bluuebunny/crossref_metadata_2025 huggingface dataset:
Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment and middle atmosphere variability
Authors: Offermann D. , Grossmann K.‐U. , Barthol P. , Knieling P. , Riese M. , Trant R.
Published: September 2002

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument was built to determine whether and to what extent small‐scale structures in global trace gas distributions and in dynamics are present in the middle atmosphere. To achieve this, trace gases were measured in the middle infrared by the limb scan technique at the highest possible horizontal and vertical resolution. CRISTA uses three telescopes (i.e., three view directions) simultaneously, and has three grating spectrometers for the middle IR (4–14 μm) and one spectrometer for the far IR (15–71 μm). The optics and detectors are cooled to cryogenic temperatures by supercritical helium or subcooled helium, respectively, in a double cryostat. An instrument overview is given, and the design guidelines are sketched. The CRISTA experiment was flown on the space shuttle STS 66 as part of NASA mission ATLAS 3 on November 3–14, 1994. Orbit altitude was 300 km, and inclination was 57°. A campaign of ground‐based, balloon, and rocket validation and complementary measurements was performed simultaneously. The CRISTA instrument performed flawlessly. A horizontal resolution of 200 km × 650 km was achieved at the equator, with higher horizontal resolution at higher latitudes. A vertical resolution of 2.5 km (or better) was obtained. The middle atmosphere was found to be highly variable at scales of <1000 km in the stratosphere. Three streamers of tropic/subtropic air extending to higher latitudes are described. Their meridional scale is ≤1000 km, while the zonal scale is of the order of 10,000 km and more. The streamers appear to be typical of specific winter conditions and to play a role in meridional transport. At mesospheric heights a strong tidal temperature oscillation was observed which extended well into the lower thermosphere.
Organic aerosol formation from the oxidation of biogenic hydrocarbons
Authors: Griffin Robert J. , Cocker David R. , Flagan Richard C. , Seinfeld John H.
Published: September 2002

A series of outdoor chamber experiments has been used to establish and characterize the significant atmospheric aerosol‐forming potentials of the most prevalent biogenic hydrocarbons emitted by vegetation. These compounds were also studied to elucidate the effect of structure on aerosol yield for these types of compounds. Because oxidation products partition between the gas and aerosol phases, the aerosol yields of the parent biogenic hydrocarbons depend on the concentration of organic aerosol into which these products can be absorbed. For organic mass concentrations between 5 and 40 μg m−3, mass‐based yields in photooxidation experiments range from 17 to 67% for sesquiterpenes, from 2 to 23% for cyclic diolefins, from 2 to 15% for bicyclic olefins, and from 2 to 6% for the acyclic triolefin ocimene. In these photooxidation experiments, hydroxyl and nitrate radicals and ozone can contribute to consumption of the parent hydrocarbon. For bicyclic olefins (α‐pinene, β‐pinene, Δ3‐carene, and sabinene), experiments were also carried out at daytime temperatures in a dark system in the presence of ozone or nitrate radicals alone. For ozonolysis experiments, resulting aerosol yields are less dependent on organic mass concentration, when compared to full, sunlight‐driven photooxidation. Nitrate radical experiments exhibit extremely high conversion to aerosol for β‐pinene, sabinene, and Δ3‐carene. The relative importance of aerosol formation from each type of reaction for bicyclic olefin photooxidation is elucidated.
Model study of a stratospheric intrusion event at lower midlatitudes associated with the development of a cutoff low
Authors: Kentarchos A. S. , Roelofs G. J. , Lelieveld J.
Published: September 2002

A tropopause folding event associated with the development of a cutoff low over southeastern Europe at the end of March 1995 enhanced the ozone concentrations in the upper troposphere. The episode has been simulated using a coupled chemistry‐general circulation model that has been “nudged” toward actual meteorology using a simple four‐dimensional assimilation technique based on European Centre for Medium‐Range Weather Forecasts data. Model results are compared with ozone observations and analyzed meteorological data. In particular, simulated geopotential heights show a realistic representation of the synoptic development of the cutoff low system. The model also succesfully reproduces the space‐time evolution of potential vorticity and specific humidity fields associated with the tropopause fold. Furthermore, modeled ozone concentrations during that period show a pronounced increase in the upper troposphere associated with downward fluxes from the stratosphere. The total amount of stratospheric ozone transferred into the troposphere during the event is estimated at 6.4 × 108 kg. The results of the present simulation indicate that the model provides a useful tool to simulate stratosphere‐troposphere exchange and its effect on global tropospheric ozone, and that the model representation of the large‐scale wave forcing is more important than the fine details of tropopause foldings.
Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) data processing and atmospheric temperature and trace gas retrieval
Authors: Riese M. , Spang R. , Preusse P. , Ern M. , Jarisch M. , Offermann D. , Grossmann K. U.
Published: September 2002

The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment aboard the Shuttle Pallet Satellite (SPAS) was successfully flown in early November 1994 (STS 66) and in August 1997 (STS 85). This paper focuses on the first flight of the instrument, which was part of the Atmospheric Laboratory for Application and Science 3 (ATLAS 3) mission of NASA. During a free flying period of 7 days, limb scan measurements of atmospheric infrared emissions were performed in the 4 to 71 μm wavelength region. For improved horizontal resolution, three telescopes (viewing directions) were used that sensed the atmosphere simultaneously. Atmospheric pressures, temperatures, and volume mixing ratios of various trace gases were retrieved from the radiance data by using a fast onion‐peeling retrieval technique. This paper gives an overview of the data system including the raw data processing and the temperature and trace gas profile retrieval. Examples of version 1 limb radiance data (level 1 product) and version 1 mixing ratios (level 2 product) of ozone, ClONO2, and CFC‐11 are given. A number of important atmospheric transport processes can already be identified in the level 1 limb radiance data. Radiance data of the lower stratosphere (18 km) indicate strong upwelling in some equatorial regions, centered around the Amazon, Congo, and Indonesia. Respective data at the date line are consistent with convection patterns associated with El Niño. Very low CFC‐11 mixing ratios occur inside the South Polar vortex and cause low radiance values in a spectral region sensitive to CFC‐11 emissions. These low values are a result of considerable downward transport of CFC‐11 poor air that occurred during the winter months. Limb radiance profiles and retrieved mixing ratio profiles of CFC‐11 indicate downward transport over ∼5 km. The accuracy of the retrieved version 1 mixing ratios is rather different for the various trace gases. In the middle atmosphere the estimated systematic error of ozone is ∼14%. Ozone data of correlative satellite measurements are well within this error bar. CRISTA agrees, for example, with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) sunset measurements typically within 5%. The random error of version 1 ozone mixing ratios is 4%. Similar values apply to other trace gases. These low random errors allow the identification of small and medium scale horizontal and vertical structures in the measured trace gas distributions. Examples of structures in mixing ratio fields of ozone, ClONO2, and CFC‐11 are given.
Frequency and distribution of forest, savanna, and crop fires over tropical regions during PEM‐Tropics A
Authors: Olson Jennifer R. , Baum Bryan A. , Cahoon Donald R. , Crawford James H.
Published: September 2002

Advanced very high resolution radiometer 1.1 km resolution satellite radiance data were used to locate active fires throughout much of the tropical region during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐Tropics (PEM‐Tropics A) aircraft campaign, held in September and October 1996. The spatial and temporal distributions of the fires in Australia, southern Africa, and South America are presented here. The number of fires over northern Australia, central Africa, and South America appeared to decrease toward the end of the mission period. Fire over eastern Australia was widespread, and temporal patterns showed a somewhat consistent amount of burning with periodic episodes of enhanced fire counts observed. At least one episode of enhanced fire counts corresponded to the passage of a frontal system which brought conditions conducive to fire to the region, with strong westerlies originating over the hot, dry interior continent. Regions that were affected by lower than normal rainfall during the previous wet season (e.g., northern Australia and southwestern Africa) showed relatively few fires during this period. This is consistent with a drought‐induced decrease in vegetation and therefore a decreased availability of fuel for burning. Alternatively, a heavier than normal previous wet season along the southeastern coast of South Africa may have contributed to high fuel loading and an associated relatively heavy amount of burning compared to data from previous years.
A Fourier transform infrared study of the adsorption of SO2 on n‐hexane soot from −130° to −40°C
Authors: Koehler Birgit G. , Nicholson Victoria T. , Roe Henry G. , Whitney Erin S.
Published: September 2002

This paper focuses on the uptake of SO2 on soot at temperatures below room temperature. Oxidation on soot may provide a mechanism for the oxidation of atmospheric SO2 under conditions when the standard gas‐phase and aqueous‐phase mechanisms cannot explain the rapid rate of H2SO4 production. An understanding of the uptake of SO2 under dry conditions provides a useful step toward understanding the uptake and oxidation of SO2 on soot under wet conditions. We find that rapid, reversible SO2 adsorption on soot occurs within a few seconds, presumably by adsorption on the outer surfaces of the spherical soot particles. Subsequently, uptake continues slowly for over an hour, presumably by diffusion into micropores within the soot particles. We focused only on the rapid adsorption. An isothermal analysis of rapid SO2 uptake revealed that a small fraction (<1%) of adsorption sites have a strong binding affinity (ΔHdes = 42±4 kJ/mol), while the majority of adsorption sites bind SO2 more weakly (26±4 kJ/mol). The lower‐limit saturation coverage of SO2 on soot is 0.3 monolayer, but the more likely value is 0.7 monolayer. The uptake coefficient is 0.002 (plus or minus factor of 2) at low coverages.
Closure of the total hydrogen budget of the northern extratropical lower stratosphere
Authors: Hurst D. F. , Dutton G. S. , Romashkin P. A. , Wamsley P. R. , Moore F. L. , Elkins J. W. , Hintsa E. J. , Weinstock E. M. , Herman R. L. , Moyer E. J. , Scott D. C. , May R. D. , Webster C. R.
Published: September 2002

Methane (CH4), molecular hydrogen (H2), and water vapor (H2O) were measured concurrently on board the NASA ER‐2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Correlations between these three main hydrogen reservoirs in the northern extratropical lower stratosphere are examined to evaluate H2O production from CH4 and H2 oxidation. The expected ratio of stratospheric H2O production (PH2O)to CH4 destruction (LCH4) = −1.973±0.003 is calculated from an evaluation of CH4 and H2 oxidation reactions and the relationship between H2 and CH4 mixing ratios measured during STRAT. Correlations between H2O and CH4 were tight and linear only for air masses with mean ages ≥3.8 years, restricting this analysis predominantly to latitudes between 40° and 90°N and potential temperatures between 470 and 540 K. The mean observed ΔH2O/CH4 (−2.15±0.18) is in statistical agreement with the expected PH2O/LCH4. The annual mean stratospheric entry mixing ratio for H2O calculated from this slope is 4.0 ± 0.3 ppm. The quantity H2O + 2·CH4 is quasi‐conserved at 7.4 ± 0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant departure of H2O + 2·CH4 from the mean value is a sensitive indicator of processes which influence H2O without affecting CH4, such as dehydration in a polar vortex or near the tropical tropopause. No significant trend is observed in ER‐2 aircraft data for H2O + 2·CH4 in the lower stratosphere from 1993 through 1997.
On the boundary between man‐made and natural emissions: Problems in defining European ecosystems
Authors: Winiwarter Wilfried , Haberl Helmut , Simpson David
Published: September 2002

Man‐made as well as natural sources contribute to the occurrence of trace constituents in the atmosphere. A strict distinction between these source types is needed to support international agreements on emission reductions. However, the nomenclatures in existing emission inventories treat several source sectors ambiguously, such that certain emissions are considered man‐made in one system and natural in the other. An investigation of the situation for Europe reveals that there are three such sources sectors that contribute significantly (more than 10%) to the total emissions of at least one pollutant: animal husbandry, forests, and possibly soils. Using qualitative parameters for distinguishing between natural and cultivated systems, it may be concluded that, at least under European conditions, both animal husbandry and soils as emission sources are controlled by humans in nearly all aspects. Forests, even if managed, as is the European practice, retain many natural characteristics. However, even for forests, emissions are certainly very different to those existing before human influence became dominant because of extensive deforestation and replanting (often of nonnative species). A “colonization” theory approach suggests that just such a difference between the natural situation and the conditions as caused by man are attributed to anthropogenic emissions. Still, a complete assessment of the respective contributions cannot be provided because of a lack of a “baseline” natural situation. Therefore the best solution, in practice, will be to formulate international agreements in terms of specific and unambiguous source sectors.
Interpolation of surface radiative temperature measured from polar orbiting satellites to a diurnal cycle: 1. Without clouds
Authors: Jin Menglin , Dickinson Robert E.
Published: September 2002

The land surface skin temperature diurnal cycle (LSTD) is an important element of the climate system. This variable, however, cannot be directly obtained globally from polar orbiting satellites because such satellites only pass a given area twice per day and because their infrared channels cannot observe the surface when the sky is cloudy. To obtain the skin temperature diurnal cycle and fully utilize satellite measurements, we have designed an efficient algorithm that combines model results with satellite and surface‐based observations and interpolates satellite twice‐daily observations into the diurnal cycle. Climatological information from a climate model, CCM3/BATS, is used to determine a normalized shape (typical pattern) of the diurnal temperature for different latitudes, seasons, and vegetation types. The satellite observations, which are by themselves inadequate, are combined with the normalized modeled diurnal typical patterns to obtain the skin temperature diurnal cycle. The normalized typical patterns depend on the parameters of the diurnal insolation, such as sunrise, sunset, and peak times, and are also affected by the type of vegetation cover and soil moisture. The underlying physical foundation of this algorithm is that the diurnal cycle of temperature can be viewed as a composite of a daily average, diurnal periodic component, and random aperiodic component (noise). With the assumption that the noise can be ignored, the daily average can be inferred from satellite twice‐per‐day measurements and the periodic part can be obtained from modeled climatologies, providing a reasonable approach for estimation of the diurnal cycle of skin temperature. The general framework of the algorithm and its application for the clear‐sky (cloud free) conditions are presented. This cloud‐free version of algorithm is evaluated using FIFE and BOREAS field experiment surface observations. Regional tests over the Mississippi River basin have also been conducted using GOES‐8 and AVHRR observations. Uncertainties of this cloud‐free algorithm have been analyzed, indicating an accuracy of about 1–2 K for monthly cloud‐free diurnal cycles at near‐pixel resolution.
Intercomparison of regional climate simulations of the 1991 summer monsoon in eastern Asia
Authors: Leung L. R. , Ghan S. J. , Zhao Z.‐C. , Luo Y. , Wang W.‐C. , Wei H.‐L.
Published: September 2002

Regional climate models have become a common research tool for downscaling global climate simulations. To further examine their usefulness for climate studies and the impacts that different physical parameterizations have on the simulations, an intercomparison experiment has been performed where three regional climate models are used to simulate an extreme flood event. Although the dynamical components of the models are almost identical, the physical parameterizations used to represent clouds, radiative transfer, turbulence transport, and surface processes are very different. The models were used to simulate the heavy precipitation during the 1991 summer which caused severe flooding over the Yangtze River in China. This extreme event is selected to highlight the differences among regional climate models. Results from the intercomparison show that all models simulated the gross flood conditions reasonably well, although each model reproduced the observed rainband to varying degrees, and significant differences are found in the simulated energy and hydrological cycles, especially over cloudy areas. Through detailed analyses of model simulations at different spatial and temporal scales, several reasons are found to cause the departure of model simulations from each other. These include the simulation of the amount and vertical distribution of clouds, the treatment of cloud‐radiative feedbacks, and the representation of land surface processes. This study suggests that aspects other than surface temperature and precipitation of the regional climate simulations need to be more carefully evaluated. One specially important evaluation criterion is the radiation balance which has serious implications for long‐term climate simulations.
Lidar and numerical studies on the different evolution of vortex pair and secondary wake in young contrails
Authors: Sussmann Ralf , Gierens Klaus M.
Published: September 2002

Vortex‐regime evolution of contrails is investigated by focusing on the role of ambient humidity. Lidar cross‐section measurements and observational analysis are combined with numerical simulations of fluid dynamics and microphysics. Contrail evolution behind four‐turbofan aircraft is classified into three different scenarios. In the case of ice‐subsaturated air, a visible pair of wingtip vortices is formed that disappears at the end of the vortex regime. In case of ice supersaturation, a diffuse secondary wake evolves above the wingtip vortices. It is due to detrainment of ice particles growing by sublimation of ambient humidity. A vertical wake‐gap opens between the wingtip vortices and the secondary wake. It is due to subsaturated air moving upward along the outer edges of the sinking vortex tubes accumulating around the upper stagnation point of the vortex system. The vertical wake‐gap preferably occurs in the wake of heavy (four turbofans) aircraft, since the vortices behind light aircraft migrate down too slowly. The secondary wake is composed of nonspherical particles larger than the ones in the wingtip vortices which are spherical particles and/or particles smaller than ≈0.5 μm. In most cases the secondary wake is the only part of a contrail that persists after vortex breakdown. This is because the ice in the vortex tubes evaporates due to adiabatic heating as the vortices travel downward. Only in the rare case of higher ambient ice supersaturation (>2%) do both parts of a contrail contribute to the persistent ice cloud. The number of ice crystals initially formed is typically reduced by a factor of 200 by evaporation (60% ambient humidity). This leads to a high population of interstitial particles. The results imply that formation of persistent contrails can be minimized by technical means.
Why is the global warming proceeding much slower than expected?
Authors: Bengtsson L. , Roeckner E. , Stendel M.
Published: September 2002

Upper air observations from radiosondes and microwave satellite instruments does not indicate any global warming during the last 19 years, contrary to surface measurements, where a warming trend is supposedly being found. This result is somewhat difficult to reconcile, since climate model experiments do indicate a reverse trend, namely, that upper tropospheric air should warm faster than the surface. To contribute toward an understanding of this difficulty, we have here undertaken some specific experiments to study the effect on climate due to the decrease in stratospheric ozone and the Mount Pinatubo eruption in 1991. The associated forcing was added to the forcing from greenhouse gases, sulfate aerosols (direct and indirect effect), and tropospheric ozone, which was investigated in a separate series of experiments. Furthermore, we have undertaken an ensemble study in order to explore the natural variability of an advanced climate model exposed to such a forcing over 19 years. The result shows that the reduction of stratospheric ozone cools not only the lower stratosphere but also the troposphere, in particular, the upper and middle part. In the upper troposphere the cooling from stratospheric ozone leads to a significant reduction of greenhouse warming. The modeled stratospheric aerosols from Mount Pinatubo generate a climate response (stratospheric warming and tropospheric cooling) in good agreement with microwave satellite measurements. Finally, analysis of a series of experiments with both stratospheric ozone and the Mount Pinatubo effect shows considerable variability in climate response, suggesting that an evolution having no warming in the period is as likely as another evolution showing modest warming. However, the observed trend of no warming in the midtroposphere and clear warming at the surface is not found in the model simulations.
Incorporation of mineralogical composition into models of the radiative properties of mineral aerosol from UV to IR wavelengths
Authors: Sokolik Irina N. , Toon Owen B.
Published: September 2002

We describe a technique to model the radiative properties of mineral aerosols which accounts for their composition. We compile a data set of refractive indices of major minerals and employ it, along with data on mineralogical composition of dust from various locations, to calculate spectral optical and radiative properties of mineral aerosol mixtures. Such radiative properties are needed for climate modeling and remote sensing applications. We consider external mixtures of individual minerals, as well as mixtures of aggregates. We demonstrate that an external mixture of individual minerals must contain unrealistically high amounts of hematite to have a single scattering albedo lower than 0.9 at 500 nm wavelength. In contrast, aggregation of hematite with quartz or clays can strongly enhance absorption by dust at solar wavelengths. We also simulate the daily mean net (solar + infrared) forcing by dust of varying compositions. We found that, for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause the positive radiative forcing while a mixture of individual minerals gives the negative forcing.
The balance of global angular momentum in a long‐term atmospheric data set
Authors: Huang Huei‐Ping , Sardeshmukh Prashant D. , Weickmann Klaus M.
Published: September 2002

The balance of global atmospheric angular momentum is examined in a long time series of “reanalysis” data generated at the National Centers for Environmental Prediction (NCEP). A systematic negative bias of about −10 Hadleys (1018 kg m2 s−2) is obtained in the total torque in all seasons. The sum of the frictional (TF) and mountain (TM) torques contributes about half of this negative bias in the northern summer and autumn. The torque due to the parameterized gravity wave drag, TG, contributes the other half of the negative bias in summer but contributes the whole of it in winter. In the annual mean the projected angular momentum imbalance due to TG exceeds that due to TM + TF. Consistent with the budget analysis of the assimilated data, losses of angular momentum are also found in an extensive set of medium‐range forecasts made with the NCEP reanalysis model. The average rates of loss of relative angular momentum in these forecasts, −7 Hadleys in January and −6 in July, are comparable to the bias of the total torque in the angular momentum budget. Further investigations are suggested to determine the model error, especially that associated with the parameterized gravity wave drag, related to the budget imbalance found in this study.
Attachment process in rocket‐triggered lightning strokes
Authors: Wang D. , Rakov V. A. , Uman M. A. , Takagi N. , Watanabe T. , Crawford D. E. , Rambo K. J. , Schnetzer G. H. , Fisher R. J. , Kawasaki Z.‐I.
Published: September 2002

In order to study the lightning attachment process, we have obtained highly resolved (about 100 ns time resolution and about 3.6 m spatial resolution) optical images, electric field measurements, and channel‐base current recordings for two dart leader/return‐stroke sequences in two lightning flashes triggered using the rocket‐and‐wire technique at Camp Blanding, Florida. One of these two sequences exhibited an optically discernible upward‐propagating discharge that occurred in response to the approaching downward‐moving dart leader and connected to this descending leader. This observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lightning strokes, these strokes being similar to subsequent strokes in natural lightning. The observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader, a length of 7–11 m, and a duration of several hundred nanoseconds. The speed of the upward connecting discharge was estimated to be about 2 × 107m/s, which is comparable to that of the downward dart leader. In both dart leader/return‐stroke sequences studied, the return stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions. This latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader/return‐stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge. The length of the upward connecting discharges (observed in one case and inferred from the height of the return‐stroke starting point in the other case) is greater for the event that is characterized by the larger leader electric field change and the higher return‐stroke peak current. For the two dart leader/return‐stroke sequences studied, the upward connecting discharge lengths are estimated to be 7–11 m and 4–7 m, with the corresponding return‐stroke peak currents being 21 kA and 12 kA, and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kV/m and 43 kV/m. Additionally, we note that the downward dart leader light pulse generally exhibits little variation in its 10–90% risetime and peak value over some tens of meters above the return‐stroke starting point, while the following return‐stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lightning channel. Our findings regarding (1) the initially bidirectional development of return‐stroke process and (2) the relatively strong attenuation of the upward moving return‐stroke light (and by inference current) pulse over the first some tens of meters of the channel may have important implications for return‐stroke modeling.
Detecting tropical convection using AVHRR satellite data
Authors: van Hees Richard M. , Lelieveld Jos , Collins William D.
Published: September 2002

A new method is introduced to identify active convective cloud regions in high‐resolution AVHRR satellite data. This method is developed to overcome several limitations of existing visible/infrared techniques. Steep temperature gradients between the convective overshoots of cumulonimbus clouds and the surrounding cirrus are used to identify the edges of active convective regions. The limitations of previous methods are shown using up to 1.1 km resolution AVHRR data of the NOAA‐11 and NOAA‐12 satellites collected during the Central Equatorial Pacific Experiment (CEPEX), a measurement campaign that took place over the central Pacific Ocean between March 7 and April 6, 1993. Differences in the identification of convection between the new algorithm and the existing algorithms are clarified by a case study and further illustrated by three collocations between NOAA‐11 and NOAA‐12 overpasses and observations made with the Massachusetts Institute of Technology (MIT) 5 cm Doppler radar onboard the research vessel R/V Vickers.
Comparisons of the TOMS aerosol index with Sun‐photometer aerosol optical thickness: Results and applications
Authors: Hsu N. C. , Herman J. R. , Torres O. , Holben B. N. , Tanre D. , Eck T. F. , Smirnov A. , Chatenet B. , Lavenu F.
Published: September 2002

A nearly 20‐year global data set (1979–1994 and 1996 to the present) of tropospheric absorbing aerosols has been developed from total ozone mapping spectrometer (TOMS) backscattered radiance measurements in the range from 331 to 380 nm. The occurrence of aerosols is derived directly from measured backscattered radiances and is represented by a quantity known as the aerosol index. Previous theoretical model simulations have demonstrated that the aerosol index depends on aerosol optical thickness (AOT), single scattering albedo, and aerosol height and that the AOT can be determined provided that the microphysical properties and height of aerosols are known. In this paper we show that the TOMS aerosol index measurements are linearly proportional to the AOT derived independently from ground‐based Sun‐photometer instruments over regions of biomass burning and regions covered by African dust. We also show how this linear relationship can be used to directly convert the aerosol index into AOT for smoke and dust aerosols for the regions near the Sun‐photometer sites and how information about aerosol height can be inferred from the results. Finally, we apply this method to the TOMS data over the last two decades and find a significant increase in the amount of biomass burning smoke in the African savanna regions during the 1990s in addition to the more obvious increase in South America.
Systematic errors in atmospheric profiles obtained from Abelian inversion of radio occultation data: Effects of large‐scale horizontal gradients
Authors: Ahmad Bilal , Tyler G. Leonard
Published: September 2002

Reduction of radio occultation data to retrieve atmospheric profiles (T‐p(r)) requires knowledge or assumption of the horizontal structure of the atmosphere. In the case of terrestrial planets the atmosphere in the vicinity of ray periapsides usually is assumed to be spherically symmetric. This assumption leads to an integral transform relationship between the profiles of refractivity versus radius and the total bending angle versus the asymptotic closest approach of rays, where the latter is directly obtainable from occultation frequency data and trajectory information. Occultation studies of the giant planets have demonstrated that departures from spherical symmetry, if not accounted for, can result in serious errors in derived T‐p(r) profiles. We analyze errors in atmospheric profiles due to large‐scale departures from spherical symmetry. For analytic convenience we represent departures from spherical symmetry as locally spherical structures with center of curvature offset in three dimensions from the center of mass, from which follow analytic expressions for errors in bending angle and impact parameter as functions of the offset and trajectory parameters. Since these expressions are not restricted to any specific occultation type, it is easy to identify the geometrical configurations and the specific trajectory parameters that enhance or suppress these errors. Errors in bending angle and impact parameter carry over into the refractivity and radius profiles, while at the same time, new errors are introduced because the bending angle versus impact parameter profile is integrated along a nonvertical path in the presence of large‐scale departures from spherical symmetry, to obtain refractivity and radius profiles. Similarly, refractivity and radius errors propagate into the temperature and pressure profiles, while a nonvertical path of integration in the presence of horizontal gradients provides another opportunity for new errors to be introduced. We estimate that fractional errors in temperature profiles can be as large as a few percent for the Martian atmosphere above 20 km, decreasing in magnitude closer to the surface. For Earth, such errors are estimated to be less than 1% above 30 km. In the lower parts of Earth's atmosphere, however, and especially in the lower troposphere, these errors can be very sensitive to horizontal gradients and hence highly variable; typically, the error magnitude remains less than 2% for the dry regions of Earth's troposphere. We have not addressed the effect on errors of water vapor gradients, or of more extreme structures such as sharp weather fronts. A small variation on this approach can incorporate errors due to imprecise knowledge of the transmitter and receiver trajectories.
Retrieval of red spectral albedo and bidirectional reflectance using AVHRR HRPT and GOES satellite observations of the New England region
Authors: d'Entremont Robert P. , Schaaf Crystal Barker , Lucht Wolfgang , Strahler Alan H.
Published: September 2002

As a prototyping exercise for the moderate‐resolution imaging spectroradiometer (MODIS) albedo/BRDF product, we demonstrate the retrieval of bidirectional reflectance distribution functions (BRDFs) and red spectral albedo measures for the New England region, United States, from merged AVHRR and GOES radiances at a 1 km2 (nominal) spatial scale. These data were acquired during a 25‐day period in early fall 1995. The spatial pattern of BRDF retrievals shows that urban, suburban, and interurban regions exhibit directional scattering that is well modeled by the geometric optics of shadow casting. The directional reflectance of more continuous forest areas is better described by volume‐scattering mechanisms. Spectral albedos are larger in urban and suburban areas than in forested regions, as might be expected from the strong absorption of leaves in the red wave band. The red spectral albedo generally increases with solar zenith angle, as has been noted in ground measurements of broadband albedo. A number of technical limitations discussed constrain the absolute accuracy of retrieved albedos presented here, although the spatial patterns of albedo and the consistency of the BRDF shapes inspire confidence. These limitations will largely be overcome with application of our algorithm to data from the MODIS and MISR instruments on the EOS AM‐1 platform.
Validation of a new prototype water vapor retrieval for the UARS Microwave Limb Sounder
Authors: Pumphrey Hugh C.
Published: September 2002

The UARS Microwave Limb Sounder (MLS) measured water vapor in the middle atmosphere between September 1991 and April 1993. The current version of the data, version 4, and its predecessor, version 3, have been used in many scientific studies. As part of the process of developing the next version of UARS MLS data, a new prototype retrieval for the stratosphere/mesosphere water vapor product was developed at the University of Edinburgh. The main improvements made were (1) corrections for systematic errors and (2) doubling of the vertical resolution of the retrieval grid. Initial results were sufficiently encouraging that the code was used to produce data for all UARS days on which the MLS 183 GHz radiometer was operational. This paper describes these data and examines their quality. Comparisons are made with the available correlative data and it emerges that the prototype agrees better with the correlative data than does version 4. Agreement with frost point hygrometers is excellent in the lower stratosphere and agreement with a ground‐based microwave is satisfactory in the upper stratosphere and mesosphere. Agreement with two solar occultation instruments, Atmospheric Trace Molecular Spectroscopy Experiment (ATMOS) and Halogen Occultation Experiment (HALOE), is better than it was for earlier MLS versions but the prototype data are uniformly drier than both of these instruments. We compare the data with version 4, showing several features which are more clearly visible in the new prototype. These include seasonal cycles in the upper mesosphere and equatorial lower stratosphere.
A thermal emission spectral library of rock‐forming minerals
Authors: Christensen Philip R. , Bandfield Joshua L. , Hamilton Victoria E. , Howard Douglas A. , Lane Melissa D. , Piatek Jennifer L. , Ruff Steven W. , Stefanov William L.
Published: September 2002

A library of thermal infrared spectra of silicate, carbonate, sulfate, phosphate, halide, and oxide minerals has been prepared for comparison to spectra obtained from planetary and Earth‐orbiting spacecraft, airborne instruments, and laboratory measurements. The emphasis in developing this library has been to obtain pure samples of specific minerals. All samples were hand processed and analyzed for composition and purity. The majority are 710–1000 μm particle size fractions, chosen to minimize particle size effects. Spectral acquisition follows a method described previously, and emissivity is determined to within 2% in most cases. Each mineral spectrum is accompanied by descriptive information in database form including compositional information, sample quality, and a comments field to describe special circumstances and unique conditions. More than 150 samples were selected to include the common rock‐forming minerals with an emphasis on igneous and sedimentary minerals. This library is available in digital form and will be expanded as new, well‐characterized samples are acquired.
Digital photogrammetric analysis of the IMP camera images: Mapping the Mars Pathfinder landing site in three dimensions
Authors: Kirk R. L. , Howington‐Kraus E. , Hare T. , Dorrer E. , Cook D. , Becker K. , Thompson K. , Redding B. , Blue J. , Galuszka D. , Lee E. M. , Gaddis L. R. , Johnson J. R. , Soderblom L. A. , Ward A. W. , Smith P. H. , Britt D. T.
Published: September 2002

This paper describes our photogrammetric analysis of the Imager for Mars Pathfinder data, part of a broader program of mapping the Mars Pathfinder landing site in support of geoscience investigations. This analysis, carried out primarily with a commercial digital photogrammetric system, supported by our in‐house Integrated Software for Imagers and Spectrometers (ISIS), consists of three steps: (1) geometric control: simultaneous solution for refined estimates of camera positions and pointing plus three‐dimensional (3‐D) coordinates of ∼103 features sitewide, based on the measured image coordinates of those features; (2) topographic modeling: identification of ∼3×105 closely spaced points in the images and calculation (based on camera parameters from step 1) of their 3‐D coordinates, yielding digital terrain models (DTMs); and (3) geometric manipulation of the data: combination of the DTMs from different stereo pairs into a sitewide model, and reprojection of image data to remove parallax between the different spectral filters in the two cameras and to provide an undistorted planimetric view of the site. These processes are described in detail and example products are shown. Plans for combining the photogrammetrically derived topographic data with spectrophotometry are also described. These include photometric modeling using surface orientations from the DTM to study surface microtextures and improve the accuracy of spectral measurements, and photoclinometry to refine the DTM to single‐pixel resolution where photometric properties are sufficiently uniform. Finally, the inclusion of rover images in a joint photogrammetric analysis with IMP images is described. This challenging task will provide coverage of areas hidden to the IMP, but accurate ranging of distant features can be achieved only if the lander is also visible in the rover image used.
Surface processes in the Venus highlands: Results from analysis of Magellan and Arecibo data
Authors: Campbell Bruce A. , Campbell Donald B. , DeVries Christopher H.
Published: September 2002

The highlands of Venus are characterized by an altitude‐dependent change in radar backscatter and microwave emissivity, likely produced by surface‐atmosphere weathering reactions. We analyzed Magellan and Arecibo data for these regions to study the roughness of the surface, lower radar‐backscatter areas at the highest elevations, and possible causes for areas of anomalous behavior in Maxwell Montes. Arecibo data show that circular and linear radar polarization ratios rise with decreasing emissivity and increasing Fresnel reflectivity, supporting the hypothesis that surface scattering dominates the return from the highlands. The maximum values of these polarization ratios are consistent with a significant component of multiple‐bounce scattering. We calibrated the Arecibo backscatter values using areas of overlap with Magellan coverage, and found that the echo at high incidence angles (up to 70°) from the highlands is lower than expected for a predominantly diffuse scattering regime. This behavior may be due to geometric effects in multiple scattering from surface rocks, but further modeling is required. Areas of lower radar backscatter above an upper critical elevation are found to be generally consistent across the equatorial highlands, with the shift in microwave properties occurring over as little as 500 m of elevation. These surfaces are not simply characterized, however, by the absence of a highly reflective component. Surface morphology and radar‐scattering properties suggest that a mantling deposit forms at the highest elevations, most likely by in situ erosion of the original rock. In Ovda Regio, this process mantles or has removed surface festoon structure at the 1‐ to 10‐m scale, implying a significant depth for the weathered layer. Similar radar‐dark areas occur in Maxwell Montes but are apparently unrelated to the current topography of the region. Possible reasons for these observations include mass wasting from areas of steep slopes, compositional or age differences within the montes, vertical tectonic shifts of relict contacts, local topographic effects on surface temperature, or errors in the Magellan topography data in the rugged terrain. While there is evidence for some of these effects in the existing data, no single model at present appears to satisfy all occurrences of high‐altitude, radar‐dark terrain. New measurements of the surface and lower atmosphere chemistry of Venus are needed to further refine these conclusions.
Origin of carbonate‐magnetite‐sulfide assemblages in Martian meteorite ALH84001
Authors: Scott Edward R. D.
Published: September 2002

A review of the mineralogical, isotopic, and chemical properties of the carbonates and associated submicrometer iron oxides and sulfides in Martian meteorite ALH84001 provides minimal evidence for microbial activity. Some magnetites resemble those formed by magnetotactic microorganisms but cubic crystals <50 nm in size and elongated grains <25 nm long are too small to be single‐domain magnets and are probably abiogenic. Magnetites with shapes that are clearly unique to magnetotactic bacteria appear to be absent in ALH84001. Magnetosomes have not been reported in plutonic rocks and are unlikely to have been transported in fluids through fractures and uniformly deposited where abiogenic magnetite was forming epitaxially on carbonate. Submicrometer sulfides and magnetites probably formed during shock heating. Carbonates have correlated variations in Ca, Mg, and 18O/16O, magnetite‐rich rims, and they appear to be embedded in pyroxene and plagioclase glass. Carbonates with these features have not been identified in carbonaceous chondrites and terrestrial rocks, suggesting that the ALH84001 carbonates have a unique origin. Carbonates and hydrated minerals in ALH84001, like secondary phases in other Martian meteorites, have O and H isotopic ratios favoring formation from fluids that exchanged with the Martian atmosphere. I propose that carbonates originally formed in ALH84001 from aqueous fluids and were subsequently shock heated and vaporized. The original carbonates were probably dolomite‐magnesite‐siderite assemblages that formed in pores at interstitial sites with minor sulfate, chloride, and phyllosilicates. These phases, like many other volatile‐rich phases in Martian meteorites, may have formed as evaporite deposits from intermittent floods.
Mars mapping with delay‐Doppler radar
Authors: Harmon John K. , Arvidson Raymond E. , Guinness Edward A. , Campbell Bruce A. , Slade Martin A.
Published: September 2002

Mars radar imaging results from Arecibo 12.6‐cm observations are presented. The images were derived from delay‐Doppler mapping using a coded‐long‐pulse technique to mitigate the effects of echo overspreading. Images of the depolarized echo are used to identify regions of high decimeter‐scale roughness. Some of the strongest echo features are located on the major shield volcanoes or on relatively young off‐shield flows such as the Olympus and Pavonis lava aprons. The shields themselves have highly irregular radar signatures suggesting complex volcanic histories. Some Mars radar features have twice the depolarized brightness of the roughest terrestrial lava flows, apparently due to higher levels of multiple scattering from surfaces of spectacular roughness or from volume scattering. Low‐brightness (smooth) areas are associated with older surfaces such as fractured and highland terra, as well as with terrain interpreted to be debris lobes, ash flows, and aureoles; in particular, a close connection was found between the 12.6‐cm counterpart of the “Stealth” feature and the Medusae Fossae Formation (postulated to be deep ignimbrite deposits). Marte Valus is anomalous in being the only outflow channel showing strongly enhanced echoes, which supports the idea that this channel and the Elysium Basin that it drained are filled with lava flows. A weak radar feature was found for the south polar residual ice cap. Comparisons with Goldstone 3.5‐cm data show that the south polar enhancement is much weaker at 12.6 cm than at 3.5 cm, indicating that the southern ice cap becomes optically thin at the longer wavelength. A north polar enhancement has also been found, which is comparable in strength to the 12.6‐cm south polar feature.