The disparate cloud size distributions obtained from aircraft and satellite observations

The central question in this study is whether simultaneous satellite observations (2D) and aircraft observations (1D) of a cumulus cloud field (essentially 3D) in principle give rise to a similar cloud size distribution, and if not, what causes this disparity.

Starting point for this study is the paper by Rodts et al. (2003) in which it was shown that aircraft measurements over Florida (the SCMS field experiment) were strongly biased towards small clouds in comparison to LANDSAT satellite observations of the region. For both aircraft and satellite measurements Rodts et al. calculated the cloud size that dominated the cloud cover and found the resulting sizes to differ by an order of magnitude. The problem, however, is that the definition of a "cloud" differs between the two measurement methods, impeding strong conclusions to be drawn from the results.

To mitigate these experimental difficulties, in this study we resort to large-eddy simulations (LES) of cumulus cloud fields. We simulate both the satellite observations (vertical projection of the cloud field) and the aircraft observations (large series of line measurements) and determine in both cases the observed cloud size distribution, as well as the cloud size that dominates the cloud cover. The simulations reproduce the large disparity between the satellite/aircraft cloud size distributions as found in the observations. Since the "real" cloud size is known in the simulations, we find that the aircraft cloud statistics is indeed significantly biased to smaller cloud sizes, whereas the satellite cloud statistics are biased to larger sizes. We show that these properties can be attributed to the particular (fractal) geometry of cumulus clouds.

S. Rodts, P. Duynkerke and H. Jonker, Size distributions and dynamical properties of shallow cumulus clouds from aircraft observations and satellite data, Journal of the Atmospheric Sciences, 2003, 60, pp. 1895-1912