4 4.5 5 5.5 6 fIF [GHz]
0 5e-14 1e-13 1.5e-13 2e-13
Radiance [W / (m2 sr Hz)
fLO = 1829.6524 GHz
10 - 16 km
17.5 km 19 km
20.5 km
22 km 23.5 km 25 km 26.5 km 28 and 29.5 km
O3 OH
Figure 6.30: A sequence of limb spectra in the OH microwindow measured by the TELIS 1.8 THz channel during the 2009 flight. The limb sequence observing an OH triplet and covering tangent heights between 10 and 29.5 km in steps of 1.5 km, is illustrated as a function of the intermediate frequencyfIF. The spectral segment of 500 MHz selected for OH retrieval is indicated by a blue rectangular box. The dedicated measurement identifier is 10890.
4 4.5 5 5.5 6 fIF [GHz]
0 5e-14 1e-13 1.5e-13
Radiance [W / (m2 sr Hz)]
fLO = 1842.2862 GHz
O3 OH O
3 O
3 16 km
18 km
20 km
22 km
24 km 26 km 28 km 30 and 32 km
Figure 6.31: A sequence of limb spectra in the OH microwindow measured by the TELIS 1.8 THz channel during the 2010 flight. The limb sequence observing an OH triplet and covering tangent heights between 16 and 32 km in steps of 2 km, is illustrated as a function of the intermediate frequency fIF. The spectral segment of 500 MHz selected for OH retrieval is indicated by a blue rectangular box. The dedicated measurement identifier is 16295.
Table 6.9: Setup for the OH retrieval from the TELIS far infrared data. Note that each microwindow covers an OH transition triplet. Information on the used spectral range and the state vector parameters is listed.
Parameter Description
Microwindow 1 2
fIF range 5–5.5 GHz 4–4.5 GHz
Target species OH
Retrieved interfering species O3
Auxiliary parameter “greybody”, baseline offset
the OH triplet located around 1837.80 GHz were recorded. As seen in Fig. 6.31, the retrieval analyzed the frequency segment of 4–4.5 GHz where an ozone signature is also present in the wing of the OH triplet. This feature suggests that inaccurate knowledge of O3 can affect the OH retrieval.
Table 6.9 summarizes the setup for the OH retrieval from the TELIS far infrared limb spectra. In both microwindows, O3 is the most important contributor to the measurement signal and has to be jointly retrieved with OH. In this case, no regularization is imposed on the O3 profile as we are interested in the retrieval of OH only.
The 2009 campaign provided observation of OH in the night up to about 35 km. Six mea-surements with the observer altitude above 25 km were analyzed and are shown in Fig. 6.32. All measurements employed the 1834.75 GHz transition triplet, excluding measurements 13942 and
0 5 10 15 20 OH VMR [ppbv]
10 15 20 25 30 35
Altitude [km]
THz 9854 THz 10890 THz 13942 THz 16249 THz 16600 THz 4757 TELIS observation; 11 March 2009
Figure 6.32: OH profiles retrieved from the TELIS balloon flight data on 11 March 2009. The solid black, red, green, blue, magenta, and orange lines correspond to the OH profiles obtained from measure-ments 9854, 10890, 13942, 16249, 16600, and 4757, respectively. The dashed lines refer to the overall accuracy of all OH profiles.
4757 which employed the 1837.80 GHz triplet. Below the highest tangent point, most profiles capture the peak around 25 km and the abundances increase with time.
The comparison of measured and modelled spectra in frequency segment 3 of the first OH microwindow is shown in Fig. 6.33. The relative differences between both spectra do not change dramatically (±1 %) for the lower tangent heights of 13 and 17.5 km, and are of about ±2 % for the tangent height of 22 km. At the higher tangent height of 26.5 km, the modelled spec-trum is roughly ±8 % off the measured spectrum and the largest difference occurs near the intermediate frequency of about 5.5 GHz. The OH feature around the intermediate frequency of approximately 5.1 GHz is not clearly noticeable at lower altitudes, but is becoming stronger with increasing altitude.
The OH observation on 24 January 2010 is displayed in Fig. 6.34. OH measurements 10318 and 16295 were taken before local sunrise and 2 h before local noon, respectively, The major differences in both retrieved profiles are located around 20–25 km and are due to the fact that OH responds very quickly to solar radiation.
Figure 6.35 shows a comparison of measured and modelled spectra in frequency segment 1.
At the tangent heights of 18 and 22 km, the relative differences in both cases range between
−2 % and 2 %, and −4 % and 4 %, respectively. For the spectra at 26 and 30 km, the relative differences near the OH triplet increase up to 15 % and 30 %, respectively. The spectra around the OH triplet are overall well fitted, as compared to those around the O3 line. The differences around the O3 line center exceeds 50 % when the spectrum is observed at the highest tangent point.
As can be noticed in Fig. 6.36, the averaging kernels corresponding to both microwindows reveal that the retrieval sensitivity is better at higher altitudes where the abundances are
sev-5 5.1 5.2 5.3 5.4 5.5 fIF [GHz]
-4 -2 0 2 4
Rel. diff. [%]
1.9e-13
Radiance [W / (m2 sr Hz)]
measured fitted
f
LO = 1829.6524 GHz; tangent: 13 km
(a)
5 5.1 5.2 5.3 5.4 5.5
fIF [GHz]
-4 -2 0 2 4
Rel. diff. [%]
1.5e-13 1.6e-13 1.7e-13
Radiance [W / (m2 sr Hz)]
measured fitted
f
LO = 1829.6524 GHz; tangent: 17.5 km
(b)
5 5.1 5.2 5.3 5.4 5.5
fIF [GHz]
-4 -2 0 2 4
Rel. diff. [%]
7.5e-14 8e-14 8.5e-14 9e-14
Radiance [W / (m2 sr Hz)]
measured fitted
fLO = 1829.6524 GHz; tangent: 22 km
(c)
5 5.1 5.2 5.3 5.4 5.5
fIF [GHz]
-8 -4 0 4 8
Rel. diff. [%]
2e-14 2.5e-14 3e-14
Radiance [W / (m2 sr Hz)]
measured fitted
fLO = 1829.6524 GHz; tangent: 26.5 km
(d)
Figure 6.33: Comparison of measured and modelled TELIS OH spectra in frequency segment 3 of the first OH microwindow during the 2009 flight. The spectra are plotted for tangent heights of (a)13,(b) 17.5,(c)22, and(d)26.5 km. The dedicated measurement identifier is 10890.
eral orders of magnitude larger. An acceptable measurement response is obtained from 20 km upwards, but the fact remains that kernels close to the location of the instrument show strong oscillations and some peaks do not correspond to the tangent heights. A high sensitivity above the instrument and large spectral noise may be the main reasons for this behaviour. In agree-ment with the averaging kernels, the OH retrieval is acceptable only above 20 km, indicating a low information content in the measurement below this altitude level.
6.5.2 Error characterization
An error budget of the OH retrieval is estimated for these two different far infrared transitions.
The results are presented in Figs. 6.37 and 6.38. For both microwindows, the overall retrieval error is less than 4 ppbv below 30 km and increases rapidly with the increasing altitude.
In 2009, MIPAS-B only measured the temperature profiles twice and there was an approx-imately 5 hours gap between both observations. This fact can be problematic for the TELIS retrieval as the a priori temperature profile is taken from the MIPAS-B data.
The pointing information and the measurement noise turn out to be the most noticeable errors around 23 and 25 km, respectively. The spectroscopy accuracy does not cause an obvious effect on the OH retrieval below the observer altitude (approximately 33 km). This result is
0 10 20 30 OH VMR [ppbv]
15 20 25 30 35
Altitude [km]
THz 10318 THz 16295 TELIS observation; 24 January 2010
Figure 6.34: OH profiles retrieved from the TELIS balloon flight data on 24 January 2010. The solid red and green lines correspond to the OH profiles obtained from measurements 10318 and 16295, respectively. The dashed lines refer to the overall accuracy of both OH profiles.
consistent with the sensitivity analysis regarding the identical transition triplet in Sect. 5.2.1.
In the case of the second OH microwindow, all error components excepting the temperature error, are smaller than 1.5–2 ppbv below 30 km. The model parameter errors due to spectroscopy and calibration stretch from 25 km upwards where OH abundances start to increase. Above 30 km, however, all errors steeply increase.
The most obvious difference between the error budgets estimated for the two OH microwin-dows is reflected by the contribution of the spectroscopic parameter and calibration errors. For example, the spectroscopy error seems to have a significant contribution above 20 km in the case of the second microwindow. This is mainly due to the strength of the OH triplet and the interfering effect from the O3 line.
For the first time, OH retrievals from TELIS measurements are presented. Because of the large measurement noise, the precision in the OH retrievals is not highly satisfactory. Unfortu-nately, there is no other instrument measuring the same transitions of OH as TELIS, and only a few far infrared observations are available. The MLS instrument has not been measuring OH regularly, because the THz module on MLS has been in standby mode most of the time after 2009, and a limited number of measurements have been acquired since 2011. Another option is to compare the retrieved profiles with ground-based measurements and sophisticated chemical models. Further investigations into cross-validations of the TELIS OH profiles are ongoing.