We conducted observations of cirrus clouds using a millimeter-wave radar as well as the MU (Middle and Upper atmosphere) radar to investigate physical structure of cirrus clouds at the Shigaraki MU Observatory in Shiga prefecture, Japan, in October, 2000, May, 2002, and, June, 2002. A millimeter-wave radar has sensitivities for small crystals of cirrus clouds because of their short wavelength (approximately 8 mm), so that it is one of the most effective instruments for cirrus cloud observations. The millimeter-wave radar we used has a steerable antenna, so that we can observe three-dimensional structure of cirrus clouds. The MU radar, which is a kind of wind-profilers, can observe atmospheric motion especially including vertical velocity.

We compared echo power obtained by the millimeter-wave radar with the atmospheric motion obtained by the MU radar, in the six cases of October 3, 5, 6, and 7, 2000, May 18, 2002, and June 28, 2002. Furthermore, we used radiosonde data launched at the radar site on October 5 and 6, 2000, May 18, 2002, and June 28, 2002. When cirrus clouds appeared, updraft was generally dominant below clouds.

Growth or decay of cirrus clouds became obvious from time variations of observed echo power with RHI (Range Height Indicator) mode of the millimeter-wave radar. Vertical shear of horizontal velocities was strong above the cloud base in all cases. Vertical shear was particularly strong and the strong shear layer was thick on October 3, 6, and 7, 2000, and May 18, 2002. In their four cases, vertical shear was enough strong to generate Kelvin-Helmholtz instability (KHI), and thick and growth clouds (over 1.5 km) appeared. When vertical shear was very strong, in addition, strong cell echoes were observed inside clouds. On the other hand, in other two cases, vertical shear was not enough strong to generate KHI, and clouds were thin (under 1.5 km). When vertical shear was enough strong to generate KHI, the strong shear layer was mixed layer where equivalent potential temperature was almost constant, and clouds were developed. On the other hand, when vertical shear was not enough strong to generate KHI, the strong shear layer was stable, and clouds were undeveloped. Furthermore, when cirrus clouds were developed, the horizontal structure of clouds, which were deduced from echo power in CAPPI (Constant Altitude Plan Position Indicator) mode, were inhomogeneous and striped. On the other hand, when cirrus clouds were undeveloped, the horizontal structure was almost homogeneous. Therefore, we consider that KHI may play an important role in the growth of cirrus clouds.