Long-term Infrared Sounder-based Temperature and Humidity Profiles

Temperature and humidity profiles for climate applications are derived based on NOAA polar orbiting satellites' High-resolution Infrared Radiation Sounder (HIRS) observations. Global HIRS observations from more than a dozen satellites during the 1978-2021 time period are inter-calibrated to a base satellite (Metop-2) to form a temporally homogeneous time series. A retrieval scheme is designed using a neural network technique to derive temperature profiles at standard pressure levels from the surface to lower stratosphere and humidity profiles from the surface to upper troposphere. The retrieval algorithm includes a two-tiered cloud screening method and bias-correction using radiosonde and Global Positioning System Radio Occultation (GPS RO) measurements. As atmospheric profiles over high surface elevations can differ significantly from those over low elevations, different neural networks are developed for two classifications of surface elevations. The significant impact from the increase of carbon dioxide in the last several decades on HIRS temperature sounding channel measurements is accounted for in the retrieval scheme. The cloud screening method incorporates Advanced Very High Resolution Radiometer (AVHRR) observations to assess the likelihood of cloudiness in HIRS pixels. Calibrating the retrievals with radiosonde and GPS RO reduces biases in retrieved temperature and humidity. The stability of the intersatellite time series is evaluated by examining sixteen pairs of satellites carrying the HIRS instrument with time periods that overlap. Correlation coefficients were calculated for the retrieval of each atmospheric pressure level and for each satellite pair. The HIRS instrument is being replaced by a new generation of hyperspectral sounders. Progress on work to extend the time series using Infrared Atmospheric Sounding Interferometer (IASI) measurements to simulate and derive HIRS-like data for the 12 HIRS longwave channels will also be presented.