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A. L. New and R. Bleck

Abstract

In a companion paper, a spinup integration of the North Atlantic Ocean with the Miami isopycnic-coordinate model was presented. The wintertime mixed layer in the central North Atlantic was subject to relatively little change in salinity or depth but cooled markedly, most probably because of heat loss associated with a partial surface relaxation to climatological sea surface temperatures in a region in which the Gulf Stream was too far to the north. This mixed layer cooling caused the isopycnic layers in the model ventilated subtropical gyre to rise and, surprisingly, to warm. While the experiment was not an attempt to simulate changes in the real Atlantic Ocean, it nevertheless appears from observations that, in recent decades, the mixed layer in nature has undergone a change similar to that exhibited by the model mixed layer. Since it is expected that changes in the ventilated subtropical gyre will be governed largely by changes in the mixed layer in the central North Atlantic, from where the ventilating water masses are subducted, one might therefore anticipate similarities between dine changes in the ventilated regions of the gyres in the model and the real world, even though the cause of the mixed layer changes in the real world may have been different from that in the model. The present paper shows that this is indeed so. In particular, the model behavior closely parallels observed changes in the ventilated subtropical gyre reported by Levitus, in a study of differences between two pentads. The degree of similarity between the model and the observations, including in particular warming of the isopycnic surfaces, leads to a proposal that the changes Levitus observed were caused largely by the subduction of water masses from a cooler mixed layer. Historical changes in the characteristics of the warm North Atlantic Central Water may also be explained by this mechanism. Changes in the wind stress or Ekman pumping fields do not necessarily need to be invoked. Overall, the model provides a framework in which observations from a number of different sources can be understood in a coherent fashion and allows new insights to be gained into the interdecadal variability of the Atlantic Ocean.

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J. Rosinski, G. Langer, and R. Bleck

Abstract

The concentrations of both deuterium and water-insoluble particles in snow collected during snowstorms in the mountains of Colorado were found to be proportional to the size of snowflakes. Large snowflakes were associated with high particle concentration and high deuterium content. Small snow crystals appear to have originated in deuterium and aerosol-particle depleted air. The small snow crystals appeared at times of highest ice-forming nuclei concentration in air. The ratios of concentrations of different sized particles present in snow collected during a brief snowshower in March reveal that the predominant particles within small snow crystals are indeed those corresponding to the size range of ice nuclei. The relationship between the number of different sized particles present in a single snow crystal and the mass of snow crystals is given. Analyses of trajectories of moisture-bearing air parcels explained the large difference between the deuterium content of snow from the two snowfalls.

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E. F. Danielsen, R. Bleck, and D. A. Morris

Abstract

A one-dimensional, time-dependent numerical model of a cumulus cloud is presented that generates hail and radar reflectivities at realistic rates. The distribution of hydrometeors evolves with time as a result of condensation, sublimation, stochastic collection through collision and coalescence, sedimentation, drop freezing, and drop breakup. A total of 40 mass categories, each twice the mass of the former, corresponding to radii from 2.5 μm to 2 cm, are used to determine the ice and hail distribution. The first 31 categories up to a radius of 2.5 mm are used for the water drop distribution. Radar reflectivities are computed from Mie scattering theory for water and ice spheres in each category, then summed to give the reflectivities that can be compared to those observed by radar. Only the updraft radius at the earth's surface, the mixing coefficients, and the initial droplet distribution at cloud base are arbitrarily specified. Four initial droplet distributions are studied separately to determine their effect on hail growth rates and the water drop and hail distributions.

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A. L. New, R. Bleck, Y. Jia, R. Marsh, M. Huddleston, and S. Barnard

Abstract

This paper describes a 30-yr spinup experiment of the North Atlantic Ocean with the Miami isopycnic-coordinate ocean model, which, when compared with previous experiments, possesses improved horizontal resolution, surface forcing functions, and bathymetry, and is extended to higher latitudes. Overall, there is a conversion of lighter to heavier water masses, and waters of densities 1027.95 and 1028.05 kg m−3 are produced in the Greenland-lceland Norwegian basin, and of density 1027.75 kg m−3 in the Labrador and Irminger basins. These water masses flow primarily southward. The main purpose of this present study, however, is to investigate the ventilation of the subtropical gyre. The role of Ekman pumping and lateral induction in driving the subduction process is examined and the relative importance of the latter is confirmed. The paper also illustrates how the mixed layer waters are drawn southward and westward into the ocean interior in a continuous spectrum of mode waters with densities ranging between 1026.40 and 1027.30 kg m−3. These are organized into a regular fashion by the model from a relatively disorganized initial state. The evolution of the model gyre during spinup is governed by mixed layer cooling in the central North Atlantic, which causes the ventilation patterns to move southwestward, the layers to rise, and surprisingly, to become warmer. This warming is explained by thermodynamic considerations. Finally, it is shown that the rate of change of potential vorticity following a fluid pathway in the subtropical gyre is governed by the diffusion of layer thickness, which represents subgrid-scale mixing processes in the model. This leads to increasing potential vorticity along pathways that ventilate from the thickest outcrop regions as fluid is diffused laterally and to decreasing potential vorticity along neighboring trajectories.

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R. D. Cadle, R. Bleck, J. P. Shedlovsky, I. H. Blifford, Jan Rosinski, and A. L. Lazrus

Abstract

An extensive series of experiments is being undertaken to determine the concentrations and transport of trace atmospheric constituents in the stratosphere and troposphere in the vicinity of jet streams during tropopause “folding” episodes. The first experiments were undertaken in March and April 1968 and the results are described and discussed. Both propeller-driven aircraft and U. S. Air Force jet aircraft were used, supplemented by ozonesonde and lidar measurements. Trace constituents investigated included ozone, sulfates, various cations, freezing nuclei, aerosol particles in general, and radioactive nuclides. A high, positive correlation was observed between the intensity of radioactivity and the sulfate concentrations. Relative concentrations were consistent with transport of air from the troposphere to the stratosphere on the anticyclonic side of the jet stream. Concentrations of freezing nuclei were on the average much lower but much more uniform in the stratosphere than in the troposphere.

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Eric P. Chassignet, Linda T. Smith, George R. Halliwell, and Rainer Bleck

Abstract

The viability of a generalized (Hybrid) Coordinate Ocean Model (HYCOM), together with the importance of thermobaricity and the choice of reference pressure, is demonstrated by analyzing simulations carried out using the World Ocean Circulation Experiment (WOCE) Community Modeling Experiment (CME) Atlantic basin configuration. The standard hybrid vertical coordinate configuration is designed to remain isopycnic throughout as much of the water column as possible while smoothly making a transition to level (pressure) coordinates in regions with weak vertical density gradients, such as the surface mixed layer, and to terrain-following coordinates in shallow-water regions. Single-coordinate (pressure or density) experiments illustrate the flexibility of the model but also bring forward some of the limitations associated with such a choice. Hybrid experiments with potential density referenced to the surface (σ θ) and to 20 MPa (∼2000 m) (σ 2) illustrate the increased influence of pressure errors with increasing distance from the reference pressure. The σ θ hybrid experiment does not properly reproduce the northward flow of Antarctic Bottom Water (AABW), and large errors in near-surface pressure gradients in the σ 2 experiment produce a wind-driven gyre circulation that is too strong, when compared with observations, and a North Atlantic Current that follows an unrealistic path. These near-surface and near-bottom pressure errors are removed when thermobaric effects are included, resulting in a more accurate representation of the upper-ocean gyre circulation, the northward AABW flow near the bottom, and the meridional overturning circulation and heat flux.

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Rainer Bleck, Howard Bluestein, Lance Bosart, W. Edward Bracken, Toby Carlson, Jeffrey Chapman, Michael Dickinson, John R. Gyakum, Gregory Hakim, Eric Hoffman, Haig lskenderian, Daniel Keyser, Gary Lackmann, Wendell Nuss, Paul Roebber, Frederick Sanders, David Schultz, Kevin Tyle, and Peter Zwack

The Eighth Cyclone Workshop was held at the Far Hills Inn and Conference Center in Val Morin, Quebec, Canada, 12–16 October 1992. The workshop was arranged around several scientific themes of current research interest. The most widely debated theme was the applicability of “potential vorticity thinking” to theoretical, observational, and numerical studies of the life cycle of cyclones and the interaction of these cyclones with their environment on all spatial and temporal scales. A combination of invited and contributed talks, with preference given to younger scientists, made up the workshop.

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