Retrieval of aerosol optical thickness for desert conditions using MERIS observations during the SAMUM campaign

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102

138

The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5‐km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper‐level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct cut‐off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.

Section: Special Samum Observationsmentioning

confidence: 99%

Dust mobilization and transport in the northern Sahara during SAMUM 2006 – a meteorological overview

Knippertz¹,

Ansmann²,

Althausen³

et al. 2009

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102

138

“…1d. For this comparison, the four 17.6 km MISR retrieval regions and (c) Tinfou von Hoyningen-Huene et al, 2008). The sun photometer green-band AOT at MISR overpass time (11:11 UTC) is indicated with an arrow.…”

Section: May 2006mentioning

confidence: 99%

Desert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging

Kahn¹,

Petzold²,

Wendisch³

et al. 2009

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T E L L U SDesert dust aerosol air mass mapping in the western Sahara, using particle properties derived from space-based multi-angle imaging A B S T R A C T Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05-0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.

“…Therefore, the first focus of this specific SAMUM contribution is the determination of phase functions of desert dust. The resulting phase functions will be used for the generation of LUTs for the aerosol retrieval in Dinter et al (2008) and the derivation of single scattering albedo in this contribution.…”

Section: Observationsmentioning

confidence: 99%

“…We explained this difference as effect of absorption, because ω 0 is a linear factor in front of all scattering terms of the radiative transfer equation. The spectral surface reflectance for the modelling is taken from retrievals by Dinter et al (2008) for the site Porte au Sahara, using atmospheric correction of measured MERIS TOA reflectance. The results of the retrieval of the surface reflectance are compared with results of spectral albedo measurements in the spectral range of 0.370-1.0 μm provided by air-borne spectral albedometer .…”

Section: Estimation Of Spectral Single Scattering Albedomentioning

confidence: 99%

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

Hoyningen‐Huene¹,

Dinter²,

Kokhanovsky³

et al. 2009

This journal is published under the terms of the Creative Commons Attribution-Noncommercial 3.0 Unported LicenseMain 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 degrees N, 5.607 degrees 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