SCIAMACHY Limb Research Articles

Comparison of the inversion algorithms applied to the ozone vertical profile retrieval from SCIAMACHY limb measurements

Abstract. This paper is devoted to an intercomparison of ozone vertical profiles retrieved from the measurements of scattered solar radiation performed by the SCIAMACHY instrument in the limb viewing geometry. Three different inversion algorithms including the prototype of the operational Level 1 to 2 processor to be operated by the European Space Agency are considered. Unlike usual validation studies, this comparison removes the uncertainties arising when comparing measurements made by different instruments probing slightly different air masses and focuses on the uncertainties specific to the modeling-retrieval problem only. The intercomparison was performed for 5 selected orbits of SCIAMACHY showing a good overall agreement of the results in the middle stratosphere, whereas considerable discrepancies were identified in the lower stratosphere and upper troposphere altitude region. Additionally, comparisons with ground-based lidar measurements are shown for selected profiles demonstrating an overall correctness of the retrievals.

Chemical heating rates derived from SCIAMACHY vibrationally excited OH limb emission spectra

The SCIAMACHY instrument on board Envisat is able to measure nearly all vibrational transitions of mesospheric hydroxyl – from the ultraviolet to the near infrared spectral region. In this paper, we analyze SCIAMACHY limb emission data in the 1000–1750 nm spectral region by means of a new vibrational non-LTE model of OH. Several hydroxyl hotbands are identified. Vibrational non-LTE model calculations applying different collisional relaxation models are able to reproduce the measured spectra. Best agreement between model calculations and measured spectra is obtained, if a combination of multiquantum and step ladder single-quantum relaxation model is applied. Emissions from the OH( v = 9) vibrational state are used to derive chemical heating rates from the SCIAMACHY spectra. Instantaneous heating rates are in the order of 10 K/day.

Latitudinal and vertical distribution of bromine monoxide in the lower stratosphere from Scanning Imaging Absorption Spectrometer for Atmospheric Chartography limb scattering measurements

Vertical profiles of stratospheric bromine monoxide (BrO) in the 15–30 km range are retrieved from SCIAMACHY limb scatter data over the globe. First validation comparisons with the balloon‐borne SAOZ‐BrO and LPMA/DOAS instruments indicate retrieval biases of ∼20% or less. Propagated spectral fitting uncertainties lead to a precision approaching ∼25% on a 2 km grid at 25 km. This worsens at higher altitudes because of reduced signal and at lower altitudes because of the reduced penetrability of the atmosphere. In terms of volume mixing ratio (VMR), the single profile precision increases from ∼4 pptv at 17 km to ∼8 pptv at 27 km. Repeatability, an alternative indicator of precision, is 2–3 pptv for SCIAMACHY retrievals and independent of altitude. The BrO stratospheric number density peak generally lies 5 ± 2 km above the tropopause. In the tropics, the stratospheric BrO VMR generally increases with increasing altitude. The observed stratospheric BrO global distribution is generally consistent with previous balloon measurements but does not agree well with results of a model that uses Bry inferred only from the observed breakdown of long‐lived bromoalkanes (i.e., methyl bromide and halons). We find best agreement with the observed vertical and latitudinal distribution of BrO for model results that include an 8.4 ± 2 pptv contribution to stratospheric Bry, most of which is expected from the breakdown of VSL (very short lived) bromocarbons, in addition to the ∼16 pptv contribution from longer‐lived sources. This suggests that stratospheric Bry exceeds 20 pptv. Profiles of Bry profiles derived from the balloon measurements of BrO also suggest Bry is in excess of 20 pptv, but the uncertainty and variability of these results do not allow us to definitively rule out this concentration. We find typical BrO VMRs of ∼4 pptv at 15 km in the tropical tropopause layer, suggesting that a significant portion of the bromine from VSL bromoalkane sources may be carried across the tropopause in the form of inorganic decomposition products. We discuss a variety of VSL bromocarbons species that may be contributing to the elevated concentrations of stratospheric BrO.

An efficient inversion algorithm for atmospheric remote sensing with application to UV limb observations

In this paper we present a retrieval algorithm for atmospheric remote sensing. The algorithm combines Tikhonov regularization and the iteratively regularized Gauss–Newton method and is devoted to the solution of multi-parameter inverse problems with simple bounds on the variables. The basic features of the algorithm: the solution of the bound-constrained minimization problem, the selection of the optimal regularization parameter, the derivation of the global regularization matrix and the characterization of the solution (error analysis) are discussed in detailed. The inversion algorithm is applied to ozone retrieval from SCIAMACHY limb scatter measurements in the ultraviolet spectral range.

Error analysis and minimum bound method for atmospheric remote sensing

In this study, we present an error analysis for Tikhonov regularization in a semi-stochastic setting. The analysis is carried out in such a way that it can be applied to any kind of inverse problem in atmospheric remote sensing. A method for selecting the optimal regularization parameter relying on the minimization of an estimator of the bound of the error between the first iterate and the exact solution is also discussed. Numerical simulations are performed for NO 2 retrieval from SCIAMACHY limb scatter measurements.

Information operator approach and iterative regularization methods for atmospheric remote sensing

In this study, we present the main features of the information operator approach for solving linear inverse problems arising in atmospheric remote sensing. This method is superior to the stochastic version of the Tikhonov regularization (or the optimal estimation method) due to its capability to filter out the noise-dominated components of the solution generated by an inappropriate choice of the regularization parameter. We extend this approach to iterative methods for nonlinear ill-posed problems and derive the truncated versions of the Gauss–Newton and Levenberg–Marquardt methods. Although the paper mostly focuses on discussing the mathematical details of the inverse method, retrieval results have been provided, which exemplify the performances of the methods. These results correspond to the NO 2 retrieval from SCIAMACHY limb scatter measurements and have been obtained by using the retrieval processors developed at the German Aerospace Center Oberpfaffenhofen and Institute of Environmental Physics of the University of Bremen.

Geophysical validation of SCIAMACHY Limb Ozone Profiles

Abstract. We discuss the quality of the two available SCIAMACHY limb ozone profile products. They were retrieved with the University of Bremen IFE's algorithm version 1.61 (hereafter IFE), and the official ESA offline algorithm (hereafter OL) versions 2.4 and 2.5. The ozone profiles were compared to a suite of correlative measurements from ground-based lidar and microwave, sondes, SAGE II and SAGE III (Stratospheric Aerosol and Gas Experiment). To correct for the expected Envisat pointing errors, which have not been corrected implicitly in either of the algorithms, we applied a constant altitude shift of -1.5 km to the SCIAMACHY ozone profiles. The IFE ozone profile data between 16 and 40 km are biased low by 3-6%. The average difference profiles have a typical standard deviation of 10% between 20 and 35 km. We show that more than 20% of the SCIAMACHY official ESA offline (OL) ozone profiles version 2.4 and 2.5 have unrealistic ozone values, most of these are north of 15° S. The remaining OL profiles compare well to correlative instruments above 24 km. Between 20 and 24 km, they underestimate ozone by 15±5%.

First results of ozone profiles between 35 and 65 km retrieved from SCIAMACHY limb spectra and observations of ozone depletion during the solar proton events in October/November 2003

Ozone density profiles between 35 and 65 km altitude are derived from scattered sunlight limb radiance spectra measured by the SCIAMACHY instrument on the Envisat satellite. The method is based on the inversion of normalized limb radiance profiles in the Hartley absorption bands of ozone at selected wavelengths between 250 and 310 nm. It employs a non-linear Newtonian iteration version of Optimal Estimation (OE) coupled with the radiative transfer model SCIARAYS. The limb scatter technique combined with a classical OE retrieval in the short-wave UV-B and long-wave UV-C delivers reliable results as shown by a first comparison with MIPAS V4.61 profiles yielding agreement within 10% between 38 and 55 km. An overview of the methodology and an initial error analysis are presented. Furthermore the effect of the solar proton storm between 28 October and 6 November 2003 on the ozone concentration profiles is shown. They indicate large depletion of ozone of about 60% at 50 km in the Northern hemisphere, a weaker depletion in the Southern hemisphere and a dependence of the depletion on the Earth’s magnetic field.

Validation of IFE-1.6 SCIAMACHY limb ozone profiles

Abstract. The IFE-1.6 scientific data set of SCIAMACHY limb ozone profiles is validated for the period August–December 2002. The data set provides ozone profiles over an altitude range of 15–45 km. The main uncertainty in the profiles is the imprecise knowledge of the pointing of the instrument, leading to retrieved profiles that are shifted in altitude direction. To obtain a first order correction for the pointing error and the remaining uncertainties, the retrieved profiles are compared to their a-priori value and ozone sondes based on absolute distance and equivalent latitude criteria. A vertical shift of the satellite profiles with 2 km downward is found to be an appropriate correction for the data set studied. A total root-mean-square difference between limb profiles and sondes of 10–15% remains for the stratospheric ozone profile after application of the correction. Small biases are left above and below the ozone maximum at mid latitudes, where the vertical gradients in the retrieved product are in general too strong.

Ozone depletion during the solar proton events of October/November 2003 as seen by SCIAMACHY

We use atmospheric ozone density profiles between 35 and 65 km altitude derived from SCIAMACHY limb measurements to quantify the ozone changes caused by the solar proton events from 26 October to 6 November 2003, known as the “Halloween storm.” Detailed maps and daily resolved time series up to 5 weeks after the first event are compared with the results from a chemistry, transport, and photolysis model of the middle atmosphere that includes NOx and HOx production due to energetic particle precipitation. The general features of the ozone loss are captured by the model fairly well. A strong ozone depletion of more than 50% even deep into the stratosphere is observed at high geomagnetic latitudes in the Northern Hemisphere, whereas the observed ozone depletion in the more sunlit Southern Hemisphere is much weaker. Reasons for these interhemispheric differences are given. Two regimes can be distinguished, one above about 50 km dominated by HOx (H, OH, HO2) driven ozone loss, one below about 50 km, dominated by NOx (NO, NO2) driven ozone loss. The regimes display a different temporal evolution of ozone depletion and recovery. We observe for the first time an establishment of two contemporaneous maxima of ozone depletion at different altitudes, which solely can be explained by these regimes.

NO 2 and BrO vertical profile retrieval from SCIAMACHY limb measurements: Sensitivity studies

A retrieval algorithm used to obtain vertical distributions of NO 2 and BrO from SCIAMACHY limb measurements is described. Furthermore, the information content of SCIAMACHY limb measurements with respect to NO 2 and BrO profile retrievals is investigated. Another investigated aspect is a sensitivity of resulting NO 2 and BrO vertical profiles to a priori trace gas vertical distributions as well as to the pressure and temperature profiles used in the forward modeling. The sensitivity region is estimated to be 12–40 km for NO 2 and 12–35 km for BrO. Larger retrieval errors are encountered below 15 km associated to the influence of the neighboring altitude levels and of a priori information. The dependence of the retrieved trace gas vertical distributions on the pressure and temperature profiles used in the forward model is found to be insignificant above 20 km, whereas it cannot be completely ignored when retrieving trace gas amounts below.

Satellite-pointing retrieval from atmospheric limb-scattering of solar UV-B radiation

We present a new algorithm for tangent height retrievals from limb-scattering observations. These observations are performed by satellite-based spectrometers operating in the UV, visible, and short-wave IR spectral ranges. They record the solar radiation scattered in Earth's atmosphere in limb-viewing geometry and aim at vertically resolved retrievals of the atmospheric composition with global coverage. Inaccuracies in the knowledge of the instrument's pointing frequently dominate the error budgets of the atmospheric composition products. Therefore, additional information on satellite pointing is crucial for the quality of all products derived from limb-scattering observations. The information is commonly expressed in terms of the tangent heights. The presented algorithm determines the tangent heights directly from the observed limb radiances by analyzing the shapes of the so-called knee in several UV-B radiance profiles. All vertical pointing information contained in a UV-B fit window is exploited by simultaneously retrieving the ozone profile. The algorithm has been implemented in the toolbox SCIARAYS and named Tangent height Retrieval by UV-B Exploitation (TRUE) knee method. We have applied it to five orbits of SCIAMACHY's limb observations. It achieves a precision of about 200 m when applied to an individual limb scan. The broadband structure of the observations can be reproduced within 1% RMS. A comparison of the retrieved tangent heights with the engineering ones delivered by ESA reveals that the engineering tangent heights exhibit a systematic error, which varies with an amplitude of about 3 km. Its origin is traced to the on-board orbit model of Envisat. PACS Nos.: 94.80.+g, 42.68.Ay, 92.60.Ta, 42.68.Wt, 07.87.+v, 92.70.Cp, and 07.60.Rd

First near‐global retrievals of OH rotational temperatures from satellite‐based Meinel band emission measurements

For the first time near‐global retrievals of mesopause OH rotational temperatures from satellite‐borne Meinel band emission measurements are presented. The measurements of the OH (3‐1) Meinel band near 1.5 micron were performed with the SCIAMACHY instrument on the European Space Agency's environmental satellite Envisat. The derived OH (3‐1) rotational temperatures are shown to be in reasonable agreement with the CIRA (1986) atmosphere temperatures for the seasons and latitudes considered. The derived temperatures are in good agreement with ground‐based measurements of the OH rotational temperature performed with a CEDAR Mesospheric Temperature Mapper (MTM) at Maui, Hawaii (21°N/204°E), with the GRound based Infrared P‐branch Spectrometer I (GRIPS‐I) at Hohenpeißenberg (47°N/11°E) and with GRIPS‐II at Wuppertal (51°N/7°E). The SCIAMACHY limb nighttime observations provide a unique data set of near‐global OH rotational temperature to study seasonal and geographical variations, dynamical processes and possibly long‐term temperature trends, if an extended data set becomes available in the future.

Theoretical precisions for sciamachy limb retrieval

The UV-vis-NIR spectrometer SCIAMACHY on ESA's Envisat will measure the back-scattered solar radiation from the Earth's atmosphere in limb geometry. From these measurements atmospheric parameter profiles will be retrieved (T, p, trace gas abundances, and aerosol and cloud parameters). Theoretical precisions for retrievals from SCIAMACHY's limb measurements are calculated with a fully spherical radiative transfer model and a realistic instrument model. The retrievals can be performed for the atmospheric layers above the top of optically thick clouds. The vertical resolution of the retrieval parameter profiles is 3 km. O 3 profiles can be measured with a theoretical precision of 1% throughout the stratosphere and in the upper troposphere. The retrievals of other trace gases (NO 2, BrO, CH 4, …) will have precisions between 2 and 100 % depending on retrieval altitude.