bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerosol optical depths over peninsular India and adjoining oceans during the INDOEX campaigns: Spatial, temporal, and spectral characteristics
Authors: Moorthy K. Krishna , Saha Auromeet , Prasad B. S. N. , Niranjan K. , Jhurry D. , Pillai Preetha S.
Published: February 2004

The spatial, temporal, and spectral characteristics of aerosol optical depths τpλ for the Indian Ocean Experiment (INDOEX) study period (January to April) are examined using data collected through a ground‐based network of multiwavelength solar radiometers (MWR) over coastal regions of peninsular India; two island locations, one in the Arabian Sea and another in the southern Indian Ocean at 20°S; in conjunction with estimates made over various locations over the Arabian Sea and Indian Ocean during the INDOEX cruises of 1996, 1998, and 1999. Spatial variations show extremely low values of τp at the shorter (visible) wavelengths (λ<750 nm) to the south of the Intertropical Convergence Zone (ITCZ), but increases substantially at locations due north of the ITCZ due to increased source impact and advection by favorable winds. An enhancement in τp is seen in the central Arabian Sea, which is attributed to air trajectory effects. Ångström parameters, deduced from optical depth spectra, reveal a high value of α (∼0.9) for north of the ITCZ, while for the south α is negative, indicating a change in the aerosol size distribution. Accumulation aerosols dominate in the north, while concentration of coarse aerosols remain nearly about the same, except very close to the coast. A north‐south gradient in aerosol optical depth, with scaling distance of ∼1000 to 2000 km at shorter wavelengths and much higher at longer wavelengths, is observed. The gradient becomes shallower at high wind speeds. The large‐scale dynamics associated with the movement of the ITCZ and its interannual variation appears to significantly influence the aerosol characteristics. As the southwest monsoon sets in over India, considerable wet removal and change in air mass characteristics cause a significant depletion in optical depths, which then became comparable to those prevailing in the southern hemisphere.
Airglow variations associated with nonideal ducting of gravity waves in the lower thermosphere region
Authors: Hickey Michael P.
Published: February 2004

A numerical full‐wave model is used to study the response of the O2 atmospheric airglow to ducted gravity waves in the mesopause region. For an isothermal, quasi‐adiabatic, and motionless background atmosphere the calculated phase differences between airglow brightness fluctuations and fluctuations of temperatures derived from the airglow, as given by Krassovsky's ratio, are in good agreement with the predictions of published theory. Significant departures from the predictions of the basic theory are obtained when we consider ducting in the presence of the eddy and molecular diffusion of heat and momentum in a nonisothermal background atmosphere. Wind shears also affect the phase difference between airglow brightness fluctuations and temperatures derived therefrom. Nonisothermal effects and the effects of diffusion and winds are largest for the slower waves we consider. Only the fastest of the ducted waves considered conform to the basic theory, while the airglow signatures associated with slower, more weakly ducted waves may be easily misinterpreted as being due to propagating waves. We conclude that for the short horizontal wavelength waves observed in the airglow, the phase of Krassovsky's ratio may be useful to identify wave ducting only for the shortest period, fastest waves. Therefore identification of ducted waves using Krassovsky's ratio will be difficult even if the required high temporal resolution measurements become available.
Aerosol optical depth measurements and their impact on surface levels of ultraviolet‐B radiation
Authors: Wenny B. N. , Saxena V. K. , Frederick J. E.
Published: February 2004

Surface measurements of total and diffuse UV irradiance at the seven narrowband wavelength channels of the ultraviolet multifilter rotating shadow‐band radiometer (UVMFR) were used to determine total column ozone and aerosol optical depth for two 6‐month periods in 1997 and 1999 at a site in the Blue Ridge Mountains of North Carolina. The retrieved column ozone displayed a seasonal dependence and consistent agreement with the Total Ozone Mapping Spectrometer (TOMS). The mean ratio of retrieved ozone to TOMS ozone was 0.98 with standard deviations of 0.02 and 0.01 for 1997 and 1999, respectively. Aerosol optical depth at 317, 325, 332, and 368 nm was derived for a 6‐month period of 1999. The seasonal trend exhibited is influenced by the persistent summertime haze that occurs in the region. The retrieved aerosol optical depths are used as input in a radiative transfer model to investigate the effect of their realistic values on the calculation of the UV index (UVI) forecasted by the National Weather Service. The percentage change in calculated surface erythemally weighted UV (versus calculations using the standard UVI aerosol inputs) ranges from a 4% increase to a nearly 50% decrease, dependent upon the aerosol optical depth and amount of absorption by aerosols. Based on our measurements, it was found that during the summertime the UV index can deviate by up to −5 index units from the forecast using the standard aerosol inputs.
Overview of the meteorological conditions and atmospheric transport processes during INDOEX 1999
Authors: Verver G. H. L. , Sikka D. R. , Lobert J. M. , Stossmeister G. , Zachariasse M.
Published: February 2004

We give an overview of the regional meteorological situation during the Indian Ocean Experiment INDOEX intensive field phase (IFP) in February and March 1999. The INDOEX domain, reaching from 30°N to 30°S and from 50°E to 100°E, was chosen because the low‐level outflow of pollution from India is carried by the northeasterly trades into the tropical convergence zone, where cloud processing modifies the properties of the aerosols. In contrast, there is also an inflow of pristine southern hemispheric air by the southeasterly trades into the convergence zone. However, during the 1999 IFP some deviations from the climatological mean were observed. In 1999 the Intertropical Convergence Zone (ITCZ) was broken into a northern convergence zone and a southern convergence zone. During February the northern zone was more active and the cross‐equatorial flow (N‐→S) was weak, both suggesting that less pollution was transported to the southern hemisphere. In February it was occasionally possible to sample a southern hemispheric air mass on the southern side of the INDOEX domain. During March 1999 the southern convergence zone became dominant and moved to a more southern position (near 5°–10°S).

It is shown that four channels carry pollution into the INDOEX domain: (1) NE trades over the western Arabian Sea, (2) NW‐NE flow along the west coast of India, (3) NE trades over the west Bay of Bengal, and (4) NE flow from SE Asia. The strength of each channel is modulated by transients moving across Pakistan and northern India (western disturbances). The heating of the Indian subcontinent in March resulted in a eastern shift of the subtropical high from central India to the Bay of Bengal, which also affected channels 2, 3, and 4. Episodes of high and low carbon monoxide concentrations as measured in Kaashidhoo (Maldives) during the 1999 IFP can qualitatively be explained by the operationality of the flow channels, determined through trajectory analyses, in combination with the intensity of the northern convergence zone.
A regional climate model coupled to ocean waves: Synoptic to multimonthly simulations
Authors: Perrie William , Zhang Yaocun
Published: February 2004

The NCAR regional climate model RegCM is coupled to the WAM ocean model, using the sea‐state‐dependent roughness parameterization derived in the HEXOS experiment. Coupled model simulations are shown to give reduced wind speeds U10 compared to uncoupled simulations. This is in accord with other recent studies. However, the wave‐atmosphere coupling is effected through the friction velocity field u*, rather than through the wind field U10. Thus because the coupling gives enhanced resistive friction, U10 is weakened, whereas u* is enhanced. Wave heights, driven by u*, are also increased in our coupled model simulations compared to uncoupled model simulations. Coupled model outputs are shown to compare favorably with air‐sea observations collected during the recent Labrador Sea Deep Convection Experiment in 1997, both for synoptic storm timescales and for seasonal timescales.
Observations of the 6.5‐day wave in the mesosphere and lower thermosphere
Authors: Talaat E. R. , Yee J.‐H. , Zhu Xun
Published: February 2004

Previous observations of atmospheric oscillations with zonal wave number 1 have consistently located waves of periods between 5 and 7 days. This study presents a global and seasonal analysis of the 6.5‐day wave in High Resolution Doppler Imager daytime mesosphere and lower thermosphere horizontal winds, temperatures, and nighttime atomic oxygen at 95 km. The horizontal structures of all these atmospheric variables are similar to the gravest symmetric wave number 1 Rossby wave, i.e., the (1,1) mode. The seasonal and spatial analysis displays possible modification by the zonal mean wind. Finally, the observed vertical structures of the 6.5‐day wave indicate that it is an internal Rossby wave, not an external or Lamb wave.
Tropospheric water vapor imaging by combination of ground‐based and spaceborne GNSS sounding data
Authors: Foelsche Ulrich , Kirchengast Gottfried
Published: February 2004

The Global Navigation Satellite System (GNSS) comprises the U.S. system GPS (Global Positioning System), its Russian pendant GLONASS, and presumably, in the future, the European system Galileo. The potential of GNSS‐based phase delay measurements for accurately estimating vertically and slant‐path‐integrated water vapor has been demonstrated recently for radio links between GPS satellites and ground‐based GPS receivers. GNSS‐based radio occultation, on the other hand, has been demonstrated via the GPS/Meteorology experiment to deliver accurate near‐vertical profiles of atmospheric variables such as temperature and humidity with high vertical resolution. Height‐resolving imaging of atmospheric water vapor becomes feasible when occultation profiles from spaceborne receivers in Low Earth Orbits (LEO) are combined with ground‐based GNSS data from a colocated receiver network. We developed a two‐dimensional, height‐resolving tomographic imaging technique following the Bayesian approach for optimal combination of information from different sources. Using simulated GNSS‐based water vapor measurements from LEO and ground, we show representative results derived from simple synthetic refractivity fields as well as from a realistic refractivity field based on a European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis. For cases located poleward of ∼40° we found a new simple mapping function to perform best within our forward model scheme, where the only free parameter is the climatological scale height in the troposphere, the exact value of which is not critical. The mapping function exploits the ratio between the straight‐line ray path length within the first two scale heights above surface and the “effective height” defined by these first two scale heights. We found our technique capable of reconstructing atmospheric features like water vapor maxima near the top of the trade wind inversion. Adjustment of the integral over the water vapor profile measurements to the horizontally averaged ground‐based vertical integrated water vapor data efficiently mitigates potential biases in the former data. Accuracies are best in areas with high absolute humidities but also over drier areas such as Finland, useful two‐dimensional information can still be obtained. Thus it is attractive to apply the developed technique in a next step to real data.
A survey of thunderstorm flash rates compared to cloud top height using TRMM satellite data
Authors: Ushio Tomoo , Heckman Stan J. , Boccippio Dennis J. , Christian Hugh J. , Kawasaki Zen‐Ichiro
Published: February 2004

The relationship between cloud height and lightning activity is examined using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Coincident data from the precipitation radar (PR) and Lightning Imaging Sensor aboard the TRMM satellite are used to examine whether lightning flash rate is proportional to the fifth power of cloud top height. This study is unique in that (1) the relationship between instantaneous rather than maximum storm height and flash rate is obtained and (2) relatively unbiased full data sets for different locations and seasons over the globe are used. The relationship between thunderstorm height and flash rate is nonlinear with large variance. The overall trend shows that flash rate increases exponentially with storm height. Some tall thunderstorms do not have large flash rates, but the reverse situation never occurs. The fifth power dependency that is derived from scaling laws is not inconsistent with, but not necessarily required by, the observed data.
Redistribution of nitric acid in the Arctic lower stratosphere during the winter of 1996–1997
Authors: Irie H. , Koike M. , Kondo Y. , Bodeker G. E. , Danilin M. Y. , Sasano Y.
Published: February 2004

Vertical profiles of HNO3, N2O, O3, and the aerosol extinction coefficient at 780 nm were observed by the Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) during the Arctic winter of 1996–1997. Irreversible redistribution of HNO3 is evaluated using HNO3‐N2O and HNO3‐O3 correlations. Denitrification and nitrification started to be observed just after the Arctic vortex cooled to below the ice frost point (TICE) on February 10. Trajectory analyses show that denitrification occurred only in air masses, which were once cooled to near TICE and were kept at temperatures below the nitric acid trihydrate saturation threshold continuously for more than 4 days. Such a temperature history provides the necessary conditions for nucleation and growth of particles causing denitrification. The average extent of denitrification at 19 km reached 43% at the center of the vortex, suggesting that stratospheric ozone could be affected by denitrification deep inside the vortex. Denitrification (>2 ppbv) and nitrification (>1 ppbv) covered 40±10% and 35±10% of the vortex area, respectively. Redistributed numbers of HNO3 molecules at each altitude were calculated by integrating the area‐weighted changes in the HNO3 concentration. The decreases in total HNO3 concentration at 17–21 km in late February and early March agreed with the increases at 12–15 km to within 25%, confirming conservation of HNO3 during sedimentation and evaporation of HNO3‐containing polar stratospheric cloud particles.
Microwave polarized signatures generated within cloud systems: Special Sensor Microwave Imager (SSM/I) observations interpreted with radiative transfer simulations
Authors: Prigerit Catherine , Pardo Juan R. , Mishchenko Michael I. , Rossow William B.
Published: February 2004

Special Sensor Microwave Imager (SSM/I) observations in cloud systems are studied over the tropics. Over optically thick cloud systems, the presence of polarized signatures at 37 and 85 GHz is evidenced and analyzed with the help of cloud top temperature and optical thickness extracted from visible and IR satellite observations. Scattering signatures at 85 GHz (TbV(85) ≤ 250 K) are associated with polarization differences ≥6 K ∼50% of the time over ocean and ∼40% over land. In addition, over thick clouds the polarization difference at 37 GHz rarely is negligible. The polarization differences at 37 and 85 GHz do not stem from the surface but are generated in regions of relatively homogeneous clouds having high liquid water content. To interpret the observations, a radiative transfer model that includes the scattering by nonspherical particles is developed, based on the T matrix approach and using the doubling and adding method. In addition to handling randomly and perfectly oriented particles this model can also simulate the effect of partial orientation of the hydrometeors. Microwave brightness temperatures are simulated at SSM/I frequencies and are compared with the observations. Polarization differences of ∼2 K can be simulated at 37 GHz over a rain layer, even using spherical drops. The polarization difference is larger for oriented nonspherical particles. The 85 GHz simulations are very sensitive to the ice phase of the cloud. Simulations with spherical particles or with randomly oriented nonspherical ice particles cannot replicate the observed polarization differences. However, with partially oriented nonspherical particles the observed polarized signatures at 85 GHz are explained, and the sensitivity of the scattering characteristics to the particle size, asphericity, and orientation is analyzed. Implications on rain and ice retrievals are discussed.
Dating the Vostok ice core by an inverse method
Authors: Parrenin Frédéric , Jouzel Jean , Waelbroeck Claire , Ritz Catherine , Barnola Jean‐Marc
Published: February 2004

Using the chronological information available in the Vostok records, we apply an inverse method to assess the quality of the Vostok glaciological timescale. The inversion procedure provides not only an optimized glaciological timescale and its confidence interval but also a reliable estimate of the duration of successive events. Our results highlight a disagreement between orbitally tuned and glaciological timescales below ∼2700 m (i.e., ∼250 kyr B.P., thousands of years before present). This disagreement could be caused by some discontinuity in the spatial variation of accumulation upstream of Vostok. Moreover, the stratigraphic datings of central Greenland ice cores (GRIP and GISP2) appear older than our optimized timescale for the late glacial. This underlines an unconsistency between the physical assumptions used to construct the Vostok glaciological timescale and the stratigraphic datings. The inverse method allows the first assessment of the evolution of the phase between Vostok climatic records and insolation. This phase significantly varies with time which gives a measure of the nonlinear character of the climatic system and suggests that the climatic response to orbital forcing is of different nature for glacial and interglacial periods. We confirm that the last interglacial, as recorded in the Vostok deuterium record, was long (16.2±2 kyr, thousands of years). However, midtransition of termination II occurred at 133.4±2.5 kyr BP, which does not support the recent claim for an earlier deglaciation. Finally, our study suggests that temperature changes are correctly estimated when using the spatial present‐day deuterium‐temperature relationship to interpret the Vostok deuterium record.
Sea‐land total ozone differences from TOMS: GHOST effect
Authors: Cuevas E. , Gil M. , Rodríguez J. , Navarro M. , Hoinka K. P.
Published: February 2004

Mean global TOMS (Total Ozone Mapping Spectrometer) ozone data, averaged in time, reveals persistent year‐to‐year differences in total ozone between continents and oceans. This feature has been named GHOST (global hidden ozone structures from TOMS). During Northern Hemisphere summer it can be seen within the latitudinal belt between 40°S and 50°N. The most pronounced land‐sea difference in total ozone with values of up to 18 Dobson units is observed between latitudes 35°N and 40°N. The gradients associated with the coastlines are stronger in summer, when transient planetary wave activity decreases, but is still observable in the winter period. The Iberian Peninsula has been selected as a case study to examine the effect of each possible contribution quantitatively. It has been found that the truncation of the lower tropospheric column due to the topography explains 26% of the land‐sea differences, while permanent differences in tropopause height distribution can account for a further 8%. After these “corrections” other structures remain. Additional contributions due to the TOMS total ozone retrieval algorithm artifact (absorbing aerosol distribution) are also explored. After considering the optical depths and absorbance of aerosols above the Iberian Peninsula, the remaining 66% is compatible with the presence of UV‐absorbing aerosols whose effects may not be correctly accounted for in the TOMS retrieval algorithm.
Interplanetary and terrestrial observations of an Earth‐directed coronal mass ejection
Authors: Vats Hari Om , Sharma Som , Oza R. , Iyer K. N. , Chandra H. , Sawant H. S. , Deshpande M. R.
Published: February 2004

In this article we report interplanetary scintillation observations at 103 and 327 MHz of an Earth‐directed coronal mass ejection (CME) which occurred near the center of the solar disk at 0435 UT on May 12, 1997. The disturbance was found to have plasma density ∼4 times more than that of the ambient plasma at a distance of ∼ 0.5 AU from the Sun. The most peculiar aspect of this CME is that it appears that the disturbance moved slightly slower than the ambient medium. Solar and Heliospheric Observatory (SOHO) and interplanetary scintillation (IPS) estimates of solar wind are quite different; it appears that the difference could be due to the projection effect of the SOHO image.Though the disturbance was not very severe, its impact on Earth's environment produced a geomagnetic storm. This event was associated with a two‐ribbon flare.The ionospheric effects of soft X rays from this solar flare were observed by a digital ionosonde at Ahmedabad in the form of excess ionization (∼1200 el cm−3) in the D region of the ionosphere.
Shallow slab detachment as a transient source of heat at midlithospheric depths
Authors: van de Zedde D. M. A. , Wortel M. J. R.
Published: February 2004

Slab detachment (or breakoff) has been proposed as a cause of temperature‐related processes associated with subduction, such as postcollisional magmatism, mineralization, and metamorphism. In this study, we quantitatively investigate the breakoff process and the subsequent thermal evolution of a plate boundary region involving continental collision after a prolonged period of oceanic lithosphere subduction. Our two‐dimensional time‐dependent thermomechanical modeling shows that the dense, oceanic part of the slab can become detached at depths as shallow as 35 km. The detached part of the slab sinks into the mantle, creating a gap in the lithospheric system which is filled with upwelling hot asthenosphere. The resulting temperature increase in the overlying material can be more than 500°C. It allows for partial melting of the asthenosphere and the overriding metasomatized lithosphere for a timespan of a few millions of years. Crustal anatexis and related magmatism and mineralization can proceed over a considerably longer period. The quantification of the conditions required for shallow slab detachment will contribute to warranted assessments concerning the role of slab detachment (relative to other proposed heat sources, such as tectonically accreted radioactive material) in the geodynamical evolution of former convergent plate boundaries.
Two‐region flow and rate‐limited sorption of uranium (VI) during transport in an unsaturated silt loam
Authors: Gamerdinger A. P. , Kaplan D. I. , Wellman D. M. , Serne R. J.
Published: February 2004

Uranium (VI) sorption during unsaturated transport was evaluated using a centrifuge method; conventional saturated columns and batch incubation procedures were used to isolate flow effects on sorption processes. Uranium (VI) sorption was dependent on flow velocity, and modeling indicated that 60% of sorption was rate‐limited. Breakthrough curves were described and independently predicted with a two‐site model; the mass transfer coefficient for sorption was assumed to be linearly related to velocity. While two‐region flow contributed to disparity in apparent Kd values (linear distribution coefficient) at different water contents, the effect was explained by increased velocity in the mobile flow domain and not a decrease in availability of sorption sites. Hydrodynamic parameters for two‐region transport, determined for a nonsorptive tracer at faster velocity (16 cm h−1), were fixed and described U(VI) transport at slower velocity (2 cm h−1). The rate‐limited parameter for mass transfer between mobile and immobile water domains was linearly related to the average pore water velocity. Effective retardation factors varied with moisture saturation and the apparent Kd value and ranged from 7 for faster velocity experiments in a saturated column to 44 at slow velocity and 40% moisture saturation.
Multisite simulation of daily precipitation and temperature in the Rhine Basin by nearest‐neighbor resampling
Authors: Buishand T. Adri , Brandsma Theo
Published: February 2004

The method of nearest‐neighbor resampling is extended to simultaneous simulation of daily precipitation and temperature at multiple locations over a large area (25 stations in the German part of the Rhine basin). Nearest neighbors refer here to historical days for which the observed weather is closest to that of the simulated weather for a given day. Resampling is done from these nearest neighbors to obtain the weather variables for the next day. The nearest neighbors are defined in terms of a weighted Euclidean distance to a feature vector containing summary statistics of the daily precipitation and temperature fields (spatial averages, fraction of stations with precipitation, and principal components). The inclusion of atmospheric circulation variables in the feature vector is also studied. There is a weak tendency to underestimate the standard deviations and autocorrelation coefficients of daily precipitation and temperature and the standard deviations of the monthly precipitation totals and monthly mean temperatures. However, the underprediction of these second‐order moment statistics is not statistically significant if the number k of nearest neighbors in the resampling procedure is small (k≈ 5) and the dimensionqof the feature vector is low (q≈ 3). A small systematic underprediction is also observed for the quantiles of the distributions of theN‐day winter maximum precipitation amounts. The spatial dependence of these extremes and the distributions ofN‐day maximum snowmelt are adequately reproduced. Long‐duration simulations show that realistic unprecedented multiday precipitation amounts can be generated.
A model for simulating the influence of a spatial distribution of large circular macropores on surface runoff
Authors: Léonard J. , Perrier E. , de Marsily G.
Published: February 2004

This paper reports the development and test, at the scale of 1 m2, of an event‐ based model that aims at simulating the influence of a spatial distribution of large circular macropores on surface runoff. The main originality of this model is that it focuses on the way macropores are supplied with water at the soil surface, by coupling an original model for water interception by individual macropores to a high‐resolution spatialized overland flow model. A three‐step evaluation of the model was carried out, involving (1) an experimental test of the model for water interception by macropores; (2) a sensitivity analysis of the model to time and space discretization; and (3) a comparison between numerical and field results in the case of runoff on a crusted soil surface with a population of large macropores made by termites in the Sahel. The model was found to accurately simulate the effect of a spatial distribution of large macropores on runoff, and it showed that small heterogeneities, like macropores or areas where a crust has been destroyed, which cover a very limited proportion of the soil surface, can have a high impact on runoff.
Relative changes in sediment supply and sediment transport capacity in a bedrock‐controlled river
Authors: Young W. J. , Olley J. M. , Prosser I. P. , Warner R. F.
Published: February 2004

Rivers can be affected by multiple natural and human‐induced changes to sediment supply and to sediment transport capacity. Assessment of the relative importance of these changes enables appropriate river management. Here assessments are made of the relative changes in sediment supply and sediment transport capacity of an 83 km section of the bedrock‐controlled Coxs River, New South Wales, Australia. These relative changes are estimated, in turn, for land degradation, historical climate variations, and dam closure. Measurements of gully erosion extent from aerial photographs indicate that since European settlement the average annual sediment supply to the river has increased by a factor of ∼20. Rainfall records and flow‐gauging records show a climate shift in the mid‐1940s. On the basis of hydrologic modeling the increased streamflow in the period since the mid‐1940s is estimated to have increased sediment transport capacity by a factor of almost 3. Closure of Lyell Dam in 1982 stopped sediment supply from the upper catchment. On the basis of hydrologic modeling the flow regulation and abstraction at the dam is estimated to have reduced the long‐term average sediment transport capacity downstream by 15%. The current sediment transport capacity after the climate shift and dam closure is 2.6 times higher that in the first half of this century. In spite of this net increase in sediment transport capacity the huge volume of sediment delivered to the channel from gully erosion has led to widespread sand deposition along much of the river. Much of the river bed that was previously dominated by bedrock is now alluvial in character. Only the steepest reaches remain bedrock‐dominated.
Non‐Darcy displacement of immiscible fluids in porous media
Authors: Wu Yu‐Shu
Published: February 2004

This paper presents a Buckley‐Leverett analytical solution for non‐Darcy displacement of two immiscible fluids in porous media. The multiphase non‐Darcy displacement is described using a Forchheimer equation or other non‐Darcy flow correlations under multiphase flow conditions. The analytical solution is used to obtain some insight into the physics of displacement involving non‐Darcy flow effects in porous media. The solution reveals how non‐Darcy displacement is controlled not only by relative permeability curves but also by non‐Darcy flow coefficients as well as injection or flow rates. This analytical solution is then applied to verify a numerical simulator for modeling multiphase non‐Darcy flow.
Long‐Term Soil Temperature Database, Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Seyfried M. S. , Flerchinger G. N. , Murdock M. D. , Hanson C. L. , Van Vactor S.
Published: February 2004

We describe long‐term soil temperature data collected at the Reynolds Creek Experimental Watershed (RCEW). These data were collected for over 15 years at five locations representing different climatic regimes and soils in the RCEW. Measurements were made at several depths at each site to a depth of 180 cm in most cases. Each site is located in close proximity to a climate station. Descriptive soil profile information is also available for each site. We illustrate two aspects of the data set, the impact of snow depth on soil temperature and the interannual variability of soil temperature at a site. These data are available to the public via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov.
Inverse modeling of the hydrological and the hydrochemical behavior of hydrosystems: Application to nitrate transport and denitrification
Authors: Pinault J.‐L. , Pauwels H. , Cann C.
Published: February 2004

An inverse model is proposed to calculate unit hydrographs as well as the impulse response of fluxes from rainfall‐runoff or rainfall‐flux data, the purpose of which is hydrograph separation. Contrary to what hydrologists have been doing for years, hydrograph separation is carried out by using transfer functions in their entirety, which enables accurate separation of fluxes. All the components that participate in the global hydrologic balance are calculated in order to fit historical and computed flow and fluxes. The effective rainfall and the effective input fluxes are calculated from inversion. Unit hydrograph as well as impulse response of fluxes are decomposed into a quick and a slow component, and, consequently, the effective rainfall is decomposed into two parts, one contributing to the quick flow (or flux) and the other contributing to the slow flow generation. A regularization method is proposed to inverse the problem, whatever the degree of stability of the numerical systems since the regularization parameters are optimized. This regularization method borrows most of its principles from the L curve theory. The inverse method is applied to understand the mechanisms affecting nitrate transport in the Coët‐Dan catchment, Brittany (France), where high levels of nitrate pollution result from intensive agriculture. The hydrograph response to dissolved species is decomposed into quick and slow components, the former being separated into two subcomponents related to shallow and deep groundwater, respectively. Variable pollution levels and efficient denitrification of deep groundwater through pyrite oxidation lead to variable chemical composition in the groundwater column. By using nitrate and sulfate fluxes, the slow component of the unit hydrograph response can be separated into nitrate‐contaminated and denitrified groundwater responses to rainfall, which enables an accurate assessment of the denitrification capacity of the catchment.
Cloud induced reduction of solar UV‐radiation: A comparison of ground‐based and satellite based approaches
Authors: Williams J. E. , den Outer P. N. , Slaper H. , Matthijsen J. , Kelfkens G.
Published: February 2004

An extensive intercomparison has been made between the daily averaged reduction factors in UV irradiance due to the presence of clouds as derived from ground‐based (pyranometer) and satellite based measurements for the summer months of six years over Europe. Two independent satellite datasets were used for this purpose (ISCCP D1 and TOMS) and the resulting correlations between daily, 10‐day and monthly averaged values are presented. In general, the correlation between the daily averaged TOMS and ground‐based values matches those for the corresponding ISCCP comparisons (with typical R2 values of 0.85 and 0.8, respectively). Moreover, the monthly averaged TOMS values exhibit a more favorable agreement with ground‐based values, with typical reduction factors ranging from between 0.61–0.77, with a typical standard deviation of <0.05 from the ideal.
Use of cloud radar observations for model evaluation: A probabilistic approach
Authors: Jakob Christian , Pincus Robert , Hannay Cécile , Xu Kuan‐Man
Published: February 2004

The use of narrow‐beam, ground‐based active remote sensors (such as cloud radars and lidars) for long‐term observations provides valuable new measurements of the vertical structure of cloud fields. These observations might be quite valuable as tests for numerical simulations, but the vastly different spatial and temporal scales of the observations and simulation must first be reconciled. Typically, the observations are averaged over time and those averages are claimed to be representative of a given model spatial scale, though the equivalence of temporal and spatial averages is known to be quite tenuous. This paper explores an alternative method of model evaluation based on the interpretation of model cloud predictions as probabilistic forecasts at the observation point. This approach requires no assumptions about statistical stationarity and allows the use of an existing, well‐developed suite of analytic tools. Time‐averaging and probabilistic evaluation techniques are contrasted, and their performance is explored using a set of “perfect” forecasts and observations extracted from a long cloud system model simulation of continental convection. This idealized example demonstrates that simple time averaging always obscures forecast skill regardless of model domain size. Reliability diagrams are more robust, though scalar scores derived from the diagrams are sensitive to the forecast probability distribution. Forecasts by cloud system and weather forecasting models then provide examples as to how probabilistic techniques might be used in a variety of contexts.
Seasonal variation of methane, water vapor, and nitrogen oxides near the tropopause: Satellite observations and model simulations
Authors: Park Mijeong , Randel William J. , Kinnison Douglas E. , Garcia Rolando R. , Choi Wookap
Published: February 2004

Seasonal variations of several trace constituents near the tropopause are analyzed based on satellite measurements, and results are compared to a recent numerical model simulation. We examine methane, water vapor, and nitrogen oxides (NOx) derived from Halogen Occultation Experiment (HALOE) satellite observations; these species have strong gradients near the tropopause, so that their seasonality is indicative of stratosphere‐troposphere exchange (STE) and circulation in the near‐tropopause region. Model results are from the Model for Ozone and Related Chemical Tracers (MOZART) stratosphere‐troposphere chemical transport model (CTM). Results show overall good agreement between observations and model simulations for methane and water vapor, whereas nitrogen oxides near the tropopause are much lower in the model than suggested by HALOE data. The latter difference is probably related to the lightning and convective parameterizations incorporated in MOZART, which produce NOx maxima not near the tropopause, but in the upper troposphere. Constituent seasonal variations highlight the imporatance of the Northern Hemisphere (NH) summer monsoons as regions for transport into the lowermost stratosphere. In MOZART, there is clear evidence that air from the monsoon region is transported into the tropics and entrained into the upward Brewer‐Dobson circulation, bypassing the tropical tropopause.
Hillslope infiltration: Planar slopes
Authors: Philip J. R.
Published: February 2004

The solution of the full nonlinear unsaturated flow equation is given for a problem of infiltration (and associated subsurface flows) into a planar hillslope of a homogeneous isotropic soil with uniform initial moisture content. The solution uses the assumption that, at some distance below the slope crest, a spatial equilibrium is approached with the moisture content (and other properties) independent of downslope coordinate x* and dependent only on the normal coordinate z* and time t. The analysis is found to be generally applicable beyond a small distance from the slope crest (or from a point of slope change). For slope angles less than 30°, infiltration normal to the slope differs relatively little from infiltration from a horizontal surface. Interesting aspects of the solution include a time‐independent total horizontal flow into the slope and a time‐dependent downslope total flow component which behaves as t1/2 at small t and as t at large t. Previously, hillslope flows in these directions have been discussed in terms of soil anisotropy and layering, but these flows in a homogeneous isotropic soil are simple physical consequences of capillarity and gravity. The analysis applies with minor modification to two simple types of hillslope anisotropy: (1) anisotropy parallel to the slope and (2) horizontal anisotropy. Type 1 yields a complicated time dependence of the horizontal flow, and type 2 affects downslope flow similarly. These flow directions will commonly reverse during the course of the infiltration process.
Solute rejection in freezing soils
Authors: Panday Sorab , Corapcioglu M. Yavuz
Published: February 2004

Dissolved compounds in the water phase are rejected during freezing of soils. This phenomenon occurs due to the inability of ice crystals to incorporate most alien molecules. The ice grows only by association with water molecules, and molecules of solutes are rejected into the unfrozen water. A mathematical model for salt rejection in freezing saline soils has been developed by employing the mass balance equations for water and solute in a saturated porous medium, and the conservation of energy equation. Additional relations introduced include a moisture retention curve as a function of ice‐water capillary pressure, the Clapeyron equation and expressions for the heat capacity and the effective thermal conductivity of the soil. The resulting set of seven equations in terms of solute concentration, soil temperature, pore water, ice, and capillary pressures, and degrees of water and ice saturation have been solved simultaneously using a Newton‐Raphson linearization, with implicit iterative treatment of nonlinearities. Results indicate the development of unfrozen high salt concentration regions trapped in a frozen zone at sufficiently high freezing rates. The numerical results are compared with experimental data obtained by some researchers. Favorable match is obtained between the theoretical and experimental results. Finally, extension of the model to situations where ice lensing and soil deformations are significant is discussed.
Multivariate tests for trend in water quality
Authors: Loftis Jim C. , Taylor Charles H. , Chapman Phillip L.
Published: February 2004

Several methods of testing for multivariate trend have been discussed in the statistical and water quality literature. We review both parametric and nonparametric approaches and compare their performance using, synthetic data. A new method, based on a robust estimation and testing approach suggested by Sen and Puri, performed very well for serially independent observations. A modified version of the covariance inversion approach presented by Dietz and Killeen also performed well for serially independent observations. For serially correlated observations, the covariance eigenvalue method suggested by Lettenmaier was the best performer.
Stochastic analysis of simulated vadose zone solute transport in a vertical cross section of heterogeneous soil during nonsteady water flow
Authors: Russo David
Published: February 2004

The problem of transport of a conservative nonreactive solute in a vertical cross section of a hypothetical partially saturated, scale‐heterogeneous soil under transient water flow was analyzed here. It was assumed that locally the water flow and the solute transport can be described by the Richards' equation and by the one‐component convection dispersion equation, respectively. The simulated water content and the solute concentration distributions in the vertical cross section of the soil at different elapsed times were quantified in terms of space averages and two‐point autocorrelation functions. The time evolution of the solute plume was quantified in terms of its first two normalized spatial moments, from which the time dependence of the longitudinal and the transverse components of the solute velocity vector, and the spatial covariance tensor, were estimated. The results of this study, which are relevant to solute transport at the local or the plume scale, demonstrated the considerable variability in the solute concentration in space and time, due to the complex heterogeneity of the soil hydraulic properties in both the vertical and the horizontal directions. Consequently, the movement of the solute plume was characterized by a compression‐expansion phenomenon, attributed to the decrease in the effective solute velocity through the zones of relatively fine‐textured soil material. It was concluded that existing stochastic vadose zone transport models may be applicable to shallow depths but may fail to describe the actual spread of a solute plume when the transport takes place at relatively large depths, mainly because of the neglect of the significant vertical heterogeneity in the soil hydraulic properties.
The Bureau of Reclamation's new mandate for irrigation water conservation: Purposes and policy alternatives
Authors: Moore Michael R.
Published: February 2004

Although the Bureau of Reclamation adopted a new mission as a water management agency, social purposes of the mission and methods of accomplishing the purposes remain undefined. A broad consensus agrees that a central feature of the agency's management program should be irrigation water conservation. This paper describes three purposes of irrigation water conservation: achieving economic efficiency of water allocation, improving environmental quality of western river systems, and satisfying outstanding Native American water claims. Five policy instruments are described as alternative methods of inducing conservation: quantity‐based regulation, price‐based regulation, transferable water use permits, conservation subsidies, and decentralization of ownership of Reclamation facilities. Two findings are: (1) price‐based regulation may not produce water conservation and (2) conservation policy instruments should be chosen with reference to their ability to achieve the purposes of federal water conservation policy. An example illustrates quantitative effects on farm income of the alternative instruments.
Formation and dissolution kinetics of Al(OH)3 (s) in synthetic freshwater solutions
Authors: Lydersen E. , Salbu B. , Polèo A. B. S. , Muniz I. P.
Published: February 2004

The precipitation of Al in aqueous solutions can be described as a two‐step process. When acidic inorganic Al solutions (pH 4.5) were titrated with NaOH to pH levels between 5.5 and 6.0, an amorphous Al(OH)3 (s) phase was formed instantaneously. During the first 5 min, the apparent half time for the reduction in dissolved Al species (t1/2) was 0.162 ± 0.07 hours (n = 4). The decrease of dissolved Al species continued during the following 24 hours, but at a far slower rate (t1/2 = 55.6 ± 25 hours (n = 4)). The highest precipitation rates were found in the solution of highest pH, and at approximately identical pH, the highest rate was found in the solution of highest temperature. The dissolution of amorphous Al(OH)3 (s) can also be described as a two‐step process. When the amorphous Al solutions (pH between 5.5 and 6.0) were acidified to pH about 4.5 (HCl), the initial dissolution was relatively fast, but not as instantaneous as in the first step of precipitation. The apparent t1/2 after 5 min was estimated to be 0.65 ± 0.12 hours (n = 4). During the first 2.67 hours (160 min) the dissolution step could be described by the equation C(t) = C0e−0.45t, i.e., t1/2 = 1.53 ± 0.60 hours (n = 20) according to which the solid Al phase should dissolve totally (97%) during 7.65 hours. However, after 24 hours of storage 40 to 50% of the original Al(OH)3 (s) remained in the solid phase. This is explained by the presence of a far lower dissolution rate in a second step, i.e., the remaining Al(OH)3 (s) was less soluble. The average dissolution rate for the rest of the 24‐hour period (21.33 hours) could be described by the exponential function C(t) = C0e−0.0095t, i.e., t1/2 = 73.2 ± 23.4 hours (n = 4). This implies that a total dissolution of Al (97%) will occur after 15.3 ± 4.9 days. The implications of these findings are discussed with reference to the current geochemical models pretending to simulate and predict aluminum chemistry in soils and surface runoff.