bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variability of near‐surface thermal structure and heat budget of the mixed layer of the tropical Indian Ocean from a new global ocean temperature climatology
Authors: Rao R. R. , Sivakumar R.
Published: September 2002

A subset of the global ocean temperature climatology is used to characterize the observed seasonal variability in the near‐surface thermal structure of the tropical Indian Ocean. In the near‐surface isothermal layer the seasonal variability is least in the warm pool region and increases poleward. Over much of the tropical Indian Ocean, local surface heat fluxes overwhelm horizontal advection in the seasonal evolution of the mixed layer temperature. Entrainment cooling is weaker by an order of magnitude than the other two processes. Several dynamical processes produce the most prominent signals in the thermocline in different geographic regions, such as coastal upwelling/downwelling with associated reversals in the flow off Arabia, off southwest India, and off the east coast of India, Ekman‐driven thermocline deepening in the central Arabian Sea, convergence of waters caused by Wyrtki Equatorial Jets in the eastern equatorial Indian Ocean, Ekman divergence of a dome of cold waters just south of the equator in the west, propagating Kelvin waves in the coastal regions of the Bay of Bengal, and propagating Rossby waves from off southwest India to Somalia and in the Indo‐Pacific throughflow region. For the spatiotemporal resolutions considered in this study the penetration of the seasonal cycle of temperature appears to be limited to 150 m depth or so. Numerical model solutions have revealed the causative mechanisms for some of the prominent signals seen in the observations, in particular, those driven by long‐period propagating waves in regions of large stratification, whose structure was not revealed by earlier climatologies to date, as revealed by this new climatology.
Neural network wind retrieval from ERS‐1 scatterometer data
Authors: Richaume P. , Badran F. , Crepon M. , Mejía C. , Roquet H. , Thiria S.
Published: September 2002

This paper presents a neural network methodology to retrieve wind vectors from ERS‐1 scatterometer data. First, a neural network (NN‐INVERSE) computes the most probable wind vectors. Probabilities for the estimated wind direction are given. At least 75% of the most probable wind directions are consistent with European Centre for Medium‐Range Weather Forecasts winds (at ±20°). Then the remaining ambiguities are resolved by an adapted PRESCAT method that uses the probabilities provided by NN‐INVERSE. Several statistical tests are presented to evaluate the skill of the method. The good performance is mainly due to the use of a spatial context and to the probabilistic approach adopted to estimate the wind direction. Comparisons with other methods are also presented. The good performance of the neural network method suggests that a self‐consistent wind retrieval from ERS‐1 scatterometer is possible.
An analysis of tropical instability waves in a numerical model of the Pacific Ocean: 1. Spatial variability of the waves
Authors: Masina S. , Philander S. G. H.
Published: September 2002

Unstable oscillations confined within the mixed layer close to the equator are generated in wind‐forced experiments performed in a multilevel general circulation model configured for the tropical Pacific Ocean. The experiments indicate that the waves develop preferentially in the eastern Pacific along the northern temperature front. However, there is clear evidence of a second unstable region along the southern temperature front in the central Pacific. In both regions the instabilities propagate westward, but in the central Pacific their phase speed is considerably smaller. The differences between the wave characteristics in the eastern and central Pacific are closely correlated to the differences in the time mean conditions of the flow. The eastern instabilities have a structure with two peaks in amplitude: one located on the equator and the other a few degrees north of it. Their dispersion characteristics show many similarities to those of tropical instability waves (TIWs) observed in the Pacific Ocean, while the instabilities which grow in the central Pacific do not have any known observed correspondents. We explore the spatial variability of the simulated waves through a wavelet analysis, which provides detailed results on how the period and wavelength of the instabilities change as a function of longitude, latitude, and depth. The wavelet analysis reveals that in the eastern Pacific and close to the surface the TIWs have a phase speed of −48 cm/s, while in the central Pacific they have a phase speed of −11 cm/s. In particular, the change in the phase speed is due to a change in the dominant period of the TIWs: The period of the central Pacific instabilities is considerably longer than the period of the instabilities present in the eastern Pacific.
Direct observations of velocity and transport in the passages between the Intra‐Americas Sea and the Atlantic Ocean, 1984–1996
Authors: Johns Elizabeth , Wilson W. Douglas , Molinari Robert L.
Published: September 2002

Shipboard acoustic Doppler current profiler observations of the velocity in the upper 200 m of the water column collected during 1984–1996 using the National Oceanic and Atmospheric Administration R/V Malcolm Baldrige are used to examine the velocity structure and transport in the passages between the Atlantic Ocean and the Intra‐Americas Sea (IAS). Data were collected during 23 cruises along the following sections: across the Straits of Florida, in the Northwest Providence Channel (NWPC), across the northern passages into the Caribbean Sea (Windward, Mona, and Anegada), across the eastern Caribbean along 63°30′ W, thereby forming a closed quadrangle, and in the Grenada Passage. The Florida Current, the eastern Caribbean, and the Grenada Passage share a similar mean velocity structure characterized by high‐velocity, surface‐intensified flows with strong vertical and horizontal shears. The northern Caribbean passages (NWPC, Windward, Mona, and Anegada) share a different common mean velocity structure, with subsurface velocity maxima directed into the IAS, and surface‐intensified counterflows along one side of each passage. On average, there is a transport balance in the upper 200 m between waters entering and exiting the IAS, with the 16.5±2.4 Sv (1 Sv = 106 m3/s) transport of the Florida Current at 27°N comprised of 0.4±0.8 Sv from the NWPC, 2.2±1.5 Sv from the Windward Passage, 2.8±2.1 and 2.4±2.8 Sv from the Mona and Anegada passages, respectively, and 9.5±4.7 Sv across the eastern Caribbean, for a total of 17.3 Sv. The four passages north of 17°N (from NWPC to Anegada Passage) have a combined transport of 7.8 Sv, nearly half of the transport of the Florida Current in the upper 200 m. Of the 9.5 Sv flowing through the eastern Caribbean between 11°N and 17°N, 4.9±2.6 Sv, or more than half, come from the Grenada Passage. This is significant to the subject of cross‐equatorial exchange of mass, heat, and salt, as the Grenada Passage is where the highest transport of waters originating in the southern hemisphere is thought to enter the Caribbean.
Circulation features in the central Arctic Ocean revealed by nuclear fuel reprocessing tracers from Scientific Ice Expeditions 1995 and 1996
Authors: Smith John N. , Ellis Katherine M. , Boyd Timothy
Published: September 2002

Measurements of the tracer radionuclides 129I and 137Cs were conducted on seawater samples collected during the Scientific Ice Expedition cruises to the Arctic Ocean of the U.S. Navy nuclear submarines, USS Cavalla and USS Pogy in 1995 and 1996, respectively. These radionuclides are derived mainly from discharges from the Sellafield (England, United Kingdom) and La Hague (France) nuclear fuel reprocessing plants and are subsequently transported with Atlantic water into the Arctic Ocean through Fram Strait and the Barents Sea. Iodine 129 results from halocline waters (water depths of 59 and 134 m) collected virtually synoptically throughout the central Arctic Ocean during the USS Cavalla cruise clearly show the front between Atlantic origin water having high 129I levels (>100×107 atoms L−1) and Pacific origin water labeled mainly by fallout (<5×107 atoms L−1). This front is aligned with the Mendeleyev Ridge and is displaced toward the Canada Basin with increasing water depth. Iodine 129 levels decrease on isohaline surfaces from stations over the continental slope and Mendeleyev Ridge to stations in the interior of the Makarov Basin. These results are consistent with circulation mechanisms in which the interior of the Makarov Basin is ventilated by lateral mixing from topographically steered boundary currents. Low 129I levels (<2×107 atoms L−1) measured in Atlantic and intermediate water in the northern Canada Basin and over the Alpha Ridge during the USS Pogy cruise indicate that ventilation rates are extremely low in this region. The much higher 129I levels (>30×107 atoms L−1) measured in central regions of the Canada Basin indicate that the interior is more efficiently ventilated than previously thought, possibly by separation from boundary currents flowing over the continental margin north of the Chukchi Plateau. The 129I and 137Cs data were interpreted using a transit time model that provided estimates of 6–7 years (±0.5 years) for the passage of halocline and Atlantic Water from the Norwegian Coastal Current (60°N) to the continental slope of the Makarov Basin and a lower limit of 8 years for transport to interior regions of the Makarov and Amundsen Basins.
Global statistical space‐time scales of oceanic variability estimated from the TOPEX/POSEIDON altimeter data
Authors: Kuragano Tsurane , Kamachi Masafumi
Published: September 2002

Four years of the TOPEX/POSEIDON altimeter data are used to produce estimates of statistical space‐time scales of ocean variabilities for the sea surface height. A three‐dimensional (space‐time) correlation function with an anisotropic directional dependence is assumed. The function has Gaussian distributions in radial directions in the space‐time coordinates and ellipsoidal contour surfaces. The function can reveal the space‐time scales and propagations of the variabilities and statistical errors in the altimeter data. We evaluated the space‐time scales using the best fit correlation function to the altimeter data. The scales show geographical differences. This means that dominant variabilities depend on regions. We show an optimum interpolation (OI) method as an example of the application of the fitted correlation function. The OI is applied to the altimeter data in the space‐time domain to make grid point data with higher accuracy than those obtained from a usual spatial (two‐dimensional) OI (2‐D OI). The values of the correlation function are used for the correlation coefficients of the first guess error in the OI. A propagation diagram obtained from the space‐time OI (3‐D OI) shows clear propagations of planetary disturbances with larger amplitude. The grid point values from the 3‐D OI show better correspondences with sea levels from tide gauges than those from the 2‐D OI. The advantage is due to the data of the different cycles being additionally adopted and the correlation function reflecting propagation of the ocean variabilities.
Wave‐breaking hydrodynamics within coral reef systems and the effect of changing relative sea level
Authors: Hearn Clifford J.
Published: September 2002

This paper considers the hydrodynamics of wave‐driven flow across a coral reef and the resultant flushing of its lagoon. Current depth coefficients are introduced that describe the fractional change of across‐reef and lagoonal current with change in the relative local sea level, i.e., the depth of water over the reef flat. The coefficients are derived from an analytical hydrodynamic model and compared with data from two reefs. The first is Ningaloo Reef in northwest Australia, which is a typical barrier reef with a narrow coastal lagoon, and the second is Kaneohe Bay in Oahu, Hawaii, where the reef is both unusually deep and wide. The current depth coefficients are shown to be sensitive to the form of the frictional law on the reef flat and can be deduced from measurements of the astronomical tidal currents in the lagoon. Both quadratic and linear friction laws are considered for the reef flat and good agreement obtained with a linear law based on the high‐frequency motion due to surface gravity waves.
On the sources of Weddell Gyre Antarctic Bottom Water
Authors: Meredith Michael P. , Locarnini Ricardo A. , Van Scoy Kim A. , Watson Andrew J. , Heywood Karen J. , King Brian A.
Published: September 2002

In March–April 1995, as part of the World Ocean Circulation Experiment section A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shelf (72.5°S, 16.5°W) to South Georgia (55°S, 34.5°W). Chlorofluorocarbon (CFC‐11, CFC‐12, and CFC‐113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Water (defined as waters with potential temperatures less than 0°C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potential temperatures less than −0.7°C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing northern Weddell Gyre than in the westward flowing southern limb. Our CFC measurements suggest that distinct sources of Weddell Sea Bottom Water exist in the western Weddell Sea, in agreement with previous descriptions based on potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0°C and −0.7°C, confirm the long‐recognized sources for this water mass in the western and southwestern Weddell Sea. In the southern Weddell Gyre at about 20°W and along the Antarctic continental slope, Weddell Sea Deep Water with potential temperatures around −0.45°C shows a deep CFC maximum about 1000 m above the seafloor. CFC concentrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30°W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea. Instead, these tracers suggest that an important portion of the Weddell Sea Deep Water in the southern Weddell Gyre originates outside the western Weddell Sea, probably near the Amery Basin and environs, around 75°E. These features of the circulation and renewal of the deep Weddell Gyre should be carefully considered in simulations dealing with fluxes, pathways, and formation rates of Antarctic Bottom Water.
Improved electromagnetic bias theory
Authors: Elfouhaily T. , Thompson D. R. , Chapron B. , Vandemark D.
Published: September 2002

In this paper we describe modifications to a previous theory for the altimeter electromagnetic bias (EM bias) of Srokosz [1986]. A major correction introduces a scaling of Srokosz's model by nonnegligible dimensionless ratios that depend on the slope variance of both long and short waves. With these modifications the EM bias is no longer simply a function of the cross skewness between surface elevation and slope but now depends on the variance ratios that represent the modulation between short and long waves. Inclusion of these ratios can reduce previous EM bias estimates by as much as 50%. Different directions for the longwave and shortwave field are also accounted for in the two‐dimensional development of our approach. A radar frequency dependence consistent with observation is also predicted by the new model. Derivation extending our development to the next higher order in wave statistics is also presented and discussed.
A semiparametric algorithm to retrieve ocean wave spectra from synthetic aperture radar
Authors: Mastenbroek C. , de Valk C. F.
Published: September 2002

A new wave retrieval method for the ERS synthetic aperture radar(SAR) wave mode is presented. The new algorithm, named semiparametric retrieval algorithm (SPRA), uses the full nonlinear mapping relations as proposed by Hasselmann and Hasselmann [1991]. It differs from previous retrieval algorithms in that it does not require a priori information on the sea state. Instead, it combines the observed SAR spectrum with the collocated wind vector from the ERS scatterometer to make an estimate of the wind sea spectrum. The residual signal in the SAR spectrum is interpreted as swell. The method has been validated by collocating over 5 years of SAR wave mode observations with spectral buoy measurements at 11 locations on the open ocean. For wave components longer than 225 m, the standard deviation between the retrieved spectra and buoy observations is 0.41 m, which corresponds to a relative RMS error of 29%. About 10% of the observed SAR spectra were rejected, in particular in light wind conditions when nonwave features such as those caused by slicks dominated the imagette. The bias and scatter in the results obtained under light wind conditions could be reduced by introducing a wind‐dependent tilt modulation. This wind‐dependent tilt formulation is derived from the empirical CMOD4 relation between the wind vector, the incidence angle, and the radar backscatter for the ERS scatterometer.
Oceanic processes associated with anomalous events in the Indian Ocean with relevance to 1997–1998
Authors: Murtugudde Raghu , McCreary Julian P. , Busalacchi Antonio J.
Published: September 2002

An anomalous climatic event occurred in the Indian Ocean (IO) region during 1997–1998, which coincided with a severe drought in Indonesia and floods in parts of eastern Africa. Cool sea surface temperature anomalies (SSTAs) were present in the eastern IO along and south of the equator. Beginning in July 1997, warm SSTAs appeared in the western IO, and they peaked in February 1998. An ocean general circulation model is employed to investigate the dynamic and thermodynamic processes that caused the SSTAs associated with this and other similar IO events. The eastern cooling resulted from unusually strong upwelling along the equator and Sumatra. The Sumatran upwelling was forced both locally by the stronger alongshore winds and remotely by equatorial and coastal Kelvin waves. By the end of 1997, weakening of the winds and the associated reduction in latent heat loss led to the elimination of the cold SST anomalies in the east. The western warming was initiated by weaker Southwest Monsoon winds and maintained by enhanced precipitation forcing, which resulted in a barrier layer structure. Analysis of the mixed layer temperature equation indicates that a downwelling Rossby wave contribution was crucial for sustaining the warming into February 1998. It is tempting to suppose that the 1997 event was related to the El Niño‐Southern Oscillation (ENSO) event that took place in the Pacific at the same time. Indeed, weaker IO events occur quite regularly in the control run that evolve similarly to the 1997 event, and they are often but not always related to ENSO. We speculate that these events represent a natural mode of oscillation in the IO, which is externally forced by ENSO but also excited by ocean‐atmosphere interactions internal to the IO.
Spreading of Red Sea overflow waters in the Indian Ocean
Authors: Beal Lisa M. , Ffield Amy , Gordon Arnold L.
Published: September 2002

As a result of its remarkably high salinity and despite its small volume input, remnants of Red Sea Water (RSW) have been identified in the Agulhas Current, over 6000 km distant of their source. This provided the motivation to investigate the long‐term mean spreading of RSW throughout the Indian Ocean, using a comprehensive set of observations, taken from the National Oceanographic Data Center archives and from the World Ocean Circulation Experiment Hydrographic Program for the Indian Ocean. After emerging from the Gulf of Aden into the Arabian Sea, RSW spreads predominantly southwestward along the African continental slope, as indicated by strongly inclined isohalines across the Arabian Sea. There is some monsoon variability, so that during the winter monsoon there is more RSW present in the Gulf of Aden and an intensification of southward spreading along the western boundary, between 12° and 5°N. Elsewhere the intermediate depth salinity field of the Indian Ocean appears relatively stationary. Between 5° and 10°S, in the region of the South Equatorial Current, isohalines of the RSW layer become quasi‐zonal across the width of the Indian Ocean, only dipping southward toward the western boundary west of 50°E. South of here, there is a strong tongue of RSW spreading southward through the Mozambique Channel and into the Agulhas Current. These conclusions concur with previous localized investigations of intermediate water properties. Using a simple mixing model, the percentage of RSW throughout the Indian Ocean was quantified. It was found that the flux of salt into the Gulf of Aden from the Red Sea is similar to that estimated to cross 32°S in the Agulhas Current. This result implies that all the RSW which is mixed into the interior of the Indian Ocean may eventually be exported at the western boundary. Furthermore, it implies that RSW is the dominant component of the salt budget for the intermediate layer and that input from the Indonesian Seas and via diapycnic processes are small.
Fortnightly variability in the transverse dynamics of a coastal plain estuary
Authors: Valle‐Levinson Arnoldo , Wong Kuo‐Chuin , Lwiza Kamazima M. M.
Published: September 2002

Current velocity and water density profiles were obtained along two cross‐estuary transects with the purpose of determining the fortnightly variability of the transverse dynamics in a partially stratified coastal plain estuary. The profiles were measured with a towed acoustic Doppler current profiler and a conductivity‐temperature‐depth recorder in the James River estuary, Virginia. The cross‐estuary transects were sampled during the spring tides of October 26–27, 1996, and the ensuing neap tides of November 2–3, 1996. The transects were ∼4 km long, featured a bathymetry that consisted of a channel flanked by shoals, and were sampled repeatedly during two semidiurnal tidal cycles (25 hours) in order to separate semidiurnal, diurnal, and subtidal signals from the observations. This work concentrates on the subtidal transverse dynamics. The transverse baroclinic pressure gradients were larger during neap tides than during spring tides. During spring tides the advective accelerations were predominantly greater than the Coriolis accelerations, most markedly over the edges of the channel. Both effects combined with frictional influences to balance the pressure gradient in the transverse direction. During neap tides, advective accelerations were not as dominant over Coriolis accelerations as during spring tides. Also, during neap tides, Coriolis played a more relevant role, compared to spring tides, in combining with friction to balance the pressure gradient. This behavior was indicative of the momentum balance approaching gravitational circulation modified by the Earth's rotation, weak friction, and nonlinear advection during neap tides. The balance became more influenced by nonlinear advection and friction and less influenced by the Earth's rotation during spring tides. These results showed that transverse dynamics of a partially stratified estuary are far from being in geostrophic balance.
On the implementation of open boundary conditions for a general circulation model: The three‐dimensional case
Authors: Palma E. D. , Matano R. P.
Published: September 2002

This article discusses the results of an experimental survey on the performance of a select group of open boundary conditions (OBCs) for three‐dimensional, primitive equation models. The focus is on passive conditions, where the model response at the open boundaries is dictated by the interior dynamics. The performance of the OBCs is tested in a series of numerical experiments conducted in a rotating channel which includes variable bottom topography and density stratification. The experiments are selected to enhance nonlinear phenomena that comprises advection and the propagation of dispersive waves. The first two experiments study coastal upwelling with constant and time‐varying wind forcing. The third experiment analyses the barotropic and baroclinic response of a coastal ocean due to the passage of a traveling storm. The open boundary condition with the best overall performance is the flow relaxation scheme on sea surface elevation and transport, a radiation condition for internal velocities, and a combined scheme of flow advection and relaxation for the temperature field. A modified gravity wave radiation scheme provides reasonable responses when combined with radiation conditions for internal velocities and an advection equation for temperature if the forcing is not changing direction rapidly at the open boundary. Schemes using the method of characteristics or traditional wave advection schemes fail when combined with radiation conditions for the baroclinic mode.
Ice growth and solar heating in springtime leads
Authors: Perovich D. K. , Richter‐Menge J. A.
Published: September 2002

The large thermal contrast between the cold atmosphere and the relatively warm ocean in springtime leads results in rapid ice growth and large fluxes of heat from the ocean to the atmosphere and of salt from the ice to the ocean. However, the magnitude of the ice growth and of the fluxes is moderated by solar radiation absorbed in the ice and upper ocean. During the Arctic lead experiment (LeadEx) we monitored ice conditions at four springtime leads during the first few days of growth. The experiment took place in March–April 1992 in the Beaufort Sea (73°N, 146°W). Two of the leads were <200 m in width, one was approximately a kilometer wide, and the fourth was quite large, more than a few kilometers wide. Ice thickness typically increased rapidly, with 15–20 cm of growth in the first few days. The crystallographic analysis of a series of ice cores taken across and along the edge of one of the smaller leads indicated that granular ice was more abundant along the edges (30%) of the lead than in the central part of the lead (10%). Observations suggest that thermodynamics processes dominated ice growth in these leads. Additional thickening of the ice at the edge of the leads was common because of rafting of the young ice and the accumulation of blowing snow. Ice temperature profiles exhibited a diurnal cycle induced by solar radiation, with daily oscillations of roughly 5°C observed in the interior of the lead ice. Theoretical simulations for one lead indicated that during the first few days of ice growth, 30% of the incident solar irradiance was absorbed in the ice and 25% was absorbed in the upper ocean. The total amount of solar energy absorbed in the ice during this period was roughly equivalent to 4 cm of ice growth. The solar heating in ice and water is roughly comparable in magnitude to the net longwave and is approximately two thirds of the turbulent fluxes.
Normalization of correlations for atmospheric species with chemical loss
Authors: Plumb I. C. , Vohralik P. F. , Ryan K. R.
Published: September 2002

In order both to measure and to model the time dependence of atmospheric trace species, it is possible to make use of the correlations that occur between the stratospheric mixing ratios for a range of atmospheric constituents. If the correlation is to have any generality, it is necessary to take into account the time dependence of the release rate from the surface of the Earth. Previous studies have done this by using the age of the air Γ. For compounds with stratospheric loss processes and rapidly changing tropospheric mixing ratios, the use of Γ when normalizing correlations is inappropriate. The correlation derived in this way may be time‐dependent and therefore of limited usefulness. In the work described here a two‐dimensional (2‐D) chemical transport model has been used to calculate Γ*, the mean arrival time for a pulsed emission of a chemically active species released from the surface of the Earth. Because chemical reactions preferentially remove molecules from the tail of the distribution, Γ* is always shorter than Γ. The tropospheric concentrations of both CH3CCl3 (methylchloroform) and CH3CFCl2 (HCFC‐141b) are currently undergoing rapid change. The results of time‐dependent 2‐D model calculations show that the use of Γ* results in unique correlations for CH3CCl3 with CFCl3 and for CH3CFCl2 with N2O. Furthermore, empirical methods are described which allow Γ* to be estimated if Γ and the mean stratospheric lifetime of the species are known. This permits the methods described here to be applied to the measurements of atmospheric constituents taken at different times if estimates of the age of the air are available.
Propagation delays induced in GPS signals by dry air, water vapor, hydrometeors, and other particulates
Authors: Solheim Fredrick S. , Vivekanandan Jothiram , Ware Randolph H. , Rocken Christian
Published: September 2002

Dry air, water vapor, hydrometeors, and other particulates (sand, dust, aerosols, and volcanic ash) in the atmosphere introduce microwave propagation delays. These delays must be properly characterized to achieve the highest accuracy in surveying and atmospheric sensing using Global Positioning System (GPS) signals. In this paper we review the theory of microwave propagation delays induced by the above atmospheric constituents and estimate their maximum delays. Because the structure of atmospheric refractivity can be highly complex and difficult to model, and because measurement tools are unavailable for characterizing most of the refractive components, we use simplified examples to illustrate its effects. Our results show that propagation delays due to water vapor, cloud liquid, rain, and sandstorms can be significant in high‐accuracy GPS applications. For instance, propagation through 1 km of heavy rain can induce 15‐mm delays in L1, and because delays due to scattering are dispersive and alias as ionospheric delays in L3processing, L3range errors are magnified to 20 mm. Depending upon the distribution of precipitation relative to the configuration of GPS satellites, such unmodeled delays can induce horizontal and vertical errors of several centimeters.
Spatial distribution of Sahelian land surface properties from airborne POLDER multiangular observations
Authors: Lacaze Roselyne , Roujean Jean‐Louis , Goutorbe Jean‐Paul
Published: September 2002

This paper presents the spatial distribution of land surface parameters in southwestern Niger, a region composed mainly of shrub and grassland fallows, millet crop, and tiger bush. The regional patterns of the surface albedo, the leaf area index, the fractional vegetation cover, and the fraction of absorbed photosynthetically active radiation are estimated through the growing season from airborne POLDER (Polarization and Directionality of Earth Reflectances) data acquired during the Hydrologic Atmospheric Pilot Experiment in Sahel (HAPEX‐Sahel). The retrieval of these parameters is via a bidirectional reflectance model, appropriate vegetation indices, and Sun‐view geometries. Comparison of the POLDER‐derived surface parameters with airborne and ground measurements shows that the procedure generally performs well, enhancing the ability to constrain soil‐vegetation‐atmosphere transfer (SVAT) models in the Sahel area by providing spatially averaged and updated information. This will enable a more valid assessment of the role of the land surface in determining the Sahelian climate, with a better determination of the scaling effect of surface processes. Although the algorithms described in this work rely primarily on multiangular observations, such as those provided by spaceborne POLDER data sets, they should be useful in a number of remote sensing applications.
Influence of biotic exchange and combustion sources on atmospheric CO2 concentrations in New England from observations at a forest flux tower
Authors: Potosnak Mark J. , Wofsy Steven C. , Denning A. Scott , Conway Thomas J. , Munger J. William , Barnes Diana H.
Published: September 2002

Hourly data for concentrations and fluxes of CO2 at 30 m in Harvard Forest (Petersham, Massachusetts) are analyzed using linear modeling to obtain regionally representative CO2 concentrations at a continental site. The time series is decomposed into contributions due to regional combustion, local canopy exchange, monthly average regional biotic exchange (as modulated by the daily cycle of growth and decay of the planetary boundary layer (PBL)), and the regional monthly background concentration. Attributions are derived using time series analysis, data for a tracer for combustion (CO or acetylene (C2H2)), and measurements of indicators of proximate canopy exchange (CO2 flux and momentum flux). Results are compared to observations at Cold Bay, Alaska. Combustion contributes on average 4–5 ppm to ambient CO2 at Harvard Forest in winter and 2–3 ppm in summer. Regional biotic emissions elevate daily mean CO2 by 4–6 ppm in winter, and the covariance of the biotic cycle of uptake and emission with PBL height enhances daily mean CO2 by 1–2 ppm in summer; minimum values in late afternoon average 10 ppm lower than at Cold Bay in summer. The study shows that regionally representative concentrations of CO2 can be determined at continental sites if suitable correlates (tracers, fluxes of CO2, and momentum) are measured simultaneously with CO2 itself.
Precipitation structure in the Sierra Nevada of California during winter
Authors: Pandey Ganesh R. , Cayan Daniel R. , Georgakakos Konstantine P.
Published: September 2002

Influences of upper air characteristics along the coast of California upon winter‐time (November‐April) precipitation in the Sierra Nevada are investigated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elevation. Most precipitation episodes in the region are associated with moist southwesterly winds (coming from the southwest direction) and also tend to occur when the 700‐mbar temperature at the upwind direction is close to −2°C. This favored wind direction and temperature signify the importance of both moisture transport and orographic lifting in augmenting precipitation in the region. By utilizing the observed dependency of the precipitation upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture transport. The skill of the model increases with timescale of aggregation, reaching more than 50% variance explained at an aggregation period of 5–7 days. This indicates that upstream air moisture transport can be used to estimate the precipitation totals in the Sierra Nevada region.
Assessment of upper tropospheric HOx sources over the tropical Pacific based on NASA GTE/PEM data: Net effect on HOx and other photochemical parameters
Authors: Crawford J. , Davis D. , Olson J. , Chen G. , Liu S. , Gregory G. , Barrick J. , Sachse G. , Sandholm S. , Heikes B. , Singh H. , Blake D.
Published: September 2002

Data for the tropical upper troposphere (8–12 km, 20°N‐20°S) collected during NASA's Pacific Exploratory Missions have been used to carry out a detailed examination of the photochemical processes controlling HOx (OH+HO2). Of particular significance is the availability of measurements of nonmethane hydrocarbons, oxygenated hydrocarbons (i.e., acetone, methanol, and ethanol) and peroxides (i.e., H2O2 and CH3OOH). These observations have provided constraints on model calculations permitting an assessment of the potential impact of these species on the levels of HOx, CH3O2, CH2O, as well as ozone budget parameters. Sensitivity calculations using a time‐dependent photochemical box model show that when constrained by measured values of the above oxygenated species, model estimated HOx levels are elevated relative to unconstrained calculations. The impact of constraining these species was found to increase with altitude, reflecting the systematic roll‐off in water vapor mixing ratios with altitude. At 11–12 km, overall increases in HOx approached a factor of 2 with somewhat larger increases being found for gross and net photochemical production of ozone. While significant, the impact on HOx due to peroxides appears to be less than previously estimated. In particular, observations of elevated H2O2 levels may be more influenced by local photochemistry than by convective transport. Issues related to the uncertainty in high‐altitude water vapor levels and the possibility of other contributing sources of HOx are discussed. Finally, it is noted that the uncertainties in gas kinetic rate coefficients at the low temperatures of the upper troposphere and as well as OH sensor calibrations should be areas of continued investigation.
Potential impacts on Colorado Rocky Mountain weather due to land use changes on the adjacent Great Plains
Authors: Chase Thomas N. , Pielke Roger A. , Kittel Timothy G. F. , Baron Jill S. , Stohlgren Thomas J.
Published: September 2002

Evidence from both meteorological stations and vegetational successional studies suggests that summer temperatures are decreasing in the mountain‐plain system in northeast Colorado, particularly since the early 1980s. These trends are coincident with large changes in regional land cover. Trends in global, Northern Hemisphere and continental surface temperatures over the same period are insignificant. These observations suggest that changes in the climate of this mountain‐plain system may be, in some part, a result of localized forcing mechanisms. In this study the effects of land use change on the northern Colorado plains, where large regions of grasslands have been transformed into both dry and irrigated agricultural lands, on regional weather is examined in an effort to understand this local deviation from larger‐scale trends. We find with high‐resolution numerical simulations of a 3‐day summer period using a regional atmospheric‐land surface model that replacing grasslands with irrigated and dry farmland can have impacts on regional weather and therefore climate which are not limited to regions of direct forcing. Higher elevations remote from regions of land use change are affected as well. Specifically, cases with altered landcover had cooler, moister boundary layers, and diminished low‐level upslope winds over portions of the plains. At higher elevations, temperatures also were lower as was low‐level convergence. Precipitation and cloud cover were substantially affected in mountain regions. We advance the hypothesis that observed land use changes may have already had a role in explaining part of the observed climate record in the northern Colorado mountain‐plain system.
Extending aircraft‐ and tower‐based CO2 flux measurements to a boreal region using a Landsat thematic mapper land cover map
Authors: Chen Jing M. , Leblanc Sylvain G. , Cihlar Josef , Desjardins Raymond L. , MacPherson J. Ian
Published: September 2002

There has been an increasing need to measure the exchange of CO2 between the atmosphere and vegetated surfaces for large areas in order to quantify the carbon budget of the terrestrial biosphere. The boreal landscape is heterogeneous owing to different forest cover types and disturbance regimes, and regional quantification of CO2 flux is difficult without numerous species‐specific flux measurements. During the Boreal Ecosystem‐Atmosphere Study in 1994 and 1996 the National Research Council of Canada operated a Twin Otter aircraft that measured CO2, sensible and latent heat fluxes, and other trace gases over boreal forests in Saskatchewan, Canada. A flux‐unmixing method was developed to calculate flux densities for eight major cover types from the aircraft‐based measurements. Using a coregistered land cover map at 30‐m resolution derived from Landsat thematic mapper data, the contribution of each cover type to the CO2 flux measured by the aircraft was estimated using a contributing area (footprint) function according to the wind direction, the atmospheric stability, the horizontal distance of each pixel from the aircraft, and aircraft height. The unmixing method uses a linear inversion method with the footprint‐weighted cover type fractions as the set coefficients for each segment of a flight line. In the inversion, various constraint strategies were used to confine the inversion results to minimize the effect of various sampling errors. It is shown that (1) mathematical constraint is critically important in the inversion, (2) a simple constraint toward the mean flux values is effective in producing reasonable inversion results, and (3) the inversion accuracy can be further improved when simultaneous tower measurements in the dominant cover types are used as tight constraints. With such constraints the estimated fluxes from the cover types without tower measurements appear to be reasonable. It is concluded that aircraft measurement adds to our ability to map the regional flux field using remote sensing images because (1) it allows the derivation of flux data for cover types without tower‐based measurements and (2) it can be used to infer the representativeness of tower measurements for the measured cover types in the landscape.
A model‐based evaluation of inversions of atmospheric transport, using annual mean mixing ratios, as a tool to monitor fluxes of nonreactive trace substances like CO2 on a continental scale
Authors: Gloor Manuel , Fan Song‐Miao , Pacala Stephen , Sarmiento Jorge , Ramonet Michel
Published: September 2002

The inversion of atmospheric transport of CO2 may potentially be a means for monitoring compliance with emission treaties in the future. There are two types of errors, though, which may cause errors in inversions: (1) amplification of high‐frequency data variability given the information loss in the atmosphere by mixing and (2) systematic errors in the CO2 flux estimates caused by various approximations used to formulate the inversions. In this study we use simulations with atmospheric transport models and a time independent inverse scheme to estimate these errors as a function of network size and the number of flux regions solved for. Our main results are as follows. (1) When solving for 10–20 source regions, the average uncertainty of flux estimates caused by amplification of high‐frequency data variability alone decreases strongly with increasing number of stations for up to ∼150 randomly positioned stations and then levels off (for 150 stations of the order of ±0.2 Pg C yr−1). As a rule of thumb, about 10 observing stations are needed per region to be estimated. (2) of all the sources of systematic errors, modeling error is the largest. Our estimates of SF6 emissions from five continental regions simulated with 12 different AGCMs differ by up to a factor of 2. The number of observations needed to overcome the information loss due to atmospheric mixing is hence small enough to permit monitoring of fluxes with inversions on a continental scale in principle. Nevertheless errors in transport modeling are still too large for inversions to be a quantitatively reliable option for flux monitoring.
Satellite observations of temporary and irreversible denitrification
Authors: Dessler A. E. , Wu J. , Santee M. L. , Schoeberl M. R.
Published: September 2002

We have investigated temporary and irreversible denitrification in the polar regions. We find that the formation of type I polar stratospheric clouds (PSCs) in both hemispheres is best described by a supercooled ternary solution model. Considering the uncertainties in the analysis, our results are also consistent with a nitric acid dihydrate composition for type I PSCs, but not a nitric acid trihydrate composition. In the Northern Hemisphere, we observe that an upper limit of 17% of HNO3 is irreversibly lost each time the temperature of an air mass drops below ∼190 K. It seems likely that in order to remove a significant fraction of the HNO3 from an air mass in the Northern Hemisphere, one must expose it to low temperatures multiple times.
Criteria for sprites and elves based on Schumann resonance observations
Authors: Huang E. , Williams E. , Boldi R. , Heckman S. , Lyons W. , Taylor M. , Nelson T. , Wong C.
Published: September 2002

Ground flashes with positive polarity associated with both sprites and elves excite the Earth's Schumann resonances to amplitudes several times greater than the background resonances. Theoretical predictions for dielectric breakdown in the mesosphere are tested using ELF methods to evaluate vertical charge moments of positive ground flashes. Comparisons of the measured time constants for lightning charge transfer with the electrostatic relaxation time at altitudes of nighttime sprite initiation (50–70 km) generally validate the electrostatic assumption in predictions made initially by Wilson [1925]. The measured charge moments (Q dS = 200–2000 C‐km) are large in comparison with ordinary negative lightning but are generally insufficient to account for conventional air breakdown at sprite altitudes. The measured charge moments, however, are sufficient to account for electron runaway breakdown, and the long avalanche length in this mechanism also accounts for the exclusive association of sprites with ground flashes of positive polarity. The association of elves with large peak currents (50–200 kA) measured by the National Lightning Detection Network in a band pass beyond the Schumann resonance range is consistent with an electromagnetic pulse mechanism for these events.
Heterogeneous formation of nitrous acid (HONO) on soot aerosol particles
Authors: Kalberer M. , Ammann M. , Arens F. , Gäggeler H. W. , Baltensperger U.
Published: September 2002

The reaction of nitrogen dioxide (NO2) to nitrous acid (HONO) on suspended soot aerosol particles was investigated using NO2 labeled with 13N (a radioactive isotope with a half‐life of 10.0 min) at low concentrations of 2–115 ppb. HONO is thought to be an important compound in the troposphere since it is accumulating during the night and photolyzed in the morning after sunrise, producing OH, the most important oxidant in the troposphere. On soot, NO2 was rapidly reduced to HONO, presumably by a reactive surface site on the soot particle surface. No HNO3 was formed as a reaction product, indicating that a disproportionation of NO2 to HONO and HNO3 with surface‐adsorbed water is not the dominant process on soot. The reaction rate is drastically reduced after the first few seconds because of consumption of the reactive surface sites giving a maximum of ∼1 × 1015 HONO molecules cm−2 particle surface area. For a reaction time of 20 s the amount of HONO increased with increasing relative humidity up to 30%, showing that H2O is necessary for the reaction. Above 40% relative humidity the HONO production decreased again because of competition of H2O adsorbing on the particle surface. In aging experiments, O3 oxidized the same particle surface sites as NO2, but simultaneous mixing of O3, NO2, and the aerosol showed that the O3 oxidation is slower than the fast reaction of NO2 to HONO. It is concluded that the NO2 to HONO reaction on soot rapidly saturates and is not the main source of HONO in the polluted boundary layer.