3. The SNAP CAWA Products

3.1. Overview

This section will give an overview of all input, intermediate and final products used and generated by the SNAP GPF TCWV and CTP processors

3.2. Input Products

3.2.1. MERIS L1b TOA Radiance Products

From the MERIS full mission (2002-2012), L1b TOA radiance reduced resolution data has been used as input data. Table Table 3.1 to Table 3.4 give an overview of MERIS L1b bands, tie point grids and L1b flag coding, respectively. A more detailed description of the MERIS standard L1b product is given in [13].


Table 3.1 MERIS bands in L1b product
Name in product Unit Type Description
radiance_<n>; n=1,..,15 mW/(m^2*sr*nm) float32 TOA radiance in band <n>
l1_flags dl (flag band) uint8 Level 1b flags
detector_index dl int16 Detector index


Table 3.2 MERIS instrument channels.
Channel Wavelength Bandwidth
1 412.5 10
2 442.5 10
3 490 10
4 510 10
5 560 10
6 620 10
7 665 10
8 681 7.5
9 709 10
10 753 7.5
11 761 3.75
12 778 15
13 865 20
14 885 10
15 900 10


Table 3.3 MERIS tie point grids in L1b product.
Name in product Unit Type Description
latitude deg float32 Latitude of the tie points
longitude deg float32 Longitude of the tie points
dem_alt m float32 Digital elevation model altitude
dem_rough m float32 Digital elevation model roughness
lat_corr deg float32 Digital elevation model latitude corrections
lon_corr deg float32 Digital elevation model longitude corrections
sun_zenith deg float32 Sun zenith angle
sun_azimuth deg float32 Sun azimuth angle
view_zenith deg float32 View zenith angle
view_azimuth deg float32 View azimuth angle
zonal_wind m/s float32 Zonal wind
merid_wind m/s float32 Meridional wind
atm_press hPa float32 Mean sea level pressure
ozone DU float32 Total ozone
rel_hum % float32 Relative humidity


Table 3.4 MERIS L1b flag coding.
Bit Flag Description
0 Cosmetic Pixel is cosmetic
1 Duplicated Pixel has been duplicated
2 Glint_Risk Pixel has glint risk
3 Suspect Pixel is suspect
4 Land_Ocean Pixel is over land,
5 Bright Pixel is bright
6 Coastline Pixel is part of a coastline


In the CAWA TCWV processing, the following bands and tie point grids from the MERIS L1b products are used:

  • radiance_13 (converted to TOA reflectance)
  • radiance_14 (converted to TOA reflectance)
  • radiance_15 (converted to TOA reflectance)
  • sun_zenith
  • sun_azimuth
  • view_zenith
  • view_azimuth

In the CAWA CTP processing, the following bands and tie point grids from the MERIS L1b products are used:

  • radiance_10
  • radiance_11
  • detector_index
  • sun_zenith
  • sun_azimuth
  • view_zenith
  • view_azimuth
  • dem_alt

3.2.2. MODIS MYD021 TOA Reflectance Products

For the ‘OLCI-like’ TCWV processing in CAWA, MODIS Aqua L1b data from MYD021KM products were used. These products contain calibrated Earth view TOA reflectance data at 1km resolution, including the 250m and 500m resolution bands aggregated to 1km resolution. The datasets are described in detail in the MODIS Level 1B Product User’s Guide [14]. Table 3.6 gives an overview of the reflective and emissive bands in the MYD021KM product.


Table 3.5 MODIS Aqua bands in L1b MYD021 product. Taken from [14].
Cryptic name Resolution Spectral bands
EV_250_RefSB 250m 1, 2
EV_500_RefSB 500m 3-7
EV_1KM_RefSB 1km 8-19, 26
EV_1KM_Emissive 1km 20-25, 27-36

Here, “RefSB” stands for “Reflective Solar Band” and “Emissive” stands for thermal emissive bands.


Table 3.6 gives an overview of the tie piont grids available in the MYD021KM product.


Table 3.6 MODIS tie point grids in L1b MYD021 product.
Name in product Unit Type Description
latitude deg float32 Latitude of the tie points (WGS-84), Greenwich origin, positive N
longitude deg float32 Longitude of the tie points (WGS-84), Greenwich origin, positive E
Height m float32 Height
Range m float32 Range
SolarZenith deg float32 Sun zenith angle
SolarAzimuth deg float32 Sun azimuth angle
SensorZenith deg float32 View zenith angle
SensorAzimuth deg float32 View azimuth angle


In the CAWA TCWV processing, the following bands and tie point grids from these products are used:

  • EV_250_Aggr1km_RefSB_2
  • EV_250_Aggr1km_RefSB_5
  • EV_1KM_RefSB_17
  • EV_1KM_RefSB_18
  • EV_1KM_RefSB_19
  • SolarZenith
  • SolarAzimuth
  • SensorZenith
  • SensorAzimuth

3.3. Intermediate Products

3.3.1. ERA-Interim Products (optional)

The CAWA TCWV algorithm uses the following prior variables:

  • temperature at 2m
  • mean sea level pressure
  • TCWV initial guess
  • windspeed at 10m, u-component
  • windspeed at 10m, v-component

As said, the way of providing these prior variables to the algorithms is somewhat arbitrary. In the CAWA TCWV processing on the BC Calvalus cluster, the variables were taken from available ERA-Interim reanalysis datasets. The ERA-Interim data extraction and preparation was done with specific scripts which were developed in the frame of other projects and which are running on Calvalus, making use of the collection of Climate Data Operators (CDO) developed at Max-Planck-Institute for Meteorology Hamburg [15]. All these components are not part of the CAWA software package. However, the content of the ERA-Interim products being resampled and collocated with the MERIS/MODIS L1b input products is given in Table Table 3.7.


Table 3.7 Bands in ERA-Interim product
Name in product Unit Type Description
t2m K float32 temperature at 2m
msl hPa float32 mean sea level pressure
tcwv kgm-2 float32 TCWV initial guess
u10 m/s float32 windspeed 10m, u-component
v10 m/s float32 windspeed 10m, v-component
latitude deg float32 latitude
longitude deg float32 longitude

3.3.2. SNAP IdePix Classification Products

The IdePix classification product is the result of the pixel classification performed on the MERIS or MODIS L1b products for both TCWV and CTP processing. In return, the IdePix product is used as input for the TCWV and CTP processing. In fact it is an ‘extended’ classification product containing the following information:

  • radiance/reflectance bands needed for TCWV/CTP retrieval
  • pixel classification flag band
  • prior variables in case of TCWV processing(for CAWA, obtained from collocation with ERA-Interim product described above)
  • L1b flags and tie point grids

The IdePix classification flag coding is given in Table Table 3.8. (Some of the flags may not be computed under certain conditions. E.g., a glint risk is not computed for land pixels.)


Table 3.8 IdePix classification flag coding.
Bit Flag Description
0 INVALID Pixel is invalid
1 CLOUD Pixel is either ‘cloud sure’ or ‘cloud ambiguous’
2 CLOUD_AMBIGUOUS Semi-transparent clouds, or cloud detection is uncertain
3 CLOUD_SURE Fully opaque clouds with full confidence of their detection
4 CLOUD_BUFFER A buffer of N pixels (user option) around a cloud
5 CLOUD_SHADOW Pixel is affected by a cloud shadow
6 SNOW_ICE Snow or ice pixel
7 GLINTRISK Pixel has glint risk (over ocean)
8 COASTLINE Pixel is part of a coastline
9 LAND Land pixel


The IdePix products are generated in NetCDF4 format. An example of the NetCDF header of an Idepix product is given in the Annex.

3.4. Final Products

3.4.1. CAWA TCWV Products

The CAWA TCWV final products are generated in CF-compliant NetCDF4 format. They just contain the TCWV, a simple TCWV flag and the pixel classification flag copied from the IdePix product (Table 3.9). An example of the NetCDF header of a TCWV product is given in the Annex.


Table 3.9 Bands in final CAWA TCWV product
Name in product Unit Type Description
tcwv mm float32 Total column of water vapour
tcwv_flags dl uint8 TCWV flags
pixel_classif_flags dl int16 Pixel classification flags


3.4.2. CAWA CTP Products

The CAWA CTP final products are generated in CF-compliant NetCDF4 format. They just contain the CTP, a simple CTP flag and the pixel classification flag copied from the IdePix product (Table 3.10). An example of the NetCDF header of a CTP product is given in the Annex.


Table 3.10 Bands in final CAWA CTP product
Name in product Unit Type Description
ctp mm float32 Cloud top pressure
ctp _flags dl uint8 CTP flags
pixel_classif_flags dl int16 Pixel classification flags