Abstract
Abstract. The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor (S5P) satellite launched in October 2017 yields a wealth of atmospheric composition data, including retrievals of total column ozone (TCO3) that are provided in near-real-time (NRT) and off-line. The NRT TCO3 retrievals (v1.0.0–v1.1.2) have been included in the data assimilation system of the Copernicus Atmosphere Monitoring Service (CAMS), and tests to monitor the data and to carry out first assimilation experiments with them have been performed for the period 26 November 2017 to 30 November 2018. The TROPOMI TCO3 data agree to within 2 % with the CAMS analysis over large parts of the globe between 60∘ N and 60∘ S and also with TCO3 retrievals from the Ozone Monitoring Instrument (OMI) and the Global Ozone Monitoring Experiment-2 (GOME-2) that are routinely assimilated by CAMS. However, the TCO3 NRT data from TROPOMI show some retrieval anomalies at high latitudes, at low solar elevations and over snow/ice (e.g. Antarctica and snow-covered land areas in the Northern Hemisphere), where the differences with the CAMS analysis and the other data sets are larger. These differences are particularly pronounced over land in the NH during winter and spring (when they can reach up to 40 DU) and come mainly from the surface albedo climatology that is used in the NRT TROPOMI TCO3 retrieval. This climatology has a coarser horizontal resolution than the TROPOMI TCO3 data, which leads to problems in areas where there are large changes in reflectivity from pixel to pixel, e.g. pixels covered by snow/ice or not. The differences between TROPOMI and the CAMS analysis also show some dependency on scan position. The assimilation of TROPOMI TCO3 has been tested in the CAMS system for data between 60∘ N and 60∘ S and for solar elevations greater than 10∘ and is found to have a small positive impact on the ozone analysis compared to Brewer TCO3 data and an improved fit to ozone sondes in the tropical troposphere and to IAGOS aircraft profiles at West African airports. The impact of the TROPOMI data is relatively small because the CAMS analysis is already well constrained by several other ozone retrievals that are routinely assimilated. When averaged over the periods February–April and September–October 2018, differences between experiments with and without assimilation of TROPOMI data are less than 2 % for TCO3 and less than 3 % in the vertical for seasonal mean zonal mean O3 mixing ratios, with the largest relative differences found in the troposphere.
Highlights
The Copernicus Atmosphere Monitoring Service (CAMS, https://atmosphere.copernicus.eu/, last access: 22 March 2019) produces daily global near-real-time (NRT) forecasts of atmospheric composition up to 5 days ahead and a range of other data sets on global and regional atmospheric composition, such as near-real-time estimates of fire emissions, reanalyses of atmospheric composition and greenhouse gas forecasts and analyses
Inness et al.: Monitoring and assimilation tests with TROPOspheric Monitoring Instrument (TROPOMI) data chemical species, including ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2) and for aerosols are improved by assimilating satellite retrievals of atmospheric composition using the 4-dimensional variations (4D-Var) data assimilation system (Benedetti et al, 2009; Inness et al, 2013, 2015; Massart et al, 2014) of the European Centre for Medium-Range Weather Forecasts (ECMWF)
TROPOMI was still in its commissioning phase until 24 April 2018, but even the early TROPOMI TCO3 data generally agreed well with the CAMS analysis over large parts of the globe and were of good enough quality to test their use in the CAMS system
Summary
The Copernicus Atmosphere Monitoring Service (CAMS, https://atmosphere.copernicus.eu/, last access: 22 March 2019) produces daily global near-real-time (NRT) forecasts of atmospheric composition up to 5 days ahead and a range of other data sets on global and regional atmospheric composition, such as near-real-time estimates of fire emissions, reanalyses of atmospheric composition and greenhouse gas forecasts and analyses. S5P carries the TROPOspheric Monitoring Instrument (TROPOMI), which provides high-resolution spectral measurements in the ultraviolet (UV), visible (Vis), near infrared (NIR) and shortwaveinfrared (SWIR) parts of the spectrum This wide spectral range allows several atmospheric trace gases to be retrieved, e.g. O3, NO2, SO2 and HCHO from the UV–Vis, and CO and CH4 from the SWIR part of the spectrum. Including TROPOMI TCO3 data passively in the CAMS system enables us (1) to carry out a continuous quality assessment of the data, (2) to detect biases between different satellite retrievals (e.g. between TCO3 from TROPOMI and the Ozone Monitoring Instrument, OMI, or the Global Ozone Monitoring Experiment-2, GOME-2) and (3) allows us to monitor instrument and algorithm stability.
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