Precipitation Enhancement:Aerosol-Cloud Interactions, Hygroscopic Seeding

National Center for Atmospheric Research

Research Applications in Weather ModificationInadequate access to fresh water is intensifying in many areas of the world. Precipitation is the primary source of fresh water in the hydrologic cycle, yet a large portion of atmospheric water present in clouds never makes it to the ground as precipitation. Therefore, enhancing precipitation through cloud seeding is an attractive possibility that has been explored by scientists and engineers for over sixty years. To assess this possibility, the physical chain of events in precipitation development must be examined so that perturbations, both intentional (via cloud seeding) and inadvertent (via aerosol-cloud interactions), can be understood and quantified.

The potential for such man–made increases in precipitation is strongly dependent on the natural microphysics and dynamics of the clouds that are to be seeded.

The National Center for Atmospheric Research (NCAR) has been engaged in scientific research to evaluate aerosol-cloud interactions and precipitation enhancement techniques for several decades. Work accomplished in the last twenty years with a hygroscopic seeding technique to stimulate rain formation in summertime convective clouds has shown promising statistical results from seeding experiments in South Africa, Mexico, and Australia. Associated cloud measurements and cloud model calculations have provided a physical basis to support the idea that seeding with small hygroscopic particles at cloud base can, at least under some conditions, augment rainfall. The potential for such man–made increases in precipitation is strongly dependent on the natural microphysics and dynamics of the clouds that are to be seeded.

Scientists in the Research Applications Laboratory (RAL) at NCAR have been involved with a variety of projects around the world related to aerosol–cloud interactions and hygroscopic cloud seeding, including efforts in Mexico, Australia, Indonesia, Italy, Saudi Arabia, West Africa, Turkey, the United Arab Emirates (UAE), India, and the United States. RAL's role has been to scientifically evaluate the potential for cloud seeding to enhance rainfall, as well as to conduct basic research on the impact of ambient and seeding aerosols on cloud and precipitation processes through field measurements and modeling.

Aerosol-Cloud InteractionsCloud formation requires the presence of aerosol particles in the atmosphere, called Cloud Condensation Nuclei (CCN), for water vapor to condense onto forming cloud droplets. Cloud droplets then grow large enough to fall out as rain by colliding and coalescing with other cloud droplets. In polluted regions, CCN concentrations can be very high and thereby yield high concentrations of very small cloud droplets, potentially reducing the precipitation formation efficiency of clouds. A variety of other factors also impact precipitation formation in clouds, such as storm dynamics, thermodynamics, and natural variability, and so all of these factors must be investigated in order to fully understand potential impacts of aerosols on clouds and precipitation.

RAL has been involved in recent research projects like the Queensland Cloud Seeding Research Program (QCSRP) and the NASA Studies of Emissions and Atmospheric

continued on reverse sideHygroscopic flare burning during Mexico seeding trials.

Radar reflectivity of a convective storm being targeted by aircraft (flight track in white). A blue outline of the storm cells identified

by the TITAN storm tracking software is also overlaid.

Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS) program in order to improve our understanding of aerosol-cloud interactions and their impact on precipitation formation. These projects have measured aerosol properties and in situ cloud microphysical properties using sophisticated aircraft instruments and weather radars to determine the relationships between aerosol properties and cloud droplet properties, and subsequent precipitation formation processes.

Hygroscopic SeedingCloud seeding is a science–based technology that aims to add particles to a cloud that will help precipitation develop more efficiently. Hygroscopic materials (particles that take on water easily, such as salts) released just below a convective cloud base can be carried into the cloud by the updraft so that water vapor condenses on them to form additional liquid cloud droplets. Adding additional particles of larger sizes may help enhance the collision and coalescence processes and convert more of the cloud water to rain.

Adding additional particles of larger sizes may help enhance collision and coalescence processes that are responsible for rain formation and convert more of the cloud water to rain.

RAL has been involved in several hygroscopic cloud seeding research programs throughout the world and in the United States. Components of these programs included many, if not all, of the following: cloud and rainfall climatology studies; randomized seeding experimental design, implementation, and statistical analysis; airborne chemistry, aerosol, and cloud physics measurements; assessment and enhancement of weather radar capabilities for collection of quantitative data; analyses of field data; and numerical modeling work.

Randomized ExperimentsRandomized seeding experiments are designed to statistically investigate the impact of a cloud seeding program, yet they often face a challenge of obtaining a large enough sample of cases to yield significant results. This has been the case for many of the hygroscopic cloud seeding programs that RAL has conducted. For example, in the UAE, results showed no significant differences between seeded and unseeded storms. Physical measurements were critical in revealing where the hygroscopic cloud seeding model did not apply for UAE clouds. However, the results from some other projects showed strong tendencies for more rain mass and longer storm durations in seeded clouds, similar to the early results from South Africa.

Physical Measurements Aerosol and microphysical measurements taken in or near the clouds by research aircraft provide the necessary information to assess the physical response from seeding. Advances in instrumentation and observational platforms, such as dual-polarization Doppler radars and new instrumentation with improved resolution, accuracy, and enhanced ability to

For More Information, Contact:Roelof Bruintjes 303-497-8909 roelof@ucar.eduNational Center for Atmospheric Research (NCAR)PO Box 3000 Boulder CO 80307-3000www.ral.ucar.edu 303-497-8401 fax

discriminate ice from liquid, bring about the potential for additional insights in aerosol and cloud seeding research, and encourages further research programs to investigate the physical effects of cloud seeding to support the statistical results. For example, dual-polarization radars, which provide additional information on particle sizes and shapes and offer superior rainfall estimation compared to traditional weather radars, have been used to study rain formation under varying aerosol conditions. Doppler radar networks can provide three-dimensional wind measurements to aid in assessing the possible dynamic effects of aerosols and cloud seeding.

Numerical Modeling Modeling studies are also useful in connecting in situ measurements with observed rainfall characteristics in a controlled framework. Cloud-resolving models with microphysics parameterizations that simulate aerosol-cloud interactions, such as the Weather Research and Forecasting (WRF) “aerosol aware” Thompson bulk microphysics scheme developed in RAL or bin microphysics schemes, can be utilized to study the cloud and precipitation response from altering the CCN properties due to inadvertent or intentional aerosol introduced into a cloud.

Randomized experiments, physical measurements and numerical modeling provide the optimal framework to study atmospheric processes that describe the range of outcomes that aerosol have on clouds and precipitation. These data allow scientists to describe the physical chain of events resulting from aerosol perturbations leading to a more precise understanding of aerosol-cloud processes in advertent and inadvertent cloud modification.

Vertical profile of aerosols over SW Saudi Arabia and the Red Sea.