A case for more multiple scattering lidar from space: Analysis of four LITE pulses returned from a marine stratocumulus deck

The Lidar-In-space Technology Experiment (LITE) was in many respects a ground-breaking NASA mission on Space Shuttle Discovery (STS-64) in September 9–20, 1994 [1]. It blazed the path for lidar systems in space, current (GLAS/ICESat, Caliop/CALIPSO) and future (ICESat-2, ADM-Aeolus, EarthCARE, ASCENDS, DESDINYL, ACE). One of LITE's lesser known technological feats was to be the first functional multiple scattering lidar system in space. Paradoxically, it preceded—by almost a decade—both ground-based [2] and airborne [3] demonstrations on a thick stratus cloud of opportunity at the DOE ARM site in Oklahoma on the night of March 25, 2003.

This success resulted from its use of a relatively wide Field-Of-View (FOV = 3.5 mrad), yielding a 900 m footprint, and a low-gain setting on the photomultiplier tube (PMT). This configuration was used during LITE's nighttime orbit #135 overflying a typical extended marine stratocumulus deck on 9/18/1994 (see figure). As expected, strong returns were recorded, most of them saturating the receiver's PMT in spite of the low-gain setting. Less expected was the stretch of the tails in the returned waveforms. What information about the clouds, beyond top height, do these signals convey? Or, as some observers first suggested, are these tails just instrumental artifacts?

Fortunately for the technology demonstration, the former hypothesis proved true. Around the same time period, early research was being performed on unconventional lidar signal modeling by the small and cohesive MUltiple SCattering Lidar Experiment (MUSCLE) community, which had by then broadened its focus from aerosol layers to optically thick clouds. This body of largely numerical work [4] was soon supplemented by analytical arguments based on the phenomenology of photon diffusion, including time-dependence, in such dense optical media as clouds [5]. Although originally motivated by passive cloud remote sensing in the solar spectrum [6], this analysis showed that both geometrical and optical cloud thicknesses determine the basic signal physics.

A preliminary search uncovered at least four unsaturated waveforms in close proximity in the 532 nm pulse train for orbit #135. These data were analyzed only recently in full detail using state-of-the-art analytical (time-domain diffusion) and numerical (Monte Carlo) methods. At the Symposium, we will report on the results of several cloud property retrievals, targeting both geometrical and optical thicknesses. The self-consistency of the outcomes is remarkable. Compatibility with the climatology of these cloud types is equally important. In particular, optical depths of 10–20 are recovered, almost an order of magnitude beyond the nominal lidar limit of ≈3 (determined by the requirement of still having detectable light after a two-way transmission).

The take-home message here is that the technology for space-based multiple-scattering/wide-FOV lidar for cloud studies is mature and signal analysis methods are at par [e.g., 7]. Should we design/propose such a system to complement future mm-wave radar probes of dense clouds? Or does this concept come to late? (It is of course not even mentioned in the Decadal Survey, which down-selected and prioritized NASA's future missions based on broad science needs.)

Indeed, at mm-wavelengths the conventional radar/lidar equation is often adequate, but sometimes questionable microphysical assumptions are required to translate radar reflectivities into climate-relevant information in the thermal and solar regions. We believe that that the proposition of new multiple-scattering lidar from space is in fact timely. An optical lidar system with a FOV wide enough to admit most of the multiply-scattered laser light would add robustness—and more immediate climate relevance—to even the most advanced mm-wave cloud radar systems under consideration for EarthCARE and ACE.

Pragmatically, it would however be unwise to push for a stand-alone cloud lidar mission for a multiple-scattering/wide-FOV instrument, in spite of the relatively high TRL. By contrast, it is very reasonable to advocate a "wide-FOV" channel for cloud purposes at the focal plane of a future lidar mission that will likely be aerosol-driven. As always since LITE, a major engineering focus has indeed been to make the receiver FOV as narrow as possible in order to boost the SNR in cloud-free columns, and thus enhance aerosol signatures. As long as the additional capability is kept in mind through the earliest stages of the design, its incremental cost would likely be quite small compared to the whole mission, and the potential return for cloud remote sensing science will be well worth it.

Indeed, a seamless transition from aerosols to clouds and back from a single remote sensing instrument will clearly support atmospheric scientists addressing the conundrum of cloud-aerosol-radiation-precipitation-climate interactions, particularly, for optically thick boundary-layer clouds. We hope to elicit at the 2010 AMS Annual Meeting a strong interest in this proven, but largely overlooked, observational technology.

References:

1. Winker, D.M., R.H. Couch, and M.P. McCormick, 1996: An overview of LITE: NASA's Lidar In-space Technology Experiment, Proc. IEEE, 84, 164-180.
2. Polonsky, I.N., S.P. Love, and A.B. Davis, 2005: The Wide-Angle Imaging Lidar (WAIL) Deployment at the ARM Southern Great Plains Site: Intercomparison of Cloud Property Retrievals, J. Atmos. and Oceanic Techn., 22, 628-648.
3. Cahalan, R.F., M.J. McGill, J. Kolasinski, T. Várnai, and K. Yetzer, 2005: THOR, cloud THickness from Offbeam lidar Returns, J. Atmos. and Oceanic Techn., 22, 605-627.
4. Flesia, C., and P. Schwendimann (Eds.), 1995: Special section on MUltiple SCattering in Lidar Experiments (MUSCLE). Applied Physics B - Lasers and Optics, B60, pp. 315-362.
5. Davis, A.B., R.F. Cahalan, J.D. Spinhirne, M.J. McGill, and S.P. Love, 1999: Off-beam lidar: An emerging technique in cloud remote sensing based on radiative Green-function theory in the diffusion domain, Phys. Chem. Earth (B), 24, 177-185 (Erratum 757-765).
6. Davis, A., A. Marshak, R.F. Cahalan, and W.J. Wiscombe, 1997: The LANDSAT scale-break in stratocumulus as a three-dimensional radiative transfer effect, Implications for cloud remote sensing, J. Atmos. Sci., 54, 241-260.
7. Hogan, R.J., and A. Battaglia, 2008: Fast lidar and radar multiple-scattering models, Part 2: Wide-angle scattering using the time-dependent two-stream approximation, J. Atmos. Sci., 65, 3636-3651.