A Rotor Circulation near the Baiu Front Observed by the MU Radar

Abstract

A rotor circulation was directly observed near the Baiu front in the lower troposphere by three-dimensional Döppler measurements with the MU radar at Shigaraki, Japan (35°N, 136°E). The temporal and vertical scale of the rotor were ∼50min and ∼2km, and the stratification observed by radiosondes was statically stable. The synoptic meteorological analysis suggests that the rotor existed just below and between Baiu-frontal banded precipitation clouds which were organized in a meso-α-scale cyclone. Precipitation echoes observed simultaneously by C/Ku-band radars were quite weak in the downdraft in front of the rotor, and became significant and tall up to ∼9km altitude at the back of the rotor circulation. The rotor was identified with a meso-β-scale depression observed by the routine meteorological network, which had a horizontal scale of ∼40km in the zonal direction and 150-200km in the meridional direction, and moved from west to east at ∼50km/h. Based on brief discussions, we conclude that the rotor circulation was locally developed from an orographic disturbance by shear instability which was occasionally induced in a weak statically-stable layer maintained by (conditional) symmetric instability.