Spectrum Climatology of the Surface Winds in Japan

Part I: The 40-60 Day Fluctuations

Abstract

The regional variations of the spectra of the surface winds in Japan are investigated from the viewpoint of spectrum climatology. For 206 stations in Japan, the spectra of the surface winds over periods from 2 hours to a few years are analyzed using 5 years of 10-min averaged, hourly data.
Characteristics of the spectra can be used as a climatic indicator. The shape of the spectrum at short periods (less than 10 days) depends on the local topography, and that at long periods (more than 10 days) is related to the geographical location. Most of the spectra are classified into three categories: The first is a strong wind type with a major synoptic peak, the second a strong wind type with a pronounced annual peak, the third a weak wind type with a major diurnal peak. But the spectra on the Nansei Islands (24-31°N, 123-130°E) are quite different from those on the main islands of Japan. The spectra on the Nansei Islands show two marked features: Firstly, the 40-60 day periodicity is predominant. Secondly, the synoptic portion of the spectra has a wide range of periods from 2 to 20 days. These features are related to the climate on the Nansei Islands, which is maritime and subtropical. The geographical distribution of the 40-60 day fluctuations is in good agreement with that of the zonal mean components of the surface winds. The 40-60 day fluctuations are predominant during summer half year and are associated with strong winds.