云南省闪电活动时大气相对湿度结构特征

利用2007年6月1日—8月31日云南省闪电定位系统监测的云-地闪电资料与NCEP/NCAR再分析资料中的相对湿度物理量,分析了云南省闪电活动时相对湿度的结构特征。结果表明:云南省闪电活动除具有夏季是高发期且日分布具有呈一峰一谷变化特征,在时间演变上还具有不均匀和阶段性的特点,这与特定的大气相对湿度环境条件密切相关;云南省闪电活动一般发生在相对湿度垂直结构具有低层湿度不高、中层高湿、高层又逐渐变干的环境大气中,大约在中层700 hPa以下相对湿度随高度增加,逐渐在中层形成高湿层,中层以上相对湿度又随高度减小,低层湿度不高、中层高湿、高层偏干的环境相对湿度条件非常有利于雷暴天气的发生和闪电的形成,一方面中层以下水汽随高度增加有利于水汽的上升运动和云的凝结形成,另一方面中高层水汽随高度迅速减小有利于形成上干下湿的大气对流不稳定,促使对流云进一步发展,从而产生闪电活动;低层相对湿度在40%～75%之间,中层600～700 hPa之间相对湿度较高,可达80%以上,一般为90%～95%,中层湿层越深厚,闪电过程越强烈,在高层250～400 hPa左右相对湿度减小到35%～60%之间。

Structural Characteristics of Atmospheric Relative Humidity During Lightning Activity in Yunnan Province

Using monitored cloud-ground lightning data of lightning detection system in Yunnan Province and relative humidity parameter in NCEP/NCAR data from 1 June to 31 August in 2007, structural character-istics of atmospheric relative humidity during lightning activity are analyzed. The results show that light-ning activity in Yunnan is most frequent in summer because of high temperature and high humidity, and there's a daily apex which appears from 16:00 to 17:00 and a valley which appears from 08.00 to 11:00 be-cause of daily variation of solar radiation. The lightening activity is uneven and discontinuous on day-to-day variation because of environmental conditions of atmospheric relative humidity.Some specific environmental humidity conditions are necessary for lightening activity. Too high hu-midity may restrain ascending motion of convective development, while low humidity is insufficient for convective cloud to develop. Moreover, convective development and intensification are aroused, and con-vective cloud developing upwards is urged by some dynamic triggering condition when unstable energy runs up to a certain extent. When electric field intensity reaches to a certain extent, the thunder phenomenon occurs.Lightning activity in Yunnan usually happens in the typical environmental atmosphere with certain vertical configuration: The relative humidity is not high at 850 hPa, but at 700 hPa it's very humid and then becomes dry at upper layers. The relative humidity increases with height below the 700 hPa, forming high moist region at the middle layer and deceases above it. This environmental condition of relative hu-midity is in favor of thunderstorm weather occurrence and thunder formation. First, relative humidity in-crease below 700 hPa is favorable for ascending motion of water vapor and cloud formation. Second, dry atmosphere at upper layers and moist atmosphere at lower layers cause convective instability so that con-vective cloud may be urged to develop and promote lightning activity. When lightening occurs, relative hu-midity at low layer is about 40%-75%, at middle layers of 600-700 hPa it is above 80% and usually rea-ches up to 90%-95%, and the humidity decreases to about 35%-60% at upper layers of 250-400 hPa.It is also found that deeper middle moist layer will probably result in stronger thunder process.