Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean

International audienceRetrieved aerosol optical thickness (AOT) based on data obtained by the Sea viewing Wide Field Sensor (SeaWiFS) is combined with surface wind speed, obtained at the European Centre for Medium-Range Weather Forecasts (ECMWFs), over the North Pacific for September 2001. In this study a cloud screening approach is introduced in an attempt to exclude pixels partly or fully covered by clouds. The relatively broad swath width for which the nadir looking SeaWiFS instrument scanned over the North Pacific means that the AOT can be estimated according to relatively large range of wind speeds for each of the scenes analyzed. The sensitivity in AOT due to sea salt and hygroscopic growth of the marine aerosols has also been investigated. The validation of the results is based on previous parameterization in combination with the environmental quantities wind speed, RH and boundary layer height (BLH), estimated at the ECMWF. In this study a factor of 2 higher mean AOT is obtained for a wind speed up to about 13 m s?1 for September 2001 over remote ocean areas. Furthermore, a factor of 2 higher AOT is more or less supported by the validation of the results. Approximately, 50% of the enhancement seems to be due to hygroscopic growth of the marine aerosols and the remaining part due to increase in the sea salt particle mass concentrations, caused by a wind driven water vapor and sea salt flux, respectively. Reasonable agreement occurs also between satellites retrieved aerosol optical thickness and AOT observed at several AERONET (Aerosol Robotic NETwork) ground-based remote sensing stations. Finally, possible reasons why relatively large standard deviations occur around the mean values of AOT estimated for a single scene are discussed

Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis

[1] Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip

Near-Bottom Observations of the Middle America Trench off Guatemala

Deep-tow data across the Middle America Trench and the lower part of the landward Trench slope off Guatemala provide high-resolution information on the geological setting of four Leg 67 drill sites. Our 6-kHz profiler data resolve stratified sediments seaward of the Trench floor, which we interpret as deposits of Trench turbidites that flowed part way up the seaward slope of the Trench. Reflectors in the Trench fill are laterally continuous and relatively undisturbed. The Trench is segmented into diamond-shaped basins by ridges that intersect it at oblique angles. The acoustic stratigraphy in the Trench fill in these sub-basins is different on either side of an oblique ridge. A mound of variable height and width occurs at the base of the lower Trench slope; the mound could be either deformed Trench strata or an accumulation of debris that slumped from the Trench slope. The lower part of the landward Trench slope is steep and is linear along strike. There is no evidence on our 6-kHz or side-scan sonar data of large gravity slides in this area. Slope sediments are ponded on a bench along the landward Trench slope. They are acoustically stratified, but are not as continuous laterally as are the Trench strata. The structure parallel to the strike of the Trench slope is variable, with many small-scale folds and faults

Jean Charcot Seabeam Survey along ODP Leg 112 Northern Transect

A marine geophysical survey that used Seabeam, multi- and single-channel seismic reflection, gravity, and magnetic profiling was conducted in two locations along the landward slope of the Peru Trench. This survey was conducted during the SeaPERC cruise of the Jean Charcot in July 1986 (Bourgois et al., 1986a; Bourgois et al., 1986b; Bourgois et al., 1987; Bourgois et al., in press). These areas were two possible drilling locations for ODP Leg 112, which was scheduled to begin in November 1986. The drilling transects center around latitudes of 5°30'S and 9°30'S, along multichannel seismic lines CDP-3 and CDP-2, respectively. The northern transect is located in the Paita Zone (Fig. 1, Box 2), and the southern transect is in the Chimbote Zone (Fig. 1, Box 6). During Leg 112, we drilled two sites (683 and 685) in the Chimbote area. This became the northern transect of Leg 112. Here, we report the Seabeam data acquired during the SeaPERC cruise. Drilling results from Sites 683 and 685, the CDP-2 multichannel seismic record, and the Seabeam data provide a threedimensional view of this region

Seabeam and seismic reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S)

Suite à une campagne géophysique réalisée au large de la côte du Perou (croisière Seaperc du R/V "Jean Charcot", juillet 1986), les auteurs proposent une nouvelle interprétation des structures caractérisant la pente continentale de la région étudiée. D'autre part, ils considèrent que cette marge active est une marge active en extension ou bien une marge d'effondrement qui développe un complexe d'accrétion induit par les effondrements de la partie médiane de la pente

The semianalytical cloud retrieval algorithm for SCIAMACHY II. The application to MERIS and SCIAMACHY data

International audienceThe SemiAnalytical CloUd Retrieval Algorithm (SACURA) is applied to the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) data. In particular, we derive simultaneously cloud optical thickness (COT) and cloud top height (CTH), using SCIAMACHY measurements in the visible (442 nm, COT) and in the oxygen A-band (755?775 nm, CTH). Some of the results obtained are compared with those derived from the Medium Resolution Imaging Spectrometer (MERIS), which has better spatial resolution and observes almost the same scene as SCIAMACHY. The same cloud algorithm is applied to both MERIS and SCIAMACHY data. In addition, we perform the vicarious calibration of SCIAMACHY at the wavelength 442 nm, using MERIS measurements at the same wavelength. Differences in the retrieved COT for the same cloud field obtained using MERIS and SCIAMACHY measurements are discussed

Arctic smoke - aerosol characteristics during a record smoke event in the European Arctic and its radiative impact

In early May 2006 a record high air pollution event was observed at Ny-Ålesund, Spitsbergen. An atypical weather pattern established a pathway for the rapid transport of biomass burning aerosols from agricultural fires in Eastern Europe to the Arctic. Atmospheric stability was such that the smoke was constrained to low levels, within 2 km of the surface during the transport. A description of this smoke event in terms of transport and main aerosol characteristics can be found in Stohl et al. (2007). This study puts emphasis on the radiative effect of the smoke. The aerosol number size distribution was characterised by lognormal parameters as having an accumulation mode centered around 165–185 nm and almost 1.6 for geometric standard deviation of the mode. Nucleation and small Aitken mode particles were almost completely suppressed within the smoke plume measured at Ny-Ålesund. Chemical and microphysical aerosol information obtained at Mt. Zeppelin (474 m a.s.l) was used to derive input parameters for a one-dimensional radiation transfer model to explore the radiative effects of the smoke. The daily mean heating rate calculated on 2 May 2006 for the average size distribution and measured chemical composition reached 0.55 K day−1 at 0.5 km altitude for the assumed external mixture of the aerosols but showing much higher heating rates for an internal mixture (1.7 K day−1). In comparison a case study for March 2000 showed that the local climatic effects due to Arctic haze, using a regional climate model, HIRHAM, amounts to a maximum of 0.3 K day−1 of heating at 2 km altitude (Treffeisen et al., 2005)

Hochschulisches Lernen – eine analytische Perspektive

Dieser Beitrag betrachtet hochschulisches Lernen unter einer analytischen Perspektive: Danach artikuliert sich hochschulisches Lernen im institutionellen Kontext der Hochschule mit ihren studiengangstrukturellen Rahmenbedingungen in einer je spezifischen, dabei stets kontingenten und polymorphen Koppelung zwischen Wissenschaft als Ort der Wissensproduktion einerseits und Studium als Ort der Erschliesung von Sinn- und Handlungsressourcen andererseits. Hochschulisches Lernen lasst sich als Moglichkeitsraum von Figurationen dieser Bezugnahmen rekonstruieren. Die Analyse des Lernens an Hochschulen erschliest auserdem Forschungsperspektiven einer bildungswissenschaftlichen Hochschulforschung und einer theoretischen Hochschuldidaktik, zu denen die Erwachsenenbildungswissenschaft substanzielle Beitrage liefern kann

Validation of SCIAMACHY top-of-atmosphere reflectance for aerosol remote sensing using MERIS L1 data

Aerosol remote sensing is very much dependent on the accurate knowledge of the top-of-atmosphere (TOA) reflectance measured by a particular instrument. The status of the calibration of such an instrument is reflected in the quality of the aerosol retrieval. Current data of the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument (operated with the data processor version 5 and earlier) give too small values of the TOA reflectance, compared e.g. to data from MERIS (Medium Resolution Imaging Spectrometer), both operating on ENVISAT (ENVIronmental SATellite). This effect causes retrievals of wrong aerosol optical thickness and disables the processing of aerosol parameters. <br><br> From an inter-comparison of MERIS and SCIAMACHY TOA reflectance, for collocated scenes correction factors are derived to improve the insufficient SCIAMACHY L1 data calibration for data obtained with the processor 5 for the purpose of aerosol remote sensing. The corrected reflectance has been used for tests of remote sensing of the aerosol optical thickness by the BAER (Bremen AErosol Retrieval) approach using SCIAMACHY data

Corrigendum to "Trend analysis of aerosol optical thickness and Ångström exponent derived from the global AERONET spectral observations" published in Atmos. Meas. Tech., 5, 1271–1299, 2012

No abstract available