39 research outputs found
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Half a century of satellite remote sensing of sea-surface temperature
Sea-surface temperature (SST) was one of the first ocean variables to be studied from earth observation satellites. Pioneering images from infrared scanning radiometers revealed the complexity of the surface temperature fields, but these were derived from radiance measurements at orbital heights and included the effects of the intervening atmosphere. Corrections for the effects of the atmosphere to make quantitative estimates of the SST became possible when radiometers with multiple infrared channels were deployed in 1979. At the same time, imaging microwave radiometers with SST capabilities were also flown. Since then, SST has been derived from infrared and microwave radiometers on polar orbiting satellites and from infrared radiometers on geostationary spacecraft. As the performances of satellite radiometers and SST retrieval algorithms improved, accurate, global, high resolution, frequently sampled SST fields became fundamental to many research and operational activities. Here we provide an overview of the physics of the derivation of SST and the history of the development of satellite instruments over half a century. As demonstrated accuracies increased, they stimulated scientific research into the oceans, the coupled ocean-atmosphere system and the climate. We provide brief overviews of the development of some applications, including the feasibility of generating Climate Data Records. We summarize the important role of the Group for High Resolution SST (GHRSST) in providing a forum for scientists and operational practitioners to discuss problems and results, and to help coordinate activities world-wide, including alignment of data formatting and protocols and research. The challenges of burgeoning data volumes, data distribution and analysis have benefited from simultaneous progress in computing power, high capacity storage, and communications over the Internet, so we summarize the development and current capabilities of data archives. We conclude with an outlook of developments anticipated in the next decade or so
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Radiometric measurements of air-sea and air-ice temperature differences in the Arctic
The polar regions are considered to be particularly sensitive to climate change. The complex interactions between the surface and the overlying atmosphere are important aspects of the local heat budget, and through the atmospheric and oceanic general circulations, to global scales. The temperature difference between the surface and the lowest layer of the air is an important parameter in the surface heat budget, but difficult to measure, especially in conditions of mixed sea-ice and open water. Both surface temperature and near-surface air temperature can be determined radiometrically from measurements of the infrared emission spectra of the surface and atmosphere. The use of a Fourier-transform infrared spectroradiometer, the M-AERI (Marine-Atmospheric Emitted Radiance Interferometer) on ships can provide such data
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Cloud forcing of surface radiation in the North Water polynya during NOW '98
Measurements taken from the CCGS Pierre Radisson in the North Water Polynya in spring and summer 1998, are used to study the effects of clouds on the longwave and shortwave components of the Arctic surface radiation budget. The clear-sky "baseline" is provided by parametrizations taken from the literature, tuned to the clear-sky measurements taken during the cruise. The effects of clouds are examined in terms of cloud cover, cloud type, solar zenith angle and time of day. A diurnal signal in the cloud cover is found, with minimum values at local noon. The cloud forcing of the surface radiation was found to be mostly negative (i.e., clouds predominantly cool the surface when the sun is in the sky), with strong dependences on the solar zenith angle, cloud amount and whether the sun is obscured by clouds or is in clear sky; the influence of cloud type was secondary to these parameters
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AIRS radiance validation over ocean from sea surface temperature measurements
Demonstrates the accuracy of methods and in situ data for early validation of calibrated Earth scene radiances measured by the Atmospheric InfraRed Sounder (AIRS) on the Aqua spacecraft. We describe an approach for validation that relies on comparisons of AIRS radiances with drifting buoy measurements, ship radiometric observations and mapped sea surface temperature products during the first six months after launch. The focus of the validation is on AIRS channel radiances in narrow spectral window regions located between 800-1250 cm/sup -1/ and between 2500 and 2700 cm/sup -1/. Simulated AIRS brightness temperatures are compared to in situ and satellite-based observations of sea surface temperature colocated in time and space, to demonstrate accuracies that can be achieved in clear atmospheres. An error budget, derived from single channel, single footprint matchups, indicates AIRS can be validated to better than 1% in absolute radiance (equivalent to 0.5 K in brightness temperature, at 300 K and 938 cm/sup -1/) during early mission operations. The eventual goal is to validate instrument radiances close to the demonstrated prelaunch calibration accuracy of about 0.4% (equivalent to 0.2 K in brightness temperature, at 300 K and 938 cm/sup -1/)
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Oceanic fronts in coastal processes — Proceedings of a workshop held at the Marine Sciences Research Center, May 25–27, 1977
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Chapter 4 Meteorology and Atmosphere–Surface Coupling in and around Polynyas
Polynyas and the overlying atmosphere interact through a series of feedback mechanisms which impart a distinctive polar maritime character to the boundary layer over and downwind of the open water area. Enhanced turbulent fluxes across the ice-free interface introduce heat and moisture into the otherwise cold, dry polar atmosphere, modifying clouds through plume formation and radiative exchanges between the atmosphere and underlying surface. Anthropogenic aerosols of remote origin and local biogenic emissions provide additional direct and indirect radiative forcing, which may also influence precipitation rates, cloud optical depth, and ozone concentration. These combined effects modulate the efficacy of polar regions' ability to act as a “heat sink” for the climate system, establishing a link between the regional polynya meteorology and global conditions. Models, gridded analyses, and remotely-sensed and validating measurements which describe the meteorology and feedback mechanisms in and around polynyas are discussed in this chapter, with an outlook toward future efforts and novel measurement and analytical techniques
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Analysis of mesoscale thermoclinicity with an example from the tropical thermocline during GATE
A method for the extension of isopycnal analysis to the mesoscale is described and illustrated with a map of the variation of vertical separation between intersecting isothermal and isopycnal surfaces at a mean depth of 41 m in the tropical Atlantic. Data were collected from a Brown conductivity-temperature-depth recorder (CTD) mounted in a Hermes Batfish that undulated with vertical speed of approximately 1 m s
−1 while being towed at approximately 4 m s
−1. Used in this way, the CTD sensors exhibit significant errors arising from thermal lags that were only partially corrected during data processing after the cruise. Estimates of the residual errors due to these lags suggest that the uncertainty in the spacing between isothermal and isopycnal surfaces would be less than ± 15 cm, which is comparable with the standard deviation of the measured spacing of the surfaces selected for purposes of illustration. However, evidence of spatial coherence of features in adjacent legs of the map suggests that the distributions are environmental rather than an instrumental artifact. To be consistent with this conclusion, the uncertainty in the measured spacing would have to be less than ±5 cm. Further evidence in favour of this view is provided by a study of the distribution of multiple incidences of the chosen density surface, which are believed to represent encounters with billows. These occur at locations of enhanced horizontal gradient of static stability, i.e. of enhanced vertical geostrophic current shear. The major feature in the map is a mesoscale tongue of disturbed water in which the spacing between the isothermal and isopycnal surfaces fluctuates by more than 1 m over horizontal distances of less than 500 m. Billows are encountered 13 times more frequently in this disturbed tongue than in the surrounding undisturbed water.
Mesoscale fluctuations of thermoclinicity limit the accuracy of internal wave investigations based on measurements of temperature or sound speed distributions, but they offer a possible signal for the investigation of turbulent motions on scales where internal waves dominate the velocity fluctuations in current meter records. The technique of isopycnal analysis described in this paper is being used to analyse this signal as recorded in the large data set (comprising over 10,000 Batfish-CTD profiles) collected by R.R.S.
Discovery during GATE
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Determination of sea surface emissivity and thermal skin layer depth using infrared interferometry
Sea surface spectral emissivity and the depth of the thermal skin boundary layer were determined using high spectral resolution measurements of the sea surface and the atmosphere from field measurements from the Marine-Atmosphere Emitted Radiance Interferometer. In order to determine the sea surface emissivity, the effective incidence angle was found by minimizing the variance in the brightness temperature spectrum retrieved from the corrected upwelling radiance spectrum. Certain wavelength regions have different absorption characteristics, allowing the temperature at different levels to be retrieved from different spectral regions. In this way, the temperature gradient of the thermal boundary layer was determined. The depth of the skin layer was then calculated by determining the depth at which the thermometrically measured bulk temperature intersects this gradient. At low wind speeds, the skin layer can be up to 0.2 mm deep, getting shallower with increased wind speed and becoming very shallow (0.01-0.07 mm) above wind speeds of 8 m s/sup -1/. These results are encouraging for application of this method to determine air-sea heat and gas fluxes in the field