bluuebunny/crossref_metadata_2025 huggingface dataset:
Derived distribution of floods based on the concept of partial area coverage with a climatic appeal
Authors: Iacobellis Vito , Fiorentino Mauro
Published: September 2002

A new rationale for deriving the probability distribution of floods and help in understanding the physical processes underlying the distribution itself is presented. On the basis of this a model that presents a number of new assumptions is developed. The basic ideas are as follows: (1) The peak direct streamflow Q can always be expressed as the product of two random variates, namely, the average runoff per unit area ua and the peak contributing area a; (2) the distribution of ua conditional on a can be related to that of the rainfall depth occurring in a duration equal to a characteristic response time тa of the contributing part of the basin; and (3) тa is assumed to vary with a according to a power law. Consequently, the probability density function of Q can be found as the integral, over the total basin area A of that of a times the density function of ua given a. It is suggested that ua can be expressed as a fraction of the excess rainfall and that the annual flood distribution can be related to that of Q by the hypothesis that the flood occurrence process is Poissonian. In the proposed model it is assumed, as an exploratory attempt, that a and ua are gamma and Weibull distributed, respectively. The model was applied to the annual flood series of eight gauged basins in Basilicata (southern Italy) with catchment areas ranging from 40 to 1600 km2. The results showed strong physical consistence as the parameters tended to assume values in good agreement with well‐consolidated geomorphologic knowledge and suggested a new key to understanding the climatic control of the probability distribution of floods.
Physical basis of time series models for water table depths
Authors: Knotters Martin , Bierkens Marc F. P.
Published: September 2002

The relationship between precipitation excess and water table depth can be described by empirical time series models such as transfer function noise models, autoregressive exogenous variable models (ARX), and threshold autoregressive self‐exciting open loop models. In this paper these models are interpreted in terms of the water balance of a soil column. A physically based ARX model is used to predict the effect of an intervention on the water table dynamics at two locations. It is shown that the physically based ARX model predicts the effect of interventions reasonably well.
Stochastic analyses of field‐scale pesticide leaching risk as influenced by spatial variability in physical and biochemical parameters
Authors: Loll Per , Moldrup Per
Published: September 2002

Field‐scale pesticide leaching risk assessments were performed by incorporating a numerical, one‐dimensional, water and pesticide transport and fate model into the two‐step stochastic modeling approach by Loll and Moldrup [1998]. The numerical model included first‐order pesticide degradation, linear equilibrium adsorption, and plant uptake of water and pesticide. Simazine was used as a model pesticide, and leaching risk was expressed as the cumulative mass fraction of applied pesticide leached below 100 cm after 1 year. Spatial variability in soil physical and biochemical data, as well as measured meteorological data from an average and a relatively wet year, was considered for two Danish field sites: (1) a coarse sandy soil, with relatively small variability in hydraulic properties, and (2) a sandy loam, with large variability in hydraulic properties. The two‐step stochastic modeling approach was used to investigate the relative impact of spatial variability in saturated hydraulic conductivity Ks, soil‐water retention through the Campbell [974] soil‐water retention parameter b, and pesticide sorption through the organic carbon content (OC). For the coarse sandy soil, field‐scale spatial variability in OC was the single most important parameter influencing leaching risk, whereas for the sandy loam, Ks was found more important than OC. The relative impact of field‐scale spatial variability in these parameters was found independent of the meteorological conditions, whereas the absolute level of leaching risk was highly dependent on the meteorological conditions. Assuming a linear dependency between pesticide half‐life and OC, a unified approach to modeling simultaneous field‐scale variability in biodegradation and adsorption was proposed. Leaching risk assessments based on this approach showed that the parts of the field with both low biological activity and low adsorption capacity contributed with a dramatic increase in leaching risk, and suggested that field‐scale spatial variability in biochemical processes can be of similar or larger importance than both hydraulic properties and meteorological conditions.
Uncertainties in cleanup times for soil vapor extraction
Authors: Massmann Joel , Shock Scott , Johannesen Lise
Published: September 2002

Uncertainties in cleanup times for soil vapor extraction systems are estimated using mass removal curves generated through Monte Carlo analyses. The primary source of uncertainty is assumed to be soil permeability. Four general model components are coupled to quantify uncertainties: (1) a geological uncertainty model, (2) a spill distribution model, (3) a vapor flow model, and (4) a mass removal model. Spatial variability in permeability has a more significant effect on cleanup time than does overall average permeability because of the effects of channeling. For the examples considered in this study, field measurements of air conductivity have a relatively small effect on uncertainties in cleanup time. The examples also suggest that surface covers can significantly reduce cleanup time in cases where the extraction wells are not immediately within the zone of contamination and for relatively large spills. If wells are completed within the contaminated zone, however, these covers can actually increase average cleanup times.
Mapping soil landscape as spatial continua: The Neural Network Approach
Authors: Zhu A.‐Xing
Published: September 2002

A neural network approach was developed to populate a soil similarity model that was designed to represent soil landscape as spatial continua for hydroecological modeling at watersheds of mesoscale size. The approach employs multilayer feed forward neural networks. The input to the network was data on a set of soil formative environmental factors; the output from the network was a set of similarity values to a set of prescribed soil classes. The network was trained using a conjugate gradient algorithm in combination with a simulated annealing technique to learn the relationships between a set of prescribed soils and their environmental factors. Once trained, the network was used to compute for every location in an area the similarity values of the soil to the set of prescribed soil classes. The similarity values were then used to produce detailed soil spatial information. The approach also included a Geographic Information System procedure for selecting representative training and testing samples and a process of determining the network internal structure. The approach was applied to soil mapping in a watershed, the Lubrecht Experimental Forest, in western Montana. The case study showed that the soil spatial information derived using the neural network approach reveals much greater spatial detail and has a higher quality than that derived from the conventional soil map. Implications of this detailed soil spatial information for hydroecological modeling at the watershed scale are also discussed.
Funneled flow mechanisms in a sloping layered soil: Laboratory investigation
Authors: Walter M. T. , Kim J.‐S. , Steenhuis T. S. , Parlange J.‐Y. , Heilig A. , Braddock R. D. , Selker J. S. , Boll J.
Published: September 2002

Artificial capillary barriers are being used to divert water away from sensitive underground regions. Conversely, funneled flow over natural capillary barriers may increase the danger of groundwater contamination by decreasing the travel time and contact area. There have been relatively few experimental studies of capillary barrier flow patterns. In this study, water was applied uniformly across the top surface of a backlit tilting chamber, 1 cm thick, 110 cm high, and 180 cm long, in which a coarse sand layer was imbedded in a fine sand. Bedding slope and water application rates were varied between 0° and 12° and 1 and 3 cm h−1, respectively. After attaining steady state, matric potential was measured along the textural interface, and photos of dye traces were taken in order to visualize streamlines. The funneled flow was characterized by three discrete regions: an initial capillary diversion, a breakthrough region, and a toe diversion. The breakthrough region consisted of a significant zone of partial breakthrough where the vertical flux into the coarse layer was less than the water application rate. The lateral distance of the capillary diversion was explained well by previously published relationships when the water entry value at the textural interface was replaced by lower, observed matric potential at which breakthrough occurred at the most upslope point. The length of the capillary diversion was overpredicted using the air entry value. Finally, the toe of the coarse layer had significant, observed effects on funneled flow patterns, which have previously received little, if any, attention. The results of this study imply that the slope of the coarse layer and infiltration rate will largely govern the effectiveness of capillary barriers and that capillary barriers are less effective than previously assumed.
Influence of ionospheric electron density fluctuations on satellite radar interferometry
Authors: Gray A. Laurence , Mattar Karim E. , Sofko George
Published: September 2002

Evidence is presented that auroral zone ionospheric disturbances can influence satellite radar interferometry (SRI) obtained with the RADARSAT, ERS and JERS‐1 satellites. Fluctuations in ionospheric electron density can lead to an azimuth shift modulation in synthetic aperture radar (SAR) imagery, which can be detected using SRI. Measurements of azimuth shift in SRI can help to differentiate ionospheric from tropospheric propagation problems, and to understand better the impact of the ionosphere on spaceborne SAR. Further, SRI azimuth shift modulation may be useful in mapping patterns of polar auroral zone ionospheric disturbances over large distances.
A global measurement of lower thermosphere atomic oxygen densities
Authors: Grossmann K. U. , Kaufmann M. , Gerstner E.
Published: September 2002

The first global measurements of the atomic oxygen fine structure emission line O(³P1) → O(³P2) at a wavelength of 63µm were performed by the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiments in November 1994 and in August 1997. The measured limb radiances were converted to atomic oxygen densities in the altitude regime from 130 km to 175 km using a radiative transfer code and MSIS thermospheric temperatures. The resultant densities are on the average about 40% below the predictions of the MSIS model for the low solar activity conditions of the CRISTA experiments and at all altitudes covered by the measurement. The densities exhibit considerable horizontal structures which are only in partial agreement with the MSIS predicted relative variations. The latitudinal distribution shows steeper decreases towards the poles than given in MSIS.
Three‐dimensional propagation of interplanetary disturbances detected with radio scintillation measurements at 327 MHz
Authors: Tokumaru Munetoshi , Kojima Masayoshi , Fujiki Ken'ichi , Yokobe Atsushi
Published: September 2002

Interplanetary scintillation (IPS) measurements at 327 MHz have been used to study the three‐dimensional propagation of interplanetary (IP) disturbances between 0.2 AU and near‐Earth distance (∼ 1 AU). IPS data of four events, September 23 and August 24, 1998, and November 4 and 6, 1997, events, in which marked IP disturbances were observed in association with energetic solar flares, have been analyzed in this study. From the analysis of these events it is found that the propagation speed of IP disturbances varies with longitude and latitude. We also found that the fast speed direction does not necessarily agree with the flare normal direction. Slow propagation speeds of IP disturbances appear to be closely associated with the slow speed region of ambient solar wind. This suggests that the large‐scale structure of the solar wind speed may play an important role in determining the propagation speed of IP disturbances, although further study is necessary.
Magnetic cloud evolution: A comparison of analytical and numerical solutions
Authors: Vandas M. , Odstrčil D.
Published: September 2002

The propagation of magnetic clouds in the interplanetary medium between the Sun and Earth is studied using two different numerical methods. No numerical method can account completely for the complexity of the physical processes that occur in the solar wind because approximations need to be made. In solving the time‐dependent MHD equations, a number of simplifications and assumptions are made. These may have significant influence on the results obtained. We compare the results of two‐dimensional simulations obtained from two different MHD codes based on different numerical methods, namely, the two‐step Lax‐Wendroff and the total variation diminishing Lax‐Friedrich numerical schemes. The two codes give similar results, which are in general agreement with observational data. The numerical results are then compared with analytical solutions of an evolution of a flux rope published by Osherovich et al. [1995] and applied to magnetic clouds. It follows from this solution that the polytropic index γ must be less than 1 in order for a cloud to expand, i.e., as is observed. Polytropic index γ > 1 allows only for oscillations of the cloud. In the present paper it is shown that under the same conditions, the analytical solution and the numerical code give the same results, but when more realistic initial conditions are used, the modeled magnetic clouds behave differently.
Role of temperature‐dependent viscosity and surface plates in spherical shell models of mantle convection
Authors: Zhong Shijie , Zuber Maria T. , Moresi Louis , Gurnis Michael
Published: September 2002

Layered viscosity, temperature‐dependent viscosity, and surface plates have an important effect on the scale and morphology of structure in spherical models of mantle convection. We find that long‐wavelength structures can be produced either by a layered viscosity with a weak upper mantle or temperature‐dependent viscosity even in the absence of surface plates, corroborating earlier studies. However, combining the layered viscosity structure with a temperature‐dependent viscosity results in structure with significantly shorter wavelengths. Our models show that the scale of convection is mainly controlled by the surface plates, supporting the previous two‐dimensional studies. Our models with surface plates, layered and temperature‐dependent viscosity, and internal heating explain mantle structures inferred from seismic tomography. The models show that hot upwellings initiate at the core‐mantle boundary (CMB) with linear structures, and as they depart from CMB, the linear upwellings quickly change into quasi‐cylindrical plumes that dynamically interact with the ambient mantle and surface plates while ascending through the mantle. A linear up welling structure is generated again at shallow depths (<200 km) in the vicinity of diverging plate margins because of the surface plates. At shallow depths, cold downwelling sheets form at converging plate margins. The evolution of downwelling sheets depends on the mantle rheology. The temperature‐dependent viscosity strengthens the downwelling sheets so that the sheet structure can be maintained throughout the mantle. The tendency for linear upwelling and downwelling structures to break into plume‐like structures is stronger at higher Rayleigh numbers. Our models also show that downwellings to first‐order control surface plate motions and the locations and horizontal motion of upwellings. Upwellings tend to form at stagnation points predicted solely from the buoyancy forces of downwellings. Temperature‐dependent viscosity greatly enhances the ascending velocity of developed upwelling plumes, and this may reduce the influence of global mantle flow on the motion of plumes. Our results can explain the anticorrelation between hotspot distribution and fast seismic wave speed anomalies in the lower mantle and may also have significant implications to the observed stationarity of hotspots.
Kelvin's age of the Earth paradox revisited
Authors: Stacey Frank D.
Published: September 2002

The age‐of‐the‐Earth paradox that beset geology toward the end of the 19th century has been widely misunderstood. Kelvin showed that if Earth were cooling by thermal diffusion, it would have reached its present state after about 20 Myr, far less time than required by sedimentary geology. The assumptions in this calculation were insecure, and the age estimate would have carried little weight if it had not been comparable to that of the Sun, deduced from its gravitational energy. It was the age limit imposed by the available solar power that inhibited serious rethinking of the terrestrial problem. Contrary to popular supposition, even the discovery of radioactivity did not solve the problem because it contributes negligibly to solar energy. The only possible escape from Kelvin's argument would have been the implausible assumption that the Sun was compositionally heterogeneous, with a strong central concentration of elements such as Si and Fe, in spite of being gaseous and presumably convecting. Since the real Earth age paradox was the inability of available solar power to explain the sedimentary record, it was resolved only by the recognition of thermonuclear fusion in the 1930s. Moreover, although Kelvin's terrestrial heat flux calculation appeared relevant when it indicated an age comparable to that of the Sun, it was not necessary to the paradox and so its invalidation did not remove the paradox.
Full interseismic locking of the Nankai and Japan‐west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS
Authors: Mazzotti Stéphane , Le Pichon Xavier , Henry Pierre , Miyazaki Shin‐Ichi
Published: September 2002

We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan‐west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan‐west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku‐Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan‐west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long‐term deformation of the Japanese islands.
Automatic inference of ice models from postglacial sea level observations: Theory and application to the British Isles
Authors: Johnston Paul J. , Lambeck Kurt
Published: September 2002

Late Pleistocene and Holocene sea level change is caused mostly by melting of ice sheets since the Last Glacial Maximum and the Earth's delayed uplift in response to changes in the surface load. Records of sea level change therefore contain information on Earth rheology and ice sheet history and have been used to constrain ice and Earth models. A method is presented which finds an ice model that simultaneously fits the sea level record and the constraints on ice elevation. It infers the ice model which maximizes the smoothness of the surface and thus produces realistic ice models with the minimum detail required to fit the observations. This is a significant improvement on previous work in which the ice model was iteratively improved by ad hoc adjustments resulting in a model that is dependent on the initial model. The method is applied to the British Isles and a model with a spatial resolution of 50 km and temporal resolution of 1000 years is obtained for the late glacial period. Glaciological data determining the maximum elevation of the ice provide an important constraint on the model as does information on the timing of formation of such trimline data. It is possible to infer ice volumes up to a few thousand years before the earliest sea level data, and therefore the availability of early sea level data is most important to constraining ice models for the time of maximum glaciation.
Postseismic deformation following the 1906 San Francisco earthquake
Authors: Kenner Shelley J. , Segall Paul
Published: September 2002

We have reevaluated triangulation data from northern California following the 1906 San Francisco earthquake, thereby increasing the temporal and spatial resolution of postseismic deformation following that event. We have calculated uniform shear strain rates and average station velocities at Point Arena using data from 1906–1907, 1929–1930, and 1973–1975 and for the Point Reyes‐Petaluma arc using data from 1929–1930, 1938–1939, and 1960–1961. With the addition of recent geodetic data we infer an effective relaxation time for long‐term, postseismic deformation following the 1906 earthquake of 36±16 years. The Point Arena data are satisfactorily fit with accelerated afterslip at depth along the San Andreas fault plane. For a 10‐km‐deep, 25‐km‐wide afterslip zone the average slip rate between 1906 and 1930 is 11.2 cm/yr. Between 1929 and 1975 it is 4.6 cm/yr. Deformation in the Point Reyes‐Petaluma arc is clearly asymmetric with respect to the trace of the San Andreas fault, especially between 1929 and 1939. After inverting for the range of acceptable accelerated deep afterslip and horizontal detachment models, a detailed analysis using geologically reasonable geometries revealed that both model types have trouble explaining the spatial variations in the deformation field through time. In particular, accelerated deep afterslip models cannot reproduce the breadth of the observed deformation field to the northeast of the San Andreas fault. As a result, more complex, time‐varying deformation mechanisms are required to explain the Point Reyes‐Petaluma arc observations.
Longitudinal convergence fronts in homogeneous rotating channels
Authors: Mied Richard P. , Handler Robert A. , Evans Thomas E.
Published: September 2002

In situ observation and remote sensing imagery indicate the presence of velocity convergences located over bathymetric channels in the mouths of tidal estuaries. In this paper we present the results of numerical simulations performed to investigate these velocity structures in a rotating channel having a single bathymetric groove. The equations of motion for a homogeneous fluid on a rotating Earth are solved using a fully spectral code in the across‐channel (i.e., the vertical or x‐z) plane. No along‐channel flow variations (in the y direction) are permitted. The bottom bathymetry is formed using a unique virtual surface approach [Goldstein et al., 1993] that generates a no‐slip bottom using feedback forcing. A Gaussian‐shaped channel is employed to simulate typical estuarine bathymetry. In the along‐channel direction a constant pressure gradient is imposed, and the flow evolves until a steady state results. The simulations are performed at high Rossby number (of order unity) based on the width of the groove and a typical surface velocity. Simulations show the development of a localized along‐channel jet colocated with an across‐channel recirculation cell. This feature results from the generation of streamwise vorticity through the tilting of planetary vorticity by the vertical shear in the along‐channel flow. The associated across‐channel surface flow above the jet exhibits convergent and divergent regions, which correlate reasonably well with features reported previously in the literature. Their number, position, and strength are seen to vary with the along‐channel Reynolds number, Ekman layer thickness, and channel aspect ratio.
Spectral characteristics of velocity and vorticity fluxes in an unstratified turbulent boundary layer
Authors: Lien R.‐C. , Sanford T. B.
Published: September 2002

Wavenumber spectral characteristics of the velocity and vorticity fluxes in an unstratified turbulent boundary layer are presented. The observed vertical and streamwise velocity spectra agree with empirical forms found in the atmospheric boundary layer. Spectral ratios of 4/3 between the vertical and streamwise velocity spectra and the agreement between the observed vorticity flux quad spectrum and that of isotropic turbulence suggest local isotropy at scales smaller thanZ. The normalized cospectrum of the momentum flux agrees remarkably well with the empirical form found in the atmospheric boundary layer. In the inertial subrange the momentum flux cospectrum shows a clear spectral slope of −7/3. The observed composite vorticity flux cospectrum has most of its variance at the streamwise wavenumberkx= (1–10)Z−1and has a spectral slope of −7/3 in the inertial subrange. The −7/3 spectral slope is consistent with a dimensional argument, assuming that the vorticity flux cospectrum is proportional to the gradient of the mean vorticity, and depends on the turbulence kinetic energy dissipation rate ϵ and the wavenumber. A model turbulent vorticity flux cospectrum is constructed based on the shape of observed spectra, a −7/3 spectral slope in the inertial subrange, and the similarity scaling of the vorticity flux in an unstratified turbulent boundary layer. The turbulence vorticity flux is directly related to the divergence of turbulence momentum flux, the force exerted by turbulence on the mean flow. Therefore our proposed empirical cospectral form of the vorticity fluxes might be useful for turbulence parameterization in numerical models.
Probability of wave breaking and whitecap coverage in a fetch‐limited sea
Authors: Xu D. , Liu X. , Yu D.
Published: September 2002

The analytical model of Snyder and Kennedy [1983] for the whitecap coverage W and that of Srokosz [1985] for the probability of wave breaking B are developed into practically applicable forms by closely combining theoretical analysis and data fitting. The database consists of two parts: the data collected from published literature and those recently measured in the Bohai Bay. A time‐averaging technique is used to estimate the fourth spectral moment of the mean Joint North Sea Wave Project spectrum. The resulting expressions are simple, in which W and B depend only on the nondimensional fetch for a fetch‐limited sea.
Upper‐stratospheric ozone trends 1979–1998
Authors: Newchurch M. J. , Bishop Lane , Cunnold Derek , Flynn Lawrence E. , Godin Sophie , Frith Stacey Hollandsworth , Hood Lon , Miller Alvin J. , Oltmans Sam , Randel William , Reinsel Gregory , Stolarski Richard , Wang Ray , Yang Eun‐Su , Zawodny Joseph M.
Published: September 2002

Extensive analyses of ozone observations between 1978 and 1998 measured by Dobson Umkehr, Stratospheric Aerosol and Gas Experiment (SAGE) I and II, and Solar Backscattered Ultraviolet (SBUV) and (SBUV)/2 indicate continued significant ozone decline throughout the extratropical upper stratosphere from 30–45 km altitude. The maximum annual linear decline of −0.8±0.2 % yr−1 (2σ) occurs at 40 km and is well described in terms of a linear decline modulated by the 11‐year solar variation. The minimum decline of −0.1±0.1% yr−1 (2σ) occurs at 25 km in midlatitudes, with remarkable symmetry between the Northern and Southern Hemispheres at 40 km altitude. Midlatitude upper‐stratospheric zonal trends exhibit significant seasonal variation (±30% in the Northern Hemisphere, ±40% in the Southern Hemisphere) with the most negative trends of −1.2% yr−1 occurring in the winter. Significant seasonal trends of −0.7 to −0.9% yr−1 occur at 40 km in the tropics between April and September. Subjecting the statistical models used to calculate the ozone trends to intercomparison tests on a variety of common data sets yields results that indicate the standard deviation between trends estimated by 10 different statistical models is less than 0.1% yr−1 in the annual‐mean trend for SAGE data and less than 0.2% yr−1 in the most demanding conditions (seasons with irregular, sparse data) [World Meteorological Organization (WMO), 1998]. These consistent trend results between statistical models together with extensive consistency between the independent measurement‐system trend observations by Dobson Umkehr, SAGE I and II, and SBUV and SBUV/2 provide a high degree of confidence in the accuracy of the declining ozone amounts reported here. Additional details of ozone trend results from 1978 to 1996 (2 years shorter than reported here) along with lower‐stratospheric and tropospheric ozone trends, extensive intercomparisons to assess relative instrument drifts, and retrieval algorithm details are given by WMO [1998].
Ozone measurements during the Airborne Polar Experiment: Aircraft instrument validation, isentropic trends, and hemispheric fields prior to the 1997 Arctic ozone depletion
Authors: Kyrö E. , Kivi R. , Turunen T. , Aulamo H. , Rudakov V. V. , Khattatov V. , MacKenzie A. R. , Chipperfield M. P. , Lee A. M. , Stefanutti L. , Ravegnani F.
Published: September 2002

The first deployment of the ECOC electro chemical ozone cell (ECOC) instrument onboard the high‐altitude research aircraft, the Geophysica M‐55, took place from Rovaniemi, northern Finland, between December 23, 1996, and January 14, 1997. The ECOC data were compared against contemporaneous data from a network of balloon‐borne ozone sondes. The comparison was carried out in potential vorticity‐potential temperature (PV, Θ) coordinates, using meteorological analyses from the European Centre for Medium‐Range Forecasts. The comparison showed that ozone mixing ratios measured by ECOC are lower than those measured by ozonesonde by a small but statistically significant bias of (−5.7±2.8)% at the cruising altitudes of the aircraft, 15 to 19 km. After establishing and removing the average bias, ECOC and ozonesonde data were analyzed together to follow the development of ozone distributions in the early winter Arctic stratosphere. The analysis showed no evidence of chemical ozone depletion at the cruising altitudes of the aircraft, that is, between 435 and 490 K. The absence of chemical depletion is in agreement with polar statospheric cloud (PSC) observations, which showed no PSCs at aircraft cruising altitudes, although from January 5 onwards, PSCs were observed above cruising altitudes. Results from a three‐dimensional chemical transport model reproduce the basic features of the reconstructed ozone fields. However, the model does not capture the observed ozone increase during the campaign, due to weak modeled ozone vertical gradients, and indicates small ozone depletion of about 3% inside the vortex at 480 K by mid January.
Temporal and spatial scales of observed soil moisture variations in the extratropics
Authors: Entin Jared K. , Robock Alan , Vinnikov Konstantin Y. , Hollinger Steven E. , Liu Suxia , Namkhai A.
Published: September 2002

Scales of soil moisture variations are important for understanding patterns of climate change, for developing and evaluating land surface models, for designing surface soil moisture observation networks, and for determining the appropriate resolution for satellite‐based remote sensing instruments for soil moisture. Here we take advantage of a new archive of in situ soil moisture observations from Illinois and Iowa in the United States, and from Russia, Mongolia, and China, to evaluate the observed temporal and spatial scales of soil moisture variations. We separate the variance into two components, the very small scale, determined by soils, topography, vegetation, and root structure, and the large scale forced by the atmosphere. This larger scale, determined by precipitation and evaporation patterns, is of interest for global climate modeling. We characterize the small scale as white noise for our analysis, keeping in mind that it is an important component of soil moisture variations for other problems. We find that the atmospheric spatial scale for all regions is about 500 km. The atmospheric temporal scale is about 2 months for the top 1‐m soil layer. The temporal scale for the top 10‐cm layer is slightly less than 2 months. The white noise component of the variance for temporal variations ranges from 50% for the top 10 cm to 20–40% for the top 1 m. For spatial variations the white noise component is the same for all depths but varies with region from 30% for Illinois to around 70% for Mongolia. Nevertheless, the red noise (atmospheric component) can be seen in all regions. These results are for Northern Hemisphere midlatitudes and would not necessarily apply to other latitudes. The results are based on observations taken from grassland or agricultural areas, and may not be similar to those of areas with other vegetation types. In China, a region with substantial latitudinal variation, the temporal scale for the top 1 m varies from 1 month in the south to 2.5 months in the north, demonstrating the control of potential evaporation on the temporal scales. Seasonal analysis of the scales of soil moisture for Illinois shows that during the winter the temporal scales are long, though the spatial scales are short. We suggest that these variations are both attributable to the seasonal cycle of potential evaporation.
Estimating cloud top height and spatial displacement from scan‐synchronous GOES images using simplified IR‐based stereoscopic analysis
Authors: Mahani Shayesteh E. , Gao Xiaogang , Sorooshian Soroosh , Imam Bisher
Published: September 2002

An efficient method for estimating cloud top heights and correcting cloud image spatial displacements was developed. The method applies stereoscopic analysis to a pair of scan‐synchronous infrared cloud images received from two GOES satellites using a piecewise linear approximation of the relationship between height and infrared brightness temperature of top of the cloud element. The algorithm solves for cloud top heights and subsequently calculates the spatial displacements of cloud images. Optimal parameterization of the piecewise linear approximation is achieved using the shuffled complex evolution (SCE) algorithm. Because the proposed method simplifies the stereoscopic analysis, it allows for an easy implementation of stereoscopic technique on desktop computers. When compared to the standard isotherm matching approaches, the proposed method yielded higher correlation between GOES 8 and GOES 9 scan‐simultaneous images after the parallax adjustment. The validity of the linear approximation was tested against temperature profiles obtained from the multiple ground sounding measurements from the Tropical Rainfall Measuring Mission/Texas and Florida Underflights (TRMM/TEFLUN) experiments. The results of this comparison demonstrated good fit, particularly within the troposphere, between the optimized relationship and atmospheric sounding measurements. The data produced by this method, including cloud top temperatures and heights, atmospheric temperature profiles for cloudy sky areas, and spatial displacement‐adjusted cloud images, can be useful for weather/climate and atmospheric studies. In particular, the displacement‐adjusted cloud images can be critical to develop high‐resolution satellite rainfall estimates, which are urgently needed by mesoscale atmospheric modeling and studies, severe weather monitoring, and heavy precipitation and flash flood forecasting. Limitations of the proposed method are also identified and discussed.
Wintertime climatology of MOZAIC ozone based on the potential vorticity and ozone analogy
Authors: Morgenstern Olaf , Marenco Alain
Published: September 2002

A climatology is compiled of wintertime ozone measured by MOZAIC aircraft. It spans the winters of 1994/1995 to 1997/1998. It is generated employing and in the process confirming the analogy between potential vorticity and ozone. The methodology consists of interpolating equivalent latitude derived from analyzed potential vorticity onto flight tracks of the aircraft. In a second step the measurements are cast into a gridded form; this generates seasonal synopses of ozone in the coordinate space spanned by time, potential temperature, and equivalent latitude. At low equivalent latitudes the irregular appearance of high‐ozone transients in combination with the relatively small measurement density renders a determination of seasonal‐mean ozone problematic. However, in middle and high equivalent latitudes of the Northern Hemisphere ozone mixing ratios are well summarized by stating seasonal means and seasonal increases only. The results derived for the four winters exhibit some marked interannual variability. The method allows the determination of “expected” ozone along the flight tracks from the gridded ozone. A comparison with measured ozone for two selected periods of time suggests that discrepancies between expected and measured ozone hint at the presence of filamentary structure in the ozone field and associated stratosphere‐troposphere exchange. In the four‐winter mean such “outliers” of measured ozone occur most frequently in the vicinity of the North Atlantic storm track, corroborating earlier studies on filamentation activity.
Influence of advection on measurements of the net ecosystem‐atmosphere exchange of CO2 from a very tall tower
Authors: Yi C. , Davis K. J. , Bakwin P. S. , Berger B. W. , Marr L. C.
Published: September 2002

In most studies of the net ecosystem‐atmosphere exchange of CO2 (NEE) using tower‐based eddy covariance (EC) systems it has been assumed that advection is negligible. In this study we use a scalar conservation budget method to estimate the contribution of advection to NEE measurements from a very tall tower in northern Wisconsin. We examine data for June–August 1997. Measured NEE0, calculated as the sum of the EC flux plus the rate of change of storage below the EC measurement level, is expected to be constant with measurement height, and we take the differences between levels as a measure of advection. We find that the average difference in total advection ΔFCadtot between 30 and 122 m is as large as 6 μmol m−2s−1 during the morning transition from stable to convective conditions and the average difference ΔFCadtot between 122 and 396 m is as large as 4 μmol m−2s−1 during daytime. For the month of July, advection between 30 and 122 m is 27% of the diurnally integrated NEE0 at 122 m, and advection between 122 and 396 m accounts for 5% of the NEE0 observed at 396 m. The observed differences of advection often have significant correlation with the vertical integral of wind speed within the same layer. This indicates that the horizontal advection contribution to NEE could be significant. Direct observations of the vertical gradient in CO2 show that ΔFCadtot cannot be explained by vertical advection alone. It is hypothesized that differing flux footprints and pooling of CO2 in the heterogeneous landscape causes the advection contribution. The magnitudes of the total advection component FCadtot of NEE at the 30 m level are roughly estimated by a linear extrapolation. A peak in FCadtot at 30 m of ∼ 3 μmol m−2 s−1 during the morning transition is predicted for all three months. The July integrated FCadtot is estimated to be 10% of the diurnally integrated NEE0 at 30 m.
Is the North Atlantic Oscillation the most important atmospheric pattern for precipitation in Europe?
Authors: Qian Budong , Corte‐Real João , Xu Hong
Published: September 2002

Signatures of the North Atlantic Oscillation (NAO) are widely detected in climatic variability, particularly in the extratropical latitudes surrounding the North Atlantic Ocean. However, it is still controversial whether the NAO is the most important pattern of nonseasonal variability of atmospheric circulation related to precipitation over Europe. This paper is an attempt to contribute to this issue. The spatial modes of nonseasonal variability of monthly fields of mean sea level pressure (MSLP) over the northeastern Atlantic and western Europe and precipitation over Europe are investigated mainly by using principal component analysis. The relationships between the two fields are studied via canonical correlation analysis (CCA). The data sets used refer to the period from 1911 to 1990. The most important spatial mode of MSLP is the NAO pattern, its corresponding principal component being closely related to the NAO index. Interestingly however, the NAO pattern seems to be responsible only for the second empirical orthogonal function in precipitation, while the most important spatial mode of precipitation corresponds to the third EOF of MSLP (North Sea pattern). Furthermore, the second EOF of MSLP (Scandinavian pattern) is highly associated with the third EOF of precipitation. Significant pairs of canonical patterns between the MSLP and precipitation fields obtained from canonical correlation analysis are coherent with the conclusions above. These results can potentially be used to assess possible changes of precipitation over Europe due to increasing greenhouse gases based on the variability of MSLP simulated by general circulation models.
Influence of air mass histories on radical species during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission
Authors: Pierson J. M. , Kawa S. R. , Salawitch R. J. , Hanisco T. F. , Lanzendorf E. J. , Perkins K. K. , Gao R. S.
Published: September 2002

The Goddard trajectory chemistry model was used with ER‐2 aircraft data to test our current knowledge of radical photochemistry during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. The results of the trajectory chemistry model with and without trajectories are used to identify cases where steady state does not accurately describe the measurements. Over the entire mission, using trajectory chemistry reduces the variability in the modeled NOx comparisons to data by 25% with respect to the same model simulating steady state. Although the variability is reduced, NOx/NOy trajectory model results were found to be systematically low relative to the observations by 20–30% as seen in previous studies. Using new rate constants for reactions important in NOy partitioning improves the agreement of NOx/NOy with the observations but a 5–10% bias still exists. OH and HO2 individually are underpredicted by 15% of the standard steady state model and worsen with the new rate constants. Trajectory chemistry model results of OH/HO2 were systematically low by 10–20% but improve using the new rates constants because of the explicit dependence on NO. This suggests that our understanding of NOx is accurate to the 20% level and HOx chemistry is accurate to the 30% level in the lower stratosphere or better for the POLARIS regime. The behavior of the NOx and HOx comparisons to data using steady state versus trajectory chemistry and with updated rate coefficients is discussed in terms of known chemical mechanisms and lifetimes.
Stratosphere‐troposphere exchange: Inferences from the isotopic composition of water vapor
Authors: Keith David W.
Published: September 2002

Air may cross the tropical tropopause either by gradual ascent or in localized episodes associated with convection. While observations demonstrate that water vapor mixing ratios of air entering the tropical stratosphere are consistent with the mean tropical tropopause temperature, they do not resolve key mechanistic questions, such as the relative contribution of gradual or episodic transport, or the role of thin cirrus. As Moyer et al. [1996] clearly argue, observations of the isotopic content of water entering the tropical stratosphere can provide a strong constraint on models of water vapor transport across the tropopause. For example, stratospheric HDO is too abundant to be compatible with the assumption that all moisture enters the stratosphere as vapor during convection. Analysis of recent H218O observations shows that kinetic effects cannot explain the HDO excess. Lofting and evaporation of cloud ice can explain the observed stratospheric water vapor content and its isotopic composition, but the relative importance of gradual or episodic transport remains unresolved.
Interannual climate variability in the Central Andes and its relation to tropical Pacific and Atlantic forcing
Authors: Vuille Mathias , Bradley Raymond S. , Keimig Frank
Published: September 2002

The main spatiotemporal modes of interannual temperature and austral summer (DJF) precipitation variability in the Central Andes are identified based on a two‐way principal component analysis (PCA) of 30‐year (1961–1990) monthly station data and related to contemporaneous tropical Pacific and Atlantic sea surface temperature anomalies (SSTAs). In addition, various meteorological fields, based on National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, NOAA‐Outgoing Longwave Radiation (OLR) and station data, are analyzed during periods of strong positive and negative SSTA and the respective composites tested for local significance using a Student's t‐test approach. Temperature variability in the Central Andes is primarily related to El Niño‐Southern Oscillation (ENSO) and closely follows SSTA in the central equatorial Pacific with a lag of 1–2 months. In the southern Altiplano, temperatures have significantly increased since the late 1970s. DJF precipitation is also primarily related to ENSO, featuring below (above) average precipitation during El Niño (La Niña). Precipitation over the dry western part of the Altiplano shows the closest relationship with ENSO, due to ENSO‐induced atmospheric circulation anomalies. Precipitation variability over the western Altiplano features a decadal‐scale oscillation, related to a similar climatic shift in the tropical Pacific domain in the late 1970s. Over the northern Altiplano the precipitation signal is reversed in the austral summer following the peak phase of ENSO, presumably due to the temporal evolution of tropical Pacific SSTA, rapidly switching from one state to the other. No evidence for a tropical Atlantic influence on DJF precipitation was found. SSTAs in the tropical NE Atlantic, however, presumably are influenced by heating and convection over the Altiplano through an upper air monsoon return flow, altering the strength of the NE trades that emanate from the Sahara High.