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

[Feng Liu]

Hi, Jimy,
You are right. I double checked the code again. I think "set equal" is a
compromise in physics reason mentioned in my last email.  I understand
now that it is possible to set the value different for each domain if
our domain definition meets the assumption of  dz <<dx (or dy),
according to your response, I would also ask why recommended to be set
equal even though the radt values are also independent? Do we have
sensitive analysis for some output variables, such as air temperature,
latent heat flux  to figure out the difference between "set equal" and
"set different" ? Thank you.
Feng


-----Original Message-----
From: Jim Dudhia [mailto:dudhia at ucar.edu] 
Sent: Saturday, April 10, 2010 5:49 PM
To: Feng Liu
Cc: Jim Dudhia; Eric Altshuler; Abdullah Kahraman;
agnes.mika at bmtargoss.com; wrf-users at ucar.edu
Subject: Re: [Wrf-users] Radiation timestep - what to use?

stepra is local to each domain, so its value will not be affected by  
the max statement.
The radt values are also independent, but recommended to be set equal,
Jimy

On Apr 9, 2010, at 6:13 PM, Feng Liu wrote:

> 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
> Fax: +31 (0)527-242016
> Web: www.bmtargoss.com
>
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