Tilman Dinter

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

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.

Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May–June 2006, airborne and ground-based solar (0.3–2.2μm) and thermal infrared (4–42μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In theSAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12Wm−2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22Wm−2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24Wm−2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.

Desert Dust Aerosol Air Mass Mapping in the Western Sahara, Using Particle Properties Derived from Space-Based Multi-Angle Imaging

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field 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.

Global Retrieval of Diatom Abundance Based on Phytoplankton Pigments and Satellite Data

Diatoms are the major marine primary producers on the global scale and, recently, several methods have been developed to retrieve their abundance or dominance from satellite remote sensing data. In this work, we highlight the importance of the Southern Ocean (SO) in developing a global algorithm for diatom using an Abundance Based Approach (ABA). A large global in situ data set of phytoplankton pigments was compiled, particularly with more samples collected in the SO. We revised the ABA to take account of the information on the penetration depth (Zpd) and to improve the relationship between diatoms and total chlorophyll-a (TChla). The results showed that there is a distinct relationship between diatoms and TChla in the SO, and a new global model (ABAZpd) improved the estimation of diatoms abundance by 28% in the SO compared with the original ABA model. In addition, we developed a regional model for the SO which further improved the retrieval of diatoms by 17% compared with the global ABAZpd model. As a result, we found that diatom may be more abundant in the SO than previously thought. Linear trend analysis of diatom abundance using the regional model for the SO showed that there are statistically significant trends, both increasing and decreasing, in diatom abundance over the past eleven years in the region.

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

Approximately 30% of the land surface is arid, having desert or semi-desert conditions. Aerosol originating from these regions plays a significant role in climate and atmospheric chemistry of the atmosphere. Retrieving aerosol properties from space-borne platforms above desert conditions, where the surface reflectance is usually very bright, is a challenging task. The proportion of the surface to top of atmosphere (TOA) reflectance can reach values over 90%, especially for wavelength above 500 nm. For these reasons detailed knowledge of aerosol and surface optical properties from these regions is required to separate atmosphere from intrinsically bright surfaces. An approach to retrieve aerosol properties over arid and semi-arid regions based on the Bremen Aerosol Retrieval (BAER) has been developed and validated within the Dust Aerosol Retrievals from Space-Born Instruments (DREAMS) Project, which is part of the Saharan Mineral Dust Experiment (SAMUM, 2006). Combining measurements of the backscattered radiation from the Medium Resolution Imaging Spectrometer (MERIS) instrument aboard Environmental Satellite (ENVISAT) and ground-based measurements in Morocco in radiation closure experiments yields the aerosol optical properties of mineral dust at selected locations.

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.

Detecting CDOM Fluorescence Using High Spectrally Resolved Satellite Data: A Model Study

Absorption and fluorescence of CDOM are widely used to characterize its concentration and composition, both in situ and with remote sensing techniques. As fluorescence is an inelastic scattering process, it can potentially be observed in filling-in of Fraunhofer lines with the DOAS method in high spectrally resolved satellite data. Here, we perform a theoretical model study and preliminary DOAS retrieval on SCIAMACHY data in order to test the feasibility of such an approach. Our results show that retrieving CDOM fluorescence is difficult, due to its relatively weak signal in global oceans (in comparison to Raman scattering of clear water) and varying broad spectral features. Detailed studies of the synergistic use of narrow and broad fluorescence features, and an appropriate scheme to account for disturbance effects of Raman scattering, are needed for further investigating the retrieval of CDOM fluorescence.

Underway spectrophotometry in the Fram Strait (European Arctic Ocean): a highly resolved chlorophyll a data source for complementing satellite ocean color

GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, VSOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at VSOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.

Synergistic Exploitation of Hyper- and Multi-Spectral Precursor Sentinel Measurements to Determine Phytoplankton Functional Types (SynSenPFT)

We derive the chlorophyll a concentration (Chla) for three main phytoplankton functional types (PFTs) – diatoms, coccolithophores and cyanobacteria – by combining satellite multispectral-based information, being of a high spatial and temporal resolution, with retrievals based on high resolution of PFT absorption properties derived from hyperspectral satellite measurements. The multispectral-based PFT Chla retrievals are based on a revised version of the empirical OC-PFT algorithm applied to the Ocean Color Climate Change Initiative (OC-CCI) total Chla product. The PhytoDOAS analytical algorithm is used with some modifications to derive PFT Chla from SCIAMACHY hyperspectral measurements. To combine synergistically these two PFT products (OC-PFT and PhytoDOAS), an optimal interpolation is performed for each PFT in every OC-PFT sub-pixel within a PhytoDOAS pixel, given its Chla and its a priori error statistics. The synergistic product (SynSenPFT) is presented for the period of August 2002 March 2012 and evaluated against PFT Chla data obtained from in situ marker pigment data and the NASA Ocean Biogeochemical Model simulations and satellite information on phytoplankton size. The most challenging aspects of the SynSenPFT algorithm implementation are discussed. Perspectives on SynSenPFT product improvements and prolongation of the time series over the next decades by adaptation to Sentinel multi- and hyperspectral instruments are highlighted.

Improving the PhytoDOAS method to retrieve coccolithophores using hyper-spectral satellite data.

Post-graduate education in Germany has changed a lot over the past decades. Formerly, PhD students generally did not have the option to attend formal classes and lectures and were expected to conduct their independent research, including occasionally teaching courses for students. Since the introduction of bachelor and masters studies with the Bologna Process in the late 90th, the higher education in Europe has been harmonized, leading to more structured and focused studies at the expense of a broad and universal disciplinary education. At this same time, special fields such as Earth System Science became more interdisciplinary. In consequence, universities and research institutes have established so-called research schools and/or graduate schools, offering specific courses and training alongside the doctorate. Especially, Earth System Science has developed from an interesting concept in Earth Sciences education to a fully integrative Science focussed on understanding the complex system Earth. This evolution is partially due to the radical and far reaching anthropogenic changes and the general feeling of helplessness with regards to the possible consequences and future impacts on the Earth System. The Helmholtz “Earth System Science Research School” (ESSReS) is a small unit of PhD students co-organized by three educational and research institutions in the city state Bremen: University of Bremen (Institute for Environmental Physics, IUP), Jacobs University (School of Engineering and Science (JU)), and Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven (AWI). ESSReS aims at the integration of research at the interface of Geology, Biology, Physics, Geophysics, Mathematics and Informatics. It is therefore multi- and interdisciplinary in every aspect. The training, curriculum, and PhD research subjects are closely located at the interfaces between the participating disciplines. This is guaranteed by interdisciplinary supervision of the PhD project. The long-term goal of ESSReS is not only to enhance exchange and interaction between these disciplines, but to enforce a newly integrated concept, where separation between disciplines becomes more and more obsolete. Now, at the end of two three-years terms of PhD student education it can be stated that ESSReS provide a solid base for a new generation of excellent scientists in Earth and Environmental Sciences.Information about past environmental conditions is preserved in the elemental signature of biogenic marine carbonates. Thus, trace element to calcium ratios (Me/Ca) of biogenic calcium carbonates, such as bivalve shells, are often used to reconstruct past environmental conditions at the time of carbonate formation (Foster et al., 2008). In this study, we examine the suitability of the long-lived (> 400 years) bivalve Arctica islandica as a high-resolution bioarchive by measuring Me/Ca ratios in the shell carbonate. Pb/Ca concentrations in A. islandica shells reflect anthropogenic gasoline lead consumption and further provide a centennial record of lead pollution for the collection site off the coast of Virginia, USA. With A. islandica shells from the North Sea we test the hypothesis that Ba/Ca and Mn/Ca ratios are indicators of the diatom abundance. Our results indicate that statistically both ratios correlate well with the diatom abundance, and yet, on a year-to-year base, there is no consistent reflection of diatom abundance patterns in the Ba/Ca and Mn/Ca annual profiles. These findings indicate that primary production affects Ba/Ca and Mn/Ca shell ratios, though we suggest that both elements are coupled to primary production through different processes and are affected by further, yet unknown processes.To date, the software package SCIATRAN (Rozanov et al. 2002; Rozanov et al., 2005, 2008) has been used for modelling radiative processes in the atmosphere for the retrieval of trace gases from satellite data from the satellite sensor SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY onboard the satellite ENVISAT). This SCIATRAN version only accounted for radiative transfer within the atmosphere and reflection of light at the earth surface. However, radiation also passes the air-water interface, proceeds within the water and is modified by the water itself and the water constituents. Therefore, SCIATRAN has been extended by oceanic radiative transfer and coupling it to the atmospheric radiative transfer model under the terms of established models for radiative transfer underwater (Kopelevich 1983; Morel et al. 1974, 2001; Shifrin 1988; Buitevald et al. 1994; Cox and Munk 1954a, 1954b; Breon and Henriot 2006; Mobley 1994) and extending the data bases to include the specific properties of the water constituents (Pope and Fry 1997; Haltrin 2006; Prieur and Sathyendranath 1981).