ABSTRACTVolcanic eruptions are natural hazards with dire consequences to life and economy. As most volcanoes are in remote areas, satellites play a vital role in providing observations and input to models used for forecasting volcanic plume evolution. Radiances from the Infrared Atmospheric Sounding Interferometer (IASI) on board the MetOp polar orbiting meteorological satellites were used to detect sulphur dioxide (SO2) from three volcanic eruptions in 2014, in different meteorological situations. Two of these eruption cases, Mount Sinabung in January and Kelut in February, are in the tropics, whereas the case in October from Bárðarbunga, is at a higher latitude. The SO2 plumes from these volcanic eruptions were identified easily and tracked using established methods from the literature, that are based on the principle that areas of increased SO2 from the volcano produce a reduction in the spectrum of the observed radiances (in the relevant absorption bands of IASI), whereas areas outside the volcanic plume do not. An estimate of the plume height was obtained in the first two cases by examination of the winds from an NWP model at different heights, and in the third, where the meteorological pattern is more complicated, by using trajectories from a Lagrangian model and matching their position with satellite observations of the plume at different times. The importance of meteorology in the detection and evolution of volcanic plumes, especially at low levels, is especially well demonstrated by the Bárðarbunga eruption. Estimates of the plume concentrations were obtained from explicit line‐by‐line calculations.

When assimilating observations into numerical weather prediction (NWP) models, it is usually assumed that there are two sources of information: the observations and the NWP background field. Current research to improve the exploitation of advanced infra-red sounder data is making use of “radiance climatologies”, i.e. large ensembles of “historical” radiances from the same instruments. The leading principal components (PCs) of the covariances of such ensembles are being used to make the processing and assimilation of these data more efficient in both the forward (radiative transfer) computation and the inverse (retrieval/assimilation) component and to reduce the noise in the measured spectra. These radiance climatologies therefore constitute a potential third source of information for retrieval/assimilation processes. We consider here, from a theoretical perspective, the status of this third source of information and its implications for retrieval/analysis accuracy. We compare two methods of applying PCs to the processing of IASI spectra: simple PC truncation and optimal radiance estimation. We show why it is possible to reduce the noise in IASI Level 1C data considerably (by a factor ~3), yet this does not lead to comparable reductions in retrieval/analysis error. We also present the theory required to consider whether it is possible, with information from the radiance climatology, to reduce the retrieval error at all.

The arrival time difference (ATD) long-range lightning location network (ATDnet) is the long-range very low frequency (VLF) lightning location network owned and operated by the Met Office, locating lightning using a waveform correlation technique. Pronounced differences in the waveform shape with distance to lightning have been observed and attributed to interference between different propagation modes within the earth-ionosphere waveguide. During the day, waveform correlations were significantly degraded at a distance of 450 km from the sensor, with the main degradations occurring during the night centered at propagation distances of 650 and 2150 km. The observation and simple modeling of modal interference spacing allowed the effective ionospheric height during summer over Europe to be estimated for day and night as 69 and 88.5 km, respectively, for 13.7 kHz. Wider distribution of sensor sites, lowering of the receiver frequency, and adaptation of the reference waveform selection criteria have been suggested to mitigate the effect of this interference on network performance.

Methodologies are discussed for the efficient representation of observations from high-resolution infrared sounders for the purposes of assimilation into numerical weather prediction models. The use of principal component analysis is explored and it is noted that, while the available information in the observations is stored efficiently, the non-locality of the Jacobians that arise may cause practical problems in an operational assimilation system. Reconstructing radiance spectra from the principal components appears to be a more realistic approach in the near term. However, initial experiments with reconstructed radiances do not appear to give significant improvements in forecast skill above that already demonstrated with the current use of advanced sounder data. Copyright (C) 2010 Royal Meteorological Society and Crown Copyright.

Principal component (PC) analysis has received considerable attention as a technique for the extraction of meteorological signals from hyperspectral infra-red sounders such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Atmospheric Infrared Sounder (AIRS). In addition to achieving substantial bit-volume reductions for dissemination purposes, the technique can also be used to generate reconstructed radiances in which random instrument noise has been reduced. Studies on PC analysis of hyperspectral infrared sounder data have been undertaken in the context of numerical weather prediction, instrument monitoring and geophysical variable retrieval, as well as data compression. This study examines the potential of PC analysis for chemistry applications.A major concern in the use of PC analysis for chemistry is that the spectral features associated with trace gases may not be well represented in the reconstructed spectra, either due to deficiencies in the training set or due to the limited number of PC scores used in the radiance reconstruction. In this paper we show examples of reconstructed IASI radiances for several trace gases: ammonia, sulphur dioxide, methane and carbon monoxide. It is shown that care must be taken in the selection of spectra for the initial training set: an iterative technique, in which outlier spectra are added to a base training set, gives the best results. For the four trace gases examined, key features of the chemical signatures are retained in the reconstructed radiances, whilst achieving a substantial reduction in instrument noise.A new regional re-transmission service for IASI is scheduled to start in 2010, as part of the EUMET-SAT Advanced Retransmission Service (EARS). For this EARS-IASI service it is intended to include PC scores as part of the data stream. The paper describes the generation of the reference eigenvectors for this new service.

The use of numerical weather prediction (NWP) models in validating and characterizing satellite sounder data is described. The next generation U.S. polar-orbiter missions will be the NPOESS Preparatory Project (NPP) and the National Polar-orbiting Operational Environmental Satellite System (NPOESS). The Met Office and ECMWF will be participating in the calibration/validation activities for these missions, and the planned contributions are described. These contributions include both pre-processing expertise and the assessment of Instrument bias and noise, using NWP models as a reference

Observations from the Infrared Atmospheric Sounding Interferometer (IASI), onboard EUMETSAT's MetOp satellite, have been assimilated at the Met Office in global and regional numerical weather‐prediction systems since 27 November 2007. Pre‐operational trials of IASI assimilation in the global model during the summer of 2007 delivered a positive impact on forecasts approximately twice as large as that shown by the Atmospheric InfraRed Sounder (AIRS) on the EOS‐Aqua satellite. A series of observing system experiments confirmed the relative performance of IASI and AIRS, and showed that impact from IASI is equivalent to a single Advanced Microwave Sounding Unit‐A (AMSU‐A) combined with a single Microwave Humidity Sounder (MHS). The results of an IASI assimilation trial for the winter of 2007 were consistent with those of the summer trial, although the impact was slightly lower overall. The assessment of impact is strongly dependent on the variables and methods chosen for verification: assimilation trials with the regional model showed similar improvements to the large‐scale fields (e.g. mean‐sea‐level pressure and geopotential height) as seen in the global model, but no forecast impact was seen for variables such as visibility and rain‐rate. © Crown Copyright 2009. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd.

The first flight models of the Advanced Microwave Sounding Unit (AMSU) were launched on the NOAA-15 satellite in 1998. This paper reviews the performance of AMSU-B to date, with particular reference to the problems experienced due to radio-frequency interference. It shows that a bias correction scheme developed by the Met Office and NOAA/NESDIS generally works well, though regular updates are necessary to keep track of long-term bias changes. The prospects for future AMSU-B flight models are discussed. The paper also outlines the aircraft-based campaigns that took place during 1999 for the purpose of validating those aspects of water-vapour measurement and spectroscopy that are important for AMSU.

In preparation for new meteorological infrared satellite sounders (such as the Infrared Atmospheric Sounding Interferometer, IASI) an interferometer has been mounted on the Meteorological Office's research aircraft. Early development of assimilation software will allow the new satellite data to be used at numerical weather prediction centres soon after launch. ARIES, the Airborne Research Interferometer Evaluation System, with a 1 cm(-1) wavenumber resolution over 600 to 3000 cm(-1) wavenumbers (wavelength 16.7 to 3.3 mu m) is described. Preliminary data are compared with results from a line by line radiative transfer model using the atmospheric profile measured by the aircraft, (which can also measure cloud in-situ).

The UKMO has developed an airborne interferometer 'ARIES' to act as simulator for future satellite-based infrared sounders. In order to characterise anomalies in ARIES observations of atmospheric scenes, methods have been developed to experimentally determine interferometer Instrument Line Shape (ILS) rather than relying on simulation.

The UKMO has developed an airborne interferometer to act as a simulator for satellite-based infrared sounders. ARIES, the Airborne Research Interferometer Evaluation System consists of a modified commercial interferometer, mounted on the UKMO C-130 aircraft. The instrument is sensitive to the wavelength range 3.3-16.6 /spl mu/m, and has a maximum optical path difference of /spl plusmn/1.037 cm.

The Advanced Microwave Sounding Unit, AMSU, is being developed to fly on the new generation of NOAA polar orbiters due to be launched in the latter half of the 1990's, The U.K. Meteorological Office (UKMO) are procuring the high frequency component of AMSU (AMSU-B) with five channels in the range 88-191 GHz, In order to determine the radiometric performance and verify the method for calibration of AMSU-B an extensive series of tests have been performed by the UKMO on the engineering and three flight models. The instruments were placed in a 3 m thermal-vacuum chamber where their temperature could be controlled over the full range expected in orbit and an Earth target and a space target could be viewed, For the first flight model the measured Ne Delta T values were all <1.1 K at the nominal instrument temperature using a 300 K target, Absolute calibration accuracy and linearity in response were measured to be well within the specification of 1 and 0.3 K, respectively, A small variation in the gain with scan angle was found and an empirical factor was derived to modify the inferred radiances to remove this effect, Measurements of the gain stability for each channel were also measured for simulated in-orbit conditions.