The landsat scale break in stratocumulus as a three-dimensional radiative transfer effect: Implications for cloud remote sensing
- Science Systems and Applications, Inc., Lanham, MD (United States)
- NASA/Goddard Space Flight Center, Greenbelt, MD (United States)
Several studies have uncovered a break in the scaling properties of Landsat cloud scenes at nonabsorbing wavelengths. For scales greater than 200-400 m, the wavenumber spectrum is approximately power law in k{sup {minus}5/3}, but from there down to the smallest observable scales (50-100 m) follows another k{sup {minus}{beta}} law with {beta} > 3. This implies very smooth radiance fields. The authors reexamine the empirical evidence for this scale break and explain it using fractal cloud models. Monte Carlo simulations, and a Green function approach to multiple scattering theory. In particular, the authors define the {open_quotes}radiative smoothing scale{close_quotes} and relate it to the characteristic scale of horizontal photon transport. The scale break was originally thought to occur at a scale commensurate with either the geometrical thickness {Delta}{sub z} of the cloud, or with the {open_quotes}transport{close_quotes} mean free path l{sub t} = [(1 {minus} g){sigma}]{sup {minus}1}, which incorporates the effect of forward scattering ({sigma} is extinction and g the asymmetry factor of the phase function). The smoothing scale is found to be approximately {radical}l{sub 1}{Delta}{sub z} at cloud top; this is the prediction of diffusion theory which applies when (1 {minus} g){tau} = {Delta}{sub z}/l{sub 1} {ge} 1 ({tau} is optical thickness). Since the scale break is a tangible effect of net horizontal radiative fluxes excited by the fluctuations of {tau}, the smoothing scale sets an absolute lower bound on the range where one can neglect these fluxes and use plane-parallel theory locally, even for stratiform clouds. In particular, this constrains the retrieval of cloud properties from remotely sensed data. Finally, the characterization of horizontal photon transport suggest a new lidar technique for joint measurements of optical and geometrical thicknesses at about 0.5-km resolution. 53 refs., 14 figs., 1 tab.
- OSTI ID:
- 530540
- Journal Information:
- Journal of the Atmospheric Sciences, Vol. 54, Issue 2; Other Information: PBD: 15 Jan 1997
- Country of Publication:
- United States
- Language:
- English
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