Hi Nan,
"surface_air_pressure" could be measured anywhere on the earth by
measuring the air pressure with a sensor placed on the ground. You will
find that a map of surface air pressure looks a lot like the
topographical height field but with opposite sign (so that the pressure
at the top of mountains is low and at sea level, relatively high).
air_pressure at sea_level can only be directly measured over the oceans
(or land surface at the same altitude as sea level). Elsewhere, sea
level pressure must be calculated, primarily accounting for the
difference in altitude between the earth's surface and sea level.
Over the oceans, surface air pressure and sea level pressure are the
same, at least for the real Earth. In spectral models the elevation of
the ocean surface undulates ("rings") a bit, especially near coasts with
high mountains (like the Andes). Since the elevation in these models is
not uniform over the ocean, the surface pressure isn't the same as sea
level pressure anywhere and you have to again calculate the sea level
pressure.
The sea level pressure is important because on a weather map, you can
infer much about the weather (unless you are close to the equator). The
sea level pressure mostly shows you where the mountains are and is less
interesting dynamically. The surface pressure, however, is usually one
of the prognostic variables in models (maybe not for z-coordinate model
though), and is needed for solving the model equations.
regards,
Karl
On 3/14/12 8:19 AM, Nan Galbraith wrote:
Hi all -
I have a question about the use of the terms 'sea_level' and
'surface' in the following definitions:
air_pressure_at_sea_level: sea_level means mean sea level, which
is close to the geoid in sea areas. Air pressure at sea level is the
quantity often abbreviated as MSLP or PMSL.
surface_air_pressure: The surface called "surface" means the
lower boundary of the atmosphere.
The question is, does 'at_sea_level' imply a Z axis value of 0? And,
is the term 'surface' the equivalent of sea_level, except that it includes
areas over land? Or, is it somehow ... less exact?
If we're measuring BPR at around 2 meters, would we need to correct
the values to what the measurement would have been if the sensor
was at the actual sea surface, to use the term air_pressure_at_sea_level?
Or, is it acceptable to provide a Z axis coordinate and use the term
at_sea_level?
Thanks -
Nan
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