bluuebunny/crossref_metadata_2025 huggingface dataset:
In‐flight measurement of aircraft CO and nonmethane hydrocarbon emission indices
Authors: Slemr F. , Giehl H. , Habram M. , Slemr J. , Schlager H. , Schulte P. , Haschberger P. , Lindermeir E. , Döpelheuer A. , Plohr M.
Published: February 2004

Emission indices were derived from in‐flight measurements of CO, nonmethane hydrocarbons (NMHCs), H2O, and nonvolatile condensation nuclei in the exhaust plumes of the Deutsches Zentrum für Luft‐ und Raumfahrt VFW 614 (ATTAS) and NASA DC‐8 experimental aircraft. CO emission indices, EIs(CO), of the ATTAS Rolls Royce M 45H Mk501 engines were determined concurrently by two independent techniques: monitoring of exhaust emissions using a customized Fourier transform infrared spectrometer (FTIR) and by simultaneous continuous fast CO and CO2 measurements. The EIs(CO) determined by FTIR were systematically 28% lower than those derived from the CO/CO2 concentration ratios. The EIs(CO) of the newer and larger CFM 56‐2C1 engines, used on DC‐8, were substantially smaller than those of the ATTAS engines. The emission behavior of CFM 56‐2C1 engines is very similar to CFM 56‐3 engines frequently used on Boeing 737 aircraft. In‐flight derived EIs(CO) of the ATTAS engines were strongly dependent on the fuel flow rate and agreed well with those calculated from ground‐based measurements. Emission indices for individual NMHCs were determined from the concentration ratios of NMHC/CO in the plume of ATTAS and DC‐8 and from the EIs(CO) determined by FTIR or derived from the concentration ratios of CO/CO2. The EIs(NMHC) are highest for alkenes and alkynes generated by a cracking of larger fuel molecules and for benzene from unburnt fuel, and they depend strongly on the power setting of the engines. As with EIs(CO), the EIs(NMHC) of the CFM 56‐2C1 engine tend to be smaller than those of the Rolls Royce M 45H Mk501.
Uncertainty in preindustrial abundance of tropospheric ozone: Implications for radiative forcing calculations
Authors: Mickley Loretta J. , Jacob Daniel J. , Rind David
Published: February 2004

Recent model calculations of the global mean radiative forcing from tropospheric ozone since preindustrial times fall in a relatively narrow range, from 0.3 to 0.5 W m−2. These calculations use preindustrial ozone fields that overestimate observations available from the turn of the nineteenth century. Although there may be calibration problems with the observations, uncertainties in model estimates of preindustrial natural emissions must also be considered. We show that a global three‐dimensional model of tropospheric chemistry with reduced NOx emissions from lightning (1–2 Tg N yr−1) and soils (2 Tg N yr−1) and increased emissions of biogenic hydrocarbons can better reproduce the nineteenth century observations. The resulting global mean radiative forcing from tropospheric ozone since preindustrial times is 0.72–0.80 W m−2, amounting to about half of the estimated CO2 forcing. Reduction in the preindustrial lightning source accounts for two thirds of the increase in the ozone forcing. Because there is near‐total titration of OH by isoprene in the continental boundary layer of the preindustrial atmosphere, isoprene and other biogenic hydrocarbons represent significant ozone sinks. The weak or absent spring maximum in the nineteenth century observations of ozone is difficult to explain within our understanding of the natural ozone budget. Our results indicate that the uncertainty in computing radiative forcing from tropospheric ozone since preindustrial times is larger than is usually acknowledged.
Acetone and acetonitrile in the tropical Indian Ocean boundary layer and free troposphere: Aircraft‐based intercomparison of AP‐CIMS and PTR‐MS measurements
Authors: Sprung Detlev , Jost Christof , Reiner Thomas , Hansel Armin , Wisthaler Armin
Published: February 2004

We have performed aircraft‐borne intercomparison measurements of acetone and acetonitrile using two different mass spectrometric techniques: A Proton Transfer Reaction Mass Spectrometer (PTR‐MS) operated and developed by the Institute of Ion Physics, University of Innsbruck, and an Atmospheric Pressure Chemical lonization Mass Spectrometer (AP‐CIMS) operated and developed by the Max Planck Institute for Chemistry, Mainz. The two devices were employed for atmospheric trace gas measurements over the Indian Ocean on board the National Center for Atmospheric Research C‐130 research aircraft during the Indian Ocean Experiment (INDOEX) intensive field phase in February and March 1999. Both instruments use reactions of ions with atmospheric trace gas molecules and mass spectrometric detection techniques for the determination of trace gas concentrations. For the detection of acetone and acetonitrile, both mass spectrometers operate in the positive ion mode and detect the protonated species of these compounds. One difference of the two systems is the operating pressure of the reaction chamber, where the ion molecule reactions take place. While the PTR‐MS uses a low‐pressure (2.2 hPa) drift tube, a much higher pressure of 300 hPa is used by the AP‐CIMS. Other differences are the determination of background concentrations and the calibration procedure. The AP‐CIMS uses a charcoal filter, and the PTR‐MS uses a Pt catalyst. For the AP‐CIMS an on‐line calibration is necessary, which is performed during the flights. The intercomparison measurements described here were carried out on two research flights along a north‐south transect along 73°E between 6°N and 8°S. Altitudes between 100 m and 6500 m above sea level were covered. Backward trajectory analyses indicate that the sampled air was taken from polluted air masses originating from India and the Bay of Bengal as well as from remote southern hemispheric air. During the two flights on February 20 and 24 the position of the Intertropical Convergence Zone was located around 1.50S in the investigated area. The measurements of both instruments of acetone and acetonitrile indicate a strong gradient in the boundary layer and a weaker one in the free troposphere. The values measured by the PTR‐MS are higher than the ones by the AP‐CIMS, but are within the combined error range of ±20–30% for each instrument. Acetone mixing ratios up to 2.5 parts per billion (ppb) and acetonitrile up to 0.4 ppb were measured in the northern hemispheric boundary layer. The measurements in the free troposphere give values between 0.3 ppb and 1.0 ppb for acetone and 0.12 ppb and 0.3 ppb for acetonitrile.
Aerosol optical properties and large‐scale transport of air masses: Observations at a coastal and a semiarid site in the eastern Mediterranean during summer 1998
Authors: Formenti P. , Andreae M. O. , Andreae T. W. , Galani E. , Vasaras A. , Zerefos C. , Amiridis V. , Orlovsky L. , Karnieli A. , Wendisch M. , Wex H. , Holben B. N. , Maenhaut W. , Lelieveld J.
Published: February 2004

Spectral measurements of the aerosol particle scattering coefficient σs and the aerosol optical depth τa were conducted at Ouranoupolis (Greece, 40°23′N, 23°57′E) and at Sde Boker (Israel, 30°51′N, 34°47′E) between June and September 1998. Measurements were related to 5‐day three‐dimensional back trajectories at 950, 850, and 550 hPa to assess the influence of long‐range transport from particular source regions to the aerosol load at the two sites. Our measurements show that the eastern Mediterranean basin is moderately to highly polluted during summer. Daily average σs values at 550 nm were typically in the range of 30–200 Mm−1 at both sites. The range obtained for the summer regional aerosol optical depth τa was 0.03–0.52 at 500 nm. Enhanced aerosol extinction was related to transport of polluted air masses from western and eastern Europe. High‐altitude transport of mineral dust from northern Africa was observed at both sites, particularly in Israel.
Determination of precipitable water and cloud liquid water over oceans from the NOAA 15 advanced microwave sounding unit
Authors: Grody Norman , Zhao Jiang , Ferraro Ralph , Weng Fuzhong , Boers Reinout
Published: February 2004

The advanced microwave sounding unit (AMSU) was finally launched in May 1998 aboard the NOAA 15 satellite. Algorithms are provided for retrieving the total precipitable water (TPW) and cloud liquid water (CLW) over oceans using the AMSU measurements at 23.8 and 31.4 GHz. Extensive comparisons are made between the AMSU retrievals of CLW and TPW and those obtained using other satellite instruments (Special Sensor Microwave Imager (SSM/I) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI)) and ground‐based radiometers. The AMSU TPW is also compared against radiosonde data, where all of the results are in good agreement with rms differences less than 3 mm and biases less than 1 mm over the range between 5 and 60 mm. The CLW comparisons show greater variability, although the time series of the AMSU and ground‐based sensors follow each other and cover the same dynamic range of 0–0.5 mm. The AMSU CLW also compares well with the other satellite measurements, although a bias exists between AMSU and TMI when the CLW exceeds 0.5 mm.
Airborne springtime IOP measurements of radiative exchange and albedos in the Barrow, Alaska, region and comparisons to growing season results
Authors: Brooks Steven B. , Hastings Steven J.
Published: February 2004

On April 8, 1999, at approximately local solar noon, airborne measurements of longwave and shortwave radiation were conducted in the Barrow, Alaska, region. Over the pack ice (100 km north of Point Barrow), albedo averaged 0.83, net radiation flux (average 32.4 W m−2) was into the surface, and surface temperatures were positively correlated to observed cloudiness and measured sky temperatures. Over the coastal “fast” ice and tundra, albedo averaged 0.90, net radiation flux (average 2.7 W m−2) was out of the surface, and surface temperatures were not significantly correlated to observed cloudiness or measured sky temperatures. Albedo was not a significant function of observed variation in cloudiness or sky temperature but may have been affected by measurable snowfall the previous day at Barrow. Compared to growing season albedos, springtime albedos had lower coefficients of variation.
Effects of simulated climate change on the hydrology of major river basins
Authors: Arora Vivek K. , Boer George J.
Published: February 2004

Changes in the climatology of precipitation, evapotranspiration, and soil moisture lead also to changes in runoff and streamflow. The potential effects of global warming on the hydrology of 23 major rivers are investigated. The runoff simulated by the Canadian Centre for Climate Modeling and Analysis (CCCma) coupled climate model for the current climate is routed through the river system to the river mouth and compared with results for the warmer climate simulated to occur towards the end of the century. Changes in mean discharge, in the amplitude and phase of the annual streamflow cycle, in the annual maximum discharge (the flood) and its standard deviation, and in flow duration curves are all examined. Changes in flood magnitudes for different return periods are estimated using extreme value analysis. In the warmer climate, there is a general decrease in runoff and 15 out of the 23 rivers considered experience a reduction in annual mean discharge (with a median reduction of 32%). The changes in runoff are not uniform and discharge increases for 8 rivers (with a median increase of 13%). Middle‐ and high‐latitude rivers typically show marked changes in the amplitude and phase of their annual cycle associated with a decrease in snowfall and an earlier spring melt in the warmer climate. Low‐latitude rivers exhibit changes in mean discharge but modest changes in their annual cycle. The analysis of annual flood magnitudes show that 17 out of 23 rivers experience a reduction in mean annual flood (a median reduction of 20%). Changes in flow duration curves are used to characterize the different kinds of behavior exhibited by different groups of rivers. Differences in the regional distribution of simulated precipitation and runoff for the control simulation currently limit the application of the approach. The inferred hydrological changes are, nevertheless, plausible and consistent responses to simulated changes in precipitation and evapotranspiration and indicate the kinds of hydrological changes that could occur in a warmer climate.
Using cloud radar polarization measurements to evaluate stratus cloud and insect echoes
Authors: Martner Brooks E. , Moran Kenneth P.
Published: February 2004

Suspended insects and other nonhydrometeor particulates are commonly detected in and near the planetary boundary layer by high‐sensitivity millimeter‐wave cloud radars. Radar reflectivity patterns of these targets are difficult to discern from those of stratus clouds, and therefore these particles represent undesirable contaminants for automated cloud‐detection algorithms. Radar polarization measurements, however, offer a means of distinguishing between cloud droplets and these contaminants, based on the shape information contained in depolarization ratio observations. A procedure is presented which allows dual‐polarization cloud radar data and simultaneous lidar and laser ceilometer data to determine the presence of cloud droplets alone, insects alone, and mixtures of the two at the Atmospheric Radiation Measurement program's Cloud and Radiation Test bed sites. Theory is also developed for estimating the contribution of cloud droplets to the observed reflectivity of an insect plus droplet mixture using dual‐polarization cloud radar data. Preliminary data indicate that the method provides more accurate estimates if the radar transmits circular rather than linear polarization because the effects of target orientation are minimized.
Positive charge in the stratiform cloud of a mesoscale convective system
Authors: Marshall Thomas C. , Stolzenburg Maribeth , Rust W. David , Williams Earle R. , Boldi Robert
Published: February 2004

A balloon sounding of electric field in the trailing stratiform cloud of a bow echo mesoscale convective system reveals only two substantial in‐cloud positive charge regions. These charge regions are located at altitudes of 5.1–5.6 km and 6.4–6.8 km, above the level of 0°C at 4.2 km. The two positive charge regions are the likely sources of six positive cloud‐to‐ground flashes with large peak currents (>32 kA) that occurred within 60 km of the balloon during its flight. The amount of charge transferred by three of these positive flashes that made Q bursts is calculated in the range of 97–196 C. Flashes of this sort are known to produce sprites and elves in the mesosphere. The positive charge regions in this stratiform cloud are substantially lower than the 10‐km altitude commonly assumed for the positive charge in many sprite modeling studies.
Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study
Authors: Stroud C. A. , Roberts J. M. , Goldan P. D. , Kuster W. C. , Murphy P. C. , Williams E. J. , Hereid D. , Parrish D. , Sueper D. , Trainer M. , Fehsenfeld F. C. , Apel E. C. , Riemer D. , Wert B. , Henry B. , Fried A. , Martinez‐Harder M. , Harder H. , Brune W. H. , Li G. , Xie H. , Young V. L.
Published: February 2004

Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK), were measured at an urban forested site in Nashville, Tennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observations were performed at Cornelia Fort Airpark for a 4 week period between June 13 and July 14. At the midday photochemical peak (1200 local standard time, LST), average mixing ratios of isoprene, MACR, and MVK were 410 parts per trillion by volume (pptv), 240 pptv, and 430 pptv, respectively. Median isoprene, MACR, and MVK mixing ratios were 400 pptv, 200 pptv, and 360 pptv, respectively, at 1200 LST. An emissions inventory calculation for Davidson County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissions. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one‐dimensional (1‐D) numerical box model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systematic manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were observed during SOS 1999 at Cornelia Fort (0.3–1.6 hours) compared to the SOS 1990 canopy study at Kinterbish (0.1–0.6 hours). This is consistent with the proximity of the tower inlets to the forest canopies during both campaigns. Isoprene had a chemical lifetime of 20 min at the average observed midday HO mixing ratio of 8×106 molecules/cm3. As a result, significant conversion of isoprene to its oxidation products was observed on the timescale of transport from the dense forest canopies surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated with a 1‐D photochemical box model. General agreement between the observations of MACR and MVK during SOS 1999 with the two chemical models suggests we have a comprehensive understanding of the first few stages of isoprene oxidation in this urban forested environment.
Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns: 1. Ground‐based observations at Cheeka Peak
Authors: Jaffe Dan , Anderson Theodore , Covert Dave , Trost Barbara , Danielson Jen , Simpson William , Blake Donald , Harris Joyce , Streets David
Published: February 2004

As part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) project, we made observations of CO, O3, NOx, peroxyacetyl nitrate (PAN), nonmethane hydrocarbons (NMHC), Rn, aerosol scattering, aerosol absorption, and aerosol number density during the springs of 1997 and 1998 at the Cheeka Peak Observatory (CPO) on the western tip of Washington State. The data have been segregated to quantify the mixing ratio of these species in the Pacific marine atmosphere. However, even in these marine air masses, there are occasionally substantial enhancements of NOx and aerosols, but not CO, which we attribute to diesel exhaust from ship traffic to and from major ports in the region. The marine air masses were further classified into four categories based on 10‐day back isentropic trajectories; high, mid, and low latitude and those which had crossed over the Asian industrial region. Mean marine mixing ratios in 1998 were significantly higher than the 1997 values for CO (1997 mean equal to 151, 1998 mean equal to 170 ppbv), ethane (1771, 1968 parts per trillion by volume (pptv)), and ethyne (306, 452 pptv). Also, segregation of the 1998 data by air mass origin produced smaller differences in the mixing ratios for most species when comparing different source regions. We attribute both of these results to elevated emissions associated with unusually large areas of biomass burning which took place in Indonesia and Siberia dunng late 1997 and 1998. The relative enhancements of CO, ethane, ethyne, and propane we observed at CPO are consistent with enhanced biomass burning and industrial sources in the spring of 1998, relative to the spring of 1997.
A priori knowledge accumulation and its application to linear BRDF model inversion
Authors: Li Xiaowen , Gao Feng , Wang Jindi , Strahler Alan
Published: February 2004

A priori knowledge can significantly improve the retrieval of surface bidirectional reflectance and spectral albedo from satellite observations. Here a priori knowledge takes the form of field measurements of bidirectional reflectance factors for various surface cover types in red and near‐infrared bands. Bidirectional reflectance and albedo retrieval refers to inversion of a kernel‐driven bidirectional reflectance distribution function (BRDF) model using surface reflectance observations derived from orbiting spacecraft. A priori knowledge is applied when noise and poor angular sampling reduce the accuracy of model inversion given a limited number of observations. In such cases, a priori knowledge can indicate when retrieved kernel weights or albedos are outside expected bounds, leading to a closer examination of data. If data are noisy, a priori knowledge can be used to smooth the data. If the data exhibit poor angular sampling, a priori knowledge can be used according to Bayesian inference theory to yield a posteriori estimates of unknown kernel weights. In the latter application, Bayes theory is applied in data space rather than in parameter space. Extensive study and simulation using 73 sets of field observations and 395 spaceborne observation sets from the POLDER instrument validates the importance of a priori information in improving inversions and BRDF retrievals.
Simulation of anomalous regional climate events with a variable‐resolution stretched‐grid GCM
Authors: Fox‐Rabinovitz Michael S.
Published: February 2004

The stretched‐grid approach provides an efficient downscaling and consistent interactions between global and regional scales due to using one variable‐resolution model for integrations. It is a workable alternative to the widely used nested‐grid approach introduced over a decade ago as a pioneering step in regional climate modeling. A variable‐resolution finite‐difference GCM (general circulation model) employing a stretched grid, with enhanced resolution over the United States as the area of interest, is used for simulating two anomalous regional climate events, the U.S. summer drought of 1988 and the U.S. summer flood of 1993. The special mode of integration using a stretched‐grid GCM and data assimilation system is developed which allows for imitating the nested‐grid framework within a stretched‐grid approach. The mode is useful for intercomparison purposes and for underlining the differences between these two approaches. The 1988 and 1993 integrations are performed for the 2 month period starting from mid‐May. Regional resolutions used in most of the experiments is 60 km. The major goal and the result of the study is obtaining the efficient downscaling over the area of interest. The monthly mean prognostic regional fields for the stretched‐grid integrations are remarkably close to those of the verifying analyses. Simulated precipitation patterns are successfully verified against gauge precipitation observations. The impact of finer 40 km regional resolution is investigated for the 1993 integration and an example of recovering subregional precipitation is presented. The obtained results show that the global variable‐resolution stretched‐grid approach is a viable candidate for regional and subregional climate studies and applications.
Major optical depth perturbations to the stratosphere from volcanic eruptions: Stellar extinction period, 1961–1978
Authors: Stothers Richard B.
Published: February 2004

A revised chronology of stratospheric aerosol extinction due to volcanic eruptions has been assembled for the period 1961–1978, which immediately precedes the era of dedicated satellite measurements. On the whole, the most accurate data consist of published observations of stellar extinction, supplemented in part by other kinds of observational data. The period covered encompasses the important eruptions of Agung (1963) and Fuego (1974), whose dust veils are discussed with respect to their transport, decay, and total mass. The effective (area‐weighted mean) radii of the aerosols for both eruptions are found to be 0.3–0.4 μm. It is confirmed that, among known tropical eruptions, Agung's dust was unique for a low‐latitude eruption in remaining almost entirely confined to the hemisphere of its production. A new table of homogeneous visual optical depth perturbations, listed by year and by hemisphere, is provided for the whole period 1881–1978, including the pyrheliometric period before 1961 that was investigated previously.
Spectral dependence in the cosine response of broadband UV instruments
Authors: Kuchinke Christopher , Nunez Manuel
Published: February 2004

The erythemal UV biometer, a product of the original Robertson‐Berger meter, is now widely used across the globe for routine UV measurements. They are employed to measure the erythemal response to radiation, however, the filter present in such instruments rarely mimics the erythemal curve. As such, the relative spectral response of each instrument is generally different from the erythemal response curve. Thus calibration of the detector is dependent upon wavelength as well as solar zenith angle. In this study, measurements were compared for the purpose of characterizing a new cosine response function (CRF) for the broadband detector. Concurrent measurements of spectral and broadband erythemal UV data have been undertaken at the University of Tasmania in Hobart, Australia. Measurements spanned solar zenith angles (SZA) of 21°‐74°, total ozone values of 220–320 Dobson units and relatively small aerosol optical depths. The CRF is both wavelength and SZA dependent and applicable to all broadband UV‐B instruments based on the original Robertson‐Berger design. A methodology is presented for incorporation of the CRF into the calibration scheme. The concept of spectral multipliers is introduced, which are unique to each broadband instrument and correct for the wavelength dependence of the CRF. A method is then described for ongoing calibration of the detector based on model data alone. The scheme is then tested with real data to validate its reliability. Overall, by nature of the simultaneous characterisation of both spectral and cosine response, the CRF allows for a more robust calibration standard by which measurements and intercomparisons can be made.
Multiwavelength lidar observation of a strange noctilucent cloud at Kühlungsborn, Germany (54°N)
Authors: Alpers Matthias , Gerding Michael , Höffner Josef , Schneider Johannes
Published: February 2004

On July 6/7, 1997, we observed a noctilucent cloud (NLC) by lidar at Kühlungsborn, Germany (54°N, 12°E) using four laser wavelengths (393, 423, 532, and 770 nm). While at the near‐ultraviolet and visible wavelengths clear backscatter signals were detected, the NLC did not produce significant backscattering at the near‐infrared wavelength 770 nm. The latter signature can not be explained by backscattering on any size distribution of homogeneous water ice spheres. In this work we discuss the spectral backscatter signatures of particles with various shapes and compositions. We found three different particle types matching the experimental results, but there are physical arguments excluding these candidates as well. Our lidar observations of this strange NLC event appear to contradict the common understanding of NLC particle formation and sublimation. We emphasize the importance of multiwavelength observations for an in‐depth interpretation of noctilucent cloud lidar backscatter data, including also the infrared part of the spectrum.
Retrieval of molecular nitrogen and molecular oxygen densities in the upper mesosphere and lower thermosphere using ground‐based lidar measurements
Authors: Mwangi M. M. , Sica R. J. , Argall P. S.
Published: February 2004

Two new techniques have been developed to estimate the densities of N2, O2, and O from Rayleigh lidar backscatter photocount profiles and from independent temperature determinations. Both techniques involve solving initial value problems for the N2, O2, and O densities. These initial value problems are derived from the lidar equation, the ideal gas law, and the assumption of hydrostatic equilibrium. Their solutions are valid in regions where the Rayleigh lidar backscatter can be fully isolated from other scattering sources, e.g., aerosol scattering and where independent temperature determinations can been made. Estimates of the N2 and O2 density profiles in the upper mesosphere and lower thermosphere have been performed on three nights using simultaneous sodium‐resonance‐fluorescence lidar measurements to independently determine the temperature profile. The available measurements are of insufficient quality to retrieve O density profiles. The resulting N2 and O2 density retrievals appear reasonable when compared with previous determinations from sounding rockets and give hope that this method will allow routine ground‐based measurements of major constituent density in the middle atmosphere. A detailed error analysis is described which shows that the primary source of random errors is the Rayleigh photocounts, while the most important systematic errors are uncertainties in the N2 and O2 Rayleigh backscatter cross sections and in the normalization of the lidar density profiles in the upper stratosphere and lower mesosphere.
Estimation of soil and vegetation temperatures with multiangular thermal infrared observations: IMGRASS, HEIFE, and SGP 1997 experiments
Authors: Menenti Massimo , Jia Li , Li Zhao‐Liang , Djepa Vera , Wang Jiemin , Stoll Marc Philippe , Su Zhongbo , Rast Michael
Published: February 2004

The potential of directional observations in the thermal infrared region for land surface studies is a largely uncharted area of research. The availability of the dual‐view Along Track Scanning Radiometer (ATSR) observations led to explore new opportunities in this direction. In the context of studies on heat transfer at heterogeneous land surfaces, multiangular thermal infrared (TIR) observations offer the opportunity of overcoming fundamental difficulties in modeling sparse canopies. Three case studies were performed on the estimation of the component temperatures of foliage and soil. The first one included the use of multi‐temporal field measurements at view angles of 0°, 23° and 52°. The second and third one were done with directional ATSR observations at view angles of 0° and 53° only. The first one was a contribution to the Inner‐Mongolia Grassland Atmosphere Surface Study (IMGRASS) experiment in China, the second to the Hei He International Field Experiment (HEIFE) in China and the third one to the Southern Great Plains 1997 (SGP 1997) experiment in Oklahoma, United States. The IMGRASS experiment provided useful insights on the applicability of a simple linear mixture model to the analysis of observed radiance. The HEIFE case study was focused on the large oasis of Zhang‐Ye and led to useful estimates of soil and vegetation temperatures. The SGP 1997 contributed a better understanding of the impact of spatial heterogeneity on the accuracy of retrieved foliage and soil temperatures. Limitations in the approach due to varying radiative and boundary layer forcing and to the difference in spatial resolution between the forward and the nadir view are evaluated through a combination of modeling studies and analysis of field data.
Climate impacts of European‐scale anthropogenic vegetation changes: A sensitivity study using a regional climate model
Authors: Heck Pamela , Lüthi Daniel , Wernli Heini , Schär Christoph
Published: February 2004

Summertime seasonal simulations with a regional climate model covering Europe and the North Atlantic are used to assess the sensitivity of the European climate with respect to continental‐scale anthropogenic vegetation changes. An ensemble of six control integrations with present‐day vegetation parameters (April until August, 1987–1992) is compared against an ensemble of six sensitivity experiments (for the same time period and years) using a potential distribution as obtained from a vegetation model. The two vegetation distributions differ strongly in the Mediterranean region, reflecting the anthropogenic deforestation that took place during the last 2000 years. The numerical simulations reveal that the additional vegetation in the sensitivity experiment leads to a moister and cooler spring, followed by a drier and warmer summer. The following key physical processes were identified: Evapotranspiration is increased in the simulation with potential vegetation during the period from April until mid‐July, thus cooling the Earth surface, moistening the boundary layer, reducing vertical stability, and eventually enhancing precipitation. In mid‐July, soil moisture values in the experiment with potential vegetation drop locally below a critical value (as a consequence of the larger springtime evapotranspiration rates), whereupon transpiration is almost completely inhibited during the following 2 months. This leads to a sign reversal of the sensitivity to vegetation cover. This contrasting behavior during spring and summer is consistently simulated for all 6 years considered. The results thus demonstrate that vegetation substantially impacts upon the summertime climate in the Mediterranean region.
Fractionation of N2O isotopomers in the stratosphere
Authors: Toyoda Sakae , Yoshida Naohiro , Urabe Taichiro , Aoki Shuji , Nakazawa Takakiyo , Sugawara Satoshi , Honda Hideyuki
Published: February 2004

The vertical distribution of isotopomers of N2O (14N15N16O, 15N14N16O, and 14N14N18O) in the lower and middle stratosphere was observed over Japan (39°N, 142°E) in 1999 using a balloon‐borne cryogenic sampler and ground‐based mass spectrometry. The abundance of the heavier isotopomers relative to 14N14N16O increased with altitude, while the mixing ratio of N2O decreased due to photochemical depletion. Maximum fractionation was observed at the highest altitude, 34.5 km, where δ15Nαair (isotopomer ratio of center nitrogen), δ15Nβair (end nitrogen), and δ18OSMOW were 144.l‰, 42.4‰, and 119.0‰, respectively. The observed distribution is mostly accounted for by isotopic fractionation during consumption processes, which is in accordance with reported simulation experiments and theoretical prediction for photolytic fractionation of N2O isotopomers. However, the apparent fractionation factors in the lower (<∼24 km) and higher regions are different, which suggests that (1) the fractionation factor for stratospheric photolysis may depend on altitude or latitude, (2) transport and mixing processes in the stratosphere can affect the vertical profile, and (3) the relative contribution of photolysis and photo‐oxidation to total N2O sink is possibly dependent on altitude.
A diagnostic analysis of the Kennedy Space Center LDAR network: 2. Cross‐sensor studies
Authors: Boccippio D. J. , Heckman S. , Goodman S. J.
Published: February 2004

Range dependencies in total (intracloud and cloud to ground) lightning observed by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network are established through cross comparison with other lightning sensors. Using total lightning observed from space by the Lightning Imaging Sensor (LIS), LDAR flash detection efficiency is shown to remain above 90% out to 90–100 km range, and to be below 25% at 200 km range. LDAR VHF source location error distributions are also determined as a function of range and are found to be asymmetric with standard deviation increasing roughly as r2. Range normalization schemes for total VHF source density are tested and shown to yield significant improvements in correlation with National Lightning Detection Network (NLDN) ground flash density at hourly, daily, monthly, and climatological timescales (up to 50% over uncorrected source densities using an exponential‐in‐range correction factor with 40–50 km e‐folding scale).
Retrieval of large‐scale wind divergences from infrared Meteosat‐5 brightness temperatures over the Indian Ocean
Authors: Bonazzola M. , Picon L. , Laurent H. , Hourdin F. , Sèze G. , Pawlowska H. , Sadourny R.
Published: February 2004

Over the tropics the atmospheric general circulation models usually fail in predicting horizontal wind divergence, which is closely related to atmospheric heating and to the vertical exchanges associated with convection. With the aim of forcing atmospheric models we present here a reconstruction of wind divergences based on the links between infrared brightness temperatures, convective activity, and large‐scale divergence. In practice, wind divergences are reconstructed from brightness temperatures using correlations obtained from numerical simulations performed with a general circulation model. When building those correlations, a distinction must be made between the brightness temperatures of opaque clouds and those of semitransparent clouds, only the former being directly associated with convection. In order to filter out semitransparent clouds we use radiative thresholds in the water vapor channel in addition to the window channel. We apply our approach to Meteosat‐5 data over the Indian Ocean. Comparison with wind divergences reconstructed independently from Meteosat water vapor winds partially validates our retrieval. Comparison with European Center for Medium‐Range Weather Forecasts analyses indicates that much can be gained by adding information on the wind divergence in the tropics to force an atmospheric model.
Assessment of simulated water balance for continental‐scale river basins in an AMIP 2 simulation
Authors: Arora Vivek K.
Published: February 2004

Streamflow, which integrates the response of the land surface to atmospheric forcing over large areas, is a useful diagnostic to assess the performance of land surface schemes over large spatial scales. This paper uses observed runoff and streamflow data to assess the performance of the Canadian land surface parameterization scheme (CLASS), when operated within the Canadian Centre for Climate modeling and analysis (CCCma) general circulation model (GCM) at 3.75° resolution, for three continental‐scale river basins. Estimates of evapotranspiration obtained using atmospheric water balance, and soil moisture obtained using the VIC‐2L model, are also used to assess the CLASS water balance simulations. Comparisons with observations of streamflow, and estimates of evapotranspiration and soil moisture, suggest that although CLASS simulates the annual cycle of evapotranspiration, streamflow, and soil moisture reasonably well, it overestimates evapotranspiration, underestimates runoff, and simulates slightly wetter soil moisture conditions.
Empirical age spectra for the midlatitude lower stratosphere from in situ observations of CO2: Quantitative evidence for a subtropical “barrier” to horizontal transport
Authors: Andrews A. E. , Boering K. A. , Wofsy S. C. , Daube B. C. , Jones D. B. , Alex S. , Loewenstein M. , Podolske J. R. , Strahan S. E.
Published: February 2004

Empirical age spectra for the northern midlatitude lower stratosphere were derived using a genetic algorithm constrained by in situ observations of CO2 and N2O. An age spectrum is the probability distribution function describing the ensemble of transit times between the tropical tropopause and a point in the stratosphere. Age spectra with two distinct maxima fit observed CO2 mixing ratios significantly better than age spectra consisting of a single peak. The mean transit time (mean age) of the younger peak increased with altitude from the tropopause to ∼1 year at 19.5 km, while the mean age of the older peak was 5.4±0.4 years throughout this altitude region. Near the tropopause, ∼10% of the air was represented by the older peak, increasing to ∼50% at 19.5 km. We argue that the younger peak represents rapid quasihorizontal transport of air from the tropics, while the older peak represents downwelling of air from higher altitudes in the mean meridional circulation. Mean ages for the younger and older peaks provide estimates of the residence times for air in the lower and middle stratosphere, respectively. The clear separation of the peaks implies a significant altitude region where there is little meridional exchange between midlatitudes and the tropics.
Climatology and modeling of quasi‐monochromatic atmospheric gravity waves observed over Urbana Illinois
Authors: Hecht J. H. , Walterscheid R. L. , Hickey M. P. , Franke S. J.
Published: February 2004

From analyzing nine months of airglow imaging observations of atmospheric gravity waves (AGWs) over Adelaide, Australia (35°S) [Walterscheid et al., 1999] have proposed that many of the quasi‐monochromatic waves seen in the images were primarily thermally ducted. Here are presented 15 months of observations, from February 1996 to May 1997, for AGW frequency and propagation direction from a northern latitude site, Urbana Illinois (40°N). As Adelaide, Urbana is geographically distant from large orographic features. Similar to what was found in Adelaide, the AGWs seem to originate from a preferred location during the time period around summer solstice. In conjunction with these airglow data there exists MF radar data to provide winds in the 90 km region and near‐simultaneous lidar data which provide a temperature climatology. The temperature data have previously been analyzed by States and Gardner [2000]. The temperature and wind data are used here in a full wave model analysis to determine the characteristics of the wave ducting and wave reflection during the 15 month observation period. This model analysis is applied to this and another existing data set recently described by Nakamura et al. [1999]. It is shown that the existence of a thermal duct around summer solstice can plausibly account for our observations. However, the characteristics of the thermal duct and the ability of waves to be ducted is also greatly dependent on the characteristics of the background wind. A simple model is constructed to simulate the trapping of these waves by such a duct. It is suggested that the waves seen over Urbana originate no more than a few thousand kilometers from the observation site.
Isentropic scaling analysis of ozone in the upper troposphere and lower stratosphere
Authors: Cho John Y. N. , Thouret Valérie , Newell Reginald E. , Marenco Alain
Published: February 2004

We examine ozone concentrations recorded by 7630 commercial flights from August 1994 to December 1997 for spatial scaling properties. The large amount of data allows an approximately isentropic analysis of ozone variability in the upper troposphere and lower stratosphere. Since ozone is a good passive tracer at cruise altitudes, the results provide a strong diagnostic for scalar advection theories and models. Calculations of structure functions and increment probability distribution functions show that ozone variability scales anomalously from ∼2 to ∼2000 km, although not continuously in this interval. We find no evidence for the simple scaling predicted for smooth advection/diffusion, even at the large scales. At mesoscales the upper tropospheric ozone field is rougher and more intermittent than in the lower stratosphere. Within the troposphere the equatorial ozone field is rougher than at higher latitudes, and the intermittency decreases with increasing latitude. In the stratosphere the intermittency and roughness are greater at high latitudes and over land than at midlatitudes and over the ocean.
Subpixel canopy cover estimation of coniferous forests in Oregon using SWIR imaging spectrometry
Authors: Lobell David B. , Asner Gregory P. , Law Beverly E. , Treuhaft Robert N.
Published: February 2004

The percent cover of vegetation canopies is an important variable for many land‐surface biophysical and biogeochemical models and serves as a useful measure of land cover change. Remote sensing methods to estimate the subpixel fraction of vegetation canopies with spectral mixture analysis (SMA) require knowledge of the reflectance properties of major land cover units, called endmembers. However, variability in endmember reflectance across space and time has limited the interpretation and general applicability of SMA approaches. In this study, a subpixel vegetation cover of coniferous forests in Oregon, United States, was successfully estimated by employing shortwave infrared reflectance measurements (SWIR2 region, 2080–2280 nm) collected by the NASA Airborne Visible Infrared Imaging Spectrometer (AVIRIS). The approach presented here, referred to as AutoSWIR [Asner and Lobell, 2000], was originally developed for semiarid and arid environments and exploits the low SWIR2 variability of materials found in most ecosystems. SWIR2 field spectra from Oregon were compared with spectra from an arid systems database, revealing significant differences only for soil reflectance. However, SWIR2 variability remained low, as indicated by field spectra and principal component analysis, and AutoSWIR was then modified to use coniferous forest spectra collected in Oregon. Subsequent high spatial resolution estimates of forest canopy cover agreed well with estimates from low‐altitude air photos (rms = 3%), demonstrating the successful extension of AutoSWIR to a coniferous forest ecosystem. The generality of AutoSWIR facilitates accurate estimates of vegetation cover that can be automatically retrieved from SWIR2 spectral measurements collected by forthcoming spaceborne imaging spectrometers such as NASA's New Millenium Program EO‐1 Hyperion. These estimates can then be used to characterize landscape heterogeneity important for land‐surface, atmospheric, and biogeochemical research.
Constraints on meridional transport in the stratosphere imposed by the mean age of air in the lower stratosphere
Authors: Jones D. B. A. , Andrews A. E. , Schneider H. R. , McElroy M. B.
Published: February 2004

The sensitivity of mean age of air in the stratosphere to the vertical structure of mass exchange between the tropics and midlatitudes is examined with a two‐dimensional model with consistent advective and diffusive transport. We use estimates of mean age of air and age spectra in the midlatitude lower stratosphere, derived from observations of CO2, to constrain transport from the tropics to midlatitudes. We show that to reproduce the latitudinal gradient in mean age in the lower stratosphere as well as the bimodal age spectra derived for the midlatitude lower stratosphere, the rate of horizontal transport from the tropics to midlatitudes between 20–30 km must be slower than that at higher and lower altitudes. The relative rates of meridional transport above and below this region determine the separation of the two peaks in the age spectra and the overall mean age of air in the lower stratosphere. Slow transport between 20–30 km can generally not be obtained by reducing only the diffusive component of transport. In the model, gravity wave drag must also be reduced in the lower stratosphere to prevent strong horizontal advective transport across the subtropics. We also show that adjusting the diffusive component of transport independently of the meridional circulation can produce mean ages different from those calculated in the fully coupled model by as much as 2 years.
April 1998 Asian dust event: A southern California perspective
Authors: Tratt David M. , Frouin Robert J. , Westphal Douglas L.
Published: February 2004

In late April 1998 an extreme Asian dust episode reached the U.S. western seaboard. This event was observed by several in situ and remote sensing atmospheric measurement stations. Dramatic reductions in boundary layer visibility were recorded and the resultant peak backscatter coefficients exceeded prevailing upper tropospheric background conditions by at least 2 orders of magnitude. An analysis of this event is given using lidar vertical backscatter profilometry, concurrent Sun photometer opacity data, and transport modeling. At San Nicolas Island the measured and modeled aerosol optical thickness at 500 nm increased dramatically from 0.15 on April 25 to 0.52 on April 26–27. Volume size distribution on April 27 exhibited a prominent coarse mode at 1–2 μm radius, and single‐scattering albedo was observed to increase from 0.90 in the blue to 0.93 in the near infrared. Concurrent lidar observations tracked the evolution of the plume vertical structure, which consisted of up to three well‐defined layers distributed throughout the free troposphere.