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J. Simpson

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Joanne Simpson and Harry J. Cooper

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H. J. Cooper, M. Garstang, and J. Simpson

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Surface wind measurements taken during the summers of 1973 and 1975 in the Florida Area Cumulus Experiments (FACE) fine-mesh networks are used to calculate surface divergence on the convective scale and mesoscale.

Examination of the daily time series of divergence averaged over a 30 km × 25 km grid showed that on days with deep convective activity over and around the network, a definable sequence in the network-averaged surface wind divergence is observed. The sequence consists of five stages: persistent convergence, peak convergence, non-divergence, peak divergence and the return to a non-divergent state.

Use is made of observations of individual cases and of radar composites built around the above sequence in the network-averaged divergence fields to demonstrate that a convective-scale feedback mechanism consisting of outflows from previous storms triggering new growth is frequently observed during both experimental periods.

It is shown that the observations taken over the FACE 1975 network are consistent with the model results of Pielke (1974) and estimates of peninsular-scale convergence. Comparison of the convective-scale vertical transports over the network at the near surface (4 m) level with the peninsular-scale forcing reveals a relationship between the two. After the initiation of convective processes by the peninsular-scale forcing, downdraft-induced convergence maintains and intensifies the convective-scale activity long after the peninsular-scale forcing has passed its peak. The daily cycle of initiation, intensification, and the eventual decline of convective activity is related to the rate of change of the peninsular-scale divergence.

The links established between the various scales are of fundamental importance to the understanding of the initiation, maintenance, and decay of deep precipitating convection and to its theoretical parameterization.

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J. J. Simpson and T. D. Dickey

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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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J. J. Simpson and T. D. Dickey

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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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James J. Simpson and Clayton A. Paulson

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James J. Simpson and Clayton A. Paulson

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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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James J. Simpson and Clayton A. Paulson

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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 σt 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 σt 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 σt 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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C. L. Simpson and J. M. Thorp

An empirical method for forecasting surface winds at an inter-mountain station, based on the differences in sea-level pressure between Hanford and five other stations, is developed. Nomograms relating the ten-hour velocity trend to the pressure gradient are presented. The method is discussed in relation to conventional methods of wind forecasting as pertaining to apparent counter-gradient winds, cyclic features of the surface winds, and variations in wind speed.

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Leo J. Fritschen and James R. Simpson

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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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