[Wrf-users] Radiation timestep - what to use?

[Feng Liu]

Hi, all,
I do not have enough time to look up previous discussion in the archive
according to Abdullah. But I think the description of how to set 'radt'
in manual is correct because
1. In the WRF code on calculation of radiation, the time step (STEPRA)
is calculated based on RADT (named as 'radt' in lower case in
namelist,input ) as below: 
    STEPRA = nint(RADT*60./DT)
    STEPRA = max(STEPRA,1)
That means only 27 works in the model run no matter how you set this
number (9,and 3) for nested domains because the calculation only takes
the maximum value of 27, therefore, "use the same value for all nests"
in the manual is correct. But reference value is for parent domain,
here, for example, is 27. 
2. All the radiation schemes in WRF are one-dimensional scheme in
vertical direction, therefore they are treated independently, and the
fluxes correspond to those in infinites horizontally uniform planes,
which works well if vertical thickness of the model layer (dz) is much
less than dx ( or dy). Therefore, using a daylight-mean cosine solar
zenith angle over a unique time interval (based on 27 in our example
here) to calculate radiation for all domains are safe (for keep above
assumption) and reasonable according to one-way and two -way nesting
simulation mode.
Feng  

-----Original Message-----
From: wrf-users-bounces at ucar.edu [mailto:wrf-users-bounces at ucar.edu] On
Behalf Of Eric Altshuler
Sent: Friday, April 09, 2010 2:27 PM
To: Abdullah Kahraman
Cc: wrf-users at ucar.edu
Subject: Re: [Wrf-users] Radiation timestep - what to use?

Dear wrf users,

I have asked this same question several times (over the course of
several years) and never gotten a definitive answer. Even among the WRF
developers, opinions differ. In one case, I had the same nesting
configuration (3 domains, 27/9/3km resolution). I got several responses
but they did not agree. Some would recommend to set radt = 27; others
would recommend using radt = 3. I chose the middle ground by setting
radt = 9, but it would be nice to settle this question once and for all.

Best regards,

Eric L. Altshuler
Assistant Research Scientist
Center for Ocean-Land-Atmosphere Studies
4041 Powder Mill Road, Suite 302
Calverton, MD 20705-3106
USA

E-mail: ela at cola.iges.org
Phone: (301) 902-1257
Fax: (301) 595-9793

----- Original Message -----
From: "Abdullah Kahraman" <havadurumu at gmail.com>
To: wrf-users at ucar.edu
Sent: Friday, April 9, 2010 4:27:41 PM
Subject: Re: [Wrf-users] Radiation timestep - what to use?


Hello Agnes, 


This question was asked before, maybe you can find out in the archive.
You are supposed to use 27 km for all domains. 


Regards, 
Abdullah KAHRAMAN 
M.Sc. Meteorological Engineer 
Turkish State Meteorological Service 
Ph.D. Student 
Istanbul Technical University, Atmospheric Sciences Program 



On Fri, Apr 9, 2010 at 11:22, Agnes Mika < agnes.mika at bmtargoss.com >
wrote: 


Dear WRF help and WRF users, 

I am confused about what value I should use for the radiation 
timestep. The recommendation in the manual is 1 minute per km of dx 
and to use the same value for all nests. 

Should I use the radt valid for the coarsest or the finest nest for 
all nests? 

E.g., I am using nests with 27, 9, and 3 km resolution. Should radt 
be set to 27 for all nests or to 3? 

Thank you in advance, 
Agnes 

-- 
Dr. A'gnes Mika 
Advisor, Meteorology and Air Quality 

Tel: +31 (0)527-242299 
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