Timeseries of relative humidity

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Timeseries of relative humidity

Postby orestis25 » Tue Jul 15, 2008 8:18 am

Hello everyone,

I am trying to create a timeseries of relative humidity at a height of 2m, so as to compare the wrf output with real measurements.

Is there a standard way of depicting relative humidity such as the T2, U10, V10 variables at this height?
If I am to calculate indirectly the relative humidity, which variables should I use?

I've tried calculating it with the formulas in rhucalc.f but I get a different result from what RIP gives me for rhu at surface pressure. I am not certain I punch in the correct variables though or if the comparison with the rhu is even correct.

I would be much obliged if you could provide me with some ideas
Thanks
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Re: Timeseries of relative humidity

Postby jimmyc » Tue Jul 15, 2008 10:10 am

the variables you need are the surface pressure, temperature at 2 meters and mixing ratio at 2 meters (PSFC, T2, Q2). standard formulas from most textbooks can be used to go from mixing ratio to vapor pressure ; and temperature converted to saturation vapor pressure. OR you can find the relevant code in RIP and duplicate it ... should be written in fortran i'd imagine.
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James Correia, Jr
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Re: Timeseries of relative humidity

Postby orestis25 » Wed Jul 16, 2008 1:05 pm

Thanks for the reply.

If I have understood correctly, Q2, T2 and PSFC are read directly from the wrf output file.
I also found the formulas for calculating relative humidity, however!

When I request through RIP the mixing ratio (qvp) at the isobaric level equal of PSFC, I get a different number from the Q2 (I do not mean hPa and Pa)
The relative humidity is therefore different but much closer to the value I get from RIP directly when I request the rhu at the PSFC level (plus, this is closer to my real surface values).

Which value should I use, the one I get from RIP at the PSFC level, the one I calculate when using Q2 or the one using qvp?

Sorry for the mess, but I am a bit at loss

Thanks again
orestis25
 
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Re: Timeseries of relative humidity

Postby jimmyc » Wed Jul 16, 2008 2:10 pm

Well I dont know RIP.

QVAPOR is the full 3d mixing ratio field. Lowest model level is what you were using for that which should be above the surface. you cant use PSFC for the computation, you have to use lowest model pressure (P + PB).

Q2 is the mixing ratio at 2m AGL so that will be closest to the surface.
The views expressed in this message do not necessarily reflect those of NOAA or the National Weather Service or the University of Oklahoma.
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Re: Timeseries of relative humidity

Postby orestis25 » Fri Jul 25, 2008 8:32 am

Although my problem was quite simple, I was confused for a while until I found out there was no
immediate way of calculating the desired relative humidity at 2m.

My approach of requesting the relative humidity at a specific isobaric level was simply wrong.
A specific isobaric level meant a specific temperature and mixing ratio, variables used in calculating RH.
In my case I had seven different stations which meant seven different surface pressures (these were the isobaric levels I used).

That meant I had to run RIP seven times to get the RH for each station since the surface pressures varies greatly due to the difference in altitude. (That's another thing I had hard-wired to myself- surface pressure is one thing and sea-level pressure of each station is another. You can only compare slp between stations but when you need to calculate a parameter-e.g. RH- you must use the surface pressure of the location at hand)

So, I requested a timeseries from rip for seven locations and three variables Q2, T2 and PSFC. I glued the txt files together so as to have the complete timeseries and inserted it in Excel.
The formula I used was in the directory /src in rhucalc.f which I found in Stull's textbook as well, slightly different in some constants. I used the wrf's and got the desired RH at 2m.

Hope my mistakes will help a bit to save time and trouble,
thanks for the help.
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Re: Timeseries of relative humidity

Postby Jumper4 » Thu Jan 27, 2011 11:20 pm

Hello Orestis,

I followed a similar method to extract RH at 2m from WRF. Did you find values of RH greater than 100% within your data set as I did? I am wondering if supersaturation at the surface is possible in the "real world" with so many CCN floating around?

Thanks in advance,
Nic.
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Re: Timeseries of relative humidity

Postby orestis25 » Mon Jan 31, 2011 5:58 am

Dear Nic,

it's been a while since I've looked at my data and I do not longer have access to them. However, I don't recall witnessing over-saturation at any point.

Do you have lots of values? Do you have any real data to compare to?
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Re: Timeseries of relative humidity

Postby ebeigi3 » Sun Jan 08, 2012 4:58 pm

Could you please send me the formula for calculating relative humidity from temperate , specific humidity and mean sea level pressure? thanks in advance.
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Re: Timeseries of relative humidity

Postby orestis25 » Mon Jan 09, 2012 8:02 am

Dear ebeigi3,

it's been over three years since I last worked with the relative humidity problem. I simply paste from one of my earier replies:

The formula I used was in the directory /src in rhucalc.f


Hope this helps, as currently I cannot help you further
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Re: Timeseries of relative humidity

Postby camila » Fri Aug 29, 2014 6:42 am

I think my problem is quite simple: WRF just doesn't go that far! (333m grid increment!)
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