MAINTENANCE MECHANISM AND THERMODYNAMIC STRUCTURE OF A BAIU FRONTAL RAINBAND RETRIEVED FROM DUAL-DOPPLER RADAR OBSERVATIONS

During the intensive observation period of the Baiu Front Heavy Rainfall Experiment in July 1988, a Baiu frontal rainband passed over the Kyushu District, and its kinematic structure was observed by dual Doppler radar. The purposes of this paper are to investigate the thermodynamic structure of the rainband using a retrieval method and to clarify its maintenance mechanism. The rainband had many similarities to those for tropical and mid-latitude squall lines. The dry rear inflow from the northeast merged with the convective downdrafts and resulted in low-level divergent outflows. The warm and moist southwesterly converged with the outflow directed toward the leading edge and caused the rearward sloping convective updrafts. The retrieved perturbation temperature field showed a positive anomaly in the convective updrafts above a height of 4 km, and a negative anomaly due to evaporation of raindrops in the dry rear inflow behind the gust front. The perturbation pressure pattern was consistent with the temperature and flow patterns and showed a meso-high behind the gust front and a meso-low beneath the warm anomaly of the convective updraft. In the low-level of the stratiform region, a meso-low was generated owing to the subsidence warming of the descending rear inflow. The distributions of temperature and pressure perturbations suggest that the convective updraft was maintained by the upward pressure gradient force below a height of 4 km, while it was maintained by a positive buoyancy above this level. The horizontal pressure gradient force associated with the mid-level meso-low in the convective region accelerated the lifted air to the rear, and the convective updraft tilted to the rear of the system. The raindrops were transferred to the rear of the system and fell into the convective downdraft region, which did not prevent the convective updraft from developing and the intense rainband was maintained.