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.     

Multi‐proxy evidence for early to mid‐Holocene environmental and climatic changes in northeastern Poland

Lauterbach, S., Brauer, A., Andersen, N., Danielopol, D. L., Dulski, P., Hüls, M., Milecka, K., Namiotko, T., Plessen, B., von Grafenstein, U. & DecLakes participants 2010: Multi‐proxy evidence for early to mid‐Holocene environmental and climatic changes in northeastern Poland. Boreas, 10.1111/j.1502‐3885.2010.00159.x. ISSN 0300‐9483. We investigated the sedimentary record of Lake Hańcza (northeastern Poland) using a multi‐proxy approach, focusing on early to mid‐Holocene climatic and environmental changes. AMS ¹⁴C dating of terrestrial macrofossils and sedimentation rate estimates from occasional varve thickness measurements were used to establish a chronology. The onset of the Holocene at c. 11 600 cal. a BP is marked by the decline of Lateglacial shrub vegetation and a shift from clastic‐detrital deposition to an autochthonous sedimentation dominated by biochemical calcite precipitation. Between 10 000 and 9000 cal. a BP, a further environmental and climatic improvement is indicated by the spread of deciduous forests, an increase in lake organic matter and a 1.7‰ rise in the oxygen isotope ratios of both endogenic calcite and ostracod valves. Rising δ¹⁸O values were probably caused by a combination of hydrological and climatic factors. The persistence of relatively cold and dry climate conditions in northeastern Poland during the first one and a half millennia of the Holocene could be related to a regional eastern European atmospheric circulation pattern. Prevailing anticyclonic circulation linked to a high‐pressure cell above the retreating Scandinavian Ice Sheet might have blocked the influence of warm and moist Westerlies and attenuated the early Holocene climatic amelioration in the Lake Hańcza region until the final decay of the ice sheet.     

Recent foraminifera in shelf sediments of the Scoresby Sund fjord, East Greenland

A study of sea-bottom samples from shallow waters in the northern part of Scoresby Sund, off Jameson Land, East Greenland, reveals four different foraminiferal assemblages between 1.3 and 35.6 m water depth. The shallowest samples. Assemblage SA (1.3–2.2 m), contain only very few foraminifera, presumably due to the annual repeated formation of an ice foot. Assemblage SB (3.7 and 4.3 m) is characterized by the calcareous species Haynesina nivea and H. orbiculare , while the arenaceous species Spiropleclammina biformis, Reophax arctica , and Recurvoides turbinatus dominate all the faunas of Assemblage SC (9.3–25.9 m depth). The change in fauna to the succeeding Portatrochammina bipolaris - Spiropleclammina biformis fauna (Assemblage SD, 29.6 and 35.6 m) presumably corresponds to the transition from the shallow Fjord water layer to the deeper and relatively more stable Polar water. Elphidiella hannai (Cushman & Grant) is recorded live for the first time in recent assemblages from the Atlantic Ocean.     

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

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) 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 the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m⁻² per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m⁻² were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m⁻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.     

Vertically resolved dust optical properties during SAMUM: Tinfou compared to Ouarzazate

Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions.     

Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006

Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9°N, –6.9°E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.