Differences between Earth's global mean all-sky outgoing longwave radiation spectrum as observed in 1970 [Interferometric Infrared Spectrometer (IRIS)], 1997 [Interferometric Monitor for Greenhouse Gases (IMG)], and 2012 [Infrared Atmospheric Sounding Instrument (IASI)] are presented. These differences are evaluated to determine whether these are robust signals of multidecadal radiative forcing and hence whether there is the potential for evaluating feedback-type responses. IASI-IRIS differences range from +2K in the atmospheric window (800-1000 cm(-1)) to -5.5K in the 1304 cm(-1) CH4 band center. Corresponding IASI-IMG differences are much smaller, at 0.2 and -0.8 K, respectively. More noticeably, IASI-IRIS differences show a distinct step change across the 1042 cm(-1) O-3 band that is not seen in IASI-IMG comparisons. This step change is a consequence of a difference in behavior when moving from colder to warmer scenes in the IRIS data compared to IASI and IMG. Matched simulations for the relevant periods using ERA reanalyses mimic the spectral behavior shown by IASI and IMG rather than by IRIS. These findings suggest that uncertainties in the spectral response of IRIS preclude the use of these data for quantitative assessments of forcing and feedback processes.

Observed far‐infrared and mid‐infrared radiance spectra measured in situ above cirrus with both the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) and the Airborne Research Interferometer Evaluation System (ARIES) are presented. Two sets of upwelling spectra between wavenumbers 100 and 1415cm −1 taken over ice cloud of visible optical thickness 0.1 and 0.3 are shown. These observations address the need for more measurements of radiance from cirrus, particularly in the far infrared, where experimental data are lacking. A test of consistency in modelling in both the far infrared and mid infrared is performed by fitting a model calculation of radiances in the infrared window region, and comparing with the measured radiances. In this consistency check, the sampling of the surrounding atmosphere and the cloud is given extensive consideration. The model was not able to consistently reproduce the observed radiance across the entire region, and was particularly poor in the 330–600cm −1 region of the far infrared. Possible causes are thought to be primarily model input uncertainties arising from inadequate sampling of cloud and atmosphere. Copyright © 2010 Royal Meteorological Society and Crown Copyright.

A polarising far infrared spectrometer has been simulated to investigate the effect of Mylar substrates on polarisers. Procedural errors were found to the calibration in certain spectral regions. Comparisons with laboratory measurements are discussed.

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.

Recent studies have highlighted the important contribution of the far-infrared (electromagnetic radiation with wavelengths greater than 12 pm) to the Earth's radiative energy budget. In a cloud-free atmosphere, a significant fraction of the Earth's cooling to space from the mid- and upper troposphere takes place via the water vapor pure rotational band between 17 and 33 pm. Cirrus clouds also play an important role in the Earth's outgoing longwave radiation. The effect of cirrus on far-infrared radiation is of particular interest, since the refractive index of ice depends strongly on wavelength in this spectral region. The scattering properties of ice crystals are directly related to the refractive index, so consequently the spectral signature of cirrus measured in the FIR is sensitive to the cloud microphysical properties [1, 2]. By examining radiances measured at wavelengths between the strong water vapor absorption lines in the FIR, the understanding of the relationship between cirrus microphysics and the radiative transfer of thermal energy through cirrus may be improved. Until recently, very few observations of FIR spectral radiances had been made. The Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) was developed by Imperial College to address this lack of observational data. TAFTS observes both zenith and nadir radiances at 0.1 cm(-1) resolution, between 80 and 600 cm(-1). During February and March 2007, TAFTS was involved in RHUBC (the Radiative Heating in Under-explored Bands Campaign), an ARM funded field campaign based at the ACRF-North Slope of Alaska site near Barrow, situated at 71 degrees latitude. infrared zenith spectral observations were taken by both TAFTS and the AERI-ER (spectral range 400 - 3300 cm(-1)) from the ground during both cloud-free and cirrus conditions. A wide range of other instrumentation was also available at the site, including a micropulse lidar, 35 GHz radar and the University of Colorado/NOAA Ground-based Scanning Radiometer (GSR). Data from these instruments, as well as from frequently launched radiosondes, were used to characterize the atmospheric state needed as input for line-by-line radiative transfer calculations. By comparing these calculations with the TAFTS and AERI-ER observations, it is possible to test the effectiveness of ice crystal size distribution parameterizations (which are generally derived from mid-latitude and tropical in-situ observations) when applied to Arctic cirrus. The influence of the assumed single scattering properties (here calculated for ice aggregates by A. Baran of the UK Met Office) on the calculated spectra is also considered in this work.

The objectives and component parts of the CAVIAR consortium are described. Preliminary data from the recent UK-based airborne field campaign are discussed, with a description of the analysis method and its expected scientific merit.

An overview of the results of recent field campaigns performed with the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) to study the radiative properties of cirrus in the far infrared spectral region is presented.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text P. D. Green, N. Humpage, C. Cox, J. E. Murray, J. E. Harries, and J. C. Pickering, "Far-IR Cirrus Cloud Radiative Properties from the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) instrument," in Fourier Transform Spectroscopy/ Hyperspectral Imaging and Sounding of the Environment, OSA Technical Digest Series (CD) (Optica Publishing Group, 2007), paper FThB1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Observed clear‐sky far‐infrared (FIR) radiances measured by the Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) are presented. These measurements were taken when flying on board the Facility for Atmospheric Airborne Measurements British Aerospace 146 aircraft over the UK on 18 September 2004 in the upper troposphere between 8.3 and 9.2 km altitude, during the European AQUA Thermodynamic Experiment (EAQUATE). Upwelling clear‐sky measurements from the TAFTS short‐wave channel (330 to 500 cm−1) are shown and these measured radiances are compared to model radiances where various dropsonde data are incorporated. The variations in the water vapour profiles below the aircraft are explored in terms of the FIR spectra. The downwelling measurements from TAFTS long‐wave channel (90 to 230 cm−1) are also compared to model spectra, produced in this case from the UK Met Office (UKMO) mesoscale model. The variation in the UKMO mesoscale modelled downwelling radiance over the flight leg is seen to have a standard deviation of 0.5 to 2 mW (m2 sr cm−1)−1, that is of the order of TAFTS detection limit in this particular campaign. Copyright © 2007 Royal Meteorological Society

Previously published work using satellite observations of the clear sky infrared emitted radiation by the Earth in 1970, 1997 and in 2003 showed the appearance of changes in the outgoing spectrum, which agreed with those expected from known changes in the concentrations of well-mixed greenhouse gases over this period. Thus, the greenhouse forcing of the Earth has been observed to change in response to these concentration changes. In the present work, this analysis is being extended to 2006 using the TES instrument on the AURA spacecraft. Additionally, simulated spectra have been calculated using LBLRTM with inputs from the HadGEM1 coupled model and compared to the observed satellite spectra.

The observation of changes in the earth's spectrally resolved outgoing longwave radiation (OLR) provides a direct method of determining changes in the radiative forcing of the climate system. An earlier study showed that satellite-observed changes in the clear-sky outgoing longwave spectrum between 1997 and 1970 from the Infrared Interferometer Spectrometer (IRIS) and Interferometic Monitor of Greenhouse Gases (IMG) instruments could be related to changes in greenhouse gas composition. The authors present a new study that extends this to 2003, through the first use of a new, independent source of global atmospheric infrared spectra, from the Atmospheric Infrared Sounder (AIRS) experiment. AIRS is a dispersion grating spectrometer, while the other two were Fourier transform spectrometers, and this is taken into account in the analysis. The observed difference spectrum between the years 2003 and 1970 generally shows the signatures of greenhouse gas forcing, and also shows the sensitivity of the signatures to interannual variations in temperature. The new 2003 data support the conclusions found in the earlier work, though, interestingly, the methane (CH4) Q branch centered at 1304 cm(-1) exhibits more complex behavior, showing a decrease in intensity in the difference spectrum between 1997 and 2003. Sensitivity analysis indicates that this is due to changes in temperature structure, superposed on an underlying increase in CH4. Radiative transfer calculations based on reanalysis data are used to simulate the changes in the OLR spectrum; limitations in such data and possible variations that could account for several observed effects are discussed.

The Geostationary Earth Radiation Budget (GERB) instrument on METEOSAT-8 is making the first measurements of the Earth's radiation budget from geostationary orbit. The first validated GERB Edition I products were released to the scientific community via the ggsps archive (http://ggsps.rl.ac.uk) earlier this year. This paper summaries the calibration accuracy and validation results for these Edition I products.

A volcanic eruption provides a natural experiment in which time constants for the onset and decay of the consequent radiative perturbation may be measured. The radiative and atmospheric responses also provide insight into how the top of atmosphere net balance of energy responds to perturbations. We analyse the response of the atmospheric humidity, temperature and radiative fields to the eruption of Mt Pinatubo to determine time constants for the growth and the decay of perturbations in these fields. We are able to distinguish between processes that respond directly to the insertion of aerosols into the atmosphere, and those, such as changes in the humidity and temperature, that involve slower dynamical processes and therefore have longer response times. The physical basis for these observations is discussed, and it is suggested that a valuable test of coupled climate models should be that they reproduce these response times, and the associated flux anomalies.

The Valencia Anchor Station (VAS) was set up by the University of Valencia at the natural region of Utiel-Requena Plateau in 2001. The plateau is a large and reasonably homogeneous area suitable for validation of low spatial resolution satellite data and products such as GERB's. In the framework of the EUMETSAT/ESA MSG-RAO Project no. 138 GIST Proposal for Calibration/Validation of SEVIRI and GERB, and of the Spanish Research Programme on Space Project SCALES (SEVIRI & GERB Cal/Val Area for Large-scale Field ExperimentS), three GERB ground validation campaigns have so far been carried out at the VAS under different land surface conditions. CERES instruments onboard NASA EOS Aqua and Terra satellites, operating in PAPS (Programmable Azimuth Plane Scanning) mode, have generously provided additional SW and LW radiance measurements to support validation efforts. These have shown to be most valuable as intermediate validation step between ground measurements and the large GERB pixel size.

The Geostationary Earth Radiation Budget experiment (GERB) is an absolute radiometer measuring the reflected shortwave (SW) and emitted longwave (LW) radiation from the Earth, from the Meteosat-8 spacecraft. From these measurements, the radiative energy balance of the part of the Earth seen from this geosynchronous vantage point are derived every 15 minutes, with a sub-satellite spatial resolution of 48km. The paper will report on the operation of the instrument, the accuracy of the radiances and fluxes obtained, the status of the instrument calibration, and results of research into convective cloud radiative forcing, and aerosol-radiation interactions over Africa and the Atlantic. Introduction The Geostationary Earth Radiation Budget (GERB) sensor is an instrument of opportunity on the Meteosat-8 spacecraft. It is a broadband radiometer, measuring the reflected shortwave (SW) and emitted longwave (LW) radiation at the top of the atmosphere (TOA). The instrument and operations are described in detail in Harries et al. (2005) and a brief summary is given here. At the heart of the instrument is a 256-element detector array, aligned in the north-south direction, and a primary mirror rotating counter to the spacecraft spin direction. The detectors are sensitive to radiation from 0.32μm to ~100μm (TOTAL channel), and a quartz filter cuts out wavelengths above 4μm when measuring the SW channel. As the Earth comes into the field of view (FOV) of the sensor, the mirror directs a frozen beam of radiation to the detectors. A scan begins by observing the area of space adjacent to the earth and the location observed by the north-south detector array is moved by one pixel width in the east-west direction on subsequent rotations until space is viewed again on the other side of the Earth’s disk. A complete scan consists of 256x282 pixels in the SW and TOTAL channels, with a nadir resolution of ~50km. A combination of the space viewed and the internal blackbody observations are used to convert the instrument voltages to filtered TOTAL and SW radiances. The radiances are geolocated and rectified to a 256x256 pixel grid. These are then unfiltered to remove effects of the sensor spectral and spatial responses and the LW radiance field is produced by removing the SW signal from the TOTAL channel. To convert the radiances to fluxes, a scene identification process retrieves surface type and cloud properties from the SEVIRI narrowband channels. Angular dependency models derived from CERES-TRMM are used in the radiance to flux conversion process. The radiance and flux data products are then resolution-enhanced using the higherresolution SEVIRI data to a ~9km nadir resolution. Figure 1. TOTAL and SW scans prior to level 1.5 processing. Each scan consists of approximately 280 lines of 256 detector observations. The GERB instrument on board Meteosat-8 has been providing almost continuous data since 2003. The validation of these data is ongoing and the current results from validation are presented below. The official release of data for science users will take place following a reprocessing of the data collected to date, and remaining data issues are indicated. Also presented are results from ongoing research at Imperial College into cloud and aerosol radiative forcing using GERB, GERB-like and SEVIRI data. Validation Results The primary means of validation of GERB radiances and fluxes is through intercomparison with the Cloud and Earth’s Radiant Energy System (CERES) instruments on board the Terra and Aqua instruments. A special scanning mode is implemented for CERES data collection on a regular basis to maximise the number of coangular, co-located data points for this purpose. The results shown below use Edition 8 (Instantaneous ERBE-like TOA estimates) from CERES FM-2 on the Terra spacecraft. The CERES sensors have higher spatial resolution than GERB, to which some of the variance in the comparisons can be attributed. Figure 2. Comparison of GERB and CERES FM2 ES8 LW (left) and SW (right) radiances. The colours correspond to scene type: ocean (blue); cloud (purple); bright vegetation (green); and bright desert (red). The agreement between the sensors in the LW is excellent, with a CERES/GERB mean ratio of 0.998+/0.007 at the 95% confidence limit. Compensating differences have been identified, however, with warmer scenes having ratios >1 and colder scenes having ratios <1. These differences are due to different LW limits applied in the data processing and this disparity will be resolved during reprocessing. For SW radiances, the agreement is scene-dependent. The best results are over deserts, where the ratio is 0.980+/-0.006. As the scene being viewed becomes bluer, the ratio reduces, down to 0.931+/-0.009 for ocean observations. A revised spectral response to be used in reprocessing is expected to improve these discrepancies. A small detectorspecific dependence has also been identified, which may be due to inter-detector response differences. As each detector observes a very small latitude range due to the scanning procedure, this could also be due to differences in the mean scene viewed by each detector. This issue is under investigation. CERES-GERB SW filtered radiance comparison 0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 All Ocean Dark Veg Bright Veg Dark Desert Bright Desert Cloudy CE RE S/ G ER B ra di an ce ra tio Figure 3. Separation of CERES/GERB SW radiance ratio according to scene type, showing agreement decreasing with blueness of scene. Radiance to flux conversion is the biggest source of error for radiation budget data. Due to GERB’s fixed geometry, any viewing angle-dependent errors in the ADMs will result in systematic biases, so analysis of ADM performance and research into improvements is ongoing to minimise this. Theoretical ADMs based on SBDART calculations and scene identification from SEVIRI IR channels are used in processing of GERB LW fluxes. Comparisons with the CERES LW fluxes show a mean ratio of 0.987 ± 0.002, with an indication of limb-darkening at the edge of the disk for the GERB fluxes and scene-dependent differences Validation studies of the GERB clear sky ocean fluxes seem to indicate diurnally varying errors in the application of the ADMs which result in a spurious diurnal signal in the fluxes (~ ±20Wm). Whether this is due to the CERES ADMs themselves, or to the way in which they are applied to the GERB data is under investigation. Data Release and Other Issues Large and time varying errors noted initially in the geolocation were due to inaccurate pointing information from Meteosat-8. The geolocation accuracy has been greatly improved by additional data made available by EUMETSAT to correct this information. Smaller systematic offsets (~1 pixel) can be corrected by tuning the instrument in-flight optical model used in processing. Planned improvements will allow the 0.1 pixel geolocation accuracy specification to be met, however it is unlikely that this will be achieved for the edition 1 release. Periods around local midnight have been shown to be affected by stray solar illumination when the sun is close to the instrument FOV. Significant contamination of Earth radiances occurs for 6-8 weeks before and after equinoxes. A new gain calculation has been introduced to minimise time periods affected by using running averages. This also removes contamination on the occasions when the moon is present in the space views used for converting voltages to radiances. A study of the straylight problem will be carried out in the future, and data affected will be flagged for the first data released. Detector response has been very stable since launch, overall. Detector 192 has not performed to specification since launch and the response of detectors 229-238 has been degraded since February 2005, due to a mechanical fault. Data affected from these detectors will also be flagged. Cloud Radiative Forcing Standard radiation budget monthly mean data products average over all cloud systems and weather regimes. This limits their application in regional scale studies of specific cloud regimes and in validation of numerical models. In order to study the effects of individual cloud types separately, previous methods include using daily averages of cloud and radiation data or radiative transfer modelling. Both cloud type and cloud radiative forcing (CRF) can vary strongly through the day, however, which can lead to incorrect attribution using diurnal mean quantities (fig. 4). Data which can resolve both day to day and diurnal variations is therefore required. The ‘GERB-like’ data shown here was produced by applying a narrowband-broadband conversion to SEVIRI channels, but the results are similar when pre-release GERB data was analysed. The EUMETSAT CLA cloud analysis product was used to identify low, mid and high-level clouds in a 15 minute snapshot at 3 hour intervals. Figure 4. a) Occurrence of low, mid and high level clouds on 1 June 2004 from SEVIRI CLA product. b) Breakdown of cloud fraction, SW CRF and LW CRF separated into cloud type for that day, showing that the large SW CRF signal would be incorrectly attributed to the more prevalent high cloud if daily mean quantities were used. The problem being addressed is summarised in figure 4 (a) and (b). Over the African convective region on the day shown (01/06/2004), a typical diurnal variation in cloud type and fraction is observed in the CLA product (fig. 4 (a)). As high clouds dominate the cloud cover in terms of time, the SW CRF effect due to low clouds present on this day would be incorrectly attributed to high clouds using the method described by Webb et al. (2001). In fact, the high clouds have a significant LW CRF, as they reduce LW TOA emission, but a smaller SW CRF than low cloud (fig. 4 (b)). Instantaneous CRF are attributed to a cloud type based on the CLA product. These are then averaged to produce a monthly time-step mean CRF corresponding to each cloud type which c