Dual Doppler Radar Analysis of an Intense Mesoscale Rainband Generated along the Baiu Front in 1988: Its Kinematical Structure and Maintenance Process@@@運動学的構造と維持過程について

During the Baiu Front Heavy Rainfall Experiment in 1988, a mesoscale rainband was generated along the Baiu front and heavy rainfall up to 178 mm occurred. Based mainly on dual Doppler radar observations, we have analyzed the three-dimensional configuration of radar echo and the circulation in the rainband and will describe its structure, focusing on its kinematical features and on the maintained for 7 hours in an environment in which the low-level horizontal temperature gradient was not intense and the thermodynamic instability was between that of the tropics and the midlatitudes. The rainband was 170 km long and consisted of a convective region and a trailing stratiform region. The orientation of the rainband was from the northwest to the southeast, being nearly parallel to the vertical wind shear between middle levels and lower levels. While existing convective-cells in the convective region moved along the rainband, new convective cells were successibely fromed at its southwestern edge where the low-level environmental wind entered the rainband. There were found several specific flows in the rainbandd: 1) a convective-scale updraft located at the front part of the rainband, 2) a convective-scale downdraft at the heaviest rainfall region, 3) a middle-level rear inflow with dry air from a reflectivity notch, 4) a mesoscale downdraft extending from the rear inflow, and 5) forward and backward divergent flows at the lowest level below the convective-scale downdraft. The lowest divergence flows produced a cold pool that was ∼4°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 layer in middle levels behind the rainband had an important role in driving the rainband, as well as the moist warm air at the lowest level ahead of the rainband. Upper sounding data showed that a negative temperature 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 are consitent with the kinematical structure deduced from the Doppler radar analysis. The rainband was generated in a midlatidude frontal zone, but in the lower troposphere the difference of thermodynamic conditions between ahead of and behind the rainband was very small, as in the 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