[Wrf-users] Question about levels in wrf

Tue Sep 6 17:12:09 MDT 2016

Jose,

The model sigma levels are there to distribute your model levels between
the model top and the surface.  Since pressure will vary at the surface,
both horizontally and temporally, then the sigma value of 1 will be your
surface level (versus the model top, with the sigma value of 0, which will
usually be a fixed pressure).  Your first model level (or any other sigma
level) above the surface should have a pressure of (model top) +
(sigma)*(surface pressure - model top).  Thus, if you have a model top of
50 mb, a surface pressure of 1000 mb, and a first sigma value above the
surface of 0.995, then the pressure of that sigma level should be 50 +
0.995 * (1000 - 50) = 995.25 mb (and from there you can calculate the
height, which would be a little less than 50 meters).  It should be pretty
easy to get 8 or more levels below 1 km in a model, though obviously be
careful to not have any sudden changes in distance between sigma levels (so
something like 0.970, 0.964, 0.933 would be bad).

Later...
Brandt

On Tue, Sep 6, 2016 at 3:05 PM, Jose Augusto Paixão Veiga <
veiga.uea at gmail.com> wrote:

> Brandt,
>
> based on your explanations (if I not loose anything), I can't choose the
> first model level, since the WRF do it during the course of run (based on
> the initial and boundary conditions). If it is true, why is there an option
> to set the model sigma levels?
>
> I want to set a run (in non-hydrostatic mode) with 8 or more levels
> between surface and 1 km of altitude to "better resolve" PBL processes (you
> already show me how to do that). But, I'd like the first model level start
> at 50 meters of altitude. In this case, what should I do?
>
> My advanced apologises for any mistake or problem of interpretation from
> your explanation on this issue.
>
> Thanks
>
>
>
> José Augusto P. Veiga
> CREA n.: 24161
>
> ======================================
> Universidade do Estado do Amazonas
> Departamento de Meteorologia
> Escola Superior de Tecnologia (EST)
> ======================================
> Av. Darcy Vargas, 1200, Manaus-AM Brasil
> Work phone:    (92) 3878 4317, Ramal 4317
> Skype: veiga_j.a.p.
> ======================================
> CV: http://lattes.cnpq.br/4027612512091565
> Site: https://scientificmet.wordpress.com
> ======================================
>
>
>
>
> On Tue, Sep 6, 2016 at 5:21 PM, Brandt Maxwell - NOAA Federal <
> brandt.maxwell at noaa.gov> wrote:
>
>> Jose,
>>
>> WRF can be run either hydrostatically or (preferred)
>> non-hydrostatically.  Any pressures at the corresponding heights/pressures,
>> including at the surface, should be determined by the WRF (based on intial
>> and boundary conditions).  Some people use an ideal/standard atmosphere
>> experimentally as input to the WRF (perhaps they add an anomaly of some
>> sort to specificially study the resulting effect), though this is far
>> different from using real data, which is almost always substantially
>> different from the standard atmosphere.
>>
>> Later...
>> Brandt
>>
>>
>> On Tue, Sep 6, 2016 at 1:04 PM, Jose Augusto Paixão Veiga <
>> veiga.uea at gmail.com> wrote:
>>
>>> Hi Brandt Naxwell,
>>>
>>> thank you very much for all your very clear explanations.
>>>
>>> It seems that, considering an ideal atmosphere we can inclusively from
>>> the hydrostatic equation set the model first level. I mean ... from the
>>> equation p(z) = po exp(-z/H), where H = RT/g0, z is given in km, and po =
>>> 101300,0 Pa (atmospheric pressure at surface), we can compute a pressure
>>> value related to any height and include it in the model. Is it correct?
>>>
>>>
>>> Thank you again.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> José Augusto P. Veiga
>>> CREA n.: 24161
>>>
>>> ======================================
>>> Universidade do Estado do Amazonas
>>> Departamento de Meteorologia
>>> Escola Superior de Tecnologia (EST)
>>> ======================================
>>> Av. Darcy Vargas, 1200, Manaus-AM Brasil
>>> Work phone:    (92) 3878 4317, Ramal 4317
>>> Skype: veiga_j.a.p.
>>> ======================================
>>> CV: http://lattes.cnpq.br/4027612512091565
>>> Site: https://scientificmet.wordpress.com
>>> ======================================
>>>
>>>
>>>
>>>
>>> On Tue, Sep 6, 2016 at 3:06 PM, Brandt Maxwell - NOAA Federal <
>>> brandt.maxwell at noaa.gov> wrote:
>>>
>>>> Jose,
>>>>
>>>> First of all, I think these should be sigma levels, even though they're
>>>> called eta levels in WRF.  The sigma levels are based on pressure, so one
>>>> can calculate approximately how many levels would be below the pressure
>>>> found at 1 km above the surface (though not this will vary based on surface
>>>> elevation, temperature in the layer, among other things).
>>>>
>>>> If you have a surface (for simplicity sake) of 1010 mb and a top of 10
>>>> mb (many of us will have a lower top), then 0,001 in the sigma coordinates
>>>> would equate to about 1 mb.  1 km above the surface would be around 900 mb
>>>> given the above surface pressure, so you can approximate anything between
>>>> sigma levels of .890 and 1.000 to be below 1 km (you can use a standard
>>>> atmosphere calculator to see how things like temperature can affect this).
>>>> If your model top is really low, like 100 mb, then your 1 km sigma level
>>>> would be more like .880.
>>>>
>>>> Later...
>>>> Brandt Maxwell
>>>> National Weather Service/San Diego
>>>>
>>>>
>>>>
>>>>
>>>> On Sun, Sep 4, 2016 at 4:03 PM, Jose Augusto Paixão Veiga <
>>>> veiga.uea at gmail.com> wrote:
>>>>
>>>>> Dear all,
>>>>>
>>>>> I am using the following eta levels in my runs with WRF (38 in this
>>>>> case).
>>>>>
>>>>>
>>>>>
>>>>> *eta_levels                          = 1.000, 0.995, 0.990, 0.985,
>>>>> 0.980, *
>>>>>
>>>>> *                                      0.970, 0.960, 0.950, 0.940,
>>>>> 0.930, *
>>>>>
>>>>> *                                       0.920, 0.910, 0.900, 0.880,
>>>>> 0.860, *
>>>>>
>>>>> *                                       0.830, 0.800, 0.770, 0.740,
>>>>> 0.710, *
>>>>>
>>>>> *                                       0.680, 0.640, 0.600, 0.560,
>>>>> 0.520, *
>>>>>
>>>>> *                                       0.480, 0.440, 0.400, 0.360,
>>>>> 0.320, *
>>>>>
>>>>> *                                       0.280, 0.240, 0.200, 0.160,
>>>>> 0.120, *
>>>>>
>>>>> *                                       0.080, 0.040, 0.000*
>>>>>
>>>>>
>>>>>
>>>>> However, I would like to know how many levels, in this example, are
>>>>> included (a) in the first km and (b) how can I identify the eta level
>>>>> representing the first km?
>>>>>
>>>>> I really appreciate any comment on this issue.
>>>>>
>>>>> Thanks.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> José Augusto P. Veiga
>>>>> CREA n.: 24161
>>>>>
>>>>> ======================================
>>>>> Universidade do Estado do Amazonas
>>>>> Departamento de Meteorologia
>>>>> Escola Superior de Tecnologia (EST)
>>>>> ======================================
>>>>> Av. Darcy Vargas, 1200, Manaus-AM Brasil
>>>>> Work phone:    (92) 3878 4317, Ramal 4317
>>>>> Skype: veiga_j.a.p.
>>>>> ======================================
>>>>> CV: http://lattes.cnpq.br/4027612512091565
>>>>> Site: https://scientificmet.wordpress.com
>>>>> ======================================
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Wrf-users mailing list
>>>>> Wrf-users at ucar.edu
>>>>> http://mailman.ucar.edu/mailman/listinfo/wrf-users
>>>>>
>>>>>
>>>>
>>>
>>
>
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