bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow of a western boundary current through multiple straits: An electrical circuit analogy for the Indonesian throughflow and archipelago
Authors: Wajsowicz Roxana C.
Published: February 2004

An analogy is drawn between the flow through multiple, frictional straits and the current through combinations of resistors acting in series and parallel in an electrical circuit. Kirchoff's laws are replaced by continuity of mass and the generalized island rule [Wajsowicz, 1993a]. Expressions are derived for the equivalent resistance of straits acting in series and in parallel. Hence the total “current” can be calculated for a given “voltage” supplied by the wind stress curl over the adjacent ocean basin. The model may be used to help understand how to parameterize the effect of the many islands within the Indonesian archipelago in a numerical general circulation model by combining islands/straits into resolvable groups with the same effective resistance. For example, from the expression for parallel straits it can be deduced that if there is at least one gap at each latitude within the archipelago, which is wide and deep enough not to block the throughflow, then the throughflow achieves its predicted wind‐driven maximum. The transport is not carried by the largest gap, but, instead, is carried by the westernmost gap up to a friction‐determined limit, and so on eastward, until a dynamically wide gap is reached. This gap carries the remainder of the transport; gaps to the east carry no transport. Hence the model explains surprising recent observations that the throughflow is as large as the Sverdrup model prediction of O (15 Sv). It also explains observations that the narrow, shallow Lombok and Savu Straits to the west carry significant transport, even though Timor Strait to the east is much wider and deeper, and similarly for Makassar Strait versus Molucca Strait.
On the slippage of deep‐drogued drifters
Authors: Krauss W.
Published: February 2004

Deep‐drogued drifters are in use to measure the near‐surface geostrophic currents. An attempt is made to study the slippage of these drifters due to wind and Ekman currents. The results are based on a data set from the unstratified North Sea obtained in winter 1991–1992, where the currents were decomposed into Ekman currents and barotropic currents. The influence of these Ekman currents on the drift performance of drifters drogued below the mixed layer in the barotropic current is determined by using quadratic drag laws. In 90% of all cases (1540 data points) the combined effect of wind drag and Ekman currents on buoy and 100‐m tether produces a slippage of less than 2 cm/s. Drifters drogued within the mixed layer show less slippage due to the reduced drag on the tether, but they are primarily designed to measure the actual near‐surface currents, which are strongly dependent on the wind conditions. It is concluded that deep‐drogued drifters are a reliable device to study weakly baroclinic geostrophic currents.
The stable isotope composition of waters of the eastern Mediterranean Sea
Authors: Gat Joel R. , Shemesh Aldo , Tziperman Eli , Hecht Artur , Georgopoulos Dimitri , Basturk Ozden
Published: February 2004

Waters of the eastern Mediterranean portray an unusual pattern of stable isotope composition compared with other evaporitic systems: an increase in 18O concentration up to value of δ18O = +2.2‰ is not matched by a commensurate increase of deuterium. It is shown that this unusual pattern is an expression of the “medi‐terranean” location of the sea, where the air‐sea interaction with relatively dry and isotopically depleted continental air masses dominates the evaporation process in winter and where the diluting meteoric waters are extremely depleted in the heavy isotopes. As a result, the slope of the evaporation line in δ(2H)‐δ(18O) space is lower than in other marine systems, whereas the mixing line between the meteoric waters and the seawater is very steep. This pattern provides an independent method for the estimation of the evaporation E and freshwater influx terms M of the water balance of the Mediterranean Sea. In winter, the ratio of evaporation to freshwater input is found to be E/M = 1.20, whereas in summer this ratio E/M = 1.83.
Kuroshio Extension dynamics from satellite altimetry and a model simulation
Authors: Mitchell J. L. , Teague W. J. , Jacobs G. A. , Hurlburt H. E.
Published: February 2004

Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy‐resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20°S and has a resolution of 0.125° latitude and 0.176° longitude. The model is synoptically driven by daily 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time‐longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasi‐permanent meanders seen in hydrographic climatology (cyclonic at 138°E and anticyclonic at 144°E, 150°E, and 160°E). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm2 s−2 along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s−1 in the region east of 155°E is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the cross‐stream differential in SSH from the simulation and Geosat observations.
Angular momentum and action in surface gravity waves: Application to wave‐current interaction
Authors: Kenyon Kern E. , Sheres David
Published: February 2004

When small‐amplitude surface gravity waves progress in deep water, all the fluid particles are observed to orbit in circles within the depth of wave influence. Each fluid particle therefore has angular momentum with respect to the center of its orbit, and the angular momentum vectors are directed parallel to the crests and troughs or perpendicular to the wave number. The angular momentum per unit volume is calculated for each fluid particle, and then, by vertical integration, the angular momentum per unit horizontal area is computed. The total energy and the magnitude of the angular momentum, both per unit area or both per unit volume, are found to be proportional, the factor of proportionality being the wave frequency; specifically, angular momentum magnitude equals energy divided by frequency. Wave action, which has become an increasingly popular quantity for interpreting surface gravity wave problems and is defined as wave energy divided by frequency, is the same thing as the magnitude of the angular momentum. A general equation for the conservation of angular momentum along wave rays is given, and it contains on the right‐hand side two torque terms; one changes the magnitude and the other changes the direction of the angular momentum. In applying this equation to the wave‐current refraction problem, there is only one torque, which changes the direction of the angular momentum, and it is explicitly determined as a function of the horizontal shear in the current. The magnitude of the angular momentum, and therefore also the wave action, will be conserved along the rays if there are no torques that could alter it, as in pure wave‐current refraction. Our conservation equation for angular momentum is easily adapted to describing wave generation and dissipation by including the appropriate torques on the right side, and this may prove to be helpful for calculations of wave evolution.
The influence of fresh‐core rings on chemical concentrations (CO2, PO4, O2, alkalinity, and pH) in the western subarctic Pacific Ocean
Authors: Rogachev Konstantin A. , Tishchenko Pavel Ya. , Pavlova Galina Yu. , Bychkov Alexander S. , Carmack Eddy C. , Wong Chi Shing , Yurasov Gennady I.
Published: February 2004

We here present conductivity‐temperature‐depth and chemical (CO2, PO4, O2, alkalinity, and pH) observations across an anticyclonic (fresh‐core) ring in the Oyashio Current near Bussol Strait, Kuril Islands. These measurements suggest that the core of the ring is a region of enhanced vertical mixing, with attendant effects on both spatial and temporal patterns of CO2 exchange. In particular, the difference between atmospheric and oceanic pCO2 at the air‐sea boundary shows a pronounced seasonal reversal in sign. In winter, pCO2 is everywhere higher in the ocean than in the atmosphere; however, because mixing within the core of the ring brings additional amounts of deep, CO2‐rich water upward the highest values of pCO2 occur above the ring. In summer, pCO2 outside the ring boundary is also higher in the ocean than in the atmosphere; however, because enhanced mixing within the ring now promotes high values of nutrient flux, primary productivity, and CO2 uptake, pCO2 values there are lower than in the atmosphere. These data suggest the importance of including mesoscale physical processes in the interpretation of oceanic CO2 exchange and demonstrate the usefulness of biogeochemical properties in studies of ring dynamics and evolution.
The effects of snowpack grain size on satellite passive microwave observations from the Upper Colorado River Basin
Authors: Josberger Edward G. , Gloersen Per , Chang Al , Rango Albert
Published: February 2004

Understanding the passive microwave emissions of a snowpack, as observed by satellite sensors, requires knowledge of the snowpack properties: water equivalent, grain size, density, and stratigraphy. For the snowpack in the Upper Colorado River Basin, measurements of snow depth and water equivalent are routinely available from the U.S. Department of Agriculture, but extremely limited information is available for the other properties. To provide this information, a field program from 1984 to 1995 obtained profiles of snowpack grain size, density, and temperature near the time of maximum snow accumulation, at sites distributed across the basin. A synoptic basin‐wide sampling program in 1985 showed that the snowpack exhibits consistent properties across large regions. Typically, the snowpack in the Wyoming region contains large amounts of depth hoar, with grain sizes up to 5 mm, while the snowpack in Colorado and Utah is dominated by rounded snow grains less than 2 mm in diameter. In the Wyoming region, large depth hoar crystals in shallow snowpacks yield the lowest emissivities or coldest brightness temperatures observed across the entire basin. Yearly differences in the average grain sizes result primarily from variations in the relative amount of depth hoar within the snowpack. The average grain size for the Colorado and Utah regions shows much less variation than do the grain sizes from the Wyoming region. Furthermore, the greatest amounts of depth hoar occur in the Wyoming region during 1987 and 1992, years with strong El Niño Southern Oscillation, but the Colorado and Utah regions do not show this behavior.
Evolution of two oceanic extratropical cyclones as observed with the special sensor microwave/imager and the Geosat and ERS 1 altimeters
Authors: Katsaros Kristina B. , Adams Jennifer Miletta , Mognard Nelly M.
Published: February 2004

The evolution of two marine cyclones across the North Atlantic and the North Pacific Oceans is followed with a combination of sequential passive and active microwave data. The special sensor microwave/imager (SSM/I) provides estimates of atmospheric parameters: integrated water vapor, precipitation, and ocean surface wind speed. The Geosat and ERS 1 altimeter data are used to derive sea state parameters along the satellite tracks: the significant wave height and the ocean surface wind speed. The SSM/I integrated water vapor gradients are used to locate and follow the evolution of the atmospheric fronts. In regions of precipitation, often located behind the atmospheric fronts, the SSM/I surface wind speed cannot be derived, while the Geosat and ERS 1 altimeter wind speed yields estimates of wind speed gradients across the fronts. It is shown that the combination of SSM/I and satellite altimeters permits additional observations of surface fronts with higher resolution both in space and time than those given in the National Meteorological Center analyses.
Satellite imagery of the onset of streaming flow of ice streams C and D, West Antarctica
Authors: Hodge Steven M. , Doppelhammer Sheila K.
Published: February 2004

Five overlapping Landsat multispectral scanner satellite images of the interior of the West Antarctic ice sheet were enhanced with principal component analysis, high‐pass filtering, and linear contrast stretching and merged into a mosaic by aligning surface features in the overlap areas. The mosaic was registered to geodetic coordinates, to an accuracy of about 1 km, using the five scene centers as control points. The onset of streaming flow of two tributaries of ice stream C and one tributary of ice stream D is visible in the mosaic. The onset appears to occur within a relatively short distance, less than the width of the ice stream, typically at a subglacial topographic feature, such as a step or ridge. The ice streams extend farther up into the interior than previously mapped. Ice stream D starts about 150 km from the ice divide, at an altitude of about 1500 m, approximately halfway up the convex‐upward dome shape of the interior ice sheet. Ice stream D is relatively much longer than ice stream C, possibly because ice stream D is currently active, whereas ice stream C is currently inactive. The grounded portion of the West Antarctic ice sheet is perhaps best conceptualized as an ice sheet in which ice streams are embedded over most of its area, with slow‐moving ice converging into fast‐moving ice streams in a widely distributed pattern, much like that of streams and rivers in a hydrologic basin. A relic margin appears to parallel most of the south margin of the tributary of ice stream D, separated from the active shear margin by about 10 km or less for a distance of over 200 km. This means there is now evidence for recent changes having occurred in three of the five major ice streams which drain most of West Antarctica (B, C, and D), two of which (B and D) are currently active.
Large‐eddy simulation of the equatorial ocean boundary layer: Diurnal cycling, eddy viscosity, and horizontal rotation
Authors: Wang Dailin , Large William G. , McWilliams James C.
Published: February 2004

Oceanic boundary layers (OBL) at the equator are studied using a large‐eddy simulation (LES) model. The model has no equatorial undercurrent, and the vertical component of Earth's rotation is zero Ωz = 0. It is forced with a constant westward zonal wind stress and with a constant surface heat flux, and, in some cases, a diurnal cycle of solar heating. Averaged over day 4, temperature and velocity are vertically mixed deeper for the diurnal cycling case, the surface temperature is cooler by 0.04°C, and the surface velocity is weaker by 0.08 m/s, compared to a constant surface heating case with the same daily averaged surface heat flux. There is no general relationship between eddy viscosity (or diffusivity) and the gradient Richardson number, especially for the case of diurnal cycling, where nonlocal transports can lead to countergradient fluxes during nighttime convection. Sharp gradients of eddy viscosity are found near the critical Richardson number, Ri = 0.25, and in the lower part of the entrainment layer, where Ri ≈ 1.0. The effect of the horizontal component of Earth's rotation Ωy on an entraining equatorial OBL is also investigated. It is found in our entraining cases that with and without Ωy, mean quantities (temperature, velocity) and turbulent fluxes differ no more than a few percent, in accord with the second‐order closure results of Galperin et al. [1989] and Kantha et al. [1989]. The LES results are used to extend the bulk mixed layer theory of Garwood et al. [1985a, b] to the entraining case. With no entrainment, Garwood et al. [1985a, b] show that Ωy effects can be substantial. The effects of Ωy are small when the entrainment heat flux is significant.
Modeling layered structure in deep Pacific circulation
Authors: Obata Atsushi , Furue Ryo , Aoki Shigeaki , Suginohara Nobuo
Published: February 2004

The layered structure of the deep Pacific circulation is investigated using a multilevel numerical model with idealized geometry. The circulation is driven by sea surface heating and Newtonian body cooling at the southwest corner of the basin. The latter forcing is intended to incorporate thermohaline forcing in the Southern Ocean. A layered structure forms in the zonally averaged meridional circulation when the reference density for the body cooling varies vertically. The density balance between vertical advection and vertical diffusion holds in the zonally averaged field as well as in the interior. This balance, combined with the reference density for the body cooling, determines the zonally averaged vertical velocity and hence the zonally averaged meridional circulation from the zonally averaged continuity equation. This two‐dimensional mechanism for describing the layered structure for this three‐dimensional, rotating fluid model is identical to that for a two‐dimensional, nonrotating fluid model. The increased vertical structure is accompanied by the enhancement of some higher vertical modes, which brings about various changes in the three‐dimensional structure, such as the formation of a broad, diffusive, deep western boundary current.
Seasonal variations in the equatorial Indian Ocean and their impact on the Lombok throughflow
Authors: Yamagata Toshio , Mizuno Keisuke , Masumoto Yukio
Published: February 2004

In order to identify the origin of the nonequilibrated pressure field which introduces semiannual variations to the Lombok throughflow, one of the major components of the Indonesian throughflow, seasonal sea level records along the eastern boundary of the Indian Ocean and subsurface temperature records in the equatorial Indian Ocean are compared with the results from ocean general circulation models forced by climatological winds. It is shown, as discussed by Clarke and Liu [1993] using a simple analytical model, that the annual component of the sea level is excited locally by monsoonal winds, whereas the semiannual component is excited remotely by zonal winds in the central equatorial Indian Ocean during the monsoon transition periods. The eastward semiannual propagation observed both in the model and in the 20°C isotherm depth anomaly along the equatorial Indian Ocean is interpreted in terms of the second mode equatorial Kelvin wave excited by the zonal winds during the monsoon transition periods. The propagation direction is westward in the off‐equatorial region along 4°N, suggesting that the equatorial Kelvin wave reflects as an equatorial Rossby wave twice a year at the coast of Sumatra. Since the seasonal winds responsible for those semiannual oceanic variations suffer short‐term climate variabilities such as El Niño‐Southern Oscillation events, they may introduce interannual modulations to the seasonal cycle of the Lombok throughflow. As a corollary, we suggest an ageostrophic limitation due to a nonequilibrated pressure field on the use of Godfrey's [1989] island rule in estimating the throughflow variations.
Numerical simulations of topographic Rossby waves along the East Greenland Front
Authors: Maslowski Wieslaw
Published: February 2004

Topographic Rossby waves propagating along the East Greenland Front between 77° and 72°N are simulated using a primitive‐equation ocean model of the Greenland Sea. The waves are prominent in both barotropic and baroclinic experiments, and their estimated wave period and wavelength are about 17.3 hours and 363 km, respectively. A possible mechanism for generation of topographic Rossby waves is presented. The dominant along‐slope component of the group velocity implies that the bulk of the energy propagates along the shelf slope of Greenland toward the northeast. Existence of the offshore component indicates that some energy is available for cross‐frontal mixing within the East Greenland Front.
Arctic Ocean deep water masses in the western Iceland Sea
Authors: Buch Erik , Malmberg Svend‐Aage , Kristmannsson Stefan S.
Published: February 2004

The Icelandic‐Danish contribution to the international Greenland Sea Project was devoted to a study of the distribution and transport of water masses in the Western Iceland Sea. Each year in September from 1987 to 1991, temperature and salinity were observed on a station net in the area off east Greenland from the Denmark Strait in the south to 72°30′N. The data reveal that the deep water column of the western Iceland Sea is occupied by a number of slightly different water masses, which all can be traced back to a formation in the Arctic Ocean. It is especially noticeable that in some years almost undiluted Eurasian Basin Deep Water can be observed as far south as the Denmark Strait.
Multiyear Stratospheric Aerosol and Gas Experiment II measurements of upper tropospheric aerosol characteristics
Authors: Kent G. S. , Wang P.‐H. , McCormick M. P. , Skeens K. M.
Published: February 2004

Measurements of aerosol extinction at wavelengths of 0.525 and 1.02 μm, made by the Stratospheric Aerosol and Gas Experiment (SAGE) II solar occultation satellite experiment, have been used to study the global‐scale characteristics of the upper tropospheric aerosol. Extinction measurements, in which only aerosols occurred along the optical path, have been separated from those that included high‐altitude cloud by examining the wavelength variation of the extinction. Data for the time period October 1984 to May 1991 show that the two main influences on the upper tropospheric aerosol were seasonal lifting of material from below and downward transfer of volcanic aerosol from the stratosphere. Maximum lifting of surface material occurs in local spring in both hemispheres and is observed at all latitudes between 20°N and 80°N and 20°S and 60°S; the data also show a strong hemispheric asymmetry with more aerosol in the northern hemisphere. Downward transfer of volcanic aerosol is particularly observed poleward of 40° latitude, where a substantial enhancement of material occurs down to altitudes 2–3 km below the tropopause. By comparing tropospheric aerosol concentrations at different times during the period of observation, it has been possible to differentiate the effects of volcanic aerosols from those of the background, or baseline, aerosols. A simple model, based on the ratio of the extinctions at the two measurement wavelengths, has been used to calculate the aerosol mass density and effective radius. It was found that in 1984–1985, approximately 15% of the volcanic aerosol still present from the eruption of El Chichón in 1982 resided in the upper troposphere. Particle sizes for the volcanic aerosol in the lower stratosphere and upper troposphere were of the order of 0.5 μm, while those for the baseline aerosol were about 0.15 μm. Slightly larger aerosol sizes, of the order of 0.25 μm, were observed at altitudes 6–8 km during the springtime enhancements. The low‐latitude aerosol enhancements in both hemispheres appear to have the characteristics of material derived from arid surface regions, while the higher‐latitude aerosol in the northern hemisphere appears more likely to be derived from anthropogenic sources.
A three‐dimensional general circulation model with coupled chemistry for the middle atmosphere
Authors: Rasch P. J. , Boville B. A. , Brasseur G. P.
Published: February 2004

We document a new middle atmosphere general circulation model that includes ozone photochemistry. The dynamical model component is based on the NCAR middle atmosphere version of the Community Climate Model. The chemistry model component simulates the evolution of 24 chemically reactive gases. The horizontal resolution is approximately 3° in latitude and 6° in longitude. It includes 44 levels, with a maximum vertical grid spacing of about 2.5 km and a top level at around 75 km. The chemical model distinguishes between species where we judge transport to be critical and those for which it may be neglected. Nine longer‐lived species (N2O, CH4, H2O, HNO3, N2O5, CO, ClONO2, HCl, and HOCl) and four chemical families (NOy, NOx, Ox and Clx) are advected. Concentrations of 15 species which are typically shorter‐lived or are members of the chemical families are diagnosed using quasi‐equilibrium assumptions ( O(1D), OH, Cl, O(3P), O3, HO2, NO2, ClO, NO, HNO4, NO3, N, OClO, Cl2O2, H2O2). Distributions for a number of other species are prescribed. Results are presented from a 2‐year simulation, which include only gas phase photochemical reactions and in which the ozone distribution forecast from the chemistry module does not affect the radiative forcing of the dynamical fields. The calculated distributions of trace species and their seasonal evolution are often quite realistic, particularly in the northern hemisphere extratropics. Distributions of long‐lived species such as N2O and CH4 correspond well to satellite observations. Some features, such as the double peak structure occurring during equinoxes, are not reproduced. The latitudinal variation and seasonal evolution of the ozone column abundance is quite realistic. The calculated vertical distribution of the ozone mixing ratio exhibits significant differences from measured values. The model underestimates significantly the ozone in the upper stratosphere (40 km) and in the extratropics, where the maximum values occur at too low an altitude. The model reproduces the key features expected in the distribution of fast reacting nitrogen and chlorine compounds. Problems in the southern polar simulation (which we attribute to the lack of a comprehensive parameterization of gravity waves) are discussed. The model currently parameterizes only stationary gravity waves generated by flow over orography. Other sources of gravity waves may be required to improve the model simulation.
Modeled impacts of stratospheric ozone and water vapor perturbations with implications for high‐speed civil transport aircraft
Authors: Rind David , Lonergan Patrick
Published: February 2004

Ozone and water vapor perturbations are explored in a series of experiments with the Goddard Institute for Space Studies climate/middle atmosphere model. Large perturbations to stratospheric ozone and water vapor are investigated, with and without allowing sea surface temperatures to change, to illuminate the nature of the dynamic and climatic impact. Then more realistic ozone and water vapor perturbations, similar to those estimated to result from aircraft emissions, are input and the equilibrium response obtained. Removing ozone in the lower stratosphere without allowing sea surface temperatures to change results in in situ cooling of up to 10°C in the tropical lower stratosphere, with radiative warming about half as large in the middle stratosphere. The temperature changes induce increases in tropospheric and lower stratospheric eddy energy and in the lower stratosphere residual circulation of the order of 10%. When sea surface temperatures are allowed to respond to this forcing, the global, annual‐average surface air temperature cools by about 1°C as a result of the decreased ozone greenhouse capacity, reduced tropospheric water vapor, and increased cloud cover. For more realistic ozone changes, as defined in the High‐Speed Research Program/Atmospheric Effects of Stratospheric Aircraft reports, the stratosphere generally cools by a few tenths degrees Celsius. In this case, the surface air temperature change is not significant, due to the conflicting influences of stratospheric ozone reduction and tropospheric ozone increase, although high‐latitude cooling of close to 0.5°C does occur consistently. Doubled stratospheric water vapor cools the middle atmosphere by 2°–3°C and warms the upper troposphere by 0.5°C. Reduced tropospheric‐stratospheric vertical stability leads to tropospheric planetary longwave energy increases of some 15% for the longest waves and stratospheric residual circulation increases of 5%. When sea surface temperatures are allowed to change, the surface air temperature warms by just a few tenths of a degree Celsius; although this change is not significant in terms of the model's natural variability, the experiment is warmer than the control in most years. The response is muted as the high altitude of energy input minimizes surface level feedbacks, and high‐level cloud cover is reduced. With a more realistic increase of stratospheric water vapor of 7%, the middle atmosphere cools by 0.5°C or less, and the surface temperature change is neither significant nor consistent. Overall, the experiments emphasize that stratospheric changes affect tropospheric dynamics in the model, that tropospheric changes can affect stratospheric dynamics, and that tropospheric feedback processes and natural variability are important when assessing the climatic response to aircraft emissions.
Airborne measurements of total sulfur gases during NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3
Authors: Farwell Sherry O. , MacTaggart Douglas L. , Chatham William H. , Everson Dale O. , Samaranayake Kumarasiri , Lim Young Taik
Published: February 2004

A metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) technique was used by investigators from the University of Idaho (UI) to measure ambient total sulfur gas concentrations from an aircraft platform during the NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3 (GTE/CITE 3) program. The MFC/FD/FPD technique allowed rapid quantitation of tropospheric background air masses using sample integration times of 1–3 min with little or no gap between measurements. The rapid and continual sampling nature of this technique yielded data covering approximately 75% of the entire CITE 3 program's air track. Ambient air measurement data obtained during northern hemisphere (NH) flights often exhibited relatively high total sulfur gas values (up to 19 ppb) and an extremely high degree of sample heterogeneity, especially in coastal locations. Data from southern hemisphere (SH) flights typically exhibited relatively low total sulfur gas concentrations and a low degree of sample heterogeneity. A bimodal interhemispheric total sulfur gas gradient was observed using data obtained during transit flights between the two CITE 3 program ground bases. Comparisons were made of UI total sulfur gas measurements with composite sulfur gas values generated using speciated sulfur gas measurements from other CITE 3 participants. Only a relatively small number of overlap periods for comparison were obtained from all the available CITE 3 data because of large differences in measurement integration times and lack of synchronization of sample start/stop times for the various investigators. These effects were compounded with extreme sample heterogeneity in the NH and the speed at which the aircraft traversed the air masses being sampled. Despite these constraints, sufficient overlapping data were available for the comparative evaluations. Comparison of NH UI total with composite sulfur gas values showed excellent correlation and linear curve fit, indicating substantial qualitative agreement. Simple linear regression of total on composite sulfur gas data yielded a slope of 1.9 for coastal NH regions and 1.2 for marine NH regions. The marine NH slope is not statistically different from one, indicating substantial quantitative agreement between UI total and composite sulfur gas values in these regions. However, a significant difference was observed when these same data were treated with a paired t test. SH data exhibited no significant correlation or linear regression slope. A paired t test showed a statistically significant difference when all SH flights were used. However, data from three SH flights that were classified into a unique group using discriminant analysis showed no significant difference between UI total and composite sulfur gas values when analyzed with the paired t test.
Characterization of vertical electric fields 500 m and 30 m from triggered lightning
Authors: Rubenstein M. , Rachidi F. , Uman M. A. , Thottappillil R. , Rakov V. A. , Nucci C. A.
Published: February 2004

Vertical electric field waveforms of leader‐return stroke sequences measured 500 m and 30 m from rocket‐triggered lightning are presented. The 500‐m data were recorded during the summer of 1986, the 30‐m data during the summer of 1991, both at the NASA Kennedy Space Center, Florida. The 40 leader‐return stroke field waveforms at 500 m and the 8 waveforms at 30 m all appear as asymmetrical V‐shaped pulses, the bottom of the V being associated with the transition from the leader to the return stroke. Only two waveforms at 30 m were suitable for quantitative analysis. The widths of the V at half of peak value for these are 1.8 and 5.0 μs, while for the 500‐m data they are 1 to 2 orders of magnitude greater, with a median value of 100 μs. Applying a widely used and simple leader model to the measured leader electric fields at 500 m, we infer, for the bottom kilometer or so of the leader channel, leader speeds between 2×106 and 2×107 m/s and leader charges per unit length of 0.02×10−3 to 0.08×10−3 C/m. From the two measured leader electric field changes at 30 m we infer, using the same leader model, for the bottom 100 meters or so of the leader channel, speeds of 3×107 and 1×107 m/s (the corresponding measured waveform half widths are 1.8 μs and 5.0 μs) and charges per unit length of 0.14×10−3 and 0.02×10−3 C/m (the corresponding measured leader field changes are 81 kV/m and 12 kV/m). The corresponding measured return stroke peak currents for the above two cases are 40 kA and 7 kA, respectively. A positive correlation is observed between the magnitude of the leader field change at 500 m and the ensuing return stroke current peak.
Aircraft measurements of O3, HNO3 and N2O in the winter Arctic lower stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) 1
Authors: Bregman A. , van Velthoven P. F. J. , Wienhold F. G. , Fischer H. , Zenker T. , Waibel A. , Frenzel A. , Arnold F. , Harris G. W. , Bolder M. J. A. , Lelieveld J.
Published: February 2004

Simultaneous in situ measurements of O3, HNO3, and N2O were performed in the Arctic (68°–74°N) lower stratosphere during February 1993 on board a Cessna Citation aircraft up to 12.5 km altitude, during the first Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) campaign. Strong variations in the concentrations, distributions, and ratios of these trace gases were found from the maximum altitude down to the tropopause. Close to the tropopause, vortex air was present with relatively low N2O concentrations. The observed N2O‐HNO3 relation agrees with earlier measurements of total nitrogen and N2O inside the vortex, suggesting subsidence of vortex air across the bottom of the vortex. This air also contained low O3 concentrations relative to N2O, indicating enhanced O3 loss by chemical reactions involving stratospheric particles. Based on trajectory calculations and assuming a potential temperature cooling rate of 0.6 K d−1, we estimate an O3 loss of 4–7 ppbv d−1 (0.9–1.2% d−1), in the Arctic lower stratosphere for the period January–February 1993. Air parcels originating from middle latitudes, containing relatively low O3 and N2O concentrations, may have originated from the vortex earlier in the winter. In addition, the results also show high HNO3 concentrations relative to O3 and N2O. Air parcels originating from high latitudes may have been enriched in HNO3 by sedimentation and evaporation of nitric acid containing particles, which would explain the relatively high HNO3 concentrations and HNO3/O3 ratios measured. Heterogeneous chemistry on sulfuric acid particles, probably enhanced in concentration by gravitational settling of the Pinatubo aerosol, is the most plausible explanation for the observed high HNO3 concentrations relative to N2O in air parcels originating from midlatitudes.
Transport of the Pinatubo volcanic aerosol to a northern midlatitude site
Authors: Langford A. O. , O'Leary T. J. , Proffitt M. H. , Hitchman M. H.
Published: February 2004

A 2‐year time series of high resolution lidar backscatter profiles at 0.532 μm, taken at the NOAA Fritz Peak Observatory (39.9°N, 105.3°W) is analyzed to investigate the evolution of the stratospheric aerosol following the eruption of Mount Pinatubo in June, 1991. Aerosol from the eruption first appeared as transient layers just above the tropopause in late summer and early fall of 1991. This was followed by a rapid increase in aerosol centered near 21 km, with an exponential risetime of ∼22 days. The maximum in late December 1991 was followed by a slow decline, punctuated by seasonal increases below 18 km and with an exponential decay timescale of ∼300 days near 20 km. Aerosol backscatter is converted to mass and a principal component analysis (PCA) is performed to explore the statistical properties of aerosol variability. More than 80% of the variability in aerosol mass is described by only three components, corresponding to variations in the layers 17–22 km (PC1, 44%), below 17 km (PC2, 27%), and above 22 km (PC3, 9%). Since most of the temporal variations occur independently in these three layers, this work provides further insight into the nature of stratospheric transport from the tropics to midlatitudes.
Chemical signatures of aged Pacific marine air: Mixed layer and free troposphere as measured during PEM‐West A
Authors: Gregory G. L. , Bachmeier A. S. , Blake D. R. , Heikes B. G. , Thornton D. C. , Bandy A. R. , Bradshaw J. D. , Kondo Y.
Published: February 2004

The Pacific Ocean is one of the few remaining regions of the northern hemisphere that is relatively free of direct anthropogenic emissions. However, long‐range transport of air pollutants is beginning to have a significant impact on the atmosphere over the Pacific. In September and October 1991, NASA conducted the Pacific Exploratory Mission‐West A expedition to study the atmospheric chemistry and background budgets of key atmospheric trace species. Aircraft sampling centered on the northern Pacific, 0° to 40°N and 115° to 180°E. The paper summarizes the chemical signature of relatively well‐aged Pacific marine air (residence time ≥10 days over the ocean). The chemical signatures show that marine air is not always devoid of continental influences. Aged marine air which circulates around the semipermanent subtropical anticyclone located off the Asian continent is influenced by infusion of continental air with anthropogenic emissions. The infusion occurs as the result of Asian outflow swept off the continent behind eastward moving cold fronts. When compared to aged marine air with a more southerly pathway, this infusion results in enhancements in the mixing ratio of many anthropogenic/continental species and typically those with lifetimes of weeks in the free troposphere. Less enhancement is seen for the short‐lived species with lifetimes of a few days as infused continental emissions are depleted during transport (about a week) around the semipermanent subtropical high.
The UV‐visible absorption spectrum and photolysis quantum yields of methylglyoxal
Authors: Staffelbach Thomas A. , Orlando John J. , Tyndall Geoffrey S. , Calvert Jack G.
Published: February 2004

Laboratory studies have been conducted to determine the rate and mechanism of methylglyoxal (MGLY, CH3COCHO) photolysis under tropospheric conditions. The UV/visible absorption cross sections of MGLY as a function of wavelength have been measured at three different temperatures, 248 K, 273 K, and 298 K. The absorption cross sections show only small variations (≤10%) over this temperature range. The results at 298 K are in agreement with the values published by Meller et al. (1991) and differ by almost a factor of 2 from earlier work published by Plum et al. (1983). The quantum yields and products of the photolysis are reported for various wavelength bands between 240 and 480 nm. The dominant pathway for photolysis at wavelengths relevant to the troposphere produces two radicals, CH3COCHO + hν → CH3CO + HCO. The measured absorption cross sections and quantum yields are used to determine a methylglyoxal lifetime for photolysis of (2.7±0.7) hours, for a solar zenith angle of 0–60°. The kinetics and mechanism for reaction of HO2 with methylglyoxal were also determined. Although this reaction is unimportant for the atmospheric oxidation of methylglyoxal, the results were used in the interpretation of the quantum yield experiments.
Observations of depleted ozone within the boundary layer of the western North Atlantic
Authors: Berkowitz Carl M. , Busness Ken M. , Chapman Elaine G. , Thorp John M. , Saylor Rick D.
Published: February 2004

Ozone measurements taken between 0.90 and 2.5 km above the surface and extending over an 800‐km radius from Halifax, Nova Scotia, are presented from aircraft flights between August 21 and September 14, 1992. The mean ozone mixing ratio was found generally to be greater above the top of the mixed layer than near the sea surface. Eleven of the 32 vertical profiles displayed an abrupt transition at the top of the boundary layer, with surface ozone mixing ratios having values of ≈15–20 ppb and values above the boundary layer increasing to ≈50–60 ppb. This transition between low and high mixing ratios was observed to occur over a vertical scale of less than 0.5 km in soundings taken within 4 hours of each other over horizontal distances of the order of several hundred kilometers. There was a well‐mixed boundary layer in all cases where these sudden transitions in the ozone profiles were observed. These profiles are associated with subsidence over land, followed by dry deposition within a hydrocarbon‐poor, well‐mixed continental boundary layer. Ozone loss through surface deposition exceeded ozone production by the time the air masses arrived at the maritime coastal waters. Two other broad categories of profiles are described, the most common having the ozone mixing ratio increasing linearly with height to the top of the sampling domain at 2.5 km. A third category had only a local maximum in ozone with much smaller values higher and lower in the atmosphere.
Investigation of the loss processes for peroxyacetyl nitrate in the atmosphere: UV photolysis and reaction with OH
Authors: Talukdar Ranajit K. , Burkholder James B. , Schmoltner Anne‐Marie , Roberts James M. , Wilson Robert R. , Ravishankara A. R.
Published: February 2004

The UV absorption cross sections of peroxyacetyl nitrate (PAN), CH3C(O)O2NO2, have been measured as a function of temperature (298,273, and 250 K) between 195 and 345 nm using a diode array spectrometer. The absorption cross sections decrease monotonically with increasing wavelength. The cross sections also decrease as the temperature is lowered. Upper limits for the rate coefficients for the reaction of OH with PAN at atmospheric temperatures were determined to be < 3 × 10−14 cm3 molecule−1 s−1 using the pulsed photolysis laser induced fluorescence technique. Photolysis becomes the most important atmospheric loss process for PAN above ∼7 km, and the OH reaction is found to be unimportant throughout the troposphere. These results are compared with previous measurements, and the significance of the revised values on the atmospheric loss rates of PAN is discussed.
The influence of ozone on light nonmethane hydrocarbons during cryogenic preconcentration
Authors: Koppmann R. , Johnen F. J. , Khedim A. , Rudolph J. , Wedel A. , Wiards B.
Published: February 2004

A number of recent measurement series of nonmethane hydrocarbons (NMHCs) based on in situ analysis report very low alkene concentrations in the remote troposphere. It was speculated that during preconcentration or thermal desorption of the sample, atmospheric ozone may react with the reactive hydrocarbons, e.g., alkenes. Therefore the behavior of ozone in different inlet systems at different conditions was investigated, in order to indicate where O3 interferences may arise. The results for the inlet and preconcentration system used for our measurements show that up to 50% of the ambient ozone is lost during passage of a heated stainless steel inlet line. The remaining ozone is preconcentrated together with the hydrocarbons. During the process of thermal desorption the remaining ozone is lost within minutes leading to a loss of reactive hydrocarbons of the order of 2–10% which is usually less than the error of measurement. These results were confirmed when different amounts of ozone were added to samples of pressurized air with moderate and low NMHC concentrations. For ozone mixing ratios of up to 100 ppb no significant change in the concentration of light alkenes was observed. The results show that our system used for cryogenic preconcentration of NMHC with subsequent thermal desorption is suitable for quantitative measurements even of reactive light alkenes in the atmosphere without an additional ozone trap.
Subsidence of the Arctic stratosphere determined from thermal emission of hydrogen fluoride
Authors: Traub Wesley A. , Jucks Kenneth W. , Johnson David G. , Chance Kelly V.
Published: February 2004

We determine subsidence in the Arctic stratospheric vortex from measurements of the column density of hydrogen fluoride, using a rotational line in thermal emission. The data were obtained with the far‐infrared spectrometer on board the NASA DC‐8 aircraft during the Arctic Airborne Stratospheric Expedition, from January through March 1992. A wide range of meteorological conditions was sampled, both inside and outside the polar vortex region. The measured values of subsidence are analyzed as a function of time of year and potential vorticity. From this analysis we draw the following conclusions: (1) Subsidence is strongly correlated with potential vorticity. However, there appear to be other factors, including meteorological conditions, which play a lesser role. (2) The gradient of subsidence with respect to potential vorticity is large and approximately constant across the vortex wall and is small elsewhere. The gradient of subsidence with respect to horizontal distance is dramatically large across the vortex wall. (3) There appears to be substantial descent in the early winter vortex, from the evidence of the observed subsidence in January and the assumption of zero subsidence at the fall equinox. (4) Descent continues to occur in late winter, with measured vertical velocities similar to that inferred for early winter. The average early and late winter vertical velocity at a representative altitude of 18 km is 0.052(±0.013) cm s−1. (5) At the measured rate of subsidence the time required to flush the stratosphere by one scale height is about 6 months.