bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of inclusions on light scattering by large ice particles
Authors: Macke Andreas , Mishchenko Michael I. , Cairns Brian
Published: February 2004

The scattering of visible light by ice crystals containing scattering and absorbing inclusions is calculated by a combination of ray‐tracing and Monte Carlo techniques. Results are shown for a randomly oriented hexagonal ice column containing ammonium sulfate particles, soot particles, and air bubbles. It is shown that a noticeable change in the ice crystal scattering properties compared to a pure crystal requires about 103 to 104 internal scatterers of a size comparable to the wavelength. While the nonabsorbing ammonium and air bubble inclusions generally decrease the asymmetry parameter g, soot provides a strong increase in g caused by the additional absorption. An independent superposition of the scattering properties of ice crystal and inclusions does not give satisfactory results because of the strong influence of internal scatterers on the characteristic ray paths inside the crystal. Multiple‐scattering calculations show that the strongest changes in the radiative fluxes are associated with the soot contaminated ice crystals.
Modeling atmospheric δ13CH4 and the causes of recent changes in atmospheric CH4 amounts
Authors: Gupta Mohan , Tyler Stanley , Cicerone Ralph
Published: February 2004

Inclusion of kinetic isotope effects (KIEs) of methane (CH4) sinks (gaseous OH and Cl, and soil microbes) has a significant effect on modeled distributions of δ13C of atmospheric CH4. For a given scenario of surface sources and corresponding δ13C values of individual CH4 sources, the KIE due to soil uptake enriched δ13C by 1.18‰ in the model's northern hemisphere (NH) (40°N) and 1.16‰ in the southern hemisphere (SH) (40°S) under steady state conditions in January. The KIE due to CH4 oxidation by stratospheric Cl radicals further enriched these δ13C values at the surface by 0.99‰ and 1.03‰, respectively. In the vertical direction, during January at 50°N, inclusion of a KIE due to Cl enriched δ13C at 18 km by 0.95‰ compared to the corresponding surface value, whereas the enrichment was only 0.31‰ when this KIE was omitted. These results suggest that modeling of δ13C distributions should include KIEs due to CH4 oxidation by soil and stratospheric chlorine radicals. It is shown that possible oxidation of CH4 in marine boundary layer by Cl radicals can significantly enrich δ13C. However, if a recent theoretical value for the KIE of the Cl and CH4 reaction is correct, then the impact of this reaction is less than the figures quoted above. In the model, monthly variations in OH concentration and interhemispheric exchange transport cannot reproduce the observed seasonal amplitude variation of δ13C in either the NH or SH. It is argued that seasonal variations in individual CH4 fluxes are primarily responsible for this discrepancy. We show that increasing Cl radical concentrations due to continued release of anthropogenic chlorocarbons enrich the δ13C values. The effects of an increase in tropospheric OH concentration due to stratospheric ozone depletion and a cooling of the troposphere due to the eruption of Mt. Pinatubo, with a lowering of water vapor concentration and reduction in isoprene emissions, on δ13C and surface CH4 mixing ratios are investigated. Other model simulations with adjusted surface CH4 fluxes have been performed to study the postulated explanations for recent changes in CH4 surface mixing ratios and δ13C values. A modified version of the Oslo two‐dimensional global tropospheric photochemical model was used for all simulations.
Trends in stratospheric and free tropospheric ozone
Authors: Harris N. R. P. , Ancellet G. , Bishop L. , Hofmann D. J. , Kerr J. B. , McPeters R. D. , Prendez M. , Randel W. J. , Staehelin J. , Subbaraya B. H. , Volz‐Thomas A. , Zawodny J. , Zerefos C. S.
Published: February 2004

Current understanding of the long‐term ozone trends is described. Of particular concern is an assessment of the quality of the available measurements, both ground and satellite based. Trends in total ozone have been calculated for the ground‐based network and the combined data set from the solar backscatter ultraviolet (SBUV) instruments on Nimbus 7 and NOAA 11. At midlatitudes in the northern hemisphere the trends from 1979 to 1994 are significantly negative in all seasons and are larger in winter/spring (up to 7%/decade) than in summer/fall (about 3%/decade). Trends in the southern midlatitudes are also significantly negative in all seasons (3 to 6%/decade), but there is a smaller seasonal variation. In the tropics, trends are slightly negative and at the edge of being significant at the 95% confidence level: these tropical trends are sensitive to the low ozone amounts observed near the end of the record and allowance must also be made for the suspected drift in the satellite calibration. The bulk of the midlatitude loss in the ozone column has taken place at altitudes between 15 and 25 km. There is disagreement on the magnitude of the reduction, with the SAGE I/II record showing trends as large as −20± 8%/decade at 16–17 km and the ozonesondes indicating an average trend of −7±3%/decade in the northern hemisphere. (All uncertainties given in this paper are two standard errors or 95% confidence limits unless stated otherwise). Recent ozone measurements are described for both Antarctica and the rest of the globe. The sulphate aerosol resulting from the eruption of Mount Pinatubo in 1991 and dynamic phenomena seem to have affected ozone levels, particularly at northern midlatitudes and in the Antarctic vortex. However, the record low values observed were partly caused by the long‐term trends and the effect on the calculated trends was less than 1.5%/decade.
Evaluation of global numerical weather prediction analyses and forecasts using DMSP special sensor microwave imager retrievals: 2. Analyses/forecasts intercomparison with SSM/I retrievals
Authors: Deblonde Godelieve , Yu W. , Garand L. , Dastoor A. P.
Published: February 2004

Satellite retrievals of environmental parameters for October 1993 were used to validate the Canadian Meteorological Center (CMC) analysis/forecast system and an improved version of the forecast model (including a prognostic equation for cloud water). In part 1 [Deblonde and Wagneur, this issure], SSM/I (special sensor microwave imager) retrievals using different algorithms were intercompared for each environmental parameter (column integrated water vapor (IWV), near‐surface oceanic wind speed (SWS), liquid water path (LWP) and surface rain rate (SRR). Here a subset of the SSM/I retrieval algorithms that performed best and other observation data sets are used to evaluate the hydrological cycle of the analysis/forecast system on a monthly timescale. Over the global oceans the agreement between analyzed IWV, forecasted IWV, and SSM/I retrievals was quite good (R ∼0.96). However, where Humsat (GOES) retrieved water vapor profiles were assimilated, significant biases were observed in large areas (up to ∼15 kgm−2). Significant differences were found between analyzed SWS and SSM/I‐retrieved SWS. In the tropics, zonally averaged analyzed SWS was underestimated by up to 1.4 ms−1 and in the southern hemisphere midlatitude differences of ∼1 ms−1 were found. Forecasted cloud fraction from the improved model was compared with that observed using the Humsat retrieval system. Over the open oceans, modeled cloud fraction was overestimated by 8%, suggesting that cloud formation was too active. By comparing SSM/I LWP retrievals with those forecasted it was possible to identify mispositioning of cloud systems that were associated with noted biases in the humidity analysis. It was also shown that the choice of function to obtain the liquid phase component of the modeled cloud water modified considerably the magnitude of the monthly mean LWP, particularly in the midlatitudes. The differences in patterns between precipitation short‐range forecasts (accumulated over a month) obtained with the operational and improved forecast models were considerably smaller than the differences between each of these forecasted fields and SSM/I retrievals or long‐term climatologies.
Is methane‐driven deglaciation consistent with the ice core record?
Authors: Thorpe R. B. , Law K. S. , Bekki S. , Pyle J. A. , Nisbet E. G.
Published: February 2004

The rapid climate changes at the terminations marking the end of the last glaciation are poorly understood. This study uses the Cambridge two‐dimensional atmospheric model to investigate the impact of a large marine sediment slump or permafrost rupture releasing 4000 Tg of CH4 into the high latitudes of the northern hemisphere, with particular emphasis on the evolution of atmospheric CH4 concentrations, which may have led to an associated direct radiative effect of the order of 1 W m−2, persisting for a few decades, because of enhanced levels of atmospheric CH4. The model‐generated CH4 concentrations were converted into synthetic ice core signals for 66°N and 85°S, representing Arctic and Antarctic cores, respectively. A range of possible averaging times was considered. The results show that irrespective of the site or enclosure time chosen, with the current sampling interval of ∼300 years, it is not possible to discount completely the existence of a spike as a result of the instantaneous release of 4000 Tg of CH4. Detection or refutation of a CH4 spike would, however, be probable if the sampling interval were reduced to around 50–100 years.
Cloud fraction errors caused by finite resolution measurements
Authors: Di Girolamo Larry , Davies Roger
Published: February 2004

The standard method of deriving cloud fraction from space simply finds the fraction of total image pixels that contains some cloud. The sensitivity of this method to sensor resolution has been examined by Shenk and Salomonson [1972], assuming the cloudy pixels are detected perfectly. Their experiment is reexamined to show that the sensitivity is much more complex than they predicted. We derive the upper and lower bounds of the true cloud fraction, At, given the standard method estimate, Ae, to show that the range of possible values for At can be, in general, very wide. By including the fraction of apparent cloud edge and cloud interior pixels, the bounds can be reduced and improvements to the standard method can be obtained. However, this improvement is also resolution limited by the misidentification of partially cloudy pixels as cloud interior rather than cloud edge. A potentially better technique for estimating the true cloud fraction is therefore explored using a pattern recognition approach. A nearest neighbor classification rule is used in two sets of experiments: one using 684 simulated cloud fields as a training set, the other using 370 cloud fields based on Advanced Very High Resolution Radiometer (AVHRR) measurements. Given the underlying distribution of At, Ae overestimates At with an overall average bias of 32% and standard error of 11% for the simulated training set, and a bias of 35% and a standard error of 3% for the AVHRR training set. The pattern recognition estimate, Ap, is essentially unbiased and has a standard error of 12% for both training sets. The relevance of these training sets to new scenes and the importance of imperfect cloud detection have yet to be investigated, but the pattern recognition technique shows considerable potential advantage over the standard technique in providing unbiased estimates of cloud fraction that are less sensitive to the effects of sensor resolution.
Comparison of surface temperature minimum and maximum and satellite measured cloudiness and radiation budget
Authors: Campbell G. Garrett , Vonder Haar Thomas H.
Published: February 2004

Mean surface temperature extremes over land areas during 1983 to 1990 are discussed in terms of satellite‐measured cloud and radiation parameters. These show that cloud fluctuations induce changes in the temperatures with the decrease of temperature maximum stronger than the increase of temperature minimum because clouds cause changes in incident shortwave surface flux as well as downward long‐wave radiation. Sensitivity to top of the atmosphere radiation is complicated by the intervening clouds and atmosphere. A comparison is also made to a sample general circulation model simulation showing the same effects.
Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA): A road map
Authors: Tuck A. F. , Brune W. H. , Hipskind R. S.
Published: February 2004

ASHOE/MAESA used instruments aboard the ER‐2 aircraft to study transport, photochemistry, radiation, and microphysics in the lower stratosphere over a range of seasons and latitudes. During the period from February to November 1994 the ER‐2 aircraft flew the suite of instruments between 60°N and 70°S in the longitude sector 115°W to 160°E. Of the 45 flights, 28 were entirely in the southern hemisphere between March and October, 6 were from Hawaii, 2 crossed the tropics between Hawaii and Fiji, and the remainder were from northern California. Data from these and associated ground‐based, balloon‐borne, and satellite instruments were combined with a variety of operational and research meteorological models to guide the flight planning and to interpret the results. The scientific rationale for the mission is given and the aircraft payload listed. A synopsis of the flights is supplied.
Retrieving land surface reflectances using the ATSR‐2: A theoretical study
Authors: Flowerdew Roland J. , Haigh Joanna D.
Published: February 2004

The measurement of absolute land reflectance from Earth‐orbiting satellites requires the separation of the signal due to the reflection of solar radiation by the surface from that due to scattering by the atmosphere. Thus for accurate but generally applicable quasi‐operational retrieval techniques it is necessary to derive atmospheric parameters, such as the aerosol type and loading, simultaneously with the surface reflectances. The assumption of large‐scale horizontal homogeneity across an image, or the presence of very low reflectance pixels within an image, is avoided, since these are not necessarily applicable in the general case. In this paper, results of simulated retrievals using the visible and near‐infrared channels of the ATSR‐2 satellite radiometer are presented on the basis of techniques which exploit the dual look and other features of the instrument. The pixel‐based retrieval scheme comprises a fast representation of atmospheric scattering and a simple surface reflectance approximation. The effect of the atmospheric approximation on the accuracy of retrieval of the surface reflectance and atmospheric optical depth is tested by using a full multiple‐scattering radiative transfer model with simpler surface model. The effect on retrieval accuracy of the surface model is tested using a full surface vegetation canopy reflectance model with inclusion of the hot‐spot effect, combined with a successive orders of scattering atmospheric radiative transfer model, taken to second order. The results indicate that the scheme could provide accurate retrieval of surface reflectances even where no extraneous information on the surface bidirectional reflectance or on the atmospheric optical thickness is available. The scheme is also applicable to the AATSR instrument to be launched on the ENVISAT (Environmental Satellite) and, more widely, to other satellite radiometers with multiple views of the Earth's surface.
Modeling sea‐salt aerosols in the atmosphere: 1. Model development
Authors: Gong S. L. , Barrie L. A. , Blanchet J.‐P.
Published: February 2004

A simulation of the processes of sea‐salt aerosol generation, diffusive transport, transformation, and removal as a function of particle size is incorporated into a one‐dimensional version of the Canadian general climate model (GCMII). This model was then run in the North Atlantic between Iceland and Ireland during the period of January‐March. Model predictions are compared to observations of sea‐salt aerosols selected from a review of available studies that were subjected to strict screening criteria to ensure their representativeness. The number and mass size distribution and the wind dependency of total sea‐salt aerosol mass concentrations predicted by the model compare well with observations. The modeled dependence of sea‐salt aerosol concentration in the surface layer (χ, μg m−3) on 10‐m wind speed (U10, m s−1) is given by. Simulations show that both a and b change with location. The value a and b range from 0.20 and 3.1 for Mace Head, Ireland to 0.26, and 1.4 for Heimaey, Iceland. The dependence of χ on surface wind speed is weaker for smaller particles and for particles at higher altitudes. The residence time of sea‐salt aerosols in the first atmospheric layer (0–166 m) ranges from 30 min for large particles (r=4–8 μm) to ∼60 hours for small particles (r=0.13–0.25 μm). Although some refinements are required for the model, it forms the basis for comparing the simulations with long‐term atmospheric sea‐salt measurements made at marine baseline observatories around the world and for a more comprehensive three‐dimensional modeling of atmospheric sea‐salt aerosols.
Behavior of ozone over Indian region after Pinatubo eruption
Authors: Chakrabarty D. K. , Peshin S. K.
Published: February 2004

To study the behavior of stratospheric ozone over the Indian region after the eruption of Mount Pinatubo in June 1991, Dobson total ozone data recorded for 20 years over the Indian region between 10 and 28 of latitude north are examined. No significant decrease in total ozone after the Mount Pinatubo eruption is apparent. Ozonesonde observations at Pune (18.3°N, 73.6°E) and balloon measurement of aerosol profile at Hyderabad (17.5°N, 78.6°E) show that the altitude of the ozone peak is about 5 km higher than the altitude of the aerosol peak. Stratospheric NO2 column observations at Ahmedabad also do not show any appreciable change in their values after Pinatubo eruption. These observations suggest that heterogeneous reactions do not play a significant role to reduce total ozone after volcanic eruption in the low‐latitude region.
Aerosol composition at Cheju Island, Korea
Authors: Carmichael Gregory R. , Hong Min‐Sun , Ueda Hiromasa , Chen Li‐Ling , Murano Kentaro , Park John K. , Lee Hogeun , Kim Young , Kang C. , Shim Shang
Published: February 2004

The seasonal variation in chemical composition of aerosols measured at Cheju Island, Korea, is presented and discussed. Daily tape filters have been obtained for the three year period analyzed, March 1992 through February 1995. The annual average non‐sea‐salt sulfate and nitrate mass concentrations of aerosols measured at Cheju, are 7.2 μg/m3 and 1.2 μg/m3, respectively; while calcium and potassium are present in nearly equal amounts of ∼0.5 μg/m3, and ammonium averages 1.3 μg/m3. The annual mean mass ratios are SO42−/NO3− = 6.2, NH4+/SO42− = 0.2, Ca2+/Na+ = 0.3, and NH4+/NO3− = 1.1. Each species is found to follow a seasonal cycle, with sulfate, nitrate, ammonium, potassium, and calcium all showing higher values in the spring. In contrast, chlorine concentrations are 10% lower in the spring. Aerosol enrichment factors relative to the composition of seawater and crustal material from the dust‐source regions in central China reveal that the aerosol measured at Cheju is enriched in sulfate, nitrate, and ammonium, but is deficient in chlorine (relative to seawater). The data from the daily tape filters are also compared with those from weekly Hi‐Vol filters located at the same site and other measurements in the region.
Carbon aerosols and atmospheric photochemistry
Authors: Lary D. J. , Lee A. M. , Toumi R. , Newchurch M. J. , Pirre M. , Renard J. B.
Published: February 2004

Carbon aerosols are produced by all combustion processes. This paper investigates some possible effects of heterogeneous reduction of atmospheric constituents on carbon aerosols. Reduction of HNO3, NO2, and O3 on carbon aerosols may be an important effect of increased air traffic that has not been considered to date. It is shown that if HNO3, NO2 and O3 are heterogeneously reduced on atmospheric amorphous carbon aerosols, then a significant, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can operate at all latitudes where amorphous carbon aerosols are present. The relative importance of the mechanism increases with nightlength. The reduction of HNO3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Owing to the very different soot levels in the two hemispheres, this implies that there should be a hemispheric assymetry in the role of these mechanisms. The renoxification leads to simulated tropospheric HNO3/NOx ratios that are close to those observed. In contrast to the stratospheric response, the tropospheric production of NOx due to the reduction of HNO3 would lead to tropospheric ozone production.
A satellite‐derived ozone climatology for balloonsonde estimation of total column ozone
Authors: McPeters R. D. , Labow G. J. , Johnson B. J.
Published: February 2004

The standard technique for computing total column ozone from a balloonsonde measurement includes an extrapolation of the measured ozone profile to altitudes above the balloon burst altitude. This total column calculation can be improved by using a monthly average ozone climatology based on ozone profile measurements from the solar backscattered ultraviolet instrument on the Nimbus 7 satellite. For each month and 10° latitude zone we provide the column ozone above 30 distinct pressure levels in the middle and upper atmosphere (from 1 through 30 mbar) that can be added to the measured balloon profile. Use of this climatology reduces the uncertainty in the column ozone estimate when the sonde data are compared with simultaneous Dobson observations at Boulder and at Mauna Loa.
Modeling studies of the effects of the heterogeneous reaction ClOOCl + HCl → Cl2 + HOOCl on stratospheric chlorine activation and ozone depletion
Authors: De Haan D. O. , Fløisand I. , Stordal F.
Published: February 2004

The heterogeneous reaction ClOOCl + HCl → Cl2 + HOOCl was introduced into a chemical trajectory model of the stratosphere. Ten‐day trajectories ending at ozonesonde stations at various northern latitudes were run to simulate the period January–March 1994. The reaction on sulfuric acid aerosol surfaces has a negligible effect on ozone chemistry if a sticking coefficient similar to that of ClONO2 + HCl is assumed. On polar stratospheric cloud (PSC) surfaces the chemical effects of the addition of this reaction depend on the fate of proposed product HOOCl: if this species photolyzes to produce either ClO + OH or Cl + HO2, then HCl is activated by the reaction with chlorine peroxide. This heterogeneous activation of chlorine by active chlorine can have a significant effect on Arctic ozone depletion rates in the days following an air parcel's encounter with PSC surfaces. The ozone depletion rate usually increased but in some cases decreased, depending on the extent of PSC processing and on the initial [HCl]/[ClONO2] ratio. Averaged over 3 months, the column ozone loss rates between 350 and 675 K were accelerated by as much as 35% for a set of 10‐day trajectories ending at an Arctic station. If, on the other hand, HOOCl decomposes at the surface into HCl and O2, the net effect of these reactions is to convert ClOOCl into Cl2. These species are functionally equivalent, and such a conversion does not perturb the model chemistry.
A three‐dimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols
Authors: Kasibhatla P. , Chameides W. L. , John J. St.
Published: February 2004

A global three‐dimensional chemical transport model is used to investigate seasonal variations of anthropogenic sulfur in the troposphere. Particular emphasis is placed on detailed comparisons of the modeled surface sulfur dioxide (SO2) and sulfate (SO4) concentrations and sulfate wet deposition fluxes with measurements from the Eulerian Model Evaluation Field Study (EMEFS) and Cooperative Program for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) field programs in North America and Europe, respectively. Initial comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2concentrations and underestimate SO4concentrations while producing a reasonable fit to measured wet deposition fluxes. Through a series of sensitivity tests we find that the addition of a nonphotochemical pathway for converting SO2to SO4in the boundary layer with a pseudo first‐order rate of constant of 1–2×10−6s−1provides the most reasonable method of bringing the model results into better agreement with the EMEFS and EMEP data sets. We propose that this additional pathway may be related to heterogeneous reactions between SO2and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the vastly improved simulation of surface SO2and SO4when this hypothetical heterogeneous oxidation pathway is included, the model is unable to simultaneously simulate the large seasonal cycle in surface SO4observed over eastern North America and the almost total absence of a seasonal cycle in surface SO4over Europe. The seasonal cycles in model‐predicted column SO4burdens are similar, but not identical, to those for surface SO4because of regional differences in transport, free tropospheric oxidation, and in‐cloud removal. We find that the summer‐to‐winter ratio in column SO4is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, where wintertime column SO4is predicted to exceed summertime column SO4.
Thermodynamic and optical properties of mixed‐salt aerosols of atmospheric importance
Authors: Tang Ignatius N.
Published: February 2004

Extensive water activity, density, and refractive index data at 25°C are reported for mixed‐salt solutions, NaCl‐KCl, NaCl‐NaNO3, NaCl‐Na2SO4, Na2SO4‐NaNO3, and (NH4)2SO4‐Na2SO4. The data are obtained from hydration experiments using the single‐particle levitation technique developed recently for measuring the thermodynamic and optical properties of microdroplets. These data, covering the whole concentration range from dilute solutions to high supersaturations, provide an opportunity to explore the light‐scattering properties of both internal and external mixtures of the chloride, sulfate, and nitrate aerosols of atmospheric importance. It is shown that for sulfate and nitrate aerosols as solution droplets, the light‐scattering properties do not differ appreciably among all mixture types and compositions, as long as the dry‐salt aerosols have the same particle‐size distribution. However, for mixed‐salt aerosols containing NaCl, the light‐scattering properties do depend upon the composition and particle‐size distribution, although not so much on the mixture type.
Precipitation characteristics in Greenland‐Iceland‐Norwegian Seas determined by using satellite microwave data
Authors: Liu Guosheng , Curry Judith A.
Published: February 2004

Precipitation characteristics and their connection with atmospheric conditions are studied for the Greenland‐Iceland‐Norwegian (GIN) Seas during wintertime using satellite microwave (Special Sensor Microwave/Imager) and SSM/T2 data. A snowfall algorithm using SSM/T2 data is presented, which makes it possible to estimate the total precipitation and the relative contribution of each precipitation component (rain or snow) over open water. The monthly mean snowfall rate distribution compares well with the snowfall frequency derived from shipboard present weather reports although no data are available for direct snowfall rate validation. The retrieved results show that more than half of the precipitation falls as snow in the GIN Seas during winter and that the monthly mean horizontal distributions of rain and snow have different patterns. By examining the relationship between surface wind and precipitation and conducting case studies, it is found that rainfall and snowfall are associated with different weather types: rainfall occurs mainly in low pressure systems, while snowfall is more likely associated with cold air outbreaks.
Comparison of the NCAR LSM1 land surface model with BOREAS aspen and jack pine tower fluxes
Authors: Bonan Gordon B. , Davis Kenneth J. , Baldocchi Dennis , Fitzjarrald David , Neumann Harold
Published: February 2004

Tower fluxes measured at the Boreal Ecosystem‐Atmosphere Study southern study area old aspen (SSA‐OA), southern study area old jack pine (SSA‐OJP), and northern study area old jack pine (NSA‐OJP) sites during the three 1994 intensive field campaigns (IFCs) (May 24 to June 16 (IFC‐1), July 19 to August 10 (IFC‐2), and August 30 to September 19 (IFC‐3)) were compared to fluxes simulated by a land surface model for the same period. Comparisons were limited to the average diurnal cycle for these periods to mitigate large day‐to‐day variability in the observations and problems with missing data. For consistency with the global implementation of the model, vegetation and soil parameters were not set to site‐specific values but rather were the generic needleleaf evergreen and broadleaf deciduous vegetation and the generic sandy and loamy soil used in the global model. Despite the use of generic vegetation and soil, the model reasonably simulated the diurnal cycle of sensible heat, latent heat, net radiation, and CO2 fluxes for the SSA‐OJP and SSA‐OA sites. The main errors were that the model did not reproduce the midday reduction in latent heat seen at the SSA‐OJP site during IFC‐1 and IFC‐2 and had less photosynthetic CO2 uptake than observed at the SSA‐OA site. Differences in vegetation structure and physiology between the two sites were important to accurately simulate the fluxes. The needleleaf evergreen vegetation resulted in higher net radiation and a higher Bowen ratio than the broadleaf deciduous vegetation. Soil differences were less important. The NSA‐OJP site was not so well simulated: midday latent heat flux was overestimated, and photosynthetic CO2 uptake was underestimated during each IFC. The only difference in the simulated southern and northern jack pine sites was in their atmospheric forcings; vegetation structure and soil types were the same. These results suggest the model is able to reproduce variability between vegetation types but not within vegetation types.
Snow ablation modeling at the stand scale in a boreal jack pine forest
Authors: Hardy J. P. , Davis R. E. , Jordan R. , Li X. , Woodcock C. , Ni W. , McKenzie J. C.
Published: February 2004

The purpose of this study is to predict spatial distributions of snow properties important to the hydrology and the remote sensing signatures of the boreal ecosystem. This study is part of the Boreal Ecosystems Atmosphere Study (BOREAS) of central Saskatchewan and northern Manitoba. Forested environments provide unique problems for snow cover process modeling due to the complex interactions among snow, energy transfer, and trees. These problems are approached by coupling a modified snow process model with a model of radiative interactions with forest canopies. Additionally, a tree well model describes the influence of individual trees on snow distribution on the ground. The snow process and energy budget model calculates energy exchange at the snow surface, in‐pack snow processes, melting and liquid water flow, heat conduction, and vapor diffusion. The surface radiation model provides input on the radiation receipt at the snow surface for model runs in the jack pine forest. Field data consisted of measured meteorological parameters above and within the canopy, spatial variability of snow properties, and variations of incoming solar irradiance beneath the forest canopy. Results show that the area beneath tree canopies accumulated 60% of the snow accumulated in forest openings. Peak solar irradiance on the snow cover was less than one half that measured above the canopy. Model runs are compared between the open and the forested sites and show the open area ablating four days before areas beneath the canopy and eight days before forest openings and compare favorably with measured data. Physically based modeling of snow ablation was successful at the forested site and nearby open area.
Annual cycle of upper air circulation and convective activity over the tropical Americas
Authors: Hastenrath Stefan
Published: February 2004

A combination of novel data sets, including satellite‐derived outgoing longwave radiation and highly reflective clouds, rain gauge records, and upper air analyses of the European Centre for Medium‐Range Weather Forecasts have been evaluated in context, to reappraise the multiyear mean convective activity and circulation and their annual cycle variations over the tropical Americas. In austral summer a maximum of 200 mbar geopotential height, anticyclonic vortex in the wind field, and extremum of stream function are located over the western part of the Amazon basin, well to the west of the extremum of the velocity potential. To the north of that and the nucleus of the most intense convection and rainfall, the major upper tropospheric divergence is found over northern South America. From this, outflow is directed primarily northward into an area of strongest convergence situated over the equatorward side of the North Atlantic surface high. In the evolution from austral to boreal summer, the high over western Amazonia vanishes, and the centers of strongest convection, upper tropospheric velocity potential, and divergence establish themselves in the realm of the Intertropical Convergence Zone over the eastern equatorial Pacific, with outflow directed mainly into a broad zone of upper tropospheric convergence in the southern hemisphere. After the height of the boreal summer, the centers of convection and velocity potential develop again over southeastern Amazonia, and the upper tropospheric high and weak anticyclone separately form again over the western portion of the Amazon basin.
Variations in the predicted spatial distribution of atmospheric nitrogen deposition and their impact on carbon uptake by terrestrial ecosystems
Authors: Holland Elisabeth A. , Braswell B. H. , Lamarque Jean‐François , Townsend Alan , Sulzman James , Müller Jean‐François , Dentener Frank , Brasseur Guy , Levy H. , Penner Joyce E. , Roelofs Geert‐Jan
Published: February 2004

Widespread mobilization of nitrogen into the atmosphere from industry, agriculture, and biomass burning and its subsequent deposition have the potential to alleviate nitrogen limitation of productivity in terrestrial ecosystems, and may contribute to enhanced terrestrial carbon uptake. To evaluate the importance of the spatial distribution of nitrogen deposition for carbon uptake and to better quantify its magnitude and uncertainty NOy‐N deposition fields from five different three‐dimensional chemical models, GCTM, GRANTOUR, IMAGES, MOGUNTIA, and ECHAM were used to drive NDEP, a perturbation model of terrestrial carbon uptake. Differences in atmospheric sources of NOx‐N, transport, resolution, and representation of chemistry, contribute to the distinct spatial patterns of nitrogen deposition on the global land surface; these differences lead to distinct patterns of carbon uptake that vary between 0.7 and 1.3 Gt C yr−1 globally. Less than 10% of the nitrogen was deposited on forests which were most able to respond with increased carbon storage because of the wide C:N ratio of wood as well as its long lifetime. Addition of NHx‐N to NOy‐N deposition, increased global terrestrial carbon storage to between 1.5 and 2.0 Gt C yr−1, while the “missing terrestrial sink” is quite similar in magnitude. Thus global air pollution appears to be an important influence on the global carbon cycle. If N fertilization of the terrestrial biosphere accounts for the “missing” C sink or a substantial portion of it, we would expect significant reductions in its magnitude over the next century as terrestrial ecosystems become N saturated and O3 pollution expands.
The gravity wave Doppler spread theory applied in a numerical spectral model of the middle atmosphere: 1. Model and global scale seasonal variations
Authors: Mayr H. G. , Mengel J. G. , Hines C. O. , Chan K. L. , Arnold N. F. , Reddy C. A. , Porter H. S.
Published: February 2004

Hines [1997a, b] has developed a Doppler spread parameterization (DSP) for the deposition of small‐scale gravity wave (GW) momentum and energy in the middle atmosphere. We have incorporated this DSP into the two‐dimensional (2‐D) version of the numerical spectral model (NSM) of Chan et al [1994a, b] which is applied to the Earth's middle atmosphere. With a globally uniform flux of (quasi) isotropically propagating GW emanating from the troposphere, the NSM has been integrated for several model years to describe seasonal variations and equatorial oscillations. Here, after a review of the NSM and DSP, we discuss numerical results that describe the temperature and wind fields during solstice and equinox conditions, emphasizing the role played by the GW spectrum. That spectrum is filtered as it ascends through the stratosphere and provides, at the solstices, a highly anisotropic wave and momentum flux at mesospheric heights. Upon further filtering there, with attendant momentum deposition, the waves decelerate and then reverse the zonal circulation. In quasi‐geostrophic balance latitudinally, this reversal is accompanied by a reversal of the latitudinal temperature gradient, one that leads to a temperature minimum in the summer polar mesopause region, as is observed. Corresponding results for equinox are obtained, and all are discussed. Our results differ only in detail from those of similar analyses that employ other GW parameterizations. They are presented here in part to exhibit the success of the DSP at this elementary level and in part to provide a point of departure against which future refinements may be judged. Our first extension of the modeling concerns the semiannual and quasi‐biennial oscillations that are produced by the DSP in the NSM at equatorial latitudes with the same, constant and uniform, incident GW flux. Initial results are presented in the companion paper and are compared there with observations.
1993 Tropospheric OH Photochemistry Experiment: A summary and perspective
Authors: Crosley David R.
Published: February 2004

The 1993 OH Tropospheric Photochemistry Experiment was performed in the forested Colorado mountains during late summer and early fall 1993. Measurements of the OH radical were made by four different techniques; one pair provided an extensive intercomparison data set. A large number of other species that control OH and HO2 concentrations were also determined. These ancillary variables have been useful both in understanding intercomparison issues and in testing detailed model predictions of OH photochemistry in this environment. The paper places this campaign in historical and scientific perspective; it discusses the results in terms of their contributions both to our ability to measure OH radicals and to our understanding of the pertinent tropospheric photochemistry; and it provides a view toward, and lessons for, future campaigns. We conclude that OH is indeed being measured and that it forms very useful tests of photochemical models but that we do not fully understand the underlying chemistry in this environment.
The average dissipation rate of turbulent kinetic energy in the neutral and unstable atmospheric surface layer
Authors: Albertson John D. , Parlange Marc B. , Kiely Gerard , Eichinger William E.
Published: February 2004

The mean rate of dissipation of turbulent kinetic energy is related to the surface fluxes of momentum and heat through the turbulent kinetic energy budget equation. This relationship may be used to estimate surface fluxes from measurements of the dissipation rates. The success of recent applications of the approach has been limited by uncertainties surrounding the functional relationship between the dimensionless dissipation rates and the atmospheric stability parameter. A pair of field experiments was designed and carried out in the atmospheric surface layer to identify this functional relationship over a broad range of neutral and convective flows, covering greater than 3 orders of magnitude in the stability parameter. Mean dissipation rates were computed using Fourier power spectra, second‐order structure functions, and third‐order structure functions. Arguments are presented for the superiority of the third‐order approach. A three‐sublayer conceptual model is invoked to guide the dimensional analysis, and the resulting dissipation rates are shown to scale uniquely in the three sublayers. Near the wall, in the dynamic sublayer, dissipation is significantly less than production, as energy is transported up to the more convective regions, where an equality between dissipation and production is achieved.
Evaluation of a commercial pulsed fluorescence detector for the measurement of low‐level SO2 concentrations during the Gas‐Phase Sulfur Intercomparison Experiment
Authors: Luke Winston T.
Published: February 2004

A modified pulsed fluorescence (PF) detector (Thermo Environmental Instruments, Model 43s) was used to measure low levels of SO2 in a rigorous, blind intercomparison experiment (Gas‐Phase Intercomparison Experiment (GASIE)). The PF detector was able to detect as little as 30 pptv SO2 in a 25‐min sampling interval. The coefficients of variation for measurements of approximately 30, 60, 200, 330, and 600 pptv were approximately 40, 9, 6.5, 3, and 3%, respectively. Overall uncertainty of the measurements at 30 pptv approaches ±100%. As inferred from GASIE results, the response of the PF detector may be reduced (quenched) by approximately 7% and 15% at water vapor mixing ratios of 1 and 1.5 mole percent (relative humidities of 35–50% at 20–25°C and 1 atm), respectively. These results are uncertain, however, due to lack of extensive data. Post‐GASIE tests point to moderate interferences from NO (rejection ratio of 35), CS2 (rejection ratio of 20), and a number of highly fluorescent aromatic hydrocarbons such as benzene, toluene, o‐xylene, m‐xylene,p‐xylene, m‐ethyltoluene, ethylbenzene, and 1,2,4‐trimethylbenzene. Rejection ratios for these compounds increase from approximately 17–123 to circa 1200–3800 as the sample flow rate is decreased from 2000 to 300 standard cubic centimeters per minute (sccm), and the hydrocarbons are more efficiently removed by the instrument's proprietary hydrocarbon “kicker” membrane. At a flow rate of 300 sccm and a pressure drop of 645 torr across the kicker, the interference from ppmv levels of many aromatic hydrocarbon was eliminated entirely. None of the tested interferants were removed by the carbonate‐impregnated paper filter used to zero the instrument during GASIE; thus they induced no net response in the PF detector. These results illustrate the importance of using a selective zeroing method to scrub SO2 without removing potential interferants from the sample flow, thus preserving the overall composition of the sampling matrix.
Radiative characteristics of the Arctic atmosphere during spring as inferred from ground‐based measurements
Authors: Pinto James O. , Curry Judith A. , Fairall Chris W.
Published: February 2004

The radiative characteristics of low‐level clouds over the Arctic ice pack in spring are inferred from ground‐based broadband flux measurements using a radiative transfer model. An informal comparison of several radiative transfer models is performed for clear‐sky conditions observed in April during LEADEX. The broadband longwave and shortwave surface fluxes obtained with Streamer and the broadband longwave surface flux obtained with the Rapid Radiative Transfer Model (RRTM) correlate well with the observations. The Streamer code is chosen to determine cloud optical and microphysical properties based on its performance under clear‐sky conditions, its sophisticated treatment of clouds, and its option for calculating fluxes using a discrete ordinate technique. The radiative properties of the Arctic atmosphere are complicated by the presence of clouds. Low clouds occurred 30% of the time during LEADEX. The longwave and shortwave optical depths of clouds based below 2 km are inferred from surface‐based measurements of cloud base height and broadband radiative fluxes by matching the observed fluxes with those obtained with the four‐stream radiative transfer code available as part of Streamer. The retrieved broadband shortwave and longwave optical depths for low clouds observed during LEADEX ranged between 0.2 and 3. An attempt is made to infer cloud phase and cloud particle size from the ratio of the retrieved broadband shortwave and longwave cloud optical depths. This ratio ranges from 1.33 to 1.75 for ice, from 0.85 to 1.9 for liquid, and from 0.85 to 1.9 for mixed‐phase clouds. Although there is much overlap between the ranges, it is shown that liquid‐water clouds are characterized by optical depth ratios that are generally smaller than those found in ice clouds. This method for inferring cloud microphysics from surface‐based measurements of broadband radiative fluxes and cloud base height may prove useful where more sophisticated observations of cloud microphysics are lacking.
Studies of the Antarctic climate with a stretched‐grid general circulation model
Authors: Krinner Gerhard , Genthon Christophe , Li Zhao‐Xin , Le Van Phu
Published: February 2004

A stretched‐grid general circulation model (GCM), derived from the Laboratoire de Météorologie Dynamique (LMD) GCM is used for a multiyear high‐resolution simulation of the Antarctic climate. The resolution in the Antarctic region reaches 100 km. In order to correctly represent the polar climate, it is necessary to implement several modifications in the model physics. These modifications mostly concern the parameterizations of the atmospheric boundary layer. The simulated Antarctic climate is significantly better in the stretched‐grid simulation than in the regular‐grid control run. The katabatic wind regime is well captured, although the winds may be somewhat too weak. The annual snow accumulation is generally close to the observed values, although local discrepancies between the simulated annual accumulation and observations remain. The simulated continental mean annual accumulation is 16.2 cm y−1. Features like the surface temperature and the temperature inversion over large parts of the continent are correctly represented. The model correctly simulates the atmospheric dynamics of the rest of the globe.
Modeling sea‐salt aerosols in the atmosphere: 2. Atmospheric concentrations and fluxes
Authors: Gong S. L. , Barrie L. A. , Prospero J. M. , Savoie D. L. , Ayers G. P. , Blanchet J.‐P. , Spacek L.
Published: February 2004

Atmospheric sea‐salt aerosol concentrations are studied using both long‐term observations and model simulations of Na+ at seven stations around the globe. Good agreement is achieved between observations and model predictions in the northern hemisphere. A stronger seasonal variation occurs in the high‐latitude North Atlantic than in regions close to the equator and in high‐latitude southern hemisphere. Generally, concentrations are higher for both boreal and austral winters. With the model, the production flux and removal flux at the atmosphere‐ocean interface was calculated and used to estimate the global sea‐salt budget. The flux also shows seasonal variation similar to that of sea‐salt concentration. Depending on the geographic location, the model predicts that dry deposition accounts for 60–70% of the total sea‐salt removed from the atmosphere while in‐cloud and below‐cloud precipitation scavenging accounts for about 1% and 28–39% of the remainder, respectively. The total amount of sea‐salt aerosols emitted from the world oceans to the atmosphere is estimated to be in the vicinity of 1.17×1016 g yr−1. Approximately 99% of the sea‐salt aerosol mass generated by wind falls back to the sea with about 1–2% remaining in the atmosphere to be exported from the original grid square (300×300 km). Only a small portion of that exported (∼4%) is associated with submicron particles that are likely to undergo long‐range transport.