During a cold-air outbreak between 25 and 27 January 2009, a broad cloud band formed along the coastal region from San-in to Hokuriku on the Sea of Japan side of the Japanese Islands. Along the mountain flank, precipitation in the cloud band was intensified. Intensification processes of the broad cloud band and microphysical characteristics of the intensified precipitation region were examined. During the lifetime of the cloud band, two low-pressure systems successively developed in the central Sea of Japan. Between the San-in and Hokuriku coastal regions located south of the low-pressure systems, westerly winds were predominant. It can be theoretically explained that the winds are blocked at least below 900 m by a high mountain region in Hokuriku. When the predominant westerly winds flowed into a high-pressure-gradient region produced by the blocking, unbalanced flow with ageostrophic winds formed. The relatively high pressure forced the westerly winds toward the left, resulting in southwesterly winds. The southwesterlies made a convergence with the predominant westerlies, the area of which corresponded to the intensified precipitation region. The correlation coefficient between vertical and horizontal polarized return signals (ρhv) averaged for 2 days indicates that melting particles were present below a height of several hundred meters in some periods and/or regions with surface temperature higher than 0°C. Above the melting level, the radar reflectivity (Zh) maximum in the intensified precipitation region was more than or equal to 35 dBZ during one-third of the 2-day lifetime of the cloud band. For portions of Zh ≥ 30 dBZ, the mode of specific differential phase (KDP) had negative values, which indicates the predominance of prolate graupel in the intensified precipitation region.
2014 by Meteorological Society of Japan