[1] Three-dimensional fog roll structure induced by Kelvin-Helmholtz instability (KHI) is studied, based on observations made with a millimeter-wave scanning Doppler radar and a rawinsonde. Roll structure in a fog layer was observed below 150 m altitude when extremely large vertical shear (exceeding 100 m s À1 km À1 ) existed at 50 -60 m altitude. The direction of the shear was perpendicular to the roll direction. The Richardson number was less than 0.25 at the shear altitude, which indicates that the fog roll structure was induced by KHI.

Observations of fogs with a millimeter-wave scanning Doppler radar were conducted at Kushiro in Hokkaido, Japan, in the summer seasons of 1999 and 2000. Three typical types of plan position indicator (PPI) displays were observed: cellular echoes with high radar reflectivity factors (∼−10 dBZ), uniformly distributed echoes with high reflectivities (∼−10 dBZ), and uniformly distributed echoes with low reflectivities (∼−30 dBZ). The authors focused on advection fog with cellular echoes observed on 5 August 1999 and 31 July 2000. Echoes showed structures of cells with a reflectivity of −10 dBZ and with intervals of about 1 km. This echo pattern moved northward (i.e., from the sea to the land). There was a vertical shear of the horizontal wind at a height around 200 m in both cases, and structures of each cell were upright above the shear line and were leaning below it. The direction and the speed of the echo pattern in both PPI and range–height indicator (RHI) displays agreed well with that of the horizontal wind at heights above the shear (200 m). In the echo cells, existence of drizzle drops is implied.

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A special fog observation campaign was conducted in the Miyoshi basin, Hiroshima prefecture, Japan during the period November 7-15, 2000. We observed the spatial distributions of fogs and their movements using a millimeterwave scanning radar. This is the first time that the distribution of basin fogs associated with fog development and decay processes has been examined. Echo intensity observed with the radar, which is mainly associated with fog particle size, was almost under −23 dBZ at levels below 200 m in height. The horizontal distribution of echo intensity changed with time. Namely, weak echoes were observed over nearly all observation areas at first, and then the echoes gradually became stronger as the fogs developed, although the echoes were weaker at higher levels. After sunrise, the echoes decayed. During the developing periods, the occurrence ratio of the echo intensity between −38 and −23 dBZ increased from the lower height, while the ratio decreased from the higher levels during the decay periods. This feature in the developing period is consistent with the results of optical measurements but the feature in the decaying period is inconsistent. It is suggested that this inconsistency is due to the difference in sensitivity between the two measurement approaches.

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