Hygroscopic Cloud Seeding Operations In Andhra Pradesh, India during 2005-07

In recent years, there has been an increased awareness that human activities may be causing climate change. Since the transfer of solar and infrared radiation represents the primary physical process that drives the circulation of the atmosphere, it is necessary to understand the radiative processes of the Earth and atmosphere in order to understand the mechanisms of climate change. These efforts require extensive infrastructure for Cloud physics measurements and analysis. Triggering of the precipitation process and scope for its enhancement in tropical regions is one of the major topics requiring extensive insitu observations and modeling efforts. The Most convective clouds in the tropics often develop precipitation via the so-called “warm rain process” where cloud droplets have sufficient time to grow to large enough sizes via condensation to start colliding and coalescing with each other and form raindrops.

In the past ten years, a new approach to hygroscopic seeding has been explored in summertime convective clouds in South Africa as part of the National Precipitation Research Program. This approach involves seeding summertime convective clouds below cloud base with pyrotechnic flares that produce particles on the order of 0.5 micron diameter in an attempt to broaden the initial cloud droplet spectrum and accelerate the coalescence process. The burning flares provide larger CCN (>0.3 micron diameter) to the growing cloud, influencing the initial condensation process and allowing fewer CCN to activate to cloud droplets. The Cloud seeding experiments carried out in Andhra Pradesh for periods varying from 90 to 120 days in each year during 2005-2007 have provided good amount of cloud data regarding cloud parameters in terms of cloud top heights, cloud centroid, precipitation flux, cloud mass, rain mass and cloud life time.

Analysis of the data clearly showed the difference in microphysical characteristics of the clouds which are subjected to hygroscopic seeding and clouds not subjected to seeding. In many cases, convective precipitation development through collision-coalescence is being considered as a two stage process involving production of large embryos that have the potential to grow into raindrops, and the subsequent growth of this embryos to precipitable sizes. The onset of the collision-coalescence process is found to be dependent on the initial cloud droplet concentrations, size distribution and the amount of liquid water content. The initial droplet concentration and size distribution is again dependent on the concentration, size and chemical composition of the cloud condensation nuclei (CCN) in the atmosphere while the CCN depends on the characteristics of the aerosols in the local region.

The essential thing in weather modification activities is the evaluation of results. The primary difficulty arises from the unavoidable fact that no two clouds or even two storm systems are exactly the same, and one cannot simply treat (seed) one, and not the other, and then observe the differences. Evaluations on a cloud or storm basis are now possible with the use of analytical numerical cloud models that can provide forecasts of specified response variables. Some evaluation of the effects of the seeding need to be carried out to achieve long-term assessment of the program. The effort spent on the evaluation will ultimately reflect the level of proof the program. Larger programs, especially those conducted with public funds, often require a greater level of proof (more evidence, either physical, statistical, or both) that the program is effective. The semi operational experimental results were sufficiently exciting, and the topic sufficiently important, that a new national international initiative should be launched to understand the physical processes taking place. In this context a major cooperative field experiment in Rayalaseema region of AP employing modern cloud physics instrumentation is being planned and carried out in the near future by research institutions like JNT University. The cloud seeding operations are conducted during July to November 2007 in 12 districts with predominant Hygroscopic seeding. About 629 Hygroscopic and Glaciogenic flares (BIPs) are burnt.The International Evaluation committee which reviewed the 2005 operations and 2006 operations has gone through the cloud seeding operations and suggested that the operations should continue at least for 5 years more so that future operations could be carried out in a fully operational manner. The continuous operations during 2005 to 2007 have given scope to improve the efficiency of seeding and our research based on cloud data analysis has clearly indicated that the flare particle size of 3 to 4 microns as used this year has improved the performance of seeding very significantly. The rainfall enhancement per flare this year is very much high compared to the earlier operations in 2004.

During this period 14 systems are formed which include 9 Low pressure areas , 2 well marked low pressure areas , 2 depressions and one deep depression. Almost 90 percent of the seeding was done in the interior of Rain shadow areas of AP. Not much has been done in coastal areas. The 12 districts were subjected to influence of these 14 systems during different times which has given scope to evaluate and quantify the influence of seeding on single cell, meso-scale cells and also synoptic scale cells. Excellent data base has been created consisting of cloud microphysical parameters as derived from TITAN and about 1656 Water samples which reflect the ambient air quality as well as the rain water quality.