Characteristics of upward lightning by interferometric measurement are discussed. The data set is obtained in winter at Hokuriku-district of Japan. We analyzed both positive and negative upward leaders which ware initiated from the top of the 50m-tall tower that is located at 4km away from the observation site. The interferometric system is operating at center frequency of 327MHz with a bandwidth of 350kHz and the time resolution of 1μs.
The comparative study of the positive and negative upward leaders shows that the intensity of UHF radiation caused by a negative leader is 10dBm higher than the upward positive leader. It also shows that a upward negative leader is preceded by a cloud discharge which may enhance the electric field at the tower top. In case of upward positive leader, no cloud discharge prior to the initiation of the upward leader, are observed.