For the purpose of the study of the mesoscale winter snow storms, the dual Doppler radar system of Nagoya University was installed in the coast of Hokuriku district during the period from December 2000 to February 2001. When an outbreak of the cold continental airmass occurs over the Sea of Japan, many cloud bands develop over the sea. Cloud bands which are almost perpendicular to the lower-level wind are referred to as ``transverse-mode clouds''. They often form in the western Sea of Japan. During the observation period, the transverse-mode clouds were observed on December 26, 2000 and January 14, 2001. Snowbands were observed by the dual Doppler radar system in the area where the transverse-mode clouds were observed by meteorological satellite. Since the snowbands are parallel to the transverse-mode clouds and almost perpendicular to the lower-level wind, they are referred to as the ``transverse-mode snowband''. Because the transverse-mode clouds and snowbands form and develop over the sea, they are difficult to be observed. Therefore, the internal structure and development process are not clarified in the past study. In this study, in order to clarify formation process of the transverse-mode snowband in the area of transverse-mode clouds, we investigated radar echo data and Doppler velocity data obtained from the dual Doppler radar system of the Nagoya University and the dual polarization Doppler radar of Hokuriku Electric Power Company.

The radar echo shows a transverse-mode snowband which has a length of 100 km blow the transverse-mode clouds. The transverse-mode snowband consisted of several short bands of a few tens kilometers. The short bands composed of one or several convective cells. We defined the short bands as "Transverse mode Band Element (TBE)." The development process of the transverse-mode snowband is development and linking the TBEs. As a result, the transverse-mode snowband extended more than 100 km.

Furthermore, three kinds of development processes of TBEs are found; extensions of TBEs to the Upshear side, to the Downshear side, and to the both sides. For these development processes ,the following two mechanisms are inferred. When the TBEs extend to the upshear side, a new cell forms on the side. On the downshear side of TBEs, a weak lower-level convergence could be present and this is favorable for development of another cell or band. When it move to the downshear side of the TBEs, they develop on the downshear side. As a result, the TBEs could extend to the downshear side.