Assessment of the Hyperspectral Infrared Atmospheric Sounder (HIRAS)

The hyperspectral infrared atmospheric sounder (HIRAS), the first Chinese hyperspectral infrared instrument, was launched in 2017 on board the fourth polar orbiter of the Feng Yun 3 series, FY-3D. The instrument is a Fourier transform spectrometer with 2275 channels covering three spectral bands (650-1136, 1210-1750, and 2155-2550 cm(-1)) with 0.625 cm(-1) spectral resolution. The first data quality assessment of HIRAS observations at full and normal spectral resolutions is presented. Comparisons with short-range forecasts from the Met Office numerical weather prediction (NWP) global system have revealed biases (standard deviation) generally less than 2.6 K (2 K) in the spectral regions mostly unaffected by trace gases where the confidence in the NWP model is largest. Of particular concern, HIRAS detector 3 seems to suffer from sunlight contamination of its calibration towards the end of the descending node. This, together with an obstruction of the detector field of view by an element of the platform, results in accentuated bias and noise in the observations from this detector. At normal spectral resolution, a background departure double difference analysis has been conducted between HIRAS and the NOAA-20 crosstrack infrared sounder (CrIS). The results show that HIRAS and CrIS are in good agreement with a mean difference across the three bands of -0.05 K (+/- 0.26 K at 1 sigma) and 75.2% of the channels within CrIS radiometric uncertainty, noting though that HIRAS is noisier than CrIS with, on average, a standard deviation 0.34 K larger.