Horizontal structure of marine boundary layer clouds from centimeter to kilometer scales

Horizontal transects of cloud liquid water content (LWC) measured at unprec edented 4-cm resolution are statistically analyzed scale-by-scale, The data were collected with a Particulate Volume Monitor (PVM) probe during the wi nter Southern Ocean Cloud EXperiment (SOCEX) on July 26, 1993, in a broken- stratocumulus/towering-cumulus cloud complex. Two scaling regimes are found in the sense that two distinct power laws, k(-beta), are needed to represe nt the wavenumber spectrum E(k) over the full range of scales r approximate to 1/k. Detailed numerical simulations show that the scale break at 2-5 m is not traceable to the normal variability of LWC in the PVM's instantaneou s sampling volume (1.25 cm(3)) driven by Poissonian fluctuations of droplet number and size. The two regimes therefore differ physically. The non-Pois sonian character of the small-scale LWC variability is consistent with a si milar finding by Baker [1992] for droplet number concentration obtained fro m Forward Scattering Spectrometer Probe (FSSP) data: at scales of a few cen timeters, spatial droplet distributions do not always follow a uniform Pois son law. With beta = 0.9 +/- 0.1, the small-scale (8-12 cm less than or sim ilar to r less than or similar to 2-5 m) regime is stationary: jumps in LWC are highly variable in size and rapidly cancel each other, leading to shor t-range correlations. By contrast, the large-scale (5 m less than or simila r to r less than or similar to 2 km) variability with beta = 1.6 +/- 0.1 is nonstationary: jumps are generally quite small, conveying a degree of pixe l-to-pixel continuity and thus building up long-range correlations in the l ow-pass filtered signal. The large-scale structure of the complex SOCEX clo ud system proves to be multifractal, meaning that large jumps do occur on a n intermittent basis, that is, on a sparse fractal subset of space. Low-ord er, hence more robust, multifractal properties of the SOCEX clouds are rema rkably similar to those of their First ISCCP Regional Experiment (FIRE) and Atlantic Stratocumulus Transition EXperiment (ASTEX) counterparts, and als o to those of passive scalars in fully developed turbulence. This is indica tive of a remarkable similarity in the microphysical and macrophysical proc esses that determine cloud structure in the marine boundary layer at very r emote locales, especially since the particular SOCEX cloud system investiga ted here was rather atypical. Interesting differences are also found: in th e scaling ranges on the one hand, and in higher-order moments on the other hand. Finally, we discuss cloud-radiative effects of the large- and small-s cale variabilities.