bluuebunny/crossref_metadata_2025 huggingface dataset:
Time scales of pattern evolution from cross‐spectrum analysis of advanced very high resolution radiometer and coastal zone color scanner imagery
Authors: Denman Kenneth L. , Abbott Mark R.
Published: February 2004

We have selected square subareas (110 km on a side) from coastal zone color scanner (CZCS) and advanced very high resolution radiometer (AVHRR) images for 1981 in the California Current region off northern California for which we could identify sequences of cloud‐free data over periods of days to weeks. We applied a two‐dimensional fast Fourier transform to images after median filtering, (x, y) plane removal, and cosine tapering. We formed autospectra and coherence spectra as functions of a scalar wavenumber. Coherence estimates between pairs of images were plotted against time separation between images for several wide wavenumber bands to provide a temporal lagged coherence function. The temporal rate of loss of correlation (decorrelation time scale) in surface patterns provides a measure of the rate of pattern change or evolution as a function of spatial dimension. We found that patterns evolved (or lost correlation) approximately twice as rapidly in upwelling jets as in the “quieter” regions between jets. The rapid evolution of pigment patterns (lifetime of about 1 week or less for scales of 50–100 km) ought to hinder biomass transfer to Zooplankton predators compared with phytoplankton patches that persist for longer times. We found no significant differences between the statistics of CZCS and AVHRR images (spectral shape or rate of decorrelation). In addition, in two of the three areas studied, the peak correlation between AVHRR and CZCS images from the same area occurred at zero lag, indicating that the patterns evolved simultaneously. In the third area, maximum coherence between thermal and pigment patterns occurred when pigment images lagged thermal images by 1–2 days, mirroring the expected lag of high pigment behind low temperatures (and high nutrients) in recently upwelled water. We conclude that in dynamic areas such as coastal upwelling systems, the phytoplankton cells (identified by pigment color patterns) behave largely as passive scalars at the mesoscale and that growth, death, and sinking of phytoplankton collectively play at most a marginal role in determining the spectral statistics of the pigment patterns.
Response of water‐leaving radiance to particulate calcite and chlorophyll a concentrations: A model for Gulf of Maine coccolithophore blooms
Authors: Ackleson Steven G. , Balch William M. , Holligan Patrick M.
Published: February 2004

A coupled atmosphere and ocean radiative transfer model, the Gulf of Maine (GOM) model, was developed to simulate water‐leaving radiance from a vertically stratified ocean containing a bloom of the coccolithophore Emiliania huxleyi. The model is based largely on atmospheric and ocean data representing the Gulf of Maine. The atmospheric submodel simulates direct sunlight and diffuse skylight illuminating the sea surface and is adjusted to account for seasonal changes in atmospheric aerosols. The optical properties of E. huxleyi, required by the ocean submodel, are derived from measurements collected in Gulf of Maine coccolithophore blooms occurring in 1989 and 1990. The modeled response of volume reflectance to the combined effects of chlorophyll and particulate calcite compares favorably with field measurements of E. huxleyi cell abundance and coastal zone color scanner (CZCS)‐derived volume reflectance representing a coccolithophore bloom in the northeast Atlantic Ocean. The GOM model was used to investigate the response of normalized water‐leaving radiance, modeled for visible CZCS and sea viewing wide field of view sensor (SeaWiFS) bands, to particulate calcite and chlorophyll a. Ranges in the concentrations of particulate calcite, chlorophyll a, and colored dissolved organic material (CDOM) are selected to represent conditions reported for coccolithophore blooms. Water‐leaving radiance increases with increasing particulate calcite concentration, primarily because of a disproportionately large amount of backscatter from detached coccoliths (about an order of magnitude larger than is predicted using Mie theory). As a result, CZCS plant pigment algorithms based upon radiance ratios may be corrupted more severely than previously estimated. As an alternative to radiance ratio‐based algorithms, an iterative procedure (also referred to as optimization) is used to invert the GOM model in order to simultaneously retrieve particulate calcite and chlorophyll a concentrations. The approach uses normalized water‐leaving radiance computed for all visible CZCS or SeaWiFS bands. Tests of the approach suggest that independent of errors associated with instrument calibration and atmospheric correction, errors in the retrieved concentrations are small, even when high concentrations of CDOM and vertical structure within the water column are neglected, i.e., with the assumptions that CDOM concentration is small and the water is vertically homogeneous. However, since there are no data sets of contemporaneous chlorophyll a concentration, particulate calcite concentration, and CZCS imagery, a rigorous test of the model and inversion technique must wait for the launch of new ocean color scanners such as the NASA SeaWiFS.
Coccolithophorid blooms in the global ocean
Authors: Brown Christopher W. , Yoder James A.
Published: February 2004

The global distribution pattern of coccolithophorid blooms was mapped in order to ascertain the prevalence of these blooms in the world's oceans and to estimate their worldwide production of CaCO3 and dimethyl sulfide (DMS). Mapping was accomplished by classifying pixels of 5‐day global composites of coastal zone color scanner imagery into bloom and nonbloom classes using a supervised, multispectral classification scheme. Surface waters with the spectral signature of coccolithophorid blooms annually covered an average of 1.4×106 km2 in the world oceans from 1979 to 1985, with the subpolar latitudes accounting for 71% of this surface area. Classified blooms were most extensive in the Subarctic North Atlantic. Large expanses of the bloom signal were also detected in the North Pacific, on the Argentine shelf and slope, and in numerous lower latitude marginal seas and shelf regions. The greatest spatial extent of classified blooms in subpolar oceanic regions occurred in the months from summer to early autumn, while those in lower latitude marginal seas occurred in midwinter to early spring. Though the classification scheme was efficient in separating bloom and nonbloom classes during test simulations, and biogeographical literature generally confirms the resulting distribution pattern of blooms in the subpolar regions, the cause of the bloom signal is equivocal in some geographic areas, particularly on shelf regions at lower latitudes. Standing stock estimates suggest that the presumed Emiliania huxleyi blooms act as a significant source of calcite carbon and DMS sulfur on a regional scale. On a global scale, however, the satellite‐detected coccolithophorid blooms are estimated to play only a minor role in the annual production of these two compounds and their flux from the surface mixed layer.
Numerical simulation of vorticity waves in the nearshore
Authors: Falqués A. , Iranzo V.
Published: February 2004

A numerical method based on spectral expansions is given for the computation of vorticity waves arising from shear instability of a longshore current. This method allows for any mean flow profile and any beach topography (remaining constant alongshore and with a straight shoreline). The shallow‐water equations are considered without any assumption about the sea surface (such as rigid lid), and dissipative terms accounting for bottom friction and/or eddy viscosity are included. A numerical simulation for some flow profiles that are quite realistic in the surf zone and for several bathymetries is presented. For inviscid flow the predictions of the Bowen and Holman (1989) analytical model for a very simplified geometry are found to give rise to the main features. However, the details in the flow and depth profiles are found to significantly influence the instability curves, especially for a barred beach. For the fastest growing mode, the wavelength is between 1.7 and 2.7 times the width of the mean current l. Frequencies of about 0.09ƒs, where ƒs is the maximum shear at the sea face of the current profile, and an e‐folding time of the exponential growth that is roughly equal to the wave period are obtained. The phase speed is between 0.5 and 0.7 of the mean current peak. Dissipation has a considerable effect on the wavenumber span and the growth rate of the instability, so reasonably constant values of the eddy viscosity and realistic values of the Chezy coefficient can entirely remove the instability. The phase speed of neutral shear waves is analytically found to be equal to the mean flow velocity at the cross‐shore location where the potential vorticity has an extremum. This velocity is found to give an estimate of the phase speed of growing modes. We found that the rigid‐lid assumption tends to overestimate the growth rates by an amount which depends on the maximum Froude number of the mean flow. The instability curves and the dispersion lines for a free surface converge towards the rigid‐lid ones when the Froude number decreases, and the rigid‐lid assumption is therefore valid for a low Froude number.
Ocean microwave backscatter distributions
Authors: Gotwols B. L. , Thompson D. R.
Published: February 2004

A probability density function, p(a), for the instantaneous amplitude of the backscattered field at moderate incidence angles is proposed. The key step is in writing the (slowly varying) backscatter cross section, σ0, as an exponential function to second order in the long wave slope component, sx, along the scatterometer look direction; that is, σ0 ∼ exp (a1sx + a2sx2). The function p(σ0) is then computed by a straightforward transformation of the normally distributed slope component, while p(a) follows from Bayes' theorem assuming p(a|σ0) to be Rayleigh distributed. The proposed probability density function was compared to Ku band dual‐polarized data taken at 20° and 45° incidence during the Synthetic Aperture and X Band Ocean Nonlinearities experiments at the Chesapeake Light Tower and at Forschungsplatform Nordsee. The function p(a) was found to follow a Weibull distribution with slope parameter v ≅ 1.9 for small amplitudes. At large amplitudes the distribution lengthens but appears to be neither Weibull nor K distributed as has been reported for neargrazing incidence data.
Comparisons of model predictions for radar backscatter amplitude probability density functions with measurements from SAXON
Authors: Thompson D. R. , Gotwols B. L.
Published: February 2004

Data ranging from L to Ka band were collected from radars mounted on the Forschungsplatform Nordsee during the Synthetic Aperture Radar and X Band Ocean Nonlinearities experiment in November 1990. In this paper we examine, for each of these radars, the total amplitude probability density function (pdf) of the field backscattered from the ocean surface. These pdfs are compared with predictions from a simulation based on our time‐dependent scattering model. We find that for lower incidence angles (∼20°), the agreement between the measured and computed pdfs is generally quite good. At these small incidence angles the behavior of the pdfs is determined by the local tilting of the long‐wave surface. No modulation of the shortwave spectral density over the long‐wave phase is needed to obtain good agreement. For larger incidence angles (∼45°) the agreement between the measured and predicted pdfs is not so good; the major discrepancy is that the tails of the predicted pdfs are somewhat too short. In this study we have attempted to account for the hydrodynamic modulation of the short‐scale waves using an approximate procedure based on the assumption that the hydrodynamic modulation is due to the interaction of the short‐scale waves with the orbital velocity of the long waves. With this procedure we are able to obtain agreement between the measured and computed pdfs at 45° incidence, although the strength of the hydrodynamic modulation needs to be adjusted. Our simulation procedure will be discussed in some detail. Also, we will show how our results are related to more conventional measurements of so‐called modulation transfer functions and give some arguments as to why in many cases the correlation between the backscattered power and the long‐wave surface velocity can be rather low.
Wave‐current interaction near the Gulf Stream during the Surface Wave Dynamics Experiment
Authors: Wang David W. , Liu Antony K. , Peng Chih Y. , Meindl Eric A.
Published: February 2004

This paper presents a case study on the wave‐current interaction near the local curvature of a Gulf Stream meander. The wave data were obtained from in situ measurements by a pitch‐roll discus buoy during the Surface Wave Dynamics Experiment (SWADE) conducted off Wallops Island, Virginia, from October 1990 to March 1991. Owing to the advection of the Gulf Stream by the semidiurnal tide, the discus buoy was alternately located outside and inside the Gulf Stream. The directional wave measurements from the buoy show the changes in wave direction, wave energy, and directional spreading when waves encountered the current in the Gulf Stream meanders. A wave refraction model, using the ray‐tracing method with an estimated Gulf Stream velocity field and meandering condition, was used to simulate wave refraction patterns and to estimate wave parameters at relative locations corresponding to buoy measurements. The numerical simulation shows that a focusing zone of wave rays was formed near the boundary and behind the crest of a simulated Gulf Stream meander. The focusing of wave rays causes changes in wave direction, increases in wave energy, and decreases in wave directional spreading, which are in good agreement with the results from the buoy measurements.
The cycling of sulfur in surface seawater of the northeast Pacific
Authors: Bates Timothy S. , Kiene Ronald P. , Wolfe Gordon V. , Matrai Patricia A. , Chavez Francisco P. , Buck Kurt R. , Blomquist Byron W. , Cuhel Russell L.
Published: February 2004

Oceanic dimethylsulfide (DMS) emissions to the atmosphere are potentially important to the Earth's radiative balance. Since these emissions are driven by the surface seawater concentration of DMS, it is important to understand the processes controlling the cycling of sulfur in surface seawater. During the third Pacific Sulfur/Stratus Investigation (PSI‐3, April 1991) we measured the major sulfur reservoirs (total organic sulfur, total low molecular weight organic sulfur, ester sulfate, protein sulfur, dimethylsulfoniopropionate (DMSP), DMS, dimethylsulfoxide) and quantified many of the processes that cycle sulfur through the upper water column (sulfate assimilation, DMSP consumption, DMS production and consumption, air‐sea exchange of DMS, loss of organic sulfur by particulate sinking). Under conditions of low plankton biomass (<0.4 μg/L chlorophyll a) and high nutrient concentrations (>8 μM nitrate), 250 km off the Washington State coast, DMSP and DMS were 22% and 0.9%, respectively, of the total particulate organic sulfur pool. DMS production from the enzymatic cleavage of DMSP accounted for 29% of the total sulfate assimilation. However, only 0.3% of sulfate‐S assimilated was released to the atmosphere. From these data it is evident that air‐sea exchange is currently only a minor sink in the seawater sulfur cycle and thus there is the potential for much higher DMS emissions under different climatic conditions.
The internal tide over the shelf inshore of Cape Point Valley, South Africa
Authors: Largier John L.
Published: February 2004

Thermistor string observations of the internal wave field over the shelf off South Africa display a whitened Garrett‐Munk spectrum (log‐log slopes of −1.7 to −2.0) with peaks in inertial, tidal, and high‐frequency (>1 cph) bands. The vertical structure is dominated by lower‐order modes, and the temporal signal is dominated by the internal tide. On propagating shoreward, the internal tide transports energy of the order of 0.04 kW m−1 onto the shelf, where it is dissipated. The resultant vertical exchange, characterized by an eddy viscosity of 10−4 m2 s−1, may maintain significant primary production in the absence of coastal upwelling. The internal tide is larger than expected, comparable with that over shelves where the forcing is twice as strong. This enhanced barotropic‐baroclinic coupling is explained in terms of resonance between multiple generation sites. A linear, two‐layer model is derived to investigate the enhancement due to the presence of Cape Point Valley, a submarine canyon that runs parallel to the shelf edge. Under resonant conditions the barotropic‐baroclinic coupling may be 3 times as large as that in the absence of the canyon; this explains an internal tide amplitude enhancement of 70%.
Effects of sill bathymetry, oscillating barotropic forcing and vertical mixing on estuary/ocean exchange
Authors: Valle‐Levinson Arnoldo , Wilson Robert E.
Published: February 2004

Intratidal and low‐pass filtered fluctuations of density stratification in eastern Long Island Sound are apparently produced by both hydraulic effects and tidally induced vertical mixing. In order to evaluate the effects of bathymetry, oscillating barotropic forcing and vertical mixing on density stratification and on the exchange between estuarine and oceanic waters, a series of numerical experiments over a simplified sill bathymetry are performed. Hydraulic effects over a sill decrease exchange by 25% relative to flat bottom. There is a fundamental change in the nature of the flow from weak to strong barotropic forcing: during weak tidal forcing, sill exchange can be subject to hydraulic control and the intratidal stratification is determined by internal wave motion on the pycnocline; during strong forcing, the density field is determined by the gradient advection of the predominant barotropic tide, the hydraulic control is broken, and exchange is determined by the tidal prism. Vertical mixing tends to decrease exchange and to break the hydraulic control.
Barotropic currents and vorticity in the central North Pacific Ocean during summer 1987 determined from long‐range reciprocal acoustic transmissions
Authors: Dushaw Brian D. , Worcester Peter F. , Cornuelle Bruce D. , Howe Bruce M.
Published: February 2004

Large‐scale depth‐integrated currents and relative vorticity were measured in the central North Pacific Ocean during summer 1987 using long‐range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Inverse techniques were used to estimate the depth‐averaged (barotropic) current bihourly at 4‐day intervals from differential travel times. Tidal constituent amplitudes and phases found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski (1980) and Cartwright et al. (1992), providing confirmation that the tomographically derived barotropic currents are correct within the expected uncertainties. The estimated low‐frequency, large‐scale currents are compared with depth‐averaged currents determined by point measurements using current meters and bottom‐mounted electrometers. Meridional and zonal currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areally averaged relative vorticity is shown to be insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large‐scale currents and vorticity determined from the Semtner and Chervin (1988) eddy‐resolving model of ocean circulation are similar to the direct measurements.
Hydrodynamic modulation of short wind‐wave spectra by long waves and its measurement using microwave backscatter
Authors: Hara Tetsu , Plant William J.
Published: February 2004

In this paper we use results of microwave backscattering experiments over the past decade to attempt to present a coherent picture of the ocean wave‐radar modulation transfer function (MTF) based on composite surface theory, short‐wave modulation, and modulated wind stress. A simplified relaxation model is proposed for the modulation of the gravity‐capillary wavenumber spectrum by long waves. The model is based on the relaxation rate and the equilibrium gravity‐capillary wavenumber spectrum. It differs from previous models by including all airflow modulation effects in the response of the equilibrium spectrum to changes in the airflow. Thus the explicit modulation of individual source functions such as wind input, short‐wave dissipation, and nonlinear interactions need not be known in order to calculate the hydrodynainic MTF. By combining this new model of the hydrodynamic MTF with microwave measurements, we attempt to determine wind shear stress modulation caused by the long waves. In order to extract the hydrodynamic MTF from the microwave data, we remove tilt and range change effects from the measured MTFs using the published analytical forms for these effects. Our results show that the inferred hydrodynamic MTF is higher for II polarization scattering than for V polarization. Since this is impossible if we have obtained the true hydrodynamic MTF, these results strongly indicate a problem with composite scattering theory as it has been traditionally applied. One explanation for this result may be the effects of intermediate‐scale waves suggested by Romeiser et al. (1993). Since these effects are much stronger for H polarization than for V polarization, they may explain our observed discrepancy and, if so, imply that V polarization return should yield an acceptable upper limit for the true hydrodynamic MTF. Thus we incorporate our V polarization results into the proposed model to estimate an upper limit for the wind shear stress modulation along the long‐wave profile. We infer that the primary source of modulation of Bragg resonant waves depends strongly on Bragg wavenumber and windspeed. For low values of these quantities, straining by long‐wave orbital velocities dominates the modulation process, while for higher values modulated wind stress becomes increasingly important. Our calculations indicate that wind stress modulation dominates the process for 3 cm Bragg waves at moderate to high wind speeds.
On surface energy balance over the northern wetlands: 1. The effects of small‐scale temperature and wetness heterogeneity
Authors: Guo Y. , Schuepp P. H.
Published: February 2004

A model is presented to calculate the surface energy balance over land surfaces covered by alternating small‐scale dry‐warm and wet‐cool patches, as might be found on northern wetlands during summer days. The model considered the effect of local heat advection, which is responsible for reallocation of a portion of surface available energy between the patches, in addition to the spatial change in primary partition of available energy between sensible and latent heat fluxes due to different physical characteristics of the patches. The modeled results showed that neglecting the effect of local heat advection could cause significant errors in calculations of both local and cumulatively regional surface heat and moisture fluxes. The predicted errors ranged up to 200% for evaporation and 67% for Bowen ratio when dry and wet patches are small and unequal in size, or up to about 35% for evaporation and 47% for Bowen ratio when dry and wet patch sizes are equal. Patch sizes with estimated nonnegligible error, i.e., with dimensions less than 100 to 200 m, are not uncommon in northern wetlands. This suggests that the currently used surface schemes (for mesoscale and global circulation models) should incorporate the effect of local heat advection for complete consideration of the sub‐grid‐scale surface heterogeneity, in order to improve their applicability to landscapes like northern wetlands. As a consequence the improvement of image resolution in aircraft and satellite remote sensing technology, for identification of sub‐grid small‐scale surface heterogeneity, seems necessary for accurate determination of surface heat and moisture flux distributions.
Carbon dioxide fluxes over a raised open bog at the Kinosheo Lake tower site during the Northern Wetlands Study (NOWES)
Authors: Neumann H. H. , den Hartog G. , King K. M. , Chipanshi A. C.
Published: February 2004

Measurements of carbon dioxide concentration and flux were made above a raised open bog at Lake Kinosheo in the southern Hudson Bay lowlands during the Northern Wetlands Study (NOWES) experiment in 1990. The flux measurements were made using micrometeorological techniques. They provide the first nondisturbing, larger‐scale CO2 flux measurements for this ecosystem and are the first to integrate the exchange over the whole 24 hours of the day. Continuous concentration measurements by infrared gas analyzers (IRGA) and spot flask samples were taken over the period July 1 to July 29. Afternoon CO2 values were only 5 to 7 parts per million by volume (ppmv) lower than measurements over the same period at Canadian background monitoring stations. This suggested that there was little draw‐down by local photosynthetic sinks. CO2 fluxes were measured at 8 and 18 m by Bowen ratio and eddy correlation methods, respectively. The methods produced comparable results on averaged data but often diverged considerably on individual half‐hour results. Fluxes were small. Daytime values averaged to −0.068 mg/m2/s by eddy correlation and −0.077 mg/m2/s by Bowen ratio over the period June 25 to July 28 (negative denotes downward flux), while at night, flux densities were +0.062 mg/m2/s and +0.085 mg/m2/s. Integration of the mean diurnal curve gave a net flux of −1.7 g/m2/d. Comparable data for this type of ecosystem were not found. However, Coyne and Kelley (1975), measuring near Barrow, Alaska, over wet meadow tundra dominated by sedges and grasses, found net fluxes of −7.2 g/m2/d. Typical net CO2 fluxes from other active temperate ecosystems have been found to be −10 to −20 g/m2/d (Monteith, 1976). Mean half hourly fluxes were almost constant at +0.06 mg/m2/s through the nighttime hours. About one half‐hour after sunrise the flux reversed direction. Uptake peaked about 0900 eastern daylight time (EDT) and then gradually declined but remained downward until near sunset. The early peak was interpreted to signify that the many plants in the bog experienced water stress during the day as evaporative demand increased and nighttime dew was evaporated.
On surface energy balance over the northern wetlands: 2. The variability of the Bowen ratio
Authors: Guo Y. , Schuepp P. H.
Published: February 2004

An energy balance model for land surfaces with small‐scale temperature and wetness heterogeneity [Guo and Schuepp, this issue] was used to study the variability of the Bowen ratio over northern wetlands. It was shown that the variation of the Bowen ratio can be caused by three factors: change in primary partition of surface available energy, local heat advection, and changes in atmosphere‐surface conditions. The first factor is the most sensitive one because it is directly proportional to the change in the ratio of dry to wet patch size. Due to its scale‐dependent, nonuniform reallocation of a portion of surface available energy between patches, the second factor adjusts the spatial variation of Bowen ratio in two aspects. On the one hand, it reduces the Bowen ratio variation caused by the change in the ratio of dry to wet patch size. On the other hand, it introduces the variation of the Bowen ratio caused by changes in absolute patch dimension, when the ratio of dry to wet patch size is constant in a given area. In model simulations for the case of equally sized dry and wet patches the latter influence was shown to generate variation in the Bowen ratio between 0.6 and 1.2, when patch dimensions varied up to 100 m. This helps explain the variability of the regionally averaged Bowen ratio observed by aircraft over different parts of the northern wetlands in the NOWES project. The third factor may affect both the absolute values and the range of the Bowen ratio by modifying directly the primary energy budgets and, indirectly, the local heat advection.
Ozone and aerosol distributions in the summertime troposphere over Canada
Authors: Browell E. V. , Fenn M. A. , Butler C. F. , Grant W. B. , Harriss R. C. , Shipham M. C.
Published: February 2004

Measurements of ozone (O3) and aerosol distributions were made with an airborne lidar system in the lowland and boreal forest regions of eastern Canada during July–August 1990 as part of the NASA Global Tropospheric Experiment/Arctic Boundary Layer Expedition (ABLE) 3B. Aerosol and O3 profiles were measured simultaneously above and below the Electra aircraft from near the surface to above the tropopause on long‐range flights over these important ecosystems. A broad range of atmospheric conditions were encountered during repeated flights over intensive study sites in the Hudson Bay lowlands near Moosonee, Ontario, and over the boreal forest near Schefferville, Quebec. The tropospheric composition in this high‐latitude region was found to be strongly influenced by stratospheric intrusions. Regions of low aerosol scattering and enhanced O3 mixing ratios were correlated with descending air from the lower stratosphere. Over 33% of the troposphere (0–12 km) along our flight track at latitudes from about 45° to 55°N had significantly enhanced O3 due to stratospheric intrusions, and in the middle to upper troposphere the extent of the enhanced O3 generally exceeded 40%. Ozone mixing ratios of 80 parts per billion by volume (ppbv) near 6 km were common in strong intrusions. In the boundary layer over the lowlands, O3 was in the 20–30 ppbv range with a vertical O3 gradient of 6.7 ppbv km−1 to about 45 ppbv at 3 km. Above 6 km the background tropospheric O3 profile was nearly constant with an average value of 53 ppbv. Due to forest fires in Canada and Alaska, plumes from biomass‐burning sources were observed on many flights. Biomass‐burning plumes influenced about 25% of the free troposphere below 4 km, and in some of the plumes, O3 was enhanced by 10–20 ppbv over ambient levels of 30–45 ppbv. Several air masses transported from the tropical Pacific were observed over Canada in the middle to upper troposphere with O3 levels 10–20 ppbv below background values of 50–55 ppbv.
Summertime distribution and relations of reactive odd nitrogen species and NOy in the troposphere over Canada
Authors: Talbot R. W. , Bradshaw J. D. , Sandholm S. T. , Singh H. B. , Sachse G. W. , Collins J. , Gregory G. L. , Anderson B. , Blake D. , Barrick J. , Browell E. V. , Klemm K. I. , Lefer B. L. , Klemm O. , Gorzelska K. , Olson J. , Herlth D. , O'Hara D.
Published: February 2004

We report here large‐scale features of the distribution of NOx, HNO3, PAN, particle NO3−, and NOy in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July–August 1990 from the NASA Wallops Electra aircraft as part of the joint United States‐Canadian Arctic Boundary Layer Expedition (ABLE) 3B‐Northern Wetlands Study. Our findings show that this region is generally NOx limited, with NOx mixing ratios typically 20–30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NOy above those in“background”air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO‐rich‐NOx‐poor smoldering local biomass‐burning fires. NOx mixing ratios in biomass‐burning impacted air masses were usually <50 pptv, but those of HNO3 and PAN were typically 100–300 pptv representing a twofold‐threefold enhancement over “background” air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America. These air masses were quite depleted in NOy (generally <150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the reactive odd nitrogen budget. Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large‐scale connection between the air masses influencing these regions.
The reaction probability of N2O5 with sulfuric acid aerosols at stratospheric temperatures and compositions
Authors: Fried Alan , Henry Bruce E. , Calvert Jack G. , Mozurkewich Michael
Published: February 2004

We have measured the rate of reaction of N2O5 with H2O on monodisperse, submicrometer H2SO4 particles in a low‐temperature flow reactor. Measurements were carried out at temperatures between 225 K and 293 K on aerosol particles with sizes and compositions comparable to those found in the stratosphere. At 273 K, the reaction probability was found to be 0.103±0.006, independent of H2SO4 composition from 64 to 81 wt %. At 230 K, the reaction probability increased from 0.077 for compositions near 60% H2S04 to 0.146 for compositions near 70% H2SO4. Intermediate conditions gave intermediate results except for low reaction probabilities of about 0.045 at 260 K on aerosols with about 78% H2SO4. The reaction probability did not depend on particle size. These results imply that the reaction occurs essentially at the surface of the particle. A simple model for this type of reaction that reproduces the general trends observed is presented. The presence of formaldehyde did not affect the reaction rate.
Stratospheric/tropospheric exchange affecting the northern wetlands regions of Canada during summer 1990
Authors: Bachmeier A. Scott , Shipham Mark C. , Browell Edward V. , Grant William B. , Klassa John M.
Published: February 2004

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted over the northern wetlands region of Canada during July and August 1990. Several Stratospheric/tropospheric exchange events were noted by zenith‐looking airborne lidar and in situ measurements of ozone and other trace gas species. Isentropic trajectories and potential vorticity analyses are utilized to determine the frequency of stratospheric inputs which would have affected the tropospheric column over the Moosonee and Schefferville regions and to describe the favored pathways of transport of stratospheric air arriving at these locations. At the 310 K potential temperature level (middle troposphere), trajectories having “aged stratospheric” values of potential vorticity at some point in their 5‐day history arrived at Moosonee or Schefferville roughly 40% of the time during the ABLE 3B study period, most often via large‐scale subsidence enroute from “stratospheric input regions” over the Arctic Ocean or northern and central Canada. At 325 K (upper troposphere), “fresh” stratospheric input was evident on about 80% of the trajectories, most often associated with jet streaks within the polar and Arctic jet streams. A case study is presented which illustrates both of these general stratospheric input processes.
Eddy correlation measurements of methane fluxes using a tunable diode laser at the Kinosheo Lake tower site during the Northern Wetlands Study (NOWES)
Authors: Edwards G. C. , Neumann H. H. , den Hartog G. , Thurtell G. W. , Kidd G.
Published: February 2004

As part of the Canadian Northern Wetlands Study (NOWES) measurements of methane flux were made at the Kinosheo Lake tower site for a 1‐month period during the 1990 summer intensive. The measurements were made with a diode‐laser‐based methane sensor using the eddy correlation technique. Measurements of the methane fluxes were made at two levels, 5 or 18 m. Approximately 900 half‐hour average methane flux measurements were obtained. Weak temporal and diurnal trends were observed in the data. Fluxes averaged over the study period showed an overall methane emission of 16 mg CH4 m−2 d−1 with a daytime average of 20 mg CH4 m−2 d−1 and a nighttime average of 9 mg CH4 m−2 d−1. The effect of emission footprint was evident in the data. A strong relationship between the daily average methane flux and wet bog temperature at 20‐cm depth was observed.
Temporal trends in surface irradiance at ultraviolet wavelengths
Authors: Justus C. G. , Murphey B. B.
Published: February 2004

To clarify recent, conflicting results about temporal trends in surface ultraviolet (UV) irradiance, a study was undertaken of UVB (280–320 nm) and UVA (320–400 nm) at Atlanta, Georgia, from 1980–1984. Results indicate that, while Robertson‐Berger (UVB) “RB” meter readings decreased about 10% over the period, Eppley “TUVR” (UVA) readings increased about the same amount. These trends appear to be due not to changes in ozone or cloud cover, but to real effects on the RB data from changes in aerosol concentrations, and to calibration drift of the TUVR sensor. Implications of these results for long‐term monitoring of UV radiation are discussed.
Role of small ice crystals in radiative properties of cirrus: A case study, FIRE II, November 22, 1991
Authors: Arnott W. Patrick , Dong Ya Yi , Hallett John , Poellot Michael R.
Published: February 2004

Aircraft observations of cirrus cloud were made near Coffeyville, Kansas, during November 1991 as part of the FIRE II project. Cloud ice particle spectra measurements were made using both a PMS 2DC probe and an ice particle replicator. Particles larger than 200 μm were column rosettes. The replicator shows the presence of large numbers of ice crystals smaller than 66 μm (two PMS size bins) that are not recorded by the PMS 2DC probe. Calculations based on the replicator data of the geometrical blocked area and absorption cross section of the cloud per unit volume show that small particles can contribute significantly to and sometimes dominate both the solar extinction and the infrared emission. Intercomparison is made of the ice particle size, area, and mass distributions determined by these different instruments. Power law relationships for area occluded by a crystal as a function of crystal maximum dimension were computed from the PMS 2DC data. The wavelength‐dependent infrared absorption cross section per volume was computed using a simple model based on anomalous diffraction and area and mass dimensional relationships for the ice crystals.
The influence of climate change and the timing of stratospheric warmings on Arctic ozone depletion
Authors: Austin John , Butchart Neal
Published: February 2004

Satellite data are presented showing the timing of sudden warmings in the lower stratosphere during the winters 1979–1992. A three‐dimensional dynamical‐radiative‐photochemical model is used to establish how Arctic ozone depletion will respond to a doubling of CO2 according to the timing of the warmings. In a series of idealized experiments the timing of the warmings is varied by specifying different geopotential wave amplitudes at the 316‐mbar model lower boundary. Results from a “transient climate change experiment” show that the chosen wave amplitudes are appropriate for both the current and the doubled CO2 atmosphere. For doubled CO2 the experiments show that any significant risk of an Arctic ozone hole will be confined to those years with only a late stratospheric warming. In all other years the results suggest that springtime total ozone over the Arctic is more likely to increase by a small amount due to a combination of slower homogeneous chemistry and changes in transport. The predictions obtained from the idealized studies are then tested by prescribing at the model lower boundary the observed geopotential wave amplitudes from two specific years with late winter warmings. Doubling CO2 amounts produced no significant increase in ozone depletion with the 1989 wave amplitudes, but with 1990 wave amplitudes, an Arctic ozone hole occurred with minimum column of 187 Dobson Units. This contrasting response is attributed to the large midwinter pulse in the 1989 wave amplitudes compared to the less dramatic and shorter timescale fluctuations in the 1990 wave amplitudes. It is concluded that under doubled CO2 conditions an Arctic ozone hole is likely to occur in years with late stratospheric warmings following winters in which there were no significant pulses in the upper tropospheric planetary wave amplitudes.
Aspects of spatial and temporal aggregation in estimating regional carbon dioxide fluxes from temperate forest soils
Authors: Kicklighter David W. , Melillo Jerry M. , Peterjohn William T. , Rastetter Edward B. , McGuire A. David , Steudler Paul A. , Aber John D.
Published: February 2004

We examine the influence of aggregation errors on developing estimates of regional soil‐CO2 flux from temperate forests. We find daily soil‐CO2 fluxes to be more sensitive to changes in soil temperatures (Q10 = 3.08) than air temperatures (Q10 = 1.99). The direct use of mean monthly air temperatures with a daily flux model underestimates regional fluxes by approximately 4%. Temporal aggregation error varies with spatial resolution. Overall, our calibrated modeling approach reduces spatial aggregation error by 9.3% and temporal aggregation error by 15.5%. After minimizing spatial and temporal aggregation errors, mature temperate forest soils are estimated to contribute 12.9 Pg C yr−1 to the atmosphere as carbon dioxide. Georeferenced model estimates agree well with annual soil‐CO2 fluxes measured during chamber studies in mature temperate forest stands around the globe.
Multiscale air quality modeling: Application to southern California
Authors: Kumar N. , Odman M. T. , Russell A. G.
Published: February 2004

A multiscale air quality model has been developed to follow accurately and efficiently the long‐range transport of pollutants emitted from urban areas. The model employs a two‐dimensional finite element scheme to follow the horizontal transport. The multiscale capability is obtained by using local finite element refinements. The model is applied to a 3‐day intensive measuring period (August 27–29, 1987) over southern California. Uniform and nonuniform grid systems are employed in the simulations. Uniform grid systems used resolutions of 5×5 km2, 10×10 km2, and 20×20 km2, horizontally. Two nonuniform grid systems are used. The first combines the 5×5 km2 grid over urban Los Angeles with the 10×10 km2 grid over the rest of the domain, while the second uses the 5×5 km2 grid followed by the 10×10 km2 grid and 20×20 km2 grid over the ocean and sparsely populated areas. Uniform coarsening of the grid impacted the urban areas by diluting NOx emissions, leading to higher levels of urban ozone predictions. The impact on regional areas was complex. The ozone being transported downwind was not followed as accurately as on the fine grid and the regional levels usually decreased. On the other hand, the artificial dilution of NOx emissions had the adverse effect of increasing regional ozone levels. Therefore grid coarsening sometimes leads to what appears to be satisfactory levels of regional ozone for the wrong reason. By using fine grids over the source areas, more satisfactory predictions of both urban and regional ozone levels were obtained. This is also computationally more efficient than using uniform fine grids over the entire domain.
Gravity waves in the upper mesosphere over Antarctica: Lidar observations at the South Pole and Syowa
Authors: Collins Richard L. , Nomura Akio , Gardner Chester S.
Published: February 2004

Lidar observations of the mesospheric Na layer were made at the south pole(90°S) and Syowa (69°S) during the winters of 1990 and 1985, respectively. These observations are used to characterize the gravity wave activity in the upper mesosphere at both sites. Strong wave activity is observed throughout the winter at both the south pole and Syowa and shows remarkable similarity with observations from several midlatitude and low‐latitude sites. The quasi‐monochromatic gravity waves exhibit the same general relationships between their wavelengths, observed periods, and amplitudes as observed at lower latitudes. The average growth length of these waves is approximately 26 km, indicating that the wave field at both Antarctic sites is strongly influenced by dissipation and saturation processes. The spectra and variances of the density perturbations associated with quasi‐random wave field at the south pole are reported. The vertical wavenumber and temporal frequency spectra follow power‐law shapes. The mean index of the vertical wavenumber spectrum is −2.4, and the mean characteristic wavelength is 14 km. The mean index of the temporal frequency spectrum is −1.7. The mean density variance at the south pole is (5.7%)2 and is similar in magnitude to that observed at a variety of lower‐latitude sites. With no tropospheric convection during the polar night and little orographic forcing over the relatively featureless Antarctic plateau, these observations suggest that nonlinear processes, rather than the source characteristics, primarily determine the characteristics of the gravity wave field in the upper mesosphere. These observations show two other distinct features. The mean Na layer over Antarctica is significantly lower and broader (centroid height ≈ 90 km and rms width ≈ 4.8 km) than at lower latitudes, reflecting the stronger downwelling and warmer winter temperatures in the mesopause region at high latitudes. Strong coherent oscillations were observed in the bottomside density contours of the Na layer with periods close to the inertial period. These oscillations were also observed in OH airglow measurements and appear to be associate with planetary scale waves.
Non‐sea‐salt sulfate and methanesulfonate at American Samoa
Authors: Savoie Dennis L. , Prospero Joseph M. , Arimoto Richard , Duce Robert A.
Published: February 2004

High‐volume bulk aerosol samples have been collected at American Samoa (14.25°S, 170.58°W) on a semicontinuous basis since the system was erected as part of the Sea/Air Exchange Program (SEAREX) in March 1983. In this report we consider those samples collected through May 6, 1992. For most of this period the sample filters were changed once a week. However, during November 1989 and from May 10 to June 10, 1990, in conjunction with the aircraft missions of the NASA Global Backscatter Experiment (GLOBE), the filters were changed daily. All of the samples were analyzed for nonsea‐salt (nss) SO4= and NO3−. Analyses for methanesulfonate (MSA) include all of the 53 daily samples, 22 weekly samples from March 19, 1983, through April 12, 1984, and 96 weekly samples from January 3, 1990, through May 6, 1992. The mean concentrations (in micrograms per cubic meter) were 0.37 for nss SO4=, 0.0229 for MSA, 0.114 for NO3−, and 5.1 for Na+. Nss SO4= and MSA are strongly linearly correlated in these 171 samples (r2 = 0.66) and the regression intercept does not differ significantly from zero. The geometric mean (GM) nss SO4=/MSA ratio, 18.1±0.9 (where ± indicates the 95% confidence interval of the GM) is about 7% higher than had previously been reported for this station. The ratio exhibits no significant seasonal variation. Although the ratio appeared to be significantly lower in the May–June 1990 daily samples (GM = 15.3±1.2), a further examination of the results indicated that the variance of the measured ratios from 18.1 (the GM for the whole data set) was attributable almost exclusively to the typical random errors in the analyses as determined from the 1σ analytical uncertainties of 5% for MSA and SO4 = and 2% for Na+.
Photodestruction rates for cometary parent molecules
Authors: Crovisier Jacques
Published: February 2004

I present new evaluations of the photodestruction rates for several molecules of cometary interest, together with a critical comparison with other estimations, and a summary of the need for future laboratory measurements.