Peter Koepke

Effective reflectance of oceanic whitecaps

The effective reflectance of the foam on the ocean surface together with the fraction of the surface covered with foam describes the optical influence of whitecaps in the solar spectral range. This effective reflectance is found to be ~22% in the visible spectral range and is presented as a function of wavelength for the solar spectral range. With the fraction of the surface covered with foam, taken from the literature, the results lead to a good agreement with satellite measured radiances and albedo values. The effective reflectance is more than a factor of 2 lower than reflectance values used to date in remote sensing and radiation budget studies. Consequently, the optical influence of whitecaps can be assumed to be much less important than formerly supposed.

Airborne measurements of dust layer properties, particle size distribution and mixing state of Saharan dust during SAMUM 2006

The Saharan Mineral Dust Experiment (SAMUM) was conducted in May/June 2006 in southern Morocco. As part of SAMUM, airborne in situ measurements of the particle size distribution in the diameter range 4 nm < Dp < 100 μm were conducted. The aerosol mixing state was determined below Dp < 2.5 μm. Furthermore, the vertical structure of the dust layers was investigated with a nadir-looking high spectral resolution lidar (HSRL). The desert dust aerosol exhibited two size regimes of different mixing states: below 0.5 μm, the particles had a non-volatile core and a volatile coating; larger particles above 0.5 μm consisted of non-volatile components and contained light absorbing material. In all cases, particles larger than 10 μm were present, and in 80% of the measurements no particles larger than 40 μm were present. The abundance of large particles showed almost no height dependence. The effective diameter Deff in the dust plumes investigated showed two main ranges: the first range of Deff peaked around 5 μm and the second range of Deff around 8 μm. The two ranges of Deff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.

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.

Comparison of Models Used for UV Index Calculations

Eighteen radiative transfer models in use for calculation of UV index are compared with respect to their results for more than 100 cloud‐free atmospheres, which describe present, possible future and extreme conditions. The comparison includes six multiple‐scattering spectral models, eight fast spectral models and four empirical models. Averages of the results of the six participating multiple‐scattering spectral models are taken as a basis for assessment. The agreement among the multiple‐scattering models is within ±0.5 UV index values for more than 80% of chosen atmospheric parameters. The fast spectral models have very different agreement, between ±1 and up to 12 UV index values. The results of the empirical models agree reasonably well with the reference models but only for the atmospheres for which they have been developed. The data to describe the atmospheric conditions, which are used for the comparison, together with the individual results of all participating models and model descriptions are available on the Internet: http://www.meteo.physik.uni‐muenchen.de/strahlung/cost/.

Uncertainties in modeled UV irradiances due to limited accuracy and availability of input data

Uncertainties in modeled spectral UV irradiances under cloud-free conditions are analyzed with respect to limited measurement accuracy of actual atmospheric input parameters or their nonavailability under the assumption that no uncertainty results from the used model or from the spectral values of the extraterrestrial solar irradiance and the gaseous absorption coefficients. The resulting mean uncertainty of spectral UV irradiance is calculated using a root-mean-square (rms) procedure for various scenarios, defined by differing qualities of the used sets of input values. The results are discussed with respect to the possibility of reducing the uncertainty in modeled UV irradiances by additional measurements of input parameters and, on the other hand, assessing which of such measurements may be redundant since greater measurement expense leads to no significant improvement in accuracy of modeled irradiances. The uncertainties in modeled UV irradiances are mainly produced by the uncertainties of the measured ozone amount, by the aerosol optical depth if it is not directly measured, and by the soot concentration of the aerosol in the haze layer. Additional uncertainties can arise where snow cover is present. If O3 and SO2 contents, spectral aerosol optical depth, and aerosol soot concentration near the ground are measured under actual conditions, the uncertainties in input parameters result in a mean uncertainty of about 5% for spectral integrals of UV irradiance. These results cannot be improved significantly, even when measured values of vertical profiles of all atmospheric constituents are used. Using only the observed visibility without the measurement of aerosol optical properties, the mean uncertainty for modeled UV integrals is about 10–15%.

Scattering functions of tropospheric aerosols: the effects of nonspherical particles.

Scattering functions, i.e., the scattered intensity as a function of angle, are modeled for tropospheric aerosol types with respect to the effect of the nonspherical shape of the particles. Scattering functions of nonspherical particles compared with those of equivalent spheres show differences increasing with particle size. Thus, for aerosol types with a relatively low amount of large particles, such as continental and urban aerosols, the effect due to uncertainty about particle shape can be ignored, compared to effects due to uncertain particle size and refractive index. In desert aerosol the nonspherical particles systematically increase side scatter with a maximum around a scattering angle of 120 degrees , while around 160 degrees the difference between scattering functions of spheres and nonspheres is small. With increasing wavelength the influence of nonspherical particles decreases.

Vicarious satellite calibration in the solar spectral range by means of calculated radiances and its application to Meteosat

The method of vicarious calibration by means of calculated radiances allows absolute calibration of satellite radiometers in orbit. It works by comparing counts from the radiometer to be calibrated with corresponding absolute radiances, calculated from actual values of the relevant optically acting parameters of the atmosphere and the earths surface. The method is applied to the VIS-channel (it measures in the visible and near IR) of the European geostationary satellite Meteosat-1. To minimize uncertainties, the procedure is carried out over different surfaces, at different atmospheric conditions, and at different sun and satellite angles. The ratio between the effective radiances (the radiances at the satellite weighted with its spectral response) and the measured 6-bit counts of the Meteosat-1-VIS-channel is the calibration constant c(sat) = 2.66 W . m(-2) . sr(-1)/count. The accuracy of the calibration is +/-6%. The inaccuracy is mainly due to the broad digitization steps of the channel. Conversion factors are presented which allow one to calculate from the effective radiance the radiance at the satellite (the radiance leaving the atmosphere).

Photolysis frequency of NO2: Measurement and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI)

[1] The photolysis frequency of NO2, j(NO2), was determined by various instrumental techniques and calculated using a number of radiative transfer models for 4 days in June 1998 at the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) in Boulder, Colorado. Experimental techniques included filter radiometry, spectroradiometry, and chemical actinometry. Eight research groups participated using 14 different instruments to determine j(NO2). The blind intercomparison experimental results were submitted to the independent experimental referee and have been compared. Also submitted to the modeling referee were the results of NO2 photolysis frequency calculations for the same time period made by 13 groups who used 15 different radiative transfer models. These model results have been compared with each other and also with the experimental results. The model calculation of clear-sky j(NO2) values can yield accurate results, but the accuracy depends heavily on the accuracy of the molecular parameters used in these calculations. The instrumental measurements of j(NO2) agree to within the uncertainty of the individual instruments and indicate the stated uncertainties in the instruments or the uncertainties of the molecular parameters may be overestimated. This agreement improves somewhat with the use of more recent NO2 cross-section data reported in the literature. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: photolysis, NO2 (nitrogen dioxide), radiative transfer, intercomparison Citation: Shetter, R. E., et al., Photolysis frequency of NO2: Measurement and modeling during the International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI), J. Geophys. Res., 108(D16), 8544, doi:10.1029/2002JD002932, 2003.

Know Your Standard: Clarifying the CIE Erythema Action Spectrum

The standard erythema action spectrum provides an internationally accepted representation of the erythema‐inducing effectiveness of wavelengths in the UV part of the spectrum. The action spectrum forms the basis of the UV index used for public health information, defines the standard erythema dose unit and the minimum erythema dose and is the default response spectrum aspired to by a range of UV radiometer manufacturers. However, there are several versions of this erythema action spectrum in use, and only one of them has been endorsed as a standard. While the differences in erythemally weighted radiation incurred by choice of action spectrum will be no more than a few percent, this uncertainty is unnecessary. Here we detail the differences in the different versions of erythema action spectra, illustrate the resulting effects in quantifying UV doses and encourage readers to use only the standard version of the action spectrum in the future.

Spectral aerosol optical depth characterization of desert dust during SAMUM 2006

The aerosol optical depth (AOD) in the range 340–1550 nm was monitored at Ouarzazate (Morocco) during the Saharan Mineral Dust Experiment (SAMUM) experiment in May–June 2006. Two different sun photometers were used for this purpose. The mean AOD at 500 nm was 0.28, with a maximum of 0.83, and the mean Ångström exponent (AE) was 0.35. The aerosol content over the site changed alternatively from very low turbidity, associated to Atlantic air masses, to moderate dust load, associated to air masses arriving in the site from Algeria, Tunisia and Libya. The dusty conditions were predominant in the measurement period (78% of data), with AOD (500 nm) above 0.15 and AE below 0.4. The spectral features of the AOD under dusty conditions are discussed. Air mass back trajectory analysis is carried out to investigate the origin and height patterns of the dust loaded air masses. The advection of dust occurred mainly at atmospheric heights below 3000 m, where east flow is the predominant. At the 5000m level, the air masses originate mainly over the Atlantic Ocean. Finally the Optical Properties of Aerosols and Clouds (OPAC) model is used to perform a set of simulations with different aerosol mixtures to illustrate the measured AOD and AE values under varying dust concentrations, and a brief comparison with other measurement sites is presented.