THE LANDSAT SCALE BREAK IN STRATOCUMULUS AS A 3-DIMENSIONAL RADIATIVE-TRANSFER EFFECT - IMPLICATIONS FOR CLOUD REMOTE-SENSING

Several studies have uncovered a break in the scaling properties of La ndsat cloud scenes at nonabsorbing wavelengths. For scales greater tha n 200-400 m, the wavenumber spectrum is approximately power law in k(- 5/3), but from there down to the smallest observable scales (50-100 m) follows another k(-beta) law with beta >3. This implies very smooth r adiance fields. The authors reexamine the empirical evidence for this scale break and explain it using fractal cloud models, Monte Carlo sim ulations, and a Green function approach to multiple scattering theory. In particular, the authors define the ''radiative smoothing scale'' a nd relate it to the characteristic scale of horizontal photon transpor t. The scale break was originally thought to occur at a scale commensu rate with either the geometrical thickness Delta(z) of the cloud, or w ith the ''transport'' mean free path l(t)=[(1-g)sigma](-1), which inco rporates the effect of forward scattering (sigma is extinction and g t he asymmetry factor of the phase Function). The smoothing scale is fou nd to be approximately root l(t) Delta(z) at cloud top; this is the pr ediction of diffusion theory which applies when (1-g)tau=Delta(z)/l gr eater than or similar to 1 (tau is optical thickness). Since the scale break is a tangizble effect of net horizontal radiative fluxes excite d by the fluctuations of tau, the smoothing scale sets an absolute low er bound on the range where one can neglect these fluxes and use plane -parallel theory locally, even for stratiform clouds. In particular, t his constrains the retrieval of cloud properties from remotely sensed data. Finally, the characterization of horizontal photon transport sug gests a new lidar technique for joint measurements of optical and geom etrical thicknesses at about 0.5-km resolution.