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In Sect. 3.4, we have presented an intercomparison of radiative transfer simulations between the two TELIS Level-2 data processing codes (PILS and AdL). The simulations are based on a limb-scanning sequence corresponding to a HCl microwindow (fLO= 619.1 GHz, fIF = 5–7 GHz) of TELIS. In this section, the inversion performances of PILS and AdL are compared by carrying out a retrieval of atmospheric HCl from real TELIS limb spectra. A TELIS submillimeter measurement for this HCl microwindow recorded on 24 January 2010 is depicted in Fig. 4.6.

The two HCl transitions are visible in the second and the fourth segments, respectively.

To rule out any factors associated with the measurement itself, both algorithms used the identical frequency segment to derive the HCl profile. Regarding PILS, we study the numerical performance with different regularization setups, i.e. by using different combinations of the regularization matrixLand a priori profilexa (identical to the initial guessx0). Table 4.2 lists the two regularization scenarios used in this comparison. In particular, we used a zero a priori profile in the case L = L0 so that the inappropriate mapping of the a priori knowledge onto the result may be avoided. The regularization parameter is determined by the SVD approach in conjunction with the discrepancy principle.

Table 4.2: Regularization setups for PILS in the retrieval intercomparison.

Scenario Regularization matrix xa

TR-1 LC AFGL subarctic winter

TR-2 L0 zero

Table 4.3: The fitted molecules for the retrieval intercomparison of the two Level-2 processing codes PILS and AdL developed by DLR and SRON, respectively.

Retrieval HCl ClO O3

H37Cl 3 – 3

H35Cl 3 3 3

The SRON retrieval algorithm for the TELIS’s 480–650 GHz channel was explicitly described in de Lange et al. [2009, 2012]. Its inversion algorithm makes use of Tikhonov regularization and the regularization parameter is chosen by the L-curve approach. The regularization matrix is set to the identity matrix (L0 =In). The a priori profile is also a zero profile, but the retrieval starts with the MLS profile as initial guess.

In addition to atmospheric profiles of species, the state vector x includes two baseline pa-rameters accounting for radiometric and physical offsets for each spectrum in the limb sequence.

The concentration profiles in the state vector for the different retrieval tests are given in Ta-ble 4.3. O3 has to be jointly retrieved as ozone produces a sloped background for both HCl lines. Besides, a weak ClO line resides in the wing of the H35Cl line and needs to be included in the state vector. O3 and ClO are not treated as the retrieval product, but rather as an improve-ment of the HCl retrieval. The systematic pointing bias (−5.4 arcmin) and the atmospheric refraction effect make the actual tangent point lower than the commanded lowest one (16 km), and the vertical FoV extends the observing area of the TELIS instrument. Thus, the bottom of the retrieval grid is set to 13 km, although the retrieval over 13–16 km has limited physical meaning.

Figure 4.7 illustrates a comparison of the retrieved HCl profile using the two inversion algo-rithms. The results of PILS correspond to the two regularization scenarios in Table 4.2 (TR-1 and TR-2). Note that the natural abundance ratio of the isotopes (Cl35/Cl37= 0.7578/0.2422) have been taken into account and that the plotted VMR profiles denote the total HCl concen-tration amounts.

In the case of the H37Cl retrieval (Fig. 4.7a), the pronounced difference is found in the altitude range of 13 to 17.5 km where the information comes mainly from the a priori. For TR-2, the regularization aims to offer smoothness of the solution, but with no a priori effect at lower altitudes. All retrieved profiles resemble the HCl absence around 23 km, which is most likely because the flight took place inside the chlorine activated air of the northern polar region.

It can be seen that the profiles derived by TR-1 and AdL are close from 25 km up to the highest tangent altitude 32.5 km.

In Fig. 4.7b, the H35Cl retrieval results are similar to the H37Cl retrieval results. The HCl profiles derived by PILS and AdL agree over almost the entire altitude range of 13 to 32.5 km.

Likewise, very little amount of HCl is observed below 25 km.

0 1 2 3 4 HCl [ppbv]

15 20 25 30 35

Altitude [km]




0 1 2 3 4

HCl [ppbv]

15 20 25 30 35

Altitude [km]




Figure 4.7: Comparison of retrieved HCl profiles delivered by the two Level-2 processing codes PILS and AdL. The inversion is carried out for the TELIS’s submillimeter measurement 20044 that was observed on 24 January 2010. The retrieval results are derived for two HCl isotopes, i.e.(a)H37Cl and(b)H35Cl.

Red and green lines indicate the results by PILS for the different regularization scenarios in Table 4.2.

Above all, the HCl profiles obtained from H37Cl and H35Cl are internally consistent over the altitude range. A strong chlorine activation occurring inside the polar vortex was detected by observations of both isotopes by TELIS, which corresponds to the altitude range with nominal HCl abundances in the stratosphere. This is supported by an intercomparison between TELIS and other limb sounders (i.e. SMILES and MLS) during that day, as stated in Sect. 6.3. The retrieval calculations implemented by both retrieval codes reach a satisfactory agreement, and the major discrepancies are estimated to be produced by the a priori information and the forward simulations.

Chapter 5


In previous chapters, the PILS’s radiative transfer model and the inversion methodology have been presented. By making use of a set of retrieval simulations stressing the application to the TELIS instrument, this chapter will serve to evaluate the inversion performance and to charac-terize the retrieval product. To compute the iterates reliably and subsequently to characcharac-terize the error budget in the solution, we conduct a feasibility study of OH detected by the 1.8 THz channel. Not only single- and multi-target retrievals, but also different regularization algorithms and techniques including the choice of the regularization matrix are studied.

Perfect knowledge of the forward and instrument model parameters is not a realistic as-sumption when dealing with the actual observations. For this reason, a sensitivity study with respect to these types of potential error sources is performed.

Furthermore, the capability of multi-channel simultaneous processing of the far infrared and submillimeter measurements for HCl retrieval is investigated. The primary objective of this implementation is to accomplish an improved use of information from observations in an extended spectral range.