bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of mesoscale ocean features in the northeast Pacific using Geosat radar altimetry data
Authors: Matthews Paul E. , Johnson Mark A. , O'Brien James J.
Published: February 2008

In this paper, Geosat altimetry data from the northeast Pacific are examined. The data are (1) binned data on 1° latitude by 2° longitude blocks and (2) collinear track data. The binned data cover the region from 20°N to 60°N from the North American coast westward to 150°W and span from December 15, 1986, to June 1, 1989. To examine the data for evidence of westward propagation of sea level anomalies, three zonal transects (45°N, 49°N, and 57°N) are made. Complex empirical orthogonal function analysis of the data transects at these latitudes reveals phase speed and amplitude information. The first eigenmode at each latitude has a very strong annual signal. Two‐dimensional spectral analysis of the first eigenmode at 45°N and 57°N shows westward propagation of sea level anomalies at 45°N only. The propagating features are annual in period and of order 1000 km in length. At 57°N, westward propagation is evident (as at 45°N), but there is also an eastward component. To examine the variability of the coastal sea level signal, collinear track data from three ascending tracks crossing the Gulf of Alaska are analyzed revealing mesoscale features of the order of 100 km that parallel the Canadian and Alaskan coast. The most notable feature is the Sitka eddy. Westward propagation of several of these features is observed. The results from the analysis of both types of Geosat data are compared with other observational data.
Annual sea level variations in the southern tropical Indian Ocean from Geosat and shallow‐water simulations
Authors: Périgaud Claire , Delecluse Pascale
Published: February 2008

Sea level variations in the Indian Ocean north of 20°S are analyzed from Geosat satellite altimeter data in 1987–1988. These observed variations are compared with numerical simulations from a reduced‐gravity model forced by observed winds over the same period. The first complex empirical orthogonal function of observed and simulated variations is an annual signal. For this signal, observations and simulations are highly correlated in both time and space. Off‐equatorial sea level variations propagate westward and poleward as Rossby waves. The strongest annual variations occur in the southeastern tropical Indian Ocean. The maximum amplitude (∼12 cm) is located at about 90°E, 12°S, although the wind stress curl is weak there and east of it. The signal propagates from the eastern boundary to the southwest across almost all the basin.
The western boundary current of the seasonal subtropical gyre in the Bay of Bengal
Authors: Shetye S. R. , Gouveia A. D. , Shenoi S. S. C. , Sundar D. , Michael G. S. , Nampoothiri G.
Published: February 2008

Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March‐April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well‐defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January‐May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.
On the role of humidity in estimating marine surface layer stratification and scatterometer cross section
Authors: Geernaert G. , Larsen S.
Published: February 2008

Estimates of any given flux (momentum, heat, or moisture) that use the full set of diabatic profile relations applicable to the marine atmospheric surface layer require an accurate representation of flux coefficients, diabatic parameters, roughness lengths, and mean values for all meteorological quantities. Calculation of the momentum flux, for example, requires that the stratification function include both temperature and humidity effects and that the solution to the equation set be based on full set iteration. We find that over warm water, particularly during cases of low humidity, the momentum flux and stratification estimates are very sensitive to the reported relative humidity. Similarly, we find that the ERS 1 radar cross section, when treated as a function of the wind stress (or momentum flux), varies significantly with relative humidity, particularly when surface temperatures are warm. This study suggests that any calibration/validation campaign using remote‐sensing observations, in particular the ERS 1 scatterometer, should utilize high‐quality ground truth relative humidity measurements in addition to the traditional suite of high‐quality wind and temperature gradient measurements.
Physical model of bathymetric effects on the Antarctic circumpolar current
Authors: Boyer Don L. , Chen Rui‐Rong , Tao Lijun , Davies Peter A.
Published: February 2008

Laboratory experiments were conducted to simulate some of the effects of the bathymetry of the southern ocean on the physical characteristics of the Antarctic Circumpolar Current (ACC). An idealized zonal wind stress, which varied inversely with the distance from the model Antarctic continent, was simulated in the laboratory model by a radially inward sink‐source flow in a thin layer along the surface of the circular test cell. The experiments demonstrated the feasibility of simulating the surface wind stress in physical models by source‐sink forcing. The present model, however, has the limitation of not accounting for such factors as the longitudinal variations in the wind shear and the decrease in wind stress on approaching the Antarctic continent from the north. Planetary beta effects were neglected because the topographic beta term can be shown to dominate over large portions of the model area. The neglect of beta effects is also a limitation of the model. In spite of these limitations, however, the simulations of the physical model for both the homogeneous and linearly stratified cases were shown to be in good agreement with observations of the ACC. These include well‐defined strong currents along the mid‐ocean ridge; strong perturbations in the vicinity of the Macquarie Ridge, Campbell Plateau, and Kerguelen Gaussberg Plateau; strong meridional transport to the east of the Drake Passage; and anomalies to the south (wave troughs) and to the north (wave ridges) of the main circumpolar current over ocean basins and mountain ridges, respectively. It was shown that the Eltanin and Udintsev fracture zones in the vicinity of 135°W are important factors in directing the ACC eastward across the Southeast Pacific Basin to the Drake Passage. The model results suggest that these fracture zones can influence the nature of the flow in the western Atlantic because of their control of the flow through the Drake Passage. The estimated volume transports through the Drake Passage based on the model results are in fair agreement with oceanic observations. Estimates of the spin‐up time of the system for homogeneous and stratified cases have been provided.
Can the weak surface currents of the Cape Verde frontal zone be measured with altimetry?
Authors: Zlotnicki Victor , Siedler Gerold , Klein Birgit
Published: February 2008

Three data types are compared in the low‐current‐velocity regime in the southeastern North Atlantic, between 12°N and 30°N, 29°W and 18°W: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic heights derived from hydrographic data from cruises 4, 6, and 9 of the research vessel Meteor. The four current meter daily time series, at depths around 200 m, were smoothed over 1 month; the altimetric geostrophic velocities were computed from sea surface slopes over 142 km every 17 days. The correlation coefficients between the current meter and altimetric geostrophic velocities range between 0.64 and 0.90 for the moorings near 29°N but between 0.32 and 0.71 for the two around 21°N; the associated rms discrepancies between the two measurement types range between 1.5 and 4.4 cm/s, which is 49% to 127% of the rms of the respective current meter time series. Dynamic heights relative to 1950 dbar for the months of November 1986 (dM4), November 1987 (dM6), and February 1989 (dM9) were computed from Meteor cruises 4, 6, and 9. Both dynamic heights and altimetric heights (hM4, hM6, hM9) were averaged over 1° boxes for the duration of each cruise. Differences dM4 − dM6 and dM9 − dM6 were computed only at bins where at least one station from both cruises existed. Assuming that dynamic heights d in dynamic centimeters are equivalent to sea level h in centimeters, the standard deviation σ of the differences ((hM4 − hM6) − (dM4 − dM6)) and corresponding M9 − M6 values was 2.1 cm. This value (squared) is only 13% of the (5.8 cm)2 variance of the dynamic height differences and is indistinguishable from the 2.7‐ to 5.6‐cm natural variability of sea level in the area expected between the times when the ship and the satellite sampled the ocean. The areally averaged discrepancy for M9 − M6 was only 0.7 cm, but the corresponding value for M4 − M6 was 5.2 cm. A systematic difference between the water vapor corrections used before and after July 1987 is responsible for the M4 − M6 difference. The average M4 − M6 discrepancy is only 0.1 cm using the Fleet Numerical Oceanography Center correction, with a standard deviation of 3.1 cm. In spite of the underlying differences in sampling and physics, including unknown barotropic components not included in our hydrographic dynamic heights, and in data errors, including water vapor, ionospheric, and orbital effects on the altimetry, consistent interannual changes of the mean sea level from the independently obtained altimetric and hydrographic data sets are obtained, and correlated seasonal changes in surface currents are observed with both altimetry and current meters.
Note on the reduction of the “full‐wave” method for rough surface scattering to the small‐height limit
Authors: Thompson D. R. , Chapman R. D.
Published: February 2008

The form of the equations for the backscattered fields used in the analysis of Thorsos and Winebrenner (Journal of Geophysical Research, 96, 17,107–17,121, 1991) and that of Bahar (Journal of Geophysical Research, 96, 17,123–17,131, 1991) in the limit as surface heights and slopes become small (SP limit) is examined. Both of these analyses start from the expression for the total full‐wave scattered field as given by Bahar (IEEE Transactions on Antennas and Propagation, AP‐28, 11–21, 1980). In his analysis, Bahar subtracts from this total field a so‐called diffraction field that he claims is important only near the specular direction. If this claim were correct, then such a subtraction should have little effect on the computed cross section for scattering directions away from the specular direction. In the SP limit, however, we show that Bahar's diffraction field contributes to the backscatter cross section in all directions. The subtraction of the diffraction field exactly cancels the second‐order moments responsible for the difference between the limiting cross sections found in the two analyses. It is because of this subtraction that Bahar is able to show agreement with the SP limit. Furthermore, we find that Bahar's expression does not yield the correct diffusely scattered field from a tilted planar surface or from a smooth cosine surface with zero mean slope. The failure of Bahar's expression for these simple cases indicates to us that even though it does reduce to the proper SP limit, it is not a correct general expression for the diffusely scattered, horizontally polarized field backscattered from a perfectly conducting, one‐dimensionally rough surface as is claimed in his article.
On improving the bed stress formulation in storm surge models
Authors: Davies A. M. , Jones J. E.
Published: February 2008

A method of improving the representation of bottom stress in storm surge models (an “enhanced” bed stress model) is tested using a fine‐mesh model of the Celtic and Irish seas and is compared with results from a “conventional” two‐dimensional model. The approach uses a convolution method to determine the bed stress, rather than relating it directly to the depth mean current, as in a “conventional” storm surge model. For the region considered, comparisons of surge elevations computed with both models, and observations, show only a slight improvement of surge elevation in the near‐coastal region with the “enhanced” bed stress model. However spatial distributions of bed stresses from the two models do exhibit significant differences which are important in considering sediment movement paths during major storms. The magnitude and direction of the bed stress vectors computed from the “enhanced” bed stress model are in excellent agreement with calculations using a full three‐dimensional model of the area. This suggests that the “enhanced” bed stress model can compute a bed stress that is not significantly different from that in a full three‐dimensional model, at a fraction of the computational cost.
A three‐dimensional numerical model of cloud dynamics, microphysics, and chemistry: 2. A case study of the dynamics and microphysics of a severe local storm
Authors: Wang Chien , Chang Julius S.
Published: February 2008

A strong convective storm has been modeled by using our three‐dimensional cloud model (Wang and Chang, this issue). The maximum updraft is about 30 m/s and the maximum Ze is 72.3 dBZ. The modeled precipitation feature and three‐dimensional radar reflectivity factors are closed to the observed data. The whole life cycle of this modeled storm can be divided into three stages: the “explosive growth” stage (∼10 ‐ 24 min of model time), the mature stage (∼24 ‐ 55 min of model time), and the static stage (after 55 min of model time). Almost all precipitated water categories grew fast during the explosive growth stage. The latent heat release during this stage produced strong updraft. In the mature stage, precipitation inside the cloud gradually changed from liquid‐phase to ice‐phase. Both graupels and hailstones initially formed around 10 km in height and inside the initial cell. Only in the later stages were they transported or fallen to other regions. Some new cores of hail were found in other regions. In the static stage, precipitation weakened. Ice crystals formed rapidly when the plume of the storm spread at the higher level. These crystals then fell into the lower levels of the storm. The modeled storm possessed a cyclonically‐rotating updraft. The inflow air of the storm came from different regions of the model domain. A short (less than 20 min) heavy precipitation period appeared during the simulation. Most of the precipitation came from ice‐phase particles, primarily the graupels and hailstones. The maximum precipitation rate occurring in a single grid point was about 989 mm/hr, while the domain‐averaged rate was about 3 mm/hr. Heavy precipitation and the downdraft also produced one high‐temperature region and two low‐temperature regions near the surface.
Measurements of atmospheric carbonyl sulfide during the NASA Chemical Instrumentation Test and Evaluation project: Implications for the global COS budget
Authors: Johnson James E. , Bandy Alan R. , Thornton Donald C. , Bates Timothy S.
Published: February 2008

Atmospheric COS concentrations were measured by three analytical systems during the Chemical Instrumentation Test and Evaluation (CITE 3) project. The three systems all used cryogenic sample preconcentration and gas chromatographic (GC) separation but differed in the method of detection. The FPD system used a flame photometric detector, the MS system used a mass selective detector, and the ECD‐S system used a fluorinating catalyst followed by an electron capture detector. With the FPD system, we found a mean COS concentration of 510 ppt over the North Atlantic and 442 ppt over the Tropical Atlantic. With the ECD‐S system, we found a mean COS concentration of 489 ppt over the North Atlantic and 419 ppt over the Tropical Atlantic. All three systems registered a latitudinal gradient in atmospheric COS of between 1.6 and 2.0 ppt per degree of latitude, with increasing COS concentrations northward which was similar to the gradient measured by Bingemer et al. (1990). It is difficult to reconcile the measured latitudinal concentration gradient with present theories of the global COS budget since the largest sink of COS is thought to be a flux to land plants, most of which are in the northern hemisphere.
Paleoclimatic tracers: An investigation using an atmospheric general circulation model under ice age conditions: 1. Desert dust
Authors: Joussaume Sylvie
Published: February 2008

Many studies with atmospheric general circulation models (AGCMs) have demonstrated their usefulness in reconstructing past climates. In a new approach, we have used an AGCM to investigate the link between tracer cycles and climate. We consider in this paper the atmospheric cycle of windblown dust material from desertic areas and in part 2 the water isotope cycles. Studies from ice cores have shown a strong increase of the dust deposits during glacial periods, both over East Antarctica and Greenland. We do not know, however, whether this past increase is global or just a local feature, where the dust came from, and what are the mechanisms yielding this increase. We try to answer to these questions by using an AGCM including a model for the desert dust cycle to simulate the present‐day and Last Glacial Maximum climates for February and August. The model simulates only a weak increase of the global atmospheric dust content. Stronger variations are obtained at a regional scale and are in good agreement with observations from deep‐sea sediments. However, the model does not reproduce the great increase of the dust concentration in snow that has been observed in ice cores. Several model deficiencies can induce this model‐data discrepancy as, for example, inaccuracies of the circulation patterns or of the dust model. However, most likely, the model fails to simulate the actual sources of dust.
A numerical study of the diurnal variation of the Adelie Land katabatic wind regime
Authors: Parish Thomas R. , Pettré Paul , Wendler Gerd
Published: February 2008

Katabatic winds occur with great frequency over the Adelie Land sector of Antarctica. Data collected by automatic weather stations (AWSs) along a string of stations from the coast of Adelie Land to the high interior plateau show a marked diurnal cycle of the wind and temperature regimes of the lower atmosphere during the short austral summer period. Numerical simulations of the katabatic wind regime have been conducted for a variety of solar forcing and synoptic conditions assuming a clear sky environment. Results suggest that the katabatic wind is a robust feature of the Antarctic boundary layer. Significant disruption of the low‐level drainage features occurs only during the midsummer months owing to the solar heating of the ice slopes and from synoptic activity along the Adelie Land coastline. During midsummer, modeled ground temperatures undergo a diurnal cycle with an amplitude of 15°C near the coast, in good agreement with the AWS observations. The katabatic wind decreases considerably during the middle of the day; wind speeds at coastal sites become reduced by more than one half of the early morning values. The numerical simulations depict a rapid summer to winter transition of katabatic wind conditions over the 1‐month period from mid‐February to mid‐March. The surface energy budget becomes negative for the entire 24‐hour period after mid‐February, corresponding to the rapid onset of the wintertime katabatic wind regime.
Optical properties of marine stratocumulus clouds modified by ships
Authors: King Michael D. , Radke Lawrence F. , Hobbs Peter V.
Published: February 2008

The angular distribution of scattered radiation deep within a cloud layer was measured in marine stratocumulus clouds modified by the emissions from ships. These observations, obtained at 13 discrete wavelengths between 0.5 and 2.3 μm, were acquired as the University of Washington C‐131A aircraft flew through a pair of roughly parallel ship track signatures produced in clouds off the coast of southern California on July 10, 1987. In the first of these ship tracks, the nadir (upwelling) intensity increased from 40 to 110 W m−2 μm−1 sr−1 at 0.744 μm. The second ship track produced a less dramatic, but more uniform, increase in the upwelling intensity. In contrast, the nadir intensity at 2.20 μm decreased from 1 to 0.13 W m−2 μm−1 sr−1 in the first ship track and to 0.6 W m−2 μm−1 sr−1 in the second ship track. The relative angular distribution of the intensity field at each wavelength was used to determine the similarity parameter, and hence single scattering albedo, of the cloud using the diffusion domain method. Besides the spectral similarity parameter, these measurements provide a good estimate of the optical depth of the cloud layer both above and below the aircraft. Results of this analysis are presented for a 120‐km section of marine stratocumulus cloud including both ship tracks. This analysis shows that the total optical thickness of the cloud layer increased in the ship tracks, in contrast to the similarity parameter which decreased. The decrease in absorption was a direct consequence of the reduction in cloud droplet size that occurred within the ship tracks.
Lidar observations of the mesopause region temperature structure at Urbana
Authors: Bills Richard E. , Gardner Chester S.
Published: February 2008

Fourteen nights of high‐resolution Na temperature lidar data were collected at the Urbana Atmospheric Observatory (40°N, 88°W) over a 6‐month period from January through June 1991. The temperature structure in the 80‐ to 105‐km region exhibited strong variability due to the influence of gravity waves, tides, and sporadic Na layers (Nas). Extreme temperatures of 140 K and 255 K at 90 km were observed. A cooling rate of 8 K/month was exhibited from winter to summer at 85–90 km, and in late May and early June a well‐defined cold mesopause formed near 85 km with temperatures typically between 150 and 165 K. This mesopause structure produced an associated enhancement in the Brunt‐Vaisala frequency near 90 km. The average Brunt‐Vaisala period decreased from 5.2 min during the winter months to 4.3 min during spring and early summer. Na density and temperature perturbations are 180° out of phase on the top side of the Na layer and in phase on the bottom side as predicted by Gardner and Shelton (1985). The rms relative temperature and horizontal wind perturbations averaged 4.7% and 19.8 m/s, respectively.
Properties of austral winter clouds derived from radiometric profiles at the South Pole
Authors: Stone Robert S.
Published: February 2008

Although clouds are known to radiatively modulate the energy budget of Antarctica, particularly during the polar night, very little is known about their physical, radiative, or microphysical properties. A unique set of measurements consisting of infrared flux profiles (radiometersonde data) collected at the south pole is analyzed in conjunction with ancillary meteorological observations to gain a better understanding of austral winter clouds. Distinct radiometric features associated with cloud boundaries are used to estimate the heights, thicknesses, and temperatures of selected cloud systems. The physical cloud properties are combined with infrared flux measurements to formulate the thermal energy budgets of the cloud layers and to derive their bulk radiative properties. By comparing these derived radiative properties with theoretically computed values for model clouds varying in microphysical characteristics the clouds' ice contents and effective particle sizes are also inferred. Austral winter clouds are found to be moderately thick and on average extend to heights greater than 3 km above the top of the surface‐based inversion layer. They are optically thin, however, and nonblack, having mean effective emissivities of about 0.6. Because they are cold (≤ −40°C) these clouds are composed of small ice particles, have low total ice contents, and contain little or no liquid water. Overall, they are similar to high‐level cirrus clouds observed in the mid‐latitudes. The characteristic radiometric features that distinguish overcast from clear sky conditions at south pole are discussed; mean profiles of infrared fluxes, temperatures, vector winds, and heating rates are presented; and a summary is provided of cloud properties that may be of use to climate modelers.
Estimation of solar backscatter ultraviolet albedo using ground‐based Umkehr measurements
Authors: DeLuisi John J. , Longenecker David U. , Mateer Carlton L. , Chu William P.
Published: February 2008

A retrieval method was developed to estimate the solar backscatter ultraviolet (SBUV) satellite albedo for the ozone profiler wavelengths using ground‐based ultraviolet measurements. For the present investigation the Umkehr was used as the ground‐based ultraviolet measurement. Simulated SBUV data and Umkehr data theoretically computed from a priori ozone profiles observed by the SAGE II satellite were used to develop the retrieval algorithm and to test its capability. The test indicated that albedos for the SBUV ozone profiler wavelengths should allow estimates to a precision of ±5% or better, depending on the accuracy of the ultraviolet measurement. Retrievals using actual Umkehr observations were also performed to provide a preliminary look at the magnitude and annual variation of retrieved albedos. A case study was performed, comparing retrieved albedos with SBUV‐measured albedos. The SBUV albedo change was seen to be approximately twice as large as the albedo changes estimated by the Umkehr method. Results of the investigation suggest that the method of estimation may be useful for determining the drift rate of the SBUV calibration.
The total reactive oxidized nitrogen levels and the partitioning between the individual species at six rural sites in eastern North America
Authors: Parrish D. D. , Buhr M. P. , Trainer M. , Norton R. B. , Shimshock J. P. , Fehsenfeld F. C. , Anlauf K. G. , Bottenheim J. W. , Tang Y. Z. , Wiebe H. A. , Roberts J. M. , Tanner R. L. , Newman L. , Bowersox V. C. , Olszyna K. J. , Bailey E. M. , Rodgers M. O. , Wang T. , Berresheim H. , Roychowdhury U. K. , Demerjiani K. L.
Published: February 2008

During the late summer and early fall of 1988, measurements of many trace species of tropospheric photochemical interest, including NO, NO2, PAN, HNO3, NO3−; NOY, and ozone were made at seven surface stations in the eastern United States and Canada. The NOY (as well as ozone) levels and its partitioning were strongly influenced by the diurnal evolution of the boundary layer at the sites that are beneath the nocturnal inversion. At the higher elevation sites the median levels of all species were much more nearly constant. During the daytime the median NOY levels were 2 to 5 ppbv at all sites, which may be representative of rural areas in the populated regions of eastern North America. Each site showed variations in the NOY levels of an order of magnitude or more. Measurements from all of the sites are consistent with the major contributors to NOY being NOX (the sum of NO and NO2), PAN, and nitric acid with a minor contribution from aerosol nitrate. At the lower elevation sites the median [NOX] to [NOY] ratios were 70% or more during the night and declined to minima of 25 to 40% during the day. During the daytime the ranges of the median contributions of PAN and HNO3 to NOY were 12 to 25% and approximately 20 to 30%, respectively. The distributions of the contributions about these medians are discussed. Results from all of the sites are consistent with the individually measured species accounting for about 90% of the simultaneously measured NOY.
Measurements of inductive charges during drop breakup in horizontal electric fields
Authors: Canosa E. Freire , List Roland
Published: February 2008

Waterdrop pairs with initial diameters of 2.1 and 1.7 mm were collided at inclinations < 3° from the vertical after attaining fall speeds of 3.4 and 2.1 m s−1, respectively, and broke up in either negligible or strong electric fields (50 k V m−1). The colliding waterdrops had initial charges < 0.1 pC. In a horizontal electric field of < 100 V m−1 the fragments were not charged measurably by noninductive processes (< 0.05 pC). However, at 50 kV m−1 the waterdrops acquired charges up to 5 pC by induction. All breakups observed between the drop pairs resulted in “disjection”, that is a separation with the largest fragment staying below the other(s). The results, based mainly on filament breakup, over mechanically limited disjection angles from 69° to 111° (given by the connecting line between the centers of the two largest fragments at the moment of separation and the horizontal) provide experimental evidence of a cosine dependence of inductive charging in the presence of horizontal electric fields. They also support application of a theoretical equation developed by Latham and Mason (1962) for rebounding rigid spherical conductors. The results can be generalized to include all raindrop‐raindrop collision/breakups involving drop pairs with diameters > (1.8;0.4 mm) and to all electrical field directions because fields up to 50 kV m−1 do not affect the breakup configurations. A particular conclusion is that breakups of colliding raindrops, in the size and voltage ranges quoted, aid in the dissipation of vertical electric fields in the atmosphere.
The radiative budgets of a tropical mesoscale convective system during the EMEX‐STEP‐AMEX experiment: 1. Observations
Authors: Wong Takmeng , Stephens Graeme L. , Stackhouse Paul W. , Valero Francisco P. J.
Published: February 2008

This paper is the first of a series of two that aim to describe the spatial and temporal variation of the radiative heating associated with tropical mesoscale convective systems (MCSs). This paper describes the analysis of data collected in and around a tropical cloud cluster system studied as part of the Equatorial Mesoscale Experiment (EMEX). The data analysis indicates that the cluster originated off the northern coast of Australia along the midlevel monsoon trough and lasted approximately 12 hours. The system moved with a velocity of about 12 m/s toward the northeast and the low‐level surface northwesterly flow at the vicinity of the premonsoon trough area seems continuously to feed the EMEX 9 cloud cluster with energetic warm, moist equatorial air. Data obtained from aircraft penetrations show features similar to tropical MCSs reported elsewhere (such as an area of strong to moderate convection surrounded by a broad region of stratiform precipitation, radar echo bright band in the stratiform region, “onion” type sounding behind the convective region). The vertical structures of the EMEX 9 cloud cluster also contain two types of imbedded convection: an upright vertical structure and a pronounced rearward slope (approximately 17°), having a vertical extent of 14.5 km and above and a horizontal scale of about 40 km. The cloud base and cloud top altitude in the stratiform region are estimated to be of the order of 4.8 km and 15 to 16 km, respectively. The composite aircraft shortwave radiation data from the stratiform region show a significant attenuation of shortwave flux through the cloud (the estimated transmission is 14% at cloud base). The upward and downward solar flux profiles are almost parallel to each other in the atmosphere inside and below the cloud base, suggesting very little solar heating in these regions. The upward and downward infrared radiation fluxes measured in the tropical MCS also show little infrared heating above and below the cloud cluster.
The tropical experiment of the Stratosphere‐Troposphere Exchange Project (STEP): Science objectives, operations, and summary findings
Authors: Russell P. B. , Pfister L. , Selkirk H. B.
Published: February 2008

The Stratosphere‐Troposphere Exchange Project Tropical Experiment (STEP Tropical) investigated stratosphere‐troposphere exchange and dehydration processes in that region and season with the coldest average tropopause temperatures, the tropical western Pacific and northern Australia during the winter monsoon (January‐February) of 1987. This is also a period of extensive convective activity and rainfall. In addition to this primary goal, STEP Tropical (1) extended stratospheric aircraft chemical tracer surveys to the southern hemisphere subtropics and mid‐latitudes and (2) sampled the anticyclone that dominates the circulation over Australia in the lower stratosphere during the winter monsoon. Advanced fast‐response instruments on NASA's ER‐2 aircraft measured meteorological variables, stratospheric and tropospheric tracers, particles, and radiative fluxes. ER‐2 sorties included six ferry flight legs across the Pacific and eleven flights in the Australian region. The 1986–1987 monsoon was atypical in that (1) its date of onset, January 14, was 3 weeks later than normal and (2) it was unusually intense and sustained once it did arrive. As a consequence, almost all the flights were conducted under monsoon conditions, with only limited sampling of premonsoon and break‐monsoon continental convection. Illustrations show flight paths for each sortie on satellite images and on 100 hPa synoptic flow charts, as well as the timing of flights with respect to overall cloudiness in the Australian region. STEP Tropical results, reported in the accompanying set of papers, include (1) observational documentation of a convective scale cold trap that dries air of recent tropospheric origin to prevailing stratospheric minimum water vapor mixing ratios (3 ppmv or less); (2) indications that this drying mechanism can be effective not only in the anvils of the tallest clouds (which occur during “break‐monsoon” conditions) but also in the anvils of tropical cyclones and monsoon mesoscale convective systems; (3) demonstration that convectively generated gravity waves account for a significant part of the momentum forcing of the semiannual and quasi‐biennial oscillations in the stratosphere; and (4) documentation of an upper tropospheric source of NOy and a significant upward flux of NOy at the tropical tropopause. For monsoon convection to be an important contributor to the net transfer of tropospheric air to the stratosphere, a mechanism is required to move air from the cloud tops to significantly greater heights. STEP Tropical results suggest that gravity‐wave‐induced small‐scale turbulence or radiative heating of anvils provides this mechanism. Observational proof of the theoretically required high heating rates remains to be obtained.
TOVS observations of a stratospheric cooling during the CHEOPS 3 campaign: February 4–6, 1990, over Scandinavia
Authors: Claud Chantal , Ovarlez Joelle , Chedin Alain , Scott Noelle A.
Published: February 2008

The Chemistry of Ozone in the Polar Stratosphere (CHEOPS 3) campaign has been conducted in Scandinavia in winter 1989–1990 for investigating the composition of the lower Arctic stratosphere during winter, that is, when the temperatures are the lowest and when polar stratospheric clouds (PSCs) are most likely to occur. Actually, on February 6, temperatures as low as 180 K at 22 km have been measured above Kiruna, while at the same time, reduced ozone amounts have been reported in some stations in Europe. Global observations from the satellites of the TIROS N series during the period from February 4 to 6, 1990, have been analyzed using the improved initialization inversion (3I) algorithm. The use of both the infrared (HIRS2) and the microwave (MSU) radiometric observations allows for the determination of the temperature profile up to about 10 hPa (about 30 km) along with other atmospheric parameters (air mass type, cloud occurrence, etc.). The impact of the stratospheric cooling on measured radiances, colocations between 3I retrieved temperature profiles and radiosonde reports, as well as comparisons with analyzed temperatures in the low stratosphere provided by the Stratospheric Research Group of the Free University of Berlin are presented. The stratospheric cooling is resolved by the TOVS but with a lower amplitude than measured by the radiosondes. Retrieved mean layer temperatures and air mass type give evidence of upper tropospheric forcing partly responsible for the ozone loss.
Thermal structure and dust loading of the Martian atmosphere during late southern summer: Mariner 9 revisited
Authors: Santee Michelle , Crisp David
Published: February 2008

The temperature structure and dust loading of the Martian atmosphere are investigated using thermal emission spectra recorded by the Mariner 9 infrared interferometer spectrometer (IRIS). The analysis is restricted to a subset of the IRIS data consisting of approximately 2400 spectra in a 12‐day period extending from LS =343° to LS =348°, corresponding to late southern summer on Mars. Simultaneous retrieval of the vertical distribution of both atmospheric temperature and dust optical depth is accomplished through an iterative procedure which is performed on each spectrum. The inclusion of dust opacity in the retrieval algorithm causes the retrieved temperatures to change by more than 20 K in some atmospheric layers. The largest column‐integrated 9 μm dust optical depths (∼ 0.4) occur over the equatorial regions. The highest atmospheric temperatures (> 260 K) are found at low altitudes near the subsolar latitude (∼ 6°S) while the coldest temperatures (< 150 K) are found at levels near 1.0 mbar over the winter pole. A comparison of temperature maps for 1400 LT and 0200 LT indicates diurnal temperature variations as large as 80 K at low altitudes near the subsolar latitude, whereas diurnal temperature changes at pressures less than 1.0 mbar are typically about 10 K. Both dayside and night side temperatures above about 0.1 mbar (∼ 40 km) are warmer over the winter (north) polar region than over the equator or the summer (south) polar region. This thermal structure suggests the existence of a net zonally averaged meridional circulation with rising motion at low latitudes, poleward flow at altitudes above 40 km, and subsidence over the poles. Because a meridional circulation transports atmospheric constituents as well as heat, it has significant implications for the net flux of dust and water into the polar regions.
Infrared spectroscopy of homogeneously nucleated hydrazine aerosols: Disordered and crystalline phases
Authors: Dunder T. , Clapp M. L. , Miller R. E.
Published: February 2008

Fourier Transform Infrared Spectroscopy has been used to study homogeneously nucleated hydrazine (N2H4) aerosols formed in a variable temperature flow cell. The spectra contain both resonant absorptions and Mie scattering features which allow detailed information to be obtained on the phase of the aerosol particles and the size distribution that is formed. In addition, the relative intensities of the absorption and scattering features can be used as a qualitative probe of the density of the particles. A disordered phase is observed at cell temperatures ranging from 230 to 180 K and a transition to a crystalline phase occurs over the temperature range 180–175 K. At lower temperatures the infrared spectra become progressively narrower, indicating that the quality of the crystals improves as the temperature is lowered. The result is previously unobserved details in the hydrazine N‐H stretch vibrations of the crystalline solid. The high quality of the spectra obtained under these conditions suggests that at the high condensation rates characteristic of the present experiments, self‐annealing yields highly crystalline particles. Mie calculations, based upon the complex refractive index data for amorphous solid hydrazine, are shown to have utility in modeling the infrared spectrum of the disordered phase, which we tentatively assign as a supercooled liquid.
Response of deep ocean circulation to initiation of northern hemisphere glaciation (3–2 MA)
Authors: Raymo M. E. , Hodell D. , Jansen E.
Published: February 2008

Carbon isotopic records from benthic foraminifera are used to map patterns of deep ocean circulation between 3 and 2 million years ago, the interval when significant northern hemisphere glaciation began. The δ18O and δ13C data from four Atlantic sites (552, 607, 610, and 704) and one Pacific site (677) show that global cooling over this interval was associated with increased suppression of North Atlantic Deep Water (NADW) formation. However, the relative strength of NADW production was always greater than is observed during late Pleistocene glaciations when extreme decreases in NADW are observed in the deep North Atlantic. Our data indicate that an increase in the equator‐to‐pole temperature gradient associated with the onset of northern hemisphere glaciation did not intensify deepwater production in the North Atlantic but rather the opposite occurred. This is not unexpected as it is the “warm high‐salinity” characteristic, rather than the “low temperature,” of thermocline waters that is critical to the deepwater formation process in this region today.
Signal power for radio acoustic sounding of temperature: The effects of horizontal winds, turbulence, and vertical temperature gradients
Authors: Lataitis Richard J.
Published: February 2008

A simple expression for the expected average signal power of a radio acoustic sounding system (RASS) comprising a monostatic pulsed Doppler radar and a continuous‐wave broadbeam acoustic source is developed. The effects of horizontal winds, atmospheric turbulence, and vertical temperature gradients are included. Under ideal conditions (i.e., in the absence of winds, turbulence, and gradients) the received signal power is a maximum and, for a broadband acoustic source, is predicted to be proportional to the radar range resolution and inversely proportional to the acoustic bandwidth and square of the range R. Turbulence‐induced distortion of the acoustic wave fronts yields the expected R−26/5 range dependence, provided winds are light. This result is based on the assumption that the turbulence strength does not vary with range. For the more realistic case of turbulence concentrated primarily within the boundary layer (R ≲ 1 km) a much weaker range dependence at longer ranges is predicted. A flattening of the acoustic wave fronts by temperature gradients typical of unstable midday conditions results in an R−6 range dependence, provided winds are light. Turbulence effects are predicted to dominate at shorter ranges (R ≲ 8 km) and gradient effects at longer ranges. The expected compensation of power loss associated with the wind‐induced lateral displacement of the acoustic wave fronts by the turbulence‐ and gradient‐induced broadening of the displaced RASS focal spot is also predicted.
Application of wave guide propagation to selection of transmitter power and frequency
Authors: Ferguson Jerry A. , Hansen Peder M.
Published: February 2008

This paper is concerned with developing reasonable criteria for the selection of frequency and power for very low and low‐frequency (VLF/LF) transmitting stations. The approach uses a wave guide model for low‐frequency propagation and accounts for the variability of the ionosphere. A sample problem involving a hypothetical transmitter is described.
Spaced antenna diversity in temperate latitude meteor burst systems operating near 40 MHz: Variation of signal cross‐correlation coefficient with time
Authors: Shukla Anil K. , Cannon Paul S. , Lester Mark
Published: February 2008

The potential for meteor burst (MB) spaced antenna diversity is studied using cross‐correlation techniques at a temperate latitude, using 37‐MHz signals scattered over an 800‐km path. Approximately 9 hours of data were analyzed from a 2‐day period in February 1990, when the antenna separation was 10λ. The correlation variations with time of signals with durations ≥ 0.75 s, which were categorized as “underdense,” “overdense,” or “not known” (NK), were investigated. The cross‐correlation coefficients of signals from underdense and overdense trails are high when correlated over their total signal envelopes, but this is not true when the signal envelope is segmented in time. NK signals are observed to be more uncorrelated than underdense or overdense signals and are, therefore, likely to be the most advantageous to a diversity system. Analysis of data from all three categories combined shows that 40% of signals with duration ≥ 0.75 s have correlation values of less than 0.6 after the first 0.25 s. The correlation‐time dependency observed for NK and underdense signals is not identified for the overdense signal category. It is proposed that for underdense and NK signals the correlation‐time dependency is due to the vector addition of other weak signal modes. These weak signal modes, however, have little effect on overdense signals which, typically, exhibit higher signal powers in the early stages of the signal envelope.
Asymptotic solutions of the wave equation for a two‐dimensional ionospheric model
Authors: Nalesso G. F. , Jacobson A. R.
Published: February 2008

The propagation of HF radio waves through a stratified ionosphere perturbed by dielectric irregularities varying both horizontally and vertically is studied in this paper. A method for obtaining two‐dimensional approximate solutions, complementary to the WKB approximation, is discussed and illustrated. Previous results of a one‐dimensional model, showing enhanced scintillation due to irregularities localized near the turning point of the wave, are confirmed. By matching the ionosphere with a uniform atmosphere the reflection coefficient for an incoming s‐polarized plane wave is calculated. The case of wave reflection by a rippled specular surface is finally derived in the limit of the ionospheric gradient length approaching zero.
Cerenkov radiation from a slow, relativistic electron moving uniformly through microwave ferrite
Authors: Mueller R. S.
Published: February 2008

The electromagnetic fields generated by a superluminal electron in longitudinally magnetized, microwave ferrite have been derived for a narrow band of frequencies in the X band. For a superluminal electron, Cerenkov radiation is emitted in two circular cones for the ordinary and extraordinary wave modes. Using a Huygens' construction of wavelets, conditions for Cerenkov radiation were related to the propagation of plane waves through the ferrite. It was found that a slow relativistic electron will radiate only in the extraordinary wave mode. It is too slow to generate radiation in the ordinary wave mode.
Statistical characterization of line‐of‐sight microwave links
Authors: Rana D. , Webster A. R. , Sylvain M.
Published: February 2008

A ray tracing approach with a statistical model of the atmospheric refractivity is used to get angle‐of‐arrival, amplitude and delay time distributions in a multipath situation. The atmospheric model considers a single superrefractive layer. Statistics of the layer parameters (height, intensity, and thickness) are derived from meteorological data. Ray tracing calculations include rays reflected from a flat ground. The influence of the various layer parameters as well as the physical origin of the rays are analyzed and discussed. Some experimental evidence relating to, and confirming, the predicted values is also considered.