Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season
Statistical Study on Cloud Top Heights of Cumulonimbi Thermodynamically Estimated from Objective Analysis Data during the Baiu Season
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DOI：
10.2151/jmsj.85.529
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发表时间：
2007-02
影响因子：
3.1
通讯作者：
T. Kato;S. Hayashi;M. Yoshizaki
中科院分区：
文献类型：
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作者：
T. Kato;S. Hayashi;M. Yoshizaki
利用水平分辨率为20 km的客观分析资料，对2001-2005年白乌季日本列岛附近的中性浮力水平进行了统计检验，以阐明积雨云形成和发展的周围大气条件。LNB出现率的垂直分布在海上和陆地上分别有三个和两个峰值。6月高层（150 hPa）峰值很弱，低层（900 hPa）峰值在陆地上没有出现。另一个峰值出现在中层（N700 hPa），理论研究表明，当低层温度变化在几度之内时，白乌季LNB强烈地依赖于中低层温度的直减率。这是因为在Baiu季节，LNB随着直减率的增加而迅速变高。统计结果表明，日本列岛周围的直减率强烈地依赖于上游地区的对流活动，特别是中国南部。因此，6月白鸥锋区的强对流活动在日本列岛附近产生中层LNB，抑制积雨云的发展。7月，强对流活动区的北移大大地解除了这种与白乌锋区有关的抑制，中层LNB也频繁出现在白乌锋区附近的其它地区，但在北太平洋西部地区很少出现。因此，表明云顶低于几公里的积雨云存在的中层LNB是白乌锋区的特征之一。
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.