The Infrared Atmospheric Sounding Interferometer (IASI) forms the main infrared sounding component of the European Organisation for the Exploitation of Meteorological Satellites's (EUMETSAT's) Meteorological Operation (MetOp)-A satellite (Klaes et al. 2007), which was launched in October 2006. This article presents the results of the first 4 yr of the operational IASI mission. The performance of the instrument is shown to be exceptional in terms of calibration and stability. The quality of the data has allowed the rapid use of the observations in operational numerical weather prediction (NWP) and the development of new products for atmospheric chemistry and climate studies, some of which were unexpected before launch. The assimilation of IASI observations in NWP models provides a significant forecast impact; in most cases the impact has been shown to be at least as large as for any previous instrument. In atmospheric chemistry, global distributions of gases, such as ozone and carbon monoxide, can be produced in near–real time, and short-lived species, such as ammonia or methanol, can be mapped, allowing the identification of new sources. The data have also shown the ability to track the location and chemistry of gaseous plumes and particles associated with volcanic eruptions and fires, providing valuable data for air quality monitoring and aircraft safety. IASI also contributes to the establishment of robust long-term data records of several essential climate variables. The suite of products being developed from IASI continues to expand as the data are investigated, and further impacts are expected from increased use of the data in NWP and climate studies in the coming years. The instrument has set a high standard for future operational hyperspectral infrared sounders and has demonstrated that such instruments have a vital role in the global observing system.

Airborne measurements from the Meteorological Research Flight's Hercules C-130 and the University of Washington's Convair C-131A during the Monterey Area Ship Track field project are used to evaluate Twomey's analytic expression for cloud susceptibility, which describes the sensitivity of cloud albedo to changes in droplet concentrations. This expression incorporates assumptions about cloud physics, such as the independence of the cloud liquid water content and the width of the droplet size distribution on droplet concentrations. Averaged over all 69 ship track penetrations, cloud liquid water content decreased slightly and the droplet size distributions broadened from the ambient values. For the 17 cases for which albedos were measured during overflights, Twomey's parameterization represents the trend of albedo changes with droplet concentrations remarkably well, passing through the midpoints of the considerable spread in the data. The fortuitous agreement results from compensating changes in cloud properties. Together with the albedo changes, the changes in cloud liquid water content and droplet size distributions imply that cloud thickness usually increased in the ship tracks. Such an increase was observed on the occasions that changes in cloud thickness were recorded (in the Sanko Peace ship track during very clean ambient conditions). Unfortunately systematic measurements of cloud thickness were not made for most of the ship tracks observed. The greatest outlier in the data corresponds to measurements made under horizontally inhomogeneous ambient conditions; possible explanations for its divergence include an increase in cloud thickness or an error in matching above-cloud albedo measurements with in-cloud microphysics measurements.

[1] In situ measurements of terrestrial radiation from the C-130 aircraft during the Saharan Dust Experiment (SHADE) are used to quantify the effect of a strong dust outbreak on radiance and brightness temperatures. The dust gives a distinct spectral signature in upwelling and downwelling terrestrial radiation when high spectral resolution data for a dusty day is compared to data from a clear day. A radiative transfer model is used together with a size distribution retrieved from Sun photometers and atmospheric profiles from dropsondes to simulate the radiance data and provide a constraint on the refractive indices of Saharan dust in the terrestrial part of the spectrum. The degree of agreement between observed and simulated brightness temperatures is dominated by the choice of refractive index, the mass loading, and the altitude of the dust layer. The uncertainties in size distribution appear to have less of an effect so long as large particles (radius greater than 1 mm) are included. In the terrestrial spectrum the dust produced a relative warming rate of up to 0.5 K/day below the dust and a relative cooling of up to 0.5 K/day within the dust layer itself. The effect on irradiance due to this dust outbreak was a decrease in upwelling terrestrial radiation at the top of the atmosphere of 6.5 Wm À2and an increase in downwelling terrestrial radiation at the surface of 11.5 Wm À2. The dust led to decreases in brightness temperature of 2-4 K in the window region, consistent with apparent features in the sea surface temperature retrieved from the advanced very high resolution radiometer.

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An overview is presented of airborne systems for in situ measurements of aerosol particles, clouds and radiation that are currently in use on research aircraft around the world. Description of the technology is at a level sufficient for introducing the basic principles of operation and an extensive list of references for further reading is given. A number of newer instruments that implement emerging technology are described and the review concludes with a description of some of the most important measurement challenges that remain. This overview is a synthesis of material from a reference book that is currently in preparation and that will be published in 2012 by Wiley.