Hierarchical Structures of Vertical Velocity Variations and Precipitating Clouds near the Baiu Frontal Cyclone Center Observed by the MU and Meteorological Radars

Abstract

We carried out a three-week observation campaign using the MU radar and meteorological (C-, X-, Ku-band) radars during the Baiu season in 1991 (17 June-8 July). A complete data set of three-dimensional atmospheric motions with high reliability and resolution was produced after removing raindrop echoes. The Baiu front was located to the south of the radar site during 17-24 June, and to the north during 25-28 June. After 29 June, surface medium-scale (meso-α-scale) cyclone centers passed near the MU observatory, and the tropopause jet stream became strong (one or two days) after the low-level jet stream appeared near the surface cyclone center. We have investigated meso-β-scale vertical velocity fluctuations and precipitating cloud clusters with temporal scales of several hours for various locations relative to the surface cyclone centers: (i) one case in the northern side of a cyclone center on the Baiu front, (ii) four cases near developing surface cyclone centers and (iii) one case in the southern side of the Baiu front far from a cyclone center. For (i) and (ii), the vertical distributions of upward-velocity regions were strongly dependent on the level of lowest stratiform turbulence near the tropopause level (LSTT) and the frontal surface. The upward velocities in the cases (ii) were associated with the developing cloud cluster on the northern side of the surface warm front, while they were dominant over the region of meso-β scale but did not always have precipitation on the northern side of the surface cold front. For (iii) some upward-velocity regions without precipitation penetrated LSTT. Each meso-β-scale fluctuation for all the cases (i)-(iii) included a number of upward-velocity peaks corresponding to meso-γ-scale disturbances. Some of them were coincident with surface rainfall (and lower-tropospheric precipitation echo) peaks. Based on all the observational evidence mentioned above, we propose a schematic picture of the ‘hierarchical structure’ of the vertical velocity fluctuations near the Baiu front. This is partly the same as the well-known multi-scale structure composed of meso-α-scale cyclones, meso-β-scale cloud clusters and meso-γ-scale precipitating clouds, but covers more broad clear regions which cannot be observed by foregoing studies based on only meteorological radars and satellites.