Understanding Climate Change
Components of the NCAR-based Community Climate System
Model (CCSM)
Details on each component are listed below | View interactive
version
Now in its third generation, the CCSM is one of the world’s most
sophisticated and widely used models of global climate. This graphic
illustrates the many components included in the CCSM, ranging from cirrus
and stratus clouds to ocean currents and soil moisture. These components
are also typical of many of the dozen or so climate models at other major
research centers around the globe. An interactive version of this graphic
is available here. (Illustration by Paul Grabhorn, ©UCAR. News
media terms of use*)
Cirrus clouds
These high, thin, icy clouds act as a warming influence on climate overall, because
they allow sunlight in but trap long-wave radiation rising from Earth’s
heated surface. The CCSM depicts these and other clouds through parameterization—tracking
the conditions that form such clouds and then specifying how much of a given
rectangle of land in the model grid is covered by each cloud type.
Stratus clouds
These low, dense, very reflective clouds act as a cooling influence
on climate overall, because they reflect a great deal of sunlight.
Subtropical oceans often feature huge areas of marine stratocumulus.
The CCSM depicts these and other clouds through parameterization—tracking
the conditions that form such clouds and then specifying how much
of a given rectangle of land in the model grid is covered by each
cloud type.
Cumulus clouds
These puffy clouds, which sometimes build in tower-like formations,
are linked to strong updrafts and the showers and thunderstorms that
result. Cumulus are difficult to represent in models because each
cloud covers only a small part of Earth’s surface, but when
taken together, cumulus clouds have a large influence on global circulation.
The CCSM depicts these and other clouds through parameterization—tracking
the conditions that form such clouds and then specifying how much
of a given rectangle of land in the model grid is covered by each
cloud type.
Precipitation and evaporation
The process of converting water vapor to water droplets or snowflakes
releases heat into the atmosphere. The CCSM simulates this process,
as well as the evaporation of water from soil, wetlands, lakes, and
oceans and the amount of rain or snow reaching the surface.
Sea ice
Sea ice helps keep polar regions cold, because it reflects most of
the sunlight that hits it. When sea ice melts, it does not raise
sea level directly (because the ice is already afloat, like a melting
ice cube in a glass of water). However, the dark surface ocean exposed
by melting sea ice absorbs most of the sunlight it receives, which
leads to further warming. Research using the CCSM indicates that
Arctic summertime sea ice may diminish greatly, as soon as the 2030s.
Winds
One of the main elements of a climate model is its depiction of winds.
The global circulation transports warm air poleward and cold air
toward the equator. This flow operates through several persistent
loops that produce trade winds in the tropics, westerly winds at
midlatitudes, and easterlies across the poles. The actual winds at
any one spot are influenced by day-to-day weather as well as climate
cycles such as El Niño. Where winds converge, rising motion
and rain or snow may develop.
Heat and salinity exchange
Because water has a higher heat capacity than soil, it takes longer
for sea surfaces to warm up or cool down relative to the continents.
Climate models must account for the transfer of heat between ocean
and atmosphere. They also need to reflect changes in salinity (salt
content) that occur as ocean water evaporates, or as fresh water
enters the oceans through increased rainfall or increased glacial
melting.
Atmospheric model layers
Contrasts in temperature and wind are much stronger vertically than
horizontally. Only a few miles above ground, temperatures are usually
frigid (even in summer), and winds may howl at 200 miles per hour
(320 kilometers per hour) or more. The CCSM divides the atmosphere
into 26 layers, tracking each layer as well as exchanges of energy
and moisture between layers. Near the ground, where vertical contrasts
are strongest, the layers (which are defined by atmospheric pressure)
can be as thin as 1,000 feet (about 300 meters) or less.
Ocean currents, temperature, and salinity
The behavior of the ocean is much more difficult to observe than the
atmosphere, and many ocean processes are still poorly understood.
As recently as the 1990s, most climate models used a “slab” ocean—one
that behaves as a single unit. Today, the CCSM and other sophisticated
models include a much more dynamic depiction of the ocean that tracks
changes in ocean currents, temperature, and salinity. These control
such phenomena as the North Atlantic’s overturning circulation,
which helps keep Europe warm for its latitude but which may be sensitive
to climate change.
Ocean model layers
Much like the atmosphere, the ocean needs to be divided into several
layers in order for a model to accurately depict the three-dimensional
flow and other qualities. The CCSM includes 40 ocean layers, ranging
in thickness from about 33 feet (10 meters) near the sea surface
to about 800 feet (250 meters) in the deep ocean.
Ocean bottom topography
In order to accurately depict the ocean circulation in three dimensions,
the CCSM includes undersea ridges, valleys, and other topographic
features of the ocean bottom.
Vertical overturning
Most of the world’s oceans outside the Arctic feature a relatively
warm sea surface and a thin region called a thermocline that
separates the warm surface layer from colder, deeper waters. In some
parts of the world, colder and deeper water regularly crosses the thermocline
to mix with warmer surface waters, or vice versa. The CCSM can simulate
these overturning processes.
Realistic geography
Large mountain chains exert a major influence on temperature, precipitation,
and wind patterns for miles around. Even when mountains are relatively
modest in size or extent, they play an important role in local climate.
Early climate models tracked the atmosphere at points separated by
hundreds of miles, so mountain ranges appeared in highly smoothed
form. The CCSM and other contemporary models operate at higher resolution
so the topography is much less smoothed.
Land surface processes
The atmosphere is strongly affected by what lies beneath it—forests,
deserts, ice sheets, mountains, and grasslands. The CCSM includes each
of these elements, tracking the exchange of energy and moisture between
them and the atmosphere. Urban areas are not yet depicted in the standard
version of the CCSM or other global climate models, although work is
under way to add them.
Soil moisture
Moisture stored near and just below ground level affects how much rain
and snow can be absorbed by the soil and how quickly a region dries
out if precipitation slackens. The CCSM includes a depiction of moisture
in 10 soil layers.
Outgoing heat energy
Virtually all of the energy that reaches our planet from the Sun leaves
the Earth system in one form or another. In order to keep their simulation
of Earth’s climate in proper balance, the CCSM and other climate
models account for this outgoing radiation.
Incoming solar energy
To create an accurate portrayal of Earth’s climate, the CCSM
calculates the amount of incoming solar radiation by location, time
of day, and time of year. When reproducing past climates, the model
can also include estimates of solar variability based on sunspot counts,
carbon dating of organic material, and other indirect evidence.
Transition from solid to vapor
Besides melting, snowpacks can erode through a process called sublimation, in
which moisture goes directly from ice crystals to water vapor in the
atmosphere. Sublimation is common in dry mountain climates, where a
snowpack may leave little or no water behind as it shrinks (even in
temperatures below freezing). The CCSM depicts sublimation as well
as snowmelt.
Evaporative and heat energy exchanges
When water evaporates, it draws heat from the lakes, rivers, or oceans
from which it came and stores that heat within its molecular bonds.
Heat can also leave the land and ocean surface directly through contact
between the surface and molecules in the atmosphere (a process called conduction).
The CCSM depicts these and other processes that move heat energy
around the Earth system.
Snow cover
Large areas of snow have a major effect on weather. Because of their
light color, they reflect large amounts of sunlight and help keep
temperatures colder than they would otherwise be, especially close
to the ground. The CCSM tracks the seasonal waxing and waning of
snowfall across mountainous and high-latitude areas. The large ice
sheets in Greenland and Antarctica are also included. Some of the
processes that control ice sheets are still being studied by scientists
and are not yet part of the CCSM and other global models.
Runoff
A key part of the global water cycle is the flow of water from rivers,
lakes, and land areas down toward the sea. The CCSM depicts runoff
more precisely than earlier models, which enhances its treatment
of the water cycle overall.
*News
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