HEAVY RAINFALL OVER THE BAIU FRONTAL ZONE AROUND JAPAN - RELATION WITH CLOUD TOP HEIGHTS OF CUMULONIMBI

摘要

The Baiu front is a specific front that is analyzed along a large equivalent potential temperature (θ_e) gradient region with the strongest convective activities over the Baiu frontal zone. This is because convective activities lift high-θ_e air flowing into the synoptically produced Baiu frontal zone and consequently a large θ_e gradient is produced there. The Baiu front is usually analyzed on the northern part of the Baiu frontal zone as "a result of convective activities", and it is not analyzed when convective activities vanish or become very weak. However heavy rainfall is often observed on the southern part of the Baiu frontal zone where the Baiu front is not analyzed on weather maps.Heavy rainfall is caused by successive formation of developed cumulonimbi. Environmental conditions that such cumulonimbi can form are often found on the southern side of the Baiu frontal zone in western Japan. There are three favorable conditions for the development of cumulonimbi: (i) the inflow of low-level high-θ_e air, (ii) less convective activities on the upstream side, and (iii) the pre-existence of updrafts over the Baiu frontal zone. Low-level high-θ_e air is transported to western Japan along the edge of the Pacific high pressure zone. On the northern part of the Baiu frontal zone, middle-level air warmed through convective activities on the upstream side (i.e., over Continental China) is often transported to the Japan Islands by westerly winds, and they stabilize the latently unstable atmospheric condition there. On the southern part of the Baiu frontal zone, since such transport of warmed air is less, the atmosphere is not usually stabilized and a relatively large lapse rate maintains there for tall cumulonimbi to develop. Moreover, higher-θ_e air is easily lifted to the level of free convection, and it usually flows from the south into the Baiu frontal zone. These are major factors to cause heavy rainfall on the southern part of the Baiu frontal zone.Statistical studies on cloud-top heights of cumulonimbi (CTOPs), using objective analysis data and simulation results of a cloud-resolving model, suggest that major CTOPs appear separately at different levels below 5 km and above 10 km. This means that either warm-rain or cold-rain type cumulonimbi (i.e., convection with a middle-level CTOP, not shallow convection, or deep convection) forms dependent on environmental conditions. Coldrain type cumulonimbi often form in western Japan in the second half of the Baiu season (i.e., July), while simulated CTOPs exceeding 10 km appear at a considerably lower rate than those below 5 km. However, the contribution rate to total rainfall amount of cold-rain type cumulonimbi is comparable with that of warm-rain types. On the other hand, in the first half of the Baiu season (i.e., June), CTOPs exceeding 10 km are rarely simulated and warm-rain type cumulonimbi contribute almost the total rainfall amount in western Japan. These results strongly depend on the lapse rate between the low and middle levels, which is mainly determined by the inflow of middle-level air warmed through convective activities on the upstream side, especially over Continental China.