The Index of Coalescence Activity (ICA) is an indicator of the microphysical state of the cloud. ICA is related to cloud base temperature and buoyancy of cloud at 500 hPa level. It is a useful parameter for making a decision on the type of seeding to be carried out. During Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) over the Indian region, aircraft operations for in situ measurements of aerosols and clouds were conducted from the base station Hyderabad to cover parts of north peninsular India falling under the rain-shadow region. Special Radiosonde flights were conducted in the afternoon hours prior to aircraft operations from the locations Hyderabad and Mahabubnagar in addition to regular India Meteorological Department (IMD) Radiosonde flights at 00 UTC from Hyderabad. The parameters like cloud base height and buoyancy at 500 hPa level obtained from Radiosonde flights have been used to estimate ICA values during the period of the experiment (June to October 2009–2011). The variability of ICA has been studied using morning and afternoon sounding profiles over the rain-shadow region. Morning hours showed highly negative ICA values, as compared to noon hours. During noon time, positive ICA values are observed before monsoon onset (in 2009, monsoon onset was 21st June over the study area) and on few occasions during post-monsoon (October). Positive ICA values represent that collision-coalescence (CC) process is not fully developed and hence, hygroscopic seeding is the proper decision. While, negative ICA values indicate active CC process, suggesting glaciogenic seeding. The soundings conducted just before the cloud seeding operations provide information about the actual thermodynamic and microphysical state of the cloud. Hence, ICA information is useful for providing guidelines to the operational cloud seeding programs.

中文翻译：

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季风期间雨影区域合并活动指数 (ICA) 的变化及其在印度人工降雨计划中的作用

聚结活动指数 (ICA) 是云的微物理状态的指标。 ICA与500 hPa云底温度和云浮力有关。它是决定要进行的播种类型的有用参数。在印度地区的云气溶胶相互作用和降水增强实验（CAIPEEX）期间，从海得拉巴基站对气溶胶和云进行了现场测量，覆盖了印度北部半岛雨影区的部分地区。除了世界标准时间 00 点从海得拉巴起飞的定期印度气象部 (IMD) 无线电探空仪飞行外，在飞机作业前的下午几个小时，还从海得拉巴和马哈布纳加尔进行了特殊无线电探空仪飞行。从无线电探空仪飞行获得的云底高度和 500 hPa 水平的浮力等参数已用于估计实验期间（2009 年 6 月至 2011 年 10 月）的 ICA 值。利用雨影区域上午和下午的探测剖面研究了 ICA 的变化。与中午相比，早上的 ICA 值呈高度负值。中午期间，在季风爆发前（2009 年，研究区域季风爆发时间为 6 月 21 日）观察到正 ICA 值，在季风后（10 月）期间也偶尔观察到 ICA 正值。正 ICA 值表示碰撞聚结 (CC) 过程尚未完全发展，因此吸湿播种是正确的决定。而负 ICA 值表明 CC 过程活跃，表明冰川形成过程。在播云操作之前进行的探测提供了有关云的实际热力学和微物理状态的信息。因此，ICA 信息对于为运营云播种计划提供指导非常有用。