SUMMARYA method for detecting cloud contamination in radiances measured by high-spectral-resolution infrared sounders is presented. It seeks to identify clear channels within a measured spectrum, rather than the locations of completely clear spectra. Applied to simulated cloudy spectra, the scheme is able to detect clear channels with residual cloud contamination better than (i.e. less than) 0.2 K in many channels, and is thus considered suf ciently stringent for numerical weather-prediction applications. The scheme has been applied to spectra measured by the Advanced InfraRed Sounder and, whilst a quantitative validation is more dif cult with real data (without true clear radiances), it is found to perform well compared with coincident imagery.

SUMMARYThe geographical distribution and persistence of meteorologically sensitive areas (derived using adjoint forecast-error tracking techniques) are investigated with respect to the occurrence of cloud. Using sensitivity patterns and cloud structures simulated from the ECMWF forecast model a high degree of correlation is found which, if realistic, could limit the ability of space-based advanced infrared sounders to measure important meteorological features. The result underlines the need to develop techniques to use infrared sounder observations in the presence of cloud.

ECMWF has been assimilating ozone-sensitive infrared (IR/O 3 ) radiances from AIRS, IASI, and HIRS in its operational system since November 2011. We present a detailed assessment of the value of assimilating these observations, as well as the steps that have been taken to successfully merge the infrared information with that provided by ozone products retrieved from UV sensors. In general, the assimilation of the IR/O 3 radiances improves the agreement between the ozone analyses and independent ozone data from MLS and sondes, especially in the UTLS and at high latitudes in the Southern Hemisphere winter. However, blending the information provided by the IR/O 3 radiances with that provided by UV instruments can lead in certain circumstances to a degradation of the ozone analyses in the region of the ozone maximum, particularly at midlatitudes in the summer hemisphere. It is shown that this problem can be alleviated by providing the assimilation system with height-resolved ozone information. The assessment of one-year long assimilation experiments has shown some additional problems in the ozone analyses related to a slow drift of adaptive observation bias correction over time. Previously, just two uncorrected infrared channels (one from AIRS and one from IASI) were used to stabilise (anchor) the bias correction of all other ozone observations against the influence of model systematic error. However, it has been found that their influence (limited primarily to the UTLS) was not sufficient to constrain the ozone assimilation system as a whole (i.e. the total column) over longer time-scales of several months to a year. This issue has been addressed by using additional anchoring observations to constrain the time evolution of the bias correction.

SUMMARYThe impact of assimilating TOVS radiance data in the European Centre for Medium-Range Weather Forecasts humidity analysis is evaluated. It has been found that the introduction of a one-dimensional variational analysis scheme (1DVAR) applied to the TOVS radiances significantly improves the representation of many aspects of the hydrological cycle. The theoretical information content of the TOVS radiance data is discussed and found to be consistent with significant changes observed in the mid upper-tropospheric moisture fields when the radiance data are assimilated. In particular, a tendency of the model (without radiance assimilation) to produce a tropical humidity structure that is far too dry is removed, and excessively moist conditions in the southern sub-tropics are improved. It is argued that the latter problem originates from the use of operational NESDIS retrieved products in the analysis. The humidity adjustments caused by the assimilation of TOVS radiances are accompanied by significant changes in the model dynamics, especially the description of the tropical Hadley circulation. One such case is described in detail, where the moistening of the tropics and drying of the sub-tropics has resulted in a stronger mean analysed meridional circulation in the Atlantic. The analysis changes are also shown to improve the medium-range forecasting of humidity, together with some associated benefit in the prediction of cloud and precipitation.

The European Centre for Medium-range Weather Forecasts (ECMWF) 4D-Var data assimilation system has been modified to allow the direct assimilation of Principal Component (PC) scores derived from spectra measured by the Infrared Atmospheric Sounding Interferometer (IASI). Testing of a prototype system where 165 IASI radiances are replaced by just 20 PC scores shows significant computational savings with no detectable loss of skill in the resulting analyses or forecasts. Indeed in some respects the assimilation of PC scores leads to marginal improvements over the traditional radiance-based assimilation.