CRISTA-NF Beobachtungen von polaren Stratosphärenwolken und Spurengasmischungsverhältnissen in der arktischen Stratosphäre im Winter
Zusammenfassung der Projektergebnisse
Polar stratospheric clouds (PSCs) play an important role for the spatial and temporal evolution of trace gases inside the polar vortex due to different processes (e.g. chlorine activation and NOy redistribution). There are still uncertainties in the representation of PSCs in model simulations and, thus, detailed observations of PSCs and information on their type (nitric acid trihydrate (NAT), supercooled ternary solution (STS), and ice) are desirable. The measurements inside PSCs by the airborne infrared limb sounder CRISTA-NF during the RECONCILE aircraft campaign showed a spectral peak at about 816 cm^-1. This peak is shifted compared to the peak at about 820 cm^-1, which is known to be caused by small NAT particles and which was used in former studies to detect NAT PSCs. In order to investigate the reason for this spectral difference of the peaks a large set of radiative transfer simulations of infrared limb emission spectra in the presence of various PSCs (NAT, STS, ice, and mixtures) for the viewing geometry of CRISTA-NF were performed. The simulation results showed that NAT particles can cause different spectral features in the region 810 – 820 cm-1. The appearance of the feature changes with increasing median radius of the NAT particles from a peak at 820 cm^-1 to a shifted peak and, finally, to a step-like feature in the spectrum. Based on this behaviour different colour indices were defined to detect PSCs containing NAT particles and to subgroup them into three size regimes: small NAT (≤ 1.0 µm), medium NAT (1.5 – 4.0 µm), and large NAT (≥ 3.5 µm). For the first time, the new/improved detection method allows such a differentiation between different NAT particle sizes. Additionally, a new method to detect the bottom altitude of the PSCs has been developed during the project. The final application of the methods to observations of the CRISTANF instrument during one local flight of the RECONCILE aircraft campaign showed the existence of STS and medium sized NAT. In a second step spectrally resolved extinction profiles for NAT particles and also for STS/H2SO4 were considered during the retrieval process. For the CRISTA-NF observations during RECONCILE a median radius of NAT of 2.0 to 2.5 µm was found to deliver the best agreement with the measurements. The spectral slope of these extinction profiles also influenced the derivation of trace gas volume mixing ratios which show the main contributions in spectral regions far away from the position of the NAT feature (810 – 820 cm-1). The new/improved retrieval setup significantly improved the results for different trace gases, especially for ClONO2, where too large volume mixing ratios (VMRs) have been derived before, and for ozone, where a large negative bias was observed when using the old retrieval setup. Comparisons between the CRISTA-NF retrieval results and in-situ observations carried out onboard the same aircraft showed a good agreement for all of the compared species: temperature, ozone, and CFC-11. In the third and last part of the project comparisons between the CRISTA-NF results and different model simulations by CLaMS were performed. With respect to the trace gases there is a good agreement between model simulations and observations. Especially for HNO3, a trace gas that is largely influenced by the PSCs because of the uptake of HNO3 by the particles, a good agreement for the spatial distribution as well as the total VMRs is observed. In the case of the NAT particle size distributions that have been derived from the model simulations the situation is different. For the first simulation run the NAT particles are slightly too large in size compared to the CRISTA-NF measurements and the number densities are very low. Concerning these differences a large improvement could be achieved in a second model simulation. Here the number densities are significantly higher and also the size of the NAT particles is slightly smaller. This is in good agreement with the expectation from the CRISTANF results. The main differences between the two simulations that led to these improvements are the use of superimposed temperature fluctuations and a higher nucleation frequency of new NAT particles (once an hour and not once a day).
Projektbezogene Publikationen (Auswahl)
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Radiative transfer simulations and observations of infrared spectra in the presence of polar stratospheric clouds: Detection and discrimination
of cloud types. 2020
Kalicinsky, C., Griessbach, S., and Spang, R.
