bluuebunny/crossref_metadata_2025 huggingface dataset:
Water‐air flux of dimethylsulfide
Authors: McGillis W. R. , Dacey J. W. H. , Frew N. M. , Bock E. J. , Nelson R. K.
Published: September 2002

The water‐air exchange of dimethylsulfide (DMS) has been measured in a laboratory wind‐wave tank in fresh and seawater. To understand the transport behavior of DMS, its exchange was measured simultaneously with that of O2, SF6, Ne, CH4, and He under varying wind speeds and hydrodynamic conditions. No unpredictable differences between fresh and seawater were found, indicating that DMS surface affinities do not exist in seawater. Results also indicate that Schmidt number corrections can be applied to DMS gas exchange. While the rate of transfer of relatively insoluble or sparingly soluble permanent gases between the ocean and the atmosphere is primarily controlled by the rate of flux through the aqueous boundary layer, interfacial mass balances indicate that increased DMS solubility increases the significance of the airside control of flux. The hypothesis that DMS transport across the water‐air interface is subject to waterside control for moderate environmental conditions is supported. However, for ocean‐atmospheric DMS exchange with low sea surface temperature or moderate wind speeds, there may be a significant influence by the atmospheric mass boundary layer. An atmospheric gradient fraction γa applied to the waterside air‐sea gas transfer velocity will correct for these effects. Estimates of ocean‐atmospheric DMS transfer velocities for these conditions are provided.
Seasonal and interannual variability of currents and temperature off central Chile
Authors: Shaffer Gary , Hormazabal Samuel , Pizarro Oscar , Salinas Sergio
Published: September 2002

Results are reported for continuous, 6 year records of current and temperature from a continental slope station at 30°S off Chile. These observations for November 1991 to November 1997 span the moderate, weak, and strong El Niño events of 1991–1992, 1994, and 1997 and the La Niña event of 1995–1996. Results for current and temperature are analyzed together with observations of wind along the South American coast from the ERS‐1 and ‐2 satellite scatterometers, wind from a local coastal site, sea level from northern and central Chile, sea surface temperature anomalies from the South Pacific region, and traditional El Niño indices. Mean poleward flow of 12.8 cm s−1 was observed for the 6 year period in the core of the Peru‐Chile Undercurrent (220 m). Mean flow in the depth range of Antarctic Intermediate Water (750 m) was equatorward at 1.1 cm s−1. Intraseasonal‐scale variability associated with coastal trapped waves was weaker during the austral winter: greatest during El Niño events and weakest during the La Niña event. Winds adjacent to the coast showed a strong annual cycle with a southward progression of maximum equatorward (upwelling favorable) winds. Alongshore flow over the slope exhibited strong semiannual and weaker annual variations. Poleward flow in the undercurrent was strongest in the austral spring and fall and weakest in the winter. Near the bottom (750 m), seasonal‐scale variability was less intense but led that higher up in the water column (220 m) by ∼1 month. Strong interannual variability of alongshore current and temperature was found at all depths over the slope, whereby changes at depth led changes higher up. A rapid warming event was observed at the end of 1996‐beginning of 1997 over the slope. This event, also seen in sea level and sea surface temperature off Chile, led El Niño indices by several months and may be best explained by local wind anomalies coupled to atmospheric teleconnections from the western tropical Pacific at the onset of El Niño. Some aspects of our current and temperature observations are consistent with results from eastern boundary current models. Our results indicate that both ocean and atmosphere pathways from the equatorial Pacific contribute significantly to interannual‐scale variability of alongshore flow and thermocline depth off Chile.
Variability in the Gulf Stream recirculation gyre
Authors: Marchese Paul J.
Published: September 2002

Using historical hydrographic data, a salinity front which marks the eastern edge of the Gulf Stream recirculation on the 10°C isotherm surface is tracked. In the region between 25° and 35°N the longitudinal position of the front exhibits considerable seasonal variability not previously detected at this depth. During January and the summer months the edge of the recirculation is at 76°W, while in spring and late autumn the front moves east to 65°W. The change in the longitudinal position of the recirculation front coincides with a meridional displacement of the Gulf Stream path. When the front is in its easternmost position, the Gulf Stream is to the south, while when the front moves to the west, the Gulf Stream shifts north. It is possible that the recirculation reacts to the meridional displacement of the Gulf Stream maintaining the volume within the recirculation constant.
High‐frequency ice motion and divergence in the Weddell Sea
Authors: Padman Laurie , Kottmeier Christoph
Published: September 2002

We describe the spatial variability of high frequency ice velocity in the Weddell Sea using satellite‐tracked ice‐mounted buoys. Ice motion is analyzed separately for “diurnal” (1/36–1/18 cph) and “semidiurnal” (1/18–1/6 cph) bands. Ice motion in both bands is caused by a combination of ocean tidal currents and wind stress. Monthly mean diurnal band ice speeds over the deep basin range from 2 to 4 cm s−1 depending on wind stress variance and ice concentration (Cice). Higher speeds (∼10 cm s−1) are found in the semidiurnal band in regions of low Cice, notably the northern Weddell Sea, where the ice velocity is dominated by the inertial response to wind stress variations. Monthly mean tidal band ice speeds over the continental slope and shelves often exceed 10 cm s−1. We use comparisons between buoy velocities, moored current meter data, and an ocean tidal model to demonstrate that ice motion is frequently a good indicator of ocean tidal currents in strongly tidal regions. The standard deviation of the divergence of ocean tidal currents estimated from an ocean‐only tidal model is small (< 0.1×10−6 s−1) over most of the Weddell Sea but has values of 1‐5×10−6 s−1 along the Ronne Ice Front and the continental shelf break. High frequency ice divergence is dominated by ice response to wind stress rather than by tides except along the shelf break and ice fronts. In these tidally dominated regions the periodic divergence maintains a mean lead (open water) area of ∼2–5%. This increased lead fraction implies an increase in area‐averaged winter ocean‐to‐atmosphere heat exchange rate of ∼4–10 W m−2 and an increase in salt flux into the upper ocean as new ice forms in the leads.
Error analysis and assimilation of remotely sensed ice motion within an Arctic sea ice model
Authors: Meier Walter N. , Maslanik James A. , Fowler Charles W.
Published: September 2002

New sea ice motion fields available from remotely sensed data are potentially useful for assessing and improving models of the polar ice pack. Here we investigate the error characteristics of the observed ice motions relative to drifting buoys and a dynamic‐thermodynamic ice model. A data assimilation approach is then used to assess the potential of the motion data for reducing model biases, as well as the potential of the model to serve as an interpolation tool to generate improved ice motion data sets. Special Sensor Microwave/Imager (SSM/I) derived and model simulated ice motions for the years 1988 through 1993 are compared with ice displacement observations from drifting buoys. Variability and biases are summarized for seasonal and regional means. SSM/I motions are assimilated into the model using an optimal interpolation method that accounts for the modeled and SSM/I motion error variances and the number and distribution of the SSM/I motions. Modeled and SSM/I‐derived motions are found to have comparable mean errors, with some notable regional and seasonal differences. Assimilation substantially reduces the error standard deviation and improves the correlation of the simulated motions relative to the buoy observations, but some biases remain. In the model framework used here, assimilation of the SSM/I data substantially alters average ice thickness in some regions of the Arctic and affects ice mass outflow through the Fram Strait but has a small effect on mean ice concentration. The assimilation yields an increase in the spatial and temporal variability in ice deformation. The observations are particularly suited for improving the simulation of specific synoptic events, where substantial differences can occur between simulated and observed ice transport.
Dynamics of mixed bottom boundary layers and its implications for diapycnal transport in a stratified, natural water basin
Authors: Gloor M. , Wüest A. , Imboden D. M.
Published: September 2002

Here we report on two field experiments from Lake Alpnach (surface area: 4.8 km2; maximum depth: 34 m) that were designed to study the process of boundary mixing and to estimate its efficiency, the ratio between the turbulent kinetic energy converted into potential energy and dissipated into heat, for diapycnal tracer transport. Lake Alpnach is particularly suited for this purpose because it is known from earlier experiments that (1) its currents follow a regular oscillatory pattern, associated with basin‐wide standing internal waves (seiches, period ∼0.5 days and 1 day, respectively), and (2) diapycnal tracer transport is mainly caused by boundary mixing. During one of the experiments reported here, strong seiches were excited regularly and damped on a timescale of the order of 3 days; during the other experiment seiching motion was comparably weak. If seiching is excited regularly, we find a persistent well‐mixed bottom layer of 4–5 m height at the deepest part of the lake. In the absence of regular seiching the layer disappears within 10–20 days. On the sloping bottom boundaries the well‐mixed layers (1) are of much more transient nature, (2) exhibit different thermal structure, and (3) decrease in thickness toward shallower depth inversely proportionally to the stability at the same depth in the lake interior. Their reduced thickness is possibly the result of repeatedly occurring intrusions of boundary mixed water masses that are observed to extend horizontally ∼100–200 m into the lake interior. As a consequence of repeated generation of intrusions, mixing on the sloping boundaries is expected to be considerably more efficient compared to mixing over flat bottom boundaries. The observed mixing efficiency, the ratio of the rate of change of potential energy below depth z caused by turbulence to the energy loss by bottom friction below depth z, increases indeed from 0.01±0.01 in the deepest well‐mixed layers to 0.15±0.04 in the upper pycnocline.
Impact of multiple submarine channels on the descent of dense water at high latitudes
Authors: Kämpf Jochen
Published: September 2002

A three‐dimensional numerical hydrodynamic model is applied to examine the impact of multiple submarine channels (<10 km across, <100 m deep), common to most continental margins of the ocean, on the descent of dense water at high latitudes. The model consists of an ocean bottom layer of constant height that follows variable bottom topography under constant vertical grid spacing. An idealized continental slope of constant bottom slope is considered, including parallel channels that run perpendicular to main isobaths. The ocean is initially homogeneous and at rest. Forcing is due to a layer of dense water prescribed along the upslope boundary. When the channel aspect ratio (ratio of width to depth) exceeds the main bottom slope, dense water is carried downslope by narrow channel plumes centered along the channel axes. Owing to a small internal Rossby radius less than the channel width the plume dynamics are governed by a geostrophic balance across the channels. Interaction of adjacent channel plumes leads to complex bottom‐parallel circulations. The net downslope density flux resulting from these circulations exceeds that of viscous (ageostrophic) flow of dense water developing without channels. When the channel aspect ratio is less than the main bottom slope, the descent of dense water is dominated by viscous flow. Narrow geostrophic circulation patterns along channels, superimposed on the mean flow, however, induce advective entrainment of lighter ambient water across the leading density front of descending water. As a result of this, the net downslope density flux is reduced as compared to that without channels. Sensitivity studies reveal that the channel‐modified dynamics are independent of the magnitude of the eddy viscosity.
An assessment of local coastal dynamics observed with high‐frequency radar
Authors: Shen Colin Y. , Fusina Robert A. , Shay Lynn K.
Published: September 2002

An assessment is made of the dominant processes governing the circulation in a 30 km by 40 km area off the coast of Cape Hatteras, North Carolina, where surface current velocity vectors have been obtained using shore‐based high‐frequency radars in the High Resolution Remote Sensing experiment in 1993. Although the currents in this region are constantly under the influence of winds and tides, frequent intrusion of energetic flows into the area, apparently mostly of Gulf Stream origin, is shown to have a strong influence on the current variability. Analysis of vorticity and divergence shows that the flow intruding into the area is strongly rotational with positive vorticity. This rotational motion is not in geostrophic balance with the surface pressure gradient, and an inertial oscillation is often excited as a result, with vorticity and divergence oscillating 90° out of phase. On average the geostrophic imbalance generates a net surface flow divergence, which, in turn, induces negative vorticity that more than offsets the positive vorticity advected into the region. However, vertical viscous transfer of positive vorticity from subsurface to surface is shown to be required to complete the surface vorticity balance; the same energetic cyclonic shear flows that intrude into the area are suggested as the source of the subsurface vorticity as these flows of Gulf Stream origin are known to be subducted under buoyant shelf water.
Lidar observations of the middle atmospheric thermal tides and comparison with the High Resolution Doppler Imager and Global Scale Wave Model: 2. October observations at Mauna Loa (19.5°N)
Authors: Leblanc Thierry , McDermid I. Stuart , Ortland David A.
Published: September 2002

Using more than 145 hours of nighttime lidar measurements obtained during October 3–16, 1996, and October 2–11, 1997, the tidal signature in the middle atmospheric thermal structure (15–95 km) at Mauna Loa, Hawaii (19.5°N), was investigated. The daytime High Resolution Doppler Imager (HRDI) temperatures taken in September and October 1993–1997 and zonally averaged at the same latitude were also used. The daytime HRDI and nighttime lidar temperature differences from their respective daytime and nighttime averages were compared to the equivalent differences predicted by the Global Scale Wave Model (GSWM) at the same latitude. Some consistent local solar time (LST)‐related structures were observed in both HRDI and lidar data, suggesting the presence of important migrating tidal components. In particular, a warm period was clearly identified, propagating downward from 105 km at 0800 LST to 65 km at 0000 LST and surrounded by two colder periods above and below. These warm/cold periods were predicted to occur 2 to 3 hours later by GSWM compared to the HRDI observations. Other LST‐related structures were observed by lidar between 30‐ and 80‐km altitude, in particular, a colder early night, warmer midnight, and colder late night around ∼70 km, suggesting a significant semidiurnal component at this altitude. As previously observed, the amplitudes predicted by GSWM were much smaller than those observed by lidar and HRDI. A new analysis “constrained wave adjustment” method described in a companion paper [Leblanc et al., this issue] was used to estimate the diurnal and semidiurnal components from the nighttime‐only lidar data. The main point of disagreement between the lidar observations and GSWM predictions occurred between 60 and 85 km. A large semidiurnal component was observed by lidar, leading to early and late cold night and warm midnight, while no such large semidiurnal component was predicted by GSWM, leading to an apparent warm early night at 60 km and an apparent cold midnight at 80 km and above. It appears that the tidal structure observed by lidar is more representative of that predicted by GSWM at 24°N, suggesting a latitudinal shift between theory and observation. It is not clear whether this shift is related to an indetermination of the tidal source and/or propagation or if the observed differences are simply due to oscillations related to local/regional local solar time obscuring the global tidal signature.
Effects of clouds on ozone column retrieval from GOME UV measurements
Authors: Koelemeijer R. B. A. , Stammes P.
Published: September 2002

The Global Ozone Monitoring Experiment (GOME) on board the European Space Agency's ERS‐2 satellite is a spectrometer measuring the Earth's reflectivity between 240 and 790 nm. The main geophysical product from GOME is the ozone vertical column density, or ozone column. To obtain the ozone column, the ozone slant column density, which is derived from the GOME reflectivity measurements between 325 and 335 nm using the Differential Optical Absorption Spectroscopy (DOAS) method, is divided by the air mass factor. The air mass factor represents the optical path length in the atmosphere. Clouds generally enhance the air mass factor; however, clouds also screen part of the ozone below them. Therefore, in an accurate ozone retrieval method the presence of clouds should be taken into account. We investigated the influence of errors in cloud parameters on the retrieved ozone column by performing a sensitivity study with a detailed radiative transfer model including polarization. We show that the influence of clouds on the retrieved ozone column depends mainly on the cloud top height, cloud albedo, and cloud fraction. For low and midlevel clouds an overestimation of the cloud fraction yields an underestimation of the ozone column; for high clouds, the opposite is true. An overestimation of the cloud top height always yields an overestimation of the ozone column. Using a cloud albedo of 0.8, and assuming errors in the GOME cloud fraction and cloud top height of ±0.2 and ±3 km, respectively, the errors in the retrieved ozone column are within 8, 6, and 5%, for solar zenith angles of 30°, 60°, and 75°, respectively.
Nitrogen species in the post‐Pinatubo stratosphere: Model analysis utilizing UARS measurements
Authors: Danilin Michael Y. , Rodriguez Jose M. , Hu Wenjie , Ko Malcolm K. W. , Weisenstein Debra K. , Kumer John B. , Mergenthaler John L. , Russell James M. , Koike Makoto , Yue Glenn K. , Jones Nicholas B. , Johnston Paul V.
Published: September 2002

We present an analysis of the impact of heterogeneous chemistry on the partitioning of nitrogen species measured by the Upper Atmosphere Research Satellite (UARS) instruments. The UARS measurements utilized include N2O, HNO3, and ClONO2 from the cryogenic limb array etalon spectrometer (CLAES), version 7 (v.7), and temperature, methane, ozone, H2O, HCl, NO and NO2 from the halogen occultation experiment (HALOE), version 18. The analysis is carried out for the UARS data obtained between January 1992 and September 1994 in the 100‐ to 1‐mbar (∼17–47 km) altitude range and over 10° latitude bins from 70°S to 70°N. The spatiotemporal evolution of aerosol surface area density (SAD) is adopted from analysis of the Stratospheric Aerosol and Gas Experiment (SAGE) II data. A diurnal steady state photochemical box model, constrained by the temperature, ozone, H2O, CH4, aerosol SAD, and columns of O2 and O3 above the point of interest, has been used as the main tool to analyze these data. Total inorganic nitrogen (NOy) is obtained by three different methods: (1) as a sum of the UARS‐measured NO, NO2, HNO3, and ClONO2; (2) from the N2O‐NOy correlation, and (3) from the CH4‐NOy correlation. To validate our current understanding of stratospheric heterogeneous chemistry for post‐Pinatubo conditions, the model‐calculated monthly averaged NOx/NOy ratios and the NO, NO2, and HNO3 profiles are compared with the UARS‐derived data. In general, the UARS‐constrained box model captures the main features of nitrogen species partitioning in the post‐Pinatubo years, such as recovery of NOx after the eruption, their seasonal variability and vertical profiles. However, the model underestimates the NO2 content, particularly in the 30‐ to 7‐mbar (∼23–32 km) range. Comparisons of the calculated temporal behavior of the partial columns of NO2 and HNO3 and ground‐based measurements at 45°S and 45°N are also presented. Our analysis indicates that ground‐based and HALOE v. 18 measurements of the NO2 vertical columns are consistent within the range of their uncertainties and are systematically higher (up to 50%) than the model results at midlatitudes in both hemispheres. Reasonable agreement is obtained for HNO3 columns at 45°S, suggesting some problems with nitrogen species partitioning in the model. Outstanding uncertainties are discussed.
On the role of nitrogen dioxide in the absorption of solar radiation
Authors: Solomon S. , Portmann R. W. , Sanders R. W. , Daniel J. S. , Madsen W. , Bartram B. , Dutton E. G.
Published: September 2002

Direct measurements of the absorption of downwelling visible radiation by nitrogen dioxide are presented. The data show that this gas can contribute significantly to local radiative forcing under certain conditions. The observed enhancements in nitrogen dioxide absorption are likely to be due both to pollution and to production by lightning in convective clouds. Case studies of several days of observations in Colorado reveal peak absorption of downwelling radiation by NO2 of up to 5–12%, corresponding to an estimated local radiative forcing that is likely to be in the range of 5–30 W/m2. The amount of local forcing associated with thunderstorm activity depends strongly upon the cloud optical depth and on where the NO2 resides within the clouds. These case studies suggest that NO2 can play a significant role in the absorption of radiation (including but not limited to anomalous cloud absorption) either under polluted conditions or when electrically active storms are considered.
Atmospheric moisture transport across the Southern Ocean via satellite observations
Authors: Slonaker Richard L. , Van Woert Michael L.
Published: September 2002

Meridional moisture transport across the Southern Ocean was measured using a combination of TIROS operational vertical sounder and special sensor microwave/imager satellite data from 1988. Satellite and radiosonde estimates of moisture flux compare favorably near Macquarie Island (54.5°S, 158.9°). The zonally averaged, vertically integrated, moisture flux values across 50°S and 60°S are −31.8 and −15.2 kg m−1 s−1, respectively. Resulting moisture convergence for the 50°–60°S latitude band is 636±50 kg m−2 yr−1, about 50% larger than indicated by contemporary studies using radiosondes or atmospheric analyses. Within this latitude band the poleward moisture transport peaks in the South Pacific sector near 135°E, with a secondary maximum in the Amundsen Sea sector close to 135°W. There is a dramatic moisture flux discontinuity across the Antarctic peninsula and the tip of South America near 45°W, with equatorward moisture transport at all latitudes in the Weddell Sea sector. A regional study of West Antarctica indicates large poleward moisture flux is accomplished through the Bellingshausen and Amundsen Seas only.
Prediction of the number of cloud droplets in the ECHAM GCM
Authors: Lohmann Ulrike , Feichter Johann , Chuang Catherine C. , Penner Joyce E.
Published: September 2002

In this paper a prognostic equation for the number of cloud droplets (CDNC) is introduced into the ECHAM general circulation model. The initial CDNC is based on the mechanistic model of Chuang and Penner [1995], providing a more realistical prediction of CDNC than the empirical method previously used. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and a shape parameter, which takes the aerosol composition and size distribution into account. The total number of aerosol particles is obtained as the sum of marine sulfate aerosols produced from dimethyl sulfide, hydrophylic organic and black carbon, submicron dust, and sea‐salt aerosols. Anthropogenic sulfate aerosols only add mass to the preexisting aerosols but do not form new particles. The simulated annual mean liquid water path, column CDNC, and effective radius agree well with observations, as does the frequency distributions of column CDNC for clouds over oceans and the variations of cloud optical depth with effective radius.
Direct deposition of subsonic aircraft emissions into the stratosphere
Authors: Gettelman Andrew , Baughcum Steven L.
Published: September 2002

The direct deposition of emissions from the 1992 aircraft fleet is studied using different aircraft emissions inventories, meteorological data from 1983–1996, and definitions of the tropopause. Globally, between 18 and 44% of current emissions of water vapor and nitrogen oxides are deposited in the stratosphere, depending on the exact definition of the tropopause and on season, in agreement with previous work. Largest emissions in the stratosphere are found to occur in January. The sensitivity of this result is tested with respect to the definition of the tropopause, the data used to analyze the tropopause, and the interseasonal and interannual variability of tropopause height. On a global basis, the stratospheric deposition is most sensitive to the definition of the tropopause. Approximately 20% of emissions are within ±1 km of the tropopause, and 35% are within ±2 km of the tropopause. Deposition into the stratosphere is not significantly affected by the choice of meteorological data set or by the interannual variability of the tropopause. These results have important implications for chemical model studies of aircraft emissions impact because they highlight the sensitivity to the exact flight altitudes, choice of tropopause height, and resolution in the tropopause region.
Exchange of methane from rice fields: National, regional, and global budgets
Authors: Sass R. L. , Fisher F. M. , Ding A. , Huang Y.
Published: September 2002

Over the past decade, several estimates have been developed to establish source strengths of various atmospheric trace gases. Interest is, in part, driven by international agreements wherein governments are required to establish emission inventories and to develop means to stabilize or reduce national emissions. Although techniques and models for quantifying gas fluxes have improved considerably for some gases and sources, the uncertainties in the national, regional, and global budgets for a number of atmospheric compounds remain. In this paper, we review estimates of the emission of methane from rice fields both on a global scale and on a country scale, using China as an example. Several reported methods of obtaining an annual methane emission value for China are reviewed. A comparison among them yields a suggested emission value of 13.0±3.3 Tg yr−1 and a range from 9.7 to 16.2 Tg yr−1. Several strategies are suggested for future requirements to obtain greater accuracy in regional and country emission values.
A one and half year interactive MA/ECHAM4 simulation of Mount Pinatubo Aerosol
Authors: Timmreck Claudia , Graf Hans‐F. , Kirchner Ingo
Published: September 2002

The Mount Pinatubo volcanic eruption in June 1991 had significant impact on stratospheric and tropospheric climate and circulation. Enhanced radiative heating caused by the aerosol absorption of solar and terrestrial radiation changed stratospheric temperature and circulation. Using the stratospheric mesospheric version of the Hamburg climate model MA/ECHAM4, we performed an interactive Pinatubo simulation with prognostic stratospheric aerosol. Interactive and noninteractive model results for the years 1991 and 1992 are compared with satellite data and in situ measurements. The on‐line calculated heating rates are in good agreement with radiation transfer models indicating maximum heating rates of about 0.3 K/d in October 1991. The dynamic feedback in the MA/ECHAM4 simulation is similar to observations. The model is able to reproduce the strengthening of the polar vortex in winter 1991/1992 and a minor warming in January. The importance of an interactive treatment of the volcanic cloud for the aerosol transport is evidenced by the analysis of effects such as aerosol lifting and meridional transport. In general, the model results agree well with observations from the northern midlatitudes, especially in the first months after the eruption. The MA/ECHAM4 model is successful in reproducing the formation of two distinct maxima in the optical depth but is unable to simulate the persistence of the tropical aerosol reservoir from the end of 1991. Better agreement may be achieved if the influence of the quasi‐biennial oscillation and ozone changes is also taken into account.
Measuring gravity wave momentum fluxes with airglow imagers
Authors: Gardner Chester S. , Gulati Kapil , Zhao Yucheng , Swenson Gary
Published: September 2002

We describe a new technique for estimating the vertical fluxes of horizontal momentum in the mesopause region using airglow imager data. The azimuthal distribution of the relative airglow intensity or rotational temperature variance associated with both monochromatic and quasi‐random waves is computed from the unambiguous two‐dimensional horizontal wave number spectrum. The variance distribution is used in a gravity wave model to predict the cross‐correlation coefficient between the vertical and horizontal wind perturbations (and hence the momentum flux) as a function of azimuth angle. The technique is applied to OH imager data collected during five nights in February and April 1995 at the Starfire Optical Range, New Mexico. To validate the technique, the cross‐correlation coefficients and momentum fluxes derived from the imager data are compared with the zonal and meridional fluxes measured by a co‐located Na wind/temperature lidar employing the standard dual‐beam Doppler technique. The mean zonal and meridional cross‐correlation coefficients and momentum fluxes inferred from the OH imager data are −0.27 ± 0.011 (−11.9 ± 0.49 m2/s2) and 0.009 ± 0.011 (0.44 ± 0.53 m2/s2), respectively. The corresponding zonal and meridional coefficients and fluxes measured by the lidar are −0.26 ± 0.09 (−11.9 ± 3.9 m2/s2) and −0.081 ± 0.10 (−3.76 ± 4.7 m2/s2), respectively. These results demonstrate that creditable estimates of momentum fluxes at mesopause heights can be derived from OH image data.
Anthropogenic influences on the chemical and physical properties of aerosols in the Atlantic subtropical region during July 1994 and July 1995
Authors: McGovern F. M. , Raes F. , Van Dingenen R. , Maring H.
Published: September 2002

Elevated pollution levels have been observed at marine boundary layer (MBL) and free troposphere (FT) sites on the island of Tenerife. These have been linked to air mass transport from continental areas, with over 40% of the measurements being directly influenced by continental sources. The influence of different meteorological patterns on this transport has been examined. Saharan dust was an important component of the FT aerosol. In the MBL, modification of aerosol light scattering and cloud nucleating properties correlated strongly with enhanced non‐sea‐salt (nss)SO42 levels, linked to transport from Europe. Regression analysis of cloud condensation nuclei (CCN) concentrations estimated at supersaturation levels of 0.15% and 0.75% gave CCN = 95 nssSO−24 + 139(0.15%) and CCN = 142 nssSO−24 + 256(0.75%), respectively. Regression analysis of submicron aerosol scattering at 530 nm gave a sulfate mass scattering efficiency value of 6.3 m2 g−1. Sea salt was the dominant supermicron scattering species in the MBL. A mass scattering efficiency of 2.9 m2 g−1 is estimated for Na+.
Role of convection in determining the budget of odd hydrogen in the upper troposphere
Authors: Collins W. J. , Stevenson D. S. , Johnson C. E. , Derwent R. G.
Published: September 2002

This paper presents a model study of the changes in upper tropospheric HOx ( = OH + HO2) due to upward convective transport of surface pollutants. The model used is a three‐dimensional global Lagrangian tropospheric chemistry transport model of 70 chemical species and 150 reactions including nonmethane hydrocarbon chemistry. It is driven by meteorological data from the U.K. Meterological Office with a 6 hour time resolution. We find that the effect of convection is to increase upper tropospheric (300–200 hPa) HOx globally by over 50%. The effect is greatest over the tropical continents where convection and VOC emissions from vegetation are colocated. The convection of isoprene, and hydroperoxides has the greatest effect. Convecting formaldehyde and acetone has a lesser effect. The contribution from isoprene depends more on the convection of its degradation products than the convection of isoprene itself. The upper tropospheric HOx budget is shown to be very sensitive to the model implementation of convective wet deposition.
Reactive uptake on hydrocarbon soot: Focus on NO2
Authors: Longfellow Cheryl A. , Ravishankara A. R. , Hanson David R.
Published: September 2002

The heterogeneous reactivity of NO2 with soot, two combustion products typically produced in close proximity, may have significant implications for the chemistry occurring in the atmosphere, especially the urban troposphere. As the soot produced in the environment is not well characterized, the interaction of NO2 with a number of different types of laboratory‐generated soot was studied. The production of HONO was observed in the interaction of NO2 with methane, propane, hexane, and kerosene soot. It was found that soot produced from hydrocarbons catalyzed the reaction of NO2 with H2O to give HONO. Reactive uptake coefficients for this process that are measured in the laboratory may not be easily applied to atmospheric calculations without more information.
Tropospheric O3 distribution over the Indian Ocean during spring 1995 evaluated with a chemistry‐climate model
Authors: de Laat A. T. J. , Zachariasse M. , Roelofs G. J. , van Velthoven P. , Dickerson R. R. , Rhoads K. P. , Oltmans S. J. , Lelieveld J.
Published: September 2002

An analysis of tropospheric O3 over the Indian Ocean during spring 1995 is presented based on O3 soundings and results from the European Centre Hamburg (ECHAM) chemistry‐general circulation model. The ECHAM model is nudged toward actual meteorology using European Centre for Medium‐Range Weather Forecasts analyses, to enable a direct comparison between model results and in situ observations. The model reproduces observed CO levels in different air mass categories. The model also reproduces the general tendencies and the diurnal variation in the observed surface pressure, although the amplitude of the diurnal variation in the amplitude is underestimated. The model simulates the general O3 tendencies as seen in the sonde observations. Tropospheric O3 profiles were characterized by low surface concentrations (<10 ppbv), midtropospheric maxima (60–100 ppbv, at 700–250 hPa) and upper tropospheric minima (<20 ppbv, at 250–100 hPa). Large‐scale upper tropospheric O3 minima were caused by connective transport of O3‐depleted boundary layer air in the intertropical convergence zone (ITCZ). Similarly, an upper tropospheric O3 minimum was caused by Cyclone Marlene south of the ITCZ. The midtropospheric O3 maxima were caused by transport of polluted African air. The ECHAM model appears to overestimate surface O3 levels and does not reproduce the diurnal variations very well. This could be related to unaccounted multiphase O3 destruction mechanisms involving low level clouds and aerosols, and missing halogen chemistry.
Concentrations and snow‐atmosphere fluxes of reactive nitrogen at Summit, Greenland
Authors: Munger J. W. , Jacob D. J. , Fan S.‐M. , Colman A. S. , Dibb J. E.
Published: September 2002

Concentrations and fluxes of NOy (total reactive nitrogen), ozone concentrations and fluxes of sensible heat, water vapor, and momentum were measured from May 1 to July 20, 1995 at Summit, Greenland. Median NOy concentrations declined from 947 ppt in May to 444 ppt by July. NOy fluxes were observed into and out of the snow, but the magnitudes were usually below 1 μmol m−2 h−1 because of the low HNO3 concentration and weak turbulence over the snow surface. Some of the highest observed fluxes may be due to temporary storage by equilibrium sorption of peroxyacetylnitrate (PAN) or other organic nitrogen species on ice surfaces in the upper snowpack. Sublimation of snow at the surface or during blowing snow events is associated with efflux of NOy from the snowpack. Because the NOy fluxes during summer at Summit are bidirectional and small in magnitude, the net result of turbulent NOy exchange is insignificant compared to the 2 μmol m−2 d−1 mean input from fresh snow during the summer months. If the arctic NOy reservoir is predominantly PAN (or compounds with similar properties), thermal dissociation of this NOy is sufficient to support the observed flux of nitrate in fresh snow. Very low HNO3 concentrations in the surface layer (1% of total NOy) reflect the poor ventilation of the surface layer over the snowpack combined with the relatively rapid uptake of HNO3 by fog, falling snow, and direct deposition to the snowpack.
An analysis of Polar Ozone and Aerosol Measurement (POAM) II Arctic polar stratospheric cloud observations, 1993–1996
Authors: Fromm Michael D. , Bevilacqua Richard M. , Hornstein John , Shettle Eric , Hoppel Karl , Lumpe Jerry D.
Published: September 2002

The Polar Ozone and Aerosol Measurement (POAM) II instrument made numerous observations of polar stratospheric clouds (PSCs) in the northern hemisphere winters of 1993/1994 through 1995/1996. An updated POAM II PSC detection algorithm, described herein, is applied to POAM II 1060 nm aerosol extinction profiles to distinguish PSCs from noncloud measurements. The impact of the updated algorithm on previously published Antarctic PSC statistics is discussed. Operating sporadically in the 1993/1994 PSC season (defined as November through April), but continuously in the following two winters, POAM II made approximately 340 PSC profile measurements. We analyze the POAM II PSC observations with respect to polar vortex location, temperature, longitude, time, and altitude. Daily PSC probability (defined as the number of PSC profiles inside the polar vortex relative to the number of all profiles located inside the vortex) exceeds 60% during the most intense PSC episodes. There is considerable year‐to‐year variability in PSC probability, preferred location, and timing of onset and final appearance. The POAM II data also reveal interannual differences in the seasonal change of PSC altitude. Certain observations of opaque clouds are used to infer Type II PSCs. The pattern of observed PSCs is discussed with respect to recent studies of Arctic ozone loss.
Sensitivity of multiparameter radar rainfall algorithms
Authors: Gorgucci Eugenio , Scarchilli Gianfranco , Chandrasekar V.
Published: September 2002

The most commonly used polarimetric radar measurements in rainfall estimation are reflectivity at one polarization (ZH), differential reflectivity (ZDR), and specific differential propagation phase (KDP). The reflectivity measurement requires knowledge of absolute power and is prone to calibration errors. Differential reflectivity is a relative power measurement and is not affected by absolute calibration errors. However, all the algorithms that use differential reflectivity also include reflectivity and therefore are prone to absolute calibration errors. Algorithms to estimate rainfall from specific differential propagation phase are immune to absolute calibration error and attenuation effects [Zrnic and Ryzhkov, 1996]. However, specific differential propagation phase is very noisy at low rain rates. In addition, specific differential propagation phase is estimated as the slope of differential propagation phase measurements over a path. Consequently, there is a trade‐off between accuracy and resolution of KDP. Thus there are advantages and disadvantages of each multiparameter radar measurement that translate into the error structure of algorithms involving multiparameter radar measurements. This paper presents a quantitative evaluation of the performance of five different polarimetric radar rainfall algorithms. The performance of the five algorithms is evaluated in the presence of physical variability in rainfall, radar measurement errors, and systematic calibration error.
Water vapor measurements from the troposphere to the lowermost stratosphere: Some signatures of troposphere to stratosphere exchanges
Authors: Ovarlez Joëlle , van Velthoven Peter , Schlager Hans
Published: September 2002

In situ water vapor measurements performed in the upper troposphere and the lower stratosphere, from an aircraft, in the North Atlantic region around 50°N, are analyzed. It is shown that the water vapor content observed in the region located slightly above the local tropopause is consistent with the transport from troposphere to stratosphere at middle and high latitudes. It appears that there is a seasonal variation with a higher water vapor content during summer. The source of this variation could be due to the difference in the relative contribution of the troposphere to stratosphere isentropic and convective transport as well as transport from the upper levels of the stratosphere due to the general circulation.
Nitric acid in the middle stratosphere as a function of altitude and aerosol loading
Authors: Jucks K. W. , Johnson D. G. , Chance K. V. , Traub W. A. , Salawitch R. J.
Published: September 2002

We present remote‐sensing measurements of the abundance of nitric acid (HNO3) in the lower and middle stratosphere (between 16 and 40 km) covering the period 1989–1997. The measurements were made with the Smithsonian Astrophysical Observatory Far‐Infrared Spectrometer (FIRS‐2) under a wide range of aerosol surface area density. We compare our measurements with the results of a photochemical steady state model to test our understanding of the chemistry of HNO3 under a variety of conditions. We find that HNO3 is significantly overestimated by the model at altitudes above 22 km, with the difference increasing with increasing altitude and decreasing aerosol surface area density. The agreement between modeled and measured HNO3 can be improved by either decreasing the rate of OH + NO2 by 35% or by using newly measured rate constants for the reactions OH + NO2 and OH + HNO3, but significant differences remain. We discuss these observations in the context of possible uncertainties in the calculated photolysis rate of HNO3 at wavelengths near 200 nm, uncertainties in the observations, errors caused by the use of constrained steady state models, and possible missing sink reactions for HNO3.
Global simulation of atmospheric mercury concentrations and deposition fluxes
Authors: Shia Run‐Lie , Seigneur Christian , Pai Prasad , Ko Malcolm , Sze Nien Dak
Published: September 2002

Results from a numerical model of the global emissions, transport, chemistry, and deposition of mercury (Hg) in the atmosphere are presented. Hg (in the form of Hg(0) and Hg(II)) is emitted into the atmosphere from natural and anthropogenic sources (estimated to be 4000 and 2100 Mg yr−1, respectively). It is distributed between gaseous, aqueous and particulate phases. Removal of Hg from the atmosphere occurs via dry deposition and wet deposition, which are calculated by the model to be 3300 and 2800 Mg yr−1, respectively. Deposition on land surfaces accounts for 47% of total global deposition. The simulated Hg ambient surface concentrations and deposition fluxes to the Earth's surface are consistent with available observations. Observed spatial and seasonal trends are reproduced by the model, although larger spatial variations are observed in Hg(0) surface concentrations than are predicted by the model. The calculated atmospheric residence time of Hg is ∼1.7 years. Chemical transformations between Hg(0) and Hg(II) have a strong influence on Hg deposition patterns because Hg(II) is removed faster than Hg(0). Oxidation of Hg(0) to Hg(II) occurs primarily in the gas phase, whereas Hg(II) reduction to Hg(0) occurs solely in the aqueous phase. Our model results indicated that in the absence of the aqueous reactions the atmospheric residence time of Hg is reduced to 1.2 from 1.7 years and the Hg surface concentration is ∼25% lower because of the absence of the Hg(II) reduction pathway. This result suggests that aqueous chemistry is an essential component of the atmospheric cycling of Hg.
Comparison of MkIV balloon and ER‐2 aircraft measurements of atmospheric trace gases
Authors: Toon G. C. , Blavier J.‐F. , Sen B. , Margitan J. J. , Webster C. R. , May R. D. , Fahey D. , Gao R. , Del Negro L. , Proffitt M. , Elkins J. , Romashkin P. A. , Hurst D. F. , Oltmans S. , Atlas E. , Schauffler S. , Flocke F. , Bui T. P. , Stimpfle R. M. , Bonne G. P. , Voss P. B. , Cohen R. C.
Published: September 2002

On May 8, 1997, vertical profiles of over 30 different gases were measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight launched from Fairbanks, Alaska. These gases included H2O, N2O, CH4, CO, NOx, NOy, HCl, ClNO3, CCl2F2, CCl3F, CCl4, CHClF2, CClF2CCl2F, SF6, CH3Cl, and C2H6, all of which were also measured in situ by instruments on board the NASA ER‐2 aircraft, which was making flights from Fairbanks during this same early May time period as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiment. A comparison of the gas volume mixing ratios in the upper troposphere and lower stratosphere reveals agreement better than 5% for most gases. The three significant exceptions to this are SF6 and CCl4 for which the remote measurements exceed the in situ observations by 15–20% at all altitudes, and H2O for which the remote measurements are up to 30% smaller than the in situ observations near the hygropause.