bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface albedo from space: Coupling bidirectional models and remotely sensed measurements
Authors: Cabot F. , Dedieu G.
Published: February 2004

The general purpose of this article is to analyze the ability of land surface bidirectional models to supplement the limited angular sampling provided by Earth‐observing sensors in order to normalize shortwave satellite data or to estimate surface albedo. Briefly, our approach relies on the fitting of directional model parameters using a set of satellite measurements, NOAA advanced very high resolution radiometer, in this study. We used the models proposed by Ross [1981], Hapke [1981], Shibayama and Wiegand [1985], Verstraete et al. [1990], Deering et al. [1990], Roujean et al. [1992a], and Rahman et al. [1993a]. These models are of three types: empirical, geometrical, and radiative transfer based. The comparison of these models with observed reflectances gave similar results, with very close statistics of adjustments (rms error of reflectance around 0.03, i.e., ∼10%). This means that these models can be used to normalize directional reflectance, provided the reference direction is close to the sampled angular domain. The most attractive reason for using bidirectional models is the assumption that they would allow the prediction of directional behavior in any direction, even if the set of angles used for parameter adjustment is limited. In order to check this assumption, fitted models were used to predict the reflectances that Meteosat would measure at the top of the atmosphere. These predicted reflectances were compared to the reflectances actually measured. This comparison gave satisfactory results for every model, with rms errors of reflectance close to 0.03 (12%). Direct surface albedos estimated from numerical integration of the models exhibit the same general behavior, but some discrepancies exist at large solar zenith angles (>60°) where no observation was available.
Comparisons of trace constituents from ground stations and the DC‐8 aircraft during PEM‐West B
Authors: Arimoto R. , Duce R. A. , Prospero J. M. , Savoie D. L. , Talbot R. W. , Dibb J. E. , Heikes B. G. , Ray B. J. , Lewis N. F. , Tomza U.
Published: February 2004

Chemical data from ground stations in Asia and the North Pacific are compared with data from the DC‐8 aircraft collected during the Pacific Exploratory Measurements in the Western Pacific Ocean (PEM‐West B) mission. Ground station sampling took place on Hong Kong, Taiwan, Okinawa, and Cheju; and at three Pacific islands, Shemya, Midway, and Oahu. Aircraft samples were collected during 19 flights, most over the western North Pacific. Aluminum was used as an indicator of mineral aerosol, and even though the aircraft did sample Asian dust, strong dust storms were not encountered. The frequency distribution for non‐sea‐salt sulfate (nss SO4=) in the aircraft samples was bimodal: the higher concentration mode (∼1 μg m−3) evidently originated from pollution or, less likely, from volcanic sources, while the lower mode, with a peak at 0.040 μg m−3, probably was a product of biogenic emissions. In addition, the concentrations of aerosol sulfate varied strongly in the vertical: arithmetic mean SO4= concentrations above 5000 m (
= 0.21±0.69 μg m−3) were substantially lower than those below (
= 1.07±0.87 μg m−3), suggesting the predominance of the surface sources. Several samples collected in the stratosphere exhibited elevated SO4=, however, probably as a result of emissions from Mount Pinatubo. During some boundary layer legs on the DC‐8, the concentrations of CO and O3 were comparable to those of clean marine air, but during other legs, several chemically distinct air masses were sampled, including polluted air in which O3 was photochemically produced. In general, the continental outflow sampled from the aircraft was substantially diluted with respect to what was observed at the ground stations. Higher concentrations of aerosol species, O3, and CO at the Hong Kong ground station relative to the aircraft suggest that much of the continental outflow from southeastern Asia occurs in the lower troposphere, and extensive long‐range transport out of this part of Asia is not expected. In comparison, materials emitted farther to the north apparently are more susceptible to long‐range transport.
Radiation field in a multilayered geophysical medium: Ice‐water‐aerosol‐vegetation‐soil (IWAVES) model
Authors: Iaquinta Jean , Pinty Bernard
Published: February 2004

Multiangular data which will be available with the upcoming satellite platforms (EOS, ENVISAT, ADEOS) offer a great potential for monitoring land surfaces on the global scale to the extent that physically based models describing the transfer of radiation can be developed. The present study constitutes an additional step toward modeling this radiative transfer with in particular the physical processes involved at the boundary between land vegetated surfaces and the atmospheric layer above. Our primary objectives are to address issues related to the perturbation by an atmospheric layer of the solar radiance field incident on the top of the vegetation canopy and the interpretation of the radiance field emerging from the atmospheric layer when isotropic scattering from the surface is a priori assumed. Indeed, the application of an inappropriate model for the interpretation of remotely sensed data can produce inaccurate retrievals of both the surface and atmosphere characteristics. In the present study the radiation transport problem in this coupled system is solved analytically for uncollided and first collided radiation and uses a discrete ordinates method for multiple‐scattered radiation. A sensitivity analysis of the multilayered ice‐water‐aerosol‐vegetation‐soil model is conducted in order to quantify the effects of atmospheric and surface perturbations within the whole system. The results are essentially reported in terms of bidirectional reflectance factors at visible and near‐infrared wavelengths, which allows the use of very different radiative properties of the vegetation layer. The consequences of assumptions made on one or the other of these media are investigated through an inversion experiment.
Occurrence frequency of thickness of annual snow accumulation layers at South Pole
Authors: Hogan Austin W. , Gow Anthony J.
Published: February 2004

This paper describes the frequency analysis of 2000 annual snow accumulation layers at the South Pole. It proposes application of this analysis to objective extraction of meteorological parameters from the South Pole snow accumulation record. We have compared the 200‐year “snow mine” accumulation record of Giovinetto and Schwerdtfeger and a 2000‐layer pit and core record obtained in 1982. Frequency analysis of the number of occurrences of layers with respect to thickness or mass of the layer showed the logarithms of thickness or mass to be normally distributed. This lognormality is quite robust. Arbitrary partitioning of the data by depth or systematic partition of the data by densification or firn‐ice transition does not destroy the lognormality of the accumulation series. The geometric standard deviation of the mass of annual layers was the same as the geometric standard deviation of the thickness of these same layers as the layers densified from firn to ice. This agrees with Kapteyn's theory of transformability of lognormal distributions. We have also compared the snow accumulation and marine aerosol (sodium) accumulation in recent annual layers with the station meteorological record and the surface aerosol record. The amounts of snow and sodium accumulated are proportional to the number of warmings (“salt storms”) in most years of this short record. The accumulation record may be transformable to provide an index of this meridional transport. We propose that the magnitude of snow accumulation, with respect to frequency of that accumulation, provides an objective criterion for comparing individual years of meteorological history and that the geometric standard deviation of accumulation provides an objective index for estimation of climatic fluctuation during the period of accumulation.
Coherency between solar UV radiation and equatorial total ozone
Authors: Bjarnason Gudmundur G. , Rögnvaldsson Örnólfur E.
Published: February 2004

Influences of solar UV variability on the total ozone column densities as measured by the total ozone mapping spectrometer (TOMS) on the Nimbus 7 satellite are investigated and distinguished from the noticeable signals of dynamical origin. We confine our analysis to the equatorial region where the purest observable periodic signals that relate to the solar UV variability at the solar cycle and solar rotational timescales appear. The Mg II index is used as a proxy for solar variability in the UV region of the solar spectrum. Applying a multiple taper method of power spectral estimation to the entire TOMS observation period (14.4 years), evidence for a solar cycle variation is obtained at the 95% confidence level. The estimated solar cycle variation is 1.5±0.3% from solar minimum to maximum, corresponding to a 7.6±0.7% change in the Mg II index with no significant phase lag. The most significant line contributions to the power spectrum of the daily total ozone values in the proximity of solar rotation frequencies have periods of 28.4±0.2 and 14.6±0.1 days during maximum solar activity in solar cycle 22. F tests show both lines to be significant at or above the 99% level. Identical analysis of the Mg II index for the same period shows the strongest signals appearing at 28.4±0.5 and 13.5±0.1 days. The latter signal is more difficult to confirm because of possible interference with other lines at close frequencies (14.8 and 12.3 days). The sensitivity of total ozone to the major solar rotational mode is comparable to the solar cycle effect, and the results are consistent with solar signals of pure photochemical origin. Strong coherency appears in a couple of narrow frequency bands around 26 and 13 days showing that total ozone responds to solar UV radiation in both of the major solar rotation modes.
An investigation of dynamical contributions to midlatitude ozone trends in winter
Authors: Hood L. L. , McCormack J. P. , Labitzke K.
Published: February 2004

On timescales less than a month, negative (positive) 100 hPa temperature anomalies (deviations from seasonal means) and positive (negative) 100 hPa height anomalies at northern midlatitudes are associated with negative (positive) total ozone anomalies owing to vertical and horizontal advective transport of ozone. We apply linear regression relationships between total ozone and 100 hPa temperature and height together with observed monthly temperature and height anomalies to estimate the component of total ozone variability at northern midlatitudes that results from month‐to‐month differences in advective transport. At 45°N, this empirical method simulates a large part of the observed monthly zonal mean total ozone variability over the period 1979–1991. In order to estimate the advective transport contribution to total ozone trends, we apply a multiple regression statistical model to empirical model time series for latitudes of 10°N to 60°N. These empirical model trends result from long‐term changes in ozone advective transport which are calculated on a month‐to‐month basis. The resulting model trend component at northern midlatitudes in February has a longitude dependence consistent with that of the observed ozone trends and has a latitude dependence that also agrees with that of the observed trends, peaking at midlatitudes and decreasing at higher latitudes. Subtracting the estimated advective contribution from the observed trends yields a residual meridional trend profile that agrees more closely in latitude dependence and amplitude with two‐dimensional stratospheric model estimates.
A 5‐year evaluation of the representativeness of the tropospheric ozone residual at nonclimatological periods
Authors: Vukovich Fred M. , Brackett Vincent , Fishman Jack , Sickles Joseph E.
Published: February 2004

Daily values, monthly averages, and seasonal averages of the tropospheric ozone residual (TOR) were estimated using the Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscattered Ultraviolet (SBUV) data for the five‐year period 1985–1989. Comparisons were made at these various timescales between TOR and tropospheric ozone using data from eight ozonesonde stations within the region 50°N to 50°S and between normalized departures of the TOR and surface ozone data averaged over four 5° latitude by 5° longitude squares within the eastern United States. These comparisons were accomplished to determine the usefulness of the TOR to represent tropospheric ozone at nonclimatological periods. The results indicated that the annual cycle of the TOR determined using monthly and seasonally averaged values provides a realistic depiction of the annual cycle of tropospheric ozone in the northern hemisphere; that is, approximately a 2% mean error and an 81% correlation. However, in a limited number of comparisons (Hilo and Natal), the annual cycle of the TOR represented the annual cycle of tropospheric ozone at tropical latitudes poorly; that is, approximately a 38% mean error and a 59% correlation.
Comparison of stratocumulus cloud modeling with satellite observations and in situ measurements
Authors: Fouilloux Anne , Iaquinta Jean
Published: February 2004

As part of the European Cloud and Radiation Experiment campaign, we conducted studies with the regional atmospheric modeling system in order to simulate stratocumulus clouds observed on April 18, 1994, over Brittany (France). This three‐dimensional model was used in its non hydrostatic mode, with bulk microphysical and radiative transfer parameterizations. The first initialization employed data provided by the European Center for Medium‐range Weather Forecasts (ECMWF) and radiosoundings obtained at the Guipavas ground station. The model outputs were then compared with the corresponding NOAA‐AVHRR (advanced very high resolution radiometer) satellite image and aircraft data collected in situ during the experiment. The relative differences between modeled and actual microphysical quantities (i.e., mixing ratio of water vapor, liquid water content, or liquid water path) were found to be less than 25% along the airplane trajectory; this disagreement was as large as 200% elsewhere in the experimental domain, principally because cloud cover was underestimated. The development of a simple time‐to‐space analysis made it possible to force the mesoscale model with satellite images combined with vertical profiles and tridimensional data of the ECMWF, leading to a more accurate spatial distribution of clouds over a large domain and diminishing these errors to 10%.
Decadal evolution of total ozone decline: Observations and model results
Authors: Tourpali Kleareti , Tie Xue Xi , Zerefos Christos S. , Brasseur Guy
Published: February 2004

In this study we examine the rate of change in the total ozone decline observed during past years over both the northern and the southern hemisphere. This is studied from the analysis of the longest available reevaluated series of total ozone observations (1964–1993), with the aid of a two‐dimensional chemical/transport model of the middle atmosphere, which takes into account microphysical formation of polar stratospheric clouds (PSCs) and heterogeneous mechanisms under different stratospheric chlorine‐loading conditions. In our analysis the zonal mean total ozone decreases during the last two decades were compared to the corresponding mean ozone levels during the undisturbed period 1964–1973. The effects of quasi‐cyclical and transient phenomena such as the quasi‐biennial oscillation, the El Nino/Southern Oscillation and the solar cycle, which are known to significantly influence the ozone variability, have been filtered out from the observations, allowing a more straightforward comparison between model results and observations. These model results are in good agreement with the observed change in the rate of total ozone decline over the middle to high latitudes of both the northern and the southern hemisphere. These model calculations indicate that the ozone depletion at high latitudes in both hemispheres is mainly caused by heterogeneous reactions on the surface of PSCs. At northern midlatitudes, though, the ozone losses are caused by heterogeneous reactions on the surface of sulfate aerosols, especially after large volcanic eruptions. From a sensitivity study performed with the model, a 2° decrease in temperature was found to affect significantly the ozone field, especially in the northern hemisphere, by facilitating the heterogeneous conversion processes on PSC surfaces through enhancement of their concentration, and by accelerating the reactions on the surface of sulfate aerosols.
Comparison of modeled backscatter using measured aerosol microphysics with focused CW lidar data over Pacific
Authors: Srivastava Vandana , Clarke Antony D. , Jarzembski Maurice A. , Rothermel Jeffry
Published: February 2004

During NASA's GLObal Backscatter Experiment (GLOBE) II flight mission over the Pacific Ocean in May‐June 1990, extensive aerosol backscatter data sets from two continuous wave, focused CO2 Doppler lidars and an aerosol microphysics data set from a laser optical particle counter (LOPC) were obtained. Changes in aerosol loading in various air masses with associated changes in chemical composition, from sulfuric acid and sulfates to dustlike crustal material, significantly affected aerosol backscatter, causing variation of about 3 to 4 orders of magnitude. Some of the significant backscatter features encountered in different air masses were the low backscatter in subtropical air with even lower values in the tropics near the Intertropical Convergence Zone (ITCZ), highly variable backscatter in the ITCZ, mid‐tropospheric aerosol backscatter background mode, and high backscatter in an Asian dust plume off the Japanese coast. Differences in aerosol composition and backscatter for northern and southern hemisphere also were observed. Using the LOPC measurements of physical and chemical aerosol properties, we determined the complex refractive index from three different aerosol mixture models to calculate backscatter. These values provided a well‐defined envelope of modeled backscatter for various atmospheric conditions, giving good agreement with the lidar data over a horizontal sampling of ∼18,000 km in the mid‐troposphere.
Time lag effects on the binary homogeneous nucleation of aerosols in the wake of an aircraft
Authors: Taleb Djamel‐Eddine , McGraw Robert , Mirabel Philippe
Published: February 2004

Steady state rates and time lags for binary homogeneous nucleation of sulfuric acid‐water aerosols in aircraft wakes are calculated for relative humidities lower than 100%, i.e., for the case in which no visible contrails form. Formation of the sulfuric acid‐water aerosol is shown to depend sensitively on the concentration of sulfuric acid derived from engine exhaust. Nucleation of the sulfuric acid‐water aerosol is found to be controlled both by the steady state rate and by the time lag on the timescales relevant to contrail formation. Only for the highest gas‐phase acid concentrations are the nucleation time lags sufficiently short that steady state conditions pertain.
Oxygen isotope composition of carbon dioxide in the middle atmosphere
Authors: Barth Volker , Zahn Andreas
Published: February 2004

We present model calculations to investigate the isotopic behavior of both heavy carbon dioxide molecules C17O16O and C18O16O in the stratosphere and mesosphere. For this purpose the existing theory of 18O transfer between O3 and CO2 via O(1D) and the electronically excited complex CO3*, as proposed by Yung et al. [1991], is extended to 17O. Our one‐dimensional model agrees well with atmospheric measurements performed till now, when reasonable assumptions for the isotopic composition of ozone in the middle atmosphere are made, and when in addition a mass‐dependent fractionation process during the transfer of oxygen atoms is included. Best agreement between model results and atmospheric CO2 isotope data is obtained by assuming (1) an averaged O3 isotope composition in the middle atmosphere of δ17O = (12.5±1.5)% and δ18O = (16±1.5)% (versus Vienna standard mean ocean water) and (2) a mass‐dependent isotope fractionation during the O atom transfer via CO3* of (32±7)‰ for 17O and (64±14)‰ for 18O, both relative to 16O. Effects of CO2 photolysis and its reformation via the reaction CO + OH → CO2 + H on the CO2 isotope composition are found to be small and may be neglected.
Performance of the HPLC/fluorescence SO2 detector during the GASIE instrument intercomparison experiment
Authors: Gallagher M. S. , King D. B. , Whung P.‐Y. , Saltzman E. S.
Published: February 2004

Sulfur dioxide (SO2) in synthetic air and diluted ambient air was measured as part of the Gas‐Phase Sulfur Intercomparison Experiment (GASIE) using the high performance liquid chromatography (HPLC)/fluorescence technique. SO2 was analyzed by equilibrating the gaseous sample with aqueous SO2, sulfite, and bisulfite, then converting the aqueous S(IV) to an isoindole derivative. The derivative was separated by reversed phase HPLC and detected via fluorescence. The system was calibrated with mixtures of SO2 in zero air prepared from an SO2 permeation device through a two‐stage dilution system. The instrument has a 4‐min. sample integration time and a measurement period of 9‐min. During the GASIE intercomparison the lower limit of detection averaged 3.6 parts per trillion by volume (pptv). The precision of replicate measurements over the entire intercomparison period was better than 5% at the 20 pptv level. Instrument performance was unaffected by the interferent gases included in the GASIE protocol (H2O, O3, NOx, DMS, CO, CO2, and CH4). During diluted ambient air tests, the HPLC/fluorescence technique exhibited an approximately 10% reduction in response relative to some other techniques. The cause of this apparent calibration change is not understood.
Application of cloud microphysics to NCAR community climate model
Authors: Ghan Steven J. , Leung L. Ruby , Hu Qi
Published: February 2004

The Colorado State University Regional Atmospheric Modeling System bulk cloud microphysics parameterization has been applied to the treatment of stratiform clouds in the National Center for Atmospheric Research community climate model. Predicted cloud properties are mass concentrations of cloud water, cloud ice, rain, and snow and number concentration of ice. Microphysical processes treated include condensation of water vapor and evaporation of cloud water and rain, nucleation of ice crystals, vapor deposition and sublimation of cloud ice and snow, autoconversion and accretion of cloud water, aggregation and collection of cloud ice, melting of ice and snow, riming on ice and snow, and gravitational settling of ice, rain, and snow. Although the parameterization is more detailed and hence more computationally demanding than other cloud microphysics parameterizations in climate models, it treats the Bergeron‐Findeisen process explicitly and hence does not require an ad hoc parameterization to distinguish liquid water and ice. A variety of simulations were performed, testing sensitivity to horizontal and vertical resolution, the treatment of ice number, droplet number, and parameterization of cumulus convection. The simulated planetary radiation balance is found to be particularly sensitive to the treatment of ice number and cumulus convection.
The effects of clouds on aerosol and chemical species production and distribution: 2. Chemistry model description and sensitivity analysis
Authors: Kreidenweis Sonia M. , Zhang Yiping , Taylor Gregory R.
Published: February 2004

A modeling study of the effects of cumulus convection on the transformation and redistribution of chemical species in the troposphere is described. A sulfate chemistry model is coupled with a two‐dimensional, mixed‐phase, two‐moment microphysics Eulerian cloud model to examine the importance of different pathways for aqueous sulfate production. The complexation of CH2O with S(IV) was found to be of minor importance in most of the model cloud, compared with the oxidation of S(IV) by H2O2 and O3, while Fe(III)‐catalyzed oxidation plays an important role in aqueous phase sulfate production. Significant redistribution of SO2 from the boundary layer to the middle and upper troposphere was predicted, with a larger percentage redistributed in cases initialized with chemical fields representative of polluted continental conditions. The sensitivity of the S(IV) production and SO2 redistribution to variations in several model parameters is also examined.
Experiments with a lunar atmospheric tidal model
Authors: Stening R. J. , Forbes J. M. , Hagan M. E. , Richmond A. D.
Published: February 2004

A lunar tidal model similar to that of Forbes [1982] is used to investigate the effects of changes in background zonal winds on the structure of the lunar tide in the upper atmosphere. It is found that changes in the winds have a greater effect than changes in the background temperature profile with latitude. Observed changes in the structure of the lunar tide from year to year at Saskatoon may be explained by introducing stratospheric warmings into the model. The addition of an associated mesospheric cooling gives an even better fit to the observed phase data. It is also seen that northern hemisphere warmings are predicted to give considerable amplitude changes in southern hemisphere lunar tides. It is expected that much of the difficulty in measuring lunar tides in the upper atmosphere is due to changes in the tidal parameters at these levels brought about by variations in the underlying atmosphere.
Combined global climate model and mesoscale model simulations of Antarctic climate
Authors: Hines Keith M. , Bromwich David H. , Liu Zhong
Published: February 2004

Simulations of high southern latitudes with the high‐resolution, limited‐area Penn State/NCAR mesoscale model, version 4 (MM4), examine the impact of a moist physics parameterization and the success of a one‐way nesting inside the global NCAR community climate model, version 2 (CCM2). Discretization is by 100 km resolution in the horizontal and 15 or 16 levels in the vertical. Initial and boundary conditions for the simulations are provided by analyses of the European Centre for Medium‐Range Weather Forecasts or CCM2 simulations. Comparisons of dry and moist simulations of an austral winter month, June 1988, are used to examine the role of clouds in the regional meteorology. A cloud‐free MM4 simulation with boundary conditions provided by CCM2 run 422 indicates that the one‐way nesting of the mesoscale model can produce significant differences in the model output, including an improvement in the location of one longwave trough, reduced cold bias, and a more detailed surface wind field. The improved location of the longwave trough over the Atlantic Ocean is hypothesized to result from blocking induced by the reduced cold bias. The nested simulation is sensitive to the forcing at the horizontal boundaries. Consequently, proper location of troughs and ridges at the boundaries is required for the model to well represent all the major troughs and ridges inside the domain. The addition of moist physics to the mesoscale simulations tends to improve the quality of the simulated fields over the Southern Ocean. In particular, the intensity of the circumpolar trough is increased. Over Antarctica, serious deficiencies are found in the simulations with moist physics. Excessive moisture is apparently stored in the simulated clouds leading to excessive atmospheric back radiation and, consequently, excessive temperatures at the surface and higher up in the troposphere.
Three‐dimensional simulations of long‐lived tracers using winds from MACCM2
Authors: Waugh D. W. , Hall T. M. , Randel W. J. , Rasch P. J. , Boville B. A. , Boering K. A. , Wofsy S. C. , Daube B. C. , Elkins J. W. , Fahey D. W. , Dutton G. S. , Volk C. M. , Vohralik P. F.
Published: February 2004

Three‐dimensional simulations of the stratospheric constituents CH4, N2O, O3, SF6, and CO2 over an annual cycle have been performed using a semiLagrangian chemical transport model [Rasch and Williamson, 1990; Rasch et al., 1994] driven by archived wind data from the Middle Atmosphere version of the National Center for Atmospheric Research Community Climate Model version 2 (MACCM2) general circulation model. The constituents undergo chemical production and loss at rates calculated by two‐dimensional photochemical models. We compare these “off‐line” simulations of CH4 and N2O with “on‐line” simulations in which the trace constituent distributions are computed interactively within the MACCM2 general circulation model and find good agreement even when daily averaged wind data and no subgrid scale parameterized mixing are used in the off‐line simulations. We also compare the model simulations to satellite, aircraft, and balloon measurements. In most regions and seasons, the zonally averaged model CH4, N2O, and O3 fields agree well with observations. Notable discrepancies are (1) a lack of a “double peak” structure in the zonally averaged mixing ratios of model CH4 and N2O at equinox, (2) an overall underestimate of CH4 and N2O in the upper stratosphere, and (3) an underestimate of the height of the mixing ratio peak in O3, particularly at high latitudes. We find good agreement between modeled CO2 and SF6 and recent aircraft observations in the lower stratosphere, and balloon measurements in the lower and middle stratosphere. From the SF6 distribution we determine the mean age of air in the model stratosphere, with values as old as 10 years in the wintertime polar upper stratosphere. In addition, we simulate the annual cycle of CO2, a stringent test of model transport, which supplements the mean age. We obtain good agreement with aircraft measurements in phase and magnitude at the tropical tropopause, and the vertical profiles of CO2 are similar to those observed. However, the amplitude of the cycle attenuates too rapidly with height in the model stratosphere, suggesting the influence of midlatitude air and/or the vertical diffusion are too large in the model tropics.
Recent intra‐annual snow accumulation at South Pole: Implications for ice core interpretation
Authors: McConnell Joseph R. , Bales Roger C. , Davis Donald R.
Published: February 2004

Spatial and temporal variability in snow accumulation can have a significant impact on interpretation of ice cores, particularly for chemical species that show a strong seasonal dependence either in atmospheric loading or in the atmosphere‐to‐snow transfer process. Using a unique, 7.25‐year record of recent, year‐round monthly snow accumulation at South Pole, we compute the number of years of averaging required to statistically ensure that an ice core record would have equivalent representation of snow from each month of the calendar year. For current South Pole meteorological conditions, averaging times of the order of 300 years are required to ensure equal representation of each month, with May being the least well‐represented month. To ensure equal representation of 3‐month seasons still requires averaging times of the order of 130 years. Through correlations of variability in monthly accumulation with potential snow transport by wind, we estimated a similar averaging time for Summit, Greenland. For studies of long‐term, inter‐annual changes in the atmospheric concentration of species that are reversibly deposited to snow such as hydrogen peroxide (H2O2), it is imperative that comparisons are made over intervals with equivalent representation of high versus low concentration periods. This is critical for H2O2 because it has a very high annual variability in the surface snow, driven by photochemical and temperature cycles.
Seasonal, semiannual, and interannual variability seen in measurements of methane made by the UARS Halogen Occultation Experiment
Authors: Ruth Sarah , Kennaugh Richard , Gray Lesley J. , Russell J. M.
Published: February 2004

Variability in atmospheric methane on semiannual, seasonal, and interannual timescales is investigated using measurements of CH4 made by the Upper Atmosphere Research Satellite Halogen Occultation Experiment (HALOE). It is shown that there is a strong correlation between the appearance of the stratopause double peak in methane mixing ratio (which is thought to be associated with the semiannual oscillation (SAO)) and the phase of the quasi‐biennial oscillation (QBO) in the equatorial lower stratosphere. In a descending westerly QBO phase the double peak is seen clearly in the HALOE methane data, but during the onset of the easterly phase the double peak is not observed. This is the opposite of that which might be expected if the cause was a straightforward modulation of the strength of the SAO due to wave filtering by the QBO. The intra‐annual variability in HALOE methane is shown to be broadly consistent with previous observations. One interesting feature is the apparent occurrence of descent in the summer high latitudes near the stratopause. It is shown that this feature is, in fact, due to enhanced photochemical destruction in this region in the presence of weak transport.
Lunar tides in the mesosphere over Christmas Island (2°N, 203°E)
Authors: Stening R. J. , Schlapp D. M. , Vincent R. A.
Published: February 2004

This paper reports the first measurement of the lunar tide in upper atmosphere winds at an equatorial site. Wind velocities have been measured at Christmas Island, in the Pacific Ocean, from 1990 to 1993 over the height range 82 to 98 km and hourly values have been used to deduce the lunar tide using a least squares method. Amplitudes are 1 to 3 m/s with only small changes in phase with season. A large phase change over the early part of 1990 was found in the northward wind. Generally, the phase difference between northward and eastward winds was about 6 hours, and good agreement was obtained with a tidal model developed from that of Forbes [1982a].
Mesoscale cyclone activity over Antarctica during 1991: 2. Near the Antarctic peninsula
Authors: Carrasco Jorge F. , Bromwich David H. , Liu Zhong
Published: February 2004

A survey of mesoscale cyclogenesis near the Antarctic peninsula has been carried out for 1991 based on digital satellite imagery collected at Palmer Station. Slightly higher cyclonic activity was found over the Bellingshausen Sea sector (BSS) than in the Weddell Sea sector (WSS). Both sides were characterized by a pronounced formation maximum in summer and a minimum in winter. Comma clouds were the dominant vortex type, and their average diameter was around 370 km. Trajectories show that many vortices in the BSS move northeastward toward the Drake Passage, with some of the most well developed moving north‐northeastward to the southern tip of South America, bringing severe weather conditions to that area; little systematic motion was noted over the WSS. The fraction of mesoscale cyclones with middle and/or high cloud on the west side of the Antarctic peninsula was much higher than on the east side. These numerous and deep mesoscale cyclones are a consequence of the frequent outbreaks of cold air associated with synoptic‐scale cyclones that move cold air northward over the relatively warm Bellingshausen Sea, with convective instability resulting. This happens much less often in the WSS, where low‐level baroclinic instability and vortex stretching are the dominant formation mechanisms and air‐sea interaction is usually much less vigorous due to the stabilizing influence of the extended sea‐ice cover.
Photochemical oxidant formation over southern Switzerland: 2. Model results
Authors: Staffelbach T. , Neftel A. , Horowitz L. W.
Published: February 2004

Measurements of photooxidants made during summer 1994 in the southern part of Switzerland are compared with results of a one‐dimensional Lagrangian model. The agreement between modeled and measured ozone concentrations is good. Concentrations of highly soluble species such as H2O2 and HNO3 are overestimated by the model. The model underestimates concentrations of isoprene, HCHO, and HONO. The impact of this discrepancy on the question of NOx versus VOC limitation of the ozone production is discussed. According to the model, reduction of the NOx emissions is more effective in reducing O3 than reduction of the VOC emissions for a large fraction of the area under investigation. This finding is in accordance with what we concluded from the data. The southern part of Switzerland is often impacted by a plume originating from Milan, which leads to additional ozone at the measuring site. We used the model to simulate the influence of this plume on the chemical regime. The plume emissions are picked up by the model at two different times. Both cases deliver the same concentrations for the end point of the calculations. If the emissions are picked up early in the morning, the ozone production becomes VOC sensitive for a few hours before switching back to being NOx sensitive. If the emissions are picked up later, the O3 production stays NOx sensitive all the time. Forcing the model to reproduce measured concentrations of HONO and isoprene shifts the O3 production toward being more NOx sensitive.
Further comment on the existence of a modified hydrogen bonding ratio in laboratory‐generated water droplets
Authors: Campillo A. J. , Eversole J. D. , Lin H‐B.
Published: February 2004

We offer further comments regarding the controversial claim of Zhang et al. [1994] that a standard vibrating orifice aerosol generator modifies the hydrogen bonding ratio of laboratory water droplets. New data reported here fail to support their observation of a surface charge induced Raman spectral change as reported by Aker et al.[1996a].
A modeling study of the effect of drizzle on cloud optical depth and susceptibility
Authors: Feingold Graham , Boers Reinout , Stevens Bjorn , Cotton William R.
Published: February 2004

This paper examines the impact of drop spectral broadening, generated by the collection process, on the optical depth, cloud albedo, and susceptibility of marine stratocumulus clouds. The results are arrived at using (1) the output from a simple box model calculation of collection and (2) the output from an eddy‐resolving model of stratocumulus clouds that explicitly represents the size distribution of the drops. It is shown that commonly used relationships for cloud optical properties developed for narrow spectra do not generally apply to spectra undergoing spectral broadening. The optical depth dependence on the drop number concentration to the one‐third power is shown to be an overestimate of the optical depth when spectra broaden through collection. In addition, the cloud susceptibility dependence on drop number is shown to be larger for spectra experiencing broadening than for narrow spectra.
A three‐dimensional simulation of the evolution of the middle latitude winter ozone in the middle stratosphere
Authors: Douglass A. R. , Rood R. B. , Kawa S. R. , Allen D. J.
Published: February 2004

Observations of O3, CH4, and N2O from the Halogen Occultation Experiment (HALOE), the Cryogenic Limb Array Etalon Spectrometer (CLAES), and the Microwave Limb Sounder on the Upper Atmosphere Research Satellite are compared with fields calculated using a three‐dimensional global chemistry and transport model which uses winds from a data assimilation system. Model and observations are found to be in generally good agreement for ozone at middle latitudes between 50 and 3 hPa. Much of the horizontal structure in the observed long‐lived tracers N2O and CH4 is seen in the model, but the model profile shapes differ systematically from those seen by CLAES (N2O and CH4) and HALOE (CH4). To understand the influence of transport on these comparisons, attention is focused on January and February 1992, when a series of minor warmings impact the ozone and tracer distributions. The model O3 at 6.8 hPa, which agrees well with observations, is shown to be influenced by transport from the tropics toward middle and high latitudes, which increases O3, and photochemical processes, through which O3 is destroyed. However, the model N2O and CH4 mixing ratios at middle latitudes at 6.8 hPa are much greater than observations. Good comparisons between modeled and observed O3 and poor comparisons between modeled and observed N2O and CH4 are explained by differences in the horizontal transport from the subtropics. Here photochemical processes control the O3 mixing ratio, and the horizontal winds transport air with realistic ozone mixing ratios to the middle latitudes. For the longer‐lived gases, the model mixing ratios in the subtropics far exceed observations, and the horizontal winds transport air with higher than observed mixing ratios to middle latitudes, leading to the poor comparisons. This analysis shows that the photochemical and transport processes that affect O3 at middle latitudes during winter are well represented in the model. However, for long‐lived tracers, errors in the model subtropics are communicated to middle latitudes by the horizontal winds.
Parameterization schemes for terrestrial water clouds in the radiative transfer model GOMETRAN
Authors: Kurosu T. , Rozanov V. V. , Burrows J. P.
Published: February 2004

Quasi‐exact and approximate methods for the parameterization of clouds inside finite‐differencing radiative transfer codes are introduced and exemplified for the radiative transfer model GOMETRAN. The quasi‐exact parameterization scheme considers the cloud as a plane‐parallel, homogeneous layer of finite vertical extent, while the approximative scheme, which holds for optically thick clouds, treats the cloud top as a non‐Lambertian reflecting surface that bounds the atmosphere from below. GOMETRAN simulations, using both cloud models, are compared for various cloud scenarios with and without atmospheric absorption. Comparison of results from the quasi‐exact parameterization with DISORT simulations yields differences of generally less than 5% for all cloud optical depths and all viewing geometries. Using this model, photon path lengths and light path enhancements inside cloud layers as well as the applicability of the developed methods to the interpretation of satellite measurements have been investigated.
An analysis of tropical transport: Influence of the quasi‐biennial oscillation
Authors: Cordero Eugene C. , Kawa S. Randolph , Schoeberl Mark R.
Published: February 2004

An analysis of over 4 years of Upper Atmosphere Research Satellite (UARS) measurements of CH4, HF, O3, and zonal wind are used to study the influence of the quasi‐biennial oscillation (QBO) on constituent transport in the tropics. At the equator, spectral analysis of the Halogen Occultation Experiment (HALOE) and Microwave Limb Sounder (MLS) observations reveals QBO signals in constituent and temperature fields at altitudes between 20 and 45 km. Between these altitudes, the location of the maximum QBO amplitude roughly corresponds with the location of the largest vertical gradient in the constituent field. Thus, at 40 km where CH4 and HF have strong vertical gradients, QBO signals are correspondingly large, while at lower altitudes where the vertical gradients are weak, so are the QBO variations. Similarly, ozone, which is largely under dynamical control below 30 km in the tropics, has a strong QBO signal in the region of sharp vertical gradients (∼28 km) below the ozone peak. Above 35 km, annual and semi‐annual variations are also found to be important components of the variability of long‐lived tracers. Therefore, above 30 km, the variability in CH4 and HF at the equator is represented by a combination of semiannual, annual, and QBO timescales. A one‐dimensional vertical transport model is used to further investigate the influence of annual and QBO variations on tropical constituent fields. QBO‐induced vertical motions are calculated from observed high resolution Doppler imager (HRDI) zonal winds at the equator, while the mean annually varying tropical ascent rate is obtained from the Goddard two‐dimensional model. Model simulations of tropical CH4 confirm the importance of both the annual cycle and the QBO in describing the HALOE CH4 observations above 30 km. Estimates of the tropical ascent rate and the variation due to the annual cycle and QBO are also discussed.
Estimating the wavelength‐dependent ocean albedo under clear‐sky conditions using NASA ER 2 spectroradiometer measurements
Authors: McLinden C. A. , McConnell J. C. , Griffoen E. , McElroy C. T. , Pfister L.
Published: February 2004

The wavelength‐dependent albedo along the flight track of a NASA ER 2 high‐altitude research aircraft has been estimated under clear‐sky conditions in the 300–775 nm spectral range. We have developed an inversion technique to estimate ocean albedo which used nadir radiance measurements obtained during recent ER 2 campaigns. These are used in conjunction with a multiple‐scattering radiative transfer model. This method is general and robust in that any type of surface can be handled as long as a functional form for the angular distribution of surface radiance can be specified. Comparisons have been made against a forward, iterative method and differences do not exceed 1% in the visible and were 1–2% in the near UV. Results are presented for clear‐sky portions of flights from two ER 2 campaigns. Specular reflection of the sunlight off the ocean surface has been treated using the Cox and Munk statistical approach. Following this, the wavelength‐dependent albedo has been estimated at selected points along various ER 2 flights. Furthermore, photolysis coefficients (or J values) for NO2 and NO3 have been calculated at these points so that the effects of accounting for both specular reflection and wavelength dependence on photochemistry can be assessed. The use of a wavelength‐dependent albedo was found to impact J values by as much as 10%. It was also determined that, in most cases, accounting for specular reflection was found to have a minor impact on J values (1–3%), but under certain conditions, differences of up to 7% are possible.
Temperature dependence of the ClO concentration near the stratopause
Authors: Ghosh S. , Pyle J. A. , Good P.
Published: February 2004

From an analysis of Microwave Limb Sounder (MLS) ClO data near the stratopause where it is reasonable to assume that ClO is in photochemical equilibrium, it is found that there is a strong temperature dependence of the order of 1000 K(±200 K) including the effects of scaling and the bias uncertainties in the retrieved MLS data. Results from a three‐dimensional model (extended UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (EUGCM)) agree well with the MLS results; we obtain a temperature dependence of 950 K in the three‐dimensional model. The temperature dependencies obtained from the MLS data and the EUGCM agree quite well with results from a purely photochemical box model which yields a temperature dependence of the order of 900 K. The generally good agreement between the temperature dependence of ClO obtained from the MLS measurements, from both the photochemical model and the three‐dimensional general circulation model does not suggest that there are major uncertainties in our understanding of the ClO chemistry in the upper stratosphere.
Comparison of regional surface fluxes from boundary‐layer budgets and aircraft measurements above boreal forest
Authors: Barr Alan G. , Betts Alan K. , Desjardins R. L. , MacPherson J. I.
Published: February 2004

Daytime surface sensible and latent heat fluxes (Hs and λEs) above boreal forest, derived independently from the boundary‐layer budgets of heat and water vapor and by eddy correlation from the Twin Otter aircraft, are compared. The aircraft and boundary‐layer budget values of Hs + λEs underestimated surface available energy by 25% and 4%, respectively, when the sum of the minor surface energy balance terms (storage and photosynthesis) was estimated as 17% of net radiation. The boundary‐layer budget estimate of the regional Bowen ratio (1.05) agreed to within 15% of the aircraft measurement (0.91). Both methods clearly have value for estimating land‐surface fluxes at a regional scale.