Capability for ozone high-precision retrieval on JEM/SMILES observation
Introduction
Ozone is one of the key constituents of the Earth’s atmosphere. It exists throughout Earth’s stratosphere and plays a major role in shielding us from solar ultraviolet rays, radiation budget and atmospheric chemistry. Accurate global measurements of O3 vertical profiles are important for accelerating the recovery of O3, and understanding the climate change and coupled processes (WMO, 2007).
There have been global, long-term and high-quality O3 measurements by a solar occultation technique with the Stratospheric Aerosol and Gas Experiment (SAGE) series from 1979 to 2005 (Rusch et al., 1997), the Halogen Occultation Experiment (HALOE) (Russell et al., 1993) from 1991 to 2005 and the Polar Ozone and Aerosol Measurement (POAM) (Bevilacqua et al., 1993) from 1993 to 2005. Also the SCISAT/ACE (Atmospheric Chemistry Experiment) mission (Bernath et al., 2005), launched in 2003, uses the solar occultation technique. Although the solar occultation technique is a powerful technique to obtain high-quality O3 measurements, the geographical coverage provided by the technique is sparse compared to emission-based techniques.
The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) launched in 2002 (Fischer and Oelhaf, 1996, Fischer et al., 2008), the Earth Observing System (EOS) Microwave Limb Sounder (MLS)(Read et al., 2006, Froidevaux et al., 2008) instrument launched in 2004, and the Submillimeter Radiometer (SMR) onboard the Odin satellite (Murtagh et al., 2002) launched in February 2001 measure the atmospheric limb emission in infrared, millimeter-wave and submillimeter-wave ranges, respectively. Those emission measurements have the advantage of independence of the solar angle. Furthermore, there are various other types of global ozone measurements. The SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) (Bovensman et al., 1999) launched in 2002 onboard ENVISAT provides nadir, limb and solar/lunar occultation measurements. Global Ozone Monitoring by Occultation of Stars (GOMOS) also onboard ENVISAT performs the stellar occultation measurements, and the OSIRIS instrument onboard the Odin satellite measures the scattered solar radiation in the visible spectral range.
The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) instrument has been put in operation as a part of the Japanese Experiment Module (JEM) on board the International Space Station (ISS) since September 2009 as a collaborative project between the Japan Aerospace Exploration Agency (JAXA) and the National Institute of Information and Communications Technology (NICT). It measures the submillimeter-wave limb emission from atmospheric minor species mainly in the stratosphere. A major feature of JEM/SMILES is the ability to receive very sensitive measurements with a low system-noise temperature (approximately 400 K) using 4-K cooled superconductor–insulator–superconductor (SIS) mixers. Table 1 shows the main specifications of SMILES. Further details are given in the SMILES mission plan (NASDA/CRL, 2002) and recent status reports are presented by Kikuchi et al., 2010, Ochiai et al., 2010.
In this paper, we refer to theoretical evaluations for the random error and the systematic error of O3 vertical profiles based on the launch-ready retrieval algorithm developed for the operational data processing system of SMILES (Takahashi et al., 2009). Since the O3 emission line at 625.37 GHz is the most intense emission line within the SMILES measurement bands and it affects the retrieval of the other target species measured together, it is important to evaluate the O3 retrieval so that other species can be retrieved accurately.
The altitude region where SMILES can detect O3 has been estimated between 20 km and 60 km (NASDA/CRL, 2002) but the altitude regions below 20 km and above 60 km have not been adequately assessed to determine the quality of O3 retrievals possible in these regions. Therefore, we will determine the measurable altitude region more strictly. It is difficult to retrieve the O3 profiles in the low- and high-altitude regions. In the low-altitude region, the uncertainty of the continuum emission by water vapour and scattering by clouds reduces the accuracy of the retrieved O3 profiles and in the high-altitude region the random error becomes worse with decreasing abundance of O3. It is important to determine the O3 distribution in these low- and high-altitude regions to improve understanding of the ozone chemistry.
The launch-ready retrieval algorithm is based on the optimal estimation method (OEM), which is discussed in Section 3. Section 2 provides an overview of SMILES, including the SMILES measurement technique and Section 4 provides O3 retrieval results. Section 5 refers to the retrieval errors caused by model parameter uncertainties such as a priori profile and uncertainty of the atmospheric temperature. In Section 6, we identify the capability for measuring O3 diurnal variation in the upper stratosphere. Finally, we present a further improvement of the retrieval algorithm in the lower stratosphere and its random error in Section 7.
Section snippets
O3 Measurements with the JEM/SMILES
SMILES measures the submillimeter-wave limb emission from the atmospheric minor species, such as O3, ClO, HCl, HNO3, HOCl, CH3CN, HO2, BrO and O3 isotopes. The nominal altitude coverage is 10–60 km while the altitude coverage of tangent altitude by the antenna scanning is typically between −10 km and 100 km. The most unique characteristics of the SMILES measurements is their high-sensitivity to atmospheric limb emission in the submillimeter-wave range thanks to the 4-K cooled SIS mixers. Overall
OEM
The retrieval algorithm is based on the OEM applied for atmospheric sounding (Rodgers, 1976, Rodgers, 1990, Rodgers, 2000). The maximum a posteriori estimate can be derived from statistical combination of a priori knowledge of a state vector x and the information about the measurement. We use a modification of the Gauss–Newton method called the Levenberg–Marquardt method (Levenberg, 1944, Marquardt, 1963). The retrieved state vector xi+1 at the iterative step i + 1 is calculated as
Setup conditions
The species included in the simulation are O3, HCl, ClO, HOCl, HNO3, CH3CN, HO2, BrO and H2O. Spectroscopic line parameters for those species are taken from the JPL catalog (Pickett et al., 1992) and H2O background continua is calculated by Liebe’s model (Liebe, 1989). The retrieval parameters are the vertical profiles of the mixing ratio for these species, the atmospheric temperature profile and a pointing offset. The profiles used in this simulation are from the US standard atmosphere.
Error analysis
Other possible error sources for O3 retrieval using SMILES are:
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Pointing error,
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Bias error from the a priori profile,
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Uncertainty of the atmospheric temperature,
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Forward model parameter errors.
An error of pointing is one of the most significant issues for atmospheric remote sensing measurements using limb-viewing instruments. We retrieve the offset of the pointing to reduce the effect from the pointing error and the details of pointing retrieval for SMILES are described in Takahashi et al. (2009).
Diurnal variation of O3 abundance in the upper stratosphere
Ozone has a strong diurnal variation in the upper stratosphere and the mesosphere (Allen et al., 1984). Since the ISS is not in a sun-synchronous orbit and its orbital plane rotates every 60 days, SMILES can produce one day datasets that can describe the diurnal variation using 60 days SMILES observation data. In this section we estimate the capability of SMILES observations for the diurnal variation of ozone by investigations of the ozone random error at two extreme times, which are the midnight
Retrieval capability in the lower stratosphere and upper troposphere
In our early estimation (NASDA/CRL, 2002), it was stated that SMILES can measure O3 profiles above 20 km (i.e. SMILES cannot measure the lower stratosphere). The results of the early estimation were based on the retrieval using a single band independently. In this section we study the retrieval capability of ozone below 20 km using two bands combinations because SMILES mounts two Acousto-Optical Spectrometer and measures the two bands simultaneously. Also we estimate effects of cirrus clouds.
Conclusion
We have estimated O3 random error with its altitude range by the SMILES observations. In the early estimations in NASDA/CRL (2002), it was estimated that the random error of O3 vertical profiles was better than 5% from 20 to 45 km and 10% from 18 to 53 km but these errors includes the random error and some systematic errors together. In this study, we reveal and classify the random error and other systematic errors. Furthermore we estimated the retrieval capability of the diurnal variation and
Acknowledgments
We thank to J. Inatani, T. Nishibori, H. Ozeki, T. Manabe and K. Kikuchi for their useful discussions and suggestions about the instrument characteristics and Y. Kasai, J. Urban, and P. Baron for useful comments about retrieval algorithms. We are also deeply grateful to T. Nagahama and A. Mizuno. Finally, we really appreciate the reviewers giving us the fruitful suggestions and comments.
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