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A. C. Poje and G. Haller

Abstract

New dynamical systems techniques are used to analyze fluid particle paths in an eddy resolving, barotropic ocean model of the Gulf Stream. Specifically, the existence of finite-time invariant manifolds associated with transient, mesoscale events such as ring detachment and merger is proved based on computer-assisted analytic results. These “Lagrangian” invariant manifolds completely organize the dynamics and mark the pathways by which fluid parcels may be exchanged across stream. In this way, the Lagrangian flow geometry of a detaching ring or a ring–jet interaction event, as well as the exact associated particle flux, is obtained.

The detaching ring geometry indicates that a significant amount of the fluid entrained by the ring originates in a long thin region on the far side of the jet and that this region extends as far upstream as the western boundary current. In the ring–stream interaction case, particle transport occurs both to and from the ring and is concentrated in thin regions on the near side of the jet and around the perimeter of the ring.

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S. A. Thorpe and A. J. Hall

Abstract

Side-scan sonars operating at 80–250 kHz have been deployed to produce narrow beams directed parallel and normal to shore on a gently sloping beach. These provide measurements of processes (such as wave propagation) seaward of the edge of the surf zone. Shoreward propagation of sound into the surf zone and hence useful information retrieval from this zone is prevented, however, by high bubble or suspended sediment absorption at its outer edge, as found in earlier Doppler sonar studies at 195 kHz by J.A. Smith. The Shoreward limit of acoustic propagation has a variable structure related to incident wave groups, the position at which waves break, and to dynamical processes within the surfzone determining the position of the bubble or suspended sediment boundary.

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P. A. Mandics and F. F. Hall Jr.

An acoustic echo sounder mounted on the NOAA ship Oceanographer during GATE proved to be a valuable tool for investigating the structure and dynamics of the tropical marine boundary layer up to 800 m in height. Under suppressed weather conditions the facsimile-recorded echo intensity returns depicted a mixed layer characterized by convective plumes rising from the surface of the water to 400 m. Disturbed weather events resulted in a substantial modification of the boundary layer; layered structures formed that at times limited the depth of the mixed layer to 100 m. The Doppler frequency shift of the acoustic returns made it possible to determine the vertical velocity field.

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J. S. Hall and L. A. Riley

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No abstract.

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Anne W. Nolin and Eileen A. Hall-McKim

Abstract

The interannual and intraseasonal variability of the North American monsoon is of great interest because a large proportion of the annual precipitation for Arizona and New Mexico arrives during the summer monsoon. Forty-one years of daily monsoon season precipitation data for Arizona and New Mexico were studied using wavelet analysis. This time-localized spectral analysis method reveals that periodicities of less than 8 days are positively correlated with mean daily precipitation during the 1 July–15 September monsoon period. Roughly 17% of the years indicate no significant periodicity during the monsoon period for either region and are associated with low monsoon precipitation. High- and low-frequency modes explain an equivalent percentage of the variance in monsoon precipitation in both Arizona and New Mexico, and in many years concurrent multiple periodicities occur. Wavelet analysis was effective in identifying the contribution of high-frequency modes that had not been discerned in previous studies. These results suggest that precipitation processes during the monsoon season are modulated by phenomena operating at synoptic (2–8 days) and longer (>8 days) time scales and point to the need for further studies to better understand the associated atmospheric processes.

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Rob A. Hall and Glenn S. Carter

Abstract

The M 2 internal tide in Monterey Submarine Canyon is simulated using a modified version of the Princeton Ocean Model. Most of the internal tide energy entering the canyon is generated to the south, on Sur Slope and at the head of Carmel Canyon. The internal tide is topographically steered around the large canyon meanders. Depth-integrated baroclinic energy fluxes are up canyon and largest near the canyon axis, up to 1.5 kW m−1 at the mouth of the upper canyon and increasing to over 4 kW m−1 around Monterey and San Gregorio Meanders. The up-canyon energy flux is bottom intensified, suggesting that topographic focusing occurs. Net along-canyon energy flux decreases almost monotonically from 9 MW at the canyon mouth to 1 MW at Gooseneck Meander, implying that high levels of internal tide dissipation occur. The depth-integrated energy flux across the 200-m isobath is order 10 W m−1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head, where internal tide energy escapes onto the shelf. Reducing the size of the model domain to exclude remote areas of high barotropic-to-baroclinic energy conversion decreases the depth-integrated energy flux in the upper canyon by 20%. However, quantifying the role of remote internal tide generation sites is complicated by a pressure perturbation feedback between baroclinic energy flux and barotropic-to-baroclinic energy conversion.

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A. Graham, D. K. Woolf, and A. J. Hall

Abstract

The population of bubbles produced by breaking waves in (10 m) winds of up to 12 m s−1 is analyzed using calibrated images from a vertical pencil-beam sonar system placed on the seabed near the Dutch coast. The structure in the images is parameterized, and the volumetric bubble backscatter is inverted to yield bubble concentrations. Data were obtained at three acoustic frequencies, with inversion effected by prescribing a bubble spectrum with two free variables, leaving a redundant measurement to test the robustness of the model. Median concentrations may in this way be obtained up to the sea surface. Measurements are multiply regressed on wind and dominant-wave variables. Bubbles penetrate to a depth of about a factor of 6γ −1 times the significant wave height H s, where γ is the wave age, or ratio of dominant-wave phase speed to wind speed. The measured mean bubble radius decreases weakly with depth, unless waves are gently sloping, at about 5% m−1. At 0.4 m, the mean radius ranges from 30 to 80 μm and is typically about two-thirds of the radius contributing most to void fraction. The total, depth-integrated surface area of the bubbles and their upward displacement of the sea surface, or “void displacement,” increase as wind speed to the powers 7 ± 1 and 8 ± 1, respectively, dependences ascribed to the preferential breaking of short, steep wind waves. It is estimated, on extrapolating trends, that the total bubble surface area on average is equal to that of the sea surface above them, and the mean void displacement is equal to the mean bubble radius, at a wind speed of about 15 m s−1.

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Tony Hall, Harold E. Brooks, and Charles A. Doswell III

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A neural network, using input from the Eta Model and upper air soundings, has been developed for the probability of precipitation (PoP) and quantitative precipitation forecast (QPF) for the Dallas–Fort Worth, Texas, area. Forecasts from two years were verified against a network of 36 rain gauges. The resulting forecasts were remarkably sharp, with over 70% of the PoP forecasts being less than 5% or greater than 95%. Of the 436 days with forecasts of less than 5% PoP, no rain occurred on 435 days. On the 111 days with forecasts of greater than 95% PoP, rain always occurred. The linear correlation between the forecast and observed precipitation amount was 0.95. Equitable threat scores for threshold precipitation amounts from 0.05 in. (∼1 mm) to 1 in. (∼25 mm) are 0.63 or higher, with maximum values over 0.86. Combining the PoP and QPF products indicates that for very high PoPs, the correlation between the QPF and observations is higher than for lower PoPs. In addition, 61 of the 70 observed rains of at least 0.5 in. (12.7 mm) are associated with PoPs greater than 85%. As a result, the system indicates a potential for more accurate precipitation forecasting.

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A. I. Flossmann, W. D. Hall, and H. R. Pruppacher

Abstract

A theoretical model is formulated which allows the processes that control the wet deposition of atmospheric pollutants to be included in cloud dynamic models. The model considers the condensation process and the collision-coalescence process which, coupled together, control the fate of atmospheric aerosol particles removed by clouds and precipitation through nucleation scavenging and impaction scavenging. The model was tested by substituting a simple parcel model for the dynamic framework. In this form the model was used to determine the time evolution of the aerosol particle mass scavenged by drops as well as the aerosol particle mass left unactivated in air as “drop-interstitial” aerosol. In the present computation all aerosol particles are assumed to have the same composition. Our study shows for inside cloud scavenging: 1) collision and coalescencence causes among the various drop size categories a redistribution of the scavenged aerosol particles in such a manner that the main aerosol particle mass is always associated with the main water mass, thus ensuring that if a cloud reaches the precipitation stage it will also return to the ground the main aerosol particle mass scavenged by the cloud; 2) although the main aerosol particle mass is contained in the large drops, the mass mixing ratio of the captured aerosol in the cloud water is larger inside smaller drops than inside larger drops, implying that smaller drops are more contaminated than larger ones; 3) through nucleation scavenging the total number concentration of aerosol particles is predicted to become reduced by 48 to 94% depending on the composition of the particles, the reduction being mainly confined to aerosol particles larger than 0.1 μm in radius. This implies that a drop interstitial aerosol exists that consists of a particle population reduced in number concentration by up to 94% and reduced in mass by several orders of magnitude, as compared to the particle concentration outside the cloud. 4) Although the aerosol particle mass scavenged by impaction scavenging cannot completely be neglected in accounting for the total amount of aerosol particle mass scavenged by clouded it is smaller by several orders of magnitude than the aerosol particle mass removed by nucleation scavenging.

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Rob A. Hall, John M. Huthnance, and Richard G. Williams

Abstract

Reflection of internal waves from sloping topography is simple to predict for uniform stratification and linear slope gradients. However, depth-varying stratification presents the complication that regions of the slope may be subcritical and other regions supercritical. Here, a numerical model is used to simulate a mode-1, M 2 internal tide approaching a shelf slope with both uniform and depth-varying stratifications. The fractions of incident internal wave energy reflected back offshore and transmitted onto the shelf are diagnosed by calculating the energy flux at the base of slope (with and without topography) and at the shelf break. For the stratifications/topographies considered in this study, the fraction of energy reflected for a given slope criticality is similar for both uniform and depth-varying stratifications. This suggests the fraction reflected is dependent only on maximum slope criticality and independent of the depth of the pycnocline. The majority of the reflected energy flux is in mode 1, with only minor contributions from higher modes due to topographic scattering. The fraction of energy transmitted is dependent on the depth-structure of the stratification and cannot be predicted from maximum slope criticality. If near-surface stratification is weak, transmitted internal waves may not reach the shelf break because of decreased horizontal wavelength and group velocity.

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