bluuebunny/crossref_metadata_2025 huggingface dataset:
On the validity of the independent pixel approximation for boundary layer clouds observed during ASTEX
Authors: Zuidema Paquita , Evans K. Franklin
Published: February 2004

The two‐dimensional radiative transfer behavior of nine marine stratocumulus clouds observed by cloud radar during the Atlantic Stratocumulus Transition Experiment is examined. The cloud radar resolves the vertical structure to 37.5 m. The method of [Frisch et al., 1995] is used to convert radar reflectivities to extinction fields. Three constructions of the same cloud field help elucidate underlying causes of variability: one is fully two‐dimensional, while the other two have vertically uniform extinction fields but possess either a flat cloud top or the original cloud top topography. Two‐dimensional solar radiative transfer results are compared with the independent pixel approximation (IPA) result.

At the scale of the domain (≈ 7km) the IPA albedo bias is small, even after including vertical structure. Locally, in contrast, the direct solar beam interaction with cloud top geometry competes with radiative smoothing as the dominant radiative process. Power spectral analysis of nadir reflectances is dominated by radiative smoothing for overhead Sun, and side illumination/shadowing of cloud top bumps for low Sun. A method that incorporates direct beam interactions with the cloud geometry, in addition to radiative smoothing, significantly improves correlations of a smoothed IPA radiance field with the 2‐D reflectances. In a remote sensing application, optical depth and albedo retrieval biases from plane‐parallel theory depend on the spatial scale chosen to emulate a satellite pixel size. For scales less than a few kilometers and with low Sun, cloud top topography can cause large positive optical depth biases even when averaged over the entire domain. A larger spatial scales the negative IPA bias always dominates. Domain‐averaged monochromatic albedo retrieval errors remain below 0.005, a relative error of less then 1%.
The texture of clouds
Authors: Jameson A. R. , Kostinski A. B. , Black R. A.
Published: February 2004

Using a precise definition of clustering, it is shown that in two tropical cumulus clouds, droplets appear to be bunched over distances ranging from at least a kilometer or more down to several centimeters. A statistical framework is proposed for quantifying clustering in terms of a Poisson probability mixture. While these observations require further substantiation in many different clouds, droplet clustering may play a role in diverse phenomena from the coalescence growth of raindrops to the scattering of radiation by clouds.
Using the Oklahoma Mesonetwork to develop and test a practical sampling error statistic for meteorological time series
Authors: Morrissey Mark L. , Greene J. Scott
Published: February 2004

The error variance based upon sampling in time which accounts for the missing values in meteorological and climatological averages in a time series is developed. This statistic can be substituted for those already developed which assume a random or uniform distribution of discrete measurements in time. The statistic is unique from other error statistics developments published in the meteorological literature in that it relates the error variance of local time averages (e.g., monthly) around the local population averages rather than the long‐term mean and allows for missing data or unevenly sampled data. A comparison of theoretical values with those derived from an autoregressive time series and a nonstationary reference data set indicate that the statistic can be appropriately used to provide researchers with a quantitative measure of the usefulness of their data.
Atmospheric methane measurements in central New England: An analysis of the long‐term trend and the seasonal and diurnal cycles
Authors: Shipham Mark C. , Bartlett Karen B. , Crill Patrick M. , Harriss Robert C. , Blaha Denise
Published: February 2004

We have compiled a unique high‐resolution ambient‐air methane data set consisting of approximately 125,000 independently measured data points for the years 1991–1995 that have been collected at a site in the northeastern United States. The annual median mixing ratio of methane for all measurements was 1808 ppbv in 1992, increasing at a variable rate to 1837 ppbv in 1995. The lower 10–30% of the data from each month were defined as representative of background air and were compared with the global Climate Monitoring and Diagnostics Laboratory (CMDL) data set. The background data exhibit a variable upward trend of 5.5 ± 2 ppbv/yr during the 4‐year time period, with most of the increase observed during 1993 and 1994. The seasonal cycle for the background data set is similar to what is observed by CMDL stations and varies from 24 to 35 ppbv. The amplitude of the seasonal cycle for the full data set was larger, ranging from 35 to 44 ppbv. Differences between the full and background mixing ratios vary on a seasonal basis and are largest in the winter and smallest in the summer. These differences appear to be controlled by changes in atmospheric stability and changes in emissions from local and regional sources throughout the year. Diurnal cycles exist in the data, with the magnitude and timing of maximum and minimum values being controlled by inputs from local sources and atmospheric stability. Nearby wetlands contribute to an overnight buildup of methane in the late spring and summer. The magnitude of the daily cycle is largest in July and August (∼23 ppbv), when inputs from wetlands are large and wind speed is generally low. In April, the daily cycle is smallest (∼6 ppbv), when inputs from local sources are low and more vigorous atmospheric mixing limits pollution buildup.
Fast Fourier synoptic mapping of UARS data
Authors: Sassi Fabrizio , Salby Murry
Published: February 2004

Fast Fourier synoptic mapping (FFSM) is used to develop an operational algorithm for mapping dynamical and chemical structure from UARS observations. The operational algorithm (1) utilizes contemporaneous measurements from different UARS instruments observing the same field property, (2) accounts for measurement error from those instruments, and (3) treats data voids that follow from operational considerations such as duty cycle, power failure, and the satellite's routine yaw maneuver. The incoherent component of space‐time variability, which includes instrument noise and random sampling error, is evaluated from the asynoptic measurements and rejected during the mapping procedure. Consolidating measurements from different instruments and interpolating low‐frequency components of the space‐time spectrum across gaps enable synoptic structure to be mapped continuously over durations of several months. Time series of synoptic maps reveal Kelvin wave activity radiating away from its convective source region in the western Pacific, localized transfers of ozone between the tropics and extratropics, and diurnal variations resolved by the UARS sampling. Dynamical and chemical structure mapped via FFSM is validated by intercomparison of chemical tracers and compared to archived grids mapped via Kalman filtering.
Performance evaluation of the U.S. National Lightning Detection Network in eastern New York: 1. Detection efficiency
Authors: Idone Vincent P. , Davis Daniel A. , Moore Paul K. , Wang Yan , Henderson Ronald W. , Ries Mario , Jamason Paul F.
Published: February 2004

The detection efficiency (DE) of the U.S. National Lightning Detection Network (NLDN) has been evaluated using a large data set of video observations of cloud‐to‐ground lightning activity in the vicinity of Albany, New York. These data were acquired during the summers of 1993, 1994, and 1995, the latter being the year of completion of a major upgrade of the network to the improved accuracy from combined technology (IMPACT) configuration. For 1993, we find a flash DE value of 67% based upon 517 cloud‐to‐ground flashes documented on video. The latter two years yielded both flash and stroke DEs: in 1994, 86% of 893 flashes and 67% of 2162 strokes were detected; in 1995, 72% of 433 flashes and 47% of 1242 strokes were detected. The higher DEs of 1994 relative to 1995 are likely due to additional sensors deployed locally during the initial stage of the IMPACT upgrade. Detection efficiencies were found to vary significantly from storm to storm in each season, likely due to the inherent variability of return stroke characteristics between storms. For a special subset of 92 strokes of known location and measured electric‐field change, peak current estimates were generated using the transmission‐line model and a return stroke speed of 1.2×l08 m/s. This speed was selected, as it is the effective speed used in present NLDN peak current estimates. For this 92‐stroke data subset, the stroke DE depended upon peak current: strokes with peak currents greater than 14 kA were almost always detected (39 of 40); below 14 kA, the DE dropped until by 6–10 kA, the stroke DE was only 18% (three of 17). None of 14 strokes with estimated peak currents below 6 kA was detected. If the IMPACT design constraint of an effective 5‐kA minimum peak current is applied to our 92‐stroke subset, the respective flash and stroke DEs are 84% and 69%; this is consistent with NLDN model predicted performance in this area. As a faster return stroke speed, possibly 1.8×108 m/s, would seem appropriate, the above cited current values would need to be scaled downward by a factor of 2/3, implying greater actual sensitivity of the NLDN to weaker strokes. However, a commensurate adjustment downward would be required of present NLDN‐derived peak current estimates as well.
Satellite climatology of African dust transport in the Mediterranean atmosphere
Authors: Moulin C. , Lambert C. E. , Dayan U. , Masson V. , Ramonet M. , Bousquet P. , Legrand M. , Balkanski Y. J. , Guelle W. , Marticorena B. , Bergametti G. , Dulac F.
Published: February 2004

A daily analysis of African dust concentrations in the Mediterranean atmosphere has been made between June 1983 and December 1994 using the International Satellite Cloud Climatology Project (ISCCP‐B2) archive of Meteosat visible (VIS) channel images. The ISCCP‐B2 archive of Meteosat infrared (IR) images has also been used to determine the frequencies of dust mobilization over the continent, north of 30°N. Despite a large daily variability, climatological results show a clear seasonal cycle with a maximum during the dry season: dust transport begins over the eastern basin in spring and spreads over the western basin in summer. These patterns are shown to be related to both cyclogenesis over North Africa and rainfall over the Mediterranean Sea. Indeed, the frequency of dust mobilization over the continent and of dust outbreaks over the sea are strongly related to the climatology of depressions affecting North Africa. Precipitations appear to be an important factor explaining both the seasonal east‐west shift in transport location and the south‐north gradients of dust concentrations over the Mediterranean.
Combustion completeness in a rainforest clearing experiment in Manaus, Brazil
Authors: Carvalho Joäo A. , Higuchi Niro , Araújo Thaís M. , Santos José C.
Published: February 2004

Results are described for a forest clearing experiment conducted in the forest reserve of the Tropical Silviculture Experimental Station of the National Institute for Research in the Amazon (INPA), located 60 km from Manaus, the capital of the Brazilian state of Amazonas. An area of 1 ha of virgin forest was cut in July 1995 and left to dry until October of the same year. It was burnt using a procedure similar to that followed by native people in the region to prepare the land for cultivation. The fresh biomass content in the test location, 684.8 t ha−1, was determined using a formula with parameters from forest inventory. The dry biomass content was 401.5 t ha−1. Combustion completeness, 20.5 %, was estimated by selecting ten 2 × 2 m2 subareas, 20 trunks with diameter at breast height (DBH) between 10 and 30 cm, and 71 trunks with DBH higher than 30 cm. Their consumption rates by fire were measured. The 2 × 2 m2 areas were used to determine the combustion completeness of smaller plant components (characteristic diameters lower than 10 cm) and the trunks to determine the efficiency of the medium and large components (characteristic diameters between 10 and 30 cm and larger than 30 cm, respectively). Combustion completeness for small, medium and large components were 88.2%, 4.39%, and 0.43%, respectively. On the basis of the biomass content (684.4 t ha−1), the average biomass moisture and carbon contents (41.6% and 47.8%, respectively, the latter on dry basis), and the obtained combustion completeness (20.5%), the average carbon, CO2 and CO mass rates released to the atmosphere were estimated to be 37.7, 121, and 8.6 t ha−1, respectively.
Change in the aerodynamic roughness height by saltating grains: Experimental assessment, test of theory, and operational parameterization
Authors: Gillette Dale A. , Marticorena Béatrice , Bergametti Gilles
Published: February 2004

Data from an experiment at Owens Lake provided an opportunity to verify the validity of Raupach's formula that predicts the apparent roughness height at equilibrium with saltation during wind erosion episodes. In addition to that verification, a simplification of Raupach's formula is presented which allows the computation of the wind friction velocity affected by saltating sand grains without the need to measure the threshold wind friction velocity and to do iterative calculations. This method estimates the increase of the wind friction velocity Δu*, above the nonsaltating wind friction velocity caused by the saltating grains: Δu* = 0.3 [U − Ut]2 where U and Ut are the wind speed and the threshold wind speed, respectively, at 10m.
Non local thermodynamic equilibrium (LTE) atmospheric limb emission at 4.6 μm: 1. An update of the CO2 non‐LTE radiative transfer model
Authors: López‐Puertas M. , Zaragoza G. , López‐Valverde M. Á. , Taylor F. W.
Published: February 2004

An update of the CO2 non local thermodynamic equilibrium (LTE) radiative transfer model of López‐Puertas et al. [1986a,b] is presented. The major improvements are (1) a revision of the modified Curtis matrix method; (2) a significant extension of the CO2 vibrational levels and bands (the current version includes 76 states and 129 transitions); and (3) a revision of the excitation mechanisms and collisional parameters. The major differences with previous results are discussed.
Glacial/interglacial variations of meridional transport and washout of dust: A one‐dimensional model
Authors: Andersen Katrine K. , Ditlevsen Peter D.
Published: February 2004

Measurements of δ18O and insoluble dust in the Greenland Ice Core Project ice core reveal that the concentration of dust is about 100 times higher in ice from the last glacial maximum than it is today. In order to understand the glacial climate it is of importance to establish to what extent this increased level was due to changes in the source areas and to what extent it was due to changes in the atmospheric transportation. We here present a one‐dimensional model evaluating the effect of changes in the zonally averaged atmospheric circulation on the atmospheric hydrological cycle and dust transportation. The main characteristics of the altered climate during glacial periods are assumed to be increased baroclinicity, an equatorward displacement of the baroclinic zone, and reduced evaporation. The model reproduces the zonally averaged hydrological cycle of the present climate reasonably well, and it produces a halving of snow accumulation in polar areas for glacial periods. From the ice core data we obtain a power law dependence between the concentration of dust in the ice and the accumulation related to long‐term climate variations. With the input of dust to the atmosphere being independent of the simulated climate, the model reproduces this power law. The obtained power is strongly dependent on the assumed position of the dust sources. For reasonable estimates of the present‐day dust sources the simulated mechanism may account for a twofold to sixfold increase in dust concentration in polar ice from interglacial to full glacial conditions. Concurrently, the atmospheric content of dust is increased at all latitudes during the glacial period.
Physical properties of stratospheric clouds during the Antarctic winter of 1995
Authors: Gobbi Gian Paolo , Di Donfrancesco Guido , Adriani Alberto
Published: February 2004

Lidar observations collected during winter 1995 at McMurdo Station, Antarctica (78°S‐167°E), are analyzed to determine polar stratospheric cloud (PSC) physical properties. A scheme to infer PSC phase from lidar depolarization and backscatter profiles is presented. Interpretation is supported by collocated temperature soundings and by isentropic back trajectories. The analysis shows that first appearance of PSC is consistent with frozen sulfates, mixing with liquid ternary solutions (H2SO4‐HNO3‐H2O) when temperature lowers. Finally, solids consistent with HNO3 mixing ratios form as mixed phases first, then followed by full solid phases. Mixed phases (i.e., coexisting solid and liquid aerosols) are detected during the whole winter. While mixed phase PSCs form particularly in the altitude range 15–20 km and are the last to disappear, full solid phases are mainly observed above 20 km and last until the end of August. Mixed phases possess the largest PSC surface areas and, as a result of selective growth, can reach large, fast settling sizes. The considerable denitrification and halogen activation observed in the Antarctic lower stratosphere, where the ozone hole takes place, appears to be well correlated with the action of this kind of PSC.
Changing lower stratospheric circulation: The role of ozone and greenhouse gases
Authors: Graf Hans‐F. , Kirchner Ingo , Perlwitz Judith
Published: February 2004

Stratospheric climate has changed significantly during the last decades. The causes of these changes are discussed on the basis of two different general circulation model experiments forced by observed greenhouse gas and ozone concentration. There is a clear and significant response of the lower stratosphere temperature and geopotential in the model simulations forced by observed ozone changes that is in accord with observed trends in summer in middle and high latitudes of the northern hemisphere. Little effect is seen in the tropics. In spring there occur the strongest anomalies/trends in both hemispheres at polar latitudes; however, the model response is late by 1 to 2 months and is much weaker than the observed effects. The ozone‐forced model in winter of both hemispheres produces slight warming or no change instead of the slight cooling observed. The effects of enhanced greenhouse gases as taken from a transient IPCC scenario AGCM run do enhance the cooling in high latitudes in spring, but the effect is much smaller than observed. Hence neither of the two forcings (reduced ozone and increased greenhouse gases) in the cold seasons is able to produce the recent stratospheric and tropospheric trend patterns alone. These trends clearly resemble a natural mode of variability both in the model and in the real world. This mode associates a strengthened polar night vortex with an enhanced North Atlantic oscillation. The excitation of this mode cannot yet be attributed to anthropogenic forcing.
A Lagrangian estimate of aircraft effluent lifetime
Authors: Schoeberl M. R. , Jackman C. H. , Rosenfield J. E.
Published: February 2004

The lifetime of aircraft exhaust is estimated using multiyear diabatic trajectory integrations using United Kingdom Meteorological Office and Goddard Space Flight Center (GSFC) Data Assimilation Office winds. Instantaneous January trajectory parcel releases in the midlatitude stratosphere at 19, 16, 14, 13, and 11 km are compared. The first two cases correspond to the proposed flight altitudes of high‐speed supersonic aircraft. The last two cases correspond to subsonic flight altitudes. Parcels descending below 250 hPa are eliminated as would occur when aircraft effluent reaches the troposphere. After a transient adjustment period the total number of parcels in most experiments decays exponentially with a decay period (lifetime) of about 1 year. This is the lifetime of the longest‐lived eigenmode as discussed by Prather [1996]. Our 1‐year lifetime is shorter than the 1.5 year GSFC two‐dimensional (2‐D) model lifetime for the same release, but in good agreement with the 1.13 year GSFC 3‐D model lifetime. We also find that the effluent lifetime is slightly longer in the southern hemisphere midlatitude stratosphere than in the northern. Both the 13 and 11 km releases are mostly within the stratospheric middle world and rapidly leave the stratosphere. We conclude that there will be a small impact of subsonic aircraft exhaust on the larger stratosphere. Finally, we find that our overall lifetime results are fairly sensitive to the lower stratospheric heating rates.
Global OH trend inferred from methylchloroform measurements
Authors: Krol Maarten , van Leeuwen Peter Jan , Lelieveld Jos
Published: February 2004

Methylchloroform (MCF) measurements taken at the Atmospheric Lifetime Experiment / Global Atmospheric Gases Experiment (ALE/GAGE) measurement stations are used to deduce the tropospheric OH concentration and its linear trend between 1978 and 1993. Global three‐dimensional fields of OH are calculated with a transport model that includes background photochemistry. Despite the large uncertainties in these OH fields, the simulated MGF concentrations at the five ALE/GAGE stations compare reasonably well to the measurements. As a next step, the OH fields are adjusted to fit the measurements optimally. An ensemble (Monte Carlo) technique is used to optimize the OH scaling factor and to derive the linear trend in OH. The optimized OH fields and trend imply a MCF lifetime in the troposphere of 4.7 years in 1978 and of 4.5 years in 1993. For CH4 these lifetimes (due to OH destruction only) are 9.2 and 8.6 years in 1978 and 1993, respectively. Uncertainties in these estimates are discussed using box‐model calculations. The optimized OH concentration is sensitive to the strength of other MCF sinks in the model and is constrained to 1.00−0.15+0.09 × 106 × molecules cm−3 in 1978 and to 1.07−0.17+0.09 × 106 molecules cm−3 in 1993. The deduced OH trend is sensitive to the trend in the MCF emissions and is confined to the interval between −0.1 and +1.1% yr−1 with a most likely value of 0.46% yr−1. Possible causes of a global increase in OH are discussed. A positive OH trend is calculated due to stratospheric ozone depletion, declining CO concentrations, increased water vapor abundance, and enhanced NOx emissions. Although the changes in the atmospheric composition are to a large extent unknown, it seems that the observed changes are consistent with significant increases in OH over the past decades.
Atmospheric CO2 simulated by the National Center for Atmospheric Research Community Climate Model: 1. Mean fields and seasonal cycles
Authors: Craig S. G. , Holmén K. J. , Bonan G. B. , Rasch P. J.
Published: February 2004

Atmospheric CO2 has been simulated on‐line in a coupled model of the atmosphere and terrestrial land surface. The atmospheric model is a precursor to version 3 of the Community Climate Model developed at the National Center for Atmospheric Research. It interacts with a mechanistic parameterization of the land surface at each time step. Photosynthesis and respiration fluxes are calculated at each time step and are linked to the tracer transport scheme that advects CO2 through the model atmosphere. For the first time in atmospheric CO2 modeling, these two fluxes are not forced to balance annually at each grid point. The model was run for 10 years with climatological sea surface temperatures, and the results were saved as monthly means. In this paper, an average year is calculated from the output of the 10 year run and then evaluated in terms of mean distributions and seasonal cycles. The model reproduces many of the observed features of the atmospheric CO2 distribution although there are discrepancies in some areas. The seasonality of tropical carbon exchange is exaggerated which leads to unrealistic CO2 fields near the equator. Carbon uptake is too strong at high northern latitudes in the boreal summer which results in a spurious annual carbon sink and exaggerated seasonal CO2 cycle amplitudes at these latitudes. The annual sink strongly affects the simulated interhemispheric CO2 gradient. Errors in the temperature and precipitation simulation cause anomalies in the CO2 field in specific regions such as the United States and India. Possible model improvements are discussed.
Thermodynamic structure and entrainment of stratocumulus over the Southern Ocean
Authors: Boers R. , Krummel P. B. , Siems S. T. , Hess G. D.
Published: February 2004

Mission 12 of the first Aerosol Characterization Experiment was flown by the National Center for Atmospheric Research C‐130 research aircraft to observe stratocumulus cloud decks over the remote Southern Ocean. The objective of the flight was the investigation of the impact of clouds on the formation and destruction of aerosol in the boundary layer. The principle focus of this paper is an examination of the thermodynamic structure and a quantification of the entrainment rate. Three methods are presented. The first method uses the thermodynamic budget of the boundary layer to calculate its evolution over time. The second uses turbulence flux observations near the inversion, while the third uses estimates of the subsidence rate together with direct observations of the rate of change of cloud top height. All three methods suffer from serious errors. Our best estimate for the entrainment rate is 4 mm s−1 with an uncertainty of at least 50%. Based on an analysis of the boundary layer evolution in a mixing diagram, entrainment rates larger than about 8 mm s−1 can probably excluded because these larger entrainment rates would impose an unrealistic thinning of the cloud layer. Similarly, entrainment rates smaller than 2 mm s−1 can be excluded as well on the basis that this would impose unrealistic thickening of the cloud layer. The boundary layer is mostly buoyancy‐driven, with shear‐generated turbulence at most only half of the buoyancy‐generated turbulence.
Equatorial diurnal variations observed in UARS Microwave Limb Sounder temperature during 1991–1994 and simulated by the Canadian Middle Atmosphere Model
Authors: Wu D. L. , McLandress C. , Read W. G. , Waters J. W. , Froidevaux L.
Published: February 2004

This paper presents a study of the equatorial diurnal tide at 27–55 km based on temperature data from the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS). The MLS temperature retrieval is significantly improved in the lower stratosphere with the recent (version 4) software and able to provide useful tidal information down to 22 hPa. The diurnal amplitude and phase, derived from MLS ascending‐descending temperature differences, show a strong annual variation in the lower stratosphere and a semiannual variation in the upper stratosphere and mesosphere, which are in good agreement with other UARS measurements and simulations from the Canadian Middle Atmosphere Model (CMAM). The largest differences between the observation and the model appear at 22 and 1 hPa, reflecting some shortcomings in current tidal modeling. It is shown in MLS observations that the (1, 1) tide suffers severe reduction in amplitude at 1 hPa which could be caused by the interference between the propagating tide and the component generated by stratospheric ozone heating. The data also indicate the possible presence of a short‐lived propagating component (i.e., the (1, 2) mode) in the lower stratosphere, which can be excited in the upper troposphere but damped quickly as it propagates upward.
Submicrosecond field variations during the onset of first return strokes in cloud‐to‐ground lightning
Authors: Willett J. C. , Krider E. P. , Leteinturier C.
Published: February 2004

Measurements of the electric fieldEanddE/dtsignatures radiated by first return strokes striking seawater in 1985 show that the average peakEand peakdE/dtare 8.6±4.4 V m−1and 42±13 V m−1μs−1, range‐normalized to 100 km, after correcting for the effects of propagation. The full width at half maximum of the initial half‐cycle ofdE/dtis 64±22 ns. The corresponding peak current and peakdI/dt, inferred using a simple semiempirical relationship, are 29±15 kA and 105±34 kA μs−1. These results are in good agreement with similar measurements made in 1984.
Direct observations of aerosol radiative forcing over the tropical Indian Ocean during the January‐February 1996 pre‐INDOEX cruise
Authors: Jayaraman A. , Lubin D. , Ramachandran S. , Ramanathan V. , Woodbridge E. , Collins W. D. , Zalpuri K. S.
Published: February 2004

Simultaneous measurements of aerosol optical depth, size distribution, and incoming solar radiation flux were conducted with spectral and broadband radiometers over the coastal Indian region, Arabian Sea, and Indian Ocean in January‐February 1996. Columnar aerosol optical depth, δa, at visible wavelengths was found to be 0.2–0.5 over the Arabian Sea and <0.1 over the equatorial Indian Ocean. Aerosol mass concentration decreased from about 80 μg/m3 near the coast to just a few μg/m3 over the interior ocean. The sub‐micron‐size particles showed more than an order of magnitude increase in number concentration near the coast versus the interior ocean. This large gradient in particle concentration was consistent with a corresponding large increase in the Sun‐photometer‐derived Angstrom exponent, which increased from 0.2 over the Indian Ocean to about 1.4 near the coast. The change in surface‐reaching solar flux with δa was obtained for both the direct and the global solar flux in the visible spectral region. The solar‐zenith‐angle‐normalized global and diffuse fluxes vary almost linearly with normalized δa. The direct visible (<780 nm) solar flux decreases by about 42±4 W m−2 and the diffuse sky radiation increases by about 30±3 W m−2 with every 0.1 increase in δa, for solar zenith angles smaller than 60°. For the same extinction optical depth the radiative forcing of the coastal aerosols is larger than the open ocean aerosol forcing by a factor of 2 or larger.
An update on reactive odd‐nitrogen measurements made during recent NASA Global Tropospheric Experiment programs
Authors: Bradshaw J. , Sandholm S. , Talbot R.
Published: February 2004

This report addresses issues that have been raised recently about the measurements of reactive odd nitrogen and, in particular, those measurements made as part of the NASA Global Tropospheric Experiment airborne field campaigns. The test results presented here demonstrate that it is unlikely that HNO3mixing ratios are underestimated by large factors (i.e., by greater than 1.5‐fold) during the Pacific Exploratory Mission‐ West A, Transport and Atmospheric Chemistry Near the Equator Atlantic, and Pacific Exploratory Mission‐ West B field campaigns and that other mechanisms are needed to explain discrepancies between measured and modeled HNO3mixing ratios. Results from NOyconversion efficiency studies indicate that potentially large artifacts may have affected the Georgia Institute of Technology measurements of total reactive odd nitrogen (NOy). Assessing the exact magnitude of these artifacts is hindered by a lack of knowledge of both tropospheric distributions of non‐NOyforms of fixed nitrogen and the efficiency with which they were detected at the time of the NOymeasurement. Indirectly, our results suggest that up to ppbv levels of non‐NOyfixed nitrogen (e.g., NH3, HCN, and perhaps nitrogeneous sulfur compounds) may exist in some regions of the remote troposphere.
Measurements of the dynamical cooling rate associated with the vertical transport of heat by dissipating gravity waves in the mesopause region at the Starfire Optical Range, New Mexico
Authors: Gardner Chester S. , Yang Weimin
Published: February 2004

Vertical heat flux and cooling rate profiles of the mesopause region resulting from dynamic processes were computed using wind and temperature measurements made with a Na lidar at the Starfire Optical Range near Albuquerque, New Mexico. Sixty five hours of observations were conducted on eight nights in 1994–1995. The mean vertical wind and temperature variances were 4.32 m2/s2 and 80.3 K2. When averaged (over seven nights) between about 92.5±6 km, the mean vertical heat flux was −2.29±0.43 Km/s, the mean cross‐correlation coefficient for the wind and temperature fluctuations was −0.12±0.02, and the mean cooling rate was −30.9±5.2 K/d. The mean heat flux and cooling rate profiles reach their maximum negative values just above 90 km where the modeled chemical and measured turbulent heating are maximum. The mean cooling rate is comparable to theoretical predictions (−36 K/d) assuming values near 320 m2/s for the vertical diffusivity. Published rocket measurements of the turbulent heating caused by breaking gravity waves are only a few K/d in this height range. Thus the net effect of dissipating gravity waves is a significant cooling of the mesopause region.
Calculated trends and the atmospheric abundance of 1,1,1,2‐tetrafluoroethane, 1,1‐dichloro‐1‐fluoroethane, and 1‐chloro‐1,1‐difluoroethane using automated in‐situ gas chromatography‐mass spectrometry measurements recorded at Mace Head, Ireland, from October 1994 to March 1997
Authors: Simmonds P. G. , O'Doherty S. , Huang J. , Prinn R. , Derwent R. G. , Ryall D. , Nickless G. , Cunnold D.
Published: February 2004

The first in‐situ measurements by automated gas chromatograph‐mass spectrometer are reported for 1,1,1,2‐tetrafluoroethane (HFC‐134a), 1,1‐dichloro‐1‐fluoroethane, (HCFC‐141b), and 1‐chloro‐1,1‐difluoroethane, (HCFC‐142b). These compounds are steadily replacing the chlorofluorocarbons (CFCs) as refrigerants, foam‐blowing agents, and solvents. The concentrations of all three compounds are shown to be rapidly increasing in the atmosphere, with 134a increasing at a rate of 2.05±0.02 ppt yr−1 over the 30 months of observations. Similarly, 141b and 142b increased at rates of 2.49±0.03 and 1.24±0.02 ppt yr−1, respectively, over the same period. The concentrations recorded at the atmospheric research station at Mace Head, Ireland, on January 1, 1996, the midpoint of the time series, were 3.67 ppt (134a), 7.38 ppt (141b), and 8.78 ppt (142b). From these observations we optimally estimate the HCFC and HFC emissions using a 12‐box global model and OH concentrations derived from global 1,1,1‐trichloroethane (CCl3CH3) measurements. Comparing two methods of estimating emissions with independent industry estimates shows satisfactory agreement for 134a and 141b, while for 142b, industry estimates are less than half those required to explain our observations.
Spatial Gradients in Aerosol Characteristics over the Arabian Sea and Indian Ocean
Authors: Satheesh S. K. , Moorthy K. Krishna , Murthy B. V. Krishna
Published: February 2004

Aerosol spectral optical depths (τpλ) estimated at 10 wavelengths in the visible and near infrared range over oceanic regions covering near and far coastal regions of the Arabian Sea and remote areas of the equatorial Indian Ocean adjoining the Indian peninsula have been used to study the spatial variation of aerosol optical depths over the ocean. Size parameters and concentrations of aerosols along the downwind direction have been derived with a view to studying the effect of the continent on aerosol properties over the ocean. The studies showed a gradual decrease in τpλ with fetch distance D with an e−l scaling distance in the range of 1700 to 2400 km under calm to moderate wind conditions. The scaling distance increases with increase in wind speed. A significant increase in the relative concentration of small (accumulation size range) particles is seen as the continent is approached from deeper oceanic regions downwind, indicating increased influence of anthropogenic activities. The coarse‐particle concentration appears to be of oceanic origin and remains nearly steady over the entire region when the winds are low to moderate.
Stratospheric and mesospheric latitudinal water vapor distributions obtained by an airborne millimeter‐wave spectrometer
Authors: Peter R.
Published: February 2004

In the last 3 years, several flights from polar regions (Spitzbergen, 78°N) to the subtropics (Cap Verde Islands, 16°N) have been performed with the Airborne Millimeter‐ and Submillimeter‐Wave Observing System aboard a Learjet of the Swiss Air Force. From the observation of the H2O rotational transition at 183.31 GHz, altitude‐latitude cross sections of the middle atmospheric water vapor abundance for August 1994, October 1995, and March 1996 have been derived from the subtropical to Arctic regions. The key findings in this new H2O data set of the latitudinal variation from the lower stratosphere to the mesosphere are (1) the unusually high H2O abundance of 6.8 ppm at a potential temperature of 700 K in the spring 1996 polar vortex indicating a strong downward descent of ∼14 km from October to March associated with air mass isolation, (2) the seasonal dependent modulations of the dry subtropical stratospheric H2O profile, (3) flat volume mixing ratio (VMR) contours between 35° and 65°N due to isentropic mixing, (4) a maximum H2O VMR of more than 7.5 ppm in the subtropical mesosphere suggesting a disturbed mesospheric hydrogen partitioning possibly due to an enhanced conversion of methane and molecular hydrogen to water vapor, and (5) a general increase of middle atmospheric water vapor VMR of ∼1.5 ppm since 1986.
Shipboard measurements of dimethyl sulfide and SO2 southwest of Tasmania during the First Aerosol Characterization Experiment (ACE 1)
Authors: De Bruyn Warren J. , Bates Timothy S. , Cainey Jill M. , Saltzman Eric S.
Published: February 2004

Measurements of seawater dimethylsulfide (DMS), atmospheric dimethylsulfide, and sulfur dioxide (SO2) were made on board the R/V Discoverer in the Southern Ocean, southeast of Australia, as part of the First Aerosol Characterization Experiment (ACE 1). The measurements covered a latitude range of 40°S–55°S during November‐December 1995. Seawater DMS concentrations ranged from 0.4 to 6.8 nM, with a mean of 1.7±1.1 nM (1σ). The highest DMS concentrations were found in subtropical convergence zone waters north of 44°S, and the lowest were found in polar waters south of 49°S. In general, seawater DMS concentrations increased during the course of the study, presumably due to the onset of austral spring warming. Atmospheric DMS concentrations ranged from 24 to 350 parts per trillion by volume (pptv), with a mean of 112±61 pptv (1σ). Atmospheric SO2 was predominantly of marine origin with occasional anthropogenic input, as evidenced by correlation with elevated 222Rn and air mass trajectories. Concentrations ranged from 3 to 1000 pptv with a mean of 48.8± 49 pptv (1σ) and a median 15.8 pptv. The mean SO2 concentration observed in undisturbed marine air was 11.9±7.6 pptv (1σ), and the mean DMS to SO2 ratio in these conditions was 13±9 (1σ). Diurnal variations in SO2 were observed, with a daytime maximum and early morning minimum in agreement with model simulations of DMS oxidation in the marine boundary layer. Steady state calculations and photochemical box model simulations suggest that the DMS to SO2 conversion efficiency in this region is 30–50%. Comparison of these results with results from warmer regions suggests that the DMS to SO2 conversion efficiency has a positive temperature dependence.
A CH4emission estimate for the Kuparuk River basin, Alaska
Authors: Reeburgh W. S. , King J. Y. , Regli S. K. , Kling G. W. , Auerbach N. A. , Walker D. A.
Published: February 2004

Integrated annual methane fluxes measured from 1994 to 1996 at sites representing specific tundra vegetation and land cover types were weighted areally using a vegetation map [Auerbach et al, 1997] for the Kuparuk River basin and subareas. Wetland and open water CH4emissions dominate the Kuparuk River basin emission estimate. Areal weighting of site fluxes resulted in a regional CH4emission estimate of 2.09×1010g CH4yr−1for the Kuparuk River basin. The global CH4emission obtained by extending areally weighted annual fluxes from this study to global tundra area (7.34×1012m2) is 5.83 Tg CH4yr−1. This is about 15% of theFung et al.[1991] atmospheric tracer model estimate and indicates that the vegetation distribution of the Kuparuk River Basin is not typical of the entire Arctic. Reconciling results from atmospheric tracer model estimates and areally weighted field flux measurements will require accurate high‐resolution circumpolar estimates of wetland and open water areas and fluxes.
Evaluation of TOVS‐derived stratospheric temperatures up to 10 hPa for a case of vortex displacement over western Europe
Authors: Claud Chantal , Ovarlez Joelle , Scott Noelle A.
Published: February 2004

At the beginning of March 1993, cold stratospheric air was displaced toward midlatitudes. In situ measurements of stratospheric temperature and water vapor obtained at Aire‐sur‐Adour (44°N, 0°E) at that time suggest that air of polar origin was indeed sampled. Corresponding observations from the satellites of the TIROS N series have been analyzed using the Improved Initialization Inversion (31) algorithm to determine temperature profiles up to ∼10 hPa (∼30 km) and to examine the evolution of the thermal fields in the stratosphere. Collocations between 3I‐retrieved temperature profiles and radiosonde reports as well as comparisons with analyzed temperatures in the low stratosphere provided by the Stratospheric Research Group of the Free University of Berlin are presented and show the satisfactory accuracy of the retrievals in this part of the atmosphere. Three layers, according to their thermal behavior, could be distinguished as follows: below 100 hPa, from 100 to 30 hPa, and above. The investigation of this new case demonstrates the potential of TIROS N observations for climatic studies.
Discriminating heavy aerosol, clouds, and fires during SCAR‐B: Application of airborne multispectral MAS data
Authors: King Michael D. , Tsay Si‐Chee , Ackerman Steven A. , Larsen North F.
Published: February 2004

A multispectral scanning spectrometer was used to obtain measurements of the reflection function and brightness temperature of smoke, clouds, and terrestrial surfaces at 50 discrete wavelengths between 0.55 and 14.2 μm. These observations were obtained from the NASA ER‐2 aircraft as part of the Smoke, Clouds, and Radiation‐Brazil (SCAR‐B) campaign, conducted over a 1500×1500 km region of cerrado and rain forest throughout Brazil between August 16 and September 11, 1995. Multispectral images of the reflection function and brightness temperature in 10 distinct bands of the MODIS airborne simulator (MAS) were used to derive a confidence in clear sky (or alternatively the probability of cloud), shadow, fire, and heavy aerosol. In addition to multispectral imagery, monostatic lidar data were obtained along the nadir ground track of the aircraft and used to assess the accuracy of the cloud mask results. This analysis shows that the cloud and aerosol mask being developed for operational use on the moderate‐resolution imaging spectroradiometer (MODIS), and tested using MAS data in Brazil, is quite capable of separating cloud, aerosol, shadow, and fires during daytime conditions over land.
The CO2 seasonal cycle as a tracer of transport
Authors: Strahan S. E. , Douglass A. R. , Nielsen J. E. , Boering K. A.
Published: February 2004

CO2 time series in the lower stratosphere have been constructed as a function of latitude and altitude using ER‐2 aircraft observations. A seasonally varying signal derived from tropical surface CO2 data was fit to these data in a manner similar to that of Boering et al. [1996]. This analysis shows how the tropospheric CO2 seasonal cycle in air entering the tropical lower stratosphere ascends and gradually loses amplitude. The midlatitude results show that in the 370‐ to 400‐K range the tropical signal is transported poleward rapidly with only a small loss in amplitude. The signal also reaches the midlatitudes rapidly at higher altitudes but with markedly less amplitude. Above ∼440 K the midlatitude CO2 data best fit a line whose slope is the secular increase in CO2. This observational analysis is used as a basis for diagnosing transport in a CO2 simulation produced using GEOS‐1 Data Assimilation System (DAS) winds in an off‐line chemistry and transport model. The model is forced at the lowest levels by a 5‐year zonal mean CO2 data set derived from surface observations. The analysis of model results as a function of height and latitude determines whether (1) the seasonally varying CO2 cycle forced in the troposphere survives in the tropical lower stratosphere and whether (2) the CO2 cycle in the tropical lower stratosphere propagates realistically to the midlatitudes. We find that the model transports a CO2 seasonal cycle from the surface to the tropical upper troposphere that agrees with the phase and amplitude of tropospheric aircraft measurements. The cycle input into the model tropical lower stratosphere attenuates with height in a reasonable way compared with observations. The phase and amplitude of the cycle transported from the tropics to the midlatitudes agree well with the data up to about 440 K, but above that the model transports more of the tropical signal to the midlatitudes than is suggested by the ER‐2 data. Overall, there is remarkable consistency between the model and observations in the upper troposphere/lower stratosphere, which supports the use of assimilated winds in assessments of the effects of aircraft exhaust.
Zonally symmetric and asymmetric features of the tropospheric Madden‐Julian oscillation
Authors: Kayano Mary T. , Kousky Vernon E.
Published: February 2004

The evolving intraseasonal (IS) modes of the zonally symmetric (ZS) part (the latitudinal profile) of the 200‐hPa stream function have been determined by performing extended empirical orthogonal function (EEOF) analyses for the northern hemisphere (NH) summer and winter. In each analysis the first EEOF mode describes an oscillation with a period of about 50 days. The prominent feature of the upper tropospheric stream function latitudinal profiles, for both seasons, is the propagation of the largest loadings in both hemispheres from near the equator toward higher latitudes. This propagation is more evident during the NH winter and throughout the year in the southern hemisphere (SH). In the NH the poleward propagation of the loadings is more conspicuous within the equator and the 30°N latitudinal band. The associated patterns for the zonally asymmetric (ZA) part of the sea level pressure and the 850‐ and 200‐hPa zonal winds have also been determined. These patterns feature, in general, the largest correlations in the tropics and a large‐scale zonal wavenumber one structure propagating continuously eastward around the globe with a 50‐day period. The ZA patterns for certain variables show strong seasonal dependence. The 850‐hPa ZA zonal wind patterns feature the largest correlations approximately along the climatological position of the Intertropical Convergence Zone, whereas the largest correlations for the upper level ZA zonal wind patterns are found near the equator, mainly in the winter hemisphere.