A. Rozanov

A numerical radiative transfer model for a spherical planetary atmosphere: combined differential–integral approach involving the Picard iterative approximation

Abstract A new radiative transfer model suitable to calculate the radiation field in a spherical planetary atmosphere has been developed. The suggested approach involves the Picard iterative approximation to solve the radiative transfer equation in its integral form. The radiation field calculated by solving the integro-differential radiative transfer equation in a pseudo-spherical atmosphere is used as an initial guess for the iterative scheme. The approach has the same advantages as the Monte-Carlo method, but is much more computationally efficient. The comparisons between the spherical model presented in this paper and a Monte-Carlo radiative transfer model for radiances at the top of the atmosphere show differences less than 1% for most situations. The accuracy of the recently developed CDI approach, which was intended to perform fast and accurate radiance computations for non-limb viewing geometry, has been estimated for limb viewing geometry.

Ozone and temperature trends in the upper stratosphere at five stations of the Network for the Detection of Atmospheric Composition Change

Upper stratospheric ozone anomalies from the satellite-borne Solar Backscatter Ultra-Violet (SBUV), Stratospheric Aerosol and Gas Experiment II (SAGE II), Halogen Occultation Experiment (HALOE), Global Ozone Monitoring by Occultation of Stars (GOMOS), and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instruments agree within 5% or better with ground-based data from lidars and microwave radiometers at five stations of the Network for the Detection of Atmospheric Composition Change (NDACC), from 45°S to 48°N. From 1979 until the late 1990s, all available data show a clear decline of ozone near 40 km, by 10%–15%. This decline has not continued in the last 10 years. At some sites, ozone at 40 km appears to have increased since 2000, consistent with the beginning decline of stratospheric chlorine. The phaseout of chlorofluorocarbons after the International Montreal Protocol in 1987 has been successful, and is now showing positive effects on ozone in the upper stratosphere. Temperature anomalies near 40 km altitude from European Centre for Medium Range Weather Forecast reanalyses (ERA-40), from National Centers for Environmental Prediction (NCEP) operational analyses, and from HALOE and lidar measurements show good consistency at the five stations, within about 3 K. Since about 1985, upper stratospheric temperatures have been fluctuating around a constant level at all five NDACC stations. This non-decline of upper stratospheric temperatures is a significant change from the more or less linear cooling of the upper stratosphere up until the mid-1990s, reported in previous trend assessments. It is also at odds with the almost linear 1 K per decade cooling simulated over the entire 1979–2010 period by chemistry–climate models (CCMs). The same CCM simulations, however, track the historical ozone anomalies quite well, including the change of ozone tendency in the late 1990s.

Comparison of radiative transfer models for limb‐viewing scattered sunlight measurements

[1] This study compares the limb scattered radiances calculated by six radiative transfer models for a variety of viewing conditions. Atmospheres that include molecular scattering, aerosol scattering, and ozone absorption are considered. All models treat single scattering accurately in full spherical geometry. Two ‘‘approximate spherical’’ models (CDI and LIMBTRAN) rely on the plane-parallel atmosphere approximation to calculate the diffuse radiance field; the remaining four ‘‘spherical’’ models (Siro, MCC++, GSLS, and CDIPI) treat multiple scattering in a spherical atmosphere. Only three of the models (Siro, MCC++, and GSLS) have vector treatment with polarization. A brief comparison of vector radiances with the limb scattered radiances measured by the SOLSE and LORE instruments demonstrates agreement usually within 15% and always within 30%. The inclusion of polarization appears to have little effect on the level of agreement among the models (which agree to within 2% for this sample case). A more general comparison among calculated scalar radiances follows, including four solar zenith angles (20� ,6 0� , 80� , and 90� ), three relative azimuth angles (20� ,9 0� , and 160� ), and two surface albedos (0 and 0.95). The single scattered radiances agree to within 1% for almost every case. Comparisons of the total radiance show larger differences, with 2–4% spread among the results of the spherical models. The approximate spherical models show a positive radiance difference relative to the other models that increases with tangent height, reaching as much as 8% at 60 km. The rule used to divide the model atmosphere into discrete layers is shown to affect the calculated radiance, causing a height-dependent difference of up to 1% for 1 km layer thickness. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0669 Electromagnetics: Scattering and diffraction; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: radiative transfer, limb scattering, model comparison

Combined differential‐integral approach for the radiation field computation in a spherical shell atmosphere: Nonlimb geometry

A new combined differential-integral approach (CDI) has been developed, which is more accurate than commonly used pseudospherical models but not so complicated and computer time-consuming as a fully spherical model. The radiative transfer equation is solved in its integral form. The multiple-scattering source function is obtained by solving the integrodifferential radiative transfer equation in a pseudospherical atmosphere. Relative differences between the new model and the pseudospherical model for a variety of solar zenith angles, viewing angles, and azimuth angles for a set of wavelengths are presented. Furthermore, the GDI model is compared with a Gauss-Seidel spherical model (GSS) and with MODTRAN. The difference between GDI and GSS is found to be less than 2% and between GDI and MODTRAN up to 12%.

SPARC Data Initiative: A comparison of ozone climatologies from international satellite limb sounders

A comprehensive quality assessment of the ozone products from 18 limb-viewing satellite instruments is provided by means of a detailed intercomparison. The ozone climatologies in form of monthly zonal mean time series covering the upper troposphere to lower mesosphere are obtained from LIMS, SAGE I/II/III, UARS-MLS, HALOE, POAM II/III, SMR, OSIRIS, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, and SMILES within 1978–2010. The intercomparisons focus on mean biases of annual zonal mean fields, interannual variability, and seasonal cycles. Additionally, the physical consistency of the data is tested through diagnostics of the quasi-biennial oscillation and Antarctic ozone hole. The comprehensive evaluations reveal that the uncertainty in our knowledge of the atmospheric ozone mean state is smallest in the tropical and midlatitude middle stratosphere with a 1σ multi-instrument spread of less than ±5%. While the overall agreement among the climatological data sets is very good for large parts of the stratosphere, individual discrepancies have been identified, including unrealistic month-to-month fluctuations, large biases in particular atmospheric regions, or inconsistencies in the seasonal cycle. Notable differences between the data sets exist in the tropical lower stratosphere (with a spread of ±30%) and at high latitudes (±15%). In particular, large relative differences are identified in the Antarctic during the time of the ozone hole, with a spread between the monthly zonal mean fields of ±50%. The evaluations provide guidance on what data sets are the most reliable for applications such as studies of ozone variability, model-measurement comparisons, detection of long-term trends, and data-merging activities.

Odin/OSIRIS observations of stratospheric BrO: Retrieval methodology, climatology, and inferred Bry

A 7+ year (2001–2008) data set of stratospheric BrO profiles measured by the Optical Spectrograph and Infra-Red Imager System (OSIRIS) instrument, a UV-visible spectrometer measuring limb-scattered sunlight from the Odin satellite, is presented. Zonal mean radiance spectra are computed for each day and inverted to yield effective daily zonal mean BrO profiles from 16 to 36 km. A detailed description of the retrieval methodology and error analysis is presented. Single-profile precision and effective resolution are found to be about 30% and 3–5 km, respectively, throughout much of the retrieval range. Individual profile and monthly mean comparisons with ground-based, balloon, and satellite instruments are found to agree to about 30%. A BrO climatology is presented, and its morphology and correlation with NO2 is consistent with our current understanding of bromine chemistry. Monthly mean Bry maps are derived. Two methods of calculating total Bry in the stratosphere are used and suggest (21.0 ± 5.0) pptv with a contribution from very short lived substances of (5.0 ± 5.0) pptv, consistent with other recent estimates.

SCIAMACHY on ENVISAT: in-flight optical performance and first results

The Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) is a contribution to the ENVISAT-1 satellite, which has been launched in March 2002. The SCIAMACHY instrument measures sunlight transmitted, reflected and scattered by the Earths atmosphere or surface simultaneously from the UV to the SWIR spectral region (214 - 2380 nm) in nadir, limb, and occultation viewing geometry. SCIAMACHY allows the characterisation of the composition of the Earth atmosphere from the ground to the mesosphere. This paper gives an overview of the SCIAMACHY instrument and its in-flight detector, spectral and radiometric performance. Furthermore first results on trace gas retrieval from limb and nadir measurement mode will be summarised.

Consistent interpretation of ground based and GOME BrO slant column data

Model computations of slant column densities (SCD) enable the comparison between ground based and satellite based absorption measurements of scattered light and are therefore a good basis to investigate the presence of tropospheric BrO amounts. In this study ground based zenith sky and GOME nadir measurements of BrO SCD are compared with simulations for the 19-21 March 1997 at Ny-Alesund. The vertical columns of tropospheric BrO amounts are estimated to be in the range 4 f 0.8 * 1Or3 [molecules/cm21 for the investigated period and location.

The SPARC Data Initiative: Comparison of upper troposphere/lower stratosphere ozone climatologies from limb‐viewing instruments and the nadir‐viewing Tropospheric Emission Spectrometer

We present the first comprehensive intercomparison of currently available satellite ozone climatologies in the upper troposphere/lower stratosphere (UTLS) (300-70hPa) as part of the Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative. The Tropospheric Emission Spectrometer (TES) instrument is the only nadir-viewing instrument in this initiative, as well as the only instrument with a focus on tropospheric composition. We apply the TES observational operator to ozone climatologies from the more highly vertically resolved limb-viewing instruments. This minimizes the impact of differences in vertical resolution among the instruments and allows identification of systematic differences in the large-scale structure and variability of UTLS ozone. We find that the climatologies from most of the limb-viewing instruments show positive differences (ranging from 5 to 75%) with respect to TES in the tropical UTLS, and comparison to a zonal mean ozonesonde climatology indicates that these differences likely represent a positive bias for p100hPa. In the extratropics, there is good agreement among the climatologies regarding the timing and magnitude of the ozone seasonal cycle (differences in the peak-to-peak amplitude of <15%) when the TES observational operator is applied, as well as very consistent midlatitude interannual variability. The discrepancies in ozone temporal variability are larger in the tropics, with differences between the data sets of up to 55% in the seasonal cycle amplitude. However, the differences among the climatologies are everywhere much smaller than the range produced by current chemistry-climate models, indicating that the multiple-instrument ensemble is useful for quantitatively evaluating these models.

Derivatives of the radiation field and their application to the solution of inverse problems

Spectral distribution of the solar radiation traveling through the Earth’s atmosphere contain an important information about numerous atmospheric and surface parameters. This information can be gained from the measured spectra employing so-called inverse theory and the problem to be solved thereby is usually referred to as the inverse problem. The first step to be done in the solution of any inverse problem is to formulate a model usually referred to as the forward model which will allow us to simulate the measured quantity assuming all relevant atmospheric and surface parameters to be known. Generally, in the case of the scattered, reflected, or transmitted solar radiance measured in the ultraviolet, visible, or near-infrared spectral range by means of satellite, airborne, or ground-based instruments, the corresponding forward model is nonlinear, i.e., there is no linear relationship between measured values of intensity and atmospheric parameters. However, the theoretical basis of the inverse problem solution is well investigated in the case of linear inverse problems only [19]. Thus, to make use of the existing numerical methods the forward model has to be linearized, i.e., a linear relationship between intensity of radiation and the atmospheric parameters has to be obtained. This can formally be done considering the intensity as a function or functional of the corresponding parameters and expanding it in the Taylor series with respect to the variations of the desired parameters.