Erik Griffioen

International Photolysis Frequency Measurement and Model Intercomparison (IPMMI): Spectral actinic solar flux measurements and modeling

[1] The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place in Boulder, Colorado, from 15 to 19 June 1998, aiming to investigate the level of accuracy of photolysis frequency and spectral downwelling actinic flux measurements and to explore the ability of radiative transfer models to reproduce the measurements. During this period, 2 days were selected to compare model calculations with measurements, one cloud-free and one cloudy. A series of ancillary measurements were also performed and provided parameters required as input to the models. Both measurements and modeling were blind, in the sense that no exchanges of data or calculations were allowed among the participants, and the results were objectively analyzed and compared by two independent referees. The objective of this paper is, first, to present the results of comparisons made between measured and modeled downwelling actinic flux and irradiance spectra and, second, to investigate the reasons for which some of the models or measurements deviate from the others. For clear skies the relative agreement between the 16 models depends strongly on solar zenith angle (SZA) and wavelength as well as on the input parameters used, like the extraterrestrial (ET) solar flux and the absorption cross sections. The majority of the models (11) agreed to within about +/-6% for solar zenith angles smaller than similar to60degrees. The agreement among the measured spectra depends on the optical characteristics of the instruments (e.g., slit function, stray light rejection, and sensitivity). After transforming the measurements to a common spectral resolution, two of the three participating spectroradiometers agree to within similar to10% for wavelengths longer than 310 nm and at all solar zenith angles, while their differences increase when moving to shorter wavelengths. Most models agree well with the measurements (both downwelling actinic flux and global irradiance), especially at local noon, where the agreement is within a few percent. A few models exhibit significant deviations with respect either to wavelength or to solar zenith angle. Models that use the Atmospheric Laboratory for Applications and Science 3 (ATLAS-3) solar flux agree better with the measured spectra, suggesting that ATLAS-3 is probably more appropriate for radiative transfer modeling in the ultraviolet.

Retrieval of stratospheric O3 and NO2 profiles from Odin Optical Spectrograph and Infrared Imager System (OSIRIS) limb-scattered sunlight measurements

Scientific studies of the major environmental questions of global warming and ozone depletion require global data sets of atmospheric constituents with relevant temporal and spatial resolution. In this paper global number density profiles of O3 and NO2 are retrieved from Odin/OSIRIS limb-scattered sunlight measurements, using the Maximum A Posteriori estimator. Differential Optical Absorption Spectroscopy is applied to OSIRIS radiances as an intermediate step, using the wavelength windows 571-617 nm for O3 and 435-451 nm for NO2. The method is computationally efficient for processing OSIRIS data on an operational basis. Results show that a 2-3 km height resolution is generally achievable between about 12 km and 45 km for O3 with an estimated accuracy of 13\% at the peak and between about 15 km and 40 km for NO2 with an estimated accuracy of 10\% at the peak. First validations of the retrieved data indicate a good agreement both with other retrieval techniques applied to OSIRIS measurements and with the results of other instruments. Once the validation has reached a confident level, the retrieved data will be used to study important stratospheric processes relevant to global environmental problems. The unique NO2 data set will be of particular interest for studies of nitrogen chemistry in the middle atmosphere.

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

LIMBTRAN: A pseudo three-dimensional radiative transfer model for the limb-viewing imager OSIRIS on the ODIN satellite

A new pseudo three-dimensional radiative transfer model has been written to calculate scattered light radiances required by inversion algorithms to retrieve minor species profile and column amount information from limb measurements. Stratospheric and mesospheric UV-visible limb radiances will be measured by the ODIN/OSIRIS (Optical Spectrograph and Infrared Imager System) satellite instrument to be launched in November 2000. This information will be used to study ozone depletion processes and transport dynamics in the Earths atmosphere through the study of trace species. Model limb radiance simulations have been carried out for a suite of solar zenith and azimuthal angles and lambertian surface albedos over a broad spectral range. The effects of Rayleigh and aerosol scattering as well as ozone absorption have been modeled. Comparisons with a spherical 3-D Monte-Carlo model show that there is generally less than 3–4% difference for single-scattering and less than 10–15% and often significantly better for multiple scattering radiances at a fraction of the computational cost.

OBSERVATIONS OF STRATOSPHERIC AEROSOL USING CPFM POLARIZED LIMB RADIANCES

Abstract The authors have used CPFM (composition and photodissociative flux measurement) polarized limb radiance measurements combined with a vector radiative transfer model to estimate stratospheric aerosol number density, extinction coefficient profiles, and size distribution. The CPFM spectroradiometer is flown on board the NASA ER-2 high-altitude research aircraft. The vertical and horizontal polarization components of limb radiance, nadir radiance, and horizontal flux are measured in the wavelength range 300–770 nm from approximately 5°–10° above to 5°–10° below the local horizon. Results from two flights during April and May 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer campaign are presented. Aerosol characteristics are determined by forcing the model radiances and polarization to match the measurements. Results indicate number densities at 20 km are roughly 5–6 cm−3 with an effective radius of 0.17–0.20 μm. Number, surface area, and volume densities compare favora...

Comparison of the Odin/OSIRIS stratospheric ozone profiles with coincident POAM III and ozonesonde measurements

We present first statistical comparison results for stratospheric ozone density profiles retrieved from Odin/OSIRIS limb scattered radiance with 1220 coincident POAM III and 205 coincident ozonesonde measurements. Profiles are compared on a monthly basis from November 2001 to October 2002. Most of the time, differences between OSIRIS mean profiles and those measured by POAM III and ozonesondes were 5-7% between 15 km and 32 km, and within 15% above 32 km. In April-July 2002, OSIRIS mean profiles appear shifted downward by ∼1 km, introducing a difference of about 10% with POAM III and about 25% with ozonesonde profiles between 15 km and 32 km. This study demonstrates that outside the April-July 2002 period, the OSIRIS ozone profiles agree well with coincident ozonesonde and POAM III ozone profiles and make a valuable addition to the international ozone database available for research into global ozone change.

Photolysis frequency of O to O( D): Measurements and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI)

R. E. Shetter, W. Junkermann, W. H. Swartz, G. J. Frost, J. H. Crawford, B. L. Lefer, J. D. Barrick, S. R. Hall, A. Hofzumahaus, A. Bais, J. G. Calvert, C. A. Cantrell, S. Madronich, M. Müller, A. Kraus, P. S. Monks, G. D. Edwards, R. McKenzie, P. Johnston, R. Schmitt, E. Griffioen, M. Krol, A. Kylling, R. R. Dickerson, S. A. Lloyd, T. Martin, B. Gardiner, B. Mayer, G. Pfister, E. P. Röth, P. Koepke, A. Ruggaber, H. Schwander, and M. van Weele

A pseudo three‐dimensional resonance line radiative transfer model with overlapping lines

A description of a resonance line Feautrier radiative transfer (RT) code which allows for coupling between closely spaced emission lines is presented. This code has been used to model closely spaced H and D Lyman α 1215.67 A and 1215.34 A emission lines, respectively, in the Jovian thermosphrre. There is no direct interaction between these lines, but coupling and energy transfer may occur as the Voigt broadened lines overlap. The RT method is pseudo three-dimensional in that a spherical shell atmosphere is accounted for in direct solar beam scattering but with one-dimensional (1-D) plane-parallel multiple scattering. In addition, allowance is made for a varying solar zenith angle along the line of sight in calculating intensities. Using a model atmosphere for thermospheric D and H produced by a 1-D photochemical-diffusion model, we present results illustrating the relative importance of including coupling of H and D Lyman α emission lines in radiative transfer calculations for both the disk and limb of Jupiter. These results suggest that the spectral contrast between the H and D Lyman α features is reduced in long limb path geometries and that the disk measurement provides better detection possibilities.

The Impact of Non-Lambertian Wavelength-Dependent Reflecting Surfaces on Stratospheric Radiation and Photochemistry

Abstract We have developed models of physically-based cloud and ocean surfacesfor use in photochemical models. These surface models are described in termsof a flux albedo and a normalized reflection function.Through these, the dependence of albedo on wavelength, solar zenithangle, cloud optical depth (cloud surfaces) and surface windspeed (ocean surfaces) are allowed for. In addition, the non-Lambertian nature of these surfaces is accounted for.We have integrated these surfacemodels into a multiple scattering radiative transfer model to assess their effects on the stratospheric radiation field and J-values. This was accomplished by comparison with results obtainedusing Lambertian, constant albedo surfaces. Comparisons of stratospheric radiation fields revealed that boththe wavelength and directional dependences of the cloud and oceansurfaces could be large effects.Differences between calculated J-values varied from 0 to 12% depending upon species, solar zenith angle, andheight.The J-values were then used as input for a chemical box model to examine the effects these surfaces had on stratospheric chemistry. Comparisons were made against box model runs using J-values fromconstant surfaces. Overall, the effect was on the order of 10%.Differences in number densities using these different surfacesvaried with latitude, height and species.Runs were made with and without heterogeneous chemistry.

Comparison of a pseudo-3D radiative transfer model with Monte Carlo simulations in limb-viewing geometry for the satellite instrument ODIN/OSIRIS

A new pseudo-spherical 3-D radiative transfer model has been written to calculate radiances and weighting functions required by inversion algorithms to retrieve minor species profile and column amount information from limb measurements. Limb radiances obtained from the ODIN/OSIRIS (Optical Spectrograph and InfraRed Imager System) satellite instrument (to be launched Spring 2000) will be used to study depletion processes and transport dynamics in the Earths atmosphere through the study of trace species. Model simulations have been carried out for a suite of tangent heights, wavelengths, tangent solar zenith and azimuthal angles and lambertian surface albedos.The effects of Rayleigh and aerosol scattering as well as ozone absorption have been modeled. Comparisons with a 3-D Monte-Carlo model show that there is generally less than 1 - 2% difference for single-scattering and less than 10% for multiple scattering radiances at a fraction of the computational cost.