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Convective clouds result from buoyant ascent, where clouds develop vertically and have fairly significant vertical motion (greater than 1.0 m/s). These clouds produce convective rainfall, as hydrometeors (condensed forms of water such as snow, fog, clouds, or rain) are lifted in the strong updraft. These hydrometeors grow primarily by accretion and other processes. (For more on the accretion process, click here to view an article in the last issue of the GPM monitor.) When the mass of a hydrometeor becomes large enough for the pull of gravity to exceed the force of air resistance keeping the particle airborne, the particle will fall to Earth as precipitation. Summer thunderstorms are very common examples of convective systems that produce convective rainfall. Convection often occurs at locations where moist, buoyant air is forced to rise abruptly or rapidly (e.g. cold fronts, sea breeze fronts, etc.). Convective cells are often found embedded in an area of lighter stratiform precipitation. Boiling water is a “real-world” example of convection. When a pot of water begins to boil, local regions or plumes of water become buoyant and rise, producing the common sequence of bubbling seen in a boiling pot. Convective clouds, like a massive Cumulonimbus cloud, are an example of the atmosphere “boiling.” |
Cumulonimbus Cloud |
Stratiform rain, on the other hand, is produced by large, broad, but relatively gentle ascent (at velocities much less than 1.0 m/s). Stratiform rain is often associated with stratus or nimbostratus (e.g., “layered”) clouds, and will typically be coupled with gentle forcing mechanisms like warm fronts. Large convective systems like squall-lines (e.g., a line of thunderstorms) will often have a trailing or surrounding stratiform region. So, that drenching convective downpour you experience on the softball field is likely to be followed by a period of lighter, stratiform rain.
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Stratus Cloud |
Rainfall, particularly in the tropics, may appear to be essentially convective in nature, but experiments over the eastern tropical Atlantic, northern Australia, and the western equatorial Pacific have shown that almost all convection occurs in association with stratiform rain (see Reference 3). The younger parts of the cumulonimbus clouds are 100% convective. Later, when convection decays, clouds become stratiform and co-exist with the embedded convective columns of rapid updraft. Stratiform rainfall generally occurs more frequently in the tropics, yet convective rainfall accounts for most (~70%) of the cumulative rainfall, because its intensity is so much higher. |
Raindrop growth in a stratiform cloud is slow, so stratiform rain consists of small drops. Convective rainfall is heavier and the drops are larger. Convective rainfall is typically characterized by sharper spatial and temporal intensity gradients of radar reflectivity or passive microwave brightness temperature. Therefore, maps of radar reflectivity or passive microwave brightness temperatures can be used to diagnostically separate areas of convective and stratiform precipitation. GPM’s Dual Frequency Precipitation Radar and Microwave Imager will provide scientists with data to construct such maps, facilitating the ability to monitor and differentiate between convective and stratiform rainfall.
A distinction between these two types of precipitation is quite useful to scientists, because the latent heat release (by condensation/deposition) peaks at different atmospheric levels for stratiform and convective systems. Latent heat release peaks at a high level in the troposphere in areas of stratiform rainfall, but a typical convective cloud system will have a peak in latent heating in the low-middle troposphere. Latent heat energy contained in the clouds is a significant variable in weather and climate models. Unfortunately we cannot see or measure this energy directly. We can, however, measure the product of the release of this latent energy—rainfall.
Responsible for three quarters of the energy that drives the global atmospheric circulation, rainfall is key fuel supply for the weather and climate engine. Both weather and climate models are very sensitive to the profile (e.g. vertical distribution), horizontal distribution, and time evolution of latent heating—especially in the tropics. Convective-Stratiform separation and measurement will be enabled and improved with GPM’s satellite instruments and complement of ground validation instruments like wind profilers and disdrometers, which measure the size of raindrops. Such improvement will allow more useful information to flow into our weather and climate models, which should lead to better weather and climate forecasts for scientists and the world’s population.
By J. Marshall Shepherd, Ph.D./GPM Deputy Project Scientist
Information and images from the website http://www-das.uwyo.edu/~geerts/cwx/notes/chap10/con_str.html were utilized to produce this article.
References:
1. Houze R.A. Jr. 1993. Cloud dynamics. Academic Press, 573pp.
2. Steiner, M. and R.A. Houze Jr., 1997. Sensitivity of estimated
monthly convective rain fraction to the choice of Z-R relation.
J Appl. Meteor., 36, 452-462.
3. Houze, R.A. 1997. Stratiform precipitation in regions of convection.
Bull. Amer. Meteor. Soc. 78, 2179-95.
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 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 Theresa Wirth (301-286-2508).
In each issue of The GPM Monitor, we will provide brief descriptions of some of the available publications in the GPM Report Series.
GPM Report 7– Bridging from TRMM to GPM to 3-Hourly
Precipitation Estimates
The primary goals of GPM are to extend the time series of data obtained
from the Tropical Rainfall Measuring Mission (TRMM), and to make
substantial improvements in precipitation observations, specifically
in terms of measurement accuracy, sampling frequency, Earth coverage,
and spatial resolution. In accomplishing these goals, GPM will provide
scientists with global precipitation data every three hours. This
report examines the reasons why GPM is needed, and explores how
GPM will improve global rain estimates. It explains the rationale
behind the need for three-hour data, and enumerates the potential
benefits of continuing and augmenting data from TRMM.
GPM Report 9– Core Coverage Trade Space Analysis
This paper summarizes a study that determined the sensitivity of
instruments aboard the GPM Core Spacecraft to spacecraft altitude
and inclination. The individual performances of three instruments—the
radiometer, Ku-band radar, and Ka-band radar—were tested at
a variety of inclinations and altitudes to determine the optimal
orbit parameters for the Core Spacecraft. Numerous color graphs
and tables displaying the study results are included in the document.
NASA Selects Precipitation Science Team
NASA recently announced the winners of its solicitation regarding research opportunities for precipitation measurement missions (NASA Research Announcement NRA-02-OES-05). Winning investigations will feature research centered on the Tropical Rainfall Measuring Mission (which launched in 1997) and focus on determining science requirements for future spaceborne precipitation missions, including GPM. The selected research projects will investigate the following questions, which are consistent with NASA’s Earth Science Enterprise objectives
1. How are global precipitation, evaporation, and the cycling of
water changing?
2. What are the effects of clouds and surface hydrologic processes
on Earth's climate?
3. How are variations in local weather, precipitation, and water
resources related to global climate variation?
4. How can weather forecast duration and reliability be improved
by new space observations, data assimilation, and modeling?
5. How well can transient climate variations be understood and predicted?
6. How well can long-term climatic trends be assessed and predicted?
Dr. Ramesh Kakar, Program Scientist for Precipitation Missions at NASA Headquarters, states, “The very successful TRMM satellite has provided a tremendous amount of information regarding the importance of precipitation measurements. A lot more work needs to be done to further analyze TRMM data, and plan for future satellite missions for measuring precipitation. We have selected a highly talented science team. I think we have the right mix of expertise in place to help us do our job.”
For a complete list of winning proposals, visit
http://research.hq.nasa.gov/code_y/nra/current/NRA-02-OES-05/winners.html
Congratulations to the entire team of investigators!