Robert B. A. Koelemeijer

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.

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...

Cirrus optical thickness and crystal size retrieval from ATSR-2 data using phase functions of imperfect hexagonal ice crystals

Along Track Scanning Radiometer 2 (ATSR-2) measurements made over a tropical cirrus anvil are analyzed on the basis of radiative transfer calculations for clouds consisting of imperfect hexagonal ice crystals. Reflectivity measurements made at two wavelengths (0.87 μm, nonabsorbing; 1.6 μm, absorbing) and two viewing directions (nadir and forward) are considered. Model calculations for a cloud consisting of single-sized imperfect hexagonal ice crystals adequately explain the gross features of the ATSR-2 reflectivity measurements. Retrieved values of optical thickness and crystal size reveal no discernible relationship between these quantities. Nadir-derived and forward-derived optical thickness and crystal size are compared for both imperfect and near-perfect hexagonal crystals. For these two crystal shapes, there appears to be a moderate trade-off in consistent retrieval of crystal size versus retrieval of optical thickness. Consistent retrieval of crystal size is found for imperfect crystals. We find an average crystal size (defined as maximum crystal dimension) of 63±4 μm for a model cloud consisting of imperfect hexagonal columns. For imperfect hexagonal plates a somewhat larger value is retrieved: 71±3 μm. Both retrieved sizes suggest that the cloud system consisted of relatively small ice crystals.

Comparison of Visible Calibrations of GOME and ATSR-2

Abstract On board ESAs second European Remote Sensing Satellite (ERS-2), launched in April 1995, two instruments perform measurements of the reflectivity of the Earth at the top of the atmosphere: the Global Ozone Monitoring Experiment (GOME) and the Along Track Scanning Radiometer-2 (ATSR-2). GOME is a spectrometer measuring radiation between 240 nm and 790 nm with a spectral resolution of 0.2–0.4 nm. The ATSR-2 is an imager with four channels in the solar part and three channels in the thermal infrared part of the spectrum. Both instruments observe the Sun via a diffuser plate for relative radiometric calibration, thereby yielding reflectivities. The ATSR-2 channels at 555 and 659 nm overlap with the wavelength range of GOME. In this article, the consistency of the reflectivity calibrations of GOME and ATSR-2 is investigated at both wavelengths, by analyzing data from collocated partly cloudy scenes over the Atlantic Ocean, acquired on 23 July 1995. The main result is that reflectivities measured by both instruments agree well; the ATSR-2 reflectivity differs from the GOME reflectivity by −4.0% at 555 nm and −2.2% at 659 nm (relative differences).

Cloud optical thickness retrieval from AVHRR data

The relation between the reflectivity of the atmosphere-surface system and the optical thickness of a homogeneous cloud layer in the atmosphere is investigated at 0.63 micrometer, which is the central wavelength of NOAA-AVHRR (advanced very high resolution radiometer) channel 1. A detailed radiative transfer model is employed, in which the multiple scattering and absorption resulting from cloud particles, molecules, aerosols, ozone and surface are fully taken into account. To estimate the sensitivity of the relation between atmospheric reflectivity and cloud optical thickness, the influence of variation of solar zenith angle, surface albedo, cloud particle effective radius and viewing geometry are examined. The relation between reflectivity and cloud optical thickness is mainly sensitive to solar zenith angle, surface albedo and viewing geometry. Therefore, these parameters have to be known for a retrieval of cloud optical thickness from reflectivity measurements. For actual retrievals, a database is prepared with atmospheric reflectivities for many solar zenith angles, viewing zenith angles, azimuth angles, surface albedos and cloud optical thicknesses. The retrieval method is applied to an AVHRR image of Sept 11, 1994. First results show a promising correlation with cloud optical thickness estimations from ground based measurements of direct solar irradiance.

Construction of Satellite Derived PM2.5 Maps using the Relationship between AOD and PM2.5 at the Cabauw Experimental Site for Atmospheric Research (CESAR) - The Netherlands

To acquire daily estimates of PM2.5 distributions based on satellite data one depends critically on a well established relation between AOD and ground level PM2.5. In this study we aimed to experimentally establish the AOD-PM2.5 relationship for the Netherlands. For that purpose an experiment was set-up at the AERONET site Cabauw. The average PM2.5 concentration during this ten month study was 18 mug/ m3, which confirms that the Netherlands are characterized by a high PM burden. A first inspection of the AERONET level 1.5 (L1.5) AOD and PM2.5 data at Cabauw showed a low correlation between the two properties. However, after screening for cloud contamination in the AERONET L1.5 data, the correlation improved substantially. When also constraining the dataset to data points acquired around noon, the correlation between AOD and PM2.5 amounted to R 2 =0.6 for situations with fair weather. This indicates that AOD data contain information about the temporal evolution of PM2.5. We used lidar observations to detect residual cloud contamination in the AERONET L1.5 data. Comparison of our cloud-screed L1.5 with AERONET L2 data that became available near the end of the study showed favorable agreement. The final relation found for Cabauw is PM2.5 = 124.5* AOD - 0.34 (with PM2.5 in mug/m3) and is valid for fair weather conditions. The relationship determined between MODIS AOD and ground level PM2.5 at Cabauw is very similar to that based on the much larger dataset from the sun photometer data, after correcting for a systematic overestimation of the MODIS data of 0.05. We applied the relationship to a MODIS composite map to assess the PM2.5 distribution over the Netherlands for the first time based on MODIS data only.

Validation of GOME cloud-cover fraction relevant for accurate ozone retrieval

The Global Ozone Monitoring Experiment (GOME), launched on board of ERS-2 in April 1995, is a spectrometer measuring the Earths reflectivity between 240 and 790 run with a spectral resolution of 0.2-0.4 nm. The spatial resolution of GOME can be varied between 40 x 40 km2 and 40 x 320 km2 . The main geophysical product from GOME is the total ozone column, which is derived from the GOME reflectivity measurements using the Differential Optical Absorption Spectroscopy method. For accurate retrievals of the total ozone column, the presence of clouds has to be taken into account. Therefore, cloud cover fraction is derived from the GOME reflectivity measurements around the 0 2 A-band using the the Initial Cloud Fitting Algorithm (ICFA). The ICFA results are input to the GOME ozone column retrieval algorithm. In this study, the ICFA cloud fraction results are validated in two ways. Firstly, a statistical approach is followed_ by comparing monthly averaged ICFA results with monthly averaged cloud fractions from the International Satellite Cloud Climatology Project (ISCCP). Secondly, a detailed comparison is performed for a limited number of data using cloud fractions derived from Along Traclc Scanning Radiometer-2 (ATSR-2) data and synoptical observations (SYNOP). The main conclusion is that the ICFA cloud fractions generally show a reasonable correlation with ISCCP, ATSR-2 and SYNOP data. However, the difference between cloud fractions over land and sea which shows up in the ISCCP results is not present in the ICFA results. Furthermore, the ICFA cloud fractions are too high over highly reflecting surfaces (e.g. desert and snow). The magnitude of errors in the GOME total ozone column due to errors in cloud fraction and cloud top height has been investigated using a detailed radiative transfer model. From this_ analysis, it is concluded that errors in these cloud parameters may introduce errors in the ozone vertical column of typically 2-33. Keywords: GOME, clouds, ozone, cloud detection