Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season

Abstract

The level of neutral buoyancy (LNB) around the Japan Islands during the Baiu season in 2001-2005, estimated by lifting a low-level air with the maximum equivalent potential temperature in a vertical direction, is statistically examined from objective analysis data with the horizontal resolution of 20 km, in order to clarify the surrounding atmospheric conditions under which cumulonimbi can form and develop. Three and two peaks are found in the vertical profiles of the appearance rate of the LNB over the sea and on the land, respectively. The high-level (∼ 150 hPa) peak is very weak in June, and the low-level (∼ 900 hPa) peak does not appear on the land. The other peak is found at the middle level (N 700 hPa).
The theoretical study shows that the LNB during the Baiu season strongly depends on the lapse rate between the low and middle levels, when the variation of the low-level temperature is within a few degrees. This is because the LNB during the Baiu season rapidly becomes higher as the lapse rate increases. The statistical results show that the lapse rate around the Japan Islands strongly depends on convective activities over upstream regions, especially the southern part of China. Therefore, strong convective activities over the Baiu frontal zone in June produce the middle-level LNB around the Japan Islands to suppress the development of cumulonimbi. In July, the northward shift of the area with strong convective activities considerably releases such suppression, associated with that of the Baiu frontal zone.
The middle-level LNB also frequently appears over the other areas around the Baiu frontal zone, although it scarcely appears over the western part of the North Pacific Ocean. Therefore, the middle-level LNB that suggests the existence of cumulonimbi with a cloud top lower than several kilometers is one of the characteristic features around the Baiu frontal zone.