bluuebunny/crossref_metadata_2025 huggingface dataset:
Frontogenesis, subduction, and cross‐front exchange at upper ocean fronts
Authors: Spall Michael A.
Published: February 2004

The subduction of water at upper ocean fronts driven by internal instabilities is investigated. A simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation, while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation. A nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front. Frontogenesis generates vertical velocities of 𝒪(30 m d−1) and ageostrophic cross front flows of 3 to 5 cm s−1. The subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front. The subducted parcels quickly develop a circulation through balanced adjustment, anticyclonic for deep surface layers and cyclonic for shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanent subduction rate is estimated to be 20 m yr−1 for frontal subduction driven by frontogenesis, although local (and temporary) subduction can be much larger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forming a baroclinic dipole that propagates at approximately 3.5 cm s−1 and is capable of transporting the anomalous water properties far from the region of formation. Similarities with observations in the North Atlantic Subtropical Convergence Zone and near the Gulf Stream are discussed.
Comparison of tropical Pacific temperature and current simulations with two vertical mixing schemes embedded in an ocean general circulation model and reference to observations
Authors: Halpern David , Chao Yi , Ma Chung‐Chun , Mechoso Carlos R.
Published: February 2004

The Pacanowski‐Philander (PP) and Mellor‐Yamada (MY) parameterization models of vertical mixing by turbulent processes were embedded in the Geophysical Fluid Dynamics Laboratory high‐resolution ocean general circulation model of the tropical Pacific Ocean. All other facets of the numerical simulations were the same. Simulations were made for the 1987–1988 period. At the equator the MY simulation produced near‐surface temperatures more uniform with depth, a deeper thermocline, a deeper core speed of the Equatorial Undercurrent, and a South Equatorial Current with greater vertical thickness compared with that computed with the PP method. Along 140°W, between 5°N and 10°N, both simulations were the same. Moored‐buoy current and temperature observations had been recorded by the Pacific Marine Environmental Laboratory at three sites (165°E, 140°W, 110°W) along the equator and at three sites (5°N, 7°N, 9°N) along 140°W. Simulated temperatures were lower than those observed in the near‐surface layer and higher than those observed in the thermocline. Temperature simulations were in better agreement with observations compared to current simulations. At the equator, PP current and temperature simulations were more representative of the observations than MY simulations.
On meltwater under ice shelves
Authors: Lane‐Serff G. F.
Published: February 2004

The basic features of the flow of meltwater under ice shelves can be described by a set of simple relations and length scales. The flow may be divided into two regions, with different basic processes dominating in each. In the first region, melting of the underside of the ice shelf is important and the temperature and salinity of the current tend toward “equilibrium” values, such that the changes due to melting of the ice shelf are balanced by changes due to entrainment of ambient seawater. The equilibrium values change with depth owing to the effect of the change in pressure on the freezing point. As the current increases in thickness, it is no longer able to adjust sufficiently rapidly to the changing equilibrium values, arid the flow moves into the second region. The extent of the first region is governed by the location of the “ambient freezing point.” In the second region, melting is less important and the current behaves as an entraining drag‐limited gravity current in a stratified ambient fluid, leaving the shelf once the current has the same density as the ambient seawater. The heights of the two regions depend mainly on the ambient conditions and only indirectly on parameters such as slope angle, entrainment constant, drag coefficient, and turbulent transfer coefficients.
Observation of a deep convection regime with acoustic tomography
Authors: Send Uwe , Schott Friedrich , Gaillard Fabienne , Desaubies Yves
Published: February 2004

In the winter of 1991–1992 a convection experiment was conducted in the western Mediterranean Gulf of Lions, combining a variety of observational techniques. An essential component was an acoustic tomography array, consisting of six moorings, designed to observe the time evolution of the large‐scale processes believed to be relevant in a convection regime. Here two‐dimensional slice inversions in three directions from the central mooring are used to estimate the volume of convected water and the mean convection depth and to observe the preconditioning and restratification processes before and after the convection. The near‐surface layer is well sampled by the acoustics, which show cooling and subsequent entrainment of the warmer Levantine Intermediate Water (LIW) from below, in agreement with mixedlayer calculations. During the 2 months prior to the main convection event the total heat loss of the large‐scale field is in approximate agreement with the surface heat fluxes, showing that little net warm‐water advection takes place from outside the convection region. The implied confinement of water by the local circulation should be an important factor in setting the location and extent of the deep convection patch. The volume of water modified by convection in this winter is estimated to correspond to an area of order 60 km radius and 1500 m depth. It is argued that the homogenized area implies an annual mean deep water replenishment of 0.3 Sv. The restratification, by the return of less dense water in the surface and LIW layers, occurs first by rapid capping in the near‐surface region. Thereafter, the southern and eastern parts of the region restratify in the deeper layers on a 40‐day timescale, while a dense core of 50–60 km diameter remains in the northwest until the end of the experiment.
Diffusion model for lightning radiative transfer
Authors: Koshak William J. , Solakiewicz Richard J. , Phanord Dieudonne D. , Blakeslee Richard J.
Published: February 2004

A one‐speed Boltzmann transport theory, with diffusion approximations, is applied to study the radiative transfer properties of lightning in optically thick thunderclouds. Near‐infrared (λ = 0.7774 μm) photons associated with a prominent oxygen emission triplet in the lighting spectrum are considered. Transient and spatially complex lightning radiation sources are placed inside a rectangular parallelepiped thundercloud geometry and the effects of multiple scattering are studied. The cloud is assumed to be composed of a homogeneous collection of identical spherical water droplets, each droplet a nearly conservative, anisotropic scatterer. Conceptually, we treat the thundercloud like a nuclear reactor, with photons replaced by neutrons, and utilize standard one‐speed neutron diffusion techniques common in nuclear reactor analyses. Valid analytic results for the intensity distribution (expanded in spherical harmonics) are obtained for regions sufficiently far from sources. Model estimates of the arrival‐time delay and pulse width broadening of lightning signals radiated from within the cloud are determined and the results are in good agreement with both experimental data and previous Monte Carlo estimates. Additional model studies of this kind will be used to study the general information content of cloud top lightning radiation signatures.
Observations of lightning phenomena using radio interferometry
Authors: Rhodes C. T. , Shao X. M. , Krehbiel P. R. , Thomas R. J. , Hayenga C. O.
Published: February 2004

A radio interferometer system is described which utilizes multiple baselines to determine the direction of lightning radiation sources with an angular resolution of a few degrees and with microsecond time resolution. An interactive graphics analysis procedure is used to remove fringe ambiguities from the data and to reveal the structure and development of lightning discharges inside the storm. Radiation source directions and electric field waveforms have been analyzed for different types of breakdown events for two lightning flashes. These include the initial breakdown and K type events of in‐cloud activity, the leaders of initial and subsequent strokes to ground, and activity during and following return strokes. Radiation during the initial breakdown of one flash was found to consist of intermittent, localized bursts of radiation that were slow moving. Source motion within a given burst was unresolved by the interferometer but was detected from burst to burst, with negative charge being transported in the direction of the breakdown progression. Radiation during initial leaders to ground was similar but more intense and continuous and had a characteristic intensity waveform. Radiation from in‐cloud K type events is essentially the same as for dart leaders; in both cases it is produced at the leading edge of a fast‐moving negative streamer that propagates along a well‐defined, often extensive, path. K type events are sometimes terminated by a fast field change that appears analogous to the field change of a return stroke. Dart leaders are sometimes observed to die out before reaching ground; these are termed “attempted leaders” and, except for their greater extent, are no different than K type events. Several modes of breakdown during and after return strokes have been documented and analyzed. One mode corresponds to the launching of a positive streamer away from the upper end of the leader channel, apparently as the return stroke reaches the leader start point. In another mode, the quenching of the dart leader radiation upon reaching ground reveals concurrent breakdown in the vicinity of the source region for the leader. In both instances the breakdown appears to establish channel extensions or branches that are followed by later activity of the flash. Finally, a new type of breakdown event has been identified whose electric field change and source development resemble those of an initial negative leader but which progresses horizontally through the storm. An example is shown which spawned a dart leader to ground.
Ground‐based measurements of column amounts of NO2 over Syowa Station, Antarctica
Authors: Kondo Y. , Matthews W. A. , Solomon S. , Koike M. , Hayashi M. , Yamazaki K. , Nakajima H. , Tsukui K.
Published: February 2004

The column amounts of NO2 have been measured using visible spectroscopy at Syowa Station, Antarctica (69°S), from March 1990. The NO2 slant column amount at a solar zenith angle of 90° exhibits a large seasonal variation reaching the minimum value of 1×1016 cm−2 or less in midwinter, and it increases to the maximum value of 17×1016 cm−2 in midsummer. The recovery of NO2 in spring is 2–3 times slower than the fall decay. The observed temperature indicates that polar stratospheric clouds (PSCs) are expected to form from midwinter to early spring. A decrease in ozone was observed from early August and continued to the end of September, which is consistent with the observed depletion in NO2 during the same period. A chemical box model has been used to interpret quantitatively these observed results. The observed NO2 values in fall are in agreement with the box model including only gas phase chemistry or with heterogeneous chemistry on background sulfuric acid aerosols. In addition, the very low NO2 amounts and slow rate of increase observed from midwinter to early spring agree well with the model results assuming heterogeneous chemistry on PSCs. From the late spring of 1991 the NO2 amounts are lower by more than 30%, presumably due to the increased rate of conversion of NOx into HNO3 via N2O5 on the enhanced amount of sulfuric acid aerosols resulting from the Pinatubo eruption. The reduction of NO2 levels in midsummer of 1991 and 1992 below those of midsummer in 1990 is probably due to the transport of air from lower latitudes, where NO2 is already reduced by the volcanic aerosols. NO2 and O3 values in October 1992 were much reduced as compared with those for 1990 and 1991, reflecting the difference in the location of the vortex boundary relative to Syowa Station.
An electrodynamic description of lightning return strokes and dart leaders: Guided wave propagation along conducting cylindrical channels
Authors: Borovsky Joseph E.
Published: February 2004

The return‐stroke breakdown pulse and the dart leader are treated as electric waves guided by conducting lightning channels; such waves are launched when current is injected into a conducting channel (producing the dart leader) or when charge on a channel begins to drain to Earth (producing the return stroke). The guided waves are self‐consistent solutions to the full set of Maxwell's equations, obeying physical boundary conditions for cylindrical channels. These waves are shown (1) to move with velocities substantially slower than c along the channel, (2) to push current inside the lightning channel, (3) to move charge and voltage along the channel, and (4) to transport energy along and into the channel via Poynting flux. The velocity of a guided wave is a function of only three parameters: the channel radius rch, the channel temperature T, and the risetime Δt of the wave front. These velocities are found to fall in the range of velocities of return strokes and of dart leaders. The wave amplitude is fixed by fixing one parameter in addition to rch, T, and Δt; thus four parameters fully determine the velocity of the wave, the voltage of the wave, the current of the wave, the electric field of the wave, the linear charge density of the wave, the power carried by the wave, and the power delivered by the wave into the channel. For an example return stroke those predicted quantities are in agreement with experimental observations and for an example dart leader those predicted quantities are consistent with the limited measurements. The dart leader and the return stroke are caused by the same type of guided electromagnetic waves: the difference in velocity is owed mostly to the difference in channel temperature. In the case of the dart leader the waves deliver Poynting flux along the outside of the channel down from a thundercloud generator to the downward‐propagating wave front. At the wave front of the dart leader the delivered energy goes into heating the channel and into storage in the form of E2/8π around the newly charged channel. In the case of the return stroke the Poynting flux is localized to the vicinity of the wave front where stored energy E2/8π is delivered radially inward onto the channel to heat the channel in the propagating front. The net result of a dart leader and return stroke is that charge is moved from the cloud to the ground and that energy is moved from the cloud onto the channel.
Physical and optical properties of the Pinatubo volcanic aerosol: Aircraft observations with impactors and a Sun‐tracking photometer
Authors: Pueschel R. F. , Russell P. B. , Allen D. A. , Ferry G. V. , Snetsinger K. G. , Livingston J. M. , Verma S.
Published: February 2004

As determined in situ by impactor samplers flown on an ER‐2 at 16.5‐ to 20.7‐km pressure altitude and on a DC‐8 at 9.5‐ to 12.6‐km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50‐fold and the particle volume up to 2 orders of magnitude. Particle composition was typical of a sulfuric acid‐water mixture at ER‐2 altitudes. Ash particles coated with sulfuric acid comprised a significant fraction of aerosol at DC‐8 altitudes. Mie‐computed light extinction increased up to 20‐fold at midvisible and >100‐fold at near‐IR wavelengths. The optical thickness measured through the aerosol layer by an autotracking Sun photometer aboard a DC‐8 aircraft at 10.7‐ to 11.3‐km pressure altitudes shows a spectral shape that is similar to the Mie‐calculated spectral extinction at ER‐2 altitudes. Surface area distributions calculated by inversion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.
A model for studying the composition and chemical effects of stratospheric aerosols
Authors: Tabazadeh Azadeh , Turco Richard P. , Jacobson Mark Z.
Published: February 2004

We developed polynomial expressions for the temperature dependence of the mean binary and water activity coefficients for H2SO4 and HNO3 solutions. These activities were used in an equilibrium model to predict the composition of stratospheric aerosols under a wide range of environmental conditions. For typical concentrations of H2O, H2SO4, HNO3, HCl, HBr, HF, and HOCl in the lower stratosphere, the aerosol composition is estimated as a function of the local temperature and the ambient relative humidity. For temperatures below 200 K, our results indicate that (1) HNO3 contributes a significant mass fraction to stratospheric aerosols, and (2) HCl solubility is considerably affected by HNO3 dissolution into sulfate aerosols. We also show that, in volcanically disturbed periods, changes in stratospheric aerosol composition can significantly alter the microphysics that leads to the formation of polar stratospheric clouds. The effects caused by HNO3 dissolution on the physical and chemical properties of stratospheric aerosols are discussed.
Clustering, randomness, and regularity in cloud fields: 4. Stratocumulus cloud fields
Authors: Lee J. , Chou J. , Weger R. C. , Welch R. M.
Published: February 2004

To complete the analysis of the spatial distribution of boundary layer cloudiness, the present study focuses on nine stratocumulus Landsat scenes. The results indicate many similarities between stratocumulus and cumulus spatial distributions. Most notably, at full spatial resolution all scenes exhibit a decidedly clustered distribution. The strength of the clustering signal decreases with increasing cloud size; the clusters themselves consist of a few clouds (less than 10), occupy a small percentage of the cloud field area (less than 5%), contain between 20% and 60% of the cloud field population, and are randomly located within the scene. In contrast, stratocumulus in almost every respect are more strongly clustered than are cumulus cloud fields. For instance, stratocumulus clusters contain more clouds per cluster, occupy a larger percentage of the total area, and have a larger percentage of clouds participating in clusters than the corresponding cumulus examples. To investigate clustering at intermediate spatial scales, the local dimensionality statistic is introduced. Results obtained from this statistic provide the first direct evidence for regularity among large (>900 m in diameter) clouds in stratocumulus and cumulus cloud fields, in support of the inhibition hypothesis of Ramirez and Bras (1990). Also, the size compensated point‐to‐cloud cumulative distribution function statistic is found to be necessary to obtain a consistent description of stratocumulus cloud distributions. A hypothesis regarding the underlying physical mechanisms responsible for cloud clustering is presented. It is suggested that cloud clusters often arise from 4 to 10 triggering events localized within regions less than 2 km in diameter and randomly distributed within the cloud field. As the size of the cloud surpasses the scale of the triggering region, the clustering signal weakens and the larger cloud locations become more random.
On the radiative coupling between mesospheric and thermospheric nitric oxide
Authors: Siskind David E.
Published: February 2004

A photochemical model of nitric oxide from 40 to 250 km has been used to study the distribution of NO in the mesosphere and lower thermosphere. The model includes a parameterization of NO photolysis that can account for the opacity of a varying thermospheric NO layer. This model is used to test the suggestion that increases in thermospheric NO which increase this opacity ”shield” NO at lower altitudes by decreasing the photolytic destruction rate. The model was ”tuned” to reproduce two recent observations of thermospheric NO, the high latitude rocket experiment of Eparvier and Barth [1992] and the midlatitude ground‐based microwave experiment of Clancy et al. [1992]. In the first case, the model indicated that the large thermospheric NO layer could have caused significant shielding and up to a factor of 2 effect on the mesospheric NO is seen. On the other hand, for the midlatitude observation the NO optical pathlength was too small to be significant. A time dependent version of the model was used to study the response of mesospheric NO to auroral energy input during winter. Under conditions which combine rapid transport and NO self‐shielding, the model suggests that NO density increases could propagate into the upper stratosphere.
Carboxylic acids in clouds at a high‐elevation forested site in central Virginia
Authors: Keene William C. , Mosher Byard W. , Jacob Daniel J. , Munger J. William , Talbot Robert W. , Artz Richard S. , Maben John R. , Daube Bruce C. , Galloway James N.
Published: February 2004

During September 1990 we sampled coarse (>18‐μm diameter) and fine (18‐ to 5.5‐μm diameter) droplets and liquid‐water content (LWC) in cloud from a tower on a forested ridge top in Shenandoah National Park, Virginia. Cloud‐water pH and aqueous‐ and vapor‐phase concentrations of carboxylic acids (HCOOH and CH3COOH) and formaldehyde (HCHO) were measured in parallel over 1‐ to 1.5‐hour intervals. Both size fractions of cloud droplets contained similar concentrations of carboxylic species and H+ during most sampling; most cloud water was in coarse droplets. The pH of coarse (3.27–4.76) and fine (3.22–4.70) droplets coupled with total LWC of 0.04–0.56 g m−3 STP (standard m3 at 0°C and 1 atm) resulted in the partitioning of carboxylic acids primarily in the vapor phase. The observed phase partitioning for CH3COOH was within the uncertainty range of thermodynamic data. However, HCOOH exhibited significant phase disequilibria, which could not be explained by artifacts from variable LWC or from mixing droplets of different acidities. We hypothesize that the large volume of liquid water deposited on the forest canopy interacted with the near‐surface cloud leading to apparent disequilibria based on time‐integrated samples. HCOOH was selectively depleted relative to CH3COOH in cloud, particularly at higher pH, suggesting rapid removal of HCOOH by cloud‐water deposition. We saw no evidence for significant production of HCOOH from the aqueous‐phase oxidation of HCHO.
Observations of water vapor by ground‐based microwave radiometers and Raman lidar
Authors: Han Yong , Snider J. B. , Westwater E. R. , Melfi S. H. , Ferrare R. A.
Published: February 2004

In November to December 1991, a substantial number of remote sensors and in situ instruments were operated together in Coffeyville, Kansas, during the climate experiment FIRE II. Included in the suite of instruments were (1) the NOAA Environmental Technology Laboratory (ETL) three‐channel microwave radiometer, (2) the NASA GSFC Raman lidar, (3) ETL radio acoustic sounding system (RASS), and (4) frequent, research‐quality radiosondes. The Raman lidar operated only at night and the focus of this portion of the experiment concentrated on clear conditions. The lidar data, together with frequent radiosondes and measurements of temperature profiles (every 15 min) by RASS allowed profiles of temperature and absolute humidity to be estimated every minute. We compared 2‐min measurements of brightness temperature (Tb) with calculations of Tb that were based on the Liebe and Lay ton (1987) and Liebe et al. (1993) microwave propagation models, as well as the Waters (1976) model. The comparisons showed the best agreement at 20.6 GHz with the Waters model, with the Liebe et al. (1993) model being best at 31.65 GHz. The results at 90 GHz gave about equal success with the Liebe and Layton (1987) and Liebe et al. (1993) models. Comparisons of precipitable water vapor derived independently from the two instruments also showed excellent agreement, even for averages as short as 2 min. The rms difference between Raman and radiometric determinations of precipitable water vapor was 0.03 cm which is roughly 2%. The experiments clearly demonstrate the potential of simultaneous operation of radiometers and Raman lidars for fundamental physical studies of water vapor.
Interpolation of missing data using nonlinear and chaotic system analysis
Authors: Amritkar R. E. , Kumar P. Pradeep
Published: February 2004

A new method for interpolating missing data in a time series is introduced. The method uses local flow approximation for interpolation and is based on a suitable modification of some recent developments in predicting nonlinear and chaotic time series. The method has been tested for various geophysical and climatic time series. We find that in many cases the method gives better results than the cubic spline. The real power of this method is seen when there are several consecutive missing values in the series.
Temperature dependent ultraviolet‐visible absorption cross sections of N2O and N2O4: Low‐temperature measurements of the equilibrium constant for 2NO2 ⇌ N2O4
Authors: Harwood M. H. , Jones R. L.
Published: February 2004

A study of the temperature dependence of the ultraviolet‐visible absorption cross sections for NO2 has been made in the temperature range 213–298 K and between 310 and 570 nm using a diode array spectrometer. Analysis of the experimental data allowed the simultaneous measurement of the NO2 and N2O4 cross sections and the equilibrium constant for the dimerization of NO2. The absorption cross sections were measured at a spectral resolution of 0.54 nm full width at half maximum (FWHM) and are presented for use in the analysis of differential absorption spectra of atmospheric NO2. Averaged cross section data for NO2 are presented for use in photolysis rate calculations. Values of cross sections for N2O4 and equilibrium constant for the association of NO2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO2 made by ultraviolet‐visible absorption techniques.
Seasonal and interannual temperature variations in the tropical stratosphere
Authors: Reid George C.
Published: February 2004

Temperature variations in the tropical lower and middle stratosphere are influenced by at least five distinct driving forces. These are (1) the mechanism of the regular seasonal cycle, (2) the quasi‐biennial oscillation (QBO) in zonal winds, (3) the semiannual zonal wind oscillation (SAO) at higher levels, (4) El Niño‐Southern Oscillation (ENSO) effects driven by the underlying troposphere, and (5) radiative effects, including volcanic aerosol heating. Radiosonde measurements of temperatures from a number of tropical stations, mostly in the western Pacific region, are used in this paper to examine the characteristic annual and interannual temperature variability in the stratosphere below the 10‐hPa pressure level (∼31 km) over a time period of 17 years, chosen to eliminate or at least minimize the effect of volcanic eruptions. Both annual and interannual variations are found to show a fairly distinct transition between the lower and the middle stratosphere at about the 35‐hPa level (∼23 km). The lower stratosphere, below this transition level, is strongly influenced by the ENSO cycle as well as by the QBO. The overall result of the interaction is to modulate the amplitude of the normal stratospheric seasonal cycle and to impose a biennial component on it, so that alternate seasonal cycles are stronger or weaker than normal. Additional modulation by the ENSO cycle occurs at its quasi‐period of 3–5 years, giving rise to a complex net behavior. In the middle stratosphere above the transition level, there is no discernible ENSO influence, and departures from the regular semiannual seasonal cycle are dominated by the QBO. Recent ideas on the underlying physical mechanisms governing these variations are discussed, as is the relationship of the radiosonde measurements to recent satellite remote‐sensing observations.
Ridge formation in the lower stratosphere and its influence on ozone transport: A general circulation model study during late January 1992
Authors: Orsolini Y. , Cariolle D. , Déqué M.
Published: February 2004

Satellite total ozone measurements showed the development of an ozone mini‐hole over northern Europe in late January 1992, during the observational phase of the European Arctic Stratospheric Ozone Experiment (EASOE). During the same period, ozone profiles, recorded with ozonesondes, showed that layers below 20 km were strongly depleted in ozone at some locations over northern and central Europe. The perturbed chemistry involving the reactive species, which play a role in the ozone‐destroying catalytic cycles, was the focus of intensive observational investigations during the EASOE campaign. These processes are likely to take place between 15 and 30 km, and the question of mixing between vortex air and midlatitude air in the lower stratosphere, especially at the base of the polar vortex, is a central one. High‐resolution (T106) 7‐day forecast simulations have been performed with a GCM, in which the ozone field was realistically initialized, in order to study the formation and evolution of the ozone mini‐hole, and the nature of large‐scale mixing in the lower stratosphere. In particular, we tried to examine whether the model could reproduce with some degree of realism ozone tongues seen in the satellite data. The study has revealed that the formation of the mini‐hole was linked to the poleward extension in the lower stratosphere of an anticyclonic ridge. The effect of tropospheric forcing was evident up to at least 40 hPa. Ozone‐poor air from subtropical latitudes was advected toward Scandinavia at the same time as a tongue of polar air extended northeasterly toward central Europe. During the course of the 1‐week simulation, the modeled ozone mixing ratio and potential vorticity (PV) revealed strong large‐scale isentropic mixing between high, middle, and low latitudes in the lower stratosphere. This mixing may occur through either the formation of narrow ozone/PV tongues, or of more vortex‐like blobs of ozone/PV also seen to be peeled off from the vortex. There is good correspondence with the structures seen in the satellite‐derived total ozone field at medium and synoptic scales.
Total ozone changes over Eurasia since 1973 based on reevaluated filter ozonometer data
Authors: Bojkov Rumen D. , Fioletov Vitali E. , Shalamjansky Arcady M.
Published: February 2004

Since the early 1960s, on the vast territory of the former USSR, 45 stations have been in continuous operation, utilizing the broadband filter M‐83 ozonometer. The quality of the ozone data during the first decade was unsatisfactory. After 1972 an improved version of the ozonometer was introduced together with improved quality control practices, including methodology of observations. The more reliable data of 1973 through March 1994 have been rigorously reexamined by applying variability analysis, comparison with lower‐stratosphere temperatures and/or nearby Dobson stations, and overpassing TOMS for identifying concurrence or discrepancies. These control procedures together with the information on instrument relocation and calibrations made it possible to reevaluate the record of all 45 stations. The accuracy of the improved ozonometer data is about 3% for direct Sun measurements and ∼5% for zenith sky observations; although not so good as that of the Dobson, in the long run it provides consistent ozone data sets. This data set is now made available to the World Ozone Data Center (WO3DC), Toronto. Thus for the first time, based on a 21‐year long record, information is deduced on the differences in the ozone annual cycle between Eastern Siberia and the European part, on the strong appearance of quasi‐biennial oscillation (QBO) signals especially pronounced as ozone deficiency during the western phase of the QBO, on the ozone variability, and on the long‐term changes over the huge territory from Central Europe to the Far East. The specifics of the ozone changes considered in concurrence with the prevailing general stratospheric circulation conditions permitted us to distinguish four broad regions with consistent ozone regimes. The appearance of the strongest northern hemisphere ozone maximum and a monthly mean of ∼470 matm cm over Siberia during winter‐spring, compared with ∼400 matm cm over Europe, the occurrence of the ozone annual minimums as early as August over Eastern Siberia, compared with October over the European part, was established and is probably related to the specifics of the atmospheric circulation patterns. The long‐term ozone changes are considered in relation to the stratospheric temperature at 100‐hPa. For each deviation from the monthly normal temperature by 1°C, there is a corresponding change by the same sign 5–6 matm cm in the monthly ozone deviations. The calculated long‐term ozone trends for 1973 through March 1994 (given in percent per decade ±2σ) are as follows: European part, −3.6±0.8; Central Asia, −2.0±0.6; Western Siberia, −3.5±0.8; and Eastern Siberia ‐ Far East, −3.2±0.8. They show a steady ozone decline similar to that deduced from the Dobson stations at the same latitudes. The decline during the past 15 years is stronger and, in general, concurs with Total Ozone Mapping Spectrometer (TOMS) trends which are, however, slightly more negative.
Measured UV spectra compared with discrete ordinate method simulations
Authors: Zeng Jun , McKenzie Richard , Stamnes Knut , Wineland Mary , Rosen James
Published: February 2004

A comprehensive radiative transfer model is used to calculate surface spectral ultraviolet irradiance under cloud‐free conditions. The results are compared with measurements made at Lauder, New Zealand (45°S, 170°E) before and after the eruption of Mount Pinatubo and including a snow‐covered surface. There is reasonable agreement between measured and calculated diffuse, direct, and global irradiances over the range 300 to 450 nm. Discrepancies may be due to calibration errors in the UV measurements, or in the extra terrestrial irradiances input to the model. Ratios of diffuse/direct irradiances are independent of such calibration uncertainties and therefore provide a sensitive test of the model. If appropriate ozonesonde data, surface albedo, and aerosol optical properties are used, the model ratios are in satisfactory agreement with measurements over a wide range of observing conditions. For cases in which the atmospheric optical properties are best known the agreement is better than 8% in the UV‐B range, and for wavelengths 320 to 450 nm the deviation is smaller. The comparison suggests that the ultraviolet radiation exposure can be computed with confidence for clear sky conditions if the appropriate atmospheric molecular density profiles, ozonesonde data, surface albedo, and aerosol optical properties are available. The UV radiation model is used to investigate the impact of changes in solar zenith angle, ozone abundance, surface albedo, and aerosol loading on UV radiation reaching the surface of the Earth. The ratios of diffuse to direct irradiance depend critically on solar zenith angle, surface albedo, and aerosol extinction. Ozone changes have pronounced effects on the global UVB irradiance but have only a minor effect on these ratios.
Observations of lightning in the stratosphere
Authors: Boeck William L. , Vaughan Otha H. , Blakeslee Richard J. , Vonnegut Bernard , Brook Marx , McKune John
Published: February 2004

An examination and analysis of video images of lightning, captured by the payload bay TV cameras of the space shuttle, provided a variety of examples of lightning in the stratosphere above thunderstorms. These images were obtained on several recent shuttle flights while conducting the Mesoscale Lightning Experiment (MLE). The images of stratospheric lightning illustrate the variety of filamentary and broad vertical discharges in the stratosphere that may accompany a lightning flash. A typical event is imaged as a single or multiple filament extending 30 to 40 km above a thunderstorm that is illuminated by a series of lightning strokes. Examples are found in temperate and tropical areas, over the oceans, and over the land.
Smithsonian stratospheric far‐infrared spectrometer and data reduction system
Authors: Johnson D. G. , Jucks K. W. , Traub W. A. , Chance K. V.
Published: February 2004

The Smithsonian far‐infrared spectrometer is a remote sensing Fourier transform spectrometer that measures the mid‐ and far‐infrared thermal emission spectrum of the stratosphere from balloon and aircraft platforms. The spectrometer has had nine successful balloon flights from 1987 to 1994, flying at float altitudes of 36–39 km and collecting 131 hours of midlatitude stratospheric limb spectra. The spectrometer also flew on a NASA DC‐8 aircraft, as part of the second Airborne Arctic Stratospheric Expedition (AASE‐II), collecting 140 hours of overhead spectra at latitudes ranging from the equator to the north pole. We present here a brief description of the instrument, a discussion of data reduction procedures, an estimation of both random and systematic errors, an outline of the procedure for retrieving mixing ratio profiles, and an explanation of the method of deriving temperature and pressure from the far‐ and mid‐infrared spectra.
Snow cover and sea ice sensitivity to generic changes in Earth orbital parameters
Authors: Gallimore R. G. , Kutzbach J. E.
Published: February 2004

A low‐resolution general circulation model is used to examine the separate and combined effects on high‐latitude climate of large changes in Earth's obliquity (3° tilt angle increase) and precession (summer versus winter perihelion in conjunction with large eccentricity (0.04)). The orbital changes, encompassing the extremes for the past 150,000 years, produced significantly increased (decreased) northern summer (winter) radiation. Increased tilt caused a much greater reduction in sea ice than the precessional change. The decreases in annual mean sea ice thickness and fall sea ice coverage are respectively 50% and 41% (21% and 0%) for the tilt increase (precessional change). The larger obliquity effect is a consequence of enhanced annual mean radiation with tilt increase; the modest response of sea ice in the precessional experiments is attributed to a seasonal interaction between processes governing model sea ice growth and melt and heat transfer through the ice. The reduced winter insolation in the experiments with increased tilt and summer versus winter perihelion produced colder conditions and up to 15% greater areal coverage of snow over the northern continents from fall to early spring. The average depth of snow on the continents increased (by 20%) only with the change from winter to summer perihelion because of a greater length of the snow accumulation season. During late spring and summer, enhanced radiation (in the experiments with increased tilt and summer perihelion) rapidly melted the snowpack so that it did not persist as long into the summer season as in the experiments with reduced summer insolation (winter perihelion and decreased tilt). The experiments indicate that orbital reduction of spring‐summer radiation could be an important factor in initiating glaciation over northeastern Canada. The duration of the snow cover‐free season was found to decrease nonlinearly with decreased spring‐summer insolation. The change from modern to 115,000 year B.P. orbital conditions reduced the snow cover‐free season over northeastern Canada by 3–4 weeks. For a more exaggerated tilt (20°) and eccentricity (0.06), snowfields occasionally survived the summer season. Overall, the model results suggest that the orbital conditions at 115,000 years B.P. greatly enhanced the likelihood of perennial snowfields over northeastern Canada.
Carbon isotopic analysis of atmospheric methane by isotope‐ratio‐monitoring gas chromatography‐mass spectrometry
Authors: Merritt Dawn A. , Hayes J. M. , Marais David J. Des
Published: February 2004

Less than 15 min are required for the determination of δ13CPDB with a precision of 0.2‰ (1σ, single measurement) in 5‐mL samples of air containing CH4 at natural levels (1.7 ppm). An analytical system including a sample‐introduction unit incorporating a preparative gas chromatograph (GC) column for separation of CH4 from N2, O2, and Ar is described. The 15‐min procedure includes time for operation of that system, high‐resolution chromatographic separation of the CH4, on‐line combustion and purification of the products, and isotopic calibration. Analyses of standards demonstrate that systematic errors are absent and that there is no dependence of observed values of δ on sample size. For samples containing 100 ppm or more CH4, preconcentration is not required and the analysis time is less than 5 min. The system utilizes a commercially available, high‐sensitivity isotope‐ratio mass spectrometer. For optimal conditions of sample handling and combustion, performance of the system is within a factor of 2 of the shot‐noise limit. The potential exists therefore for analysis of samples as small as 15 pmol CH4 with a standard deviation of <1‰.
Simultaneous lidar measurements of mesospheric Ca, Na, and temperature profiles at Urbana, Illinois
Authors: Qian Jun , Gardner Chester S.
Published: February 2004

Simultaneous common volume measurements of mesospheric Ca, Na, and temperature profiles were conducted at Urbana, Illinois (40°N, 88°W), from late October 1992 to January 1993. The Ca layer abundance varied between 2.1–10.7 × 107 cm−2, with an average of 6.3×107 cm−2. The Na abundance averaged 7.2×109 cm−2, or approximately 110 times that of Ca. The Ca layer centroid height varied between 90.8 and 92.5 km, with an average value of 91.8 km, and was on average 0.2 km higher than the Na layer centroid height. The rms widths of the Ca and Na layers averaged 3.5 and 4.7 km, respectively. No substantial sporadic Ca or sporadic Na layers were detected during our observations. Considerable gravity wave activity was observed in the Ca, Na, and temperature profiles on December 21–22, 1992. The relative Ca and Na density perturbations are in phase with temperature perturbations on the bottom side of the layers, and out of phase on the topside, consistent with theoretical predictions. Relative Ca density perturbations are larger than those of Na by a factor of 2 on this night. Our measurements are compared with the only previous lidar measurements of mesospheric Ca conducted in France by Granier et al. [1989].
Influence of tropical cloud systems on the relative humidity in the upper troposphere
Authors: Udelhofen Petra M. , Hartmann Dennis L.
Published: February 2004

The influence of tropical clouds on the humidity in the upper troposphere is investigated. Upper tropospheric humidity (UTH) in the 550–200 mbar layer is retrieved from GOES 7 6.7‐μm brightness temperatures and validated against humidity profiles from radio soundings and European Centre for Medium‐Range Weather Forecasts analyses. UTH decreases from values of 44–74% near the edges of upper tropospheric clouds to 11–15% at a distance of about 500 km from the cloud edge and remains relatively constant for greater distances. The average UTH at the cloud edge is lower in land regions and in regions of suppressed convection than in the Intertropical Convergence Zone (ITCZ). The spread of UTH from cloudy to clear regions is least in the regions of suppressed convection and greatest in the ITCZ. It is shown that the UTH at the cloud edge and its spread into cloud‐free regions both increase with the average cloud top altitude. Using particular geographical regions as examples, it is demonstrated that UTH seems to increase linearly with the amount of upper level cloud. A diurnal cycle of UTH in cloud‐free areas is observed and related to diurnal variations in cloud amount. The diurnal variation in UTH lags behind the diurnal cycle in upper tropospheric clouds by about 8 hours.
Influence of lower tropospheric ozone on total column ozone as observed over the Pacific Ocean during the 1991 PEM‐West A expedition
Authors: Gregory Gerald L. , Anderson Bruce E. , Browell Edward V.
Published: February 2004

Pacific Exploratory Mission (PEM‐West A) aircraft ozone data and concurrently measured TOMS atmospheric total column ozone data are examined. The focus of the paper is to determine, for relatively remote Pacific regions (110°–180°E longitude, 0°–40°N latitude), what fraction of total column ozone can be attributed to lower‐tropospheric ozone (surface to 9‐km altitude) and how this fraction varies with time. At ≈30° to 40°N the surface to 9‐km ozone column contributes about 10% to total column ozone. Of this 10%, about half is attributed to the surface to 3‐km column. Nearer the equator (0°–15°N), the contributions are less, about 5 (0–9 km) and 1% (0–3 km) of the total column, respectively. Variations in ozone within the 3‐km and 9‐km columns over periods of days tend to be larger at northern latitudes. At 40° to 50°N, ozone variations within the columns can result in changes in each column's contribution to the total column that approach >50% of the column's contribution to the total column. Nearer the equator, variations are ≤1 Dobson unit which is generally <10% of the surface to 9‐km column's contribution to total column ozone. The results provide insight into the relative importance of lower‐tropospheric ozone to observed variations in atmospheric total column ozone and thus are important to the interpretation of total column measurements, especially for tropospheric studies.
Stratospheric and mesospheric pressure‐temperature profiles from rotational analysis of CO2 lines in atmospheric trace molecule spectroscopy/ATLAS 1 infrared solar occultation spectra
Authors: Stiller G. P. , Gunson M. R. , Lowes L. L. , Abrams M. C. , Raper O. F. , Farmer C. B. , Zander R. , Rinsland C. P.
Published: February 2004

A simple, classical, and expedient method for the retrieval of atmospheric pressure‐temperature profiles has been applied to the high‐resolution infrared solar absorption spectra obtained with the atmospheric trace molecule spectroscopy (ATMOS) instrument. The basis for this method is a rotational analysis of retrieved apparent abundances from CO2 rovibrational absorption lines, employing existing constituent concentration retrieval software used in the analysis of data returned by ATMOS. Pressure‐temperature profiles derived from spectra acquired during the ATLAS 1 space shuttle mission of March–April 1992 are quantitatively evaluated and compared with climatological and meteorological data as a means of assessing the validity of this approach.