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J. Simpson

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J. J. Simpson
and
T. D. Dickey

Abstract

The effect of solar flux divergence on upper ocean dynamics and energetics under both low and high wind speeds was determined using four different parameterizations of downward irradiance. The first (case I) involved only one attenuation length, the second (case II) involved two attenuation lengths, the third (case III) used a spectral decomposition of the incident solar flux over nine wavelength bands, and the fourth (case IV) used an arctangent model of downward irradiance. The Mellor-Yamada turbulence closure scheme (level 2½) was used for the simulations. Cases II–IV predict the existence of an intensified shallow shear zone which is consonant with recent observations. At low wind speeds, the turbulent energy budget is dominated by shear production, dissipation and the diffusion of turbulent kinetic energy, regardless of parameterization. At high wind speeds, shear production is balanced by dissipation. Specific recommendations are made for parameterizing the downward irradiance in the context of numerical studies of upper ocean dynamics, general circulation and climate studies.

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J. J. Simpson
and
T. D. Dickey

Abstract

The relationship between downward irradiance and upper ocean structure has been studied using a numerical model. Two general classes of irradiance parameterizations were utilized. The first (case I) employed a single attenuation length while the second (cast II) involved two attenuation lengths. The latter formulation provided for enhanced absorbance in the upper few meters. Wind speeds of 0, 1, 2, 3, 4, 5, 10 and 20 m.s−1 were used for the simulations in order to characterize heat versus wind dominated regimes. A one-dimensional second moment turbulent closure model was selected for the study so that heat could be treated differentially with depth. The case II results indicated warmer surface temperatures, shallower mixed layers, and more intense thermoclines than case I for wind speeds <10 m s−1. Results converged for higher wind speeds. There was considerably greater sensitivity to wind speed for case II when compared with case I. Mean horizontal velocity as well as thermal structure was sensitive to the empirical formulation of downward irradiance. For low wind speeds the turbulent energy budget is dominated by shear production, dissipation and the diffusion of turbulent kinetic energy, regardless of parameterization. For high wind speeds, shear production is balanced by dissipation. The results of this study provide strong indications that downward irradiance and its proper parameterization are important in determining upper ocean structure and may have implications for large-scale climate studies.

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Joanne Simpson
and
Harry J. Cooper

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Leo J. Fritschen
and
James R. Simpson

Abstract

Surface evaluation of sensible and latent heat flux densities and the components of the radiation balance were desired for various vegetative surfaces during the ASCOT84 experiment to compare with modeled results and to relate these values to drainage winds. Five battery operated data systems equipped with sensors to determine the above values were operated for 105 station days during the ASCOT84 experiment. The Bowen ratio energy balance technique was used to partition the available energy into the sensible and latent heat flux densities. A description of the sensors and battery operated equipment used to collect and process the data is presented. In addition, improvements and modifications made since the 1984 experiments are given. Details of calculations of soil heat flow at the surface and an alternate method to calculate sensible and latent heat flux densities are provided.

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James J. Simpson
and
Clayton A. Paulson

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James J. Simpson
and
Clayton A. Paulson

Abstract

Observations of sea surface temperature and wave height were made from a large, manned spar buoy (R/P FLIP) ∼100 km off the coast of Baja California. Surface temperature was measured with a radiation thermometer which viewed a disc on the surface 12 cm in diameter. The instrument responded to frequencies up to 3 Hz. Wave height was measured with a resistance gage located close to the field of view of the radiometer.

Log-log plots of spectra of sea surface temperature exhibit a plateau between 0.05 and 0.5 Hz, followed by a rapid decrease in energy at frequencies >1 Hz. A coherence of 0.5 between waves and surface temperature occurs at the same frequency as the peak in the wave spectrum. Phase spectra show that warm temperatures associated with the thinning of the surface viscous layer occur systematically upwind of the crests of the dominant gravity waves and downwind of the crests of steeply sloping, shorter period gravity waves. The warm temperatures are hypothesized to be caused by enhanced wind stress upwind from the crests and by surface instability and surface convergence downwind from the crests.

The magnitude of the mean temperature difference between the surface and the warmer, well-mixed water below is estimated from the surface temperature record. It is assumed that the warmest surface temperatures observed are associated with thinning of the viscous layer and are representative of the well-mixed water below. The dimensionless constant in a formula due to Saunders (1967), which relates the temperature difference to wind stress and heat flux, is found to be seven.

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R. H. SIMPSON
and
PAUL J. HEBERT

Abstract

A general overview of the 1972 hurricane season in the North Atlantic is presented together with detailed accounts of all named tropical cyclones.

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Clayton A. Paulson
and
James J. Simpson

Abstract

Observations were made of downward solar radiation as a function of depth during an experiment in the North Pacific (35°N, 155°W). The irradiance meter employed was sensitive to solar radiation of wavelength 400–1000 nm arriving from above at a horizontal surface. Because of selective absorption of the short and long wavelengths, the irradiance decreases much faster than exponential in the upper few meters, falling to one-third of the incident value between 2 and 3 m depth. Below 10 m the decrease was exponential at a rate characteristic of moderately clear water of Type IA. Neglecting one case having low sun altitude, the observations are well represented by the expression I/I 0=Re z/ζ1 +(1−R)e 2 , where I is the irradiance at depth −z, I 0 is the irradiance at the surface less reflected solar radiation, R=0.62, ζ1 and ζ2 are attenuation lengths equal to 1.5 and 20 m, respectively, and z is the vertical space coordinate, positive upward with the origin at mean sea level. The depth at which the irradiance falls to 10% of its surface value is nearly the same as observations of Secchi depth when cases with high wind speed or low solar altitude are neglected. Parameters R, ζ1, and ζ2 are computed for the entire range of oceanic water types.

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James J. Simpson
and
Clayton A. Paulson

Abstract

Mid-ocean observations (35°N, 155°W) of temperature and salinity were made from R/P Flip during the period 28 January-14 February 1974 as part of the NORPAX POLE Experiment.

Autocorrelations for the time series of depth of several σ t surfaces confirm the presence of a semidiurnal internal tide whose amplitude is about 10 m. The period of 12.7 h determined from the autocorrelation analysis is not statistically significantly different from the period of the M2 semidiurnal tide (12.4 h). The coherence between pairs of time series of the depth of the σ t surfaces is high, ranging from 0.97 to 0.91 at the frequency of the peak in the spectrum corresponding to the semi-diurnal tide. The coherence between a given σ t surface and deeper lying surfaces decreases slowly with the mean separation between surfaces. The vertical coherence scale suggests that most of the energy of the semi-diurnal internal tide is in the low-order modes. The data show that the phase difference between surfaces increases with the mean separation between surfaces at the approximate rate of +35° (100 m). Estimates of the vertical and horizontal wavelengths of the observed semi-diurnal internal tide are 1 km and 35 km, respectively.

One-dimensional mixed-layer deepening models fail to predict the mixed-layer depths and temperatures observed during POLE. Horizontal advection, as evidenced from the salinity maximum frequently occurring at the bottom of the mixed layer and other near-surface changes in salinity and temperature not associated with local surface forcing, are responsible for the failure. During the one period in which the one-dimensional models may be applicable a value of the mixing energy flux coefficient m = 0.0017 was obtained.

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