field merging Options

The purpose of the merging functionaltiy is to produce a combined field from two input fields, typically one with low horizontal resolution covering a large area (base or outer ), and one with high horizontal resolution covering a small area (top or inner). The merge process has two steps: first, the transition between the outer border of the high-resolution field and the low-resolution field is smoothed; second, both are interpolated to the final grid, using the smoothed high-resolution field where defined, and the low-resolution field elsewhere.

When using the fimex program, input.file/type/config specify the outer "low resolution" file, while merge.inner.file/type/config specify the inner "high resolution" file. All compatible variables will be merged, where compatibilty means that names agree, and shapes match except for length-1 dimensions and horizontal axes. To keep all other variables from the base-file, on can specify merge.keepOuterVariables.

The final grid may be specified with merge.projString,x/yAxisValues,x/yAxisUnit,x/yAxisType; if not specified, it will be derived automatically by extending the high resolution grid until it covers the outer grid.

Ofen, one needs to manipulate heavily both the outer as well as the inner inner data to match each other. The outer data can be manipulated with the usual fimex-commands, while the inner can be modified with a fimex-setup file and the –merge.inner.cfg option.

A complex example is shown below, run as

  fimex -c ecPlusArome.cfg --output.file=out.nc4

It joins global EC-data with Norwegian local AROME data. EC-data comes in 3hourly timesteps, while Arome has hourly data. EC precipitation is split into convective and large-scale precipitation, Arome has only one type of precipitation (arome precipitation will be put into convective precipitation, while large-scale precipitation will be set to 0). Arome contains only surface data, while the output should contain all model-level data from EC model (keepOuterVariables).

# file: ecPlusArome.cfg

[input]
file=ec_n1d_20151208_00.nc


# extract only certain parameters used for dispersion modelling
[extract]
selectVariables=air_temperature_2m
selectVariables=lwe_thickness_of_convective_precipitation_amount
selectVariables=lwe_thickness_of_stratiform_precipitation_amount
selectVariables=x_wind_10m
selectVariables=y_wind_10m
selectVariables=air_temperature_ml
selectVariables=x_wind_ml
selectVariables=y_wind_ml
selectVariables=air_pressure_at_sea_level
selectVariables=surface_air_pressure
selectVariables=surface_geopotential


# merge with arome 2.5km data
[merge]
inner.file=arome_metcoop2_5km_20151208_00.nc
# manipulate names with arome2EcPrecip.ncml
inner.config=arome2EcPrecip.ncml
# further configure data with arome2hprecip.cfg
inner.cfg=arome3hprecip.cfg
keepOuterVariables=1
smoothing=LINEAR(3,2)
method=bilinear
# ec - 0.05deg
projString=+proj=longlat +R=6.371e+06 +no_defs
xAxisUnit=degree_east
yAxisUnit=degree_north

# create 3hourly data
[timeInterpolate]
timeSpec=2015-12-08T00:00:00Z,2015-12-08T03:00:00Z,...,2015-12-10T12:00:00

#file: arome2precip.ncml

<?xml version="1.0" encoding="UTF-8"?>
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2 http://www.unidata.ucar.edu/schemas/netcdf/ncml-2.2.xsd">

<!-- change from kg/m2 (mm) to m and rename -->
<variable orgName="precipitation_amount_acc" name="lwe_thickness_of_convective_precipitation_amount">
  <attribute name="units" type="String" value="m" />
  <attribute name="scale_factor" type="float" value="0.001" />
</variable>

<!-- create a dummy with everything, will be made 0 in quality extractor -->
<variable orgName="surface_air_pressure" name="lwe_thickness_of_stratiform_precipitation_amount">
  <attribute name="units" type="String" value="m" />
  <attribute name="scale_factor" type="float" value="0.001" />
</variable>


</netcdf>

# file: arome3hprecip.cfg
[extract]
# this is a dummy 0 genereated with ncml
selectVariables=lwe_thickness_of_stratiform_precipitation_amount
# was precipitation_amount_acc before ncml
selectVariables=lwe_thickness_of_convective_precipitation_amount

# select same period as in ec
[timeInterpolate]
timeSpec=2015-12-08T00:00:00Z,2015-12-08T03:00:00Z,...,2015-12-10T12:00:00

[qualityExtract]
config=noConvectivePrec.xml

# file: noConvectivePrec.xml

<?xml version="1.0" encoding="UTF-8"?>
<cdmQualityConfig>

<!-- set all values to 0 (no negative precip) -->
<variable name="lwe_thickness_of_stratiform_precipitation_amount" fillValue="0">
   <status_flag_variable name="lwe_thickness_of_stratiform_precipitation_amount">
      <allowed_values use="max:0" />
   </status_flag_variable>
</variable>

</cdmQualityConfig>

See also: MetNoFimex::CDMMerger fimex Program Options