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S. SethuRaman

Abstract

Turbulence fluctuations over water are found to change for wind speeds more than 10–12 m s−1. Increase in turbulence level beyond this critical wind speed may be attributed to the formation of helical roll vortices. Integral scales of turbulence estimated over water are found to be several times larger than corresponding values over land.

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S. Sethuraman

Abstract

A simple empirical model is developed based on physical and dimensional considerations to predict the height of air mass modification due to a change in surface characteristics. Most of the input parameters can be obtained from surface weather maps. The results of the empirical model are found to be in good agreement with observations made in the atmosphere over ocean with cooler downwind temperatures. The model is then used with appropriate parameters to predict flow modification over a heated surface in a wind tunnel for different upwind and downwind surface conditions.

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S. SethuRaman

Abstract

Momentum flux measurements made from an instrumented ocean buoy indicate that the surface drag coefficient CD is strongly dependent on changes in mean wind direction. A change in mean wind direction is accompanied by a change in wave propagation direction and associated variations in wave steepness and stage of wave development. From simultaneous wind-stress and wave measurements a critical value for the relative motion of air and surface waves is suggested beyond which the dominant waves reach the fully developed stage and the drag decreases.

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S. Sethuraman

Abstract

Mean and fluctuating winds wore measured within the atmospheric surface layer at three locations across Long Island during the landfall of Hurricane Belle on 9 August 19176. An order of magnitude increase in wind shear was observed. A maximum friction velocity of ∼133 cm s−1 and a maximum energy dissipation rate of ∼130 cm−2 s−3 were estimated. Mean wind speeds at the beach were found to he 3–5 times the corresponding wind speeds inland. A periodicity in rainfall associated with bands of thundershowers was observed. A storm surge of ∼125 cm was estimated from water level records near Shinnecock Inlet. The records indicated the three successive stages, forerunner, hurricane surge and resurgence, associated with the hurricane.

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S. SethuRaman and Joyce Tichler

Abstract

Chi-square goodness-of-fit is used to test the hypothesis that the medium scale of turbulence in the atmospheric surface layer is normally distributed. Coefficients of skewness and excess are computed from the data. If the data are not normal, these coefficients are used in Edgeworth's asymptotic expansion of Gram-Charlier series to determine an alternate probability density function. The observed data are then compared with the modified probability densities and the new chi-square values computed.

Seventy percent of the data analyzed was either normal or approximately normal. The coefficient of skewness g 1 has a good correlation with the chi-square values. Events with |g 1| < 0.21 were normal to begin with and those with 0.21 < |g 1| < 0.43 were approximately normal. Intermittency associated with the formation and breaking of internal gravity waves in surface-based inversions over water is thought to be the reason for the non-normality.

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S. SethuRaman and G. S. Raynor

Abstract

Observations of mean wind speed and longitudinal turbulence at a height of 8 m over the Atlantic ocean, 5 km off Long Island, New York, were compared with simultaneous observations at the beach. Results were grouped into wind direction classes characteristic of changes in roughness and fetch. Mean winds over the ocean were 15–100% higher than those at the beach. Changes in turbulence seem to depend on variations in the aerodynamic roughness of the sea surface and the thermal processes that take place over the water. A decrease in turbulence over the ocean relative to that at the beach due to a decrease in sea surface roughness for alongshore flows could be predicted reasonably well with a simple logarithmic wind profile relationship.

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S. SethuRaman and G. S. Raynor

Abstract

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S. SethuRaman and W. A. Tuthill

The design and construction of a bivane to measure horizontal and elevation angle fluctuations in the atmosphere are presented. Wind tunnel tests indicated a reasonably high-frequency response. Field comparisons with a commercially available instrument gave good results. A unique feature in the design of this bivane is its ruggedness in combination with good frequency response.

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S. Sethuraman and R. M. Brown

Abstract

A three-dimensional hot-film probe, a Vector Vane, and an Aerovane were used to measure the mean wind speed and turbulence structure in the atmospheric surface layer at a location on the south shore of Long Island. A comparison was recently made of the characteristics of the three instruments to determine their capabilities in measuring the various meteorological parameters of interest. Results from the comparison indicated that the mean wind speed measured by the three instruments was the same.

The estimated spectral densities of the Vector Vane were approximately equal to those of the hot-film probe to a cyclic frequency of 1 Hz. The standard deviations of the velocity fluctuations were equal.

Comparison of the longitudinal velocity fluctuations measured by the Aerovane and Vector Vane were not significantly different to a frequency of 0.3 Hz. The Aerovane underestimated the lateral velocity fluctuations.

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S. SethuRaman, C. Nagle, and G. S. Raynor

Abstract

Atmospheric internal gravity waves that formed over a coastal and an inland site were identified from analog records of wind speed and direction. Internal gravity waves occurred at all hours at the coastal site but only during nights inland. More waves formed during the spring and summer at the coastal site as compared to other seasons. The frequency distribution of internal gravity waves inland showed no preference in seasons.

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