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 Post subject: Question about Pair
PostPosted: Sat Mar 26, 2011 3:54 pm 
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Joined: Tue Aug 21, 2007 5:44 pm
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Location: Jeju National University
Dear Users,

There are two different surface pressure variable from atmospheric model such as NCEP.

The one is surface pressure and the other is sea level pressure.

I was wondering which one should I use when I use 'Pair' in the bulk formulae.

Regards,

-Peter


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 Post subject: Re: Question about Pair
PostPosted: Sun Mar 27, 2011 10:46 pm 
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Joined: Tue Jul 01, 2003 4:12 am
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Location: NIWA
susonic wrote:
Dear Users,

There are two different surface pressure variable from atmospheric model such as NCEP.

The one is surface pressure and the other is sea level pressure.

I was wondering which one should I use when I use 'Pair' in the bulk formulae.

Regards,

-Peter


The variable I have used is surface pressure, however I discovered to my surprise some time ago that the surface height on ocean grid points can be significantly non-zero, to the tune of several hundred metres. (This is because the NCEP model uses a spectral horizontal decomposition.) So I correct the surface pressure for the non-zero surface height. I presume that sea level pressure has this correction already done and is the better one to use.

I expect that for the bulk formulae it's not going to be a big effect.


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 Post subject: Re: Question about Pair
PostPosted: Mon Mar 28, 2011 12:59 am 
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Location: Jeju National University
Thank you for your reply, Mr.Hadfield.

So, if I can get the sea level pressure, then I'd better use it.

But if I can not get the sea level pressure, I can also use the surface pressure (or surface air

pressure) as an alternative since it is expected showing negligible effect.


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 Post subject: Re: Question about Pair
PostPosted: Mon Mar 28, 2011 12:50 pm 
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Joined: Wed Dec 31, 2003 6:16 pm
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Location: USGS, USA
i googled "surface pressure vs sea level pressure" and got this.
http://san.hufs.ac.kr/~gwlee/session3/sealev1calc.html
I do not know this site, and am not advocating it. But it gives a calc to determine the difference.

When we coupled WRF to ROMS, we use mean sea level pressure (MSLP) from WRF, not surface pressure (PSFC).


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 Post subject: Re: Question about Pair
PostPosted: Tue Mar 29, 2011 6:37 am 
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Location: Jeju National University
Thank you for your answer, Mr.Warner.

I downloaded sea level pressure and surface pressure from NCEP reanalysis version 1.

I plotted same time step of both dataset(see attached)

The general features of the ocean looks about the same although there is a small wave like motion in the surface pressure.

Significant differences occur in the land area especially in high altitude area.

The press_sfc_mask_2010 figure denotes surface pressure data in the ocean.

We can see there is a difference between sea level pressure and surface pressure in the coastal area.


I looked up the bulk_flux.F file and found that Pair affects not only heat flux but also wind stress calculation.

Therefore, if one have to use the surface pressure data especially near the coastal area with high altitude such as Taiwan, Vladivostok,

Gulf of Alaska, they'd better convert it to sea level pressure.


Attachments:
press_sfc_mask_2010.png
press_sfc_mask_2010.png [ 64.79 KiB | Viewed 2595 times ]
press_sfc_2010.png
press_sfc_2010.png [ 73.7 KiB | Viewed 2595 times ]
slp_2010.png
slp_2010.png [ 55.42 KiB | Viewed 2595 times ]
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