# [netcdfgroup] How to define hybrid sigma pressure coordinates?

```Hi there,

```
I am confused about how to define hybrid sigma pressure coordinates. I am aware of the CF1.4 metadata conventions, and tried to adhere to them. I attached the header of the file I created.
```
```
From my understanding, everything should be okay. However, the variables levm and levi don't show any values? I would have thought that NetCDF would calculate the pressures automatically, because all the information necessary is there?
```
```
I took a look at one of the example files (http://www.unidata.ucar.edu/software/netcdf/examples/cami_0000-09-01_64x128_L26_c030918.nc) shown on the NetCDF homepage. However, there, the lev and levi variables are only 1d, even though according to the formula_terms they also depend on PS, which in turn is 3d ... I'm lost.
```
--
Andreas Hilboll
PhD Student

Institute of Environmental Physics
University of Bremen

U3145
Otto-Hahn-Allee 1
D-28359 Bremen
Germany

+49(0)421 218 62133 (phone)
+49(0)421 218 98 62133 (fax)
http://www.iup.uni-bremen.de/~hilboll

```
```netcdf sat_2007-06_comp {
dimensions:
lat = 64 ;
lon = 128 ;
levm = UNLIMITED ; // (60 currently)
levi = UNLIMITED ; // (61 currently)
time = UNLIMITED ; // (720 currently)
variables:
float lat(lat) ;
lat:standard_name = "latitude" ;
lat:units = "degrees_north" ;
lat:long_name = "latitude" ;
lat:label = "Latitude [deg]" ;
float lon(lon) ;
lon:standard_name = "longitude" ;
lon:units = "degrees_east" ;
lon:long_name = "longitude" ;
lon:label = "Longitude [deg]" ;
float hyam(levm) ;
hyam:long_name = "hybrid A coefficient at layer midpoints" ;
float hybm(levm) ;
hybm:long_name = "hybrid B coefficient at layer midpoints" ;
float hyai(levi) ;
hyai:long_name = "hybrid A coefficient at layer interfaces" ;
float hybi(levi) ;
hybi:long_name = "hybrid B coefficient at layer interfaces" ;
float levm(time, levm, lat, lon) ;
levm:long_name = "hybrid level at midpoints" ;
levm:units = "hPa" ;
levm:standard_name =
"atmosphere_hybrid_sigma_pressure_coordinate" ;
levm:formula_terms = "a: hyam b: hybm ps: surface_pressure" ;
float levi(time, levi, lat, lon) ;
levi:long_name = "hybrid level at interfaces" ;
levi:units = "hPa" ;
levi:standard_name =
"atmosphere_hybrid_sigma_pressure_coordinate" ;
levi:formula_terms = "ap: hyai b: hybi ps: surface_pressure" ;
float time(time) ;
time:standard_name = "time" ;
time:units = "hours since 1990-01-01 00:00:00 +0:00" ;
time:long_name = "hours since 1990-01-01 00:00:00 +0:00" ;
time:calendar = "gregorian" ;
time:label = "Date" ;
float surface_pressure(time, lat, lon) ;
surface_pressure:units = "hPa" ;
surface_pressure:long_name = "surface pressure" ;
surface_pressure:label = "surface pressure [hPa]" ;
float tropopause_height(time, lat, lon) ;
tropopause_height:units = "m" ;
tropopause_height:standard_name = "tropopause_altitude" ;
tropopause_height:long_name = "tropopause height above surface
from ECMWF-IFS potential vorticity" ;
tropopause_height:label = "tropopause height above surface [m]"
;
float no2(time, levm, lat, lon) ;
no2:units = "m^3/m^3" ;
no2:standard_name =
"volume_mixing_ratio_of_nitrogen_dioxide_in_air" ;
no2:long_name = "volume mixing ratio NO2" ;
no2:label = "VMR NO2 [m^3/m^3]" ;
float hcho(time, levm, lat, lon) ;
hcho:units = "m^3/m^3" ;
hcho:standard_name =
"volume_mixing_ratio_of_formaldehyde_in_air" ;
hcho:long_name = "volume mixing ratio HCHO" ;
hcho:label = "VMR HCHO [m^3/m^3]" ;
float so2(time, levm, lat, lon) ;
so2:units = "m^3/m^3" ;
so2:standard_name =
"volume_mixing_ratio_of_sulfur_dioxide_in_air" ;
so2:long_name = "volume mixing ratio SO2" ;
so2:label = "VMR SO2 [m^3/m^3]" ;
float chocho(time, levm, lat, lon) ;
chocho:units = "m^3/m^3" ;
chocho:standard_name = "volume_mixing_ratio_of_glyoxal_in_air" ;
chocho:long_name = "volume mixing ratio CHOCHO" ;
chocho:label = "VMR CHOCHO [m^3/m^3]" ;
float altitude(time, levm, lat, lon) ;
altitude:units = "m" ;
altitude:standard_name = "altitude" ;
altitude:long_name = "altitude above surface" ;
altitude:label = "altitude above surface [m]" ;
float temperature(time, levm, lat, lon) ;
temperature:units = "K" ;
temperature:standard_name = "air_temperature" ;
temperature:long_name = "air temperature" ;
temperature:label = "air temperature [K]" ;

// global attributes:
:description = "Test dataset" ;
:Conventions = "CF-1.4" ;
:title = "2007/06" ;
:institution = "Institute of Environmental Physics, University
of Bremen, Germany." ;
:history = "This file was created on Thu Aug 19 14:17:10 2010
by test-compression.py" ;
:contact = "Andreas Hilboll <hilboll@xxxxxxxxxxxxxxxxxxxxxxxx>"
;
}

```
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