|
|
|
|
|
JOURNAL OF GEOPHYSICAL RESEARCH, 113, D14S10, doi:10.1029/2007JD009666, 2008.
Multiple-scattering lidar from both sides of the clouds: Addressing internal structure.
Anthony B. Davis
Abstract
Multiple-scattering (a.k.a. "off-beam") lidar is an emerging technology in cloud
remote sensing. It delivers, as in classic lidar ceilometry, cloud base altitude but also the
cloud’s physical thickness H as well as its optical depth t (averaged over horizontal
scales on the order of H). The value of t in fact must lie beyond the range accessible by
standard (i.e., single-scattering/on-beam) lidar profiling, namely, up to 3 – 4. A refined
diffusion-theoretical model is presented here for signals from multiple-scattering lidar and
applied, on the one hand, to retrieval algorithm development and, on the other hand,
signal-to-noise ratio (SNR) estimation. SNRs are computed for LANL’s ground-based
Wide-Angle Imaging Lidar (WAIL) system and NASA’s space-based Lidar-In-space
Technology Experiment (LITE). The refinements are threefold and all about internal
structure. First, the laser source is modeled as a collimated anisotropic exponentially
distributed internal source rather than an isotropic point source at the cloud boundary; this
opens the possibility of using d-Eddington rescaling to capture the forward peaked phase
function more effectively within the diffusion framework. Second, stratification of the
scattering coefficient is modeled as an increasing function of distance to cloud base; this
strongly differentiates the signals when observed from above or from below. Finally,
Cairns’ rescaling is applied to this conservative scattering problem to account for the
systematic effects of random (turbulence-driven) internal variability at scales up to a few
mean free paths.
| |
|
