HIERARCHICAL STRUCTURES OF VERTICAL VELOCITY VARIATIONS AND PRECIPITATING CLOUDS NEAR THE BAIU FRONTAL CYCLONE CENTER OBSERVED BY THE MU AND METEOROLOGICAL RADARS

We carried out a three-week observation campaign using the MU radar an d meteorological (C-, X-, Ku-band) radars during the Baiu season in 19 91 (17 June-8 July). A complete data set of three-dimensional atmosphe ric motions with high reliability and resolution was produced after re moving 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. Aft er 29 June, surface medium-scale (meso-alpha-scale) cyclone centers pa ssed near the MU observatory and the tropopause jet stream became stro ng (one or two days) after the low-level jet stream appeared near the surface cyclone center. We have investigated meso-beta-scale vertical velocity fluctuations and precipitating cloud clusters with temporal s cales of several hours for various locations relative to the surface c yclone centers: (i) one case in the northern side of a cyclone center on the Baiu front, (ii) four cases near developing surface cyclone cen ters and (iii) one case in the southern side of the Baiu front far fro m a cyclone center. For (i) and (ii): the vertical distributions of up ward-velocity regions were strongly dependent on the level of lowest s tratiform 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-beta scale but did not always have precipitation on the northern side of the surface cold front. For (iii) some upward-velocity regions without precipitat ion penetrated LSTT. Each meso-beta-scale fluctuation for all the case s (i)-(iii) included a number of upward-velocity peaks corresponding t o meso-gamma-scale disturbances. Some of them were coincident vith sur face rainfall (and lower-tropospheric precipitation echo) peaks. Based on all the observational evidence mentioned above, we propose a schem atic 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 mese-alpha-scale cyclones, mes o-beta-scale cloud clusters and meso-gamma-scale precipitating clouds, but covers more broad clear regions which cannot be observed by foreg oing studies based on only meteorological radars and satellites.