(Invited Speaker) Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over India: An overview

Surrounded by warm oceans leading to high level of humidity coupled with high level of aerosol loading over the region, makes the modification clouds by aerosols over the Asian Monsoon region intriguing and challenging to understand. Lack of adequate simultaneous high resolution observations of aerosols and cloud microphysical properties leaves estimation of indirect effect of aerosols on climate uncertain. The Cloud Aerosol Interaction and Precipitation Enhancement (CAIPEEX) is being conducted in India by the Indian Institute of Tropical Meteorology (IITM) with the objective to understand the pathways through which aerosols modify clouds and precipitation processes. Under this program an instrumented aircraft was used for the measurements of aerosols and cloud parameters together with supporting ground based observations. During first phase of CAIPEEX, intensive survey of cloud and aerosol observations were carried out all over India during May to September 2009. During pre-monsoon as well as monsoon conditions high aerosol concentrations were observed at surface as well as at higher altitudes and were found to control the rain processes in the convective clouds. Super cooled water was found till -18° C in the growing convective clouds. Ice particles were found at -4° C over the Indian sub-continent. The cloud droplets were found to trigger the collision coalescence process at effective radius of 10µm, more significantly above 12µm. Over the sea, sea salt was found to naturally seed the convective clouds resulting in early rain formation. Over land, high concentrations of cloud condensation nuclei push the cloud depth even up to 6 km for onset of precipitation, indicating the formation of super cooled water. Over the rain shadow areas aerosols were found to suppress warm rain process and precipitation was observed to start in mixed phase.

In a country which is largely dependent on agriculture, cloud seeding is considered to be an alternative technique for the source of water. Second phase of CAIPEEX is aimed to study the impacts of natural and artificially seeded aerosols on cloud microstructure, precipitation forming processes and rainfall during August to October 2010. In addition, randomized precipitation enhancement experiment will be carried out with respective type of aerosols under the suitable conditions.