Total water vapour columns derived from Sentinel 5P using the AMC-DOAS method
Authors: Küchler Tobias , Noël Stefan , Bovensmann Heinrich , Burrows John Philip , Wagner Thomas , Borger Christian , Borsdorff Tobias , Schneider Andreas 
Published: January 2022

Abstract. Water vapour is the most abundant natural greenhouse gas in the Earth's atmosphere,
and global data sets are required for meteorological applications and climate research.
The Tropospheric Monitoring Instrument (TROPOMI) on board Sentinel-5 Precursor (S5P) launched
on 13 October 2017 has a high spatial resolution of around 5 km and a daily global coverage.
Currently, there is no operational total water vapour product for S5P measurements.
Here, we present first results of a new scientific total column water
vapour (TCWV) product for S5P using the so-called air-mass-corrected differential optical absorption spectroscopy (AMC-DOAS) scheme.
This method analyses spectral data between 688 and 700 nm and has
already been successfully applied to measurements from the Global
Monitoring Experiment (GOME) on ERS-2,
the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY
(SCIAMACHY) on Envisat and GOME-2 on MetOp. The adaptation of the AMC-DOAS method to S5P data requires an additional post-processing procedure to correct
the influences of surface albedo, cloud height and cloud
fraction. The quality of the new AMC-DOAS S5P water vapour product is
assessed by comparisons with data from GOME-2 on MetOp-B
retrieved also with the AMC-DOAS algorithm and with four
independent data sets, namely reanalysis data from the
European Centre for Medium range Weather Forecast (ECMWF
ERA5), data obtained by the Special Sensor Microwave Imager
and Sounder (SSMIS) flown on the Defense Meteorological
Satellite Program (DMSP) platform 16 and two scientific S5P TCWV products
derived from TROPOMI measurements. Both are recently published
TCWV products for S5P provided by the Max Planck Institute for
Chemistry (MPIC) in Mainz and the Netherlands Institute for Space Research (SRON), Utrecht.
The SRON TCWV is limited to clear-sky scenes over land. These comparisons reveal a good agreement between the various
data sets but also some systematic differences between all of them.
On average, the daily derived offset between AMC-DOAS S5P
TCWV and AMC-DOAS GOME-2B TCWV
is negative (around −1.5 kg m−2) over land and positive over ocean
surfaces (more than 1.5 kg m−2).
In contrast, SSMIS TCWV is on average
lower than AMC-DOAS S5P TCWV by about
3 kg m−2. Monthly averaged ERA5 TCWV and AMC-DOAS S5P TCWV
comparison shows spatial features over both land and water
surface.
Over land, there are systematic spatial structures. There are
larger differences between AMC-DOAS S5P TCWV and
ERA5 TCWV in tropical regions.
Over sea, AMC-DOAS S5P TCWV is slightly lower than
ERA5 TCWV by around 2 kg m−2.
The AMC-DOAS S5P TCWV and S5P
TCWV from MPIC agree on average within
1 kg m−2 over both land and ocean.
TCWV from SRON shows daily global averaged differences to AMC-DOAS S5P TCWV of around 1.2 kg m−2.
All of these differences are in line with the accuracy of
these products and with the typical range of differences of 5 kg m−2
obtained when comparing different TCWV data sets. The AMC-DOAS TCWV product for S5P provides therefore a
valuable new and independent data set for atmospheric
applications which also has a higher spatial coverage than
the other S5P TCWV products.