bluuebunny/crossref_metadata_2025 huggingface dataset:
Boussinesq modeling of a rip current system
Authors: Chen Qin , Dalrymple Robert A. , Kirby James T. , Kennedy Andrew B. , Haller Merrick C.
Published: December 2003

In this study, we use a time domain numerical model based on the fully nonlinear extended Boussinesq equations [Wei et al., 1995] to investigate surface wave transformation and breaking‐induced nearshore circulation. The energy dissipation due to wave breaking is modeled by introducing an eddy viscosity term into the momentum equations, with the viscosity strongly localized on the front face of the breaking waves. Wave run‐up on the beach is simulated using a moving shoreline technique. We employ quasi fourth‐order finite difference schemes to solve the governing equations. Satisfactory agreement is found between the numerical results and the laboratory measurements ofHaller et al.[1997], including wave height, mean water level, and longshore and cross‐shore velocity components. The model results reveal the temporal and spatial variability of the wave‐induced nearshore circulation, and the instability of the rip current in agreement with the physical experiment. Insights into the vorticity associated with the rip current and wave diffraction by underlying vortices are obtained.
Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific
Authors: Bromirski Peter D. , Flick Reinhard E. , Graham Nicholas
Published: December 2003

Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near‐coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land‐based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site‐specific, empirically derived seismic‐to‐wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near‐coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.
Ocean feedback to wind‐driven coastal polynyas
Authors: Chao Shenn‐Yu
Published: December 2003

For lack of observations, a coupled numerical model of an ice layer and a coastal ocean is used to conduct thought experiments on the evolution of wind‐driven coastal polynyas. Attention is focused on the possible ocean feedback to polynya developments before and after wind relaxation. The coupled system is initially motionless, having an ice layer of uniform concentration and thickness. In the ocean, the salinity is initially uniform and the temperature is at the freezing point. Subsequent cold air outbreaks of several days in duration push the ice layer offshore. Frazil is produced and collected in the polynya owing to continuous heat loss to the atmosphere. Ocean feedback is particularly effective after wind relaxation. Under large‐scale seaward winds, the displaced ice edge does not return landward after wind abatement if the wind event is strong and long. Over a sizable coastal bottom obstacle, however, anticyclonic circulation develops and its landward arm may force a landward return of the ice edge after wind relaxation. Further, dipole vortices in the ocean may develop under a mesoscale seaward wind event. Landward ocean currents around the rim of a dipole may also force a postwind return of the ice edge. The ocean feedback is generally stronger and faster in shallower basins; this can be demonstrated by a one‐dimensional, stress‐driven coastal ocean model.
Direct velocity measurements in the Malvinas Current
Authors: Vivier Frédéric , Provost Christine
Published: December 2003

We present a set of current meter measurements collected in the southwest Atlantic from December 1993 to June 1995. Four moorings were set out to monitor the Malvinas (Falkland) Current at 40°–41°S, and one mooring was located within the Brazil‐Malvinas Front area at 38°S. The mean flow of the Malvinas Current has an equivalent barotropic structure, whereas the vertical structure of the variable part of the flow is dominated by a barotropic‐like empirical mode for regions shallower than 2500 m, surface intensified, which appears as a linear combination of barotropic and baroclinic modes, suggesting mode coupling due to the steep topography. This mode appears to gradually become nearly exactly barotropic toward shallower depths. Higher empirical modes, bottom intensified and associated with higher frequencies, suggest waves trapped by the conjugated effects of topography and stratification. The time variability is discussed in terms of along‐ and cross‐isobath flows. The along‐shelf flow features substantial energy in the 50 to 70‐day band. There is no evidence of an annual cycle nor of a distinct semiannual cycle, but there is a suggestion of significant energy at periods a little shorter (135 days). The cross‐shelf variability is important, with a quite clear annual cycle, associated with excursions of the subantarctic front. Its phase coincides with that of the Brazil Current transport, suggesting that annual migrations of the front are predominately determined by the strength of the latter, whereas the intensity of the Malvinas Current would only have marginal impact, a conclusion that may not hold at other timescales.
Stratified Ekman layers
Authors: Price James F. , Sundermeyer Miles A.
Published: December 2003

Under fair weather conditions the time‐averaged, wind‐driven current forms a spiral in which the current vector decays and turns clockwise with increasing depth. These spirals resemble classical Ekman spirals. They differ in that their rotation depth scale exceeds the e‐folding depth of the speed by a factor of 2 to 4 and they are compressed in the downwind direction. A related property is that the time‐averaged stress is evidently not parallel to the time‐averaged vertical shear. We develop the hypothesis that the flat spiral structure may be a consequence of the temporal variability of stratification. Within the upper 10–20 m of the water column this variability is associated primarily with the diurnal cycle and can be treated by a time‐dependent diffusion model or a mixed‐layer model. The latter can be simplified to yield a closed solution that gives an explicit account of stratified spirals and reasonable hindcasts of midlatitude cases. At middle and higher latitudes the wind‐driven transport is found to be trapped mainly within the upper part of the Ekman layer, the diurnal warm layer. At tropical latitudes the effects of diurnal cycling are in some ways less important, and Ekman layer currents are likely to be significant to much greater depths. In that event the lower part of the Ekman layer is likely to be affected by stratification variability that may be nonlocal.
On the use of high‐resolution satellite data to describe the spatial and temporal variability of sea surface temperatures in the New Zealand region
Authors: Uddstrom Michael J. , Oien Niles A.
Published: December 2003

A new high‐resolution (1.1 km) regional data set of satellite‐derived sea surface temperatures (SST) is developed and validated against drifting and moored buoy data. Analysis of 5 years of monthly means and related statistics suggests that there is often a coherent relationship between the SST patterns and oceanographic features. Identifiable relationships are detailed, then used to indicate the temporal and spatial characteristics of watermasses, fronts, and currents. For example, a gradient analysis suggests that the Subtropical Front east of New Zealand (and west of 177°W) is bathymetrically locked to the southern flank of the Chatham Rise (near 44°S). Beyond 177°W it loops southward to a mean position close to 47°S on the northern edge of the Bounty Plateau, adjacent to the Subantarctic Front (near 48°S). Harmonic analysis of the 60 months of data suggests that spatial variability in the amplitude of the SST annual cycle is related to circulation (water mass) features and varies from less than 1°C in Subantarctic Waters to more than 4°C in New Zealand nearshore neritic waters. The Antarctic Circumpolar, East Auckland, and East Cape Currents also display coherent annual cycle amplitude signatures that differ from adjacent water masses. However, the amplitude of the annual cycle in Subantarctic Waters south of the Chatham Rise and north of the Pukaki Rise and Bounty Plateau is significantly different from that of this water mass at other (similar) latitudes. An empirical orthogonal function (EOF) analysis of seasonal anomalies shows that 90% of SST variance can be associated with the annual cycle. However, four EOFs are required to explain 50% of the variance in SST monthly anomalies. The first of these (25.4% of the variance) indicates that anomalies north and east of New Zealand are negatively correlated with those in the Subantarctic. The second EOF represents a Tasman Sea mode and suggests that the Tasman Sea has been in a warming phase since December 1996 (to at least April 1998). This might explain the exceptional size (and sign) of the SST anomaly in New Zealand waters during the 1998 El Niño, where in February 1998 it reached +3.5°C in northern New Zealand waters, similar to the amplitude of the annual cycle.
Statistical analysis of atmospheric forcing of North Pole ice draft variations
Authors: Shy Timothy L. , Walsh John E.
Published: December 2003

A series of correlation experiments and regression models are used to explore the atmospheric forcing of observed interannual variability in North Pole ice drafts as measured by U.S. Navy submarine sonar from 1982 to 1992. Mean ice drafts as well as ice draft distributions are used in the analysis. Nearby drifting buoys are used to approximate a full 2‐year motion history of the North Pole ice parcels prior to submarine underpass. Drawing from theoretical work and empirical observation, predictors of North Pole ice draft variation are selected which embody accumulated ice exposure to subfreezing air temperature, ice divergence, and patterns of ice motion. The timescales of the predictors are based on systematic evaluations of the lag dependence of the predictor‐predictand relationships. Linear correlation of each predictor with the observed ice draft distribution shows accumulated ice exposure to subfreezing air temperature is weakly correlated with reductions in shallow (0 to 2.5‐m) drafts. Ice divergence a week prior to draft measurement correlates most closely with the redistribution of ice drafts shallower than 3.5 m. Ice divergence and ice deflection from its wind‐forced motion during the 6 months prior to observation correlate with deeper ice drafts (deeper than 6.8 and 4.6 m, respectively). Together, these two parameters explain more than 80% of the variance of the observed mean ice draft. The accumulated ice exposure to subfreezing air temperatures colder than the 1980–1992 mean in the 2 years prior to ice arrival at the North Pole adds little to an explanation of the interannual variance of the mean ice draft. The results imply that (1) a realistic formulation of sea ice dynamics is a first‐order priority for model simulations of ice draft and (2) North Pole ice thickness is not a simple function of Arctic air temperature.
Relationship between GPS signals reflected from sea surfaces and surface winds: Modeling results and comparisons with aircraft measurements
Authors: Lin Bing , Katzberg Stephen J. , Garrison James L. , Wielicki Bruce A.
Published: December 2003

Theoretical calculations of global positioning system (GPS) signals reflected from rough sea surfaces are discussed and compared with the aircraft measurements. The theoretical model is based on the assumption that rough sea surfaces are composed of facets, and the reflection of GPS signals from the facets can be calculated on the basis of geometric optics. In order to determine the slopes of the sea surface facets, the statistical model of Cox and Munk [1954] for ocean surface slopes is used. Since much of the sea surface roughness observed is at scales much less than the GPS wavelength, the dependence of the mean square slopes on frequency is taken into consideration. Model results agree well with aircraft measurements: the differences between model results and observations are within the level of experimental errors. For calm sea surfaces (near‐sea‐surface wind speeds <1 m s−1), the correlation powers of the GPS signals are narrow functions of time delay with peak values of ∼0—−3 dB. In stronger wind cases (wind speeds >7 m s−1), the correlation powers decrease their peak values to ∼4—−8 dB, significantly widen time delay responses, and shift toward the delays corresponding to path lengths longer than those of specular points. Sensitivity tests show that there is considerable potential for current or advanced GPS receiving systems to estimate near‐sea‐surface wind speeds: the errors in the wind speed estimates could be smaller than 2 m s−1. For near‐sea‐surface wind directions, the maximum changes in the correlation powers of the GPS signals as a function of azimuth appear to be ∼0.5 dB for current GPS receiving systems, which may be too small for wind direction estimation. New designs of the GPS receivers or a higher signal‐to‐noise ratio than that of current aircraft receivers may be needed.
Origin and ages of mode waters in the Brazil‐Malvinas Confluence region during austral winter 1994
Authors: Maamaatuaiahutapu Keitapu , Provost Christine , Andrié Chantal , Vigan Xavier
Published: December 2003

The ventilation of the main thermocline associated with the formation of Subtropical Mode Water (STMW) and Subantarctic Mode Water (SAMW) within the Brazil/Malvinas Confluence region is investigated using satellite data, hydrographic data, and dissolved CFCs as transient tracers. During the winter of 1994, two types of STMW were observed. Both types, 26.24 σθ and 26.46 σθ were formed within the warm pool of the South Atlantic Central Water (SACW). The SAMW is found on the 27.10 σθ density surface. This surface lies in the minimum salinity layer of the Antarctic Intermediate Water (AAIW). The mode waters are dated using CFC ratios. To our knowledge, this is the first estimate of the age of the mode waters in the region using the CFC‐113:CFC‐11 ratio method. The lighter STMW was being formed during the cruise. The heavier STMW was found to be 6±3 years old and is thought to have circulated in the subtropical gyre without losing its primary properties. The SAMW, or the lightest AAIW has an age of 3±2 years. This water is very young compared with the oldest (10±5 years) and heaviest component of the AAIW, which circulates within the subtropical gyre. The formation of the STMW in the confluence region is dependent on the southward extension of the Brazil Current.
Relationship of ENSO to snowfall and related cyclone activity in the contiguous United States
Authors: Kunkel Kenneth E. , Angel James R.
Published: December 2003

The relationship of the El Niño‐Southern Oscillation to the contiguous U.S. snowfall was investigated using data from 3841 cooperative observer stations for the period 1951–1997. Snowfall for seven strong El Niño and five strong La Niña events was compared with snowfall occurring during the other 34 winters. The average snowfall for the seven El Niño winters exhibits a coherent large‐scale pattern with below‐average snowfall across approximately the northern half of the United States and above‐average snowfall over the southwestern United States. Application of the t‐test indicates that these snowfall anomalies are significantly different (at the 10% level) from the base period snowfall in parts of the mid‐Mississippi River basin, the northern Ohio River basin, the western Great Lakes, and the northwestern United States. During strong La Niña winters, above‐normal snowfall occurs in the northwest United States and northern Great Lakes. Although these anomalies do not pass tests of statistical significance (at the 10% level), the difference between La Niña and El Niño years is quite notable in the northwest United States. The results suggest that useful long‐range outlooks of snowfall anomalies are possible for certain areas of the contiguous United States during strong El Niño events. An analysis of the frequency of strong cyclones (central pressure <992 hPa) shows that the El Niño winters were characterized by above‐normal frequency in the extreme southern United States and below‐normal frequency along the U.S.‐Canadian border, with the pattern approximately reversed in the La Niña winters.
Vertical mixing and chemistry of isoprene in the atmospheric boundary layer: Aircraft‐based measurements and numerical modeling
Authors: Doskey Paul V. , Gao Weigang
Published: December 2003

Vertical profiles of isoprene, methanol, and ozone (O3) concentrations were measured between the middle and upper atmospheric boundary layer (ABL) from a research aircraft and were numerically simulated for the ABL and a deciduous forest canopy with a one‐dimensional model coupling turbulence diffusion and atmospheric chemistry. Isoprene emissions from the deciduous forest canopy were estimated by coupling an existing biogenic emission algorithm with estimates of canopy leaf density inferred from satellite remote sensing observations. Numerical simulations predicted low isoprene concentrations in the middle and upper ABL; however, the agreement between the simulations and the measured values was poor for two of the three profiles, indicating that a three‐dimensional transport model might be necessary in future simulations. Chemical oxidation of isoprene by O3 and hydroxyl radical (OH), particularly in the middle and upper ABL, tends to reduce the isoprene concentrations and influences the vertical fluxes in that layer; however, chemical reactions have little effect on fluxes of isoprene near the emission source, where turbulent mixing is much faster than chemical reactions and where the emission process controls the vertical flux. The isoprene flux decreases rapidly with increasing height, with little isoprene escaping from the ABL. Vertical profiles of methanol concentrations were simulated with the biogenic emission algorithm used for isoprene; these vertical profiles were similar to the measured values for the well‐mixed ABL but were much lower than the measured concentrations in the lower layers of the growing ABL because of weaker calculated mixing in the upper ABL during the morning. The results of this investigation indicate that chemical oxidation of isoprene is rapid enough to allow O3 and other oxidants to accumulate in the ABL on a regional scale if sufficient levels of nitrogen oxides are present; however, methanol is much more stable, and biogenic emissions of this compound have the potential to form O3 and other oxidants in areas distant from the emission source.
Ozone in the remote marine boundary layer: A possible role for halogens
Authors: Dickerson Russell R. , Rhoads Kevin P. , Carsey Thomas P. , Oltmans Samuel J. , Burrows John P. , Crutzen Paul J.
Published: December 2003

On the spring 1995 cruise of the National Oceanic and Atmospheric Administration research vessel Malcolm Baldrige, we measured very large diurnal variations in ozone concentrations in the marine boundary layer. Average diurnal variations of about 32% of the mean were observed over the tropical Indian Ocean. We simulated these observations with the Model of Chemistry in Clouds and Aerosols, a photochemical box model with detailed aerosol chemistry. The model was constrained with photolysis rates, humidity, aerosol concentrations, NO, CO, and O3 specified by shipboard observations and ozonesondes. Conventional homogeneous chemistry, where ozone photolysis to O(1D) and HOx chemistry dominate ozone destruction, can account for a diurnal variation of only about 12%. On wet sea‐salt aerosols (at humidities above the deliquescence point), absorption of HOBr leads to release of BrCl and Br2, which photolyze to produce Br atoms that may provide an additional photochemical ozone sink. After 8 days of simulation, these Br atoms reach a peak concentration of 1.2×107 cm−3 at noon and destroy ozone through a catalytic cycle involving BrO and HOBr. Reactive Br lost to HBr can be absorbed into the aerosol phase and reactivated. The model predicts a diurnal variation in O3 of 22% with aerosol‐derived Br reaction explaining much, but not all, of the observed photochemical loss. The lifetime of ozone under these conditions is short, about 2 days. These results indicate that halogens play an important role in oxidation processes and the ozone budget in parts of the remote marine boundary layer.
Spatial heterogeneity of albedo over a snow‐covered agricultural landscape
Authors: Kunkel Kenneth E. , Isard Scott A. , Hollinger Steven E. , Gleason Byron , Belding Mark
Published: December 2003

Measurements of albedo were obtained along a 100 km transect in east central Illinois using a combination of airborne and ground‐based radiometers. Analysis of the measurements focused on three different land use categories: agricultural cropland, prairie grass, and woodland. The value of albedo over the woodland after a fresh snowfall was 0.30, which is somewhat higher than what has been measured in a previous experiment over an aspen forest but similar to previous measurements over a deciduous forest in the eastern United States. The albedo values over the grassland were similar to past experiments and similar to the values over cropland. The drifting that accompanied one of the snow events caused large differences in albedo during snowmelt compared to another event with minimal drifting but similar average snow depth. The albedo was lower over the melting wind‐blown snow cover than for a snow cover of similar average depth that fell under light wind conditions. These large differences suggest that inclusion of meteorological conditions during snowfall in snow parameterizations may appreciably increase their accuracy. Different tillage practices used in the agricultural fields also caused measurable differences in albedo during the snowmelt phase.
Advanced hydrologic prediction system
Authors: Connelly Brian A. , Braatz Dean T. , Halquist John B. , DeWeese Michael M. , Larson Lee , Ingram John J.
Published: December 2003

As our Nation's population and infrastructure grow, natural disasters are becoming a greater threat to our society's stability. In an average year, inland flooding claims 133 lives and resulting property losses exceed $4.0 billion. Last year, 1997, these losses totaled $8.7 billion. Because of this blossoming threat, the National Weather Service (NWS) has requested funding within its 2000 budget to begin national implementation of the Advanced Hydrologic Prediction System (AHPS). With this system in place the NWS will be able to utilize precipitation and climate predictions to provide extended probabilistic river forecasts for risk‐based decisions. In addition to flood and drought mitigation benefits, extended river forecasts will benefit water resource managers in decision making regarding water supply, agriculture, navigation, hydropower, and ecosystems. It's estimated that AHPS, if implemented nationwide, would save lives and provide $677 million per year in economic benefits. AHPS is used currently on the Des Moines River basin in Iowa and will be implemented soon on the Minnesota River basin in Minnesota. Experience gained from user interaction is leading to refined and enhanced product formats and displays. This discussion will elaborate on the technical requirements associated with AHPS implementation, its enhanced products and informational displays, and further refinements based on customer feedback.
Improving first‐order snow‐related deficiencies in a regional climate model
Authors: Liston Glen E. , Pielke Roger A. , Greene Ethan M.
Published: December 2003

A climate version of the Regional Atmospheric Modeling System (RAMS) is used to simulate snow‐related land‐atmosphere interactions in the Great Plains and Rocky Mountain regions of the United States. The availability of observed snow‐distribution products allow snow‐water‐equivalent distribution data to be assimilated directly into the RAMS simulations. By performing two kinds of model integrations, one with and one without assimilating the snow‐distribution observations, the differences between the model runs are used to highlight model deficiencies and limitations and thus identify areas of possible improvement in the atmospheric model. The need to simulate subgrid snow distributions is identified and addressed by implementing a snow submodel that accounts for subgrid variations in air temperature and precipitation. This subgrid snow model is found to significantly improve the model's simulation of snow‐related processes.
Heterogeneous sulfate production in the remote marine environment: Cloud processing and sea‐salt particle contributions
Authors: Gurciullo C. , Lerner B. , Sievering H. , Pandis S. N.
Published: December 2003

Heterogeneous sulfate production in the remote marine boundary layer is studied by combining measurements collected during the first Southern Hemisphere marine Aerosol Characterization Experiment (ACE 1) with two numerical models: one simulating heterogeneous sulfate production in sea‐salt aerosol water (SSAW) and the other simulating cloud processing. The models calculate oxidation of SO2 by O3 and H2O2 via aqueous‐phase reactions both in SSAW and in cloud droplets. Model calculations indicate that as much as 50–75% of the observed non‐sea‐salt (NSS) sulfate in particles in the ambient diameter size range of 0.9–16 μm during the ACE 1 measurement period is due to production in SSAW, with cloud processing producing the remainder. The initial alkalinity of the wind‐generated sea‐salt aerosols can strongly influence sulfate production. Changing the effective alkalinity of SSAW from a value typical of seawater to a value based on the measurements taken at Cape Grim results in a better match to the observed NSS sulfate size distribution with up to 75% of the predicted supermicrometer NSS sulfate being due to heterogeneous conversion in SSAW. The average rates of removal of sulfate due to dry and wet deposition for the 0.4–16 μm size range are 38 and 14 ng m−3 d−1 respectively.
Generality of drought flow characteristics within the Arkansas River Basin
Authors: Eng Ken , Brutsaert Wilfried
Published: December 2003

Drought flow measurements from 11 river basins, contained within and surrounding the Cooperative Atmosphere‐Surface Exchange Study (CASES) in the Walnut River watershed in Kansas, were subjected to a recession slope analysis. This analysis was based on the Dupuit‐Boussinesq groundwater formulation and made use of the Historical Streamflow Daily Values published by the U.S. Geological Survey. The long‐ and short‐term response features of the riparian aquifers were found to be practically identical to those observed in an earlier study in the Washita River basin in Oklahoma. As both study areas lie within the Osage section of the Central Lowland physiographic province, these findings support the applicability of regionalization and scaling of low‐streamflow‐related basin characteristics within the same geomorphic domain.
Mercury flux measurements in a naturally enriched area: Correlation with environmental conditions during the Nevada Study and Tests of the Release of Mercury From Soils (STORMS)
Authors: Poissant Laurier , Pilote Martin , Casimir Alain
Published: December 2003

An international intercomparison of micrometerological techniques and dynamic flux chamber methods applied to measure mercury fluxes was conducted from September 1 to 4, 1997, during the Nevada Study and Tests of the Release of Mercury From Soils (STORMS) in Reno, Nevada. Nine research groups from four countries met in the Steamboat Springs, Nevada Geothermal Area, to participate in the first international flux intercomparison ever attempted for mercury. The highly heterogeneous soil Hg concentrations and complex landscape within the study area (4 ha) were unfavorable for spatial intercomparison of Hg fluxes between the research groups. However, reliable and correlated Hg fluxes were measured between our micrometerological technique and a dynamic flux chamber method (r2 = 0.29), run side by side (5 m). Hg fluxes and their relationships with environmental factors were complex. After ∼90 days of dry condition, a series of storm events impacted the site and increased the soil moisture from <0.5 to 6.6%. This appeared to promote strong Hg evasion during the transition period from dry to wet soil conditions. However, the subsequent relationship between soil moisture and Hg flux was significantly negatively correlated. Multivariate analysis was applied to extract the principal components (principal component analysis). Three principal components were extracted (explained up to 79% of the total variance) and discussed with respect to their environmental signification. Environmental conditions under southern wind sectors were optimal to promote Hg fluxes. Turbulence rather than Hg air concentrations seemed to be the main factor promoting the determined Hg fluxes during this study.
Spatial modeling and prediction of snow‐water equivalent using ground‐based, airborne, and satellite snow data
Authors: Carroll Steven S. , Carroll Thomas R. , Poston Robert W.
Published: December 2003

In this research we modify existing spatial interpolation methodologies so that we can use ground‐based and remotely sensed (airborne and satellite) snow data to characterize the spatial distribution of snow‐water equivalent (SWE) and obtain optimal gridded SWE predictions in the upper Mississippi River basin. We developed and tested the models using ground‐based, airborne, and satellite snow data collected over North and South Dakota, Minnesota, Wisconsin, Iowa, Illinois, and Michigan between March 3 and 6, 1996. Using these data and the spatial models, we obtained optimal gridded predictions of SWE and the associated root mean square prediction errors over a 5 min by 5 min grid covering Minnesota and parts of Wisconsin, North and South Dakota, Iowa, Michigan, and Canada. Because we use an optimal interpolation technique and incorporate satellite areal extent of snow cover data, the predictions are expected to be more accurate than those that would be obtained from interpolation procedures currently used by the National Weather Service. Maps of the gridded snow water equivalent predictions and of the associated error estimates provide a means to investigate the spatial distributions of the predictions and of the associated error estimates. Our research enables hydrologists and others not only to examine these spatial distributions but also to generate optimal gridded predictions of snow‐water equivalent that will aid flood forecasting and water resource management efforts.
Estimating soil moisture from satellite microwave observations: Past and ongoing projects, and relevance to GCIP
Authors: Owe M. , Van de Griend A. A. , de Jeu R. , de Vries J. J. , Seyhan E. , Engman E. T.
Published: December 2003

On the basis of a series of studies conducted in Botswana and preliminary results from an ongoing study in Spain, developments in microwave remote sensing by satellite, which can be used to monitor near‐real‐time surface moisture and also study long‐term soil moisture climatology, are described. A progression of methodologies beginning with single‐polarization studies and leading to both dual polarization and multiple frequency techniques are described. Continuing analysis of a 9 year data set of satellite‐derived surface moisture in Spain is ongoing. Preliminary results from this study appear to provide some evidence of long‐term desertification in certain parts of this region. The methodologies developed during these investigations can be applied easily to other regions such as the GCIP area and could provide useful databases for simulation and validation studies. Additionally, they have strong potential for global applications such as climate change studies.
Nonmethane hydrocarbons in surface waters, their sea‐air fluxes and impact on OH in the marine boundary layer during the First Aerosol Characterization Experiment (ACE 1)
Authors: Pszenny A. A. P. , Prinn R. G. , Kleiman G. , Shi X. , Bates T. S.
Published: December 2003

Concentrations of six abundant alkanes (propane, 2‐methylpropane, butane, 2‐methylbutane, pentane, cyclopentane) and three abundant alkenes (2‐methylpropene, cis‐2‐butene, 1,3‐butadiene) were determined in air equilibrated with surface seawater in real time during the International Global Atmospheric Chemistry First Aerosol Characterization Experiment (ACE 1) campaign to the Southern Ocean in November‐December 1995. Concentrations in ocean surface water inferred from these measurements generally lay within the ranges observed by other investigators at other times in other regions of the global ocean. Concentrations tended to decrease with increasing latitude. Comparison with a limited number of dissolved organic carbon (DOC) data obtained during transit from Hawaii to Hobart, Tasmania, suggested that higher hydrocarbon concentrations were associated with higher DOC levels. During the ACE 1 intensive study period west and south of Tasmania the differences in NMHC concentrations among the three major water masses sampled, while frequently significant, were not related in a simple direct or inverse way to surface water temperature, concentrations of nitrate, chlorophyll a, or dimethylsulfide, or to calculated sea‐to‐air transfer velocity or solar radiation intensity. The only correlations that showed some consistency within water masses were with temperature (negative) and nitrate (positive), an essential nutrient for phytoplankton growth. Deduced sea‐to‐air fluxes indicated modest open‐ocean emissions of C3–C5 NMHC in this part of the global ocean. Assuming that the deduced fluxes were balanced by reaction with hydroxyl radical, OH, we estimated that the nine measured NMHC together accounted for negligible removal of OH compared to that due to the dominant OH + CO reaction and about 5 times less than that due to the DMS + OH reaction in the region of the marine boundary layer studied during ACE 1.
An evaluation of NCEP Eta model predictions of surface energy budget and cloud properties by comparison with measured ARM data
Authors: Hinkelman Laura M. , Ackerman Thomas P. , Marchand Roger T.
Published: December 2003

Time‐series output from the Eta forecast model of the National Weather Service's National Centers for Environmental Prediction was evaluated by comparison with measured values for a location in Oklahoma. The measured data were drawn from the archives of the Department of Energy's Atmospheric Radiation Measurement program for the southern Great Plains site. Surface energy budget components and cloud indicators were examined for the first half of 1997. Overall, the Eta surface energy budget was found to be nearly balanced, as intended by the model physics, except for one instance when light snowfall occurred in the spring. Despite this balance, an average 50 W m−2 excess in incoming solar radiation was found. Approximately half of this excess was attributed to deficient extinction of shortwave radiation by water vapor and aerosols in the model, while the remainder appeared to be due to cloud treatment errors. The excess shortwave flux was offset by a smaller negative bias in the downwelling infrared flux and the use of slightly high albedos, in addition to errors of lesser magnitude in the latent and sensible heat fluxes. The upwelling infrared flux and ground heat flux were closer to measured values. Ambiguity in the definition of a cloud as well as measurement limitations hindered the analysis of cloud prediction. However, a rough cloud comparison indicated that the Eta model has more difficulty predicting convection than the movement of large storm systems. In addition, cirrus clouds were predicted too frequently in the winter, while low and midlevel clouds were often missed in the spring. This study demonstrates that single‐point time‐series data can be used effectively both to ascertain the quality of model output and to investigate the treatment of individual physical processes within models.
Effect of temporal sampling of precipitation on hydrologic model calibration
Authors: Holman‐Dodds Jennifer K. , Bradley A. Allen , Sturdevant‐Rees Paula L.
Published: December 2003

Although the effects of spatial and temporal variability of precipitation on hydrologic modeling results have been well established, there has been very little attention given to sampling impacts on model calibration. To examine the effects of temporal sampling of rainfall, a combination of numerical simulations and calibration experiments have been carried out. A “true” runoff scenario, which is used for calibration of models using longer sampling periods, is established using very high resolution precipitation data and parameters derived from the physical properties of the soils. Three point infiltration models were chosen to isolate the effects of temporal sampling. A simple empirical model, a more complex empirical model, and a physics‐based model were tested and compared. Numerical simulations using parameters based directly on soil properties demonstrated an undersimulation of runoff as the precipitation signal is smoothed at lower sampling frequencies. Calibration of selected parameters may compensate for this undersimulation effect. This impact becomes more pronounced for higher infiltration capacities, as calibration is required to compensate for greater underestimation of runoff. The adjustments that are necessary to compensate for the sampling effect illustrate how some of the physical meaning of the parameters is lost in the calibration process. Since the calibrated model parameters correspond to physical properties of the system, these results demonstrate that it may not be possible to make a priori parameter estimates based solely on physical properties of the system or to use parameters calibrated using data with different temporal or spatial sampling.
Intercomparisons of ECMWF ERA and TOGA data with observations for the 1993 Great Flood
Authors: Hwu Wenje , Sorooshian Soroosh , Gao Xiaogang , Famiglietti James S.
Published: December 2003

The 1993 summer rainfall over the Midwestern United States caused major flooding in the Upper Mississippi and Lower Missouri River basins. In this paper, a monthly comparative study of the analysis products from the European Centre for Medium‐Range Weather Forecasts (ECMWF) with observations is conducted for the period September 1992 to August 1993. The model products include the ECMWF analysis for the Tropical Ocean and Global Atmosphere (TOGA) project and ECMWF reanalysis (ERA) project. Overall, the large‐scale hydrological components (precipitation, evaporation, precipitable water, surface temperature, and relative humidity) appear partially improved from TOGA to ERA. Some internal inconsistencies exist in the assimilation of the ERA data from October 1992 to March 1993, in association with the dramatic prevalence of large‐scale moisture divergence at 0000 UT. The separation of moisture flux into meridional and zonal components indicates that the correct description of the meridional moisture flow is of crucial importance for simulation of anomalously wet conditions. While the spatially averaged monthly fields in ERA and TOGA are reasonable (in particular, the models maintain very good mean flow), the fine‐scale extremes are less accurate. The models do not reproduce observed mesoscale convective complexes due to limitations on resolution and the parameterization of precipitation processes. The new soil hydrology and boundary layer schemes of ERA work better in warm months but with no consistent improvement over TOGA. The terrestrial water budget in ERA is unrealistic, with accumulated model runoff being largely deviated from the observed streamflow. In addition to modeling internal water balance, the lack of the routing procedure and the low spatial resolution of surface hydrology are the other major reasons for this imbalance.
An evaluation of NEXRAD precipitation estimates in complex terrain
Authors: Young C. Bryan , Nelson Brian R. , Bradley A. Allen , Smith James A. , Peters‐Lidard Christa D. , Kruger Anton , Baeck Mary Lynn
Published: December 2003

Next Generation Weather Radar (NEXRAD) precipitation estimates are used for hydrological, meteorological, and climatological studies at a wide range of spatial and temporal scales. The utility of radar‐based precipitation estimates in such applications hinges on an understanding of the sources and magnitude of estimation error. This study examines precipitation estimation in the complex mountainous terrain of the northern Appalachian Mountains. Hourly digital precipitation (HDP) products for two WSR‐88D radars in New York state are evaluated for a 2‐year period. This analysis includes evaluation of range dependence and spatial distribution of estimates, radar intercomparisons for the overlap region, and radar‐gage comparisons. The results indicate that there are unique challenges for radar‐rainfall estimation in mountainous terrain. Beam blockage is a serious problem that is not corrected by existing NEXRAD algorithms. Underestimation and nondetection of precipitation are also significant concerns. Improved algorithms are needed for merging estimates from multiple radars with spatially variable biases.
Assessment of land‐surface energy budgets from regional and global models
Authors: Berbery Ernesto Hugo , Mitchell Kenneth E. , Benjamin Stanley , Smirnova Tatiana , Ritchie Harold , Hogue Richard , Radeva Ekaterina
Published: December 2003

The surface energy budgets estimated from the 0‐ to 12‐hour forecasts of three operational model suites and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis are analyzed at local to continental scales. The models are (1) the Eta model, (2) the Mesoscale Analysis and Prediction System (MAPS), and (3) the Global Environmental Multiscale (GEM) model. The first two are regional, while the third one is global with a variable grid with a resolution over North America that is equivalent to that of the regional models. This assessment of one summer month (August 1997) and one winter month (January 1998) has the purpose of estimating the reliability of the surface energy budgets within the context of the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP) goals. Satellite estimates were used to evaluate the downward short wave radiation at the surface, while measurements from the southern Great Plains region were used to evaluate the model computed surface energy budget estimates. The results show that the surface short wave radiation biases of the models are widespread and of the order of 25–50 W m−2 and averaging over larger areas does not help reduce the differences. These biases are compensated by the other long and short wave radiation terms so that the resulting errors in the net radiation are smaller. During August 1997, continental east‐west gradients of latent heat flux and Bowen ratio were surprisingly dissimilar among models. Still, the Bowen ratio estimated from the Eta and GEM models was close to observations over the southern Great Plains region, while both the Reanalysis and MAPS had ratios that at least doubled the observed ones. In the case of MAPS a revised latent heat flux formulation was introduced in fall 1997, and, subsequently, for January 1998, estimates were closer to the other models' estimates. However, during January 1998 all models had difficulties reproducing the Bowen ratios from observations. Further, daily time series showed that models' estimates also tended to miss the amplitude of the day‐to‐day variability. It is conceivable that this may be the result of difficulties in parameterizing the total cloud cover, and, particularly, attenuation by clouds may still be insufficient.
Nitric oxide emissions from a southern African savanna
Authors: Otter Luanne B. , Yang WenXing X. , Scholes Mary C. , Meixner Franz X.
Published: December 2003

NO fluxes from soils of a periodically flooded tropical savanna in southern Africa were investigated and modeled. In the laboratory, NO production rates, NO consumption rate constants, NO mixing ratios, relationships between NO emissions and soil temperature and moisture were determined for nutrient‐poor, nutrient‐rich savanna soils and a floodplain soil. The NO production rate and consumption rate constants of the floodplain soil (1.96 ng N s−1 per kilogram of soil and 2.04×10−5 m3 s−1 per kilogram of soil, respectively) were significantly higher than those of the savanna soils (average of 1.28 ng N s−1 per kilogram of soil and 1.47×10−5 m3 s−1 per kilogram of soil, respectively), but there were no significant difference between the nutrient‐rich and nutrient‐poor soils. NO flux rates increased exponentially with soil temperature. NO flux rates increased with soil moisture reaching a maximum near the field capacity (7.5–10% and 31.2% gravimetric water content for savanna and floodplain soils, respectively), after which the NO flux rate declined. These laboratory data were used in a model to estimate field NO flux rates, which were compared with actual field NO emission measurements. The NO model was modified to incorporate NO “pulsing” after the first rains of the season. Correlation between the modeled and field NO fluxes from the nutrient‐poor savanna, nutrient‐rich savanna, and the floodplain soils showed r2 values of 0.91, 0.82, and 0.74, respectively. The NO model was linked with a soil moisture and temperature model to predict annual NO emission estimates from savannas. Annual NO flux from the nutrient‐poor and nutrient‐rich savannas was estimated to be 0.16×10−3 and 0.14×10−3 kg N m−2 yr−1, respectively, which agree well with estimates from other savanna studies.
Parameter estimation of a land surface scheme using multicriteria methods
Authors: Gupta H. V. , Bastidas L. A. , Sorooshian S. , Shuttleworth W. J. , Yang Z. L.
Published: December 2003

Attempts to create models of surfaceߚ;atmosphere interactions with greater physical realism have resulted in land surface schemes (LSS) with large numbers of parameters. The hope has been that these parameters can be assigned typical values by inspecting the literature. The potential for using the various observational data sets that are now available to extract plot‐scale estimates for the parameters of a complex LSS via advanced parameter estimation methods developed for hydrological models is explored in this paper. Results are reported for two case studies using data sets of typical quality but very different location and climatological regime (ARM‐CART and Tucson). The traditional single‐criterion methods were found to be of limited value. However, a multicriteria approach was found to be effective in constraining the parameter estimates into physically plausible ranges when observations on at least one appropriate heat flux and one properly selected state variable are available.