bluuebunny/crossref_metadata_2025 huggingface dataset:
Covariance of biophysical data with digital topographic and land use maps over the FIFE site
Authors: Davis F. W. , Schimel D. S. , Friedl M. A. , Michaelsen J. C. , Kittel T. G. F. , Dubayah R. , Dozier J.
Published: February 2012

Sampling design is critical in locating ground sampling stations for large‐scale climatological field experiments. In the stratified sampling design adopted for the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), the study region was stratified into 14 different terrain units based on land use/land cover and topographic variables that were hypothesized to have a strong influence on surface biophysical properties. Digital terrain maps were produced to facilitate ground data integration and extrapolation. This paper describes the biophysical stratification of the FIFE site, implementation of the stratification using geographic information system (GIS) techniques, and validation of the stratification with respect to field measurements of biomass, soil moisture, Bowen ratio (β), and the greenness vegetation index (GVI) derived from thematic mapper satellite data. Maps of burning and topographic position were significantly associated with variation in biomass, GVI, and β. The effects of burning and topography were stronger for the Konza Prairie Long‐Term Ecological Research (KPLTER) site than for the rest of the FIFE site, where cattle grazing was a major confounding effect. The stratified design did not appreciably change the estimated site‐wide means for surface climate parameters but accounted for between 25 and 45% of the sample variance depending on the variable. The design was weakened by undersampling of several strata, by high within‐station variance in soil and vegetation data, and by failure to account for diverse land management practices on private lands surrounding KPLTER. We recommend that future large‐scale climatological studies include the development of a digital terrain data base well in advance of field campaigns and that multitemporal imagery be used to obtain preliminary estimates of spatial and temporal variance in surface biophysical properties. We also recommend that sampling for the most heterogeneous biophysical variables be conducted in the framework of a multistage estimation scheme incorporating remotely sensed data. Although this means that ground‐based estimation of regional fluxes cannot be made independent of aircraft or satellite data, it may well be the only means of obtaining reliable estimates of these variables over large areas.
Radiometric calibration of a polarization‐sensitive sensor
Authors: Ahmad Suraiya P. , Markham Brian L.
Published: February 2012

The radiometric accuracy of a sensor is adversely affected by scene polarization if its optical system is sensitive to polarization. Tests performed on the reflective bands of the NS001 thematic mapper simulator, an aircraft multispectral scanner, show that it is very sensitive to the polarization state of the incoming radiations. For 100% linearly polarized light, errors in the measured intensity vary from −40 to +40%, depending on the scan angle and spectral band. To estimate polarization‐induced errors in the intensity measured at aircraft level, the intensity and polarization of the atmospheric radiances were simulated using a realistic Earth‐atmosphere radiative transfer model. For the polarization of atmospheric radiances in the solar meridian plane over a vegetated target, intensity errors may range from −10 to +10%. The polarization‐induced errors are highest in the shortest NS001 spectral band (0.450–0.525 μm) because of large atmospheric polarizations contributed by Rayleigh particles and small diluting effects caused by the small contributions of weakly polarized radiations coming from aerosols and the surface. Depending on the illumination and view angles, the errors in derived surface reflectance due to the radiance errors can be very large. In particular, for large off‐nadir view angles in the forward scattered direction when the Sun is low, the relative errors in the derived surface reflectance can be as large as 4 to 5 times the relative error in the radiances. Polarization sensitivity errors cannot be neglected for the shorter wavelengths when the surface reflectance contribution to atmospheric radiances is very small.
Atmospheric correction of remotely sensed image data by a simplified model
Authors: Wrigley R. C. , Spanner M. A. , Slye R. E. , Pueschel R. F. , Aggarwal H. R.
Published: February 2012

The interaction of radiation with the atmosphere is complex and has proved difficult to calculate without reference to measurements made at or close to the time and location of interest. In this paper we describe the use of data from an airborne‐tracking sunphotometer mounted on a NASA C‐130 aircraft to derive optical properties of the atmospheric aerosols above the aircraft when it is on the ground as well as when the aircraft is at altitude collecting remote sensing data. Furthermore, we describe the use of these optical properties for quantitative atmospheric correction of Landsat thematic mapper (TM) and NS00l TM simulator radiances in a simplified radiative transfer model appropriate for each pixel of entire images. Both qualitative and quantitative comparisons of results from use of the model show it provides good to excellent correction for atmospheric effects in TM and NS00l TM simulator data. Quantitative comparisons with measurements of surface radiances measured near the time of the TM overpass showed agreement within a few percent for bands 2, 3, and 4. Band 1 yielded radiances within 15% of the surface radiances. We think the simplified atmospheric correction model shows great promise for applying atmospheric corrections to entire images instead of individual points within images to yield surface radiances within a few percent in a computationally efficient manner.
Measured and calculated optical property profiles in the mixed layer and free troposphere
Authors: Rosen James M. , Bodhaine Barry A. , Boatman Joe F. , DeLuisi John J. , Post M. J. , Kim Young , Schnell Russell C. , Sheridan Patrick J. , Garvey Dennis M.
Published: February 2012

Nearly simultaneous measurements of the physical and optical properties of mixed layer and free tropospheric aerosols near Boulder, Colorado, were made on several occasions using aircraft, balloon, and ground‐based sensors. This effort (Front Range Lidar, Aircraft, and Balloon experiment (FRLAB)) was conducted with the purpose of obtaining a diverse, self‐consistent data set that could be used for testing optical model calculations based on measured physical characteristics such as apparent size distribution, composition, and shape. It was found that even with the uncertainties involved, the model predictions are in good agreement with the measurements in the visible and near infrared wavelength regions. At CO2 lidar wavelengths there is considerably more uncertainty in both the calculated and measured values; however, within the estimated errors there appears to be satisfactory agreement except for the highest free tropospheric layer studied. The results also indicate that during FRLAB the aerosol in the boundary layer and free troposphere behaved as spherical particles for optical modeling purposes. The utility of the observations for determining the extinction‐to‐backscatter ratio relevant to aerosols in the boundary layer and free troposphere is described with typical measured values being in the 20 to 30 sr range.
Emission factors for particles, elemental carbon, and trace gases from the Kuwait oil fires
Authors: Laursen Krista K. , Ferek Ronald J. , Hobbs Peter V. , Rasmussen Rei A.
Published: February 2012

Emission factors are presented for particles, elemental carbon (i.e., soot), total organic carbon in particles and vapor, and for various trace gases from the 1991 Kuwait oil fires. Particle emissions accounted for ∼2% of the fuel burned. In general, soot emission factors were substantially lower than those used in recent “nuclear winter” calculations. Differences in the emissions and appearances of some of the individual fires are discussed. Carbon budget data for the composite plumes from the Kuwait fires are summarized; most of the burned carbon in the plumes was in the form of CO2. Fluxes are presented for several combustion products.
Radiative effects of the smoke clouds from the Kuwait oil fires
Authors: Pilewskie Peter , Valero Francisco P. J.
Published: February 2012

The radiative effects of the smoke from the Kuwait oil fires were assessed by measuring downwelling and upwelling solar flux, as well as spectral solar extinction beneath, above, and within the smoke plume. Radiative flux divergence measurements were made to determine smoke‐induced heating and cooling rates. Seven radiation flight missions were undertaken between May 16 and June 2, 1991, to characterize the plume between the source region in Kuwait and approximately 200 km south, near Manama, Bahrain. We present results from one flight representative of conditions of the composite plume. On May 18, 1991, in a homogeneous, well‐mixed region of smoke approximately 100 km downstream of the fires, visible optical depths as high as 2 were measured, at which time transmission to the surface was 8%, while 78% of the solar radiation was absorbed by the smoke. The calculated instantaneous heating rate inside the plume reached 24 K/d. While these effects are probably typical of those regions in the Persian Gulf area directly covered by the smoke, there is no evidence to suggest significant climatic effects in other regions.
Optical extinction properties of smoke from the Kuwait oil fires
Authors: Weiss Ray E , Hobbs Peter V.
Published: February 2012

Aircraft‐based measurements of optical extinction, optical scattering, and particle mass concentrations were obtained in the smoke from the Kuwait oil fires during May and June 1991. These measurements were used to derive optical absorption, single‐scattering albedo (
), specific absorption and the amount of soot in the smoke. Measurements were made in smoke from individual oil wells, pool fires and in composite smoke plumes. The value of
for smoke from the individual fires was either 0.35–0.4 (for the black smoke) or 0.85–0.95 (for the white smoke). For the aged composite plume from all of the fires,
ranged from 0.52 to 0.6. The specific absorption of the composite smoke varied from about 2 m2 g−1 near the fires to about 1.5 m2 g−1 well downwind.
Trends in column ozone based on TOMS data: Dependence on month, latitude, and longitude
Authors: Niu Xufeng , Frederick John E. , Stein Michael L. , Tiao George C.
Published: February 2012

On the basis of the TOMS satellite column ozone data in latitudes 70°S–70°N from November 1978 to May 1990, we use a statistical model to estimate the trends in ozone as a function of latitude, longitude, and month. The trends in the TOMS ozone data are highly seasonal and dependent on location. Near the equator, the estimated monthly trends are not significantly different from zero. For high latitudes, most of the estimated monthly trends are negative. In January, February, and March, there are some positive trend estimates in the western hemisphere around latitude 60°N. The most negative trends for these 3 months also appear in the high latitudes of the northern hemisphere. Starting in June, more negative trends appear in the latitudes 50°S–70°S than the trends in the rest of the world considered. A large depletion develops during the spring time (September to November) in the southern high‐latitude region, and the area of peak ozone decline is moving eastward during the period. The largest negative trends (about −29% per decade) for the area considered in this study appear in October around the latitude 70°S and longitudes 20°W–100°W region. Since the magnitudes of the estimated trends in the southern hemisphere increase toward the pole, more negative trends occur beyond the latitude 70°S. For the northern hemisphere, the year‐round trend estimates for latitudes 30°N–70°N range from −0.96% to −7.43% per decade. In the latitudes 30°N–50°N, the winter trend estimates are more negative than those for the summer and the fall. However, this pattern did not hold for latitudes 50°N–70°N.
A bidirectional reflectance model of the Earth's surface for the correction of remote sensing data
Authors: Roujean Jean‐Louis , Leroy Marc , Deschamps Pierre‐Yves
Published: February 2012

A surface bidirectional reflectance model has been developed for the correction of surface bidirectional effects in time series of satellite observations, where both sun and viewing angles are varying. The model follows a semiempirical approach and is designed to be applicable to heterogeneous surfaces. It contains only three adjustable parameters describing the surface and can potentially be included in an algorithm of processing and correction of a time series of remote sensing data. The model considers that the observed surface bidirectional reflectance is the sum of two main processes operating at a local scale: (1) a diffuse reflection component taking into account the geometrical structure of opaque reflectors on the surface, and shadowing effects, and (2) a volume scattering contribution by a collection of dispersed facets which simulates the volume scattering properties of canopies and bare soils. Detailed comparisons between the model and in situ observations show satisfactory agreement for most investigated surface types in the visible and near‐infrared spectral bands. The model appears therefore as a good candidate to reduce substantially the undesirable fluctuations related to surface bidirectional effects in remotely sensed multitemporal data sets.
A coordinated study of a storm system over the South American continent: 2. Lightning‐related data
Authors: Pinto I. R. C. A. , Pinto O. , Gin R. B. B. , Diniz J. H. , de Araújo R. L. , Carvalho A. M.
Published: February 2012

On December 13, 1989, a coordinated campaign was conducted in Brazil to study the electrical signatures associated with a large storm system over the South American continent. Balloon‐borne VLF electric field data showed more than a hundred lightning‐related stratospheric vertical electric field (sferics) signatures during approximately 6 hours. The sferic signatures were characterized by a rapid transient with peak amplitudes exceeding 7 V/m which would be expected by the removal of positive charge below followed by a slow recovery. This signature indicates that the lightning strokes experienced by the balloon were either cloud discharges or positive ground discharges. The peak amplitudes indicate that the positively charged region is of the order of tens of coulombs. The recovery curves are quite variable, reflecting the complex nature of the propagating phenomena in the atmosphere. A lightning position and tracking system (LPATS) network recorded 326 ground strokes at distances less than 100 km of the balloon. In contrast with previous observations during the summer in the northern hemisphere it was found that most lightning flashes (60%) were positive. In addition, only a small fraction of them occurred simultaneously with a balloon sferic signature, indicating that most balloon sferics were a result of cloud discharges. An analysis of wind and temperature data during the event showed that the predominance of positive ground flashes is apparently not supported by the tilted dipole hypothesis, being necessary to search for other mechanisms to explain it. Although this observation cannot be considered as usual in this region, it clearly indicates that the flash polarity should depend on many factors besides the season. These factors should be probably related to the geographical location and the meteorological environment.
Summertime tropospheric observations related to Nx Oy distributions and partitioning over Alaska: Arctic Boundary Layer Expedition 3A
Authors: Sandholm S. T. , Bradshaw J. D. , Chen G. , Singh H. B. , Talbot R. W. , Gregory G. L. , Blake D. R. , Sachse G. W. , Browell E. V. , Barrick J. D. W. , Shipham M. A. , Bachmeier A. S. , Owen D.
Published: February 2012

Measurements of the reactive odd nitrogen compounds NO, NO2, peroxyacetyl nitrate (PAN), and NOy are presented for the summertime middle/lower troposphere (6.1–0.15 km) over northern high latitudes. In addition, the chemical signatures revealed from concurrent measurements of O3, CO, C2H2, C2H6, C3H8, C2Cl4, and H2O are used to further characterize factors affecting the budget and distribution of NxOy, in the Arctic and sub‐Arctic tropospheric air masses sampled over Alaska during the NASA Arctic Boundary Layer Expedition (ABLE 3A) field campaign. Many of the compounds listed above exhibited a general trend of median mixing ratios increasing in proportion with altitude within the lower 6‐km column. However, median mixing ratios of NO and NOx (NO + NO2) were nearly independent of altitude, having values of about 8.5 and 25 pptv, respectively. Median mixing ratios of NOy varied from about 350 pptv within the lowest altitudes to about 600 pptv within the highest altitudes sampled. PAN constituted the largest fraction of NOy (∼50%) at the highest altitudes. In addition, PAN mixing ratios accounted for all of the approximate 60 pptv/km altitudinal dependency in NOy. The analyses presented implicate biomass burning in Siberia as the probable source of about one‐third of the NOy abundance within the middle/lower troposphere over Alaska. These analyses also implicate the downward transport of air from altitudes in the vicinity of the tropopause as a major contributor to the abundance of NOy, (∼30–50%) within the lower 6‐km column over Alaska. However, the exact origin of this high‐altitude NOy remains uncertain. The impact of lower latitude industrial/urban pollution also remains largely uncertain, although various chemical signatures imply inputs from these regions would have been relatively well aged (15–30 days).
GPS meteorology: Remote sensing of atmospheric water vapor using the global positioning system
Authors: Bevis Michael , Businger Steven , Herring Thomas A. , Rocken Christian , Anthes Richard A. , Ware Randolph H.
Published: February 2012

We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground‐based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time‐varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground‐based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.
Aircraft‐ and tower‐based fluxes of carbon dioxide, latent, and sensible heat
Authors: Desjardins R. L. , Hart R. L. , MacPherson J. I. , Schuepp P. H. , Verma S. B.
Published: February 2012

Fluxes of carbon dioxide, water vapour, and sensible heat obtained over a grassland ecosystem, during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE), using an aircraft‐ and two tower‐based systems are compared for several days in 1987 and in 1989. The tower‐based cospectral estimates of CO2 (WC), sensible heat (WT), water vapour (Wq), and momentum (WU), expressed as a function of wavenumber K times sampling height z, are relatively similar to the aircraft‐based estimates for K × z > 0.1. A measurable contribution to the fluxes is observed by tower‐based systems at K × z < 0.01 but not by the aircraft‐based system operating at an altitude of approximately 100 m over a 15 × 15 km area. Using all available simultaneous aircraft and tower data, flux estimates by both systems were shown to be highly correlated. As expected from the spatial variations of the greenness index, surface extrapolation of airborne flux estimates tended to lie between those of the two tower sites. The average fluxes obtained, on July 11, 1987, and August 4, 1989, by flying a grid pattern over the FIFE site agreed with the two tower data sets for CO2, but sensible and latent heat were smaller than those obtained by the tower‐based systems. However, in general, except for a small underestimation due to the long wavelength contributions and due to flux divergence with height, the differences between the aircraftand tower‐based surface estimates of fluxes appear to be mainly attributable to differences in footprint, that is, differences in the area contributing to the surface flux estimates.
An overview of surface radiance and biology studies in FIFE
Authors: Blad B. L. , Schimel D. S.
Published: February 2012

The use of satellite data to study and to understand energy and mass exchanges between the land surface and the atmosphere requires information about various biological processes and how various reflected or emitted spectral radiances are influenced by or manifested in these processes. To obtain such information, studies were conducted by the First ISLSCP Field Experiment (FIFE) surface radiances and biology (SRB) group using surface, near‐surface, helicopter, and aircraft measurements. The two primary objectives of this group were to relate radiative fluxes to biophysical parameters and physiological processes and to assess how various management treatments affect important biological processes. This overview paper summarizes the results obtained by various SRB teams working in nine different areas: (1) measurement of bidirectional reflectance and estimation of hemispherical albedo; (2) evaluation of spatial and seasonal variability of spectral reflectance and vegetation indices; (3) determination of surface and radiational factors and their effects on vegetation indices and PAR relationships; (4) use of surface temperatures to estimate sensible heat flux; (5) controls over photosynthesis and respiration at small scales; (6) soil surface CO2 fluxes and grassland carbon budget; (7) landscape variations in controls over gas exchange and energy partitioning; (8) radiometric response of prairie to management and topography; and (9) determination of nitrogen gas exchanges in a tallgrass prairie.
Satellite validation of GCM‐simulated annual cycle of the earth radiation budget and cloud forcing
Authors: Bony Sandrine , Le Treut Hervé , Duvel Jean‐Philippe , Kandel Robert S.
Published: February 2012

Earth Radiation Budget Experiment (ERBE) data are used to validate radiative fluxes and cloud radiative forcing (CRF) simulated by the Laboratoire de Météorologie Dynamique (LMD) general circulation model (GCM). The emphasis of the work is on the development of new tests to obtain more significant elements of comparison between model simulations and satellite observations. These tests are applied to the clear‐sky fluxes and the cloud radiative forcing. The validation of the CRF described by a model requires to test the consistency between the solar or shortwave (SW: 0.2 to 5 μm) and longwave (LW: 5 to 50 μm) cloud forcing. For this purpose, we compute the mean cloud perturbation of the planetary albedo as a function of the LW cloud forcing for both model results and ERBE observations. In the SW spectral domain, the consideration of total fluxes does not provide very constraining elements of validation because most of the observed variations are prescribed (incoming solar radiation, solar zenith angle). We therefore distinguish the part of the SW seasonal variations related only to the variation of external parameters (mainly the insolation) from the part which arises from the combined variation of internal climate parameters (mainly cloud albedo and snow/ice cover) with the insolation. Fourier analysis is used to study the seasonal amplitude and phase of the CRF. The seasonal variation of the cloudiness is, respectively, out of phase (in phase) with the insolation in mid‐latitudes (in low and high latitudes). We show that this acts to enhance (to reduce) the seasonal amplitude of the absorbed SW flux in mid‐latitudes (in low and high latitudes). Finally, we show that the impact of the seasonal variation of the cloudiness on the variation of the net CRF is less than 10 W m−2.
A model study of the formation of cloud condensation nuclei in remote marine areas
Authors: Lin Xing , Chameides W. L. , Kiang C. S. , Stelson A. W. , Berresheim H.
Published: February 2012

A theoretical study is presented of the mechanisms by which freshly nucleated sulfuric acid particles can be grown to the size of cloud condensation nuclei (CCN) in remote marine areas. Three mechanisms are considered: (1) growth by the scavenging and subsequent aqueous phase oxidation of atmospheric SO2, (2) growth by coagulation, and (3) growth by scavenging of atmospheric sulfuric acid. While model calculations indicate that the first two mechanisms are too slow to represent a significant source of CCN in the remote marine atmosphere, scavenging of sulfuric acid can be an effective mechanism for transforming tiny sulfuric acid particles into CCN provided the concentration of preexisting CCN is less than some critical concentration. For conditions typical of the marine boundary layer with ambient SO2 concentrations of about 30 parts per trillion by volume (pptv), we estimate this critical concentration to be quite low (that is, about 1 cm−3). However, under conditions with enhanced SO2 the value of this critical concentration also increases and is estimated to reach a level of about 10 cm−3 when SO2 is equal to 100 pptv. Our calculations suggest that coupling between dimethyl sulfide (DMS) emissions and CCN production in the marine boundary layer can only exist when the existing CCN concentrations at a specific locale fall below this critical concentration.
Free tropospheric ozone production following entrainment of urban plumes into deep convection
Authors: Pickering Kenneth E. , Thompson Anne M. , Scala John R. , Tao Wei‐Kuo , Dickerson Russell R. , Simpson Joanne
Published: February 2012

It is shown that rapid vertical transport of air from urban plumes through deep convective clouds can cause substantial enhancement of the rate of O3 production in the free troposphere. Simulation of convective redistribution and subsequent photochemistry of an urban plume from Oklahoma City during the 1985 PRESTORM campaign shows enhancement of O3 production in the free tropospheric cloud outflow layer by a factor of almost 4. In contrast, simulation of convective transport of an urban plume from Manaus, Brazil, into a pristine free troposphere during GTE/ABLE 2B (1987), followed by a photochemical simulation, showed enhancement of O3 production by a factor of 35. The reasons for the different enhancements are (1) intensity of cloud vertical motion; (2) initial boundary layer O3 precursor concentrations; and (3) initial amount of background free tropospheric NOx. Convective transport of ozone precursors to the middle and upper troposphere allows the resulting O3 to spread over large geographic regions, rather than being confined to the lower troposphere where loss processes are much more rapid. Conversely, as air with lower NO descends and replaces more polluted air, there is greater O3 production efficiency per molecule of NO in the boundary layer following convective transport. As a result, over 30% more ozone could be produced in the entire tropospheric column in the first 24 hours following convective transport of urban plumes.
Peroxy radicals in the ROSE experiment: Measurement and theory
Authors: Cantrell C. A. , Lind J. A. , Shetter R. E. , Calvert J. G. , Goldan P. D. , Kuster W. , Fehsenfeld F. C. , Montzka S. A. , Parrish D. D. , Williams E. J. , Buhr M. P. , Westberg H. H. , Allwine G. , Martin R.
Published: February 2012

The concentrations of the HO2‐RO2 species measured during July 11, 1990, in the ROSE (Rural Oxidants in the Southern Environment) study in Alabama are compared to those expected in theory from calculations based upon detailed hourly measurements of a variety of trace gases including the hydrocarbons, NO, NO2, carbonyl compounds, CO, PAN (peroxyacetylnitrate) and calculated jO3 values. The measurements are also compared with the [HO2] + [RO2] as estimated from deviations from the NO2 + hv (+O2) ⇄ NO + O3 photostationary state. Within the error of the measurements all of the data appear to be in reasonable accord.
Airborne flux measurements of trace species in an Arctic boundary layer
Authors: Ritter John A. , Barrick John D. W. , Sachse Glen W. , Gregory Gerald L. , Woerner Mary A. , Watson Catherine E. , Hill Gerald F. , Collins James E.
Published: February 2012

In situ airborne measurements of the turbulent flux and mean values for O3, CO, and CH4 were obtained in the boundary layer over selected wetland systems in Alaska. These measurements were obtained in July–August 1988 as part of the NASA Global Tropospheric Experiment Program's Arctic Boundary Layer Expedition (ABLE 3A). The flux measurements obtained from this study provide information on the source/sink distribution of O3 and CH4 over the Yukon‐Kuskokwim Delta (YKD) and Alaskan North Slope (ANS) regions of Alaska. The source/sink distribution over the YKD is qualitatively correlated with surface vegetation type, identified from multispectral scanner imagery. Direct measurements of the spatial variation in the CH4 source strength were obtained over the YKD. The CH4 source strength over the YKD ranged from 25 to 85 mg m−2 d−1 during a flux survey flight which spanned a considerable portion of the YKD. A spatially averaged, seasonally adjusted source strength of 51 mg m−2 d−1 was established for the YKD. Indirect CH4 flux estimates obtained over the ANS indicate a much lower (∼10 mg m−2 d−1) source strength. The global CH4 emissions from tundra were estimated to be 44 Tg/a based on (1) the spatially averaged source strength obtained over the YKD, (2) current estimates of the global coverage of tundra, and (3) assuming a similarity between other tundra areas and that of the YKD. This estimate is taken to be an upper limit due to possible sampling inadequacies and because the spatial distribution of the CH4 source function over the YKD may not extend to all other northern wetland regions. This estimate is, however, in reasonable agreement with previous estimates. Airborne CO flux measurements over the YKD indicated low negative flux values over the coastal areas, while some positive fluxes were observed in the inland, sparsely forested regions. An inspection of the cospectrum of CO with vertical velocity for sample runs in coastal areas indicated a minimum at wavelengths which were noticeably shorter (70–400 m) than where any prominent feature could be found for similar cospectrums of heat, moisture, O3, or CH4 with vertical velocity. Similar analyses for transects over inland areas indicated occasional peaks in this same, short wavelength band. These features indicate the possibility of in situ photochemical destruction/production of CO, although the identification of a possible chemical mechanism was not attempted at this time. Tundra surfaces are estimated to be responsible for ∼32% of the total deposition loss of O3 poleward of 60°N. Data from the airborne O3 flux measurements made during this study indicate similar values of Rc for the ANS and YKD regions.
A one‐dimensional modeling study of carbonaceous haze effects on the springtime Arctic environment
Authors: Emery Christopher A. , Haberle Robert M. , Ackerman Thomas P.
Published: February 2012

A one‐dimensional three‐component numerical model of the Arctic environment is developed to study the effect of Arctic haze on the surface energy balance. The model represents an improvement over earlier studies in that it accounts for the transfer of infrared as well as solar radiation through the atmosphere, turbulent mixing within the boundary layer, and heat conduction through snow‐capped sea ice. In a control simulation, the model is used to calculate the steady state response of the springtime ocean‐ice‐atmosphere system to the presence of an airborne carbonaceous aerosol with an idealized (but realistic) vertical distribution representative of spring. Additional simulations are then carried out to examine the sensitivity of the response to humidity and surface albedo. In general, the results indicate that surface temperatures increase when haze is present, provided the relative humidity through the column does not change. Except in our control simulation, the increase is due to an increase in the absorbed radiation at the surface. This result is consistent with previous studies which indicated that the increased infrared emission to the surface would more than compensate for the haze‐induced reduction in solar radiation at the surface. As in previous studies, the increase in infrared emission is due to the increase in atmospheric temperatures because of the absorption of solar radiation by the haze, and the increase in atmospheric emissivity because of the greater amount of atmospheric water vapor. The latter follows from the assumption that the relative humidity does not change. However, in the present study the change in the net radiation at the surface is small such that the magnitude of surface warming is also small (several degrees Kelvin or less). Indeed, in the control simulation the absorbed radiation at the surface decreases and the warming is actually brought about by an increase in the sensible heat flux. Furthermore, if the specific humidity rather than relative humidity is assumed constant, then surface temperatures decrease rather than increase. These results suggest that haze‐induced surface temperature changes in the Arctic are difficult to determine, both in sign and magnitude, and that the Arctic environment is less sensitive than previously assumed.
A zonal average model of the stratospheric contributions to the tropospheric ozone budget
Authors: Follows Michael J. , Austin Jillian F.
Published: February 2012

A zonally averaged transport and chemistry model for atmospheric trace species is developed, based on an existing statistical‐dynamical climate model. The modeled region is a single hemisphere from 1000 mbar to 240 mbar. The model is applied to a study of the budget of tropospheric ozone, in particular to an estimation of the relative importance of the stratospheric and in situ photochemical sources. A parameterization of cross‐tropopause exchange is developed based on the concept of tropopause folding. It is used to provide transfers of air and ozone across the upper boundary. A basic tropospheric chemistry scheme is also incorporated, featuring methane oxidation and subsequent reactions, carbon monoxide oxidation and oxygen‐hydrogen reactions. Nitrogen oxide reactions are reduced to a minimum by specification of NOx distribution in space and time. Nonmethane hydrocarbon reactions are omitted. Transport and surface deposition of relevant species are performed.

The model produces ozone distributions which are in general agreement with observations and other modeling studies. These are analyzed by considering, χχ, the mixing ratio of ozone of stratospheric origin with respect to the “complete” ozone mixing ratio. The sources and sinks of χχ are parameterized in terms of the relevant processes, and it can be treated as a quasi‐conservative variable with respect to transports. Zonally averaged fields of this mixing ratio are produced and show the influence of ozone of stratospheric origin to be very small (5 % at most) close to the surface, in this zonally averaged analysis. At 300 mbar, in middle to high latitudes, the influence is more significant (up to about 50 % in high‐latitude winter). These results are in accord with the very limited observational evidence.
Sampling errors in the vertical fluxes of potential temperature and moisture measured by aircraft during FIFE
Authors: Grossman Robert L.
Published: February 2012

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) was carried out over a 15 × 15 km area in central Kansas [Sellers et al., this issue]. The site size was constrained by land use characteristics, topography, and, importantly, the ability to field a reasonable network of surface observations of plant physiology, soil moisture, and radiative characteristics as well as surface observations of meteorological observations, including vertical fluxes of sensible heat and moisture. As described by Kelly [this issue], aircraft flying within the atmospheric boundary layer over the FIFE site played an important role: they provided direct measurements of the vertical fluxes of sensible heat and moisture above the FIFE site. Potential temperature flux and sensible heat flux differ by the constant ρdcp, where ρd is the dry air density (which is nearly constant in the atmospheric boundary layer) and cp is the specific heat of dry air at constant pressure.
Aqueous‐phase chemical processes in deliquescent sea‐salt aerosols: A mechanism that couples the atmospheric cycles of S and sea salt
Authors: Chameides W. L. , Stelson A. W.
Published: February 2012

The aqueous‐phase chemistry of deliquescent sea‐salt aerosols in the remote marine boundary layer is investigated with a steady state box model. The model simulates the scavenging of soluble and reactive gaseous species by the sea‐salt aerosols, the chemical reactions of these species and sea‐salt ions in the deliquescent solution, and changes in the aerosol composition that occur as a result of these processes. The calculations indicate that deliquescent sea‐salt aerosols are strongly buffered with a pH that remains close to 8 until the amount of acid added to the aerosol solution exceeds the alkalinity of sea salt. The oxidation of chloride by O3 and by free radicals is found to proceed at extremely slow rates, and thus these reactions cannot explain the high‐chloride deficits recently observed over the North Atlantic Ocean. On the other hand, the oxidation of dissolved SIV by O3 in sea‐salt aerosols is found to proceed at rates approaching 0.1 eq L−1 hr−1 and appears to be sufficiently rapid to qualitatively explain the observations of nss‐SO4= in sea‐salt aerosols over the North Atlantic Ocean. The high rate of SIV oxidation is found to proceed until the amount of nss‐SO4= generated in the aerosol is sufficiently large to overwhelm the buffering capacity of the deliquescent solution and lower the pH below 6. As a result, the calculations suggest the existence of a removal mechanism for atmospheric S that is largely controlled by the alkalinity of seawater and the flux of this alkalinity into the atmosphere in sea salt. It is estimated that this process will generate about 0.75 neq m−3 of nss‐SO4= associated with sea salt in the marine boundary layer and ultimately remove about (1–4) × 1011 moles of SO2 from the atmosphere annually. Comparison of this loss rate with other elements of the atmospheric S cycle suggests that sea salt may remove a significant amount of S from the marine atmosphere and thereby depress the SO2 concentration in the marine boundary layer and limit the number of cloud condensation nuclei generated from the oxidation of SO2.
Mixing ratios of carbon monoxide in the troposphere
Authors: Novelli Paul C. , Steele L. Paul , Tans Pieter P.
Published: February 2012

Carbon monoxide (CO) mixing ratios were measured in air samples collected weekly at eight locations. The air was collected as part of the CMDL/NOAA cooperative flask sampling program (Climate Monitoring and Diagnostics Laboratory, formerly Geophysical Monitoring for Climatic Change, Air Resources Laboratory/National Oceanic and Atmospheric Administration) at Point Barrow, Alaska (71°N), Niwot Ridge, Colorado (40°N), Mauna Loa and Cape Kumakahi, Hawaii (19°N), Guam, Marianas Islands (13°N), Christmas Island (2°N), Ascension Island (8°S) and American Samoa (14°S). Half‐liter or 3‐L glass flasks fitted with glass piston stopcocks holding teflon O rings were used for sample collection. CO levels were determined within several weeks of collection using gas chromatography followed by mercuric oxide reduction detection, and mixing ratios were referenced against the CMDL/NOAA carbon monoxide standard scale. During the period of study (mid‐1988 through December 1990) CO levels were greatest in the high latitudes of the northern hemisphere (mean mixing ratio from January 1989 to December 1990 at Point Barrow was approximately 154 ppb) and decreased towards the south (mean mixing ratio at Samoa over a similar period was 65 ppb). Mixing ratios varied seasonally, the amplitude of the seasonal cycle was greatest in the north and decreased to the south. Carbon monoxide levels were affected by both local and regional scale processes. The difference in CO levels between northern and southern latitudes also varied seasonally. The greatest difference in CO mixing ratios between Barrow and Samoa was observed during the northern winter (about 150 ppb). The smallest difference, 40 ppb, occurred during the austral winter. The annually averaged CO difference between 71°N and 14°S was approximately 90 ppb in both 1989 and 1990; the annually averaged interhemispheric gradient from 71°N to 41°S is estimated as approximately 95 ppb.
The overshoot and equilibration of saltation
Authors: Shao Y. , Raupach M. R.
Published: February 2012

Downwind of the leading edge of a saltating surface, the saltation process develops toward an eventual stream wise equilibrium. The nature of this development is investigated in a portable wind tunnel by measuring the vertically integrated streamwise flux Q of saltating sand grains as a function of distance x from the leading edge of the erodible surface, at wind speeds between 8 and 13 m s−1. It is found that Q(x) increases from zero at x = 0 to a maximum value at x around 7 m, before decreasing toward an eventual equilibrium value (about half the maximum value) when x is greater than about 15 m. These results are in qualitative agreement with two recent numerical simulations of the approach of saltation to equilibrium. The experiment indicates the minimum length of wind tunnel required for studying the saltation process in its equilibrium state, and the order of magnitude of the measurement errors if shorter tunnels must be used.
Correlative measurements of the stratospheric aerosols
Authors: Santer R. , Brogniez C. , Herman M. , Diallo S. , Ackerman M.
Published: February 2012

Joint experiments were organized or available during stratospheric flights of a photopolarimeter, referred to as RADIBAL (radiometer balloon). In May 1984, RADIBAL flew simultaneously with another balloon‐borne experiment conducted by the Institut d'Aéronomie Spatiale de Belgique (IASB), which provides multiwavelength vertical profiles of the aerosol scattering coefficient. At this time, the El Chichon layer was observable quite directly from mountain sites. A ground‐based station set up at Pic du Midi allowed an extensive description of the aerosol optical properties. The IASB and the Pic du Midi observations are consistent with the aerosol properties derived from the RADIBAL measurement analysis. The ability of RADIBAL to retrieve the vertical profile of the aerosol extinction coefficient was also demonstrated during a winter arctic campaign by comparison to Stratospheric Aerosol Measurement (SAM II) data and to airborne lidar data. As a result, RADIBAL appears to be a good candidate for in situ validations of the Stratospheric Aerosol and Gas Experiment (SAGE II) and its derived products.
Impact of heterogeneous chemistry on model predictions of ozone changes
Authors: Granier Claire , Brasseur Guy
Published: February 2012

A two‐dimensional chemical/transport model of the middle atmosphere is used to assess the importance of chemical heterogeneous processes both in the polar regions (on polar stratospheric clouds (PSCs)) and at other latitudes (on sulfate aerosols). When conversion on type I and type II PSCs of N2O5into HNO3and of ClONO2into reactive forms of chlorine is taken into account, enhanced ClO concentrations lead to the formation of a springtime “ozone hole” over the Antarctic continent. No such major reduction in the ozone column is found in the Arctic region. When conversion of nitrogen and chlorine compounds is assumed to occur on sulfate particles present in the lower stratosphere at all latitudes, significant perturbations in the chemistry are also found. For background aerosol conditions, the concentration of nitric acid is enhanced and agrees with observed values, while that of nitrogen oxides is reduced and agrees less than if heterogeneous processes are ignored in the model calculations. The concentration of the OH radical is significantly increased. Ozone number density appears to become larger between 16 and 30 km but smaller below 16 km, especially at high latitudes. The ozone column is only slightly modified, except at high latitudes where it is substantially reduced if the ClONO2conversion into reactive chlorine is taken into account. After a large volcanic eruption such as that of Mount Pinatubo in June 1991, these changes are further exacerbated. The ozone budget in the lower stratosphere becomes less affected by nitrogen oxides but is largely controlled by the ClOxand HOxchemistries. A substantial decrease in the ozone column is predicted as a result of the Pinatubo eruption, mostly in winter at midlatitudes and high latitudes. The predicted values depend on the assumption made for the evolution of the aerosol surface area density but is expected to be of the order of 10% at mid‐latitudes in February and March 1992. An enhanced mixing ratio of NO2of the order of 100 parts per trillion by volume and a reduced mixing ratio of NO2below 25 km should be detected outside the polar vortex, especially in air masses with high levels of volcanic aerosols.