An improved fast radiative transfer model for assimilation of satellite radiance observations

Abstract

To assimilate atmospheric and surface radiance measurements from satellites in a numerical weather prediction model, a fast radiative transfer model is required to compute radiances from the model first guess fields at every observation point. Such a model for satellite infrared and microwave radiance measurements is used operationally for the assimilation of TIROS operational vertical sounder radiances at the European Centre for Medium-Range Weather Forecasts. An improved version of this model has been developed which requires ozone, in addition to temperature and water vapour, in the input profile and it has been generalized to compute radiances for other satellite radiometers using the same code. Instruments such as the high resolution infrared radiation sounder and the advanced microwave sounding unit on the National Oceanic and Atmospheric Administration polar orbiters, the METEOSAT water vapour imager and the Geostationary Operational Environmental Satellite infrared sounder have been simulated. It is demonstrated, by comparisons with line-by-line model computed radiances, that the fast model can reproduce the line-by-line model radiances for the TIROS operational vertical sounder stratospheric temperature sounding channels to an accuracy below the instrumental noise. The tropospheric, surface sensing, water vapour and ozone channel radiances cannot be predicted to such an accuracy, but still accurately enough for numerical weather prediction assimilation. A comparison of measured TIROS operational vertical sounder radiances with predicted values from numerical weather prediction model analyses gives larger differences than would be expected from the combination of the fast model and instrument related errors for most channels. The validity of the tangent linear approximation of the model gradient for typical radiance departures is also explored, with several examples, for the high resolution infrared radiation sounder/advanced microwave sounding unit instrument combination. The tangent-linear approximation is valid for temperature but significant departures from linearity about the first guess profile are observed for water vapour and ozone. Cloud affected infrared radiances have a highly non-linear response.

Recent changes to the hydrological cycle of an Arctic basin at the tundra–taiga transition

“…They argued that decreases in short-wave irradiance are driven by changes in atmospheric composition, such as aerosols and greenhouse gases, producing a decreasing in the calculated daily Clearness Index. However, the ERA-I irradiance model calculation (Saunders et al, 1999) does not include the effect of aerosol scattering, but it does include the effect of greenhouse gasses, such as water vapour and carbon monoxide. Mean annual long-wave irradiance shows an increasing trend of 2.9 W m −2 decade −1 (Table 2) or 7.3 % over 1960-2016 with respect to 1960.…”

The vertical distribution of volcanic SO<sub>2</sub> plumes measured by IASI

“…The forward model, based on the Radiative Transfer model for TOVS (RTTOV) (Saunders et al, 1999) but extended to include SO 2 explicitly, uses ECMWF temperatures interpolated to the measurement time and location. The retrieval technique uses an error covariance matrix, based on a climatology of differences between the IASI measurements and SO 2 -free forward modelled spectra.…”

Retrieval of ash properties from IASI measurements

“…The chosen model to simulate clear-sky radiances (i.e. containing gaseous absorbers but not cloud or aerosol/ash) is RTTOV (Saunders et al, 1999;Hocking et al, 2014), and its output is combined with an ash layer using the same scheme as that for the Optimal Retrieval of Aerosol and Cloud (ORAC) algorithm; see Sect. 5.1.1.…”