Clear-sky temperature and water vapor retrievals utilizing collocated microwave and infrared hyperspectrometers

Accurately monitoring atmospheric temperature and water vapor profiles with high spatial and temporal resolutions is crucial for weather forecasting and climate research. Hyperspectral radiance measurements offer a promising opportunity to retrieve these profiles due to the distinct absorption features of various atmospheric compositions.

Three clear-sky field campaigns were conducted in Ottawa to collect collocated hyperspectral measurements from two sophisticated instruments: the High Spectral Resolution Airborne Microwave Sounder (HiSRAMS) and the Atmospheric Emitted Radiance Interferometer (AERI). HiSRAMS operates within the microwave oxygen band (49.6-58.3 GHz) and microwave water vapor band (175.9-184.6 GHz), while AERI operates in the infrared spectral range (520-1800 cm^-1). Both instruments possess high spectral resolution, enabling the detection of subtle changes in temperature and water vapor profiles. Radiosonde measurements were simultaneously taken during each field campaign to serve as the truth for this study.

Initially, we performed a simultaneous assessment of the radiometric accuracy of HiSRAMS and AERI through radiative closure tests. A persistent warm radiance bias was detected in AERI observations in the window band. Correcting this bias improved radiative closure within the same band. HiSRAMS observations, when directed towards the nadir, displayed a smaller brightness temperature bias compared to zenith observations. We diagnosed and compared the radiometric accuracy of both instruments based on the relationship between radiometric bias and optical depth. HiSRAMS exhibited similar radiometric accuracy to AERI in nadir-pointing measurements but demonstrated comparatively poorer accuracy in zenith-pointing measurements, necessitating further characterization.

Subsequently, clear-sky temperature and water vapor concentration profiles were successfully retrieved from collocated HiSRAMS flight measurements and AERI ground measurements. These retrieved profiles were validated against radiosonde measurements, demonstrating good agreement. When both instruments were positioned on the ground for zenith-pointing measurements, infrared hyperspectral measurements provided higher information content and better vertical resolution for temperature and water vapor retrievals compared to microwave hyperspectral measurements. Combining airborne nadir-pointing microwave measurements and ground-based zenith-pointing infrared measurements, termed the “sandwich” sounding approach, exhibited increased information content and reduced retrieval uncertainty for temperature and water vapor concentrations across all retrieval levels.