P. Stammes

The ozone monitoring instrument

The Ozone Monitoring Instrument (OMI) flies on the National Aeronautics and Space Administrations Earth Observing System Aura satellite launched in July 2004. OMI is a ultraviolet/visible (UV/VIS) nadir solar backscatter spectrometer, which provides nearly global coverage in one day with a spatial resolution of 13 km/spl times/24 km. Trace gases measured include O/sub 3/, NO/sub 2/, SO/sub 2/, HCHO, BrO, and OClO. In addition, OMI will measure aerosol characteristics, cloud top heights, and UV irradiance at the surface. OMIs unique capabilities for measuring important trace gases with a small footprint and daily global coverage will be a major contribution to our understanding of stratospheric and tropospheric chemistry and climate change. OMIs high spatial resolution is unprecedented and will enable detection of air pollution on urban scale resolution. In this paper, the instrument and its performance will be discussed.

A fast method for retrieval of cloud parameters using oxygen A band measurements from the Global Ozone Monitoring Experiment

The Global Ozone Monitoring Experiment (GOME) on board the ERS-2 is designed to measure trace gas column densities in the Earths atmosphere. Such retrievals are hindered by the presence of clouds. The most important cloud parameters that are needed to correct trace gas column density retrievals for the disturbing effects of clouds are the (effective) cloud fraction and cloud top pressure. At present, in the operational GOME data processor an effective cloud fraction is derived for each pixel, but cloud top pressure is assumed a priori and is deduced from a climatological database. Here we report an improved cloud retrieval scheme, which simultaneously retrieves the effective cloud fraction and cloud top pressure from GOME data. This algorithm, called Fast Retrieval Scheme for Clouds from the Oxygen A band (FRESCO), makes use of reflectivities as measured by GOME inside and outside the oxygen A band (758–778 nm). For validation, the results of FRESCO are compared to effective cloud fractions and cloud top pressures derived with standard methods from colocated measurements made by the Along Track Scanning Radiometer-2 (ATSR-2). The brightness temperatures of the cloudy pixels as measured by ATSR-2 are related to cloud top pressures using temperature profiles from the European Center for Medium-range Weather Forecasts model. Generally, the results from FRESCO and ATSR-2 agree reasonably well. For the effective cloud fractions the average difference (based on a comparison of 322 points) is 0.04; the standard deviation is 0.09. For the cloud top pressures, only points with an effective cloud fraction larger than 0.1 have been compared. For these 236 points the average difference in cloud top pressure is 65 hPa, and the standard deviation is 92 hPa.

Scattering matrices of imperfect hexagonal ice crystals

Abstract Scattering matrices of perfect and imperfect hexagonal ice crystals are presented and compared to those of so-called polycrystals, proposed by Macke et al. [1] . Scattering matrices of imperfect hexagonal crystals are calculated using statistical deviations of ray paths during the ray tracing in perfect hexagonal crystals. At a certain degree of deviation, the resulting scattering matrix becomes similar to that of polycrystals. Therefore, this method forms a link between perfect hexagonal columns and polycrystals. The optical properties of these imperfect crystals are sensitive to the aspect ratio (and thus to particle size) as well as to the allowed deviation from the perfect shape. This approach to simulate imperfect ice crystals introduces new possibilities for interpretation of satellite measurements.

Regional Distribution of Aerosol over Land, Derived from ATSR-2 and GOME

Abstract Two aerosol optical depth retrieval algorithms, using different instruments and different methods, are compared. The first method uses both the directional and the spectral information of the Along Track Scanning Radiometer 2 (ATSR-2) to compute the aerosol optical depth in the visible and near-infrared ranges. The second algorithm uses data in the wavelength range between 0.340 μm and 0.400 μm from the Global Ozone Monitoring Experiment (GOME) to determine the aerosol optical depth in the ultraviolet. Both ATSR-2 and GOME are onboard the ERS-2 satellite. The two methods are applied to data from the ERS-2 overpass over northwestern Europe on 25 July 1995. The retrieved aerosol optical depths compare favorably. Also, there is good comparison between satellite retrievals and ground-based measurements. Optical depth images show a large aerosol plume over Belgium and northern France. Back-trajectories indicate that the sources for this aerosol plume are the industrialized regions in Germany and Belgium.

Clear‐sky shortwave radiative closure for the Cabauw Baseline Surface Radiation Network site, Netherlands

[1] In this paper a clear-sky shortwave closure analysis is presented for the Baseline Surface Radiation Network (BSRN) site of Cabauw, Netherlands (51.97N, 4.93E). The analysis is based on an exceptional period of fine weather during the first half of May 2008, resulting in a selection of 72 comparisons, on 6 days, between BSRN measurements and Doubling Adding KNMI (DAK) model simulations of direct, diffuse, and global irradiances. The data span a wide range of aerosol properties, water vapor columns, and solar zenith angles. The model input consisted of operational Aerosol Robotic Network (AERONET) aerosol products and radiosonde data. The wavelength dependence of the aerosol optical thickness, single scattering albedo, and asymmetry parameter was taken into account. On the basis of these data, excellent closure was obtained: the mean differences between model and measurements are 2 W/m 2 (+0.2%) for the direct irradiance, 1 W/m 2 (+0.8%) for the diffuse irradiance, and 2 W/m 2 (+0.3%) for the global irradiance. The good results were obtained because of proper specification of the DAK model input and the high quality of the AERONET and BSRN measurements. The sensitivity of the achieved closure to uncertainties in the aerosol optical thickness, single scattering albedo, and asymmetry parameter was examined. Furthermore, several sensitivity experiments related to the wavelength dependence of the aerosol optical properties and the treatment of water vapor were performed. It appeared that a correct description of the wavelength dependence of the aerosol optical properties is important for achieving broadband closure. However, broadband closure can also be obtained by means of using spectrally averaged values of the single scattering albedo and the asymmetry parameter. Cancellation of errors in different parts of the solar spectrum also contributes to the achieved closure.

Degree of lineair polarization of light emerging form the cloudless atmosphere in de oxygen A-band

We used radiative transfer calculations and model atmospheres for a theoretical investigation of the behavior of the degree of linear polarization, P, of light emerging from the top or the bottom of the cloudless atmosphere in the wavelength region of the O2 A absorption band, between 755 and 775 nm. Results of P are shown for four model atmospheres and for various albedos of the underlying Lambert surface. One of the model atmospheres contains only molecules, whereas the other atmospheres contain also aerosols. It is shown that when the molecular absorption optical thickness of the atmosphere is much smaller than one, which represents the continuum and the weak absorption lines in the band, the state of polarization of the emerging radiation is mainly determined by low-order scattering in the lowest atmospheric layers and by reflection by the surface. In the strong absorption lines, where the molecular absorption optical thickness is much larger than one, we distinguish the three following situations: (1) P of the reflected light is mainly determined by single scattering by molecules and aerosol in the upper atmospheric layers, (2) for small solar zenith and/or viewing angles, P of the diffusely transmitted light is mainly determined by single scattering, and (3) for other geometries, P of the diffusely transmitted light is predominantly due to second-order scattering, with the first scattering taking place in the upper atmospheric layers and the second in the lower layers. For a given surface albedo, the variation of P across an absorption line thus depends on the scattering properties of the atmospheric particles and on their vertical distribution. The surface albedo and P of light emerging from the atmosphere in the principal plane are shown to be related through a simple formula at each wavelength within the absorption band. It is concluded that high-resolution spectropolarimetry in wavelength regions with strongly varying molecular absorption optical thickness can provide valuable information on aerosol at various altitudes in the atmosphere.

Effect of aerosol microphysical properties on polarization of skylight: sensitivity study and measurements

We analyze the sensitivity of the degree of linear polarization in the Suns principal plane as a function of aerosol microphysical parameters: the real and imaginary parts of the refractive index, the median radius and geometric standard deviation of the bimodal size distribution (both fine and coarse modes), and the relative number weight of the fine mode at a wavelength of 675 nm. We use Mie theory for single-scattering simulations and the doubling-adding method with the inclusion of polarization for multiple scattering. It is shown that the behavior of the degree of linear polarization is highly sensitive to both the small mode of the bimodal size distribution and the real part of the refractive index of aerosols, as well as to the aerosol optical thickness; whereas not all parameters influence the polarization equally. A classification of the importance of the input parameters is given. This sensitivity study is applied to an analysis of ground-based polarization measurements. For the passive remote sensing of microphysical and optical properties of aerosols, a ground-based spectral polarization measuring system was built, which aims to measure the Stokes parameters I, Q, and U in the visible (from 410 to 789 nm) and near-infrared (from 674 to 995 nm) spectral range with a spectral resolution of 7 nm in the visible and 2.4 nm in the near infrared. We compare polarization measurements taken with radiative transfer simulations under both clear- and hazy-sky conditions in an urban area (Cabauw, The Netherlands, 51.58 degrees N, 4.56 degrees E). Conclusions about the microphysical properties of aerosol are drawn from the comparison.

Spectral fine-structure in the polarisation of skylight

High-spectral resolution polarisation measurements of the clear daylight sky reveal an unexpected rich spectral fine-structure in the polarisation super-imposed on the more familiar broad-band continuum. This polarisation spectrum shows the spectral fingerprints of scattering and absorption processes which is illustrated with radiative transfer calculations. In particular, radiative transfer calculations suggest the potential of high-spectral resolution polarimetry within absorption bands for remote sensing of aerosol vertical distribution, which is relevant to the Earths radiation budget and ozone hole chemistry. In addition, the presence of spectral fine-structure in the polarisation of skylight also has relevance to radiance measurements performed with polarisation sensitive spectrometers, such as the space-based GOME instrument.

Modeling total and polarized reflectances of ice clouds: evaluation by means of POLDER and ATSR-2 measurements

Four ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earths Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 microm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > approximately 16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 microm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 microm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs.

Cloud Thermodynamic-Phase Determination From Near-Infrared Spectra of Reflected Sunlight

Abstract A simple method for the determination of the thermodynamic phase of clouds over ocean from near-infrared spectra of reflected sunlight is presented. The method is based on thresholding the parameter S1.67 (in percent), which is defined as the ratio of the difference between the spectral reflectivities at 1.70 and 1.64 μm to the reflectivity at 1.64 μm. Radiative transfer calculations for different cloudy atmospheres over ocean are presented to show that S1.67 ≈ 0 for water clouds and S1.67 > 0 for ice clouds and mixed-phase clouds. It is shown that S1.67 is sensitive to the presence of ice particles in clouds, and depends primarily on ice-cloud optical thickness and crystal size. The method is relatively independent of viewing and solar geometry because it is based on spectral absorption properties rather than scattering properties of clouds. The method is thoroughly analyzed using near-infrared reflectivity spectra made by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) over a we...