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Hi! Sure, here is the dump:
S:\>ncdump -h -s aermr04_2017_1_1_0.nc
netcdf aermr04_2017_1_1_0 {
dimensions:
longitude = 2880 ;
latitude = 1441 ;
level = 21 ;
time = UNLIMITED ; // (1 currently)
variables:
float longitude(longitude) ;
longitude:units = "degrees_east" ;
longitude:long_name = "longitude" ;
float latitude(latitude) ;
latitude:units = "degrees_north" ;
latitude:long_name = "latitude" ;
int level(level) ;
level:units = "millibars" ;
level:long_name = "pressure_level" ;
int time(time) ;
time:units = "hours since 1900-01-01 00:00:0.0" ;
time:long_name = "time" ;
time:calendar = "gregorian" ;
short aermr04(time, level, latitude, longitude) ;
aermr04:scale_factor = 1.84160928156453e-012 ;
aermr04:add_offset = 6.03421697197432e-008 ;
aermr04:_FillValue = -32767s ;
aermr04:missing_value = -32767s ;
aermr04:units = "kg kg**-1" ;
aermr04:long_name = "Dust Aerosol (0.03 - 0.55 um) Mixing
Ratio" ;
// global attributes:
:Conventions = "CF-1.6" ;
:history = "2017-10-30 22:48:34 GMT by grib_to_netcdf-2.5.0:
grib_to_netcdf
/data/data04/scratch/_mars-atls02-a82bacafb5c306db76464bc7e824bb75-6HA6Rj.grib
-o
/data/data03/scratch/_grib2netcdf-atls00-98f536083ae965b31b0d04811be6f4c6-AYhwQY.nc
-utime" ;
:_Format = "64-bit offset" ;
}
From: Ed Hartnett <edwardjameshartnett@xxxxxxxxx>
Date: Saturday, 4 November 2017 at 20:13
To: Richard Cloete <richard.cloete@xxxxxxxxxxx>
Cc: Ketan Kulkarni <ketan.kulkarni@xxxxxxxxx>, Daniel Neumann
<daniel.neumann@xxxxxxxxxxxxxxxxx>, "netcdfgroup@xxxxxxxxxxxxxxxx"
<netcdfgroup@xxxxxxxxxxxxxxxx>
Subject: Re: [netcdfgroup] Negative dust values
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<mailto: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<mailto:netcdfgroup-bounces@xxxxxxxxxxxxxxxx>>
on behalf of Ketan Kulkarni
<ketan.kulkarni@xxxxxxxxx<mailto:ketan.kulkarni@xxxxxxxxx>>
Date: Saturday, 4 November 2017 at 15:26
To: Daniel Neumann
<daniel.neumann@xxxxxxxxxxxxxxxxx<mailto:daniel.neumann@xxxxxxxxxxxxxxxxx>>
Cc: "netcdfgroup@xxxxxxxxxxxxxxxx<mailto:netcdfgroup@xxxxxxxxxxxxxxxx>"
<netcdfgroup@xxxxxxxxxxxxxxxx<mailto: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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