A 16-year global climate data record of total column water vapour generated from OMI observations in the visible blue spectral range
Authors: Borger Christian , Beirle Steffen , Wagner Thomas 
Published: July 2023

Abstract. We present a long-term data set of
1∘×1∘ monthly mean total column water
vapour (TCWV) based on global measurements of the
Ozone Monitoring Instrument (OMI) covering the time
range from January 2005 to December 2020. In comparison to the retrieval algorithm of
Borger et al. (2020), several modifications and filters
have been applied accounting for instrumental issues
(such as OMI's “row anomaly”) or the inferior quality
of solar reference spectra. For instance,
to overcome issues related to low-quality reference spectra,
the daily solar irradiance spectrum is replaced by
an annually varying mean earthshine radiance obtained in
December over Antarctica. For the TCWV
data set, we only consider measurements with an effective cloud fraction less than 20 %,
an air mass factor (AMF) greater than 0.1, a snow- and
ice-free ground pixel, and an OMI row that is not affected by the
row anomaly over the complete time range of the data set.
The individual TCWV measurements are then gridded
to a regular 1∘×1∘ lattice, from which
the monthly means are calculated. The investigation of sampling errors in the OMI
TCWV data set shows that these are dominated by
the clear-sky bias and cause on average deviations
of around −10 %, which is consistent with the findings
of previous studies. However, the spatiotemporal
sampling errors and those due to the row-anomaly
filter are negligible. In a comprehensive intercomparison study, we
demonstrate that the OMI TCWV data set is in good
agreement with the global reference data sets of ERA5 (fifth-generation ECMWF atmospheric reanalysis), RSS SSM/I (Remote Sensing Systems Special Sensor Microwave Imager),
and CM SAF/CCI TCWV-global (COMBI): over ocean
the orthogonal distance
regressions indicate slopes close to unity with
very small offsets and high coefficients of
determination of around 0.96.
However, over land, distinctive positive
deviations of more than +10 kg m−2
are obtained for high TCWV values. These overestimations
are mainly due to extreme overestimations of high
TCWV values in the tropics, likely caused by
uncertainties in the retrieval input data (surface
albedo, cloud information) due to frequent cloud
contamination in these regions. Similar results are
found from intercomparisons with in situ radiosonde
measurements from version 2 of the Integrated Global Radiosonde Archive (IGRA2) data set. Nevertheless, for
TCWV values smaller than 25 kg m−2, the
OMI TCWV data set shows very good agreement with
the global reference data sets. Furthermore, a temporal stability analysis proves that
the OMI TCWV data set is consistent with the temporal
changes in the reference data sets and shows no
significant deviation trends. As the TCWV retrieval can be easily applied to
further satellite missions, additional TCWV data sets
can be created from past missions, such as the Global Ozone Monitoring Experiment-1 (GOME-1) or the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY); under consideration of systematic
differences (e.g. due to different observation times), these data sets
can be combined with the OMI TCWV data set in order
to create a data record that
would cover a time span from 1995 to the present.
Moreover, the TCWV retrieval will also work for all
missions dedicated to NO2 in the future, such as
Sentinel-5 on MetOp-SG. The Max Planck Institute for Chemistry (MPIC) OMI total column water vapour (TCWV) climate data
record (CDR) is available at https://doi.org/10.5281/zenodo.7973889 (Borger et al., 2023).