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- Author or Editor: Yukihiro Mori x
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Abstract
The daily variation of the surface wind at Marcus Island (24°N, 154°E) in the subtropical Pacific area has been investigated with respect to the semidiurnal atmospheric tide. The semidiurnal oscillations of scalar wind speed and vector components of wind have been analyzed harmonically. The phase of the zonal wind component in local time agrees with the observed values at Bermuda and in the Atlantic trades.
Using the Ekman model the semidiurnal tidal wind oscillation in the planetary boundary layer has been investigated, and an analytic solution has been obtained. In order to fit the theoretical tidal wind oscillation to the observed one, the eddy viscosity K should be smaller than 0.1 m2 s−1. This value of K appears smaller than the representative value in the trades. This suggests the limitation of the simple model.
Abstract
The daily variation of the surface wind at Marcus Island (24°N, 154°E) in the subtropical Pacific area has been investigated with respect to the semidiurnal atmospheric tide. The semidiurnal oscillations of scalar wind speed and vector components of wind have been analyzed harmonically. The phase of the zonal wind component in local time agrees with the observed values at Bermuda and in the Atlantic trades.
Using the Ekman model the semidiurnal tidal wind oscillation in the planetary boundary layer has been investigated, and an analytic solution has been obtained. In order to fit the theoretical tidal wind oscillation to the observed one, the eddy viscosity K should be smaller than 0.1 m2 s−1. This value of K appears smaller than the representative value in the trades. This suggests the limitation of the simple model.
Abstract
Spectra of the surface wind at Marcus Island (24°N, 154°E) in the subtropical Pacific area wore constructed over a wide frequency range from 10−1 to 5 × 10−5 Hz (periods from 6 h to about three years). The major kinetic energy peak is found at a period of about 6 days. Besides this peak, for the lower frequency range the significant peak is found at a period of 14–22 days. The latter peak is not found in the spectra at other places in the middle latitudes reported before and is significant in the warm season. This means that equatorial planetary-scale waves exert influence on the surface wind at this island. The diurnal and semidiurnal peaks are small but clearly found. The daily variation of the surface wind may be due to the diurnal and semidiurnal tidal waves of atmospheric pressure. Small peaks are found in some cases at or near the inertial period. Rotary spectra are calculated and also shown.
Abstract
Spectra of the surface wind at Marcus Island (24°N, 154°E) in the subtropical Pacific area wore constructed over a wide frequency range from 10−1 to 5 × 10−5 Hz (periods from 6 h to about three years). The major kinetic energy peak is found at a period of about 6 days. Besides this peak, for the lower frequency range the significant peak is found at a period of 14–22 days. The latter peak is not found in the spectra at other places in the middle latitudes reported before and is significant in the warm season. This means that equatorial planetary-scale waves exert influence on the surface wind at this island. The diurnal and semidiurnal peaks are small but clearly found. The daily variation of the surface wind may be due to the diurnal and semidiurnal tidal waves of atmospheric pressure. Small peaks are found in some cases at or near the inertial period. Rotary spectra are calculated and also shown.
Abstract
In a recent paid, the author reviewed methods for estimating the mean and the standard deviation of wind direction. Some comments on this subject are now added. If large sporadic eddy fluctuations do not occur in a sampling duration, we can exactly calculate an arithmetic mean and standard deviation of wind direction by using an approach suggested by Mitsuta. This is via a single-pass procedure without considering the discontinuity in wind direction scale. By using the Mitsuta approach it is found that Mardia's method provides a good estimator of the standard deviation of wind direction even in the case when the range of fluctuations of wind direction is wide.
Abstract
In a recent paid, the author reviewed methods for estimating the mean and the standard deviation of wind direction. Some comments on this subject are now added. If large sporadic eddy fluctuations do not occur in a sampling duration, we can exactly calculate an arithmetic mean and standard deviation of wind direction by using an approach suggested by Mitsuta. This is via a single-pass procedure without considering the discontinuity in wind direction scale. By using the Mitsuta approach it is found that Mardia's method provides a good estimator of the standard deviation of wind direction even in the case when the range of fluctuations of wind direction is wide.
Abstract
Proposed single-pass methods for estimating the mean (D̄) and the standard deviation (σ d ) of wind direction and other problems in wind statistics have been evaluated using extensive field data. It can be concluded that Mardia's methods for estimating D̄ and σ d provide good estimators in practical application. Yamartino's empirical method also provides a good estimator of σ d .
The relationship between the vector mean wind speed Vv and the scalar mean wind speed Va has been investigated in connection with σ d and the standard deviation of the crosswind velocity σ v . It is found that Kampé de Fériet-Frenkiel's equation, which represents the relation for Vv and Va in terms of σ v , holds for the field data, so we can get an estmator of σ v from observed Vv and Va. The ratio of Vv/Va is roughly related to the standard deviation of wind direction, and the relation can be expressed as Vv/Va=exp(−0.5σ d 2) for σ d <20 deg.
Abstract
Proposed single-pass methods for estimating the mean (D̄) and the standard deviation (σ d ) of wind direction and other problems in wind statistics have been evaluated using extensive field data. It can be concluded that Mardia's methods for estimating D̄ and σ d provide good estimators in practical application. Yamartino's empirical method also provides a good estimator of σ d .
The relationship between the vector mean wind speed Vv and the scalar mean wind speed Va has been investigated in connection with σ d and the standard deviation of the crosswind velocity σ v . It is found that Kampé de Fériet-Frenkiel's equation, which represents the relation for Vv and Va in terms of σ v , holds for the field data, so we can get an estmator of σ v from observed Vv and Va. The ratio of Vv/Va is roughly related to the standard deviation of wind direction, and the relation can be expressed as Vv/Va=exp(−0.5σ d 2) for σ d <20 deg.
Abstract
Using the winds in Japan as an example, the validity of the vectorial treatment of the surface wind in climatological investigations has been demonstrated, and the expressions for climatological mean and variability of the wind have been reviewed. The annual resultant surface winds tend to be westerly at latitudes higher than 30° and easterly at lower latitudes. At lower latitudes, the mean surface winds and the mean 850 mb winds are almost opposite to each other in direction at times.
Climatological mean wind directions according to different definitions have been investigated for the surface wind. The means of wind direction agree with the directions of the vector mean wind within 30° where the magnitudes of the vector mean wind are larger than 1.5 m s−1. The relationships between the two measures of the constancy of the wind are investigated.
Abstract
Using the winds in Japan as an example, the validity of the vectorial treatment of the surface wind in climatological investigations has been demonstrated, and the expressions for climatological mean and variability of the wind have been reviewed. The annual resultant surface winds tend to be westerly at latitudes higher than 30° and easterly at lower latitudes. At lower latitudes, the mean surface winds and the mean 850 mb winds are almost opposite to each other in direction at times.
Climatological mean wind directions according to different definitions have been investigated for the surface wind. The means of wind direction agree with the directions of the vector mean wind within 30° where the magnitudes of the vector mean wind are larger than 1.5 m s−1. The relationships between the two measures of the constancy of the wind are investigated.
Abstract
Climatelogical aspects of the geostrophic wind calculated from geopotential heights in the planetary boundary layer in Japan have been investigated. The mean geostrophic winds at the 850 mb level nearly agree with the observed 850 mb winds in both magnitude and direction, with some exceptions. The mean geostrophic wind vectors at the 850 mb and 1000 mb levels for each site are significantly different from each other, especially in winter. This suggests that the large thermal wind exists in the 1000–850 mb layer. It is suggested that the mean geostrophic wind shear is nearly constant with height in this layer.
Abstract
Climatelogical aspects of the geostrophic wind calculated from geopotential heights in the planetary boundary layer in Japan have been investigated. The mean geostrophic winds at the 850 mb level nearly agree with the observed 850 mb winds in both magnitude and direction, with some exceptions. The mean geostrophic wind vectors at the 850 mb and 1000 mb levels for each site are significantly different from each other, especially in winter. This suggests that the large thermal wind exists in the 1000–850 mb layer. It is suggested that the mean geostrophic wind shear is nearly constant with height in this layer.
