Polarimetric doppler radar analysis of organization of a stationary rainband with changing orientations in July 2010

A stationary rainband brought heavy rainfall across Gifu and Aichi prefectures, Japan, on 15 July 2010. The orientation of the rainband was initially southwest–northeast, and then changed from west-southwest to east-northeast, before reverting to its original orientation, although the rainband remained stationary over the same area. This study analyzes the organization of the rainband during these three orientation periods using polarimetric Doppler radar. The rainband was maintained by south-southwesterly inflows of high equivalent potential temperature (≥340 K) below 2 km, while southwesterly winds prevailed at middle level during the rainfall. It is suggested that these rainband orientations were determined by the travel directions of the convective cells and positions of cell generation relative to the rainband, which in turn were governed by intensities of low-level inflow and cell-origin outflow. During the first orientation period, convective cells formed over a wide area within the rainband and traveled northeastward. Low-level outflows from deep convective cells in the northern section of the rainband shifted the cell-generation area southward, and enhanced south-southwesterly inflows caused the southwestern portion of the rainband to drift slightly to the north; hence, the rainband was oriented from west-southwest to east-northeast. The convective cells were deeper during the second period and low-level outflows were stronger than those in the first stage. The strong outflows formed a cell generation area on the southern lateral side of the rainband, while enhanced low-level inflows contributed to the north-northeastward motion of the generated cells crossing the rainband at an angle of 45°. The outflows and south-southwesterly inflows then weakened, and convective cells formed successively on the southwestern edge of the rainband and moved to the northeast. As a result, the rainband reverted to its original southwest-northeast orientation.