Turbulence characteristics and micrometeorological structure of Atmospheric surface layer in stable stratification in Antarctica
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概要
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The author, as a meteorological research member of the Wintering Party of the Eleventh Japanese Antarctic Research Expedition in 1969-1971, carried out micrometeorological observations at Syowa Station in Antarctica during the period from February 1970 to February 1971. The results have been reported in the following papers: MAKI (1971); MAKI (1972a, b, c, d); and MAKI (1974a, b). The observations on the structure of the atmospheric turbulence near the ground were made with the sonic anemometer-thermometer for the first time as the main theme of the meteorological research project. This investigation was carried out for the first year in the four year project of "Explanation of Formation and Structure of Antarctic Anticyclone". An atmospheric turbulence in stable stratification was measured on the 21 m-high tower built on the sloping terrain of Syowa Station during the period from February to December, 1970. Its characteristics are analyzed in relation to the stability (Richardson number, Ri) in the atmospheric surface layer. The vertical mean wind speed at the height of 20 m is observed to be about one- third of the longitudinal mean wind speed at the same height (U_<20>) above the slope with the inclination of about 10 degrees. The standard deviations of the longitudinal, lateral and vertical wind speed fluctuations decrease exponentially with the increase of Ri. The ratios of the standard deviations of the longitudinal, lateral and vertical wind speed fluctuations to the friction velocity are independent of Ri. The energy dissipation rate increases roughly in proportion to the 0.5 power of Ri. The scale of the atmospheric turbulence seems to be a solid body of a long and narrow cubic vortex having the dimensions of length : breadth : height=6.3:1.9:1. The charac- teristic scale of the atmospheric turbulence which is the largest turbulon (INOUE et al., 1955) is independent of the stability (Ri). Furthermore, the scale of the smallest turbulon increases exponentially with the value of Ri. The nondimensional frequency at the maximum spectral density of the longitudinal wind speed increases with the increase of Ri. On the other hand, the peak frequency of the lateral wind speed is independent of Ri, and that of the vertical wind speed decreases with the increase of Ri. The interrelations among the wind direction, the wind velocity, the air tempera 2 Atmospheric Turbulence and Structure in Stable Layer ture and the vertical air temperature gradient have become clear as follows: In winter, the frequency of the prevailing wind direction is not higher than that in other seasons. The frequencies of the southern wind directions of S and SSE, the frequency of the dead calm (below 0.1 m/s) and the wind speed of the prevailing wind in winter are higher than those in other seasons. The air temperatures are high in the wind directions of NNE to E, and low in the wind directions of SSW and SW. The vertical air temperature gradient in stable stratification is large in the wind directions of ESE to SSW and decreases rapidly with the increase of the wind velocity below about 10 m/s and gently above about 10 m/s. The warming ratio which is defined as the ratio of the increase of air tempera- ture to the increase of wind velocity shows a significant annual variation. The warm- ing ratio is 2.9℃ per 10 m/s in spring, 1.3℃ in summer, 3.0℃ in autumn and 5.0℃ in winter, particularly 5.8℃ in May and August and 0.5℃ in December. The vertical air temperature gradient decreases 0.0057℃/m with the 1℃ increase of air temperature. The frequency of stable condition is about 88% in winter, about 30% in summer and about 75% throughout the year. The maximum of the frequency distribution of the vertical air temperature gradient shifts from the maximum of the normal distribution to the unstable side in winter, and vice versa in summer. The Kernlose-type phenomenon also appears in the annual variations of the tropospheric maximum air temperature an