Jacqueline E. Russell

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...

Spectral Signatures of Earth’s Climate Variability over 5 Years from IASI

AbstractInterannual variability in spectrally resolved longwave radiances is quantified at a variety of spatial scales using 5 yr of IASI observations. Maximum variability is seen at the smallest scales investigated (10° zonal means) at northern and southern high latitudes across the center of the 15-µm CO2 band. As the spatial scale increases, the overall magnitude of interannual variability is reduced across the spectrum and the spectral shape of the variability changes. In spectral regions sensitive to conditions in the upper troposphere, the effect of increasing spatial scale is relatively small and at the global scale these parts of the spectrum show the greatest year-to-year variability. Conversely, the atmospheric window (8–12 µm), which is sensitive to variations in surface temperature and cloud, shows a marked reduction in interannual variability with increasing spatial scale. Over the 5 yr studied, at global scales the standard deviation in annual mean brightness temperature is less than 0.17 K ...

Assessing the Errors in Shortwave Radiative Fluxes Inferred from the Geostationary Earth Radiation Budget (GERB) Instrument in the Presence of Dust Aerosol

Abstract The Geostationary Earth Radiation Budget (GERB) instruments flying on the Meteosat Second Generation series of satellites provide a unique tool with which to monitor the diurnal evolution of top-of-atmosphere broadband radiation fields. GERB products, which have recently been released to the scientific community, include aerosol information in addition to the observed radiances and inferred fluxes. However, no account of the anisotropic characteristics of aerosol has been incorporated in the radiance-to-flux conversion, which uses angular distribution models developed for clear or cloudy conditions. Here an attempt is made to quantify the impact of this omission in the shortwave (SW), focusing on dust-contaminated scenes. An observationally based representation of dust is used to develop a theoretical angular distribution model, which is tested through comparison with observed GERB radiances. For dusty scenes that have been processed as clear ocean, applying the dust model to convert GERB radianc...

Enhancing weak transient signals in SEVIRI false color imagery: Application to dust source detection in southern Africa

A method is described to significantly enhance the signature of dust events using observations from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI). The approach involves the derivation of a composite clear-sky signal for selected channels on an individual time step and pixel basis. These composite signals are subtracted from each observation in the relevant channels to enhance weak transient signals associated with either (a) low levels of dust emission or (b) dust emissions with high salt or low quartz content. Different channel combinations, of the differenced data from the steps above, are then rendered in false color imagery for the purpose of improved identification of dust source locations and activity. We have applied this clear-sky difference (CSD) algorithm over three (globally significant) source regions in southern Africa: the Makgadikgadi Basin, Etosha Pan, and the Namibian and western South African coast. Case study analyses indicate three notable advantages associated with the CSD approach over established image rendering methods: (i) an improved ability to detect dust plumes, (ii) the observation of source activation earlier in the diurnal cycle, and (iii) an improved ability to resolve and pinpoint dust plume source locations.

Traceable radiometry underpinning terrestrial and heliostudies (truths): a bencmark mission for climate

Sound policymaking requires high confidence in climate predictions verified against decadal change observations with robustly known accuracy. Yet, our ability to monitor and predict the future of the climate is inadequate as we currently do not possess sufficient accuracy in our observing capability to confidently observe the small but critical climate change signals that are expected to occur over decadal time scales. These signals are fundamental to assessing the accuracy of climate change projections made by models and for the unambiguous attribution of climate change.

The Direct Cloud‐free Longwave Radiative Effect of Saharan Dust as observed by the Geostationary Earth Radiation Budget (GERB) Experiment

The Geostationary Earth Radiation Budget (GERB) instruments flying on the Meteosat Second Generation (MSG) series of satellites provide a unique tool with which to monitor the diurnally resolved evolution of the top of atmosphere broad‐band radiation fields. In addition, coincident narrow band observations from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instruments, also flying on the MSG platforms, can be used to provide information about the key atmospheric parameters that influence these broad‐band radiative fluxes. One such parameter which can cause a large radiative perturbation, and is commonly seen within the GERB field of view is airborne Saharan dust. In this paper we briefly recap the algorithms that we have developed to identify and quantify Saharan dust loading over North Africa and Arabia using the SEVIRI observations, and to simultaneously diagnose the cloud‐free longwave dust direct radiative effect (LW DRE) from GERB. Focussing on spring and early summer 2006, we obtain ini...

A Geostationary Earth Radiation Budget Dataset For Climate Research: First Intercomparison Results Between Data From the GERB‐1 & ‐2 Instruments

Initial results from a study comparing filtered radiance data from the first two Geostationary Earth Radiation Budget (GERB) instruments on board the METEOSAT‐8 and ‐9 satellites are presented. The differences seen between longwave filtered radiances from GERB‐1 and ‐2 are in line with those predicted when considering the known differences in the ground measured spectral responses between the two instruments. However, the filtered shortwave radiances are found to differ by several percent from those expected. It is possible that in‐orbit calibration updates, which have yet to be determined for GERB‐1, may explain the discrepancies between the observed and predicted differences.

Instrumentation and Sampling Considerations for Geostationary Earth Radiation Budget Measurements

Resolving uncertainties surrounding the nature of future climate change is currently one of the greatest challenges facing mankind. Validation of global climate model (GCM) predictions of the currently much miss-represented cloud radiative feedback requires measurements made from orbit of the Earth Radiation Budget (ERB), specifically targeted at clouds. The ERB parameters for measure are the scattered solar or short wave (SW, 0.3 5µm) and the emitted thermal or long wave radiance (LW, 5 100µm). Such measurements map out the heat source/sink locations that drive all weather and climate, which acts like a complex network of coupled heat engines. The Clouds and the Earth’s Radiant Energy System (CERES), as part of NASA’s Earth Observing System (EOS), uses thermistor bolometer detectors to provide global high spatial resolution ERB measurements from Low Earth Orbit (LEO) space platforms. However, comprehensive validation of GCM prediction of cloud processes requires ERB measurements on a sub-hourly timescale to sample throughout the cloud formation/dissipation process. This is not practically possible with the use of LEO platforms, so ERB measurements are needed from synchronous orbits such as geostationary in order to increase the frequency of measurements. The Geostationary Earth Radiation Budget (GERB) experiment is a European Space Agency (ESA) project on board the spin stabilized Meteosat second Generation (MSG) platform. Location in geostationary orbit and the use of an array of thermopile detectors enables sampling of ERB radiances from the entire Earth disc at an optimum 15 minute temporal resolution. This study describes the instrumentation and sampling capabilities of the current GERB mission. Given its success it is proposed that efforts be made to expand the project and place GERB-like instruments on multiple high orbit platforms. This will provide climatologists with much needed global ERB data at sub-hourly time resolution, making it more likely that model predictions of future climate change can be properly validated.