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J. W. Hutchings

Abstract

For the calendar year 1956, daily observations of wind and humidity at eight atmospheric levels extending from the earth's surface up to the 400-mb level are used to compute average annual and seasonal vertically integrated horizontal water-vapor transfers at seventeen serological stations on and near the Australian continent.

Computations include both total and eddy transfer and as a by-product of the investigation the total water-vapor content (precipitable water) at each station is obtained. Results are presented in the form of annual and seasonal charts showing the fields of water-vapor content and transfer. The vertical structure of these fields is discussed by means of graphs showing the distribution of water-vapor content and transfer with height at selected individual stations.

Integrated total water-vapor transfers are used to estimate the convergence (net inflow) of water vapor over the eastern part of the continent and, with due allowance for changes in atmospheric storage, this is compared with the excess of measured precipitation over estimated evapotranspiration. Annual figures show good agreement between these quantities but when corresponding monthly values are compared the agreement is less satisfactory.

The various uncertainties occurring in the basic data and in the methods of computation are briefly discussed, it being found that the most significant uncertainty is that due to the frequent occurrence of open-circuit (motorboating) humidity values over much of the Australian region.

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J. W. Hutchings

Abstract

An outline is given of a preliminary attempt to apply the methods which have proved serviceable in the treatment of small-scale turbulence to the wider problem of large-scale turbulence in the atmosphere. In particular, application is made of theoretical results giving the form of the correlation function for velocity fluctuations in the inertial range according to the theory of Kolmogoroff. It appears that some of these results may be valid over a considerably greater range of eddy sizes than might at first have been anticipated. Some attempt is also made to apply these methods to the determination of the cprrelation functions for larger-scale fluctuations of pressure and temperature in the atmosphere. Actual data taken at the earth's surface and in the free atmosphere are used to test the validity of these deductions.

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J. W. Hutchings

Abstract

Radiosonde data have been used to construct atmospheric cross sections along meridian 170°E for the winter and summer months. This southern-hemisphere section is briefly compared with northern-hemisphere data and some new features are pointed out.

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M. J. Yelland
,
B. I. Moat
,
P. K. Taylor
,
R. W. Pascal
,
J. Hutchings
, and
V. C. Cornell

Abstract

A large dataset of wind stress estimates, covering a wide range of wind speed and stability conditions, was obtained during three cruises of the RRS Discovery in the Southern Ocean. These data were used by Yelland and Taylor to determine the relationship between 10-m height, neutral stability values for the drag coefficient, and the wind speed, and to devise a new formulation for the nondimensional dissipation function under diabatic conditions. These results have been reevaluated allowing for the airflow distortion caused by the ship. The acceleration and vertical displacement of the flow have been modeled in three dimensions using computational fluid dynamics (CFD). The CFD modeling was tested, first by comparison with wind tunnel measurements on models of two Canadian research ships and second, by analysis of data from four anemometers on the foremast of the RRS Charles Darwin. Originally, the four anemometers gave drag coefficient values that differed by up to 20% from one to another and were all unexpectedly high. The CFD results showed that the airflow had been decelerated by 4%–14% and displaced vertically by about 1 m. These effects caused the original drag coefficient results to be overestimated by up to 60%. After correcting for flow distortion effects, the results from the different anemometers became consistent, which gave confidence in the quantitative CFD-derived corrections.

The CFD modeling showed that the anemometer position on the RRS Discovery was much less affected by airflow distortion. For a given wind speed the CFD corrections reduced the drag coefficient by about 6%. The resulting mean drag coefficient to wind speed relationship confirmed that suggested by Smith from a more limited set of open ocean data.

The effects of flow distortion are sensitive to changes in the relative wind direction. It is shown that much of the scatter in drag coefficient estimates may be due to variations in airflow distortion rather than to the effect of changing sea states. The Discovery wind stress data is examined for evidence of a sea-state dependence: none is found. It is concluded that a wave-age-dependent wind stress formulation is not applicable to open ocean conditions.

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