Alessandro Ipe

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.

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.

A Spectral Aging Model for the Meteosat-7 Visible Band

Formorethan30years,theMeteosatsatelliteshavebeeninageostationaryorbitaroundtheearth.Because ofthe hightemporalfrequencyof thedata andthelong timeperiod,this database isan excellentcandidatefor fundamental climate data records (FCDRs). One of the prerequisites to create FCDRs is an accurate and stable calibration over the full data period. Because of the presence of contamination on the instrument in space, a degradation of the visible band of the instruments has been observed. Previous work on the Meteosat First Generation satellites, together with results from other spaceborne instruments, led to the idea that there is a spectral component to this degradation. This paper describes the model that was created to correct the Meteosat-7 visible (VIS) channel for these spectral aging effects. The model assumes an exponential temporal decay for the gray part of the degradation and a linear temporal decay for the wavelength-dependent part. The effect of these two parts of the model is tuned according to three parameters; 253 clear-sky stable earth targets with different surface types are used together with deep convective cloud measurements to fit these parameters. The validationof themodelleadsto anoverallstabilityoftheMeteosat-7reflectedsolarradiation data record of about 0.66 W m 22 decade 21 .

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.