CAIRT, a proposed satellite mission to chart the middle atmosphere

2023-2024
CAIRT is a satellite instrument project to observe the Earth’s middle atmosphere, to diagnose how climate change may increase ozone transport from the stratosphere to the troposphere, where ozone has its largest warming potential.

Proposed for ESA’s flagship satellite Earth Explorer 11, we simulated CAIRT ozone data and estimated how they could constrain a model of the middle atmospheric composition. Compared with NASA MLS instrument, results confirm that CAIRT will be a step forward for upper tropospheric ozone observations.

Why do we need to monitor the middle atmosphere?

The ozone layer, part of the middle atmosphere, is an essential component of the Earth’s system. By absorbing ultraviolet solar light, it protects the Earth’s surface from harmful radiation. While the manmade production of halogen compounds responsible for the destruction of the ozone layer was banned by the Montreal protocol in 1987, the ozone layer is still a concern for the scientific community.

Ozone is a greenhouse gas with its largest warming potential below the tropopause (around 10 and 15 km). With climate change, the atmospheric circulation is predicted to intensify, increasing the ozone exchange between the stratosphere and the troposphere.

One of the goals of CAIRT is to monitor ozone in this region and BIRA-IASB in showing that this instrument will be a step forward compared to previous satellite observations.

What is CAIRT?

CAIRT (Changing Atmosphere Infra-Red Tomography explorer) proposes to measure the infrared emission of the Earth’s atmosphere in limb geometry, i.e. looking at the horizon of the Earth from space.

This geometry allows us to retrieve the vertical distribution – or vertical profile – of chemical compounds, which is not possible with the nadir geometry when the instrument is looking toward the Earth’s surface. Combined with infrared emission measurements, many atmospheric quantities can be observed, like ozone (but also temperature or water vapour).

While satellite infrared emission instruments are not new, CAIRT proposes for the first time to be based on an imager allowing a spatial resolution never achieved so far.

This is highlighted in Figure 1, which compares the daily coverage of the NASA MLS instrument with CAIRT, MLS being the current standard for ozone profiles. While MLS retrieves an ozone profile every 167 km along the orbit, CAIRT will provide 4 profiles every 50 km, this is more than 12 times more profiles.

Value of CAIRT Ozone in the Upper Troposphere

We have evaluated the value of CAIRT to monitor ozone in the upper troposphere using an Observing System Simulation Experiment (OSSE).

To do so, we have taken reference ozone from the EU model CAMS, one of the best for ozone. CAMS ozone had then been resampled at the CAIRT simulated observed point and perturbed according to CAIRT expected uncertainty. These profiles have been ingested in the BIRA-IASB model BASCOE providing CAIRT ozone. Using CAMS ozone as well, MLS profiles have also been simulated, again ingested in BASCOE and providing MLS ozone.

A BASCOE control run, without ingesting any observations, has also been done to evaluate the influence of the model on the results.

Figure 2 compares the differences between the three BASCOE runs and the CAMS reference ozone for the period of the OSSE, between Oct. 2021 and Feb. 2022. As one can see, CAIRT displays a much better agreement with the reference ozone than the other runs. While MLS resolved most of the bias at and above the tropopause, CAIRT ozone profiles give a much larger constrain in upper tropospheric ozone.

 

Reference

Earth Explorer 11 Candidate Mission CAIRT: Report for Mission Selection, European Space Agency, Noordwijk, The Netherlands, ESA-EOPSM-CAIR-RP-4797, 230pp. https://esamultimedia.esa.int/docs/EarthObservation/ESA_EE11_Report_for…

Daily coverage of ozone from MLS
Daily coverage of ozone from MLS compared to the expected coverage of CAIRT
Figure 1: Daily coverage of ozone from MLS (a) compared to the expected coverage of CAIRT (b). Each blue point represents the geolocation of an ozone profile. While MLS provides one profile per measurement every 167 km along the orbit, CAIRT will deliver 4 profiles per measurement every 50 km along the orbit.
Difference between Control, MLS and CAIRT runs against Reference ozone
Figure 2: Difference between the Control (left), MLS (centre) and CAIRT (right) runs against the Reference ozone for the period Oct. 2021 – Feb 2022. The dotted black lines represent the region of the tropopause.