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

Mon Nov 5 11:03:00 MST 2018

HI Barry,

I'm not entirely sure, but I'd guess that ice mixing ratio is needed to
detect upper level cirrus, but anywhere that snow mixing ratio is being
produced, the water vapor mixing ratio already has an optical depth of 1.0
above where that's occurring. The author probably just kept the algorithm
simple. Keep in mind that this is just a diagnostic, which is simplified
with things like "optical depth = 1.0". For some cloud types, an optical
depth of 1.0 might be too thick (e.g. cirrus). A satellite observation is
going to be more accurate.

Bill

On Sat, Nov 3, 2018 at 9:46 AM Barry Lynn <barry.h.lynn at gmail.com> wrote:

> Hi Bill:
>
> Thanks.  Much appreciated.
>
> Is there any particular reason why the scheme does not include snow in its
> calculations? Some schemes produce more snow than cloud ice....
>
> Barry
>
> On Fri, Nov 2, 2018 at 7:31 PM Bill Ladwig <ladwig at ucar.edu> wrote:
>
>> 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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>>>>> ncl-talk at ucar.edu
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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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>>>
>>
>
> --
> 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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