Radiosondes are important for calibrating satellite sensors and assessing sounding retrievals. Vaisala RS41 radiosondes have mostly replaced RS92 in the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) and the conventional network. This study assesses RS41 and RS92 upper tropospheric humidity (UTH) accuracy by comparing with Infrared Atmospheric Sounding Interferometer (IASI) upper tropospheric water vapor absorption spectrum measurements. Using single RS41 and RS92 soundings at three GRUAN and DOE Atmospheric Radiation Measurement (ARM) sites and dual RS92/RS41 launches at three additional GRUAN sites, collocated with cloud-free IASI radiances (OBS), we compute Line-by-Line Radiative Transfer Model radiances for radiosonde profiles (CAL). We analyze OBS-CAL differences from 2015 to 2020, for daytime, nighttime, and dusk/dawn separately if data is available, for standard (STD) RS92 and RS41 processing, and RS92 GRUAN Data Processing (GDP; RS41 GDP is in development). We find that daytime RS41 (even without GDP) has ~1% smaller UTH errors than GDP RS92. RS41 may still have a dry bias of 1–1.5% for both daytime and nighttime, and a similar error for nighttime RS92 GDP, while standard RS92 may have a dry bias of 3–4%. These sonde humidity biases are probably upper limits since “cloud-free” scenes could still be cloud contaminated. Radiances computed from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses match better than radiosondes with IASI measurements, perhaps because ECMWF assimilates IASI measurements. Relative differences between RS41 STD and RS92 GDP, or between radiosondes and ECMWF humidity profiles obtained from the radiance analysis, are consistent with their differences obtained directly from the RH measurements.

Abstract. This study examines the trustworthiness

Abstract. The launch of ENVISAT in 2002 and the launch of MetTop-A in 2006 put two highly accurate instruments in space to measure Top of Atmosphere (TOA) radiances. These instruments are the AATSR and IASI. While the AATSR, by design is a climate accurate (i.e. accuracy within 0.1 K and stability within 0.05 K dec−1) instrument, the IASI is a hyperspectral instrument that has a stated accuracy of within 0.5 K. This accuracy and stability are used in producing climate CDR's from these instruments and also aids in using these instruments as benchmarks for inter-comparison studies that aim at measuring stability and accuracy of instruments that are concurrently flying with them. The GSICS (Global Space Based Inter-Calibration System) has extensively exploited the IASI by comparing its measurements with Polar as well as Geostationary satellite instruments and measuring the in-orbit stability and accuracy of these instruments. More recent re-calibration efforts, such as the NOAA CDR project that is aimed at recalibrating the AVHRR uses the IASI and the AATSR as references. However to trust the recalibrated radiances it is vital that the in-orbit accuracy of the reference sources is known and critical issues such as scan angle dependence, and temporal variation of the accuracy are fully evaluated across a large temperature range (200–300 K). In order to better understand the accuracy and asses the trustworthiness of these references we present here a comprehensive analysis of the AATSR–IASI bias derived from their collocated pixels, over the period January 2008 through March 2011. Our analysis indicates that generally the AATSR (Nadir View) and IASI can act as good reference instruments and IASI is much more accurate than its design specification. In fact, taking into account a small bias the AATSR–IASI bias is close to the AATSR pre-launch bias implying that IASI can get close to pre-launch levels of accuracy. We also examine temperature dependent bias in the AATSR at low (< 240 K) temperatures which seems to appear after orbit was lowered of the ENVISAT satellite and its inclination control was discontinued. In addition, a very small scan angular dependence of AATSR–IASI bias indicates that the AVHRR has a scan angle dependent bias. We also examine the bias problem with the 12 μm channel of the AATSR in detail. We show that this bias not only has a temperature dependence (it grows up to 0.4 K at low temperatures) but also has a seasonal dependence in the SST (265–300 K) temperature range and is highly correlated to instrument temperature in the cold temperature range. We then discuss a possible method to correct the 11 and the 12 μm bias so as to use the corrected radiances for re-calibration of AVHRR.