bluuebunny/crossref_metadata_2025 huggingface dataset:
On the origin of polar vortex air
Authors: Rosenfield Joan E. , Schoeberl Mark R.
Published: February 2004

Forward and backward three‐dimensional stratospheric polar vortex trajectory computations between fall and spring for each of the years 1992–1993 through 1999–2000 in the Northern Hemisphere and the years 1992–1999 in the Southern Hemisphere have been carried out. We find that the forward and backward trajectories give very different pictures of polar vortex descent. The backward trajectories show a complex distribution of parcels in which one population originates in the upper stratosphere and mesosphere and experiences considerable descent in the polar regions, while the remaining parcels originate at lower altitudes of the middle and lower stratosphere and are mixed into the polar regions during vortex formation. The forward trajectory calculations do not show this second population since the parcels in the forward calculation originate within the protovortex. The amount of descent experienced by the first population shows little variability from year to year, while the computed descent and mixing of the remaining parcels show considerable interannual variability due to the varying polar meteorology. Spring methane values reconstructed from Halogen Occultation Experiment (HALOE) fall data and the back trajectories compare fairly well with typical HALOE spring values. These results imply that using a comparison between prewinter and postwinter tracer profiles to estimate the amount of descent over the fall‐to‐spring time period might result in unrealistically large values of descent, since the fall profiles are not representative of the same air as the spring profiles. Additional back trajectory calculations of descent in the lower stratosphere over 3‐month time periods from spring to midwinter show that very little mixing occurs at these times and altitudes in the Northern Hemisphere during winters when the vortex is undisturbed, and in the Southern Hemisphere for all years studied. Thus, for this shorter time period, tracking constant mixing ratios of tracers within the vortex should give reasonable estimates of descent amounts, except for years when there are meteorological disturbances in the Northern Hemisphere.
A chronology of annual mean effective radii of stratospheric aerosols from volcanic eruptions during the twentieth century as derived from ground‐based spectral extinction measurements
Authors: Stothers Richard B.
Published: February 2004

Stratospheric extinction can be derived from ground‐based spectral photometric observations of the Sun and other stars (as well as from satellite and aircraft measurements available since 1979) and is found to increase after large volcanic eruptions. This increased extinction shows a characteristic wavelength dependence that gives information about the chemical composition and the effective (or area‐weighted mean) radius of the particles responsible for it. Known to be tiny aerosols constituted of sulfuric acid in a water solution, the stratospheric particles at midlatitudes exhibit a remarkable uniformity of their column‐averaged effective radiireffin the first few months after the eruption. Considering the seven largest aerosol‐producing eruptions of the twentieth century,reffat this phase of peak aerosol abundance is ∼0.3 μm in all cases. A year later,reffeither has remained about the same size (almost certainly in the case of the Katmai eruption of 1912) or has increased to ∼0.5 μm (definitely so for the Pinatubo eruption of 1991). The reasons for this divergence in aerosol growth are unknown.
Stratospheric N2O–NOy system: Testing uncertainties in a three‐dimensional framework
Authors: Olsen S. C. , McLinden C. A. , Prather M. J.
Published: February 2004

Nitrous oxide (N2O) is an important greenhouse gas and the major source of stratospheric reactive nitrogen (NOy), an active participant in the stratospheric chemistry controlling ozone depletion. Tropospheric N2O abundances are increasing at nearly 0.3% yr−1 and this increase is expected to continue in the near future as are direct stratospheric NOy perturbations, for example, from aircraft. In order to test and gain confidence in three‐dimensional (3‐D) model simulations of the stratospheric N2O‐NOy system, a simplified photochemistry for N2O and NOy is developed for use in chemistry transport models (CTMs). This chemical model allows for extensive CTM simulations focusing on uncertainties in chemistry and transport. We compare 3‐D model simulations with measurements and evaluate the effect on N2O and NOy of potential errors in model transport, in column and local ozone, and in stratospheric temperatures. For example, with the three different 3‐D wind fields used here, modeled N2O lifetimes vary from 173 to 115 years, and the unrealistically long lifetimes produce clear errors in equatorial N2O profiles. The impact of Antarctic denitrification and an in situ atmospheric N2O source are also evaluated. The modeled N2O and NOy distributions are obviously sensitive to model transport, particularly the strength of tropical upwelling in the stratosphere. Midlatitude, lower‐stratospheric NO2/N2O correlations, including seasonal amplitudes, are well reproduced by the standard model when denitrification is included. These correlations are sensitive to changes in stratospheric chemistry but relatively insensitive to model transport. The lower stratospheric NOy/N2O correlation slope gives the correct net NOy production of about 0.5 Tg N yr−1 (i.e., the cross‐tropopause flux as in the Plumb‐Ko relation) only when N2O values from 250 to 310 ppb are used. As a consequence, the Synoz calibration of the flux of O3 from the stratosphere to the troposphere needs to be corrected to 550±140 Tg O3 yr−1.
Production of stabilized Criegee intermediates and peroxides in the gas phase ozonolysis of alkenes: 1. Ethene, trans‐2‐butene, and 2,3‐dimethyl‐2‐butene
Authors: Hasson Alam S. , Orzechowska Grazyna , Paulson Suzanne E.
Published: February 2004

Ozone‐alkene reactions generate stabilized Criegee intermediates (of the form R1R2COO), which are believed to react with water molecules to form organic hydroperoxides, hydrogen peroxide and carboxylic acids. These reactions are thought to be significant sources of these environmentally important compounds, yet both the yields of stabilized Criegee intermediates and the branching ratios from their reaction with water are not well known. The formation of hydrogen peroxide and organic hydroperoxides was investigated in the gas phase ozonolysis of ethene, trans‐2‐butene, and 2,3‐dimethyl‐2‐butene for relative humidities (RH) from 0 and 80% by gas chromatography with flame ionization detection and high‐performance liquid chromatography with fluorescence detection. Additionally, yields of acetaldehyde and acetic acid from trans‐2‐butene and acetone from 2,3‐dimethyl‐2‐butene were measured. The reactions of stabilized Criegee intermediates with water were found to proceed almost entirely via organic hydroperoxide or hydrogen peroxide formation with little acid formation. Stabilized Criegee intermediate yields of 0.39, 0.24, and 0.10 were obtained for ethene, trans‐2‐butene, and 2,3‐dimethyl‐2‐butene, respectively.
A linearized radiative transfer model for ozone profile retrieval using the analytical forward‐adjoint perturbation theory approach
Authors: Landgraf Jochen , Hasekamp Otto P. , Box Michael A. , Trautmann Thomas
Published: February 2004

For the retrieval of ozone profiles from space‐borne radiance measurements, a new linearized radiative transfer model LIRA is presented. The model enables an effective linearization of the reflectance at the top of the atmosphere with respect to both the ozone density in the different layers of the model atmosphere and the Lambertian surface albedo in the UV of the solar spectrum. The linearization of the model is based on the forward‐adjoint perturbation theory, where the forward and adjoint solution of the scalar radiative transfer equation in its plane‐parallel form are achieved by employing the Gauss‐Seidel iteration technique. For clear sky and aerosol‐loaded atmospheres the model provides the reflectance as well as its derivatives with respect to ozone density with an accuracy of better than 0.02%. The derivatives with respect to surface reflection can be calculated with an error of less than 0.05%. The suitability of the model for ozone profile retrieval is demonstrated. Therefore ozone profiles are retrieved from 156 modeled radiance measurements, simulating real radiance measurements of the Global Ozone Monitoring Experiment (GOME) spectrometer in the UV. The comparison of the retrieved profiles using the proposed model LIRA with a reference retrieval shows small deviations in the stratosphere and upper troposphere of less than 1% and tolerable differences in the middle and lower troposphere of up to 10% in the mean profile at ground level.
Analysis of smoke impact on clouds in Brazilian biomass burning regions: An extension of Twomey's approach
Authors: Feingold Graham , Remer Lorraine A. , Ramaprasad Jaya , Kaufman Yoram J.
Published: February 2004

Satellite remote sensing of smoke aerosol‐cloud interaction during the recent Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) experiment is analyzed to explore the factors that determine the magnitude of the cloud response to smoke aerosol. Analysis of 2 years worth of data revealed that the response is greatest in the north of Brazil where aerosol optical depth is smallest, and tends to decrease as one moves southward, and as aerosol optical depth increases. Saturation in this response occurs at an aerosol optical depth of 0.8 in 1987 and 0.4 in 1995. To explore the reasons for this, a framework is developed in which the satellite‐measured response can be compared to simple analytical models of this response and to numerical models of smoke aerosol‐cloud interaction. Three types of response are identified: (1) cloud droplet concentrations increase with increasing aerosol loading, followed by saturation in the response at high concentrations; (2) as in type 1, followed by increasing droplet concentrations with further increases in aerosol loading. This increase in droplet concentration is due to the suppression of supersaturation by abundant large particles, which prevents the activation of smaller particles. This enables renewed activation of larger particles when smoke loadings exceed some threshold; (3) as in type 1, followed by a decrease in droplet number concentrations with increasing aerosol loading as intense competition for vapor evaporates the smaller droplets. The latter implies an unexpected increase in drop size with increasing smoke loading. The conditions under which each of these responses are expected to occur are discussed. It is shown that although to first‐order smoke optical depth is a good proxy for aerosol indirect forcing, under some conditions the size distribution and hygroscopicity can be important factors. We find no evidence that indirect forcing depends on precipitable water vapor.
Asian chemical outflow to the Pacific in spring: Origins, pathways, and budgets
Authors: Bey Isabelle , Jacob Daniel J. , Logan Jennifer. A. , Yantosca Robert M.
Published: February 2004

We analyze the Asian outflow of CO, ozone, and nitrogen oxides (NOx) to the Pacific in spring by using the GEOS‐CHEM global three‐dimensional model of tropospheric chemistry and simulating the Pacific Exploratory Mission‐West (PEM‐West B) aircraft mission in February–March 1994. The GEOS‐CHEM model uses assimilated meteorological fields from the NASA Goddard Earth Observing System (GEOS). It reproduces relatively well the main features of tropospheric ozone, CO, and reactive nitrogen species observed in PEM‐West B, including latitudinal and vertical gradients of the Asian pollution outflow over the western Pacific although simulated concentrations of CO tend to be too low (possibly because biogenic sources are underestimated). We use CO as a long‐lived tracer to diagnose the processes contributing to the outflow. The highest concentrations in the outflow are in the boundary layer (0–2 km), but the strongest outflow fluxes are in the lower free troposphere (2–5 km) and reflect episodic lifting of pollution over central and eastern China ahead of eastward moving cold fronts. This frontal lifting, followed by westerly transport in the lower free troposphere, is the principal process responsible for export of both anthropogenic and biomass burning pollution from Asia. Anthropogenic emissions from Europe and biomass burning emissions from Africa make also major contributions to the Asian outflow over the western Pacific; European sources dominate in the lower troposphere north of 40°N, while African sources are important in the upper troposphere at low latitudes. For the period of PEM‐West B (February–March) we estimate that fossil fuel combustion and biomass burning make comparable contributions to the budgets of CO, ozone, and NOx. in the Asian outflow. We find that 13% of NOx emitted in Asia is exported as NOx or PAN, a smaller fraction than for the United States because of higher aerosol concentrations that promote heterogeneous conversion of NOx to HNO3. Production and export of ozone from Asia in spring is much greater than from the United States because of the higher photochemical activity.
Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation
Authors: Bey Isabelle , Jacob Daniel J. , Yantosca Robert M. , Logan Jennifer A. , Field Brendan D. , Fiore Arlene M. , Li Qinbin , Liu Honguy Y. , Mickley Loretta J. , Schultz Martin G.
Published: February 2004

We present a first description and evaluation of GEOS‐CHEM, a global threedimensional (3‐D) model of tropospheric chemistry driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Data Assimilation Office (DAO). The model is applied to a 1‐year simulation of tropospheric ozone‐NOx‐hydrocarbon chemistry for 1994, and is evaluated with observations both for 1994 and for other years. It reproduces usually to within 10 ppb the concentrations of ozone observed from the worldwide ozonesonde data network. It simulates correctly the seasonal phases and amplitudes of ozone concentrations for different regions and altitudes, but tends to underestimate the seasonal amplitude at northern midlatitudes. Observed concentrations of NO and peroxyacetylnitrate (PAN) observed in aircraft campaigns are generally reproduced to within a factor of 2 and often much better. Concentrations of HNO3 in the remote troposphere are overestimated typically by a factor of 2–3, a common problem in global models that may reflect a combination of insufficient precipitation scavenging and gas‐aerosol partitioning not resolved by the model. The model yields an atmospheric lifetime of methylchloroform (proxy for global OH) of 5.1 years, as compared to a best estimate from observations of 5.5 +/− 0.8 years, and simulates H2O2 concentrations observed from aircraft with significant regional disagreements but no global bias. The OH concentrations are ∼20% higher than in our previous global 3‐D model which included an UV‐absorbing aerosol. Concentrations of CO tend to be underestimated by the model, often by 10–30 ppb, which could reflect a combination of excessive OH (a 20% decrease in model OH could be accommodated by the methylchloroform constraint) and an underestimate of CO sources (particularly biogenic). The model underestimates observed acetone concentrations over the South Pacific in fall by a factor of 3; a missing source from the ocean may be implicated.
Solar contamination effects on the infrared channels of the advanced very high resolution radiometer (AVHRR)
Authors: Cao Changyong , Weinreb Michael , Sullivan Jerry
Published: February 2004

Solar impingement on the advanced very high resolution radiometers (AVHRRs) near the terminator can contaminate the onboard radiometric calibration system and degrade AVHRR data. The solar contamination causes disagreement between the sensor‐measured radiometric output of the onboard blackbody versus its bulk temperature measured by the platinum resistance thermometers. Despite the sun shield installed on the latest AVHRR unit, solar contamination can still contribute errors of more than 0.5 K in the 3.7 μm channel and 0.25 K in the long‐wave infrared channels. The timescale and spectral characteristics of the contamination are analyzed to find the possible causes. In addition, stray light in Earth scenes is found whenever the radiometric calibration is contaminated. These effects occur as the spacecraft moves out of the shadow of the Earth at spacecraft sunrise. The intensity of the effects varies by orbit and season and is related to the solar zenith and azimuth angles at the spacecraft.
Introduction to special section: Vostok
Authors: Kotlyakov V. M. , Lorius C. , Palais J. , Raynaud D.
Published: February 2004

The Russian Vostok Station is located on the high Antarctic plateau, at 78°29′S, 106°49′E, in one of the most remote parts of the world. Vostok Station was established in 1957 during the International Geophysical Year (IGY), and ice‐drilling activities started there in 1970. The environmental conditions at Vostok Station are very severe (elevation, 3490 m; mean annual temperature, −55.4°C), and the working conditions are extremely difficult. Adding to the extreme climatic conditions, delays in acquiring a complete climate record were related to problems with drilling (i.e., stuck drills and the need to start a new hole when one drill became stuck) and the infrastructure support needed from the station (i.e., a fire over the winter in the power station delayed drilling for 1 year).
OH and HO2 in the tropical Pacific: Results from PEM‐Tropics B
Authors: Tan D. , Faloona I. , Simpas J. B. , Brune W. , Olson J. , Crawford J. , Avery M. , Sachse G. , Vay S. , Sandholm S. , Guan H.‐W. , Vaughn T. , Mastromarino J. , Heikes B. , Snow J. , Podolske J. , Singh H.
Published: February 2004

OH and HO2 data collected on NASA's Pacific Exploratory Mission ‐ Tropics B (PEM‐Tropics B) are presented here and compared to results from a photochemical box model. PEM‐Tropics B took place in the tropical Pacific in March and April of 1999 and examined photochemistry and sulfur chemistry in the remote tropical atmosphere. Altitude‐resolved HOx budgets are presented. The model showed good overall agreement with the data, with a mean model to observed ratio of 0.86 for OH and 1.03 for HO2. The model tends to underpredict OH at higher altitudes and overpredict at low altitudes. The model agrees well with the HO2 observations at middle altitudes but tends to overpredict slightly at high and low altitudes.
HOx budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign
Authors: Tan D. , Faloona I. , Simpas J. B. , Brune W. , Shepson P. B. , Couch T. L. , Sumner A. L. , Carroll M. A. , Thornberry T. , Apel E. , Riemer D. , Stockwell W.
Published: February 2004

Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NOx levels and high isoprene emissions. Detailed HOx budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO2, however, is in good agreement with the measured HO2. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO2 with this added source is 15% to 30% higher than the measured HO2. Moreover, the HO2/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO2 is poorly described by the model. We discuss possible reasons for the discrepancies.
What is the concentration footprint of a tall tower?
Authors: Gloor Manuel , Bakwin Peter , Hurst Dale , Lock Loreen , Draxler Roland , Tans Pieter
Published: February 2004

Studies that have attempted to estimate sources and sinks of trace gases such as CO2 with inverse calculations unanimously identify the lack of continental stations as a prime obstacle. Continental stations have traditionally been avoided because of the difficulty of interpretation due to large time‐variability of trace substance mixing ratios. Large variability is caused by the proximity to the strongly variable sources in space and time and the complicated airflow within the lowermost 100–200 m of the planetary boundary layer. To address the need for continental stations and to overcome the problems associated with them, the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory started in 1992 to measure CO2 and other trace gases on tall television transmission towers [Bakwin et al., 1995]. An essential question in connection with these tower measurements is the area around the tower from which fluxes substantially contribute to the observed short‐term variability of trace gas mixing ratios. We present here a simple data and back trajectory‐based method to estimate the fraction of the observed short‐term variability explained by a localized flux around a tall television transmission tower in Wisconsin in dependence of its location relative to the tower (the concentration “footprint”). We find that the timescale over which the imprint of surface fluxes on air parcels before its arrival at the tower are still discernible in the mixing ratio variations observed at the tower is of the order of 1.5 days. Based on this timescale and the characteristics of air parcel trajectories, we infer a spatial extent of the footprint of the order of 106 km2, or roughly a tenth of the area of the United States. This is encouraging evidence that tall tower measurements may be useful in global inversions and may also have implications for monitoring fluxes of anthropogenic trace substances on regional scales.
Measurements of isoprene nitrates above a forest canopy
Authors: Grossenbacher John W. , Couch Tara , Shepson Paul B. , Thornberry Troy , Witmer‐Rich Maria , Carroll Mary Anne , Faloona Ian , Tan Dave , Brune William , Ostling Kristi , Bertman Steven
Published: February 2004

Measurements of atmospheric organic nitrates derived from isoprene, i.e., “isoprene nitrates”, were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measurements were conducted using on‐line chromatography in conjunction with a nitrate‐selective detection scheme. Measured concentrations of isoprene nitrates ranged from 0.5 parts per trillion (ppt), the detection limit of the method employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy, which was typically 0.5–1.5% of total odd nitrogen, but up to ∼4% for well‐aged air. Concentrations of isoprene nitrates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemistry of the precursor peroxy radicals and the NOx dependence of isoprene nitrate formation.
Evaluation of Polar MM5 simulations of Greenland's atmospheric circulation
Authors: Cassano John J. , Box Jason E. , Bromwich David H. , Li Lin , Steffen Konrad
Published: February 2004

A complete annual cycle over the Greenland ice sheet is simulated with the Polar MM5, a mesoscale model optimized for use over extensive ice sheets. These simulations are compiled from a series of short duration (48 hour), forecast mode, simulations. The model output is compared to observations primarily from the Greenland Climate Network automatic weather station (AWS) array. The model simulations show a high degree of skill for all variables evaluated with the AWS data (pressure, temperature, water vapor mixing ratio, wind speed and direction, downwelling shortwave radiation, and net radiation) for all seasons, although the use of a fixed albedo in the Polar MM5 leads to large errors in the simulated net radiation budget over melting ice surfaces during the summer months. The modeled precipitation distribution agrees with available observations in the interior of the ice sheet but is excessive along the steep margins of the island. A discussion of possible future applications of the Polar MM5 is presented.
Distribution of vegetation in wind‐dominated landscapes: Implications for wind erosion modeling and landscape processes
Authors: Okin Gregory Stewart , Gillette Dale A.
Published: February 2004

Dust emission and wind erosion from arid and semiarid environments provide a major source of global atmospheric aerosols. Well‐known relations between wind stress and saltation sand flux for sand sheets and relations between sand flux and dust emission by sandblasting have enabled construction of dust models that have only been partly successful in predicting atmospheric mineral dust concentrations. Most models of wind erosion assume that vegetation is evenly distributed. Through the use of field, Fourier transform, and semivariogram analysis, we show that mesquite dunelands in the Chihuahuan Desert of southern New Mexico, United States, have anisotropic shrub distributions. Elongated areas of bare soil, “streets,” which are aligned with the prevailing winds may partially explain discrepancies between observed and predicted atmospheric dust concentrations. Soils in the streets are not protected from winds blowing down the streets and may therefore produce more dust than if vegetation were more evenly distributed. Currently, few desert landscape evolution models take the role of wind explicitly into account. The existence of streets implies that wind plays a major role in the evolution of vegetated arid and semiarid landscapes with wind‐erodible soils. Here wind acts in tandem with water to enforce islands of fertility centered around individual shrubs and may provide an explanation for reduced soil fertility observed in shrublands. Furthermore, in order for mathematical models of dust flux to be successful in these landscapes, new landscape models are required which incorporate the existence and orientation of streets.
Trends in sea level pressure across western Canada
Authors: Nkemdirim Lawrence C. , Budikova Dagmar
Published: February 2004

This paper examines recent trends in sea level pressure (SLP) in western Canada. Time series of mean monthly SLP from each of 51 stations from 1956 to 1993 were analyzed for temporal and spatial trends using a number of analytical techniques, such as simple regression and correlation, trend surface analyzes, and geographical information system. Over the period, annual SLP declined at an average rate of 0.4 hPa/100 yr. Seasonal trends were clearly marked. In winter (December, January, February) and spring (March, April, May), SLP decreased at an average rate of 1.6 hPa/100 yr and 3.3 hPa/100 yr, respectively. The opposite trend was measured in the summer (+1.1 hPa/100 yr) and the fall (+2.3 hPa/100 yr). Trends in January and March were strongly negative (−9.7 hPa/100 yr and −4.3 hPa/100 yr, respectively). Equivalent values for July and October were +0.30 hPa/100 yr and +5.9 hPa/100 yr, respectively. All trends and their statistical significance levels amplified poleward. In general, trends were larger and more consistent in the Arctic and sub‐Arctic, and least in British Columbia and the southern prairies. These observations suggest a weakening of the Arctic High in the winter and spring which, if correct, is in agreement with recent mild winters in the Canadian Prairies and the Arctic.
Scattering matrices of mineral aerosol particles at 441.6 nm and 632.8 nm
Authors: Volten H. , Muñoz O. , Rol E. , de Haan J. F. , Vassen W. , Hovenier J. W. , Muinonen K. , Nousiainen T.
Published: February 2004

We present measured scattering matrices as functions of the scattering angle in the range 5°–173° and at wavelengths of 441.6 nm and 632.8 nm for seven distinct irregularly shaped mineral aerosol samples with properties representative of mineral aerosols present in the Earth's atmosphere. The aerosol samples, i.e., feldspar, red clay, quartz, loess, Pinatubo and Lokon volcanic ash, and Sahara sand, represent a wide variety of particle size (typical diameters between 0.1 and 100 μm) and composition (mainly silicates). We investigate the effects of differences in size and complex refractive index on the light‐scattering properties of these irregular particles. In particular, we find that the measured scattering matrix elements when plotted as functions of the scattering angle are confined to rather limited domains. This similarity in scattering behavior justifies the construction of an average aerosol scattering matrix as a function of scattering angle to facilitate, for example, the use of our results for the interpretation of remote sensing data. We show that results of ray‐optics calculations, using Gaussian random shapes, are able to describe the experimental data well when taking into account the high irregularity in shape of the aerosols, even when these aerosols are rather small. Using the results of ray‐optics calculations, we interpret the differences found between the measured aerosol scattering matrices in terms of differences in complex refractive index and particle size relative to the wavelength. The importance of our results for studies of astronomical objects, such as planets, comets, asteroids, and circumstellar dust shells is discussed.
Serial soundings of electric field through a mesoscale convective system
Authors: Stolzenburg Maribeth , Marshall Thomas C. , Rust W. David
Published: February 2004

Five partial balloon soundings of electric field and thermodynamics were made in a mesoscale convective system (MCS) in 1998. Data were gathered in an updraft and outside updrafts within the convective region, in the transition zone, and in the stratiform cloud region. In the first sounding the maximum and average inferred updraft speeds (27 and 15 m s−1) are the largest found to date in soundings through MCS convection. As in previous measurements the electric field profile is simpler and fewer charge regions are inferred within the updraft than elsewhere. Together the first four soundings support in most details a previously published conceptual model of MCS electrical structure from the convective region updraft to the transition zone. Two exceptions within the soundings were found; these involve opposite polarity of the lowest two charge regions in the transition zone cloud and additional complexity above the main negative charge region. The earlier conceptual model also showed possible connections between charge regions across an MCS, but those were based on soundings through many different MCSs. The first four soundings presented herein provide stronger evidence for three to five similar charge regions across the front of this MCS. The sounding through the stratiform cloud region was atypical and does not fit the conceptual model, and no connections to charge regions in the rest of the MCS are obvious. Two soundings presented from a second MCS indicate that the upper three charge regions in the transition zone are connected to the stratiform cloud, as depicted in earlier conceptual models.
Seasonal differences in the photochemistry of the South Pacific: A comparison of observations and model results from PEM‐Tropics A and B
Authors: Olson J. R. , Crawford J. H. , Davis D. D. , Chen G. , Avery M. A. , Barrick J. D. W. , Sachse G. W. , Vay S. A. , Sandholm S. T. , Tan D. , Brune W. H. , Faloona I. C. , Heikes B. G. , Shetter R. E. , Lefer B. L. , Singh H. B. , Talbot R. W. , Blake D. R.
Published: February 2004

A time‐dependent photochemical box model is used to examine the photochemistry of the equatorial and southern subtropical Pacific troposphere with aircraft data obtained during two distinct seasons: the Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) field campaign in September and October of 1996 and the Pacific Exploratory Mission‐Tropics B (PEM‐Tropics B) campaign in March and April of 1999. Model‐predicted values were compared to observations for selected species (e.g., NO2, OH, HO2) with generally good agreement. Predicted values of HO2 were larger than those observed in the upper troposphere, in contrast to previous studies which show a general underprediction Of HO2 at upper altitudes. Some characteristics of the budgets Of HOx, NOx, and peroxides are discussed. The integrated net tendency for O3 is negative over the remote Pacific during both seasons, with gross formation equal to no more than half of the gross destruction. This suggests that a continual supply of O3 into the Pacific region throughout the year must exist in order to maintain O3 levels. Integrated net tendencies for equatorial O3 showed a seasonality, with a net loss of 1.06×1011 molecules cm−2 s−1 during PEM‐Tropics B (March) increasing by 50% to 1.60×1011 molecules cm−2 s−1 during PEM‐Tropics A (September). The seasonality over the southern subtropical Pacific was somewhat lower, with losses of 1.21×1011 molecules cm−2 s−1 during PEM‐Tropics B (March) increasing by 25% to 1.51×1011 molecules cm−2 s−1 during PEM‐Tropics A (September). While the larger net losses during PEM‐Tropics A were primarily driven by higher concentrations of O3, the ability of the subtropical atmosphere to destroy O3 was ∼30% less effective during the PEM‐Tropics A (September) campaign due to a drier atmosphere and higher overhead O3 column amounts.
Effects of 1997 Indonesian forest fires on tropospheric ozone enhancement, radiative forcing, and temperature change over the Hong Kong region
Authors: Chan C. Y. , Chan L. Y. , Zheng Y. G. , Harris J. M. , Oltmans S. J. , Christopher S.
Published: February 2004

Tropospheric ozone enhancements were measured over Hong Kong (22.2°N, 114.3°E) by electrochemical concentration cell ozonesondes during the 1997 period when many forest fires were burning in Indonesia. The enhancements have a maximum ozone concentration of up to 130 ppbv and an ozone‐enhanced layer depth of 10 km. We used Total Ozone Mapping Spectrometer, advanced very high resolution radiometer satellite image, and back air trajectory to identify the source region and the transport pattern of ozone. The strong tropospheric ozone enhancements covered all of tropical Southeast Asia and subtropical south China. They were due to photochemical ozone buildup from the biomass burning emissions from the Indonesian fires. The ozone‐rich air mass was transported to Hong Kong following the east Asia local Hadley circulation and an abnormal anticyclonic flow related to the El Niño phenomenon in the tropical western Pacific. A rough estimation of the radiative forcing due to the ozone enhancement was carried out for two cases in October and December in 1997 using a normalized tropospheric ozone radiative forcing parameter derived from the Unified Chemistry‐Climate model [Mickley et al., 1999]. The ozone enhancements induced an additional radiative forcing of 0.26 and 0.48 Wm−2 compared to the normal total forcing of 0.48 and 0.39 Wm−2 in October and December over the Hong Kong region. Estimation of the associated surface temperature change suggests that enhanced ozone from biomass burning on the scale of the 1997 Indonesian fires may have significant impact on regional surface temperature change.
Origin of extreme ozone minima at middle to high northern latitudes
Authors: Hood L. L. , Soukharev B. E. , Fromm M. , McCormack J. P.
Published: February 2004

Extreme ozone minima represent localized and temporally brief (several days) reductions in column ozone amounts below some chosen absolute level. Although such minima at middle to high northern latitudes are known to be primarily dynamical in origin, a remaining issue is whether heterogeneous chemical loss processes may also contribute significantly to their formation. A case in point is the record low 165 Dobson units (DU) minimum occurring on November 30, 1999, when temperatures near 30 hPa at the location of the minimum were lower than the threshold for the formation of type I polar stratospheric clouds (PSC). An examination of Polar Ozone and Aerosol Measurement III data for times surrounding the event shows that PSCs were indeed present in the vicinity where the ozone minimum was observed. However, archived data show that a similar extreme minimum of 167 DU with characteristics comparable to those of the November 30, 1999, minimum occurred on October 30, 1985, when no PSCs were present. An ensemble of 71 extreme ozone minima with amplitudes under 215 DU exhibit a nearly linear relationship between ozone minimum deviations from the zonal mean and corresponding 30‐hPa temperature deviations. Such a relationship is predicted by analytic transport models which assume that vertical motions (i.e., upwelling) are responsible for the ozone minima. Temperature deviations near 30‐hPa were unusually large for both the November 30, 1999, and the October 30, 1985, events, implying unusually rapid upward transport for these events. All 71 minima occur in regions where deviations from the zonal mean of 330 K potential vorticity are negative, implying an additional contribution to their formation by quasi‐horizontal transport. The timescale for column ozone reductions during extreme ozone minima events is also determined and found to be at least 20 times more rapid than expected from known chemical loss processes. The data are therefore most consistent with a purely dynamical origin for extreme ozone minima in general and the November 30, 1999, event in particular. As was shown by earlier work, the basic dynamical process involves a combination of isentropic transport of ozone‐poor air from the tropical upper troposphere and rapid upwelling over upper tropospheric anticyclonic disturbances resulting from poleward Rossby wave breaking events.
Seasonal variation of gravity wave activity at 5–20 km observed with VHF radar at White Sands Missile Range, New Mexico
Authors: Hansen Anthony R. , Nastrom Gregory D. , Eaton Frank D.
Published: February 2004

Wind observations at altitudes from 5 to 20 km obtained from the VHF wind profiler radar at the White Sands Missile Range (WS), New Mexico, are used to construct a climatology of gravity wave intensities over WS during the nearly 6‐year period from January 1991 through September 1996. The kinetic energy densities for both high‐frequency gravity waves (ground‐based period of 6 min to 2 hours) and low‐frequency gravity waves (period from 2 to 21 hours) are considered. These results are compared to earlier results from the middle and upper atmosphere (MU) radar in Japan. Comparisons of gravity wave variability at high and low frequency reveal both similarities and differences between the two sites. The primary differences occur at high frequency. The seasonal cycle in the high‐frequency gravity waves is less pronounced and less regular at WS compared to MU. The magnitude of the high‐frequency kinetic energy density is generally larger at WS compared to MU, and the relationship between the upper tropospheric jet stream winds and the high‐frequency gravity wave activity is not nearly so strong at WS as at MU. The primary high‐frequency gravity wave excitation mechanism at WS appears to change with season. There is evidence of a connection in winter and spring between the wind speeds in the middle troposphere and the high‐frequency gravity wave activity, suggesting a topographic source for these waves. Strong summertime high‐frequency variability at WS in the upper troposphere is most likely linked to waves launched by deep convection during the monsoon season in New Mexico. The results for low‐frequency gravity waves are more consistent between WS and MU. Similarities appear in the kinetic energy density, the nature of the seasonal cycle, and the correlation between the kinetic energy density and the mean winds.
Influence of cloud‐radiation interaction on simulating tropical intraseasonal oscillation with an atmospheric general circulation model
Authors: Lee Myong‐In , Kang In‐Sik , Kim Jong‐Khun , Mapes Brian E.
Published: February 2004

The influence of cloud‐radiation interaction in simulating the tropical intraseasonal oscillation (ISO) is examined using an aqua planet general circulation model (GCM). Two types of simulation are conducted: one with prescribed zonal mean radiation and the other with fully interactive clouds and radiation. In contrast to the fixed radiation case, where the ISO is simulated reasonably well, the cloud‐radiation interaction significantly contaminates the eastward propagation of the ISO by producing small‐scale disturbances moving westward with the easterly basic winds. The small‐scale disturbances are persistently excited by a strong positive feedback through interaction between cumulus‐anvil clouds and radiation. The longwave interaction is shown to play a bigger role in contaminating the ISO than the shortwave interaction does. The anvil clouds reduce the longwave cooling significantly in the lower troposphere while releasing latent heating in the upper troposphere. To moderate the strong cloud‐radiation feedback, the large‐scale condensation scheme in the GCM is modified by reducing the autoconversion timescale, needed for cloud condensates to grow up to rain drops. In addition, upper air ice cloud contents are reduced to change the cloud albedo. These modifications make a more realistic simulation of the ISO similar to the observed.
On different approaches to calculating lightning electric fields
Authors: Thottappillil Rajeev , Rakov Vladimir A.
Published: February 2004

Three different approaches to the computation of lightning electric fields are compared. These approaches are the traditional dipole (Lorentz condition) technique and two versions of the monopole (continuity equation) technique. The latter two techniques are based on two different formulations of the continuity equation, one used by Thottappillil et al. [1997] and the other by Thomson [1999], the difference between the formulations being related to different treatments of retardation effects. The three approaches involve the same expression for the vector potential but different expressions for the scalar potential. It is analytically shown that the three different expressions for the scalar potential are equivalent and satisfy the Lorentz condition. Further, the three approaches yield the same total fields and the same Poynting vectors. However, expressions in the three approaches for the individual electric field components in the time domain, traditionally identified by their distance dependence as electrostatic, induction, and radiation terms, are different, suggesting that explicit distance dependence is not an adequate identifier. It is shown that the so identified individual field components in the electric field equation in terms of charge density derived by Thottappillil et al. [1997] are equivalent to the corresponding field components in the traditional equation for electric field in terms of current based on the dipole technique. However, the individual field components in the electric field equation based on Thomson's [1999] approach are not equivalent to their counterparts in the traditional dipole technique equation. Further, in Thottappillil et al.'s [1997] technique and in the traditional dipole technique, the gradient of scalar potential contributes to all three electric field components, while in Thomson's [1999] technique it contributes only to the electrostatic and induction components. Calculations of electric fields at different distances from the lightning channel show that the differences between the corresponding field components identified by their distance dependence in different techniques are considerable at close ranges but become negligible at far ranges.
A general climate model simulation of the aerosol radiative effects of the Laacher See eruption (10,900 B.C.)
Authors: Graf H.‐F. , Timmreck C.
Published: February 2004

A simulation of radiative forcing and zonal mean temperature effects is presented for the Laacher See eruption (10,900 B.C.). A climate model which also covers the middle atmosphere is forced with background conditions representative of the Younger Dryas period. The Laacher See eruption is simulated by the injection of 15 Mt of SO2 to the lower stratosphere. The resulting sulfate loading in the stratosphere is comparable with post‐Pinatubo (1991) observations, but clearly restricted to the Northern Hemisphere. The sulfate aerosol decays with an e‐folding time of approximately 11 months, that is, also similar to recent observations. The high concentration of aerosol in the high latitudes leads to strongly intensified cooling in the polar night and to an intensified polar vortex. This produces midlatitude “continental winter warming” in the troposphere as observed after tropical eruptions. After tropical eruptions, strong absorption of terrestrial and near‐infrared solar radiation in the lower latitudes is responsible for the strengthening of the polar vortex. However, for midlatitude and high‐latitude eruptions it is the intense irradiance of longwave radiation from the aerosol‐containing layer at polar latitudes that produces the meridional thermal gradient enhancing the polar vortex.
Modeling spatially distributed ecosystem flux of boreal forest using hyperspectral indices from AVIRIS imagery
Authors: Rahman Abdullah F. , Gamon John A. , Fuentes David A. , Roberts Dar A. , Prentiss Dylan
Published: February 2004

Correct estimation of spatially distributed CO2flux is of utmost importance for regional and global carbon balance studies. Tower‐based instruments provide flux data from a small footprint area and may not be suitable for spatial extrapolation over areas not represented by the towers. In this study we developed a method of combining optical indices from remotely sensed hyperspectral images with flux data from towers covering different vegetation types to make spatially continuous maps of gross CO2fluxes. Using a simple light‐use efficiency model, we tested the ability of spectral indices derived from Airborne Visible Infrared Imaging Spectrometer (AVIRIS) imagery to estimate photosynthetic fluxes of several boreal forest stands. Because CO2flux from terrestrial ecosystems is dependent on both vegetation cover and physiological state, we hypothesized that measures of both forest structure and physiology were important for flux estimation. Consequently, the modeled fluxes considered both the normalized difference vegetation index (NDVI) and a scaled value of the photochemical reflectance index (PRI), both derived from narrowband reflectance. NDVI alone was of limited use in describing the variation in ecosystem fluxes (R2= 0.26). Addition of the PRI, which is related to xanthophyll cycle pigment activity and unrelated to NDVI, improved the agreement between modeled and measured fluxes (R2= 0.82). Our results also indicated that simple extrapolation of point‐based flux tower data to represent the large‐area fluxes of boreal forest may lead to an underestimation of the spatially distributed fluxes, at least for the vegetation types studied in this analysis.