DUAL-DOPPLER RADAR ANALYSIS OF AN INTENSE MESOSCALE RAINBAND GENERATED ALONG THE BAIU FRONT IN 1988 - ITS KINEMATICAL STRUCTURE AND MAINTENANCE PROCESS

During the Baiu Front Heavy Rainfall Experiment in 1988, a mesoscale r ainband was generated along the Baiu front and heavy rainfall up to 17 8 mm occurred. Based mainly on dual Doppler radar observations, we hav e analyzed the three-dimensional configuration of radar echo and the c irculation in the rainband and will describe its structure, focusing o n its kinematical features and on the maintenance process. The mesosca le rainband was generated on 17 July 1988 and was maintained for 7 hou rs in an environment in which the low-level horizontal temperature gra dient was not intense and the thermodynamic instability was between th at of the tropics and the midlatitudes. The rainband was 170 km long a nd consisted of a convective region and a trailing stratiform region. The orientation of the rainband was from the northwest to the southeas t, being nearly parallel to the vertical wind shear between middle lev els and lower levels. While existing convective-cells in the convectiv e region moved along the rainband, new convective cells were successiv ely formed at its southwestern edge where the low-level environmental wind entered the rainband. There were found several specific flows in the rainband: 1) a convective-scale updraft located at the front part of the rainband, 2) a convective-scale downdraft at the heaviest rainf all region, 3) a middle-level rear inflow with dry air from a reflecti vity notch, 4) a mesoscale downdraft extending from the rear inflow, a nd 5) forward and backward divergent flows at the lowest level below t he convective-scale downdraft. The lowest divergence flows produced a cold pool that was similar to 4 degrees C colder than the environment, and a gust front was formed between the cold pool and the moist, warm southwesterly flow ahead of the rainband. The synoptic-scale dry laye r in middle levels behind the rainband had an important role in drivin g the rainband, as well as the moist warm air at the lowest level ahea d of the rainband. Upper sounding data showed that a negative temperat ure deviation, which was made by evaporative cooling of precipitation particles, existed in the convective-scale downdraft and the mesoscale downdraft. The dry layer associated with the rear inflow reached the lowest level of the convective region. These thermodynamic features ar e consistent with the kinematical structure deduced from the Doppler r adar analysis. The rainband was generated in a midlatidude frontal zon e, but in the lower troposphere the difference of thermodynamic condit ions between ahead of and behind the rainband was very small, as in th e tropics. The rainband could be called a 'self-maintaining convective system' as well as tropical or midlatitude squall lines, rather than mesoscale systems as observed along the polar fronts of Western Europe or the Pacific coast of North America.