bluuebunny/crossref_metadata_2025 huggingface dataset:
Microphysical modeling of the 1999–2000 Arctic winter: 1. Polar stratospheric clouds, denitrification, and dehydration
Authors: Drdla K. , Schoeberl M. R. , Browell E. V.
Published: December 2002

The freezing processes that may lead to the formation of solid phase polar stratospheric clouds (PSCs) have been examined to assess their winter‐long effects, especially denitrification, in a coupled microphysical/photochemical model. Trajectory simulations spanned from November 1999 to April 2000, using a large set of trajectories which provided representative coverage of the entire Arctic vortex through the period of PSC formation and ozone depletion. A freezing process occurring at temperatures above the ice frost point is shown to be necessary to explain both the occurrence of solid phase PSCs early in the winter and denitrification, especially without dehydration. If freezing only occurs below the ice frost point the primary contributor to denitrification is actually sedimentation of liquid phase PSC particles. The mechanism of a second freezing process, occurring above the ice frost point, can not yet be conclusively determined. Of the cases considered, heterogeneous freezing of the aerosol to form nitric acid trihydrate (NAT) particles best reproduced solid phase PSC formation and observations of widespread denitrification with limited dehydration. The simulations constrain the number of frozen particles to be near either 0.02% or 1% of the total aerosol number; values in between 0.02% and 1% produce more intense denitrification than observed, demonstrating that small changes in the number of frozen particles could exacerbate denitrification. However, this result was contingent upon assuming that the heterogeneous nuclei remain active, producing PSCs, throughout the winter. An idealized homogeneous freezing process was also able to produce NAT PSCs and denitrification (rates of 106–107cm−3s−1compared favorably with data) but differed from observations in one key aspect: denitrification was more frequently accompanied by dehydration. Nitric acid dihydrate (NAD) particles were less effective than NAT at denitrification, but heterogeneous freezing of 0.1% of the aerosol yielded results marginally consistent with measurements. An important limitation, however, of all the scenarios considered is that they produced more intense and more widespread dehydration than was observed. This suggests that model minimum temperatures (from UK Meteorological Office analyses) were too cold by 1 to 3 K.
Patterns and dynamics of treeline advance on the Seward Peninsula, Alaska
Authors: Lloyd Andrea H. , Rupp T. Scott , Fastie Christopher L. , Starfield Anthony M.
Published: December 2002

Boreal tree species are expected to invade tundra ecosystems as climate warms. Because forested ecosystems differ from tundra ecosystems in a number of climatically relevant characteristics, this advance of the altitudinal and latitudinal tree limit may ultimately feedback on regional climate. We used tree rings to reconstruct the response of the treeline ecotone on the Seward Peninsula in northwestern Alaska to 20th century warming. We further explored the sensitivity of the treeline ecotone to climate change using the spatially explicit, frame‐based model ALFRESCO. Our reconstructions of forest response to past warming indicate that in upland tundra spruce have successfully established progressively farther from the forest limit since the 1880s. Shrub tundra has thus been converted to low‐density forest–tundra within a band extending approximately 10 km from the forest limit. In lowland sites, where soil thermal and hydrologic properties are the primary constraint on tree distribution, trees began establishing in tundra after 1920, and establishment was restricted to areas that had experienced thermokarst activity in the past. Modeling experiments suggested that changes in disturbance regime and spruce growth response to climate may introduce strongly nonlinear responses to climate change at treeline. Field data and model experiments thus both indicate that large and nearly instantaneous responses to warming are likely at the treeline ecotone but that sensitivity to warming is likely to vary substantially over space and time.
Climate/chemistry effects of the Pinatubo volcanic eruption simulated by the UIUC stratosphere/troposphere GCM with interactive photochemistry
Authors: Rozanov E. V. , Schlesinger M. E. , Andronova N. G. , Yang F. , Malyshev S. L. , Zubov V. A. , Egorova T. A. , Li B.
Published: December 2002

The influence of the sulfate aerosol formed following the massive Pinatubo volcanic eruption in June 1991 on the chemical composition, temperature, and dynamics of the atmosphere has been investigated with the University of Illinois at Urbana‐Champaign (UIUC) stratosphere–troposphere General Circulation Model (GCM) with interactive photochemistry (ST‐GCM/PC). Ensembles of five runs have been performed for the unperturbed (control) and perturbed (experiment) conditions. The simulated repartitioning within the chlorine and nitrogen groups, as well as the ozone changes, are in reasonable quantitative agreement with observations and theoretical expectations. The simulated ozone changes in the tropics reveal the ozone mixing ratio decreases below 28 km and increases in the stratosphere above this level. However, these changes are not statistically significant in the lowermost stratosphere. The simulated total ozone loss reached 15% over the northern middle and high latitudes in winter and early spring. However, the simulated changes are statistically significant only during early winter. The magnitude of the simulated total ozone depletion is generally less than that observed, but some members of the experiment ensemble are in better agreement with the observed ozone anomalies. The model simulates a pronounced stratospheric warming in the tropics, which exceeds the warming derived from observations by 1–2 K. The model matches well the intensification of the polar‐night jet (PNJ) in December 1991 and 1992, the statistically significant cooling of the lower stratosphere and warming of the surface air in boreal winter over the United States, northern Europe, and Russia, and the cooling over Greenland, Alaska, and Central Asia.
Vortexwide denitrification of the Arctic polar stratosphere in winter 1999/2000 determined by remote observations
Authors: Kleinböhl Armin , Bremer Holger , von König Miriam , Küllmann Harry , Künzi Klaus F. , Goede Albert P. H. , Browell Edward V. , Grant William B. , Toon Geoffrey C. , Blumenstock Thomas , Galle Bo , Sinnhuber Björn‐Martin , Davies Stewart
Published: December 2002

Denitrification has been studied using measurements of stratospheric HNO3 and N2O by the Airborne Submillimeter Radiometer (ASUR), operated on board the NASA DC‐8 during SOLVE/THESEO 2000. Lidar measurements taken on board the same aircraft have been used to distinguish between temporary uptake of HNO3 in polar stratospheric clouds (PSCs) and denitrification events. To derive an NOy budget, ClNO3 data by balloonborne and ground‐based Fourier transform infrared measurements and a model estimate of NOx + 2N2O5 have been considered. The HNO3 profiles of sporadic ASUR measurements without PSC coverage in January suggest that denitrification had started in the vortex core region by then. Vortexwide denitrification was found in mid‐March 2000. Corrected for diabatic descent using the N2O measurements, a vortex‐averaged NOy deficit between 1.2 ± 0.9 ppb at about 16 km altitude and 5.3 ± 2.7 ppb at about 20.5 km altitude was derived compared to December 1999, based on an observed decrease in HNO3 between 2.2 and 3.5 ppb during this time period. A shift in the NOy partitioning from HNO3 toward ClNO3 of about 0.4 to 0.7 ppb was observed in mid‐March compared to December, indicating that chlorine deactivation was occurring. Comparisons with the SLIMCAT three‐dimensional chemical transport model applying denitrification schemes based on ice and nitric acid trihydrate particles in equilibrium, respectively, reveal agreement within the error bars at higher altitudes (∼19 km) but show discrepancies at lower altitudes (∼16 km). It is suggested that more sophisticated denitrification schemes are needed to generally describe denitrification processes.
Multidecadal trends in tropical convective available potential energy
Authors: Gettelman A. , Seidel D. J. , Wheeler M. C. , Ross R. J.
Published: December 2002

Time series of convective available potential energy (CAPE) calculated from 15 tropical radiosonde stations indicate mostly positive trends in CAPE during 1958–1997. Increases in CAPE are associated with increases in near‐surface temperature and water vapor, consistent with previous studies. The predominantly positive trends appear mostly as a shift in the middle 1970s, consistent with the time of an apparent shift of the background state of the climate system, as documented by others. A general circulation model of the atmosphere forced by observed sea surface temperatures does not reproduce these overall increases in CAPE, although it does reproduce the temperature trends. The observed changes imply significant changes to the tropical atmosphere over the last 40 years, and potential limitations of climate model simulations.
Two case studies of winter continental‐type water and mixed‐phase stratocumuli over the sea 2. Absorption of solar radiation
Authors: Asano Shoji , Uchiyama Akihiro , Yamazaki Akihiro , Gayet Jean‐François , Tanizono Masayo
Published: December 2002

We have carried out airborne experiments on cloud‐radiation interactions for wintertime boundary layer clouds over the East China Sea and the Japan Sea in January 1999 as part of the Japanese Cloud and Climate Study (JACCS) program. By means of collocated and synchronized flights of two instrumented aircraft, flying above and below the cloud layer, respectively, we directly measured the visible and near‐infrared (IR) solar absorption for two cases of stratiform clouds: one featuring supercooled water stratocumulus cloud polluted by continental aerosols observed on 21 January and one featuring highly inhomogeneous, mixed‐phase stratocumulus clouds observed on 30 January. The former cloud layer, with a geometrical thickness of about 500 m (optical thickness of about 30), absorbed substantial (6%) and significant (21%) amounts of solar radiation in the visible and near‐IR bands, respectively. For the latter mixed‐phase cloud, on an average over a long flight distance, the visible‐band solar absorption was almost zero, while the near‐IR‐band absorption was about 24% for the cloud layer with a mean thickness of 1.3 km. The results indicate no sign of the so‐called anomalous solar absorption for either cloud case. Through radiative transfer simulations for reasonable aerosol‐cloud models, along with back trajectory analysis, the visible‐band solar absorption by the aerosol‐polluted cloud on 21 January was attributed to absorbing aerosols from continental East Asia. The simulation study also suggests that anthropogenic water‐soluble aerosols, as well as soil‐dust particles, may cause appreciable visible absorption by cloud particles, resulting in an enhanced solar heating in the boundary layer cloud.
Numerical study of a very intensive eastern Mediterranean dust storm, 13–16 March 1998
Authors: Tsidulko M. , Krichak S. O. , Alpert P. , Kakaliagou O. , Kallos G. , Papadopoulos A.
Published: December 2002

Presented herein is an analysis of an exceptionally intensive central and eastern Mediterranean dust intrusion of 15–16 March, 1998. The intrusion has been associated with development of an intense cyclone over Africa. The weather and dust development processes were simulated with the Eta weather and dust prediction model. Also presented is a comparison of the model results with the Total Ozone Mass Spectrometer Aerosol Index (TOMS AI) pictures as well as with those of the surface and weather observations. The roles of the main processes responsible for the dust plume development and associated dust intrusion to the eastern Mediterranean (EM) are studied. The observation data as well as the model‐simulated parameters are jointly analyzed. The analysis also includes the data from the back‐trajectory computations. With the aid of the model‐produced dust profiles we studied the significant variation in the altitudes of the dust layers within the cyclone sectors. In the warm sector of the cyclone located over the Mediterranean Sea the dust layer extended up to 8–10 km. Relatively low dust concentration values were found here. In the area of the cold front over Africa the dust was restricted to the lower troposphere and the planetary boundary layer. Here the model simulated high values of the dust concentrations. The sharp TOMS AI increase over Israel and the eastern Mediterranean up to 5.0 units, further away from the dust sources, is explained by the strong winds, by the increased cyclone convergence, and the formation of a two‐layer dust plume.
Footprint analysis using event‐based isotope data for identifying source area of precipitated water
Authors: Yamanaka Tsutomu , Shimada Jun , Miyaoka Kunihide
Published: December 2002

Precipitation samples were collected during 45 events at 18 stations in the Kanto Plain, Japan, during June–September 1997. The hydrogen and oxygen isotope ratios were measured for each event and then the deuterium‐excess parameter d was determined. Spatial variability (a few per mil) of the deuterium excess was significantly less than its temporal variability (20‰). Spatially averaged d was highly correlated with vapor pressure, temperature, and wind vector in the lower atmosphere. The observed d (except for that influenced by evaporation from falling raindrops) was compared to d estimated using meteorological data from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. Estimation of d was based on a simple relationship between the deuterium excess and humidity condition over sea surface. The source area of precipitated water, as indicated by its isotopic footprint, was recognized over seas near Japan as an area where excellent agreement was found between estimated and observed values (e.g., root mean square difference <1). The location of the estimated source area appears to be quite reasonable considering with the direction of the water vapor advection in the lower atmosphere. These results suggest that event‐based d values are useful for tracing the hydrological cycle in the atmosphere.
Net fluxes of CO2 in Amazonia derived from aircraft observations
Authors: Chou Wendy W. , Wofsy Steven C. , Harriss Robert C. , Lin John C. , Gerbig C. , Sachse Glenn W.
Published: December 2002

A conceptual framework is developed using atmospheric measurements from aircraft to determine fluxes of CO2 from a continental land area. The concepts are applied to measurements of CO2, O3, and CO concentrations from the Amazon Boundary Layer Experiment (ABLE‐2B, April–May 1987) to estimate fluxes of CO2 for central and eastern Amazonia late in the wet season of 1987. We observed that column amounts of CO2 from 0 to 3 km decreased during the day over Amazonia at the average rate of −6.3 ± 1 μmol m−2 s−1, corresponding to an uptake flux modestly smaller than the daytime uptake (−10.2 μmol m−2 s−1) at a flux tower in the study area. The estimated net flux of CO2, integrated over 24 hours, was −0.03 ± 0.2 μmol m−2 s−1, indicating that the carbon budget of a substantial area of central Amazonia was close to balance in April 1987. We argue that net CO2 fluxes on the continental scale of Amazonia, with its heterogeneous landscape and large areas of inundation, are strongly modified by the influence of seasonal hydrological factors that enhance respiration and decomposition in forests and wetlands, offsetting growth of forest trees in the wet season.
Relative source contribution analysis using an air trajectory statistical approach
Authors: Lin Ching‐Ho , Chang Len‐Fu W.
Published: December 2002

An air trajectory statistical approach was developed to estimate the relative contributions of various sources within a planning domain for a receptor site during high air pollution. The proposed approach is based on the coupling of residence time analysis and a known emission inventory. The theoretical basis of the approach is detailed. The approach was applied to investigate the relative contributions of various anthropogenic volatile organic compound (VOC) sources to the ozone formation potential of a receptor site in southern Taiwan. One hundred and ninety‐seven ozone events (defined as those with an hourly ozone concentration that exceeded 120 ppb standard) over 1994–1998 were selected to undergo the residence time analysis. The VOC emission inventory was adjusted to reflect the different ozone formation potentials of the various sources by considering the source VOC profiles and the maximum incremental reactivity scales of each VOC compound. The results show that the sources that influenced the evaluated receptor site were located in northwestern coastal regions; the relative contributions of the point, line, and area sources were in the ration of approximately 5:2:3. Two districts, Kaohsiung City and Kaohsiung Hsien, were the dominant contributors of the five investigated districts. The proposed method may act as a preliminary tool to efficiently select the potential source region/category, such that mitigation and control strategies can be targeted, and/or used to guide a comprehensive modeling study.
STAAARTE‐MED 1998 summer airborne measurements over the Aegean Sea 1. Aerosol particles and trace gases
Authors: Formenti Paola , Reiner Thomas , Sprung Detlev , Andreae Meinrat O. , Wendisch Manfred , Wex Heike , Kindred Dave , Dewey Ken , Kent Joss , Tzortziou Maria , Vasaras Alexandros , Zerefos Christos
Published: December 2002

We performed airborne measurements of sulfur dioxide (SO2), carbon monoxide (CO), and aerosol chemical/microphysical properties over the Aegean Sea. The data were collected on board the Met Office C‐130 research aircraft in August 1998. High SO2 mixing ratios up to 18 ppb and aerosol sulfate levels up to 500 ppt were measured near Thessaloniki in northern Greece. The highest concentrations were observed at altitudes between 1 and 2 km, while near the surface much lower SO2 mixing ratios between 1 and 4 ppb were found. The pollution was transported southward over the Aegean Sea as far south as Crete, where SO2 mixing ratios up to 5 ppb were observed. The combined results of the SO2, aerosol sulfate, and particle size distribution measurements indicate air masses containing emissions with different ages, which range from a few hours to several days. Fossil fuel pollution observed over northern Greece is transported from eastern Europe (Bulgaria, Romania, and Turkey). Additionally, our measurements indicate significant contributions from forest fires, in particular from haze layers that originated ten days earlier from fires over the Northwestern Canadian territories, and had crossed the Atlantic and passed over Europe. CO layers 1 to 2 km deep were observed above 1 km. Often a double‐layer structure with a secondary CO layer above 3.5 km was also observed. We estimate that the contribution of biomass burning to the CO measured over Greece is fourfold that of fossil fuel. The lower layer had a significant number of particles in the accumulation mode (Np, size range 0.1–1 μm, ratio Np/CO of the order of 2–6 cm−3 (STP) ppb−1), while the upper layers were particle‐depleted by convection and wet scavenging. The Aitken particle mode (size range 5–100 nm) was depleted both in the lower and in the upper layers. This suggests that the Aitken particles originally present in the lower layers had already grown to the accumulation mode size when detected.
Summertime tropospheric ozone over China simulated with a regional chemical transport model 2. Source contributions and budget
Authors: Ma Jianzhong , Zhou Xiuji , Hauglustaine Didier
Published: December 2002

A regional three‐dimensional chemical transport model is used to simulate the contributions of various sources to the NOx and O3 distributions in the troposphere over China during a summertime period. O3 tracers are added in the model to simulate the distribution of O3 that originates from stratosphere‐troposphere exchange (STE), lateral boundary import (LBI) and in situ photochemistry via NO, respectively. Tracers for reactive nitrogen species are further incorporated into the model to simulate the distributions of NOx that originate from surface emissions, aviation, lightning, STE and LBI, respectively. Meanwhile, the O3 formed by chemical reactions via the NO tracer originating from these sources is also tagged. The model results indicate that surface emissions are the dominant contributor to the NOx and hence O3 concentrations in the polluted boundary layer of eastern China. At higher altitudes the O3 produced via surface‐emitted NOx accounts for 10–20% of the O3 concentrations over these polluted regions. The impact of aircraft NOx emissions on tropospheric O3 over China is small, contributing up to 2% of the O3 concentrations at cruise altitudes. The net O3 production via surface‐emitted NOx over the region is estimated to be 132 Gg d−1 in the boundary layer and 33 Gg d−1 in the free troposphere. Because there is much more O3 transported into than out of the region and high relative humidity, in situ photochemistry acts as a sink for O3 in the troposphere over the region.
Twentieth century increase of atmospheric ammonia recorded in Mount Everest ice core
Authors: Kang Shichang , Mayewski Paul A. , Qin Dahe , Yan Yuping , Zhang Dongqi , Hou Shugui , Ren Jiawen
Published: December 2002

An NH4+ record covering the period A.D. 1845‐1997 was reconstructed using an 80.4 m ice core from East Rongbuk Glacier at an elevation of 6450 m on the northern slope of Mount Everest. Variations in NH4+ are characterized by a dramatic increase since the 1950s. The highest NH4+ concentrations occur in the 1980s. They are about twofold more than those in the first half of twentieth century. Empirical orthogonal function (EOF) analysis on the eight major ion (Na+, K+, Mg2+, NH4+, Ca2+, NO3−, SO42− and Cl−) series from this core indicates that NH4+ is loaded mainly on EOF3 (60% of NH4+ variance), suggesting that NH4+ has a unique signature. Instrumental sea level pressure (SLP) and regional temperatures are used to explore the relationship between NH4+ variations and both atmospheric circulation and natural source strength over Asia. Higher NH4+ concentrations are associated with an enhanced winter Mongolian High and a deepened summer Mongolian Low. A positive relationship also exists between NH4+ concentrations and regional temperature changes of the GIS Box 36 (Indian subcontinent), indicating that an increase in temperature may contribute to the strengthening of natural ammonia emissions (e.g., from plants and soils). A close positive correlation between NH4+ and acidic species (SO42− plus NO3−) concentrations suggests that a portion of the increase in NH4+ concentrations could be contributed by enhanced atmospheric acidification. Anthropogenic ammonia emissions from enhanced agricultural activities and energy consumption over Asia in concert with population increase since the 1950s appear also to be a significant factor in the dramatic increase of NH4+ concentrations during the last few decades.
Interaction between dynamics and chemistry of ozone in the setup phase of the Northern Hemisphere polar vortex
Authors: Kawa S. R. , Bevilacqua R. M. , Margitan J. J. , Douglass A. R. , Schoeberl M. R. , Hoppel K. W. , Sen B.
Published: December 2002

The morphology and evolution of the stratospheric ozone (O3) distribution at high latitudes in the Northern Hemisphere (NH) are examined for the late summer and fall seasons of 1999. This time period sets the O3 initial condition for the SOLVE/THESEO field mission performed during winter 1999/2000. In situ and satellite data are used along with a three‐dimensional model of chemistry and transport (CTM) to determine the key processes that control the distribution of O3 in the lower‐to‐middle stratosphere. O3 in the vortex at the beginning of the winter season in late November is found to be nearly constant from 500 to above 800 K with a value at 3 ppmv ± ∼10%. Values outside the vortex above 550 K are up to a factor of 2 higher and increase significantly with potential temperature. The seasonal time series of data from POAM shows that the relatively low O3 mixing ratios, which characterize the vortex in late November, are already present at high latitudes at the end of summer in mid‐September before the vortex circulation sets up. Analysis of the CTM output shows that the minimum O3 and increase in variance in the middle stratosphere in late summer are the result of (1) stirring of polar concentric O3 gradients by nascent wave‐driven transport and (2) an acceleration of net photochemical loss with decreasing solar illumination. The segregation of low O3 mixing ratios into the vortex as the circulation strengthens through the fall suggests a possible feedback role between O3 chemistry and the vortex formation dynamics that may need to be better understood in order to make confident predictions of the recovery of NH O3 at high latitudes.
Conservative and oscillation‐less atmospheric transport schemes based on rational functions
Authors: Xiao F. , Yabe T. , Peng X. , Kobayashi H.
Published: December 2002

In this paper, we present two practical schemes for advection transport equation. The schemes, namely Conservative Semi‐Lagrangian with Rational function (CSLR0) and CSLR1, are two new variants of the Constrained Interpolation Profile‐Conservative Semi‐Lagrangian (CIP‐CSL) type methods [Yabe et al., 2001]. In these schemes, the subgrid profile is approximated within a single cell by rational interpolation functions. Both the cell‐integrated average and the values at the two cell interfaces are employed for the interpolation construction. The interface value is computed through a semi‐Lagrangian procedure, while the cell‐integrated quantity is advanced via a flux form formulation that is equivalent to a mass conservative remapping. The schemes are exactly conservative with regards to the cell integration of the advected quantity and have quite small dispersion and diffusion errors. The rational interpolations suppress spurious oscillation, and a continuous profile with a smoothness of at least C0 can be obtained over the whole computational domain. Without any explicit computation of flux or slope limiter, the schemes are more computationally efficient than the piecewise parabolic method (PPM) scheme in the context of 1D. Our numerical tests show that the presented schemes are comparable to the PPM method with regards to numerical dispersion, clipping, and shape preserving.
STAAARTE‐MED 1998 summer airborne measurements over the Aegean Sea 2. Aerosol scattering and absorption, and radiative calculations
Authors: Formenti Paola , Boucher Olivier , Reiner Thomas , Sprung Detlev , Andreae Meinrat O. , Wendisch Manfred , Wex Heike , Kindred Dave , Tzortziou Maria , Vasaras Alexandros , Zerefos Christos
Published: December 2002

Chemical, physical, and optical measurements of aerosol particle properties within an aged biomass‐burning plume were performed on board a research aircraft during a profile descent over a ground‐based site in northeastern Greece (40°24′N, 23°57′E; 170 m asl) where continuous measurements of the spectral downwelling solar irradiance (global, direct, and diffuse) are being made. The aerosol optical depth measured at the ground during the time of overflight was significantly enhanced (0.39 at a wavelength of 500 nm) due to a haze layer between 1 and 3.5 km altitude. The dry particle scattering coefficient within the layer was around 80 Mm−1, and the particle absorption coefficient was around 15 Mm−1, giving a single scattering albedo of 0.89 at 500 nm (dry state). The black carbon fraction is estimated to account for 6–9% of the total accumulation mode particle mass (<1 μm diameter). The increase of the particle scattering coefficient with increasing relative humidity at 500 nm is of the order of 40% for a change in relative humidity from 30 to 80%. The dry, altitude‐dependent, particle number size distribution is used as input parameter for radiative transfer calculations of the spectral short‐wave, downwelling irradiance at the surface. The agreement between the calculated irradiances and the experimental results from the ground‐based radiometer is within 10%, both for the direct and the diffuse components (at 415, 501, and 615 nm). Calculations of the net radiative forcing at the surface and at the top of the atmosphere (TOA) show that due to particle absorption the effect of aerosols is much stronger at the surface than at the TOA. Over sea the net short‐wave radiative forcing (daytime average) between 280 nm and 4 μm is up to −64 W m−2 at the surface and up to −22 W m−2 at the TOA.
Estimating downward cross‐tropopause ozone flux using column ozone and potential vorticity
Authors: Olsen Mark A. , Douglass Anne R. , Schoeberl Mark R.
Published: December 2002

A new method of estimating the downward ozone flux across the midlatitude tropopause is introduced. The algorithm derives the estimate from total column ozone observations. Vertical information is given by analysis potential vorticity fields. This method yields an annual estimate of 500 ± 140 Tg yr−1 stratospheric injection of ozone into the northern hemisphere, midlatitude troposphere. The downward ozone flux exhibits the expected spring maximum and autumn minimum. The annual distribution of the cross‐tropopause ozone transport by latitude is consistent with the seasonal frequency and distribution of baroclinic systems. This algorithm also produces localized results and can thus be applied to a single case or global studies.
Stability of the Martian climate system under the seasonal change condition of solar radiation
Authors: Nakamura Takasumi , Tajika Eiichi
Published: December 2002

Previous studies on stability of the Martian climate system used essentially zero‐dimensional energy balance climate models (EBMs) under the condition of annual mean solar radiation income. However, areal extent of polar ice caps should affect the Martian climate through the energy balance and the CO2 budget, and results under the seasonal change condition of solar radiation will be different from those under the annual mean condition. We therefore construct a one‐dimensional energy balance climate model with CO2‐dependent outgoing radiation, seasonal changes of solar radiation income, changes of areal extent of CO2 ice caps, and adsorption of CO2 by regolith. We have investigated behaviors of the Martian climate system and, in particular, examined the effect of the seasonal changes of solar radiation by comparing the results of previous studies under the condition of annual mean solar radiation. One of the major discrepancies between them is the condition for multiple solutions of the Martian climate system. Although the Martian climate system always has multiple solutions under the annual mean condition, under the seasonal change condition, existence of multiple solutions depends on the present amounts of CO2 in the ice caps and the regolith.
On the origin of a wedge crack within the icy crust of Europa
Authors: Schulson Erland M.
Published: December 2002

We suggest that a crack that developed into a wedge‐shaped band within the icy crust of Europa (at ∼28°S, 170°W [see Schenk and McKinnon, 1989, Figure 4; Prockter et al., 1999, Figure 1]) originated under an applied compressive stress through the operation of a frictional sliding mechanism. We analyze this suggestion using a scale‐independent, sliding crack model and obtain an estimate of the maximum compressive stress to initiate the crack that compares reasonably well with earlier estimates [Helfenstein and Parmentier, 1985; McEwen, 1986; Greenberg et al., 1998] of Europan crustal stresses that are based upon elastic deformation of a shell. We then show through an application of both power law dislocation creep and diffusion creep that colder, near‐surface Europan ice appears to be capable of supporting the deduced stresses with little relaxation over a period from 104 to 105 years.
Apollo 17 landing site: Topography, photometric corrections, and heterogeneity of the surrounding highland massifs
Authors: Robinson Mark S. , Jolliff Bradley L.
Published: December 2002

A high‐resolution digital elevation model for the Apollo 17 landing site (20°N, 31°E) and surrounding region is used to correct the photometry of Clementine ultraviolet‐visible multispectral data to standard viewing and illumination geometry on a pixel‐by‐pixel basis. FeO and TiO2 concentrations were derived from these topographic‐photometrically corrected data, and steep‐sloped highland massifs are used to assess the magnitude of the effects of the correction. The effects are significant, yielding errors as high as 5 wt% FeO and 4 wt% TiO2 (absolute) on 30° slopes. The magnitude of the correction varies for Sun‐facing versus anti‐Sun slopes, with greater changes on anti‐Sun slopes. Because most of the Apollo 17 sample stations were on surfaces with low slope values (<5°), the topography has only a small effect on the use of the Apollo 17 site for calibration of FeO and TiO2 parameters. Topographically corrected data and derived compositional information are used to show accurately the variation in composition of the highland units such as the massifs and the Sculptured Hills on a fine scale. Massif compositions are consistent with mixtures of noritic impact melt and feldspathic granulitic material, plus variable amounts of high‐Ti basalt on flanks at low elevations and pyroclastic deposits at high elevations, as surmised from previous studies. An unexpected enrichment in FeO at intermediate elevations in some places where massif surfaces have not been covered by mass wasting of impact‐melt rich material signals an older volcanic component not previously recognized. This component may be correlative with an old, high‐plains volcanic unit exposed northeast of the landing site and north of the Sculptured Hills. Downslope movement of regolith on steep slopes appears to have resulted in compositional or grain size sorting, leading to an apparent enrichment in FeO and TiO2 from volcanic glass in valleys and draws. The Sculptured Hills have highly variable compositions, ranging from feldspathic patches to exposures of mafic rocks. A strong but localized mafic anomaly within the Sculptured Hills may be a small, postbasin volcanic feature consisting of very low Ti basalt.
South Polar Layered Deposits of Mars: The cratering record
Authors: Koutnik Michelle , Byrne Shane , Murray Bruce
Published: December 2002

Data from the Mars Orbiter Laser Altimeter (MOLA) and Mars Orbiter Camera (MOC) instruments aboard the Mars Global Surveyor (MGS) were used in a detailed search of a selected part of the South Polar Layered Deposits (SPLD) for impact craters. Impact craters with diameters from 0.8 to 5 km were identified from a MOLA‐derived shaded relief map and were primarily validated using individual MOLA tracks and, in select cases, MOC narrow angle images. The resultant crater population determined in this study is at least four times the density of the crater population previously recognized. From these new statistics, we estimate the mean apparent surface age of the SPLD to be 30–100 Ma, depending on the established production model isochrons used. All of these craters are considerably shallower than other Martian craters in the same diameter range. We attribute this shallowness to be the cause of the lower detection rates of previous studies. There is a correlation between crater depth and rim height, which suggests that both erosion and infilling have affected the crater forms. A similar study of the north polar layered deposits uncovered no craters in this diameter range. A limited population of craters smaller than 800 m was uncovered in higher‐resolution MOC narrow angle images. These do not appear to have been degraded to the same degree. This separate population implies a surface exposure age of only 100,000 years and perhaps indicates an event that erased all small craters and degraded and infilled the larger ones.
Development and application of GPS signal multipath simulator
Authors: Byun Sung H. , Hajj George A. , Young Lawrence E.
Published: December 2002

A ray tracing GPS signal multipath simulator which takes into account the signal reflection and diffraction from surrounding objects has been developed. By properly modeling the environment around an antenna and the GPS receiver's tracking loop, this simulator can assess the GPS signal multipath error. Thus, it can be used in the early design phase of an experiment to foretell hazardous environmental configurations that can cause severe multipath. It can also aid in finding the best antenna type, location, and orientation within a given environment, and provides a quantitative estimate of multipath errors on GPS measurements. The capability of the simulator is demonstrated by using the International Space Station (ISS) environment.
Three‐dimensional planar surface‐current equivalence theorem with application to receiving antennas as linear differential operators
Authors: Yaghjian A. D.
Published: December 2002

A surface‐current equivalence theorem that states that the electromagnetic fields outside a three‐dimensional (3‐D) source region can be generated (to any degree of accuracy) by electric and magnetic surface currents that lie in a single plane within the source region is proven by showing that the fields of each spherical electromagnetic multipole can be generated by delta‐function electric and magnetic surface currents in an arbitrarily oriented plane of infinitesimal area located at the origin of the spherical multipole. The theorem can be used to justify integral equation methods based on equivalent surface currents to represent radiated and scattered fields. Application of the theorem to antennas reveals that the output of an arbitrary linear receiving antenna can be expressed in terms of just the transverse spatial derivatives of the transverse components of the incident electric and magnetic fields at a single point in space. The coefficients of the transverse linear differential operator are expressed in terms of the spherical multipole coefficients of the antenna's complex receiving pattern.
Interplay between subduction retreat and lateral extrusion: Tectonics of the Western Carpathians
Authors: Sperner Blanka , Ratschbacher Lothar , Nemčok Michal
Published: December 2002

Analysis of brittle microstructures reveals three Tertiary deformation events in the Western Carpathians: (1) Late Paleogene bedding‐parallel extension; (2) Oligocene (?) to middle Miocene NNE‐SSW compression and ESE‐WNW extension; (3) post‐mid‐Miocene NW‐SE extension. These regional deformations, applied to crustal blocks in the intra‐Carpathian region, resulted from forces which were induced by the coupled plate tectonic processes of subduction retreat beneath the Carpathian arc and lateral extrusion from the Eastern Alps toward the Carpathian region. Material flow from the Alps spread toward east and NE, guided by sinistral strike‐slip zones along the NE trending continental margin. Subduction retreat was terminated by mid‐Miocene “soft” collision of the Inner Western Carpathians with the European foreland. Ongoing subduction retreat beneath the Eastern Carpathians caused post‐mid‐Miocene back arc extension forming the Pannonian basin. Extension spread into the Western Carpathians by reactivating the extrusion‐related strike‐slip faults as normal faults.
Preferential flow and pesticide transport in a clay‐rich till: Field, laboratory, and modeling analysis
Authors: Jørgensen Peter R. , Hoffmann Martin , Kistrup Jens P. , Bryde Claus , Bossi Rossana , Villholth Karen G.
Published: December 2002

This study investigates vertical flow and pesticide transport along fractures in water saturated unoxidized clayey till. From two experimental fields, each 40 m2, 96% and 98%, respectively, of total vertical flow was conducted along fractures in the till, while the remaining 2–4% of flow occurred in the clay matrix at very slow flow rate. An applied dye tracer was observed only along 10–26% of the total fracture length measured on the horizontal surface of the experimental fields. In vertical sections the dyed fracture portions constituted root channels, which penetrated the till vertically along the fractures into the local aquifer at 5 m depth. No dye tracer was observed in the fractures without root channels or in the unfractured clay matrix, suggesting that root growth along the fracture surfaces was the principal agent of fracture aperture enhancement. Using hydraulic fracture aperture values determined from large undisturbed column (LUC) collected from one of the experimental fields, it was estimated that 94% of flow in the fractures was conducted along the fracture root channels, while only 6% of flow was conducted along the fracture sections without root channels. For natural vertical hydraulic gradients (0.8–2.3 at the site), flow rates of 0.8–2 km/d were determined for a fracture root channel, while fracture sections without root channels revealed flow rates of 9–22 m/d. Corresponding flow rates in the unfractured matrix were 7–19 mm/yr. For infiltrated bromide (nonreactive tracer) and mobile pesticides mecoprop (MCPP) and metsulfuron, very rapid migration (0.28–0.5 m/d) and high relative breakthrough concentrations (30–60%) into the aquifer were observed to occur along the fracture root channels using a constant hydraulic gradient of 1. Only traces were measured from infiltration of the strongly sorbed pesticide prochloraz. The concentrations of the bromide and pesticides in the monitoring wells were modeled with a discrete fracture matrix diffusion (DFDM) model coupled with a single porosity model (SP) for the till and aquifer, respectively. Using effective fracture spacings and mean fracture apertures for the fracture channel sections as modeling input parameters for the till, the concentrations observed in the wells of the aquifer could be reasonably approximated.
Particle shape characterization using angle of repose measurements for predicting the effective permittivity and electrical conductivity of saturated granular media
Authors: Friedman Shmulik P. , Robinson David A.
Published: December 2002

The particle shape of sediments, soils, and rocks affects the packing of the material and the subsequent pore geometry; it therefore influences important transport properties, such as electrical conductivity, dielectric permittivity, diffusion coefficient, thermal conductivity, and hydraulic conductivity. It is difficult to quantify the “average” shape characteristics of a granular material so a method, which would capture the three‐dimensional shape characteristics of a granular material in regards to its packing geometry, would be of great benefit. In this article we examine the use of the angle of repose and the maximum angle of stability of a slope of granular material poured into water, as a simple and rapid method of quantifying its “effective” particle shape. A two‐stage concept is discussed relating the measured slope angles to the aspect ratio of an equivalent oblate ellipsoidal particle and using this aspect ratio for predicting the effective permittivity and bulk electrical conductivity of isotropic packings of granular materials. The slope angle‐aspect ratio relationships were established using measurements of bulk electrical conductivity of monosized spherical glass beads and nonspherical sand and tuff grains packed in water to different porosities. This empirical relationship, the choice of an oblate geometry to represent particle shape, and a heuristic mixing model accounting for the effect of neighboring particles on the internal electrical field were tested using measurements of the effective permittivity for those granular media. Good agreement was found between the model predictions and measurements, indicating the physical significance of the above relationships and quantifiers. We have also tested and found high correlations between slope angles and a shape factor determined by image analysis of two‐dimensional particle micrographs. We do not, however, propose to use it for routine particle shape characterization because of the insufficiently informative viewing of oblates from above and because of the expensive equipment and extensive work involve with it. Instead, we propose to use the easily measurable slope angles for routine particle shape characterization because of its simplicity and also because of its beneficial sensitivity to surface roughness and particle angularity effects beyond the aspect ratio of a smooth equivalent ellipsoid.
Mobility of bed material in Harris Creek
Authors: Church Michael , Hassan Marwan A.
Published: December 2002

The mobility of bed material is investigated in a small gravel bed river in which the bed is weakly bimodal in texture and relatively stable. The bed surface remained intact throughout the freshet, and scour was limited to small areas of the bed. Observations of bedload caught in pit traps and displacement of magnetically tagged particles during two spring freshets permit analysis of the mobility of each size fraction. Fractional transport rates plot up to three orders of magnitude below the reference transport rate suggested in the literature. At low flows, no size fraction was fully mobile. At intermediate flows (shear stress >27 Pa) sand was fully mobile while larger material remained partially mobile. At high flows (>43 Pa) the division between the full and partial mobility regimes occurred around 16 mm, which remains finer than the median size of the subsurface material. The largest material scarcely moves at mean annual flood. Magnetic tracers confirmed results obtained from the pit traps. Three distinct zones of mobility can be defined in fractional transport plots: partial mobility, full mobility, and overpassing/suspended. Critical shear stress for incipient motion varied over an order of magnitude; a qualitatively similar result for bimodal sediment was reported by Wilcock and McArdell [1993]. Incipient motion analysis based on the largest grain observed to move yielded an upper envelope implying transport similarity for sizes found on the bed surface, and a lower envelope that corresponds with our (nonsimilar) entrainment criterion.