bluuebunny/crossref_metadata_2025 huggingface dataset:
The impact of the wet tropospheric correction on the interpretation of altimeter‐derived ocean topography in the northeast Pacific
Authors: Phoebus Patricia A. , Hawkins Jeffrey D.
Published: February 2008

Atmospheric water vapor data derived from the special sensor microwave/imager (SSM/I) are used to make time‐coincident, wet tropospheric range corrections to Geosat altimeter data in the northeast Pacific. The original and corrected sea surface height residuals along numerous tracks are examined to determine the impact of water vapor on the altimeter signal. Mesoscale feature analyses of corrected and uncorrected altimeter data are used to assess the impact of water vapor path lengthening in areas of low sea surface height variability. Results indicate that the horizontal spatial variations in the water vapor height corrections are frequently similar to true oceanographic gradients. Interpretation of altimeter data is affected in several ways. The unaccounted‐for presence of atmospheric water vapor may mimic or mask the true ocean features, and even small changes in the water vapor over short spatial scales can enhance a partially obscured feature. Analyses of all Geosat tracks crossing the area of interest in September 1987 clearly illustrate that water vapor frequently contaminates the ocean topography measurements, making the water vapor adjustment critical before the altimeter data can be successfully used to locate and identify mesoscale ocean features. Furthermore, the SSM/I and Geosat data must be closely matched in both space and time, a difficult task since it takes 3.5 days to obtain global SSM/I coverage with one operational sensor. To optimize the mesoscale oceanographic application of altimeter data, a bore‐sighted radiometer should be included on all altimeter spaceborne platforms.
A preliminary estimate of geoid‐induced variations in repeat orbit satellite altimeter observations
Authors: Brenner Anita C. , Koblinsky C. J. , Beckley B. D.
Published: February 2008

Altimeter satellites are often maintained in a repeating orbit to facilitate the separation of sea height variations from the geoid. However, atmospheric drag and solar radiation pressure cause a satellite orbit to drift. For Geosat this drift causes the ground track to vary by ±1 km about the nominal repeat path. This misalignment leads to an error in the estimates of sea surface height variations because of the local slope in the geoid. We have estimated this error globally for the Geosat Exact Repeat Mission using a mean sea surface constructed from Geos 3 and Seasat altimeter data. Over most of the ocean the geoid gradient is small, and the repeat track misalignment leads to errors of only 1 to 2 cm. However, in the vicinity of trenches, continental shelves, islands, and seamounts, errors can exceed 20 cm. The estimated error is compared with direct estimates from Geosat altimetry, and a strong correlation is found in the vicinity of the Tonga and Aleutian trenches. This correlation increases as the orbit error is reduced because of the increased signal‐to‐noise ratio.
Gulf Stream surface transport and statistics at 69°W from the Geosat altimeter
Authors: Kelly Kathryn A. , Gille Sarah T.
Published: February 2008

Geosat altimeter height measurements for the Gulf Stream for three subtracks from the Exact Repeat Mission were processed to remove the geoid and mean Gulf Stream height. A Gaussian model for the Gulf Stream velocity profile was used to predict the form of the residual height profiles seen by Geosat and the mean height profile, using estimates of the width and position derived from the height residuals. Estimates of the maximum surface velocity and the surface transport were obtained by a least squares fit of the synthetic height profiles to the data. Location statistics from the Geosat data were in good agreement with values obtained from infrared images of the Gulf Stream. Maximum surface velocities were typically between 1.2 and 2.0 m s−1. The time series showed that surface transport is correlated primarily with the magnitude of the surface velocities, although an anomalously large increase in surface transport in the winter of 1987–1988 was caused by an increase in the width of the Gulf Stream. An analysis of the average annual signal in transport showed maximum values in the late fall and minimum transport in the late spring. Maximum (minimum) surface transport generally occurred when the Gulf Stream was north (south) of its mean position.
Tides in the Strait of Gibraltar
Authors: Candela Julio , Winant Clinton , Ruiz Antonio
Published: February 2008

The Strait of Gibraltar is where the transition occurs between two distinct tidal regimes: the North Atlantic, where tidal ranges are in excess of 2 m, and the western Mediterranean, where tidal ranges are less than 1 m. Within the strait the tide is principally semidiurnal: observations indicate that on average, 96% of the bottom pressure and 74% of the current variability are contained in the semidiurnal band (2 cpd). The structure of the local cotidal chart of the M2 tidal constituent is complicated but can be explained in terms of the along‐ and across‐strait momentum balances. Along the strait, the pressure gradient is mainly balanced by the acceleration of the flow, although friction can be of appreciable magnitude, corresponding to a decay time scale of 3–16 hours. Across the strait the momentum balance appears mostly geostrophic, but better observations would be required to explain residual terms. Available observations indicate that the M2 tide has no net energy flux through the strait. At the sill the tidal transport in the upper layer exceeds that in the lower layer by nearly a factor of 2. At the eastern end of the strait, between Algeciras and Ceuta, the tidal transport is larger in the lower layer. This apparent shift in the location of maximum tidal transport is explained in terms of the kinematics of the interface between Atlantic and Mediterranean waters. A mean transport of 0.21 Sv, due to subinertial and tidal flows, carries Atlantic water through the strait, compensated by an approximately equal return in the lower layer.
The influences of atmospheric half‐yearly cycle on the sea ice extent in the Antarctic
Authors: Enomoto Hiroyuki , Ohmura Atsumu
Published: February 2008

The relationship between sea ice and weather, one of the least known components of the climatic system, could be an important factor for the climate of high latitudes. The annual cycle of the sea ice extent is characterized by a asymmetric development, with the sea ice area slowly advancing toward the equator in the winter and rapidly retreating in summer. In this study the seasonal asymmetric behavior of ice extent and the changes in sea ice concentration are shown to be linked to the atmospheric convergence line (ACL) around Antarctica. It is found that the relative positions of the ACL characterized by the half‐year cycle exert a strong influence upon the mean movement of the sea ice. It is also observed from the investigations of the areal concentration of the sea ice that a decrease in ice concentration prior to the sea ice retreat is needed for a rapid retreat.
The influence of atmospheric stratification on scatterometer winds
Authors: Hoffman Ross N. , Louis Jean‐Francois
Published: February 2008

Scatterometers measure surface roughness which is empirically related either to surface stress or to the equivalent neutral stability wind. We study the importance of atmospheric stability effects for the analysis of these data. For low wind speeds and neutral to slightly stable conditions, neutral stability wind is quite sensitive to stability. We develop a variational analysis procedure for the scatterometer data, which adjusts both the near‐surface veclocity and temperature. In simulation tests, temperature analysis increments are found to be small. Also, the differences in the wind analyses due to differences in the temperature background field are small. However, if stability effects are not accounted for, there will be small systematic errors in the wind analysis.
The geometry of turbulent patches from a numerical simulation of internal wave critical layer events
Authors: Dewitt R. J. , Henyey F. S. , Wright J. A.
Published: February 2008

We model the development of turbulent patches in the ocean as due to internal wave breaking at critical layers. Numerical simulations of small‐scale internal wave packets moving in a large‐scale background are performed using the Eikonal technique. The wave packets are found to break in long, thin horizontal layers that resemble the turbulent patches seen in experimental records. Statistical analysis of the final positions of the wave packets yields values for typical thicknesses of between 0.75 m and 2.5 m, which agree with the predominance of the experimental data on patches. The simulation suggests that patches with thicknesses much greater than 2.5 m should be quite rare events. The statistically computed tilt of the patches from the horizontal computed from the simulation is 0.014, which is in agreement with typical aspect ratios for the background internal waves in our study and with experimental observations. We conclude that the patches created by internal wave breaking are not created as thick patches that then collapse by some mechanism into long, thin pancake shapes, but instead are created in their observed form by the very nature of the critical layer events.
Seasonal and interannual variability of pigment concentrations across a California Current frontal zone
Authors: Thomas A. C. , Strub P. T.
Published: February 2008

Previously published physical and biological data document a zonally oriented frontal region within the California Current system separating colder and more eutrophic water north of ≈ 33° N from warmer, more stratified, and oligotrophic water farther to the south. Satellite images of phytoplankton pigment from the coastal zone color scanner from 1979–1983 and 1986 are used to examine the seasonal and interannual variability of both the latitudinal position of this front and the pigment concentrations associated with it. Many temporal and spatial characteristics of the pigment structure are repeated in different years, and a general seasonal cycle is described. Variations in the frontal structure are controlled primarily by changes in pigment concentration north of the front. Seasonality is minimum south of the front where concentrations remain low (< 0.5 mg m−3) throughout the spring, summer, and fall. The frontal gradient is typically strongest from late March until early June when higher concentrations (> 2.0 mg m−3) are present north of the front. Lower pigment concentrations within the sampled region (0.5–1.0 mg m−3) north of the front in mid‐late summer (June–August), resulting from a seasonal shift in the cross‐shelf distribution of pigment, reduce and often eliminate the pigment gradient forming the front. Concentrations greater than 1.0 mg m−3 typically extend 150–250 km farther offshore in spring (April) than in summer (June–July). Superimposed on this general seasonality is strong interannual variability in the magnitude of the frontal gradient, its latitudinal position, and the seasonal development of higher biomass in regions north of the front. Pigment concentrations during the El Niño year of 1983 are distinctly lower than those of other years. The patterns evident in the satellite data are compared with available in situ measured hydrographic data and nutrient and phytoplankton concentrations. A comparison of the seasonal and interannual variability of these patterns to surface wind shows little direct relation between frontal strength or position and wind forcing.
Examples of, reasons for, and consequences of the poor quality of wind data from ships for the marine boundary layer: Implications for remote sensing
Authors: Pierson Willard J.
Published: February 2008

Wind reports by data buoys are used to demonstrate that these reports have in the past provided useful values for the synoptic scale winds and that at present these reports provide very reliable values for the synoptic scale winds. Past studies of wind reports by ships have revealed that the data are of poor quality, but the causes for this poor quality are not identified. Examples of the poor quality of wind data from ships are obtained by comparing ship reports with buoy reports and comparing reports of different kinds of ships with each other. These comparisons identify many different reasons for the poor quality of wind data from ships. Suggestions are made for improving the quality of ship data. The consequences of the poor quality of ship winds are described in terms of the effects on weather and wave forecasts. The implications for remotely sensed winds are discussed.
Oceanic adjustment in the presence of mean currents on an equatorial β plane
Authors: Chang Ping
Published: February 2008

The oceanic adjustment to variable winds in the presence of mean currents is studied by means of a shallow water model on an equatorial β plane. The waves that are available to affect the adjustment fall into two groups: a discrete, finite or infinite number of equatorially trapped modes, and a continuum of modes that are bounded by turning latitudes and/or critical latitudes. In response to variable winds, adjustment near the equator is very similar to the adjustment in the absence of the mean currents except for modifications to the speeds and structures of the waves that are involved. Off the equator, where the continuum of modes come into play critical layer absorption is a factor provided that the waves have a sufficiently large meridional group velocity to propagate to the critical layers in a reasonable time (before they cross the ocean basin, for example). Winds that are suddenly switched on excite waves with small zonal wave numbers and small meridional group velocities. The waves therefore succeed in effecting the adjustment. In the case of periodic winds, linear critical layer absorption modifies the shadow zones (Ekman response region) and regions of Sverdrup response that characterize the response in the absence of mean currents. In response to the annual harmonic of the wind the western part of the North Equatorial Countercurrent is unlikely to by affected by Rossby waves, because that is a shadow zone. The region between 10°N and 15°N approximately, the southern flank of the North Equatorial Current, is an interesting region because it is effectively a waveguide for modes with a turning latitude to the south and a critical latitude to the north. Wave trains are likely to be evident in that region.
Coupling of coastal zone color scanner data to a physical‐biological model of the southeastern U.S. continental shelf ecosystem: 3. Nutrient and phytoplankton fluxes and CZCS data assimilation
Authors: Ishizaka Joji
Published: February 2008

Phytoplankton and nutrient fluxes across the 40‐ and 75‐m isobaths at a nominal depth of 17 m were calculated from the distributions obtained with a physical‐biological model of the southeastern U.S. continental shelf ecosystem. These flux estimates represent integrated values over a 200‐km region of the southeastern U.S. continental shelf and cover the period of April 1980. The phytoplankton and nutrient fluxes showed considerable variability with time; however, the fluxes across the 40‐m isobath were primarily onshore while the fluxes across the 75‐m isobath were primarily offshore. The fluctuations in the nutrient flux were associated with the passage of Gulf Stream frontal eddy events. The effect of frontal eddy events on the phytoplankton fluxes was less apparent. Comparison of the time average of the two fluxes over one month with similar estimates for the fluxes at 37–45 m (Hofmann, 1988) showed a clear pattern of strong up welling between the 40‐ and 75‐m isobaths and an associated increase in phytoplankton concentration in the upper water column. In general, the pattern of the flux estimates was insensitive to the values chosen for the biological model parameters. The chlorophyll distributions obtained from nine Coastal Zone Color Scanner (CZCS) images from April 1980 were assimilated into the physical‐biological model to improve the phytoplankton flux estimates. Three techniques were tested for CZCS data assimilation into physical‐biological models, all of which gave stable model results. In general, the accuracy of the simulated chlorophyll fields was improved by assimilation of the CZCS data; however, the simulated fields rapidly converged to the fields obtained with no CZCS data assimilation after a few days. The time change of the phytoplankton carbon fluxes from the model with CZCS data assimilation were very similar to the fluxes obtained from the model with no CZCS data assimilation. However, the magnitude of the phytoplankton fluxes was less than the one obtained with no data assimilation because of the overestimation of the chlorophyll concentrations by the original model. The assimilation of CZCS chlorophyll into the physical‐biological model significantly improved the predictive capability of the model for phytoplankton distributions and associated phytoplankton fluxes. However, it is difficult to determine if the accuracy of other components of the model, such as the nutrient fields, is also improved by the assimilation of CZCS data.
On the noncorrelation of the vertical structure of light scattering and chlorophyll α in case I waters
Authors: Kitchen James C. , Zaneveld J. Ronald V.
Published: February 2008

The relationships between beam attenuation, absorption, suspended particle concentration, size distribution and pigment content are examined for a region where the particle concentration and pigment maxima are widely separated. Mie scattering analyses are performed on this data to predict the profiles of backscattering. It was found that absorption and scattering are not and have no theoretical reason to be well correlated in a vertical section even if the particle concentration is perfectly correlated with phytoplankton biomass. Due to the rapid change in irradiance with depth, pigment concentration and therefore paniculate absorption are not expected to be correlated with biomass or productivity as calculated from published laboratory results and theoretical models of phytoplankton pigment content. The beam attenuation is a good indicator of particle concentration and its vertical distribution relative to the pigment distribution is consistent with models of phytoplankton growth. An examination of data from a meridional transect of the North Pacific and from the Northeast Pacific coastal upwelling region suggested the universality of these findings. The remotesensing algorithms that predict backscattering from chlorophyll concentration were developed using near‐surface data. Use of these algorithms for the prediction of vertical structure of backscattering is discouraged.
Thermal advection and stratification effects on surface winds and the low level meridional mass transport
Authors: Levy Gad , Tiu Felice S.
Published: February 2008

Statistical tests are performed on the Seasat scatterometer observations to examine if and to what degree thermal advection and stratification effects manifest themselves in these remotely sensed measurements of mean wind and wind stress over the ocean. On the basis of a two layer baroclinic boundary layer model which is presented, it is shown that the thermal advection and stratification of the entire boundary layer as well as the geostrophic forcing influence the modeled near surface wind and wind stress profiles. Evidence of diurnal variation in the stratification under barotropic conditions is found in the data, with the daytime marine boundary layer being more convective than its nighttime counterpart. The temporal and spaciai sampling pattern of the satellite makes it impossible to recover the full diurnal cycle, however. The observed effects of the thermal advection are shown to be statistically significant during the day (and presumed more convective) hours, causing a systematic increase in the poleward transport of mass and heat. The statistical results are in a qualitative agreement with the model simulations and cannot be reproduced in randomized control tests.
On the behavior of boundary undercurrents near canyons
Authors: Hughes Roger L. , Ofosu Korantang N. , Hickey Barbara
Published: February 2008

A simple finite Froude number, quasi‐geostrophic, frictionless model is developed to study the response of a boundary undercurrent, such as the California Undercurrent, to variations in the underlying topography. Despite its baroclinic nature, the flow tends to follow contours of total depth, although this pattern of response is not fully achieved. Over deep, narrow canyons the flow separates from the bottom. Another simple model containing stratification and frictional effects predicts upwelling to occur in the underlying water within the canyon.
The effect of curvature on turbulence in stratified fluids
Authors: Kantha L. H. , Rosati A.
Published: February 2008

The influence of streamline curvature on small‐scale turbulence and vertical mixing in stratified fluids is the subject of this study. The roles of curvature and stratification in enhancing and suppressing turbulent mixing are explored using second‐moment closure for turbulence. Governing equations for second moments are expressed in generalized orthogonal curvilinear coordinates, from which, through a series of approximations, simplified expressions are derived for second moments in the limit of small streamline curvature. The governing equations are then used to obtain a quasi‐equilibrium turbulence model suited for application to atmospheric and oceanic mixed layers. A typical model application is illustrated by simulation of stratified flows over two‐dimensional, idealized mountains and valleys. The limit of local equilibrium is further invoked to derive semi‐analytical results for the enhancement and suppression of vertical turbulent mixing under the combined influence of stratification and curvature. It is shown that stabilizing curvature can drastically suppress turbulence even when the stratification is strongly destabilizing. Conversely, under strong stable stratification that would otherwise lead to total suppression of turbulence, destabilizing curvature can keep turbulence alive. Streamline curvature is also shown to significantly modify the Monin‐Obukhov similarity laws for momentum and heat fluxes in the constant flux region of the atmospheric boundary layer. Finally, the need for observational data on curvature effects on mixing in stratified flows either in the laboratory or in flows over topography in the oceans and the atmosphere is highlighted.
Ageostrophy and pressure work in the Gulf Stream at 73°W
Authors: Kontoyiannis Harilaos , Watts D. Randolph
Published: February 2008

During 1984, velocity, bottom pressure and thermocline depth data were obtained from deployments of tall current meter moorings, bottom pressure gauges and inverted echo sounders at five sites on the anticyclonic side of the Gulf Stream at 73° W. An additional number of inverted echo sounders, deployed in several nearby sites, allowed us to construct daily regional maps of the thermocline topography. Using this data base we examined the momentum balance and the pressure work in the eddy kinetic energy balance at 500 m above the seafloor and at a depth of 400 m. At 500 m off the bottom, the correlations between fluctuating geostrophic currents, inferred from pressure differences between sites, and suitably averaged observed currents provide observational confirmation of the geostrophic balance in the deep Gulf Stream, within our ability to measure it. At 400 m depth, the ageostrophic current was computed using velocity measurements, curvature estimates from the thermocline maps, and the momentum equation in natural coordinates. Comparison of observed and ageostrophic velocities showed occasional departures from geostrophy of O(30% to 60%) with duration from 2 to 5 days. These ageostrophic events were caused by tangential and centripetal accelerations, mostly during the coalescence of two cold‐core rings with the stream. The mean departure from geostrophy at 400 m during the deployment period was 7%. The mean effect of the pressure work, on the anticyclonic side of the stream at 400 m, was to decrease the eddy kinetic energy at a rate of −3×10−3 ergs cm−3 s−1; a value with magnitude as large as that of the barotropic instability term. The pressure work record is very episodic with peaks, during the ageostrophic events, close to 30 times larger than the mean estimate and with both positive and negative values. At 500 m off the bottom, the pressure work was found to be insignificantly different from zero.
Evidence for 18.6‐year lunar nodal drought in western North america during the past millennium
Authors: Currie Robert G.
Published: February 2008

Analysis of 38 tree ring chronologies yields evidence for enhanced drought conditions every 18.6 years in the North American interior for the past millennium. Cross spectra between a drought area index and 26 temperature records confirm the analysis of Currie (1979, 1981d) as regards a lunar nodal MN term in these data. Discussion of the MN tidal constituent, which appreciably modulates amplitude and phase of four main constituents (M2, K1, O1, Mf) is presented, and implications for agriculture are surveyed.
The frequency response of an electrochemical ozone sonde and its application to the deconvolution of ozone profiles
Authors: De Muer Dirk , Malcorps Henri
Published: February 2008

The frequency response of the combined sensor and air‐sampling system of an electrochemical ozone sonde has been determined by means of a digital Fourier analyzer. The measured frequency response can be represented by a first‐order process and a diffusion process in series. If the mean ozone concentration at the input is below a certain level, a fraction of the ozone passes through an additional diffusion process. The variation of the frequency response as a function of the solution temperature and the background ozone level was determined for the ranges of interest. A satisfactory physical interpretation for the behavior of the frequency response is given. If the frequency response of the sensor is known, the ozone soundings may be compensated for it by deconvolution; this results in a highly improved correspondence between ascent and descent profiles.
Stratospheric odd nitrogen: Measurements of HNO3, NO, NO2, and O3 near 54°N in winter
Authors: Ridley B. A. , Luu Son Ha , Hastie D. R. , Schiff H. I. , McConnell J. C. , Evans W. F. J. , McElroy C. T. , Kerr J. B. , Fast H. , O'Brien R. S.
Published: February 2008

Data obtained during three stratospheric measurement campaigns from Cold Lake, Alberta (100.0°W, 54.4°N), in February 1977, 1978, and 1979 are presented. Altitude profiles of NO, HNO3, O3, CFM‐11, CFM‐12, and N2O and ground‐based total column measurements of NO2 were obtained and are compared with similar measurements made at 51°N in summer. The winter data demonstrate enhanced variability when compared with summer conditions, but in general in winter (1) there is a greater abundance of HNO3 and the stratospheric layer is thicker, (2) there is less nitric oxide particularly in the 18‐ to 28‐km region and the vertical distributions are characterized by strong mixing ratio gradients, and (3) the column abundance of NO2 is lower and exhibits a diurnal change qualitatively similar to that observed in summer. The difference between the summer and winter observations is not solely due to changes in photochemistry but requires consideration of stratospheric dynamics. We correlate the reduction in NOx in winter with the production of N2O5 in regions of little or no insolation followed by transport to Cold Lake. The unusual profiles are shown to result from air masses at different altitudes having either different origins, for example, polar or mid‐latitude, or different transit times from the source to the sampling point.
Spectroscopy and transmittances for the LIMS Experiment
Authors: Drayson S. Roland , Bailey Paul L. , Fischer Herbert , Gille John C. , Girard Andre , Gordley Larry L. , Harries John E. , Planet Walter G. , Remsberg Ellis E. , Russell James M.
Published: February 2008

The Limb Infrared Monitor of the Stratosphere (LIMS) instrument on the NIMBUS 7 spacecraft sensed limb emission profiles for 7 months starting in October 1979. Vertical temperature profiles are inferred from measurements in two CO2 channels, and constituent profiles are obtained from the O3, HNO3, NO2, and water vapor channels. The line parameters used to calculate transmittances in the retrieval algorithm are presented. Their adequacy for LIMS and for other applications are assessed, and recommendations for future investigations are outlined.
Oxygen absorption cross sections in the herzberg continuum and between 206 and 327 K
Authors: Johnston Harold S. , Paige Mark , Yao Francis
Published: February 2008

The ultraviolet absorption cross sections for molecular oxygen have been determined for wavelengths between 205 and 225 nm, for temperatures between 206 and 327 K, and at a pressure from 100 to 750 torr. No temperature effect on the O2 cross sections was observed in the Herzberg continuum. It was necessary to separate the extremely weak O2 absorption from the stronger collision‐induced absorption. These experimental data and similar data by others were interpreted in terms of a theoretical expression for the shape of the Herzberg continuum absorption spectrum. The O2 absorption cross sections are in good agreement with the values measured in the stratosphere by Herman and Mentall (1982).
The rainout parameterization in a photochemical model
Authors: Giorgi F. , Chameides W. L.
Published: February 2008

Photochemical models of tropospheric trace species typically simulate the effects of wet removal of soluble species from the atmosphere through a first‐order rainout parameterization, which is independent of the species solubility and constant in time. In this work a modified first‐order rainout parameterization is proposed that is a function of the species' solubility and takes into account the intermittent nature of atmospheric wet removal. In the case of a highly soluble species, such as HNO3, the inclusion of the effects of intermittent rainout in the parameterization leads to averaged rainout lifetimes considerably higher than ones used in most photochemical models. For illustrative purposes this parameterization is applied to a one‐dimensional steady state model with parameterized eddy diffusion, although in principle it can be used in more sophisticated multidimensional models. The model results are compared to analagous calculations obtained with a 3‐D tracer transport model.
A theoretical study of electrostatic field wave shapes from lightning leaders
Authors: Thomson E. M.
Published: February 2008

Electrostatic dE/dt field wave shapes are calculated for various lightning leader models over a perfectly conducting ground plane. These provide an improved theoretical basis for the characterization and interpretation of observed leader wave shapes for close lightning. Since dE/dt at a particular time is a function of how the leader tip is propagating, whereas the total electric field change at the same time is a function of the aggregate behavior of the leader up to this time, dE/dt is the preferable parameter for leader characterization. In particular, whether or not dE/dt changes polarity in the course of the leader is independent of the channel charge distribution, unlike the polarity of the net field change. The dE/dt wave shape is also more easily interpreted in terms of channel geometry than the electric field wave shape for leader channels that have branches or are not vertical. Features of dE/dt are derived for a vertical, one‐dimensional leader channel assuming an electrostatic relation, and deviations from this model are studied for both inclined channels and three‐dimensional charge distributions. The existence of a turning point (dE/dt = 0) in the electric field wave shape is investigated, and the distance dependence of the leader tip height corresponding to an electric field turning point is presented. For a vertical channel the final leader current IL0 is shown to be related to the final dE/dt, (dE/dt)0, by IL0 = −2πε0[H2 + D2]3/2 (dE/dt)0/H, where H is the height above ground level and D is the horizontal distance for the charge center which is the origin of the leader charge. The physical validity of the leader models used is discussed, and some applications of the derived plots are considered.
Some observations on the concentrations of short‐lived decay products of radon and thoron in the monsoon rains of Bombay, India
Authors: Rangarajan C. , Eapen C. D.
Published: February 2008

The concentrations of radon decay products 214Pb(RaB) and 214Bi(RaC) and thoron daughter 212Pb(ThB) have been measured in rainfall at Bombay. The presence of short‐lived 218Po(RaA) is indicated in a few samples. The levels of 214Pb varied from 300 to 7000 pCi (11–260 Bq) per liter, while the activity ratios of 214Bi/214Pb are in the range of 0.5–1.5. From these values of the ratios a cloud drop lifetime, from nucleation to deposition, of 20–60 min is estimated. The concentrations of 212Pb varied from 2 to 30 pCi (0.075–1.1 Bq) per liter. The levels of radon and thoron daughter products are lower in Bombay summer rainfall compared with other areas due to their reduced concentrations in the maritime monsoon winds. Approximate estimates of scavenging ratios based on the surface air concentrations are 85±65 (1 standard deviation) and 30±20 (1 standard deviation) for radon and thoron daughters, respectively.
The coupling between melting and convective air motions in stratiform clouds
Authors: Moore G. W. Kent , Stewart Ronald E.
Published: February 2008

Unlike evaporation and condensation that can occur throughout the depth of the atmosphere, the melting of snowflakes can only occur at temperatures near 0°C. The cooling of the atmosphere because of this melting often results in the development of an isothermal layer at 0°C and a region of instability below it. In this paper we consider the effect that the cooling caused by melting of snowflakes has on the resultant convective activity. This cooling results in upward vertical velocities and upward penetration depths that are significantly smaller than those anticipated from consideration of saturated adiabatic processes and particle drag alone.
Estimates of nitrate formation in rain and snow systems
Authors: Chang T. Y.
Published: February 2008

Starting from gas‐phase and heterogeneous chemical reaction mechanisms of NOy species and conservation equations for chemical species and hydrometeors (cloud droplets, raindrops, and snowflakes), approximate expressions for estimating NOy species concentrations and nitrate formation in storm systems have been derived. These expressions are intended for moderately polluted, regional areas and for the average situations of large rain or snow systems where there is widespread weakly ascending air motion. With input parameters which are representative of average summer rainstorms and winter snowstorms in the northeastern United States, HNO3 generated within storm systems (by scavenging of NO3, N2O5, and HNO3) is shown to contribute up to 10–30 μM L−1 in the ground level precipitation water. In this estimate, contributions from the moderately polluted, lowest layer (1–2 km) alone have been included, and N2O5 and NO3 as well as HNO3 have been assumed to be irreversibly scavenged by hydrometeors to produce aqueous nitrate.
Observations of the diurnal variation of nitrogen dioxide in the stratosphere
Authors: Solomon S. , Russell J. M. , Gordley L. L.
Published: February 2008

Observations of NO2 from the limb infrared monitor of the stratosphere (LIMS) experiment in high‐latitude summer are presented. Appropriate selection of latitude and days during the month of May yields NO2 observations obtained during the afternoon and evening at solar zenith angles ranging from about 35° to 110°. These data show that the NO2 abundances in the stratosphere become strongly dependent on the solar zenith angle for angles exceeding about 80°. Model calculations and other observations suggest that from about 80° to 95° this dependence probably reflects variations in the propagation of the visible radiation that photodissociates NO2. The data obtained at zenith angles greater than about 95° provide clear evidence for the night decay of NO2 to form N2O5, and the observed decay rate is shown to be consistent with present theory.
Chemistry of marine aerosol over the tropical and equatorial Pacific
Authors: Raemdonck Hans , Maenhaut Willy , Andreae Meinrat O.
Published: February 2008

Aerosols from the marine boundary layer were collected during a cruise from the Peru/Ecuador shelf through the equatorial and tropical Pacific to the Hawaiian Islands. A variety of samplers of different design and independent analytical techniques were used to validate the results obtained. On large particles, little or no chloride deficiency relative to seawater composition was observed, while on submicrometer particles, as much as 40% of chloride had been lost. A silicate component with a composition similar to the crustal average was associated with sea‐salt particles as an internal mixture. Along the Peru coast, substantial concentrations of Cu, Zn, As, and Pb were found which must be related to large emissions from the metallurgical industry in that region. High levels of selenium in the remote equatorial region suggest a biological source of volatile selenium in this area of high productivity. The concentrations of excess sulfate were found to be consistent with the flux of dimethylsulfide determined on the same cruise. The presence of soot carbon, particulate organic carbon, and fine excess potassium showed evidence of long‐range transport of combustion‐derived aerosols to this remote region. Enhanced soot and crustal elements in the Intertropical Convergence Zone suggest downward mixing of materials transported in the upper troposphere in this region of active vertical exchange.
Analysis of remote measurements of tropospheric carbon monoxide concentrations made during the 1979 summer monsoon experiment (MONEX)
Authors: Doherty G. Mark , Newell Reginald E. , Reichle Henry G.
Published: February 2008

Mixing ratios of tropospheric CO as measured by an aircraft‐mounted radiometer over Saudi Arabia, the Arabian Sea, and northern India during May and June 1979 are reported. During early May, exceptionally high CO levels were detected over Saudi Arabia, and strong horizontal gradients in CO mixing ratios were seen to develop over a period of several days. Over the Arabian Sea, mixing ratios of the order of 150 parts per billion by volume were observed before the monsoon onset, and a pronounced decrease in CO was detected toward the equator. Subsequent measurements after the monsoon had become established revealed a consistent decrease in CO mixing ratio across this region. Analysis of aircraft dropsonde data and constant pressure daily streamline charts lend strong support to the hypothesis that this reduction is associated with the influx of CO‐poor southern hemisphere air in the monsoon southwesterlies.
Zonal wave number variance spectra of stratospheric microwave brightness temperatures
Authors: Martin Russell L. , Stanford John L.
Published: February 2008

Well‐known theoretical predictions suggest that variances of large‐scale atmospheric fluctuations, for scales somewhat shorter than those of the forcing mechanisms, should obey a power law, k−b, where k is the horizontal wave number. The invariance of the exponent b with season, hemisphere, and latitude is tested using satellite‐measured brightness temperatures. Global grids of TIROS‐N Microwave Sounding Unit channel 4 measurements, which closely approximate the 30‐ to 150‐mbar layer mean temperature, are constructed for January, March, and August 1979. These grids are zonally Fourier‐transformed, and the resulting spectra are averaged over four 18°‐wide latitude bands. Fits of a power law to these spectra over wave numbers 10–36 and 10–26 give values of b which are independent of season, hemisphere, and latitude band, to within statistical uncertainties. The observed values of b are about 3.6, as compared to theoretical predictions of 3.
Rainout lifetimes of highly soluble aerosols and gases as inferred from simulations with a general circulation model
Authors: Giorgi Filippo , Chameides William L.
Published: February 2008

The rainout‐determined lifetimes of highly soluble particulate and gaseous atmospheric compounds are investigated using general circulation model simulations in which removal is explicitly calculated in terms of the local, model‐produced precipitation rates. Our calculations indicate that because of the episodic and asymmetric nature of rainout, species' lifetimes depend not only on the amount of precipitation but also on the characteristics of the precipitation regime (such as duration and frequency of the precipitation events) and on the direction of the tracer main flow (determined by the species' average mixing ratio gradient). For this reason, averaged rainout lifetimes of tracers flowing downward from the stratosphere are found to differ substantially from those of tracers of surface origin flowing upward or tracers of a more ubiquitous tropospheric source. These results imply that the use of a first‐order parameterization to simulate rainout in a photochemical model that does not explicitly calculate precipitation can be inadequate in representing this process. A computationally efficient parameterization that includes the effects of intermittence and asymmetry of rainout is proposed, and it is shown how this parameterization can be used to estimate rainout‐determined tropospheric residence times from observational data sets. A review of published estimates of submicron aerosol tropospheric residence times based on observations shows that these are consistent with our model results.
Wave‐associated density disturbances at space vehicle reentry heights
Authors: Chimonas G.
Published: February 2008

Space vehicle operations are sensitive to density disturbances in the thermosphere. The quantification of such disturbances is therefore a matter of importance. In this paper we discuss the contribution from mesoscale waves and conclude that “external” waves cause much larger disturbances than the internal gravity waves. The internal waves are restricted to relatively small amplitudes by convective overturning, while the external waves, typified by the Lamb wave, do not suffer from this limitation. The energy in a Lamb wave is largely confined to the lower atmosphere, which is also its expected source region. But we demonstrate that the exponentially small penetration of this wave to high altitudes produces an exponentially increasing relative disturbance in the more tenuous upper atmosphere. We conclude that the Lamb wave is a significant disturbance in the upper atmosphere and that it may provide density variations that are problems for space vehicle reentry and performance.
Physical interpretation of variations in total solar irradiance
Authors: Foukal P.
Published: February 2008

Radiometry from the Solar Maximum Mission and Nimbus 7 satellites has demonstrated that the solar constant varies at a peak‐to‐peak level of up to 0.2% on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disc accounts for most of that variation. Reasonable explanations have been put forward to explain how the spot‐blocked heat flow might be stored and to explain the source of the intense radiation that gives rise to the increased irradiance produced by the bright magnetic faculae. Time‐dependent models of the response of the solar convection zone to small perturbations also indicate that slower variations in total solar irradiance of comparable magnitude are likely. More precise observations of the total solar irradiance and radius over long time scales are required to demonstrate the existence of such climatologically relevant changes and to test models that would enable us to interpret and, possibly, to predict these changes.
Solar ultraviolet radiation induced variations in the stratosphere and mesosphere
Authors: Hood L. L.
Published: February 2008

Solar ultraviolet induced perturbations of the middle atmosphere occurring on the solar cycle time scale have received the most theoretical attention in the past because of the need for comparison with predicted anthropogenic trends or for evaluation of possible climatological consequences. However, short‐term perturbations occurring on time scales comparable to the solar rotation period are more readily observable at present. Studies of these perturbations allow basic tests of our understanding of the relevant physics and chemistry that are needed for more accurate long‐term model predictions. Detection of short‐term solar UV induced ozone and/or temperature responses is hindered even at low latitudes by endogenic dynamical forcing which results in an inverse phase relationship (for either ozone or temperature) with higher‐latitude variations for many events. Nevertheless, consistent correlative evidence for contributions of solar UV variability to ozone temporal behavior in the upper stratosphere and lower mesosphere has been obtained in recent years. The mean amplitude of the ozone response at low latitudes reaches a maximum near the 3‐mbar level of approximately 0.5% for a 1% change in the solar flux at 205 nm. The phase lag of the ozone response relative to the 205‐nm flux increases with decreasing altitude and is positive below 3 mbar. Above 3 mbar, increasingly negative lags are measured (i.e., the ozone maximum leads the UV maximum). The increasingly negative lags imply the existence of a positive temperature perturbation following the UV maximum that acts to reduce the amplitude of the ozone response during the latter part of the ozone response cycle. Weak correlative evidence for the presence of the inferred temperature perturbations has been obtained in some studies. It is shown that these temperature perturbations have approximately the correct amplitudes and phase lags needed to explain the negative ozone lags. The derived positive temperature phase lags are approximately twice as large as would be predicted by one‐dimensional models that consider only radiative‐photochemical coupling. The importance of dynamical coupling in producing these temperature perturbations is therefore indicated. A possible major source of dynamical coupling is alteration of the reflection‐transmission properties of planetary waves, a process for which some observational evidence exists on the solar rotation time scale. Finally, in regions where transport effects on ozone concentration may be neglected, simultaneous measurements of ozone, temperature, and solar UV flux can be combined to calculate parameters that are directly relatable to photochemical theory. These are the odd oxygen photochemical relaxation time (approximately half of the conventional odd oxygen lifetime) and the chemical sensitivity of odd oxygen to local temperature changes. It is therefore possible that observed upper stratospheric and lower mesospheric responses to measured solar irradiance changes will be useful to constraining photochemical theory.