The structure of a polar low developed over Chukchi Sea during the Arctic cruise of the R/V Mirai of October 2009 was investigated using Dopper radar, radiosondes and satellite observation data. A well-defined 800 km wide comma cloud was identified by satellite imagery. The R/V Mirai passed just below the centre of the polar low with continuous Doppler radar and radiosonde observations. Three-hourly radiosonde observations showed there was a cold dome in the vicinity of the central region of the polar low from middle to upper troposphere, suggesting the intrusion of the upper-level potential vorticity anomaly and tropopause fold. From the vertical structure of horizontal wind field derived from the Doppler velocity, easterly winds prevailed in the northern convective clouds from 2 to 5 km level, while westerly winds dominated in its southern region. This is likely a cyclonic circulation induced by the upper-level potential vorticity anomaly. A role of lower diabatically generated positive potential vorticity on the rapid intensification of the polar low will be discussed.