Hi Nan,
I must admit that I've always regarded any oceanographic barometric pressure
from ships or met buoys to be 'sea level pressure'. The only time I've given
the z co-ordinate a thought was for some oil platform data until I discovered
the data had been corrected to sea level, as are many terrestrial barometers
through a correction built into the instrument.
Cheers, Roy.
From: [email protected]
[mailto:[email protected]] On Behalf Of Karl Taylor
Sent: 14 March 2012 23:46
To: [email protected]
Cc: [email protected]
Subject: Re: [CF-metadata] surface_air_pressure vs air_pressure_at_sea_level
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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