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N. K. JOHNSON

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NELSON K. JOHNSON

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Todd K. Schaack and Donald R. Johnson

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Three-dimensional global distributions of atmospheric heating are estimated for January and July of the 3-year period 1986–88 from the ECMWF/TOGA assimilated datasets. Emphasis is placed on the interseasonal and interannual variability of heating both locally and regionally. Large fluctuations in the magnitude of heating and the disposition of maxima/minima in the Tropics occur over the 3-year period. This variability, which is largely in accord with anomalous precipitation expected during the ENSO cycle, appears realistic. In both January and July, interannual differences of 1.07−1.5 K day−1 in the vertically averaged heating occur over the tropical Pacific. These interannual regional differences are substantial in comparison with maximum monthly averaged Heating rates of 2.0−2.5 K day−1. In the extratropics, the most prominent interannual variability occurs along the wintertime North Atlantic cyclone track.

Vertical profiles of heating from selected regions also reveal large interannual variability. Clearly evident is the modulation of the heating within tropical regions of deep moist convection associated with the evolution of the ENSO cycle. The beating integrated over continental and oceanic basins emphasizes the impact of land and ocean surfaces on atmospheric energy balance and depicts marked interseasonal and interannual large-scale variability.

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Donald R. Johnson and William K. Downey

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The azimuthally averaged transport and budget equations for a translating storm volume are derived in generalized coordinates. The mean and eddy lateral modes of transport by rotational and irrotational motion are contrasted in symmetric and asymmetric vortices. By contrasting the transport relations in isobaric, cartesian, and isentropic coordinates, the results establish that hydrostatic-rotational regimes of atmospheric motion are typified by eddy modes of transport in isobaric and cartesian coordinates, while both mean and eddy modes may be present in isentropic coordinates. This requirement for a “handover” from an eddy mode of transport in the hydrostatic-rotational environment of a vortex to a mean mode of transport via irrotational motion within a vortex is discussed.

Evidence for the existence of mean meridional circulation in isentropic coordinates for the Midwest extratropical cyclone of 22–24 April 1968 is presented. The inward mass transport in the lower troposphere and outward mass transport in the upper troposphere are coupled to vertical mass transport through isentropic surfaces associated with the release of latent heat in the middle troposphere.

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J. J. Stephens and K. W. Johnson

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An algorithm for the exact determination of potentials for the divergent and rotational wind components is presented and illustrated. The algorithm is based on the discrete Fourier transform of consistent difference approximations of the potential equations.

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Donald R. Johnson and William K. Downey

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The transport and budget formulations developed for a translating system are used to study the mass and absolute angular momentum budgets of a Midwest cyclone. Using isentropic coordinates a mass circulation is isolated which transports angular momentum into the lower half of the tropospheric vortex and out of the upper half during the cyclone’s development and maturation. The occlusion process, characterized by the vertical development of the cyclone vortex into the middle and upper troposphere, occurred through a redistribution of angular momentum by an upward transport of angular momentum associated with the diabatic release of latent heat and an upward redistribution by internal pressure torques.

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William K. Downey and Donald R. Johnson

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The budgets of mass, absolute angular momentum and kinetic energy for two model-generated cyclones and one anticyclone are examined using a sigma-coordinate framework which moves with the center of the MSL pressure extremum. The mass budgets for all three cases show a concentration of lateral mass transport in the surface boundary layer and at a level near 200 mb. The spin up of the low troposphere during cyclogenesis results from the dominance of the mean mode of lateral transport of absolute angular momentum. The spin up of the upper troposphere results from the combined influence of an inward eddy mode of lateral transport and vertical transport of absolute angular momentum. The eddy mode of lateral transport is determined by the configuration of the upper level flow (particularly jet streaks) and is enhanced by frontogenesis in the low and mid-troposphere as these regions spin up. The increase of kinetic energy in the low troposphere during cyclogenesis results from the dominance of local generation by cross-isobar flow toward the center of the developing vortex. In the upper troposphere the kinetic energy budget is not related uniquely to the development or decay of the surface cyclone. While the anticyclone, to a large extent, displays similar behavior to the cyclone, the eddy mode of lateral transport of angular momentum in the upper troposphere is not enhanced by lower level frontogenetic effects, as in the case of the cyclone.

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Donald R. Johnson and William K. Downey

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The concept of absolute angular momentum and its time rate of change is developed for a translating vortex. Storm absolute angular momentum is defined to be the moment of the velocity about an origin translating with the center of the vortex. With this development in generalized coordinates, sources by transport and by internal torques are isolated. An integration over a storm volume reveals that for hydrostatic atmospheres, pressure, viscous, gravitational and inertial torques sum to boundary integrals.

After the vector relations are established for storm absolute angular momentum, the component along the storm axis of rotation through the vortex is determined. By a systematic analysis, the physical basis for a geostrophic torque in an asymmetric baroclinic vortex is established. The role of the geostrophic torque is to transfer angular momentum vertically in isentropic coordinates. Angular momentum is extracted from an isentropic layer with an inward geostrophic mode of mass transport and given to a layer with an outward geostrophic mode. The vertical transfer across the isentropic layer occurs through pressure stresses. Two examples for the Midwest cyclone of 23 April 1968 are presented. Finally, the modes of mean and eddy transport of earth and relative angular momentum as well as sources for the azimuthally averaged storm absolute angular momentum are studied in isobaric, cartesian, and isentropic coordinates.

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A. K. Wåhlin and H. L. Johnson

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The Atlantic overturning circulation has conventionally been pictured in the meridional–vertical plane, but a significant densification of the water masses involved also occurs as the surface branch of the circulation flows in boundary currents around the subpolar gyre and northern marginal seas. Here an analytical model of the heat and salt budget for an idealized coastal boundary current in a marginal sea is presented. The boundary current exchanges heat and freshwater with the atmosphere as well as with the interior of the basin through eddy and Ekman transports. Its along-coast volume transport is assumed to be constant and independent of buoyancy; it is set, for example, by the wind forcing. Because the atmospheric fluxes of heat and freshwater are different, the temperature and salinity of the boundary current adjust on different length scales. The size of these length scales compared with the circumference of the basin determines the properties of the water that flows over the sill. Furthermore, the relative size of the two length scales determines the evolution of the density as the current moves around the basin. If temperature and salinity adjust on the same length scale (or if the density forcing is represented by a single component), then the density will increase or decrease monotonically from the inflow to the outflow. However, when the adjustment length scale for temperature is shorter than that for salinity, a warm and salty inflow can cool significantly before it freshens. As a result, the density first increases to a local maximum before decreasing again. Therefore, when salinity as well as temperature is included in the buoyancy forcing, the outflow from the basin can be significantly denser than for the equivalent single-component density forcing and can be more sensitive to the forcing parameters. The relevance and implications for the Nordic seas are discussed.

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H. K. Johnson and H. J. Vested

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With the goal of improving the formulation of the wind shear stress used in numerical current modeling, a new method for including the effect of water waves on sea roughness is presented. The method is a hybrid of earlier sea roughness models by Kitaigorodskii and Volkov and Donelan, with two distinctions: 1) the roughness height is assumed to be proportional to a wave height representing high-frequency waves (k>1.5k p) in the spectrum, and 2) the hybrid model is calibrated to give drag coefficients obtained by Smith and Banke for their site conditions and verified by comparisons with other independent datasets. In deep water, this model gives a sea roughness that increases with wave age (C p/U *) in the early stages of wave growth (up to C p/U *=5), after which the sea roughness decreases with wave age. This method yields wave-modified C d values for several test cases and can be used directly in numerical current models.

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