[ncl-talk] Calculating Cloud Top Temperatures (lack of sensitivity)

[Bill Ladwig]

Hi Barry,

In an upcoming release (or you can use wrf-python now), you can adjust the
optical thickness to trigger when a cloud is detected, so that might help
some. By default, that value is 1.0. The next version will also allow you
to fill the no-cloud regions with fill values instead of the surface
temperature, which is how it works now. Here is the source code for the
algorithm:

https://github.com/NCAR/wrf-python/blob/develop/fortran/wrf_fctt.f90

The algorithm itself is pretty basic. It looks from the top down, and
integrates optical thickness until the threshold is met (1.0), then the
cloud top temperature is set to the temperature of that level. Optical
thickness at each level (if cloud ice mixing ratio is present in your files
[note: converted to g/kg]) is:

opdepthd = opdepthd + (ABSCOEF*qcw(i,j,ripk) + ABSCOEFI*qci(i,j,ripk))*dp/G
>

with the absorption coefficients:

REAL(KIND=8), PARAMETER :: ABSCOEFI = .272D0  ! cloud ice absorption
> coefficient in m^2/g
> REAL(KIND=8), PARAMETER :: ABSCOEF = .145D0   ! cloud water absorption
> coefficient in m^2/g


I'm not sure what the source is for those absorption constant values, but I
imagine the author chose something safe, since it seems like something that
could depend on cloud type, number concentration, etc. So, aside from
changing the optical thickness threshold to trigger a cloud detection, the
absorption coefficients could be changed from constants to variables, but
that's really the only way to modify this algorithm in to something that
uses more ice.

Hopefully this new behavior will be out in NCL in the next few months.

Hope this helps,

Bill



On Fri, Nov 2, 2018 at 8:18 AM Barry Lynn <barry.h.lynn at gmail.com> wrote:

> Hi:
>
> I have used the "ctt" wrf get user calculation on two different case
> studies with WRF with 4 km grid spacing). In both, the cloud top height is
> much lower than the observations, much lower than the GEFS (and much lower
> than with WRF with a CU parameterization on a 12 km grid).
>
> Hence, I think that there might be an error when calculating cloud top
> temperatures with (4 km) explicit microphysics.
>
> Is there some easy way to modify the source code to be more sensitive to
> ice concentration?
>
> Thanks,
>
> Barry
>
> On Tue, Jul 24, 2018 at 11:02 PM Mary Haley <haley at ucar.edu> wrote:
>
>> Barry,
>>
>> You can always get the latest version of the NCL source code from our
>> github repo:
>>
>> https://github.com/ncar/ncl
>>
>> If you click on the "Find File" button, then you can search for
>> WRFUserARW.ncl individually.
>>
>> From there you should be able to right click on the file name to
>> "download linked file", or you can click on it to view the contents.
>>
>> If you need the file from a particular version of NCL, like NCL V6.4.0,
>> then from the github.com/ncar/ncl page click on the downward triangle in
>> the "Branch:master" button and select "Tags". You will then see different
>> versions of NCL and can search the file in any one of those.
>>
>> --Mary
>>
>>
>> On Sun, Jul 22, 2018 at 5:40 AM, Barry Lynn <barry.h.lynn at gmail.com>
>> wrote:
>>
>>> Hi:
>>>
>>> I have been searching the web in vain to find the latest version of this
>>> file.
>>>
>>> I must have an old version because it doesn't contain "ctt."
>>>
>>> Could someone please point me in the direction of the download for this
>>> file.
>>>
>>> (I am using version 6.4, but I must still be using an old file as it is
>>> not present.)
>>>
>>> Thanks
>>>
>>> --
>>> Barry H. Lynn, Ph.D
>>> Senior Associate Scientist, Lecturer,
>>> The Institute of the Earth Science,
>>> The Hebrew University of Jerusalem,
>>> Givat Ram, Jerusalem 91904, Israel
>>> Tel: 972 547 231 170
>>> Fax: (972)-25662581
>>>
>>> C.E.O, Weather It Is, LTD
>>> Weather and Climate Focus
>>> http://weather-it-is.com
>>> Jerusalem, Israel
>>> Local: 02 930 9525
>>> Cell: 054 7 231 170
>>> Int-IS: x972 2 930 9525
>>>
>>>
>>> _______________________________________________
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>>>
>>
>
> --
> Barry H. Lynn, Ph.D
> Senior Associate Scientist, Lecturer,
> The Institute of the Earth Science,
> The Hebrew University of Jerusalem,
> Givat Ram, Jerusalem 91904, Israel
> Tel: 972 547 231 170
> Fax: (972)-25662581
>
> C.E.O, Weather It Is, LTD
> Weather and Climate Focus
> http://weather-it-is.com
> Jerusalem, Israel
> Local: 02 930 9525
> Cell: 054 7 231 170
> Int-IS: x972 2 930 9525
>
> _______________________________________________
> ncl-talk mailing list
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>
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