bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical modeling of cloud and precipitation chemistry associated with two rainbands and some comparisons with observations
Authors: Barth Mary C. , Hegg Dean A. , Hobbs Peter V.
Published: February 2012

A diagnostic modeling study is described of the chemistry of a pair of rainbands, consisting of a narrow and a wide cold‐frontal rainband, which were associated with a cold front on the mid‐Atlantic coast of the United States. Precipitation growth in the narrow cold‐frontal rainband (NCFR) was dominated by collection processes. The wide cold‐frontal rainband (WCFR), which followed the NCFR, had most of its precipitation associated with frontal lifting. Surface sulfate and nitrate deposition rates predicted by the model for the WCFR were of the same order as those obtained from measurements on the ground. When the two rainbands were allowed to interact in the model, allowing the NCFR to process the air that entered the WCFR, sulfate deposition from the WCFR decreased, but nitrate deposition remained unchanged. The total sulfate deposition from the two bands, with and without interaction, differed by only ∼2%. The effects on the sulfate and nitrate deposition of the initial concentrations of their gaseous precursors in the clear air ahead of each rainband were examined. For most cases modeled, the sulfate and nitrate depositions responded approximately linearly to variations in the concentrations of the precursors. Examination of the pathways for oxidizing aqueous sulfur dioxide shows that iron‐catalyzed aerobic oxidation may be an important mechanism for converting sulfur dioxide to sulfate.
Intercomparison of eddy correlation carbon dioxide sensors during FIFE 1989
Authors: Moncrieff J. B. , Verma S. B. , Cook D. R.
Published: February 2012

An intercomparison was made of the infrared gas analyzers used in the First ISLSCP Field Experiment (FIFE) 1989 to measure surface fluxes of carbon dioxide immediately following intensive field campaign (IFC) 5. Fluxes of CO2 agreed to within 15% when all instruments are compared; when models of the same type of analyzer were compared, the agreement was within 5%. The stability of the calibration coefficients is examined, and it is shown that frequent calibration of the gas analyzers is required. Differences in the physical design of the analyzer models discussed here, such as sampling volume and sensitivity of the electronics to thermal drift, influence the precision of the flux estimates. Spectra and cospectra reveal the degree to which instrumental noise limits the complete sampling of all frequencies of eddies contributing to flux transport.
Characterization of ozone at high elevation in the eastern United States: Trends, seasonal variations, and exposure
Authors: Aneja Viney P. , Li Zheng
Published: February 2012

Ozone measurements at five high‐elevation (≳ 1000 m msl) Mountain Cloud Chemistry Program (MCCP) sites in the eastern United States for the period from May to October 1986–1988 have been analyzed. In order to characterize and compare the exposure to ozone at these sites, several ozone exposure indices are used. The sum of all hourly mean ozone concentrations equal to or greater than 0.07 ppm (SUM07) for 1988 indicates a north‐south gradient in ozone exposure: Southeastern sites (Mount Mitchell, North Carolina, 105 parts per million‐hours (ppm‐h), Whitetop Mountain, Virginia, 85.7 ppm‐h, Shenandoah Park, Virginia, 51.6 ppm‐h); Northeastern sites (Whiteface Mountain, New York, 49.2 ppm‐h, Mount Moosilauke, New Hampshire, 34.2 ppm‐h). This shows that forest areas in the southeastern high‐elevation sites experienced higher ozone exposure than their northeastern counterparts during the hot and dry 1988 measurement season. Ozone episodes (concentrations greater than 70 ppbv lasting 8 hours or more) were more frequent during the summer months at all sites. This suggests a correlation with the seasonal photochemical cycle and perhaps with local biogenic hydrocarbon emissions. Seasonal analysis suggests that ozone maximum occurs during spring coincident with spring maximum at Mauna Loa, Hawaii and at Alpine stations in Europe, suggestive of an underlying hemispheric phenomena. A reversed diurnal cycle (nighttime maximum) pattern was also observed from May to October at all the five high‐elevation sites. Back trajectory analysis, during high‐ozone episodes, indicates that most MCCP sites were influenced by urban and industrial sources from the midwestern region of the United States, suggesting large‐scale transport patterns.
Solar photolysis of ozone to singlet D oxygen atoms
Authors: Blackburn Thomas E. , Bairai Solomon T. , Stedman Donald H.
Published: February 2012

The ground level photolysis frequency of ozone J(O3) to produce metastable singlet D oxygen atoms (O (1D)) has been measured using a novel instrumental technique involving electrical conductivity. The O(1D) atoms produced react with nitrous oxide (N2O) carrier gas to form higher oxides of nitrogen (NOx). These oxides (NOx) were detected by mixing with methanol and determining the increase in electrical conductivity with a continuous flow dual conductivity cell. Over 70 days of data were collected under varying sky conditions. The effect of temperature on J(O3) was measured. The results agree with model predictions. The effects of atmospheric aerosols, changes in overhead ozone column, and local cloudiness are discussed.
A model of nitrous oxide evolution from soil driven by rainfall events: 2. Model applications
Authors: Li Changsheng , Frolking Steve , Frolking Tod A.
Published: February 2012

Simulations of nitrous oxide (N2O) and carbon dioxide (CO2) emissions from soils were carried out with a rain‐event model of nitrogen and carbon cycling processes in soils (Li et al., this issue). Model simulations were compared with five field studies: a 1‐month denitrification study of a fertilized grassland in England; a 2‐month study of N2O emissions from a native and fertilized grassland in Colorado; a 1‐year study of N2O emissions from agricultural fields on drained, organic soils in Florida; a 1‐year study of CO2 emissions from a grassland in Germany; and a 1‐year study of CO2 emissions from a cultivated agricultural site in Missouri. The trends and magnitude of simulated N2O (or N2O + N2) and CO2 emissions were consistent with the results obtained in field experiments. The successful simulation of nitrous oxide and carbon dioxide emissions from the wide range of soil types studied indicates that the model, DNDC, will be a useful tool for studying linkages among climate, land use, soil‐atmosphere interactions, and trace gas fluxes.
Annual, quasi‐biennial, and El Niño‐Southern Oscillation (ENSO)time‐scale variations in equatorial total ozone
Authors: Shiotani Masato
Published: February 2012

Equatorial total ozone variations with time scales of annual, quasi‐biennial, and about 4‐year periodicities are described by paying attention to their longitudinal structure. Analyses are made for 11 years from 1979 to 1989, using the global total ozone data derived from the total ozone mapping spectrometer on board the Nimbus 7 satellite. Over the equator an annual cycle in total ozone is conspicuous. Zonal mean values are maximum around September and minimum around January. The longitudinal structure shows a zonal wavenumber 1 pattern with minimum values around 140°E to the date line all year‐round, indicating a close relationship to a region where the convective cloud activity is vigorous. By removing the climatological annual cycle from the original data, there appears the quasi‐biennial oscillation in total ozone. This variation is characterized by zonally uniform phase changes and is strongly coupled with the quasi‐biennial oscillation of the equatorial zonal wind in the lower stratosphere. Moreover, subtracting zonal mean values from the anomaly data mentioned above, we see an east‐west seesaw variation with a nodal longitude around the date line. This east‐west variation, having a characteristic time scale of about 4 years, is clearly related to the El Niño and the Southern Oscillation cycle. During El Niño events the longitudinal anomaly field in total ozone is positive in the western Pacific and negative in the eastern Pacific; the anomaly pattern is reversed during anti‐El Niño events. Because the active region of convective clouds is located relatively in the eastern Pacific sector during El Niño events, it is suggested that the stronger upwelling and the higher tropopause associated with the convective cloud activity bring about less total ozone.
Solar variation 1979–1987 estimated from an empirical model for changes with time in the sensitivity of the solar backscatter ultraviolet instrument
Authors: Schlesinger Barry M. , Cebula Richard P.
Published: February 2012

Variations with time in the solar irradiance have been derived from a new empirical model for change in the sensitivity of the solar backscatter ultraviolet (SBUV) instrument aboard Nimbus 7. The SBUV sensitivity change is modeled as the product of two separate exponential decays: one with instrument exposure to the Sun, the other with time. Time constants for these exponentials are calculated from functional fits to the measured UV irradiances for three time intervals, short compared with the instrument lifetime. At nearly all wavelengths the degradation coefficients for exposure dependence (rλ) do not vary significantly from interval to interval, but those for time dependence (sλ) vary significantly with time at the shorter wavelengths. A new instrument sensitivity function is defined to be the product of an exponential decay with exposure with constant rλ and an exponential decay with time with time‐varying sλ. The time dependence of sλ is constrained by the values derived for the three fitting intervals. Use of this functional form to correct the measured irradiances for instrument changes yields solar irradiance variations between 175 and 400 nm within 3% of those predicted by the Mg core‐to‐wing ratio, appropriately scaled with wavelength. This result strongly suggests that the instrument parameters derived during the short fit intervals can be used to improve the representation of instrument behavior during those intervals and can serve as constraints on any function designed to characterize long‐term changes in the SBUV instrument. The new characterization yields an estimate for the variation of solar irradiance between maximum and minimum of 5–8% at 205 nm and 1–4% between 210 and 260 nm, consistent with the 8–9 and 3–4% predicted using the Mg index.
A 2XCO2 climate change scenario over Europe generated using a limited area model nested in a general circulation model 2. Climate change scenario
Authors: Giorgi F. , Marinucci M. R. , Visconti G.
Published: February 2012

In this paper we discuss a regional climate change scenario over Europe induced by doubling of carbon dioxide concentration (what we call the 2XCO2 experiment) as simulated with a limited area model (LAM) nested in a general circulation model (GCM). The models used are a 4.5×7.5 latitude×longitude resolution (R15) version of the comrnunity climate model (CCM) of the National Center for Atmospheric Research (NCAR) and a 70‐km resolution version of the NCAR/Pennsylvania State University mesoscale model (MM4). Overall, the model produces a 2XCO2 warming over Europe in all seasons, which is in the range of 1.5°–7°. The response of the large scale circulation patterns, and therefore precipitation, over various European subregions varies from season to season. For example, the mid‐latitude jet stream over Europe is weaker in the perturbed climate run than in the control run in January and October, while it is stronger in July and April. As a consequence, simulated 2XCO2 changes in regional precipitation amounts also vary for different seasons, e.g., from −20% of present‐day values over the Alpine region in October to +177% over the western Mediterranean in July. Because of its finer representation of European topography and coastlines the nested MM4 produces a temperature and precipitation change scenario which shows finer spatial detail than that of the driving CCM. The 2XCO2‐induced average temperature modifications predicted by the two models differ locally by up to a few degrees, and predicted precipitation changes differ in some instances not only in magnitude but also in sign. Similarly, the nested MM4 predicts changes in snow depth and other components of the surface hydrology, such as surface runoff and soil water drainage, which show a great deal of sub‐GCM grid‐scale detail. These results emphasize the inadequacy of simply interpolating coarse resolution GCM output to obtain estimates of local changes in surface climatic variables for impact assessments.
Atmospheric chemistry in the Arctic and subarctic: Influence of natural fires, industrial emissions, and stratospheric inputs
Authors: Wofsy S. C. , Sachse G. W. , Gregory G. L. , Blake D. R. , Bradshaw J. D. , Sandholm S. T. , Singh H. B. , Barrick J. A. , Harriss R. C. , Talbot R. W. , Shipham M. A. , Browell E. V. , Jacob D. J. , Logan J. A.
Published: February 2012

Haze layers with perturbed concentrations of trace gases, believed to originate from tundra and forest wild fires, were observed over extensive areas of Alaska and Canada in 1988. Enhancements of CH4, C2H2, C2H6, C3H8, and C4H10were linearly correlated with CO in haze layers, with mean ratios (mole hydrocarbon/mole CO) of 0.18 (± 0.04 (1 σ)), 0.0019 (± 0.0001), 0.0055 (± 0.0002), 0.0008 (± 0.0001), and 1.2 × 10−4(±0.2× 10−4), respectively. Enhancements of NOy, were variable, averaging 0.0056 (± 0.0030) mole NOy/mole CO, while perturbations of NOxwere very small, usually undetectable. At least 1/3 of the NOyin the haze layers had been converted to peroxyacetyl nitrate (PAN), representing a potential source of NOxto the global atmosphere; much of the balance was oxidized to nitrate (HNO3and paniculate). The composition of sub‐Arctic haze layers was consistent with aged emissions from smoldering combustion, except for CH4, which appears to be partly biogenic. Inputs from the stratosphere and from biomass fires contributed major fractions of the NOyin the remote sub‐Arctic troposphere. Analysis of aircraft and ground data indicates relatively little influence from mid‐latitude industrial NOyin this region during summer, possibly excepting transport of PAN. Production of O3was inefficient in sub‐Arctic haze layers, less than 0.1 O3molecules per molecule of CO, reflecting the low NOx/CO emission ratios from smoldering combustion. Mid‐latitude pollution produced much more O3, 0.3 – 0.5 O3molecules per molecule of CO, a consequence of higher NOx/CO emission ratios.
Effects of management and topography on the radiometric response of a tallgrass prairie
Authors: Turner C. L. , Seastedt T. R. , Dyer M. I. , Kittel T. G. F. , Schimel D. S.
Published: February 2012

Bidirectional reflectance measurements were obtained on grazed, burned ungrazed, and unburned ungrazed tallgrass prairie in eastern Kansas. Observations were also made on experimental plots on which vegetation height and biomass were manipulated by mowing. Foliage biomass and productivity (including off‐take estimates) were measured concurrently at all sites. While productivity of mowed or grazed sites was either equal to or greater than that on unmowed or ungrazed sites, individual or cumulative normalized difference vegetation index (NDVI) values tended to be positively correlated with biomass, not productivity. NDVI on grazed prairie was lower than on burned and unburned prairie during the first half of the growing season. After midseason, NDVI on grazed prairie was higher than on unburned prairie but no different than on burned prairie. Strong linear relationships among NDVI and canopy nitrogen and biomass existed early and late in the growing season but not at midseason. These relationships suggest that plant physiological processes associated with regrowth following defoliation are dominant influences on reflectance early in the season, whereas the accumulation of senescent material is the dominant process affecting reflectance in the latter half of the growing season. Canopy temperature was related to canopy nitrogen and biomass at midseason. The use of NDVI to estimate plant productivity and vegetation‐atmosphere exchange is complicated by changes in plant characteristics induced by grazing or mowing. Grazing tends to homogenize potential landscape‐induced differences in vegetation activity. Factors otherwise useful in estimating plant production, such as burning treatment and soil depth, are not strongly correlated with aboveground biomass or NDVI under grazing conditions. Results presented here suggest that concurrent use of thermal information provided by some satellite sensors may improve this relationship.
Relationship between satellite‐derived vegetation indices and aircraft‐based CO2 measurements
Authors: Cihlar Josef , Caramori Paulo H. , Scnuepp Peter H. , Desjardins Raymond L. , MacPherson J. Ian
Published: February 2012

The objective of this study was to analyze the relationship between satellite‐derived vegetation indices and CO2 uptake, as an initial step in exploring the possibility of using a satellite‐derived vegetation index as a measure of net photosynthesis. The study area included the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site located on the Konza prairie and adjacent area as well as a transect between Manhattan and Salina. One third of the transect exhibited vegetation and terrain characteristics similar to those on the FIFE site, whereas cultivated land predominated in the remaining portion of the 75‐km‐long flight line. In June, July, August, and October 1987, several CO2 data sets were obtained using the National Research Council of Canada's Twin Otter research aircraft. The normalized difference vegetation index (NDVI) and the simple ratio (SR) were computed from NOAA advanced very high resolution radiometer (AVHRR) data acquired as part of FIFE. Aircraft and satellite data were processed to obtain spatially coincident and locally representative flux values. Results show a linear relationship between NDVI and CO2 uptake during a single day; however, a nonlinear relationship emerged when all data sets were combined. The data from FIFE and the regional transect were consistent for one date but differed for other periods. Overall, about 60% of total variability in CO2 flux was accounted for by the NDVI and 74% by the SR.
Biophysical properties affecting vegetative canopy reflectance and absorbed photosynthetically active radiation at the FIFE site
Authors: Walter‐Shea E. A. , Blad B. L. , Hays C. J. , Mesarch M. A. , Deering D. W. , Middleton E. M.
Published: February 2012

Leaves of the dominant grass species of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site reflect and transmit radiation in a similar manner to other healthy green leaves. Visible reflectance factors (RFs) and transmittance factors (TFs) were lower for older leaves than younger leaves except during senescence, when RF and TF values were higher. Near‐infrared (NIR) RF values increased and TF values decreased with leaf age, with the reverse occurring as the leaf underwent senescence. Leaf optical properties were not found to be dependent on leaf water potential in the range from −0.5 to −3.0 MPa. Canopy bidirectional reflectance factor (BRF) values generally increased with increasing view zenith angle (θυ). Maximum values were in the backscatter direction, whereas BRF values in the visible region were lowest at oblique off‐nadir θυ in the forward scatter direction and at or near nadir in the NIR region. Solar principal plane BRF values varied most at large solar zenith angles (θs). Visible and mid‐infrared canopy BRF values decreased and NIR BRF values increased with leaf area index (LAI). Soil BRF distributions in the solar principal plane varied slightly with θs and θυ and varied considerably for wet and dry surfaces. Spectral vegetation indices (SVIs) varied with θs and θυ; values were lowest in the backscatter direction and highest in the forward scatter direction. The fraction of absorbed photosynthetically active radiation (APAR) increased with increasing θs. APAR had a strong linear relationship to nadir‐derived SVI values but not to oblique off‐nadir‐derived SVI values. The relatively small dependence of off‐nadir SVI values on θs should allow daily APAR values to be estimated from measurements made at any time of the day.
Comparison of the observed and calculated clear sky greenhouse effect: Implications for climate studies
Authors: Kiehl J. T. , Briegleb B. P.
Published: February 2012

The clear sky greenhouse effect is defined in terms of the outgoing longwave clear sky flux at the top of the atmosphere. Recently, interest in the magnitude of the clear sky greenhouse effect has increased due to the archiving of the clear sky flux quantity through the Earth Radiation Budget Experiment (ERBE). The present study investigates to what degree of accuracy this flux can be analyzed by using independent atmospheric and surface data in conjunction with a detailed longwave radiation model. The conclusion from this comparison is that for most regions over oceans the analyzed fluxes agree to within the accuracy of the ERBE‐retrieved fluxes (±5 W m−2). However, in regions where deep convective activity occurs, the ERBE fluxes are significantly higher (10–15 W m−2) than the calculated fluxes. This bias can arise from either cloud contamination problems or variability in water vapor amount. It is argued that the use of analyzed fluxes may provide a more consistent clear sky flux data set for general circulation modeling validation. Climate implications from the analyzed fluxes are explored. Finally, results for obtaining longwave surface fluxes over the oceans are presented.
Two‐dimensional assessment of the impact of aircraft sulphur emissions on the stratospheric sulphate aerosol layer
Authors: Bekki S. , Pyle J. A.
Published: February 2012

We have developed a two‐dimensional sulphate aerosol model, which includes most of the important microphysical processes governing the formation and evolution of stratospheric sulphate particles. Simulations of the background aerosol layer shows that important aerosol properties (surface area density, optically active particle concentration, mass mixing ratio) are strongly latitude dependent. This model is used to investigate if the sulphur aircraft emissions may have caused the 60% increase in large aerosol particles observed over the 1979–1990 time period. Results suggest that although aircraft may represent a substantial source of sulphate below 20 km, the rise in air traffic is insufficient to account for the apparent trend in stratospheric aerosol particles.
Electric field pulses in K and M changes of lightning ground flashes
Authors: Rakov Vladimir A. , Thottappillil Rajeev , Uman Martin A.
Published: February 2012

From electric field records of 27 ground flashes near Tampa, Florida, and 19 ground flashes at the NASA Kennedy Space Center (KSC) the occurrence and waveshape of microsecond‐scale electric field pulses associated with both millisecond‐scale steplike K changes and millisecond‐scale hook‐shaped M changes are examined to test and disprove the following two hypotheses: (1) that K changes contain a microsecond‐scale pulse component which can be described by the characteristic pulse waveform proposed by Arnold and Pierce (1964) and (2) that there is essentially no difference between K and M processes, as argued by Kitagawa et al. (1962). Microsecond‐scale electric field variations exceeding by at least 50% the system noise level were observed in 23% of 135 K changes from Tampa and in 25% of 128 K changes from KSC, while such field variations were found in 44% of 88 M changes from Tampa and in 77% of 30 M changes from KSC. In the majority of the K changes having microsecond‐scale pulse activity, that activity did not occur at the beginning of the K step, while in most cases the pulses associated with M changes occurred at the initial portion of the M hook. These results can be interpreted to imply that K changes and M changes are associated with dissimilar physical processes, in refutation of hypothesis (2) above. The microsecond‐scale pulse activity during K changes and M changes was highly variable and sometimes irregular in waveshape. Not all the pulses had the same polarity as the K step or the initial portion of the M hook on which they were superimposed. The relation of the microsecond‐scale variations to the overall K changes in ground flashes as regards the frequency of occurrence, the position of pulses within the slower field change, and the shape of the pulses is similar to that reported by Bils et al. (1988) for K changes in cloud flashes. The observed microsecond‐scale field variations associated with K changes are not consistent with the characteristic electric field pulse waveform attributed by Arnold and Pierce (1964) and some other investigators to K changes, in refutation of hypothesis (1) above. No relation was observed between the magnitude of a K change and the presence or absence of corresponding microsecond‐scale field variations. M changes during continuing‐current field changes of relatively short duration (less than 20 ms or so) are more likely to have pulses than M changes during continuing‐current field changes of longer duration.
Some characteristics of cloud‐to‐ground lightning in tropical northern Australia
Authors: Petersen Walter A. , Rutledge Steven A.
Published: February 2012

Peak currents and polarities for the first return strokes of approximately 5000 cloud‐to‐ground lightning flashes have been studied. The flashes were associated with eight mesoscale convective systems (MCSs) that occurred in tropical northern Australia in 1989 during the second field phase of the Down Under Doppler and Electricity Experiment (Dundee). Statistical analysis of the lightning data produced an average peak current (independent of polarity) of 39 kA. When considered with the average cloud‐top heights of the tropical MCSs examined herein, the 39‐kA peak current average appears to be in general agreement with Orville's (1990) hypothesis of latitudinal variation in peak current. In six of the Dundee MCSs consisting of both convective and stratiform precipitation components, we found that positive peak current maxima tended to occur in the trailing stratiform region while positive peak current minima were generally situated in convective precipitation. We also observed this pattern in a middle‐latitude MCS that occurred on June 10–11, 1985. The magnitude of the positive peak current maxima in the tropical MCSs and the middle‐latitude MCS increased coincidentally in time with the growth of the stratiform regions, and reached peak values when the stratiform regions were most intense. Given the apparent dependence of positive peak current magnitude on the life‐cycle of the stratiform region and the spatial separation between positive peak current maxima and the convective line, we offer further support for a noninductive charging mechanism being responsible for the electrical charging and subsequent lightning observed in the trailing stratiform regions of MCSs.
Aqueous‐phase photochemical sources of peroxyl radicals and singlet molecular oxygen in clouds and fog
Authors: Faust Bruce C. , Allen John M.
Published: February 2012

An investigation was carried out to identify some of the oxidants formed during the sunlight photolysis of authentic cloud and fog water samples from North Carolina, Virginia, New York, Washington, and Oregon. Constituents (chromophores) present in the atmospheric water samples absorb solar ultraviolet radiation, and thereby initiate formation of peroxyl radicals (RO2· and HO2·), and singlet molecular oxygen, O2(1Δg). In the sunlit cloud and fog waters studied, (1) photostationary state total concentrations of unspeciated peroxyl radicals ranged from 1 to 30 nanomolar (nM), and (2) photostationary state concentrations and aqueous‐phase photochemical formation rates of O2(1Δg) ranged from 27 to 1100 femtomolar (fM) and 5 to 200 nanomolar/second (nM/s), respectively. The chromophores responsible for initiating these photochemical reactions have not yet been identified. Although these studies contain uncertainties, we conclude that aqueous‐phase photochemical reactions are a significant, and in some cases dominant, source of peroxyl radicals and O2(1Δg) in tropospheric cloud and fog drops. This represents a heretofore unrecognized source of these oxidants in atmospheric water drops and the first discussion and report of their detection in authentic condensed phases of the atmosphere.
Catastrophic loss of stratospheric ozone in dense volcanic clouds
Authors: Prather Michael
Published: February 2012

Rapid, localized loss of ozone is predicted to occur in the mid‐latitude and tropical stratosphere in the presence of very large concentrations of sulfate aerosols. Volcanic eruptions can increase the effective surface area of sulfuric acid so that heterogeneous reactions involving ClONO2, and secondarily N2O5, are able to suppress NOx abundances by more than a factor of 10 relative to gas phase chemistry. When NOx levels fall below a threshold, e.g., 0.6 ppb at 24 km in mid‐latitudes, the chlorine‐catalyzed loss of O3 proceeds at rates comparable to those during the formation of the Antarctic ozone hole, more than 50 ppb per day. If such losses occurred following the eruption of Mount Pinatubo in the most volcanically perturbed regions over the tropics and mid‐latitudes, this model predicts that they are driven primarily by the suppression Of NOx below these critical levels. The increase in stratospheric chlorine since El Chichon has made Mount Pinatubo more than twice as effective in causing rapid O3 loss. Overall global losses associated with a volcanic eruption are approximately linear in the amount of sulfate surface area, but depend critically on the rate of the CIONO2‐sulfate reaction.
Comparison of ground‐ and satellite‐based measurements of the fraction of photosynthetically active radiation intercepted by tallgrass prairie
Authors: Demetriades‐Shah T. H. , Kanemasu E. T. , Flitcroft I. D. , Su H.
Published: February 2012

Concurrent measurements of the fraction of photosynthetically active radiation intercepted by vegetation canopies, FIPAR, and ground‐, helicopter‐, and satellite‐based reflectance measurements were taken at 13 tallgrass prairie sites. The sites were subjected to various combinations of burning and grazing managements. The ground‐ and helicopter‐based reflectance measurements were taken on the same day or within a few days of the time the Landsat and SPOT satellites passed over the sites. Sun photometer readings taken at the times of the satellite overpasses were used to correct for atmospheric attenuation. Hand‐held radiometer spectral indices were strongly correlated with the helicopter‐based values (r = 0.94) and satellite‐based values (r = 0.92 for Landsat thematic mapper (TM) and 0.86 for SPOT). However, the ground‐, the helicopter‐, and the satellite‐based normalized difference spectral vegetation indices showed low sensitivity to changes in FIPAR. Reflectance measurements were only moderately well correlated with measurements of FIPAR (r = 0.82 for the hand‐held radiometer, 0.84 for helicopter measurements, 0.75 for SPOT, and 0.73 for the Landsat TM). Improved spectral indices that can compensate for site differences are needed to monitor FIPAR more reliably.
Volume‐imaging lidar observations of the convective structure surrounding the flight path of a flux‐measuring aircraft
Authors: Eloranta Edwin W. , Forrest Daniel K.
Published: February 2012

The University of Wisconsin volume imaging lidar has been used to portray images of the three‐dimensional structure of clear air convective plumes in the atmosphere surrounding the flight path of the instrumented Twin Otter aircraft operated by the National Aeronautical Establishment (NAE) of Canada. Lidar images provide a context for interpretation of the aircraft measurements. The position of data points within a convective element can be determined and the temporal development of the plume can be observed to time the observation with respect to the life cycle of the plume. Plots of the vertical flux of water vapor, q′w′, superimposed on lidar images clearly demonstrate the well‐known sampling difficulties encountered when attempting to measure fluxes near the top of the convective layer. When Loran was used to determine average aircraft velocity, flight‐leg‐averaged horizontal winds measured by the aircraft and area‐averaged winds measured by lidar agree to within 0.2 m s−1 in speed and 1° in direction.
Adjustment of footprint correction for airborne flux mapping over the FIFE site
Authors: Schuepp P. H. , MacPherson J. I. , Desjardins R. L.
Published: February 2012

A meaningful interpretation of airborne flux estimates over nonuniform terrain must consider local advection from nonhomogeneous source distributions (footprint correction). An empirical procedure for footprint correction of airborne flux data obtained in the First ISLSCP Field Experiment (FIFE) 1989 is presented, based on simple analytical solutions of the diffusion equation which have been adjusted to approximate more realistic solutions. Optimized estimates of surface flux “maps” derived from airborne observations are then constructed for maximum correlation between flux estimates and independently observed surface characteristics. This paper also includes an analysis of resolution for the given procedure, in terms of amplitude recovery of hypothetical square‐wave distributions of surface flux density, and discusses its implications for the interpretation of the optimized estimates of FIFE surface flux maps. Results show that corrected spatial sink distributions are obtained by the current empirical procedure at the cost of considerable reduction in amplitude recovery of small‐scale variations in surface flux density. The high spatial correlation observed between corrected flux maps and surface characteristics such as greenness or surface temperature excess is ascribed to the approximately correct positioning of the main flux gradients across the site, which can be expected to have been reproduced with amplitude recovery of ≥60%.
Evidence for long‐range transport of aerosol from the Kuwaiti oil fires to Hawaii
Authors: Lowenthal D. H. , Borys R. D. , Chow J. C. , Rogers F. , Shaw G. E.
Published: February 2012

To detect long‐range transport of Kuwaiti oil‐fire smoke, fine‐particle aerosol samples were collected on a weekly basis from May through July 1991 at the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostic Laboratory's Mauna Loa Observatory (MLO, 19.5°N, 155.6°W) at an altitude of 3.4 km in the free troposphere and at a sea level site in the marine boundary layer on the island of Oahu (21.4°N, 157.7°W). Samplers were sector controlled by wind speed and direction to operate only during on‐shore flow at the coastal Oahu site and during downslope flow at Mauna Loa. Cloud and rainwater samples were also collected at a windward site on the island of Hawaii. A hand‐held sun photometer was used at MLO to determine aerosol optical depths at three wavelengths. Aerosol samples were analyzed for trace elements and elemental (EC) and organic (OC) carbon. EC concentrations and temporal variations were similar at both sites. At MLO, concentrations of S, Pb, Zn, As, Sb, and Si covaried with that of EC. MLO vanadium crustal enrichment factors ranged from 1 to 2.5. The noncrustal V/Zn ratios of several samples indicated a higher level of oil‐combustion emissions than would be expected from regional emissions from Japan or China. Cloud and rainwater measurements indicated a preferential fractionation of V, Mn, and I to the cloud water. The results of this experiment are evidence for (1) long‐range transport of pollution and crustal aerosol from Asia and/or North America to Mauna Loa and (2) the possible influence of the Kuwaiti oil fires at Mauna Loa and Oahu.
Simulations of condensation and cloud condensation nuclei from biogenic SO2 in the remote marine boundary layer
Authors: Raes Frank , Van Dingenen Rita
Published: February 2012

An aerosol dynamics model is presented describing the formation and growth of H2SO4‐H2O droplets in the homogeneous gas phase. The model describes the evolution in time of the particle size distribution and hence predicts the number densities of the detectable fraction of the aerosol (CN) as well as the number of cloud condensation nuclei (CCN) active at various supersaturations. The model is applied to CN and CCN formation from biogenic SO2 in the remote marine boundary layer (MBL). In the calculations the MBL is characterized by the following average conditions: [SO2] < 30 pptv, [OH]max = 2 × 106 molecules cm−3, the relative humidity is 80%, T = 10°C, the MBL height is 1000 m. The calculations suggest that under these conditions, biogenic SO2 will commonly lead to the observed levels of CN but not of CCN active at 2% supersaturation; in order to obtain realistic levels of both CN and CCN (2%), more than 3 days of undisturbed homogeneous gas‐phase conditions are required. A number of alternative scenarios and mechanisms are therefore discussed that might facilitate the formation of CCN(2%) in the MBL. Of those, two are likely to be relevant: (1) the formation of new particles outside the MBL, with subsequent transport to the MBL, and (2) the occurrence of heterogeneous SO2 → SO4 conversion on newly formed H2SO4‐H2O droplets. Inclusion of cloud‐induced and ion‐induced nucleation in the model does not favor CCN formation.
Trace gas measurements in the Kuwait oil fire smoke plume
Authors: Luke W. T. , Kok G. L. , Schillawski R. D. , Zimmerman P. R. , Greenberg J. P. , Kadavanich M.
Published: February 2012

We report trace gas measurements made both inside and outside the Kuwait oil‐fire smoke plume during a flight of an instrumented research aircraft on May 30, 1991. Concentrations of SO2, CO, and NOx averaged vertically and horizontally throughout the plume 80 km downwind of Kuwait City were 106, 127, and 9.1 parts per billion by volume (ppbv), respectively, above background concentrations. With the exception of SO2, trace gas concentrations were far below typical U.S. urban levels and primary national ambient air quality standards. Ambient ozone was titrated by NO in the dark, dense core of the smoke plume close to the fires, and photochemical ozone production was limited to the diffuse edge of the plume. Photochemical O3 production was noted throughout the plume at a distance of 160 km downwind of Kuwait City, and averaged 2.3 ppbv per hour during the first 3 hours of transport. Little additional photochemical production was noted at a downwind range of 340 km. The fluxes of sulfur dioxide, carbon monoxide, and reactive nitrogen from the roughly 520 fires still burning on May 30, 1991 are estimated at 1.4 × 107 kg SO2/d, 6.9 × 106 kg CO/d, and 2.7 × 105 kg N/d, respectively. Generally low concentrations of CO and NOx indicate that the combustion was efficient and occurred at low temperatures. Low total nonmethane hydrocarbon concentrations suggest that the volatile components of the petroleum were burned efficiently.
A three‐parameter approximation for radiative transfer in nonhomogeneous atmospheres: Application to the O3 9.6‐μm band
Authors: Fu Qiang , Liou K. N.
Published: February 2012

A three‐parameter scaling approximation is developed in conjunction with random models for the computation of transmittance along nonhomogeneous atmospheres. In addition to the scaled path length and pressure, a third parameter is introduced to account for intermediate absorption. With reference to the results derived from line‐by‐line calculations, we assess the accuracy of this scaling approximation in different atmospheric conditions. For the O3 9.6‐μm band where the Curtis‐Godson and other conventional scaling methods usually produce large errors, the present model can achieve relative accuracies within ∼1% in transmittance and the downward surface flux. Accuracy of ∼0.15 K/d can be obtained in cooling rate calculations. The present three‐parameter approximation is well suited for radiative transfer parameterizations in which the demand for high accuracy is required in the transmittance and flux calculations.
Focused sun observations using a Brewer ozone spectrophotometer
Authors: Josefsson Weine A. P.
Published: February 2012

Accurate measurements of total ozone at high latitudes during winter have become increasingly important for studies of polar ozone depletion. The direct Sun measurements are the most direct and physically well defined type of measurement of total ozone with Dobson and Brewer spectrophotometers along with the focused Sun measurements. The direct Sun and focused Sun method are influenced by many sources of error at low solar elevations. A large error arises from the addition of radiation scattered from air along the path of the solar beam. A method to correct for this error using the Brewer instrument is described. The method gives reasonably accurate values down to solar elevations of 5° compared to 10°–20° without corrections. This is especially important for high‐latitude stations, where the low solar elevations are a limiting factor for accurate measurements of the total ozone particularly during the winter.
Calculation of area‐averaged vertical profiles of the horizontal wind velocity from volume‐imaging lidar data
Authors: Schols J. L. , Eloranta E. W.
Published: February 2012

Area‐averaged horizontal wind measurements are derived from the motion of spatial inhomogeneities in aerosol backscattering observed with a volume‐imaging lidar. Spatial averaging provides high precision, reducing sample variations of wind measurements well below the level of turbulent fluctuations, even under conditions of very light mean winds and strong convection or under the difficult conditions represented by roll convection. Wind velocities are measured using the two‐dimensional spatial cross correlation computed between successive horizontal plane maps of aerosol backscattering, assembled from three‐dimensional lidar scans. Prior to calculation of the correlation function, three crucial steps are performed: (1) the scans are corrected for image distortion by the wind during a finite scan time; (2) a temporal high pass median filtering is applied to eliminate structures that do not move with the wind; and (3) a histogram equalization is employed to reduce biases to the brightest features.
On the determination of cloud cover from satellite sensors: The effect of sensor spatial resolution
Authors: Wielicki Bruce A. , Parker Lindsay
Published: February 2012

Landsat thematic mapper (TM) data are used to provide a very high spatial resolution source of cloud “truth.” TM atmospheric window channels at wavelengths of 0.83 and 11.5 μm are used to simulate the visible and infrared channels on meteorological satellites. The TM data are spatially averaged to provide spatial resolutions (that is, pixel sizes) ranging from the full resolution 28.5‐m data to the 1‐, 4‐, and 8‐km resolutions typical of AVHRR and GOES data. These data are then used to examine the sensitivity of satellite‐derived estimates of cloud fractional coverage to changing sensor spatial resolution. Seven different cloud retrieval algorithms are examined, including the International Satellite Cloud Climatology Project (ISCCP), new cloud for Earth (NCLE) radiation budget, hybrid bispectral threshold method (HBTM), spatial coherence, box counting based on fractal theory, reflectance threshold, and temperature threshold methods. The analysis is carried out for 24 cloud fields of varying cloud type: cumulus, stratocumulus, altocumulus, cirrus, and multilayered cloud. Results indicate that estimates of cloud fraction vary greatly as a function of sensor spatial resolution and cloud algorithm assumptions. Even for 28.5‐m spatial resolution data, current cloud algorithms give cloud fractions that vary by as much as 0.25. In general, cloud algorithms are sensitive to either sensor resolution (threshold methods such as ISCCP or HBTM) or to assumptions about cloud optical depth (NCLE, spatial coherence). When present, the effect of sensor resolution is small for satellite pixel sizes less than 0.25 km. The effects are large for pixel sizes of 1 km or larger. Results are shown to be a strong function of cloud type. The effect of sensor resolution is strongest for boundary layer clouds and is very weak for cirrus clouds. At the 4‐ to 8‐km spatial resolutions typical of meteorological satellite data, the ISCCP algorithm overestimates cloud fraction for boundary layer cloud by about 0.05 but underestimates thin cirrus by 0.05. The NCLE algorithm underestimates all cloud types by an average of 0.32, and spatial coherence underestimates boundary layer cloud fraction an average of 0.18. For boundary layer clouds the errors are traced to the assumption of cloud‐filled pixels for ISCCP, the assumption of optically thick clouds for spatial coherence, and the assumption of a typical cloud albedo for the NCLE method. Using 8‐km data, the HBTM and ISCCP methods provide the most accurate cloud fraction, although the HBTM method underestimates thin cirrus by 0.20 and ISCCP overestimates all cloud types but cirrus by about 0.05. The 24 cloud fields examined were chosen to explore some of the more difficult cloud retrieval cases, so the results should not be extrapolated to global average conditions. Nevertheless, the results suggest a critical need for a clearer understanding of the performance of satellite‐derived cloud properties.
On the downward transfer of tritium to the ocean by a cloud model
Authors: Lipps Frank B. , Hemler Richard S.
Published: February 2012

Observational evidence analyzed by Eriksson [1965] and Weiss and Roether [1980] suggests that over the globe the ratio of tritium deposition into the ocean by vapor diffusion to that by rainfall should be near or slightly greater than two, while Koster et al. [1989] found in a general circulation model study that the diffusion to rainout ratio was closer to one. This study investigates the convective transport of tritium from the atmosphere to the ocean using a two‐dimensional warm‐rain cloud model. It is found that the deposition ratio is strongly dependent on the frequency and duration of rain events, with typical values for a monotonic tritium profile being about 1.0, but with values as low as 0.7 when long‐duration events occur frequently and as high as 1.9 when convective events are short‐lived and infrequent. On the basis of this study it appears that explicit treatment of convection in the general circulation model would not resolve the discrepancy in the deposition ratio between the model and the observations. It is also shown that the process of isotopic adjustment of tritium between the rain and the vapor phases is a key factor in determining the deposition ratio, since this process allows tritium to escape from the raindrops and ultimately diffuse to the surface. When tritium is “frozen” in the droplets, deposition ratios are reduced significantly.