bluuebunny/crossref_metadata_2025 huggingface dataset:
Methane emission and transport by arctic sedges in Alaska: Results of a vegetation removal experiment
Authors: King Jennifer Y. , Reeburgh William S. , Regli Shannon K.
Published: February 2004

Methane flux and below‐ground methane profile studies were conducted in a wet meadow vegetation manipulation site at the Toolik Lake Long‐Term Ecological Research (LTER) site during the summers of 1995 and 1996. Control plots, moss‐removal plots, and sedge‐removal plots were studied to determine the role of these vegetation types in wetland methane emission and to study the gas transport mechanism. Methane emission was greatest from plots with intact sedges. Depth distributions of root density collected in 1995 showed a strong inverse relationship to pore water methane concentration. Results on insertion of arrays of gas‐permeable silicone rubber tubing into the soil indicate that they are reasonable analogs for the physical process of gaseous diffusion through plants. The observed differences in flux between plots with and without sedges cannot be fully explained by differences in methane production or dissolved organic carbon concentrations in our measurements.
Measurements of NOx and PAN and estimates of O3 production over the seasons during Mauna Loa Observatory Photochemistry Experiment 2
Authors: Ridley B. , Walega J. , Hübler G. , Montzka D. , Atlas E. , Hauglustaine D. , Grahek F. , Lind J. , Campos T. , Norton R. , Greenberg J. , Schauffler S. , Oltmans S. , Whittlestone S.
Published: February 2004

Measurements of peroxyacetyl nitrate (PAN) and NOx and a variety of other constituents were made over approximately 1‐month‐long intensives in the autumn of 1991 and the winter, spring, and summer of 1992 during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). PAN and NOx in the free troposphere had maximum abundances in spring in concert with the well‐known maximum in O3. The ratio of the spring to summer averages was a factor of 4.1 for PAN, a factor of 1.6 for O3, and only a factor of 1.4 for NOx. During most intensives, variations over periods of a few days to a week were often larger than the average seasonal amplitude. In free tropospheric air masses local to Hawaii, average PAN/NOx ratios were a maximum in winter through spring but in the range of 0.25–0.86 in all intensives. PAN decomposition is unlikely to be the major net source of NOx in local air masses in summer and fall. The low HNO3/NOx ratios determined during MLOPEX 1 were confirmed during MLOPEX 2. Intensive average ratios of 1.6–3.8 over the year are lower than some model predictions. Both the low ratio and the magnitude of NOx imply a shortcoming in our understanding of the transformations and sources of NOy constituents in the central Pacific, The 3‐ to 4‐km altitude region near Hawaii was a net importer of O3, on average, over the year. The average net rate of production of O3 in free tropospheric air was near zero in winter, −0.4 to −0.8 ppbv/d in spring, −1.4 ppbv/d in summer, and −0.6 ppbv/d in autumn. Thus the spring maximum in O3 is not due to local photochemistry. We believe, as has been concluded from the long‐term measurements of long‐lived constituents by the Climate Monitoring and Diagnostics Laboratory, that the variation of ozone precursors over the year and on shorter timescales of a few days to a week is controlled predominantly by changes in long‐range transport: more frequent sampling of higher‐latitude and higher‐altitude air masses in winter and spring versus more frequent sampling of well‐aged air from lower altitudes and latitudes in summer and autumn.
Comparison of model‐calculated and ERBE‐retrieved clear‐sky outgoing longwave radiation
Authors: Ho Chang‐Hoi , Chou Ming‐Dah , Suarez Max , Lau Ka‐Ming , Yan Michael M.‐H.
Published: February 2004

In order to validate radiative transfer models and identify sources of errors in the satellite retrieval of radiation budgets, model calculations of clear‐sky outgoing longwave radiation (OLR) over oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). The NASA Goddard Earth Observing System (GEOS‐1) and the National Centers for Environmental Prediction and National Center for Atmospheric Research reanalyses of temperature and humidity and the Stratospheric Aerosol and Gas Experiment retrieval of the stratospheric humidity are used in the model calculations. Averaged over time (1985–1989) and space (60°S‐60°N), the model‐calculated clear‐sky OLR has a positive bias of 1.9–2.3 W m−2 when compared with that of ERBE. Nearly all of the bias can be accounted for by the inclusion of the absorption due to CO2 in the 4.3‐μm band and to the weak CO2 and O3 molecular lines distant from band centers. The use of the two different reanalyses has only a small effect on the flux calculations (∼0.4 W m−2). Consistent with suggestions by previous studies, ERBE is found to overestimate (underestimate) the clear‐sky OLR over high humid (dry) regions due to incorrect identification of clear scenes in the ERBE retrievals. The importance of the upper tropospheric humidity in affecting the Earth radiation budget is also investigated. Although only ∼15% of the atmospheric humidity is contained in the region above the 600‐hPa level, the upper troposphere is as important as the lower troposphere in contributing to the clear‐sky OLR.
Non local thermodynamic equilibrium (LTE) atmospheric limb emission at 4.6 μm: 2. An analysis of the daytime wideband radiances as measured by UARS improved stratospheric and mesospheric sounder
Authors: López‐Puertas M. , Zaragoza G. , López‐Valverde M. Á. , Taylor F. W.
Published: February 2004

An analysis of the measurements taken by the improved stratospheric and mesospheric sounder (ISAMS) in its carbon monoxide wideband channel around 4.6 μm at daytime is presented. The radiances show a good signal to noise ratio up to the lower thermosphere (about 120 km) and have been shown to be mainly due to emission from the weak CO2 4.3 μm isotopic and hot bands. They exhibit a very clear dependence with the solar illumination at tangent heights above about 60 km, where they have been found to be almost exclusively determined by the solar elevation. Below about 50 km they are dominated by the variations of the kinetic temperature. The measurements have been analyzed in the 50–100 km range by using a detailed non local thermodynamic equilibrium (LTE) model of the CO2 states emitting in the 4.3 μm spectral region and the GENLN2 line‐by‐line radiance code. A large number (up to 32) of CO2 isotopic and hot bands emit significantly in this spectral region. The N2O(ν3 = 1) and two O3(ν1 + ν3) bands also give contributions in the stratosphere and lower mesosphere. The CO(1→0) band is of relative importance only in the lower thermosphere. The absolute radiances as well as the solar zenith angle dependence are well reproduced by the model. The dependence on the solar zenith angle is due to the absorption of solar radiation in the CO2 near‐infrared bands. A sensitivity study of the radiances was also conducted. The major conclusions are (1) the inclusion of the excitation of N2(1) from the electronic energy of O(1D) was required to explain the radiances in the lower mesosphere; (2) the value for the rate of the vibrational exchange between CO2(ν1,ν2,1) and N2(1) is very similar to the laboratory measurements and to that used in the analysis of the Spectral Infrared Rocket Experiment (SPIRE) 4.3 μm CO2 atmospheric limb radiances; and (3) the CO2 volume mixing ratio (vmr) in the 70–100 km region is significantly smaller than that measured in rocket experiments and similar to that deduced from the atmospheric trace molecule spectroscopy (ATMOS) absorption measurements. A reanalysis of the SAMS 4.3 μm limb radiance measurements has also been conducted with the same non‐LTE model and the GENLN2 line‐by‐line radiance code. The values for the parameters derived from the ISAMS data also explain the SAMS measurements very satisfactorily.
Measurements of C2‐C7 hydrocarbons during the Polar Sunrise Experiment 1994: Further evidence for halogen chemistry in the troposphere
Authors: Ariya P. A. , Jobson B. T. , Sander R. , Niki H. , Harris G. W. , Hopper J. F. , Anlauf K. G.
Published: February 2004

Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground‐based sites: Alert, Northwest Territories (82.5°N, 62.3°W) and Narwhal ice camp, an ice floe 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2‐day aerial survey conducted on Julian days 89 and 90, 1994 over the Arctic archipelago. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed at Alert and Narwhal ice camp. At Narwhal, a long period (≥7 days) of ozone depletion was observed during which a clear decay of alkane concentration occurred. A kinetic analysis led to a calculated Cl atom concentration of 4.5×103 cm−3 during this period. Several low‐ozone periods concurrent with NMHC concentration decreases were observed over a widespread region of the Arctic region (82°–85°N, and 51°–65°W). Hydrocarbon measurements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with Cl atom reactions. The changes in ethyne concentration from its background level were in excess of those expected from Cl atom kinetics alone and are attributed to additional Br atom reactions. A box modeling exercise suggested that the Cl and particularly Br atom concentrations required to explain the hydrocarbon behavior are also sufficient to destroy ozone.
Uncertainty analysis of satellite rainfall algorithms over the tropical Pacific
Authors: Morrissey Mark L. , Greene J. Scott
Published: February 2004

This paper describes the uncertainty analysis applied to comparisons between satellite monthly rainfall estimates and twenty 2.5° × 2.5° box rainfall estimates constructed from Pacific atoll‐sited rain gauge sites. This work was done in conjunction with NASA's WETNET Precipitation Intercomparison Project 3 (PIP 3), and was conducted by personnel at the Environmental Verification and Analysis Center (EVAC) at the University of Oklahoma. Motivation for this work stemmed from the need to supply surface areal rainfall estimates of known accuracy for comparison with satellite rainfall estimates and the need to use these estimates to quantify the uncertainty of the comparisons. This allowed for the discrimination of the value of various statistics associated with regression analysis, using different satellite algorithms, and for the quantification of the regression statistics computed by accounting for rain gauge uncertainty. To our knowledge this type of work has not been performed for previous satellite rainfall intercomparison programs.
Model interpretation of Jovian lightning activity and the Galileo Probe results
Authors: Yair Yoav , Levin Zev , Tzivion Shalva
Published: February 2004

An axisymmetric numerical cloud model with detailed microphysics is used to evaluate the development of water clouds under various Jovian conditions. A formulation of the noninductive ice‐ice charge separation mechanism is used to calculate the electrical structure of the clouds and to identify the charged regions at the altitudes of the Galileo entry probe trajectory. The results show that for midlatitude conditions, with reasonable assumptions, the water clouds in the deep Jovian troposphere have an electrical dipole structure, with a large positive charge center located at the 3 bar pressure level and a lower negative charge center at the 4.5 bar level. This structure is modified as the cloud matures and the breakdown electric field is exceeded and lightning occurs. Our model predicts that the weak convection, which occurs at the stable equatorial region, is capable of producing water clouds with 10 m s−1 updrafts and water and ice content of 5 g kg−1. Charge separation and lightning generation in these clouds is less intensive and lightning frequency is lower than at midlatitudes. For a low oxygen abundance of 0.2 times solar, we show that ice clouds form at temperatures where charge separation is weak, and no lightning can occur. The results suggest a very low probability that there was lightning activity near the path of the descending probe.
Density and surface temperature of graupel and the charge separation during ice crystal interactions
Authors: Jayaratne E. R.
Published: February 2004

Significant amounts of charge are separated when vapor‐grown ice crystals interact with riming graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well‐known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of −10°C and a CWC of 0.4 g m−3 the charging current to the graupel reversed from positive to negative as the surface temperature of the graupel exceeded −6°C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m−3 at −10°C to less than 0.9 g m−3 at −20°C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.
Ducting of acoustic‐gravity waves in a nonisothermal atmosphere around a spherical globe
Authors: Liang Jun , Wan Weixing , Yuan Hong
Published: February 2004

The ducting mechanism of acoustic‐gravity waves in a thermally stratified atmosphere with spherical geometry is investigated analytically. By introducing a reasonable atmospheric sound speed model, the Green's function of Navier‐Stokes equations is derived. The dispersion relation of ducted acoustic‐gravity wave modes is studied. Calculations reported here show various ducted modes, namely, the quasi‐surface modes, the fully ducted modes, and the imperfectly ducted mode. The ducting mechanism of the “leaky” gravity wave mode, usually referred to as mesospherically ducted mode, is reexamined. We find that the sudden rise of temperature at the lower thermosphere can itself contribute to the ducting of the lower atmospheric gravity waves.
Harvard Forest regional‐scale air mass composition by Patterns in Atmospheric Transport History (PATH)
Authors: Moody J. L. , Munger J. W. , Goldstein A. H. , Jacob D. J. , Wofsy S. C.
Published: February 2004

We calculated 4 years (1990–1993) of back trajectories arriving at Harvard Forest and used them to define patterns in atmospheric transport history. This information was used to assess the degree to which regional‐scale transport modulates the chemical composition of air masses sampled at Harvard Forest. Different seasonal signals in trace‐gas concentration are derived for different flow patterns. Throughout the year, high‐speed transport of cool, dry, cloud‐free air from the north and northwest represents background conditions for the Harvard Forest site. These synoptic conditions describe the atmosphere after passage of a cold front. The most polluted conditions in each season occurred under SW flow, with warmer temperatures, higher water vapor mixing ratios, low mixed‐layer depths at the site, and a higher frequency of cloudy conditions. These regional‐scale air mass characteristics describe synoptic conditions of warm sector transport. In addition to average air mass characteristics, we have analyzed the covariation of species (e.g., O3 versus NOy‐NOx; O3 versus CO) to address chemical processes based on transport history. For summer daytime measurements, we show that relatively fresh pollutants arrive in SW flow while the most aged air masses with higher O3 to NOz slopes arrive with W flow, suggesting a Midwestern contribution to regional high‐oxidant episodes. These observations of patterns in chemical characteristics related to patterns in transport are corroborated with probability maps indicating the likelihood of transport from upwind regions using trajectories selected for chemical distribution end‐members (10th and 90th percentiles).
POLDER observations of cloud bidirectional reflectances compared to a plane‐parallel model using the International Satellite Cloud Climatology Project cloud phase functions
Authors: Descloitres J. , Buriez J. C. , Parol F. , Fouquart Y.
Published: February 2004

This study investigates the validity of the plane‐parallel cloud model and in addition the suitability of water droplet and ice polycrystal phase functions for stratocumulus and cirrus clouds, respectively. To do that, we take advantage of the multidirectional viewing capability of the Polarization and Directionality of the Earth's Reflectances (POLDER) instrument which allows us to characterize the anisotropy of the reflected radiation field. We focus on the analysis of airborne‐POLDER data acquired over stratocumulus and cirrus clouds during two selected flights (on April 17 and April 18, 1994) of the European Cloud and Radiation Experiment (EUCREX'94) campaign. The bidirectional reflectances measured in the 0.86 μm channel are compared to plane‐parallel cloud simulations computed with the microphysical models used by the International Satellite Cloud Climatology Project (ISCCP). Although clouds are not homogeneous plane‐parallel layers, the extended cloud layers under study appear to act, on average, as a homogeneous plane‐parallel layer. The standard water droplet model (with an effective radius of 10 μm) used in the ISCCP analysis seems to be suitable for stratocumulus clouds. The relative root‐mean‐square difference between the observed bidirectional reflectances and the model is only 2%. For cirrus clouds, the water droplet cloud model is definitely inadequate since the rms difference rises to 9%; when the ice polycrystal model chosen for the reanalysis of ISCCP data is used instead, the rms difference is reduced to 3%.
Studies of quasi horizontally propagating gravity waves in the troposphere using the Piura ST wind profiler
Authors: Liziola Luis E. , Balsley Ben B.
Published: February 2004

VHF profiler observations of the troposphere obtained over a flat desert in northern Perú show the presence of strong, quasi‐sinusoidal oscillations in the vertical wind velocity with periods of around 8–18 min. These oscillations are a very typical feature of the vertical velocities over Piura. This paper presents an overview of the wave characteristics obtained at Piura until this point, including results obtained using two additional antennas built a few hundred meters to the west and south of the main antenna to measure horizontal wavelengths and phase velocities. Briefly, the Piura waves appear to have horizontal wavelengths of 1–3 km, with either westward or eastward phase velocities (in a ground‐based reference frame) of 0.5–4 m s−1. Their occurrence appears to be causally related to the magnitude of the easterly wind, which blows over the Andean Cordillera some 60 km to the east of the site.
Detailed analysis of the error associated with the rainfall retrieved by the NOAA/NESDIS SSM/I algorithm: 1. Tropical oceanic rainfall
Authors: Li Qihang , Ferraro Ralph , Grody Norman
Published: February 2004

A general formulation has been established for the total error associated with passive microwave retrieval of space‐time rainfall. The total error consists of two independent components: the sampling error and the algorithm error. The former arises from the discrete sampling scheme of a polar‐orbiting satellite, and the later is the result of the imperfect algorithm used to convert the measured radiation to instantaneous rain rate. Rainfall data over the Tropical Ocean and Global Atmosphere/Coupled Ocean and Atmosphere Response Experiment (TOGA/COARE) region has been used to represent oceanic rainfall conditions to evaluate the error of the National Oceanic and Atmospheric Administration/National Environmental Satellite, Data, and Information Service (NOAA/ NESDIS) Special Sensor Microwave/Imager (SSM/I) rainfall algorithm. Rainfall statistics that affect the sampling error have been obtained from radar data, which include mean, variance, and autocorrelation of areal rain. The magnitude of the variance and the strength of the autocorrelation are quantitatively related to the size of the averaging area. Instantaneous measurement errors on the FOV scale and the large scale (2.5° box) are obtained by analyzing simultaneous observations of SSM/I and radar. Based on these parameter values, the sampling error, the algorithm error, and the total error are computed as a function of sampling frequency, size of averaging area, and other factors. This study has demonstrated the feasibility of a procedure to quantify the total retrieval error of space‐time rainfall by focusing on TOGA/COARE area (equivalent in size to a 2.5° box). The same procedure can potentially be applied to other grid boxes to produce a global error field if rain statistics and measurement errors are estimated for those boxes. The accuracy of the global error field will depend on the accuracy of the rain statistics and the measurement error values used in the individual grid boxes.
Atmospheric circulation anomalies associated with 1996/1997 summer precipitation events on Sajama Ice Cap, Bolivia
Authors: Vuille Mathias , Hardy Douglas R. , Braun Carsten , Keimig Frank , Bradley Raymond S.
Published: February 2004

The analysis of atmospheric circulation anomalies related to snowfall events on Sajama volcano (Bolivian Andes) provides important information for the calibration of an ice core, recently recovered from the summit. Seventeen precipitation episodes were recorded on Sajama volcano during the 1996/1997 summer season (November 1996 to March 1997) by snow depth sensors and additional measurements of an automatic weather station located on the summit. The analysis of atmospheric circulation patterns during these events is based on zonal and meridional wind, air temperature, relative humidity, geopotential height and horizontal divergence at three pressure levels (400, 500, and 700 hPa levels), atmospheric thickness (700 hPa‐400 hPa), and precipitable water (vertically integrated), all extracted from the National Centers for Environmental Prediction (NCEP) data set. Highly convective situations prevailed through most of December and January, with strong vertical motion over the Bolivian Altiplano. In February and March, increased moisture advection from the east occurred in midtropospheric levels. These results are confirmed by isobaric 5‐day back trajectories and transit time analysis at the 400 hPa level. The extremely southern position of the upper air high‐pressure system (“Bolivian High”) in February and March is the main reason for the unusually high precipitation amounts on the Altiplano in 1996/1997. Highly variable patterns of atmospheric circulation can lead to snowfall on Sajama during the summer months.
Production of sulfate aerosol precursors in the turbine and exhaust nozzle of an aircraft engine
Authors: Lukachko S. P. , Waitz I. A. , Miake‐Lye R. C. , Brown R. C. , Anderson M. R.
Published: February 2004

Recent in‐flight measurements of aircraft engine exhaust have suggested much higher conversions of fuel sulfur to sulfuric acid aerosols than can be explained by gas‐phase oxidation within the exhaust plume. This paper describes the effects of turbine and exhaust nozzle aerodynamics and chemical kinetics on the production of sulfate aerosol precursors in engine exhaust. Results from both one‐dimensional (1‐D) and two‐dimensional (2‐D) numerical simulations are presented for a range of flow and chemistry conditions. One‐dimensional calculations for the entire postcombustor flow path of an advanced subsonic engine resulted in up to 6.3% sulfur oxidation through the turbine and exhaust nozzle. Results were most sensitive to species concentrations at the combustor exit, combustor exit temperature, and cooling flow mass addition in the turbine. Two‐dimensional calculations for a representative turbine demonstrated that intraengine fluid mechanics can increase sulfur oxidation by a factor of 3 across a single blade row because of cooling‐induced temperature nonuniformities. Comparisons of averaged 2‐D results with 1‐D simulations of the same blade row further showed that while 1‐D simulations produce the correct trends, the magnitude of change in sulfur oxidation may be underpredicted by as much as 47% over a single blade row if spatial nonuniformities in flow field temperature are not included.
Simultaneous measurements of CO2, CH4, and N2O in air extracted by sublimation from Antarctica ice cores: Confirmation of the data obtained using other extraction techniques
Authors: Güllük T. , Slemr F. , Stauffer B.
Published: February 2004

A sublimation technique has been developed to extract air samples from polar ice cores for subsequent simultaneous measurement of several trace gases by frequency‐modulated tunable diode laser absorption spectroscopy. This extraction and analysis technique is shown to be suitable as an extraction method for the determination of concentrations of the greenhouse gases CO2, CH4, and N2O in air samples of ∼1–5 cm3 recovered from ice samples of 10–50 g. Air samples from the Siple ice core have been analyzed covering the period between 1772 and 1973. In addition, a few samples from two different ice cores from Vostok station have been analyzed. Our results are in a good agreement with results obtained by other researchers using melting and crushing extraction techniques. This agreement indicates that processes connected with the formation of clathrates in ice under high pressure at greater depths and their destruction after drilling are not affecting the CO2, CH4, and N2O measurements significantly.
First geometrical pathlengths probability density function derivation of the skylight from spectroscopically highly resolving oxygen A‐band observations: 1. Measurement technique, atmospheric observations and model calculations
Authors: Pfeilsticker K. , Erle F. , Funk O. , Veitel H. , Platt U.
Published: February 2004

First measurements of the probability density function of geometrical pathlengths (PDF‐GP) for the skylight transmitted from clear and cloudy skies to the terrestrial surface are reported. The measurements are performed using a novel technique, which includes spectroscopically highly resolving observations of the O2 A‐band (760–780 nm) and a Laplacian back transformation of the intensity ratios measured for the set of individual O2 A‐band rotational absorption lines. The technique provides a new and powerful tool to study the radiative transfer (RT) of the atmosphere for different kinds of atmospheric aerosol loadings and cloudiness. First measurements show, as expected, that the photons transmitted by clouds to ground generally experience longer geometrical paths inside the cloud than expected for a corresponding clear sky condition. The comparison of the measured oxygen A‐band absorption system with plane‐parallel discrete ordinate radiative transfer (DISORT) model calculations shows a reasonable agreement for direct Sun and clear sky zenith observations. For cloudy sky the same comparison shows a good agreement between the observed and the modeled weak absorption lines but systematically larger observed than modeled optical densities for strong absorption lines. Our data also suggest that this significant discrepancy is most likely caused by shortcomings in the physical description of the RT by nonstatistical models, which do not properly account for the fractal nature of the terrestrial cloud cover. In particular our study reveals that the photon path statistics involved with the multiple Mie scattering inside clouds is not properly modeled using the DISORT code. Closing this gap in our understanding of the cloud sky RT may possibly provide much of the explanation for the frequently lower modeled than measured cloudy sky absorption of the solar radiation.
Comparisons of techniques for measuring shortwave absorption and black carbon content of aerosols from biomass burning in Brazil
Authors: Reid Jeffrey S. , Hobbs Peter V. , Liousse Catherine , Martins J. Vanderlei , Weiss Ray E. , Eck Thomas F.
Published: February 2004

Six methods for measuring the shortwave absorption and/or black carbon (BC) content of aerosols from biomass burning were compared during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment. The methods were the optical extinction cell (OEC), integrating plate (IP), optical reflectance (OR), particle soot/absorption photometer (PSAP), thermal evolution (TE), and remote sensing (RS). Comparisons were made for individual smoke plumes and for regional hazes dominated by smoke. Taking the OEC as a primary standard, measurements of the absorption coefficient (σa) showed that the OR method had the lowest uncertainty (17%) in σa. The other optical methods had uncertainties ranging from 20 to 40%. However, with sufficient sample size, the values of σaderived from the optical methods converged to within 20% of each other. For biomass burning aerosols in regional hazes over Brazil, this led to systematic differences of ±0.02 in the values of the single‐scattering albedo derived from the various in situ techniques. It was found also that the BC content of the aerosol and σawere poorly correlated. This is likely due to a large uncertainty in the BC content of the aerosol measured by TE, and/or a high variability in the mass absorption efficiency of BC in biomass burning aerosol. Hence there is a high uncertainty in inferring σafrom the BC content of smoke aerosol.
Modeling study of cloud droplet nucleation and in‐cloud sulfate production during the Sanitation of the Atmosphere (SANA) 2 campaign
Authors: Liu Xiaohong , Seidl Winfried
Published: February 2004

Based upon the measurements of vertical profiles of gaseous SO2, H2O2, O3, and meteorological parameters from aircraft and of the aerosol chemical composition and gaseous NH3, HNO3, and SO2 at the surface in southeastern Germany (Melpitz) during the Sanitation of the Atmosphere (SANA) 2 campaign, realistic modeling of cloud droplet nucleation and in‐cloud sulfate production was performed with an explicit microphysical cloud model with size‐resolved chemistry and cloud top entrainment. For the fair weather cumulus observed during the measurements, the calculated cloud droplet number concentrations could be as high as 2000 cm−3 (and precloud aerosol sulfate up to 9.1 μg m−3), indicating strong sulfur pollution at Melpitz during the campaign. The in‐cloud sulfate production is within 1.5–5.0 μg m−3, depending on the initial gaseous NH3 concentration in the parcel. This result shows the necessity of gaseous NH3 vertical profile measurements. Entrainment can reduce the cloud droplet number concentration and cause the distribution of in‐cloud produced sulfate to shift toward larger particle sizes. Under the cases we studied, we do not find a significant effect of cloud top gaseous H2O2 entrainment on the in‐cloud sulfate production. For the adiabatic cases the departure of bulk water H2O2 from the Henry's law equilibrium is very small. When entrainment included, however, bulk water H2O2 concentrations could be clearly less than the equilibrium values, and the deficiencies are higher (>20%) for droplets larger than 10 μm radius. Our results suggest that entrainment could be one of the important factors to account for the measured H2O2 deficiency in cloud water.
Overview of atmospheric conditions during the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) field experiment
Authors: Nobre Carlos A. , Mattos Luiz F. , Dereczynski Claudine P. , Tarasova Tatiana A. , Trosnikov Igor V.
Published: February 2004

The Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) field experiment was conducted in central Brazil and southern Amazonia during the period August 15 to September 20, 1995. This paper presents an overview of atmospheric conditions during the SCAR‐B period. A meteorological office was established in Brasília to support the mission flights and field activities. All the meteorological data collected during the SCAR‐B have been archived and are available to the scientific community. The meteorological conditions throughout the SCAR‐B period of August and September 1995 were near climatological conditions. Few synoptic‐scale waves traveling from southeastern Pacific reached Brazil, therefore the SCAR‐B period was characterized by long periods of low humidity, dryness, little cloudiness, or rain, and haze events. The stable situation was interrupted due to the penetration of a frontal system on September 20, 1995, and a second and more rain‐producing one on September 28, 1995. Those two frontal systems in a period of 10 days brought an end to the dry season. Air particle trajectories show that most of the smoke below 4 km from Amazonia was transported to the south and then to the east exiting the continent at 25°S–30°S. Radiative transfer calculations carried out for the clear sky gaseous and aerosol atmosphere show the decrease of the solar radiation absorption in the atmosphere‐surface system due to smoke aerosol loading at the value from 5 to 50 W m−2 depending on the aerosol optical thickness, single‐scattering albedo, and solar zenith angle.
Global trends and emission estimates of CCl4 from in situ background observations from July 1978 to June 1996
Authors: Simmonds P. G. , Cunnold D. M. , Weiss R. F. , Prinn R. G. , Fraser P. J. , McCulloch A. , Alyea F. N. , O'Doherty S.
Published: February 2004

Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment/Advanced Global Atmospheric Gases Experiment (ALE/GAGE/AGAGE) measurements of CCl4 at five remote surface locations from 1978 to 1996 are reported. The Scripps Institution of Oceanography (SIO) 1993 absolute calibration scale is used, reducing the concentrations by a factor of 0.77 compared to previous ALE/GAGE reports. Atmospheric concentrations of CCl4 reached a peak in 1989–1990 of 104.4±3.1 parts per trillion (ppt) and have since been decreasing 0.7±0.1 ppt yr−1. Assuming an atmospheric lifetime of 42±12 years, the emissions averaged 94−11+22 × 106 kg from 1979 to 1988 and 49−13+26 × 106 kg frorn 1991 to 1995. The reduction in the emissions in 1989–1990 coincided with a substantial decrease in the global production of the chlorofluorocarbons (CFCs). The total emission of CCl4 from countries that report annual production is estimated to have declined from 11% in 1972 to 4% in 1995 of the CCl4 needed to produce the CFC amounts reported. This implies that nonreporting countries released substantial amounts of CCl4 into the atmosphere in the 1980s and that their releases have exceeded those from the reporting countries since 1991.
Significant reduction in the stratospheric ozone deficit using a three‐dimensional model constrained with UARS data
Authors: Khosravi Rashid , Brasseur Guy P. , Smith Anne K. , Rusch David W. , Waters Joe W. , Russell James M.
Published: February 2004

We present a near‐global analysis of the ozone deficit problem by constraining a fully diurnal, three‐dimensional, chemical, radiative, transport model of the middle atmosphere with colocated UARS measurements of ClO, NOx, H2O, and CH4. The domain of the study covers a wide range of altitudes (37.4–49.6 km) and latitudes (62.5°S–27.5°N), for the period of January–February 1992. In this domain, the baseline (no constraints with measurements) model temperatures are mostly warmer than U.K. Meteorological Office (UKMO) observations (by up to 5 K), and the baseline O3 mixing ratios are underestimated by 10 to 25% relative to HALOE measurements. Also, in this domain the model/data discrepancies in concentrations of the ozone‐relevant species are as follows: [H2O] and [NOx] are mostly in good agreement, [CH4] is underestimated by 10–60%, and [ClO] is overestimated by 1.3 to 3 times. We find the following: (1) Constraining the model with UKMO temperatures eliminates about 3–10% of the deficit in the 40–48 km altitude range. (2) Constraining the model with observed NOx or H2O (in addition to temperature) has minimal effect on the ozone deficit in most parts of the domain. (3) When the model temperature and ClO profiles are constrained with observations, the deficit is reduced to about 5–15%, bringing the model ozone predictions in the 40 km region to within the uncertainties of HALOE ozone measurements. (4) A 40% reduction in the rate constant of HO2 + O → OH + O2, in addition to constraining T, NOx, and ClO, eliminates the deficit in portions of the 40 km region and in the upper stratosphere, but it results in 5–10% excess ozone near the equatorial stratopause. (5) When the model methane profile is constrained with HALOE observations and a 6% HCl + O2 channel for the ClO + OH reaction is included in the chemistry, the model ClO abundance agrees well with MLS measurements in most parts of the domain. Further improvement in the ClO abundance can be obtained by decreasing [OH] through reducing the rate constant of the HO2 + O → OH + O2 reaction.
Comparison of directly measured CCN with CCN modeled from the number‐size distribution in the marine boundary layer during ACE 1 at Cape Grim, Tasmania
Authors: Covert David S. , Gras John L. , Wiedensohler Alfred , Stratmann Frank
Published: February 2004

Cloud condensation nucleus concentration (CCN) was measured directly at a supersaturation of 0.5% with a thermal gradient diffusion cloud chamber at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1) field study in November and December of 1995. Number‐size distributions N(Dp) from 3 to 800 nm diameter and the hygroscopic properties of the aerosol in the 30 to 300 nm diameter range (which contains most of the CCN active at 0.5%) were measured concurrently at the same location. This data set provides a basis to compare measured and modeled CCN concentrations. A critical particle diameter that would form cloud droplets at 0.5% supersaturation was derived from the hygroscopic growth data including consideration of the hydration of the size distribution measurement. This empirically derived diameter incorporates the effects of soluble and insoluble mass as well as an effective van't Hoff factor and surface tension as described by Köhler theory for heterogeneous nucleation of cloud droplets. The size distributions were integrated for diameters greater than the critical value and compared to the directly measured CCN concentrations. The modeled CCN concentration was 95 cm−3 during baseline sector periods and 128 cm−3 overall. This was about 20% greater than the directly measured CCN concentration and well correlated (R2 = 0.7) with measured CCN. Two thirds of the CCN at 0.5% supersaturation derive from an accumulation mode (80 nm < Dp < 200 nm); with one sixth from each of the other two modes, a larger mode (Dp > 200 nm) and the Aitken mode (Dp < 80 nm). The data include air masses from continental, anthropogenic, and biomass burning sources as well as the more dominant marine source.
An improved rainfall disaggregation technique for GCMs
Authors: Onof C. , Mackay N. G. , Oh L. , Wheater H. S.
Published: February 2004

Meteorological models represent rainfall as a mean value for a grid square so that when the latter is large, a disaggregation scheme is required to represent the spatial variability of rainfall. In general circulation models (GCMs) this is based on an assumption of exponentiality of rainfall intensities and a fixed value of areal rainfall coverage, dependent on rainfall type. This paper examines these two assumptions on the basis of U.K. and U.S. radar data. Firstly, the coverage of an area is strongly dependent on its size, and this dependence exhibits a scaling law over a range of sizes. Secondly, the coverage is, of course, dependent on the resolution at which it is measured, although this dependence is weak at high resolutions. Thirdly, the time series of rainfall coverages has a long‐tailed autocorrelation function which is comparable to that of the mean areal rainfalls. It is therefore possible to reproduce much of the temporal dependence of coverages by using a regression of the log of the mean rainfall on the log of the coverage. The exponential assumption is satisfactory in many cases but not able to reproduce some of the long‐tailed dependence of some intensity distributions. Gamma and lognormal distributions provide a better fit in these cases, but they have their shortcomings and require a second parameter. An improved disaggregation scheme for GCMs is proposed which incorporates the previous findings to allow the coverage to be obtained for any area and any mean rainfall intensity. The parameters required are given and some of their seasonal behavior is analyzed.
Intercomparison of pulsed lidar data with flight level CW lidar data and modeled backscatter from measured aerosol microphysics near Japan and Hawaii
Authors: Cutten D. R. , Spinhirne J. D. , Menzies R. T. , Bowdle D. A. , Srivastava V. , Pueschel R. F. , Clarke A. D. , Rothermel J.
Published: February 2004

Aerosol backscatter coefficient data were examined from two flights near Japan and Hawaii undertaken during NASA's Global Backscatter Experiment (GLOBE) in May‐June 1990. During each of these two flights the aircraft traversed different altitudes within a region of the atmosphere defined by the same set of latitude and longitude coordinates. This provided an ideal opportunity to allow flight level focused continuous wave (CW) lidar backscatter measured at 9.11‐μm wavelength and modeled aerosol backscatter from two aerosol optical counters to be compared with pulsed lidar aerosol backscatter data at 1.06‐ and 9.25‐μm wavelengths. The best agreement between all sensors was found in the altitude region below 7 km, where backscatter values were moderately high at all three wavelengths. Above this altitude the pulsed lidar backscatter data at 1.06‐ and 9.25‐μm wavelengths were higher than the flight level data obtained from the CW lidar or derived from the optical counters, suggesting sample volume effects were responsible for this. Aerosol microphysics analysis of data near Japan revealed a strong sea‐salt aerosol plume extending upward from the marine boundary layer. On the basis of sample volume differences, it was found that large particles were of different composition compared with the small particles for low backscatter conditions.
Electric fields produced by lightning discharges
Authors: Nickolaenko Alexander P. , Hayakawa Masashi
Published: February 2004

The time domain electric field produced by the model lightning discharges in the neutral atmosphere is computed and its modifications due to geometrical changes of the stroke are examined. The combined effect of the motion of the current wave and the bending of the lightning channel results in multiple pulses. The vector of the electric field in the atmosphere acquires transient vertical inward/outward and sideways components. Therefore the vector “scans” the sky at the onset of the pulse. The effects are unobservable at the ground surface or right above the stroke. To detect these features, the airborne measurements should be performed at a distance of tens of kilometers from the discharge and at as high an altitude as possible. Quasi‐static, induction, and radiation field components of a severe stroke may trigger the modifications in the atmosphere. Simultaneously, disturbances above the storm may lead to the air density and temperature fluctuations that facilitate the initiation of the plasma processes. Since both the atmospheric disturbances and the focusing electric fields are enhanced at the periphery of a thunderstorm the luminous structures should be expected there at some distance from the parent strokes.
Airborne spectral measurements of surface anisotropy during SCAR‐B
Authors: Tsay Si‐Chee , King Michael D. , Arnold G. Thomas , Li Jason Y.
Published: February 2004

During the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) deployment, angular distributions of spectral reflectance for vegetated surfaces and smoke layers were measured using the scanning cloud absorption radiometer (CAR) mounted on the University of Washington C‐131A research aircraft. The CAR contains 13 narrowband spectral channels between 0.3 and 2.3 μm with a 190° scan aperture (5° before zenith to 5° past nadir) and 1° instantaneous field of view. The bidirectional reflectance is obtained by flying a clockwise circular orbit above the surface, resulting in a ground track ∼3 km in diameter within about 2 min. Although the CAR measurements are contaminated by minor atmospheric effects, results show distinct spectral characteristics for various types of surfaces. Spectral bidirectional reflectances of three simple and well‐defined surfaces are presented: cerrado (August 18, 1995) and dense forest (August 25, 1995), both measured in Brazil under nearly clear‐sky conditions, and thick smoke layers over dense forest (September 6 and 11, 1995). The bidirectional reflectances of cerrado and dense forest revealed fairly symmetric patterns along the principal plane, with varying maximal strengths and widths spectrally in the backscattering direction. In the shortwave‐infrared region the aerosol effect is very small due to low spectral optical depth. Also, these backscattering maxima can be seen on the bidirectional reflectance of smoke layer over dense forest. These detailed measurements of the angular distribution of spectral reflectance can be parameterized by a few independent variables and utilized to retrieve either surface characteristics or aerosol microphysical and optical properties (e.g., size distribution and single‐scattering parameters), if proper physical and radiation models are used. The spectral‐hemispherical albedo of these surfaces is obtained directly by integrating all angular measurements and is compared with the measured nadir reflectance. Using CAR nadir reflectance as a surrogate for spectral‐hemispherical albedo can cause albedos to be underestimated by 10–60%, depending on solar zenith angle.
Reducing uncertainties in δ13C analysis of tree rings: Pooling, milling, and cellulose extraction
Authors: Borella Silvio , Leuenberger Markus , Saurer Matthias , Siegwolf Rolf
Published: February 2004

Recent developments of on‐line methods have provided another boost to the determination of stable isotope ratios in organic material. Along with a significant increase in sample throughput, the sample sizes decrease, both of which are necessary conditions to acquire long time series from limited wood amounts. In view of this new technique we reconsidered the most important factors influencing the measured isotopic signature which are (1) pooling, (2) homogeneity, and (3) cellulose extraction. In most cases, pooling (i.e., mixing wood of the same year from different trees) can be made in a simple way by mixing the whole wood available because mass‐weighted and unweighted isotope measurements were the same within the error. More attention must be paid in homogenizing the sample. Theoretical considerations underpinned by experimental results suggest a fineness of 0.15 mm (115 mesh) if cellulose is extracted and 0.1 mm (165 mesh) for direct wood analysis. Many of previous studies did not achieve this fineness. We find that wood is as good a climate proxy as cellulose. This is shown by comparing correlations of wood and corresponding cellulose isotope values with meteorological data, which are identical within the uncertainty.