bluuebunny/crossref_metadata_2025 huggingface dataset:
Macroscale water fluxes 3. Effects of land processes on variability of monthly river discharge
Authors: Milly P. C. D. , Wetherald R. T.
Published: December 2002

A salient characteristic of river discharge is its temporal variability. The time series of flow at a point on a river can be viewed as the superposition of a smooth seasonal cycle and an irregular, random variation. Viewing the random component in the spectral domain facilitates both its characterization and an interpretation of its major physical controls from a global perspective. The power spectral density functions of monthly flow anomalies of many large rivers worldwide are typified by a “red noise” process: the density is higher at low frequencies (e.g., <1 y−1) than at high frequencies, indicating disproportionate (relative to uncorrelated “white noise”) contribution of low frequencies to variability of monthly flow. For many high‐latitude and arid‐region rivers, however, the power is relatively evenly distributed across the frequency spectrum. The power spectrum of monthly flow can be interpreted as the product of the power spectrum of monthly basin total precipitation (which is typically white or slightly red) and several filters that have physical significance. The filters are associated with (1) the conversion of total precipitation (sum of rainfall and snowfall) to effective rainfall (liquid flux to the ground surface from above), (2) the conversion of effective rainfall to soil water excess (runoff), and (3) the conversion of soil water excess to river discharge. Inferences about the roles of each filter can be made through an analysis of observations, complemented by information from a global model of the ocean‐atmosphere‐land system. The first filter causes a snowmelt‐related amplification of high‐frequency variability in those basins that receive substantial snowfall. The second filter causes a relatively constant reduction in variability across all frequencies and can be predicted well by means of a semiempirical water balance relation. The third filter, associated with groundwater and surface water storage in the river basin, causes a strong reduction in high‐frequency variability of many basins. The strength of this reduction can be quantified by an average residence time of water in storage, which is typically on the order of 20–50 days. The residence time is demonstrably influenced by freezing conditions in the basin, fractional cover of the basin by lakes, and runoff ratio (ratio of mean runoff to mean precipitation). Large lake areas enhance storage and can greatly increase total residence times (100 to several hundred days). Freezing conditions appear to cause bypassing of subsurface storage, thus reducing residence times (0–30 days). Small runoff ratios tend to be associated with arid regions, where the water table is deep, and consequently, most of the runoff is produced by processes that bypass the saturated zone, leading to relatively small residence times for such basins (0–40 days).
A mixing layer theory for flow resistance in shallow streams
Authors: Katul Gabriel , Wiberg Patricia , Albertson John , Hornberger George
Published: December 2002

A variety of surface roughness characterizations have emerged from nineteenth and twentieth century studies of channel hydraulics. When the water depth h is much larger than the characteristic roughness height ks, roughness formulations such as Manning's n and the friction factor f can be explicitly related to the momentum roughness height zo in the log‐law formulation for turbulent boundary layers, thereby unifying roughness definitions for a given surface. However, when h is comparable to (or even smaller than) ks, the log‐law need not be valid. Using a newly proposed mixing layer analogy for the inflectional velocity profile within and just above the roughness layer, a model for the flow resistance in shallow flows is developed. The key model parameter is the characteristic length scale describing the depth of the Kelvin‐Helmholtz wave instability. It is shown that the new theory, originally developed for canopy turbulence, recovers much of the earlier roughness results for flume experiments and shallow gravel streams. This study is the first to provide such a unifying framework between canopy atmospheric turbulence and shallow gravel stream roughness characterization. The broader implication of this study is to support the merger of a wealth of surface roughness characterizations independently developed in nineteenth and twentieth century hydraulics and atmospheric sciences and to establish a connection between roughness formulations across traditionally distinct boundary layer types.
Uncertainty estimation of well catchments in heterogeneous aquifers
Authors: Stauffer Fritz , Attinger Sabine , Zimmermann Stephanie , Kinzelbach Wolfgang
Published: December 2002

The uncertainty in the boundary of two‐dimensional, steady state well catchments due to the uncertainty of the spatially variable hydraulic conductivity field is investigated. The well discharge rate and the areal recharge rate are assumed constant. The catchment boundary is traced by backward particle tracking in the velocity field. The uncertainty bandwidth of the catchment boundary is approximated in first order by formulating the time‐dependent longitudinal and transversal second moments of the particle displacements along and normal to the mean particle trajectory. Applications of the approach are presented for a set of simple configurations. The results are compared with the results from unconditional numerical Monte Carlo simulations. The comparison allows an assessment of the accuracy, the applicability, and the limits of the method. The approximation corresponds quite well with the Monte Carlo simulations provided that the distance to the domain boundary is sufficiently large.
Hydraulic conductivity, velocity, and the order of the fractional dispersion derivative in a highly heterogeneous system
Authors: Herrick Matt G. , Benson David A. , Meerschaert Mark M. , McCall Katherine R.
Published: December 2002

A one‐dimensional, fractional order, advection‐dispersion equation accurately models the movement of the core of the tritium plume at the highly heterogeneous MADE site. An a priori estimate of the parameters in that equation, including the order of the fractional dispersion derivative, was based on the assumption that the observed power law (heavy) tail of the hydraulic conductivity (K) field would create a similarly distributed velocity field. Monte Carlo simulations were performed to test this hypothesis. Results from the Monte Carlo analysis show that heavy tailedKfields do give rise to heavy tailed velocity fields; however, the exponent of the power law (the tail parameter) describing these two distributions is not necessarily the same. The tail parameter that characterizes a velocity distribution is not solely dependent on the tail parameter that characterizes theKdistribution. TheKfield must also have long‐range dependence so that water may flow through relatively continuous high‐Kchannels.
Sources of alluvium in a coastal plain stream based on radionuclide signatures from the 238U and 232Th decay series
Authors: Yeager Kevin M. , Santschi Peter H. , Phillips Jonathan D. , Herbert Bruce E.
Published: December 2002

Discerning alluvial sources and their change over time or distance is a fundamental question in hydrology and geology, often critical in identifying impacts of human and natural perturbations on fluvial systems. Surfaces of upland interfluves and subsoils, sources of alluvium in the lower Loco Bayou basin, Texas, were distinguished using the isotope ratios 226Ra/232Th, 226Ra/230Th, and 230Th/232Th. Channel alluvium indicates a transition from interfluve surface to subsoil sources during flood (subsoil ∼34% to ∼91%, over about 8 km) and bank‐full stages (subsoil ∼9% to ∼74%, over about 12 km), with distance downstream. These results indicate strong coupling between hillslope and channel processes, reflecting land use change from forested to agricultural, concentrated in lower Loco Bayou. This methodology shows that sediment sources can be differentiated based upon landscape placement where lithologic contrast is absent. The geochemistry, long half‐lives, and fractionation of 238U and 232Th decay series radionuclides during pedogenic and fluvial processes in humid climates suggest that these methods are applicable in a wide variety of fluvial systems.
Intensity‐duration effects on drainage: Column experiments at near‐zero pressure head
Authors: Torres Raymond , Alexander Leslee J.
Published: December 2002

We conducted column experiments to examine the effects of rapidly changing upper boundary flux conditions upon an unsaturated, near‐zero pressure head flow region. An entire 0.75 m long medium sand column was driven to a quasi‐steady pressure head and uniform water content with low‐intensity irrigation. We then spike increased the irrigation rate by 2–9 times the background intensity but 3–4 orders of magnitude below the saturated hydraulic conductivity and for various durations. Thereafter, the irrigation rate was reduced to the prespike value for 1 hour and then stopped. Results show that during the postspike irrigation the pressure head, water content, and column storage decreased to values that were lower than those observed during the prespike interval, despite having the same steady irrigation rate. This observation indicates that a rapid decrease in surface flux upon a near‐zero pressure head region could lead to an enhanced drainage of an unsaturated profile. The conceptual model explaining these observations relies on the “release” of stored water from the bulk soil water flow.
A global multilevel coordinate search procedure for estimating the unsaturated soil hydraulic properties
Authors: Lambot S. , Javaux M. , Hupet F. , Vanclooster M.
Published: December 2002

We present a new inverse modeling procedure to characterize the hydraulic properties of partially saturated soils from soil moisture measurements during a natural transient flow experiment. The inversion of the governing one‐dimensional Richards equation is carried out using the Global Multilevel Coordinate Search optimization algorithm in sequential combination with the local Nelder‐Mead Simplex algorithm (GMCS‐NMS). We introduce this optimization method in the area of unsaturated zone hydrology since it is adapted for solving accurately and efficiently complex nonlinear problems. Several numerical experiments have been conducted to evaluate the proposed inversion method using synthetic error‐free and error‐contaminated data for different textured soils. Inversion of the simulated error‐free data and examination of the related response surfaces demonstrated the uniqueness of the inverse solution and the suitability of the GMCS‐NMS strategy when identifying four key parameters of the hydraulic functions described by the Mualem‐van Genuchten model. Inversion of the error‐contaminated data proved further the good stability of the inverse solution that is consistent with the needs required by real experiments.
Large rate of uptake of atmospheric carbon dioxide by planted forest biomass in Korea
Authors: Choi Sung‐Deuk , Lee Kitack , Chang Yoon‐Seok
Published: December 2002

The Republic of Korea, henceforth referred to as Korea, has successfully implemented intensive programs of reforestation and forest management over the last 30 years to restore its once‐rich forests. This nationwide effort has resulted in a massive accumulation of less than 30‐year‐old tree biomass, which now accounts for about 72% of the total forest biomass in Korea. Here we use a forest tree inventory data set for Korea to calculate the effectiveness of these planted trees in absorbing excess carbon dioxide from the atmosphere during the period 1954–2000. The forest carbon density in Korea has increased from 5–7 megagrams of carbon per hectare (Mg C ha−1, Mg = 106 grams) in the period 1955–1973 to more than 30 Mg C ha−1 in the late 1990s. The calculated carbon uptake has increased from a mean rate of 0.001 petagrams of carbon per year (Pg C yr−1, Pg = 1015 grams) in the period 1955–1973 to as high as 0.012 Pg C yr−1 in recent years, largely due to the 30‐year implementation of reforestation and forest management projects. The contemporary rate of carbon uptake by the total Korean tree biomass is approximately one‐half of the 1994–1998 mean rate of carbon uptake by the total Chinese forest biomass of 0.026 Pg C yr−1 [Fang et al., 2001]; the Chinese forest biomass has recently been found to be a significant carbon sink in northern temperate regions. The observed uptake rate for Korea is remarkably high, considering the fact that the total area of Korean forests is approximately 16 times smaller than that of Chinese forests. Our results show that long‐term rates of carbon sequestration by nationwide forests can be increased substantially through reforestation and forest management.
Weighing magnetic masses or in magnetic fields: A cautionary note
Authors: Rochette P.
Published: December 2002

The magnetization of a weighed sample can influence the magnetic sensor of standard electronic balances. The corresponding weight bias is demonstrated on several meteoritic and terrestrial basalt samples previously exposed to large fields, as well as on small permanent magnets in the range 10–100 g. The mass bias is only a few mg (except for magnets which should never be brought near to an electronic balance), but it can be troublesome in case of differential weighting. Moreover, a similar magnetic bias is observed near laboratory variable field sources with significant gradient (such as electromagnets).
Petrological layering induced by an endothermic phase transition in the Earth's mantle
Authors: Mambole Axel , Fleitout Luce
Published: December 2002

We investigate the effect of an endothermic phase transition on the average petrology of the mantle as function of depth. We study chemical differentiation using a numerical model in which the oceanic lithosphere, comprised of a crustal layer overlying depleted harzburgite, is injected at the ridges. Three families of tracers are used, each one with its own buoyancy, to distinguish oceanic crust, harzburgite and peridotite. The depth of the phase transition and the Clapeyron slope are assumed to be the same whatever the petrological component of the mantle. Velocities are imposed at the surface to mimic plate tectonics. The viscosity is depth and temperature dependent. The negative Clapeyron slope tends to induce a segregation of the heavy material below the phase transition and of the light material above the phase transition with a compositional jump across the transition zone.
Sounding‐rocket observation of O II 83.4‐nm emission over the polar ionosphere
Authors: Yamazaki A. , Tashiro S. , Nakasaka Y. , Yoshikawa I. , Miyake W. , Nakamura M.
Published: December 2002

Results of recent satellite observations indicate that a large amount of O+ are escaping from the terrestrial ionosphere into the magnetosphere. However, either the global distribution or the temporal variation of such O+ escape has not been understood well yet. A 2‐Dimensional observation of oxygen ion emission (e.g., O II 83.4 nm) provides crucial information on the oxygen escape processes. In order to establish the basic technology required for such a 2‐D (i.e., imaging) observation, we have developed an eXtreme Ultra‐Violet (XUV) sensor sensitive to the O II 83.4‐nm emission. In this study we present recent results of the observation of O+ ion escape from the ionosphere in the polar cap region using our XUV sensor carried by the sounding rocket SS‐520‐2. The XUV sensor successfully obtained an altitudinal profile of the intensity of the O II emission from 150 km through 1100 km of altitudes and detected 5–6 Rayleighs of the O II emission from the uppermost part of the ionosphere. The observation suggests that O+ ions convected from the cusp/cleft region exist over the polar ionosphere.
Frozen ground monitoring using DC resistivity tomography
Authors: Hauck C.
Published: December 2002

Time‐lapse DC resistivity tomography is shown to be a useful method for permafrost and frozen ground monitoring in high‐mountain areas. Resistivity changes are related to freezing and thawing processes and monitor the permafrost evolution over monthly to seasonal time scales. A fixed‐electrode array allows measurements independent of the snow cover thickness. The 2‐dimensional tomographic approach yields information about spatially variable transient processes, such as the advance and retreat of freezing fronts. In combination with borehole temperature data, differences in total water content at different depths could be estimated. In addition, the temporal evolution of the unfrozen water content was calculated showing a strong decrease during the winter months in the near‐surface layer and a quasi‐sinusoidal behaviour at greater depths. This approach seems promising for future long‐term monitoring programmes of the permafrost evolution at low cost.
On protons, iron and the high‐pressure behavior of ringwoodite
Authors: Kleppe Annette K. , Jephcoat Andrew P. , Smyth Joseph R. , Frost Daniel J.
Published: December 2002

Raman spectra of a single crystal of hydrous Fo89‐ringwoodite have been measured in a diamond‐anvil cell at room temperature to 60 GPa. Strong peaks corresponding to asymmetric and symmetric stretching vibrations of the SiO4 tetrahedra are observed at 796 and 841 cm−1 respectively. Additional bands occur in the range 709–939 and 100–250 cm−1. The SiO4 stretching modes of the spinel structure shift continuously up to 60 GPa. Near 40 GPa new Raman bands emerge in the region 550–680 cm−1 gaining intensity to the maximum pressure, a change that is reversible on pressure release. This pressure‐induced modification in the structure appears to be related to structural complexity generated by both Fe and proton substitution.
The early major warming in December 2001 – exceptional?
Authors: Naujokat B. , Krüger K. , Matthes K. , Hoffmann J. , Kunze M. , Labitzke K.
Published: December 2002

The early major warming in December 2001 is described and compared to the two other December major warmings in 1998 and 1987, showing a strong tropospheric‐stratospheric coupling in all three cases. We argue that the occurrence of free westward propagating Rossby waves interacting with a forced quasi‐stationary wave number 1 led to these three early events. The possible excitation of these waves is discussed with respect to the tropospheric circulation, which showed strong blockings over the northern Atlantic prior to the early major warmings.
Effects of a turbid hydrothermal plume on the sedimentation rates in a karstic lake
Authors: Serra T. , Colomer J. , Gacia E. , Soler M. , Casamitjana X.
Published: December 2002

The formation of a hydrothermal plume has been recently found in the main basin of a karstic lake. The hydrothermal plume carries a concentration of particles in suspension that are transported up to an equilibrium depth. At the equilibrium level, particles are transported horizontally as a turbidity current. The particle volume concentration in the hypolimnion, where the turbidity current occurs, has been found to be homogeneous for points close to the source of the plume. However, there is a decrease in the particle concentration points from close to the source of the plume to the points far from the source, which indicates that the sedimentation of particles from the turbidity current occurs. This sedimentation process has been attributed mainly to double diffusive sedimentation rather than to single particle sedimentation. Here we demonstrate the effect of this turbidity current on the zonal sedimentation rates at the southern lobe of the lake.
First hydrographic evidence of the southeast Madagascar upwelling cell
Authors: Machu E. , Lutjeharms J. R. E. , Webb A. M. , Van Aken H. M.
Published: December 2002

Hydrographic sections were conducted south of Madagascar during the Agulhas Current Sources EXperiment (ACSEX‐2) cruise survey in March 2001. The East Madagascar Current (EMC), presumably one of the sources of the Agulhas Current, was crossed four times over the Madagascar ridge. The upwelling cell, at the southeastern tip of Madagascar (inshore of the EMC), was hydrographically highlighted for the very first time during ACSEX‐2. The behavior of the EMC south of Madagascar is studied using both hydrographic data and satellite imagery. At the southeastern end of Madagascar, the EMC turns westward and the dramatic change in the shape of the shelf favours the development of a cyclonic eddy that is embedded between the EMC core and the coast. The upwelling is then associated to the presence of this eddy and to wind favorable conditions.
Regression of the Mountains of Mitchel polar ice after the onset of a global dust storm on Mars
Authors: Bonev Boncho P. , James Philip B. , Bjorkman Jon E. , Wolff Michael J.
Published: December 2002

Data from the Mars Orbiter Camera experiment on Mars Global Surveyor show that the 2001 spring regression of the Mountains of Mitchel, a bright region in the south polar cap of Mars, was faster than in 1999. Those two years differed in that a large spring dust storm occurred in 2001, while 1999 was relatively dust free. We have modeled the effects of atmospheric dust on the CO2 sublimation rate, using a Monte Carlo radiative transfer model. We explicitly accounted for the two major effects of atmospheric dust: 1) reduction of the direct solar heating and 2) the opposing increase in the absorption of IR thermal dust emission by the surface. Our modeling supports the hypothesis that this year's faster regression of the Mountains of Mitchel was related to the early spring global dust storm.
Predictability of atmospheric boundary‐layer flows as a function of scale
Authors: Basu S. , Foufoula‐Georgiou E. , Porté‐Agel F.
Published: December 2002

Predictability analysis based on the maximum Lyapunov exponent considers infinitesimal perturbations, which are associated with errors in the smallest fastest‐evolving scales of motion. However, these errors become irrelevant for the predictability of larger scale motions. In this study we employ the newly developed Finite Size Lyapunov Exponent (FSLE) analysis to assess predictability of atmospheric boundary layer flows as a function of scale. We demonstrate the expected enhanced predictability at large scales and quantify the dependence of predictability on the stability of the atmospheric environment.
Fe‐ and C‐self‐diffusion in liquid Fe3C to 15 GPa
Authors: Dobson David P. , Wiedenbeck Michael
Published: December 2002

Iron‐ and C‐self diffusion have been measured in Fe3C composition liquids at 8 and 15 GPa. Diffusivities fall within the range of values for molten metals and scale inversely to the atomic radius of each species. This supports models such as the Stokes‐Einstein relation and the free volume model which relate transport properties to the atomic radius. Along the melting curve, diffusivity is predicted to be constant and we tentatively predict outer core diffusivities at the inner core boundary of 5 × 10−9 m2 s−1 and 7 × 10−9 m2 s−1 for iron and carbon, respectively. This would correspond to a viscosity of around 15 mPas.
High resolution velocity structure beneath Mount Vesuvius from seismic array data
Authors: Scarpa Roberto , Tronca Fabrizio , Bianco Francesca , Del Pezzo Edoardo
Published: December 2002

A high resolution P‐wave image of Mt. Vesuvius edifice has been derived from simultaneous inversion of travel times and hypocentral parameters of local earthquakes, land based shots and small aperture array data. The results give details down to 300–500 m. The relocated local seismicity appears to extend down to 5 km below the central crater, distributed in a major cluster, centered at 3 km below the central crater and in a minor group, with diffuse hypocenters inside the volcanic edifice. The two clusters are separated by an anomalously high Vp region at around 1 km depth. A zone with high Vp/Vs in the upper layers is interpreted as produced by the presence of intense fluid circulation. The highest energy quakes (up to M = 3.6) are located in the deeper cluster, in a high P‐wave velocity zone. Our results favor an interpretation in terms of absence of shallow magma reservoirs.
Pacific warm pool excitation, earth rotation and El Niño southern oscillations
Authors: Yan Xiao‐Hai , Zhou Yonghong , Pan Jiayi , Zheng Dawei , Fang Mingqiang , Liao Xinhao , He Ming‐Xia , Liu W. Timothy , Ding Xiaoli
Published: December 2002

The interannual changes in the Earth's rotation rate, and hence in the length of day (LOD), are thought to be caused by the variation of the atmospheric angular momentum (AAM). However, there is still a considerable portion of the LOD variations that remain unexplained. Through analyzing the non‐atmospheric LOD excitation contributed by the Western Pacific Warm Pool (WPWP) during the period of 1970–2000, the positive effects of the WPWP on the interannual LOD variation are found, although the scale of the warm pool is much smaller than that of the solid Earth. These effects are specifically intensified by the El Niño events, since more components of the LOD‐AAM were accounted for by the warm pool excitation in the strong El Niño years. Changes in the Earth's rotation rate has attracted significant attention, not only because it is an important geodetic issue but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere and solid Earth, and hence the global changes.
Possible existence of small‐scale convection under the back arc
Authors: Honda S. , Saito M. , Nakakuki T.
Published: December 2002

We discuss a possible existence of small‐scale convection under the back arc, which may be a cause of recently proposed along‐arc variation of thermal structure under northeastern Japan. Based on numerical simulations, we find that, under the situation of opening of Japan Sea, a possible existence of low viscosity layer (LVL) below the back arc makes the existence of small‐scale convection be likely. A shear produced by the movement of subducting slab aligns the axes of convection roll perpendicular to the arc. To explain the observed wavelength of along‐arc variation (∼50 to 80 km), our preferred model requires a similar thickness of the LVL whose viscosity is two‐orders of magnitude less than that of surrounding. Another notable feature is the upward movement of the mantle associated with the small‐scale convection. We discuss the water released from the subducting crust as a possible origin of the LVL.
A contribution of slab‐melts to the formation of high‐Mg andesite magmas; Hf isotopic evidence from SW Japan
Authors: Hanyu Takeshi , Tatsumi Yoshiyuki , Nakai Shun'ichi
Published: December 2002

Hf isotope ratios were measured for high‐Mg andesites (HMA) and basalts from the Setouchi volcanic belt, Southwest Japan, in order to examine whether fluid or melt from subducted slabs act as metasomatic agents for HMA magma genesis. Since Hf is one of the incompatible elements that may not be readily transported by aqueous fluids, Hf isotopes should provide strong constraints on the origin of such slab‐derived components. Setouchi HMAs and basalts are distinct in their lower 176Hf/177Hf (εHf < +10.8) than MORBs and Izu‐Bonin‐Mariana arc basalts (>+13), suggesting significant contribution of subducted sediments to Setouchi magma genesis. It should be further stressed that Hf isotopes are well correlated with Sr‐Nd isotopes and La/Nb ratios among the Setouchi magmas. Geochemical modeling suggests that such characteristics can be best explained by addition of slab‐derived components with sediment melts rather than aqueous fluids to the HMA source.
Interpretation of non‐specular radar meteor trails
Authors: Dyrud Lars P. , Oppenheim Meers M. , Close Sigrid , Hunt Stephen
Published: December 2002

Radar data of non‐specular meteor trails shows two clear and consistent features: (1) non–specular meteor trails are observed from a narrower altitude range than are head echoes and (2) an approximately 20 ms delay between meteor head echoes and trail radar scatter. This paper shows that both features can result from meteor trail plasma instability. Simulations have demonstrated that trails often develop Farley‐Buneman/gradient‐drift (FBGD) waves which become turbulent and generate field aligned irregularities (FAI). Plasma stability analysis shows that trails are only unstable within a limited altitude range, matching the observed altitudes of non–specular trails to within 1–2 km. The simulations show that instability develops into turbulence in ∼20 ms and appears to be the only meteor trail process that can explain both the observed delay between head and trail echoes and generate coherent scatter at both UHF and VHF wavelengths.
Composition of individual particles in the wakes of an Athena II rocket and the space shuttle
Authors: Cziczo D. J. , Murphy D. M. , Thomson D. S. , Ross M. N.
Published: December 2002

The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument was used to obtain the first in situ measurements of the composition of particles in the wakes of solid rocket motor (SRMs) launch vehicles. PALMS acquired mass spectra of over 2300 exhaust particles within the plumes of an Athena II rocket and the Space Shuttle. The majority of positive spectra indicated the presence of primary and trace components of the aluminum fuel and the combustion catalyst. Negative spectra showed chlorine from the oxidizer. Nitrate and phosphate fragments and water were common features of spectra acquired during the Space Shuttle encounters. Elemental carbon (EC) was a significant particle type observed in the Athena II plume. The data show that particles emitted by SRMs are more diverse and probably more reactive than previously considered.
Convectively generated mesoscale gravity waves simulated throughout the middle atmosphere
Authors: Horinouchi Takeshi , Nakamura Takuji , Kosaka Jun‐ichi
Published: December 2002

A three‐dimensional simulation was conducted with a cloud‐resolving model to investigate mesoscale gravity waves generated by cumulus convection that propagate to the mesospheric and lower thermospheric (MLT) region. It is found that both individual convective turrets and mesoscale convective systems excite gravity waves, resulting in a broad scale distribution. Waves excited by the former have conically shaped phase surfaces as previously reported and are conspicuous up to around the stratopause. Waves excited by the latter dominate in the MLT region. Frequent wave breaking is found above 85 km, where convective instability is found typically in the initial stage, leading to unstable shear with billows resembling the ripple‐type structures observed frequently in airglow imaging.
Statistical features of substorm indicators during geomagnetic storms
Authors: Lee D.‐Y. , Min K. W.
Published: December 2002

This paper reports statistical features of two substorm indicators, magnetic dipolarization and energetic particle injection at the geosynchronous orbit, during the storm evolution period, which is divided into three phases: main, early recovery, and late recovery. Two separate sets of data for each of the indicators are used from two different sources: GOES and Los Alamos National Laboratory (LANL) spacecraft data for dipolarization and injection, respectively. We find that the statistical features of the two indicators are remarkably similar and well correlated to each other but clearly distinguish between three phases of storms in several aspects: (1) During the main phase, both substorm indicators are strong and most frequent in its occurrence. The occurrence probability of their onsets spreads out over wide local time sectors, ∼82% of onsets being scattered over 20–01 LT. (2) During the early recovery phase, the occurrence frequency of both indicators overall substantially reduces, but their intensity around the midnight is still significant as compared to the main phase. The onset probability now becomes concentrated to more limited sectors, ∼83% of them being concentrated over 21–24 LT with a distinct peak at 23–24 LT. (3) During the late recovery phase, both substorm indicators become much weaker than in the earlier two phases, least frequent in their occurrence, and over 92% of onsets are concentrated to 22–01 LT with a well prominent peak at 23–24 LT again. (4) The proton flux enhancement is greater at higher energy channels for most events of main and early recovery phases while it is prominent only at first two lowest energy channels for the late recovery phase events.
A statistical study on the geoeffectiveness of Earth‐directed coronal mass ejections from March 1997 to December 2000
Authors: Wang Y. M. , Ye P. Z. , Wang S. , Zhou G. P. , Wang J. X.
Published: December 2002

We have identified 132 Earth‐directed coronal mass ejections (CMEs) based on the observations of the Large Angle Spectroscopic Coronagraph (LASCO) and Extreme Ultraviolet Imaging Telescope (EIT) on board of Solar and Heliospheric Observatory (SOHO) from March 1997 to December 2000 and carried out a statistical study on their geoeffectiveness. The following results are obtained: (1) Only 45% of the total 132 Earth‐directed halo CMEs caused geomagnetic storms with Kp ≥ 5; (2) The initial sites of these geoeffective halo CMEs are rather symmetrically distributed in the heliographic latitude of the visible solar disc, while asymmetrical in longitude with the majority located in the west side of the central meridian; (3) The frontside halo CMEs accompanied with solar flares (identified from GOES‐8 satellite observations) seem to be more geoeffective; (4) Only a weak correlation between the CME projected speed and the transit time is revealed. However, for the severe geomagnetic storms (with Kp ≥ 7), a significant correlation at the confidence level of 99% is found.
A survey of streamer and diffuse glow dynamics observed in sprites using telescopic imagery
Authors: Gerken Elizabeth A. , Inan Umran S.
Published: December 2002

While the fine structure in sprites can assume a wide variety of shapes, sizes, and timescales, certain patterns such as upward branching, downward branching, beading, columns, bidirectional streamers, and propagating large‐scale diffuse glow regions are repeatedly observed. Example cases of these streamer and diffuse glow dynamics observed in sprites are presented using video data obtained by a telescopic imaging system in July–August 1998 and are compared to predictions of current sprite and streamer theories. The previously unreported propagating diffuse glows move slowly (∼104 m/s) and are broader than that predicted for a streamer formation at the same altitude. Sudden brightening of slowly developing negative streamers may be indicative of a return stroke process in which the streamers connect with charge in a lowered ionosphere. Meteoric dust particles in the upper atmosphere may be responsible for the fine beading that exists in many negative streamers and may cause plasma enhancements that initiate double‐headed streamers. Beads at the base of columns can glow for over 100 ms while slowly drifting upward (∼104 m/s). Columns may initiate from downward branching positive streamers. Faint positive streamers are observed at the base of and/or preceding large bright sprite events. Some sprites may initiate as double‐headed streamers formed in localized regions of enhanced electron density. A transition between the streamer formation region and the diffuse glow region is observed at ∼80‐km altitude. No fine structure is observed in telescopic images of the diffuse glow region also referred to as the “sprite halo” [Barrington‐Leigh and Inan, 2001].
Ion and neutral density variations during ionospheric storms in September 1974: Comparison of measurement and models
Authors: Richards P. G.
Published: December 2002

This paper examines the ability of the field line interhemispheric plasma (FLIP) physical ionospheric model to reproduce the O+, NO+, O2+, and N2+ ion densities, and nitric oxide density in the ionosphere and thermosphere over Australia in September 1974. The study includes ionospheric electron density data from Australian ionosonde stations as well as ion and neutral density composition data from elliptical orbits of the Atmosphere Explorer‐C (AE‐C) satellite. The mass spectrometer and incoherent scatter (MSIS) neutral atmosphere model accurately reproduces the O and N2 densities during quiet periods in September 1974, and this enables the FLIP model to also accurately reproduce the measured ion and NO densities as well as ion and electron temperatures during quiet periods. However, the MSIS model produces poor results during the disturbed period of 15 and 16 September because it underestimates the increase in neutral temperature by several hundred degrees and the depletion of the O column density by a factor of 2. When the measured neutral densities are used in the FLIP model instead of MSIS densities, there is fair agreement between the measured and modeled O+, O2+, and NO+ densities above 200 km on 15 September, but the FLIP model overestimates the O2+ density below 200 km. The FLIP model is also in fair agreement with the NmF2 from the ionosonde and reproduces the measured O+ density on 16 September but severely overestimates the satellite measured O2+ and NO+ densities at all altitudes. As found in previous studies, the FLIP model was able to accurately reproduce a positive ionospheric storm that occurred during this period. An important result to emerge from this study is the observation that even during these severely disturbed periods the altitude distributions of the measured O and N2 densities can be reasonably well represented by hydrostatic equilibrium profiles. This suggests that the basic philosophy of the MSIS model remains intact and that it should be possible to improve the MSIS model storm behavior just by improving the specification of the neutral boundary densities and the exospheric temperature. The assumption of hydrostatic equilibrium also simplifies the task of deducing neutral composition from remote sensing.
Imaging radar observations and theory of type I and type II quasi‐periodic echoes
Authors: Hysell D. L. , Yamamoto M. , Fukao S.
Published: December 2002

Midlatitude E region plasma irregularities have been investigated using the Middle and Upper Atmosphere (MU) radar in Shigaraki, Japan, and the Clemson 30 MHz radar in South Carolina, USA. A new in‐beam imaging technique has been incorporated in the data analysis. Radar images reveal that the coherent backscatter associated with quasi‐periodic (QP) echoes mainly arrives from spatially localized, elongated scattering regions and that striations in radar range‐time‐intensity (RTI) maps are the signatures of the migrations of these regions through the sparsely filled illuminated volume. The scattering regions in question appear to maintain altitude as they drift. Refraction and finite aspect angle sensitivity permit field‐aligned irregularities in the regions to be detected by radars over a broad range of zenith angles. Circulation observed within the scattering regions is consistent with simulations of polarized E region plasma clouds described in a companion paper, as is the occasional appearance of type I echoes. Several long, continuous bands of scatterers are also evident in the radar images.
Effect of solar wind velocity on magnetic cloud‐associated magnetic storm intensity
Authors: Wu Chin‐Chun , Lepping R. P.
Published: December 2002

We investigate geomagnetic storm activity during periods of 135 magnetic clouds identified with hourly averages of OMNI data from 1965 to 1998. It is found that the storm Dst index correlates well with both the Bz component (correlation coefficient, c.c. = −0.86) and the “rectified” electric field VBS (c.c. = −0.88) but does not correlate well with solar wind speed (c.c. = −0.58), indicating that the role of magnetic cloud speed in predicting storm intensity is a minor one. Solar wind speed does become important in predicting Dst for the studied cloud events when high solar wind speeds are considered. The correlation coefficient (c.c.) for Dst versus Bz increases dramatically when the solar wind speed exceeds 600 km/s. For example, the c.c. for Dst versus Bz is 0.99 for speeds between 600 and 750 km/s (15 events). This implies that solar wind velocity is also important indirectly for predicting the storm intensity when using Bz as a direct predictor. Specially, over the next 2 or 3 years, cloud speed is expected to increase, on average, as has already been observed. This provides us a good opportunity to estimate the intensity of cloud‐associated geomagnetic storms by using the observed upstream Bz.
Seasonal and local time dependences of the interhemispheric field‐aligned currents deduced from the Ørsted satellite and the ground geomagnetic observations
Authors: Yamashita S. , Iyemori T.
Published: December 2002

The precise magnetic field measurements by the Ørsted satellite revealed a distinct spatial variation of the eastward component residual (ΔBϕ) at mid and low latitudes on the dayside. This magnetic field variation was attributed to the interhemispheric field‐aligned currents (IHFACs) at mid and low latitudes based on its comparison with ground geomagnetic field variations. These IHFACs were found to flow from the summer hemisphere to the winter hemisphere in the dawn sector, and from the winter hemisphere to the summer hemisphere in the noon and the dusk sectors. These IHFACs flip their direction not at equinoxes but in May and November. The current intensity of the IHFACs is largest not at solstices but in February and August. The ground‐based geomagnetic field observations at 17 low latitude stations also support the seasonal dependence and local time distribution of the IHFACs.
Annual CO2 balance of a subarctic fen in northern Europe: Importance of the wintertime efflux
Authors: Aurela Mika , Laurila Tuomas , Tuovinen Juha‐Pekka
Published: December 2002

The annual balance of CO2 was measured on a subarctic flark fen located in northern Finland (69°08′N, 27°17′E). Measurements were conducted using the micrometeorological eddy covariance method during the period from April 1998 to April 1999. Special emphasis was put on the importance of the wintertime CO2 balance as a component of the annual budget. The CO2 fluxes observed during the winter were relatively small (0.0055 mg CO2 m−2 s−1), but due to the long duration of the snow cover period, they play an important role in the annual balance of CO2. The efflux during the coldest periods, when soil surface temperatures fell below −10 °C, was enabled by markedly higher peat temperatures (close to 0 °C) in deeper layers. The typical nighttime respiration observed in summer was 0.10 mg CO2 m−2 s−1. The net photosynthesis had a typical daytime peak value of about −0.20 mg CO2 m−2 s−1 in July. Corresponding with the instantaneous CO2 fluxes, the highest daily sink balances of about −9 g CO2 m−2 d−1 were observed in July, while the highest daily respiration balances of about 2.5 g CO2 m−2 d−1 were observed before and after the sink period. The CO2 balance for the winter period (105 g CO2 m−2) was greater than the absolute value of the total annual balance (−68 g CO2 m−2), which shows the importance of the wintertime efflux. After taking into account estimates for CH4 efflux and carbon leaching, an annual carbon balance of −7 ± 5 g C m−2 was obtained for the fen.
Parameterization of shortwave cloud optical properties for a mixture of ice particle habits for use in atmospheric models
Authors: Chou Ming‐Dah , Lee Kyu‐Tae , Yang Ping
Published: December 2002

On the basis of single‐scattering optical properties precomputed with an improved geometric optics method, the bulk absorption coefficient, single‐scattering albedo, and asymmetry factor of ice particles have been parameterized as a function of the effective particle size of a mixture of ice habits, the ice water amount, and spectral band. The parameterization has been applied to computing fluxes for sample clouds with various particle size distributions and assumed mixtures of particle habits. It is found that flux calculations are not overly sensitive to the assumed particle habits if the definition of the effective particle size is consistent with the particle habits on which the parameterization is based. Otherwise, the error in the flux calculations could reach a magnitude unacceptable for climate studies. Different from many previous studies, the parameterization requires only an effective particle size representing all ice habits in a cloud layer but not the effective size of individual ice habits.
A 10‐year spatial and temporal trend of sulfate across the United States
Authors: Malm William C. , Schichtel Bret A. , Ames Rodger B. , Gebhart Kristi A.
Published: December 2002

Legislative and regulatory mandates have resulted in reduced sulfur dioxide (SO2) emissions in both the eastern and western United States with anticipation that concurrent levels of ambient SO2, SO42−, and rainwater acidity would decrease. This paper examines spatial and temporal trends in ambient SO42− concentration from 1988 to 1999, SO2 emissions from 1990 to 1999, and the relationship between these two variables. The SO42− concentration data came from combining data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) and the Clean Air Status and Trends Network (CASTNet). Over 70 sites spread across the continental United States are considered in this analysis. From a spatial perspective, the 90th percentile summer sulfate concentrations are highest along the Ohio River Valley and in central Tennessee where the emission density of SO2 is greatest. These concentrations are a factor of 2 greater than the Northeast, northern Michigan, and coastal areas of the Southeast and about a factor of 15 greater than the central western United States. In the East, the largest SO42− decreases in the 80th percentile concentrations occurred north of the Ohio River Valley, while most monitoring sites south of Kentucky and Virginia showed increasing and decreasing trends that were not statistically significant. Big Bend National Park, Texas, Cranberry, North Carolina, and Lassen Volcanic National Park, California, are the only areas that show a statistically significant increase in SO42− mass concentrations. The 1990–1999 annual 80th percentile SO42− time series were compared to the annual SO2 emissions over four broad United States regions. Each region had a unique time series pattern with the SO42− concentrations and SO2 emissions closely tracking each other over the 10‐year period. Both the SO42− and SO2 emissions decreased in the Northeast (28%) and the West (15%), while there was little change in the Southeast and a 15% increase over Texas, New Mexico, and Colorado.
Atmospheric electric conductivity measurements over the Indian Ocean during the Indian Antarctic Expedition in 1996–1997
Authors: Deshpande C. G. , Kamra A. K.
Published: December 2002

The atmospheric electric conductivity measurements made over the Indian Ocean with a Gerdien's apparatus mounted aboard MV Polarbird during the XVI Indian Scientific Expedition in 1996–1997 are reported. Simultaneous three‐hourly measurements of aerosol concentrations of 13–1000 nm size and some meteorological parameters are also reported. Latitudinal variation of conductivity along the cruise route shows a minimum at ∼28°S. Further, the variations in conductivity in the 10°N–20°S and 60°–70°S latitudinal belts show opposite trends on the outward and return cruises, which fall near to the onset and withdrawal phases of the northeast monsoon season, respectively. The results are explained on the basis of the well‐known northward shift of the subsidence leg of the southern Hadley cell and of the position of the Intertropical Convergence Zone during the months of March–April in this region and the observations of the cyclonic systems near the continent of Antarctica during the period of outward cruise.