bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral characteristics of East Antarctica meteorological parameters during 1994
Authors: Argentini Stefania , Petenko Igor V. , Mastrantonio Giangiuseppe , Bezverkhnii Viacheslav A. , Viola Angelo P.
Published: February 2004

A data set recorded during 1994 from the Dome Charlie and D‐80 automatic weather stations and the Dumont d'Urville meteorological station has been used to investigate the time variation and the spatial distribution of pressure, temperature, and wind velocity over East Antarctica. The advanced procedures of wavelet analysis, high‐resolution spectral estimation, and their combination were used for this study. Special attention was devoted to the temperature behavior during winter. The surface temperature inside the continent shows strong oscillations with a period of ∼10 days and a sharp increase ranging from 20°C to 40°C during the winter season. Cross‐covariance functions between temperature and pressure at different sites were analyzed to estimate the correlation between warming events and their temporal and spatial distribution. The warming is more often associated with high‐pressure anomalies and follows the variation of the pressure with a delay varying from some hours to 1 day. Sharp changes in wind speed and direction generally occur when the temperature increases. In such cases the wind has the tendency to blow from the W‐NW sector, deflecting from the dominant katabatic flow direction.
Transport of boreal forest fire emissions from Canada to Europe
Authors: Forster Caroline , Wandinger Ulla , Wotawa Gerhard , James Paul , Mattis Ina , Althausen Dietrich , Simmonds Peter , O'Doherty S. , Jennings S. Gerard , Kleefeld Christoph , Schneider Johannes , Trickl Thomas , Kreipl Stephan , Jäger Horst , Stohl Andreas
Published: February 2004

In August 1998, severe forest fires occurred in many parts of Canada, especially in the Northwest Territories. In the week from August 5 to 11, more than 1000 different fires burned >1 × 106 ha of boreal forest, the highest 1‐week sum ever reported throughout the 1990s. In this study we can unambigously show for the first time that these fires caused pronounced large‐scale haze layers above Europe and that they influenced concentrations of carbon monoxide and other trace gases at the surface station Mace Head in Ireland over a period of weeks. Transport took place across several thousands of kilometers. An example of such an event, in which a pronounced aerosol layer was observed at an altitude of 3–6 km over Germany during August 1998, is investigated in detail. Backward trajectories ending at the measured aerosol layer are calculated and shown to have their origin in the forest fire region. Simulations with a particle dispersion model reveal how a substantial amount of forest fire emissions was transported across the Atlantic. The resulting aerosol lamina over Europe is captured well by the model. In addition, the model demonstrates that the forest fire emissions polluted large regions over Europe during the second half of August 1998. Surface measurements at Mace Head are compared to the model results for an anthropogenic and a forest fire carbon monoxide tracer, respectively. While wet deposition removed considerable amounts of aerosol during its transport, forest fire carbon monoxide reached Europe in copious amounts. It is estimated that during August 1998, 32%, 10%, and 58% of the carbon monoxide enhancement over the background level at Mace Head were caused by European and North American anthropogenic emissions and forest fire emissions, respectively.
A dust outbreak episode in sub‐Sahel West Africa
Authors: Pinker R. T. , Pandithurai G. , Holben B. N. , Dubovik O. , Aro T. O.
Published: February 2004

Wind blown dust is a major contributor to the tropospheric aerosol mass loading and has a significant effect on the local radiative forcing. Information on aerosol optical properties and their temporal and spatial distribution is very limited. Attempts to derive such information from space, in particular, over land, are in a preliminary stage. Recently, information on aerosol optical properties is becoming available from ground networks within one day from the time the observations are made. In this study, use was made of such “real‐time” aerosol observations, to characterize their optical properties, during a heavy dust event in the sub‐Sahel in January 2000. Aerosol optical depths at all wavelengths showed a sharp increase when compared to the average for the season, reaching values up to 3.5 at 500 nm. The Angstrom exponent was reduced from 1.2 to 0.3, and a sharp increase in the single‐scattering albedo was found. The aerosol optical properties differed significantly from climatologically available information on dust aerosols, and therefore their timely assimilation into transport models or weather prediction models could be of great relevance. Developments in the ways aerosol are presently observed could influence future treatment of aerosols in climate research.
Improving the precision of simulated hydrologic fluxes in land surface models
Authors: Dong Jiarui , Salvucci Guido D. , Myneni Ranga B.
Published: February 2004

The numerical method for estimating soil moisture transport coefficients in land surface models (LSMs) is evaluated through comparison to a weighted‐average method based on successive steady states [Warrick, 1991] and a proposed new scheme [Dong and Wang, 1997]. The focus is on situations with large gradients of soil moisture. The reference cases used to evaluate the performance of the various methods are based on numerical integrations with fine spatial resolution and small time steps. A series of tests were performed to detect sources of discrepancies and to assess the impact of various numerical methods on the simulation of evaporation, transpiration, and drainage. Off‐line LSM simulations driven by observed atmospheric forcings were analyzed to understand the propagation of errors between the various hydrologic processes. We suggest a simple modification to LSM that can improve the precision of simulated hydrologic fluxes without increasing vertical resolution.
Variations with elevation in the surface energy balance on the Pasterze (Austria)
Authors: Greuell Wouter , Smeets Paul
Published: February 2004

This paper presents the surface energy balance at five locations on the Pasterze, a glacier in Austria, during a period of 46 days in the summer of 1994. The computations are based on local measurements of radiative fluxes, wind speed, atmospheric temperature and humidity, and precipitation. These measurements served as input for a numerical mass‐balance model that computes the surface temperature by simulation of subsurface processes. The surface temperature was then used for the computation of the outgoing long‐wave radiative flux and for the computation of the turbulent fluxes by means of the “bulk method.” Values of the roughness length for momentum were computed by profile analysis. They ranged from 1.2 to 5.8 mm. At all locations, net short‐wave radiation constituted the dominant energy flux, the latent heat flux was small, and the net long‐wave radiative flux and the sensible heat flux were of intermediate magnitude. The energy balances at the three stations on the glacier tongue were similar to each other but differed from those at the two stations in the accumulation basin, which were also similar to each other. These similarities within the two areas are due mainly to the fact that during the experiment there was hardly any variation in temperature and albedo within the two areas. The ablation rate at U3 (2420 m above sea level (asl)) was even slightly higher than the ablation rate at Al (2205 m asl). At all stations the calculated mass balance agrees within 5% with the measured mass balance, which is within the margins determined by the uncertainties in the input variables and the model parameters and equations. This gives some confidence in the method used to compute the turbulent fluxes, but on the other hand, the agreement may also be caused by cancellation of errors. Because of the high temperatures during PASTEX the calculated mass balance would have been almost identical if the “zero‐degree assumption” had been used instead of the subsurface module.
On the accuracy of the independent column approximation in calculating the downward fluxes in the UVA, UVB, and PAR spectral ranges
Authors: Scheirer Ronald , Macke Andreas
Published: February 2004

In order to investigate the accuracy of simplification in modeling the radiative transfer in those solar spectral regions with major impacts on bio‐organisms, i.e., the UVA (0.32–0.4 μm), the UVB (0.28–0.32 μm), and the photosynthetically active radiation (PAR, 0.4–0.7 μm), radiative transfer calculations with varying treatments of cloud geometries (plane‐parallel homogeneous (PPHOM), independent column approximation (ICA), and three‐dimensional (3‐D) inhomogeneous) have been performed. The complete sets of atmospheric information for 133 cloud realizations are taken from the three‐dimensional nonhydrostatic mesoscale atmospheric model (GESIMA). A Monte Carlo radiative transfer model (GRIMALDI) has been developed that simulates scattering and absorption for arbitrarily three‐dimensional distributions of cloud hydrometeors, air molecules, and water vapor. Results are shown for domain‐averaged direct and total transmission (and so, implicitly, diffuse transmission) at the ground surface. In the UVA the PPHOM assumption leads to an underestimation in direct (total) downward flux by as much as 43 (28) W m−2, which is about 49% (32%) of the incoming irradiation, whereas results based on the ICA are almost identical to the 3‐D case, except for convective clouds where the error in the UVA for direct (total) downward flux reaches 5 (2) W m−2, or 6% (2%) of the incoming solar irradiation.
A neural network model for visibility nowcasting from surface observations: Results and sensitivity to physical input variables
Authors: Pasini Antonello , Pelino Vinicio , Potestà Sergio
Published: February 2004

A neural network model recently developed for fog nowcasting from surface observations is summarized in its features, paying attention to its particular learning structure (weighted least squares training), introduced because of the nonconstant errors associated with the estimation of visibility values. We apply it to a winter forecast of meteorological visibility in Milan (Italy). The performance of this model is presented and shown to be always better than persistence and climatology. Finally, we introduce a bivariate analysis and a network pruning scheme and discuss the possibility of identifying the more significant physical input variables for a correct very short‐range forecast of visibility.
Altitude effects on UV spectral irradiance deduced from measurements at Lauder, New Zealand, and at Mauna Loa Observatory, Hawaii
Authors: McKenzie Richard L. , Johnston Paul V. , Smale Dan , Bodhaine Barry A. , Madronich Sasha
Published: February 2004

Measurements from Lauder, New Zealand, and from the high‐altitude Mauna Loa Observatory, Hawaii, are used to determine the altitude effects on spectral UV irradiance and to relate these altitude differences to other factors that influence UV radiation. The measured ratios UVMauna Loa/UVLauder are complex functions of both wavelength and solar zenith angle (SZA). Spectrally, the ratios tend to increase toward shorter wavelengths through most of the UV‐A region. For small SZA (SZA < ∼40°) the ratios continue to increase as wavelength decreases throughout the UV‐B region. For SZA = 60° a local maximum occurs in the UV‐B region. As the SZA increases, this turning point moves to longer wavelengths and its peak value decreases. For SZA >∼80°, local minima in the ratios are seen at shorter wavelengths in the UV‐B region. For biologically weighted irradiances, the peak ratios occur near SZA = 70°, where UV‐A, erythemally weighted UV, UV‐B, and DNA‐weighted UV irradiances at Mauna Loa Observatory exceeded those at Lauder by ∼17%, 26%, 27%, and 29% respectively. The ratios of irradiances at the two altitudes, as functions of SZA and wavelength, were related to differences expected from radiative transfer calculations. For small SZA, modeled and measured ratios agreed within the limits of experimental uncertainty without taking differences in altitude distributions of ozone and temperature into account. However, for larger SZA and shorter wavelengths these profile shapes had a significant effect. In the model calculations, satisfactory agreement with the measurements was achieved only when the contribution from radiation scattered from air or cloud tops below the observation height at Mauna Loa Observatory was included. To model this accurately, a three‐dimensional radiative transfer code should be used in conjunction with a topographical model of the surrounding terrain.
Algorithm development strategies for retrieving the downwelling longwave flux at the Earth's surface
Authors: Zhou Yaping , Cess Robert D.
Published: February 2004

Algorithm development strategies for retrieving the surface downwelling longwave flux (SDLW) have been formulated on the basis of detailed studies with radiative transfer models and observational data. The model sensitivity studies were conducted with the column radiation model from the National Center for Atmospheric Research Community Climate Model Version 3 and the Moderate‐Resolution Transmittance radiation model. The studies show the clear‐sky SDLW can be largely determined from only two parameters: the surface upwelling longwave flux and the column precipitable water vapor. Cloudy‐sky sensitivity tests show that, as would be expected, cloud base height is an important factor in determining the SDLW, especially for low clouds. However, when considering broken clouds as occur in reality, there is no way of logically defining an average cloud base height. Instead, cloud liquid water path is shown to be a preferable parameter for use in an all‐sky algorithm, not because it serves as a direct cloud input parameter, but rather that it serves as a useful surrogate for cloud base height. Observational data from the Atmospheric Radiation Measurements Program at the U.S. Southern Great Plains (SGP) Oklahoma Central Facility and the Tropical Western Pacific (TWP) Manus Island were used in deriving and validating an illustrative algorithm. The observations show similar relations as found in the model sensitivity tests and suggest that a single algorithm could be applicable for both clear and cloudy conditions as well as for diverse geographical locations. For example, when applied to the TWP data, an algorithm based on a regression of SGP all‐sky data produces a relative bias error in SDLW of only 1.4% under all‐sky conditions and −0.2% for clear skies.
A methodology for initializing soil moisture in a global climate model: Assimilation of near‐surface soil moisture observations
Authors: Walker Jeffrey P. , Houser Paul R.
Published: February 2004

Because of its long‐term persistence, accurate initialization of land surface soil moisture in fully coupled global climate models has the potential to greatly increase the accuracy of climatological and hydrological prediction. To improve the initialization of soil moisture in the NASA Seasonal‐to‐Interannual Prediction Project (NSIPP), a one‐dimensional Kalman filter has been developed to assimilate near‐surface soil moisture observations into the catchment‐based land surface model used by NSIPP. A set of numerical experiments was performed using an uncoupled version of the NSIPP land surface model to evaluate the assimilation procedure. In this study, “true” land surface data were generated by spinning‐up the land surface model for 1987 using the International Satellite Land Surface Climatology Project (ISLSCP) forcing data sets. A degraded simulation was made for 1987 by setting the initial soil moisture prognostic variables to arbitrarily wet values uniformly throughout North America. The final simulation run assimilated the synthetically generated near‐surface soil moisture “observations” from the true simulation into the degraded simulation once every 3 days. This study has illustrated that by assimilating near‐surface soil moisture observations, as would be available from a remote sensing satellite, errors in forecast soil moisture profiles as a result of poor initialization may be removed and the resulting predictions of runoff and evapotranspiration improved. After only 1 month of assimilation the root‐mean‐square error in the profile storage of soil moisture was reduced to 3% vol/vol, while after 12 months of assimilation, the root‐mean‐square error in the profile storage was as low as 1% vol/vol.
Incorporation of a permafrost model into a large‐scale ecosystem model: Evaluation of temporal and spatial scaling issues in simulating soil thermal dynamics
Authors: Zhuang Q. , Romanovsky V. E. , McGuire A. D.
Published: February 2004

This study evaluated whether a model of permafrost dynamics with a 0.5‐day resolution internal time step that is driven by monthly climate inputs is adequate for representing the soil thermal dynamics in a large‐scale ecosystem model. An extant version of the Goodrich model was modified to develop a soil thermal model (STM) with the capability to operate with either 0.5‐hour or 0.5‐day internal time steps and to be driven with either daily or monthly input data. The choice of internal time step had little effect on the simulation of soil thermal dynamics of a black spruce site in Alaska. The use of monthly climate inputs to drive the model resulted in an error of less than 1°C in the upper organic soil layer and in an accurate simulation of seasonal active layer dynamics. Uncertainty analyses of the STM driven with monthly climate inputs identified that soil temperature estimates of the upper organic layer were most sensitive to variability in parameters that described snow thermal conductivity, moss thickness, and moss thermal conductivity. The STM was coupled to the Terrestrial Ecosystem Model (TEM), and the performance of the coupled model was verified for the simulation of soil temperatures in applications to a black spruce site in Canada and to white spruce, aspen, and tundra sites in Alaska. A 1°C error in the temperature of the upper organic soil layer had little influence on the carbon dynamics simulated for the black spruce site in Canada. Application of the model across the range of black spruce ecosystems in North America demonstrated that the STM‐TEM has the capability to operate over temporal and spatial domains that consider substantial variation in surface climate given that spatial variability in key structural characteristics and physical properties of the soil thermal regime are described.
Validation of the Integrated Biosphere Simulator over Canadian deciduous and coniferous boreal forest stands
Authors: El Maayar Mustapha , Price David T. , Delire Christine , Foley Jonathan A. , Black T. Andrew , Bessemoulin Pierre
Published: February 2004

Data collected during the Boreal Ecosystem‐Atmosphere Study (BOREAS) at four different forest stands were used to test surface energy and carbon fluxes simulated by the Integrated Biosphere Simulator (IBIS). These stands included deciduous and conifer species and were located in both the BOREAS northern and southern study areas. Two runs were made: one using the original IBIS model and the other using a version modified to consider an organic soil layer (OSL) covering the mineral soil surface. Results show that the inclusion of the OSL substantially improved the simulation of soil heat flux, as well as of temperature and moisture in the topmost soil layer. Simulations show that latent and sensible heat fluxes, and net ecosystem exchange of carbon, were not affected appreciably by the presence of a thin (10 cm or less) OSL covering the forest floor. With a thick (50 cm) OSL, however, simulation of latent heat flux and net ecosystem exchange of carbon was substantially improved. Consideration of the OSL in the model also led to better simulation of the onsets of soil thawing. Correct estimation of heat diffusion to deep soil through thick organic layers requires a parameterization that accounts for the state of the organic material decomposition. Simulations presented here also show the necessity for using detailed information on soil physical properties for better evaluation of model performance.
Surface climatology of the Greenland Ice Sheet: Greenland Climate Network 1995–1999
Authors: Steffen Konrad , Box Jason
Published: February 2004

The Greenland climate network has currently 18 automatic weather stations (AWS) distributed in most climate regions of the ice sheet. The present network captures well the regional climates and their differences in the accumulation region of the ice sheet. An annual mean latitudinal temperature gradient of −0.78°C/1° latitude was derived from the AWS data for the western slope of the ice sheet, and an annual mean latitudinal temperature gradient of −0.82°C/1° latitude was derived for the eastern slope. The mean annual lapse rate along the surface slope is 0.71°C/100 m, with monthly mean lapse rates varying between 0.4°C/100 m in summer and 1.0°C/100 m in winter. The annual range of monthly mean temperatures is between 23.5°C and 30.3°C for the western slope of the ice sheet, with increasing ranges from south to north and with increase in elevation. The annual mean air temperature was found to be 2°C warmer for the central part of Greenland for the time period 1995–1999, as compared to the standard decade 1951–1960. This annual mean temperature change decreased to approximately 1°C for the elevation 1000–2000 m, whereas at lower elevations, no AWS data are available with sufficient spatial and temporal coverage to verify any temperature trend. Firn temperatures (10‐m depth) at high‐elevation sites were found to be colder than the mean annual air temperature of the preceding year for the central part and northern Greenland by as much as 2.5°C. In the percolation zone and at the equilibrium line altitude the firn and ice temperatures at 10 m were consistently warmer than the annual mean air temperature because of percolation of meltwater and the isolation effect of the snow cover. The wind speed and direction are affected by the katabatic outflow of the cold air along the slope of the ice sheet, whereas at higher elevations the large‐scale synoptic condition is the dominant factor that governs the wind field. The surface height change at high elevations (accumulation minus sublimation) can be approximated with a linear model over an annual cycle using AWS data, whereas in the ablation region and along the equilibrium line altitude the surface height change shows a strong annual cycle.
Experimental and modeling studies of secondary organic aerosol formation and some applications to the marine boundary layer
Authors: Gao Song , Hegg Dean A. , Frick Glendon , Caffrey Peter F. , Pasternack Louise , Cantrell Chris , Sullivan William , Ambrusko John , Albrechcinski Thomas , Kirchstetter Thomas W.
Published: February 2004

A series of controlled experiments were carried out in the Calspan Corporation's 600 m3 environmental chamber to study some secondary organic aerosol formation processes. Three precursor‐ozone systems were studied: cyclopentene‐ozone, cyclohexene‐ozone, and α‐pineneozone. Additionally, SO2 was added to the initial gas mixture in several instances and was likely present at trace levels in the ostensibly organic‐only experiments. It was found that all three systems readily formed new submicron aerosols at very low reactant levels. The chemical composition of formed aerosols was consistent with some previous studies, but the yields of organic products were found to be lower in the Calspan experiments. A three‐step procedure is proposed to explain the observed particle nucleation behavior: HO · production → H2SO4 formation → H2SO4‐H2O (perhaps together with NH3) homogeneous nucleation. It is also proposed that some soluble organic products would partition into the newly formed H2SO4‐H2O nuclei, enhance water condensation, and quickly grow these nuclei into a larger size range. While the observations in the two cycloolefin‐ozone systems could be well explained by these proposed mechanisms, the exact nature of the nucleation process in the α‐pinene‐ozone system remains rather opaque and could be the result of nucleation involving certain organics. The results from three simple modeling studies further support these proposals. Their applicability to the marine boundary layer (MBL) is also discussed in some detail. Particularly, such a particle nucleation and growth process could play an important role in secondary aerosol formation and, quite likely, CCN formation as well in certain MBL regions.
Investigation of cattle methane production and emission over a 24‐hour period using measurements of δ13C and δD of emitted CH4 and rumen water
Authors: Bilek R. S. , Tyler S. C. , Kurihara M. , Yagi K.
Published: February 2004

We have conducted a series of experiments to determine the effects different diets, including the addition of unsaturated fatty acids, had on isotopic signatures of CH4 emissions from dairy cattle. Measurements of emitted δ13CH4, δ13CO2, and δD‐CH4 were made on gases collected over a 24‐hour period from animal chambers containing individual Holstein cows. Some measurements of δD‐H2O from samples collected directly from the rumen were also made. We observed variation in δ13CH4 values with time after feeding and a correlation in δ13C between emitted gases and diet. The average δ13CH4 value, which includes all samples of emitted gases, was −70.6±4.9‰ (n = 57). Measurements of δD‐CH4 over the 24‐hour sampling period had an average value of −357.8±15.0‰ (n = 56). These δD measurements are among the lightest reported for CH4 produced by cattle. Our cattle data indicate that hydrogen incorporated into the CH4 produced by CO2 reduction in high H2 concentration environments is fractionated to a greater degree than that incorporated in systems with relatively low H2 conditions, such as wetlands. Our results support bacterial studies that have demonstrated large hydrogen fractionation in high H2 concentration systems during methanogenesis.
Influence of sublimation on stable isotope records recovered from high‐altitude glaciers in the tropical Andes
Authors: Stichler W. , Schotterer U. , Fröhlich K. , Ginot P. , Kull C. , Gäggeler H. , Pouyaud B.
Published: February 2004

Sublimation dominates the ablation process on cold, high‐altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30°08′S, 69°55′W) revealed a strong enrichment in the 2H and 18O content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of δ2H and δ18O help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2–4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.
Greenland Ice Sheet melt extent: 1979–1999
Authors: Abdalati Waleed , Steffen Konrad
Published: February 2004

Analysis of melt extent on the Greenland ice sheet is updated to span the time period 1979–1999 and examined along with its spatial and temporal variability using passive microwave satellite data. To acquire the full record, the issue of continuity between the previous passive microwave sensors (SMMR, SSM/I F‐8, and SSM/I F‐1l) and the most recent SSM/I F‐13 sensor is addressed. The F‐13 cross‐polarized gradient ratio melt‐classification threshold is determined to be −0.0154. Results show that for the 21‐year record, a positive melt trend of nearly 1%/yr is observed, but this trend falls just below the 90% significance level. The observed melt increase does appear to be driven by conditions in the western portion of the ice sheet, rather than the east where melt appears to have decreased slightly. Moreover, the eruption of Mount Pinatubo in 1991 is likely to have had some impact on the melt but not so much as previously suspected. The 1992 melt anomaly is 1.7 standard deviations from the mean. Finally, the relationship between coastal temperatures and melt extent suggests an increase in surface runoff contribution to sea level of 0.31 mm/yr for a 1°C temperature rise.
Outlet glacier and margin elevation changes: Near‐coastal thinning of the Greenland ice sheet
Authors: Abdalati W. , Krabill W. , Frederick E. , Manizade S. , Martin C. , Sonntag J. , Swift R. , Thomas R. , Wright W. , Yungel J.
Published: February 2004

Repeat surveys by aircraft laser altimeter in 1993/1994 and 1998/1999 have revealed significant thinning along many parts of the Greenland ice sheet at elevations below about 2000 m. In this paper we examine elevation changes from 29 repeat aircraft surveys over the lower portions of some of the larger outlet glaciers and parts of the ice sheet margin. Here thinning rates in excess of 1 m/yr are common in the lower sections of the flight lines, but in some cases, this rate is measured at elevations as high as 1500 m. Warmer summers along parts of the coast may have caused a few tens of cm/yr additional melting, but the magnitudes and character of the elevation changes suggest that in many cases they are more likely a result of glacier dynamics and creep thinning. The most extreme thinning was observed near the terminus of the Kangerdlugssuaq Glacier in southeastern Greenland where rates as high as 10 m/yr were measured. There are a few areas of significant thickening (over 1 m/yr), which is probably related to higher than normal accumulation rates during the observation period; but one location L, Bistrup Brae, had local regions of thickening of 8 to 9 m/yr. Three glaciers in the northeast show patterns of thickness change that may suggest surging behavior, and one has been independently documented as a surging glacier. Overall, the lowest reaches of the outlet glaciers and ice sheet edges appear to be changing significantly, with thinning observed more frequently than thickening.
Impact of interannual meteorological variability on total ozone in northern middle latitudes: A statistical approach
Authors: Krzyścin Janusz W. , Degórska Malgorzata , Rajewska‐Więch Bonawentura
Published: February 2004

The midwinter (January‐February) and summer (June‐July‐August) means of the total O3 derived from ground‐based measurements taken at northern midlatitudes are analyzed for the period 1963–1999. We use a simple, multiple, linear regression model to examine the ozone changes related to interannual meteorological variability. The explanatory variables of the model are selected from many variables characterizing the atmospheric circulation in the troposphere, the lowermost stratosphere, and the stratospheric overworld. The modeled time series of the midwinter and summer total ozone over Europe and North America show significant year‐to‐year variations corresponding to observed variations. Moreover, the model reproduces correlation between the summer (June‐July‐August) midlatitudinal ozone and its preceding midwinter (January‐February) value. The model calculates its lowest ozone amount during winter 1993. This follows the Mount Pinatubo eruption, but the ozone behavior can be explained as a result of dynamical variations only. The model‐observation agreement provides support for the hypothesis that the long‐term variations in total ozone over northern midlatitudes are strongly modulated by transport processes. It is estimated that interannual changes in the dynamical processes account for ∼70–80% of the long‐term ozone decline over the northern midlatitudes for the 1970–1997 period.
Relative roles of secondary sulfate and organics in atmospheric cloud condensation nuclei production
Authors: Kerminen Veli‐Matti
Published: February 2004

Atmospheric production of cloud condensation nuclei (CCN) becomes increasingly important when going to rural or more remote locations. A preliminary investigation on the relative roles of two of the most important compound classes contributing to atmospheric CCN production, sulfate and secondary organic matter, was performed. The investigation was made using a Lagrangian air parcel model that simulates the time evolution of a particle population within a marine or a continental boundary layer. Model simulations demonstrated that for a system containing small (<40–50 nm diameter) primary or secondary particles, production of new CCN can be thought of as a two‐step process: (1) condensational growth of very small particles to a so‐called “potential activation zone,” in which these particles can be activated to cloud droplets at high supersaturations associated with strong updrafts, and (2) growth of particles from the “potential activation zone” to sizes where they form droplets at more typical cloud supersaturations. Since the exact location and width of the “potential activation zone” depend on both cloud physics and the chemical properties of the smallest particles able to act as CCN, the border between these two steps is not very sharp under real atmospheric conditions. According to the simulations, the first step of the above steps relies heavily on the presence of nonvolatile organic vapors, with their required gas phase source rate being of the order 0.1–0.5 μg m−3 d−1 or greater. Sulfuric acid formed in the gas phase can give significant contribution under very specific conditions only. While condensational growth associated with secondary organics may also be involved in the second step, in‐cloud sulfate production associated with strong updrafts appears to be the most effective way to move particles from the “potential activation zone” to larger sizes. In general, the second step was shown to depend on the amount of hygroscopic material transported to particles in the “potential activation zone,” on the variability of the cloud supersaturation between successive cloud events and on the amount of SO2 available for in‐cloud oxidation.
Effects of aerosols on tropospheric oxidants: A global model study
Authors: Tie Xuexi , Brasseur Guy , Emmons Louisa , Horowitz Larry , Kinnison Douglas
Published: February 2004

The global distributions of sulfate and soot particles in the atmosphere are calculated, and the effect of aerosol particles on tropospheric oxidants is studied using a global chemical/transport/aerosol model. The model is developed in the framework of the National Center for Atmospheric Research (NCAR) global three‐dimensional chemical/transport model (Model for Ozone and Related Chemical Tracers (MOZART)). In addition to the gas‐phase photochemistry implemented in the MOZART model, the present study also accounts for the formation of sulfate and black carbon aerosols as well as for heterogeneous reactions on particles. The simulated global sulfate aerosol distributions and seasonal variation are compared with observations. The seasonal variation of sulfate aerosols is in agreement with measurements, except in the Arctic region. The calculated vertical profiles of sulfate aerosol agree well with the observations over North America. In the case of black carbon the calculated surface distribution is in fair agreement with observations. The effects of aerosol formation and heterogeneous reactions on the surface of sulfate aerosols are studied. The model calculations show the following: (1) The concentration of H2O2 is reduced when sulfate aerosols are formed due to the reaction of SO2 + H2O2 in cloud droplets. The gas‐phase reaction SO2 + OH converts OH to HO2, but the reduction of OH and enhancement of HO2 are insignificant (<3%). (2) The heterogeneous reaction of HO2 on the surface of sulfate aerosols produces up to 10% reduction of hydroperoxyl radical (HO2) with an uptake coefficient of 0.2. However, this uptake coefficient could be overestimated, and the results should be regard as an upper limit estimation. (3) The N2O5 reaction on the surface of sulfate aerosols leads to an 80% reduction of NOx at middle to high latitudes during winter. Because ozone production efficiency is low in winter, ozone decreases by only 10% as a result of this reaction. However, during summer the N2O5 reaction reduces NOx by 15% and O3 by 8–10% at middle to high latitudes. (4) The heterogeneous reaction of CH2O on sulfate aerosols with an upper limit uptake coefficient (γ = 0.01) leads to an 80 to 90% decrease in CH2O and 8 to 10% reduction of HO2 at middle to high latitudes during winter. Many uncertainties remain in our understanding of heterogeneous chemical processes and in the estimate of kinetic parameters. This model study should therefore be regarded as exploratory and subject to further improvements before final conclusions can be made.
Lidar measurements during Aerosols99
Authors: Voss Kenneth J. , Welton Ellsworth J. , Quinn Patricia K. , Johnson James , Thompson Anne M. , Gordon Howard R.
Published: February 2004

The Aerosols99 cruise (January 14 to February 8, 1999) went between Norfolk, Virginia, and Cape Town, South Africa. A Micropulse lidar system was used almost continually during this cruise to profile the aerosol vertical structure. Inversions of this data illustrated a varying vertical structure depending on the dominant air mass. In clean maritime aerosols in the Northern and Southern Hemispheres the aerosols were capped at 1 km. When a dust event from Africa was encountered, the aerosol extinction increased its maximum height to above 2 km. During a period in which the air mass was dominated by biomass burning from southern Africa, the aerosol layer extended to 4 km. Comparisons of the aerosol optical depth (AOD) derived from lidar inversion and surface Sun photometers showed an agreement within ±0.05 RMS. Similar comparisons between the extinction measured with a nephelometer and particle soot absorption photometer (at 19 m altitude) and the lowest lidar measurement (75 m) showed good agreement (±0.014 km−1). The lidar underestimated surface extinction during periods when an elevated aerosol layer (total AOD > 0.10) was present over a relatively clean (aerosol extinction < 0.05 km−1) surface layer, but otherwise gave accurate results.
Comparison of albedos computed by land surface models and evaluation against remotely sensed data
Authors: Wei Xue , Hahmann Andrea N. , Dickinson Robert E. , Yang Zong‐Liang , Zeng Xubin , Schaudt Kimberly J. , Schaaf Crystal B. , Strugnell Nicholas
Published: February 2004

The albedos of two land surface models, the Biosphere‐Atmosphere Transfer Scheme (BATS) and the NCAR Land Surface Model (LSM), are compared with remotely sensed data and each other. The model albedos differ primarily because of their assumptions about and model differences in soil moisture content, soil color, snow albedo, shading of snow by canopy, and prescribed parameters for each land cover type. Global albedo maps for February and July 1995, developed from the advanced very high resolution radiometer (AVHRR) data, are used to evaluate model albedos. The models display a high bias as compared to the remotely sensed values in desert and semidesert regions. Over North Africa, LSM, whose albedos were previously tuned to data from the Earth Radiation Budget Experiment (ERBE), has the highest albedos. Elsewhere, and overall, BATS has the highest bias for desert and semidesert regions. Both models demonstrate a high bias over regions of winter snow, where the AVHRR data are expected to have a negative bias. LSM has especially high winter albedos, apparently because of intercepted snow increasing its canopy albedo.
Surface and tethered‐balloon observations of actinic flux: Effects of arctic stratus, surface albedo, and solar zenith angle
Authors: de Roode Stephan R. , Duynkerke Peter G. , Boot Wim , Van der Hage Jeroen C. H.
Published: February 2004

As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment actinic flux measurements were made above the Arctic Sea ice during May 1998. The actinic flux, which is also referred to as the 4π radiative flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane‐parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the actinic flux. Continuous actinic flux measurements were made just above the snow‐covered ice surface by a UV‐A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the actinic flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C‐130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear‐sky conditions the diurnal variation of the magnitude of actinic flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus cloud layer the actinic flux was found to be almost the same as during clear‐sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. In the arctic stratus clouds the actinic flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the actinic flux increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the actinic flux was found to increase gradually from cloud base to cloud top. A delta‐Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.
Lightning and precipitation relationship in coastal thunderstorms
Authors: Seity Yann , Soula Serge , Sauvageot Henri
Published: February 2004

Some differences between lightning and precipitation characteristics of thunderstorms over sea and over land are analyzed. The rain field is observed by a meteorological radar. The cloud‐to‐ground (CG) lightning activity is observed by the French network METEORAGE, and the total lightning activity is observed by a Surveillance et Alerte Foudre par Interférométrie Radioélectrique (SAFIR) interferometric system. Several parameters are compared for 21 stormy days over land and over the ocean in different meteorological conditions. The parameters considered for precipitation are cumulative rainfall and rain rate. Those for the lightning activity are the density, rate, proportion, peak current, and multiplicity of CG flashes, and the density and rate of the total flashes and of the VHF sources. The whole data set is statistically analyzed, and some individual convective systems are discussed. The storm activity and some parameters like the CG peak current clearly seem to be affected by the surface characteristics (sea or land). Others parameters, like the positive CG flash percentage, are not affected. The land‐sea ratio is found to be 1.8 for cumulative rainfall and 2.36 for CG flash number. Most of the thunderstorms observed over sea develop close to the coastline. A substantial decrease of lightning activity is observed during the lifetime of a hail‐bearing thunder cell. The rain volume per CG flash, averaged for all situations and for the whole domain, is 68 × 103 m3 per CG flash. For individual cases this volume increases with the positive CG proportion. Moreover, the low values of the rain volume per CG flash correspond to a relatively strong lightning activity and are often associated with a large cloud vertical development. Assuming that a high probability of lightning flash occurrence corresponds to particularly efficient charging processes, this last observation suggests that the role of strong vertical velocities in these electrical processes is essential.
Present and past nonanthropogenic CO2 degassing from the solid earth
Authors: Kerrick Derrill M.
Published: February 2004

Global carbon cycle models suggest that CO2 degassing from the solid Earth has been a primary control of paleoatmospheric CO2 contents and through the greenhouse effect, of global paleotemperatures. Because such models utilize simplified and indirect assumptions about CO2 degassing, improved quantification is warranted. Present‐day CO2 degassing provides a baseline for modeling the global carbon cycle and provides insight into the geologic regimes of paleodegassing. Mid‐ocean ridges (MORs) discharge 1–3 × 1012 mol/yr of CO2 and consume ∼3.5 × 1012 mol/yr of CO2 by carbonate formation in MOR hydrothermal systems. Excluding MORs as a net source of CO2 to the atmosphere, the total CO2 discharge from subaerial volcanism is estimated at ∼2.0–2.5 × 1012 mol/yr. Because this flux is lower than estimates of the global consumption of atmospheric CO2 by subaerial silicate weathering, other CO2 sources are required to balance the global carbon cycle. Nonvolcanic CO2 degassing (i.e., emission not from the craters or flanks of volcanos), which is prevalent in high heat flow regimes that are primarily located at plate boundaries, could contribute the additional CO2 that is apparently necessary to balance the global carbon cycle. Oxidation of methane emitted from serpentinization of ultramafics and from thermocatalysis of organic matter provides an additional, albeit unquantified, source of CO2 to the atmosphere. Magmatic CO2 degassing was probably a major contributor to global warming during the Cretaceous. Metamorphic CO2 degassing from regimes of shallow, pluton‐related low‐pressure regional metamorphism may have significantly contributed to global warming during the Cretaceous and Paleocene/Eocene. CO2 degassing associated with continental rifting of Pangaea may have contributed to the global warming that was initiated in the Jurassic. During the Cretaceous, global warming initiated by CO2 degassing of flood basalts, and consequent rapid release of large quantities of CH4 by decomposition of gas hydrates (clathrates), could have caused widespread extinctions of organisms.