Question about cloud amount

[Joong]

Hello,

I've noticed that the cloud fraction (cldamt) values range from 0 to
500%. Is this expected? And if you look at 2009 year data (basic data),
cloud fraction in America region is above 1000% during first few days!
Is this normal?

Thank you.

- Joong

[Ken Knapp - NOAA Federal]

Hi Joong

Thanks for using ISCCP and we appreciate the question.

The ISCCP data are stored using CF conventions. I would recommend reading some of their documents to be familiar with it. In particular, they describe how the data are encoded using the variable attributes. For example, the cldamt attributes are:

        short cldamt(time, lat, lon) ;
                cldamt:long_name = "Mean cloud amount" ;
                cldamt:units = "percent" ;
                cldamt:valid_min = -500s ;
                cldamt:valid_max = 500s ;
                cldamt:scale_factor = 0.1f ;
                cldamt:_FillValue = 32767s ;
                cldamt:standard_name = "isccp_cloud_area_fraction" ;
                cldamt:add_offset = 50.f ;
                cldamt:cell_methods = "area: mean time: mean within days time: mean over days" ;

(This is from the HGM file for Jan 2009). 

The data are stored as short integers (2 bytes). They are scaled back to floats using scale_factor and add_offsets using: Value=short * scale_factor + add_offset.

I think the 0-500 that you're seeing is the range of the stored data, which should range from -500 to 500. The large values that you note as 1000+ appear to be the missing data.

Please test out these concepts and see if that helps.

If you continue to have problems, please provide the name of the file that you're trying to read so that we can check it on our side.

Thanks-

-Ken

 

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Ken Knapp, Ph.D.

Meteorologist, Center for Weather and Climate

NOAA National Centers for Environmental Information

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