Oxygen A-band Spectroscopy: An Overlooked Resource for Ground-Based Inference of Physical and Radiative Properties of Clouds

It is less risky to infer climatically-relevant properties of clouds by remote sensing using modalities that exploit climatically-important wavelengths. For instance, to study the energy budget a retrieval of cloud optical depth performed in the solar spectrum is more credible than one that uses microwaves, largely because it is sensitive to the 2nd moment of the particle size distribution (PSD) that determines scattering properties---hence cloud reflectivity and transmitivity. On the other hand, estimates of cloud and precipitable water paths based on passive microwave radiometry are more appropriate for hydrological cycle studies, largely because they are primarily sensitive to the key 3rd-order moment of the PSD. Although highly attractive due to superior spatial resolution, mm-wave cloud radar delivers the 6th moment of the PSD, which is not of any immediate use. This active measurement can be processed into information about the desired lower-order PSD moments, but at the cost of making assumptions about the cloud microphysics that may sometimes be questionable. From this risk management standpoint, we will argue that the O₂ A-band spectroscopy (759--771~nm) is an under-exploited resource in cloud remote sensing that can constrain retrievals of cloud optical depth or pressure thickness from ground stations such as the US DOE's ARM facilities. In other words, it should work well as a cloud remote sensing asset in synergy with more common ground-based instrumentation, including multi-spectral shortwave radiometers, hyper-spectral thermal IR spectrometers, multi-channel microwave radiometers, and mm-wave radars. But O₂ A-band can bring to the table more unique information about clouds. At high enough spectral resolution, A-band spectra have been shown to respond strongly to deviations from the single/unbroken cloud layer scenario, i.e., fully 3D clouds. In particular, A-band has the surprising capability (for a passive sensor) of detecting the presence of more than one cloud layer of significance to the solar energy budget. This feature also ensures that the above-mentioned retrievals of single-layer cloud properties are reliable. As the spatial complexity of the cloudiness increases, ground-based O₂ A-band spectroscopy morphs from a 1D (quasi-slab geometry) cloud remote sensing tool into a fully 3D cloud radiative property diagnostic tool. In this new capacity, O₂ A-band observations can be brought to bear on the challenging issue of accurately predicting mean shortwave heating rates over large domains, as required in GCM parameterizations of fast physics. Specifically, the performance of GCM cloud and/or shortwave radiation schemes can be evaluated against O₂ A-band data, once separated in to their broadband spectral integration and spatial radiation transport parts. In this presentation, we will (1) survey the published technical literature on the cloud remote sensing applications of O₂ A-band spectroscopy and (2) describe the emerging ideas of how it can be used to test some key GCM physics, namely, clouds and solar radiation, at the component level. Key differences between ground- and space-based observations will be highlighted.