The development of next generation hyperspectral microwave radiometers for the planetary boundary layer observation

The 2017-2027 Decadal Survey for Earth Science and Applications from Space (ESAS 2017) [1] has recommended the development of new technologies, retrieval approaches and observational techniques for remotely sensing the Planetary Boundary Layer (PBL) with an unprecedented sensitivity. The panel has identified the accurate observation of PBL as one of the top ranked Earth science questions to better advance our understanding to the topography of the earth’s surface.

The high-resolution measurements of PBL are essential toward improving the state of numerical weather prediction (NWP) models and climate forecast sensitivity. The current passive microwave systems are not optimized for near surface sensing. The current operational sounders only sample about few percent of the microwave spectrum below 200 GHz and significantly sub-sample the boundary layer atmospheric temperature and water vapor structure. Most all of the boundary layer information is contained in the spectrum between the strong absorption lines in the so-called window regions. Software defined spectrometers have been identified as a key technology enabling high resolution spectral sampling (<1MHz) for sensing of the fine high-altitude absorption lines with lower spectral resolution (1GHz) wide bandwidth spectromters filling in the window regions. Such a sounder adds re-configurability on the ground or even in flight that we expect will lead to lower development costs and increased resiliency for our sounding systems as well as reduce profile retrieval errors ([2] and [3]).

In this talk, we will describe two radiometer technology development and demonstration activities ongoing at JPL NASA/Caltech that leverage software defined spectrometer capabilities. The first is a wide bandwidth three channel airborne atmospheric microwave sounder (AMS) to be installed onto the NOAA G550 aircraft and operating near the two Oxygen (O₂) absorption lines at 60 and 118 GHz and the water vapor absorption line at 183 GHz [Fig.1a]. The second is the development of a high-altitude balloon (~20 km) instrument based on the existing high-frequency airborne microwave and millimeter-wave radiometer (HAMMR-HD) [Fig.1b] [4] for acquisition of up to 30 days of continuous data. The key innovations from existing microwave sounders are increasing the number of channels around the oxygen and water-vapor absorption lines with high-resolution spectral sampling and wide-coverage to further improve accuracy of observation at the boundary layer. The hyperspectral microwave concept leads to a multiplexed system with high number of Intermediate Frequency (IF) window channels. The HAMMR balloon experiment will provide a rich data set sampling the full microwave spectrum from 18-200 GHz over a variety of atmospheric scenes. The resulting data will be valuable for testing our ability to extract PBL structure from observed microwave spectra and support mission/instrument concept development for future PBL focused missions at NASA.

References

[1] "Thriving on Our Changing Planet: A Decadal Strategy for Earth Observation from Space," The National Academics Press, Science, Engineering, and Medicine, 2018.

[2] J.-F. Mahfouf, C. Birman, F. Aires, O. E. Prigent and M. Milz, "Information content on temperature and water vapor from a hyper-spectral microwave sensor," Quarterly Journal of the Royal Meteorological Society, vol. 141, no. 693, pp. 3268-3284, 2015.

[3] F. Aires, C. Prigent, E. Orlandi, M. Milz, P. Eriksson, S. Crewell, C.-C. Lin and V. Kangas, "Microwave hyperspectral measurements for temperature and humidity atmospheric profiling from satellite: The clear‐sky case," Geophisical Research: Atmospherers, vol. 120, no. 21, pp. 11-334, 2015.

[4] X. Bosch-Lluis et al., "Instrument design and performance of the high-frequency airborne microwave and millimeter-wave radiometer", IEEE J. Sel. Topics Appl. Earth Observ. Remote Sens., vol. 12, no. 11, pp. 4563-4577, Nov. 2019.