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HRLDAS (High Resolution Land Data Assimilation System)
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Calculation of Specific Humidity #203

Closed Shuaichen-He closed 3 months ago

Shuaichen-He commented 3 months ago

https://github.com/NCAR/hrldas/blob/087f1fd78601b37eaeead93da9f30253441f7cc2/hrldas/HRLDAS_forcing/run/examples/ERA5/Prepare_ERA5-Land_Forcing.sh#L165

Hello, is there an issue with the calculation of specific humidity? 🤔 When I using other forcing data, the specific humidity is measured in kg/kg, but the results calculated using this bash script for ERA5 seem to have a different magnitude. Or does using ERA5-land as the input require the units to be in g/kg ? 🥲

https://archive.eol.ucar.edu/projects/ceop/dm/documents/refdata_report/eqns.html

According to Bolton 1980, is the specific humidity calculated using this formula in kg/kg?

cenlinhe commented 3 months ago

@CharlesZheZhang Could you please take a look? The specific forcing input variable should be in unit of kg/kg.

CharlesZheZhang commented 3 months ago

Thank you for pointing out this issue. I think the issue is with the "/1000." in this line. The q is already in kg/kg, so don't need to divide it again by 1000, which can give you a very small number of the specific humidity. I will create a PR to correct this today.

CharlesZheZhang commented 3 months ago

Thank you for pointing out this issue. I think the issue is with the "/1000." in this line. The q is already in kg/kg, so don't need to divide it again by 1000, which can give you a very small number of the specific humidity. I will create a PR to correct this today.

Please see this pull request: https://github.com/NCAR/hrldas/pull/204

cenlinhe commented 3 months ago

So is this "/1000" related to the use of ERA5-Land vs. ERA5 atmospheric data ? Just want to make sure if this bug fix would work for both ERA5-Land and ERA5 atmospheric data. Have you checked if the updated code produces reasonable specific humidity magnitude?

CharlesZheZhang commented 3 months ago

So is this "/1000" related to the use of ERA5-Land vs. ERA5 atmospheric data ? Just want to make sure if this bug fix would work for both ERA5-Land and ERA5 atmospheric data. Have you checked if the updated code produces reasonable specific humidity magnitude?

We only have pre-processing code for ERA5-Land for now.

cenlinhe commented 3 months ago

OK, I see. Does the specific humidity magnitude looks reasonable after bug fix?

CharlesZheZhang commented 3 months ago

OK, I see. Does the specific humidity magnitude looks reasonable after bug fix?

Yes, the range of specific humidity is about 0~0.025 [kg/kg] after this fix, which is typical range for global values: https://geography.name/how-does-specific-humidity-vary-globally-and-seasonally/

Shuaichen-He commented 3 months ago

I sincerely appreciate your attention to this issue I’ve raised. 🤗 I’m uncertain whether HRLDAS automatically converts the units when generating forcing data.

cenlinhe commented 3 months ago

This bug has been fixed in this PR: https://github.com/NCAR/hrldas/pull/204

cenlinhe commented 3 months ago

I sincerely appreciate your attention to this issue I’ve raised. 🤗 I’m uncertain whether HRLDAS automatically converts the units when generating forcing data.

  • After decumulate the three cumulative variables in the script, would it be necessary to also convert their units?
  • For instance, should precipitation, currently measured in m, be converted to mm/s, and J/m^2 for radiation to W/m^2.🤔

I do not think we need further unit conversion after running all the ERA5-Land pre-processing scripts provided here. @CharlesZheZhang is this correct?

Shuaichen-He commented 3 months ago

After reviewing the units in the Vtable, I’ve realized that these variables indeed do not require conversion. I appreciate your assistance and attention to these matter.🤗