Rain Is Not Simply Rain
Have you ever been caught in a heavy downpour of rain with lightning
flashing and thunder clapping? If so, you were likely experiencing
convective rainfall. On the other hand, have you ever had a day
spoiled by a constant, persistent light rainfall that was not particularly
threatening, but was somewhat annoying? That was probably stratiform
rainfall.
GPM will help scientists detect and discern these different types
of precipitation on Earth. But why do we care about differentiating
convective and stratiform rain? Isn’t rain just rain? We can
find the answers to these questions by examining the different processes
associated with the formation of these two types of precipitation.
Convection is the transport of energy due to density differences
when not in a free-fall environment (i.e. when under the influence
of gravity). As a liquid or gas is heated, it expands and becomes
less dense and therefore lighter. If a cooler denser material is
above a hotter layer, the warmer material will rise through the
cooler material. The rising material will dissipate its heat (energy)
into the surrounding environment, become more dense (cooler), and
then will sink to start the process over.
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.
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.
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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 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.
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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!
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