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T. J. Smith

Abstract

A model for the representation of the Reynolds-stress tensor in three-dimensional hydrodynamic models of shallow water flows is derived which combines the accuracy of turbulence-energy closure schemes with the computational efficiency of algebraic eddy viscosity models. The proposed model assumes, the eddy-viscosity tensor to have structural similarity, from which it is shown that its magnitude is scaled on the depth-mean turbulence energy and the depth-mean turbulence-energy dissipation rate, while the vertical structure is described by a suitable similarity function, two alternatives of which are derived. The similarity assumptions used in the analysis are verified and the model is tested by application to steady and tidal flows.

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Phillip J. Smith and John T. Snow
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ÁH. Flosadóttir, J. C. Larsen, and J. T. Smith

Abstract

Motionally induced voltage differences offer one of the few observational methods sensitive to changes in large-scale ocean transports. They present a useful contrast to most oceanographic data by virtue of their natural spatial integration, temporal continuity, and potentially long duration. However, widespread oceanographic use of the voltages observable with seafloor cables has been impeded by uncertainties of interpretation. Interpretation in terms of volume transport fluctuations has proved successful in the Straits of Florida and for a short cable in the easternmost part of the Bering Strait. Still, a number of older experimental studies resulted in disappointment, the Bering Strait work has been little known, and the Florida success might be a special case. The question considered in this paper is: Does a linear relationship between net transport and voltage difference fluctuations hold for long, open-ocean cables? This question is addressed by using a numerical model based on two years of results from the WOCE Community Modeling Effort, which simulated the wind-driven and thermohaline circulation in the North Atlantic using mean monthly winds and realistic topography with a resolution sufficient to permit mesoscale eddies. The model includes the effects of spatial and temporal variations of seawater temperature and salinity, electric current loops, the effects associated with the meandering of ocean currents over realistic topography and sediment thickness, realistic earth conductivity, and the spatially varying geomagnetic field. The main result is that the relationship between voltage and net cross-cable transport fluctuations can be remarkably linear over long distances. In view of the difficulties of long-term, large-scale transport monitoring by other methods, the implication of this work is that well chosen and carefully interpreted voltage observations hold great promise. This should be explored through renewed modeling, observation, and interpretation efforts.

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T. M. L. Wigley, S. J. Smith, and M. J. Prather

Abstract

Reactive gas emissions (CO, NOx, VOC) have indirect radiative forcing effects through their influences on tropospheric ozone and on the lifetimes of methane and hydrogenated halocarbons. These effects are quantified here for the full set of emissions scenarios developed in the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. In most of these no-climate-policy scenarios, anthropogenic reactive gas emissions increase substantially over the twenty-first century. For the implied increases in tropospheric ozone, the maximum forcing exceeds 1 W m−2 by 2100 (range −0.14 to +1.03 W m−2). The changes are moderated somewhat through compensating influences from NOx versus CO and VOC. Reactive gas forcing influences through methane and halocarbons are much smaller; 2100 ranges are −0.20 to +0.23 W m−2 for methane and −0.04 to +0.07 W m−2 for the halocarbons. Future climate change might be reduced through policies limiting reactive gas emissions, but the potential for explicitly climate-motivated reductions depends critically on the extent of reductions that are likely to arise through air quality considerations and on the assumed baseline scenario.

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Harry J. Cooper, Eric A. Smith, and Michael T. Rubes

Abstract

Analysis of surface latent heat flux measurements taken within the sea-breeze front of the coast of Florida during active thunderstorm periods demonstrates an important effect of the timing of coastal storms on the seasonal surface water budget. Historical records document a systematic cross-peninsula water runoff gradient across Florida, with total runoff greater on the east coast (Atlantic side) than on the west coast (gulf side). This situation persists even though convective rainfall tends to be greater in the summertime on the gulf side. In this paper, the authors examine the effect of the time of day that summer thunderstorms occur at a given location on poststorm evaporation of rainfall and place these effects into the context of the annual runoff at the coasts and seasonal rainfall in order to assess their possible significance.

A surface water exchange analysis, based on datasets obtained during the 1991 summertime Convection and Precipitation Electrification Experiment, finds that part of the runoff gradient can be explained by an indirect atmospheric mechanism. Results indicate that differences in the diurnal timing of thunderstorms between the two coasts and the associated differences in postthunderstorm evapotranspiration can account for a significant portion of the annual differential in runoff. During the summer months, gulf coast storms often occur earlier in the day than Atlantic coast storms because of the combined effects of the mesoscale sea-breeze convergence and synoptic-scale flow around the Bermuda high. Under these conditions, once the later-day east coast thunderstorms dissipate, there is no longer any net solar radiation source to drive evapotranspiration, so that rainwater not taken up by ground filtration tends to go into runoff. On the west coast, when thunderstorms occur earlier and dissipate in midafternoon, there is still enough net surface radiation to drive significant rates of evapotranspiration, which reduces the amount of water available for runoff.

The difference in available rainfall that results from the increased evaporation after the earlier storms is found to be about 2 mm, which over the summer season amounts to some 50 mm of water not made available for runoff on the west coast. This is significant when compared to the annual cross-peninsula runoff gradient of 250 mm. It is also found that it takes 4.5 days of clear-sky latent heat fluxes to reevaporate average storm rainfall back into the atmosphere. In addition, areas that are not close to the center of storm outflows tend to be neutral in terms of daily surface water exchange, evaporating as much as they receive, while cloudy areas with no rain evaporate at rates close to 90% of the clear-sky rates on a daily basis. This paper addresses the details of these processes and quantifies the surface water exchange in south Florida as a function of the proximity to the summertime thunderstorm outflows.

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W. A. M. Nimmo Smith, J. Katz, and T. R. Osborn

Abstract

Six sets of particle image velocimetry (PIV) data from the bottom boundary layer of the coastal ocean are examined. The data represent periods when the mean currents are higher, of the same order, and much weaker than the wave-induced motions. The Reynolds numbers based on the Taylor microscale (Reλ) are 300–440 for the high, 68–83 for the moderate, and 14–37 for the weak mean currents. The moderate–weak turbulence levels are typical of the calm weather conditions at the LEO-15 site because of the low velocities and limited range of length scales. The energy spectra display substantial anisotropy at moderate to high wavenumbers and have large bumps at the transition from the inertial to the dissipation range. These bumps have been observed in previous laboratory and atmospheric studies and have been attributed to a bottleneck effect. Spatial bandpass-filtered vorticity distributions demonstrate that this anisotropy is associated with formation of small-scale, horizontal vortical layers. Methods for estimating the dissipation rates are compared, including direct estimates based on all of the gradients available from 2D data, estimates based on gradients of one velocity component, and those obtained from curve fitting to the energy spectrum. The estimates based on vertical gradients of horizontal velocity are higher and show better agreement with the direct results than do those based on horizontal gradients of vertical velocity. Because of the anisotropy and low turbulence levels, a −5/3 line-fit to the energy spectrum leads to mixed results and is especially inadequate at moderate to weak turbulence levels. The 2D velocity and vorticity distributions reveal that the flow in the boundary layer at moderate speeds consists of periods of “gusts” dominated by large vortical structures separated by periods of more quiescent flows. The frequency of these gusts increases with Reλ, and they disappear when the currents are weak. Conditional sampling of the data based on vorticity magnitude shows that the anisotropy at small scales persists regardless of vorticity and that most of the variability associated with the gusts occurs at the low-wave-number ends of the spectra. The dissipation rates, being associated with small-scale structures, do not vary substantially with vorticity magnitude. In stark contrast, almost all the contributions to the Reynolds shear stresses, estimated using structure functions, are made by the high- and intermediate-vorticity-magnitude events. During low vorticity periods the shear stresses are essentially zero. Thus, in times with weak mean flow but with wave orbital motion, the Reynolds stresses are very low. Conditional sampling based on phase in the wave orbital cycle does not show any significant trends.

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R. J. Hung, T. Phan, and R. E. Smith

Abstract

Penetrative convection, thunderstorms, squall lines, etc., all generate atmospheric gravity waves which can be observed by a ground-based ionospheric Doppler sounder array. Sources of these waves can be determined from reverse ray tracing computations. Case studies of gravity waves associated with isolated tornadic storms on 13 January 1976 were summarized to establish the minimum data sampling time required for correct spectral analysis and ray tracing computations. It was concluded that the data sampling time can be reduced to two to three times the wave period while still obtaining a reasonably good power spectral density. It was also demonstrated that the data sampling time can be reduced to two to three times the time delay of the wave arrival between two station pairs while still obtaining a justifiably good cross-spectral analysis. Computed source locations of the observed gravity waves are compared with conventional and satellite meteorological data.

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E. J. Smith, E. E. Adderley, and D. T. Walsh

Abstract

A cloud-seeding experiment was conducted in the Snowy Mountains of Australia from 1955–1959 inclusive. The objective was to determine if silver-iodide smoke released from an aircraft into clouds could increase the precipitation over a selected area. The method involved a comparison of the precipitation in a target area and that in a control area during randomized periods of seeding and no seeding. Over the five years, the ratio of the precipitation in the target to that in the control area was higher in seeded than in unseeded periods. Three statistical tests are presented which show that the seeded periods are different from the unseeded periods at significance levels of 0.03, 0.09 and 0.03 (one sided). This supports a positive seeding effect. Other analyses both detract from and support this contention. The net result is that the experiment in inconclusive. Further, improved experiments are proposed.

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J. L. Haferman, W. F. Krajewski, and T. F. Smith

Abstract

Several multifrequency techniques for passive microwave estimation of precipitation based on the absorption and scattering properties of hydrometeors have been proposed in the literature. In the present study, plane-parallel limitations are overcome by using a model based on the discrete-ordinates method to solve the radiative transfer equation in three-dimensional rectangular domains. This effectively accounts for the complexity and variety of radiation problems encountered in the atmosphere. This investigation presents results for plane-parallel and three-dimensional radiative transfer for a precipitating system, discusses differences between these results, and suggests possible explanations for these differences.

Microphysical properties were obtained from the Colorado State University Regional Atmospheric Modeling System and represent a hailstorm observed during the 1986 Cooperative Huntsville Meteorological Experiment. These properties are used as input to a three-dimensional radiative transfer model in order to simulate satellite observation of the storm. The model output consists of upwelling brightness temperatures at several of the frequencies on the Special Sensor Microwave/Imager. The radiative transfer model accounts for scattering and emission of atmospheric gases and hydrometeors in liquid and ice phases.

Brightness temperatures obtained from the three-dimensional model of this investigation indicate that horizontal inhomogeneities give rise to brightness temperature fields that can be quite different from fields obtained using plane-parallel radiative transfer theory. These differences are examined for various resolutions of the satellite sensor field of view. In addition, the issue of boundary conditions for three-dimensional atmospheric radiative transfer is addressed.

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R. J. Small, T. Campbell, J. Teixeira, S. Carniel, T. A. Smith, J. Dykes, S. Chen, and R. Allard

Abstract

In situ experimental data and numerical model results are presented for the Ligurian Sea in the northwestern Mediterranean. The Ligurian Sea Air–Sea Interaction Experiment (LASIE07) and LIGURE2007 experiments took place in June 2007. The LASIE07 and LIGURE2007 data are used to validate the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) developed at the Naval Research Laboratory. This system includes an atmospheric sigma coordinate, nonhydrostatic model, coupled to a hydrostatic sigma-z-level ocean model (Navy Coastal Ocean Model), using the Earth System Modeling Framework (ESMF).

A month-long simulation, which includes data assimilation in the atmosphere and full coupling, is compared against an uncoupled run where analysis SST is used for computation of the bulk fluxes. This reveals that COAMPS has reasonable skill in predicting the wind stress and surface heat fluxes at LASIE07 mooring locations in shallow and deep water. At the LASIE07 coastal site (but not at the deep site) the validation shows that the coupled model has a much smaller bias in latent heat flux, because of improvements in the SST field relative to the uncoupled model. This in turn leads to large differences in upper-ocean temperature between the coupled model and an uncoupled ocean model run.

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