Abstract
Abstract. Since data from the Infrared Atmospheric Sounding Interferometer (IASI) became available in 2007, a number of papers have appeared in the literature which have reported relatively large discrepancies between IASI spectra and forward calculations in the centre of the CO2 Q-branch at 667 cm−1. In this paper we show that these discrepancies are primarily due to errors in the temperature profiles used in the forward calculations. In particular, we have used forecasts of temperature profiles from the European Centre for Medium-Range Weather Forecasts (ECMWF) to demonstrate that, for the case study considered in this paper, these profiles are affected by systematic errors of the order of ≈10 K at the level of the stratopause. To derive the magnitude and the spatial location of the systematic errors in the temperature profile, we have carried out forward/inverse calculations for a number of clear-sky, daytime, IASI tropical soundings over the sea. The forward calculations have been performed using atmospheric state vectors which have been obtained either from the direct inversion of the IASI radiances or from space-time co-located profiles derived from radiosonde observations and from the ECMWF model. To rule out any effect due to the accuracy of the forward model, we have performed the forward calculations using two independent models. The sensitivity of the temperature biases to the variability of the CO2 profile and to spectroscopy errors has also been studied.
Highlights
The Infrared Atmospheric Sounding Interferometer (IASI) is providing data of unprecedented spectral resolution and accuracy for an operational infrared sounding instrument (see e.g. the recent IASI special issue edited by Richter and Wag-ner (2009))
This result has been documented in a number of unpublished presentations made during the second workshop of the EUMETSAT IASI Sounding Science Working Group (ISSWG-2) and in several papers published in the open literature, e.g. see Shephard et al (2009); Matricardi (2009); Masiello et al (2009)
The two forward models used in this study yield very similar results. Both models respond in the same way to the new input state vectors. Based on these results and on the arguments presented in the previous sections, we suggest that the residuals observed around 667 cm−1 are most likely due to the inconsistency of the temperature profile in the upper stratosphere
Summary
The Infrared Atmospheric Sounding Interferometer (IASI) is providing data of unprecedented spectral resolution and accuracy for an operational infrared sounding instrument As shown, we found that large discrepancies between observations and simulations are observed in the spectra of the Atmospheric Infrared Sounder (AIRS) instrument In this case simulations were performed using the RTTOV (Matricardi et al (2004)) forward model and input fields obtained from the ECMWF short-range forecasts. Given the difficulty (if not the impossibility) of a direct validation of the ECMWF forecasts of temperature in the upper stratosphere, we can gain some insight into the behaviour of the ECMWF temperature fields in this region by using the stratospheric/mesospheric temperature sounding channels of the Special Sensor Microwave Imager/Sounder (SSMI/S) instrument To this end, the statistics of the (largely negative) differences between observations and radiances simulated using profiles from the ECMWF forecasts, suggests the presence of an increasing ECMWF temperature bias between 45 and 85 km We add an index i to the residual vector, δRi, where i can run over the set of n available soundings
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