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James J. O'Brien

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James J. O'Brien

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James J. O'Brien

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The kinematic method for determining vertical velocity ω in pressure coordinates is reviewed. Alternative objective procedures are derived for obtaining ω, and an analytical solution to the pressure-differentiated continuity equation is found. A variational formulation leads to a generalized objective adjustment for divergence estimates which yields improved, physically realistic estimates of ω. Case studies for intense mesoscale convection demonstrate the utility of an adjustment scheme based on the simplest hypothesis, namely, that the errors in divergence estimates are a linear function of pressure.

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James J. O'Brien

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This paper is a continuation of a theoretical description of upwelling and mixing induced in a stratified, rotating, two-layer ocean by momentum transfer from an intense, stationary, axially-symmetric atmospheric vortex. A second model which includes mixing is considered. The dynamic internal response of the ocean is assumed to be axially symmetric which permits consideration of the solution in two independent variables, radial distance and time. Numerical integration via the method of characteristics is utilized to obtain values of depth-averaged radial and tangential velocities, depth of the upper layer, and density contrast for a period of two days. Transfer of momentum between the air and the sea and between the upper and lower layers of the ocean is included. Transfer of heat and salt between the two ocean layers is simulated. Transfer of heat and moisture with the atmosphere is not considered.

The mechanism of energy transfer to and from the atmosphere and to and from the lower layer is examined in detail. This indicates that the total energy varies only with the inertial period. The energy associated with the effect of mixing is an order of magnitude smaller than that associated with turbulent dissipation. However, turbulent mixing of heat and salt modifies the density structure throughout the wind-forced region of the ocean, while intense upwelling is confined to within twice the radius of maximum hurricane winds.

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JAMES J. O'BRIEN

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James J. O'Brien

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Andrew Johnson Jr.
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James J. O'brien

Abstract

A series of meteorological observations including aircraft, pilot balloon (pibal), rawinsonde, surface buoy, and special land-based surface observations was taken on 23–24 August 1972, on the central Oregon coast, to investigate the mesoscale thermal and kinematic responses of the lowest 4 km of the atmosphere during a sea breeze event.

A description of those field observations is given. Vertical cross sections of the wind field on a line perpendicular to the coast, extending 60 km inland from data obtained at three pibal stations, are presented and discussed. Time sections of the wind field and temperature fields at the coast are discussed. Mesoscale features are presented and related to prevailing synoptic-scale changes occurring aloft during the observational period.

The sea breeze event on 23 August exhibited the following important characteristics: 1) a sea breeze front, distinguishable in the zonal wind field, which penetrated more than 60 km inland; 2) a distinct wind maximum which followed the front inland; 3) the surface onshore flow at the coast which took place below the main inversion, deepening the marine layer at the onset; and 4) a return flow above the inversion which appeared in quasi-periodic surges in response to surges in the sea breeze flow.

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Arnold Gruber
and
James J. O'Brien

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An objective technique for adjusting wind data, such that the total mass divergence in a volume of the atmosphere is zero, is developed. The adjustment is obtained by applying a least-squares smoothing with a Lagrangian multiplier to constrain the total mass divergence to a specified amount. The computational details are derived and the method is applied to several examples. Both for theoretical wind profiles and for actual data, a very satisfactory adjustment is achieved without destroying the physical information contained in the data.

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Jiayan Yang
and
James J. O'Brien

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A coupled atmosphere-ocean model is used to investigate some important effects of a steep sloping thermocline in the, central Pacific. It is found that the coupled modes are very sensitive to the steepness of the thermocline in the central Pacific Ocean. The wave reflection and modal decomposition processes play an important role in the initial development of the unstable mode and dramatically affect the fates of the oscillation. A sensitivity test is conducted to test the role of western boundary reflection in this particular model. The insensitivity of the western boundary reflection seems to agree with the results of previous research that showed that coupled unstable modes do not necessarily depend on Rossby wave reflections.

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James J. O'brien
and
Fred Parham

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In the last several years many scientists have been using poorly resolved coupled models to study the El Niño- Southern Oscillation (ENSO). It has been very common to state that an ENSO cycle found in a model cannot have oceanic Kelvin waves as a mechanism because such waves do not exist in an ocean model with coarse grid spacing. In this note we demonstrate that equatorial Kelvin waves can exist in models with coarse grids.

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