Airflow structure of clear-air and precipitation echoes observed by an X-band Doppler radar over the Nobi Plain in Japan in the summer of 2010

Detection of airflow and turbulence in a clear air, before formation of precipitation particles in a convective cloud and after dissipation of precipitation echo, is important for understanding formation and structure of convective clouds, as well as airflow associated with precipitation clouds. Clear-air echoes that caused by fraction of air or turbulence is detectable by precipitation radars in moist environments. During a summer season from August to September of 2010, we made an attempt of clear air and precipitation observation using an X-band Doppler radar over the Nobi Plain. The Nobi Plain is located on the Pacific side in the central parts of Japan and faces the Ise Bay to the south. In this study, we clarified wind field in clear air and precipitation echoes on the Nobi Plain during a summer season and discussed possibility of clear-air observation using an X-band Doppler radar.

The clear-air observation was carried out using a Nagoya University's X-band polarimetric radar (NUPol). The NUPol radar was placed at Nagoya University located on the Nobi Plain. The radar was operated in high pulse repetition frequency (PRF) mode to gain a large number of pulses for integration. Settings of the high PRF mode are followings. The PRF was 20000 ppm, and the Nyquist velocity was 10 m s-1. The number of pulse integration was 40000, and the antenna rotation rate was 0.1 rpm. The pulse width was 1.0 micro second. For the observation reported here, the NUPol performed plan position indicator (PPI) scans at an elevation of 4 degrees. The observation range is 4.2 km in radius. In the high PRF mode, the radar transmits only horizontally polarized wave. The production data by the radar in the high PRF mode was spectrum data of Doppler velocity for each range bin. The received intensity, Doppler velocity, and velocity dispersion were calculated from each of the spectrum. Ground clatter signals were removed from the spectrum. Data of wind profiler operated by Japan Meteorological Agency (JMA) were used for validation of Doppler velocity measured by the NUPol. The wind profiler is 1.9 km north-northwest of the radar along the 343 azimuth angle. Data of X-band polarimetric radars operated by Ministry of Land, Infrastructure and Transport (MLIT) were subsidiarily used to detect convective precipitation echoes.

On 25 August 2010, the NUPol radar showed distinguished echoes in a no-precipitation field between 1000 LST (= UTC + 9) and 1400 LST over the observation area. Doppler velocity observed as clear-air echo by the radar showed that prevailing wind shifted from easterly wind to southerly wind between 1000 LST and 1230 LST. The southerly wind from ocean was approximately 3.5 m s^-1. The time when the southerly wind predominated in the radar observation range is in good agreement with a beginning time of a development of southerly-wind layer shown in a vertical distribution of wind observed by a JMA wind profiler (1200-1300 LST). The southerly wind corresponds to sea breeze, and the shift from easterly wind to southerly wind is considered to be inflow of the sea breeze. When the southerly wind from ocean was distinguished, a precipitation cell formed in an observation range of the NUPol. The cell possibly formed and developed in the sea breeze front. In and around the precipitation echo, there were four turbulence flow regions embedded in a southerly wind at least. The horizontal scale of turbulence was approximately 5-10 km. After the precipitation echo dissipated, Doppler velocity observed by the NUPol showed turbulence flow regions even though there was no precipitation echo by MLIT radars in the NUPol's observation area.

On 17 September 2010, north-westerly wind dominated over the Nobi Plain, and no precipitation echo was detected by weather radars. The Doppler velocity of clear-air echoes observed by the NUPol radar showed north-westerly wind of approximately 8 m s^-1. This value is in good agreement with that of the JMA wind profiler. Although the radar detected Doppler velocity of clear-air echoes, its signal was weak and was not clear. This suggests that in the north-westerly wind clear-air echoes were hard to detect by the X-band radar. This is because the north-westerly wind was considered to include less water vapor compared to that of southerly wind of a case on 25 August 2010.

This study showed a detailed distribution of turbulence associated with a convective precipitation cloud forming in a southerly wind and a probability of detection of airflow structure in clear air in a summer season by an X-band Doppler radar. The clear-air observation is found to be useful for southerly wind of moist air over the Nobi Plain during the summer season.