A ROTOR CIRCULATION NEAR THE BAIU FRONT OBSERVED BY THE MU RADAR

A rotor circulation was directly observed near the Baiu front in the l ower troposphere by three-dimensional Doppler measurements with the MU radar at Shigaraki, Japan (35-degrees-N, 136-degrees-E). The temporal and vertical scale of the rotor were approximately 50 min and approxi mately 2 km, and the stratification observed by radiosondes was static ally stable. The synoptic meteorological analysis suggests that the ro tor existed just below and between Baiu-frontal banded precipitation c louds which were organized in a meso-alpha-scale cyclone. Precipitatio n echoes observed simultaneously by C/Ku-band radars were quite weak i n the downdraft in front of the rotor, and became significant and tall up to approximately 9 km altitude at the back of the rotor circulatio n. The rotor was identified with a meso-beta-scale depression observed by the routine meteorological network, which had a horizontal scale o f approximately 40 km in the zonal direction and 150-200 km in the mer idional direction, and moved from west to east at approximately 50 km/ h. Based on brief discussions, we conclude that the rotor circulation was locally developed from an orographic disturbance by shear instabil ity which was occasionally induced in a weak statically-stable layer m aintained by (conditional) symmetric instability.