Intercomparison of integrated IASI and AATSR calibrated radiances at 11 and 12 μm

S M Illingworth,R J Parker,J J Remedios

doi:10.5194/acp-9-6677-2009

Abstract

Abstract. The mission objectives of the Infrared Atmospheric Sounding Interferometer (IASI) are driven by the needs of the Numerical Weather Prediction (NWP) and climate monitoring communities. These objectives rely upon the IASI instrument being able to measure top of atmosphere radiances accurately. This paper presents a technique and first results for the validation of the radiometric calibration of radiances for IASI, using a cross-calibration with the Advanced Along Track Scanning Radiometer (AATSR). The AATSR is able to measure Brightness Temperature (BT) to an accuracy of 30 mK, and by applying the AATSR spectral filter functions to the IASI measured radiances we are able to compare AATSR and IASI Brightness Temperatures. By choosing coincident data points that are over the sea and in clear sky conditions, a threshold of homogeneity is derived. It is found that in these homogenous conditions, the IASI BTs agree with those measured by the AATSR to within 0.3 K, with an uncertainty of order 0.1 K. The agreement is particularly good at 11 μm where the difference is less than 0.1 K. These first results indicate that IASI is meeting its target objective of 0.5 K accuracy. It is believed that a refinement of the AATSR spectral filter functions will hopefully permit a tighter error constraint on the quality of the IASI data and hence further assessment of the climate quality of the radiances.

Highlights

The Infrared Atmospheric Sounding Interferometer (IASI) is a nadir sounding Fourier transform spectrometer flying operationally on the MetOp-A satellite, and which will operate on the two subsequent satellites of the MetOp pro-gramme; the resulting record of spectrally resolved radiances should span at least 15 years
Having defined the match-ups between the IASI and Advanced Along Track Scanning Radiometer (AATSR) data sets, the IASI spectrally resolved TOA radiances were integrated to “AATSR-like” radiances, and converted to radiometric brightness temperatures (BTs), which are the equivalent form of the AATSR TOA radiances
Results for the differences between IASI equivalent BTs and the AATSR mean BTs are shown in Fig. 3, for each classification and for the 11 μm channel as a function of the AATSR BT standard deviation within each IASI pixel

Summary

Introduction

The Infrared Atmospheric Sounding Interferometer (IASI) is a nadir sounding Fourier transform spectrometer flying operationally on the MetOp-A satellite, and which will operate on the two subsequent satellites of the MetOp pro-. It is important to intercompare wellcalibrated, long-term instruments, where possible, providing independent verification of radiances. It is a valuable and necessary exercise to undertake such tests as part of a long-term testing strategy which can evaluate trends in instrument calibration. This paper is directed towards the development of a methodology for the former, and the provision of first results for this aspect for the IASI instrument. This paper is organised as follows: the description of the IASI and AATSR instruments used in the cross-calibration is provided in Sect.

The instruments

Overview of concept

Cross-calibration match-ups

Radiance comparisons

Cross-calibration results

Conclusions