bluuebunny/crossref_metadata_2025 huggingface dataset:
Decadal‐scale chemical variability in the subtropical North Atlantic deduced from nutrient and oxygen data
Authors: Garcia Hernan , Cruzado Antonio , Gordon Louis , Escanez Jose
Published: February 2004

In August 1992 the R/V Hesperides reoccupied a transatlantic section nominally along 24.5°N previously sampled in April 1957 and in August 1981. Observation of significant warming over the 35‐year period has already been reported for the ocean layer between 800 and 2500 m depth (∼0.01°C yr−1). We examine decadal‐scale variability in the nutrient and O2 measurements primarily comparing the 1981 and 1992 sections. The basin‐scale water mass structure remained relatively unchanged from 1981 to 1992. Zonally averaged differences of the chemical data show that the layer of maximum increase in temperature and salinity at 1100 m is roughly coincident with an apparent decrease in O2 concentration of about 7 μmol kg−1. The O2 decrease is equivalent to an average rate of decrease of 0.6 μmol kg−1 yr−1. The vertically integrated water column O2 utilization rate is 0.9 mol m2 yr−1. This is equivalent to 23–33% of the new production rate estimates in the Sargasso Sea. The O2 decrease at 1100 m cannot be explained by using oxidative ratios relating regeneration of nutrients and O2 consumption. The O2 decrease is not highly correlated with zonally averaged changes in the nutrient measurements. The nutrients show small or insignificant changes near 1100 m over the 11‐year period. Part of the O2 decrease might be explained by measurement error (30%) and changes in solubility (20%) due to warming. The chemical changes are likely caused by changes in the characteristics of the source waters entering 24.5°N near 700–1700 m depth.
Statistical analysis and intercomparison of WAM model data with global ERS‐1 SAR wave mode spectral retrievals over 3 years
Authors: Heimbach P. , Hasselmann S. , Hasselmann K.
Published: February 2004

Ocean wave spectra were retrieved from a set of ERS‐I synthetic aperture radar (SAR) wave mode (SWM) spectra between January 1993 and December 1995. An assessment is given of the SWM data quality and the retrieval performance as well as the operational feasibility of the retrieval algorithm. Sensitivity studies are performed to demonstrate the weak residual dependence of the retrieval on the first‐guess input spectrum. The mean spectral parameters of the SWM retrievals are compared with spectral parameters from collocated wave model (WAM) spectra. The time series of SWM‐retrieved and WAM‐derived monthly mean significant wave heights Hs in various ocean basins show good overall agreement but with a small systematic underestimation of Hs by the WAM. A decomposition of the wave spectra into wind sea and swell reveals an average 10% overprediction of the wind sea by the WAM while swell is underpredicted by 20–30%. The positive wind‐sea bias exhibits no clear wave height dependence, while the negative swell bias decreases with swell wave height. This could be due to a too strong damping in the WAM at low frequencies. Detailed regional investigations point to the existence of smaller‐scale phenomena, which may not be adequately reproduced by the WAM at the present resolution of the wind forcing. Finally, an intercomparison is made of the observed and modeled azimuthal cutoff length scales, and global distributions are investigated. Ratios of the observed azimuthal cutoff wavenumber to the mean azimuthal wavenumber component indicate that about 75% of the swell can be directly resolved by the SAR, while about 70% of the wind sea lies at least partially beyond the cutoff.
Crest pairing predicted by modulation theory
Authors: Trulsen Karsten
Published: February 2004

Crest pairing of water surface waves is a consequence of linear dispersion alone and is predicted by the linear Schrödinger equation for the slow evolution of a wave train. Linear theory also explains why crest pairing is observed in front of a wave group. The role of nonlinearity is to redistribute sufficient energy to the sidebands such that the linear dispersive mechanism can become effective. There is also quantitative agreement with experimental observations for the propagation of a wave group faster than the linear group velocity.
Flow variability at the continental shelf break of the Mackenzie Shelf in the Beaufort Sea
Authors: Kulikov E. A. , Carmack E. C. , Macdonald R. W.
Published: February 2004

Observations from four, yearlong current meter deployments made in 1987/1988 near the shelf break in the Canadian Beaufort Sea are analyzed by statistical and spectral methods to characterize the structure and variability (vertical and horizontal) of the velocity field, and to compare measurements obtained during summer (ice‐free) and winter (ice‐covered) conditions. Three main frequency bands are defined: low frequencies (0.01–0.07 cpd), intermediate or synoptic‐scale frequencies (0.07–0.7 cpd), and high frequencies (>0.7 cpd; including tides and inertial currents). Within this division, about 65% of the kinetic energy is in the low‐frequency band, 15% in the intermediate band, and 20% is in the high‐frequency band; the latter is dominated by tidal motions. Low‐frequency variability is well correlated both vertically and horizontally along the shelf for separation distances exceeding 200 km. Variability in this band appears to be related to wind forcing and topographic wave propagation over large horizontal scales. Low‐frequency variability is highest during October and November in association with the occurrence of intense autumn storms, and lowest during periods of ice cover due to wind shielding and increased frictional damping by ice cover. Intermediate‐scale motions have significant correlation in the vertical, but poor correlation in the horizontal. Motions in this frequency band appear to be associated with mesoscale eddies advected by the mean flow. There is also evidence of local generation of intermediate‐frequency current fluctuations by storms in late autumn and, again, an apparent damping during winter periods of ice cover. Finally, high‐frequency motions show significant peaks in diurnal and semidiurnal tidal bands which are well correlated horizontally and vertically. However, other high‐frequency motions in this band show no significant correlation in either the vertical or horizontal direction. There is no clear evidence in the current meter records of coupling between high‐frequency motions and direct atmospheric forcing.
Ozone depletion in the late winter lower Arctic stratosphere: Observations and model results
Authors: Bregman A. , van den Broek M. , Carslaw K. S. , Müller R. , Peter T. , Scheele M. P. , Lelieveld J.
Published: February 2004

Ozone loss rates in the lowermost part of the Arctic stratosphere (at potential temperature levels ≤375 K) in the period January and February 1993 are calculated using a chemistry‐trajectory model and 30‐day back trajectories. The results were compared with observations carried out during the first Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM) in February 1993 in the Arctic lower stratosphere. Relatively low N2O and low O3 concentrations were measured during STREAM, and O3 loss rates of 8.0 (±3.6) ppbv d−1 were calculated from O3‐N2O STREAM data in the vortex area. The average O3 loss rate calculated by the model is 8.6 ppbv d−1 (1.3% d−1), in agreement with observations. However, the calculated O3 loss rate decreases to the lower value of the observed loss rates when taking into account N2O‐Cly, interrelations from different studies. Heterogeneous reactions on liquid sulfuric acid aerosols, in particular those involving the chlorine reservoir species ClONO2 and HCl, must be considered to explain the observed O3 loss rates. Complete conversion of ClONO2 and HCl to active chlorine by heterogeneous reactions in the model occurs at temperatures ≤205 K under conditions with enhanced aerosol loading, and at temperatures ≤200 K with background aerosol levels. Since the trajectory temperatures were frequently below 205 K and occasionally below 200 K, the model results are (1) sensitive to the Cly level but relatively insensitive to the initial chlorine partitioning within Cly and (2) show significant O3 loss at background aerosol levels, being only 1–2 ppbv d−1 less compared to conditions with enhanced aerosol loading. We conclude that future O3 loss in the Arctic lower stratosphere is quite sensitive to temperature changes, while it appears to be less sensitive to enhanced aerosol loading (e.g., by volcanic sulfate particles).
Ab initio study of XOClO3 (X = Cl, F, and H): Implications for formation of ClOClO3 in the stratosphere
Authors: Parthiban S. , Raghunandan B. N. , Sumathi R.
Published: February 2004

Although OClO photolysis was one of the first photochemical reactions to be studied, the primary photochemistry of OClO still remains controversial. Several studies have demonstrated that Cl2O4 is one of the major photolysis products of OClO, in addition to ClO + O and ClOO. Thus there is a need to analyze the chemical properties of Cl2O4 and its related species. In the present study we have investigated the equilibrium structures and harmonic vibrational frequencies of chlorine perchlorate (Cl2O4), fluorine perchlorate (FOClO3), and hydrogen perchlorate (HOClO3) at Hartree‐Fock and MP2 levels using 6–31G* basis set. Normal mode analysis has been carried out using ab initio derived force fields at the correlation level for the 6–31G* basis set. The calculated frequencies of XOClO3 (X = Cl, F, and H) molecules are discussed in comparison with those determined from experiment and with the corresponding vibrations of Cl2O7. In addition, the paper draws attention to the possible role of C12O4 formation in the polar stratosphere with relevance to ozone layer depletion.
Infrared spectroscopy of sulfuric acid/water aerosols: Freezing characteristics
Authors: Clapp M. L. , Niedziela R. F. , Richwine L. J. , Dransfield T. , Miller R. E. , Worsnop D. R.
Published: February 2004

A low‐temperature flow cell has been used in conjunction with a Fourier transform infrared (FT‐IR) spectrometer to study sulfuric acid/water aerosols. The aerosols were generated with a wide range of composition (28 to 85 wt%), including those characteristic of stratospheric sulfate aerosols, and studied over the temperature range from 240 K to 160 K. The particles exhibited deep supercooling, by as much as 100 K below the freezing point in some cases. Freezing of water ice was observed in the more dilute (<40 wt% sulfuric acid) particles, in agreement with the predictions of Jensen et al. and recent observations by Bertram et al. In contrast with theoretical predictions, however, the entire particle often does not immediately freeze, at least on the timescale of the present experiments (seconds to minutes). Freezing of the entire particle is observed at lower temperatures, well below that characteristic of the polar stratosphere.
Tracer lamination in the stratosphere: A global climatology
Authors: Appenzeller Christof , Holton James R.
Published: February 2004

Vertical soundings of stratospheric ozone often exhibit laminated tracer structures characterized by strong vertical tracer gradients. The change in time of these gradients is used to define a tracer lamination rate. It is shown that this quantity can be calculated by the cross product of the horizontal temperature and horizontal tracer gradients. A climatology based on UARS satellite‐borne ozone data and on ozone‐like pseudotracer data is presented. Three stratospheric regions with high lamination rates were found: the part of the stratospheric overworld which is influenced by the polar vortex, the part of the lowermost stratosphere which is influenced by the tropopause and a third region in the subtropical lower stratosphere mainly characterized with strong vertical shear. High lamination rates in the stratospheric overworld were absent during summer, whereas in the lowermost stratosphere high lamination rates were found year‐round. This is consistent with the occurrence and seasonal variation of the horizontal tracer gradient and vertical shear necessary for tilting the tracer surfaces. During winter, high lamination rates associated with the stratospheric polar vortex are present down to ∼100 hPa. Several features of the derived climatology are roughly consistent with earlier balloon‐borne studies. The patterns in the southern and northern hemisphere are comparable, but details differ as anticipated from a less disturbed and more symmetric southern polar vortex.
Measurements of sulfur dioxide during GASIE with the mist chamber technique
Authors: Talbot R. W. , Scheuer E. M. , Lefer B. L. , Luke W. T.
Published: February 2004

This paper highlights the performance of the mist chamber/ion chromatography (MC/IC) technique for measuring atmospheric sulfur dioxide (SO2) during the Gas‐Phase Sulfur Intercomparison Experiment (GASIE). The technique was found to be free of interference from CO, CO2, CH4, NOx, O3, CH3SCH3, and H2O vapor. Repeated measurements at various mixing ratios of SO2 indicated that the coefficient of variation in the MC/IC measurement is 3–5% at 300–500 parts per trillion by volume (pptv), 4–7% at 150 pptv, 10% at 50 pptv, and as great as 20% near 20 pptv. In ambient air diluted five‐ to tenfold with zero air, the MC/IC technique tracked the other methods over the range of 30–3500 pptv. This agreement reinforced the conclusions obtained during the other test phases of GASIE. The MC/IC method has desirable features such as simplicity, small size, and weight of required equipment, sample integration times of 10 min or less, low pptv detection capabilities, and relatively modest implementation costs. In addition, a suite of important soluble gases can be measured simultaneously including nitric, formic, and acetic acids.
Removal of tropospheric path length variations in very long baseline interferometry with measurement of tropospheric emission
Authors: Bower Geoffrey C. , Backer D. C. , Plambeck R. L. , Wright M. C. H.
Published: February 2004

We present our first results of the application of the measurement of atmospheric emission to correct for atmospheric phase variations in very long baseline interferometry. This technique corrects for the fluctuations in the phase path length through the atmosphere by measuring the water vapor content through its emissivity. Two epochs of 3‐mm wavelength (λ) observations under different atmospheric conditions show that total power monitoring at Hat Creek and Kitt Peak Observatories can be used to reduce the rms interferometer phase when liquid water is not present in the beam. In the best cases, the root‐mean‐square (rms) phase is reduced by a factor of 2 to 1 rad. Instability in the receiver gain at the level of one part in 103 creates the 1‐rad limit in the rms phase. The technique is currently most successful at eliminating variability on timescales greater than 10 s. However, it is not always successful, and we discuss sources for the residual variability in the phase. We also derive the expected value of the scale factor relating delay to brightness temperature at 86 and 230 GHz. As this technique is improved, future experiments will have greater sensitivity to weak sources, accurate astrometry and geodesy will be possible, and imaging at 1‐mm λ may be explored.
Role of spatial and temporal variations in the computation of radiative forcing and GWP
Authors: Myhre Gunnar , Stordal Frode
Published: February 2004

We investigate the role of spatial and temporal resolution for estimation of radiative forcing due to SF6 and a range of halocarbons as well as CO2. A broadband model, which is used in the calculations, is described. Some comparative calculations have also been performed with a line‐by‐line model. The most detailed horizontal resolution used is 2.5°×2.5° in latitude and longitude. A variety of resolutions up to global averages are investigated. The effects of variations on diurnal, monthly, and seasonal scales are also studied. Spatial and temporal variation in the radiative forcing due to variations in temperature, humidity, and cloudiness has been taken into account on the basis of observed data. Inaccuracies due to temporal variations are small in all cases (up to about 1%). Deviations in forcings due to spatial averaging are also small (less than 1%) as long as latitudinal variations are resolved, but significant inaccuracies are introduced when global averaged conditions are assumed. The forcing due to CO2 responds somewhat differently to spatial averaging compared to SF6 and the halocarbons, so global warming potential (GWP) values for SF6 and halocarbons with CO2 as a reference gas are less accurate. Resolution of latitudinal variations in input parameters is shown to increase the accuracy of the GWP values for SF6 and the halocarbons. The choice of tropopause level, where radiative forcing is determined, is shown to be crucial, with differences up to 10% in the global average radiative forcing for different assumptions.
Climatology of the semiannual oscillation of the tropical middle atmosphere
Authors: Garcia Rolando R. , Dunkerton Timothy J. , Lieberman Ruth S. , Vincent Robert A.
Published: February 2004

We have used a variety of satellite, ground‐based, and in situ observations to construct a climatology of the semiannual oscillation (SAO) of the tropical middle atmosphere. The sources of data include rocketsonde observations of winds and temperature, MF radar wind observations, and observations of winds and temperatures made from space by the High Resolution Doppler Imager (HRDI) and the Solar Mesosphere Explorer (SME). These data sets provide a generally consistent picture of the SAO, of the relationship between its stratospheric and mesospheric manifestations, and of its apparent modulation by the stratospheric quasi‐biennial oscillation (QBO). In agreement with earlier studies, we find that the first cycle of the stratospheric SAO (which begins with the stratopause easterly phase in northern winter) is stronger than the second cycle (beginning with the easterly phase in southern winter). Similar behavior is apparent in the mesosphere, where the easterly phase is stronger during the first cycle than during the second cycle. HRDI and MF radar are capable of observing the seasonal cycle well into the lower thermosphere. Data from these two sources indicate that a strong SAO is present up to about 90 km, giving way above this altitude to time mean easterly winds with a weak semiannual variation. Between 105 and 110 km, HRDI data indicate the presence of a westerly wind layer with almost no seasonal variation. Apparent modulation of the stratospheric SAO by the QBO is found in rocketsonde data, while HRDI and MF radar observations suggest a correlation between the QBO and the easterly phase of the mesospheric SAO. We discuss the implications of these observations for the wave processes that drive the SAO.
Magnitudes and seasonal patterns of energy, water, and carbon exchanges at a boreal young jack pine forest in the BOREAS northern study area
Authors: McCaughey J. Harry , Lafleur Peter M. , Joiner David W. , Bartlett Paul A. , Costello Andrew M. , Jelinski Dennis E. , Ryan Michael G.
Published: February 2004

Seasonal patterns of the energy balance of a young jack pine site near Thompson, Manitoba, in the summer of 1994 are reported. The experiment was part of the Boreal Ecosystem‐Atmosphere Study (BOREAS), and it ran from May 24 to September 19. The average tree density was 4.4 trees m−2, but there was substantial spatial variation associated with three primary vegetation patterns: dense cover of short trees, sparse cover of tall trees, and a mixture of short and tall trees. The frequency distribution of tree heights was bimodal with peaks at 1 and 2 m and a range from 0.3 to 5.7 m. The average tree height was 2.3 m. The daily average photosynthetically active radiation albedo was conservative, varying from 0.054 under clear‐sky conditions to 0.051 under cloudy skies. The shortwave albedo was 0.136 under clear‐sky conditions and decreased by 1% following rain; the presence of smoke over the site increased it by 1%. Heat storage in the soil, trees, and air was an important component of the energy balance throughout the season with soil heat flux comprising the bulk of the total storage. On the average, for the whole experimental period, the sensible heat flux was approximately twice the latent heat flux (Bowen ratio ≈2). The average daily energy balance closure varied from 85% before day‐of‐year (DOY) 200 to 95% after DOY 200. As a result of problems with the measurement of the net CO2 flux above the canopy under stable nighttime conditions, all nighttime values were modeled. The typical diurnal pattern of net CO2 flux shows maximum uptake by the surface in the morning and a gradual decrease through the afternoon. For the whole field season of 118 days, the site fixed 224 g m−2 of carbon.
Techniques for analyzing lead condition in visible, infrared and microwave satellite imagery
Authors: Fett Robert W. , Englebretson Ronald E. , Burk Stephen D.
Published: February 2004

The condition of a lead or polynya, whether open, partially refrozen, or completely ice covered, is extremely important from many points of view. From a meteorological and climatological perspective the moisture and heat flux resulting from air and sea interaction over leads during cold weather periods is crucial. Enormous cloud plumes have been observed emanating from relatively small openings in leads and polynyas. Modeling studies have shown that plume development is extremely sensitive to changes in humidity, air temperature, and the nature of near‐surface atmospheric lapse rates. The studies indicate that plumes can exist, even in the refreezing process, until the new ice becomes sufficiently thick so that its temperature approaches the temperature of the surrounding ice. Heat balance studies for modeling the arctic environment depend upon knowledge of such distinctions for accurate assessment of the radiation balance over the area at any given time. This paper describes a new visible channel differencing technique shown to have some capabilities in distinguishing between open and refrozen leads and, in larger leads, distinguishing the part that is open from that which is partially or newly refrozen. Output from this technique is correlated with infrared data in two examples over the Beaufort Sea and in a polynya example with additional microwave data over the Laptev Sea. Although in situ evidence for direct documentation of lead condition is limited, the existence of cloud plumes emanating from open leads provides some verification and establishment of remote sensing parameters indicative of an open or closed condition. A logical consistency of values from independent sensors over varying conditions of open water, mixed ice and water, new ice, and first or multiyear ice also provides contributing remote sensing evidence of lead condition.
Ozone production and transports in the tropical Atlantic region during the biomass burning season
Authors: Roelofs Geert‐Jan , Lelieveld Jos , Smit Herman G. J. , Kley Dieter
Published: February 2004

Tropospheric O3 distributions over the Atlantic Ocean have been calculated with a coupled chemistry‐general circulation model. Photochemically produced O3, mainly from biomass burning emissions, dominates O3 abundances over the tropical Atlantic Ocean in the southern hemisphere (SH) during September/October. On the other hand, O3 of stratospheric origin strongly contributes to the tropospheric O3 column at latitudes poleward of 30°S. Simulated tropospheric flow patterns are in good agreement with European Centre for Medium‐Range Weather Forecasts analyses and trajectory studies. Tracer transports over the tropical South Atlantic Ocean are strongly influenced by wind shear between the boundary layer and the free troposphere, leading to a stratification of O3. At the surface relatively O3‐poor air is transported from SH middle latitudes. At higher altitudes, relatively O3‐rich air is transported from the African biomass burning regions between 2 and 5 km and from South American biomass burning regions in the mid‐to‐upper troposphere. The model simulates O3 production rates of 10–50 parts per billion by volume (ppbv) O3 d−1 in the lower troposphere over the biomass burning regions and 2–6 ppbv O3 d−1 in the middle and upper troposphere. Photochemical destruction of O3 prevails in the lower troposphere over the ocean, maximizing in the African and South American outflow regions. In the northern hemisphere, in situ photochemically produced O3 dominates throughout most of the troposphere. Calculated O3 volume mixing ratios are compared with a latitude‐altitude O3 distribution measured during an Atlantic ship cruise along 30°W, between 55°N and 30°S, in September/October 1988, and with ozone sonde measurements from Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry Near the Equator—Atlantic (SAFARI/TRACE A) in September/October 1992. Calculated O3 levels agree reasonably well with the ship cruise data, except for the tropical SH lower troposphere where the model significantly underestimates O3. However, modeled vertical O3 distributions are underpredicted compared to TRACE A sonde measurements. Simulated O3 columns over the ocean are somewhat lower compared to values retrieved from satellite data, in particular over the tropical Atlantic. The underestimation is probably due to the neglect of higher hydrocarbon chemistry in the model.
Polar stratospheric clouds observed by lidar over Spitsbergen in the winter of 1994/1995: Liquid particles and vertical “sandwich” structure
Authors: Shibata Takashi , Iwasaka Yasunobu , Fujiwara Motowo , Hayashi Masahiko , Nagatani Masahiro , Shiraishi Ko‐ichi , Adachi Hiroshi , Sakai Tetsu , Susumu Kazumi , Nakura Yoshinobu
Published: February 2004

Polar stratospheric clouds (PSCs) were observed by lidar at Ny‐Ålesund, Spitsbergen, in December 1994 and January 1995. The backscattering coefficient at wavelengths of 1064 and 532 nm and the depolarization ratio at 532 nm of PSCs were measured by the lidar system. The stratospheric temperature was below the estimated frost point of nitric acid tri‐hydrate (NAT) in the winter of 1994/1995. PSCs were observed more frequently in this low‐temperature period than in previous winters since 1991. The characteristics of the PSCs were very variable but had a noticeable vertical “sandwich” structure in January in which a layer of liquid PSC particles at the altitude around 20 km existed between the two solid particle layers. The wavelength dependence of the backscattering shows that the size of both liquid and solid particles was larger than the average size of background stratospheric aerosols. Lidar observations of the liquid layer particles show characteristics in qualitative agreement with those expected from model PSC particles grown in ternary solutions of H2SO4, HNO3, and H2O with a temperature decrease. However, the observed backscattering coefficient and its wavelength dependence indicate that PSC particles require further growth than that predicted by the ternary solution model at temperature where most HNO3 molecules in the surrounding atmosphere are considered to be condensed on PSCs.
Production of stratospheric HNO3 by different ion‐molecule reaction mechanisms
Authors: Aikin A. C.
Published: February 2004

Precipitation of energetic charged particles into the polar winter stratosphere leads to an enhancement of nitric acid. Three different ion‐molecule reaction mechanisms have been suggested as the cause of the HNO3 enhancement: 1) OH production by ion‐molecule reactions and subsequent reaction with NO2 to produce HNO3, 2) positive ion‐negative ion recombination, and 3) positive hydrated ion reaction with N2O5. It is shown that the OH reaction can be disregarded. While H+(H2O)n + N2O5 may be important below 40 km, it accounts for less than half the HNO3 production at 45 km and is negligible in comparison with ion‐ion recombination at higher altitudes. The inclusion of ion‐ion recombination greatly reduces the ion pair production requirements for HNO3 production.
Validation of POAM ozone measurements with coincident MLS, HALOE, and SAGE II observations
Authors: Rusch D. W. , Bevilacqua R. M. , Randall C. E. , Lumpe J. D. , Hoppel K. W. , Fromm M. D. , Debrestian D. J. , Olivero J. J. , Hornstein J. H. , Guo F. , Shettle E. P.
Published: February 2004

Ozone measurements from the Polar Ozone and Aerosol Measurement (POAM) experiment launched in September 1993 are compared with those from the Microwave Limb Sounder (MLS), the Halogen Occultation Experiment (HALOE), and the Stratosphere Aerosol and Gas Experiment II (SAGE II). The comparisons extend over the period from February 1994 to April 1995. POAM is in excellent agreement with the comparison instruments above 22 km. The agreement is generally in the 5–7% range and is substantially smaller than the instrumental errors. Below 22 km, POAM tends to have lower ozone densities than the other instruments, reaching differences of nearly 20% at the lowest altitudes. We discuss the implications of these low‐altitude differences in the context of systematic measurement errors in the comparison instruments. The accuracy of the POAM ozone measurements is demonstrated by comparing zonal means and standard deviations of the three data sets. The large number of comparisons with satellite instruments demonstrate that the POAM ozone data are valid within the instrumental errors and that the precision and accuracy of the data are adequate for scientific investigation and interpretation.
Retrieval of NO2 vertical profiles from ground‐based UV‐visible measurements: Method and validation
Authors: Preston K. E. , Jones R. L. , Roscoe H. K.
Published: February 2004

Vertical profiles of NO2 are retrieved from ground‐based UV‐visible slant columns by sequential estimation, using a forward model that consists of a stacked box photochemical model and a radiative transfer model. The retrieval method is characterized, and a rigorous error analysis is presented. The vertical resolution of the retrieved profiles is shown to vary from 5 to 10 km, depending on the retrieval altitude. The retrieved profiles are found to be moderately sensitive to the assumed vertical profiles of ozone and aerosol, to the range of solar zenith angles of the observations, and to the error in the slant column. However, in the altitude range 10 to 35 km they are relatively insensitive to a priori information and agreed well with profiles from simultaneous balloon‐borne measurements.
The lightning activity of a hailstorm as a function of changes in its microphysical characteristics inferred from polarimetric radar observations
Authors: López Raúl E. , Aubagnac Jean‐Pierre
Published: February 2004

The progression of cloud‐to‐ground (CG) lightning activity in an Oklahoma hailstorm with well‐developed supercell organization is related to changes in the microphysical characteristics of the storm as inferred from polarimetric radar observations. A decision tree based on the observed values of radar reflectivity ZH, differential reflectivity ZDR, and specific differential phase KDP is used to identify regions in the storm containing graupel, hail, supercooled liquid water, and rain. Mixing ratios of graupel/hail and liquid water are obtained in terms of ZH, ZDR, and KDP. One of the objectives of the study is to relate the development of graupel aloft, and the descent of graupel and hail, to fluctuations in CG lightning production. The results indicate that the development of graupel above the freezing level is related to the overall increase and decrease in the production of CG flashes (probably by providing the negative charge necessary for the flashes). However, it is the fluctuating descent of small hail below the height of the negatively charged graupel region that is correlated with the three or four pulses in the CG lightning activity (apparently by producing the electric bias needed for the discharge of the graupel layers aloft).
The wind field around Mauna Loa derived from surface and balloon observations
Authors: Ryan Steven
Published: February 2004

The mountain wind field in the vicinity of Mauna Loa Observatory is derived by comparing vertical profiles of wind, ozone, and water vapor in the free troposphere to measurements made at the observatory. The wind field near the surface is described by two components: a radiation wind caused by the diurnal heating and cooling of the mountain slope, and a barrier wind caused by the free tropospheric wind flowing around the mountain barrier. The radiation wind is the primary factor in transporting air from different source altitudes in the free‐troposphere to the observatory at 3400 m. At midday, air typically arrives from near the top of the marine boundary layer at 2500 m. After midnight, the average source altitude is 3400 m. The barrier wind field consists of a windward stagnation point, strong cross‐slope and downslope flow in the flanks, and moderate downslope flow in the leeward sectors. The barrier wind field is effective at disrupting the surface temperature inversion and the radiation wind at night. A simple model is presented which relates the average properties and statistical variation of these winds to the vertical transport of air from the free troposphere to the observatory by the mountain wind field.
Atmospheric angular momentum fluctuations during 1979–1988 simulated by global circulation models
Authors: Hide R. , Dickey J. O. , Marcus S. L. , Rosen R. D. , Salstein D. A.
Published: February 2004

Changes in major global dynamical phenomena in the Earth's atmosphere are manifested in the time series of atmospheric angular momentum (AAM), as determined directly from meteorological observations and indirectly from geodetic observations of small fluctuations in the rotation of the solid Earth which are proportional to length of day. AAM fluctuations are intimately linked with energetic processes throughout the whole atmosphere and also with the stresses at the Earth's surface produced largely by turbulent momentum transport in the oceanic and continental boundary layers and by the action of normal pressure forces on orographie features. A stringent test of any numerical global circulation model (GCM) is therefore provided by a quantitative assessment of its ability to represent AAM fluctuations on all relevant timescales, ranging from months to several years. From monthly data provided by the Atmospheric Model Intercomparison Project (AMIP) of the World Climate Research Programme, we have investigated seasonal and interannual fluctuations and the decadal mean in the axial component of AAM in 23 AMIP GCMs over the period 1979–1988. The decadal means are generally well simulated, with the model median value (1.58×1026 kg m2 s−1) being only 3.5% larger than the observed mean and with 10 of the models being within 5% of the observed. The seasonal cycle is well reproduced, with the median amplitude of the models' seasonal standard deviations being only 2.4% larger than observed. Half the seasonal amplitudes lie within 15% of the observed, and the median correlation found between the observed and model seasonal cycles is 0.95. The dominant seasonal error is an underestimation of AAM during northern hemisphere winter associated with errors in the position of subtropical jets. Less robust are the modeled interannual variations, although the median correlation of 0.61 between model simulations and observed AAM is statistically significant. The two El Niño‐Southern Oscillation events that occurred during the AMIP decade 1979–1988 have the expected positive AAM anomalies, although the AAM signature of the 1982–1983 event tends to be underestimated and that of the 1986–1987 event overestimated.
The 41Ca bomb pulse and atmospheric transport of radionuclides
Authors: Zerle L. , Faestermann T. , Knie K. , Korschinek G. , Nolte E. , Beer J. , Schotterer U.
Published: February 2004

For the first time, the 41Ca signal from the nuclear weapon tests has been measured. Calcium 41 concentrations have been determined in alpine ice of the Fiescherhorn glacier (Switzerland) with accelerator mass spectrometry. The peak concentrations have been observed to be about 3×106 atoms of 41Ca per kilogram of ice in. the 1950s. It has been found that 41Ca is produced essentially by the atoll bombs. A universal box model, able to describe atmospheric transport of radionuclides that are in gaseous form or attached to aerosols, has been developed. The model has been applied to calculate the bomb pulses of 14C, 36Cl, 41Ca, 90Sr, and 137Cs, For the transport of radionuclides that are attached to aerosols such as 41Ca, 90Sr, and 137Cs, sedimentation (gravitational settling) in the upper stratosphere has been taken into account. It has been found that the deposition of bomb‐produced 36Cl on the Earth's surface is delayed compared to that of 90Sr by about 1 year because 36Cl stays gaseous in the stratosphere. The model can also be used to calculate the deposition of cosmogenic radionuclides, e.g., 36Cl and 10Be, in their natural archives, such as polar ice sheets.
Statistical analysis of gravity waves observed with the middle and upper atmosphere radar in the middle atmosphere: 2. Waves propagated in different directions
Authors: Gavrilov N. M. , Fukao S. , Nakamura T. , Tsuda T.
Published: February 2004

The statistical method developed in the first part of the paper is applied to obtain parameters of internal gravity wave (IGW) harmonics with periods from 5 min to 6 hours propagating upward and downward, and in the direction of the mean wind and opposite to it, from the observations of motions in the middle atmosphere with the middle and upper atmosphere radar. Usually, wave amplitudes and modulus of momentum fluxes are larger for IGWs having upward energy fluxes and propagated opposite to the mean wind (eastward propagation in summer and westward propagation in winter). Most of IGWs have momentum fluxes directed to the northeast in winter and to the east in summer.
Major features of glaciochemistry over the last 110,000 years in the Greenland Ice Sheet Project 2 ice core
Authors: Yang Qinzhao , Mayewski Paul A. , Twickler Mark S. , Whitlow Sallie
Published: February 2004

Major chemical species (Cl−, NO3−, SO42−, Na+, NH4+, K+, Mg2+, Ca2+) and δ18O covering the last 110,000 years from the Greenland Ice Sheet Project 2 (GISP2) ice core were utilized in this study in order to reconstruct the soluble chemistry of the atmosphere over Greenland and interpret major climate events that have affected the region. During the Holocene the major chemical species and δ18O do not display any significant relationship. However, a strong inverse correlation was found between concentrations of the major chemical species and δ18O (a proxy for temperature) during the last glacial period, suggesting that in general during periods of decreased temperature, there is an increase in atmospheric chemical loading. Examination of changes in major chemical composition over the last 110,000 years of the GISP2 ice core reveals that during the Holocene, the atmosphere was acidic; during inters t adials the atmosphere was neutral or alkalescent; and during stadials the atmosphere was alkaline. In addition, the relative abundance of major chemical species varied during the Holocene, stadials, and interstadials. During the Holocene, NH4+ and NO3− are the dominant cations and anions; while Ca2+ and SO42− are the dominant cations and anions during the stadials and inters t adials. This suggests that source regions or types differed between the Holocene and the last glacial period. In addition, changes in chemical composition and changes in chemical ratios also indicate that source regions differed during the Holocene, stadials, and interstadials. Twenty‐four previously identified Dansgaard‐Oeschger (stadial/interstadial) events [Dansgaard et al., 1993] were in the GISP2 chemical series. The duration of the stadials is inversely correlated with variations in sea level over the last glacial period (i.e., the more extensive the northern hemisphere ice sheet, the longer the duration of the stadial). There is also a close correspondence between the duration of interstadials and the timing of Heinrich events (massive icebergs discharged into the ocean) in the GISP2 ice core. Long (up to 2000 years) warm periods follow each Heinrich event, suggesting perhaps that enhanced deep‐water circulation is re‐initiated following Heinrich events.
Observations of Antarctic polar stratospheric clouds by POAM II: 1994–1996
Authors: Fromm Michael D. , Lumpe Jerry D. , Bevilacqua Richard M. , Shettle Eric P. , Hornstein John , Massie Steven T. , Fricke K. H.
Published: February 2004

The Polar Ozone and Aerosol Measurement (POAM) II solar occultation instrument has made extensive measurements of polar stratospheric clouds (PSCs) since launch in September 1993. In a polar orbit similar to that of the Stratospheric Aerosol Measurement (SAM) II experiment but measuring to within 2 latitude of the south pole, POAM II observations of PSCs provide a valuable geographic and temporal extension of the SAM II PSC climatology. The cloud detection algorithm used to identify PSCs from POAM II measurements is described. POAM II PSC data are also examined in comparison with coincident lidar PSC observations. Results from the 1994 to 1996 Antarctic fall/winter/spring seasons are presented and related qualitatively to the SAM II PSC climatology. The frequency of PSC occurrence increases during the Antarctic winter, reaching a maximum of about 71% of all POAM II measurements in August. There is a strong longitudinal variation in the cloud frequency, which is closely related to longitudinal temperature patterns. A broad minimum in PSC frequency is centered near the international dateline and a broad maximum is centered about 315°E, in the lee of the Antarctic Peninsula, where the PSC frequency is about twice that near the minimum. In May, PSCs are observed at an average altitude of 24 km, with the altitudes moving downward as the altitude of the coldest air descends within the polar vortex during the winter. By October the average PSC altitude is 17 km.
The relationship between the transient, meridional eddy sensible and latent heat flux
Authors: Raphael Marilyn N.
Published: February 2004

The relationship between the transient, meridional eddy sensible heat and latent heat fluxes during January and July at 850 mbar is examined using band‐pass‐filtered, European Center for Medium‐Range Weather Forecasts data for the period 1979–1989. The fluxes are shown to be strongly positively correlated both temporally and spatially. They are most strongly correlated in the the midlatitude storm tracks, and this relationship varies from January to July and from the northern to the southern hemisphere. The climatic variables that affect this relationship are the local temperature gradient, moisture availability and land/sea distribution. The temperature gradient is the most important influence, such that the relationship is strongest when the temperature gradient in the storm track is strong and advection by the meridional wind is strong. When the gradient weakens, moisture availability and the distribution of land and sea exert a greater influence upon the fluxes, and the relationship between them is weaker.
A model investigation of turbulence‐driven pressure‐pumping effects on the rate of diffusion of CO2, N2O, and CH4 through layered snowpacks
Authors: Massman W. J. , Sommerfeld R. A. , Mosier A. R. , Zeller K. F. , Hehn T. J. , Rochelle S. G.
Published: February 2004

Pressure pumping at the Earth's surface is caused by short‐period atmospheric turbulence, longer‐period barometric changes, and quasi‐static pressure fields induced by wind blowing across irregular topography. These naturally occurring atmospheric pressure variations induce periodic fluctuations in airflow through snowpacks, soils, and any other porous media at the Earth's surface. Consequently, the uptake or release of trace gases from soils and snowpacks is a combination of molecular diffusion and advection forced by pressure pumping. Using model‐estimated fluxes, this study attempts to quantify the influence that turbulent pressure fluctuations with periods between 0.1 and 1000 s can have on the rate of exchange of CO2, N2O, and CH4 through a seasonal snowpack. Data for this study were collected at a forested subalpine meadow site in the Rocky Mountains of southern Wyoming, during February 1995 when the snowpack was distinctly layered and approximately 1.4 m deep. The data include mole fraction of CO2, N2O, and CH4 just above and at the base of the snowpack, several profiles of CO2, N2O, and CH4 mole fraction in the top l m of the snowpack, and a profile of snowpack density and tortuosity. Turbulent atmospheric pressure‐pumping fluctuations, sampled at approximately 11 Hz for several hours, were obtained with a fast response differential pressure sensor. A one‐dimensional steady state diffusion model and one‐ and three‐dimensional time‐dependent pressure‐pumping models are used to estimate the gas fluxes through the snowpack. Boundary conditions are provided by grab samples just above the snowpack and at the soil/snow interface. The pressure‐pumping models are driven by the observed pressure fluctuations, and all models include the observed layering of the snowpack. As with previous studies the present results indicate that the effects of pressure pumping are diminished with increasingly strong gradients. Furthermore, we conclude that unless pressure pumping influences the gas concentrations at the boundaries of the snowpack, it appears unlikely that it can significantly impact the rate of gaseous diffusion through the snowpack. Even two‐ and three‐dimensional effects, which can have a significant short‐term impact on the fluxes and concentration profiles, are nearly eliminated when averaged over a period of hours. It is also suggested that vertical layering is important for three‐dimensional pressure‐pumping studies and that the time‐dependent temperature term, which is traditionally ignored when modeling dynamic pressure variations, may in fact be dominant in some situations and probably should be incorporated in future modeling studies of pressure pumping.
The influence of gas transport and porosity on methane oxidation in soils
Authors: Ball B. C. , Dobbie K. E. , Parker J. P. , Smith K. A.
Published: February 2004

Two porosity‐dependent parameters that affect gas transport in soils, gas diffusivity and air permeability, were assessed as possible indicators of methane oxidation rates. Soil gas diffusivity was measured in intact cores in the laboratory and in situ, and air permeability was measured in intact cores. An in situ method of measuring gas diffusivity was specifically modified for this purpose to use Freon in a portable probe. A laboratory (core) method of measuring gas diffusivity, using krypton 85, was also employed. Measurements were made at the soil surface and at a range of depths within the topsoil, in conjunction with in situ measurements of CH4 oxidation, in forest, arable, and set‐aside soils at a lowland site and in a forest soil at an upland site, in southeast Scotland. The surface layer of soil caused marked variations in diffusivity measurements, particularly with the in situ method. At the upland site, where a 50‐mm‐thick surface organic layer was present, the in situ technique revealed a sharp decrease in diffusivity with depth. Only where gas transport was low, in the set‐aside soil, was methane oxidation rate influenced by gas transport changes associated with increasing soil water content. Differences in CH4 oxidation rates related better to core gas diffusivities than to in situ diffusivities. The relationship was best at 50–150 mm depth where diffusivities were lower than at the surface. Air permeability, which is affected more by soil structure than diffusivity, appeared to be as relevant as the latter parameter to CH4 oxidation rate, particularly for land use changes associated with agriculture. Thus CH4 oxidation rate appears to be influenced by gas transport properties and soil structure, either at the surface in the litter layer or below the surface where the oxidizing microorganisms are likely to occur.