bluuebunny/crossref_metadata_2025 huggingface dataset:
Remote sensing of the plasmapause during substorms: Geotail observation of nonthermal continuum enhancement
Authors: Kasaba Y. , Matsumoto H. , Hashimoto K. , Anderson R. R. , Bougeret J.‐L. , Kaiser M. L. , Wu X. Y. , Nagano I.
Published: September 2002

The continuum enhancement, another type of terrestrial nonthermal continuum radiation, has been frequently observed by the Geotail spacecraft. This radiation is a short‐lived enhancement generated at the plasmapause from the midnight to dawnside sector. Simultaneous Geotail and Wind observation shows that usual nonthermal continuum radiation (the normal continuum) generated at the dayside plasmapause appears to follow the continuum enhancement. This suggests that both radiations are generated by a series of electrons injected at the midnight sector associated with the same substorm. The continuum enhancement regularly consists of “fast” and “main” components. The fast component has faster rising rate of average frequency (+50∼+100 kHz/h) and shorter duration of 0.5–1 hours. The main component has slower rising rate of average frequency (+10∼+20 kHz/h) and longer duration of 1–3 hours. We suggest that the former is generated by the lower‐energy electrons at the midnight plasmapause, while the latter is generated by the higher‐energy electrons at the dawnside plasmapause. However, the harmonic structure of the continuum enhancement indicates the radius of the source on the plasmapause close to the magnetic equatorial plane. We find that the radius of the plasmapause first reduces at the rate of −1.0 ∼ −0.5 RE/h in the first 1 hour after the substorm onset. This shows a typical scale of the peeling off of plasma from the outer plasmasphere associated with individual substorms. When geomagnetic activity is low we also find the reversal expansion of the plasmapause at the rate of +0.1 ∼ +0.5 RE/h in the next 1 hour. The expansion rate is faster than the value expected from the upwelling from the upper ionosphere. This suggests that fast compression and recovery processes should also affect the radial motion of the plasmapause.
MU radar observations of H+ ions in the topside ionosphere
Authors: Kawamura S. , Otsuka Y. , Oliver W. L. , Balan N. , Fukao S.
Published: September 2002

We report the detection of H+ ions in the topside ionosphere above the middle and upper atmosphere (MU) radar in Japan, the first such detection by a mesosphere‐stratosphere‐troposphere radar extended to perform as an incoherent scatter radar. Owing to the very limited signal‐to‐noise ratios of ∼1% achievable in the topside ionosphere with this radar, long integration times of 45 min and statistical analysis of 10 years of data are employed to yield useful results. This sensitivity restriction limits the top altitude of observation to only ∼400 km during solar‐minimum winter but up to ∼650 km during solar‐maximum summer. These measurements are possible only at night, when we may assume that the electrons and ions have the same temperature. While the data scatter is large, climatological averages constructed for various solar‐activity, seasonal, and magnetic activity cases show less than ∼5% H+ at most under conditions of detect ability, although it is exactly those conditions of undetectability which are expected to correspond to the highest proportions of H+. At solar maximum we see no detectable amounts of H+ up to 650 km altitude, except perhaps for a faint amount in winter. At solar minimum the data are good up to ∼550 km during summer and equinox but to only ∼400 km during winter. The H+ profiles for solar‐minimum summer and equinox closely resemble each other and reach ∼5% at 550 km altitude. We see no clearly discernible H+ in winter up to our 400 km altitude limit of detectability; we expect that there is substantial H+ present above 400 km, but the total ion density is so low that we cannot measure it. We see no detectable change in H+ between magnetically quiet and disturbed periods. The H+ proportions seen above the MU radar are much smaller than those seen above Arecibo, which has a similar magnetic latitude but much lower geographic latitude, and are much more comparable to those amounts seen by the higher‐latitude radars at Saint Santin and Millstone Hill. Comparisons with models suggest that at these low altitudes, H+ may well be near its charge‐exchange‐equilibrium value.
Electromagnetic proton/proton instabilities in the solar wind
Authors: Daughton William , Gary S. Peter
Published: September 2002

Electromagnetic proton/proton instabilities are excited by the relative streaming parallel to the magnetic field of two distinct proton components, the more dense core and the more tenuous beam. Here linear Vlasov theory is used to study these instabilities in a homogeneous plasma model. Under conditions often observed in the high‐speed solar wind, both magnetosonic and Alfvén modes become proton/proton unstable; for various parameter domains there are two unstable regimes of the magnetosonic mode and three unstable regimes of the Alfvén mode. In a dimensionless parameter model representing typical high‐speed solar wind conditions, the most strongly unstable modes are the magnetosonic instability with maximum growth rate in the direction of the background magnetic field and the Alfvén mode at propagation oblique to that field. Although the former mode has been regarded by several previous researchers to be the dominant proton/proton instability in the solar wind, the results described here indicate that the strongly unstable regime of the Alfvén mode, which previously has not been studied in a solar wind model, has the lower threshold at sufficiently large beam density and/or sufficiently small core β. Earlier studies of proton/proton instabilities in the solar wind based on the assumption that the magnetosonic mode is most important may need reconsideration.
Climatology and variability of the semidiurnal tide in the lower thermosphere over Millstone Hill
Authors: Goncharenko L. P. , Salah J. E.
Published: September 2002

Data obtained by the Millstone Hill incoherent scatter radar during Lower Thermosphere Coupling Study campaigns in 1987–1997 were analyzed in order to determine the characteristics of the semidiurnal variations in horizontal neutral winds and temperatures at midlatitudes in the altitude range of 95–130 km. We present altitude profiles of amplitude and phase of the semidiurnal variations for climatological mean data, seasonally averaged data, as well as individual multiday campaigns. The climatologically mean semidiurnal amplitude attains ∼50m/s for eastward wind, ∼35m/s for the northward wind, and ∼27 K for the temperature. At most altitudes, the standard deviation is 50–70% of the magnitude of the climatological mean value, and a large part of the observed variability is due to seasonal differences. The strongest semidiurnal oscillations are found for equinox and summer winds, reaching 80–90 m/s, while winter amplitudes are generally smaller. Summer winds are characterized by a consistent pattern in amplitude and phase; however, winter winds are more variable from campaign to campaign. Similarly, temperatures reveal low variability in phase for the summer season and high variability in winter, but the amplitudes of the 12‐hour oscillations in temperature are variable for all seasons. Comparison with other experimental data suggests good agreement in general but larger variability. We discuss the possible sources of the observed variability and link it to processes in the mesosphere.
Two‐dimensional simulations of gradient‐drift turbulence in the daytime equatorial electrojet
Authors: Hu S. , Bhattacharjee A.
Published: September 2002

Two‐dimensional numerical simulations of type II irregularities, attributed to gradient‐drift instabilities, are carried out with observed profiles in the daytime equatorial electrojet. Earlier work has shown that these profiles are linearly unstable to gradient‐drift modes with peak growth rates at wavelengths of the order of 1 km. The nonlinear evolution of these instabilities leads to saturated turbulent structures, with the linear drive at kilometer‐scale wavelengths quenched by a direct energy cascade to short wavelengths that are effectively damped by velocity shear and diffusion. The saturated turbulent structures show quasi‐steady 1 – 2 km horizontal waves, and appear to be quite isotropic in the plane perpendicular to the ambient magnetic field. The electron vertical velocity spectra, constructed from the simulation output, show 1–2 km vertical structures which are qualitatively in accord with recent high‐resolution radar observations at the Jicamarca Observatory in Peru.
A 2.5‐dimensional MHD parametric study of interplanetary shock interactions with the heliospheric current sheet/heliospheric plasma sheet
Authors: Smith Zdenka , Odstrcil Dusan , Dryer Murray
Published: September 2002

A study of the interaction of interplanetary shock waves with the heliospheric current sheet/heliospheric plasma sheet (HCS/HPS) and the disturbed flow behind them is made with a 2.5 dimensional MHD code that uses improved computational techniques. The aim is to determine if such interactions for typical shocks can produce sufficiently large changes in the interplanetary plasma and field to be geoeffective. The mutual interaction of the shock waves with the HCS/HPS is examined for several angles between the shock normal and the HCS/HPS. We separate the effects of the HCS from those due to angular distance of observer from the initial shock center. The shocks are initiated at 0.1 AU by a finite‐duration increase in initial radial velocity. We examine the transit times of the resultant shocks to 1‐AU and their strengths along their peripheries. We also examine the 1‐AU signatures of the polar component of the magnetic field, Bθ, in the interplanetary disturbance at and following the leading fast mode MHD shock. Note that Bθ = −Bz. We show the differences that result from the presence of the HCS, both qualitatively and quantitatively. The largest effects occur in Bθ, which reaches its largest excursions within 10° of the edge of the HPS. For the moderate shocks simulated in this study, deflections of the field lines produce values of Bθ between 4 and 5 nT in the presence of a HCS. These values are at the lower end of the range considered necessary for interplanetary events to initiate moderate geomagnetic storms. Further study is required to investigate the nature of input pulses and HCS that would give larger Bθ variations.
Evidence for enhanced deformation in two‐phase rocks: Experiments on the rheology of calcite‐anhydrite aggregates
Authors: Bruhn D. F. , Olgaard D. L. , Dell'Angelo L. N.
Published: September 2002

We investigated the deformational behavior of fine‐grained calcite‐anhydrite aggregates at conditions where the end‐member phases are of similar strength and deform by dislocation creep and/or diffusion creep. Samples of various volume proportions were made by hot isostatically pressing CaCO3 and CaSO4 powders into dense aggregates. Grain sizes are 5 and 8 μm for two different anhydrites (An) and 8 μm for calcite (Cc). In all mixtures, grain sizes are 2–4 μm for both phases. The stress difference σ, strain rate
and grain size d data are fit to a power law with
. At 500°C, grain flattening accounts for most of the sample strain for all mixtures, suggesting that dislocation creep is the dominant deformation mechanism. However, the mixtures, especially the 50:50, are often weaker than the end‐member phases. At 600°C the mixtures deformed dominantly by diffusion creep, as evidenced by n = 1, the sensitivity of aggregate strength to the grain size of at least one component, little evidence is found for grain flattening and change in microstructures compared to the undeformed materials, and the mixtures are generally weaker than both end‐members. However, the grain size difference is not enough to account for the strength difference. For a small number of experiments, the variability in the strengths appears to correlate with the fraction of interphase boundaries; weaker samples have higher fractions of boundaries between unlike phases (Cc‐An) than stronger samples. We suggest that the relative weakness of the mixtures is at least in part due to enhanced boundary diffusion rates between unlike phases relative to like phases.
Variability in the southeast Indian Ocean from altimetry: Forcing mechanisms for the Leeuwin Current
Authors: Morrow Rosemary , Birol Florence
Published: September 2002

Seasonal and interannual variability in the southeastern Indian Ocean is investigated with the aid of Topex/Poseidon (T/P) altimeter data for the 3‐year period 1993–1995. The annual Rossby wave signal around 10°S is clearly marked, and consistent with modelling and Geosat results from earlier periods. A band of higher mesoscale variability between 20° and 35°S extends across the entire Indian Ocean, with characteristic timescales between 120 and 180 days and length scales of order 500 km. Sea level anomalies are shown to propagate at around 1.5 to 2 times the theoretical linear Rossby wave speed, with an associated signal in sea surface temperature (SST) anomalies, and the propagation suggests that the variability is not locally forced but originates near the eastern boundary. Altimeter data is also used to examine variations in the alongshore pressure gradient, thought to be the principal mechanism forcing the Leeuwin Current poleward against the prevailing equatorward winds. The T/P data confirms that the alongshore pressure gradient is maximum in May when the Leeuwin Current is strongest, but we find a consistent secondary peak in November which is not evident in the climatological data. There is also significant interannual variation, related to large interannual variations on the Australian Northwest Shelf. The seasonal and interannual variations also influence the thermal structure of the instabilities associated with the Leeuwin Current, which may be the source of the westward propagating anomalies between 20°S and 35°S. The results compare well with satellite SST and in situ XBT data at the eastern boundary and suggest that satellite data can be used to monitor the variability of the Leeuwin Current.
A two‐channel laterally averaged estuarine circulation model (LAECIM)
Authors: Wang Jia
Published: September 2002

A two‐dimensional, two‐channel, laterally averaged estuarine circulation model (LAECIM) with a sigma coordinate in the vertical is developed for tides, tidal currents, and salinity in a two‐channel estuary with any arbitrary angle. The model is forced by oceanic tides composed of 16 major constituents and freshwater discharge. The two channels are governed by the same dynamic equations. A control volume at the junction between the two channels is designed to conserve mass and volume transport and to guarantee the continuity of momentum in the finite difference schemes, based on the physical principle in continuum medium of fluid. An explicit leapfrog scheme in time and centered differencing in space are employed, together with a time filter to remove the computational mode. An Euler backward scheme is periodically used to correct (retard) the phase frequency, which is overestimated due to the explicit leapfrog scheme used. The model is applied to the entire San Francisco Bay with its two channels: the main north‐south channel and the Golden Gate channel, which is perpendicular to the former (i.e., with a right angle to the former). The simulated time series of tides, tidal currents, and salinities compare favorably with the observed tidal gauge data and moored current and salinity time series during the summer 1980. The simulated along‐channel salinity section also compares well to the conductivity‐temperature‐depth sectional observations during the same period. The model simulated Eulerian residual current is consistent with two‐layer flow theory.
Numerical experiments of nonlinear energy transfer within the oceanic internal wave spectrum
Authors: Hibiya Toshiyuki , Niwa Yoshihiro , Fujiwara Kayo
Published: September 2002

From the fact that the Garrett‐Munk‐like (GM‐like) internal wave spectrum is maintained even in regions of weak local energy sources, it is believed that energy is continuously supplied to the local wave spectrum by internal waves propagating from source regions where they are generated by wind stress fluctuations or tide‐topography interactions. In order to examine how the energy thus supplied by propagating internal waves cascades through the local wave spectrum down to small dissipation scales, we carry out three sets of numerical experiments where the quasi‐equilibrium internal wave spectrum obtained by Hibiya et al. [1996] is perturbed with forcing applied to different parts of the low‐frequency low‐wavenumber portion. The evolution of the internal wave spectrum is examined over eight inertial periods after the forcing is applied. First, in experiment I the forcing is applied to the low‐vertical‐wavenumber inertial‐frequency (ω=f) portion of the spectrum. In this case, no significant increase or decrease of spectral intensity can be seen within the two‐dimensional wavenumber spectrum. Next, in experiment II the forcing is applied at low‐vertical wavenumbers in the frequency range of 2f<ω<3f. In contrast to the result of experiment I, high‐vertical‐wavenumber near‐inertial spectral values are seen to increase, exceeding the GM level as time progresses. Finally, in experiment III the forcing is applied at low‐vertical wavenumbers in the frequency range of 1.6f<ω<2f. Although the spectral location of the forcing is very close to that assumed in experiment II, no appreciable energy transfer to high‐vertical wavenumbers occurs in this case. From the results of these numerical experiments it is shown that the energy transfer to the small dissipation scales is dominated by parametric subharmonic instability which transfers energy from low‐vertical‐wavenumber waves with frequencies over 2f to high‐vertical‐wavenumber near‐inertial (f<ω<2f) waves. This supports the model for the dynamic balance of the internal wave spectrum proposed by Hibiya et al. [1996] that with the increase (or decrease) of energy supply to the local internal wave spectrum, high‐vertical‐wavenumber near‐inertial current shear is enhanced (or diminished) leading to an increase (or decrease) in the rate of energy dissipation at critical layers.
El Niño‐Southern Oscillation‐related ocean‐atmosphere coupling in the western equatorial Pacific
Authors: Mayer D. A. , Weisberg R. H.
Published: September 2002

Using 43 years of Comprehensive Ocean‐Atmosphere Data Set and related data for the period 1950–1992, an examination is made into the regional dependence of ocean‐atmosphere coupling in relation to the El Niño‐Southern Oscillation (ENSO). The cross correlation between sea surface temperature (SST) and sea level pressure (SLP) anomalies over the global tropics shows two patterns of significant negative correlation consistent with a local hydrostatic response of SLP to SST: (1) the eastern Pacific, where the correlation is symmetric about and largest on the equator, and (2) the western Pacific, where symmetric regions of negative correlation are found off the equator, separated by a region of positive correlation on the equator. Anomalies within these two patterns vary out of phase with each other. While the SLP anomalies on both sides of the basin are of similar magnitude, the SST anomalies in the east are much larger than those in the west. Despite this disparity in the SST anomaly magnitudes between the eastern and western Pacific we argue that the ocean‐atmosphere couplings in the western and west‐central Pacific are important for ENSO. The off‐equator SST anomalies in the west enhance the SLP anomalies there, and they appear to initiate easterly wind anomalies over the far western Pacific during the peak El Niño phase of ENSO. As these easterlies evolve, their effect upon the ocean tends to oppose that of the westerly wind anomalies found over the west‐central Pacific. These competing effects suggest a mechanism that may contribute to coupled ocean‐atmosphere system oscillations. The west‐central equatorial Pacific (the region separating the eastern and western patterns), while exhibiting large momentum and heat flux exchanges, shows minimum correlation between SST and SLP. Thus neither the SST and SLP anomaly magnitudes nor the correlation between them is alone indicative of ocean‐atmosphere coupling, and the regional dependence for such coupling in relation to ENSO appears to be more complicated than mechanistic interpretations of ENSO would suggest.
Winter convective events and bottom water warming in the Greenland Sea
Authors: Budéus Gereon , Schneider Wolfgang , Krause Gunther
Published: September 2002

From observations on yearly cruises to the central Greenland Sea between 1993 and 1996, conclusions are drawn with respect to winter convection and bottom water renewal. The data indicate that winter convection was extremely weak after 1993, not even ventilating the intermediate waters. This is remarkable, since the salinities in the upper layers increased considerably between 1993 and 1995, thus providing presumably favorable conditions for winter convection. With the absence of deep reaching winter convective events, the temperatures in the deeper waters of the Greenland Gyre increased steadily by about 0.03 K between 1993 and 1996. We conclude from the development of mainly the thermal structure on a zonal transect that an explanation for the temperature increase can be given by a large‐scale downward water movement of about 150 m/yr in the central Greenland Sea. The data indicate that this process is independent of changes in the dynamically induced density distribution. It is therefore possible that a downward movement, perhaps masked by other processes, may continue for many years. If this is the case, resulting flushing times would be of the order of 20–30 years only. The presence of a large‐scale circulation cell with downward movement in the central Greenland Gyre would explain the observed warming of the bottom waters without the demand for an actually active heat source. It is also in accordance with the observed increase of chemical tracer concentrations in the deep waters.
Long‐term equilibrium tides
Authors: Shaffer John A. , Cerveny Randall S.
Published: September 2002

Extreme equilibrium tides, or “hypertides,” are computed in a new equilibrium tidal model combining algorithms of a version of the Chapront ELP‐2000/82 Lunar Theory with the BER78 Milankovitch astronomical expansions For the rerent past, a high correspondence exists between computed semidiurnal tide levels and a record of coastal flooding demonstrating that astronomical alignment is a potential influence on such flooding. For the Holocene and near future, maximum tides demonstrate cyclic variations with peaks at near 5000 B.P. and 4000 A.P. On the late Quaternary timescale, variations in maximum equilibrium tide level display oscillations with periods of approximately 10,000, 100,000 and 400,000 years, because of precessional shifts in tidal maxima between vernal and autumnal equinoxes. While flooding occurs under the combined effects of tides and storms via “storm surges,” the most extensive flooding will occur with the coincidence of storms and the rarer hypertides and is thus primarily influenced by hypertides. Therefore we suggest that astronomical alignment's relationship to coastal flooding is probabilistic rather than deterministic. Data derived from this model are applicable to (1) archaeological and paleoclimatic coastal reconstructions, (2) long‐term planning, for example, radioactive waste site selection, (3) sealevel change and paleoestuarine studies or (4) ocean‐meteorological interactions.
Sea ice extent mapping using Ku band scatterometer data
Authors: Remund Quinn P. , Long David G.
Published: September 2002

Although spaceborne scatterometers such as the NASA scatterometer have inherently low spatial resolution, resolution enhancement techniques can be used to increase the utility of scatterometer data in monitoring sea‐ice extent in the polar regions, a key parameter in the global climate. The resolution enhancement algorithm produces images of A and B, where A is the normalized radar backscatter coefficient σO at 40° incidence and B is the incidence angle dependence of σO. Dual‐polarization A and B parameters are used to identify sea ice and ocean pixels in composite images. The A copolarization ratio and vertically polarized B are used as primary classification parameters to discriminate between sea ice and open ocean. Estimates of the sea‐ice extent are obtained using linear and quadratic (Mahalanobis distance) discriminant boundaries. The distribution parameters needed for the quadratic estimate are taken from the linear estimate. The σO error variance is used to reduce errors in the linear and Mahalanobis ice/ocean classifications. Noise reduction is performed through binary image region growing and erosion/dilation techniques. The resulting edge closely matches the NASA Team algorithm special sensor microwave imager derived 30% ice concentration edge. A 9‐month data set of global sea‐ice extent maps is produced with one 6‐day average map every 3 days.
Methane photooxidation in the atmosphere: Contrast between two methods of analysis
Authors: Johnston Harold , Kinnison Douglas
Published: September 2002

It is our opinion that over the past 20 years, some atmospheric scientists have used a scientifically incorrect method for finding the contribution of methane to changes in ozone. In any volume element of the atmosphere, the differential equation for ozone D is equal to gross production P minus gross loss L plus transport, which may be written D = P (methane) − L (methane) + P(not methane) − L (not methane) + transport. If both P(not methane) and L (not methane) are effectively zero, then the full differential equation is equal to the differential equation for the effect of methane, D (total) = P(methane) − L (methane) + transport. An approach was introduced around 1980 that has been widely used since then, with the valid assumption that in the lower troposphere the methane smog reactions are the only source of gross ozone production, P(methane). It concluded that the differential rate equation for total ozone change in such regions gives the net ozone change by the methane process (DE method). However, in the lowest troposphere, there are processes not caused by methane that destroy ozone at all altitudes (notably O1D + H2O = 2OH), so that L (not methane) is not zero, and thus D = P (methane) − L (methane) − L (not methane) + transport, making the DE method conceptually wrong everywhere. The object of this paper is to solve for how methane changes ozone in the troposphere and stratosphere. Crutzen [1973] gave the first treatment of ozone formation in the global troposphere via methane photooxidation. He considered radical attack on methane to produce CH3 followed by three reaction sequences that go from CH3 to CO2, referred to here as the sequence (SEQ) method. The present study substantially extends Crutzen's sequence method: There are five significant branching points as methane is consumed and 34 sequences (different reaction paths) between CH4 and CO, and more reactions lead to CO2. A detailed derivation is given here, branching ratios are evaluated, and the results are presented as two complicated, closed, algebraic equations, which are valid from the surface up to the middle stratosphere. The SEQ method contains only ozone changes caused by the presence of methane. This method is implemented with output from the 1997 version of the Lawrence Livermore National Laboratory two‐dimensional model. Assuming we carried out the SEQ method correctly, we find the DE method to be a fairly good numerical approximation in some areas and a poor numerical approximation in other areas. Averaged over the troposphere, the net effect of the methane smog process is to destroy OH at a rate 32% as fast as it is formed by the nonmethane process, O1D + H2O = 2OH. The integral over the global troposphere finds that the net rate of ozone production from methane by the DE is 2.2‐fold slower than the SEQ method.
A comparison of airborne and surface trace gas measurements during the Southern Oxidants Study (SOS)
Authors: Luke Winston T. , Watson Thomas B. , Olszyna Kenneth J. , Gunter R. Laureen , McMillen Robert T. , Wellman Dennis L. , Wilkison Stan W.
Published: September 2002

The NOAA Twin Otter conducted more than a dozen overflights of ground‐level air quality monitoring stations during the 1995 Southern Oxidants Study (SOS) Nashville/Middle Tennessee Ozone Project Field Intensive. Surface and aircraft observations of ozone and ozone precursors were examined to identify systematic sampling errors, and to assess the degree to which surface measurements may be considered representative of the larger planetary boundary layer (PBL). Overall agreement between surface and aircraft trace gas measurements was excellent in the well developed mixed layer, especially in rural‐regional background air and under stagnant conditions, where surface concentrations change only slowly. On July 2, surface level measurements were representative of the larger mixed layer over distances as far as 70 km in background air, and 30 km in the weakly advected urban plume. Vertical variations in trace gas concentrations were often minimal in the well‐mixed PBL, and measurements at the surface always agreed well with aircraft observations up to 460 m above ground level. Under conditions of rapidly varying surface concentrations (e.g., during episodes of power plant plume fumigation and early morning boundary layer development), agreement between surface and aloft is dependent upon the spatial (aircraft) and temporal (ground) averaging intervals used in the comparison. Under these conditions, surface sites may be representative of the PBL only to within a few kilometers. Under clear skies in the well‐mixed PBL, regression of aircraft trace gas data collected within 5 km of the ground sites against 15–20 min average surface concentrations centered on the times of the overflights yielded the following relationships: [O3]aircraft = ([O3]surface × 0.9374) + 4.86 ppbv (r2 = 0.9642); [CO]aircraft = ([CO]surface × 0.8914) + 16.4 ppbv (r2=0.9673); [SO2]aircraft = ([SO2]surface × 0.9414) − 0.069 ppbv (r2 = 0.9945). Although O3, CO, and SO2 measurements at the surface and aloft generally agreed well, isolated examples of unexplained measurement discrepancies emerged, illustrating the need for side‐by‐side instrument comparisons. NOY measurements agreed poorly between surface and aircraft: [NOY]aircraft = ([NOY]surfacex × 0.9184) + 4.56 ppbv (r2 = 0.9188).
Chemical characteristics of Pacific tropospheric air in the region of the Intertropical Convergence Zone and South Pacific Convergence Zone
Authors: Gregory G. L. , Westberg D. J. , Shipham M. C. , Blake D. R. , Newell R. E. , Fuelberg H. E. , Talbot R. W. , Heikes B. G. , Atlas E. L. , Sachse G. W. , Anderson B. A. , Thornton D. C.
Published: September 2002

The Pacific Exploratory Mission (PEM)‐Tropics provided extensive aircraft data to study the atmospheric chemistry of tropospheric air in Pacific Ocean regions, extending from Hawaii to New Zealand and from Fiji to east of Easter Island. This region, especially the tropics, includes some of the cleanest tropospheric air of the world and, as such, is important for studying atmospheric chemical budgets and cycles. The region also provides a sensitive indicator of the global‐scale impact of human activity on the chemistry of the troposphere, and includes such important features as the Pacific “warm pool,” the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and Walker Cell circulations. PEM‐Tropics was conducted from August to October 1996. The ITCZ and SPCZ are major upwelling regions within the South Pacific and, as such, create boundaries to exchange of tropospheric air between regions to the north and south. Chemical data obtained in the near vicinity of the ITCZ and the SPCZ are examined. Data measured within the convergent zones themselves are not considered. The analyses show that air north and south of the convergent zones have different chemical signatures, and the signatures are reflective of the source regions and transport histories of the air. Air north of the ITCZ shows a modest urban/industrialized signature compared to air south of the ITCZ. The chemical signature of air south of the SPCZ is dominated by combustion emissions from biomass burning, while air north of the SPCZ is relatively clean and of similar composition to ITCZ south air. Chemical signature differences of air north and south of the zones are most pronounced at altitudes below 5 km, and, as such, show that the ITCZ and SPCZ are effective low‐altitude barriers to the transport of tropospheric air. At altitudes of 8 to 10 km, chemical signatures are less dissimilar, and air backward trajectories (to 10 days) show cross‐convergent‐zone flow. At altitudes below about 5 km, little cross‐zonal flow is observed. Chemical signatures presented include over 30 trace chemical species including ultrafine, fine, and heated‐fine (250°C) aerosol.
Airborne observations of the tropospheric CO2 distribution and its controlling factors over the South Pacific Basin
Authors: Vay S. A. , Anderson B. E. , Conway T. J. , Sachse G. W. , Collins J. E. , Blake D. R. , Westberg D. J.
Published: September 2002

Highly precise measurements of CO2 mixing ratios were recorded aboard both the NASA DC‐8 and P3‐B aircraft during the Pacific Exploratory Mission‐Tropics conducted in August‐October 1996. Data were obtained at altitudes ranging from 0.1 to 12 km over a large portion of the South Pacific Basin representing the most geographically extensive CO2 data set recorded in this region. These data along with CO2 surface measurements from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) and the National Institute of Water and Atmospheric Research (NIWA) were examined to establish vertical and meridional gradients. The CO2 spatial distribution in the southern hemisphere appeared to be largely determined by interhemispheric transport as air masses with depleted CO2 levels characteristic of northern hemispheric air were frequently observed south of the Intertropical Convergence Zone. However, regional processes also played a role in modulating background concentrations. Comparisons of CO2 with other trace gases indicated that CO2 values were influenced by continental sources. Large scale plumes from biomass burning activities produced enhanced CO2 mixing ratios within the lower to midtroposphere over portions of the remote Pacific. An apparent CO2 source was observed in the NOAA/CMDL surface data between 15°N and 15°S and in the lower altitude flight data between 8°N and 8.5°S with a zone of intensity from 6.5°N to 1°S. Inferred from these data is the presence of a Southern Ocean sink from south of 15°S having two distinct zones seasonally out of phase with one another.
Hydrogen peroxide dry deposition lifetime determined from observed loss rates in a power plant plume
Authors: Jobson B. T. , Frost G. J. , McKeen S. A. , Ryerson T. B. , Buhr M. P. , Parrish D. D. , Trainer M. , Fehsenfeld F. C.
Published: September 2002

Hydrogen peroxide measurements made aboard the NOAA WP‐3D aircraft in the summer of 1995 as part of the Southern Oxidants Study revealed loss of hydrogen peroxide within fossil fuel power plant plumes and the Nashville urban plume. On July 7 the Cumberland power plant plume was intercepted at five different downwind distances. H2O2 mixing ratios within the plume decreased with increasing distance from the plant. Conversely, ozone mixing ratios increased. The H2O2 loss was likely a result of the lowered rate of formation due to suppression of HO2 radical concentrations in the high NOx environment and subsequent removal of H2O2 by photolysis, reaction with HO, and dry deposition. From steady state model estimates of the chemical loss rates in the plume and an analytical approximation of the influx of H2O2 into the plume center due to horizontal dispersion, a simple budget for H2O2 was formulated, from which a dry deposition lifetime of 19 hours ±55% was inferred.
Loss rate of NOy from a power plant plume based on aircraft measurements
Authors: Gillani N. V. , Luria M. , Valente R. J. , Tanner R. L. , Imhoff R. E. , Meagher J. F.
Published: September 2002

This study was motivated by the recent work of Buhr et al. [1996] which reported losses of NOy from large power plant plumes as high as 0.25 hour−1, much higher than generally accepted values. If true, conclusions pertaining to the efficiency of ozone and nitrate production in the lower troposphere would need major revisions. The results of Buhr et al. were based on aircraft measurements in four TVA (Tennessee Valley Authority) power plant plumes on July 7, 1995, as part of the Nashville/Middle Tennessee Ozone Study, a measurement program of the Southern Oxidants Study (SOS), whereas the results reported in this paper are also based on measurements made in the same SOS study aboard another instrumented aircraft (the TVA helicopter), in plumes of one of these power plants (the Cumberland Steam Plant in northwestern Tennessee) during five different days in 1994 and 1995. Between the 1994 and 1995 sampling periods, emissions of SO2 at the Cumberland plant were reduced by nearly 95% by installation of scrubbers. Our data from the one 1994 day show that the ratio of excess SO2 to NOy, in the plume core increased significantly with plume age, indicating a potentially high differential loss rate of NOy (excess loss of NOy relative to SO2) of about 0.12 hour−1. However, results based on the larger 1995 data set indicate a low differential NOy loss rate of only 0.00±0.03 hour−1, consistent with accepted low loss rates. Because the SOS‐Nashville/Middle Tennessee Ozone Study was not specifically designed to explore the NOy loss issue, the question of NOy loss rates in plumes is not currently resolved and additional focused field studies are needed.
Measurements comparison of oxygenated volatile organic compounds at a rural site during the 1995 SOS Nashville Intensive
Authors: Apel E. C. , Calvert J. G. , Riemer D. , Pos W. , Zika R. , Kleindienst T. E. , Lonneman W. A. , Fung K. , Fujita E. , Shepson P. B. , Starn T. K. , Roberts P. T.
Published: September 2002

Simultaneous measurements of oxygenated volatile organic compounds (OVOCs) by four independent research groups at the Youth Incorporated (YI) site during the 1995 Southern Oxidants Study Nashville Intensive, between July 4 and 30, 1995, provided a unique opportunity to compare results from different techniques. 2,4‐Dinitrophenylhydrazine (DNPH) coated C18 (C18AtmAA) and Si‐Gel (Si‐GelMTE) cartridges were compared with each other and with two sorbent‐based preconcentration gas chromatographic (GC) techniques coupled with mass spectrometric (MS) detection (GC/MSUM and GC/MSPU) or flame ionization detection GC/FIDUM· The experiment consisted of both a laboratory (part A) and an ambient air comparison (part B). In part A of the study, standard mixtures of OVOCs were diluted in a flowing gas stream of humidified, purified air, both with and without addition of O3, and distributed to participant's instrumentation. Concentrations were compared with the expected values based on known cylinder concentrations and dilution factors. In part B of the study, the instruments sampled ambient air from a common glass manifold. Species intercompared were formaldehyde, acetaldehyde, acetone, and propanal. The C18AtmAA data were typically higher than the Si‐GelMTE data for the four intercompared compounds, and possible explanations are given. Agreement between the cartridge systems and the GC/MS, GC/FID systems for comeasured compounds was poorer than expected but improved towards the end of the experiment. The GC/MS systems tracked each other well for acetone, but there were differences in the absolute concentration values. These results show that improvements in the accuracy and comparability of techniques should be of high priority.
Convective injection and photochemical decay of peroxides in the tropical upper troposphere: Methyl iodide as a tracer of marine convection
Authors: Cohan Daniel S. , Schultz Martin G. , Jacob Daniel J. , Heikes Brian G. , Blake Donald R.
Published: September 2002

The convective injection and subsequent fate of the peroxides H2O2 and CH3OOH in the upper troposphere is investigated using aircraft observations from the NASA Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) over the South Pacific up to 12 km altitude. Fresh convective outflow is identified by high CH3I concentrations; CH3I is an excellent tracer of marine convection because of its relatively uniform marine boundary layer concentration, relatively well‐defined atmospheric lifetime against photolysis, and high sensitivity of measurement. We find that mixing ratios of CH3OOH in convective outflow at 8–12 km altitude are enhanced on average by a factor of 6 relative to background, while mixing ratios of H2O2 are enhanced by less than a factor of 2. The scavenging efficiency of H2O2 in the precipitation associated with deep convection is estimated to be 55–70%. Scavenging of CH3OOH is negligible. Photolysis of convected peroxides is a major source of the HOx radical family (OH + peroxy radicals) in convective outflow. The timescale for decay of the convective enhancement of peroxides in the upper troposphere is determined using CH3I as a chemical clock and is interpreted using photochemical model calculations. Decline of CH3OOH takes place on a timescale of a 1–2 days, but the resulting HOx converts to H2O2, so H2O2 mixing ratios show no decline for ∼5 days following a convective event. The perturbation to HOx at 8–12 km altitude from deep convective injection of peroxides decays on a timescale of 2–3 days for the PEM‐Tropics A conditions.
Introduction to special section: Regional Climate Modeling Revisited
Authors: Giorgi Filippo , Mearns Linda O.
Published: September 2002

This paper provides an introduction to the special issue of the Journal of Geophysical Research on “New Developments and Applications With the NCAR Regional Climate Model (RegCM).” In the first part of the paper we revisit and discuss outstanding issues in regional climate modeling in view of the progress achieved in this area of research during the last decade. We discuss issues of simulation length, spin‐up, model physics, domain and resolution, lateral boundary conditions, multiple and two way nesting, and variable resolution approaches. In the second part we introduce the papers included in this issue. Among the primary model developments that occurred in the last few years are inclusions of the radiative transfer package and cumulus convection scheme from the National Center for Atmospheric Research (NCAR) global model CCM3, a simplified explicit moisture scheme including direct interaction with cloud radiation, testing of a variable resolution model configuration, improvements in the coupled lake model, and interactive coupling with radiatively active atmospheric aerosols. The papers in the issue illustrate a wide range of applications over different regions, such as the United States, East Asia, central Asia, eastern Africa. The main model limitations and areas in need of improvement are indicated.
Regional stretched grid generation and its application to the NCAR RegCM
Authors: Qian Jian‐Hua , Giorgi Filippo , Fox‐Rabinovitz Michael S.
Published: September 2002

A method for the generation and application of a regional variable resolution stretched grid system is developed. In the modeling domain, a stretched grid buffer zone surrounds the central uniform grid region. To be comparable with current nesting methods, two integers are specifically considered in the design of the grid system. The first integer is the total stretching rate, which is the ratio of the maximum grid distance to the minimum grid distance, and the other integer is the ratio of the width of the buffer zone to the maximum grid distance. The grid distance is uniformly reduced from the outer lateral boundary of the model domain to the edge of the central uniform grid region. The final purpose of this grid is to facilitate the two‐way interaction between global and regional models. However, the primary objective of this paper is to present the theoretical framework of the stretched grid system and its implementation within the NCAR regional climate model (RegCM). We provide an example of the performance of the stretched grid model configuration by applying it to an adiabatic version of the RegCM and completing a 3‐day simulation over a domain encompassing the continental United States. The stretched grid version of the model produces results comparable to a control run with uniform resolution. The computational cost of running the stretched grid system is about 40% lower than that of the uniform grid system because of the smaller number of grid points in the lateral buffer area. These preliminary numerical tests with the RegCM illustrate the robustness of our new regional stretched grid method.
Impact of tundra ecosystems on the surface energy budget and climate of Alaska
Authors: Lynch Amanda H. , Bonan Gordon B. , Chapin F. S. , Wu W.
Published: September 2002

The sensitivity of regional terrestrial climate to the characteristics of tundra ecosystems has been investigated by a series of sensitivity experiments concentrating on the summer of 1995. Validation of the NCAR Land Surface Model and the Arctic Regional Climate System Model for this season indicate their adequacy for this study. Comparisons of the simulated climate in response to a wet meadow tundra or a dry heath tundra results in an expected cooling and moistening of both the local area and the adjoining sea ice and forested regions. The impact of atmospheric cloud‐radiation feedbacks is to reduce the cooling as the summer progresses, although moistening continues, associated with increased precipitation in some areas. The spatial variability of the response is dependent upon prevailing synoptic conditions, which act to enhance moisture advection in certain areas. This study indicates that vegetation variation within the Arctic has substantial climatic effects that extend beyond the Arctic. In addition, the perturbations in the summer season could have profound implications of Arctic wintertime climate and issues of snow‐albedo feedback and spring melt.
Aerosol optical depth retrieval using ATSR‐2 and AVHRR data during TARFOX
Authors: Veefkind J. Pepijn , de Leeuw Gerrit , Durkee Philip A. , Russell Philip B. , Hobbs Peter V. , Livingston John M.
Published: September 2002

Satellite retrieved aerosol optical properties are compared to aircraft measurements for a case study during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). Two satellite instruments are used: the Along Track Scanning Radiometer 2 (ATSR‐2) and the advanced very high resolution radiometer (AVHRR). The aerosol optical depth in the mid‐visible (0.555 μm) retrieved from the ATSR‐2 data agrees within 0.03 with colocated sunphotometer measurements. Also, the spectral behavior of the aerosol optical depth is retrieved accurately. Good correlation is found between aerosol optical depths for AVHRR channel 1 (0.64 μm) and sunphotometer derived values, but the satellite retrieved values are 0.05 to 0.15 lower. The Ångström wavelength exponent is determined both from the ATSR‐2 and the AVHRR data. The ATSR‐2 derived Ångström exponents are in good agreement with the values computed from the sunphotometer data. The Angstrom exponents determined from AVHRR data show very large variations. Both the ATSR‐2 and the AVHRR aerosol optical depth images show a large gradient. Vertical profile data of temperature, relative humidity, and particle scattering indicate that this gradient is probably caused by changes in the dry aerosol properties, rather than a change in the relative humidity.