Wide Angle Imaging Lidar: Validation for Clouds at Night and Advances Toward Aerosol and Daytime Capabilities

Cloud effects on climate are a major source of uncertainty and to better understand their role we have to better know their properties: structural (top and bottom altitudes, aspect ratio, internal stratification, turbulent mixing), microphysical (temperature, water phase, droplet or crystal size, precipitation, etc.), and optical (e.g., extinction and scattering coefficients, phase function). The Wide-Angle Imaging Lidar (WAIL) was developed at LANL to deliver several of these important characteristics. WAIL has a huge field of view by common lidar standards, 52º (subdivided currently into ≈(π/4)128² spatial pixels), and fully programmable time binning. WAIL "movies" of optically thick clouds are produced, one "frame" at a time, by exposing for a few minutes total a gated/intensified CCD detector to highly-scattered 532 nm photons originating from the 0.5 mJ laser pulses produced at a 4 kHz rep rate. The result is the remotely observable part of the cloud's Green function for a boundary source [Davis et al., 1999]. We demonstrate that this new signal can be accurately modeled using photon transport theory in the diffusion approximation. By weighting and nonlinear fitting to observations, this forward model also provides us with a simple and efficient technique for retrieving certain cloud properties.

WAIL was deployed as a part of multi-instrument campaign in March 2002 at the ARM Climatge Research Facility in Oklahoma. A more recent analysis [Polonsky et al., 2005] of the collected data demonstrated good accuracy for the retrieved cloud parameters, presently, geometrical and optical thicknesses as well as the classic laser ceilometry product, cloud base height. Using the WAIL data, we find: cloud top and bottom altitudes at ±20%, and the cloud extinction coefficient at ±6%. This makes WAIL an efficient and cost-effective solution for probing optically thick clouds. In view for certain suboptimal settings during the 2002 campaign, this is a worse-case accuracy for nighttime operation. We have investigated mitigating strategies for the solar background noise that complicates daytime operation [Love et al., 2002]. We are currently upgrading the photon diffusion model to incorporate, hence access in retrieval mode, the in-cloud stratification of the extinction coefficient. We thus plan to restore full profiling capability for lidar remote sensing in the presence of a dense cloud layer, a situation where it was previously reduced to an active ceilometry method.

Finally, the imaging capabilities of WAIL offer the possibility to explore the aerosol layer situated between the probed cloud and the sensor, or under clear skies for that matter. Similarly to how it has been exploited in Multiple Field of View (MFoV) lidar [Bissonnette et al., 2002], the angular dependence of the signal allows us to determine aerosol characteristics such as extinction coefficient and phase function as a function of altitude. In the case of WAIL however, much larger zenith angles can be accessed and the analytical small-angle approximation is replaced by a numerical model.

References:

Bissonnette, L., G. Roy, L. Poutier, S. Cober, and G. Isaac, 2002: Multiple-Scattering Lidar Retrieval Method: Tests on Monte Carlo Simulations and Comparisons with in situ Measurements, Appl. Opt., 41, 6307-6324.

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).

Love, S. P., A. B. Davis, C. A. Rohde, L. Tellier, and C. Ho, 2002: Active Probing of Cloud Multiple Scattering, Optical Depth, Vertical Thickness, and Liquid Water Content using Wide-Angle Imaging Lidar, S.P.I.E. Proceedings, vol. 4815: "Atmospheric Radiation Measurements and Applications in Climate," J. A. Shaw (ed.), pp. 129-138.

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.