W. von Hoyningen-Huene

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and ...

Possibility of refractive index determination of atmospheric aerosol particles by ground-based solar extinction and scattering measurements

Abstract A method is developed to infer the optical properties of aerosol particles from ground-based solar extinction and scattering measurements under cloudless conditions. On the basis of the spectral aerosol optical thickness (extinction measurement) radiative transfer calculations are carried out yielding the diffusely scattered radiances at the ground. These calculated radiances are compared with the scattering measurement. The unknown refractive index and consequently all the other optical aerosol properties are varied in the model calculations until best agreement to the measurement is achieved. Thus a complete set of aerosol characteristics is deduced describing the measurements in an optically equivalent sense. The method is examined by means of aerosol models. These tests show that the method works well if many optically effective fine particles are present and in the case of slight absorption. The imaginary part of the refractive index cannot be obtained with the present version of the method. The application to real atmospheric measurements stresses the need to consider the nonsphericity of the aerosol particles, especially if the relative humidity is low and the portion of large particles is high. A comparison with the results of independent nucleopore filter measurements yields good agreement. Furthermore typical values of the real part of the refractive index for different geographical regions are given.

RADIATIVE PROPERTIES OF DESERT DUST AND ITS EFFECT ON RADIATIVE BALANCE

Abstract A complete set of experimentally derived climate-relevant radiative parameters of mineral dust from a measurement campaign made in Senegal are presented and applied to estimate the shortwave radiative forcing by the desert aerosol. The experimental results are used for the validation of existing aerosol models for mineral dust.

Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM

Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May–10 June 2006) at the site Porte au Sahara (30.237◦N, 5.607◦W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.

The Intercomparison of Cloud Parameters Derived Using Multiple Satellite Instruments

Cloud optical thickness (COT) has been retrieved using multiple optical instruments onboard ENVISAT and compared for consistency for a single cloud field over central Europe. To match the spatial resolution of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), the results of retrievals from higher resolution instruments have been averaged on the scale of 30 times 60 km. It was found that the Medium Resolution Imaging Spectrometer (MERIS), Advanced Along Track Scanning Radiometer (AATSR), and SCIAMACHY (all onboard ENVISAT) give close values of COT and, therefore, cloud albedo. Similar results have been obtained for Moderate Resolution Imaging Spectroradiometer onboard the Terra satellite. This suggests that these instruments can be used for synergetic retrievals of cloud properties from space. For instance, the high spectral resolution of SCIAMACHY can be used to enhance MERIS or AATSR retrievals of cloud top height and other cloud characteristics

The Intercomparison of Top-of-Atmosphere Reflectivity Measured by MERIS and SCIAMACHY in the Spectral Range of 443–865 nm

This letter is aimed at better understanding of Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) reflectance radiometric calibration errors using the Medium Resolution Imaging Spectrometer (MERIS) onboard ENVISAT. Earlier investigations showed that the SCIAMACHY calibration error can reach 20% in the visible bands, which prevents aerosol retrievals using the SCIAMACHY data. Recent improvements of the SCIAMACHY calibration are discussed. It is found that the differences in reflectances for the wavelengths 443, 560, 665, 754, and 865 nm between MERIS and improved Processor 6 SCIAMACHY data are close to the MERIS radiometric calibration error, which is below 4%

The determination of the atmospheric optical thickness over Western Europe using SeaWiFS imagery

The first results obtained from the aerosol-cloud retrieval algorithm (developed at the University of Bremen) are presented. The algorithm enables the observation of the regional characteristics of aerosol and cloud optical thickness both over land and ocean surfaces. The aerosol and cloud optical thickness over Western Europe is derived from the high-resolution SeaWiFS data for October 11, 2001 (11:30 UTC). The most probable value of the aerosol optical thickness was found to be equal approximately 0.25. The frequency distributions of the aerosol and cloud optical thickness are skewed and have long tails for larger optical thickness. It was found that retrieved values of the aerosol optical thickness at wavelengths 0.412 and 0.440 /spl mu/m are close to those measured by five ground-based instruments placed at different locations. The problems related to the retrieval of the atmospheric optical thickness from space are discussed.

The determination of cloud altitudes using SCIAMACHY onboard ENVISAT

This letter shows first results for the application of a recently developed semianalytical cloud retrieval algorithm for the determination of cloud top heights from space. The technique is based on the measurements of the top-of-atmosphere reflectance in the oxygen A-band. The depth of the band depends on the cloud top height and its geometrical thickness. The data obtained are compared to ground-based measurements of the cloud top height using a cloud-profiling radar.

Determination of the cloud fraction in the SCIAMACHY ground scene using MERIS spectral measurements

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) is a passive remote sensing spectrometer observing backscattered radiation from the atmosphere and the Earths surface, in the wavelength range between 240 and 2380 nm. The instrument is onboard ENVironmental SATellite (ENVISAT) which was launched on 1 March 2002. The Medium Resolution Imaging Spectrometer (MERIS) is also one of the 10 instruments onboard the ENVISAT satellite. MERIS is a 68.5° field-of-view nadir-pointing imaging spectrometer which measures the solar radiation reflected by the Earth in 15 spectral bands (visible and near-infrared). It obtains a global coverage of the Earth in three days. Its main objective is to measure sea colour and quantify ocean chlorophyll content and sediment, thus providing information on the ocean carbon cycle and thermal regime. It is also used to derive the cloud top height, aerosol and cloud optical thickness, and water vapour column. The ground spatial resolution of the instrument is 260 m × 290 m. This paper is aimed at determining the cloud fraction in SCIAMACHY pixels (typically, 30 km × 60 km ground scenes) using MERIS observations and number of thresholds for MERIS top-of-atmosphere reflectances and their ratios. Thresholds utilize the fact that clouds are bright white objects having similar reflectances in the blue and red. The MERIS cloud fraction has been derived for a number of SCIAMACHY states with area of 916 km × 400 km. The results are compared with correspondent cloud fractions obtained using SCIAMACHY polarization measurement devices (PMDs). Large differences are found between cloud fractions derived using SCIAMACHY and MERIS measurements. It is recommended to use highly spatially resolved MERIS observations instead of SCIAMACHY PMD measurements to retrieve cloud fractions in SCIAMACHY pixels. The improvements advised will enhance SCIAMACHY trace gas and cloud retrievals in the presence of broken cloud fields.

Sounding The Troposphere From Space: A New Era For Global Atmospheric Chemistry

TROPOSAT: the Project and the Scientific Highlights.- An Overview of the Scientific Activities and Achievements.- Development of Algorithms.- Use of Satellite Data to Understand Atmospheric Processes.- Synergistic Use of Different Instrumentation and Platforms for Tropospheric Measurements.- Validation and Data Assimilation for Tropospheric Satellite Data Products.