Re: [netcdfgroup] Negative dust values

• To: Richard Cloete <richard.cloete@xxxxxxxxxxx>
• Subject: Re: [netcdfgroup] Negative dust values
• From: Ed Hartnett <edwardjameshartnett@xxxxxxxxx>
• Date: Sat, 4 Nov 2017 14:13:15 -0600

Howdy!

I suggest you run ncdump -h -s <FILENAME> and post the results to the
mailing list.

Thanks,
Ed Hartnett

On Sat, Nov 4, 2017 at 2:02 PM, Richard Cloete <richard.cloete@xxxxxxxxxxx>
wrote:

> Thanks everyone!
>
>
>
> It turns out that there is an attribute called “_FillValue”. There are
> also other attributes such as “scale_factor”, “add_offset” and
> “missing_value”
>
> Curiously, FillValue == missing_value. The negative values are actually in
> the NetCDF file (I used Panoply to open it). The negative values found in
> the NetCDF file are different from the _FillValue, though, which is odd:
> there cannot be a negative Dust Aerosol (0.03 - 0.55 um) Mixing Ratio.
>
>
>
> Kind regards,
>
> Richard
>
>
>
> *From: *<netcdfgroup-bounces@xxxxxxxxxxxxxxxx> on behalf of Ketan
> Kulkarni <ketan.kulkarni@xxxxxxxxx>
> *Date: *Saturday, 4 November 2017 at 15:26
> *To: *Daniel Neumann <daniel.neumann@xxxxxxxxxxxxxxxxx>
> *Cc: *"netcdfgroup@xxxxxxxxxxxxxxxx" <netcdfgroup@xxxxxxxxxxxxxxxx>
> *Subject: *Re: [netcdfgroup] Negative dust values
>
>
>
> Check if the data is in packed netcdf format which has scale and offset
> values. I remember era interim data is in such format.
>
>
>
> On Nov 4, 2017 4:17 PM, "Daniel Neumann" <daniel.neumann@xxxxxxxxxxxxxxxxx>
> wrote:
>
> Hi Richard,F
>
> If you find strange/unexpected values in your data you should contact the
> data creator. Possibly a contact person is noted in the global attributes
> of your netCDF file. I am not sure whether ECMWF provides only their own
> data or also data of third parties.
>
> What might be the reason for the negative values: If you look into the
> original netCDF file (e.g. with "ncdump -h FILENAME") you will possibly
> find an attribute "_fillvalue". Depending on the software you used to
> process the netCDF file, all values equal to "_fillvalue" are ignored or
> not (they should be ignored and kept untouched). Because _fillvalue is
> often negative, it came to my mind that your software possibly treats the
> _fillvalues like normal values. Because the atmospheric pressure is not
> constant, you will get slightly different negative values in space and time
> (where the _fillvalues have been before). At least this might be a reason
> for getting the negative values.
>
> Cheers,
> Daniel
>
> On 04.11.2017 15:07, Richard Cloete wrote:
>
> Hi all,
>
>
>
> Pretty new to NetCDF and ECMWF data.
>
>
>
> Just a quick (probably with an obvious answer) question: What do I do with
> negative values? For example, I am using aermr04, aermr05 and aermr06
> (dust) data from ECMWF. I have noticed many negative values.
>
> What do I do with these data? Shall I just treat them as zeros? I ask
> because I am calculating the concentrations using the following formula:
> https://software.ecmwf.int/wiki/display/CKB/How+to+
> compute+concentration+from+a+mixing+ratio+field
>
>
>
> Many thanks,
>
> Richard
>
>
>
>
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    • From: Richard Cloete
  • Re: [netcdfgroup] Negative dust values
    • From: Daniel Neumann
  • Re: [netcdfgroup] Negative dust values
    • From: Ketan Kulkarni
  • Re: [netcdfgroup] Negative dust values
    • From: Richard Cloete