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There are many physical processes occuring within the atmosphere such
as radiation, convection and boundary layer exchanges and these processes
need to be represented numerically. Often, these processes are occuring
on scales that are too small to be directly resolved by the numerical
model. The representation of these processes is called parametrization.
Parametrization schemes may make assumptions due to computational restraints
or have shortcomings due to our lack of fully understanding the processes
involved.
Development of physical parametrization schemes involves close co-operation
between several groups in different divisions of the Met Office. Groups
within Atmospheric Processes
perform observation studies and standalone modelling to help us gain
a better understanding of the physical processes that occur within
the atmosphere. Groups within Numerical Weather Prediction and the
Hadley Centre work
together to devise the numerical scheme that is eventually used within
the Unified
Model.
Below you will find a short description of some of the processes
represented in the Unified Model.
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The atmosphere is driven by radiation received from the sun and thus
an accurate representation is essential for a numerical model of the
atmosphere.
More about radiation
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| Surface and sub-surface processes |
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There is a constant exchange of heat, moisture and momentum between
the atmosphere and the underlying surface and these can have a crucial
impact on the temperature, wind and humidity of the atmosphere, not
just at the surface but extending right up the air column.
More about surface and sub-surface
processes
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| Large-scale cloud and precipitation |
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Large-scale cloud and precipitation results from the development of
synoptic-scale features such as depressions and fronts.
More about large-scale cloud
and precipitation
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| Convection and convective precipitation |
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Convection is ascent on a local scale, often resulting in showers or
thunderstorms.
More about convection and
convective precipitation
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In certain conditions of atmospheric stability and wind shear, air passing
over mountains may set up lee waves which break much like sea waves
breaking on a beach. This breaking sets up a host of small-scale tuSub-Heading
Goes Hererbulent eddies over and to the lee of mountains with the result
that when the air resumes relatively smooth motion it is permanently
deprived of this momentum over a considerable depth. The effect is that
mountain ranges act as a sink for momentum and the process is called
gravity wave drag.
More about gravity wave drag
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Near the earth's surface the wind speed decreases due to interaction
with the surface, the direction also veers. This effect is called drag
and is dependent upon the roughness of the surface.
More about orographic drag
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