Re: A question about processing HDF-eos when using ncl-6.2.0

Hi wei, thanks for the help.

I don’t understand why same code yields different results for var_f (in my case) 

under ncl-6.1.0 and 6.2.0? 

As commented in the code, using 6.1.0, the var_f prints my desired one, which is -99,

but in 6.2.0,  var_f will print -9900.

 原始邮件 

发件人: Wei Huang<huangwei@ucar.edu>

收件人: Cheung<zuibeidemei@126.com>

抄送: NCL<ncl-talk@ucar.edu>

发送时间: 2014年4月29日(周二) 21:41

主题: Re: [ncl-talk] A question about processing HDF-eos when using ncl-6.2.0

Cheung,

Your code:

 filein = "2009200040119_17152_CS_2B-TAU_GRANULE_P_R04_E02.hdf"

 eos_file = addfile(filein+".he2","r")

;read using eos file

 var = eos_file->layer_optical_depth_2B_TAU

 var_f = short2flt(eos_file->layer_optical_depth_2B_TAU)/100

which var read for the he2 directly, and NCL show it as short:

Variable: var (subsection)

Type: short

Total Size: 8 bytes

            4 values

Number of Dimensions: 1

Dimensions and sizes:   [nray_2B_TAU | 4]

Coordinates: 

Number Of Attributes: 9

  factor :      100

  offset :       0

  long_name :   Layer cloud optical depth

  units :       -

  valid_range : ( 0, 5000 )

  missing :     -9900

  missop :      ==

  _FillValue :  0

  hdfeos_name : layer_optical_depth

(0)     -9900

(1)     -9900

(2)     -9900

(3)     -9900

When you use short2flt, NCL has the (scale-)factor multiplied to the var, and then return as float.

see the print:

Variable: var_f (subsection)

Type: float

Total Size: 16 bytes

            4 values

Number of Dimensions: 1

Dimensions and sizes:   [4]

Coordinates: 

Number Of Attributes: 1

  _FillValue :   0

(0)     -9900

(1)     -9900

(2)     -9900

(3)     -9900

In most the case, when the data is short, and has factor and offset,

you can get the original value as:

new_var = ori_var * tofloat(factor) + tofloat(offset)

That is what short2flt does.

See document for short2flt at:

http://www.ncl.ucar.edu/Document/Functions/Contributed/short2flt.shtml

In your case, you may want to do:

new_var = ori_var / tofloat(factor)

You do not need the offset, as it is zero for your case.

Hope this help:

Wei

On Apr 28, 2014, at 9:37 PM, Cheung <zuibeidemei@126.com> wrote:

Hi Wei:

              Thanks for your response.

              I put an example online https://github.com/slashgithub/test_2btau,
   
              which contains data and test script, please download and check it.

---

Regards

Cheung

At 2014-04-29 00:53:57,"Wei Huang" <huangwei@ucar.edu> wrote:

Cheung,

Can you point us a data you are using, so we can take a look?

Thanks,

Wei

On Apr 23, 2014, at 6:23 AM, Cheung <zuibeidemei@126.com> wrote:

 Dear all:
                  I just came across a different  result when I use ncl-6.2.0 processing the HDF-eos file from CloudSat, before I was using 6.1.0.

        This particular variable is layer_optical_depth  from 2b-tau as documented http://www.cloudsat.cira.colostate.edu/dataSpecs.php?prodid=2.

        I used to use:

        var = short2flt(eos_file->layer_optical_depth_2B_TAU)/100 to retreive the "actual" value in float from this file due to a factor of 100.
      
        However, in this new version, I found I need to "xxxxx/10000" to  retreive the "actual" value in float.

        I'm not sure what happen during this process, any comment is appreciated.

Regards

Cheung


       

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