bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of the polarization of light reflected from sea ice
Authors: Perovich Donald K.
Published: February 2004

As part of a large, interdisciplinary program investigating the electromagnetic properties of sea ice, we made spectral measurements of the albedo, reflectance, and Stokes vector of the reflected radiance field. The overall program encompassed observations of sea ice physical properties, optical properties and microwave properties, plus an extensive modeling effort. Measurements were made of an evolution sequence including young sea ice, pancake ice, snow‐covered ice, first‐year ice, and ponded ice. The effects of surface roughness were investigated by artificially roughening part of a 31‐cm‐thick, smooth, young ice sheet. Spectral and total albedos were sensitive to surface conditions, consistent with earlier studies. Stokes vector observations exhibited the greatest variability in the plane of incidence of the solar beam. Reflectances at 0° azimuth increased sharply with zenith angle. Smoother surfaces, such as melt ponds, pancakes and bare ice, exhibited a larger increase than the snow‐covered cases with their “rougher” surfaces. Since the solar zenith angle was close to Brewster's angle, specularly reflected light was highly polarized. In the “smooth” ice cases, there was significant polarization associated with the increase in reflectance, implying a substantial contribution from specular reflection. This contribution was greater at longer wavelengths, where specular reflection was a larger component of the reflected radiance. Away from the plane of incidence, the Stokes vector showed little angular variation, and the reflected light at these angles was largely unpolarized.
Vertical profiles of bromoform in snow, sea ice, and seawater in the Canadian Arctic
Authors: Sturges William T. , Cota Glenn F. , Buckley Paul T.
Published: February 2004

Bromoform (CHBr3) was measured in vertical profiles from the snow surface through the snowpack, sea ice, and water column to the seafloor at Resolute Bay, Canada, in the spring of 1992. Elevated concentrations of bromoform were observed in both the ice (32–266 ng L−1 by liquid water volume) and seawater (∼20 ng L−1) at the ice‐water interface, associated with bromoform emission from ice microalgae. A surprising finding was a second horizon of high bromoform concentrations (336–367 ng L−1) in sea ice at the snow‐ice interface. Chlorophyll and salinity were also elevated in this upper ice layer, although chlorophyll was much lower than in the basal ice microalgal layer. We speculate that this upper bromoform‐enriched layer may have originated from scavenging of the surface water layer by frazil ice during initial ice formation in the preceding autumn. Equally unexpected was the occurrence of yet higher bromoform concentrations in snowpack immediately overlying the sea ice (492–1260 ng L−1), declining in concentration (by about a factor of 2 or more) toward the snow surface. Snow of very recent origin, however, contained as little as 2 orders of magnitude less bromoform than the older snowpack. Possible origins for elevated bromoform in the snowpack include diffusion out of the bromoform‐enriched upper ice layer and gradual concentration of bromoform out of the atmosphere by adsorption on to ice crystals. These are considered in turn. In one scenario, photolysis of bromoform from snow is considered, which might help account for atmospheric bromine‐ozone chemistry. The possible contributions from snow, sea ice, and seawater to atmospheric bromoform levels during both the winter and spring are also considered, and it is concluded that surface seawater presents the most significant reservoir for atmospheric bromoform.
Observation of tropical cyclones by high‐resolution scatterometry
Authors: Quilfen Y. , Chapron B. , Elfouhaily T. , Katsaros K. , Tournadre J.
Published: February 2004

Unprecedented views of surface wind fields in tropical cyclones (hereafter TCs) are provided by the European Remote Sensing Satellite (ERS) C band scatterometer. Scatterometer measurements at C band are able to penetrate convective storms clouds, observing the surface wind fields with good accuracy. However the resolution of the measurements (50×50 km2) limits the interpretation of the scatterometer signals in such mesoscale events. The strong gradients of the surface wind existing at scales of a few kms are smoothed in the measured features such as the intensity and location of the wind maxima, and the position of the center. Beyond the ERS systems, the scatterometers on‐board the ADEOS and METOP satellites, designed by the Jet Propulsion Laboratory and by the European Space Agency, respectively, will be able to produce measurements of the backscattering coefficient at about 25×25 km2resolution. A few sets of ERS‐1 orbits sampling TC events were produced with an experimental 25×25 km2resolution. Enhancing the resolution by a factor of 2 allows location of the wind maxima and minima in a TC with a much better accuracy than at 50 km resolution. In addition, a better resolution reduces the geophysical noise (variability of wind speed within the cell and effect of rain) that dominates the radiometric noise and hence improves the definition of the backscattering measurements. A comprehensive analysis of the backscattering measurements in the case of high winds and high sea states obtained within TCs is proposed in order to refine the interpretation of the wind vector derived from a backscattering model that is currently only calibrated up to moderate winds (<20 m/s) in neutral conditions. Observations of the TOPEX‐POSEIDON dual‐frequency altimeter are also used for that purpose. Patterns of the surface winds in TCs are described and characteristic features concerning asymmetries in the maximum winds and in the divergence field are discussed.
Annual cycle of sea surface temperature in the tropical Atlantic Ocean
Authors: Carton James A. , Zhou Zengxi
Published: February 2004

The strongest temporal variation of sea surface temperature (SST) in the Atlantic occurs at the annual period. In this paper we examine the causes of the annual cycle of SST through a series of modeling studies. In the subtropics we find that the annual cycle of SST follows the annual cycle of net shortwave radiation, except near the eastern boundary. Off northwest Africa, the annual cycle results from coastal upwelling induced by annual variations in alongshore winds. Off southwest Africa, the annual cycle results from a combination of coastal upwelling and effects caused by changes in the trade winds along the equator. In the band of latitudes between 3°N and 10°N, changes in latent heat loss due to the annual variation in wind speed is most important. The dominance of latent heat loss in this region explains why SST reaches a maximum in boreal fall even though the depth of the thermocline is at its annual minimum.
The general circulation of the Mediterranean Sea from a 100‐year simulation
Authors: Wu Peili , Haines Keith
Published: February 2004

Results from a 120‐year simulation of the Mediterranean sea with a 1/4°, 19‐level eddy‐permitting Cox model with monthly varying wind and buoyancy forcing are presented. Mean and seasonal currents are examined along with water distributions on isopycnals. Detailed equilibrated thermohaline water pathways, broadly consistent with observed hydrographic data, are determined. The most important factor permitting a long model run is low vertical mixing, which maintains Levantine intermediate water (LIW) above the level of the Sicily sill so that it can reach the western basin. A new feature is Tyrrhenian deep water, which is formed after 40 years by deep water transfer from the Balearic basin south of Sardinia. A water mass analysis reveals the modification of water properties in T, S space through the mixing processes that take place at straits and through the action of mesoscale eddies. The surface heat and freshwater fluxes derived a posteriori are in very good agreement with observations, showing that basin average fluxes are closely tied to the generation of a realistic thermohaline circulation. A complete analysis of surface and strait fluxes demonstrates that equilibrium conditions have been reached. A study of the heat budget in the intermediate water layer at 280 m depth reveals that the dominant diabatic terms are at localized convection sites, suggesting that at this depth the thermocline is under advective rather than diffusive control. The eddy and mean flow heat transports balance to a large degree away from these sites, with a negligible role for diffusion.
Complex empirical orthogonal functions analysis of ERS‐1 and TOPEX/POSEIDON combined altimetric data in the region of the Algerian current
Authors: Bouzinac Catherine , Vazquez Jorge , Font Jordi
Published: February 2004

Maps of sea level anomalies (SLA) relative to the 1993 annual mean sea level combine the data from the two altimetric missions, ERS‐1 and TOPEX/POSEIDON, during the overlap period (October 1992 to December 1993). These regular maps in space and time of residual sea level every 10 days on a 0.2° regular grid are used in the region of the Algerian current where the mesoscale eddies are of primary importance to the circulation of all the Mediterranean water masses. They are first compared with ERS‐1 along‐track scanning radiometer sea surface temperature images to get information on two anticyclonic eddies produced by instabilities of the Algerian current and visible in both infrared and altimetric data sets. Then, an analysis of complex empirical orthogonal functions (CEOFs) is performed on the SLA data set to see the correlation of the different dynamic features of the observed variability. The CEOF analysis is applied to the complex time series formed from the original SLA time series and their Hubert transforms to separate the variability into spatially coherent modes. The spatially correlated signal in the study area (0–15°E and 35°–40°N) was found to be dominated by the first two CEOFs. These first two modes explain nearly 85% of the variability, with 80% of the total variance for the first one and 5% of the total variance for the second one. The temporal phase of the first mode indicates that a constant frequency of one cycle per year is clearly dominant, corresponding to the seasonal signal. The strongest amplitude is obtained in the southern part of the channel of Sardinia and south of the Strait of Sicily. The temporal amplitude and the temporal phase of the second mode show a periodicity of about 6 months which appears to be associated with the variability of the Algerian current as the phase isolines are parallel to the mean current path along the Algerian coast. The strongest amplitude of the second mode is located near the African coast at ∼4°E and 8°E. These two points of high variability could correspond to eddy detachments from the main current.
Field observations of bar‐trapped edge waves
Authors: Bryan K. R. , Howd P. A. , Bowen A. J.
Published: February 2004

Edge waves which appear to be topographically trapped by surf‐zone bars are observed in data collected during the DELILAH experiment, at Duck, North Carolina, in October 1990. Edge waves are theoretically trapped on bars when the phase speed is between (ghbar)1/2 and (ghtrough)1/2, where the effective depth h is the actual depth modified by the longshore current [Howd et al., 1992]. Spectral analysis of the longshore component of orbital velocity data from two longshore instrument arrays, one in the trough and one seaward of the bar, showed continuous, distinct, diagonal lines of variance, extending into incident wave frequencies. These lines of variance have the same frequency‐wavenumber coordinates as theoretical calculations of bar‐trapped edge waves. When the instrument array is on the top of the bar, the bar‐trapped waves with the simplest shape are dominant; however, when the array is farther from the bar, more complex bar‐trapped waves dominate. In the latter case, the simplest bar‐trapped edge waves may exist on top of the bar and may have decayed to an insignificant size at the instrument location. Isolated cases exist where the simplest bar‐trapped waves are still dominant even in the trough. These instances correspond to days with a strongly bimodal incident wave spectrum, which is broadbanded in both frequency and incident direction. These field observations show that bar‐trapped edge waves can be the dominant edge wave modes in the longshore component of velocity data collected near or on the bar over a wide range of frequencies, including incident frequencies; their potential contribution to nearshore hydrodynamics should be addressed in future studies of barred beaches.
A basin‐scale oceanic instability event in the Gulf of Alaska
Authors: Thomson Richard E. , Gower James F. R.
Published: February 2004

A composite NOAA 14 satellite advanced very high resolution radiometer image of the Gulf of Alaska for early March 1995 reveals a sequence of six anticyclonic oceanic eddies spanning more than 1500 km of the continental margin of North America from central British Columbia to central Alaska. Although isolated mesoscale eddies have previously been documented for this eastern boundary region, the composite image provides the first evidence that trains of such eddies can develop “simultaneously” along the entire coast. The 80 km radius and 250 km spacing of these warm‐core eddies are consistent with a baroclinic instability event in the poleward coastal current that prevails along the northwest continental margin of North America in “winter” (roughly October through March). Wind data from offshore meteorological buoys, combined with partial cloud‐free thermal images for January and February, suggest that the event was triggered in mid January by an abrupt, coast‐wide reversal in the prevailing poleward wind. The instability event disrupted the alongshore transport of relatively warm, high‐salinity water and presumably resulted in a greatly enhanced cross‐shore flux of brackish, nutrient‐rich coastal water into the open ocean. On the basis of its basin‐scale extent and more than 2 month duration it is feasible that the event had a major impact on the early spring recruitment and survival of zooplankton and pelagic fish within the highly productive fishery zones of coastal British Columbia and Alaska. Less extensive eddy sequences found in several earlier satellite images for the region suggest that these instability events are fairly common. If so, this indicates that the poleward current that forms over the eastern Pacific continental slope in late fall (herein, the “Northeast Pacific Coastal Current”) is marginally stable and likely to deform into a series of mesoscale eddies following abrupt, basin‐scale reversals in the seasonally dominant wind. Provided these wind events occur early enough in the season, the coastal current could reform with the onset of poleward winter winds.
Evolution of Langmuir circulation during a storm
Authors: Smith Jerome A.
Published: February 2004

Wind stress, waves, stratification, velocity profiles, and surface fields of radial velocity and acoustic backscatter intensity were measured along a drift track 50 to 150 km off Point Arguello, California. On March 8, 1995, the wind increased from calm to 12 m/s from the SE, opposing swell from the NW. It increased to 15 m/s at noon UTC on March 9, remained steady over the next 12 hours, briefly dropped and veered by 60°, then returned. A mixed layer deepened quickly to 25 m, then held roughly steady through the next 2 days, in spite of gusty winds continuing at 15–25 m/s. A phased‐array Doppler sonar system took measurements covering 250 m by 150 m of the surface, with 5 m by 10 m spatial resolution. Averages over 6 min removed surface waves, permitting continuous assessment of strength, orientation, spacing, and degree of organization of features associated with surface motion (e.g., Langmuir circulation), even when conditions were too rough for visual assessment. Several results stand out: (1) As found previously, most wind mixing arises from inertial shear across the thermocline. (2) Consistent with wind/wave forcing of Langmuir circulation,Plueddemann et al. [1996] suggest that surface velocity variance  scales like (u*Us), whereu*is friction velocity andUsis the surface Stokes' drift; however, the measurements here scale with (Us)1 alone, once Langmuir circulation is established. (3) The velocity variance is weaker here than expected, given the magnitudes of wind and waves, leading to a smaller estimated mixing effect. (4) Large vacillations in LC strength are seen just before the brief veering of the wind; it is suggested that bubble buoyancy could play a dynamic role. (5) Mean orientation and spacing can differ for intensity versus radial velocity features.
Potential atmospheric sources and sinks of nitrous oxide: 3. Consistency with the observed distributions of the mixing ratios
Authors: Prasad Sheo S. , Zipf E. C. , Zhao Xuepeng
Published: February 2004

This paper discusses the idea that the potential atmospheric sources and sinks of nitrous oxide (N2O) discussed by Prasad [1994, this issue] may not be inconsistent with the observed vertical profiles of the N2O mixing ratios. For this purpose the vertical distributions of N2O mixing ratios were calculated in one‐dimensional model atmosphere with and without the new sources and sinks. It was found that the inclusion of the new sources and sinks in the model atmosphere predicts very nearly the same mixing ratios as are predicted by the use of only the classical chemistry of N2O. This means that these sources and sinks are in accord with atmospheric observations also, given the very good agreement between the classical chemistry predictions and the observations. The important issue of the observed correlations of the N2O mixing ratios with those of other long‐lived tracers such as CH4 and CF2Cl2 is also discussed. The key point is that while similar chemistries (i.e., similar nature and spatial distributions of sources and sinks) ensure good cross‐species correlations, the reverse is not necessarily true. It is therefore possible that N2O may be correlated with CH4 or chlorofluorocarbons despite differences in their chemistry suggested by the potential new sources and sinks of N2O.
Origins of African dust transported over the northeastern tropical Atlantic
Authors: Chiapello I. , Bergametti G. , Chatenet B. , Bousquet P. , Dulac F. , Soares E. Santos
Published: February 2004

On the basis of 3 years of daily measurements of mineral dust concentrations at Sal Island (Cape Verde), this study focuses on the origin of the dust transported over the northeastern tropical Atlantic during the dusty season (October to April). Using backward trajectories, we have identified three different sectors of origin of the transported dust, covering the Sahara (north/west and central/south) and the Sahel regions. Time variations of the dust transport from these different areas can be explained by the seasonal shift of the subtropical high‐pressure center. A geochemical approach highlights that the calcium amount and the Si/Al ratio of the transported dust differ according to these source regions, in relation to their respective soil compositions.
Seasonal variations of atmospheric trace gases in the high Arctic at 79°N
Authors: Notholt J. , Toon G. , Stordal F. , Solberg S. , Schmidbauer N. , Becker E. , Meier A. , Sen B.
Published: February 2004

Since March 1992 the total column abundances of several tropospheric and stratospheric trace gases have been monitored year‐round from the Network for Detection of Stratospheric Change station in Ny Ålesund, Spitsbergen (78.9°N, 11.9°E). A ground‐based Fourier transform infrared (FTIR) spectrometer performed these measurements using the Sun as a light source during the summer, and the Moon during the winter. In situ measurements of C2H2, C2H6, and CCl2F2, made from the top of a nearby mountain, were combined with the FTIR column data to infer additional information about the variation of the volume mixing ratio profiles with altitude and season. The short‐lived tropospheric trace gases C2H2, C2H6, and CO exhibit large seasonal variations with a summer minimum, caused by reaction with OH. CH2O shows a second maximum during the summer, caused by its formation by methane oxidation. For the long‐lived gases HF, N2O, and CH4 the seasonal cycle is less pronounced and is forced mainly by wintertime stratospheric diabatic descent, which starts in early November and reaches a maximum in March. The total columns of the stratospheric trace gases indicate that the chemical repartitioning of HCl into ClONO2 starts in November, before the widespread production of polar stratospheric clouds. The total columns of the sum of HCl plus ClONO2 suggests that between December and March they are converted into their active counterparts. Photolysis of HNO3 gives rise to its summer minimum, and its winter maximum, with no evidence for a strong winter denitrification.
Spatiotemporal analysis of radar‐estimated precipitation during the BOREAS summer 1994 field campaigns
Authors: Schnur Reiner , Krauss Terry W. , Eley F. Joe , Lettenmaier Dennis P.
Published: February 2004

A truck‐mounted C‐band precipitation radar was operated in the Boreal Ecosystem‐Atmosphere Study southern study area (SSA) north of Prince Albert, Saskatchewan, for the duration of the intensive field campaigns, May to September 1994. Radar reflectivities were transformed to rainfall intensities using the Marshal‐Palmer relationship at 10‐min intervals for 2×2 km pixels. Hourly accumulations of this product are analyzed for an area that coincides roughly with the southern modeling subarea (SMSA), a 50×40 km rectangle inside the SSA where the greatest focus of field activities took place. For validation, rainfall measurements from a network of 12 tipping bucket and Belfort gauges are used. Although there are significant differences between the rainfall measurements of single gauges and the overlying radar pixels, the area‐averaged (SMSA) radar rainfall coincides fairly well with the average rainfall at the gauges. The conditional mean rainfall intensity over the SSA is shown to be closely related to its standard deviation, whereas the area‐averaged rain rate is related to its frequency of occurrence and rainfall coverage by power laws. The fractional area covered by rainfall above certain thresholds follows a lognormal distribution. It is demonstrated how such relationships can be used by modelers to determine the sub‐grid‐scale characteristics of precipitation. Using a spatially distributed hydrology‐vegetation model, the effect of the spatial resolution of the rainfall fields on moisture and energy fluxes between the surface and the atmosphere is investigated. There are large differences in the modeled fluxes if only the rainfall information from the gauge network is used to produce model precipitation field inputs, as compared to using the 2×2 km resolution radar rainfall fields. Using aggregated radar precipitation at decreasing resolutions results in increasing differences of the area‐averaged fluxes from the 2×2 km base run. In general, the mean absolute differences for area‐averaged latent and sensible heat fluxes and surface runoff are less than 10%. Locally though, the effect of using lower‐resolution rainfall input fields on surface energy fluxes can be quite large.
Algorithm for atmospheric and glint corrections of satellite measurements of ocean pigment
Authors: Fraser Robert S. , Mattoo Shana , Yeh Eueng‐Nan , McClain C. R.
Published: February 2004

An algorithm is developed to correct satellite measurements of ocean color for atmospheric and surface reflection effects. The algorithm depends on taking the difference between measured and tabulated radiances for deriving water‐leaving radiances. The tabulated radiances are related to the measured radiance where the water‐leaving radiance is negligible (670 nm). The tabulated radiances are calculated for rough surface reflection, polarization of the scattered light, and multiple scattering. The accuracy of the tables is discussed. The method is validated by simulating the effect of different wind speeds than that for which the lookup table is calculated, and aerosol models different from the maritime model for which the table is computed. The derived water‐leaving radiances are accurate enough to compute the pigment concentration with an error of less than ±15% for wind speeds of 6 and 10 m/s and an urban atmosphere with aerosol optical thickness of 0.20 at λ443 nm and decreasing to 0.10 at λ670 nm. The pigment accuracy is less for wind speeds less than 6 m/s and is about 30% for a model with aeolian dust. On the other hand, in a preliminary comparison with coastal zone color scanner (CZCS) measurements this algorithm and the CZCS operational algorithm produced values of pigment concentration in one image that agreed closely.
Lidar observations of polar stratospheric clouds at the South Pole: 1. Stratospheric unperturbed conditions, 1990
Authors: Cacciani Marco , Fiocco Giorgio , Colagrande Paola , Di Girolamo Paolo , di Sarra Alcide , Fuà Daniele
Published: February 2004

Observations of polar stratospheric clouds (PSCs) carried out at the Amundsen Scott South Pole Station by lidar in the winter of 1990 are reported. Echoes attributable to PSCs began to appear in late May, were a persistent feature till early September, and from that time until the end of October were sporadically observed. Values of the backscatter ratio as high as 16 were recorded in late August at 13 km. Analyses based on the sensitivity of the backscattering to temperature confirm that the attribution of the echoes to PSC type I or II is possible in some cases. Those that could be ascribed to type II were observed mostly during July and August, while those attributable to type I appeared only at the beginning of the winter. The comparison of water vapor mixing ratios derived from PSC type II occurrence temperatures and frost point hygrometer measurements indicates a fast dehydration in the beginning of the winter.
Evolution of chlorine and nitrogen species in the lower stratosphere during Antarctic spring: Use of tracers to determine chemical change
Authors: Mickley L. J. , Abbatt J. P. D. , Frederick J. E. , Russell J. M.
Published: February 2004

Observations of O3, HCl, NO, and NO2 from the Halogen Occultation Experiment (HALOE) provide a means to investigate chemical change in the lower stratosphere over Antarctica during the first 23 days of October 1992. Two long‐lived species also observed by HALOE, HF and CH4, are used as tracers to identify a series of air parcels having similar Cly and NOy abundances. The set of parcels chosen using tracer analysis show uniformly low O3 mixing ratios, less than 1 ppm on the 480 K surface (about 18–20 km). HCl mixing ratios for those parcels with less than 1 ppm of ozone nearly tripled during the time period, and NO+NO2 abundances rose sharply, by a factor of 6 or 7. These trends in HCl, NO, and NO2 agree qualitatively with model calculations which show that the formation of HCl proceeds quickly when O3 levels fall so low that (1) the rate of the reaction Cl+O3→ClO+O2 slows and (2) the rate of the reaction NO+ClO→NO2+Cl becomes faster than the rate of the competing reaction NO+O3→NO2+O2. Under these conditions, Cl increases at the expense of ClO, and HCl is formed via the reaction Cl+CH4→HCl+CH3. Stratospheric chlorine is thus shifted from reactive species to the long‐lived, reservoir molecule HCl. The repartitioning of the active chlorine family in favor of HCl halts the processes that destroy ozone and makes available active nitrogen in the form of NO and NO2. The investigation confirms earlier results and validates tracer analysis as a reliable method to probe chemical change in the stratosphere.
Feasibility of using sand dunes as archives of old air
Authors: Severinghaus Jeffrey P. , Keeling Ralph F. , Miller Benjamin R. , Weiss Ray F. , Deck Bruce , Broecker Wallace S.
Published: February 2004

Large unaltered samples of the atmosphere covering the past century would complement the history of atmospheric gases obtained from bubbles in ice cores, enabling measurement of geochemically important species such as O2, 14CH4, and 14CO. Sand dunes are a porous media with interstitial air in diffusive contact with the atmosphere, somewhat analogous to the unconsolidated layer of firn atop glaciers. Recent studies have demonstrated the value of firn as an archive of old air [Battle et al., 1996; Bender et al., 1994a]. Unlike firn, sand dunes are incompressible and so remain permeable to greater depths and may extend the firn record into the past century. To evaluate the feasibility of using sand dunes as archives of old air, we drilled 60 m deep test holes in the Algodones Dunes, Imperial Valley, California. The main objective was to see if the air in a sand dune is as old as predicted by a diffusion model, or if the dune is rapidly flushed by advective pumping during windstorms and barometric pressure changes. We dated the air with chlorofluorocarbons and krypton‐85, anthropogenic tracers whose atmospheric concentrations are known and have been increasing rapidly in the past half century. These tracer data match the pure diffusion model well, showing that advection in this dune is negligible compared to diffusion as a transport mechanism and that the mean age of the air at 61 m depth is ∼10 years. Dunes therefore do contain old air. However, dunes appear to suffer from two serious drawbacks as archives. Microbial metabolism is evident in elevated CO2 and N2O and depressed CH4 and O2 concentrations in this dune, corrupting the signals of interest in this and probably most dunes. Second, isotopic analyses of N2 and O2 from the dune show that fractionation of the gases occurs due to diffusion of water vapor, complicating the interpretation of the O2 signal beyond the point of viability for an air archive. Sand dunes may be useful for relatively inert gases with large atmospheric concentration changes such as chlorofluorocarbons.
Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides
Authors: Roehl Coleen M. , Burkholder James B. , Moortgat Geert K. , Ravishankara A. R. , Crutzen Paul J.
Published: February 2004

The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (σmax at 270 nm). The spectrum of CH2I2 is further red‐shifted, reaching a maximum of 3.85×10−18 cm2 molecule−1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10−6 s−1 for CH3I to 5×10−3 s−1 for CH2I2 at a solar zenith angle of 40°.
Cloud overlap effects on general circulation model climate simulations
Authors: Liang Xin‐Zhong , Wang Wei‐Chyung
Published: February 2004

The vertical distribution of clouds significantly affects the radiative heating/cooling distributions. Consequently, the general circulation model (GCM) simulated climate can be sensitive to the treatment of cloud overlap, which is related to the subgrid scale cloud vertical geometric association. Here we developed and tested a “mosaic” treatment that can explicitly consider the cloud association in the GCM radiation parameterization. The effects of this treatment on radiative forcing and climate simulations were studied by comparing the results with those using the “random” overlap, which assumes clouds are independent and therefore yields a larger effective cloudiness. The mosaic treatment calculates a significantly different atmospheric radiative heating/cooling distribution caused mainly by the changes in infrared radiation. In the tropics it yields a heating in the upper troposphere and a cooling in the lower troposphere especially near the surface; opposite changes are calculated in the middle to high latitudes. Because of a smaller effective cloudiness the mosaic treatment calculates less infrared downward radiation reaching the surface, which is partially compensated by increased incident solar radiation. Differences in the climate responses are substantial, with several major model biases corrected by the mosaic treatment For example, the middle to upper troposphere of the tropics and subtropics are warmed by more than 3°C throughout the year, and the polar night northern stratosphere becomes much warmer, up to a maximum of 15°C. A more realistic distribution of tropical precipitation is simulated especially over central to east Pacific oceans. Other notable improvements include the decreased convective rainfall and surface evaporation and the increased atmospheric moisture content. The mosaic treatment also enhances significantly the daily variability of surface air temperature and radiation fluxes in the low latitudes.
Improved surface and boundary layer models for the Goddard Institute for Space Studies general circulation model
Authors: Hartke Gregory J. , Rind David
Published: February 2004

Improved surface and boundary layer models have been developed and implemented in the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The new boundary layer model retains the methodology of the Model 2 version in which the surface winds are interpolated using a model profile but amends the approximations that were previously used to keep the model analytical for ease of computation. The new surface layer model is entirely different than that previously used in the GISS GCM and uses similarity theory to compute the transport coefficients, drag coefficient, Dalton number, and Stanton number. The combination yields distinct improvements in the general circulation characteristics of the GISS GCM. The new boundary layer and surface models were chosen to allow surface properties to be precomputed, resulting in minimal impact to the overall execution time. This is particularly important in a GCM used for climate experiments with timescales spanning centuries.
Trajectories in the Antarctic lower troposphere
Authors: Kottmeier C. , Fay B.
Published: February 2004

The statistics of air mass transport to the Neumayer Antarctic station (8°22′W, 70°7′S) are derived from three‐dimensional 5‐day backward trajectories on the basis of a numerical weather prediction model, the Global Model of the German Weather Service, for the period April 1993 to March 1996. Trajectory errors are discussed in detail and errors of individual trajectories may sum up to from a few hundred to more than a thousand kilometers during 5 days' traveling time despite the high quality of the Global Model and trajectory module described. The most frequent easterly flow at Neumayer consists of advection from oceanic and continental regions. During southeasterly flow, mean trajectories document oceanic air passing coastal continental regions during the 3 days preceding arrival. Air arriving from above the Antarctic continent south of 75°S usually descends over the marginal plateau and especially over the slope region. Advection from the central plateau regions poleward of 80°S is rare. Marine trajectories from latitudes north of 60°S mostly start in 0 – 3 km height. The modeled transport for the arrival levels at 950 hPa and 850 hPa at Neumayer station document that the frequency of oceanic source regions increases with height. Trajectories for 15 other coastal destinations show that near‐surface easterly winds with a downslope component dominate along the eastern shore of the Weddell Sea. The prevailing transport from the south at the Filchner Ronne ice shelf arises from deflected barrier winds along the Transantarctic Mountains. At the northern tip of the peninsula, westerly winds prevail, and the air usually originates from the Bellingshausen Sea. The probability of contact with air masses from ice‐free continents is small for all coastal sites.
Emissions of organic trace gases from savanna fires in southern Africa during the 1992 Southern African Fire Atmosphere Research Initiative and their impact on the formation of tropospheric ozone
Authors: Koppmann R. , Khedim A. , Rudolph J. , Poppe D. , Andreae M. O. , Helas G. , Welling M. , Zenker T.
Published: February 2004

CO, CH4, and organic trace gases were measured in air samples collected during several flights with a DC‐3 aircraft through the plumes from savanna fires and agricultural fires during the SAFARI 92 campaign in southern Africa in September and October 1992. In all samples a variety of higher molecular weight organic compounds was found, most of which are very reactive. More than 70 of the roughly 140 major components present could be identified. Typically, mixing ratios of several hundred parts per billion carbon of organic compounds were measured inside the plumes, corresponding to an emission ratio of total organic carbon relative to CO2of up to 1%. About 50% of these emissions were in the form of oxygenated and unsaturated compounds. The contributions of still unknown compounds to the total emission of organic compounds add up to another 20–30%. The observed emission ratios relative to CO2show a considerable variation depending on the fuel type and the burning stages of the fire. The lowest value of the emission ratio of the sum of all identified organic compounds relative to CO2was found for a sugar cane fire with (1.7±0.7)×10−3(ppb C/ppb CO2). For a large savanna fire in Kruger National Park the ratio was (7.4±1.6)×10−3(ppb C/ppb CO2). The highest value was (13.7±0.9)×10−3(ppb C/ppb CO2) for an uncontrolled fire of mainly wood and shrub in the Drakensberg region. Results of model calculations show that in biomass‐burning plumes, reactive organic compounds contribute significantly to the formation of ozone, especially during the initial phase of photochemical processing.
Comparison of the seasonal change in cloud‐radiative forcing from atmospheric general circulation models and satellite observations
Authors: Cess R. D. , Zhang M. H. , Potter G. L. , Alekseev V. , Barker H. W. , Bony S. , Colman R. A. , Dazlich D. A. , Del Genio A. D. , Déqué M. , Dix M. R. , Dymnikov V. , Esch M. , Fowler L. D. , Fraser J. R. , Galin V. , Gates W. L. , Hack J. J. , Ingram W. J. , Kiehl J. T. , Kim Y. , Le Treut H. , Liang X.‐Z. , McAvaney B. J. , Meleshko V. P. , Morcrette J. J. , Randall D. A. , Roeckner E. , Schlesinger M. E. , Sporyshev P. V. , Taylor K. E. , Timbal B. , Volodin E. M. , Wang W. , Wang W. C. , Wetherald R. T.
Published: February 2004

We compare seasonal changes in cloud‐radiative forcing (CRF) at the top of the atmosphere from 18 atmospheric general circulation models, and observations from the Earth Radiation Budget Experiment (ERBE). To enhance the CRF signal and suppress interannual variability, we consider only zonal mean quantities for which the extreme months (January and July), as well as the northern and southern hemispheres, have been differenced. Since seasonal variations of the shortwave component of CRF are caused by seasonal changes in both cloudiness and solar irradiance, the latter was removed. In the ERBE data, seasonal changes in CRF are driven primarily by changes in cloud amount. The same conclusion applies to the models. The shortwave component of seasonal CRF is a measure of changes in cloud amount at all altitudes, while the longwave component is more a measure of upper level clouds. Thus important insights into seasonal cloud amount variations of the models have been obtained by comparing both components, as generated by the models, with the satellite data. For example, in 10 of the 18 models the seasonal oscillations of zonal cloud patterns extend too far poleward by one latitudinal grid.
Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene
Authors: Chappellaz Jérôme , Blunier Thomas , Kints Sophie , Dällenbach André , Barnola Jean‐Marc , Schwander Jakob , Raynaud Dominique , Stauffer Bernhard
Published: February 2004

High‐resolution records of atmospheric methane over the last 11,500 years have been obtained from two Antarctic ice cores (D47 and Byrd) and a Greenland core (Greenland Ice Core Project). These cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution; this together with a good relative chronology, provided by unequivocal CH4 features, allows a direct comparison of the synchronized Greenland and Antarctic records, and it reveals significant changes in the interpolar difference of CH4 mixing ratio with time. On the average, over the full Holocene records, we find an interpolar difference of 44±7 ppbv. A minimum difference of 33±7 ppbv is observed from 7 to 5 kyr B.P. whereas the maximum gradient (50±3 ppbv) took place from 5 to 2.5 kyr B.P. A gradient of 44±4 ppbv is observed during the early Holocene (11.5 to 9.5 kyr B.P). We use a three‐box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere. The model results support the previous interpretation that past natural CH4 sources mainly lay in tropical regions, but it also suggests that boreal regions provided a significant contribution to the CH4 budget especially at the start of the Holocene. The growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the Holocene. Finally, our model results suggest a large source increase in tropical regions from the late Holocene to the last millennium, which may partly be caused by anthropogenic emissions.
Measurements of the H2SO4 mass accommodation coefficient onto polydisperse aerosol
Authors: Jefferson A. , Eisele F. L. , Ziemann P. J. , Weber R. J. , Marti J. J. , McMurry P. H.
Published: February 2004

The loss rate of H2SO4 vapor onto submicron particles was measured for three different particle substrates. The experimental technique involved direct flow tube measurements of H2SO4 decay rates onto a polydisperse aerosol using chemical ionization mass spectroscopic detection. The aerosols of this study were partially hydrated crystalline salts with diameters in the size range of 20 to 400 nm. The mass accommodation coefficients, α, were calculated from the first‐order rate constants for H2SO4 loss to be 0.73±0.21 and 0.79±0.23 for loss onto (NH4)SO4 and NaCl, respectively. Measurements of the loss rate of H2SO4 onto a NaCl aerosol coated with stearic acid resulted in lower mass accommodation coefficients with values of 0.31 and 0.19 for aerosol with high and low stearic acid coverage, respectively. The observed decrease in α on an aerosol with a hydrocarbon coating suggests that aerosol composition is a key factor in H2SO4 adsorption on to a particle surface.
Retrieval of aerosol size distributions from satellite extinction spectra using constrained linear inversion
Authors: Steele H. M. , Turco R. P.
Published: February 2004

We examine the retrievability of stratospheric aerosol size distributions from stratospheric aerosol and gas experiment (SAGE) II extinction measurements. Analytical size distributions, which have been fitted to in situ measurements, are used as a model in conjunction with a Mie scattering code to produce synthetic extinction spectra at the SAGE II observational wavelengths. These synthetic data are analyzed using a constrained linear inversion procedure to deduce the variety of particle size distributions which can satisfy them. Fidelity of the retrieval depends on the range of radii over which the inversion is carried out and the range of wavelengths over which measurements are available, but is less sensitive to the number of wavelength channels. As expected, solutions are not unique and depend upon the weighting function used to constrain the solution and the degree of “smoothness” imposed. A given set of extinction measurements may be attributed to either a unimodal or bimodal size distribution, and the selection of a single solution is always subjective to some degree. For bimodal distributions the larger particle mode can be recovered reasonably well; however, the smaller particle mode of a bimodal distribution or the single mode of the background aerosol, which lie below the experiment's sensitivity range, cannot be retrieved accurately. The retrieval of any physically reasonable solution becomes increasingly difficult in the presence of experimental error. In many simulations in which a 10% random error was added to the extinction measurements, only solutions which were strongly influenced by the weighting function could be obtained. Integrated aerosol properties, such as total surface area and volume, can be retrieved to within 25 and 15%for particles in the size range between 0.1 and 1.0 μm from the SAGE II data using this technique. However, these error estimates must be increased to account for particles lying outside this range. It is these latter parameters which are most directly applicable to global chemistry and climate change studies.
NOx from lightning and the calculated chemical composition of the free troposphere
Authors: Flatøy Frode , Hov Øystein
Published: February 2004

In the free troposphere, injection from the stratosphere, emissions at the surface transported upward by dynamic processes, aircraft emissions and production in lightning strokes are the main sources of oxides of nitrogen (NOx). The global source of NOx production by lightning is not well known and estimates vary from 3 to 650 Mt NO2/yr. In this paper the role of regional and episodic emissions of NOx from lightning is examined with a three‐dimensional chemistry transport model, and a parameterization of the emissions is proposed, linking it to the intensity of latent heat generation in convection calculated in a numerical weather prediction model. The parameterization is scaled to give an annual global emission of 16 MtNO2/yr. The resulting distribution of the concentration of NOx, ozone, OH and nonmethane hydrocarbons is compared for the free troposphere over the northern hemisphere with the results of a calculation where the total NOx emissions from lightning were about the same but were fixed in time and space over the 18 days' calculation (June 18 to July 5, 1995). The maximum differences in the free tropospheric concentrations are significant: for NOx, OH and nonmethane hydrocarbon of the same order of magnitude as the concentrations themselves, for ozone 1 order of magnitude less than the ozone concentration. This means that NOx emissions from lightning may cause a variability in free tropospheric composition which makes it quite difficult to distinguish the importance of other sources of free tropospheric NOx from that of lightning in periods when lightning occurs.
Black carbon measurements at Mace Head, 1989–1996
Authors: Cooke William F. , Jennings S. G. , Spain T. G.
Published: February 2004

Black carbon mass concentrations have been measured using an aethalometer at Mace Head on the west coast of Ireland on an almost continuous basis from February 1989 to June 1996. The purpose of this paper is to report on the monthly averaged black carbon concentration at this site over the 7 year period and to examine the influence of air mass on the black carbon mass concentration. The seasonal variation of black carbon mass concentration for clean marine and continental air masses is also investigated.