to HNO3 in the boundary layer. HNO3 is subsequently removed rapidly by either dry deposition or precipitation. The characteristic (e‐folding) time for NOx oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ∼1.5 days in the winter, when heterogeneous reactions are dominant and O3 concentrations are lowest. The characteristic time for removal of NOx oxidation products (defined as NOy minus NOx) from the boundary layer by wet and dry deposition is ∼1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first‐order rates for oxidation of NOx, followed by deposition, predicts that 45% of NOx in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.
Influence of nonlocal chemistry on tracer distributions: Inferring the mean age of air from SF6
Authors: Hall Timothy M. , Waugh Darryn W.
Published: February 2004

Sulfur hexafluoride (SF6) is nearly inert in the troposphere and stratosphere and has a documented, steady increase in the troposphere due to industrial sources, making it a useful tracer of atmospheric circulation. Studies using SF6 to estimate the mean age of stratospheric air have assumed the influence of mesospheric photochemical destruction is negligible. However, the mean age of an air parcel may be sensitive to small fractions of the air that have resided for long times in the upper atmosphere. Here we use two three‐dimensional chemical transport models to estimate the influence of mesospheric SF6 loss on mean age inferences in the stratosphere. Because the mechanisms of SF6 loss are uncertain, we perform a number of simulations employing a range of magnitudes of a simple constant mesospheric loss frequency, as well as a range of scenarios for SF6 time variation in the troposphere. Using loss rates producing plausible global lifetimes of SF6 (1000 to 3000 years), and tropospheric time variation matching observations, we find that age estimates inferred from SF6 mixing ratios may be significantly biased and that the bias is increasing in time. For example, the present‐day time lag of SF6 mixing ratio from the troposphere overestimates the mean age by up to 18% at 68°S and 20 km, and up to 65% at 68°S and 30 km, depending on the loss rate and model. Correcting for this bias would bring recent comparisons of modeled and measured mean age closer in line.
Apparent breakdown of reciprocity in reflected solar radiances
Authors: Di Girolamo Larry , Várnai Tamás , Davies Roger
Published: February 2004

Observations of reflected solar radiation measurements from natural surfaces (e.g., clouds and forested biomes) are often noted as disobeying the principle of reciprocity. In these contexts the application of reciprocity has been in its directional form. We note that the general principle of reciprocity also encompasses spatial attributes and reduces to a directional form only when the areas of illumination and measurement are the same. In either form a proper reciprocal set of reflected solar radiation measurements can never be obtained due to our inability to control the area illuminated by the Sun. This may appear as a breakdown in the directional reciprocity of the observations, which has serious implications for remote sensing. Using Monte Carlo radiative transfer simulations, we demonstrate that the magnitude of the apparent reciprocity breakdown depends on three factors: (1) measurement resolution, with larger deviations occurring for higher resolutions; (2) Sun‐view geometry, with larger deviations occurring for larger Sun/view zenith angles and differences between Sun and view angles; and (3) the distribution of scatterers within and surrounding the measurement area, making the deviations depend on scene type.
A cumulus parameterization with multiple cloud base levels
Authors: Ding Ping , Randall David A.
Published: February 2004

We have generalized the Arakawa‐Schubert cumulus parameterization to allow multiple cloud‐base levels. A spectrum of cloud‐top levels is allowed for each cloud‐base level. We use a linear (rather than exponential) mass flux profile, because this greatly simplifies the determination of the entrainment rate and reduces the computational requirements of the parameterization. Cloud‐top entrainment is included. The linear mass flux profile and the multiple cloud bases are separately tested in a general circulation model. Results show that the linear mass flux profile tends to lead to smoother precipitation distribution with a decrease in the depth and intensity of deep convection and that cumuli with cloud bases above the boundary layer tend to occur at night over land.
Physically based modeling of atmosphere‐to‐snow‐to‐firn transfer of H2O2 at South Pole
Authors: McConnell Joseph R. , Bales Roger C. , Stewart Richard W. , Thompson Anne M. , Albert Mary R. , Ramos Ricardo
Published: February 2004

Quantitative interpretation of ice core chemical records requires a detailed understanding of the transfer processes that relate atmospheric concentrations to those in the snow, firn, and ice. A unique, 2 year set of year‐round surface snow samples at South Pole and snow pits, with associated accumulation histories, were used to test a physically based model for atmosphere‐to‐firn transfer of H2O2. The model, which extends our previous transfer modeling at South Pole into the snowpack, is based on the advection‐dispersion equation and spherical diffusion within representative snow grains. Required physical characteristics of the snowpack, such as snow temperature and ventilation, were estimated independently using established physical models. The surface snow samples and related model simulations show that there is a repeatable annual cycle in H2O2 in the surface snow at South Pole. It peaks in early spring, and surface snow concentration is primarily determined by atmospheric concentration and temperature, with some dependence on grain size. The snow pits and associated model simulations point out the importance of accumulation timing and annual accumulation rate in understanding the deposition and preservation of H2O2 and δ18O at South Pole. Long‐term snowpack simulations suggest that the firn continues to lose H2O2 to the atmosphere for at least 10–12 years (∼3 m) after burial at current South Pole temperatures and accumulation rates.
Evidence for solar‐cycle forcing and secular variation in the Armagh Observatory temperature record (1844–1992)
Authors: Wilson Robert M.
Published: February 2004

A prominent feature of previous long‐term temperature studies has been the appearance of warming since the 1880s, this often being taken as evidence for anthropogenic‐induced global warming. In this investigation, the long‐term, annual, mean temperature record (1844–1992) of the Armagh Observatory (Armagh, North Ireland), a set of temperature data based on maximum and minimum thermometers that predates the 1880s and correlates well with northern hemispheric and global standards, is examined for evidence of systematic variation, in particular, as related to solar‐cycle forcing and secular variation. Indeed, both appear to be embedded within the Armagh data. Removal of these effects, each contributing about 8% to the overall reduction in variance, yields residuals that are randomly distributed. Application of the 10‐year moving average to the residuals, furthermore, strongly suggests that the behavior of the residuals is episodic, inferring that (for extended periods of time) temperatures at Armagh sometimes were warmer or cooler (than expected), while at other times they were stable. Comparison of cyclic averages of annual mean temperatures against the lengths of the associated Hale cycles (i.e., the length of two, sequentially numbered, even‐odd sunspot cycle pairs) strongly suggests that the temperatures correlate inversely (r = −0.886 at <2% level of significance) against the length of the associated Hale cycle. Because sunspot cycle 22 ended in 1996, the present Hale cycle probably will be shorter than average, implying that temperatures at Armagh over this Hale cycle will be warmer (about 9.31±0.23°C at the 90% confidence level) than average ( = 9.00°C).
Chemical air mass differences near fronts
Authors: Bethan S. , Vaughan G. , Gerbig C. , Volz‐Thomas A. , Richer H. , Tiddeman D. A.
Published: February 2004

Two case studies are presented of aircraft measurements (ozone, NOy, CO, and meteorological parameters) in the vicinity of fronts located over the eastern side of the North Atlantic Ocean during spring 1994. The aim of these studies was twofold: (1) to investigate whether frontal circulations can transport ozone from the boundary layer to the free troposphere in well‐defined layer; and (2) to ascertain whether or not conveyor belts associated with extratropical cyclones exhibit well‐defined chemical signatures. The first case study (March 2, 1994) sampled a well‐defined ozone‐enhanced layer within the free troposphere. It is demonstrated that this air was transferred from the boundary layer to the free troposphere during the development of a baroclinic wave. Two warm conveyor belts sampled within this flight (one associated with the developing baroclinic wave and the other with a mature low‐pressure system) displayed clear and contrasting chemical signatures, a consequence of their geographically different origins. During the second case study (April 25, 1994), both the dry intrusion and the warm conveyor belt of a mature, occluded low‐pressure system were sampled. Their chemical signatures (in particular, that of the dry intrusion) showed that interleaving of the two airstreams had occurred, probably in the vicinity of the occluded front. It is thus demonstrated that chemical measurements within conveyor belts provide valuable information on the nature, history, and extent of these coherent flows.