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Jack A. C. Kaiser and Kingsley G. Williams

Abstract

By measuring the time rate of change of temperature in the upper 65 m of the sea at night with a precision sounding device, the amount of heat transported upward at various depths and through the sea surface as a function of time during the night was determined. The heat flux through any surface of depth z was given by e −αt (1−Az ) for z<z max (40–65 m). The amount of heat released from the sea surface ranged from 1.34 to 0.311 ly min−1, the release rate decreasing with time after sunset.

The data also allowed estimates of the spatially averaged thermal boundary layer thickness in the sea surface, 0.2 cm or less.

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J. Williams, G. Krömer, and A. Gilchrist

Abstract

Experiments were made with the Meteorological Office general circulation model (GCM) to investigate the response of the simulated atmospheric circulation to the addition of large amounts of waste heat in localized areas. The concept of large-scale energy parks determined the scenarios selected for the five perturbation experiments. Waste heat totaling 150 or 300 TW was added to the sensible heat exchange between the surface and air at energy parks in the Atlantic and Pacific Oceans in four experiments. In a fifth experiment, 300 TW were added to a 10 m deep “ocean box” simulated beneath the energy parks. Forty-day averages of meteorological fields from the five waste heat experiments and from three control cases are compared. Model variability is estimated on the basis of the three control cases. The regional and hemispheric responses of the atmospheric circulation are discussed, with emphasis on the magnitude of the heating rates and 500 mb height changes. The main conclusions that can be drawn are that the model exhibits a nonlinear response to the waste heat input and that, in middle latitudes, the spatial scale of the response is large even though the heat input scale is small.

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A. J. George Nurser and Richard G. Williams

Abstract

The effect of cooling on the separated boundary current predicted by the model of Parsons is studied. The separating current is found to strengthen and to move southwards and eastwards. The model is also robust to limited heating. in which case the separating current weakens and moves northwards.

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A. J. G. Nurser, Robert Marsh, and Richard G. Williams

Abstract

The formation rate of water masses and its relation to air–sea fluxes and interior mixing are examined in an isopycnic model of the North (and tropical) Atlantic that includes a mixed layer. The diagnostics follow Walin’s formulation, linking volume and potential density budgets for an isopycnal layer.

The authors consider the balance between water mass production, mixing, and air–sea fluxes in the model in the context of two limit cases: (i) with no mixing, where air–sea fluxes drive water mass formation directly, and (ii) a steady state in a closed basin, where air–sea fluxes are balanced by diffusion. In such a steady state, since mixing always acts to reduce density contrast, surface forcing must act to increase it.

Considered over the whole basin, including the Tropics, the model is in steady state apart from the densest layers. Most of the mixing is achieved by diapycnal diffusion in the strong density gradients within upwelling regions in the Tropics, and by entrainment into the tropical mixed layer. Mixing from entrainment associated with the seasonal cycle of mixed layer depth in mid and high latitudes and lateral mixing of density within the mixed layer are less important than this tropical mixing. These model results as to the relative importance of the different mixing processes are consistent with a simple scaling analysis.

Outside the Tropics, the upwelling-linked mixing is no longer important, and a first-order estimate of water mass formation rates may be made from the surface fluxes. Lateral mixing of density within the mixed layer and seasonal entrainment mixing are as important as the remaining thermocline mixing within this domain.

An apparent vertical diffusivity is diagnosed over both the full and extratropical domain. It reaches 10−4 m2 s−1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.

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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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A. G. Williams, J. M. Hacker, and H. Kraus

Abstract

The structure of the intertropical convergence zone ITCZ cloud-topped marine atmospheric boundary layer away from the most intense mesoscale convective systems during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) is investigated. Eight vertical profiles taken by the Australian Cessna research aircraft are analyzed, representing the successive influence of a growing small cluster of precipitating cumulus upon the subcloud layer. On the basis of conclusions from a spectral analysis in Part I of this study, results are partitioned into contributions from three distinct categories: (a) small-scale (<2 km) processes, corresponding to small eddies contained and forced mainly within the subcloud layer and weakly active cumulus motions; (b) cloud-scale (>2 km) processes, corresponding to meso-γ-scale motions associated mainly with the action of precipitating cumulus clouds and larger motions; and (c) extreme processes, corresponding to contributions from events at the tail of the small-scale statistical flux distribution. Such events are associated with downdrafts below precipitating cumulus, updrafts at gustfronts, and the effects of moisture contamination on thermodynamic data, and can act to significantly skew the flux distribution. In the presence of vigorous cumuli, cloud root circulations (including compensating downdrafts) force significant cloud-scale fluxes in the upper subcloud layer. When conditions become highly disturbed, these fluxes dominate and the vast majority of small-scale humidity transport is concentrated into the cloud root regions. Precipitation produces strong downdrafts and outflows of evaporatively cooled air in the lower subcloud layer, markedly increasing temperature and velocity variances. Neither cloud root circulations nor outflows are supported by cloud-scale buoyancy, with the former being fed by pressure and momentum forces while the latter are formed via small-scale (extreme) buoyancy effects. Small-scale (surface forced) processes moisten and slow the subcloud layer as a whole, while cloud processes cause drying and often acceleration due to enhanced cloud–subcloud-layer exchanges. Processes on all scales lead to net warming of the subcloud layer in the present dataset. Although in zero or low precipitation cases the mean structure of the mixed layer may still be represented to some degree by existing simple zero-order jump models, significant adjustments are required to such models in order to account for the effects of cloud-scale processes under disturbed conditions. In particular, the enhancement of cloud–subcloud-layer exchanges by cloud root processes and the effects of increased horizontal wind variances upon surface fluxes requires attention. A new velocity scale is suggested, based on large-scale vertical velocity at cloud base, which may be useful in the formulation of newparameterizations.

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A. G. Williams, H. Kraus, and J. M. Hacker

Abstract

Spectral analysis of high-resolution turbulence data from the South Australian Cessna research aircraft is performed in an investigation of the multiscale nature of vertical transport processes in the atmospheric boundary layer (ABL) during TOGA COARF. The flights were conducted in the vicinity of large cloud cluster systems in the intertropical convergence zone, but away from the most intense mesoscale (100s of km) convective systems within the clusters. A number of very long (up to 430 km) and low (20-70 m) continuous data runs, composing an excellent dataset for studying the spectral composition of near-surface fluxes, are complemented by eight “stack” patterns providing important information regarding vertical variations. The ABL in these regions is found to be highly horizontally heterogeneous, due to the intrusion of cool air masses associated with precipitating cumulus and cumulonimbus clouds, and the action of lines of convention on a range of scales. Not only does this lead to large variations in the surface turbulent flux field, but it can also generate significant direct fluxes in a submesoscale (20–50 km) range at low altitudes, which are not expected to be controlled by ABL parameters. That is, enhanced motions resulting from the action of precipitating cumulus clouds in the presence of wind shear can lead to strong entrainment of air into the subcloud layer, and, in addition, gravity waves generated above the ABL can also influence subcloud motion. Analysis of the form and consistency of the cospectra suggests that, despite the absence of a clear “gap” in the power spectra of the major variables, it is nevertheless possible to achieve a reasonable partitioning between “ABL turbulence” and the larger-scale processes via a simple spectral separation with a crossover wavelength at around 2 km. This useful characteristic appears to reflect an ability of the ABL turbulence to maintain a high degree of coherency in spite of the changing conditions imposed by the mesoscale disturbances.

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Earle R. Williams, Spiros G. Geotis, and A. B. Bhattacharya

Abstract

Radar measurements and model studies are combined to investigate the plasma condition and the physical structure of lightning in thunderclouds. The lightning radar target is inferred to be an arclike plasma whose temperature exceeds 5000 K, thereby implying overdense plasma at all meteorological wavelengths. Lightning echoes are treated as volume targets and are modeled as treelike assemblages of conductive channels which are each long and thin compared to the radar wavelength. The channel lengths per unit volume deduced from more than one thousand reflectivity measurements at 11 cm wavelength range from 10−3 to 102 km km−3. Comparisons with more than 200 measurements at 5 cm wavelength show that the wavelength dependence is highly variable. On the average, the apparent dependence is λ−2 but this is unreliable because of the masking effects of precipitation. The infrequent detection of lightning at short wavelengths (λ ≥ 5 cm) is also attributed to masking rather than to an underdense plasma condition.

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John C. Marshall, Richard G. Williams, and A. J. George Nurser

Abstract

The annual rate at which mixed-layer fluid is transferred into the permanent thermocline—that is, the annual subduction rate S ann and the effective subduction period 𝒯eff—is inferred from climatological data in the North Atlantic. From its kinematic definition, S ann is obtained by summing the vertical velocity at the base of the winter mixed layer with the lateral induction of fluid through the sloping base of the winter mixed layer. Geostrophic velocity fields, computed from the Levitus climatology assuming a level of no motion at 2.5 km, are used; the vertical velocity at the base of the mixed layer is deduced from observed surface Ekman pumping velocities and linear vorticity balance. A plausible pattern of S ann is obtained with subduction rates over the subtropical gyre approaching 100 m/yr—twice the maximum rate of Ekman pumping.

The subduction period 𝒯eff is found by viewing subduction as a transformation process converting mixed-layer fluid into stratified thermocline fluid. The effective period is that period of time during the shallowing of the mixed layer in which sufficient buoyancy is delivered to permit irreversible transfer of fluid into the main thermocline at the rate S ann. Typically 𝒯eff is found to be 1 to 2 months over the major part of the subtropical gyre, rising to 4 months in the tropics.

Finally, the heat budget of a column of fluid, extending from the surface down to the base of the seasonal thermocline is discussed, following it over an annual cycle. We are able to relate the buoyancy delivered to the mixed layer during the subduction period to the annual-mean buoyancy forcing through the sea surface plus the warming due to the convergence of Ekman heat fluxes. The relative importance of surface fluxes (heat and freshwater) and Ekman fluxes in supplying buoyancy to support subduction is examined using the climatologist observations of Isemer and Hasse, Schmitt et al., and Levitus. The pumping down of fluid from the warm summer Ekman layer into the thermocline makes a crucial contribution and, over the subtropical gyre, is the dominant term in the thermodynamics of subduction.

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Richard G. Williams, John C. Marshall, and Michael A. Spall

Abstract

Stommel argued that the seasonal cycle leads to a bias in the coupling between the surface mixed layer and the main thermocline of the ocean. He suggested that a “demon” operated that effectively only allowed fluid at the end of winter to pass from the mixed layer into the main thermocline. In this study, Stommel's hypothesis is examined using diagnostics from a time-dependent coupled mixed layer-primitive equation model of the North Atlantic (CME). The influence of the seasonal cycle on the properties of the main thermocline is investigated using two methods. In the first, the rate and timing of subduction into the main thermocline is diagnosed using kinematic methods from the 1° resolution CME fields. In the second, tracer diagnostics of the CME and idealized experiments using a “date” tracer identifying the timing of subduction are performed. Over the subtropical gyre, both approaches generally support Stommel's hypothesis that fluid is only transferred from the mixed layer into the main thermocline over a short period, ∼1 month, in late winter/early spring. Tracer date experiments are also conducted using the eddy-resolving ⅓° CME fields. Eddy stirring is found to enhance the rate at which the tracer spreads into unventilated regions, but does not alter the seasonal bias of the Stommel demon mechanism.

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