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Jürgen Fischer and Friedrich A. Schott

Abstract

Fifteen profiling floats were injected into the deep boundary current off Labrador. They were ballasted to drift in the core depth of Labrador Sea Water (LSW) at 1500-m depth and were deployed in two groups during March and July/August 1997. Initially, for about three months, the floats were drifting within the boundary current, and the flow vectors were used to determine the mean horizontal structure of the Deep Labrador Current, which was found to be about 100 km wide with an average core speed of 18 cm s−1. North of Flemish Cap the boundary current encounters complicated topography around “Orphan Knoll,” and there the LSW outflow splits up into different routes. One obvious LSW path is eastward through the Charlie Gibbs Fracture Zone and another route is a narrow recirculation toward the central Labrador Sea. A surprising result was that none of the floats were able to follow the boundary current southward to the Grand Banks area and exit into the subtropics. Trajectories and temperature profiles of the eastward drifting floats indicate the importance of the North Atlantic Current for dispersing the floats, even at the level of LSW.

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Martina Kästner, Herbert Fischer, and Hans-Jürgen Bolle

Abstract

Temperature Humidity Infrared Radiometer (THIR) data of adjacent Nimbus 5 orbits have been used to derive winds in the upper troposphere from the radiance measured in a wide spectral interval within the 6.3 μm water vapor band. The radiation in this spectral channel stems primarily from water vapor emission at ∼400 mb and/or emission of high clouds. The horizontal resolution at the subsatellite point is ∼23 km.

For this purpose the digital data in overlapping swaths of consecutive orbits first have been enhanced to detect clear contours using the gradient method and the algorithm of Smith and Davis. Wind vectors computed from the displacements of the contours agree well with conventional data. The rms deviation between the derived wind velocities and the radiosonde data is 5–6 m s−1 for a mean wind speed of 30 m s−1. The rms deviation of the wind direction is about 21°.

Systematic deviations can be explained by interfering cirrus clouds and by time differences between radiosonde and satellite measurements. The method is especially limited by the low spatial resolution of the THIR. The conditions can be improved by using Meteosat data available now.

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Gerhard Peters, Bernd Fischer, and Hans Jürgen Kirtzel

Abstract

A Doppler sodar and an ultrasonic anemometer–thermometer, which are operated continuously at a nuclear research center in Germany, are described. Mean values and second moments including turbulent surface fluxes recorded over 1 yr are analyzed in order to assess their credibility. Particularly, the effects of rain on the data quality and the statistical bias caused by a reduced data availability—typical for remote sensing measurements—are investigated. Mean profiles of the vertical wind variance for different classes of Monin–Obukhov length, the mean vertical wind component versus wind direction, and the annual variation of 24-h averages of the surface heat flux are presented.

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Peter Brandt, Angelo Rubino, and Jürgen Fischer

Abstract

The analysis of high-resolution oceanographic data referring to velocity measurements carried out by means of a vessel-mounted acoustic Doppler current profiler on 12 November 2000 in the equatorial Atlantic, at 44°W between 4.5° and 6°N, reveals the presence of three large-amplitude internal solitary waves superimposed on the velocity field associated with the North Equatorial Countercurrent (NECC). These waves were found in the deep ocean, more than 500 km off the continental shelf and far from regions of topographic variations. They propagated toward the north-northeast, strongly inclined with respect to the main axis of the NECC and perpendicular to the Brazilian shelf, as well as to the North Brazil Current, and were characterized by maximum horizontal velocities of about 2 m s−1 and maximum vertical velocities of about 20 cm s−1. The large magnitudes of the measured velocities indicate that the observed waves represent disturbances evolving in a strongly stratified ocean. The distance separating the waves (about 70 km) indicates that the observed features cannot be considered as elements of a single train of internal solitary waves. The waves consist, instead, of truly disconnected, pulselike intense solitary disturbances. This behavior, which strongly differs from that typically observed for trains of tidally generated internal solitary waves, indicates that different mechanisms were possibly involved in their generation and/or evolution.

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Friedrich A. Schott, Jürgen Fischer, and Lothar Stramma

Abstract

The mean warm water transfer toward the equator along the western boundary of the South Atlantic is investigated, based on a number of ship surveys carried out during 1990–96 with CTD water mass observations and current profiling by shipboard and lowered (with the CTD/rosette) acoustic Doppler current profiler and with Pegasus current profiler. The bulk of the northward warm water flow follows the coast in the North Brazil Undercurrent (NBUC) from latitudes south of 10°S, carrying 23 Sv (Sv ≡ 106 m3 s−1) above 1000 m. Out of this, 16 Sv are waters warmer than 7°C that form the source waters of the Florida Current. Zonal inflow from the east by the South Equatorial Current enters the western boundary system dominantly north of 5°S, adding transport northwest of Cape San Roque, and transforming the NBUC along its way toward the equator into a surface-intensified current, the North Brazil Current (NBC). From the combination of moored arrays and shipboard sections just north of the equator along 44°W, the mean NBC transport was determined at 35 Sv with a small seasonal cycle amplitude of only about 3 Sv. The reason for the much larger near-equatorial northward warm water boundary current than what would be required to carry the northward heat transport are recirculations by the zonal current system and the existence of the shallow South Atlantic tropical–subtropical cell (STC). The STC connects the subduction zones of the eastern subtropics of both hemispheres via equatorward boundary undercurrents with the Equatorial Undercurrent (EUC), and the return flow is through upwelling and poleward Ekman transport. The persistent existence of a set of eastward thermocline and intermediate countercurrents on both sides of the equator was confirmed that recurred throughout the observations and carry ventilated waters from the boundary regime into the tropical interior. A strong westward current underneath the EUC, the Equatorial Intermediate Current, returns low-oxygen water westward. Consistent evidence for the existence of a seasonal variation in the warm water flow south of the equator could not be established, whereas significant seasonal variability of the boundary regime occurs north of the equator: northwestward alongshore throughflow of about 10 Sv of waters with properties from the Southern Hemisphere was found along the Guiana boundary in boreal spring when the North Equatorial Countercurrent is absent or even flowing westward, whereas during June–January the upper NBC is known to connect with the eastward North Equatorial Countercurrent through a retroflection zone that seasonally migrates up and down the coast and spawns eddies. The equatorial zone thus acts as a buffer and transformation zone for cross-equatorial exchanges, but knowledge of the detailed pathways in the interior including the involved diapycnal exchanges is still a problem.

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Jürgen Fischer, Friedrich A. Schott, and Marcus Dengler

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The deep circulation and related transports of the southern Labrador Sea are determined from direct current observations from ship surveys and a moored current-meter array. The measurements covered a time span from summer 1997 to 1999 and show a well-defined deep boundary current extending approximately out to the 3300-m depth contour and weak reverse currents farther offshore. The flow has a strong barotropic component, and significant baroclinic flow is only found in the shallow Labrador Current at the shelf break and associated with a deep core of Denmark Strait Overflow Water. The total deep-water transport below σ Θ = 27.74 kg m−3 was 26 ± 5 Sv (Sv ≡ 106 m3 s−1) comprising Labrador Sea Water (LSW), Gibbs Fracture Zone Water (GFZW), and Denmark Strait Overflow Water (DSOW). Intraseasonal variability of the flow and transport was high, ranging from 15 to 35 Sv, and the annual means differed by 17%. A seasonal cycle is confined to the shallow Labrador Current; in its deeper part, where the mean flow is still strong, no obvious seasonality could be detected. The transport of the interior anticyclonic recirculation was estimated from lowered acoustic Doppler current profiler stations and geostrophy, yielding about 9 Sv. Thus, the net deep-water outflow from the Labrador Sea was about 17 Sv. The baroclinic transport of GFZW and DSOW referenced to the depth of the isopycnal σ Θ = 27.80 kg m−3 is only about one-third of the total transport in these layers. Longer-term variations of the total transports are not represented well by the baroclinic contribution.

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Rasmus Lindstrot, Rene Preusker, and Jürgen Fischer

Abstract

Spaceborne spectrometers like the Medium Resolution Imaging Spectrometer (MERIS) on board the Environmental Satellite (Envisat) are widely used for the remote sensing of atmospheric and oceanic properties and make an important contribution to the monitoring of the earth’s atmosphere system. To enable retrievals with high accuracy, the spectral and radiometric properties of the instruments have to be characterized with high precision. One of the main sources of radiometric errors is stray light caused by multiple reflections and scattering at the optical elements within the instruments. If not corrected for properly, the stray light–induced offsets of measured intensity can lead to significant errors in the derived parameters. The effect of stray light is particularly momentous in the case of measurements inside strong absorption bands like the oxygen A band at 0.76 μm or the ρστ absorption band of water vapor around 0.9 μm. For example, the retrieval of surface and cloud-top pressure from MERIS measurements in the O2 A band can be biased because of an insufficient correction of stray light in the operational processing chain.

To correct for the residual stray light influence after the operational stray light correction in the O2 A-band channel of MERIS, an empirical stray light correction of the measured radiance at 0.76 μm has been developed based on optimizing the coefficients of a simple brightness-dependent stray light model. The optimal model coefficients were found by adjusting the retrievals of surface and cloud-top pressure to accurate reference data for several selected scenes. To account for the limited accuracy of the MERIS spectral calibration, the center wavelength of the O2 A-band channel was additionally adjusted within a ±0.1-nm tolerance range. The correction was tested on a variety of clear and cloudy scenes at different locations by applying the surface and cloud-top pressure retrieval algorithms to data recorded over the whole lifetime of MERIS. The results indicate the potential to greatly improve the accuracy of the retrieved pressure values using the proposed correction factors.

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Rasmus Lindstrot, Rene Preusker, and Jürgen Fischer

Abstract

Measurements of the Medium-Resolution Imaging Spectrometer (MERIS) on the Environmental Satellite (Envisat) are used for the retrieval of surface pressure above land and ice surfaces. The algorithm is based on the exploitation of gaseous absorption in the oxygen A band at 762 nm. The strength of absorption is directly related to the average photon pathlength, which in clear-sky cases above bright surfaces is mainly determined by the surface pressure, with minor influences from scattering at aerosols.

Sensitivity studies regarding the influences of aerosol optical thickness and scale height and the temperature profile on the measured radiances are presented. Additionally, the sensitivity of the retrieval to the accuracy of the spectral characterization of MERIS is quantified. The algorithm for the retrieval of surface pressure (SPFUB) is presented and validated against surface pressure maps constructed from ECMWF sea level pressure forecasts in combination with digital elevation model data. The accuracy of SPFUB was found to be within 10 hPa above ice surfaces at Greenland and 15 hPa above desert and mountain scenes in northern Africa and southwest Asia. In a case study above Greenland the accuracy of SPFUB could be enhanced to be better than 3 hPa by spatial averaging over areas of 40 km × 40 km.

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Rasmus Lindstrot, Rene Preusker, and Jürgen Fischer

Abstract

A novel and unique algorithm for the retrieval of multilayer cloud-top pressure is presented, relying on synergetic observations of the Medium Resolution Imaging Spectrometer (MERIS) and Advanced Along Track Scanning Radiometer (AATSR) on board the Environmental Satellite (Envisat). The retrieval is based on the exploitation of the differing signals observed in the thermal infrared spectral region (AATSR) and the oxygen A band at 0.76 μm (MERIS). Past studies have shown that the cloud-top pressure retrieved from MERIS measurements is highly accurate in the case of low single-layered clouds. In contrast, in the presence of multilayered clouds like cirrus overlying water clouds, the derived cloud height is biased. In this framework, an optimal estimation algorithm for the correction of the measured O2 A transmission for the influence of the upper cloud layer was developed. The algorithm is best applicable in cases of optically thin cirrus (1 ≤ τ ≤ 5) above optically thick water clouds (τ > 5), as found frequently in the vicinity of convective or frontal cloud systems. The split-window brightness temperature difference technique is used for the identification of suitable cases. The sensitivities of the AATSR and MERIS measurements to multilayered clouds are presented and discussed, revealing that in the case of dual-layered clouds, the AATSR-derived cloud height is close to the upper cloud layer, even if it is optically thin. In contrast, the cloud height retrieved from MERIS measurements represents the optical center of the cloud system, which is close to the lower layer in cases where the upper layer is optically thin. Two case studies of convective, multilayered cloud systems above the northern Atlantic Ocean are shown, demonstrating the plausibility of the approach. The presented work is relevant especially in view of the upcoming Global Monitoring for Environment and Security Sentinel-3 satellite to be launched in 2012 that will carry the respective MERIS and AATSR follow-up instruments Ocean and Land Colour Instrument (OLCI) and Sea and Land Surface Temperature Radiometer (SLSTR).

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Lothar Schüller, Ralf Bennartz, Jürgen Fischer, and Jean-Louis Brenguier

Abstract

Algorithms are now currently used for the retrieval of cloud optical thickness and droplet effective radius from multispectral radiance measurements. This paper extends their application to the retrieval of cloud droplet number concentration, cloud geometrical thickness, and liquid water path in shallow convective clouds, using an algorithm that was previously tested with airborne measurements of cloud radiances and validated against in situ measurements of the same clouds. The retrieval is based on a stratified cloud model of liquid water content and droplet spectrum. Radiance measurements in visible and near-infrared channels of the Moderate Resolution Imaging Spectroradiometer (MODIS), which is operated from the NASA platforms Terra and Aqua, are analyzed. Because of uncertainties in the simulation of the continental surface reflectance, the algorithm is presently limited to the monitoring of the microphysical structure of boundary layer clouds over the ocean. Two MODIS scenes of extended cloud fields over the North Atlantic Ocean trade wind region are processed. A transport and dispersion model (the Hybrid Single-Particle Lagrangian Integrated Trajectory Model, HYSPLIT4) is also used to characterize the origin of the air masses and hence their aerosol regimes. One cloud field formed in an air mass that was advected from southern Europe and North Africa. It shows high values of the droplet concentration when compared with the second cloud system, which developed in a more pristine environment. The more pristine case also exhibits a higher geometrical thickness and, thus, liquid water path, which counterbalances the expected cloud albedo increase of the polluted case. Estimates of cloud liquid water path are then compared with retrievals from the Special Sensor Microwave Imager (SSM/I). SSM/I-derived liquid water paths are in good agreement with the MODIS-derived values.

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