Nicolas Clerbaux

The Geostationary Earth Radiation Budget Project

This paper reports on a new satellite sensor, the Geostationary Earth Radiation Budget (GERB) experiment. GERB is designed to make the first measurements of the Earths radiation budget from geostationary orbit. Measurements at high absolute accuracy of the reflected sunlight from the Earth, and the thermal radiation emitted by the Earth are made every 15 min, with a spatial resolution at the subsatellite point of 44.6 km (north–south) by 39.3 km (east–west). With knowledge of the incoming solar constant, this gives the primary forcing and response components of the top-of-atmosphere radiation. The first GERB instrument is an instrument of opportunity on Meteosat-8, a new spin-stabilized spacecraft platform also carrying the Spinning Enhanced Visible and Infrared (SEVIRI) sensor, which is currently positioned over the equator at 3.5°W. This overview of the project includes a description of the instrument design and its preflight and in-flight calibration. An evaluation of the instrument performance after ...

The EarthCARE Satellite: The Next Step Forward in Global Measurements of Clouds, Aerosols, Precipitation, and Radiation

AbstractThe collective representation within global models of aerosol, cloud, precipitation, and their radiative properties remains unsatisfactory. They constitute the largest source of uncertainty in predictions of climatic change and hamper the ability of numerical weather prediction models to forecast high-impact weather events. The joint European Space Agency (ESA)–Japan Aerospace Exploration Agency (JAXA) Earth Clouds, Aerosol and Radiation Explorer (EarthCARE) satellite mission, scheduled for launch in 2018, will help to resolve these weaknesses by providing global profiles of cloud, aerosol, precipitation, and associated radiative properties inferred from a combination of measurements made by its collocated active and passive sensors. EarthCARE will improve our understanding of cloud and aerosol processes by extending the invaluable dataset acquired by the A-Train satellites CloudSat, Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and Aqua. Specifically, EarthCARE’s c...

Outgoing longwave flux estimation: improvement of angular modelling using spectral information

A radiance-to-flux conversion is needed to estimate radiative fluxes at the top of the atmosphere from directional measurements made by broadband (BB) radiometers on satellites. Such a conversion is known to be one of the major sources of error in the resulting instantaneous shortwave and longwave fluxes. This paper analyzes the possibility to improve the radiance-to-flux conversion for the longwave radiation when spectral information about the radiation is available through a set of narrow-band (NB) measurements. The study is based on a database of spectral radiance fields at the top of the atmosphere built using radiative transfer computation. The analysis of this database shows that there exists a certain degree of correlation between the angular and the spectral behaviors of the radiation field. According to the type and the accuracy of the spectral information, this correlation allows a 25–55% reduction of the error introduced by the radiance-to-flux conversion with respect to a simple model that uses only broadband information. The method discussed in this paper might be used when broadband radiometer and spectral imager data are available together like the combination of Geostationary Earth Radiation Budget (GERB) and Spinning Enhanced Visible and Infrared Radiometer Imager (SEVIRI) or the combination of CERES and MODIS.

Unfiltering of the Geostationary Earth Radiation Budget (GERB) Data. Part II: Longwave Radiation

Abstract The method used to estimate the unfiltered longwave broadband radiance from the filtered radiances measured by the Geostationary Earth Radiation Budget (GERB) instrument is presented. This unfiltering method is used to generate the first released edition of the GERB-2 dataset. This method involves a set of regressions between the unfiltering factor (i.e., the ratio of the unfiltered and filtered broadband radiances) and the narrowband observations of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument. The regressions are theoretically derived from a large database of simulated spectral radiance curves obtained by radiative transfer computations. The generation of this database is fully described. Different sources of error that may affect the GERB unfiltering have been identified and the associated error magnitudes are assessed on the database. For most of the earth–atmosphere conditions, the error introduced during the unfiltering processes is well under 0.5% (RMS error of abo...

Estimation of the 2002 Mount Etna eruption cloud radiative forcing from Meteosat-7 data

Abstract On 27th October 2002, after 15 months of small activity, the Mount Etna located on the island of Sicily, Italy (37.73°N, 15.00°E) erupted undergoing one of its most vigorous eruptions in years. During a few days, Europes highest and most active volcano hurled lava and spewed significant amounts of ash and trace gases into the atmosphere. The smoke and ash plume originating from the volcano stretched from the Sicily to the north African coast. The plume from the volcano has been identified and tracked using half-hourly Meteosat-7 visible, infrared, and water vapor imagery in order to estimate the radiative forcing produced by the introduction of the volcanic cloud in a previously clear sky. Our results indicate that, while the volcanic cloud has introduced a well-discernable radiative perturbation, the magnitude of the shortwave volcanic forcing appears in the range of the one introduced by large clouds above the Mediterranean Sea. By contrast, the perturbation generated in the longwave spectrum at the top of the atmosphere is larger than the one introduced by large meteorological clouds.

The CM SAF TOA Radiation Data Record Using MVIRI and SEVIRI

The CM SAF Top of Atmosphere (TOA) Radiation MVIRI/SEVIRI Data Record provides a homogenised satellite-based climatology of TOA Reflected Solar (TRS) and Emitted Thermal (TET) radiation in all-sky conditions over the Meteosat field of view. The continuous monitoring of these two components of the Earth Radiation Budget is of prime importance to study climate variability and change. Combining the Meteosat MVIRI and SEVIRI instruments allows an unprecedented temporal (30 min/15 min) and spatial (2.5 km/3 km) resolution compared to, e.g., the CERES products. It also opens the door to the generation of a long data record covering a 32 years time period and extending from 1 February 1983 to 30 April 2015. The retrieval method used to process the CM SAF TOA Radiation MVIRI/SEVIRI Data Record is discussed. The overlap between the MVIRI and GERB instruments in the period 2004–2006 is used to derive empirical narrowband to broadband regressions. The CERES TRMM angular dependency models and theoretical models are respectively used to compute the TRS and TET fluxes from the broadband radiances. The TOA radiation products are issued as daily means, monthly means and monthly averages of the hourly integrated values (diurnal cycle). The data is provided on a regular grid at a spatial resolution of 0.05 degrees and covers the region 70 ∘ N–70 ∘ S and 70 ∘ W–70 ∘ E. The quality of the data record has been evaluated by intercomparison with several references. In general, the stability in time of the data record is found better than 4 Wm − 2 and most products fulfill the predefined accuracy requirements.

Method for comparison of GERB and CERES radiances

The Geostationary Earth Radiation Budget (GERB) instrument aboard the Meteosat Second Generation Satellite has 256 channels which measure total radiance and 256 channels which measure solar radiation reflected from the Earth. In order to validate the calibration of these channels, the Clouds and Earth Radiant Energy System (CERES) instrument aboard the Terra spacecraft is operated in such a way as to view Earth scenes from the same direction as the GERB, so as to measure the same total and reflected solar radiances. The method uses the capability to program the azimuth of the CERES scan plane, such that the scan plane includes the GERB.

Angular distribution models, anisotropic correction factors, and mixed clear-scene types: a sensitivity study

Because radiometers do not measure the earths outgoing flux directly, angular distribution models (ADMs) are used to invert measured radiances at the top of atmosphere (TOA) to flux. However, data used to build ADMs are generally not sorted for mixed scene types, and anisotropic correction factors for such scenes are not usually available. In the present study, we have analyzed shortwave (SW) flux values retrieved over nine areas representative of a junction between two different ADM scene types in the Meteosat-7 field of view. The Clouds and the Earths Radiant Energy System (CERES) broadband SW ADMs were used to perform the radiance-to-flux conversion. Because of the large anisotropy difference that can exist between ADMs, use of the ADM that corresponds to the scene type with the highest percent coverage over footprints containing a mixture of scene types generates instantaneous as well as systematic errors in the retrieved SW flux values. Nevertheless, in the absence of available mixed scene type ADMs, we show that the CERES on the Tropical Rainfall Measuring Mission satellite SW ADMs can be combined together to provide reliable mixed scene types anisotropic correction factors. The use of such anisotropic factors appears to be especially well suited along the coastline of continents.

Spectral Aging Model Applied to Meteosat First Generation Visible Band

The Meteosat satellites have been operational since the early eighties, creating so far a continuous time period of observations of more than 30 years. In order to use this data for climate data records, a consistent calibration is necessary between the consecutive instruments. Studies have shown that the Meteosat First Generation (MFG) satellites (1982–2006) suffer from in-flight degradation which is spectral of nature and is not corrected by the official calibration of EUMETSAT. Continuing on previous published work by the same authors, this paper applies the spectral aging model to a set of clear-sky and cloudy targets, and derives the model parameters for all six MFG satellites (Meteosat-2 to -7). Several problems have been encountered, both due to the instrument and due to geophysical occurrences, and these are discussed and illustrated here in detail. The paper shows how the spectral aging model is an improvement compared to the EUMETSAT calibration method with a stability of 1%–2% for Meteosat-4 to -7, which increases up to 6% for ocean sites using the full MFG time period.

Evidence of pre-launch characterization problem of Meteosat-7 visible spectral response

Since the early 1980s, the geostationary Meteosat instruments have been observing the earth at a high temporal and spatial resolution. The data record contains more than 30 years of observations and has become very useful for climate monitoring. With a compilation of 10 satellites, it is crucial to reach the highest possible consistency between the instruments. However, the quality of the level 1 data records is still far from what is needed to generate homogeneous climate data records, due to both temporal ageing as well as discontinuities between successive instruments. In this letter, a method is proposed to assess the pre-launch Meteosat-7 visible spectral response curve and a model of its ageing. The results not only validate the ageing model, but also provide evidence that the official pre-launch spectral response curve of Meteosat-7 poorly represents the sensitivity of the instrument in the blue part of the visible spectrum. Until the Meteosat-7 visible spectral response curve has been reassessed, it is suggested to replace the Meteosat-7 curve with the curve of the high resolution visible channel of Meteosat-8. Quantitatively, the error introduced when unfiltering the data (i.e. conversion from filtered to unfiltered radiance), due to the uncertainty on the Meteosat-7 spectral response, is estimated at 4.5% using the Meteosat-7 spectral response curve, but could be reduced to 2.1% by following the recommendations in this letter.