bluuebunny/crossref_metadata_2025 huggingface dataset:
Ice thickness variability, isostatic balance and potential for snow ice formation on ice floes in the south polar Pacific Ocean
Authors: Adolphs Ute
Published: February 2004

Spatial and temporal characteristics of snow and ice thickness were determined for data, acquired during five cruises into the Bellingshausen/Amundsen and Ross Seas between September 1993 and 1995. When sorted into distance classes from the ice edge, snow and ice thicknesses show a pattern of spatial and temporal variability that reflects the relative regional importance of different sea ice growth mechanisms, motion and deformation as well as of oceanographic processes. The potential for snow ice formation was directly investigated for the drilling data. Flooding, measured prior to drilling, has a strong correspondence with negative freeboard, while moist snow shows a lesser correspondence. Wetness and flooding are strongly correlated with the ratio of snow load and ice thickness while the relation to the amount of ridging is less conclusive. Using an isostatic argument, the contribution of snow ice formation to the overall ice growth was shown to be considerably higher than that of congelation ice growth. It was found that one length unit of snow accumulation or bottom ice melt on an ice floe leads to approximately the same amount of subsidence for the floe. Further, rates of subsidence, due to snow load and ice melt, appear to be similar to uplift from congelation ice growth. The degree of isostatic balance on different spatial scales was evaluated by introducing a generalized density‐weighted snow/ice thickness ratio (GSI) for the drilling data. The GSI includes slush densities in order to account for the suggested importance of snow ice formation. Deviations from isostatic balance occur locally but decrease quickly on spatial averaging scales of a few meters. The closest fit to isostatic equilibrium is reached by choosing a maximum value for the slush density.
Upper ocean salinity balance in the western equatorial Pacific
Authors: Cronin Meghan F. , McPhaden Michael J.
Published: February 2004

The upper ocean salinity balance in the western equatorial Pacific warm pool was evaluated using up to 2.5 years of data (September 1991 through April 1994) from the Coupled Ocean‐Atmosphere Response Experiment's enhanced monitoring array of moorings. At the central mooring site (0°, 156°E), precipitation had a record‐length mean of 4.5 m yr−1, while evaporation had a mean of 1.4 m yr−1. This excess surface freshwater flux was balanced primarily by vertical mixing (estimated as the residual of a salt budget calculation) and by zonal advection. For timescales between a month and 2.5 years, surface salinity variability was dominated by zonal advection and only weakly correlated with precipitation, consistent with the concept of a zonally migrating “fresh pool.” The effects of precipitation on local surface salinity variations were more apparent for timescales shorter than a month. Shallow rain “puddles” tended to form in a matter of hours. However, owing to the combination of mixing and advection, these precipitation‐generated freshwater puddles were typically short‐lived.
Current and tide observations in the southern Yellow Sea
Authors: Teague W. J. , Perkins H. T. , Hallock Z. R. , Jacobs G. A.
Published: February 2004

Determination of mean currents and tides are particularly difficult in coastal regions. Flows are often nongeostrophic and short lived. Bathymetry is frequently not adequately known and can greatly influence coastal dynamics. To better quantify tides and currents in the southern Yellow Sea, three pressure gauges and three acoustic Doppler current profilers (ADCPs) were bottom moored for 4–6 month intervals at depths ranging from 77 to 89 m. Tidal range is over 2 m, and maximum current velocities are between 50 and 80 cm/s at the mooring locations. The present data are analyzed for 13 significant tidal constituents which are found to account for ∼85% of the sea surface height variability. M2 is the dominant constituent, followed by S2 and K1. Flow is dominated by the tides. Mean currents are relatively small, ranging from ∼1.5 cm/s at the northern mooring to ∼4 cm/s at the southern mooring. Delineations of barotropic and depth‐dependent currents are made from the ADCP measurements in terms of mean and eddy kinetic energy. Currents are found to be most depth dependent in the near‐surface layers. Approximately 85–90% of the eddy kinetic energy is depth independent.
Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters: Analysis and implications for bio‐optical models
Authors: Bricaud Annick , Morel André , Babin Marcel , Allali Karima , Claustre Hervé
Published: February 2004

Spectral absorption coefficients of total particulate matter ap(λ) were determined using the in vitro filter technique. The present analysis deals with a set of 1166 spectra, determined in various oceanic (case 1) waters, with field chl a concentrations (〈chl〉) spanning 3 orders of magnitude (0.02–25 mg m−3). As previously shown [Bricaud et al., 1995] for the absorption coefficients of living phytoplankton aø(λ), the ap(λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link ap(λ) and aø(λ) to 〈chl〉 show striking similarities. Despite large fluctuations, the relative contribution of nonalgal particles to total absorption oscillates around an average value of 25–30% throughout the 〈chl〉 range. The spectral dependence of absorption by these nonalgal particles follows an exponential increase toward short wavelengths, with a weakly variable slope (0.011±0.0025 nm−1). The empirical relationships linking ap(λ) to (〈chl〉) can be used in bio‐optical models. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient based on in situ measurements. This agreement is worth noting as independent methods and data sets are compared. It is stressed that for a given (〈chl〉), the ap(λ) coefficients show large residual variability around the regression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflectance of the ocean, are examined, according to whether or not these variations in ap are associated with concomitant variations in particle scattering. In most situations the deviations in ap actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.
Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths
Authors: Sokolik Irina N. , Toon Owen B. , Bergstrom Robert W.
Published: February 2004

We explore the importance of the composition of airborne mineral aerosols for assessments of their direct radiative forcing at infrared wavelengths. Our calculations employing Mie theory and data on spectral refractive indices show that the existing variations in refractive indices can cause large changes in the major aerosol optical characteristics. Calculations of IR radiative forcings at the top of the atmosphere and IR downward and upward fluxes, based on an one‐dimensional radiation transfer code, give a wide range of results for varying optical models of the mineral aerosols. We estimate that for a “low dust loading” scenario the changes in IR downward flux at the surface relative to dust free conditions are in the range from 7 to 14 W/m2 depending upon the mineral aerosol selected. Under “dry tropics” atmospheric conditions the IR forcing at the top of the atmosphere is in the range from 2 to 7 W/m2. In turn, for a “high dust loading” scenario the calculated changes, relative to dust free conditions, in IR downward flux at the surface vary from 50 to 80 W/m2, and the IR forcing at the top of the atmosphere varies from 15 to 25 W/m2. Therefore, we conclude that incorporation of regionally and temporally varying dust mineralogical composition into general circulation models could be beneficial for decreasing the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosols. Also the use of appropriate mineralogical data is required for remote sensing of the atmospheric aerosols using satellite infrared observations.
Evaluation of snow extent and its variability in the Atmospheric Model Intercomparison Project
Authors: Frei Allan , Robinson David A.
Published: February 2004

Simulations of monthly mean northern hemisphere snow extent from 27 atmospheric general circulation models (GCMs), run under the auspices of the Atmospheric Model Intercomparison Project (AMIP), are compared to observations. AMIP model runs have common values for sea surface temperatures specified from observations for the decade 1979 through 1988. Here AMIP GCMs are evaluated in terms of their simulations of (1) snow extent over northern hemisphere lands and (2) synoptic conditions associated with extremes in snow extent over particular regions. Observations of snow extent are taken from digitized charts of remotely sensed snow extent from visible imagery provided by the National Oceanic and Atmospheric Administration. In general, AMIP models reproduce a seasonal cycle of snow extent similar to the observed cycle. However, GCMs tend to underestimate fall and winter snow extent (especially over North America) and overestimate spring snow extent (especially over Eurasia). The majority of models display less than half of the observed interannual variability. No temporal correlation is found between simulated and observed snow extent, even when only months with extremely high or low values are considered. These poor correlations indicate that in the models, interannual fluctuations of snow extent are not driven by sea surface temperatures. GCMs are inconsistent in their abilities to simulate synoptic‐scale tropospheric circulation patterns associated with extreme snow extent over North American regions, although some models are able to capture many of the observed teleconnection patterns.
On the climatic implications of volcanic cooling
Authors: Lindzen Richard S. , Giannitsis Constantine
Published: February 2004

A simple energy balance model is used to investigate the response to a volcanic‐type radiative forcing under different assumptions about the climatic sensitivity of the system. Volcanic eruptions are used as control experiments to investigate the role of the ocean‐atmosphere coupling and of diffusive heat uptake by the thermocline. The effect of varying equilibrium climate sensitivity by varying the coupling of the atmosphere and ocean is examined, high sensitivity being associated with weak coupling. A model representing a coupled land‐ocean system, with a reasonably realistic representation of the large‐scale physics is used. It is found that systems with large equilibrium sensitivities not only respond somewhat more strongly to radiative perturbations but also return to equilibrium with much longer timescales. Based on this behavior pattern, we examine the model response to a series of volcanic eruptions following Krakatoa in 1883. Comparison between the model results and past temperature records seems to suggest that use of small sensitivity parameters is more appropriate. Despite the uncertainties associated with both the physics and the quantitative characteristics of the radiative forcing and the temperature anomalies produced by volcanic eruptions, the present study constitutes a possible test of different assumptions about the sensitivity of the climate system.
A Combined TOA/MDF Technology Upgrade of the U.S. National Lightning Detection Network
Authors: Cummins Kenneth L. , Murphy Martin J. , Bardo Edward A. , Hiscox William L. , Pyle Richard B. , Pifer Alburt E.
Published: February 2004

The U.S. National Lightning Detection NetworkTM (NLDN) has provided lightning data covering the continental United States since 1989. Using information gathered from more than 100 sensors, the NLDN provides both real‐time and historical lightning data to the electric utility industry, the National Weather Service, and other government and commercial users. It is also the primary source of lightning data for use in research and climatological studies in the United States. In this paper we discuss the design, implementation, and data from the time‐of‐arrival/magnetic direction finder (TOA/MDF) network following a recent system‐wide upgrade. The location accuracy (the maximum dimension of a confidence region around the stroke location) has been improved by a factor of 4 to 8 since 1991, resulting in a median accuracy of 500 m. The expected flash detection efficiency ranges from 80% to 90% for those events with peak currents above 5 kA, varying slightly by region. Subsequent strokes and strokes with peak currents less than 5 kA can now be detected and located; however, the detection efficiency for these events is not quantified in this study because their peak current distribution is not well known.
Performance evaluation of the U.S. National Lightning Detection Network in eastern New York: 2. Location accuracy
Authors: Idone Vincent P. , Davis Daniel A. , Moore Paul K. , Wang Yan , Henderson Ronald W. , Ries Mario , Jamason Paul F.
Published: February 2004

We have evaluated the location accuracy of the U.S. National Lightning Detection Network (NLDN) via comparison of individual NLDN stroke locations with their corresponding stroke terminations as determined from multiple‐site video recordings of local cloud‐to‐ground lightning flashes. The video records used in this analysis were acquired in the vicinity of Albany, New York, during the summers of 1994 and 1995, seasons corresponding to the initial and final phases of the upgrade of the NLDN to the improved accuracy from combined technology (IMPACT) configuration. For 1994 a total of 751 strokes were located in common between the video and NLDN data sets, yielding median and mean values of the NLDN‐video location separation of 2.21 km and 3.74 km, respectively. Constraint of the 1994 data to the best video locations (those with a semimajor axis value of the 50% error ellipse less than 500 m) yielded a subset of 53 strokes with median and mean values of the NLDN‐video location separation of 2.61 km and 4.74 km, respectively. For 1995 a total of 219 strokes were located in common, yielding median and mean values of the NLDN‐video location separation of 442 m and 865 m, respectively. Constraint of the 1995 data to that year's best video locations (those with a semimajor axis value of the 50% error ellipse less than 200 m) yielded a subset of 79 common strokes with median and mean values of the NLDN‐video location separation of 435 m and 625 m, respectively. The IMPACT upgrade of the NLDN apparently has resulted in about a fivefold increase in location accuracy. A special subset of 11 strokes occurred in 1995; these were located by the NLDN and observed to terminate on local structures of accurately known location. For these 11 strokes, the median and mean values of the NLDN location error are 518 m and 484 m, respectively, a result quite consistent with the overall NLDN‐video location comparison and the claimed 500‐m median location accuracy for the upgraded network. Indeed, we argue that removal of an apparent residual bias in the NLDN locations ultimately could yield a median location accuracy in this region approaching 250 m. Finally, we find that the NLDN location error is a function of the estimated stroke peak current, with the weakest peak currents having the poorest locations for both years. For the 1995 data, strokes with peak currents of intermediate magnitude (20–30 kA) were the most accurately located.
Physical and optical properties of young smoke from individual biomass fires in Brazil
Authors: Reid Jeffrey S. , Hobbs Peter V.
Published: February 2004

Physical and optical characteristics of particles in smoke from 19 fires were measured in Brazil during the 1995 burning season as part of the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) project. The University of Washington C‐131A measured particle sizes and absorption and scattering properties in very young smoke (<4 min old). These properties are related to fuel type, fire intensity, combustion efficiency, and particle composition. The count median diameter (CMD) of particles from tropical forest fires were strongly and positively correlated with the combustion efficiency. The particle volume median diameter (VMD) of the particles from forest fires did not correlate well with combustion efficiency, but it was highly correlated with the emission factors of particles and unsaturated hydrocarbons. The median diameter and standard deviation of the particle size spectra for smoke from grass and cerrado fires did not correlate with either the combustion efficiency or any emission factor. The measured particle radiative properties correlated well with the measured particle sizes and compositions, and the relationships between these parameters are described fairly well by Mie theory. The optical properties of smoke from individual biomass fires in Brazil differ significantly from those of smoke from biomass burning in North America. In particular, the total light‐scattering coefficient for smoke particles in Brazil is, on average, 15% less than for smoke particles in North America. Also, the average values of the single‐scattering albedos of smoke particles in Brazil are 0.05 to 0.1 less than those in North America.
An evaluation of the temperature, water vapor, and vertical velocity structure of aircraft contrails
Authors: Baumgardner Darrel , Miake‐Lye R. C. , Anderson M. R. , Brown R. C.
Published: February 2004

Measurements made in contrails formed in the wake of a Lear 35 have been analyzed with respect to their temperature, water vapor, and vertical velocity structure. The National Center for Atmospheric Research (NCAR) Sabreliner traversed the Lear's wake at distances varying from 150 m to 5 km and penetrated the contrails 243 times over the course of seven flights. The width of the plume is documented as a function of the distance between the leading and chase aircraft and the temperature, humidity and vertical motion components inside the plume have been analyzed and compared with a three dimensional wake vortex model. The analysis shows that the width increases very little within the first 5 km, in agreement with the vortex model. The increases in temperature and water vapor in the contrail 500 m from the exhaust were ≈0.6° and 8 ppmv, respectively. The vertical velocity change across the contrail was ≈4 m s−1. The agreement between observations and model predictions is quite good in comparisons of temperature. The model predicts larger water vapor mixing ratios, but the differences can be explained within the expected measurement uncertainties and lack of particle condensation in the vortex model. The model predicts larger excursions in vertical velocity but the trends are in excellent agreement between model and observations.
Trends in stratospheric humidity and the sensitivity of ozone to these trends
Authors: Evans Simon J. , Toumi Ralf , Harries John E. , Chipperfield Martyn R. , Russell James M.
Published: February 2004

Measurements of stratospheric water vapor and methane from the Halogen Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research Satellite (UARS) are used to investigate changes in stratospheric water vapor over the period 1992–1996 inclusive. An increase in water vapor mixing ratio is found at levels between 30 km and 65 km across the globe which fit, to first order, a linear trend varying with altitude from 40 parts per billion by volume per year (ppbv yr−1) to a maximum of 90 ppbv yr−1 at 45 km. These trends appear to be greater than that expected due to the growth in tropospheric methane over the past several decades, and possible mechanisms accounting for this are discussed. The trend of the combined budget of 2 × CH4 + H2O is approximately constant with altitude with a global mean value of 61±4 ppbv yr−1. On the basis of these estimates, sensitivity studies have been performed using a two‐dimensional (2‐D) radiative‐chemical‐dynamical model to assess the impact on concentrations of stratospheric ozone of this degree of change in stratospheric water vapor over timescales consistent with doubling CO2 scenarios. We find that the impact of increased stratospheric water vapor is to enhance the ozone increase in the midstratosphere by ∼1–2% compared to the response due to a doubling of CO2 itself of ∼5–10%. In the upper stratosphere the destruction of ozone is enhanced and the changeover from production to loss is lowered to ∼50 km (from ∼70 km). A chemical mechanism for these processes involving enhanced OH and NO2 is identified.
Measurement of alkyl nitrates at Chebogue Point, Nova Scotia during the 1993 North Atlantic Regional Experiment (NARE) intensive
Authors: Roberts James M. , Bertman Steven B. , Parrish David D. , Fehsenfeld Fredrick C. , Jobson B. Thomas , Niki Hiromi
Published: February 2004

Measurements of six alkyl nitrates, methyl‐, ethyl‐, 1‐propyl‐, 2‐propyl‐, 2‐butyl, and 3‐pentyl nitrates, and two peroxycarboxylic nitric anhydrides, peroxyacetic nitric anhydride (PAN) and peroxypropionic nitric anhydride (PPN) were made at Chebogue Point, Nova Scotia, during the 1993 North Atlantic Regional Experiment campaign. The correlation of alkyl nitrates (RONO2) with carbon monoxide makes it clear that the chemistry at this site is being driven by continental emissions, since RONO2 compounds are known to be by‐products of HC‐NOx photochemistry. Comparison of the ratios of alkyl nitrates to their parent hydrocarbons to that of 2‐butyl nitrate/butane showed significant deviations from trends predicted from rate constants, branching ratios, and loss rates. For ethyl nitrate this deviation is likely due to a combination of many additional pathways to the intermediate peroxy radical from the decomposition of larger alkoxy radicals. The propyl nitrates showed trends that differed from one another; 2‐propyl nitrate was a factor of 2 to 3 higher than predicted, while 1‐propyl nitrate was a factor of 8 to 10 higher than predicted. The trend of 3‐pentyl nitrate/pentane was relatively close (about 50% lower) to the predicted trend. The relationship between PAN and PPN showed unexpectedly high ratios of PPN to PAN in air masses coming from the south and west of the site, implying a slight local source of PPN.
Physical retrieval of land surface temperature using the special sensor microwave imager
Authors: Weng Fuzhong , Grody Norman C.
Published: February 2004

This study developed a physical algorithm for retrieving land surface temperatures (LST) by using the special sensor microwave imager (SSM/I) brightness temperature measurements at 19.35 and 22.23 GHz. A nonlinear algebraic equation is obtained for the LST and is solved through the Newtonian iterative procedure. The technique does not require any surface classification because the surface emissivity variations are essentially eliminated by using two closely spaced channels. In addition, the algorithm is less influenced by ice scattering from precipitation and snow cover because of the use of low frequencies. The retrieval accuracy is first examined using simulated measurements. It is found that the root‐mean‐square (RMS) error is about 3.8 K, with the largest errors occurring in dry atmospheres having total precipitable water of 5.0 mm or less. The RMS error slightly increases to 4.4 K when the algorithm is applied for the actual SSM/I measurements. Large biases were found for cold temperatures where the atmospheric water vapor content may be very low.
Age of air in a zonally averaged two‐dimensional model
Authors: Bacmeister Julio T. , Siskind David E. , Summers Michael E. , Eckermann Stephen D.
Published: February 2004

The distribution of age‐of‐air in a two‐dimensional, zonally averaged model of the atmosphere is examined. The age‐of‐air is determined from mixing ratios of a fictitious tracer with a linearly increasing tropospheric source. The impact of horizontal mixing is isolated by performing parallel calculations with and without horizontal mixing terms in the tracer evolution equation. We examine the evolution of the tracer over several model years, using different tropical heat sources as well as different amounts of planetary wave mixing and drag. Typical ages in the midstratosphere when horizontal mixing is included range from 3 to over 5 years, depending primarily on the strength of the model's prescribed tropical heat source. These values of age are within the range of observational estimates based on long‐lived tracers with increasing tropospheric mixing ratios, such as HF, CO2 and SF6. Age determined from the horizontally unmixed tracer is reduced by 1 to 2 years. However, the evolution of the unmixed tracer is consistent with observed behavior of tropical water vapor oscillations in data from the Halogen Occultation Experiment (HALOE). We found stronger than expected sensitivity of middle atmospheric age to prescribed heating in the tropical troposphere. This sensitivity in our model arises from small resulting changes to the residual vertical velocity in the tropics. These vertical velocity changes are in turn associated with net heating changes of 0.05–0.10 K d−1 in the tropical lower stratosphere. We argue that changes of this magnitude are within the uncertainties imposed by natural variations in the tropics.
Electrostatic force on saltating sand
Authors: Schmidt D. S. , Schmidt R. A. , Dent J. D.
Published: February 2004

In blizzards and sandstorms, wind transport of particles is associated with separation of electrostatic charge. Moving particles develop charge of sign opposite the electrostatic charge on stationary surface particles. This electrification produces forces in addition to the gravitational and fluid friction forces that determine trajectories for particles being transported in saltation. Evaluating electrostatic forces requires the electric field strength very near the saltation surface and charge‐to‐mass ratios for the moving particles. In a low‐level blowing sand event we measured an average charge‐to‐mass ratio of +60 μC kg−1 on the saltating particles at 5‐cm height and a maximum electric field of +166 kV m−1 at 1.7‐cm height, in wind gusts near 12 m s−1 at 1.5‐m height. The electrostatic force estimated from these measurements was equal in magnitude to the gravitational force on the saltating particles. Including electrostatic forces in the equations of motion for saltating particles may help explain discrepancies between measurements and models of saltation transport.
Optical characteristics of red sprites produced by runaway air breakdown
Authors: Yukhimuk V. , Roussel‐Dupré R. A. , Symbalisty E. M. D. , Taranenko Y.
Published: February 2004

The results of numerical calculations of intensity and spectra of optical emissions from red sprites produced by runaway air breakdown in the atmosphere are presented. It is shown that the optical emissions from red sprites consist of two components: (1) short‐term (t ≈ 0.3–2 ms) emissions produced as a result of dissipation of an energetic electron beam in air; 2) long‐term (t ≈ 2–10 ms) emissions produced by a population of low‐energy electrons in an electric field. The long‐term optical emissions are calculated for all low‐energy electrons, including the secondary low‐energy electrons produced by the relativistic electron beam, ambient background electrons, and electrons produced as a result of regular breakdown. The theoretical results are compared with observational data.
UARS Microwave Limb Sounder HNO3 observations: Implications for Antarctic polar stratospheric clouds
Authors: Santee M. L. , Tabazadeh A. , Manney G. L. , Salawitch R. J. , Froidevaux L. , Read W. G. , Waters J. W.
Published: February 2004

We present Microwave Limb Sounder (MLS) measurements of gas‐phase HNO3 obtained at the beginning of five southern hemisphere winters: 1992–1996. To investigate the composition of the polar stratospheric clouds (PSCs) that formed in early winter each year, the observed evolution of HNO3 at 465 K is compared against that predicted using nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and liquid ternary solution models of PSC formation and correlated with temperature histories from three‐dimensional back trajectory calculations. The MLS HNO3 observations suggest that the initial composition of PSCs depends on the physical state of the background sulfate aerosols. If the preexisting aerosols are liquid, then the formation of ternary solutions is initiated as the temperature drops below about 192 K, followed by a gradual conversion to NAD after exposure to low temperatures for several days. HNO3 uptake into ternary solutions occurs at higher temperatures, and the conversion to NAD is delayed, under conditions of enhanced aerosol loading from the Mount Pinatubo eruption. If a majority of the background aerosols are frozen, the growth of ternary solutions is inhibited, but formation of a metastable, water‐rich, HNO3‐containing solid phase characterized by a relatively high HNO3 vapor pressure (type Ic PSC) may occur. In general, MLS HNO3 measurements obtained during early southern winter indicate a strong correspondence between the area of gas‐phase HNO3 loss and the area of temperatures below 192 K but only a weak correspondence between the area of gas‐phase HNO3 loss and the area of temperatures below 195 K, the value commonly assumed as the threshold for PSC formation. Although temperatures were low enough to maintain NAT PSCs, the MLS data show that they were not forming, at least not over spatial scales comparable to or larger than the ∼400 × 100 × 5 km MLS field of view.
Measurements of irradiance attenuation and estimation of aerosol single scattering albedo for biomass burning aerosols in Amazonia
Authors: Eck Thomas F. , Holben Brent N. , Slutsker Ilya , Setzer Alberto
Published: February 2004

Investigation of the effects of biomass burning aerosols on the surface irradiance were conducted as a part of the Smoke, Clouds, and Radiation ‐ Brazil (SCAR‐B) experiment during August–September 1995. Measurements of broadband and spectral irradiance, in conjuction with measurements of aerosol physical and optical properties (optical depth, phase function, and size distribution) were made under varying conditions of aerosol loading during the SCAR‐B field campaign. Estimates of aerosol single scattering albedo (ω0) were made from matching of the measured irradiance values to the model computed irradiances by varying ω0, for observations made under cloudless conditions. Values of ω0, at approximately 550 nm, estimated from this technique using broadband 400–700 nm irradiance measurements, ranged from approximately 0.82 to 0.94 for the dates and times of these SCAR‐B measurements. Utilizing spectral irradiance data, the model retrieved values of ω0decreased with increasing wavelength, with the change of ω0as a function of wavelength differing on different days. Reductions in photosynthetically active radiation (PAR; 400‐700 nm) incident at the surface were computed to range from about 20 to 45% compared to background aerosol conditions for the 2 month biomass burning season at several locations in the southern Amazon Basin. These large reductions in incident PAR at the surface due to the heavy aerosol loadings could have implications for primary production of sensitive ecosystems. In addition, reductions of total incident solar radiation from aerosol direct radiative effects may have significant impact on reducing surface heating and increasing aerosol layer heating from absorption.
Real‐time measurement of absorbing material in contrail ice using a counterflow virtual impactor
Authors: Ström Johan , Ohlsson Sofia
Published: February 2004

Near‐field in situ measurements of absorbing material contained in contrail ice has been measured using a particle soot/absorption photometer connected to a counterflow virtual impactor. Vertical profiles made through exhaust plumes from commercial aircraft show a difference in content of absorbing material between the upper and the lower half of the contrail. In the upper part, average concentrations of absorbing material ranged between 0.006 and 0.045 μg m−3, while much less or nothing was detected in the lower half of the exhaust plume. The observed concentrations of absorbing material after normalization with fuel flow and plume dilution show a considerable consistency between exhaust from different aircraft.
The interaction of cumulus convection with soil moisture distribution: An idealized simulation
Authors: Emori Seita
Published: February 2004

In order to investigate the interaction between cumulus convection and soil moisture distribution, two‐dimensional numerical experiments using a regional atmospheric model are performed. The model roughly resolves each convective cell and represents cloud processes by a microphysics parameterization. Two long‐term (60‐day) integrations with relatively dry and wet conditions are made with the atmosphere‐land system in a quasi‐equilibrium state. Though the initial and boundary conditions are horizontally homogeneous, horizontal contrasts in soil moisture spontaneously develop due to the spotty nature of convective precipitation. When intense soil moisture contrasts develop, they cause surface temperature contrasts through a change in evaporation. As a result, thermally induced local circulations occur in the daytime, with upward branches over the dry and hot regions and downward branches over the wet and cool regions. Most cumulus convection events are initiated by the upward motion of this local circulation over the dry region. They mostly occur in the afternoon (1300–1700 LT), while convection that forms over regions that are wet throughout may occur at any time during the day. The intense precipitation over the dry region “overdamps” the soil moisture contrast, which results in the maintenance of a heterogeneous distribution of soil moisture.
Specification of regional sea surface temperatures in atmospheric general circulation model simulations
Authors: Kumar Arun , Hoerling Martin P.
Published: February 2004

A hierarchy of atmospheric general circulation model simulations of the 1950–1994 period that employ different prescriptions of the oceanic boundary conditions are analyzed. It is demonstrated that contrary to expectations, increased spatial coverage of the sea surface temperature specification can degrade the underlying simulated climate signals. The cause for this is related to an inability to accurately model the diversity of air‐sea interaction regimes that exist across the global oceans. Such failure is shown to generate spurious sources of atmospheric energy that exert global influences on interannual and decadal timescales.
Mesospheric HOx chemistry from diurnal microwave observations of HO2, O3, and H2O
Authors: Sandor Brad J. , Clancy R. Todd
Published: February 2004

Microwave (λ ∼1.2 mm) spectral emission line observations of HO2, O3, and H218O from the upper stratosphere and mesosphere (45–80 km) were obtained in April 1992, January 1993, and April and December 1996, using the Kitt Peak National Radio Astronomy Observatory (NRAO). Significant improvements to this, the only method for making mesospheric HO2 measurements, have been made with hardware upgrades allowing observation of a stronger emission line than described in previous work. Data analysis has been improved by (1) recalibration of the (primarily astronomical) instrument for atmospheric studies, (2) incorporation of updated laboratory spectroscopic work, and (3) use of an improved photochemical model. Diurnal HO2 data analyses employing 30 min to 2 hour observations show measured HO2 abundances at 50–80 km altitude are 23–47% higher than photochemical model values at mid day, agree with model values prior to 0900 local time (LT), and exceed model mixing ratios by 70–100% immediately after sunset. Coordinated diurnal measurements of mesospheric O3 find a model O3 deficit consistent with model underprediction of ozone seen in all comparable data sets. HO2 observations are critical to understanding mesospheric chemistry, in that simultaneous model underpredictions of this ozone‐destroying species and of ozone place stringent constraints on proposed sources of model error. The ozone model deficit and 0900–1800 HO2 model deficit are simultaneously resolved with a 40% reduction in the rate coefficient k(HO2 + O → OH + O2). We review other modeling studies of this rate change, indicating it has negligible effect in the lower stratosphere and improves a diverse set of model results from the upper stratosphere to mesopause.
Long‐term MF radar observations of solar tides in the low‐latitude mesosphere: Interannual variability and comparisons with the GSWM
Authors: Vincent R. A. , Kovalam S. , Fritts D. C. , Isler J. R.
Published: February 2004

Long‐term MF radar wind measurements in the 80–100 km height range made at three equatorial and subtropical sites (Adelaide, Christmas Island, and Kauai) are used to produce climatologies and to study interannual variability of solar tides. Twelve years of data were available for Adelaide and up to 6 years at the other sites and are analyzed in 30‐day intervals. The climatological values are compared with the Global Scale Wave Model (GSWM). Good agreement between the measured and model amplitudes and phases is found for the diurnal tide, but the semidiurnal model values agree less well with the observations. The diurnal tidal amplitudes and phases show strong seasonal variability. Maximum amplitudes are attained in March, and subsidiary maxima are observed in July/August and October, while the phase shows an annual cycle at Adelaide and Kauai, with the phase advancing by ∼4–6 hours from summer to winter. Amplitudes of the semidiurnal tide rarely exceed 10 m s−1. The phases undergo rapid shifts around the equinoxes at Adelaide and Kauai, but there is a more complicated phase variation at Christmas Island. The diurnal tide shows strong interannual variability in amplitude, especially near the March equinox. There appears to be an association with the quasi‐biennial oscillation (QBO) in zonal winds in the equatorial stratosphere, with the amplitudes being larger than the climatological average in years when the stratospheric winds are eastward and smaller than average when the QBO is in its westward phase. In contrast, the phase of the diurnal tide, as well as the semidiurnal tide, shows little systematic interannual variability.
Dynamics associated with total aboveground biomass, C, nutrient pools, and biomass burning of primary forest and pasture in Rondônia, Brazil during SCAR‐B
Authors: Guild Liane S. , Kauffman J. Boone , Ellingson Lisa J. , Cummings Dian L. , Castro Elmar A. , Babbitt Ron E. , Ward Darold E.
Published: February 2004

Burning of slashed tropical forests and pastures is among the most significant global sources of atmospheric emissions, yet the composition of the fuels and fires that creates these emissions is not well characterized. As part of the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment, we measured total aboveground biomass (TAGB) as well as carbon, nitrogen, and sulfur pools in one cattle pasture and two slashed primary forests in Rondônia, Brazil. These pools were measured before and immediately after fires. From these data, we calculated the quantities of biomass and elements lost to the atmosphere during biomass burning. Prefire biomass in the pasture was 66 Mg ha−1; fire consumed 31% of this mass. Woody debris from the forest that occupied this site 12 years previously comprised 81% of the pasture prefire TAGB. Elemental inputs into the atmosphere (site losses) from the pasture fire were 9 Mg C ha−1, 88 kg N ha−1, and 5 kg S ha−1. Combining previous studies with this one, we calculate that the mean TAGB of Amazonian pastures is 74 Mg ha−1with a mean combustion factor of 46%. Mean nutrient losses from pasture fires in Amazonia are 14 Mg C ha−1, 199 kg N ha−1, and 16 kg S ha−1. The TAGB of the two slashed primary forests before fire was 355 and 399 Mg ha−1and following fire was 188 and 185 Mg ha−1(i.e., a combustion factor of 47 and 54%), respectively. Combining this study with other studies of Amazon slashed primary forests, we calculate that the mean TAGB is 349 Mg ha−1and the mean combustion factor is 48%. Total elemental losses arising from the primary forest slash fires in this study were notably higher than losses from the pasture site: 79 and 102 Mg C ha−1; 1019 and 1196 kg N ha−1; and 87 and 96 kg S ha−1. From this study combined with previous research in Rondônia and Pará, we calculate that mean nutrient losses from primary forest slash fires are 88 Mg C ha−1, 1181 kg N ha−1, and 107 kg S ha−1. As rates of deforestation are remaining high in the Brazilian Amazon and pastures are the most frequent end product, it can be expected that these will remain the dominant sources of atmospheric emissions from Amazonia in the future.
Radiative forcing from the 1991 Mount Pinatubo volcanic eruption
Authors: Stenchikov Georgiy L. , Kirchner Ingo , Robock Alan , Graf Hans‐F. , Antuña Juan Carlos , Grainger R. G. , Lambert Alyn , Thomason Larry
Published: February 2004

Volcanic sulfate aerosols in the stratosphere produce significant long‐term solar and infrared radiative perturbations in the Earth's atmosphere and at the surface, which cause a response of the climate system. Here we study the fundamental process of the development of this volcanic radiative forcing, focusing on the eruption of Mount Pinatubo in the Philippines on June 15, 1991. We develop a spectral‐, space‐, and time‐dependent set of aerosol parameters for 2 years after the Pinatubo eruption using a combination of SAGE II aerosol extinctions and UARS‐retrieved effective radii, supported by SAM II, AVHRR, lidar and balloon observations. Using these data, we calculate the aerosol radiative forcing with the ECHAM4 general circulation model (GCM) for cases with climatological and observed sea surface temperature (SST), as well as with and without climate response. We find that the aerosol radiative forcing is not sensitive to the climate variations caused by SST or the atmospheric response to the aerosols, except in regions with varying dense cloudiness. The solar forcing in the near infrared contributes substantially to the total stratospheric heating. A complete formulation of radiative forcing should include not only changes of net fluxes at the tropopause but also the vertical distribution of atmospheric heating rates and the change of downward thermal and net solar radiative fluxes at the surface. These forcing and aerosol data are available for GCM experiments with any spatial and spectral resolution.
Effects of aerosols on the wavelength dependence of atmospheric transmission in the ultraviolet and visible: 1. A “single‐scattering‐separate” delta‐Eddington model
Authors: Erlick Carynelisa , Frederick John E.
Published: February 2004

A modification to the conventional delta‐Eddington radiative transfer scheme was implemented in order to create a simple and efficient model particularly suitable for handling the highly anisotropic scattering properties exhibited by cloud drops and aerosol particles in the atmosphere. The modification includes separating out the singly scattered radiation and applying the conventional Eddington approximation to the multiply scattered radiation alone. The singly scattered irradiance is computed analytically without approximation, preserving more of the angular dependence in the radiance than conventional two‐stream style solutions. The single‐scattering‐separate delta‐Eddington model was found to handle strong absorption in the stratosphere and the anisotropic scattering within optically thin cloud layers better than both the simple two‐stream and conventional Eddington models, and to produce results similar to a 22‐stream discrete ordinates model.
On the climate forcing of carbon monoxide
Authors: Daniel John S. , Solomon Susan
Published: February 2004

Carbon monoxide plays a primary role in governing OH abundances in the troposphere. It is likely that through this chemical interaction, CO plays an important role in climate forcing by affecting CH4 concentrations. We use a photochemical box model to estimate the indirect global warming potential (GWP) of tropospheric carbon monoxide resulting from its effect on methane abundances. We also consider indirect GWPs of CO due to carbon dioxide production and possible CO GWP ranges due to ozone production to estimate a total CO indirect GWP range over various time horizons. We estimate that for current emission levels, the short‐term (<15 years) cumulative radiative forcing due to the direct anthropogenic emission of CO may be larger than the cumulative forcing due to anthropogenically emitted N2O.
Mass‐related aerosol properties over the Leipzig Basin
Authors: Heintzenberg Jost , Müller Konrad , Birmili Wolfram , Spindler Gerald , Wiedensohler Alfred
Published: February 2004

The present study comprises a large body of physical and chemical mass‐related aerosol data which were collected over the industrialized basin of Leipzig, Germany, during the time period 1983 to 1997. For the period 1996/1997 consistent mass size distributions over the range 0.01 to 10 μm (PM10) were determined with a differential mobility analyzer and an aerodynamic particle sizer. The modal size distribution parameters were compared to similar lognormal data from the United States of the 1970s, yielding lower concentrations and coarse particle geometric diameters. In both accumulation and coarse mode, narrower distributions than over the United States were found which may be real or due to instrumental limitations in the 1970s. Within the range of measurement uncertainties and with reasonable assumptions about particle densities, chemical mass closure could be achieved with results of daily PMl0 samples. However, the results of a harmonic analysis of the physical and chemical time series indicated that there may be significant aerosol components which were not covered by our analyses, possibly organic compounds. Since 1990 a significant downward trend in gravimetric mass concentration and since 1993 clear changes in aerosol composition were found which are consistent with similar findings in precipitation samples over eastern Germany.