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<mailto: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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