L. Gonzalez

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.

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.

Angular distribution models anisotropic correction factors and sun glint: a sensitivity study

Because radiometers do not measure the Earths outgoing fluxes directly, angular distribution models (ADMs) are used to invert measured radiances at the top of atmosphere (TOA) to fluxes. In this study, we have investigated if the use of the newly developed clear ocean Clouds and the Earths Radiant Energy System (CERES) broadband short wave (SW) ADMs from the Tropical Rainfall Measuring Mission (TRMM) satellite will allow reliable estimation of the instantaneous reflected SW fluxes at the TOA when the measured radiances are contaminated by sun glint. Using SW fluxes estimations computed from Meteosat‐7 visible radiances as a surrogate of the forthcoming Geostationary Earth Radiation Budget SW fluxes, our results indicate that while CERES‐TRMM ADMs angular resolution presents an advance over the previously built Earth Radiation Budget Experiment (ERBE) ADMs, the angular resolution is still too coarse to suitably define anisotropic correction factors in the sunlight region. SW fluxes are overestimated in the strong sun glint region and underestimated in the surrounding regions. Nevertheless, we show that by combining the high temporal sampling of the sun glint regions afforded by the geostationary orbit of the instrument with information contained in the clear ocean wind‐speed‐dependent CERES‐TRMM ADMs, an improved estimation of the reflected SW flux at the TOA is possible by comparison to a simple ADM flux interpolation.

The GERB Edition 1 products SEVIRI scene identification

The first Geostationary Earth Radiation Budget (GERB) instrument was launched during the 2002 summer together with the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board of the Meteosat-8 satellite. This broadband radiometer aims to deliver near real-time estimates of the top of the atmosphere solar and thermal radiative fluxes at high temporal resolution thanks to the geostationary orbit. Such goal is achieved with the L20 GERB processing which generates these fluxes at several spatial resolutions from the directional filtered radiance measurements of the instrument. This processing consists of successive components, one of them being a radiance-to-flux conversion. Such conversion is carried out in the solar region by using the shortwave angular dependency models (ADMs) developed from the Tropical Rainfall Measuring Mission (TRMM) Clouds and the Earths Radiant Energy System (CERES) experiment. As these ADMs are stratisfed according to specific scene properties, the GERB ground segment will have to rely on a scene identification of SEVIRI data which allows us to select the proper ADM. In this paper, we will briefly justify and describe the implementation of a specific GERB scene identification for the offcial Edition 1 release of the L2 products. Preliminary comparisons between GERB and CERES scene identifications both applied to SEVIRI data will follow. Finally, we will suggest possible improvements based on limitations which could be found.

Evidence of azimuthal anisotropy for the thermal infrared radiation leaving the Earth's atmosphere

The analysis of one year of Cloud and the Earths Radiant Energy System (CERES) data from the Earth Observation System (EOS)-Terra satellite provides evidence that the longwave radiation escaping from the atmosphere exhibits significant variability according to the azimuthal angle of observation. A regional analysis of this variability shows that the anisotropy in azimuth is maximum over mountain and desert areas and under cloud-free conditions. A relative difference between north and south views of about 5% in annual average is observed over the Himalayan region in the 8-14 µm infrared (IR) window. The remote sensing community should be aware of this variability, in particular when analysing IR data provided by instruments on geostationary orbits. Indeed, in this case, the azimuthal anisotropy may lead to systematic overestimation of the outgoing longwave radiation and to biases on estimated quantities such as the surface temperature.

First TOA fluxes from the Geostationary Earth Radiation Budget (GERB) instrument

On 29th January 2004 the first Meteosat Second Generation satellite MSG-1, renamed Meteosat-8 (MS-8), commenced routine operations. MS-8 carries the new Spinning Enhanced Visible and Infra Red Imager (SEVIRI) and a Geostationary Earth Radiation Budget (GERB) radiometer. GERB provides valuable short- and long wave broadband measurements of the Earth in order to estimate the top-of-atmosphere radiation budget accurately. The unique feature of GERB in comparison with previous measurements of the Earths radiation budget is its very fast temporal sampling (15 minutes) afforded by geostationary orbit. On the other hand, the GERB instrument only accounts for a crude spatial resolution (about 50 km at the sub-satellite point). Taking advantage of the synergy between the data from GERB and SEVIRI, we propose at the Royal Meteorological Institute of Belgium to merge the two data streams to produce near real-time estimates of the radiation budget for limited geographical regions at a 3x3 SEVIRI pixel resolution (the SEVIRI resolution is 3 km at satellite sub-point). Such fluxes aim to be used by the climate and numerical weather prediction (NWP) scientific communities through climate studies and validation/evaluation of the performance of NWP models over the region covered by MS-8.

Validation and homogenization of cloud property retrievals for RMIB GERB/SEVIRI scene identification

The Geostationary Earth Radiation Budget (GERB) instrument has been launched this summer together with the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) on board of the Meteosat Second Generation (MSG) satellite. This broadband radiometer will aim to deliver near real-time estimates of the top of the atmosphere (TOA) radiative fluxes at the high temporal resolution due to the geostationary orbit. In order to infer these fluxes, a radiance-to-flux conversion based on Clouds and the Earths Radiant Energy System (CERES) angular dependency models (ADMs) need to be performed on measured radiances. Due to the stratification of these ADMs according to some CERES scene identification (SI) features such as cloud optical depth and cloud fraction, the GERB ground segment must include some SI on SEVIRI data which mimic as close as possible the one from CERES in order to select the proper ADM. In this paper, we briefly present the method we used to retrieve cloud optical depth and cloud fraction on footprints made of several imager pixels. We then compare the retrieval of both features on the same targets using nearly time-simultaneous Meteosat-7 imager and CERES Single Satellite Footprint (SSF) data. The targets are defined as CERES radiometer footprints. We investigate the possible discrepancies between the two datasets according to surface type and, if they exist, suggest some strategies to homogenize GERB retrievals based on CERES ones.

Diurnal Asymmetry in the GERB SW Fluxes

The launch of the Geostationary Earth Radiation Budget (GERB) instrument onboard the Meteostat 8 allows a diurnal sampling of the Earths Radiation Budget for the first time, providing a unique and important addition to polar-orbiting measurements. However, preliminary data from the GERB instrument exhibit systematic asymmetry in the short-wave (SW) flux diurnal variation. Such asymmetries are not found in the Clouds and the Earths Radiant Energy System angular distribution models used to convert the directional broad-band GERB SW radiances to fluxes. Comparison between angularly matched estimations of reflected SW flux at the top of the atmosphere from the Spinning Enhanced Visible and Infra Red Imager (SEVIRI) and GERB data indicates that the SEVIRI spectral modeling could be a major issue. In addition, the results indicate that other factors such as the GERB SW geolocation, the GERB detector spectral response functions, the GERB nominal footprint resolution, and cloud cover could also potentially have an influence on the diurnal evolution of the GERB SW fluxes, as they can erroneously impact on the GERB SW correction factor