Calculate the maximum cloud fraction between pressure levels

[Xin Zhang]

Dear users,

I'm trying to calculate the maximum cloud fraction between 350-400 hPa. I find that variables of the wrfout file include 'CLDFRA'.

Is it correct to use the maximum of 'CLDFRA' in the wrfout* file? Do I need to generate the maximum cloud fraction by other algorithms like mentioned by the other post?

Best regards,

Xin

[Bill Ladwig]

Hi Xin,

If you use CLDFRA that is calculated by some WRF radiation schemes, I think it mainly produces a 1 or 0 for the presence or absence of a cloud. There are other schemes that produce CLDFRA2, but I don't know if it's better. 

Instead, if you use wrf-python's 'cloudfrac' diagnostic via getvar, you can set the thresholds for where the cloud fraction is calculated and it takes the maximum value for that range. By default, when using pressure as your vertical coordinate, the thresholds are 970 hPa - 800 hPa for low clouds, 800 hPa - 450 hPa for mid clouds, and less than 450 hPa for high clouds. In your case, you want to change the mid and high thresholds for your range, then use the 'mid' (index 1) for your result. 

my_cfrac = getvar(f, 'cloudfrac', vert_type='pressure', mid_thresh=350, high_thresh=400)[1]

 

[Note: WRF-Python's cloud fraction diagnostic is a simple calculation using relative humidity, and is specified as:

lowc(i,j)  = 4.0*RH(i,j)/100. - 3.0 ! For the low threshold to mid threshold

midc(i,j)  = 4.0*RH(i,j)/100. - 3.0 ! For the mid threshold to high threshold

highc(i,j) = 2.5*RH(i,j)/100. - 1.5 ! Above the high threshold

]

Hope this helps,

Bill

 

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[Xin Zhang]

Hi Bill,

After checking module_radiation_driver.F in WRFV3/phys, I find that the calculation of CLDFRA has been updated from WRF V3.6. You can check the update of icloud.

Unfortunately, I'm using WRF-Chem V3.5 which is modified for my research. So, it's difficult to update the whole radiation module. Now, I'm using formula of cal_cldfra1 to calculate CLDFRA  which is a 3D variable.

Best regards,

Xin

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