Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over India: Overview and Results

The Cloud Aerosol Interaction and Precipitation Enhancement (CAIPEEX) was 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 and also to experimentally test the seeding hypothesis in research mode. Under this program an instrumented aircraft, seeding aircraft and weather radars were used for the measurements of aerosols and cloud parameters together with supporting ground based observations. CAIPEEX was conducted during the years 2009 to 2011 and about 670 hrs of aircraft observations were carried out. During the 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 -18C in the growing convective clouds. Ice particles were found at -4C 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 was aimed to study the impacts of natural and artificially seeded aerosols on cloud microstructure, precipitation forming processes and rainfall during the monsoon and post-monsoon seasons of 2010 and 2011. In addition, randomized precipitation enhancement experiment also has been carried out with both hygroscopic aerosols filled in flares as well as salt under the suitable conditions and in suitable warm clouds during the monsoon and post monsoon periods. Attempts have been made to see the seeding effects on the cloud microphysics by taking in-situ observations with aircraft sampling the clouds prior and after seeding. Seeded cloud volume exhibited increase in the number and size of the cloud droplets. Apart from this some case studies of cloud seeding simulation have also been carried out with regional scale models. The results of the same will be discussed.