bluuebunny/crossref_metadata_2025 huggingface dataset:
High‐frequency type II radio emissions associated with shocks driven by coronal mass ejections
Authors: Reiner M. J. , Kaiser M. L.
Published: August 2003

We investigate the causal relationship between type II radio emissions in the frequency range from 1 to 14 MHz observed by the WAVES experiment on the Wind spacecraft and metric type II bursts observed by ground‐based radio telescopes. We do this by extrapolating, to lower radio frequencies, fits to the frequency drift rates of metric type II bursts. These fits, which describe the shock propagation through the solar corona, are based on coronal density laws derived from white light observations. In order to establish whether the type II radio emissions were generated from the same shock, we explicitly chose events for which the type II radio emissions in the frequency range from 1 to 14 MHz were temporally separated from the associated metric type II bursts. The analyses indicate that some type II radio events observed by WAVES may not be low‐frequency continuations of the apparently associated metric type II bursts. We conclude that two temporally separated shocks are required to account for the radio observations for these events, one shock being temporally well associated with the flare maximum. In two cases the observed GOES X‐ray flux confirms the presence of two closely spaced solar events. We argue that the type II radio emissions observed by Wind/WAVES are likely generated by coronal mass ejection‐driven shocks that form as low as 2 RS in the solar corona.
Alfvén wave phase mixing driven by velocity shear in two‐dimensional open magnetic configurations
Authors: Ruderman M. S. , Goldstein M. L. , Roberts D. A. , Deane A. , Ofman L.
Published: August 2003

Phase mixing of torsional Alfvén waves in axisymmetric equilibrium magnetic configurations with purely poloidal magnetic field and stationary flow along the field lines in resistive viscous plasmas is studied. The characteristic wavelength along the magnetic field lines is assumed to be much smaller than the characteristic scale of inhomogeneity in the magnetic field direction, and the WKB method is used to obtain an analytic solution describing phase mixing. The general solution is applied to a particular configuration with the radial magnetic field and flow under the assumptions that the magnetic field and density are independent of the polar angle in the spherical coordinates and the flow velocity is independent of the radial coordinate. The only source of phase mixing in this configuration is velocity shear. The analytical solution is compared with a numerical simulation of the fully nonlinear resistive MHD equations. The numerical and analytical results are in good agreement. Consequences for wave energy deposition into the solar corona and solar wind and for the evolution of the Alfvén wave energy spectrum are discussed.
Generalized Walén tests through Alfvén waves and rotational discontinuities using electron flow velocities
Authors: Scudder J. D. , Puhl‐Quinn P. A. , Mozer F. S. , Ogilvie K. W. , Russell C. T.
Published: August 2003

A new formulation of the Walén test [Walén, 1944] to locate rotational discontinuities (RD) or Alfvén wave trains is constructed that is more appropriate for use within and across the current‐carrying layers of these structures. This generalized test is to be distinguished from the traditional rigorous jump condition that is only appropriate for connecting asymptotic, current free states on either side of the discontinuity or wave train. The generalized test is most simply formulated as a vector difference equation relating changes in the electron flow velocity vector and changes of the magnetic field vector, ΔUe(x) = αeΔB(x) with a prescribed scalar constant of proportionality. For an electron proton plasma this paper shows that αe is precisely the same constant of the jump condition that has long been in use. Small corrections to αe are computed for minor helium populations. This new formulation is used with Polar plasma, E, and B measurements to search for rotational discontinuities at the magnetopause. Of the 44 cases where electrons gave acceptable theoretical correlations and coefficients αe predicted by the theory, nearly 95% of the simultaneous ion fits gave corresponding values of at that were smaller in magnitude than implied by simple ideal MHD jump conditions. Possible explanations for this discrepancy are discussed. In addition, these data have been used to provide the first empirical evidence of the importance of the electron pressure gradient in these RD current layers and represent the first direct determination of the sizes EnHT ≃ 0.3−5 mV/m of the deHoffmann‐Teller electric field in RD current layers at the magnetopause.
The N2 + O+ charge‐exchange reaction and the dayglow N2+ emission
Authors: Broadfoot A. L. , Stone T.
Published: August 2003

Dayglow spectra were recorded by the Arizona Airglow Experiment from the payload bay of the shuttle, STS‐74. These spectra are used to reexamine the role of the prominent N2+ first negative emission from the dayglow thermosphere. Many reports of the N2+ emissions identify problems in validating the intensity of the emission. Also, an extended vibrational and rotational structure of the bands remains unexplained in the historical analysis. These anomalies appear to be due to the charge‐exchange reaction, N2+O+(2D,2P)→N2++O, which is the dominant source of N2+ ions in the sunlit atmosphere at high altitudes. In the present work the N2+ emission was considered to originate from two separate ion sources. First are those emissions originating from ions produced by photoionizaton and electron bombardment; these emissions can be modeled. Second are the emissions originating from ions produced by the charge‐exchange reaction; these emissions cannot be modeled. Synthetic emission profiles due to the first ion source were modeled and subtracted from the observed spectrum, leaving emission profiles resulting from the charge‐exchange ion source. These residual vibrational and rotational profiles were analyzed to retrieve resonance scattering rates for these ions. These scattering rates can be used to estimate the N2+ first negative emission rate expected from the thermosphere with a model of the atmosphere. It is suggested that measurements of the N2+ emission rate can be used to determine the daytime concentration of the oxygen ion, O+(2D,2P). Although the present work appears to resolve the question of the extended vibrational and rotational band structure, it does not help the excess intensity problem significantly. It does point out that O+ must play an important role in intensity problems.
Biological response to mesoscale eddies in the Algerian Basin
Authors: Taupier‐Letage I. , Puillat I. , Millot C. , Raimbault P.
Published: August 2003

The Eddies and Leddies Interdisciplinary Study in the Algerian Basin (ELISA) experiment (1997–1998, MAST‐3/MTP2/MATER program) was a multidisciplinary and multiplatform experiment designed to study the anticyclonic Algerian Eddies (AEs) generated by the instability of the Algerian Current and their influence on the general circulation and biological phenomena. This paper presents preliminary results of the data obtained over the four year‐round cruises ELISA‐1 to 4. Two AEs (called 96‐1 and 97‐1) were tracked with satellite images over their counterclockwise circuit in the eastern Algerian Basin, from the Algerian to the Sardinian slope, and then to the open sea. They have been sampled over different periods and positions. Associated biological response was analyzed considering the hydrodynamical structure and the distribution of chlorophyll and nitrate concentration in the upper 300 m. In summer both AEs (96‐1 located along slope and 97‐1 located offshore) corresponded to highly oligotrophic areas. The deep chlorophyll maximum was ∼90–110 m, with concentrations up to ∼1 mg m−3, the nitrate concentrations were low down to ∼250 m in the AEs' central zone. The downward entrainment of chlorophyll along isopycnals in the AEs' peripheral zone was well observed down to ∼250 m. In spring the maximum integrated chlorophyll concentrations were found offshore, in 96‐1, where the upper ∼150 m were quasi‐homogeneous. Lower integrated chlorophyll concentrations were found inshore in 97‐1, which was embedded in an Algerian Current meander and remained stratified throughout wintertime. AEs generate secondary phenomena such as small‐scale cyclonic shear eddies, where the highest chlorophyll concentrations (∼4 mg m−3) were found. We show that through the AEs it generates, the Algerian Current can be responsible for producing areas in the coastal zone that are at least as oligotrophic as the eastern Mediterranean and, alternately, for productive areas offshore. As AEs generally follow a counterclockwise circuit in the Algerian Basin, it is inferred that they play an important role in the redistribution of matter on a basin scale. However, the biological response associated with AEs varies according to their history, a combination of trajectory, location, and season, in ways that are not yet clear.
Melt supply variations to a magma‐poor ultra‐slow spreading ridge (Southwest Indian Ridge 61° to 69°E)
Authors: Cannat Mathilde , Rommevaux‐Jestin Céline , Fujimoto Hiromi
Published: August 2003

The Southwest Indian Ridge (SWIR) to the east of the Melville Fracture zone receives anomalously low volumes of melt on average. However, a small number of ridge segments appear to receive more melt than this regional average. We use off‐axis bathymetry, gravity, and magnetic data to show that this melt distribution pattern, quite distinct from what is observed at the Mid‐Atlantic Ridge (MAR), has been a characteristic of the easternmost SWIR for at least the past 10 myr. We also show that segments of the easternmost SWIR are substantially shorter lived than most segments of the MAR. Melt distribution in our SWIR study area is therefore both more focused and more variable in time than at the MAR. We tentatively propose a mechanism by which strong and transient melt‐focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere, causing thermal thinning of this lithosphere and along‐axis melt migration. These two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments. Rapid melt extraction by dikes that feed large volcanic constructions on the seafloor, followed by tectonic disruption of these volcanic constructions by deep‐reaching faults, may then cool the axial lithosphere back to its original thickness and end the melt‐focusing events. The easternmost SWIR is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks. At the faster spreading MAR, similar characteristics are found near the ends of ridge segments. We propose that spreading at the ultra‐slow SWIR during periods when the melt supply is low (i.e., most of the time for the easternmost SWIR) is dominated by large offset asymmetric normal faulting, with significant flexural uplift of the footwalls. Faults face either north or south, and changes in fault polarity are frequent, both along axis and along flow lines (i.e., with time). Producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick, a predictable characteristic for this ultra‐slow ridge, which has an anomalously low regionally averaged melt supply.
Active deformation and shallow structure of the Wagner, Consag, and Delfín Basins, northern Gulf of California, Mexico
Authors: Persaud Patricia , Stock Joann M. , Steckler Michael S. , Martín‐Barajas Arturo , Diebold John B. , González‐Fernández Antonio , Mountain Gregory S.
Published: August 2003

Oblique rifting began synchronously along the length of the Gulf of California at 6 Ma, yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern Gulf. Instead, multichannel seismic data show a broad shallow depression, ∼70 × 200 km, marked by active distributed deformation and six ∼10‐km‐wide segmented basins lacking well‐defined transform faults. We present detailed images of faulting and magmatism based on the high resolution and quality of these data. The northern Gulf crust contains a dense (up to 18 faults in 5 km) complex network of mainly oblique‐normal faults, with small offsets, dips of 60–80° and strikes of N‐N30°E. Faults with seafloor offsets of tens of meters bound the Lower and two Upper Delfín Basins. These subparallel basins developed along splays from a transtensional zone at the NW end of the Ballenas Transform Fault. Twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone. A structural connection between the two Upper Delfín Basins is evident in the switching of the center of extension along axis. Sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material, atypical of mid‐ocean ridges. On the basis of the near‐surface manifestations of active faulting and magmatism, seafloor spreading will likely first occur in the Lower Delfín Basin. We suggest the transition to seafloor spreading is delayed by the lack of strain‐partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover.
Late Carboniferous to Permian remagnetization of Devonian limestones in the Ardennes: Role of temperature, fluids, and deformation
Authors: Zegers T. E. , Dekkers M. J. , Bailly S.
Published: August 2003

To test various remagnetization mechanisms in unmetamorphosed to very low grade metamorphic carbonates, we integrated paleomagnetic and rock magnetic data of Devonian carbonates with geological observations in the well‐studied Rhenohercynian Belt of France and Belgium. Paleomagnetic results from 42 sites show that there are three separate remanence components. Hysteresis measurements that are consistent with unremagnetized limestones and the low paleotemperature of the Boulonnais area (<55°C) indicate that component B is a primary Middle Devonian natural remanent magnetization (NRM). Prefolding to postfolding Carboniferous component C and postfolding Early Permian component P both occur throughout the Ardennes and Brabant Massif. Rock magnetic results indicate that component P is carried by pyrrhotite and component C is carried by a magnetite mix that straddles the single‐domain‐multidomain grain‐size range (10–30 nm). The relative intensity factor, Ri, defined as ln(intensity P/intensity C), shows a marked spatial correlation with the southern of two Mississippi Valley‐type (MVT) ore districts. This, in combination with the timing of remagnetization, suggests that component P pyrrhotite formed as a result of percolation of MVT fluids through these carbonates. Fine‐grained magnetite formation, as a by‐product of smectite‐to‐illite conversion in the presence of host‐rock‐buffered internal fluids during deformation, is the most likely remagnetization mechanism for component C. Pressure solution deformation processes may have enhanced the smectite‐to‐illite conversion and hence remagnetization.
Magma supply dynamics at Westdahl volcano, Alaska, modeled from satellite radar interferometry
Authors: Lu Zhong , Masterlark Timothy , Dzurisin Daniel , Rykhus Russell , Wicks Charles
Published: August 2003

A group of satellite radar interferograms that span the time period from 1991 to 2000 shows that Westdahl volcano, Alaska, deflated during its 1991–1992 eruption and is reinflating at a rate that could produce another eruption within the next several years. The rates of inflation and deflation are approximated by exponential decay functions having time constants of about 6 years and a few days, respectively. This behavior is consistent with a deep, constant‐pressure magma source connected to a shallow reservoir by a magma‐filled conduit. An elastic deformation model indicates that the reservoir is located about 6 km below sea level and beneath Westdahl Peak. We propose that the magma flow rate through the conduit is governed by the pressure gradient between the deep source and the reservoir. The pressure gradient, and hence the flow rate, are greatest immediately after eruptions. Pressurization of the reservoir decreases both the pressure gradient and the flow rate, but eventually the reservoir ruptures and an eruption or intrusion ensues. The eruption rate is controlled partly by the pressure gradient between the reservoir and surface, and therefore it, too, decreases with time. When the supply of eruptible magma is exhausted, the eruption stops, the reservoir begins to repressurize at a high rate, and the cycle repeats. This model might also be appropriate for other frequently active volcanoes with stable magma sources and relatively simple magma storage systems.
Observations of Black Sea mesoscale eddies and associated horizontal mixing
Authors: Zatsepin Andrei G. , Ginzburg Anna I. , Kostianoy Andrey G. , Kremenetskiy Vyacheslav V. , Krivosheya Vladimir G. , Stanichny Sergey V. , Poulain Pierre‐Marie
Published: August 2003

A large (80–90 km in diameter) anticyclonic eddy centered at 43°N, 37–38°E has been the subject of complex investigation in the summer‐autumn of 1999 based on measurements carried out during the “Black Sea'99” expedition on board of R/V Akvanavt (CTD surveys, deployment of Argos‐tracked drifters), analysis of satellite imagery, and using altimetric sea level anomaly maps from merged TOPEX/POSEIDON and ERS‐2 satellite data. The eddy was formed as a nearshore anticyclonic eddy (NAE) in the Sochi‐Sukhumi region, separated from the coast on 6–9 April 1999, stayed at the center of the eastern basin, which is usually characterized by cyclonic circulation, during about 8 months and decayed near the Turkish coast in December 1999. A compilation of hydrodynamic situations of different years (1993, 1997–1999) suggests that similar open sea anticyclonic eddies are frequently the elements of the circulation in the eastern Black Sea in the warm season (April–December). A positive correlation appears to be between eddies' formation and weak macroscale circulation associated with low atmospheric wind forcing. Long lifetime of open sea anticyclones is likely determined by their interaction with neighboring eddies. NAEs' separation from the coast and their transformation into open sea eddies provide horizontal mixing of the upper layer waters and results in deflection of the Rim Current offshore, formation of large meanders of the current around the eddies, and its branching when rounding such features. An example of calculation of cross‐shelf water transport relating to an offshore Rim Current branch is presented. It is pointed out that an estimation of the shelf/open sea water exchange based on the box balance model does not contradict the assumption that such exchange is considerably determined by NAEs' separation from the coast.
Sources of global warming of the upper ocean on decadal period scales
Authors: White Warren B. , Dettinger Michael D. , Cayan Daniel R.
Published: August 2003

Recent studies find global climate variability in the upper ocean and lower atmosphere during the twentieth century dominated by quasi‐biennial, interannual, quasi‐decadal and interdecadal signals. The quasi‐decadal signal in upper ocean temperature undergoes global warming/cooling of ∼0.1°C, similar to that occurring with the interannual signal (i.e., El Niño–Southern Oscillation), both signals dominated by global warming/cooling in the tropics. From the National Centers for Environmental Prediction troposphere reanalysis and Scripps Institution of Oceanography upper ocean temperature reanalysis we examine the quasi‐decadal global tropical diabatic heat storage (DHS) budget from 1975 to 2000. We find the anomalous DHS warming tendency of 0.3–0.9 W m−2 driven principally by a downward global tropical latent‐plus‐sensible heat flux anomaly into the ocean, overwhelming the tendency by weaker upward shortwave‐minus‐longwave heat flux anomaly to drive an anomalous DHS cooling tendency. During the peak quasi‐decadal warming the estimated dissipation of DHS anomaly of 0.2–0.5 W m−2 into the deep ocean and a similar loss to the overlying atmosphere through air‐sea heat flux anomaly are balanced by a decrease in the net poleward Ekman heat advection out of the tropics of 0.4–0.7 W m−2. This scenario is nearly the opposite of that accounting for global tropical warming during the El Niño. These diagnostics confirm that even though the global quasi‐decadal signal is phase‐locked to the 11‐year signal in the Sun's surface radiative forcing of ∼0.1 W m−2, the anomalous global tropical DHS tendency cannot be driven by it directly.
Analysis of measurements of Saharan dust by airborne and ground‐based remote sensing methods during the Puerto Rico Dust Experiment (PRIDE)
Authors: Reid Jeffrey S. , Kinney James E. , Westphal Douglas L. , Holben Brent N. , Welton Ellsworth J. , Tsay Si‐Chee , Eleuterio Daniel P. , Campbell James R. , Christopher Sundar A. , Colarco P. R. , Jonsson Haflidi H. , Livingston John M. , Maring Hal B. , Meier Michael L. , Pilewskie Peter , Prospero Joseph M. , Reid Elizabeth A. , Remer Lorraine A. , Russell Philip B. , Savoie Dennis L. , Smirnov Alexander , Tanré Didier
Published: August 2003

For 26 days in mid‐June and July 2000, a research group comprised of U.S. Navy, NASA, and university scientists conducted the Puerto Rico Dust Experiment (PRIDE). In this paper we give a brief overview of mean meteorological conditions during the study. We focus on our findings on African dust transported into the Caribbean utilizing a Navajo aircraft and AERONET Sun photometer data. During the study midvisible aerosol optical thickness (AOT) in Puerto Rico averaged 0.25, with a maximum >0.5 and with clean marine periods of ∼0.08. Dust AOTs near the coast of Africa (Cape Verde Islands and Dakar) averaged ∼0.4, 30% less than previous years. By analyzing dust vertical profiles in addition to supplemental meteorology and MPLNET lidar data we found that dust transport cannot be easily categorized into any particular conceptual model. Toward the end of the study period, the vertical distribution of dust was similar to the commonly assumed Saharan Air Layer (SAL) transport. During the early periods of the study, dust had the highest concentrations in the marine and convective boundary layers with only a weak dust layer in the SAL being present, a state usually associated with wintertime transport patterns. We corroborate the findings of Maring et al. [2003] that in most cases, there was an unexpected lack of vertical stratification of dust particle size. We systematically analyze processes that may impact dust vertical distribution and speculate that dust vertical distribution predominately influenced by flow patterns over Africa and differential advection coupled with fair weather cloud entrainment, mixing by easterly waves, and regional subsidence.
GOES 8 retrieval of dust aerosol optical thickness over the Atlantic Ocean during PRIDE
Authors: Wang Jun , Christopher Sundar A. , Reid Jeffrey S. , Maring Hal , Savoie Dennis , Holben Brent N. , Livingston John M. , Russell Philip B. , Yang Shi‐Keng
Published: August 2003

Using 30 days of half‐hourly, high temporal resolution GOES 8 imager data and radiative transfer calculations, dust aerosol optical thickness (AOT) was retrieved over the Atlantic Ocean (14°N ∼ 26°N, 73°W–63°W) during the Puerto Rico Dust Experiment (PRIDE). Dust aerosol size distributions and complex index of refraction inferred from ground‐based measurements (1.53–0.0015i at 0.55 μm), which were used in Mie calculations and a plane‐parallel discrete ordinate radiative transfer model (DISORT) to compute look up tables for AOT retrievals. Using a combination of spectral, spatial, and temporal tests, a dust detection algorithm was developed from the GOES 8 imager data. The degradation of the signal response relative to the prelaunched calibration of the GOES 8 visible channel was 39% in July 2000 and the GOES 8 AOT detection limit was estimated to be 0.04 in AOT (0.67 μm). The satellite‐retrieved AOT were then compared with AOT values derived from ground‐based Sun photometer (SP) sites. The comparison showed that GOES 8 retrieved AOT are in good agreement with the SP derived values, with linear correlation coefficient of 0.91 and 0.80 for the two sites. The GOES 8 monthly mean 0.67 μm AOT (0.19 ± 0.13, 0.22 ± 0.12) over the two SP sites matched the monthly mean SP AOT values (0.23 ± 0.13, 0.22 ± 0.10). The linear correlation between the GOES 8 retrieved AOT and the aircraft derived values from particle probe data and airborne Sun photometer AATS‐6 measurements were 0.88 and 0.83, respectively. Besides the uncertainties from the nonspherical effect of dust aerosols, sensitivity studies showed that the uncertainties (Δτ) of the GOES 8 retrieved AOT values were mainly from the uncertainties due to the imaginary part of refractive index (Δτ = ±0.05) and surface reflectance [Δτ = ±(0.02 ∼ 0.04)]. This paper demonstrates the application of geostationary satellites to detect and retrieve dust AOT even at low to moderate AOTs. The GOES 8 imager with high temporal resolutions also captures aerosol diurnal variation in this study that can further reduce the uncertainties in the current aerosol forcing estimations caused by the high temporal variations of AOT, thereby playing a complementary role with global AOT retrievals from polar orbiting satellites.
An observational study of drizzle formation in stratocumulus clouds for general circulation model (GCM) parameterizations
Authors: Pawlowska Hanna , Brenguier Jean‐Louis
Published: August 2003

Climate model parameterization of precipitation formation in boundary layer stratocumulus clouds is a challenge that needs to be carefully addressed for simulations of the aerosol impact on precipitation and on cloud life time and extent, the so‐called second indirect effect of aerosol on climate. Existing schemes are generally tuned against global observations of the liquid water path, as very few in situ observations are available for their validation. This issue is addressed here with data collected during the second Aerosol Characterization Experiment. The methodology is different from previous experimental studies in the sense that each case study is first analyzed for retrieving properties that are representative of the observed cloud system as a whole, such as the cloud system geometrical thickness, droplet concentration, precipitation flux, etc. Special attention is given to the characterization of the droplet number concentration by deriving a value that is representative of the aerosol activation process instead of the mean value over the cloud system. The analysis then focuses on the variability of these cloud system values for eight case studies with different aerosol backgrounds. The data set reveals that precipitation forms when the maximum mean volume droplet radius in the cloud layer reaches values >10 μm, the same critical value as previously used in cloud resolving models. This maximum radius can be predicted with an adiabatic diagnostic on the basis of cloud geometrical thickness and droplet number concentration. The measured reduction rate of drizzle water content by precipitation is also compared to predictions of auto‐conversion and accretion production rates derived from existing bulk parameterizations initialized with the measured values of cloud droplet and drizzle water content. The good agreement with the parameterizations suggests that the cloud layer reaches a nearly steady state characterized by a balance between the production and reduction rates of cloud and drizzle water content. Finally, it is shown that the cloud system precipitation rate can be expressed as a power law of cloud geometrical thickness and cloud droplet number concentration, hence providing a simple large‐scale parameterization of the precipitation process in boundary layer clouds.
Analysis of 1970–1995 trends in tropospheric ozone at Northern Hemisphere midlatitudes with the GEOS‐CHEM model
Authors: Fusco Andrew C. , Logan Jennifer A.
Published: August 2003

The causes of trends in tropospheric ozone at Northern Hemisphere midlatitudes from 1970 to 1995 are investigated with the GEOS‐CHEM model, a global three‐dimensional model of the troposphere driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS). This model is used to investigate the sensitivity of tropospheric ozone with respect to (1) changes in the anthropogenic emission of nitrogen oxides and nonmethane hydrocarbons, (2) increases in methane concentrations, (3) variations in the stratospheric source of ozone, (4) changes in solar radiation resulting from stratospheric ozone depletion, and (5) increases in tropospheric temperatures. Model results indicate that local increases in NOx emissions have caused most of the increases seen in lower tropospheric ozone over Europe and Japan. Increases in methane are responsible for roughly one fifth of the anthropogenically induced increase in tropospheric ozone at northern midlatitudes. However, changes in ozone precursors do not adequately explain either the spatial differences in observed ozone trends across midlatitudes or the observed decreases in ozone over Canada throughout the troposphere. We argue that ozone depletion in the lowermost stratosphere is likely to have reduced the stratospheric source by as much as 30% from the early 1970s to the mid 1990s. Model simulations that account for such a reduction along with reported changes in anthropogenic emissions show steep declines of ozone in the upper troposphere and variable increases in the lower troposphere that are more consistent with observations. Differential temperature trends in summer between North America and Europe may account for at least some of the remaining spatial variation in tropospheric ozone trends. Increases in ultraviolet (UV) radiation due to stratospheric ozone depletion do not appear to significantly reduce tropospheric ozone, except at midlatitudes in the Southern Hemisphere following the breakup of the ozone hole.
Multiple oxygen and sulfur isotopic analyses on water‐soluble sulfate in bulk atmospheric deposition from the southwestern United States
Authors: Bao Huiming , Reheis Marith C.
Published: August 2003

Sulfate is a major component of bulk atmospheric deposition (including dust, aerosol, fog, and rain). We analyzed sulfur and oxygen isotopic compositions of water‐soluble sulfate from 40 sites where year‐round dust traps collect bulk atmospheric deposition in the southwestern United States. Average sulfur and oxygen isotopic compositions (δ34S and δ18O) are 5.8 ± 1.4 (CDT) and 11.2 ± 1.9 (SMOW) (n = 47), respectively. Samples have an oxygen 17 anomaly (Δ17O), with an average value of 1.0 ± 0.6‰. Except for a weak positive correlation between δ18O and Δ17O values (r2 ≈ 0.4), no correlation exists for δ18O versus δ34S, Δ17O versus δ34S, or any of the three isotopic compositions versus elevation of the sample site. Exceptional positive Δ17O values (up to 4.23‰) are found in samples from sites in the vicinity of large cities or major highways, and near‐zero Δ17O values are found in samples close to dry lakes. Comparison of isotopic values of dust trap sulfate and desert varnish sulfate from the region reveals that varnish sulfate has average isotopic values that are ∼4.8‰ lower for δ18O, ∼2.1‰ higher for δ34S, and ∼0.3‰ lower for Δ17O than those of the present‐day bulk deposition sulfate. Although other factors could cause the disparity, this observation suggests a possibility that varnish sulfate may have recorded a long‐term atmospheric sulfate deposition during the Holocene or Pleistocene, as well as the differences between sulfur and oxygen isotopic compositions of the preindustrial bulk deposition sulfate and those of the industrial era.
Wet and dry deposition of sulfur associated with the eruption of Miyakejima volcano, Japan
Authors: Fujita Shin‐ichi , Sakurai Tatsuya , Matsuda Kazuhide
Published: August 2003

Since July 2000 the activity of Miyakejima volcano has become brisk. Sulfur emitted from the volcano has flowed into the Japanese Archipelago and brought high‐concentration episodes of SO2 over a wide area. The daily average emission of SO2 during the period from August 2000 to March 2001 was the same level as the recent emission of SO2 originating from fossil fuel combustion in east Asia. Estimated total (wet plus dry) deposition of sulfur at Tanzawa before the eruption for the most remarkable high‐concentration episodes was almost the same level as the recent total deposition of sulfur over Japan. Estimated total deposition of sulfur after the eruption, on the other hand, was more than three times higher than the level before the eruption. The contribution of wet deposition increased with increasing daily precipitation amount, which indicates that precipitation scavenging had an instant effect in the removal of the volcanic sulfur. In the warm season, in which the subtropical high‐pressure system over the Pacific is dominant, high‐concentration episodes at monitoring stations were sometimes linked together with a time lag, which was explained by the fact that volcanic plumes from Miyakejima were transported north by the prevailing wind system in this season. In the cold season, however, the predominant wind system over the Japanese Archipelago is northwesterly from the Asian continent; most of the volcanic sulfur was thought to flow over the Pacific Ocean. Although no direct correspondence was found between the SO2 dry deposition and the SO2 emission, there was a close parallel temporal variation between the nss‐SO42− wet deposition and the SO2 emission. It was thought that the interaction between the precipitation cloud and the volcanic cloud took place just after the eruption of Miyakejima and that wet deposition plays an important role in the removal of the volcanic sulfur inflow outside the region.
Application of the NARCM model to high‐resolution aerosol simulations: Case study of Mexico City basin during the Investigación sobre Materia Particulada y Deterioro Atmosférico‐Aerosol and Visibility Research measurements campaign
Authors: Munoz‐Alpizar Rodrigo , Blanchet Jean‐Pierre , Quintanar Arturo I.
Published: August 2003

The Northern Aerosol Regional Climate Model (NARCM) is used to study the visibility and three‐dimensional (3‐D) evolution of aerosol distributions within the Mexico City basin. NARCM simulates transport, diffusion, deposition, and size distributions of sulphur aerosol particles in the region. The model assumes only simple sulphur oxidation, not taking explicitly into account the urban air chemistry. Rather, it focuses on detailed aerosol microphysics and 3‐D optical properties. The simulation performance is compared with upper air and ground‐based observations for the following specific days of intensive measurement: 2, 4, and 14 March 1997. Time series at Mexico City airport shows lower values of visibility in the morning due to a shallow mixed layer and higher values in the afternoon following the evolution of the mixed layer depth. Modeled visibility shows large dependence on cardinal direction and size distribution of particles. It is found that better resolution of particle size leads to better representation of coagulation processes and to realistic size distributions which show a characteristic accumulation mode around 0.3 μm. As a result, visibility simulations are closer to those observed at the airport location. Comparing visibility is a stringent test for the model because it requires an accurate representation of 3‐D meteorological fields together with a realistic aerosol simulation.
Upper limits of stratospheric IO and OIO inferred from center‐to‐limb‐darkening‐corrected balloon‐borne solar occultation visible spectra: Implications for total gaseous iodine and stratospheric ozone
Authors: Bösch H. , Camy‐Peyret C. , Chipperfield M. P. , Fitzenberger R. , Harder H. , Platt U. , Pfeilsticker K.
Published: August 2003

We report upper limits of lower stratospheric IO (inferred lowest values: 0.10 ppt, 0.07 ppt, and 0.06 ppt at 20, 15, and 12.5 km, respectively) and OIO (inferred lowest values: 0.10 ppt, 0.06 ppt, and 0.04 ppt at 20, 15, and 12.5 km, respectively) inferred from balloon‐borne solar occultation UV/visible spectroscopy. The spectra were recorded during a series of Laboratoire de Physique Moléculaire et Applications/differential optical absorption spectroscopy (LPMA/DOAS) balloon flights that were conducted at different geophysical conditions, i.e., inside the Arctic winter vortex, at midlatitudes, and at high latitudes in spring, summer, and fall. Photochemical modeling that accounts for the iodine partitioning during the observations allows us to infer upper limits of total inorganic gas‐phase iodine (Iy), i.e., Iy (<20 km) of ≤0.10 ± 0.02 ppt and ≤0.07 ± 0.01 ppt, taking into account and neglecting OIO photolysis, respectively. For the middle stratosphere, the inferred upper limits of total Iy are larger because of the smaller detection sensitivity there. These observations suggest that either much less iodine enters the stratosphere than the amount expected from the stratospheric entry level iodine concentrations (primarily the tropical upper troposphere) or it resides in other minor gaseous species or, eventually, in a nongaseous, i.e., particulate, form in the stratosphere. The implications of our iodine measurements for stratospheric ozone are also briefly assessed using a two‐dimensional model. For the assumed loading of 0.1 ppt Iy the modeled ozone reduction was small compared to a model run without iodine, with a maximum decrease of around 1% in midlatitudes to high latitudes when OIO photolysis is included.
Application of empirical orthogonal functions to evaluate ozone simulations with regional and global models
Authors: Fiore Arlene M. , Jacob Daniel J. , Mathur Rohit , Martin Randall V.
Published: August 2003

Empirical orthogonal functions are used together with standard statistical metrics to evaluate the ability of models with different spatial resolutions to reproduce observed patterns of surface ozone (O3) in the eastern United States in the summer of 1995. We examine simulations with the regional Multiscale Air Quality Simulation Platform model (horizontal resolution of 36 km2) and the global GEOS‐CHEM model (2° × 2.5° and 4° × 5°). As the model resolution coarsens, the ability to resolve local O3 maxima (O3 ≥ 90 ppbv) is compromised, but the spatial correlation improves. This result shows that synoptic‐scale processes modulating O3 concentrations are easier to capture in models than processes occurring on smaller scales. Empirical orthogonal functions (EOFs) derived from the observed O3 fields reveal similar modes of variability when averaged onto the three model horizontal resolutions. The EOFs appear to represent (1) an east‐west pattern associated with frontal passages, (2) a midwest‐northeast pattern associated with migratory high‐pressure systems, and (3) a southeast stagnation pattern linked to westward extension of the Bermuda High. All models capture the east‐west and southeast EOFs, but the midwest‐northeast EOF is misplaced in GEOS‐CHEM. GEOS‐CHEM captures the principal components of the observational EOFs when the model fields are projected onto these EOFs, implying that it can resolve the contribution of the EOFs to the observed variance. We conclude that coarse‐resolution global models can successfully simulate the synoptic conditions leading to high‐O3 episodes in the eastern United States.
Use of dual‐frequency microwave links for measuring path‐averaged rainfall
Authors: Rahimi A. R. , Holt A. R. , Upton G. J. G. , Cummings R. J.
Published: August 2003

Results are presented from an experiment designed to test the use of dual‐frequency microwave attenuation for measuring path‐averaged rainfall. In general, the relation between attenuation and path‐averaged rainfall is nonlinear. However, for carefully chosen pairs of frequencies, the relation between the difference in attenuations and path‐averaged rainfall is well represented by a straight line through the origin. The slope of this line is essentially unaffected by variations in the (unknown) drop size distribution, drop shape, and drop temperature. This paper describes the results obtained from a pair of links operating at 12.8 GHz and 17.6 GHz on a 23.3 km path. Results are shown for three individual events, including comparisons with rainfall estimates obtained from more traditional sources: a local rain gauge network and an adjacent weather radar. We report summary results for 112 events recorded on this path during almost 2 years of observation, and also for 52 events recorded on a nearby 13.9 km path operating at 13.9 GHz and 22.9 GHz. In both cases the results suggest that dual‐frequency links could serve as useful additional tools for the measurement of rainfall, particularly in locations such as urban areas and obscured mountain valleys, where traditional measuring instruments struggle to provide accurate estimates.
Phreatomagmatic explosive origin of Hrad Vallis, Mars
Authors: Wilson Lionel , Mouginis‐Mark Peter J.
Published: August 2003

Hrad Vallis is a ∼370 m deep, ∼800 km long depression located at 34°N, 218°W, in Elysium Planitia, Mars. A distinctive deposit, present on both sides of the source area of Hrad Vallis, has a maximum width of ∼70 km along the strike of the depression, a lower unit extends for nearly 100 km away from the depression on either side, and an upper unit extends for ∼50 km. Viking and Mars Orbiter Camera (MOC) images show lobate deposit boundaries, rheomorphic deformation, and numerous craters 270–580 m in diameter within the deposit. The deposit coincides with a broad topographic rise ∼30 km wide and ∼100–150 m high extending along the sides of the valley for at least 100 km along strike. We propose that these features formed when a dike underlying the axis of the valley generated a shallow sill, which in turn is partly responsible for the rise topography. The remainder of the rise consists of a mud‐like deposit of ejecta from a large‐scale phreatomagmatic explosion due to violent mechanical and thermal mixing between the central part of the sill and ice‐rich rock layers in the upper few hundred meters of the crust in a fuel‐coolant‐type interaction. The observed range and thickness of ejecta and the depth of the Hrad depression imply explosion steam pressures up to 360 MPa, ejecta speeds of ∼400 m s−1, and a sill at least 150 m thick intruded at a depth of ∼350–400 m below the original surface.
Glacial North Atlantic: Sea‐surface conditions reconstructed by GLAMAP 2000
Authors: Pflaumann U. , Sarnthein M. , Chapman M. , d'Abreu L. , Funnell B. , Huels M. , Kiefer T. , Maslin M. , Schulz H. , Swallow J. , van Kreveld S. , Vautravers M. , Vogelsang E. , Weinelt M.
Published: August 2003

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX‐28) modern analog technique with 947 modern analog samples and 119 well‐dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice‐free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45° and 60°N, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12°C. In the subtropical ocean gyres the new reconstruction supports the glacial‐to‐interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2° and 6°N suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6°–8°C during Northern Hemisphere summer. Differences between these SIMMAX‐based and published U37k‐ and Mg/Ca‐based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.
Polarimetric passive microwave remote sensing of wind vectors with foam‐covered rough ocean surfaces
Authors: Zhou Lin , Tsang Leung , Chen Dong
Published: August 2003

In this paper, polarimetric microwave emissions from wind‐generated foam‐covered ocean surfaces are investigated. The foam is treated as densely packed air bubbles coated with thin seawater coating. The absorption, scattering and extinction coefficients are calculated by Monte Carlo simulations of solutions of Maxwell equations of a collection of coated particles. The effects of boundary roughness of ocean surface are included by using the second‐order small perturbation method (SPM) describing the reflection coefficients between foam and ocean. An empirical wavenumber spectrum is used to represent the small‐scale wind‐generated sea surfaces. The iterative method is employed to solve dense media radiative transfer (DMRT) equations, and is applied to calculate results of all four Stokes parameters of rough ocean surfaces. The theoretical results of four Stokes brightness temperatures with typical parameters of foam in passive remote sensing at 10.8 GHz, 19 GHz and 36.5 GHz are illustrated. The azimuth variations of polarimetric brightness temperature are calculated. Emission with various wind speed and foam layer thickness is studied. The results are also compared with those based on Quasi‐Crystalline Approximation (QCA).
Application of the transient storage model to analyze advective hyporheic exchange with deep and shallow sediment beds
Authors: Zaramella Mattia , Packman Aaron I. , Marion Andrea
Published: August 2003

Hydrodynamic exchange between a stream and its bed plays an important role in solute transport in rivers. Stream‐subsurface exchange is known to occur due to several different mechanisms and different approaches have been used to model the resulting solute transport, but there has been little investigation of the ability of the various models to represent specific exchange processes. This work evaluates the ability of the semiempirical transient storage model (TSM) to represent advective hyporheic exchange driven by bed form‐induced pore water flows. The TSM is based on the idealized hypothesis that the flux of contaminants is proportional to the difference in concentration between the bed and the stream. To evaluate the ability of this simplified mass transfer relationship to reproduce advective hyporheic exchange, we apply the TSM to data sets for the exchange of conservative solutes with sand beds in laboratory flumes where bed form‐induced pumping is the dominant exchange mechanism. The results show that the simplified expressions used in the TSM can represent some but not all aspects of the pumping process. The TSM can represent advective exchange with shallow beds that have a defined exchange layer restricted by the presence of an impermeable boundary. In this case, transient storage parameters can be directly related to the streamflow conditions and the channel geometry. However, the TSM does not do a good job of representing exchange with a relatively deep sediment bed, where flow along different advective paths in the bed yields a wide distribution of exchange timescales.