Radiometric and Spectral Characteristics of the ScaRaB-3 Instrument on Megha-Tropiques: Comparisons with ERBE, CERES, and GERB

International audienceThe Indian-French Megha-Tropiques mission, scheduled to be launched in 2010, will carry radiation and microwave sensors to study the energy and water cycle in the tropics. The radiation sensor, the third model of the Scanner for Radiation Budget (ScaRaB-3), is dedicated to the earth's radiation budget, the difference between the solar absorbed flux and the terrestrial emitted flux. These fluxes are calculated from satellite measurements of outgoing shortwave (SW) and longwave (LW) radiances using angular distribution models (ADMs). For practical reasons, the LW radiation is calculated from the difference between a total (T) channel (0.2-100 mu m) and an SW channel (0.2-4 mu m). With the ADM application, the radiance calibration remains the most critical issue in the radiation budget estimation. The 1% accuracy goal is difficult to achieve, specifically in the SW domain. The authors explain their efforts to improve the radiometric calibration of ScaRaB-3. The internal calibration module is improved: the sensor is switched between SW and T channels by rotating the filter wheel on which the SW filter is now installed. Because the pyroelectric detector is sensitive to the thermal effect of the electromagnetic radiation independently of its spectral range, this plan allows calibrating the SW channel as a T channel by viewing a blackbody. Indeed, the transfer of the T calibration to the SW domain requires perfect knowledge of the total spectral response and of the transmittance of the SW filter, which is discussed in the article. Spectral errors are calculated with updated data. In the SW domain, they are found to be the smallest compared to those of the Earth Radiation Budget Experiment (ERBE), the Clouds and the Earth's Radiant Energy System (CERES), and the Geostationary Earth Radiation Budget (GERB)

Observation of the Earth's radiation budget from space

International audienc

Planetary Radiation Budgets

International audienceThe energy state of a planet depends fundamentally on its radiation budget. Measurements made from space over past decades have led to significant revisions of ground-based estimates, both of the reflected fraction (the Bond albedo) of solar radiative flux and of the emitted thermal infrared radiation flux, for the Earth as well as for the other planets. After a brief survey of methods and difficulties in accurately determining planetary radiation budgets, we note contradictions in existing tabulations of global parameters, in particular Bond albedo. For the Earth, such contradictions are unjustified, considering that global and annual means as well as the seasonal cycle of Earth Radiation Budget components have now been determined with high accuracy. The Earth's Bond albedo is close to 0.3. Net storage of energy in the Earth-ocean system is close to zero, with a well-established annual cycle of amplitude close to ±12 Wm-2. Some contradictions remain for the other terrestrial planets. For the giant planets, modern reduced values of the Bond albedo imply reduced but still significant internal energy generation

Restitutions et analyse des paramètres climatiques mesurés par satellites sur l'Afrique et l'Océan Atlantique pour les deux dernières décennies (Tendances et variabilité des flux radiatifs en liaison avec les autres paramètres)

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Monthly means of reflected solar flux from POLDER (ADEOS-1) and comparison with ERBE, ScaRaB and CERES

International audienceA diurnal cycle model is applied to POLDER instantaneous albedo determinations (Nov. 1996-June 1997) in order to compute daily and monthly means of the reflected flux at the top of the atmosphere. These results are compared to the ERBE, ScaRaB and CERES records. The quantitative comparison of the tropical means shows that the POLDER reflected flux density is on average lower by about 7 Wm−2 compared to ERBE (1985-1989) and ScaRaB-1 (1994-1995) and by 2.7 Wm−2 compared to ScaRaB-2 (Nov. 1998-March 1999) and CERES/Terra (2000-). The maps of POLDER albedo reveal strong deviations in the tropical Pacific. They correspond to the beginning of the 1997-1998 ENSO event

Télédétection par satellite du matériel particulaire en suspension en Manche orientale

Visible, infra-red and micro-wave satellite images emphasize important features of oceanography, sometimes unknown. As an exemple, The Coastal Zone Color Scanner reveals the spatial distribution of suspended matter in the Eastern Channel. These images require calibration works at sea, which are also presented.Les images visibles, infrarouges et micro-ondes des satellites dévoilent de grands traits souvent méconnus de l'océanographie. C'est le cas de la répartition des matières en suspension en Manche Orientale, perçue par le Coastal Zone Color Scanner. Ces images appellent des travaux de calibration à la mer, qui sont également présentés.Viollier Michel, Dupont Jacques, Balois J.Y. Télédétection par satellite du matériel particulaire en suspension en Manche orientale. In: Hommes et Terres du Nord, 1985/3. Télédétection. pp. 230-233

Analyse comparative des réponses spectrales dans le domaine optique des cibles d'un marais tropical à diverses résolutions spatiales et spectrales

Spectral signatures of the same tropical salt marsh targets, adquired on the ground (Sept 20th 1986) and from altitude (helicopter survey sept 17th 1986) with three looking angles are compared to the SPOT HRV data of sept. 18th 1986 and correlated to some physical properties of the barren salt flats. Those observations give coherent results but, on dense vegetation the satellite response appears under estimated when, on barren flats, it appears over estimated. An atmospheric environmental effect explanation is proposed. Significant correlation between vegetation and brightness and humidity indexes and organic matter of soils are found.Les réponses spectrales des mêmes cibles d'un marais salé tropical, acquises sur le terrain le 20 septembre 1986 et par un survol hélicoptère le 17 septembre 1986 avec 3 angles différents de prise de vue sont comparées à celles de l'acquisition SPOT HRV du 18 septembre 1986 et corrélées à plusieurs propriétés physiques des sols nus de ce marais salé. Ces observation offrent des résultats cohérents mais, au dessus de la végétation dense, la réponse du satellite apparait sous estimée alors qu'au dessus des sols nus elle est surestimée. L'explication de l'influence d'un effet environnemental atmosphérique est avancée. Des corrélations entre les indices de brillance des sols et d'humidité avec la teneur des sols en matière organique sont mises en évidence. Résumé

Megha-Tropiques : un satellite hydrométéorologique franco-indien

Une forte variabilité caractérise les systèmes météorologiques qui conditionnent les échanges d'énergie et d'eau dans les tropiques. L'objectif de Megha-Tropiques est de mesurer avec une bonne répétitivité des observables liés aux flux radiatifs, à la vapeur d'eau, aux précipitations. L'orbite du satellite a une inclinaison de 20 degrés sur l'équateur, et la charge utile comporte trois instruments : un imageur hyperfréquences pour les nuages et la pluie (Madras), un sondeur hyperfréquences de vapeur d'eau (Saphir) et un instrument à bandes larges destiné à la mesure des flux radiatifs (Scarab). Les caractéristiques de ces instruments et de l'échantillonnage sont présentées, ainsi que les grandes lignes du traitement et de l'utilisation envisagé des données. Le lancement de ce satellite est prévu à la fin de l'année 2009.Tropical systems driving water and energy exchanges present a large space-time variability. Megha-Tropiques aims to measure with a high repetitivity radiances linked to radiative fluxes, water vapour and precipitation. The satellite orbit has an inclination of 20 degrees and the payload is composed by three instruments: a microwave imager for clouds and rain (Madras), a microwave sounder for water vapour (Saphir) and a wide band instrument for radiative fluxes (Scarab). These instruments and the sampling are presented, as well as the leading principles of the data processing and use. The launch is foreseen in the end of 2009

An improved derivation of the top-of-atmosphere albedo from POLDER/ADEOS-2: 2. Broadband albedo

International audienceThe narrowband albedos derived from Polarization and Directionality of Earth Reflectances (POLDER) measurements have been described in a companion paper (Buriez et al., 2005). Here, they are used to estimate the broadband shortwave albedo. Except for the gaseous absorption, the albedos at 443 and 670 nm are considered as representative of the UV-visible range, and the albedo at 865 nm is considered as representative of the near infrared. The gaseous absorption is estimated from TOMS data and from the POLDER 910- to 865-nm reflectance ratio. In a previous approach, the respective weights of the three narrowband albedos were based on radiative transfer simulations. Now, we take advantage of spatiotemporal coincidences between the second Advanced Earth Observing Satellite (ADEOS-2) and Terra to adjust these weights from the comparison between POLDER narrowband and CERES broadband reflectances. With no adjustment, the POLDER albedos are underestimated by 2% (in relative value) whereas the associated reflectances are underestimated by 4%. With an adjustment to the CERES reflectances, the POLDER shortwave albedos are overestimated by 2%. With or without adjustment the RMS difference between the POLDER and the CERES 1°-regional instantaneous shortwave albedos is 6%; this is quite satisfactory since it is comparable to the CERES SW albedo consistency between nadir and oblique viewing zenith angles. When considering only homogeneous areas, the agreement between POLDER and CERES estimates was rather less good in the previous approach; it is now more than twice better. The difference between POLDER and CERES shortwave albedo estimates appears, for a large part, due to our plane-parallel assumption that is crucial for the heterogeneous liquid water clouds. It results in a bias in monthly mean shortwave flux around 2 W m−2 that is found to present only small variations in latitude

Direct Comparisons of Radiances Measured by Independent Contemporary ERB Instruments

Comparisons of radiance measurements from overlapping independent Earth and cloud radiation budget (ERB) missions are an important contribution to the validation process of the these missions and are essential to the construction of a consistent long-term record of ERB observations. Measurements from the CERES instrument on TRMM are compared to ScaRaB on Resurs (Jan-Mar 1999) and CERES on Terra (Mar-Apr 2000)