bluuebunny/crossref_metadata_2025 huggingface dataset:
Primary production in Southern Ocean waters
Authors: Arrigo Kevin R. , Worthen Denise , Schnell Anthony , Lizotte Michael P.
Published: February 2004

The Southern Ocean forms a link between major ocean basins, is the site of deep and intermediate water ventilation, and is one of the few areas where macronutrients are underutilized by phytoplankton. Paradoxically, prior estimates of annual primary production are insufficient to support the Antarctic food web. Here we present results from a primary production algorithm based upon monthly climatological phytoplankton pigment concentrations from the coastal zone color scanner (CZCS). Phytoplankton production was forced using monthly temperature profiles and a radiative transfer model that computed changes in photosynthetically usable radiation at each CZCS pixel location. Average daily productivity (g C m−2 d−1) and total monthly production (Tg C month−1) were calculated for each of five geographic sectors (defined by longitude) and three ecological provinces (defined by sea ice coverage and bathymetry as the pelagic province, the marginal ice zone, and the shelf). Annual primary production in the Southern Ocean (south of 50°S) was calculated to be 4414 Tg C yr−1, 4–5 times higher than previous estimates made from in situ data. Primary production was greatest in the month of December (816 Tg C month−1) and in the pelagic province (contributing 88.6% of the annual primary production). Because of their small size the marginal ice zone (MIZ) and the shelf contributed only 9.5% and 1.8%, respectively, despite exhibiting higher daily production rates. The Ross Sea was the most productive region, accounting for 28% of annual production. The fourfold increase in the estimate of primary production for the Southern Ocean likely makes the notion of an “Antarctic paradox” (primary production insufficient to support the populations of Southern Ocean grazers, including krill, copepods, microzooplankton, etc.) obsolete.
Evolution of new ice and turbulent fluxes over freezing winter leads
Authors: Alam Afshan , Curry Judith A.
Published: February 2004

A model is presented that describes the coevolution of surface turbulent fluxes and new ice growth during the freezing of leads. The model includes a sophisticated parameterization of the surface sensible and latent heat fluxes. The new ice growth model distinguishes between the congelation and frazil regimes. During frazil growth, heat loss from the open water part of the lead results in formation of new ice which is advected to the downwind edge. With time, the ice edge extends to the upwind lead edge, and the lead is gradually covered with ice. Over the ice‐covered portions, the turbulent heat loss results in ice consolidation, and thereafter ice growth occurs. The turbulent heat flux depends on the surface characteristics which vary across the lead surface during frazil growth. Therefore in the frazil regime, ice concentration, ice thickness, surface temperature, and the surface turbulent flux vary across the lead surface. Even after consolidation, frazil ice has a different surface roughness length from congelation ice for the same ice thickness up to an ice thickness of 10 cm. We have used this model to determine the evolution of surface turbulent heat fluxes under various atmospheric conditions and for different lead widths. In the frazil regime, there is a considerable fetch dependence of the surface characteristics, as the ice is advected to the downwind edge and slowly covers the entire lead. This fetch dependence is greatest for the higher wind speeds and larger lead widths. There is significantly higher ice production under conditions when frazil formation occurs because the ice transport to the downwind edge leaves the surface of the lead open, allowing the warmer sea surface to exchange heat with the atmosphere. The rapid growth rates result in large salt release to the ocean, with implications for ocean dynamics. We have done a sensitivity study to investigate the effect of oceanic heat flux at the underside of the ice, which results from the salt rejection upon ice formation in freezing leads, on the evolution of new ice and turbulent fluxes.
Thermomechanics of pack ice
Authors: Lewis James K.
Published: February 2004

A thermodynamic formulation for pack ice is presented that includes a means of relating stresses in floes to thermally induced strain rates. The formulation combines some basic thermodynamics as detailed in previous research. The mechanics component includes viscous creep and mechanically induced strain rates due to typical floes being too large to bend or twist in response to spatially varying strain rates. Moreover, the rheology quantifies the impact of existing cracks in the floe on the overall strain rate in the floe and the resulting stress state. A paradigm is proposed which specifies tensile fracturing and the resulting stress relief as proportional to the amount of existing cracks in the floe and some measure of the tensile strength of the ice. This allows the thermomechanical equations to be used for both first‐year and multiyear floes by differentiating the two by the extent of existing cracks. The thermomechanical equations are applied in an inverse fashion to solve for flaw structure for a first‐year floe as well as a multiyear floe. This is done by matching stresses predicted by the thermomechanical equations to observed stresses in the floes. The results indicate that the top two thirds of the ice in the first‐year floe was relatively free of cracks while the bottom third of the floe had cracks covering ∼50% of the ice. In the multiyear floe, cracks covered 70% to 90% of the ice in the top two‐thirds of the floe while the bottom third of the floe had cracks covering ∼30% of the ice.
Large‐scale sea ice drift and deformation: Comparison between models and observations in the western Weddell Sea during 1992
Authors: Geiger Cathleen A. , Hibler William D. , Ackley Stephen F.
Published: February 2004

Statistical comparisons between numerical sea ice models and an observed large‐scale strain array in the western Weddell Sea during 1992 are used to evaluate the performance of three of the more generally utilized sea ice rheology formulations. Results show that sea ice velocity is reproduced with relatively high accuracy (90% coherence, >80% normalized cross correlation) in models having high‐quality atmospheric forcing fields (e.g., the European Centre for Medium‐Range Weather Forecasts). On the other hand, temporal and spatial variability of the velocity field, as exemplified by progressive vector plots and ice deformation, respectively, are reproduced less accurately (coherence and normalized cross correlation <50%). In terms of model sensitivity, this means that deformation and temporal variability are more discriminating in terms of elucidating specifics about the constitutive relation and mechanical properties of sea ice on a large scale. For example, inclusion of both compressive and shear stresses is important in attaining a proper probability distribution of deformation relative to observations. Additional analysis shows that adjustments to specific model parameters improve the model results for either drift or select deformation components, but no best solution could be found, given the models examined here. Results suggest that inclusion of more physically based processes, such as subdaily tidal and inertial oscillations, reconsideration of the boundary layer formulation, and consideration of anisotropy, may be necessary to include in next‐generation sea ice models, especially those that are intended for coupling with high‐resolution (eddy resolving) ocean models.
Motion‐induced stresses in pack ice
Authors: Lewis James K. , Richter‐Menge J. A.
Published: February 2004

We consider motion‐induced stresses in pack ice through the analyses of a variety of observations collected during the Sea Ice Mechanics Initiative study conducted in the Beaufort Sea during 1993. Motion‐induced components of in situ stress from stress gauge data are compared to stresses calculated as residuals based on a force balance argument using observed wind, current, and ice motion data. There is reasonable qualitative and quantitative agreement between the observed and calculated motion‐induced stresses in the north‐south direction if the residual stress is assumed to be distributed over a horizontal distance of ∼10 m. To obtain a general agreement with the magnitudes of the observed and calculated stresses in the east‐west direction, the residual stress must be considered to be distributed over a horizontal distance of ∼50 m. There are three significant stress events determined by the force balance calculations, but only the one event in the north‐south direction has a strong corresponding signal in the stress gauge data. There is very little indication of the two events in the east‐west direction in the gauge data. Numerical simulations of the distribution of motion‐induced stresses within a floe show that significant variations in the character of the stresses can occur over short horizontal distances throughout the floe. Hence a seeming lack of a clear correspondence between the observed and calculated stress may be due to our inability to properly recognize the modified signature of the event at the specific locations of the stress gauges. The results suggest that to effectively develop an understanding of the role that point stress measurements can play in developing our understanding of the process of ice deformation, it may be necessary to couple the stress measurements with models of the patterns of motion‐induced stresses within a floe. Finally, we consider the relationship between the residual stress and the differential motion of the ice pack as seen by a cluster of drifters on various floes. The three main stress events seen in the residual stresses all occurred during periods of convergence of the floes. However, we have tested various relationships between stress and strain, and they indicate that there should have been additional stress events as a result of other periods of substantial convergences of the ice pack. This suggests the possibility that the residual stresses were not locally generated.
Edge wave observation using remote seismoacoustic sensing of ice events in the Arctic
Authors: Dudko Yuriy V. , Schmidt Henrik , von der Heydt Keith , Scheer Edward K.
Published: February 2004

As part of the Office of Naval Research Sea Ice Mechanics Initiative, a real‐time monitoring and processing program for acoustic emission from ice fracture and ridge‐building events was established. A wide‐aperture, horizontal hydrophone array was used in combination with a vertical line array to record the acoustic signals, which were then passed through a focused beam former for real‐time generation of ice seismicity maps. A number of rapidly deployable geophone arrays were used in active zones to measure the acoustic emissions in the near field for detailed seismic event analysis. During one such deployment, a highly regular transient arrival structure was recorded on all sensors located near a major lead, with a transient appearing every 5 s. These data have been processed using frequency‐wavenumber analysis to show that the transients correspond to “edge waves” propagating forth and back along the edges of the lead, with the probable source being a “stick‐slip” mechanical phenomenon toward the ends of the lead.
Physicobiological oceanographic remote sensing of the East China Sea: Satellite and in situ observations
Authors: Ning X. , Liu Z. , Cai Y. , Fang M. , Chai F.
Published: February 2004

The satellite remote sensing on the NOAA Advanced Very High Resolution Radiometer (AVHRR, 1981 to 1986) and the Nimbus7 coastal zone color scanner (CZCS) (1978 to 1986) data sets were used to study the physicobiological characteristics of the East China Sea. The oceanographic dynamics of the East China Sea are greatly influenced by a counterclockwise circulation system that consists of the Kuroshio ‐ Tsushima Current ‐ Yellow Sea Warm Current on the eastern side of the Sea, and the Coastal Current on the western side. The former, coming from tropical open ocean with high temperature and salinity, brings oligotrophic water with very low chlorophyll concentrations; the latter has a low salinity but high nutrient and chlorophyll concentrations. Our analysis demonstrated that variation of the physicobiological features shifted systematically from each subarea to the next, as exemplified by the temperature increase and the pigment decrease from northwest to southeast. This was matched by spatial and seasonal distributions of dissolved oxygen in the East China Sea. We also found that the CZCS pigment images clearly indicated the positions of the biological productivity front in the Changjiang Estuary, which was just beyond the boundary of the turbid zone along the coastal areas of the East China Sea. They also showed the seasonal variation of the direction of the Changjiang River discharge tongue. The ocean color and infrared images complemented each other, and they were very useful in the interpretation of the spatial and monthly variations of the circulation patterns in the East China Sea.
Transport of Middle Atlantic Bight shelf water to the Gulf Stream near Cape Hatteras
Authors: Churchill James H. , Berger Thomas J.
Published: February 2004

Flowing along the U.S. eastern seaboard from Nantucket Shoals to Cape Hatteras is a low‐salinity (<34 practical salinity units) water mass known as Middle Atlantic Bight (MAB) shelf water. It has long been recognized that a large quantity of this water is entrained into the Gulf Stream near Cape Hatteras. We examine this process using moored instrument, hydrographic, and drifter track data acquired in the Cape Hatteras region over 1992–1994. These measurements reveal that the cross section and volume transport of MAB shelf water tend to shrink considerably going southward toward and past Cape Hatteras. In particular, our hydrographic data show shelf water cross section to be reduced by about a factor of 8 over a region stretching 80 km north of Diamond Shoals (between 35° 20′N and 36°10′N). There appear to be two fairly distinct zones of export in this region. Our data suggest that shelf water loss in the northern zone, over 35°40′N–36°10′N, occurs principally seaward of the shelf edge. Seaward movement of shelf water into Gulf Stream meanders and into the circulation of water discharged from the Gulf Stream are identified as important agents effecting this loss. Export of shelf water in the southern zone occurs over middle and outer shelf, as indicated by 5–10 cm s−1 mean off shelf velocities measured by moored current meters. Contributing to this export is a seaward flow of shelf water in a strong current at the edge of the front separating MAB shelf water from the more saline shelf water of the South Atlantic Bight. The offshore transport of shelf water at the edge of this front is principally observed when the front is migrating shoreward across the shelf. This intermittency in the seaward transport of shelf water at the front may be partly responsible for the variability of the presence and transport of shelf water seen at the edge of the Gulf Stream.
Evolution of an isolated turbulent region in a stratified fluid
Authors: Fonseka S. V. , Fernando H. J. S. , van Heijst G. J. F.
Published: February 2004

This paper reports an experimental study on the evolution of an isolated turbulent region in an otherwise quiescent linearly stratified fluid. A turbulent patch was generated by pulsed horizontal injection of a small volume of fluid. It was found that the turbulent blob thus produced initially grows as in a nonstratified fluid for a nondimensional time period of Nt ≈ 4–5 (with N the buoyancy frequency), attains a maximum height, and then physically collapses slowly to form a planar, quasi‐two‐dimensional dipole pattern. The timescale for the physical collapse of the blob was found to be much larger than that of the turbulence collapse. Scaling arguments are presented to explain the evolution of the turbulent region, and the experimental results are compared with the model predictions.
Model results of flow instabilities in the tropical Pacific Ocean
Authors: Donohue Kathleen A. , Wimbush Mark
Published: February 2004

A two‐and‐a‐half‐layer model of the tropical Pacific Ocean is used to investigate the energy source for the intraseasonal dynamic‐height variability observed near 6°N. A simulation of equatorial circulation is produced by forcing the model with mean‐monthly wind‐stress climatology. Two westward‐propagating waves appear in the upper layer in the central and eastern portion of the model basin. These two waves are distinguished by period and meridional structure. An off‐equatorial wave with period of 30 days and wavelength of 1100 km has a meridional sea‐level maximum near 6°N similar to that of the 30–50 day intraseasonal wave observed in the ocean. The meridional velocity signal also is asymmetric with respect to the equator, with maximum near 4°N. The second wave with period of 15 days has a strong meridional velocity signal centered on the equator. The sea‐level and zonal velocity signals associated with this equatorial wave have maxima near 1.5°N and 1.5°S. The eddy‐energy budget reveals strong conversions from the mean‐flow to eddy field through baroclinic and upper‐layer barotropic conversion terms. Conversion terms north of the equator exhibit a bimodal structure: one maximum between the equator and 3°N is dominated by upper‐layer barotropic conversion spatially coincident with the cyclonic shear along the equatorward edge of the South Equatorial Current (SEC), and a second smaller maximum between 3°N and 5°N is a combination of upper‐layer barotropic conversion along the poleward edge of the SEC (anticyclonic shear) and baroclinic conversion near the core of the SEC. The two peaks in the conversion terms, combined with similar structure in the flux‐divergence terms in the model eddy‐energy budget, provide evidence that two wave processes are generated at the different source regions: one near the equator and a second between 2°N and 5°N.
Bar‐trapped edge waves and longshore currents
Authors: Bryan K. R. , Bowen A. J.
Published: February 2004

Edge waves react to the presence of a mean longshore current as though the current was a change in bottom topography. In this way, the cross‐shore shape of the edge wave can be amplified over the current, relative to the shoreline, in the same way as the edge wave shape can be amplified over bars. On a beach with both longshore currents and bars, the degree of edge wave amplification (or trapping) depends on the strength and location of the longshore current shear relative to the steepness of the bottom slope and also on the direction of edge wave propagation relative to the longshore current. Weak longshore current shear over well‐developed bars does not significantly alter edge wave trapping over bars simply enhancing or reducing the existing bar trapping effect. Conversely, strong current shears over gentle topography can govern the existence and location of edge wave trapping, the extreme example being edge wave trapping on plane beaches where the amplification is entirely due to the existence of mean longshore currents. In reality the possibilities are restricted, since the shape of the longshore current profile is primarily determined by the pattern of incident wave breaking, which itself depends on the beach profile. The importance of realistic current shear relative to realistic bottom slope in determining edge wave shape is demonstrated using frequency‐wavenumber spectra taken during the Duck Experiment on Low‐Frequency and Incident‐band Longshore and Across‐shore Hydrodynamics (DELILAH) at Duck, North Carolina. These frequency‐wavenumber spectra show a general dominance of edge waves traveling with the longshore current. In cases where the current shear is strong relative to the bottom slope, the location of edge wave trapping changes as the longshore current profile changes with the tide. When the bar was well defined, the longshore current shear was rarely strong enough to influence the existence or location of edge wave trapping, but could have a enhancing effect. While past work [i.e. Schönfeldt, 1995] has suggested that edge waves, trapped and amplified on longshore currents, may cause bars to form on plane beaches and bars to move on barred beaches, these observations suggest that under most conditions, edge wave trapping on longshore currents is a subtle effect, modulated by sea level changes, and is unlikely to produce significant morphological change unless the current is very strong.
Spreading of juvenile freshwater in the Baltic proper
Authors: Eilola Kari , Stigebrandt Anders
Published: February 2004

In low‐saline estuaries, “juvenile freshwater,” that is, freshwater trapped by the seasonal thermal stratification, may be used as an almost neutrally buoyant natural tracer for dispersion studies. Most of the juvenile freshwater supplied to the Baltic proper enters the northern parts by runoff from rivers and outflow of low‐saline water from the Bothnian Sea. The mean monthly spatial distribution of juvenile freshwater in the Baltic proper in the period April to August is studied using historical hydrographic data. Month‐to‐month changes of the distribution reveal a fast southward spreading that only partly can be explained by the quite slow mean circulation in the Baltic proper. Most of the spreading is probably due to currents forced by fluctuating winds. In early spring, when the surface temperature is around the temperature of maximal density, the spreading of juvenile freshwater may initiate the seasonal stratification, which in turn starts the spring bloom. The spreading route of juvenile freshwater in early spring, as deduced in this paper, is in qualitative agreement with the spatial progress of the observed time for the start of the spring bloom of phytoplankton. The use of juvenile freshwater as a tracer should help in the interpretation of the spreading of tracers (e.g., pollutants) from local sources, as well as helping to verify recipes for calculating the spread of tracer clouds by different mechanisms. The monthly averaged spatial distributions of juvenile freshwater in the Baltic proper presented in this paper should be useful for testing dispersion properties of state‐of‐the‐art three‐dimensional ocean circulation models.
Detection of the fast Kelvin wave teleconnection due to El Niño‐Southern Oscillation
Authors: Meyers Steven D. , Melsom Arne , Mitchum Gary T. , O'Brien James J.
Published: February 2004

Previous analyses of the ocean state along the western American coast have often indicated unexpectedly slow and limited propagation of coastally trapped Kelvin waves associated with the El Niño‐Southern Oscillation. In contrast, theoretical and numerical ocean models demonstrate that these Kelvin waves are a rapid and long‐range teleconnection between the low‐ and high‐latitude Pacific Ocean, strongly impacting both the surface coastal currents and nutrient upwelling. Sea level variations along the western coast of North America are reexamined under the assumption that tropically forced Kelvin waves are produced in bursts of several months duration. A cross‐correlation analysis, restricted to mid‐1982 to mid‐1983, is performed between Galapagos Island and stations along western Central and North America. A coastally trapped Kelvin wave is revealed to propagate at a speed of 2–3 m s−1 from the tropical Pacific to the Aleutian Island Chain. The observed phase speed agrees with the estimated speed of a Kelvin wave based on the average density profile of the ocean near the coast. Weaker El Niño events in 1986/1987 and 1991/1992 appear to contain a combination of this remote signal and local wind forcing. The wave propagation speed calculated from the spectral phase is shown to be sensitive to the presence of other (noise) processes in the observations. This is demonstrated through an analysis of a synthetic sea level data set that contains many of the essential features of the real sea level data. A relatively small level of red noise can give a 100% expected error in the estimated propagation speed. This suggests a new explanation for this important inconsistency within dynamical oceanography.
Methane efflux from high‐latitude lakes during spring ice melt
Authors: Phelps Allan R. , Peterson Kim M. , Jeffries Martin O.
Published: February 2004

Ice cores removed from shallow ice‐covered tundra lakes near Barrow, Alaska, and taiga lakes near Anchorage, Alaska, exhibit increasing concentrations of methane with depth. Methane concentrations in the ice cores increased from 0 μM in the top 15 cm sections to a maximum of 23 μM in the lowest 15 cm sections of tundra lake ice and to a maximum of 147 μM in taiga lake ice. Methane concentrations in the water beneath the ice reflect a similar pattern, with values near 5 μM early in the ice‐covered season, increasing up to 42 μM in the tundra lakes, and up to 730 μM in the taiga lakes. Methane levels increase in the water beneath the ice during the course of the winter due to decreasing water volume, exclusion from growing ice, and continued methane production in thawed sediments. Since the ice layer prohibits gas exchange with the atmosphere, the methane is not oxidized, as it would be during the summer months, allowing the winter accumulation and storage of methane in the ice and lake waters. Efflux measurements, taken with floating chambers on the taiga lakes, indicated a large pulse of methane released during the period of ice melt and spring turnover. The efflux from one lake ranged from 2.07 g CH4m−2in 1995 to 1.49 g CH4m−2in 1996 for the 10 day period immediately after ice melt. Estimation of methane efflux using a boundary layer diffusion model and surface water concentrations during the entire ice‐free period in 1996 predicted an efflux of 1.79 g CH4m−2during the same 10 day period, compared with 2.28 g CH4m−2for the remainder of the summer season. This observation suggests that almost as much methane efflux can occur during a brief period immediately after ice melt as occurs during the remainder of the ice‐free season. Since measurements of methane efflux from high‐latitude‐lakes are generally made after this breakup period, the overall contribution to atmospheric methane from high‐latitude lakes may be twice that of current estimates.
Regional measurements and modeling of windblown agricultural dust: The Columbia Plateau PM10 Program
Authors: Claiborn Candis , Lamb Brian , Miller April , Beseda Jim , Clode Bryan , Vaughan Joe , Kang Liping , Newvine Carol
Published: February 2004

The Columbia Plateau PM10 Program [CP3] is a multi‐investigator study of windblown dust in the Pacific Northwest with an emphasis upon the role of agricultural lands in regional dust storms. Ambient measurements of PM10 within the source areas of the central basin of Washington during several autumn dust periods show that typical background concentrations [nonwind‐event periods] decrease from an average of 34 μg m−3 in early fall to 10 μg m−3 in late fall. During wind events, ambient concentrations at downwind urban receptors can exceed 500 μg m−3 on an hourly basis, with 24 hour averaged values as high as 300 μg m−3. Particle counts during wind events are enhanced by as much as a factor of 5 for particle sizes greater than 5 μm, and also for sizes between 1 and 5 μm compared to nonwindy periods. Analysis of the synoptic conditions which exist during these dust storms showed a common situation where a surface low is moving rapidly across British Columbia while a surface high is positioned in the Great Basin of Nevada. A regional windblown dust air quality model, developed for the CP3 study, predicts large dust plumes stretching across eastern Washington with maximum concentrations in the source regions exceeding 10,000 μg m−3. Total mass emissions during a storm are estimated to equal 100 Gg dy−1, which represents about 1% of recent estimates of the global annual dust emission rate. In the initial applications of the model, available PM10 observations are used to calibrate the dust emission algorithm. Changes in the dust constant for two modeled events are consistent with changes in soil cover and accumulated precipitation between an early fall event and a late fall event. The estimated fluxes are in a range similar to those in the literature but appear to be much less than estimated from global modeling of recently disturbed soils.
Hydrophobic particles can activate at lower relative humidity than slightly hygroscopic ones: A Köhler theory incorporating surface fixed charge
Authors: Wexler Anthony S. , Ge Zhaozhu
Published: February 2004

A number of laboratory experiments indicate that Hydrophobic carbonaceous particles activate at lower supersaturations than is predicted by Köhler theory. Since a significant fraction of the global energy balance uncertainty is due to the so‐called indirect effect, that of clouds, quantifying which particles activate is crucial to accurate prediction of climate. Most material surfaces obtain a fixed charge in water, and this is the case for many materials found in atmospheric aerosols. This fixed charge generates a charge double layer near the material surface which lowers the water activity. In the presence of this surface fixed charge, hydrophobic particles activate at low supersaturations. Since a small number of soluble ions in the particles causes them to only activate at higher supersaturations, surface charge activation is not relevant to activation of particles in the atmosphere. Thus laboratory experiments that measure the growth and activation of hydrophobic particles are measuring an effect, the surface charge effect, that is probably not relevant to the atmosphere.
Lightning phenomenology in the Tel Aviv area from 1989 to 1996
Authors: Yair Yoav , Levin Zev , Altaratz Orit
Published: February 2004

We present the results of a continuing survey of lightning characteristics in Tel‐Aviv, Israel, for the period 1989–1996, based on daily registrations of a CGR3 lightning flash counter [Mackerras, 1985]. The lightning season in Israel lasts from October to April, and the long‐term average of the annual flash density in the Tel‐Aviv area was found to be 4.7±2.3 km−2 y−1. The mean intracloud/cloud‐to‐ground flash ratio was found to be 2.5±1.3, with maxima in the autumn months. This may be attributed to the higher altitudes of the −10°C and −25°C isotherms (which signify the locations of charge centers) and to the weaker wind shears that occur in these months. The average fraction of positive ground flashes (PGF) in Tel‐Aviv thunderstorms was F = 0.16±0.08. Storms that exhibited larger than average PGF fraction were found to be subjected to a strong shear of the horizontal wind. The observed empirical relation between the PGF fraction and the intensity of the wind shear W (in m s−1 km−1) was log F = 0.0305W + 0.073.
Ocean‐atmosphere exchange of methyl bromide: NW Atlantic and Pacific Ocean studies
Authors: Groszko Wayne , Moore Robert M.
Published: February 2004

Measurements of methyl bromide partial pressure and concentration in surface water and air samples in the NW Atlantic Ocean in July 1995 and the Pacific Ocean in October 1995 are presented. Mean atmospheric mixing ratios were found to be 11.4±0.7 parts per trillion by volume (pptv) for the northern hemisphere and 10.0±0.5 pptv for the southern hemisphere. Cold, high‐latitude water in the Labrador Sea and warm water in the central Pacific were undersaturated in methyl bromide, and some supersaturated waters were found in the Gulf Stream in the Atlantic and around 35°S in the South Pacific. By a simple extrapolation, the global ocean is estimated to be a net sink of 10 (3 to 13) Gg of methyl bromide per year from the atmosphere, with the range including a factor of 2 uncertainty in the piston velocity.
Global simulation of tropospheric O3‐NOx‐hydrocarbon chemistry: 2. Model evaluation and global ozone budget
Authors: Wang Yuhang , Logan Jennifer A. , Jacob Daniel J.
Published: February 2004

Results from a global three‐dimensional model for tropospheric O3‐NOx‐hydrocarbon chemistry are presented and evaluated with surface, ozonesonde, and aircraft measurements. Seasonal variations and regional distributions of ozone, NO, peroxyacetylnitrate (PAN), CO, ethane, acetone, and H2O2 are examined. The model reproduces observed NO and PAN concentrations to within a factor of 2 for a wide range of tropospheric regions including the upper troposphere but tends to overestimate HNO3 concentrations in the remote troposphere (sometimes several fold). This discrepancy implies a missing sink for HNO3 that does not lead to rapid recycling of NOx; only in the upper troposphere over the tropical South Atlantic would a fast conversion of HNO3 to NOx improve the model simulation for NOx. Observed concentrations of acetone are reproduced in the model by including a large biogenic source (15 Tg C yr−1), which accounts for 40% of the estimated global source of acetone (37 Tg C yr−1). Concentrations of H2O2 in various regions of the troposphere are simulated usually to within a factor of 2, providing a test for HOx chemistry in the model. The model reproduces well the observed concentrations and seasonal variations of ozone in the troposphere, with some exceptions including an underestimate of the vertical gradient across the tropical trade wind inversion. A global budget analysis in the model indicates that the supply and loss of tropospheric ozone are dominated by photochemistry within the troposphere and that NOx. emitted in the southern hemisphere is twice as efficient at producing ozone as NOx emitted in the northern hemisphere.
Global simulation of tropospheric O3‐NOx‐hydrocarbon chemistry: 1. Model formulation
Authors: Wang Yuhang , Jacob Daniel J. , Logan Jennifer A.
Published: February 2004

We describe a global three‐dimensional model for tropospheric O3‐NOx‐hydrocarbon chemistry with synoptic‐scale resolution. A suite of 15 chemical tracers, including O3, NOx, PAN, HNO3, CO, H2O2, and various hydrocarbons, is simulated in the model. For computational expediency, chemical production and loss of tracers are parameterized as polynomial functions to fit the results of a detailed O3‐NOx‐hydrocarbon mechanism. The model includes state‐of‐the‐art inventories of anthropogenic emissions and process‐based formulations of natural emissions and deposition that are tied to the model meteorology. Improvements are made to existing schemes for computing biogenic emissions of isoprene and NO. Our best estimates of global emissions include among others 42 Tg N yr−1for NOx(21 Tg N yr−1from fossil fuel combustion, 12 Tg N yr−1from biomass burning, 6 Tg N yr−1from soils, and 3 Tg N yr−1from lightning), and 37 Tg C yr−1for acetone (1 Tg C yr−1from industry, 9 Tg C yr−1from biomass burning, 15 Tg C yr−1from vegetation, and 12 Tg C yr−1from oxidation of propane and higher alkanes).
Vertical velocity, vertical diffusion, and dilution by midlatitude air in the tropical lower stratosphere
Authors: Mote Philip W. , Dunkerton Timothy J. , McIntyre Michael E. , Ray Eric A. , Haynes Peter H. , Russell James M.
Published: February 2004

Air passing upward through the tropical tropopause is “marked” by an annually varying water vapor mixing ratio much as a tape recorder marks a magnetic tape; as the air ascends in the tropical stratosphere, these marks are effaced by a combination of vertical diffusion within the tropics and dilution of tropical air by sideways (isentropic) mixing‐in of midlatitude air. We represent these processes using a one‐dimensional advection‐diffusion‐dilution model, which we inverse‐solve for the vertical profiles of three unknowns (vertical advection velocity, vertical diffusion coefficient, and dilution rate coefficient) after prescribing the vertical profiles of time mean methane [CH4] and of amplitude and phase of the annually varying tape recorder signal in 2[CH4]+[H2O]. When tested on synthetic data generated by forward solving the same model, the method for inverse solution proved to be well conditioned and to give accurate results above 18 km. Applying the method to 5 years of smoothed data from the Halogen Occultation Experiment, we find a vertical advection velocity with a minimum of about 0.2 mm s−1 near 20 km, and both dilution rate coefficient and vertical diffusion coefficient with remarkably low minima near 22 km, 1/(6–7 year) and roughly 0.02 m2s−1, respectively. Our derived profile of vertical advection velocity agrees well, between 18 and 24 km, with an independent, radiatively derived, mass‐budget‐constrained transformed Eulerian mean calculation. Despite the relatively modest values of the diffusion coefficient, vertical diffusion plays a significant role in attenuating the tape recorder signal, according to our model. The minimum value of the dilution rate coefficient corresponds to a relaxation timescale of 6–7 years, much longer than the timescales found in other studies. The long relaxation timescale at 20–24 km is, however, consistent with (1) the minimum in vertical velocity, (2) a reduced attenuation rate in the tape recorder signal, and (3) a decrease, hitherto unremarked, in the tropical vertical gradient of [CH4] there.
An evaluation using 14C and N2O simulations of three‐dimensional transport driven by United Kingdom Meteorological Office and Goddard Space Flight Center assimilated winds
Authors: Kindler Thomas P. , Cunnold Derek M. , Alyea Fred N. , Chameides William L. , Lou Guang Ping , Schwan Karsten
Published: February 2004

We have used two different sets of assimilated input wind fields (United Kingdom Meteorological Office (UKMO), NASA) to drive the transport in a three‐dimensional global chemical transport model. In a first experiment the distribution of carbon 14 after nuclear bomb testing in the early 1960s was simulated. The second, more comprehensive experiment consists of a comparison of simulated nitrous oxide (N2O) distributions for both input wind fields against CLAES satellite measurements. Both experiments show that the NASA winds are producing too much upward transport in the equatorial area. By analyzing the Eulerian‐mean conservation equation for zonal mean mixing ratios, we were able to show that these model differences are initially caused by differences in the mean residual circulations in the two different wind field sets but that because the latitudinal gradients remain steeper in the NASA results, differences in isentropic mixing rates are also evident. The NASA residual mean circulation in the equatorial lower stratosphere in the model is higher than the UKMO residual mean circulation by about a factor of 2. However, there is also evidence that isentropic mixing rates are too large in the subtropics in both models, and there is evidence from the N2O comparisons of overly large amplitudes of wavenumbers 4 to 7 in the tropics and subtropics in the models. Filtering the incoming wind fields to eliminate wavenumbers larger than 10 resulted in only minor improvements in the model comparisons against CLAES observations. Because of the significant discrepancies between the model simulations, using either one of the wind field sets, and the observations, we recommend care in using these assimilated winds for long‐term model studies in general. Both wind field sets, however, produced good simulations of strong intrusions of tropical air into midlatitudes of the upper stratosphere in September 1992.
A modeling study of tropospheric species during the North Atlantic Regional Experiment (NARE)
Authors: Lin X. , Trainer M. , Hsie E.‐Y.
Published: February 2004

A three‐dimensional regional chemistry model is used to simulate distributions of a number of tropospheric species over eastern North America and the western North Atlantic for August of 1993. The model is tested by comparisons of the results with observations obtained at two ground stations in Nova Scotia, Canada, and on board two aircraft. The modeled mixing ratios of CO and O3 as well as the ratio of ΔO3/ΔCO at the two ground sites are in satisfactory agreement with the observations. More than 90% of the aircraft‐observed O3 and CO and 75% of NOy can be reproduced by the model with relative differences less than 50 and 100%, respectively. The model‐simulated correlations among O3, CO, and NOy are close to the aircraft observations. Most of the model‐calculated vertical distributions of these species have the same pattern as the observations. The calculated profiles coincide with the observations with mean discrepancies of about 25, 30, and 100% for O3, CO, and NOy, respectively. The model‐calculated wind speeds are in good agreement with the observations. Continental plumes within the model may be spatially off by a few hundred kilometers or temporally off by several hours. Possible inaccuracy in side boundary conditions and some difficulties in capturing small scale processes like small scale intrusion from the stratosphere and subgrid scale convective mixing are possible explanations for discrepancies between chemical data and model results.
Determination of the passing efficiency for aerosol chemical species through a typical aircraft‐mounted, diffuser‐type aerosol inlet system
Authors: Sheridan Patrick J. , Norton Richard B.
Published: February 2004

To assess the particle transmission efficiency of a conventional aircraft‐mounted, diffuser‐type inlet (CI), a new design inlet containing an internal filter basket assembly (aerosol filter inlet, or AFI) was constructed. All interior surfaces of the AFI were covered with filter material, and air was actively pulled through these filter walls during aerosol sampling. The AFI was demonstrated in the laboratory to trap nearly all particles entering its nozzle orifice, so it was considered usable as a baseline to judge the performance of other inlets. Wind tunnel studies were conducted at three different wind velocities that approximated typical research aircraft speeds. As wind velocity increased, particle transmission through the CI relative to the AFI decreased, as evidenced by chemical analysis of the filter deposits. Aircraft studies of the two inlets showed that particle transmission varied significantly with the measured species. Typical coarse‐particle species such as Ca++, Mg++, Na+ and K+ showed 50–90% mass losses through a conventional diffuser‐type inlet/curved intake tube system. Predominantly fine particle species such as SO4= and NH4+ passed the CI system with much higher efficiencies, with aerosol mass losses of 0–26% for most flights. Since the AFI traps nearly all particles aspirated into its nozzle orifice, these values indicate that on average, 80–90% of the SO4= and NH4+ aerosol mass passes through the CI and curved intake tube during airborne sampling. This finding suggests that the capability to sample fine (i.e., submicrometer) aerosols from aircraft is perhaps not as bad as has been previously reported, given that adequate attention is paid to inlet design, location, and orientation issues.
Climatic context of the First Aerosol Characterization Experiment (ACE 1): A meteorological and chemical overview
Authors: Hainsworth A. H. W. , Dick A. L. , Gras J. L.
Published: February 2004

During the intensive field operations period (November 15 to December 14, 1995) of the First Aerosol Characterization Experiment (ACE 1) cold front activity was generally above average, resulting in below average temperatures, pressures, and rainfall. The principal cause was the presence for much of the experiment of a long wave trough. This trough was mobile, traversing the ACE area during the project, with some warm anomalies evident in the areas under the influence of the long wave ridges. There is evidence of greater convective activity than normal possibly leading to a slightly deeper than average mixing layer. A greater west to northwesterly component to the air flow than average during November appears to have led to higher than average concentrations of radon and particles in the clean, marine or “baseline” sector at Cape Grim (190° to 280°). This is likely to have resulted from inclusion of continental air from western parts of the Australian mainland in the baseline sector winds. Although aerosol‐bound sulfur species were generally near their normal concentrations across the ACE 1 area, the overall pattern including atmospheric dimethylsulfide suggests slightly higher than usual sulfur species levels in the southern part of the region and lower concentrations in the northern part during November. This could be related to changes in marine biogenie productivity, air‐sea exchange, or atmospheric removal. In December, the changing long wave pattern brought an increase in south and southwesterly flow over the entire region. The baseline sector became less affected by continental species, but it appears that the colder conditions brought by this pattern have led to lower than usual atmospheric concentrations of biogenie species, as the region went into one of the coldest summers on record.
Effects of extended growing season and soil warming on carbon dioxide and methane exchange of tussock tundra in Alaska
Authors: Oberbauer Steven F. , Starr Gregory , Pop Eric W.
Published: February 2004

The active season of tussock tundra was extended during two growing seasons (1995 and 1996) by snow removal in early season and prevention of snow accumulation in late season to test the effects of a longer growing season on tundra carbon exchange. Three treatments were established: extended season, extended season + soil warming, and controls. Soil warming was accomplished using cold‐frame, resistance heating wire installed the year prior to the initiation of treatments. Diurnal courses of CO2 exchange were measured weekly using infrared gas analysis with enclosed chamber techniques. Methane fluxes were measured two to three times a season also using enclosure methods. In 1995, snowmelt occurred unusually early, and snow removal treatments increased the season only 9–10 days. In 1996 the early season was increased approximately 24 days. As expected, thaw depth, soil temperature, and plant growth were greater earlier in the extended season and extended season + soil heating plots. Methane fluxes in both seasons were low but tended to be higher in the extended season and soil heated plots. Net ecosystem CO2 fluxes were similar among treatments early in the season, with a tendency toward more positive fluxes (system loss) for the snow removal and wanned plots, possibly due to higher belowground respiration. During midseason, fluxes were similar among the treatments. Later in the season, fluxes of extended season and warmed plots tended to be lower (less carbon loss) than controls, especially in 1995. Totaled over the season, however, the fluxes of the three treatments did not statistically differ and represented losses to the atmosphere. Measurements of dark respiration in 1996 indicate that both respiration and uptake were increased on the extended season plots, resulting in similar net fluxes to controls.
Active‐layer thickness in north central Alaska: Systematic sampling, scale, and spatial autocorrelation
Authors: Nelson F. E. , Hinkel K. M. , Shiklomanov N. I. , Mueller G. R. , Miller L. L. , Walker D. A.
Published: February 2004

Active‐layer thickness was determined in late August 1995 and 1996 at 100 m intervals over seven 1 km2 grids in the Arctic Coastal Plain and Arctic Foothills physiographic provinces of northern Alaska. Collectively, the sampled areas integrate the range of regional terrain, soil, and vegetation characteristics in this region. Spatial autocorrelation analysis indicates that patterns of active‐layer thickness are governed closely by topographic detail, acting through near‐surface hydrology. On the coastal plain, maximum variability occurs at scales involving hundreds of meters, and patterns were similar in the two years. Substantially less spatial structure and interannual correspondence were found within the foothill sites, where high variability occurs over smaller distances. The divergence in patterns of thaw depth between the two provinces reflects the scale of local terrain features, which predetermines the effectiveness of fixed sampling intervals. Exploratory analysis should be performed to ascertain the scale(s) of maximum variability within representative areas prior to selection of sampling intervals and development of long‐term monitoring programs.