Planetary Boundary Layer Sensing, Information Content, And The Next Generation Of Ultra-wideband Passive Microwave Sensor

The 2017 National Academies Decadal Survey (ESAS17) has identified PBL measurements as a new frontier for remote sensing, as it is one of the most poorly modeled and measured regions of the atmosphere yet a critical geophysical component for energy exchange. Its proximity to the surface and rapid air property changes make PBL a challenging measurement from space.

Passive microwave systems are critical for a PBL observing system as they provide thermodynamic information in both cloudy and clear air. Traditional passive microwave systems suffer from limited bandwidths, center frequency, spectral resolution resulting in sub-optimal observation of atmospheric variables near the surface. These systems miss much of the boundary layer information encoded in the wider microwave spectrum.

Boundary layer temperature and water vapor structure is observed in the microwave spectrum around oxygen and water vapor absorption lines, and microwave spectrum's window regions adjacent to these lines contain near-surface thermodynamic data. Variations in boundary layer temperature/water vapor structure and layer depth are uniquely reflected in the spectral shape of the top-of-atmosphere related to the pressure/temperature where the water vapor is concentrated.

Many studies demonstrate the value from improved spectral resolution along the wings of oxygen and water vapor absorption lines in terms of information content (Mahfouf et al. (2015), Aires et al. (2015)). Despite vertical resolution limitations of microwave radiometry, the ultra-wideband microwave system offers maximum measurement bandwidth and information content.

Ultra-wideband width microwave radiometer systems can provide optimal measurements required for hard-to-measure atmospheric retrievals. This recommendation was also made in the recently released NASA PBL white paper study. The key enabling technology to allow ultra-wideband measurements is the combination of low-noise wide band RF radiometer subsystems with RF Photonics backend sub-systems, allowing never achieved before ability to spectrally resolve the complete microwave lines. These wideband systems will be able to parse out the thermodynamic information encoded within the spectral shapes of the microwave spectrum.

The full system when developed will be able to sample from 20-200 GHz, with the photonic backend spectrometer capable of ingesting sets of 40 GHz bands and channelize them at 1 GHz. Select 1 GHz bands can be further channelized to a much higher resolution (< 10 MHz) where required (around water vapor lines or oxygen lines). The instrument will fully-resolve the shape and magnitude of the window and sounding channel spectrum.

We will present the current instrument development status and discuss the ability to maximize retrieved information from the ultra-wideband sensor.

This research was partially carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. (c) 2023. California Institute of Technology. Government sponsorship acknowledged.