How Does Precipitation Form in a Cloud?

The “rain-making” processes in a typical cloud are fascinating and mysterious to meteorologists. As I child, I often wondered, “Why don’t the clouds fall to the ground like the rain drops?” After all, clouds are simply composed of tiny liquid cloud drops, ice crystals or both. Yet, the processes that allow clouds to produce rainfall and other forms of precipitation are far from simple. Precipitation that falls from clouds usually results from large water drops, ice crystals that have melted, or ice particles that stay frozen (e.g. snow or sleet). To understand precipitation processes, we must begin with basic physics.

Clouds are created by air that rises until it cools to the dew point. At this point, the air is saturated and vapor molecules seek to condense to form small water droplets. However, in the atmosphere, condensation from vapor to gas is not a straightforward process. In the atmosphere, water vapor molecules need small particles called condensation nuclei (CN) to cling to. CN are tiny particles like sea salt, clay, and dust. CN are usually 0.1 micron to 10 microns in diameter. A typical human hair is 100 microns thick. When droplets freeze in the atmosphere to form ice crystals, tiny ice nuclei (IN) are also required to speed the freezing process along. In fact, the concept of “cloud seeding” or “man-made rainmaking” seeks to introduce CN like dry ice or silver iodide into clouds to stimulate growth.

Once cloud droplets begin to form, the stability of the air determines how the air will continue to rise, and what type of cloud shape will take place. Air is unstable when a rising air parcel is warmer than surrounding air. Conversely, air is stable when a rising air parcel is colder than surrounding air. Instability promotes continued rising motion needed for cloud development. Instability, moist air, and a source of lift are critical for cloud development. More unstable air promotes development of convective type clouds like cumulus or cumulonimbus (e.g., thunderstorms). More stable air tends to produce either no cloud development or layered clouds like stratus, depending on the amount of stability. The primary lesson to this point is that CN, IN, instability, saturated air, and rising air are all critical ingredients for that recipe called “precipitation-making.” But, what happens in the clouds to produce precipitation? First, gravity pulls cloud droplets toward the Earth just like a baseball that you might toss into the air. Like the baseball, cloud droplets have a “terminal velocity,” the speed at which upward force of air resistance matches gravitational pull. Cloud droplets have very small terminal velocities (about 0.02 mph) so it is very easy for a cloud environment to suspend droplets. To fall to the ground, drops must grow larger (and so does their terminal velocity, e.g., 14 mph). But, how do the drops grow larger?

At first, you might guess that condensation occurs until the drops become big enough to fall. However, scientists realized in the early part of last century that condensation alone was too slow and could not produce raindrops in 30 minutes or more. Between 1911 and 1930, scientists theorized that ice was the key. If ice crystals and cloud water droplets are mixed together in a cloud, the ice crystals grow at the expense of water droplets. Eventually, ice crystals grow large enough to fall, melt on the way down (or stay frozen as snow), and hit the ground. This process is called the ice-crystal theory or “cold rain process”. Until World War II, this process was assumed to account for most precipitation. However, during the war, scientists in the Tropics found that much of the precipitation falls from clouds that never produce below-freezing temperatures. Therefore, the ice-crystal theory was not always valid.

An alternative process, the so-called “warm rain process” was introduced. In this process, cloud droplets grow to raindrop sizes (~ 2 mm) by coalescence and accretion. In other words, cloud droplets collide with each other and stick together. As they grow larger and begin to fall, larger droplets can collect smaller droplets in the same way that droplets grow as the trickle down a car window. There are problems with this process, too. Scientists are still uncertain about the efficiency of the accretion-coalescence process, the impact of drop aerodynamics on growth, and the role of turbulence on growth.

Schematic illustrating the coalescence (warm rain) and ice crystal (cold rain) processes. (Figure courtesy of Texas A&M University)

Precipitation processes are still very active areas of research in the scientific community. Research seems to suggest multiple answers. Some precipitation forms under either the cold or warm process, while a significant portion of precipitation likely involves both processes. In fact, on a hot summer day in Washington, D.C., it is very possible that a raindrop that hits your face in the 85 degree heat began as an ice crystal in the sub-freezing levels of a cloud. The so-called phase state of liquid, size/shape of drops and crystals, and their interactive processes in the clouds are often called cloud microphysics. Many of the GPM mission’s precipitation and latent heating estimation algorithms and validation/calibration efforts depend on better understanding of cloud microphysics and how they vary in different precipitation regimes around the world.

By Dr. J. Marshall Shepherd
Deputy Project Scientist for GPM

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GPM Report Series Now Online

The GPM Report Series, edited by Eric A. Smith and W. James Adams, is a collection of conference publications, technical memoranda, and technical reports documenting key issues pertaining to the formulation of the GPM. The publications are designed to serve as informational and reference material for interested scientists, engineers, industry, educators, and the public.

The subject material of the reports varies widely. For example, current report topics range from a summary of GPM Workshop Proceedings, to an explanation of the benefits of international partnership with GPM, to a discussion of potential tropical open ocean precipitation validation sites. Presently, there are six reports available, with several others in the works.

The GPM Report Series is accessible via the online GPM Library at http://gpm.gsfc.nasa.gov/library.html. If you prefer to receive a hard copy of any of the reports, please contact Leslie Cusick (301-286-9094).

In each issue of The GPM Monitor, we will provide a brief summary of some of the available publications in the GPM Report Series.

GPM Report 2– Benefits of Partnering with GPM Mission
Analyses indicate that at least eight satellites will be required in the GPM constellation to provide the data needed to produce the three-hour precipitation products that scientists desire. GPM’s success, therefore, depends upon the participation of numerous domestic and international partners to configure such a constellation. This report contains a summary of the incentives that NASA can provide to potential GPM partners. The report identifies and elaborates upon seven specific benefits that GPM partners will be able to reap:

· The opportunity to help determine the direction of GPM science investigations and data products
· Access to NASA’s Tracking Data and Relay Satellite System (TDRSS) to obtain real-time data from partner spacecraft
· NASA launch support
· Access to GPM data directly from the GPM processing system server instead of the distribution system
· The ability to host a regional archive and data center for GPM
· The opportunity to execute specialized software in the GPM data processing system
· Access to software developed for GPM, reducing partner’s software development costs

While the above incentives offer potential GPM partners significant advantages, most of them will not contribute to NASA’s cost or risk in a substantial manner.

GPM Report 5– Potential Tropical Open Ocean Precipitation Validation Sites
GPM will require a number of tropical open ocean sites for precipitation validation purposes. There exist a number of candidate locations that could be used for this purpose; a cost/benefit analysis will be used to choose the best sites. This report provides some of the data required as input to such an analysis. It examines fourteen geographically dispersed candidate islands, and contains pertinent information about each site including physical location, topography, land area, historical background, the state of affairs on the island and its nation, and the precipitation climatology. The authors summarize the advantages and disadvantages of each site, taking into consideration precipitation climatology, infrastructure, economic and political stability, availability of transportation, health and entertainment options, and labor sources. The report also includes graphs depicting average rainfall rates for each site.

By Lena Braatz/Booz Allen Hamilton

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GPM Precipitation Processing System Demonstration Conducted

The Precipitation Processing System (PPS) is a key component of the GPM architecture. From the data it receives, the PPS will generate global rain products to aid NASA in the pursuit of answers to science questions relating to Earth’s water cycle, precipitation rates, climate changes, and more. PPS products will include global maps of rain rate and latent heat (the energy released into the atmosphere as water vapor condenses to form clouds and raindrops).

The PPS will process data from the GPM Core Spacecraft and NASA’s GPM Constellation Spacecraft. In addition, the PPS will receive data streams from other contributing entities such as the National Oceanographic and Atmospheric Administration (NOAA), the Department of Defense (DoD), as well as a number of foreign partners. The PPS will merge these data streams to produce global precipitation products, including error characterizations, for the world’s science community.

As you might imagine, this data processing endeavor will be quite complex. The PPS must feature a flexible design—it will be receiving data from an indeterminate number of sources, in numerous formats. The “rolling wave” concept of GPM dictates that data streams coming into the PPS will continually change, as new data collection programs are initiated over the years, and established programs are phased out. Some of the data streams the PPS receives will be processed already, using a variety of error correction schemes; others will be “raw” and will require processing.

PPS designers are currently working to ensure that the system is adaptable, and can accept varying types of data without requiring major modifications either to the system or software. To maximize the value of NASA’s investment, the PPS must be an enduring, flexible system that will execute its precipitation mission for many years. Development of the PPS is one of the initiatives NASA is undertaking in an effort to transform the focus of NASA research activities from a concentration on specific missions, to the resolution of important science questions.

The PPS is being designed utilizing architecture, knowledge, and experience inherited from the Tropical Rainfall Measuring Mission (TRMM) Science Data and Information System (TSDIS). TSDIS successfully processes rainfall data from the TRMM satellite, producing and distributing precipitation data products to archival centers, where the science community can access them.

TRMM Map of Average Rainfall Data from GPM's PPS will be used to produce similar data products

The main component of the PPS will reside at Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, with support components located at the Global Hydrology Climate Center (GHCC) in Huntsville, Alabama. Since the PPS must interface with all of the GPM partners who will provide the various data streams, the PPS can be considered to require additional components worldwide.

In November 2002, the PPS team conducted a demonstration using new, cost-saving hardware and software that will be employed in the PPS evolution. As scientists and representatives from NASA Headquarters looked on, the system processed an entire day of TRMM data in approximately 30 minutes. The test system demonstrated several key data processing procedures and generated data in several different formats, proving the PPS will be capable of interpreting the diverse data streams it will receive from GPM partners.

By Lena Braatz/Booz Allen Hamilton

For further information regarding the PPS, please contact Erich Stocker/PPS Manager and Chief Architect (Erich.F.Stocker@nasa.gov).

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