A Preliminary Survey of Clear-Air Echo Appearances over the Kanto Plain in Japan from July to December 1997

Abstract

A preliminary survey of clear-air echo appearances over the Kanto plain in Japan from July to December 1997 is presented. A distinct contrast was seen between clear-air echoes observed over land and their absence over the ocean. During July to late November, clear-air echoes were observed 40%?80% of the time during days when no precipitation occurred. The clear-air echo was not observed between late November and the end of the study period (December). The drop-off of clear-air echo was associated with daily average surface temperatures dipping below 10°C. During this period, temperatures would often increase above 10°C for short periods but were not associated with clear-air return. The diurnal variation of clear-air echo appearances had two peaks at the periods of the daytime and the twilight. During July to late November, the correlations of clear-air echo appearances with surface temperature, wind direction, and sunshine duration were significantly low. In July and August, there appeared to be a strong correlation with surface wind speed. During the daytime, the appearance and depth of clear-air return is related to the evolution of the mixed boundary layer. However, during early morning and evening hours, the thermodynamic structure of the boundary layer is unrelated to the structure of the clear-air return. For operational use, it is shown that the clear-air boundary layer is rich in these echoes and new scan strategies and algorithms for them would be worth developing. On the other hand, there are some limitations due to the asymmetrical area coverage, diurnal and seasonal variations, and sensitive dependence on boundary layer conditions. The accuracy operational use of the echoes requires data care and frequent monitoring of boundary layer conditions. A possible relationship between clear-air echo appearances and the behavioral habits of aerial plankton is suggested.